Eccentric Birth Rings, Asymmetric Debris Disks, and HD 15115

Details Eccentric Birth Rings, Asymmetric Debris Disks, and HD 15115 Eccentric Birth Rings, Asymmetric Debris Disks, and HD 15115

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dda....40.0612h&link_type=abstract

American Astronomical Society, DDA meeting #40, #6.12; Bulletin of the American Astronomical Society, Vol. 41, p.901

Astronomy and Astrophysics

Astronomy

Scientific paper

Debris disks are the dusty circumstellar disks observed orbiting a number of nearby stars. These dust disks must also be replenished, since collisions and radiation forces tend to drain them of their dust. Many disks appear to be sustained by an unseen `birth ring' of planetesimals whose collisions generate dust that is lofted outwards into wide orbits ( 100's of AU) by radiation pressure (Strubbe & Chiang 2006). Note that when the birth ring is circular and unperturbed, the disk's optical depth is axisymmetric. However, when the birth ring is eccentric (like Fomalhaut's), then the debris disk's optical depth can be quite non-axisymmetric. This is because the size threshold for dust blow-out by radiation pressure becomes sensitive to longitude in the eccentric ring. Note that the colliding planetesimals, which are the source of this dust, are traveling faster/slower at the ring's periapse/apoapse, which causes the blow-out threshold there to be larger/smaller. Also note that the bound dust grains tend to loiter at their apoapse, which is 180 degrees away from where they originated in the birth ring. Consequently, longitudes in the disk near the birth ring's peripase also tends to be overdense with the smaller dust grains that provide most of the optical depth. This asymmetry can be quite large, factors of 2-10, when the birth ring has a modest eccentricity (e>0.1) and a steep enough differential dust-size distribution (q>4). Thus an eccentric birth ring might account for the asymmetry observed in the debris disk at HD 15115, whose western ansa is 3 times brighter than its eastern ansa (Kalas etal 2007). The Fomalhaut birth ring might also be eccentric (Chiang etal 2008), so any asymmetry seen in its debris disk could be used to measure or constrain its dust size distribution, too.

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