A GLIMPSE into the Nature of Galactic Mid-IR Excesses

We investigate the nature of the mid-IR excess for 31 intermediate-mass stars that exhibit an 8 � m excess in either theGalacticLegacyInfraredMid-PlaneSurveyExtraordinaireortheMid-CourseSpaceExperimentusinghigh-reso- lutionopticalspectratoidentifystarssurroundedbywarmcircumstellardust.Fromthesedatawedetermineprojected stellarrotationalvelocitiesandestimatestellareffectivetemperaturesforthesample.Weestimatestellaragesfromthese temperatures,parallacticdistances,andevolutionarymodels.UsingMIPS(24)measurementsandstellarparameterswe determine the nature of the infrared excess for 19 GLIMPSE stars. Wefind that 15 stars exhibit Hemission and four exhibitHabsorption.Assuming thatthemid-IR excessesarise incircumstellar disks,weusethe Hfluxestomodel and estimate the relative contributions of dust and free-free emission. Six stars exhibit Hfluxes that imply free-free emission can plausibly explain the infrared excess at (24). These stars are candidate classical Be stars. Nine stars ex- hibitHemission,buttheirHfluxesareinsufficienttoexplain theinfraredexcessesat(24),suggestingthepresence ofacircumstellar dustcomponent.After theremoval of thefree-free componentinthese sources,we determineprob- able disk dust temperatures of Tdisk ' 300 800 K and fractional infrared luminosities of LIR/L� '10 � 3 . These nine stars may be pre-main-sequence stars with transitional disks undergoing disk clearing. Three of the four sources showing Habsorption exhibit circumstellar disk temperatures '300-400 K, LIR/L� '10 � 3 , IR colors K �½ 24� 3:3 implying an op- tically thick outer disk and is a transition disk candidate. Subject headingg circumstellar matter — line: profiles — stars: emission-line, Be — stars: fundamental parameters — stars: rotation — techniques: spectroscopic