The Dust and Gas Around beta Pictoris

TL;DR

This study presents detailed infrared spectroscopy of beta Pictoris's debris disk, revealing crystalline silicate features at wavelengths longer than 10 microns and setting upper limits on gas content, indicating a predominantly dust-rich environment.

Contribution

First detection of silicate emission bands beyond 10 microns in beta Pictoris's disk, providing new insights into dust composition and gas content.

Findings

Crystalline silicate emission bands at 28 and 33.5 microns detected.

No molecular hydrogen or atomic gas emission detected, with gas mass limits set.

Dust modeled as fluffy cometary and crystalline olivine aggregates.

Abstract

We have obtained Spitzer IRS 5.5 - 35 micron spectroscopy of the debris disk around beta Pictoris. In addition to the 10 micron silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 micron and 33.5 micron. This is the first time that the silicate bands at wavelengths longer than 10 micron have ever been seen in the beta Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 sigma upper limit of <17 Earth masses on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around beta Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.