Identification of Main-Sequence Stars with Mid-Infrared Excesses Using GLIMPSE: β Pictoris Analogs?

SpitzerIRAC3.6–8 � mphotometryobtainedaspartoftheGLIMPSEsurveyhasrevealedmid-infraredexcesses for33fieldstarswithknownspectraltypesina1.2deg 2 fieldcenteredonthesouthernGalacticHiiregionRCW49. These stars comprise a subset of 184 stars with known spectral classification, most of which were preselected to have unusually red IR colors. We propose that the mid-IR excesses are caused by circumstellar dust disks that are either very late remnants of stellar formation or debris disks generated by planet formation. Of these 33 stars, 29 appear to be main-sequence stars on the basis of optical spectral classifications. Five of the 29 main-sequence stars are O or B stars with excesses that can be plausibly explained by thermal bremsstrahlung emission, and four are post–main-sequencestars.TheloneOstarisanO4V((f))ataspectrophotometricdistanceof 3233 þ540 � 535 pcandmay be the earliest member of the Westerlund 2 cluster. Of the remaining 24 main-sequence stars, 18 have spectral energydistributions thatare consistentwithhotdustydebrisdisks,a possible signatureofplanetformation.Modeling the excesses as blackbodies demonstrates that the blackbody components have fractional bolometric disk-to-star luminosity ratios, LIR/L� , ranging from 10 � 3 to 10 � 2 with temperatures ranging from 220 to 820 K. The inferred temperatures are more consistent with asteroid belts than with the cooler temperatures expected for Kuiper belts. Mid-IR excesses are found in all spectral types from late B to early K. Subject headingg circumstellar matter — open clusters and associations: individual (Westerlund 2) — planetary systems: formation