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

dWe 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 mu m we are able to look for excesses as small as similar to 2% of photospheric levels, with particular sensitivity to weak spectral features. For stars with no excess, the 3 sigma upper limit in a band at 30 - 34 mu m corresponds to similar to 75 times the brightness of our zodiacal dust cloud. Comparable limits at 8.5 - 13 mu m correspond to similar to 1400 times the brightness of our zodiacal dust cloud. These limits correspond to material located within the < 1 to similar to 5 AU region that, in our solar system, originates predominantly from debris associated with the asteroid belt. We find excess emission longward of similar to 25 mu m fromfive stars, of which four also show excess emission at 70 mu m. This emitting dust must be located in a region starting around 5 - 10 AU. One star has 70 mu m 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 mu m, where spectral features reveal the presence of a population of small, hot dust grains emitting in the 7 - 20 mu m 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 mu m and shorter are rare: only 1 out of 40 stars older than 1 Gyr or similar to 2.5% shows an excess. Asteroid belts 10 - 30 times more massive than our own appear are rare among mature, solar-type stars.