The dust and gas around β Pictoris

We have obtained Spitzer IRS 5.5-35 mu m spectroscopy of the debris disk around beta Pictoris. In addition to the 10 mu m silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 and 33.5 mu m. This is the first time that the silicate bands at wavelengths longer than 10 mu m 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 < 17M(circle plus) on the H-2 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 beta Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.