Spectral survey of the star-forming region W51 e1/e2 at 3 mm

Abstract

A spectral survey of the W51 e1/e2 star-forming region at 84-115 GHz has yielded detections of 105 molecules and their isotopic species, from simple diatomic or triatomic molecules, such as CO, CS, HCN, up to complex organic compounds, such as CH3OCH3, CH3COCH3, and C2H5OOCH. Ninety-three lines that are absent from the Lovas list of molecular lines observed from space were detected, and approximately half of these were identified. A significant number of the detectedmolecules are typical for hot cores. These include the neutral molecules CH3OCHO, C2H5OH, CH3COCH3 etc., which are currently believed to exist in the gas phase only in hot cores and shock-heated gas. In addition, vibrationally excited SiO, C4H, HCN, l-C3H, HCCCN, CH3CN, CH3OH, H2O, and SO2 lines with upper-level temperatures of several hundred Kelvin were found. Such lines can arise only in hot gas with temperatures of the order of 100 K or higher. Apart from neutral molecules, various molecular ions were also detected. Some of these (N2H+, HCO+, HCS+) usually exist in molecular clouds with high visual extinctions A (V) . At the same time, the CF+ ion should be observed in photon-dominated regions with A (V) values of about unity or lower. An interesting result is the tentative detection of two molecules that have thus far been observed only in the atmospheres of late-type giant stars-MgCN and NaCN. This suggests that the conditions in the hottest W51 regions (probably, in the vicinities of protostars) are close to those in the atmospheres of giant stars. It would be desirable to search for other lines of these molecules to verify these tentative detections. Analysis of the radial velocities of the detected molecules suggests that the contribution from the e2 core dominates the emission of some O-bearing molecules (CH3OCHO, CH3CH2OH), while the contribution of the e1 core dominates the emission of some N-bearing molecules (e.g., CH3CH2CN). Thus, the molecular composition of the e2 core may be closer to the composition of the "Compact Ridge" in OMC-1, while the composition of the e1 core is closer to that for the "Hot Core" in the same cloud.