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Dust sedimentation in protoplanetary disks with polycyclic aromatic hydrocarbons

Dullemond, C. P. and Henning, Th. and Visser, R. and Geers, V. C. and van Dishoeck, E. F. and Pontoppidan, K. M. (2007) Dust sedimentation in protoplanetary disks with polycyclic aromatic hydrocarbons. Astronomy and Astrophysics, 473 (2). pp. 457-466. ISSN 0004-6361. http://resolver.caltech.edu/CaltechAUTHORS:20100205-093834042

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Abstract

Context. Dust sedimentation is known to affect the infrared spectra and images of disks. In particular the far-infrared emission may be reduced by strong sedimentation. However, dust grains of different sizes sediment to different depths in the disk. Spectral features of one species may thus be enhanced, while those of other species may be suppressed. Aims. Polycyclic aromatic hydrocarbons (PAHs) are among the smallest “dust grains”. We investigate how the presence of PAHs in protoplanetary disks affects the disk’s spectral energy distribution (SED) and feature strengths when the thermal (large) grains are allowed to sediment. Methods. We calculate the vertical distribution of dust grains for both the PAH “dust” and the thermal dust grains.We include vertical settling and vertical mixing via turbulence. The results are inserted into a Monte-Carlo radiative transfer code to compute the SEDs. Results. For high turbulence the sedimentation barely affects the spectrum. For low turbulence, however, the PAHs still stay well-mixed in the disk’s surface layer, while the 0.1 μm size grains sediment deep into the disk. This strongly enhances the PAH features relative to the continuum (by factors of 2 to 10), while the far-infrared flux is reduced. This predicts that sources with weak far-infrared flux have stronger PAH features, which is – at least among Herbig Ae stars – opposite to what is observed, suggesting that sedimentation is not the only factor responsible for the weak mid- to far-infrared excess in some disks. We speculate that coagulation might be a solution, reducing both the mid- to far-infrared flux and the PAH features.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1051/0004-6361:20077581DOIArticle
ORCID:
AuthorORCID
van Dishoeck, E. F.0000-0001-7591-1907
Pontoppidan, K. M.0000-0001-7552-1562
Additional Information:© ESO 2007. Received 1 April 2007. Accepted 28 June 2007. We thank Carsten Dominik for useful comments. Astrochemistry in Leiden is supported by a NWO Spinoza grant and a NOVA grant, and by the European Research Training Network “The Origin of Planetary Systems” (PLANETS, contract number HPRN-CT-2002-00308).
Funders:
Funding AgencyGrant Number
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
Nederlandse Onderzoekschool voor de Astronomie (NOVA)UNSPECIFIED
European Research Training NetworkHPRN-CT-2002-00308
Subject Keywords:accretion; accretion disks; circumstellar matter; stars : formation; stars : pre-main-sequence; infrared : stars
Issue or Number:2
Record Number:CaltechAUTHORS:20100205-093834042
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20100205-093834042
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17394
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:10 Feb 2010 18:47
Last Modified:22 Aug 2017 20:17

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