The characterization of gas in the inner disks around young stars is of particular interest because of its connection to planet formation. In order to study the gas in inner disks, we have obtained high-resolution K-band and M-band spectroscopy of 14 intermediate mass young stars. In sources that have optically thick inner disks, i.e. E(K-L)>1, our detection rate of the rovibrational CO transitions is 100% and the gas is thermally excited. Of the five sources that do not have optically thick inner disks, we only detect the ro-vibrational CO transitions from HD 141569. In this case, we show that the gas is excited by UV fluorescence and that the inner disk is devoid of gas and dust. We discuss the plausibility of the various scenarios for forming this inner hole. Our modeling of the UV fluoresced gas suggests an additional method by which to search for and/or place stringent limits on gas in dust depleted regions in disks around Herbig Ae/Be stars.Subject headings: accretion, accretion disks ---circumstellar matter---line: profiles---molecular processes---planetary systems: protoplanetary disks---stars: pre-main sequence 1 Michelson Postdoctoral Fellow A complementary diagnostic of gas in the inner disk are the ro-vibrational transitions of CO. Previous work has shown that the ro-vibrational lines of CO are sensitive probes of circumstellar gas and are well suited to exploring conditions within the inner, planet-forming regions of disks These lines can be excited thermally or by UV fluorescence. Infrared observations can detect an amount of CO much smaller than an Earth mass, well below the threshold necessary to circularize the orbits of terrestrial planets. Although CO is not a useful probe of the total mass of circumstellar gas at large column densities, its presence can be used to trace the "skin" of the gas disk.An observational clarification of how the gas evolves in the inner disks of young stars is central to an understanding of early planet formation. If planets form by a two-step process where grains first agglomerate into a large rocky core followed by the accretion of a gaseous envelope (e.g. Lissauer 1993), then the disk should go through a gas-rich/dust-poor stage. Alternatively, if planets form by a gravitational instability (e.g. Boss 1998), then the gas and dust ObservationsWe have observed a heterogeneous sample of 14 young intermediate mass stars (2-10 M  ) to measure the emission from H I (Br γ) and CO (Δv=1) ( Table 1). The targets include two intermediate mass T Tauri stars (IMTTS), seven Herbig AeBe (HAeBe) stars, four transitional

We present a comparison of CO fundamental rovibrational lines (observed in the M band near 4.7 lm) from the inner circumstellar disks around the Herbig AeBe stars AB Aur and HD 141569. The CO spatial profiles and temperatures constrain the location of the gas for both stars to a distance of less than 50 AU. The CO emission from the disk of the $4 Myr star AB Aur shows at least two temperature components, the inner disk at a rotational temperature of 1540 AE 80 K and the outer disk at 70 AE 10 K. The hot gas is located near the hot bright inner rim of the disk and the cold gas is located in the outer disk from 8-50 AU. The relative intensities of low-J lines suggest that the cold gas is optically thick. The excitation of CO in both temperature regimes is dominated by infrared fluorescence (resonant scattering). In the more evolved disk around HD 141569, the CO is excited by UV fluorescence. The relative intensity of the CO emission lines implies a rotational temperature of 190 AE 30 K. The resulting column density is $10 11 cm À2 , indicating approximately 10 19 g of CO. The observed line profiles indicate that the inner disk has been cleared of CO gas by stellar radiation out to a minimum of 17 AU. The residual mass of CO suggests that the inner disk of HD 141569 is not in an active phase of planet building but it does not rule out the possibility that giant planet building has previously occurred.

In the current paradigm, Oort cloud comets formed in the giant planets' region of the solar nebula, where temperatures and other conditions varied greatly. The measured compositions of four such comets (Halley, Hyakutake, Hale-Bopp, and Lee) are consistent with formation from interstellar ices in the cold nebular region beyond Uranus. The composition of comet C/1999 S4 (LINEAR) differs greatly, which suggests that its ices condensed from processed nebular gas, probably in the Jupiter-Saturn region. Its unusual organic composition may require reevaluation of the prebiotic organic material delivered to the young Earth by comets.

We present the first high-resolution, ground-based observations of HCN and C 2 H 2 toward the T Tauri binary star system GV Tau. We detected strong absorption due to HCN ν 3 and weak C 2 H 2 (ν 3 and ν 2 + (ν 4 + ν 5 ) 0 + ) absorption toward the primary (GV Tau S) but not the infrared companion. We also report CO column densities and rotational temperatures, and present abundances relative to CO of HCN/CO ∼ 0.6% and C 2 H 2 /CO ∼ 1.2% and an upper limit for CH 4 /CO < 0.37% toward GV Tau S. Neither HCN nor C 2 H 2 were detected toward the infrared companion and results suggest that abundances may differ between the two sources.