Inferring the evolutionary stages of the internal structures of NGC 7538 S and IRS1 from chemistry

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: syfeng@mpe.mpg.de
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
³ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA

Received: 3 September 2014
Accepted: 11 May 2016

Abstract

Context. Radiative feedback of young (proto)stars and gas dynamics including gravitational collapse and outflows are important in high-mass star-forming regions (HMSFRs), for the reason that they may leave footprints on the gas density and temperature distributions, the velocity profile, and the chemical abundances.

Aims. We unambiguously diagnose the detailed physical mechanisms and the evolutionary status of HMSFRs.

Methods. We performed 0.4′′ (~1000 AU) resolution observations at 1.37 mm towards two HMSFRs, NGC 7538 S and IRS1, using the Plateau de Bure Interferometre (PdBI). The observations covered abundant molecular lines, including tracers of gas column density, hot molecular cores, shocks, and complex organic molecules. We present a joint analysis of the 1.37 mm continuum emission and the line intensity of 15 molecular species (including 22 isotopologues). Assuming local thermal equilibrium (LTE), we derived molecular column densities and molecular abundances for each internal gas substructure that is spatially resolved. These derived quantities are compared with a suite of 1D gas-grain models.

Results. NGC 7538 S is resolved into at least three dense gas condensations. Despite the comparable continuum intensity of these condensations, their differing molecular line emission is suggestive of an overall chemical evolutionary trend from the northeast to the southwest. Line emission from MM1 is consistent with a chemically evolved hot molecular core (HMC), whereas MM3 remains a prestellar candidate that only exhibits emission of lower-excitation lines. The condensation MM2, located between MM1 and MM3, shows an intermediate chemical evolutionary status. Since these three condensations are embedded within the same parent gas core, their differing chemical properties are most likely due to the different warm-up histories, rather than the different dynamic timescales. Despite remaining spatially unresolved, in IRS1 we detect abundant complex organic molecules (e.g. NH₂CHO, CH₃OH, HCOOCH₃, CH₃OCH₃), indicating that IRS1 is the most chemically evolved HMC presented here. We observe a continuum that is dominated by absorption features with at least three strong emission lines, potentially from CH₃OH. The CH₃OH lines which are purely in emission have higher excitation than the ones being purely in absorption. Potential reasons for this difference are discussed.

Conclusions. This is the first comprehensive comparison of observations of the two high-mass cores NGC 7538 S and IRS1 and a chemical model. We have found that different chemical evolutionary stages can coexist in the same natal gas core. Our achievement illustrates the strength of chemical analysis for understanding HMSFRs.