A Chemical Exploration of Massive Star-Forming Regions: Unraveling the Molecular Complexity of Hot Cores

Author: ORCID icon orcid.org/0000-0002-1577-5322
El-Abd, Samer, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Brogan, Crystal, Astronomy, University of Virginia / National Radio Astronomy Observatory
McGuire, Brett, Chemistry, Massachusetts Institute of Technology

The short-lived massive star formation process is difficult to observe in its early stages and therefore an ill-understood mechanism. Of the regions we have been able to observe, much of the information we are able to glean comes from analysis of their molecular emission spectra, particularly in the radio regime. Star-forming regions are hotbeds of interstellar chemistry; the physical conditions in these regions are conducive to producing a large number of complex interstellar molecules. Attaining a better understanding of this unique chemistry will in turn better inform us on the ongoing physical processes. In this work I describe my efforts to better understand and characterize the molecular emission from massive star-forming regions, and subsequently the physical conditions in which they are found.

The relative column densities of the structural isomers methyl formate, glycolaldehyde, and acetic acid are first derived for a dozen positions towards the massive star-forming regions MM1 and MM2 in the NGC 6334I complex, which are separated by about 4000 au. Relative column densities of these molecules are also gathered from the literature for 13 other star-forming regions. In this combined dataset, a clear bi-modal distribution is observed in the relative column densities of glycolaldehyde and methyl formate. No such distribution is evident with acetic acid. The two trends are comprised of star-forming regions with a variety of masses, suggesting that there must be some other common parameter that is heavily impacting the formation of glycolaldehyde. This is indicative of some demonstrable differentiation in these cores; studying the abundances of these isomers may provide a clue as to the integral chemical processes ongoing in a variety of protostellar environments.

The time-consuming nature of fitting synthetic spectra to observations interactively for such line-rich sources often results in such analysis being limited to data extracted from a single-dish observation or a handful of pixels from an interferometric observation. Yet, star-forming regions display a wide variety of physical conditions that are difficult, if not impossible, to accurately characterize with such a limited number of spectra. I have developed an automated fitting routine that visits every pixel in the field of view of an ALMA data cube and determines the best-fit physical parameters, including excitation temperature and column densities, for a given list of molecules. In this proof-of-concept work, I provide an overview of the fitting routine and apply it to 0.26", 1.1 km/s resolution ALMA observations of two sites of massive star-formation in NGC 6334I. Parameters were found for 21 distinct molecules by generating synthetic spectra across 7.48 GHz of spectral bandwidth between 280 and 351 GHz. Spatial images of the derived parameters for each of the >8000 pixels are presented with special attention paid to the C2H4O2 isomers and their relative variations. I highlight the greater scientific utility of the column density and velocity images of individual molecules compared to traditional moment maps of single transitions.

I then apply this routine - the Simultaneous Autonomous Molecular Emission Resolver (SAMER) - to a new high-mass star-forming region, G34.41+0.24. This source is part of the Complex Chemistry in hot Cores with ALMA (CoCCoA) survey, which aims to improve our understanding of the massive star formation process through the observation of twenty-five hot cores in a self-consistent manner across a common spatial resolution, spectral resolution, and frequency range. Preliminary images of the excitation temperature, velocities, and various molecules will be presented, as will interpretations on how they inform us on the physical processes driven by the hot core.

Future directions and applications following the results of this work will also be discussed.

PHD (Doctor of Philosophy)
Star Formation, Astrochemistry
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