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

Abstract

We report the results of a spectroscopic search for debris disks surrounding 41 nearby solar type stars, including 8 planet-bearing stars, using the {\\it Spitzer Space Telescope}. With accurate relative photometry using the Infrared Spectrometer (IRS) between 7-34 $\\micron$ we are able to look for excesses as small as $\\sim$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 $\\sim$ 75 times the brightness of our zodiacal 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 correspond to material located within the $<$1 to $\\sim$5 AU region that, in our solar system, originates from debris associated with the asteroid belt. We find excess emission longward of $\\sim$25 $\\mu$m from five stars of which four also show excess emission at 70 $\\mu$m. This emitting dust must be located around 5-10 AU. One star has 70 micron 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 $\\micron$ where spectral features reveal the presence of a population of small, hot dust grains emitting in the 7-20 $\\mu$m band. The data presented here strengthen the results of previous studies to show that excesses at 25 $\\micron$ and shorter are rare: only 1 star out of 40 stars older than 1 Gyr or $\\sim 2.5$% shows an excess. Asteroid belts 10-30 times more massive than our own appear are rare among mature, solar-type stars.