Kinematic Diagnostics of Disks around Young Stars: CO Overtone Emission from WL 16 and 1548C27

We report high spectral resolution observations of the CO vibrational overtone emission from the young stellar object 1548C27; our observations include both the v = 2-0 and v = 5-3 band head regions. These data and similar observations of the young stellar object WL 16, reported in a previous contribution to this journal, provide some of the most compelling evidence to date for the existence of inner disks around young stars. We describe the simple procedure that we use to synthesize band head emission from disks including the effect of thermal dissociation of CO and non-LTE excitation of the vibrational levels. Using this spectral synthesis procedure to extract the kinematics and physical properties of the emitting gas from the overtone data, we show how these high signal-to-noise ratio data are also powerful probes of the stellar and inner disk properties of these systems. Our modeling is consistent with the identification of WL 16 and 1548C27 as Herbig AeBe stars with stellar masses of approximately 2 and 4 M<SUB>sun</SUB>, respectively. Thus, the kinematic signature of rotating disks in the overtone spectra of these sources provides strong support for the role of accretion disks in the formation of intermediate-mass stars. For both WL 16 and 1548C27, we interpret our modeling results as indicating that the overtone emission arises from a temperature inversion region in the inner disk atmosphere. We also find evidence for suprathermally broadened lines and are able to place useful constraints on the radial temperature and column density distributions of the CO line formation region of the disk atmosphere. Given these deduced properties, we discuss the constraints that our observations place on the physical processes responsible for the overtone emission in these sources.