Volume 417, Number 1, April I 2004
|Page(s)||159 - 168|
|Section||Interstellar and circumstellar matter|
|Published online||16 March 2004|
Flaring vs. self-shadowed disks: The SEDs of Herbig Ae/Be stars
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: firstname.lastname@example.org
Corresponding author: C. P. Dullemond, email@example.com
Accepted: 19 December 2003
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
© ESO, 2004
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