Spectral-line Survey of the Region of Massive Star Formation W51e1/e2 in the 4 mm Wavelength Range

Abstract

We present the results of a spectral-line survey of the W51e1/e2 star-forming region at 68-88 GHz. 79 molecules and their isotopologues were detected, from simple diatomic or triatomic molecules, such as SO, SiO, and CCH, to complex organic compounds, such as CH${}_3$OCH${}_3$ or CH${}_3$COCH${}_3$. A number of lines that are absent from the Lovas list of molecular lines observed in space were detected, and most of these were identified. A significant number of the detected molecules are typical for hot cores. These include the neutral molecules HCOOCH${}_3$, CH${}_3$CH${}_2$OH, CH${}_3$COCH${}_3$ etc, which are currently believed to exist in the gas phase only in hot cores and shock-heated gas. In addition, vibrationally excited C${}_4$H and HC${}_3$N lines with upper-level energies of several hundred Kelvins were found. Such lines can arise only in hot gas with temperatures on the order of 100 K or higher. Apart from neutral molecules, various molecular ions were also detected. Some of these (HC${}^{18}$O${}^{+}$, H${}^{13}$CO${}^{+}$, and HCS${}^{+}$) usually exist in molecular clouds with high visual extinctions. Potential formation pathways of complex organic molecules (COMs) and of hydrocarbons, along with nitriles, are considered. These formation routes are first discussed in the context of laboratory experiments elucidating the synthesis of organic molecules in interstellar ices in cold molecular clouds, followed by sublimation into the gas phase in the hot core stage. Thereafter, we discuss the predominant formation of hydrocarbons and their nitriles in the gas phase through bimolecular neutral-neutral reactions.

22 pages, 8 figures, 5 tables, Accepted for publication in ApJ

References in corpus (6)

• Detection of Two Interstellar Polycyclic Aromatic Hydrocarbons via Spectral Matched Filtering
• Early Science from GOTHAM: Project Overview, Methods, and the Detection of Interstellar Propargyl Cyanide (HCCCH${}_2$CN) in TMC-1
• The ALMA-PILS survey: propyne (CH${}_3$CCH) in IRAS 16293$-$2422
• Analytical Methods for Measuring the Parameters of Interstellar Gas Using the Data of Methanol Observations
• Uniform Silicon Isotope Ratios Across the Milky Way Galaxy
• First detection of CS masers around a high-mass young stellar object, W51 e2e

Cited by in corpus (4)

• Observational studies on S-bearing molecules in massive star forming regions
• Revisiting rotationally excited CH at radio wavelengths: A case study towards W51
• Looking for evidence of high-mass star formation at core scale in a massive molecular clump
• Spectral Survey of the Star Formation Region DR21OH in the 4 mm Wavelength Range