Constraining the physical structure of the inner few 100 AU scales of deeply embedded low-mass protostars^⋆,⋆⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: magnusp@strw.leidenuniv.nl
² Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik (ITA), Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
³ Max-Planck Institute für extraterrestrische Physik (MPE), Giessenbachstrasse, 85748 Garching, Germany
⁴ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 København K, Denmark
⁵ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark
⁶ Institute of Astronomy and Department of Physics, National Tsing Hua University, 101 Section 2 Kuang Fu Road, 30013 Hsinchu, Taiwan
⁷ Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, 10617 Taipei, Taiwan

Received: 30 October 2015
Accepted: 2 March 2016

Abstract

Context. The physical structure of deeply embedded low-mass protostars (Class 0) on scales of less than 300 AU 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 100 AU) 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 (~1000 AU) and investigated further for one of the sources on smaller scales.

Results. The radii of the fitted disk-like structures range from ~90−170 AU, and the derived masses depend on the method. Using the Gaussian disk model results in masses of 54−556 × 10^-3 M_⊙, and using the power-law disk model gives 9−140 × 10^-3 M_⊙. 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 100 K varies from 7% to 30%.

Conclusions. A thin disk model can approximate the emission and physical structure in the inner few 100 AU 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 6.2 × 10^-5 (IRAS 2A), 0.33 × 10^-5 (IRAS 4A-NW), 1.8 × 10^-7 (IRAS 4B), and < 2 × 10^-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.