2016A&A...590A..33P

Context. The physical structure of deeply embedded low-mass protostars (Class 0) on scales of less than 300AU is still poorly constrained. While molecular line observations demonstrate the presence of disks with Keplerian rotation toward a handful of sources, others show no hint of rotation. Determining the structure on small scales (a few 100AU) is crucial for understanding the physical and chemical evolution from cores to disks.
Aims. We determine the presence and characteristics of compact, disk-like structures in deeply embedded low-mass protostars. A related goal is investigating how the derived structure affects the determination of gas-phase molecular abundances on hot-core scales.
Methods. Two models of the emission, a Gaussian disk intensity distribution and a parametrized power-law disk model, are fitted to subarcsecond resolution interferometric continuum observations of five Class 0 sources, including one source with a confirmed Keplerian disk. Prior to fitting the models to the de-projected real visibilities, the estimated envelope from an independent model and any companion sources are subtracted. For reference, a spherically symmetric single power-law envelope is fitted to the larger scale emission (∼1000AU) and investigated further for one of the sources on smaller scales.
Results. The radii of the fitted disk-like structures range from ∼90-170AU, and the derived masses depend on the method. Using the Gaussian disk model results in masses of 54-556x10^–3M_☉, and using the power-law disk model gives 9-140x10^–3M_☉. While the disk radii agree with previous estimates the masses are different for some of the sources studied. Assuming a typical temperature distribution (r^–0.5), the fractional amount of mass in the disk above 100K varies from 7% to 30%.
Conclusions. A thin disk model can approximate the emission and physical structure in the inner few 100AU scales of the studied deeply embedded low-mass protostars and paves the way for analysis of a larger sample with ALMA. Kinematic data are needed to determine the presence of any Keplerian disk. Using previous observations of p-H₂¹⁸O, we estimate the relative gas phase water abundances relative to total warm H₂ to be 6.2x10^–5 (IRAS 2A), 0.33x10^–5 (IRAS 4A-NW), 1.8x10^–7 (IRAS 4B), and <2x10^–7 (IRAS 4A-SE), roughly an order of magnitude higher than previously inferred when both warm and cold H₂ were used as reference. A spherically symmetric single power-law envelope model fails to simultaneously reproduce both the small- and large-scale emission.