The Dust cloud around the White Dwarf G 29-38. 2. Spectrum from 5-40 microns and mid-infrared variability

Abstract

We model the mineralogy and distribution of dust around the white dwarf G29-39 using the infrared spectrum from 1-35 microns and combining a wide range of materials based on spectral studies of comets and debris disks. In order of their contribution to the mid-infrared emission, the most abundant minerals for G29-38 are amorphous carbon, amorphous and crystalline silicates, water ice, and metal sulfides. The amorphous C can be equivalently replaced by other materials (like metallic Fe) with featureless infrared spectra. The best-fitting crystalline silicate is Fe-rich pyroxene. In order to absorb enough starlight to power the observed emission, the disk must either be much thinner than the stellar radius (so that it can be heated from above and below) or it must have an opening angle wider than 2 degrees. A `moderately optically thick' torus model with mass 2x10^19 g fits the spectrum well. A physically thin (less than the white dwarf radius) and optically thick disk can contribute to the near-infrared continuum only; such a disk cannot explain the longer-wavelength continuum or strong silicate features. The silicate composition contains minerals found from cometary spectra and meteorites, but Fe-rich pyroxene is more abundant than enstatite (Mg-rich pyroxene) or forsterite (Mg-rich olivine) in G29-38, in contrast to what is found in most comet or meteorite mineralogies. Enstatite meteorites may be the most similar solar system materials to the G29-38 dust. Finally, we suggest the surviving core of a `hot jupiter' as an alternative (neither cometary nor asteroidal) origin for the debris, though further theoretical work is needed to determine if this hypothesis is viable.