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A&A 455, 577-593 (2006)
DOI: 10.1051/0004-6361:20065311
On the internal structure of starless cores
II. A molecular survey of L1498 and L1517B
M. Tafalla1, J. Santiago-García1, P. C. Myers2, P. Caselli2, 3, C. M. Walmsley3 and A. Crapsi41 Observatorio Astronómico Nacional, Alfonso XII 3, 28014 Madrid, Spain
e-mail: m.tafalla@oan.es
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
3 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
4 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
(Received 29 March 2006 / Accepted 12 May 2006 )
Abstract
Context.Low mass starless cores present an inhomogeneous chemical composition.
Species like CO and CS deplete at their dense interiors, while
N2H+ and NH3 survive in the gas phase. As molecular line
observations are used to determine the physical conditions and kinematics
of the core gas, chemical inhomogeneities can introduce a serious bias.
Aims.We have carried out a molecular survey towards two
starless cores, L1498 and L1517B.
These cores have been selected for their relative isolation and
close-to-round shape. They have been observed in a number of lines of
13 molecular species in order to determine a self-consistent set of
abundance profiles.
Methods.In a previous paper we modeled the physical structure
of L1498 and L1517B. Here we use this work
together with a spherically-symmetric Monte Carlo
radiative transfer code to determine the radial profile
of abundance for each species in the survey. Our model
aims to fit simultaneously the radial profile of integrated
intensity and the emerging spectrum from the core center.
Results.L1498 and L1517B present similar abundance patterns,
with most species suffering a significant drop
toward the core center. This occurs for
CO, CS, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO,
HCO+, and DCO+, which we fit with profiles
having a sharp central hole. The size of this hole varies with
molecule: DCO+, HCN, and HC3N have the smallest holes,
while SO, C2S and CO have the largest holes. Only N2H+
and NH3 seem present in the gas phase at the core centers.
Conclusions.From the different behavior of molecules, we select
SO, C2S, and CH3OH as the most sensitive tracers of molecular
depletion. Comparing our abundance determinations with the predictions
from current chemical models we find order of
magnitude discrepancies. Finally, we show how the
"contribution function" can be used to study the formation
of line profiles from the different regions of a core.
Key words: ISM: abundances -- ISM: clouds -- ISM: molecules -- stars: formation -- ISM: individual objects: L1498 -- ISM: individual objects: L1517B
© ESO 2006