Showing papers by "Thomas G. Phillips published in 1987"

Molecular abundances in OMC-1 - the chemical composition of interstellar molecular clouds and the influence of massive star formation

Abstract: We present here an investigation of the chemical composition of the various regions in the core of the Orion molecular cloud (OMC-1) based on results from the Caltech Owens Valley Radio Observatory (OVRO) millimeter-wave spectral line survey (Sutton et al.; Blake et al.). This survey covered a 55 GHz interval in the 1.3 mm (230 GHz) atmospheric window and contained emission from over 800 resolved spectral features. Of the 29 identified species 14 have a sufficient number of detected transitions to be investigated with an LTE "rotation diagram" technique, in which large numbers of lines are used to estimate both the rotational excitation and the overall abundance. The rotational temperatures and column densities resulting from these fits have then been used to model the emission from those remaining species which either have too few lines or which are too weak to be so analyzed. When different kinematic sources of emission are blended to produce a single feature, Gaussian fits have been used to derive the individual contributions to the total line profile. The uniformly calibrated data in the unique and extensive Caltech spectral line survey lead to accurate estimates of the chemical and physical parameters of the Orion molecular cloud, and place significant constraints on models of interstellar chemistry. A global analysis of the observed abundances shows that the markedly different chemical compositions of the kinematically and spatially distinct Orion subsources may be interpreted in the framework of an evolving, initially quiescent, gas-phase chemistry influenced by the process of massive star formation. The chemical composition of the extended Orion cloud complex is similar to that found in a number of other objects, but the central regions of OMC-1 have had their chemistry selectively altered by the radiation and high-velocity outflow from the young stars embedded deep within the interior of the molecular cloud. Specifically, the extended ridge clouds are inferred to have a low (subsolar) gas-phase oxygen content from the prevalence of reactive carbon-rich species like CN, CCH, and C_3H_2 also found in more truly quiescent objects such as TMC-1. The similar abundances of these and other simple species in clouds like OMC-1, Sgr B2, and TMC-1 lend support to gas-phase ion-molecule models of interstellar chemistry, but grain processes may also play a significant role in maintaining the overall chemical balance in such regions through selective depletion mechanisms and grain mantle processing. In contrast, the chemical compositions of the more turbulent plateau and hot core components of OMC-1 are dominated by high-temperature, shock-induced gas and grain surface neutral-neutral reaction processes. The high silicon/sulfur oxide and water content of the plateau gas is best modeled by fast shock disruption of smaller grain cores to release the more refractory elements followed by a predominantly neutral chemistry in the cooling postshock regions, while a more passive release of grain mantle products driven toward kinetic equilibrium most naturally explains the prominence of fully hydrogenated N-containing species like HCN, NH_3 , CH_3CN, and C_2H_5CN in the hot core. The clumpy nature of the outflow is illustrated by the high-velocity emission observed from easily decomposed molecules such as H_2CO. Areas immediately adjacent to the shocked core in which the cooler, ion-rich gas of the surrounding molecular cloud is mixed with water/oxygen rich gas from the plateau source are proposed to give rise to the enhanced abundances of complex internal rotors such as CH_30H, HCOOCH_3 , and CH_30CH_3 whose line widths are similar to carbon-rich species such as CN and CCH found in the extended ridge, but whose rotational temperatures are somewhat higher and whose spatial extents are much more compact.

Molecular gas in the starburst nucleus of M82

Abstract: High-resolution interferometric observations of the CO(1-0) emission from the central one arcmin region of the peculiar galaxy M82 are presented. The observations show that the molecular gas can be divided into two components: a high concentration in the central 700 pc x 200 pc and extended regions that may be shell-like and filamentary in structure. The properties of the interstellar medium in the nuclear region of M82 implied by these and other observations are discussed, and the implications of the observations for star formation in M82 and other galaxies are considered. 76 references.

CO emission from Centaurus A

Abstract: CO(2-1) emission has been detected along the major axis of the dust lane in Centaurus A out to a distance of 2-arcmin from the active nucleus. The derived radial distribution of molecular gas (H2) is centrally peaked with a scale length of about 1.5 kpc and is similar to that expected from a small Sc galaxy. The total H2 mass is about a billion solar masses, comparable to the mass of atomic gas. The molecular gas appears to be concentrated mostly at inner radii, while the atomic gas is found predominantly at larger radii. At current linear resolution of 1 kpc, infalling molecular gas near the nucleus could not be detected; the kinematics of the CO emission from the nuclear region is consistent with circular rotation. The systematic velocity of the molecular disk is found to be 547 km/s, similar to previous values determined for Centaurus A from H I and H II regions in the disk, and to recent values for stars in the elliptical component, indicating that, if it were to be considered as a disk galaxy-elliptical galaxy merger system, Centaurus A would be well relaxed. 16 references.

Independent determinations of temperature and ionization balance in a laser-produced plasma by use of L-shell x-ray spectra.

Abstract: Methode pour determiner independamment la temperature et la distribution des etats de charge dans des plasmas en utilisant les rapports des raies satellite de type sodium et des raies RX de resonance du type neon. La technique est appliquee a un plasma de brome produit par laser. Les resultats sont compatibles avec la formation, dans la couronne, d'un plasma se recombinant. La temperature diminue avec la distance dans la couronne

Co maps of the OMC-1 outflow

Abstract: CO and (C-13)O emission from OMC-1 have been mapped with a resolution of 6 x 9 arcsec. These maps of the hot core and low-velocity flow suggest that essentially all the material near IRc2 is taking part in a turbulent outflow, the densest clumps of which form the hot core. The high-velocity flow is weakly bipolar and is offset to the north of IRc2. Analysis of energy sources for the outflow suggests that it must be driven close to the surface of IRc2, rather than by an orbiting disk. 50 references.

Comparison of submillimeter and ultraviolet observations of neutral carbon toward Zeta Ophiuchi

Abstract: We have observed the ^3P_1 → ^3P_0 ground state transition of C_I emission toward ζ Oph. We compare this observation with predictions made from Copernicus ultraviolet absorption measurements of the population of the ^3P_1 level and with millimeter wave observations of CO.

Observations of the Cooling of the Interstellar Gas

Abstract: Cooling of the gas of the dense interstellar clouds takes place primarily at submillimeter wavelengths. At about 1mm cooling is due to the many rotational lines of heavy moleules, but at shorter wavelengths light molecules (hydrides) become the dominant species, with some assistance from the fine structure transitions of atomic carbon. Photodissociation regions at the surfaces of molecular clouds are cooled by fine structure transitions of atoms and atomic ions. A presentation is given of some aspects of the observed emission due to heavy molecules, light molecules, atoms and ions.

Lawrence Livermore National Laboratory X-Ray Laser Research: Recent Results

Abstract: Since the successful demonstration of gain in neon-like selenium using an exploding foil amplifier, the x-ray laser group at Lawrence Livermore National Laboratory has investigated further the exploding foil amplifier concept for use in XUV lasers. Results are reported of the characteristics of selenium amplifiers up to 50 mm in length. Observation of at least 16 gain lengths for the 206 A line of selenium is reported. Output powers in excess of 1 MW have been measured in pulses of approximately 200 picoseconds. The effects of refraction on the performance of long amplifiers have been studied. The occurrence time of the x-ray laser output relative to the input heating pulse has been measured and found to be in disagreement with a recent model that suggests three-body recombination driven by rapid radiative cooling as the inversion process in the selenium plasma.