Herschel observations of extraordinary sources: full Herschel/HIFI molecular line survey of Sagittarius B2(M)

Context. We present a full analysis of a broadband spectral line survey of Sagittarius B2 (Main), one of the most chemically rich regions in the Galaxy located within the giant molecular cloud complex Sgr B2 in the central molecular zone. Aims. Our goal is to derive the molecular abundances and temperatures of the high-mass star-forming region Sgr B2(M) and thus its physical and astrochemical conditions. Methods. Sgr B2(M) was observed using the Heterodyne Instrument for the Far-Infrared (HIFI) on board the Herschel Space Observatory in a spectral line survey from 480 to 1907 GHz at a spectral resolution of 1.1 MHz, which provides one of the largest spectral coverages ever obtained toward this high-mass star-forming region in the submillimeter with high spectral resolution and includes frequencies >1 THz that are unobservable from the ground. We modeled the molecular emission from the submillimeter to the far-infrared using the XCLASS program, which assumes local thermodynamic equilibrium. For each molecule, a quantitative description was determined taking all emission and absorption features of that species across the entire spectral range into account. Because of the wide frequency coverage, our models are constrained by transitions over an unprecedented range in excitation energy. Additionally, we derived velocity resolved ortho/para ratios for those molecules for which ortho and para resolved molecular parameters are available. Finally, the temperature and velocity distributions are analyzed and the derived abundances are compared with those obtained for Sgr B2(N) from a similar HIFI survey. Results. A total of 92 isotopologues were identified, arising from 49 different molecules, ranging from free ions to complex organic compounds and originating from a variety of environments from the cold envelope to hot and dense gas within the cores. Sulfur dioxide, methanol, and water are the dominant contributors. Vibrationally excited HCN (v(2) = 1) and HNC (v(2) = 1) are detected as well. For the ortho/para ratios, we find deviations from the high temperature values between 37 and 180%. In total 14% of all lines remain unidentified. Conclusions. Compared to Sgr B2(N), we found less complex molecules such as CH3OCH3, CH3NH2, or NH2CHO, but more simple molecules such as CN, CCH, SO, and SO2. However some sulfur bearing molecules such as H2CS, CS, NS, and OCS are more abundant in N than in M. The derived molecular abundances can be used for comparison to other sources and for providing further constraints for astrochemical models.

Automated summary

A comprehensive analysis of the broadband spectral line survey of Sagittarius B2 (Main) using HIFI revealed the presence of a variety of chemical species, with sulfur dioxide, methanol, and water being the dominant contributors. Deviations in ortho/para ratios ranged from 37% to 180%, and 14% of all lines remained unidentified.