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

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.

ApJ in press

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• Simulating Observations of Ices in Protoplanetary Disks
• The water-ice feature in near-infrared disk-scattered light around HD 142527: Micron-sized icy grains lifted up to the disk surface?
• Multi-Epoch Detections of Water Ice Absorption in Edge-on Disks around Herbig Ae Stars: PDS 144N and PDS 453
• Constraining the detectability of water ice in debris disks
• Detection of Near-Infrared Water Ice at the Surface of the (pre)Transitional Disk of AB Aur: Informing Icy Grain Abundance, Composition, and Size
• Scattering polarization of 3-$\mu$m water-ice feature by large icy grains