Trends in Silicates in the $\beta$ Pictoris Disk

TL;DR

This study re-analyzed the beta Pictoris debris disk spectrum to identify trends in silicate dust properties with stellocentric distance, revealing increased crystallinity, irregularity, and decreasing Fe/Mg ratio in grains farther from the star.

Contribution

It presents new spectral features and uncovers systematic variations in silicate grain properties as a function of distance from beta Pictoris, enhancing understanding of disk composition.

Findings

Identification of new 18 and 23 micron forsterite emission features.

Small grains are more crystalline and irregular farther from the star.

Fe/Mg ratio in crystalline silicates decreases with distance.

Abstract

While beta Pic is known to host silicates in ring-like structures, whether the properties of these silicate dust vary with stellocentric distance remains an open question. We re-analyze the beta Pictoris debris disk spectrum from the Spitzer Infrared Spectrograph (IRS) and a new IRTF/SpeX spectrum to investigate trends in Fe/Mg ratio, shape, and crystallinity in grains as a function of wavelength, a proxy for stellocentric distance. By analyzing a re-calibrated and re-extracted spectrum, we identify a new 18 micron forsterite emission feature and recover a 23 micron forsterite emission feature with a substantially larger line-to-continuum ratio than previously reported. We find that these prominent spectral features are primarily produced by small submicron-sized grains, which are continuously generated and replenished from planetesimal collisions in the disk and can elucidate their parent bodies' composition. We discover three trends about these small grains: as stellocentric distance increases, (1) small silicate grains become more crystalline (less amorphous), (2) they become more irregular in shape, and (3) for crystalline silicate grains, the Fe/Mg ratio decreases. Applying these trends to beta Pic's planetary architecture, we find that the dust population exterior to the orbits of beta Pic b and c differs substantially in crystallinity and shape. We also find a tentative 3-5 micron dust excess due to spatially unresolved hot dust emission close to the star. From our findings, we infer that the surfaces of large planetesimals are more Fe-rich and collisionally-processed closer to the star but more Fe-poor and primordial farther from the star.