Transitional Disks: Theories and Observations
First Author:
Eugene Chiang
Email: echiang AT astro.berkeley.edu
UC Berkeley
601 Campbell Hall
Berkeley, CA, 94720, USA
Coauthors:
Abstract
What physical processes operate in transitional disks to clear and maintain their central cavities? The enormous reductions in optical depth implied by observations require consolidation of dust into larger objects---possibly planet-sized as an extreme hypothesis. We discuss recent developments in our understanding of how solids coagulate, highlighting gravitational instability at the dust-rich midplane. It is shown how the traditional objection to gravitational instability---turbulence triggered by vertical shearing instabilities---might be circumvented, by having the disk be of supersolar metallicity. Once the inner disk initially rids itself of opaque dust, there is the further problem of maintaining the central clearing against ongoing accretion from the outer disk. Planets, though they may exist in the hole and be responsible for its initial clearing, cannot easily gate the accretion flow by themselves. An alternative means by which accretion can be regulated involves the magneto-rotational instability (MRI) operating at the disk rim. Transitional disks present promising environments for the MRI because they are devoid of dust. In this scheme, gas is leached from the rim at a rate set by the depth to which stellar X-rays ionize the rim wall. Blown out by radiation pressure, dust fails to accrete with gas. We present a testable prediction of this mechanism.
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