A GLIMPSE into the Nature of Galactic Mid‐IR Excesses

Abstract

We investigate the nature of the mid-IR excess for 31 intermediate-massstars that exhibit an 8 {μ}m 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 exhibitH{α} emission and four exhibit H{α} absorption. Assuming thatthe mid-IR excesses arise in circumstellar disks, we use the H{α}fluxes to model and estimate the relative contributions of dust andfree-free emission. Six stars exhibit H{α} fluxes that implyfree-free emission can plausibly explain the infrared excess at [24].These stars are candidate classical Be stars. Nine stars exhibitH{α} emission, but their H{α} 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 T_{disk}~=300-800 K and fractional infrared luminosities ofL_{IR}/L_{*}~=10^{-3}. These nine stars may bepre-main-sequence stars with transitional disks undergoing diskclearing. Three of the four sources showing H{α} absorption exhibitcircumstellar disk temperatures ~=300-400 K,L_{IR}/L_{*}~=10^{-3}, IR colors K-[24]{\lt}3.3,and are warm debris disk candidates. One of the four H{α} absorptionsources has K-[24]{\gt}3.3 implying an optically thick outer disk and isa transition disk candidate.