A Comparative Astrochemical Study Of The High-Mass Protostellar Objects NGC 7538 IRS 9 and IRS 1

TL;DR

This study compares the chemical compositions of two high-mass protostellar objects, NGC 7538 IRS 9 and IRS 1, revealing differences in their environments and providing insights into their evolutionary stages through high-resolution mid-infrared spectroscopy.

Contribution

It presents the first detailed spectroscopic comparison of these objects, analyzing molecular lines to understand their chemical and physical evolution.

Findings

Differences in spectral lines suggest varying radiation and thermal environments.

Temperatures and column densities are similar, but line widths differ.

Upper limit of 2000 AU for the continuum-emitting region.

Abstract

We report the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 and compare our observations to published data on the nearby object NGC 7538 IRS 1. Both objects originated in the same molecular cloud and appear to be at different points in their evolutionary histo- ries, offering an unusual opportunity to study the temporal evolution of envelope chemistry in objects sharing a presumably identical starting composition. Observations were made with the Texas Echelon Cross Echelle Spectrograph (TEXES), a sensitive, high spectral resolution (R = {\lambda}/{\Delta}{\lambda} \simeq 100,000) mid-infrared grating spectrometer. Forty-six individual lines in vibrational modes of the molecules C2H2, CH4, HCN, NH3 and CO were detected, including two isotopologues (13CO, 12C18O) and one combination mode ({\nu}4 + {\nu}5 C2H2). Fitting synthetic spectra to the data yielded the Doppler shift, excitation temperature, Doppler b parameter, column density and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radiation and thermal environments. Temperatures and column densities for the two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (\sim2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9.