HD 145263: Spectral Observations of Silica Debris Disk Formation via Extreme Space Weathering?

Abstract

We report here time-domain infrared spectroscopy and optical photometry of the HD 145263 silica-rich circumstellar-disk system taken from 2003 through 2014. We find an F4V host star surrounded by a stable, massive 10²²–10²³ kg (M _Moon to M _Mars) dust disk. No disk gas was detected, and the primary star was seen rotating with a rapid ~1.75 day period. After resolving a problem with previously reported observations, we find the silica, Mg-olivine, and Fe-pyroxene mineralogy of the dust disk to be stable throughout and very unusual compared to the ferromagnesian silicates typically found in primordial and debris disks. By comparison with mid-infrared spectral features of primitive solar system dust, we explore the possibility that HD 145263's circumstellar dust mineralogy occurred with preferential destruction of Fe-bearing olivines, metal sulfides, and water ice in an initially comet-like mineral mix and their replacement by Fe-bearing pyroxenes, amorphous pyroxene, and silica. We reject models based on vaporizing optical stellar megaflares, aqueous alteration, or giant hypervelocity impacts as unable to produce the observed mineralogy. Scenarios involving unusually high Si abundances are at odds with the normal stellar absorption near-infrared feature strengths for Mg, Fe, and Si. Models involving intense space weathering of a thin surface patina via moderate (T < 1300 K) heating and energetic ion sputtering due to a stellar super-flare from the F4V primary are consistent with the observations. The space-weathered patina should be reddened, contain copious amounts of nanophase Fe, and should be transient on timescales of decades unless replenished.