\begin{table}%t4
\caption{HNCO abundance with respect to H$_2$ in different astronomical environments.}
\label{tab:comp}
%\centering
\par
\begin{tabular}{lcc}
\hline
\hline
Source        & $X$(HNCO)   & References\\  
              & 10$^{-9}$ &\\
\hline
L1157-B2               & 25--96      &1\\
Translucent clouds & 0.2--5      &2  \\  
Orion bar PDR      & $<$$10^{-2}$ &3 \\
Dense cores       & 0.2--8.7    &4  \\
Orion Hot core     & 5          &5\\
SgrB2M             & 1          &6\\
W3(H$_2$O)         & 5.0--27     &7    \\
G34.3+0.15        & 1.4        &8 \\
IRAS 16293        & 0.17--4.0   & 9, 10   \\
GC clouds         & 3--48       & 11,12\\
Starburst galaxies & 1--6.3      & 13\\
\hline
\end{tabular}\\
\tablebib{(1)~This work; (2)~\citet{Turner99}; (3)~\citet{Jansen95}; 
(4)~\citet{Zinchenko00}; (5)~\citet{Blake87}; (6)~\citet{Minh98};
(7)~\citet{Helmich97}; (8)~\citet{Bockelee-Morvan00}, using the HNCO  column density given by \cite{MacDonald96} and the H$_2$ column density given by \cite{Hatchell98}; 
(9)~\citet{vanDishoeck95}; (10)~\citet{Bisschop08};
 (11)~\citet{Rizzo00}; (12)~\citet{Martin08}, and references within; 
(13)~\citet{Martin09} 
}
\end{table}