
\section{Introduction} \label{sec:intro}
The solar corona dynamically expands into interplanetary space in the form of the continuous solar wind \citep{Parker1958,Coleman1966}, the birth and the acceleration mechanism of which are still not well understood \citep{Tu2005,He2007,Cranmer2019}. The solar wind flowing into interplanetary space carries information about its source region, and on the other hand, involves diversified nonlinear physical processes \citep{Tu1995,Bruno2013}. It is essential to investigate the nature of near-sun fluctuations in order to analyze and understand these nonlinear physical processes as well as the heating and acceleration mechanisms of solar wind.

The statistical properties of the solar wind generally vary with speed, location and type of source region and heliocentric distance \citep{Bavassano1982,Tu1989,He2013,Matteini2014,Stansby2018,Horbury2018,Wang2019,Perrone2019,Bandyopadhyay2020,Chen2020,Chhiber2020,Duan2020,Qudsi2020}. \citet{Tu1989} contrast the properties of MHD turbulence between high speed and low speed solar wind at 0.3AU using the spectra of Els\"asser variables, cross helicity, residual energy, Alfv\'en ratio and Els\"asser ratio. They consider that, compared to the high speed wind, the turbulence evolves in an advanced state in slow wind, due to the longer expansion time. The $z^+$ and $z^-$ are close to a balanced state with an approximate -1.67 spectral index. The mode composition therein is dominated by the Alfv\'en mode and slow mode in the limit of incompressibility \citep{Dobrowolny1980}. \citet{Bavassano1982} study the variation of the nature of the fluctuations with heliocentric distance and scale in the trailing edge of a stream interaction region. The anisotropy defined with respect to the direction corresponding to the minimum eigenvalue decreases as the heliocentric distance increases and the scale decreases. The magnetic field closer to the sun is more compressed. However, \citet{Chen2020} report the evolution of solar wind turbulence from 0.17AU to 1AU, recently, and find at 0.17AU that: (1) the spectra of magnetic field, velocity and Els\"asser variables present a -3/2 slope at MHD scales; (2) the magnetic field is less compressed and (3) the outward propagating Alfv\'en waves are more dominant than at 1AU. Fast solar wind is characterised by highly Alfv\'enic fluctuations, although a new type of Alfv\'enic slow solar wind, possibly coming from quiet-Sun regions or coronal-hole boundaries, has been reported at distances from 0.3 AU to 1 AU. \citep{D'Amicis2015,Wang2019,Perrone2020,Parashar2020}. It is of interest to study this kind of solar wind, on account of its distinct properties, which differ from the classical slow solar wind. 

Even if we relax the assumption of a pure superposition of linear waves, nonlinear turbulent fluctuations still retain certain polarization and correlation properties of linear modes \citep{Tu1995}. When we use the term 'wave', we refer to the mode composition of the fluctuations within this wave-turbulence paradigm. The composition of wave modes in the solar wind near 1 AU has been extensively studied and controversially discussed. There are many means to diagnose the wave modes: correlation analysis between velocity and magnetic field fluctuations \citep{Wang2012,Safrankova2019}, cross helicity analysis \citep{Roberts1987}, comparison of the MHD dispersion relations derived from measurements with theory predictions \citep{Shi2015}, and mode recognition methods \citep{Glassmeier1995,Narita2015,Chaston2020}. According to these studies, non-compressive outward Alfv\'en modes dominate the fluctuations in the solar wind especially in fast streams \citep{Bruno2013}. Compressive waves likely suffer strong Landau damping \citep{Barnes1966}, resulting in their suppression in the overall fluctuations. Correlations among variables (e.g. magnetic pressure, thermal pressure, density and temperature), show that the compressive component simultaneously exists of magnetosonic waves and pressure balanced structures (PBSs) \citep{Kellogg2005,Yao2011,Yang2017}. The majority of the compressive fluctuations is slow-mode-like rather than fast-mode-like in polarization \citep{Howes2012,He2015,Shi2015}. In situ observations show that the Alfv\'enicity decreases with heliocentric distance, which might be caused by the increased contribution of inward propagating Alfv\'en waves or the compressive fluctuations \citep{Bruno1993}.

To further comprehend the underlying multi-scale nature and evolution of near-sun turbulence, we systematically study the variation of the fluctuations' properties with scale and heliocentric distance within 0.3AU. The properties include the propagation direction of the wave, the mode composition, and the characteristic of anisotropy on average. In Section \ref{sec:data}, we briefly introduce the data sets we use. In Section \ref{sec:result}, we present our methods and analysis results, and give our summary and conclusions in Section \ref{sec:sum}. Our observations can provide observational evidence for the verification of existing theoretical models at closer heliocentric distances, and also impose constraints on the improvement of existing theoretical models and the proposal of new models.


\section{ Data Sets and Data Deduction} \label{sec:data}
We conduct our analysis using the data obtained from {\em Parker Solar Probe} (PSP), which is the closest human-built satellite to the sun up to now \citep{Fox2016}. We use the Level-2 magnetic field data supplied by the Flux-gate Magnetometer (MAG; \cite{Bale2016}) and the Level-3i proton data provided by the Solar Probe Cup (SPC; \cite{Kasper2016}). The time interval investigated spans from UTC2018-10-31/20:00:00 to UTC2018-11-10/15:00:00 in which period PSP cruised between 0.166AU (35.78 solar radii)and 0.243AU (56.37 solar radii). The interval we choose is shorter than the high-cadence interval around the first perihelion, because there are several sampling gaps longer than 30 minutes in the other intervals from which the SPC data are unavailable. We analyze time periods of the fluctuations in the range from 10s to 1000s, corresponding to MHD scales in the plasma frame. We do not exclude the so-called `switchback' patterns that exist among various scales (see \cite{Bale2019,Kasper2019,Dudokdewit2020}).

For the analysis of propagation direction and fluctuation anisotropy, we use the Singular Value Decomposition (SVD) method to resolve the frequencies and wavevectors of the waves based on Faraday's law. We estimate the three singular values of the spectral matrix (Eq.(8) of \citet{Santolik2003}), based on the principle of divergence-free magnetic field. We estimate the electric field at MHD scales for our SVD analysis as $E=-V_i\times B$ \citep{Shi2015}, where $V_i$ is the proton bulk velocity obtained from SWEAP/SPC. Note that only one wavevector is solved for every specific frequency with the SVD method. Therefore, the resolved frequency and wavevector can be regarded as the frequency and wavevector of the major wave mode. In reality, it is possible that multiple wave modes exist in the turbulence at the same time and scale. For the mode composition analysis, we use the method suggested by \citet{Glassmeier1995}, and get the contributions of the six MHD modes (parallel and anti-parallel propagating Alfv\'en mode, fast mode and slow mode) to the fluctuations. We estimate the spectral energy density of each mode as $e_i^TS(f_{sc},t)e_i$, where $S(f_{sc},t)$ is the spectral density matrix as defined by \citet{Glassmeier1995} and $e_i$ is the eigenvector of the corresponding mode.

\begin{figure}
\centerline{\includegraphics[width=20cm, clip=]{Figure1.eps}}
\caption{Time sequences overview of magnetic and plasma measurements during PSP's first encounter. Panel (a): magnetic field strength ($|B|$) and proton density ($N_{\rm p}$). Panel (b\&c\&d): magnetic fields ($B_{\rm R}, B_{\rm T}, B_{\rm N}$) and proton bulk velocities ($V_{\rm R,p}, V_{\rm T,p}, V_{\rm N,p}$) in RTN coordinates. Panel (e): proton density ($N_{\rm p}$) and thermal velocity ($W_{\rm p}$). Panel (f): plasma beta ($\beta$) and heliocentric distance ($R$) of spacecraft's position.
\label{fig:fig1}}
\end{figure}

\section{Analysis Results} \label{sec:result}
We present an overview of the observed magnetic field and plasma measurements in Figure \ref{fig:fig1}. To highlight the correlated fluctuations of the variables over such a long duration of about 10 days, we smooth all the measurements with a running window of 30 min. Figure 1(a) shows that the proton density ($N_{\rm p}$) and the magnetic field strength ($|B|$) decrease with increasing heliocentric distance. The three components of the magnetic field ($B_{\rm R}, B_{\rm T}, B_{\rm N}$) and the proton velocity ($V_{\rm R,p}, V_{\rm T,p}, V_{\rm N,p}$) in the RTN coordinates are positively correlated, respectively, which suggests that the large-scale outward-propagating Alfv\'enic fluctuations are dominant during this encounter. The proton thermal velocity ($W_{\rm p}$) varies between 50 km/s and 100 km/s and there is no global correlation between the proton density and the thermal velocity. The plasma beta ($\beta_{\rm p}$) is around 2, which does not exhibit a significant variation with heliocentric distance ($R$).

We solve the wavevector, $\bm{k}(\tau, R(t))$, at different heliocentric distances ($R(t)$) and periods ($\tau$), where $R$ is a function of time ($t$). The local background magnetic field, $\bm{B_0}(\tau, R(t))$, is acquired by Gaussian-weighting of the magnetic-field time series at time $t$, where the width of the Gaussian profile is defined by the period $\tau$ \citep{Podesta2009}. We then calculate the angles between $\bm{k}$ and $\bm{B_0}$, $\theta_{{\rm k,B_0}}(\tau, R(t))$. Figure \ref{fig:fig2} (a1), (b1) and (c1) show the probability distribution functions (PDFs) of wavevectors in three distance ranges. For $kd_{\rm i}>0.02$, the wavevectors cluster around the quasi-perpendicular direction. For $kd_{\rm i}<0.02$, the most probable wavevectors are quasi-parallel, relative to the local background magnetic field. Figure \ref{fig:fig2} (a2), (b2) and (c2) show the PDFs of $\theta_{\rm k,B_0}$ depending on $kd_{\rm i}$, in three different distance ranges. The propagation angles are close to $160^{\circ}$ for $kd_{\rm i}<0.02$, and close to $90^{\circ}$ for $kd_{\rm i}<0.02$. This indicates that the propagation angles are scale-dependent and turn from quasi-parallel at large scales to quasi-perpendicular at small scales.

\begin{figure}
\centerline{\includegraphics[width=20cm, clip=]{Figure2.eps}}
\caption{Panel (a1\&b1\&c1): Probability distribution functions of the wavevector in $|k_{\parallel}d_{\rm i}|-|k_{\perp}d_{\rm i}|$ space in the range of 0.180AU-0.185AU, 0.209AU-0.214AU, and 0.238AU-0.243AU, respectively. The white solid lines represent the relation between $k_{\parallel}$ and $k_{\perp}$ as predicted from the phenomenology of critical balance, $k_{\parallel}\sim k_{\perp}^{\frac{2}{3}}k_0^{\frac{1}{3}}$, where $k_0$ is the wavenumber of the outer scale. Panel(a2\&b2\&c2): PDFs of the propagation angle ($\theta_{{\rm k,B_0}}$) at differing scales ($kd_{\rm i}$), in the corresponding distance ranges. 
\label{fig:fig2}}
\end{figure}

We carry out a mode composition diagnosis \citep{Glassmeier1995} and directly obtain the fractions of the six MHD wave modes, at different heliocentric distances and periods. According to the radial component of the local background magnetic field, $B_{0r}=\bm{B_0}\cdot\bm{\hat{e}_r}$, we transform the parallel and anti-parallel modes into outward/anti-sunward modes when $\bm{k}\cdot\bm{B_0}>0$, and inward/sunward modes when $\bm{k}\cdot\bm{B_0}<0$, respectively. The variation results of the averaged fractions of the transformed MHD modes are shown in Figure \ref{fig:fig3}. The upper three panels show the variation of the fractions of the MHD modes with period averaged over the distance ranges of 0.180AU-0.185AU, 0.209AU-0.214AU and 0.238AU-0.243AU, respectively. The wave mode occupying the highest spectral proportion is the outward Alfv\'en mode at most scales in these three $R$-intervals. The outward fast mode, the inward Alfv\'en mode and the outward slow mode represent the modes with the lowest fractional proportions throughout the whole MHD range at these distances. The fractional proportions of these three modes slightly increase with increasing distance. On average, from 0.180AU to 0.185AU, the inward fast mode is the second-most abundant mode, while the inward slow mode is in third place. From 0.209AU to 0.214AU, the inward slow mode and the inward fast mode have approximately equivalent proportions. From 0.238AU to 0.243AU, the inward slow mode is in second place, followed by the inward fast mode. We also find this change of mode composition with distance in the radial variation of the period-averaged fraction of mode compositions (see lower panel of Figure \ref{fig:fig3}). The outward Alfv\'en mode dominates throughout the whole near-Sun region under investigation. The fractional contribution of the fast mode decreases with increasing distance, while the contribution from inward slow modes increase with distance.

\begin{figure}
\plotone{Figure3.eps}
\caption{(Top) The period-depending variation of the spectral fractions of the six MHD modes i.e., outward/anti-sunward ({\it solid line}) and inward/sunward ({\it dashed line}) propagating Alfv\'en modes ({\it green}), fast modes ({\it blue}) and slow modes ({\it red}), as averaged over different distance ranges: 0.180AU-0.185AU ({\it Left}), 0.209AU-0.214AU ({\it Middle}) and 0.238AU-0.243AU ({\it Right}), respectively. (Bottom) The heliocentric distance variation of the spectral fractions of the six MHD modes as averaged over the time scale (period) from 10 to 1000s. The lime shadow sections correspond to the distance ranges used for the averaging of the intervals in the upper three panels.
\label{fig:fig3}}
\end{figure}

To further verify this composition diagnosis results, we reconstruct the dispersion relations of Alfv\'en waves and slow waves, as shown in Figure \ref{fig:fig4}. We first demonstrate a benchmark test to verify the ability of the SVD method to resolve the MHD dispersion relations. The preset basic parameters are: bulk velocity, $V_0=400{\rm km/s}$, background magnetic field, $B_0=90{\rm nT}$, proton number density, $n_p=300{\rm cm^{-3}}$, proton thermal velocity, $60{\rm km/s}$, $\theta_{{\rm k,B_0}}=20^{\circ}$. Based on the polarization relations of the Alfv\'en mode and slow mode, we set up the corresponding magnetic field and velocity disturbances of the two modes respectively, and use these disturbances as an artificial data input of the SVD method. In order to test the robustness of the SVD method, we also add 0.1\% level of noise for each wave at all scales to the virtual data input. As a result, we obtain a solution in terms of the wavevector ($kd_{\rm i}$) at every frequency ($\omega/\omega_{\rm ci}$) and during every local time interval, and $\omega_{{\rm ci}}$ is the ion cyclotron frequency. Furthermore, we construct the PDF($kd_{\rm i}$, $\omega/\omega_{\rm ci}$) statistically based on the information of $kd_{\rm i}$($\omega/\omega_{\rm ci}$, $t$). The PDFs($kd_{\rm i}$, $\omega/\omega_{\rm ci}$) for the benchmark tests of the Alfv\'en mode and slow mode are illustrated in Figure \ref{fig:fig4}(a) and \ref{fig:fig4}(b). The dispersion relations as indicated by the ridges with high PDF values are fully consistent with the theoretical dispersion relations, which means that the SVD method is well able to resolve the MHD dispersion relations from our observations. Thereafter, we apply the SVD method to the observational measurements to examine whether the dispersion relations of Alfv\'en waves and slow waves prevail. The results are shown in Figure \ref{fig:fig4}(c) and Figure \ref{fig:fig4}(d). The green patches corresponding to high levels of PDF concentrate on and around the theoretical dispersion relations of MHD Alfv\'en and slow modes. These results further confirm the existence of incompressible Alfv\'en waves (the most prevalent component) and compressible slow waves (the sub-dominat component).

\begin{figure}
\plotone{Figure4.eps}
\caption{Panel (a)\&(b): The PDFs of the normalized wavenumbers, $kd_{\rm i}$, for Alfv\'en waves and slow waves, at each normalized angular frequency, $\omega/\omega_{{\rm ci}}$ in the plasma frame, resolved by a benchmark test of the SVD method, with MHD Alfv\'en-mode and MHD slow-mode fluctuations. The theoretical MHD dispersion relations of Alfv\`en mode, fast mode and slow mode are plotted in black, blue and red solid lines, respectively. Panel(c)\&(d): The PDFs of $kd_{\rm i}$ for Alfv\'en waves and slow waves, at each $\omega/\omega_{{\rm ci}}$, obtained from application of the SVD method to the magnetic and velocity measurements from PSP in [20:00, 21:00] UT on 2018-11-05 (panel c) and [18:20, 18:25] UT on 2018-11-04 (panel d), consistent with the dispersion relations of Alfv\'en and slow modes, respectively. Unlike in panels (a) and (b), we use the averaged plasma parameters over the corresponding time intervals in panels (c) and (d).
\label{fig:fig4}}
\end{figure}

Lastly, we investigate the variation of the fluctuation anisotropy with period and distance in Figure \ref{fig:fig5}. The ratio of the middle and maximum singular values of the spectral matrix (Eq.(8) of \citet{Santolik2003}), $\lambda_{{\rm mid}}/\lambda_{{\rm max}}$, is adopted to represent the anisotropy of the magnetic field fluctuations in the plane perpendicular to the propagation direction, which we take to be oriented along the direction with the minimum singular value $\lambda_{{\rm min}}$. $\lambda_{{\rm mid}}/\lambda_{{\rm max}}$ is also known as the ellipticity. The ratio increases from around 0.3 to over 0.37 as the wave period increases, throughout the distance range under investigation. According to the above analysis, the dominant Alfv\'en mode increases in its degree of circular or arc polarization with increasing period.

\begin{figure}
\plotone{Figure5.eps}
\caption{(a) The distance profiles of the ratio between the middle and maximum singular values of the magnetic spectral matrix ($\lambda_{{\rm mid}}/\lambda_{{\rm max}}$) for the magnetic fluctuations at different periods from 10 to 1000 s. (b) The variations of $\lambda_{{\rm mid}}/\lambda_{{\rm max}}$ with period for the magnetic fluctuations at different distances from 0.17 to 0.24 AU.
\label{fig:fig5}}
\end{figure}

\section{Conclusion}\label{sec:sum}
The diversity, complexity and evolution of solar wind turbulence have always been important research topics in heliospheric physics. Hence, we statistically study mode propagation, mode composition, and fluctuation anisotropy of the solar wind MHD turbulence as measured by PSP. We find that:

(1) At 0.166AU$<$R$<$0.243AU: The propagation angles ($\theta_{{\rm k,B_0}}$) of wave-like turbulent fluctuations for $kd_{\rm i}<0.02$ are greater than $135^{\circ}$, mainly concentrating around $160^{\circ}$, while the distribution gradually shifts its center to $\theta_{{\rm k,B_0}}\sim90^{\circ}$ for $0.02<kd_{\rm i}<0.1$.

(2) The distance variations of the scale-averaged fractions of the MHD modes show that: (a) the outward/anti-sunward propagating Alfv\'en mode dominates the mode composition throughout the whole investigated  range of distances, while the outward slow mode, the inward/sunward Alfv\'en mode and the outward fast mode represent the three smallest proportions; (b) the fraction of the inward fast mode decreases with distance, whereas the fraction of the inward slow mode increases with distance; (c) at 0.166AU$<$R$<$0.215AU, the inward fast mode takes the second place and the inward slow mode is in third place; at 0.215AU$<$R$<$0.243AU, the inward slow mode is in third place followed by the inward fast mode.

(3) The ellipticity increases with spacecraft-frame period in the heliocentric distance range studied.

According to the critical balance hypothesis \citep{Goldreich1995,Horbury2008}, the anisotropy of the power spectrum follows from the condition that the nonlinear time and the propagation time are approximately equal ($k_{\parallel}V_A\sim k_{\perp}\delta v$) in strong MHD turbulence with balanced Els\"asser fluxes. However, in solar wind turbulence with imbalanced fluxes dominated by outward Alfv\'en waves, our probability distribution function of wave propagation in $k_{\parallel}-k_{\perp}$ space (see color maps in Figure \ref{fig:fig2})) is inconsistent with this prediction of critical balance theory (see white solid lines in Figure \ref{fig:fig2}). For $kd_{\rm i}<0.02$, the most probable wavevector is more parallel, while for $kd_{\rm i}>0.02$, the most probable wavevector is closer to the quasi-perpendicular direction. This observational result will help to enlighten and promote the theory of turbulence anisotropy characterized by a transition of propagation direction from quasi-parallel to quasi-perpendicular with a large angular jump at a certain scale. After integrating the ideas of both "slab+2D" and "critical balance" scenarios, an upgraded turbulence phenomenology in Fourier space was proposed to involve "quasi-parallel wavelike fluctuations" and "quasi-2D fluctuations" as well as energy transfer between them and within themselves \citep{Oughton2015}. The observed transition from quasi-parallel to quasi-perpendicular propagation with increasing wavenumber shows a way how to improve the turbulence model in the future.

The transition of the dominant outward Alfv\'en mode from $\theta_{{\rm k,B_0}}\sim160^{\circ}$ to $\theta_{{\rm k,B_0}}\sim90^{\circ}$, as the period decreases from 1000s to 10s, may also indicates the geometry of the Alfv\'en waves at kinetic scales in the near-sun solar wind. Quasi-perpendicular Alfv\'en waves are more likely to dominate at scales closer to ion scale. Accordingly, quasi-perpendicular modes (e.g. kinetic Alfv\'en waves) may participate in the turbulent cascade and further dissipation, energizing and shaping the non-thermal ion distributions, which may develop temperature anisotropic and feed back to excite the ion-cyclotron waves reported during this interval \citep{Bowen2020}. In the future, we will study such chain of energy conversion process: damping of quasi-perpendicular kinetic waves $\longrightarrow$ energization of particles $\longrightarrow$ growth of quasi-parallel waves.

In some respect, our mode composition diagnosis results differ from the results of \citet{Chaston2020}. They study the spectral energy density fractions of six MHD modes inside and outside the field reversal regions, separately. They report that the three outward (anti-sunward) modes are dominant at MHD scales on average. This difference may lie in the calculation of the propagation angle, which is an input parameter of the mode-recognition method \citep{Glassmeier1995}. \citet{Chaston2020} obtained the propagation direction using the spectral matrix of the magnetic field only as suggested by \citet{Samson1980}, while we use both the magnetic and the electric field based on the Faraday's law \citep{Santolik2003}. This aspect may lead to the different results of mode composition.

The ellipticity serves here as an indicator to distinguish if the polarization is circular ($\lambda_{{\rm mid}}/\lambda_{{\rm max}}\sim1$), arc ($0<\lambda_{{\rm mid}}/\lambda_{{\rm max}}<1$), or linear ($\lambda_{{\rm mid}}/\lambda_{{\rm max}}\sim0$). The ellipticity increases with the period of the fluctuations from 0.3 to 0.37, which indicates that the magnetic fluctuations tend to be more and more circular-polarized as the period increases. Based on Figure \ref{fig:fig2}, the waves are also quasi-parallel propagating at larger periods. This observation is consistent with the prediction that the Alfv\'en branch of the MHD solutions is circular-polarized when $\theta_{{\rm k,B_0}}\sim0^{\circ}$.

\section{Acknowledgements}
This work at Peking University (PKU) is supported by NSFC under contracts 41574168, 41674171, 41874200, and 41421003. The team from PKU is also supported by CNSA under contract Nos. D020301 and D020302. D.V. is supported by the STFC Ernest
Rutherford Fellowship ST/P003826/1 and STFC Consolidated Grant ST/S000240/1. S.D.B. acknowledges the
support of the Leverhulme Trust Visiting Professorship program. The authors acknowledge the contributions of the Parker Solar Probe mission operations and spacecraft engineering teams at the Johns Hopkins University Applied Physics Laboratory as well as the FIELDS and SWEAP teams for use of the data. PSP data is available on SPDF (https://cdaweb.sci.gsfc.nasa.gov/index.html/).




\section{Introduction} \label{sec:intro}

\latex\ \footnote{\url{http://www.latex-project.org/}} is a document markup
language that is particularly well suited for the publication of
mathematical and scientific articles \citep{lamport94}. \latex\ was written
in 1985 by Leslie Lamport who based it on the \TeX\ typesetting language
which itself was created by Donald E. Knuth in 1978.  In 1988 a suite of
\latex\ macros were developed to investigate electronic submission and
publication of AAS Journal articles \citep{1989BAAS...21..780H}.  Shortly
afterwards, Chris Biemesdefer merged these macros and more into a \latex\
2.08 style file called \aastex.  These early \aastex\ versions introduced
many common commands and practices that authors take for granted today.
Substantial revisions
were made by Lee Brotzman and Pierre Landau when the package was updated to
v4.0.  AASTeX v5.0, written in 1995 by Arthur Ogawa, upgraded to \latex\ 2e
which uses the document class in lieu of a style file.  Other improvements
to version 5 included hypertext support, landscape deluxetables and
improved figure support to facilitate electronic submission.  
\aastex\ v5.2 was released in 2005 and introduced additional graphics
support plus new mark up to identifier astronomical objects, datasets and
facilities.

In 1996 Maxim Markevitch modified the AAS preprint style file, aaspp4.sty,
to closely emulate the very tight, two column style of a typeset
Astrophysical Journal article.  The result was emulateapj.sty.  A year
later Alexey Vikhlinin took over development and maintenance.  In 2001 he
converted emulateapj into a class file in \latex\ 2e and in 2003 Vikhlinin
completely rewrote emulateapj based on the APS Journal's RevTEX class.

During this time emulateapj gained growing acceptance in the astronomical
community as it filled an author need to obtain an approximate number of
manuscript pages prior to submission for cost and length estimates. The
tighter typeset also had the added advantage of saving paper when printing
out hard copies.

Even though author publication charges are no longer based on print pages
\footnote{see Section \ref{sec:pubcharge} in the Appendix for more details
about how current article costs are calculated.} the emulateapj class file
has proven to be extremely popular with AAS Journal authors.  An 
analysis of submitted \latex\ manuscripts in 2015 revealed that $\sim$65\%
either called emulateapj or have a commented emulateapj classfile call
indicating it was used at some stage of the manuscript construction.
Clearly authors want to have access to a tightly typeset version of the
article when corresponding with co-authors and for preprint submissions.

When planning the next \aastex\ release the popularity of emulateapj played
an important roll in the decision to drop the old base code and adopt and
modify emulateapj for \aastex\ v6.+ instead.  The change brings \aastex\
inline with what the majority of authors are already using while still
delivering new and improved features.  \aastex\ v6.0 through v6.3 were
written by Amy Hendrickson. The release dates were January 2016 (v6.0),
October 2016 (v6.1), January 2018 (v6.2), and June 2019 (v6.3),
respectively.

The new features in the recent releases includes:

\begin{itemize}
 \item{v6.0}
   \begin{enumerate}
      \item line numbering and watermarking,
      \item improved citations for third party data repositories and software,
      \item easier construction of matrix figures consisting of multiple 
encapsulated postscript (EPS) or portable document format (PDF) files,
      \item figure set mark up for large collections of similar figures,
      \item color mark up to easily enable/disable revised text highlighting,
      \item improved url support, and
      \item numerous table options such as the ability to hide columns, column decimal alignment, automatic column math mode and numbering, plus splitting of wide tables (see Section \ref{subsec:tables} for details).
    \end{enumerate}
  \item{v6.1}
    \begin{enumerate}
      \item ORCID support for preprints,
      \item improved author, affiliation and collaboration mark up,
      \item reintroduced the old AASTeX v5.2 {\tt\string\received}, {\tt\string\revised}, {\tt\string\accepted}, and {\tt\string\published} commands plus added the new {\tt\string\submitjournal} command to document which AAS Journal the manuscript was submitted to, plus
      \item new typeset style options including {\tt\string modern}.
    \end{enumerate}
  \item{v6.2}
    \begin{enumerate}
      \item A new RNAAS style option for Research Note manuscripts,
      \item Titles no longer put in all caps,
      \item No page skip between the title page and article body,
      \item re-introduce RevTeX's widetext environment for long lines in two column style formats, and
      \item upgrade to the {\tt\string\doi} command.
    \end{enumerate}
  \item{v6.3}
    \begin{enumerate}
      \item New {\tt\string interactive} environment to highlight interactive figures (see Section \ref{animation}),
      \item Improved collaboration commands, 
      \item New {\tt\string anonymous} style to keep the authors, affiliations and acknowledgments from showing in the compiled pdf for dual anonymous review, and
      \item Adoptions of IAU approved syntax for nominal units, see Section \ref{nominal}.
    \end{enumerate}
\end{itemize}

The rest of this article provides information and examples on how to create
your own AAS Journal manuscript with v6.3.  Special emphasis is placed on
how to use the full potential of \aastex\ v6+.  The next section describes
the different manuscript styles available and how they differ from past
releases.  Section \ref{sec:floats} describes table and figure placement. 
Specific examples of tables, Section
\ref{subsec:tables}, and figures, Section \ref{subsec:figures}, are also
provided. A special emphasis is placed on interactive figures.
Section \ref{sec:displaymath} discusses how to display math and
incorporate equations in a manuscript while Section \ref{sec:highlight}
discuss how to use different ways to highlight revisions.  The last section,
\ref{sec:cite}, shows how recognize software and external data as first
class references in the manuscript bibliography.  An appendix is included
to show how to construct one and provide some information on how article
charges are calculated.  Additional information is available both embedded
in the comments of this \latex\ file and in the online documentation at
\url{http://journals.aas.org/authors/aastex.html}.

\section{Manuscript styles} \label{sec:style}

The default style in \aastex\ v6.3 is a tight single column style, e.g.  10
point font, single spaced.  The single column style is very useful for
article with wide equations. It is also the easiest to style to work with
since figures and tables, see Section \ref{sec:floats}, will span the
entire page, reducing the need for address float sizing.

To invoke a two column style similar to the what is produced in
the published PDF copy use \\

\noindent {\tt\string\documentclass[twocolumn]\{aastex63\}}. \\

\noindent Note that in the two column style figures and tables will only
span one column unless specifically ordered across both with the ``*'' flag,
e.g. \\

\noindent{\tt\string\begin\{figure*\}} ... {\tt\string\end\{figure*\}}, \\
\noindent{\tt\string\begin\{table*\}} ... {\tt\string\end\{table*\}}, and \\
\noindent{\tt\string\begin\{deluxetable*\}} ... {\tt\string\end\{deluxetable*\}}. \\

\noindent This option is ignored in the onecolumn style.

Some other style options are outlined in the commented sections of this 
article.  Any combination of style options can be used.

Two style options that are needed to fully use the new revision tracking
feature, see Section \ref{sec:highlight}, are {\tt\string linenumbers} which 
uses the lineno style file to number each article line in the left margin and 
{\tt\string trackchanges} which controls the revision and commenting highlight
output.

There is also a new {\tt\string modern} option that uses a Daniel
Foreman-Mackey and David Hogg design to produce stylish, single column
output that has wider left and right margins. It is designed to have fewer
words per line to improve reader retention. It also looks better on devices
with smaller displays such as smart phones.

For a Research Note use the {\tt\string RNAAS} option. This will produce a
manuscript with no abstract and in the {\tt\string modern} style.

The {\tt\string anonymous} option will prevent the author and affiliations
from being shown in the compiled pdf copy. This option allows the author 
to keep this critical information in the latex file but prevent the reviewer
from seeing it during peer review if dual anonymous review is requested. 
Likewise, acknowledgments can also be hidden if placed after the 
{\tt\string\acknowledgments} command.

\section{Floats} \label{sec:floats}

Floats are non-text items that generally can not be split over a page.
They also have captions and can be numbered for reference.  Primarily these
are figures and tables but authors can define their own. \latex\ tries to
place a float where indicated in the manuscript but will move it later if
there is not enough room at that location, hence the term ``float''.

Authors are encouraged to embed their tables and figures within the text as
they are mentioned.  Please do not place the figures and text at the end of
the article as was the old practice.  Editors and the vast majority of
referees find it much easier to read a manuscript with embedded figures and
tables.

Depending on the number of floats and the particular amount of text and
equations present in a manuscript the ultimate location of any specific
float can be hard to predict prior to compilation. It is recommended that
authors textbf{not} spend significant time trying to get float placement
perfect for peer review.  The AAS Journal's publisher has sophisticated
typesetting software that will produce the optimal layout during
production.

Note that authors of Research Notes are only allowed one float, either one
table or one figure.

For authors that do want to take the time to optimize the locations of
their floats there are some techniques that can be used.  The simplest
solution is to placing a float earlier in the text to get the position
right but this option will break down if the manuscript is altered.
A better method is to force \latex\ to place a
float in a general area with the use of the optional {\tt\string [placement
specifier]} parameter for figures and tables. This parameter goes after
{\tt\string \begin\{figure\}}, {\tt\string \begin\{table\}}, and
{\tt\string \begin\{deluxetable\}}.  The main arguments the specifier takes
are ``h'', ``t'', ``b'', and ``!''.  These tell \latex\ to place the float
\underline{h}ere (or as close as possible to this location as possible), at
the \underline{t}op of the page, and at the \underline{b}ottom of the page.
The last argument, ``!'', tells \latex\ to override its internal method of
calculating the float position.  A sequence of rules can be created by
using multiple arguments.  For example, {\tt\string \begin\{figure\}[htb!]}
tells \latex\ to try the current location first, then the top of the page
and finally the bottom of the page without regard to what it thinks the
proper position should be.  Many of the tables and figures in this article
use a placement specifier to set their positions.

Note that the \latex\ {\tt\string tabular} environment is not a float.  Only
when a {\tt\string tabular} is surrounded by {\tt\string\begin\{table\}} ...
{\tt\string\end\{table\}} is it a true float and the rules and suggestions
above apply.

In AASTeX v6.3 all deluxetables are float tables and thus if they are
longer than a page will spill off the bottom. Long deluxetables should
begin with the {\tt\string\startlongtable} command. This initiates a
longtable environment.  Authors might have to use {\tt\string\clearpage} to
isolate a long table or optimally place it within the surrounding text.

\subsection{Tables} \label{subsec:tables}

Tables can be constructed with \latex's standard table environment or the
\aastex's deluxetable environment. The deluxetable construct handles long
tables better but has a larger overhead due to the greater amount of
defined mark up used set up and manipulate the table structure.  The choice
of which to use is up to the author.  Examples of both environments are
used in this manuscript. 

Tables longer than 200 data lines and complex tables should only have a
short example table with the full data set available in the machine
readable format.  The machine readable table will be available in the HTML
version of the article with just a short example in the PDF. Authors are
required to indicate in the table comments that the data in machine 
readable format in the full article.
Authors are encouraged to create their own machine
readable tables using the online tool at
\url{http://authortools.aas.org/MRT/upload.html}.

\aastex\ v6 introduced five new table features that were designed to make
table construction easier and the resulting display better for AAS Journal
authors.  The items are:

\begin{enumerate}
\item Declaring math mode in specific columns,
\item Column decimal alignment, 
\item Automatic column header numbering,
\item Hiding columns, and
\item Splitting wide tables into two or three parts.
\end{enumerate}

Full details on how to create each type are given in the following 
sections. Additional details are available in the AASTeX
guidelines at \url{http://journals.aas.org/authors/aastex.html}

\subsubsection{Column math mode}

Both the \latex\ tabular and \aastex\ deluxetable require an argument to
define the alignment and number of columns.  The most common values are
``c'', ``l'' and ``r'' for \underline{c}enter, \underline{l}eft, and
\underline{r}ight justification.  If these values are capitalized, e.g.
``C'', ``L'', or ``R'', then that specific column will automatically be in math
mode meaning that \$s are not required.  Note that having embedded dollar
signs in the table does not affect the output. 

\subsubsection{Decimal alignment}

Aligning a column by the decimal point can be difficult with only center,
left, and right justification options.  It is possible to use phantom calls
in the data, e.g. {\tt\string\phn}, to align columns by hand but this can
be tedious in long or complex tables.  To address this \aastex\ introduces
the {\tt\string\decimals} command and a new column justification option,
``D'', to align data in that column on the decimal.  In deluxetable the
{\tt\string\decimals} command is invoked before the {\tt\string\startdata}
call but can be anywhere in \latex's tabular environment.  

Two other important thing to note when using decimal alignment is that each
decimal column \textit{must end with a space before the ampersand}, e.g.
``\&\&'' is not allowed.  Empty decimal columns are indicated with a decimal,
e.g. ``.''.  Do not use deluxetable's {\tt\string\nodata} command.

The ``D'' alignment token works by splitting the column into two parts on the
decimal.  While this is invisible to the user one must be aware of how it
works so that the headers are accounted for correctly.  All decimal column
headers need to span two columns to get the alignment correct. This can be
done with a multicolumn call, e.g {\tt\string\multicolumn2c\{\}} or
{\tt\string\multicolumn\{2\}\{c\}\{\}}, or use the new
{\tt\string\twocolhead\{\}} command in deluxetable.  Since \latex\ is
splitting these columns into two it is important to get the table width
right so that they appear joined on the page.  You may have to run the
\latex\ compiler twice to get it right.  

\subsubsection{Automatic column header numbering} \label{subsubsec:autonumber}

The command {\tt\string\colnumbers} can be included to automatically number
each column as the last row in the header. Per the AAS Journal table format
standards, each column index numbers will be surrounded by parentheses. In
a \latex\ tabular environment the {\tt\string\colnumbers} should be invoked
at the location where the author wants the numbers to appear, e.g. after
the last line of specified table header rows. In deluxetable this command
has to come before {\tt\string\startdata}.  {\tt\string\colnumbers} will
not increment for columns hidden by the ``h'' command, see Section
\ref{subsubsec:hide}. 

Note that when using decimal alignment in a table the command 
{\tt\string\decimalcolnumbers} must be used instead of 
{\tt\string\colnumbers} and {\tt\string\decimals}. 

\subsubsection{Hiding columns} \label{subsubsec:hide}

Entire columns can be \underline{h}idden from display simply by changing
the specified column identifier to ``h''.  In the \latex\ tabular environment
this column identifier conceals the entire column including the header
columns.   In \aastex's deluxetables the header row is specifically
declared with the {\tt\string\tablehead} call and each header column is
marked with {\tt\string\colhead} call.  In order to make a specific header
disappear with the ``h'' column identifier in deluxetable use 
{\tt\string\nocolhead} instead to suppress that particular column header.

Authors can use this option in many different ways.  Since column data can
be easily suppressed authors can include extra information and hid it
based on the comments of co-authors or referees.  For wide tables that will
have a machine readable version, authors could put all the information in
the \latex\ table but use this option to hid as many columns as needed until
it fits on a page. This concealed column table would serve as the
example table for the full machine readable version.  Regardless of how
columns are obscured, authors are responsible for removing any unneeded
column data or alerting the editorial office about how to treat these
columns during production for the final typeset article.

Table \ref{tab:messier} provides some basic information about the first ten
Messier Objects and illustrates how many of these new features can be used
together.  It has automatic column numbering, decimal alignment of the
distances, and one concealed column.  The Common name column
is the third in the \latex\ deluxetable but does not appear when the article
is compiled. This hidden column can be shown simply by changing the ``h'' in
the column identifier preamble to another valid value.  This table also
uses {\tt\string\tablenum} to renumber the table because a \latex\ tabular
table was inserted before it.

\begin{deluxetable*}{cchlDlc}
\tablenum{1}
\tablecaption{Fun facts about the first 10 messier objects\label{tab:messier}}
\tablewidth{0pt}
\tablehead{
\colhead{Messier} & \colhead{NGC/IC} & \nocolhead{Common} & \colhead{Object} &
\multicolumn2c{Distance} & \colhead{} & \colhead{V} \\
\colhead{Number} & \colhead{Number} & \nocolhead{Name} & \colhead{Type} &
\multicolumn2c{(kpc)} & \colhead{Constellation} & \colhead{(mag)}
}
\decimalcolnumbers
\startdata
M1 & NGC 1952 & Crab Nebula & Supernova remnant & 2 & Taurus & 8.4 \\
M2 & NGC 7089 & Messier 2 & Cluster, globular & 11.5 & Aquarius & 6.3 \\
M3 & NGC 5272 & Messier 3 & Cluster, globular & 10.4 & Canes Venatici &  6.2 \\
M4 & NGC 6121 & Messier 4 & Cluster, globular & 2.2 & Scorpius & 5.9 \\
M5 & NGC 5904 & Messier 5 & Cluster, globular & 24.5 & Serpens & 5.9 \\
M6 & NGC 6405 & Butterfly Cluster & Cluster, open & 0.31 & Scorpius & 4.2 \\
M7 & NGC 6475 & Ptolemy Cluster & Cluster, open & 0.3 & Scorpius & 3.3 \\
M8 & NGC 6523 & Lagoon Nebula & Nebula with cluster & 1.25 & Sagittarius & 6.0 \\
M9 & NGC 6333 & Messier 9 & Cluster, globular & 7.91 & Ophiuchus & 8.4 \\
M10 & NGC 6254 & Messier 10 & Cluster, globular & 4.42 & Ophiuchus & 6.4 \\
\enddata
\tablecomments{This table ``hides'' the third column in the \latex\ when compiled.
The Distance is also centered on the decimals.  Note that when using decimal
alignment you need to include the {\tt\string\decimals} command before
{\tt\string\startdata} and all of the values in that column have to have a
space before the next ampersand.}
\end{deluxetable*}

\subsubsection{Splitting a table into multiple horizontal components}

Since the AAS Journals are now all electronic with no print version there is
no reason why tables can not be as wide as authors need them to be.
However, there are some artificial limitations based on the width of a
print page.  The old way around this limitation was to rotate into 
landscape mode and use the smallest available table font
sizes, e.g. {\tt\string\tablewidth}, to get the table to fit.
Unfortunately, this was not always enough but now along with the hide column
option outlined in Section \ref{subsubsec:hide} there is a new way to break
a table into two or three components so that it flows down a page by
invoking a new table type, splittabular or splitdeluxetable. Within these
tables a new ``B'' column separator is introduced.  Much like the vertical
bar option, ``$\vert$'', that produces a vertical table lines 
the new ``B'' separator indicates where to \underline{B}reak
a table.  Up to two ``B''s may be included.

Table 2
shows how to split a wide deluxetable into three parts with
the {\tt\string\splitdeluxetable} command.  The {\tt\string\colnumbers}
option is on to show how the automatic column numbering carries through the
second table component, see Section \ref{subsubsec:autonumber}.

\begin{splitdeluxetable*}{lccccBcccccBcccc}
\tabletypesize{\scriptsize}
\tablewidth{0pt} 
\tablenum{5}
\tablecaption{Measurements of Emission Lines: two breaks \label{tab:deluxesplit}}
\tablehead{
\colhead{Model} & \colhead{Component}& \colhead{Shift} & \colhead{FWHM} &
\multicolumn{10}{c}{Flux} \\
\colhead{} & \colhead{} & \colhead{($\rm
km~s^{-1}$)}& \colhead{($\rm km~s^{-1}$)} & \multicolumn{10}{c}{($\rm
10^{-17}~erg~s^{-1}~cm^{-2}$)} \\
\cline{5-14}
\colhead{} & \colhead{} &
\colhead{} & \colhead{} & \colhead{Ly$\alpha$} & \colhead{N\,{\footnotesize
V}} & \colhead{Si\,{\footnotesize IV}} & \colhead{C\,{\footnotesize IV}} &
\colhead{Mg\,{\footnotesize II}} & \colhead{H$\gamma$} & \colhead{H$\beta$}
& \colhead{H$\alpha$} & \colhead{He\,{\footnotesize I}} &
\colhead{Pa$\gamma$}
} 
\colnumbers
\startdata 
{       }& BELs& -97.13 &    9117$\pm      38$&    1033$\pm      33$&$< 35$&$<     166$&     637$\pm      31$&    1951$\pm      26$&     991$\pm 30$&    3502$\pm      42$&   20285$\pm      80$&    2025$\pm     116$& 1289$\pm     107$\\ 
{Model 1}& IELs& -4049.123 & 1974$\pm      22$&    2495$\pm      30$&$<     42$&$<     109$&     995$\pm 186$&      83$\pm      30$&      75$\pm      23$&     130$\pm      25$& 357$\pm      94$&     194$\pm      64$& 36$\pm      23$\\
{       }& NELs& \nodata &     641$\pm       4$&     449$\pm 23$&$<      6$&$<       9$&       --            &     275$\pm      18$& 150$\pm      11$&     313$\pm      12$&     958$\pm      43$&     318$\pm 34$& 151$\pm       17$\\
\hline
{       }& BELs& -85 &    8991$\pm      41$& 988$\pm      29$&$<     24$&$<     173$&     623$\pm      28$&    1945$\pm 29$&     989$\pm      27$&    3498$\pm      37$&   20288$\pm      73$& 2047$\pm     143$& 1376$\pm     167$\\
{Model 2}& IELs& -51000 &    2025$\pm      26$& 2494$\pm      32$&$<     37$&$<     124$&    1005$\pm     190$&      72$\pm 28$&      72$\pm      21$&     113$\pm      18$&     271$\pm      85$& 205$\pm      72$& 34$\pm      21$\\
{       }& NELs& 52 &     637$\pm      10$&     477$\pm 17$&$<      4$&$<       8$&       --            &     278$\pm      17$& 153$\pm      10$&     317$\pm      15$&     969$\pm      40$&     325$\pm 37$&
     147$\pm       22$\\
\enddata
\tablecomments{This is an example of how to split a deluxetable. You can
split any table with this command into two or three parts.  The location of
the split is given by the author based on the placement of the ``B''
indicators in the column identifier preamble.  For more information please
look at the new \aastex\ instructions.}
\end{splitdeluxetable*}

\subsection{Figures\label{subsec:figures}}

\begin{figure}[ht!]
\plotone{cost.pdf}
\caption{The subscription (squares) and author publication (asterisks) 
costs from 1991 to 2013. Subscription cost are on the left Y axis while
the author costs are on the right Y axis. All numbers in US dollars and
adjusted for inflation. The author charges also account for the change
from page charges to digital quanta in April 2011.  \label{fig:general}}
\end{figure}

Authors can include a wide number of different graphics with their articles
but encapsulated postscript (EPS) or portable document format (PDF) are
encouraged. These range from general figures all authors are familiar with
to new enhanced graphics that can only be fully experienced in HTML.  The
later include figure sets, animations and interactive figures.  All
enhanced graphics require a static two dimensional representation in the
manuscript to serve as the example for the reader. All figures should
include detailed and descriptive captions.  These captions are absolutely
critical for readers for whom the enhanced figure is inaccessible either
due to a disability or offline access.  This portion of the article
provides examples for setting up all these types in with the latest version
of \aastex.

\subsection{General figures\label{subsec:general}}

\aastex\ has a {\tt\string\plotone} command to display a figure consisting
of one EPS/PDF file.  Figure \ref{fig:general} is an example which shows
the approximate changes in the subscription costs and author publication
charges from 1991 to 2013 in the AAS Journals.  For a general figure
consisting of two EPS/PDF files the {\tt\string\plottwo} command can be
used to position the two image files side by side.

Both {\tt\string\plotone} and {\tt\string\plottwo} take a
{\tt\string\caption} and an optional {\tt\string\figurenum} command to
specify the figure number\footnote{It is better to not use
{\tt\string\figurenum} and let LaTeX auto-increment all the figures. If you
do use this command you need to mark all of them accordingly.}.  Each is
based on the {\tt\string graphicx} package command,
{\tt\string\includegraphics}.  Authors are welcome to use
{\tt\string\includegraphics} along with its optional arguments that control
the height, width, scale, and position angle of a file within the figure.
More information on the full usage of {\tt\string\includegraphics} can be
found at \break
\url{https://en.wikibooks.org/wiki/LaTeX/Importing\_Graphics\#Including\_graphics}.

\subsection{Grid figures}

Including more than two EPS/PDF files in a single figure call can be tricky to
easily format.  To make the process easier for authors \aastex\ v6 offers
{\tt\string\gridline} which allows any number of individual EPS/PDF file
calls within a single figure.  Each file cited in a {\tt\string\gridline}
will be displayed in a row.  By adding more {\tt\string\gridline} calls an
author can easily construct a matrix X by Y individual files as a
single general figure.

For each {\tt\string\gridline} command a EPS/PDF file is called by one of
four different commands.  These are {\tt\string\fig},
{\tt\string\rightfig}, {\tt\string\leftfig}, and {\tt\string\boxedfig}.
The first file call specifies no image position justification while the
next two will right and left justify the image, respectively.  The
{\tt\string\boxedfig} is similar to {\tt\string\fig} except that a box is
drawn around the figure file when displayed. Each of these commands takes
three arguments.  The first is the file name.  The second is the width that
file should be displayed at.  While any natural \latex\ unit is allowed, it
is recommended that author use fractional units with the
{\tt\string\textwidth}.  The last argument is text for a subcaption.

Figure \ref{fig:pyramid} shows an inverted pyramid of individual
figure constructed with six individual EPS files using the
{\tt\string\gridline} option.

\begin{figure*}
\gridline{\fig{V2491_Cyg.pdf}{0.3\textwidth}{(a)}
          \fig{HV_Cet.pdf}{0.3\textwidth}{(b)}
          \fig{LMC_2009.pdf}{0.3\textwidth}{(c)}
          }
\gridline{\fig{RS_Oph.pdf}{0.3\textwidth}{(d)}
          \fig{U_Sco.pdf}{0.3\textwidth}{(e)}
          }
\gridline{\fig{KT_Eri.pdf}{0.3\textwidth}{(f)}}
\caption{Inverted pyramid figure of six individual files. The nova are
(a) V2491 Cyg, (b) HV Cet, (c) LMC 2009, (d) RS Oph, (e) U Sco, and (f) 
KT Eri. These individual figures are taken from \citet{2011ApJS..197...31S}.
\label{fig:pyramid}}
\end{figure*}

\subsection{Enhanced graphics}

Enhanced graphics have an example figure to serve as an example for the
reader and the full graphical item available in the published HTML article.
This includes Figure sets, animations, and interactive figures. The 
Astronomy Image Explorer (\url{http://www.astroexplorer.org/}) provides 
access to all the figures published in the AAS Journals since they offered
an electronic version which was in the mid 1990s. You can filter image
searches by specific terms, year, journal, or type. The type filter is 
particularly useful for finding all published enhanced graphics. As of
June 2019 there are over 3000 videos, 1000 figure sets, and 65 interactive
figures. The next sections describe how to include these types of graphics
in your own manuscripts.

\subsubsection{Figure sets}

The grid commands given above works great for a limited set of individual
figure files but what do you do if you have many 10s or 100s or even 1000s of
individual figure files? Figure sets represents a virtual flip book of a
large group of similar style figures.  The derived PDF article will only
shows an example figure while the enhanced content is available in the
figure set in the HTML edition.  The advantage of a figure set gives the
reader the ability to easily sort through a large collection to find
individual component figures.  The advantage to the author is that grouping
similar figures into a figure set can result in significant cost savings in
terms of reduced publication charges, see Appendix B. All of the figure set
components, along with their html framework, are also available to the reader
for download in a single .tar.gz package.

Special \latex\ mark up is required to create a figure set.  Prior to
\aastex\ v6 the underlying mark up commands had to be inserted by hand
but is now included.  Note that when an article with figure set is compiled
in \latex\ none of the component figures are shown and a floating Figure
Set caption will appear in the resulting PDF.

\figsetstart
\figsetnum{4}
\figsettitle{Swift X-ray light curves}

\figsetgrpstart
\figsetgrpnum{1.1}
\figsetgrptitle{KT Eri}
\figsetplot{KT_Eri.pdf}
\figsetgrpnote{The Swift/XRT X-ray light curve for the first year after
outburst of KT Eri.}
\figsetgrpend

\figsetgrpstart
\figsetgrpnum{1.2}
\figsetgrptitle{RS Oph}
\figsetplot{RS_Oph.pdf}
\figsetgrpnote{The Swift/XRT X-ray light curve for the first year after
outburst of RS Oph.}
\figsetgrpend

\figsetgrpstart
\figsetgrpnum{1.3}
\figsetgrptitle{U Sco}
\figsetplot{U_Sco.pdf}
\figsetgrpnote{The Swift/XRT X-ray light curve for the first year after
outburst of U Sco.}
\figsetgrpend

\figsetgrpstart
\figsetgrpnum{1.4}
\figsetgrptitle{V2491 Cyg}
\figsetplot{V2491_Cyg.pdf}
\figsetgrpnote{The Swift/XRT X-ray light curve for the first year after
outburst of V2491 Cyg.}
\figsetgrpend

\figsetgrpstart
\figsetgrpnum{1.5}
\figsetgrptitle{Nova LMC 2009}
\figsetplot{LMC_2009.pdf}
\figsetgrpnote{The Swift/XRT X-ray light curve for the first year after
outburst of nova LMC 2009.}
\figsetgrpend

\figsetgrpstart
\figsetgrpnum{1.6}
\figsetgrptitle{HV Cet}
\figsetplot{HV_Cet.pdf}
\figsetgrpnote{The Swift/XRT X-ray light curve for the first year after
outburst of HV Cet.}
\figsetgrpend

\figsetend

\begin{figure}
\plotone{KT_Eri.pdf}
\caption{The Swift/XRT X-ray light curve for the first year after
outburst of the suspected recurrent nova KT Eri. At a maximum count rate of 
328 ct/s, KT Eri was the brightest nova in X-rays observed to date. All 
the component figures (6) are available in the Figure Set. Note that
these components that are {\bf not} shown in the compiled pdf. The figure
set consists of the same figures as shown in Figure \ref{fig:pyramid}. 
The example figure shown for figure sets can be one component or many. 
\label{fig:fig4}}
\end{figure}

Authors are encouraged to use an online tool at
\url{http://authortools.aas.org/FIGSETS/make-figset.html} to generate their
own specific figure set mark up to incorporate into their \latex\ articles.

\subsubsection{Animations \label{animation}}

Authors may, and are in fact encouraged, to include animations in their
manuscripts. The video will stream inline with the published article and
also be available for download.  When writing the manuscript, a stand alone
figure is necessary to serve as an example for the reader.  Ideally, this
is a single still frame from the animation but in some case the animation
may only represent a small portion of the example figure, say one many
panels as shown in Figure \ref{fig:video}. Regardless, it is very
important that the author provide descriptive text in the figure caption
including start and stop times and the video duration. Authors should
review the AAS animation guidelines in the graphics guide at
\url{https://journals.aas.org/graphics-guide/#animations}.

\begin{figure}
\begin{interactive}{animation}{movie.mp4}
\plotone{f4.pdf}
\end{interactive}
\caption{Figure 1 from \citet{2018ApJ...868L..33L}. AIA 171\AA (a,b), 
AIA 131\AA (c), and AIA 304\AA images are shown. The red rectangle 
in (a) shows the field of view of the other panels. An animation of 
panels (b-d) is available. It covers 8 hours of observing beginning 
at 01:00 UT on 2012 January 19. The video duration is 20 seconds. 
\label{fig:video}}
\end{figure}

Animations and interactive figures (Section \ref{sec:interactive}) should 
use the {\tt\string\begin{interactive}} environment in the figure call. This 
environment
places a blue border around the figure to indicate that the figure is 
enhanced in the published HTML article. The
command also serves to alert the publisher what files are used to generate
the dynamic HTML content. {\tt\string\interactive} takes two arguments. The
first details the type and currently only three are allowed. The types are
{\tt\string js} for generic javascript interactive figures, 
{\tt\string animation} for inline videos, and 
{\tt\string timeseries} for interactive light curves produced
by astropy \citet{2013A&A...558A..33A}\footnote{To be release in the 
summer of 2019}. If these types are not provide the compiler will issue an
error and quit. The second argument is the file that produces the enhanced
feature in the HTML article.

\subsubsection{Interactive figures \label{sec:interactive}}

Interactive figures give the reader the ability to manipulate the
information contained in an image which can add clarity or help further the
author's narrative.  These figures consist of two parts, a static 
representative figure for the manuscript and the dynamic javascript plus
HTML framework that allows for interactive control.

An example of an interactive figure is a 3D model.
The underlying figure is a X3D file while x3dom.js is the javascript driver
that displays it. An author created interface is added via a html wrapper.
The first 3D model published by the AAS Journals using this technique was
\citet{2014ApJ...793..127V}.  

Figure \ref{fig:interactive} provides an interactive example which can be
run locally to demonstrate how a simple javascript plus html interface
allows a reader to switch between figures. The necessary files for this
particular interactive figure are in the {\tt\string interactive.tar.gz}
file included with this package. Unpack the file and point the browser to
the local html file. In this case, the javascript that runs the interactive
buttons is embedded in the html file but it could just as easily be calls
to external javascript libraries. Ideally, the javascript should be
included with the submitted package of interactive files to minimize
external dependencies within the published article.

\begin{figure}
\begin{interactive}{js}{interactive.tar.gz}
\plotone{f5.pdf}
\end{interactive}
\caption{Figure 4 from \citet{2018AJ....156...82C}. \emph{Upper panel}: the
cumulative median observing time to measure the $3\sigma$ RV masses of TESS
planets as a function of host star spectral type and up to $10^3$ hours.
The \emph{dashed blue curves} represent the results from the optical
spectrograph whereas the \emph{solid red curves} represent the near-IR
spectrograph. \emph{Lower panel}: the time derivative of the cumulative
observing time curves used to indicate the RV planet detection efficiency.
The \emph{horizontal dashed line} highlights the value of the detection
efficiency at 20 hours per detection.  Note that unlike the lower panels,
the upper panels do not share a common ordinate due to the differing number
of planet detections around stars in each spectral type bin. The
interactive version has two buttons that allows one to turn the optical and
NIR layers. \label{fig:interactive}}
\end{figure}

Authors should consult the online tutorials at 
\url{https://journals.aas.org/graphics-guide/#interactive_figures}
for more information on what is currently supported and links to 
tutorials and examples.

\section{Displaying mathematics} \label{sec:displaymath}

The most common mathematical symbols and formulas are in the amsmath
package.  \aastex\ requires this package so there is no need to
specifically call for it in the document preamble.  Most modern \latex\
distributions already contain this package.  If you do not have this
package or the other required packages, revtex4-1, latexsym, graphicx,
amssymb, longtable, and epsf, they can be obtained from 
\url{http://www.ctan.org}

Mathematics can be displayed either within the text, e.g. $E = mc^2$, or
separate from in an equation.  In order to be properly rendered, all inline
math text has to be declared by surrounding the math by dollar signs (\$).

A complex equation example with inline math as part of the explanation
follows.

\begin{equation}
\bar v(p_2,\sigma_2)P_{-\tau}\hat a_1\hat a_2\cdots
\hat a_nu(p_1,\sigma_1) ,
\end{equation}
where $p$ and $\sigma$ label the initial $e^{\pm}$ four-momenta
and helicities $(\sigma = \pm 1)$, $\hat a_i=a^\mu_i\gamma_\nu$
and $P_\tau=\frac{1}{2}(1+\tau\gamma_5)$ is a chirality projection
operator $(\tau = \pm1)$.  This produces a single line formula.  \latex\ will
auto-number this and any subsequent equations.  If no number is desired then
the {\tt\string equation} call should be replaced with {\tt\string displaymath}.

\latex\ can also handle a a multi-line equation.  Use {\tt\string eqnarray}
for more than one line and end each line with a
\textbackslash\textbackslash.  Each line will be numbered unless the
\textbackslash\textbackslash\ is preceded by a {\tt\string\nonumber}
command.  Alignment points can be added with ampersands (\&).  There should be
two ampersands per line. In the examples they are centered on the equal
symbol.

\begin{eqnarray}
\gamma^\mu  & = &
 \left(
\begin{array}{cc}
0 & \sigma^\mu_+ \\
\sigma^\mu_- & 0
\end{array}     \right) ,
 \gamma^5= \left(
\begin{array}{cc}
-1 &   0\\
0 &   1
\end{array}     \right)  , \\
\sigma^\mu_{\pm}  & = &   ({\bf 1} ,\pm \sigma) , 
\end{eqnarray}

\begin{eqnarray}
\hat a & = & \left(
\begin{array}{cc}
0 & (\hat a)_+\\
(\hat a)_- & 0
\end{array}\right), \nonumber \\
(\hat a)_\pm & = & a_\mu\sigma^\mu_\pm 
\end{eqnarray}


\section{Revision tracking and color highlighting} \label{sec:highlight}

Authors sometimes use color to highlight changes to their manuscript in
response to editor and referee comments.  In \aastex\ new commands
have been introduced to make this easier and formalize the process. 

The first method is through a new set of editing mark up commands that
specifically identify what has been changed.  These commands are
{\tt\string\added\{<text>\}}, {\tt\string\deleted\{<text>\}}, and
{\tt\string\replaced\{<old text>\}\{<replaced text>\}}. To activate these
commands the {\tt\string trackchanges} option must be used in the
{\tt\string\documentclass} call.  When compiled this will produce the
marked text in red.  The {\tt\string\explain\{<text>\}} can be used to add
text to provide information to the reader describing the change.  Its
output is purple italic font. To see how {\tt\string\added\{<important
added info>\}}, {\tt\string\deleted\{<this can be deleted text>\}},
{\tt\string\replaced\{<old data>\}\{<replaced data>\}}, and \break
{\tt\string\explain\{<text explaining the change>\}} commands will produce
\added{important added information}\deleted{, deleted text, and }
\replaced{old data}{and replaced data,} toggle between versions compiled with
and without the {\tt\string trackchanges} option.\explain{text explaining
the change}

A summary list of all these tracking commands can be produced at the end of
the article by adding the {\tt\string\listofchanges} just before the
{\tt\string\end\{document\}} call.  The page number for each change will be
provided. If the {\tt\string linenumbers} option is also included in the
documentclass call then not only will all the lines in the article be
numbered for handy reference but the summary list will also include the
line number for each change.

The second method does not have the ability to highlight the specific
nature of the changes but does allow the author to document changes over
multiple revisions.  The commands are {\tt\string\edit1\{<text>\}},
{\tt\string\edit2\{<text>\}} and {\tt\string\edit3\{<text>\}} and they
produce {\tt\string<text>} that is highlighted in bold red, italic blue and
underlined purple, respectively.  Authors should use the first command to
\edit1{indicated which text has been changed from the first revision.}  The
second command is to highlight \edit2{new or modified text from a second
revision}.  If a third revision is needed then the last command should be used 
\edit3{to show this changed text}.  Since over 90\% of all manuscripts are
accepted after the 3rd revision these commands make it easy to identify
what text has been added and when.  Once the article is accepted all the
highlight color can be turned off simply by adding the
{\tt\string\turnoffediting} command in the preamble. Likewise, the new commands
{\tt\string\turnoffeditone}, {\tt\string\turnoffedittwo}, and
{\tt\string\turnoffeditthree} can be used to only turn off the 
{\tt\string\edit1\{<text>\}}, {\tt\string\edit2\{<text>\}} and 
{\tt\string\edit3\{<text>\}}, respectively.

Similar to marking editing changes with the {\tt\string\edit} options there
are also the {\tt\string\authorcomments1\{<text>\}}, 
{\tt\string\authorcomments2\{<text>\}} and
{\tt\string\authorcomments3\{<text>\}} commands.  These produce the same
bold red, italic blue and underlined purple text but when the
{\tt\string\turnoffediting} command is present the {\tt\string<text>}
material does not appear in the manuscript.  Authors can use these commands
to mark up text that they are not sure should appear in the final
manuscript or as a way to communicate comments between co-authors when
writing the article.

\section{Software and third party data repository citations} \label{sec:cite}

The AAS Journals would like to encourage authors to change software and
third party data repository references from the current standard of a
footnote to a first class citation in the bibliography.  As a bibliographic
citation these important references will be more easily captured and credit
will be given to the appropriate people.

The first step to making this happen is to have the data or software in
a long term repository that has made these items available via a persistent
identifier like a Digital Object Identifier (DOI).  A list of repositories
that satisfy this criteria plus each one's pros and cons are given at \break
\url{https://github.com/AASJournals/Tutorials/tree/master/Repositories}.

In the bibliography the format for data or code follows this format: \\

\noindent author year, title, version, publisher, prefix:identifier\\

\citet{2015ApJ...805...23C} provides a example of how the citation in the
article references the external code at
\doi{10.5281/zenodo.15991}.  Unfortunately, bibtex does
not have specific bibtex entries for these types of references so the
``@misc'' type should be used.  The Repository tutorial explains how to
code the ``@misc'' type correctly.  The most recent aasjournal.bst file,
available with \aastex\ v6, will output bibtex ``@misc'' type properly.

\acknowledgments

We thank all the people that have made this AASTeX what it is today.  This
includes but not limited to Bob Hanisch, Chris Biemesderfer, Lee Brotzman,
Pierre Landau, Arthur Ogawa, Maxim Markevitch, Alexey Vikhlinin and Amy
Hendrickson. Also special thanks to David Hogg and Daniel Foreman-Mackey
for the new "modern" style design. Considerable help was provided via bug
reports and hacks from numerous people including Patricio Cubillos, Alex
Drlica-Wagner, Sean Lake, Michele Bannister, Peter Williams, and Jonathan
Gagne.


\vspace{5mm}
\facilities{HST(STIS), Swift(XRT and UVOT), AAVSO, CTIO:1.3m,
CTIO:1.5m,CXO}


\software{astropy \citep{2013A&A...558A..33A},  
          Cloudy \citep{2013RMxAA..49..137F}, 
          SExtractor \citep{1996A&AS..117..393B}
          }




\subsubsection*{#1}}

\pagestyle{headings}
\markright{Reference sheet: \texttt{natbib}}

\usepackage{shortvrb}
\MakeShortVerb{\|}

\begin{document}
\thispagestyle{plain}

\newcommand{\textsc{Bib}\TeX}{\textsc{Bib}\TeX}
\newcommand{\texttt{#1}\def\filedate{#2}\def\fileversion{#3}}}{\texttt{#1}\def\filedate{#2}\def\fileversion{#3}}}
\begin{center}{\bfseries\Large
    Reference sheet for \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\ usage}\\
    \large(Describing version \fileversion\ from \filedate)
\end{center}

\begin{quote}\slshape
For a more detailed description of the \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\ package, \LaTeX\ the
source file \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\texttt{.dtx}.
\end{quote}
\head{Overview}
The \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\ package is a reimplementation of the \LaTeX\ |\cite| command,
to work with both author--year and numerical citations. It is compatible with
the standard bibliographic style files, such as \texttt{plain.bst}, as well as
with those for \texttt{harvard}, \texttt{apalike}, \texttt{chicago},
\texttt{astron}, \texttt{authordate}, and of course \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}.

\head{Loading}
Load with |\usepackage[|\emph{options}|]{|\texttt{#1}\def\filedate{#2}\def\fileversion{#3}}|}|. See list of
\emph{options} at the end.

\head{Replacement bibliography styles}
I provide three new \texttt{.bst} files to replace the standard \LaTeX\
numerical ones:
\begin{quote}\ttfamily
 plainnat.bst \qquad abbrvnat.bst \qquad unsrtnat.bst
\end{quote}
\head{Basic commands}
The \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\ package has two basic citation commands, |\citet| and
|\citep| for \emph{textual} and \emph{parenthetical} citations, respectively.
There also exist the starred versions |\citet*| and |\citep*| that print
the full author list, and not just the abbreviated one.
All of these may take one or two optional arguments to add some text before
and after the citation.
\begin{quote}
\begin{tabular}{l@{\quad$\Rightarrow$\quad}l}
  |\citet{jon90}| & Jones et al. (1990)\\
  |\citet[chap.~2]{jon90}| & Jones et al. (1990, chap.~2)\\[0.5ex]
  |\citep{jon90}| & (Jones et al., 1990)\\
  |\citep[chap.~2]{jon90}| & (Jones et al., 1990, chap.~2)\\
  |\citep[see][]{jon90}| & (see Jones et al., 1990)\\
  |\citep[see][chap.~2]{jon90}| & (see Jones et al., 1990, chap.~2)\\[0.5ex]
  |\citet*{jon90}| & Jones, Baker, and Williams (1990)\\
  |\citep*{jon90}| & (Jones, Baker, and Williams, 1990)
\end{tabular}
\end{quote}
\head{Multiple citations}
Multiple citations may be made by including more than one
citation key in the |\cite| command argument.
\begin{quote}
\begin{tabular}{l@{\quad$\Rightarrow$\quad}l}
  |\citet{jon90,jam91}| & Jones et al. (1990); James et al. (1991)\\
  |\citep{jon90,jam91}| & (Jones et al., 1990; James et al. 1991)\\
  |\citep{jon90,jon91}| & (Jones et al., 1990, 1991)\\
  |\citep{jon90a,jon90b}| & (Jones et al., 1990a,b)
\end{tabular}
\end{quote}

\head{Numerical mode}
These examples are for author--year citation mode. In numerical mode, the
results are different.
\begin{quote}
\begin{tabular}{l@{\quad$\Rightarrow$\quad}l}
  |\citet{jon90}| & Jones et al. [21]\\
  |\citet[chap.~2]{jon90}| & Jones et al. [21, chap.~2]\\[0.5ex]
  |\citep{jon90}| & [21]\\
  |\citep[chap.~2]{jon90}| & [21, chap.~2]\\
  |\citep[see][]{jon90}| & [see 21]\\
  |\citep[see][chap.~2]{jon90}| & [see 21, chap.~2]\\[0.5ex]
  |\citep{jon90a,jon90b}| & [21, 32]
\end{tabular}
\end{quote}
\head{Suppressed parentheses}
As an alternative form of citation, |\citealt| is the same as |\citet| but
\emph{without parentheses}. Similarly, |\citealp| is |\citep| without
parentheses. Multiple references, notes, and the starred variants
also exist.
\begin{quote}
\begin{tabular}{l@{\quad$\Rightarrow$\quad}l}
  |\citealt{jon90}| & Jones et al.\ 1990\\
  |\citealt*{jon90}| & Jones, Baker, and Williams 1990\\
  |\citealp{jon90}| & Jones et al., 1990\\
  |\citealp*{jon90}| & Jones, Baker, and Williams, 1990\\
  |\citealp{jon90,jam91}| & Jones et al., 1990; James et al., 1991\\
  |\citealp[pg.~32]{jon90}| & Jones et al., 1990, pg.~32\\
  |\citetext{priv.\ comm.}| & (priv.\ comm.)
\end{tabular}
\end{quote}
The |\citetext| command
allows arbitrary text to be placed in the current citation parentheses.
This may be used in combination with |\citealp|.
\head{Partial citations}
In author--year schemes, it is sometimes desirable to be able to refer to
the authors without the year, or vice versa. This is provided with the
extra commands
\begin{quote}
\begin{tabular}{l@{\quad$\Rightarrow$\quad}l}
  |\citeauthor{jon90}| & Jones et al.\\
  |\citeauthor*{jon90}| & Jones, Baker, and Williams\\
  |\citeyear{jon90}|   & 1990\\
  |\citeyearpar{jon90}| & (1990)
\end{tabular}
\end{quote}
\head{Forcing upper cased names}
If the first author's name contains a \textsl{von} part, such as ``della
Robbia'', then |\citet{dRob98}| produces ``della Robbia (1998)'', even at the
beginning of a sentence. One can force the first letter to be in upper case
with the command |\Citet| instead. Other upper case commands also exist.
\begin{quote}
\begin{tabular}{rl@{\quad$\Rightarrow$\quad}l}
  when & |\citet{dRob98}| & della Robbia (1998) \\
  then & |\Citet{dRob98}| & Della Robbia (1998) \\
     &   |\Citep{dRob98}| & (Della Robbia, 1998) \\
     &   |\Citealt{dRob98}| & Della Robbia 1998 \\
     &   |\Citealp{dRob98}| & Della Robbia, 1998 \\
     &   |\Citeauthor{dRob98}| & Della Robbia
\end{tabular}
\end{quote}
These commands also exist in starred versions for full author names.

\head{Citation aliasing}
Sometimes one wants to refer to a reference with a special designation,
rather than by the authors, i.e. as Paper~I, Paper~II. Such aliases can be
defined and used, textual and/or parenthetical with:
\begin{quote}
\begin{tabular}{lcl}
  |\defcitealias{jon90}{Paper~I}|\\
  |\citetalias{jon90}| & $\Rightarrow$ & Paper~I\\
  |\citepalias{jon90}| & $\Rightarrow$ & (Paper~I)
\end{tabular}
\end{quote}
These citation commands function much like |\citet| and |\citep|: they may
take multiple keys in the argument, may contain notes, and are marked as
hyperlinks.
\head{Selecting citation style and punctuation}
Use the command |\bibpunct| with one optional and 6 mandatory arguments:
\begin{enumerate}
\item the opening bracket symbol, default = (
\item the closing bracket symbol, default = )
\item the punctuation between multiple citations, default = ;
\item the letter `n' for numerical style, or `s' for numerical superscript
      style, any other letter for
      author--year, default = author--year;
\item the punctuation that comes between the author names and the year
\item the punctuation that comes between years or numbers when common author
      lists are suppressed (default = ,);
\end{enumerate}

The optional argument is the character preceding a post-note, default is a
comma plus space. In redefining this character, one must include a space if
one is wanted.

Example~1, |\bibpunct{[}{]}{,}{a}{}{;}| changes the output of
\begin{quote}
  |\citep{jon90,jon91,jam92}|
\end{quote}
into [Jones et al. 1990; 1991, James et al. 1992].

Example~2, |\bibpunct[; ]{(}{)}{,}{a}{}{;}| changes the output of
\begin{quote}
  |\citep[and references therein]{jon90}|
\end{quote}
into (Jones et al. 1990; and references therein).

\head{Other formatting options}
Redefine |\bibsection| to the desired sectioning command for introducing
the list of references. This is normally |\section*| or |\chapter*|.

Define |\bibpreamble| to be any text that is to be printed after the heading but
before the actual list of references.

Define |\bibfont| to be a font declaration, e.g.\ |\small| to apply to
the list of references.

Define |\citenumfont| to be a font declaration or command like |\itshape|
or |\textit|.

Redefine |\bibnumfmt| as a command with an argument to format the numbers in
the list of references. The default definition is |[#1]|.

The indentation after the first line of each reference is given by
|\bibhang|; change this with the |\setlength| command.

The vertical spacing between references is set by |\bibsep|; change this with
the |\setlength| command.

\head{Automatic indexing of citations}
If one wishes to have the citations entered in the \texttt{.idx} indexing
file, it is only necessary to issue |\citeindextrue| at any point in the
document. All following |\cite| commands, of all variations, then insert
the corresponding entry to that file. With |\citeindexfalse|, these
entries will no longer be made.

\head{Use with \texttt{chapterbib} package}

The \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\ package is compatible with the \texttt{chapterbib} package
which makes it possible to have several bibliographies in one document.

The package makes use of the |\include| command, and each |\include|d file
has its own bibliography.

The order in which the \texttt{chapterbib} and \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\ packages are loaded
is unimportant.

The \texttt{chapterbib} package provides an option \texttt{sectionbib}
that puts the bibliography in a |\section*| instead of |\chapter*|,
something that makes sense if there is a bibliography in each chapter.
This option will not work when \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\ is also loaded; instead, add
the option to \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}.

Every |\include|d file must contain its own
|\bibliography| command where the bibliography is to appear. The database
files listed as arguments to this command can be different in each file,
of course. However, what is not so obvious, is that each file must also
contain a |\bibliographystyle| command, \emph{preferably with the same
style  argument}.
\head{Sorting and compressing citations}
Do not use the \texttt{cite} package with \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}; rather use one of the
options \texttt{sort} or \texttt{sort\&compress}.

These also work with author--year citations, making multiple citations appear
in their order in the reference list.

\head{Long author list on first citation}
Use option \texttt{longnamesfirst} to have first citation automatically give
the full list of authors.

Suppress this for certain citations with |\shortcites{|\emph{key-list}|}|,
given before the first citation.

\head{Local configuration}
Any local recoding or definitions can be put in \texttt{#1}\def\filedate{#2}\def\fileversion{#3}}\texttt{.cfg} which
is read in after the main package file.

\head{Options that can be added to \texttt{\char`\\ usepackage}}
\begin{description}
\item[\ttfamily round] (default) for round parentheses;
\item[\ttfamily square] for square brackets;
\item[\ttfamily curly] for curly braces;
\item[\ttfamily angle] for angle brackets;
\item[\ttfamily colon] (default) to separate multiple citations with
     colons;
\item[\ttfamily comma] to use commas as separaters;
\item[\ttfamily authoryear] (default) for author--year citations;
\item[\ttfamily numbers] for numerical citations;
\item[\ttfamily super] for superscripted numerical citations, as in
     \textsl{Nature};
\item[\ttfamily sort] orders multiple citations into the sequence in
     which they appear in the list of references;
\item[\ttfamily sort\&compress] as \texttt{sort} but in addition multiple
     numerical citations are compressed if possible (as 3--6, 15);
\item[\ttfamily longnamesfirst] makes the first citation of any reference
     the equivalent of the starred variant (full author list) and subsequent
     citations normal (abbreviated list);
\item[\ttfamily sectionbib] redefines |\thebibliography| to issue
     |\section*| instead of |\chapter*|; valid only for classes with a
     |\chapter| command; to be used with the \texttt{chapterbib} package;
\item[\ttfamily nonamebreak] keeps all the authors' names in a citation on
     one line; causes overfull hboxes but helps with some \texttt{hyperref}
     problems.
\end{description}
\end{document}

