Diagnosing Circumstellar Debris Disks

TL;DR

This paper develops a numerical model to analyze circumstellar debris disks, applied to beta Pictoris, revealing the disk's properties, dust production, and potential planetesimal destruction in the 75-150 AU region.

Contribution

The paper introduces a comprehensive model that accounts for dust production, destruction, and radiation effects, providing new insights into debris disk structure and evolution.

Findings

Disk radius estimated between 75-150 AU

Dust mass approximately 11 lunar masses

High dust production rate of about 9 Earth-masses per Myr

Abstract

(Abridged) A numerical model of a circumstellar debris disk is developed and applied to observations of the circumstellar dust orbiting beta Pictoris. The model accounts for the rates at which dust is produced by collisions among unseen planetesimals, and the rate at which dust grains are destroyed due to collisions. The model also accounts for the effects of radiation pressure, which is the dominant perturbation on the disk's smaller but abundant dust grains. Solving the resulting system of rate equations then provides the dust abundances versus grain size and over time. Those solutions also provide the dust grains' collisional lifetime versus grain size, and the debris disk's optical depth and surface brightness versus distance from the star. Comparison to observations then yields estimates of the unseen planetesimal disk's radius, and the rate at which the disk sheds mass due to planetesimal grinding. The model is then applied to optical observations of the edge-on dust disk orbiting beta Pictoris, and good agreement is achieved when the unseen planetesimal disk is broad, with 75<r<150 AU. If it is assumed that the dust grains are bright like Saturn's icy rings, then the cross section of dust in the disk is A_d~2x10^20 km^2 and its mass is M_d~11 lunar masses. In this case the planetesimal disk's dust production rate is quite heavy, dM_d/dt~9 earth-masses per Myr, implying that there is or was a substantial amount of planetesimal mass there, at least 110 earth-masses. But if the dust grains are darker than assumed, then the planetesimal disk's mass-loss rate and its total mass are heavier. In fact, the apparent dearth of any major planets in this region, plus the planetesimal disk's heavy mass-loss rate, suggests that the 75<r<150 AU zone at beta Pic might be a region of planetesimal destruction, rather than a site of ongoing planet formation.