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TwitterA&A 418, 185-202 (2004)
DOI: 10.1051/0004-6361:20035769
On the origin of H
CO abundance enhancements in low-mass
protostars
F. L. Schöier1, 2, J. K. Jørgensen1, E. F. van Dishoeck1 and G. A. Blake3
1 Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
2 Stockholm Observatory, AlbaNova, 106 91 Stockholm, Sweden
3 Division of Geological and Planetary Sciences, California Institute of Technology, MS 150-21, Pasadena, CA 91125, USA
(Received 1 December 2003 / Accepted 23 January 2004)
Abstract
High angular resolution H
2CO 218 GHz line observations
have been carried out toward the low-mass protostars
IRAS 16293-2422
and
L1448-C
using the Owens Valley
Millimeter Array at ~2
resolution. Simultaneous 1.37 mm
continuum data reveal extended emission which is compared with that
predicted by model envelopes constrained from single-dish data. For
L1448-C
the model density structure works well down to the 400 AU scale to which the interferometer is sensitive. For
IRAS 16293-2422
, a known proto-binary object, the
interferometer observations indicate that the binary has cleared much
of the material in the inner part of the envelope, out to the binary
separation of ~800 AU. For both sources there is excess
unresolved compact emission centered on the sources, most likely due
to accretion disks
200 AU in size with masses of
0.02
(L1448-C) and
0.1
(IRAS 16293-2422). The H
2CO data for both sources are dominated
by emission from gas close to the positions of the continuum peaks.
The morphology and velocity structure of the H
2CO array data have
been used to investigate whether the abundance enhancements inferred
from single-dish modelling are due to thermal evaporation of ices or
due to liberation of the ice mantles by shocks in the inner envelope.
For
IRAS 16293-2422
the H
2CO interferometer observations
indicate the presence of rotation roughly perpendicular to
the large scale CO outflow. The H
2CO distribution differs from
that of C
18O, with C
18O emission peaking near MM1 and H
2CO stronger near MM2. For
L1448-C
, the region of
enhanced H
2CO emission extends over a much larger scale
>1
'' than
the radius of
50-100 K (
) where thermal
evaporation can occur. The red-blue asymmetry of the emission is
consistent with the outflow; however the velocities are significantly
lower. The H
2CO
322-221/303-202 flux ratio derived
from the interferometer data is significantly higher than that found
from single-dish observations for both objects, suggesting that the
compact emission arises from warmer gas. Detailed radiative transfer
modeling shows, however, that the ratio is affected by abundance
gradients and optical depth in the
303-202 line. It is
concluded that a constant H
2CO abundance throughout the envelope
cannot fit the interferometer data of the two H
2CO lines
simultaneously on the longest and shortest baselines. A scenario in
which the H
2CO abundance drops in the cold dense part of the
envelope where CO is frozen out but is undepleted in the outermost
region provides good fits to the single-dish and interferometer data
on short baselines for both sources. Emission on the longer baselines
is best reproduced if the H
2CO abundance is increased by about an order
of magnitude from ~
10-10 to ~
10-9
in the inner parts of the envelope due to
thermal evaporation when the temperature exceeds ~50 K. The
presence of additional H
2CO abundance jumps in the innermost hot
core region or in the disk cannot be firmly
established, however, with the present sensitivity and resolution.
Other scenarios, including weak outflow-envelope
interactions and photon heating of the envelope, are discussed and
predictions for future generation interferometers are presented,
illustrating their potential in distinguishing these competing
scenarios.
Key words: astrochemistry -- stars: formation -- stars: circumstellar matter -- stars: individual: IRAS 16293-2422 , L1448-C -- ISM: abundances
Offprint request: F. L. Schöier, fredrik@astro.su.se
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