\begin{table}%t6
\caption{\label{mass_env}CR ionization rate toward massive protostellar envelopes.}
%\centerline
{
\begin{tabular}{llll}
\hline\hline\noalign{\smallskip}
    & $N({\rm H}_2)$ & $\zeta^{{\rm H}_2}$ \\
    & (cm$^{-2}$)    &  (s$^{-1}$)         \\
\hline\noalign{\smallskip}
NGC~2264~IRS    & $8.3\times 10^{22~a}$  & $(4\pm 2)\times 10^{-16~a}$  \\  
GL~2136         & $1.2\times 10^{23~ b}$  & $7.6\times 10^{-17~c}$       \\
W3~IRS5         & $1.3\times 10^{23~ a}$  & $(4\pm 2)\times 10^{-17~a}$  \\  
GL~2591         & $9.6\times 10^{22~ d}$  & $1.3\times 10^{-16~c}$       \\
GL~490          & $2.0\times 10^{23~ b}$  & $1.5\times 10^{-17~ c}$       \\
W~33A           & $6.2\times 10^{23 ~b}$  & $3.0\times 10^{-17~ c}$       \\
W3~IRS5         & $2.3\times 10^{23 ~b}$  & $6.4\times 10^{-17~ c}$       \\
S~140           & $1.4\times 10^{23~ b}$  & $8.5\times 10^{-17~c}$       \\
DR21(OH)        & $1.1\times 10^{23~e}$  & $3.1\times 10^{-18~e}$       \\
\hline
\end{tabular}}
\medskip
 $^a$ From de Boisanger  et~al. (\cite{deBoisanger1996}). \\
 $^b$ CO column density and CO/H$_2$ ratio from van der Tak et~al.~(\cite{vanderTak2000b}). \\
 $^c$ CR rate from van der Tak \& van Dishoeck~(\cite{vanderTak2000a}). \\
 $^d$ From Doty et~al.~(\cite{Doty2002}). \\
 $^e$ From Hezareh et~al.~(\cite{Hezareh2008}). \\
\end{table}