**Table 9:** Preferred chemical scenarios for the different sources^a.
$\begin{table} \par\begin{displaymath} \begin{array}{p{0.45\linewidth}p{0.47\lin... ...300 \\ \noalign{\smallskip } \hline \end{array} \end{displaymath}\end{table}$

: a(b) means $a\times10^b$ .
^a: The sources are ordered by a decreasing temperature of the warm gas (Lahuis & van Dishoeck 2000).
^b: H₂O ice column densities from Gibb & Whittet (2002). H₂O ice column density for AFGL 2591 from Smith et al. (1989) and for NGC 7538 IRS1 from Gerakines et al. (1999).
^c: C₂H₂ excitation temperature from Lahuis & van Dishoeck (2000) and Boonman et al. (2003), which is a good tracer of the warm gas. For MonR2 IRS3 and S 140 IRS1 the ¹³CO temperatures from Giannakopoulou et al. (1997) and Mitchell et al. (1990) respectively, are listed.