Thermal Formaldehyde Emission in NGC7538 IRS1

TL;DR

This study reports the detection and analysis of rare thermal formaldehyde emission at cm wavelengths in NGC7538 IRS1, revealing dense gas conditions and extending understanding of molecular emissions in star-forming regions.

Contribution

It provides new observations of thermal H2CO emission at multiple wavelengths and constrains its spatial extent, offering insights into the physical conditions of dense star-forming gas.

Findings

Detected 2 cm formaldehyde emission with a size of ~50 arcseconds.

Non-LTE analysis indicates emission from dense gas (~10^5 to 10^6 cm^-3).

Observed extended emission of multiple molecules like CH3OH, NH3, and SO.

Abstract

Spectral lines from formaldehyde (H2CO) molecules at cm wavelengths are typically detected in absorption and trace a broad range of environments, from diffuse gas to giant molecular clouds. In contrast, thermal emission of formaldehyde lines at cm wavelengths is rare. In previous observations with the 100m Robert C. Byrd Green Bank Telescope (GBT), we detected 2 cm formaldehyde emission toward NGC7538 IRS1 - a high-mass protostellar object in a prominent star-forming region of our Galaxy. We present further GBT observations of the 2 cm and 1 cm H2CO lines to investigate the nature of the 2 cm H2CO emission. We conducted observations to constrain the angular size of the 2 cm emission region based on a East-West and North-South cross-scan map. Gaussian fits of the spatial distribution in the East-West direction show a deconvolved size (at half maximum) of the 2 cm emission of 50" +/- 8". The 1 cm H2CO observations revealed emission superimposed on a weak absorption feature. A non-LTE radiative transfer analysis shows that the H2CO emission is consistent with quasi-thermal radiation from dense gas (~10^5 to 10^6 cm^-3). We also report detection of 4 transitions of CH3OH (12.2, 26.8, 28.3, 28.9 GHz), the (8,8) transition of NH3 (26.5 GHz), and a cross-scan map of the 13 GHz SO line that shows extended emission (> 50").