\begin{table} %T7
\caption{\label{ShockAge}Proposed schema of the formation and destruction of~SO, SO$_2$, H$_2$S, SiO and H$_2$CO as function of time. 
The times are indicative and are meant to represent a sequence.}
%\centerline
{
\begin{tabular}{ccc} 
\hline\hline
Time (yr)& Formation & Destruction \\
\hline 
0$^a$    & H$_2$S, H$_2$CO, SiH$_2$, SiH$_4$ & \\
$10^3$   & SO, SO$_2$, H$_2$S, SiO           & \\
$3 \times 10^4$   &  & SO$_2$, H$_2$CO\\
$10^5$   &   & SiO \\
\hline 
\end{tabular}}
\smallskip
$^a$~Injection in the gas phase of the molecules sputtered 
from the grain mantles. 
\end{table}