ALBERT

Description: Results of a search for extended CO(7-6) line emission in the Orion A molecular cloud using the University of Texas submillimeter laser heterodyne receiver are presented. The large-scale distribution of quiescent CO(7-6) emission is mapped along a cut across the Trapezium cluster extending more than 13 arcmin (nearly 2 pc) in R.A. The receiver, observing method, and calibration techniques are described, and the CO(7-6) spectra and maps are presented. The physical parameters and energetics of the molecular gas are discussed. The findings are compared with observations of other high-luminosity star-forming regions.

Description: The results of a combined millimeter-spectral-line and continuum survey of cold far-infrared sources selected to favor embedded young stars in the Galaxy are presented. The spectral-line observations were performed with the 5 meter antenna of the University of Texas Millimeter-Wave Observatory. High resolution continuum observations were obtained with the Owens Valley (OVRO) Millimeter-Wave Interferometer. The goal of the survey was to gain insight into the mass, temperature, and distribution of cold dust which envelopes stars during the earliest stages of their evolution. The first phase of our survey involved 1.2 arcmin resolution observations of CO-12 and CO-13 emission lines toward each source. All but two sources had detectable CO emission. We found that 40% of the sources appear to be associated with star formation as evidenced by the presence of enhanced CO-12 line widths or broad wings. At least five of these objects are associated with bipolar molecular outflows. The second phase of our survey involves high resolution 2.7 mm continuum observations with 3 interferometer baselines ranging from 15 to 55 m in length. Preliminary results indicate that about 25% of the sources in our sample have detectable continuum emission on scales less than 30 arcsec. The high percentage of sources with enhanced CO-12 line widths or broad wings indicates that a significant fraction of our samples, 40%, are likely to be young stars. The lower detection percentage in the continuum observations, 25%, suggest that such objects are not always surrounded by large concentrations of gas and dust. The continuum detection percentage for actual dust emission could be lower than that given above since emission from ionized gas could be responsible for the observed 2.7 mm emission in some objects. To get an understanding of the type of object detected in our survey, a map of one of the survey sources, L1689N, has been made using the OVRO mm interferometer.

Description: We mapped the distribution of atomic far-IR line emission from (O I) and (C II) over parsec scales in the Galactic star-forming regions L1630, M17, and W3 using the MPE Far-Infrared Fabry-Perot Imaging spectrometer (FIFI) on board the NASA Kuiper Airborne Observatory. The lines mapped include (O I) 63 microns, (O I) 146 microns, and (C II) 158 microns. Comparison of the intensities and ratios of these lines with models of photodissociation regions (e.g., Tielens & Hollenbach 1985, ApJ, 344, 770) allows us to derive temperatures and densities of the primarily neutral atomic gas layers lying on the surfaces of UV-illuminated molecular gas. In general, the (C II) line arises ubiquitously throughout the molecular clouds while the (O I) lines are mainly confined to warm, dense gas (T is greater than 100 K, n is greater than 10(exp 4)/cu cm) near the sites of O and B stars. The distribution of (C II) in the star-forming clouds implies that the (C II) emission arises on the surfaces of molecular clumps throughout the clouds, rather than only at the boundary layer between molecular gas and H II regions.

Description: The performance of silica heat shields in the outer planet atmospheres is analyzed and described in a set of differential equations. Results are presented and discussed.

Description: The construction of a large scale map of the 158 micrometer C+ line toward the L1630/Orion B molecular cloud, covering an approximately 35' by 45' area which includes the NGC 2024 H II region, zeta-Ori, the reflection nebula NGC 2023, and the Horsehead nebula, is reported. Emission in the C II line is very widespread. The line was detected at levels in excess of a few 0.0001 erg/sq cm/s/sr over almost the entire mapped region. Extended emission associated with the NGC 2024 H II region and its envelope accounts for more than half of the C II flux. Over this approximately 1.5 by 2.5 pc region, the amount of gas phase carbon in the form of C+ is comparable to the amount of carbon in CO. This result, together with the C II distribution implies that C II emission arises on the surface of clumps throughout the cloud rather than in a single layer at the H II region boundary. Away from the H II region, most of the C II emission comes from the western edge of the L1630 cloud and probably results from excitation by external OB stars. The overall extent of the C II emission is comparable to that of millimeter molecular lines but the distributions are different in detail. The difference in C II and molecular line distributions, in particular, the larger extent of the C II emission west of NGC 2024 implies large variations in the ratio of the C II and J = 1 towards 0 intensities. Models of photon dominated regions can explain the relation between C II and CO intensities only if the cloud edges and cloud interior are considered separately. A method for using C II and radio continuum emission to characterize the relationship between OB stars and photon dominated regions is proposed.

Description: Dense gas at the ionized/neutral boundaries of molecular clouds illuminated by far-UV photons plays an important role in the appearance of the neutral interstellar medium. It also is a laboratory for the study of UV-photochemistry and of a number of heating and cooling phenomena not seen elsewhere. Fine structure lines of neutral and low ionization potential species dominate the cooling in the outer part of the photodissociation regions. Observations of these lines show that the regions are dense and highly clumped. Observations of H2 and CO show that heating by UV photons plays a significant role in the excitation of molecular lines near the H II/neutral boundary. Warm CO is more abundant in these regions than predicted by the standard theoretical models. Optical reflection nebulas provide an ideal laboratory for the study of photodissocciation region phenomena.

Description: A photodissociation region (PDR) with a known UV excitation source was studied by examining emission from atomic and molecular material. In agreement with models of externally excited PDRs, the photodissociation region southwest of HD 37903 contains both atomic and excited molecular material and arises close to the exciting star than does the bulk of the molecular cloud. The C(+) emission arises closest to the star but is 1.5 x 10 to the 17th cm or less from the peaks in the excited CO and fluorescent H2 emission regions. The (C-18)O J = 2 - 1 line, which traces molecular column density, has its emission peak about 4 x 10 to the 17th cm beyond the C(+) peak. Even in a source with an incident UV flux as low as about 1000 times the mean interstellar radiation field, there is a significant amount of CO 7 - 6 emission arising from regions with T(gas) greater than 85 K which is substantially greater than T(dust).