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

TL;DR

This study presents an extensive molecular line survey of the star-forming region LIRS 36 in the Small Magellanic Cloud, revealing molecular abundances, physical conditions, and chemical differences compared to Galactic regions.

Contribution

It provides the most comprehensive molecular multiline analysis of an interstellar medium with significant heavy element depletion, highlighting unique chemical and physical properties.

Findings

Detected multiple molecules including CO, CS, SO, C2H, HCO+, HCN, HNC, H2CO, C3H2.

Identified a high-density core with n(H2) = 10^5 - 10^7 cm-3.

Found molecular abundances about 10 times lower than in Galactic disk clouds.

Abstract

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(H_2)/I_{CO} conversion factor is ~ 4.8 x 10^{21} 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 10^{21} cm-2 and the density n(H2) ~ 100 cm-3. I(\HCOp)/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) = 10^5 - 10^7 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.