The dust and gas around β pictoris

We have obtained Spitzer IRS 5.5-35 μm spectroscopy of the debris disk around β Pictoris. In addition to the 10 μm silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 and 33.5 μm. This is the first time that the silicate bands at wavelengths longer than 10 μm have ever been seen in the β 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 σ upper limit of < 17 M _⊕ on the H₂ S(1) gas mass, assuming an excitation temperature of T_ex = 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 β Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.