IRS spectra of solar-type stars: A search for asteroid belt analogs

We report the results of a spectroscopic search for debris disks surrounding 41 nearby solar-type stars, including eight planet-bearing stars, using the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. With the accurate relative photometry of the IRS between 7 and 34 μm we are able to look for excesses as small as ∼2% of photospheric levels, with particular sensitivity to weak spectral features. For stars with no excess, the 3 σ upper limit in a band at 30-34 μm corresponds to ∼75 times the brightness of our zodiacal dust cloud. Comparable limits at 8.5-13 μm correspond to ∼1400 times the brightness of our zodiacal dust cloud. These limits correspond to material located within the < 1 to ∼5 AU region that, in our solar system, originates predominantly from debris associated with the asteroid belt. We find excess emission longward of ∼25 μm from five stars, of which four also show excess emission at 70 μm. This emitting dust must be located in a region starting around 5-10 AU. One star has 70 μm emission but no IRS excess. In this case, the emitting region must begin outside 10 AU; this star has a known radial velocity planet. Only two stars of the five show emission shortward of 25 μm, where spectral features reveal the presence of a population of small, hot dust grains emitting in the 7-20 μm band. One of these stars, HD 72905, is quite young (300 Myr), while the other, HD 69830, is older than 2 Gyr. The data presented here strengthen the results of previous studies to show that excesses at 25 μm and shorter are rare: only 1 out of 40 stars older than 1 Gyr or ∼2.5% shows an excess. Asteroid belts 10-30 times more massive than our own appear are rare among mature, solar-type stars.