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Volume 377, Number 2, October II 2001
Page(s) 566 - 580
Section Formation, structure and evolution of stars
DOI https://doi.org/10.1051/0004-6361:20011137
Published online 15 October 2001

A&A 377, 566-580 (2001)
DOI: 10.1051/0004-6361:20011137

Submillimeter lines from circumstellar disks around pre-main sequence stars

G.-J. van Zadelhoff1, E. F. van Dishoeck1, W.-F. Thi1 and G. A. Blake2

1  Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
2  Division of Geological and Planetary Sciences, California Institute of Technology, MS 150-21, Pasadena, CA 91125, USA

(Received 13 November 2000 / Accepted 9 August 2001)


Observations of submillimeter lines of CO, HCO+, HCN and their isotopes from circumstellar disks around low mass pre-main sequence stars are presented. CO lines up to $J=6\to5$, and HCO+ and HCN lines up to $J=4\to3$, are detected from the disks around LkCa 15 and TW Hya. These lines originate from levels with higher excitation temperatures and critical densities than studied before. Combined with interferometer data on lower excitation lines, the line ratios can be used to constrain the physical structure of the disk. The different line ratios and optical depths indicate that most of the observed line emission arises from an intermediate disk layer with high densities of 106-108 cm-3 and moderately warm temperatures in the outer regions. The data are compared with three different disk models from the literature using a full 2D Monte Carlo radiative transfer code. The abundances of the molecules are constrained from the more optically thin 13C species and indicate depletions of $\approx$ 1-30 for LkCa 15 and very high depletions of >100 for TW Hya with respect to dark cloud abundances. Evidence for significant freeze-out (factors of 10 or larger) of CO and HCO+ onto grain surfaces at temperatures below 22 K is found, but the abundances of these molecules must also be low in the warmer upper layer, most likely as a result of photodissociation. A warm upper layer near the surface of a flaring disk heated by stellar and interstellar radiation is an appropriate description of the observations of TW Hya. LkCa 15 seems to be cooler at the surface, perhaps due to dust settling. The density constraints are also well fitted by the flared disk models.

Key words: stars: circumstellar matter -- stars: pre-main sequence -- stars: planetary systems: protoplanetary disks -- accretion, accretion disks

Offprint request: G.-J. van Zadelhoff, zadelhof@strw.leidenuniv.nl

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© ESO 2001

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