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Issue A&A Volume 417, Number 1, April I 2004 Page(s) 159 - 168 Section Interstellar and circumstellar matter DOI 10.1051/0004-6361:20031768

A&A 417, 159-168 (2004)
DOI: 10.1051/0004-6361:20031768

Flaring vs. self-shadowed disks: The SEDs of Herbig Ae/Be stars

C. P. Dullemond¹ and C. Dominik<sup>2

1</sup>  Max Planck Institut für Astrophysik, PO Box 1317, 85741 Garching, Germany
²  Sterrenkundig Instituut "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
    e-mail: dominik@science.uva.nl

(Received 11 June 2003 / Accepted 19 December 2003)

Abstract
Isolated Herbig Ae stars can be divided into two groups (Meeus et al. 2001): those with an almost flat spectral energy distribution in the mid-infrared ("group I"), and those with a strong decline towards the far-infrared ("group II"). In this paper we show that the group I vs. II distinction can be understood as arising from flaring vs. self-shadowed disks. We show that these two types of disks are natural solutions of the 2D radiation-hydrostatic structure equations. Disks with high optical depth turn out to be flaring and have a strong far-IR emission, while disks with an optical depth below a certain threshold drop into the shadow of their own puffed-up inner rim and are weak in the far-IR. In spite of not having a directly irradiated surface layer, self-shadowed disks still display dust features in emission, in agreement with observations of group II sources. We propose an evolutionary scenario in which a disk starts out with a flaring shape (group I source), and then goes through the process of grain growth, causing the optical depth of the disk to drop and the disk to become self-shadowed (group II source). We show that this scenario predicts that the (sub-)millimeter slope of the disk changes from steep (small grains) to Rayleigh-Jeans-like (large grains) in the early stages of evolution, so that all group II sources are expected to have Rayleigh-Jeans-like slopes, while some group I sources may still have steep (sub-)millimeter slopes.

Key words: accretion, accretion disks -- stars: circumstellar matter -- stars: formation -- stars: pre-main-sequence -- infrared: stars

Offprint request: C. P. Dullemond, dullemon@mpa-garching.mpg.de

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