Effect of Photodesorption on Snow Line at the Surface of Optically Thick Circumstellar Disks around Herbig Ae/Be Stars

TL;DR

This study examines how photodesorption influences the snow line position on the surface of protoplanetary disks around Herbig Ae/Be stars, revealing that UV radiation can significantly shift the ice condensation front.

Contribution

It provides a new analytic formula for the critical temperature at which photodesorption affects the snow line, and classifies Herbig Ae/Be stars based on this effect.

Findings

Photodesorption shifts the surface snow line outward significantly.

The critical effective temperature depends on stellar and disk properties.

Estimated snow line at HD142527 matches water ice observations.

Abstract

We investigate the effect of photodesorption on the snow line position at the surface of a protoplanetary disk around a Herbig Ae/Be star, motivated by the detection of water ice particles at the surface of the disk around HD142527 by Honda et al. For this aim, we obtain the density and temperature structure in the disk with a 1+1D radiative transfer and determine the distribution of water ice particles in the disk by the balance between condensation, sublimation, and photodesorption. We find that photodesorption induced by the far-ultraviolet radiation from the central star depresses the ice-condensation front toward the mid-plane and pushes the surface snow line outward significantly when the stellar effective temperature exceeds a certain critical value. This critical effective temperature depends on the stellar luminosity and mass, the water abundance in the disk, and the yield of photodesorption. We present an approximate analytic formula for the critical temperature. We separate Herbig Ae/Be stars into two groups on the HR diagram according to the critical temperature; one is the disks where photodesorption is effective and from which we may not find ice particles at the surface, and the other is the disks where photodesorption is not effective. We estimate the snow line position at the surface of the disk around HD142527 to be 100--300 AU which is consistent with the water ice detection at >140 AU in the disk. All results depend on the dust grain size by a complex way and this point requires more work in future.