The SOMA MM Survey. I. An Astrochemical Census of Massive Protostars

TL;DR

This study conducts an astrochemical census of 22 massive protostars from the SOMA survey using ALMA ACA+TP Band 6 at 1.3 mm to derive column densities, line widths, and excitation temperatures for 35 molecular species through LTE modeling with MADCUBA. The sample includes seven line-rich cores with >100 transitions and reveals TEX distributions that separate hot-core-like gas (Tex > 100 K) from lukewarm, chemically simple gas (Tex < 50 K). By comparing chemistry to SED-derived evolutionary indicators, notably $L_ ext{bol}/M_ ext{env}$, the authors identify tentative correlations of line widths, Tex, and molecular column densities with evolutionary stage, and they detect H$30 extalpha$ and He$30 extalpha$ recombination lines across a range of chemistries. The results provide valuable constraints for chemodynamical models of massive protostellar cores and motivate higher-resolution follow-up to resolve internal multiplicity and gas motions.

Abstract

During massive star formation, dense gas undergoes chemical evolution, producing both simple and complex organic molecules (COMs) characteristic of hot molecular cores. How this evolution depends on protostellar physical properties remains unclear. We investigate the chemical content of 22 well-studied massive protostars from the SOFIA Massive (SOMA) Star Formation survey, aiming to identify correlations between chemical and physical parameters. We analyzed Atacama Compact Array and Total Power 1.3 mm (Band 6) data, deriving column densities, line widths, and excitation temperatures of multiple molecular species by modeling detected lines under local thermodynamic equilibrium (LTE) using MADCUBA. Spectra show 35 species, from simple molecules (e.g., CO, SO, SiO) to complex organic molecules (COMs), with seven sources exhibiting high chemical complexity (> 100 transitions). Average excitation temperatures vary across the sample: $T_\text{ex}>100~\text{K}$ for eight sources, $50-100~\text{K}$ for four, and $T_\text{ex} < 50~\text{K}$ for the remainder. Sources with $T_\text{ex} < 50~\text{K}$ trace lukewarm, chemically simple gas, while those with $T_\text{ex}>100~\text{K}$ indicate the presence of typical hot cores where thermal desorption is efficient, resulting in line-rich spectra. Comparing these chemical properties with the bolometric luminosity to envelope mass ratio ($L_\text{bol}/M_\text{env}$), an evolutionary tracer, we find tentative correlations with line widths, excitation temperature, and column densities. These data provide important constraints for chemodynamical models of massive protostellar cores.

Figures (31)

• Figure 1: The 1.33 mm continuum emission maps of the SOMA MM sample obtained with ALMA at $\sim225.7~$GHz. Contour levels start at 3$\sigma$ (1$\sigma$ level is shown in Table \ref{['tab:observations']} for each source) and steps are in 3$\sigma$. The red-dashed circles represent the aperture size from which we extracted the spectra. The black stars show the peaks of emission. The name of the region is shown in the upper-left corner of each map, and a letter is added as a label if more than one source is detected in the same region. The ellipses at the bottom-left corner of the images show the synthesized beam (see Table \ref{['tab:observations']}), and a linear scale is displayed at the bottom right.
• Figure 2: Top: Number of sources with detection for each species over the 22 sources studied. Middle: Number of detected molecules for each source. Bottom: Number of detected transitions for each source, including separate values of the main and rarer isotopologues.
• Figure 3: Summary of the detected species observed in the sample.
• Figure 4: Correlation between the column densities ($N_\text{tot}$) of chemically linked pairs of molecules. A linear fit is drawn on the plot, and in the top-right corner, the Pearson correlation coefficient ($\rho$) for each dataset is displayed. Each data point represents a single targeted source.
• Figure 5: Correlation between the line width (FWHM) of chemically linked pairs of molecules. Each data point represents a single targeted source. The Pearson correlation coefficient ($\rho$) for each dataset is displayed.