Letter "c" appearing inside pmatrix environment with aastex

Question

My journal recently updated its official class file, aastex, from version 5.2 to version 6.0. Apparently this was a major rewrite, as the journal abandoned its own class and based 6.0 off the ancient third-party emulateapj made before the journal published class files.

Now I'm getting an italix "c" at the beginning of my pmatrix with the new version. Here's an example:

\documentclass{aastex}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
  \begin{pmatrix}
    -1 & 0 \\
    0  & 1
  \end{pmatrix}
\end{equation*}
\end{document}

With 5.2 I get this:

while with 6.0 I get this:

Where is this coming from, and how do I get rid of it?

Answer 1

The problem is the internal usage of the array environment by amsmath's pmatrix. At some point

\array{*\c@MaxMatrixCols c}

is executed which prepares a table with MaxMatrixCols columns (default is 10).

(This makes it that you do not have to insert the number of columns in your table/matrix on your own. Some work around it is done to ensure that the parentheses are correctly spaced.)

If one defines \array{*{\c@MaxMatrixCols}c} instead, the code snipped works.

I don't know what aastex6 changes but it uses a lot of \newcolumntypes …

Code

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\def\env@matrix{\hskip -\arraycolsep % taken from amsmath.sty lines 895ff
  \let\@ifnextchar\new@ifnextchar
  \array{*{\c@MaxMatrixCols}c}}
\makeatother
\begin{document}
\begin{equation*}
  \begin{pmatrix}
    -1 & 0 \\
    0  & 1
  \end{pmatrix}
\end{equation*}
\end{document}