R. F. Knacke

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.

The Silicates in the Disk of beta Pictoris

We have obtained intermediate-resolution (R≃50) infrared (2.6-13.5 μm) spectra of the particles in the circumstellar disk of β Pic. The silicate dust feature near 10 μm is broader and contains more structure than interstellar and most circumstellar emission features. The silicate feature in β Pic is remarkably similar to those in comets Halley, Bradfield 1987s, and Levy 1990 XX which have emission features characteristic of crystalline silicates. This result supports the inference based on IRAS results that cometary bodies resupply the grains in the β Pic disk. Detailed models of the dust disk and grains are used to derive plausible disk temperature and density gradients

Deep 10 and 18 Micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles and Tentative Evidence for a Brightness Asymmetry

We present new 10.8 and 18.2 km images of HR 4796A, a young A0 V star that was recently dis- covered to have a spectacular, nearly edge-on, circumstellar disk prominent at D20 km (Jayawardhana and coworkers ; Koerner and coworkers). These new images, obtained with OSCIR (the University of Florida Observatory Spectrometer/Camera for the Infrared) at Keck II, show that the disks size at 10 km is comparable to its size at 18 km. Therefore, the 18 kmemitting dust may also emit some, or all, of the 10 km radiation. Using these multiwavelength images, we determine a ii characteristic ˇˇ diameter of 2¨3 km for the mid-infraredemitting dust particles if they are spherical and composed of astronomical silicates. Particles this small are expected to be blown out of the system by radiation pressure in a few hundred years, and therefore these particles are unlikely to be primordial. Rather, as inferred in a com- panion paper (Wyatt and coworkers), they are probably products of collisions that dominate both the creation and the destruction of dust in the HR 4796A disk. Dynamical modeling of the disk, the details of which are presented in the companion paper, indicates that the disk surface density is relatively sharply peaked near 70 AU, which agrees with the mean annular radius deduced by Schneider and coworkers from their NICMOS images. Interior to 70 AU, the model density drops steeply by a factor of 2 between 70 and 60 AU, falling to zero by 45 AU, which corresponds to the edge of the previously discovered central hole ; in the context of the dynamical models, this ii soft ˇˇ edge for the central hole occurs because the dust particle orbits are noncircular. The optical depth of mid-infraredemitting dust in the hole is D3% of the optical depth in the disk, and the hole is therefore relatively very empty. We present evidence (D1.8p signi—cance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the disk particle orbits by the low-mass stellar companion or a planet. This ii pericenter glow,ˇˇ which must still be con—rmed, results from a very small (a few AU) shift of the disks center of symmetry relative to the central star HR 7496A ; one side of the inner bound- ary of the annulus is shifted toward HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dynamical eUects of planets on disks is within reach. Subject headings : circumstellar matterinfrared : starsstars : individual (HR 4796A)

The abundances of ethane and acetylene in the atmospheres of Jupiter and Saturn

Abstract Infrared spectra near 780 cm −1 of Jupiter and Saturn have been obtained to determine the stratospheric abundances of ethane (C 2 H 6 ) and acetylene (C 2 H 2 ). Atmospheric models using Voyager thermal profiles and density profiles with constant mixing ratios result in the mixing ratios, X (C 2 H 2 ) = 1.0(±0.3) × 10 −7 and X (C 2 H 6 ) = 5.5(±1.5) × 10 −6 for Jupiter. The results for Saturn are X (C 2 H 2 ) = 3.0(±1.0) × 10 −7 and X (C 2 H 6 ) = 7.0(±1.5) × 10 −6 . The ratio of ethane to acetylene, n [C 2 H 6 ]/ n [C 2 H 2 ], is found to be insensitive to model atmosphere assumptions. The ratio is 55 ± 31 for Jupiter and 23 ± 12 for Saturn from models with uniform mixing ratios. Atmospheric models with densitu profiles adapted from the theoretical photochemical models also result in a higher ratio of ethane to acetylene (by a factor of 2 at the 1-mbar level) on Jupiter. The lower abundance of acetylene on Jupiter suggests that the rate of vertical transport in the stratosphere may be more rapid on Saturn than on Jupiter.

Collision of comet Shoemaker-Levy 9 with Jupiter observed by the NASA infrared telescope facility

The National Aeronautics and Space Administration (NASA) Infrared Telescope Facility was used to investigate the collision of comet Shoemaker-Levy 9 with Jupiter from 12 July to 7 August 1994. Strong thermal infrared emission lasting several minutes was observed after the impacts of fragments C, G, and R. All impacts warmed the stratosphere and some the troposphere up to several degrees. The abundance of stratospheric ammonia increased by more than 50 times. Impact-related particles extended up to a level where the atmospheric pressure measured several millibars. The north polar near-infrared aurora brightened by nearly a factor of 5 a week after the impacts.

Detection of Extended Thermal Infrared Emission around the Vega-like Source HD 141569

We report the detection of extended IR emission at 10.8 and 18.2 µm around the Vega-like source HD 141569. Mid-IR imaging with OSCIR on Keck II shows emission from dust extending out to 100 AU from the B9.5 Ve star. Our modeling of the dust places an upper limit of approximately 2 µm on the diameter of the mid-IR-emitting particles if they are Mie spheres of astronomical silicates. Comparison of our mid-IR images to the near-IR (1.1 µm) NICMOS images of HD 141569 (Weinberger et al. 1999) shows that the mid-IR emission originates at smaller distances from the star than the scattered near-IR light, as also previously observed for the archetype Vega-like source beta Pictoris.

The abundances of CH4, CH3D, NH3, and PH3 in the troposphere of Jupiter derived from high-resolution 1100-1200/cm spectra

High-resolution (..delta..v = 1 cm/sup -1/) spectra of the 1110--1200 cm/sup -1/ region of the central part of Jupiter were obtained in 1980 March and 1981 April. The best fit NH/sub 3/ distribution curve shows a higher than solar mixing ratio, (NH/sub 3/)/(H/sub 2/) = (3.3 +- 1.7) x 10/sup -4/, below the 147 K layer (>0.6 atmosphere). If we introduce NH/sub 3/ ice particles as an opacity source, the NH/sub 3/ mixing ratio below the 147 K layer can be lowered, but the fit is worse than that given by the model excluding NH/sub 3/ ice particles. The best fit PH/sub 3/ distribution curve has a (PH/sub 3/)/(H/sub 2/) mixing ratio of (8.3 +- 2.0) x 10/sup -7/ in the troposphere. We also found a (CH/sub 4/)/(H/sub 2/) mixing ratio of (2.5 +- 0.4) x 10/sup -3/ in the troposphere. The derived D/H ratio is 3.0/sup +1.1//sub -0.8/ x 10/sup -5/. Most of the flux in this spectral region comes from layers above the 170 K level (<1 atmosphere).

The origin and vertical distribution of carbon monoxide in Jupiter

Six clearly-resolved lines of the CO 1-0 vibration-rotation band near 4.7 microns have been observed in Jupiter at a resolution of 0.07/cm. CO is not found to be concentrated in the stratosphere, but is shown to be present in the troposphere at a mole fraction of 1.6 + or - 0.3 x 10 to the -9th, suggesting that rapid vertical mixing is the source of CO. Results indicate that the global oxygen abundance in Jupiters gaseous envelope below the cloud-forming regions must be near the solar value, and that intervening clouds with an optical depth of 0.5-4 are present above the line-forming region.

Observation of interstellar ammonia ice

An absorption band probably due to solid ammonia on interstellar grains has been detected in the infrared spectrum at 2.97 ..mu..m of the Becklin-Neugebauer object and probably in NGC 2264-IR. An ammonia-water amorphous ice mixture can explain the structure of the new band and of the 3.07 ..mu..m interstellar absorption. Laboratory data suggest that a long wavelength wing extending to 3.5 ..mu..m in interstellar dust spectra may be absorption by NH/sub 3/ x H/sub 2/O complexes in the ices. In the molecular cloud obscuring the BN object, about 20 times as much NH/sub 3/ is frozen in grains as exists in the gas phase, suggesting that gas-grain interactions may be important in the ammonia chemistry of molecular clouds. Arguments are given that interstellar features at 6.0 and 6.8 ..mu..m are also ammonia-related absorptions.

Ethane and acetylene abundances in the Jovian atmosphere

New observations of Jupiter in the 755--850 cm/sup -1/ spectral range have been used in conjunction wth radiative-transfer calculations to obtain mixing ratios of ethane and acetylene. The monochromatic absorption coefficient of the central Q branch of the ..nu../sub 9/ fundamental of ethane was measured in the laboratory. The largest mixing ratios for the region above the Jovian tropopause consistent with the observations are (n (C/sub 2/H/sub 6/))/(n (H/sub 2/))=3 x 10/sup -5/ and (n (C/sub 2/H/sub 2/))/(n (H/sub 2/))< or =7.5 x 10/sup -8/. (AIP)