Photoelectric cross-sections of gas and dust in protoplanetary disks

TL;DR

This paper provides polynomial fits for X-ray photoelectric cross-sections in protoplanetary disks, accounting for gas and dust separately, and discusses how dust evolution affects X-ray opacity.

Contribution

It introduces simple polynomial models for X-ray cross-sections in gas and dust mixtures in protoplanetary disks, considering dust evolution effects.

Findings

Grain growth reduces dust opacity at ~1 keV.

Dust settling significantly decreases dust opacity.

Gas opacity dominates at 1-10 keV when dust is depleted.

Abstract

We provide simple polynomial fits to the X-ray photoelectric cross-sections (0.03 < E < 10keV) for mixtures of gas and dust found in protoplanetary disks. Using the solar elemental abundances of Asplund et al. (2009) we treat the gas and dust components separately, facilitating the further exploration evolutionary processes such as grain settling and gain growth. We find that blanketing due to advanced grain-growth (a_max > 1 micron) can reduce the X-ray opacity of dust appreciably at E_X ~ 1keV, coincident with the peak of typical T Tauri X-ray spectra. However, the reduction of dust opacity by dust settling, which is known to occur in protoplanetary disks, is probably a more significant effect. The absorption of 1-10keV X-rays is dominated by gas opacity once the dust abundance has been reduced to about 1% of its diffuse interstellar value. The gas disk establishes a floor to the opacity at which point X-ray transport becomes insensitive to further dust evolution. Our choice of fitting function follows that of Morrison & McCammon (1983), providing a degree of backward-compatibility.