Solar performance evaluation test program of the 9.5-ft-diam. electroformed nickel concentrator S/N 1 at Table Mountain, California

Optical and calorimetric tests of nickel mirrors for use as power source for thermionic generator

The Cosmochemistry of Protostellar Matter

The different processes that can affect the chemical composition of matter as it evolves from quiescent molecular clouds into protostellar regions is discussed. Millimeter observations of molecules at high angular resolution in cold, dark clouds such as TMC-1 and L134N reveal large chemical gradients on scales of a few tenths of a pc, which are not well understood. Further, the abundances of the dominant oxygen- (H_2O, O_2, O), and nitrogen-bearing (N_2, N) species are ill determined, both observationally and theoretically, and little is known about some important carbon-bearing molecules such as CH_4, CO_2 and C_2H_2 . Observations of the distribution of molecular material in disks surrounding newly-formed low-mass stars such as IRAS 16293 -2422 are just starting to become available, and reveal a complex chemistry on scales of 500-10,000 AU. Remarkable similarities are found with the chemistry observed in the highmass star forming region Orion/KL, despite a factor of 1000 difference in stellar luminosity. A brief comparison with the chemical composition comets is made

Chemistry in circumstellar disks: CS toward HL Tauri

High-resolution millimeter-wave aperture synthesis images of the CS J = 2 → 1 and dust continuum emission toward the young star HL Tauri have been combined with single-dish spectra of the higher J CS transitions in order to probe the chemical and physical structure of circumstellar material in this source. We find that the extended molecular cloud surrounding HL Tau is similar to other Taurus dark cloud cores, having T_(kinetic) ≈ 10-20 K, n_(H2) ≈ 10^4-10^5 cm^(-3) , and x(CS) = N(CS)/N(H_2) ≈ (1-2) x 10^(-8). In contrast, the gas-phase CS abundance in the circumstellar disk is depleted by factors of at least 25-50, and perhaps considerably more. These results are consistent with substantial depletion onto grains, or a transition from kinetically controlled chemistry in the molecular cloud to thermodynamically controlled chemistry in the outer regions of the circumstellar disk. Dust continuum emission at 3.06 mm, although unresolved in a 3".0 beam, appears centered on the stellar position; combined with other millimeter-wave measurements its intensity indicates an emissivity index of β = 1.2 ± 0.3. This β may reflect grain growth via depletion and aggregation or compositional evolution, and suggests that the 3.06 mm dust opacity exceeds unity within 8-10 AU of HL Tauri. Even at millimeter and submillimeter wavelengths, observational studies of other high dipole moment molecules in circumstellar disks may also be hampered by the combination of grain mantle depletion and dust opacity structure in sources viewed nearly edge-on

On the origin of H_2CO abundance enhancements in low-mass protostars

High angular resolution H_2CO 218 GHz line observations have been carried out toward the low-mass protostars IRAS 16293-2422 and L1448-C using the Owens Valley Millimeter Array at ~2" resolution. Simultaneous 1.37 mm continuum data reveal extended emission which is compared with that predicted by model envelopes constrained from single-dish data. For L1448-C the model density structure works well down to the 400 AU scale to which the interferometer is sensitive. For IRAS 16293-2422 , a known proto-binary object, the interferometer observations indicate that the binary has cleared much of the material in the inner part of the envelope, out to the binary separation of ~800 AU. For both sources there is excess unresolved compact emission centered on the sources, most likely due to accretion disks ≾200 AU in size with masses of ≳0.02 M_☉ (L1448-C) and ≳0.1 M_☉ (IRAS 16293-2422). The H_2CO data for both sources are dominated by emission from gas close to the positions of the continuum peaks. The morphology and velocity structure of the H_2CO array data have been used to investigate whether the abundance enhancements inferred from single-dish modelling are due to thermal evaporation of ices or due to liberation of the ice mantles by shocks in the inner envelope. For IRAS 16293-2422 the H_2CO interferometer observations indicate the presence of rotation roughly perpendicular to the large scale CO outflow. The H_2CO distribution differs from that of C^(18)O, with C^(18)O emission peaking near MM1 and H_2CO stronger near MM2. For L1448-C, the region of enhanced H_2CO emission extends over a much larger scale >1" than the radius of 50-100 K (0."6-0".15) where thermal evaporation can occur. The red-blue asymmetry of the emission is consistent with the outflow; however the velocities are significantly lower. The H_2CO 3_(22)-2_(21)/3_(03)-2_(02) flux ratio derived from the interferometer data is significantly higher than that found from single-dish observations for both objects, suggesting that the compact emission arises from warmer gas. Detailed radiative transfer modeling shows, however, that the ratio is affected by abundance gradients and optical depth in the 3_(03)-2_(02) line. It is concluded that a constant H_2CO abundance throughout the envelope cannot fit the interferometer data of the two H_2CO lines simultaneously on the longest and shortest baselines. A scenario in which the H_2CO abundance drops in the cold dense part of the envelope where CO is frozen out but is undepleted in the outermost region provides good fits to the single-dish and interferometer data on short baselines for both sources. Emission on the longer baselines is best reproduced if the H_2CO abundance is increased by about an order of magnitude from ~ 10^(-10) to ~ 10^(-9) in the inner parts of the envelope due to thermal evaporation when the temperature exceeds ~50 K. The presence of additional H_2CO abundance jumps in the innermost hot core region or in the disk cannot be firmly established, however, with the present sensitivity and resolution. Other scenarios, including weak outflow-envelope interactions and photon heating of the envelope, are discussed and predictions for future generation interferometers are presented, illustrating their potential in distinguishing these competing scenarios

Clathrate type 2 hydrate formation in vacuo under astrophysical conditions

The properties of clathrate hydrates were used to explain the complex and poorly understood physical processes taking place within cometary nuclei and other icy solar system bodies. Most of all the experiments previously conducted used starting compositions which would yield clathrate types I hydrates. The main criterion for type I vs. type II clathrate hydrate formation is the size of the guest molecule. The stoichiometry of the two structure types is also quite different. In addition, the larger molecules which would form type II clathrate hydrates typically have lower vapor pressures. The result of these considerations is that at temperatures where we identified clathrate formation (120-130 K), it is more likely that type II clathrate hydrates will form. We also formed clathrate II hydrates of methanol by direct vapor deposition in the temperature range 125-135 K

Ground-based searches for interstellar H_(2)D^+

We present ground-based searches for the 1_(10) - 1_(11) line of interstellar H_(2)D^(+) at 372 GHz which are more sensitive than those obtained from the Kuiper Airborne Observatory by factors of 3-4 for extended sources and by more than two orders of magnitude for compact sources. The line was not detected in a variety of interstellar clouds, including NGC 2264 toward which a possible detection had been suggested previously. The inferred H_(2)D^(+) abundance limits of 10^(-10) - 10^(-11) are still consistent with, but approach the abundances predicted by chemical models. Simultaneous observations of the DCO^(+) 3-2 and N_(2)H^(+) 4-3 lines have been used to place additional limits on the H_(3)^(+) abundance, and suggest 10^(-11) < x(H_(3)^(+))< 10^(-9). The N_(2)H^(+) data also indicate that for NGC 2264, but perhaps not for the other sources, gas-phase N_2 contains a substantial fraction of the available nitrogen in the cloud

Structure and Evolution of the Envelopes of Deeply Embedded Massive Young Stars

The physical structure of the envelopes around a sample of fourteen massive (1000-100,000 solar L) young stars is investigated on 100- 100,000 AU scales using maps and spectra in submillimeter continuum and lines of C17O, CS and H2CO. The total column densities and the temperature profiles are obtained by fitting self-consistent dust models to submillimeter photometry. Both the molecular line and dust emission data indicate density gradients ~r^{-alpha}, with alpha=1.0-1.5, significantly flatter than the alpha=2.0 generally found for low-mass objects. This flattening may indicate that in massive young stellar objects, nonthermal pressure is more important for the support against gravitational collapse, while thermal pressure dominates for low-mass sources. We find alpha=2 for two hot core-type sources, but regard this as an upper limit since in these objects, the CS abundance may be enhanced in the warm gas close to the star.Comment: To be published in The Astrophysical Journal. 54 pages including 14 figures Revised version with references adde