Alan T. Tokunaga

Detection of absorption by H2 in molecular clouds: A direct measurement of the H2:CO ratio

Vibrational absorption by H2 and CO has been searched for toward infrared sources embedded in molecular clouds. H2 was detected toward NGC 2024 IRS 2 and possibly toward NGC 2264 (GL 989). CO was detected toward both sources. The results are consistent with the H2 ortho:para ratio being equilibrated at the cloud temperature. Toward NGC 2024, H2:CO = (3700(sub -2600)(sup +3100)) (2 sigma limits), and toward NGC 2264, H2:CO less than 6000. Approximately one-third of all carbon is in gas-phase CO.

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 CO(2-0) and CO(5-3) bandhead regions. These data and similar observations of the young stellar object WL16, 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 bandhead 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 data are also powerful probes of the stellar and inner disk properties of these systems. Our modeling is consistent with the identification of WL16 and 1548C27 as Herbig AeBe stars with stellar masses of approximately 2 and 4 solar masses, 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 WL16 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.

Spectroscopy of emission features near 3 microns in comet Wilson (1986l)

Emission from C-H groups at 3.4 microns and an emission feature at 2.8 microns were detected in comet Wilson (1986l). A high-resolution spectrum confirms features at 3.52 microns. There is a possible emission feature at 3.04 microns. The spectra are strikingly similar to those of comet Halley. Derived abundances of the organic compounds are comparable. A model of the fluorescence spectrum of formaldehyde does not match the structure longward of 3.5 microns. The polymerized form of formaldehyde, polyoxymethylene, is a possible contributor to the 3.4-micron emission. The similarity between the 3.4-micron emission in a dynamically old (Halley) and a new (Wilson) comet suggests that these organics were incorporated into comets at the time of their formation. 38 references.