Issue |
A&A
Volume 470, Number 1, July IV 2007
|
|
---|---|---|
Page(s) | 221 - 230 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20077613 | |
Published online | 10 May 2007 |
Observing the gas temperature drop in the high-density nucleus of L 1544*
1
Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: crapsi@arcetri.astro.it
2
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
3
Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MS 42, Cambridge, MA 02138, USA
4
Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain
Received:
5
April
2007
Accepted:
27
April
2007
Context.The thermal structure of a starless core is crucial for our understanding of the physics in these objects and hence for our understanding of star formation. Theory predicts a gas temperature drop in the inner ~5000 AU of the pre-stellar core L 1544, but there has been no observational proof of this.
Aims.We performed VLA observations of the NH3 (1, 1) and (2, 2)
transitions towards L 1544 in order to
measure the temperature gradient between the high density core
nucleus and the surrounding core envelope. Our VLA observation for the first time
provide measurements of gas temperature in a core with a resolution smaller than 1000 AU.
We have also obtained high resolution Plateau de Bure
observations of the 110 GHz para–NH2D line in order to further constrain
the physical parameters of the high density nucleus.
Methods.We combine our interferometric NH3 and NH2D observations
with available single dish measurements in order to estimate
the effects of flux loss from extended components upon our data.
We have estimated the temperature
gradient using a model of the source to fit our data in the plane.
As the NH3(1, 1) line is extremely optically thick, this also
involved fitting a gradient in the NH3 abundance.
In this way, we also measure the [ NH2D] /[ NH3] abundance ratio in
the inner nucleus.
Results.We find that indeed the temperature decreases toward the core nucleus
from 12 K down to 5.5 K resulting in an increase of a factor of 50% in the estimated density of the core from the
dust continuum if compared with the estimates done with constant temperature of 8.75 K.
Current models of the thermal equilibrium can describe consistently the observed temperature and
density in this object, simultaneously fitting our temperature profile and the continuum emission.
We also found a remarkably high abundance of deuterated ammonia with respect to the ammonia abundance (50% ± 20%),
which proves the persistence of
nitrogen bearing molecules at very high densities ( cm-3) and shows that high-resolution
observations yield higher deuteration values than single-dish observations. The NH2D observed transition, free of the optical depth
problems that affect the NH3 lines in the core center, is a much better probe of the high-density nucleus and, in fact, its map peak at the
dust continuum peak. Our analysis of the NH3 and NH2D kinematic fields
shows a decrease of specific angular momentum from the large scales to the small scales.
Key words: ISM: clouds / ISM: evolution / ISM: individual objects: L1544 / ISM: molecules / stars: formation / techniques: interferometric
Based on observations carried out with the IRAM Plateau de Bure Interferometer and the Very Large Array. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
© ESO, 2007
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