**Table 4:** Observed and modeled column densities N in AB Aur and DM Tau.
Molecule	$\chi ^2$ -minimization method			Chemical model		DM Tau
	N	1 $\sigma$	N/N(¹³CO) $^{\rm 1}$	N	N/N(¹³CO) $^{\rm 2}$	N/N(¹³CO) $^{\rm 1}$
	[cm^-2]	error		[cm^-2]
H₂	6 $\times$ 10²²	$\rm 1\times10^{22}$	$1.5\times10^{6}$	$5\times10^{22}$	$1.3\times10^{6}$	$1\times10^{7}$
¹³CO^(*3)	$4\times10^{16}$	$5\times10^{15}$	1	$4\times10^{16}$	1	1
HCO⁺	$6\times10^{12}$	$3\times10^{11}$	$1.5\times10^{-4}$	$1.5\times10^{13}$	$4\times10^{-4}$	$2\times10^{-3}$
HCN	$5\times10^{11}$	$3\times10^{11}$	$1.3\times10^{-5}$	$4\times10^{11}$	10^-5	$7\times10^{-4}$
CS	$3\times10^{12}$	$3\times10^{12}$	< $8\times10^{-5}$	$2\times10^{11}$	$5\times10^{-6}$	$3\times10^{-4}$
C₂H	$2\times10^{13}$	$2\times10^{13}$	< $5\times10^{-4}$	10¹⁰	$2.5\times10^{-7}$	10^-3
CH₃OH	0	$7\times10^{15}$	< $2\times10^{-1}$	0	0	0

$^{\rm 1}$ Relative to the ¹³CO column density at 250 AU obtained by the $\chi ^2$ -minimization method.
$^{\rm 2}$ Relative to the ¹³CO column density at 250 AU obtained by the chemical modeling.
$^{\rm 3}$ See results reported by Pietu et al. (2005).

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