Dust amorphization in protoplanetary disks

TL;DR

This study investigates how stellar X-ray emission influences dust crystallinity in protoplanetary disks, revealing an anti-correlation that suggests X-ray driven amorphization of dust grains during early planet formation stages.

Contribution

It provides the first observational evidence linking stellar X-ray activity to dust amorphization in protoplanetary disks, highlighting the role of energetic stellar winds in disk evolution.

Findings

Significant anti-correlation between X-ray luminosity and dust crystallinity.

X-ray fluxes are sufficient to amorphize dust grains efficiently.

X-ray activity may erase other correlations between crystallinity and stellar parameters.

Abstract

High-energy irradiation of the circumstellar material might impact the structure and the composition of a protoplanetary disk and hence the process of planet formation. In this paper, we present a study on the possible influence of the stellar irradiation, indicated by X-ray emission, on the crystalline structure of the circumstellar dust. The dust crystallinity is measured for 42 class II T Tauri stars in the Taurus star-forming region using a decomposition fit of the 10 micron silicate feature, measured with the Spitzer IRS instrument. Since the sample includes objects with disks of various evolutionary stages, we further confine the target selection, using the age of the objects as a selection parameter. We correlate the X-ray luminosity and the X-ray hardness of the central object with the crystalline mass fraction of the circumstellar dust and find a significant anti-correlation for 20 objects within an age range of approx. 1 to 4.5 Myr. We postulate that X-rays represent the stellar activity and consequently the energetic ions of the stellar winds which interact with the circumstellar disk. We show that the fluxes around 1 AU and ion energies of the present solar wind are sufficient to amorphize the upper layer of dust grains very efficiently, leading to an observable reduction of the crystalline mass fraction of the circumstellar, sub-micron sized dust. This effect could also erase other relations between crystallinity and disk/star parameters such as age or spectral type.