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We have completed an optical spectroscopic survey of an unbiased,extinction-limited sample of candidate young stars covering 1.3 square degreesof the Rho Ophiuchi star forming region. While infrared, X-ray, and opticalsurveys of the cloud have identified many young stellar objects (YSOs), thesesurveys are biased towards particular stages of stellar evolution and are notoptimal for studies of the disk frequency and initial mass function.We haveobtained over 300 optical spectra to help identify 135 association membersbased on the presence of H-alpha in emission, lithium absorption, X-rayemission, a mid-infrared excess, a common proper motion, reflection nebulosity,and/or extinction considerations. Spectral types along with R and I bandphotometry were used to derive effective temperatures and bolometricluminosities for association members to compare with theoretical tracks andisochrones for pre-main-sequence stars. An average age of 3.1 Myr is derivedfor this population which is intermediate between that of objects embedded inthe cloud core of Rho Ophiuchi and low mass stars in the Upper Scorpiussubgroup. Consistent with this age we find a circumstellar disk frequency of27% plus or minus 5%. We also constructed an initial mass function for anextinction-limited sample of 123 YSOs (A_v less than or equal to 8 mag), whichis consistent with the field star initial mass function for YSOs with masses >0.2 M_sun. There may be a deficit of brown dwarfs but this result relies oncompleteness corrections and requires confirmation.

The Initial Mass Function and Disk Frequency of the Rho Ophiuchi Cloud: An Extinction-Limited Sample