期刊
ASTRONOMY & ASTROPHYSICS
卷 417, 期 1, 页码 159-168出版社
E D P SCIENCES
DOI: 10.1051/0004-6361:20031768
关键词
accretion, accretion disks; stars : circumstellar matter; stars : formation; stars : pre-main-sequence; infrared : stars
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 If 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.
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