Problem displaying degrees celsius with siunitx and xelatex

Question

I'm trying to discuss a temperature in a document where I'm using xelatex and siunitx.

       The temperature was further increased to \SI{200}{\degreeCelsius}.

When using xelatex this causes a hollow box to appear in place of the unit. When using pdflatex and a distribution supplied font it works fine. Is there a solution to this that I have missed?

This is a link to a character guide for the font -- ITC Baskerville Std Roman -- I'm using. http://store2.adobe.com/cfusion/store/html/index.cfm?store=OLS-UK&event=displayFont&code=BASQ10005000

I've also tried Times New Roman with no joy.

I'm using a fully updated texlive 2011.

Many thanks,

MWE

 \documentclass{article}

 \usepackage{fontspec}                              
 \setmainfont[Mapping=tex-text]{Times New Roman}
 %\usepackage{xkeyval}
 %\usepackage{polyglossia}                          
 %\setdefaultlanguage[variant=british]{english}             
 %\usepackage{etoolbox} 
 \usepackage{siunitx}

 \begin{document}
 The temperature was \SI{200}{\degreeCelsius}.
 \end{document}

Answer 1

Expanding on Joseph's answer, you can find a matching font that has the required symbol; for instance, if your Times New Roman hasn't the ℃ symbol, then you can do

\newfontfamily{\tgtermes}[Ligatures=TeX]{TeX Gyre Termes}

\usepackage{siunitx}

\usepackage{newunicodechar}
\newunicodechar{℃}{{\tgtermes ℃}}
\sisetup{text-celsius = ℃}

thereby using the different font only for the character you need. Alternatively, you can also try faking the symbol:

\sisetup{text-celsius = $^\circ\mkern-1mu$C}

(adjust the amount of kerning).

Answer 2

The siunitx package has a set of basic symbol choices built in for working with standard LaTeX fonts. However, these may or may not work when loading system fonts with XeLaTeX or LuaLaTeX. Fonts do not cover all symbols, and it is simply not realistic to detect every possible combination. Thus when using non-standard fonts it is down to the end user to make an appropriate choice, using for example

\sisetup{text-celsius = <symbol or appropriate macro>}

The log file may show

LaTeX Font Warning: Some font shapes were not available, defaults substituted.

if the appropriate symbols are not available. (This will depend on the font in use.)

Answer 3

use the TeX Gyre Times font, is has a degree symbol:

\documentclass{article}    
\usepackage{fontspec}      
\fontspec[Extension=.otf,
          BoldFont=texgyretermes-bold,
          ItalicFont=texgyretermes-italic,
          BoldItalicFont=texgyretermes-bolditalic]{texgyretermes-regular}
\addfontfeatures{Mapping=tex-text}
\usepackage{siunitx}

\setmainfont{TeX Gyre Termes}

\begin{document}
 The temperature was \SI{200}{\degreeCelsius}.
\end{document}

If you are using Linux, then pay attention, that xelatex finds the open type fonts of TeXLive, located at $texmf-dist/fonts/opentype/public/texgyre/. With lualatex it is no problem.