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A GLIMPSE into the Nature of Galactic Mid-IR Excesses
We investigate the nature of the mid-IR excess for 31 intermediate-massstars that exhibit an 8 μm excess in either the Galactic LegacyInfrared Mid-Plane Survey Extraordinaire or the Mid-Course SpaceExperiment using high-resolution optical spectra to identify starssurrounded by warm circumstellar dust. From these data we determineprojected stellar rotational velocities and estimate stellar effectivetemperatures for the sample. We estimate stellar ages from thesetemperatures, parallactic distances, and evolutionary models. Using MIPS[24] measurements and stellar parameters we determine the nature of theinfrared excess for 19 GLIMPSE stars. We find that 15 stars exhibitHα emission and four exhibit Hα absorption. Assuming thatthe mid-IR excesses arise in circumstellar disks, we use the Hαfluxes to model and estimate the relative contributions of dust andfree-free emission. Six stars exhibit Hα fluxes that implyfree-free emission can plausibly explain the infrared excess at [24].These stars are candidate classical Be stars. Nine stars exhibitHα emission, but their Hα fluxes are insufficient to explainthe infrared excesses at [24], suggesting the presence of acircumstellar dust component. After the removal of the free-freecomponent in these sources, we determine probable disk dust temperaturesof Tdisk~=300-800 K and fractional infrared luminosities ofLIR/L*~=10-3. These nine stars may bepre-main-sequence stars with transitional disks undergoing diskclearing. Three of the four sources showing Hα absorption exhibitcircumstellar disk temperatures ~=300-400 K,LIR/L*~=10-3, IR colors K-[24]<3.3,and are warm debris disk candidates. One of the four Hα absorptionsources has K-[24]>3.3 implying an optically thick outer disk and isa transition disk candidate.
| The Frequency of Mid-Infrared Excess Sources in Galactic Surveys
We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8μm mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSEsurveys. Of these, 183 are either OB stars earlier than B8 in which theexcess plausibly arises from a thermal bremsstrahlung component orevolved stars in which the excess may be explained by an atmosphericdust component. The remaining 47 stars have spectral classifications B8or later and appear to be main-sequence or late pre-main-sequenceobjects harboring circumstellar disks. Six of the 47 stars exhibitmultiple signatures characteristic of pre-main-sequence circumstellardisks, including emission lines, near-IR K-band excesses, and X-rayemission. Approximately one-third of the remaining 41 sources haveemission lines suggesting relative youth. We modeled the excesses in 26stars having two or more measurements in excess of the expectedphotospheres as single-component blackbodies. We determine probable disktemperatures and fractional IR luminosities in the range 191K
| UVBY photometry of HIPPARCOS stars
Photoelectric data in the uvby system have been obtained for 367southern stars of the Hipparcos Input Catalogue. Most of the stars are Fand G main sequence and fall in the magnitude range V = 9 to 10.
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