The SOFIA massive (SOMA) star formation survey. I. Overview and first results

Abstract

We present an overview and first results of the Stratospheric Observatory For Infrared Astronomy Massive (SOMA) Star Formation Survey, which is using the FORCAST instrument to image massive protostars from similar to 10 to 40 mu m. These wavelengths trace thermal emission from warm dust, which in Core Accretion models mainly emerges from the inner regions of protostellar outflow cavities. Dust in dense core envelopes also imprints characteristic extinction patterns at these wavelengths, causing intensity peaks to shift along the outflow axis and profiles to become more symmetric at longer wavelengths. We present observational results for the first eight protostars in the survey, i.e., multiwavelength images, including some ancillary ground-based mid-infrared (MIR) observations and archival Spitzer and Herschel data. These images generally show extended MIR/FIR emission along directions consistent with those of known outflows and with shorter wavelength peak flux positions displaced from the protostar along the blueshifted, near-facing sides, thus confirming qualitative predictions of Core Accretion models. We then compile spectral energy distributions and use these to derive protostellar properties by fitting theoretical radiative transfer models. Zhang and Tan models, based on the Turbulent Core Model of McKee and Tan, imply the sources have protostellar masses m(*) similar to 10-50M circle dot accreting at similar to 10(-4)-10(-3) M circle dot yr (1) inside cores of initial masses M-c similar to 30-500M circle dot embedded in clumps with mass surface densities Sigma(cl) similar to 0.1-3 g cm(-2). Fitting the Robitaille et al. models typically leads to slightly higher protostellar masses, but with disk accretion rates similar to 100x smaller. We discuss reasons for these differences and overall implications of these first survey results for massive star formation theories.