We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 \mu m mid-infrared extraphotospheric excesses in the MidCourse Space Experiment ( MSX ) and Spitzer Space Telescope Galactic Legacy MidPlane Survey Extraordinaire ( 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-infrared K-band excesses , and X-ray emission .
Approximately one-third of the remaining 41 sources have emission lines suggesting relative youth .
Of the 25 GLIMPSE stars with SST data at \lambda \geq 24 \mu m , 20 also show an excess at 24 \mu m. Three additional objects have 24 \mu m upper limits consistent with possible excesses , and two objects have photospheric measurements at 24 \mu m. Six MSX sources had a measurement at \lambda > 8 \mu m. 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 infrared luminosities in the range 191 < T < 787 and 3.9 \times 10 ^ { -4 } < \frac { L _ { IR } } { L _ { * } } < 2.7 \times 10 ^ { -1 } .
Six stars exhibit fractional infrared luminosities less than or equal to 10 ^ { -3 } and are consistent with having debris disks similar to that around \beta Pictoris .
Six stars have \frac { L _ { IR } } { L _ { * } } > 5 \times 10 ^ { -2 } and are almost certainly Class II protostars .
The majority of our sample ( 14 stars ) have 10 ^ { -3 } < \frac { L _ { IR } } { L _ { * } } < 10 ^ { -2 } and are consistent either with transition disks in late-stage protostars or massive debris disks .
These objects have fractional infrared luminosities and temperatures between those of \beta -Pic type debris-disk systems ( \frac { L _ { IR } } { L _ { * } } \leq 10 ^ { -3 } ) and Class II pre-main-sequence systems ( \frac { L _ { IR } } { L _ { * } } \simeq 10 ^ { -1 } ) .
The modeled temperatures for excesses in emission-line stars are all > 430 Â K , while the best-fit temperatures for excesses in non emission-line stars are almost exclusively < 430 Â K. 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 \pm 0.3 \% .