**Table 4:** ${\rm C^{18}O}$ column density and abundance from (J=1-0) and (J=2-1) transitions.
Source	$T_{\rm ex}$	${N_{\rm C^{18}O}}$	$N({\rm H_2}$ )^a	$\chi_{\rm C^{18}O}^{b}$	$\eta^{c}$
	K	$10^{16} (10^{15})~\rm cm^{-2}$	10²³ cm^-2	10^-8
G8.13+0.25	11.8	1.6 (0.1)	2.6	6.2	2.7
G8.68-0.37	8.5	2.4 (0.9)	7.9	3.0	5.6
G8.71-0.37	4.2	1.8 (0.5)	3.5	5.2	3.2
G10.15-0.34	19.1	2.1 (0.4)	3.8	5.4	3.1
G10.21-0.31	8.4	1.4 (0.2)	2.2	6.4	2.7
G10.21-0.32	8.0	2.6 (0.3)	7.4	3.6	4.8
G10.61-0.33	9.1	1.0 (0.3)	2.7	3.8	4.5
G11.11-0.12P1	6.4	0.6 (0.1)	1.1	5.5	3.1
G11.11-0.12P3	6.9	0.5 (0.3)	0.8	6.2	2.7
G11.11-0.12P4	6.6	0.5 (0.2)	1.4	3.5	4.8
G12.19-0.12	11.6	1.2 (0.2)	2.4	5.1	3.3
G13.18+0.06	15.7	1.2 (0.6)	6.3	1.8	9.3
G15.01-0.67	32.5	6.0 (0.4)	16.6	3.6	4.7
G15.01-0.69	52.2	4.5 (0.6)	11.6	3.9	4.4
G15.03-0.65	51.7	5.5 (0.6)	14.2	3.9	4.4
G18.17-0.30	14.3	1.5 (0.8)	3.3	4.6	3.7
G18.21-0.34	10.9	0.7 (0.4)	2.5	3.0	5.8
G23.41-0.23	8.4	3.4 (0.8)	9.3	3.6	4.7
G23.42-0.23	9.0	3.1 (0.3)	3.8	8.0	2.1
G23.44-0.18	9.2	3.0 (0.5)	7.7	3.9	4.3
G27.29+0.15	8.8	1.2 (0.2)	1.7	6.9	2.5
G27.31+0.18	8.3	0.5 (0.2)	1.7	2.7	6.3
G28.34+0.06P1	4.8	1.3 (0.5)	1.5	8.6	2.0
G28.34+0.06P2	8.7	1.4 (0.5)	7.8	1.8	9.6
G29.97-0.05	9.3	1.3 (0.7)	2.6	4.8	3.5
G79.34+0.33	8.4	0.7 (0.1)	2.1	3.5	4.9
G81.74+0.59	16.0	1.8 (0.2)	4.7	3.8	4.4

^a ${\rm H_2}$ column density derived from the 850 $\mu$ m dust continuum flux measured using SCUBA. The dust temperature is assumed to be equal to the gas temperature derived from ${\rm NH_3}$ (1, 1) and (2, 2) measurements as given in Table 3. ^b abundance of ${\rm C^{18}O}$ relative to H₂. ^c depletion factor compared to the canonical value of 1.7 $\cdot 10^{-7}$ (Frerking et al. 1982).

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