The frequency of mid-infrared excess sources in galactic surveys

We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 mu m mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSE surveys. Of these, 183 are either OB stars earlier than B8 in which the excess plausibly arises from a thermal bremsstrahlung component or evolved stars in which the excess may be explained by an atmospheric dust component. The remaining 47 stars have spectral classifications B8 or later and appear to be main-sequence or late pre-main-sequence objects harboring circumstellar disks. Six of the 47 stars exhibit multiple signatures characteristic of pre-main-sequence circumstellar disks, including emission lines, near-IR K-band excesses, and X-ray emission. Approximately one-third of the remaining 41 sources have emission lines suggesting relative youth. We modeled the excesses in 26 stars having two or more measurements in excess of the expected photospheres as single-component blackbodies. We determine probable disk temperatures and fractional IR luminosities in the range 191 K < T < 787 K and 3.9 x 10(-4) < L-IR/L-* < 2.7; 10(-1). The majority of our modeled sample (14 stars) have 10(-3) < L-IR/L-* < 10(-2) and are consistent with either transition disks or massive debris disks. These objects have fractional IR luminosities and temperatures between those of beta Pic-type debris disk systems (L-IR/L-* <= 10(-3)) and Class II pre-main-sequence systems (L-IR/L* similar or equal to 10(-1)). We estimate a lower limit on the fraction of Tycho-2 Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses to be 1.0% +/- 0.3%.