**Table A.1:** Molecular parameters used to estimate $N_{\rm tot}$ .
Transition	$S\mu^2_g$ ^a	Q(15)^b
	D
${\rm NH_{2}D}$ 1₁₁-1₀₁ (para)	11.9	47.8^c
${\rm NHD_2}$ 1₁₀-1₀₁ (ortho)	0.722	50.2^d
${\rm H^{13}CN}$ J=1-0, F=2-1	14.84	22.8^e
${\rm HC^{15}N}$ J=1-0	8.909	7.6^e
${\rm C^{18}O}$ J=1-0	0.012	6.1^e
${\rm C^{18}O}$ J=2-1	0.024	6.1^e

^a S is the line strength and $\mu_{g}$ is the dipole moment along the molecular g-axis. ^b the partition function corresponding to a temperature of 15 K, the typical temperature of the sources in the present sample. ^c the partition function obtained by the summation over the contribution of the different energy levels from J=0 to J=2. ^d the partition function based on data from the CDMS catalog (Müller et al. 2005). ^e the partition function based on data from the JPL catalog (Pickett et al. 1998).

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