Image Profile (IMPRO) Fitting of Massive Protostars. I. Method Development and Test Cases of Cepheus A and G35.20-0.74N

TL;DR

This paper introduces IMPRO, a new image profile fitting method that enhances the measurement of physical properties of massive protostars by combining MIR imaging with SED fitting, tested on Cepheus A and G35.20-0.74N.

Contribution

The paper develops and demonstrates a novel IMPRO fitting pipeline that improves protostellar property constraints by integrating MIR image profiles with SED fitting.

Findings

IMPRO fitting yields tighter constraints on protostar properties.

Spatial resolution limits the method's effectiveness for distant sources.

Higher resolution MIR data will expand the method's applicability.

Abstract

Massive stars play a critical role in the evolution of galaxies, but their formation remains poorly understood. One challenge is accurate measurement of the physical properties of massive protostars, such as current stellar mass, envelope mass, outflow cavity properties, and system orientation. Spectral energy distribution (SED) fitting is widely-used to test models against observations. The far-infrared SED traces cold dust in envelopes, while the near- and mid-infrared (MIR) probes emission from outflow cavities and/or the inner envelope. However, SED fitting has degeneracy limiting its ability to yield accurate measurements of protostellar properties. Here, we develop image profile (IMPRO) fitting as a method to improve the characterization of protostars. We utilize brightness distributions from multi-wavelength MIR images of massive protostars taken by SOFIA/FORCAST as part of the SOFIA Massive Star Formation (SOMA) survey to constrain protostellar properties via comparison to a grid of radiative transfer models. We develop a fitting pipeline to extract information along the outflow axis, which is then combined with the SED fitting to yield improved constraints on protostellar properties. We apply the IMPRO fitting method on the nearby massive protostar Cepheus A, finding that its properties become more tightly constrained compared to SED fitting, especially in the inclination of the source. However, for the more distant G35.20-0.74N, we find that the spatial resolution of SOFIA/FORCAST limits the utility of this combined fitting pipeline. However, higher resolution MIR observations, e.g., with JWST, are expected to greatly expand the applicability of this fitting technique to protostars across the Galaxy.