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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

THE GALACTIC CENSUS OF HIGH- AND MEDIUM-MASS PROTOSTARS. III. 12 CO MAPS AND PHYSICAL PROPERTIES OF DENSE CLUMP ENVELOPES AND THEIR EMBEDDING GMCs

Barnes, Peter J., Hernandez, Audra K., O’Dougherty, Stefan N., Schap III, William J., Muller, Erik 27 October 2016 (has links)
We report the second complete molecular line data release from the Census of High-and Medium-mass Protostars (CHaMP), a large-scale, unbiased, uniform mapping survey at sub-parsec resolution, of millimeter-wave line emission from 303 massive, dense molecular clumps in the Milky Way. This release is for all (CO)-C-12 J = 1 -> 0 emission associated with the dense gas, the first from Phase II of the survey, which includes (CO)-C-12, (CO)-C-13, and (CO)-O-18. The observed clump emission traced by both (CO)-C-12 and HCO+ (from Phase I) shows very similar morphology, indicating that, for dense molecular clouds and complexes of all sizes, parsec-scale clumps contain. similar to 75% of the mass, while only 25% of the mass lies in extended (>10 pc) or "low density" components in these same areas. The mass fraction of all gas above a density of 10(9) m(-3) is xi(9) greater than or similar to 50%. This suggests that parsec-scale clumps may be the basic building blocks of the molecular interstellar medium, rather than the standard GMC concept. Using (CO)-C-12 emission, we derive physical properties of these clumps in their entirety, and compare them to properties from HCO+, tracing their denser interiors. We compare the standard X-factor converting I (CO)-C-12 to N-H2 with alternative conversions, and show that only the latter give whole-clump properties that are physically consistent with those of their interiors. We infer that the clump population is systematically closer to virial equilibrium than when considering only their interiors, with perhaps half being long-lived (10s of Myr), pressure-confined entities that only terminally engage in vigorous massive star formation, supporting other evidence along these lines that was previously published.
2

NEW DETECTIONS OF HNC IN PLANETARY NEBULAE: EVOLUTION OF THE [HCN]/[HNC] RATIO

Schmidt, D. R., Ziurys, L. M. 19 January 2017 (has links)
New detections of HNC have been made toward 11 planetary nebulae (PNe), including K4-47, K3-58, K3-17, M3-28, and M4-14. These sources, which represent a wide range of ages and morphologies, had previously been observed in HCN by Schmidt & Ziurys. Measurements of the J = 1 -> 0 and J = 3 -> 2 transitions of HNC near 90 and 271 GHz were conducted using the new 12 m and the Sub-Millimeter Telescope of the Arizona Radio Observatory. HCN and HNC were also identified via their J = 1 -> 0 lines toward eight positions across the Helix Nebula (NGC 7293). Column densities for HNC, determined from radiative transfer modeling, were N-tot(HNC) similar to (0.06-4.0) x 10(13) cm(-2), corresponding to fractional abundances with respect to H-2 of f similar to (0.02-1.4) x 10(-7). The HCN and HNC column densities across the Helix were found to be N-tot (HCN) similar to (0.2-2.4). x. 10(12) cm(-2) and Ntot (HNC) similar to (0.07-1.6). x. 1012 cm(-2), with fractional abundances of (0.2-3.2) x 10(-7) and (0.09-2.2) x 10(-7). The [ HCN]/[ HNC] ratio varied between similar to 1-8 for all PNe, with [ HCN]/[ HNC] similar to 1-4 across the Helix. These values are greatly reduced from what has been found in asymptotic giant branch stars, where the ratio is typically > 100. Both the abundance of HNC and the [ HCN]/[ HNC] ratio do not appear to vary significantly with nebular age across a time span of similar to 10,000 years, in contrast to predictions of chemical models. The increase in HNC appears to arise in the proto-planetary stage, but becomes " frozen" once the PN phase is reached.
3

New Identifications of the CCH Radical in Planetary Nebulae: A Connection to C-60?

Schmidt, D. R., Ziurys, L. M. 22 November 2017 (has links)
New detections of CCH have been made toward nine planetary nebulae (PNe), including K4-47, K3-58, K3-17, M3-28, and M4-14. Measurements of the N = 1 -> 0 and N = 3 -> 2 transitions of this radical near 87 and 262 GHz were carried out using the new 12 m and the Sub-Millimeter Telescope (SMT) of the Arizona Radio Observatory (ARO). The presence of fine and/or hyperfine structure in the spectra aided in the identification. CCH was not observed in two PNe which are sources of C-60. The planetary nebulae with positive detections represent a wide range of ages and morphologies, and all had previously been observed in HCN and HNC. Column densities for CCH in the PNe, determined from radiative transfer modeling, were N-tot(CCH) similar to 0.2-3.3 x 10(15) cm(-2), corresponding to fractional abundances with respect to H-2 of f similar to 0.2-47 x 10(-7). The abundance of CCH was found to not vary significantly with kinematic age across a time span of similar to 10,000 years, in contrast to predictions of chemical models. CCH appears to be a fairly common constituent of PNe that are carbon-rich, and its distribution may anti-correlate with that of C-60. These results suggest that CCH may be a product of C-60 photodestruction, which is known to create C-2 units. The molecule may subsequently survive the PN stage and populate diffuse clouds. The distinct, double-horned line profiles for CCH observed in K3-45 and M3-28 indicate the possible presence of a bipolar flow oriented at least partially toward the line of sight.
4

DISCOVERY OF BROAD MOLECULAR LINES AND OF SHOCKED MOLECULAR HYDROGEN FROM THE SUPERNOVA REMNANT G357.7+0.3: HHSMT, APEX, SPITZER , AND SOFIA OBSERVATIONS

Rho, J., Hewitt, J. W., Bieging, J., Reach, W. T., Andersen, M., Güsten, R. 22 December 2016 (has links)
We report a discovery of shocked gas from the supernova remnant (SNR) G357.7+0.3. Our millimeter and submillimeter observations reveal broad molecular lines of CO(2-1), CO(3-2), CO(4-3), (CO)-C-13 (2-1), and (CO)-C-13 (3-2), HCO+, and HCN using the Heinrich Hertz Submillimeter Telescope, the Arizona 12 m Telescope, APEX, and the MOPRA Telescope. The widths of the broad lines are 15-30 km s(-1), and the detection of such broad lines is unambiguous, dynamic evidence showing that the SNR G357.7+0.3 is interacting with molecular clouds. The broad lines appear in extended regions (>4'.5 x 5'). We also present the detection of shocked H-2 emission in the mid-infrared but lacking ionic lines using Spitzer/IRS observations to map a few-arcminute area. The H2 excitation diagram shows a best fit with a two-temperature local thermal equilibrium model with the temperatures of similar to 200 and 660 K. We observed [C II] at 158 mu m and high-J CO(11-10) with the German Receiver for Astronomy at Terahertz Frequencies (GREAT) on the Stratospheric Observatory for Infrared Astronomy. The GREAT spectrum of [C II], a 3 sigma detection, shows a broad line profile with a width of 15.7 km(-1) that is similar to those of broad CO molecular lines. The line width of [C II] implies that ionic lines can come from a low-velocity C-shock. Comparison of H2 emission with shock models shows that a combination of two C-shock models is favored over a combination of C- and J-shocks or a single shock. We estimate the CO density, column density, and temperature using a RADEX model. The best-fit model with n(H-2) = 1.7 x 10(4) cm(-3), N(CO) = 5.6 x 10(16) cm(-2), and T = 75 K can reproduce the observed millimeter CO brightnesses.

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