摘要：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.
