\begin{table}%t4
\caption{\label{tab3}Fractional abundances relative to \cch.   The assumed  column densities of \cch~are recalled in the first row. CP stands for Cyanopolyyne Peak. References are: $^{(a)}$~Pratap et~al. \cite{Pratap1997}, $^{(b)}$~Foss\'e et~al. \cite{Foss2001}, $^{(c)}$~Gu\'elin et~al. 1982, $^{(d)}$~Foss\'e \cite{Foss2003}, $^{(e)}$~Dickens et~al. \cite{Dickens2000}, $^{(f)}$~Takano et~al. \cite{Takano1998}. {  Since no \csh~and HC$_{3}$N measurements were available at the Horsehead IR peak, the corresponding abundances relative to \cch~are given for the hydrocarbon peak.}}
%\centering
\par
\begin{tabular}{l  c c c c c} 
\hline \hline  
Molecule		& Horsehead         	& IC~63 				& \roph 				& TMC-1     	& L134N \\ 
			& IR peak area		&(0\arcsec, $-$0\arcsec)		& ($\Delta \alpha=-21$\arcsec)		& (CP)		&	\\\hline
\cch~($10^{13}$~cm$^{-2}$)&    16            	&		5.3		&	3.0				&   7.2~$^{(a)}$&      15~$^{(d)}$\\ \hline
$c$-\ccch             	&	0.024		&		$<$0.023	&	--				& 0.140~$^{(b)}$&  0.050~$^{(d)}$\\
$l$-\ccch              	&	0.013		&		$<$0.110	&	--				& 0.012~$^{(b)}$&  0.040~$^{(d)}$\\
$c$-\ccchh             	&	0.058		&		0.085		&	0.100				& 0.800~$^{(b)}$&  0.330~$^{(d)}$\\
$l$-\ccchh             	&	$<$0.002	&		--		&	--				& 0.030~$^{(b)}$&  0.004~$^{(d)}$\\
\chhhcch              	&	$<$0.069	&		--		&	--				&   1.4~$^{(a)}$&  0.070~$^{(d)}$\\
\cccch               	&	0.069		&		0.062		&	0.500				&   2.8~$^{(c)}$&  0.735~$^{(d)}$\\
\csh     		&	0.004		&		--		&	--				& 0.115~$^{(b)}$&  0.008~$^{(d)}$\\
HN$^{13}$C             	&	0.002		&		$<$0.002	&	--				& 0.056~$^{(a)}$&  0.054~$^{(e)}$\\
HC$_{3}$N             	&	0.003		&		--      	&	--				&   2.2~$^{(f)}$&  0.127~$^{(e)}$ \\
\hline
\end{tabular}
\end{table}