Molecular abundances in the Magellanic Clouds III. LIRS 36, a star-forming region in the SMC

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Detections of CO, CS, SO, C2H, HCO+, HCN, HNC, H2CO, and C3H2 reported from LIRS 36, a star-forming region in the Small Magellanic Cloud. C18O, NO, CH3OH, and most notably CN have not been detected, while the rare isotopes 13CO and, tentatively, C34S are seen. This is so far the most extensive molecular multiline study of an interstellar medium with a heavy element depletion exceeding a factor of four. The X = N(H₂)/ICO conversion factor is 4.8 x 10²¹ cm-2 (Kkms-1)⁻¹, slightly larger than the local Galactic disk value. The CO (1--0) beam averaged column density then becomes N(H2) 3.7 x 10²¹ cm-2 and the density n(H2) 100 cm-3. I()/I(HCN) and I(HCN)/I(HNC) line intensity ratios are > 1 and trace a warm (Tkin > 10 K) molecular gas exposed to a high ionizing flux. Detections of the CS J=2--1, 3--2, and 5--4 lines imply the presence of a high density core with n(H2) = 10⁵ - 10⁷ cm-3. In contrast to star-forming regions in the LMC, the CN 1--0 line is substantially weaker than the corresponding ground rotational transitions of HCN, HNC, and CS. CO, CS, HCO+, and H2CO fractional abundances are a factor 10 smaller than corresponding values in Galactic disk clouds.