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Submillimeter lines from circumstellar disks around pre-main sequence stars

van Zadelho, G.-J. and van Dishoeck, E. F. and Thi, W.-F. and Blake, G. A. (2001) Submillimeter lines from circumstellar disks around pre-main sequence stars. Astronomy and Astrophysics, 377 (2). pp. 566-580. ISSN 0004-6361.

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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 → 5, and HCO^+ and HCN lines up to J = 4 → 3, 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 10^6−10^8 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 ^(13)C species and indicate depletions of ≈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.

Item Type:Article
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URLURL TypeDescription DOIArticle
van Dishoeck, E. F.0000-0001-7591-1907
Blake, G. A.0000-0003-0787-1610
Additional Information:© 2001 ESO. Received 13 November 2000. Accepted 9 August 2001. The authors are very grateful to P. D'Alessio, R. Bell and E. Chiang for sending and discussing the models used in the paper. They thank M. Hogerheijde and F. van der Tak for useful discussions and providing their radiative transfer code, and are grateful to the staff of the CSO and JCMT for their support. Astrochemistry in Leiden is supported by a SPINOZA grant from The Netherlands Organization for Scientific Research (NWO). This paper is dedicated to Fred Baas, who died on April 4, 2001. His expert, generous support at the JCMT was essential to make these observations possible.
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Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
Subject Keywords:stars: circumstellar matter -- stars: pre-main sequence -- stars: planetary systems: protoplanetary disks -- accretion, accretion disks
Issue or Number:2
Record Number:CaltechAUTHORS:20120917-130527589
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Official Citation:Submillimeter lines from circumstellar disks around pre-main sequence stars G.-J. van Zadelhoff, E. F. van Dishoeck, W.-F. Thi and G. A. Blake A&A 377 (2) 566-580 (2001) DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:34144
Deposited By: Tony Diaz
Deposited On:17 Sep 2012 21:25
Last Modified:03 Oct 2019 04:16

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