EDP Sciences
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Volume 420, Number 3, June IV 2004
Page(s) 957 - 974
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20035915

A&A 420, 957-974 (2004)
DOI: 10.1051/0004-6361:20035915

Observations of L1521F: A highly evolved starless core

A. Crapsi1, 2, P. Caselli3, C. M. Walmsley3, M. Tafalla4, C. W. Lee5, T. L. Bourke6 and P. C. Myers2

1  Università degli Studi di Firenze Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 5, 50125 Firenze, Italy
2  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3  INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
4  Observatorio Astronómico Nacional (IGN), Alfonso XII, 3, 28014 Madrid, Spain
5  Taeduk Radio Astronomy Observatory, Korea Astronomy Observatory, 61-1 Hwaam-dong, Yusung-gu, Daejon 305-348, Korea
6  Harvard-Smithsonian Center for Astrophysics, Submillimeter Array Project, 645 N. A'ohoku Place, Hilo, HI 96720, USA

(Received 19 December 2003 / Accepted 19 March 2004)

We observed the pre-stellar core L1521F in dust emission at 1.2 mm and in two transitions each of , , and  in order to increase the sample of well studied centrally concentrated and chemically evolved starless cores, likely on the verge of star formation, and to determine the initial conditions for low-mass star formation in the Taurus Molecular Cloud. The dust observation allows us to infer the density structure of the core and together with measurements of CO isotopomers gives us the CO depletion. and lines are good tracers of the dust continuum and thus they give kinematic information on the core nucleus. We derived in this object a molecular hydrogen number density n( $)\sim 10^6$  $\hbox{{\rm cm}}^{-3}$ and a CO depletion factor, integrated along the line of sight, $f_{\rm D} \equiv 9.5\times10^{-5}/x_{\rm obs}({\rm CO})\sim 15$ in the central 20´´, similar to the pre-stellar core L1544. However, the  $N(\hbox{${\rm N}_2{\rm D}^+$ })/N(\hbox{${\rm N}_2{\rm H}^+$ })$ column density ratio is ~0.1, a factor of about 2 lower than that found in L1544. The observed relation between the deuterium fractionation and the integrated CO depletion factor across the core can be reproduced by chemical models if  is slightly (factor of ~2 in fractional abundance) depleted in the central 3000 AU. The  and  linewidths in the core center are ~0.3  $\hbox{{\rm km s}}^{-1}$, significantly larger than in other more quiescent Taurus starless cores but similar to those observed in the center of L1544. The kinematical behaviour of L1521F is more complex than seen in L1544, and a model of contraction due to ambipolar diffusion is only marginally consistent with the present data. Other velocity fields, perhaps produced by accretion of the surrounding material onto the core and/or unresolved substructure, are present. Both chemical and kinematical analyses suggest that L1521F is less evolved than L1544, but, in analogy with L1544, it is approaching the "critical" state.

Key words: radio lines: ISM -- radio continuum: ISM -- submillimeter

Offprint request: P. Caselli, caselli@arcetri.astro.it

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