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

Detections of CO, CS, SO, C2H,HCO, HCN,HNC, H2CO, and C3H2 are 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 × 1021 cm−2 (K km s−1)−1, slightly larger than the local Galactic disk value. The CO (1–0) beam averaged column density then becomesN (H2)≈ 3.7×1021 cm−2 and the density n (H2) ≈ 100 cm−3. A comparison with X-values from Rubio et al. (1993a) shows that on small scales (R ≈ 10 pc) X-values are more similar toGalactic disk values than previously anticipated, favoring a neutral interstellar medium of predominantly molecular nature in the cores. The I(13CO)/I(C18O) line intensity ratio indicates an underabundance of 12C18O relative to 13C16O w.r.t. Galactic clouds. I(HCO+)/I(HCN) and I(HCN)/I(HNC) line intensity ratios are > 1 and trace a warm (Tkin > 10K) 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, theCN1–0 line is substantiallyweaker 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. Fractional abundances of HCN, HNC, and likely CN are even two orders of magnitude below their ‘normal’, Galactic disk values. The CN/CS abundance ratio is <∼ 1. Based on Send offprint requests to: Y.-N. Chin, ASIAA, Taiwan, [email protected] ? Based on observations with the Swedish-ESO Submillimeter Telescope (SEST) at the European Southern Observatory (ESO), La Silla, Chile chemical model calculations, we suggest that this is because of the small metallicity of the SMC, which affects the destruction of CN but not CS, and because of the high molecular core density which also favors CN destruction.