IRS Spectra of Solar‐Type Stars: A Search for Asteroid Belt Analogs

We report the results of a spectroscopic search for debris disks surrounding41 nearby solar type stars, including 8 planet-bearing stars, using the {\itSpitzer Space Telescope}. With accurate relative photometry using the InfraredSpectrometer (IRS) between 7-34 $\micron$ we are able to look for excesses assmall as $\sim$2% of photospheric levels with particular sensitivity to weakspectral features. For stars with no excess, the $3\sigma$ upper limit in aband at 30-34 $\mu$m corresponds to $\sim$ 75 times the brightness of ourzodiacal dust cloud. Comparable limits at 8.5-13 $\mu$m correspond to $\sim$1,400 times the brightness of our zodiacal dust cloud. These limits correspondto material located within the $<$1 to $\sim$5 AU region that, in our solarsystem, originates from debris associated with the asteroid belt. We findexcess emission longward of $\sim$25 $\mu$m from five stars of which four alsoshow excess emission at 70 $\mu$m. This emitting dust must be located around5-10 AU. One star has 70 micron emission but no IRS excess. In this case, theemitting region must begin outside 10 AU; this star has a known radial velocityplanet. Only two stars of the five show emission shortward of 25 $\micron$where spectral features reveal the presence of a population of small, hot dustgrains emitting in the 7-20 $\mu$m band. The data presented here strengthen theresults of previous studies to show that excesses at 25 $\micron$ and shorterare rare: only 1 star out of 40 stars older than 1 Gyr or $\sim 2.5$% shows anexcess. Asteroid belts 10-30 times more massive than our own appear are rareamong mature, solar-type stars