NEAR-INFRARED SPECTRA OF FLAT-SPECTRUM PROTOSTARS - EXTREMELY YOUNG PHOTOSPHERES REVEALED

We present new high resolution (R similar or equal to 21,000) near-inf rared (lambda=2 mu m) spectroscopic observations of a sample of flat-s pectrum and Class II young stellar objects in the rho Ophiuchi dark cl oud. Our observations of the hat-spectrum sources are the first to rev eal details about the atmospheres of objects so young that they are st ill in the protostellar stages of early stellar evolution. The flat-sp ectrum objects exhibit molecular (GO) and atomic (Na, Sc, Si) line sha pes which are consistent with those expected for rotating stellar phot ospheres. No clear spectroscopic signatures of Line formation in Keple rian circumstellar disks are found. The 2.294 mu m nu=0-2 CO spectra s how that the flat-spectrum objects are rotating more rapidly ([upsilon sin i]similar or equal to 26 km s(-1)) than the Class II pre-main-seq uence stars in our sample ([upsilon sin i]similar or equal to 12 km s( -1)). Analysis of the relative strengths of the atomic Lines of the tw o flat-spectrum sources GY21 and VSSG17 indicates that these objects h ave late spectral types, MO and M2 (or later), respectively. Their sur face ,gravities are more similar to those of late-type dwarfs or pre-m ain-sequence subgiant stars than to those of late-type stellar giants or FU Ori stars. These characteristics can be understood if the absorp tion lines of flat-spectrum sources arise in stellar (rather than disk ) atmospheres that are similar to those of pre-main-sequence subgiant T Tauri stars but are accreting mass from their circumstellar disks at higher rates. (C) 1997 American Astronomical Society.