Photoevaporation of Circumstellar Disks Revisited: The Dust-Free Case

TL;DR

This paper revisits the photoevaporation of dust-free circumstellar disks by solving 2D radiative transfer, revealing that direct stellar radiation dominates and providing a new mass loss rate formula dependent on disk radius.

Contribution

The study introduces a more accurate 2D radiative transfer model, showing the importance of direct stellar radiation and deriving a new, radius-dependent photoevaporation rate for dust-free disks.

Findings

Direct stellar radiation is more important than diffuse radiation at the disk surface.

The radial density distribution follows a single power-law with index -3/2.

The new mass loss rate depends on the outer disk radius, not the gravitational radius.

Abstract

Photoevaporation by stellar ionizing radiation is believed to play an important role in the dispersal of disks around young stars. The mass loss model for dust-free disks developed by Hollenbach et al. is currently regarded as a conventional one and has been used in a wide variety of studies. However, the rate in this model was derived by the crude so-called 1+1D approximation of ionizing radiation transfer, which assumes that diffuse radiation propagates in a direction vertical to the disk. In this study, we revisit the photoevaporation of dust-free disks by solving the 2D axisymmetric radiative transfer for steady-state disks. Unlike that solved by the conventional model, we determine that direct stellar radiation is more important than the diffuse field at the disk surface. The radial density distribution at the ionization boundary is represented by the single power-law with an index -3/2 in contrast to the conventional double power-law. For this distribution, the photoevaporation rate from the entire disk can be written as a function of the ionizing photon emissivity, Phi_EUV, from the central star and the disk outer radius, r_d, as follows: Mdot_PE = 5.4 x 10^-5 x (Phi_EUV/10^49 sec^-1)^1/2 x (r_d/1000 AU)^1/2 Msun/yr. This new rate depends on the outer disk radius rather than on the gravitational radius as in the conventional model, caused by the enhanced contribution to the mass loss from the outer disk annuli. In addition, we discuss its applications to present-day as well as primordial star formation.