H2 Volume Density Research Articles

Ground State Rotational Lines of Doubly Deuterated Ammonia as Tracers of the Physical Conditions and Chemistry of Cold Interstellar Medium

We report the first detection of the N = 111 → 000 and 110 → 000 ground state rotational lines of o-ND2H at 335.5 and 388.7 GHz, obtained in the Lynds 1689N, Barnard 1, and Lynds 1544 molecular clouds using the Caltech Submillimeter Observatory (CSO). The submillimeter ND2H lines have moderate opacities and simple hyperfine patterns, which allow accurate determination of the excitation temperature, H2 volume density, and molecular column density. Both transitions have high critical densities. The 389 GHz line, in particular, traces molecular material with densities above a few × 106 cm-3. The strong 389 GHz ND2H emission in LDN 1689N implies a high fraction of dense gas in this source, ~30%, as compared to ~15% in B1 and LDN 1544. All these regions are sites of strong molecular depletion and heavy deuteration. Nonaccreting molecules, H and its isotopologues, are difficult to study, but in the sources studied here it appears that ammonia and its isotopologues are not completely frozen out, even in the high density gas. In the well-studied case of LDN 1544, the volume probed by the ND2H emission has densities of ~106-107 cm-3, within the range where the complete freezeout has been predicted to occur. The critical density of the 389 GHz ND2H line is close to that of the 309 GHz ND3 line. Observations of these two transitions thus provide an accurate measure of the [ND3]/[ND2H] fractionation ratio in the very dense gas. The [ND3]/[ND2H] ratio in LDN 1689N (~3%) appears lower than the values measured in B1 and LDN 1544 (~7%-10%), indicating that different chemical processes may be at work in these environments. The submillimeter lines of deuteroammonia are relatively strong and detectable from good sites, such as Mauna Kea or Chajnantor. Interferometric observations of these lines with the Submillimeter Array (SMA), and subsequently the Atacama Large Millimeter Array (ALMA), will provide new opportunities to study the physics and chemistry of cold, dense ISM, where most molecules are depleted onto dust grains.

Dense Molecular Clumps in the Orion Bar Photon-dominated Region

We present high angular resolution observations of the Orion Bar photon-dominated region (PDR) in optically thin H13CN and H13CO+ (1-0) lines, obtained using the IRAM Plateau de Bure interferometer. At least 10 spatially resolved molecular condensations are identified in the H13CN image with virial masses in the range 0.5-1.5 M☉. The median value of their H2 volume density, ~6 × 106 cm-3, is a factor of ~4 higher than the estimate based on previous PDR modeling of the main isotopomers of HCN and HCO+. Since optically thin H13CN emission is likely to trace the densest gas in the clump interiors, as compared to the main isotopomer, the H13CN clumps appear to be close to virial equilibrium. The H13CN fractional abundance is a factor of ~8 lower than that in the Orion ridge, well shielded from the far-ultraviolet (FUV) photons (~1 × 10-10). The H13CN condensations can be described in the framework of models of photoevaporating clumps exposed to an intense flux of FUV photons. The derived clump parameters are consistent with models of clumps of turbulent origin that evolve, so that their column densities are equal to the critical value determined by the incident FUV field. In this case, the column densities of the H13CN clumps seem high enough so that gravitational collapse can be triggered by the FUV-driven shock wave compression. The clumps may thus be collapsing to form low-mass stars. The observed H13CN clump parameters are also consistent with pressure-confined clump models. However, in this case the clumps would not be virialized and susceptible to gravitational collapse.

Morphology and Energetics of the Molecular Gas within a Core and a Diffuse Region in the Filamentary Dark Cloud GF 9

We have performed a CO, 13CO, and CS survey of a dense core region (GF 9–Core) and a diffuse filamentary region (GF 9–Fila) within the dark cloud GF 9 (LDN 1082). Spectra in each line were obtained toward 120 positions within each region covering areas of 8' × 10'. GF 9–Core is associated with the Class 0 protostar IRAS PSC 20503+6006, while GF 9–Fila has no associated IRAS point sources. The median CO excitation temperature of the core region is 7.2 ± 0.5 K; for the filament region, the median temperature is 7.8 ± 0.5 K. The mass derived from the LTE isothermal analysis of 13CO is 53 ± 8 and 40 ± 6 M⊙ for GF 9–Core and GF 9–Fila, respectively. Using near-infrared extinction data to trace the H2 column density, the isothermal LTE assumptions for 13CO appear to break down at AV 3 mag. The near-infrared extinction data are used to correct the derived 13CO column densities, yielding mass estimates that are ~20% larger than the LTE-derived masses. The average H2 volume densities for GF 9–Core and GF 9–Fila are ~5000 ± 700 and ~1700 ± 200 cm-3. Each region contains a 15 ± 3 M⊙ centrally condensed CS core, which is approximately 3 times more dense than the ambient 13CO regions. In GF 9–Core, the high-density gas core appears to be physically associated with the IRAS point source. Both high-density cores appear to be in virial equilibrium; however, the CS line widths in GF 9–Core are almost twice as large as those measured in GF 9–Fila. Because GF 9–Core is associated with the Class 0 protostar PSC 20503+6006, the CS line widths may be enhanced by outflow or infall motion. The additional nonturbulent line-broadening component caused by outflow/infall motion required to explain the line width implies a supersonic velocity of at least 0.3 km s-1 (Mach 1.5).

The W51 Giant Molecular Cloud

We present 45''–47'' angular resolution maps at 50'' sampling of the 12CO and 13CO J = 1–0 emission toward a 139 × 133 region in the W51 H II region complex. These data permit the spatial and kinematic separation of several spectral features observed along the line of sight to W51 and establish the presence of a massive (1.2 × 106 M⊙), large (Δl × Δb = 83 × 114 pc) giant molecular cloud (GMC), defined as the W51 GMC and centered at (l, b, V)c ≈ (495, -02, 61 km s-1). A second massive (1.9 × 105M⊙), elongated (22 × 136 pc) molecular cloud is found at velocities of ~68 km s-1 along the southern edge of the W51 GMC. Of the five radio continuum sources that classically define the W51 region, the brightest source at λ6 cm (G49.5-0.4) is spatially and kinematically coincident with the W51 GMC and three (G48.9-0.3, G49.1-0.4, and G49.2-0.4) are associated with the 68 km s-1 cloud. Published absorption-line spectra indicate that the fifth prominent continuum source (G49.4-0.3) is located behind the W51 molecular cloud. The W51 GMC is among the upper 1% of clouds in the Galactic disk by size and the upper 5%–10% by mass. While the W51 GMC is larger and more massive than any nearby molecular cloud, the average H2 column density is not unusual given its size, and the mean H2 volume density is comparable to that in nearby clouds. The W51 GMC is also similar to other clouds in that most of the molecular mass is contained in a diffuse envelope that is not currently forming massive stars. We speculate that much of the massive star formation activity in this region has resulted from a collision between the 68 km s-1 cloud and the W51 GMC.

Understanding the Star Formation Process in the Filamentary Dark Cloud GF 9: Near-Infrared Observations

We have performed a near-infrared JHK survey of a dense core and a diffuse filament region within the filamentary dark cloud GF 9 (LDN 1082). The core region is associated with the IRAS point source PSC 20503+6006 and is suspected of being a site of star formation. The diffuse filament region has no associated IRAS point sources and is likely quiescent. We find that neither the core nor the filament region appears to contain a Class I or Class II young stellar object. As traced by the dust extinction, the core and filament regions contain 26 and 22 solar mass, respectively, with an average H2 volume density for both regions of approximately 2500/cu cm. The core region contains a centrally condensed extinction maximum with a peak extinction of A(sub v) greater than or approximately equal to 10 mag that appears to be associated with the IRAS point source. The average H2 volume density of the extinction core is approximately 8000/cu cm. The dust within the filament, however, shows no sign of a central condensation and is consistent with a uniform-density cylindrical distribution.

A multitransitional CO study of the Cepheus C cloud core

Strip maps of (12)CO, (13)CO, and C(18)O J = 2-1 and J = 1-0 emissions from the Cep C dense core have been obtained to determine its density distribution and kinematic properties. Using large velocity gradient models to estimate gas conditions, we find that the H2 volume density ranges between 2500/cu cm at the core edge to 7000/cu cm at the central position. Absorption features are found in the (12)CO lines which are redshifted with respect to the cloud core velocities. Self-absorption is also identified in the profiles of (13)CO J = 2-1 emission but at velocities much closer to those of the central core of the line. Such a velocity structure denotes different kinematics and thermal conditions within several layers of material along the line of sight.

The molecular gas associated with the Orion bright bar

view Abstract Citations (10) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Molecular Gas Associated with the Orion Bright Bar Omodaka, Toshihiro ; Hayashi, Masahiko ; Hasegawa, Telsuo ; Hayashi, Saeko S. Abstract Detailed studies of the shocked region associated with the Orion bright bar are presented. The millimeter wave spectral lines of (12)CO (J = 1-0), (13)CO (J = 1-0), CS (J = 2-1), HCO(+) (J = 1-0), and H51 alpha have been observed across the bright bar. The intensity of all the molecular species shows a rapid increase close to the ionization front and a significant falloff at a distance of approximately 50 sec farther out from it. This suggests the existence of a layer of dense molecular gas just outside the ionization front. This layer has a velocity redshifted by 1-2 km s-1 relative to the ambient molecular cloud, which can be due to acceleration by thermal and kinetic pressure from the H II region or due to a passage of a shock. The high-density molecular layer associated with the bar is probably a shock-compressed layer driven by the ionization front of M42. A multitransitional anaylsis of the CS emission shows that the H2 volume density of the molecular gas is larger than that of the ambient gas by a factor of 3. The apparent density enhancement of a factor of 3 in the shocked gas is too small for a radiative shock in a homogeneous medium; density inhomgeneities or clumpiness in the pre- and postshocked layer may account for this apparently small compression ratio. This layer is exposed to intense UV radiation from the Trapezium stars and a photodissocited region is formed between neutral layer and the ionization front. The similarity in distribution of the thermally excited H2 emissions arising from shock fronts and or dense phtotodissociation regions and the millimeter-wave molecular line emissions originating from cooled shock-compressed regions also supports the idea of homogeneity or clumpiness is the shocked cloud. The gas temperature of this shocked layer is about 100 K and is very high compared with other molecular clouds without an embedded heat source. Both shock heating and radiative heating may contribute to maintain this high temperature. Publication: The Astrophysical Journal Pub Date: July 1994 DOI: 10.1086/174400 Bibcode: 1994ApJ...430..256O Keywords: Interstellar Gas; Line Spectra; Molecular Clouds; Molecular Gases; Orion Nebula; Radio Spectra; Shock Fronts; Carbon Monoxide; Gas Temperature; H Ii Regions; Hydrogen; Shock Waves; Astrophysics; ISM: INDIVIDUAL NAME: ORION NEBULA; ISM: MOLECULES; RADIO LINES: ISM; SHOCK WAVES full text sources ADS | data products SIMBAD (4)