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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

Millimeter-wavelength spectral line surveys of evolved circumstellar envelopes

Chau, Wayne C, 周穎鏘 January 2013 (has links)
Evolved stars including asymptotic giant branch stars, proto-planetary nebulae and planetary nebulae are well known to be sources of molecules for the interstellar medium. Circumstellar envelopes of asymptotic giant branch stars are known to be sites of active molecular synthesis. These molecular species are then distributed to the interstellar medium as the envelope is gradually ejected in the proto-planetary nebula and planetary nebula phases. As carbon that is dredged-up from the stellar core by convectional instabilities facilitates rapid formation and reprocessing of molecules, the chemical abundances differ significantly between objects at different stages of stellar evolution. Changes in the physical conditions in the envelope also allow different chemical processes to take place throughout the evolutionary sequence. Conducting spectral line surveys on a large sample of objects at different stages of stellar evolution will allow one to track those chemical and physical changes along the evolutionary track. Presented in this thesis are the results from a series of unbiased systematic line surveys targeting the circumstellar envelopes of evolved stars. As part of an on-going effort, these surveys aim to expand the coverage both in terms of objects at different evolutionary stages and wavelength ranges in order to paint a more complete picture in our understanding of the chemical evolution in objects at the final stages of stellar evolution. All surveys included in the present study are conducted using the 45m radio telescope at the Nobeyama Radio Observatory, and the same instrumental settings were used when observing a given frequency region to minimize the systematic uncertainties. The four objects surveyed in this study, the asymptotic giant branch star CIT6, the proto-planetary nebulae CRL 2688 and CRL 618, and the planetary nebula NGC7027, represent a healthy sample from all key phases of late-stage stellar evolution. Since the surveys are conducted in millimeter-wavelengths, they are mostly targeting rotational transitions from molecules. In particular, rotational transitions associated with the cyanopolyyne chain HC7N reported in the survey of CIT6 and CRL 2688 are novel detections. Spectra from the series of surveys are analyzed using previously obtained spectra of the prototypical asymptotic giant branch star IRC+10216 as a benchmark. Results from fractional abundance computations suggested that CIT6 is a more evolved specimen relative to IRC+10216 and supported chemical models pertaining to the synthesis of cyanopolyyne chains. Intensity ratios in CRL 2688 showed that the cyanopolyyne enrichment towards conclusion of the asymptotic giant branch phase is no longer present in proto-planetary nebulae. Rotational analysis provided evidence to support the hypothesis that the ammonia absorption in CRL 618 originates from two distinct components. Flux ratios in NGC7027 also showed that the recombination lines in the planetary nebula behave according to theoretical predictions. Altogether, these findings will complement previous results and enable one to trace and understand the interplay between chemistry and physical conditions within the circumstellar envelope as objects proceeds on the evolutionary track. / published_or_final_version / Physics / Master / Master of Philosophy
2

The environments in which stars and circumstellar discs form /

Poulton, Christopher John. January 2008 (has links)
Thesis (Ph.D.) - University of St Andrews, August 2008.
3

What Sets the Radial Locations of Warm Debris Disks?

Ballering, Nicholas P., Rieke, George H., Su, Kate Y. L., Gáspár, András 18 August 2017 (has links)
The architectures of debris disks encode the history of planet formation in these systems. Studies of debris disks via their spectral energy distributions (SEDs) have found infrared excesses arising from cold dust, warm dust, or a combination of the two. The cold outer belts of many systems have been imaged, facilitating their study in great detail. Far less is known about the warm components, including the origin of the dust. The regularity of the disk temperatures indicates an underlying structure that may be linked to the water snow line. If the dust is generated from collisions in an exo-asteroid belt, the dust will likely trace the location of the water snow line in the primordial protoplanetary disk where planetesimal growth was enhanced. If instead the warm dust arises from the inward transport from a reservoir of icy material farther out in the system, the dust location is expected to be set by the current snow line. We analyze the SEDs of a large sample of debris disks with warm components. We find that warm components in single-component systems (those without detectable cold components) follow the primordial snow line rather than the current snow line, so they likely arise from exo-asteroid belts. While the locations of many warm components in two-component systems are also consistent with the primordial snow line, there is more diversity among these systems, suggesting additional effects play a role.
4

A search for debris disks with a dual channel adaptive optics imaging polarimeter

Potter, Daniel E. January 2003 (has links)
Thesis (Ph. D.)--University of Hawaii at Manoa, 2003. / Includes bibliographical references.
5

The SPHERE view of the planet-forming disk around HD 100546

Garufi, A., Quanz, S. P., Schmid, H. M., Mulders, G. D., Avenhaus, H., Boccaletti, A., Ginski, C., Langlois, M., Stolker, T., Augereau, J.-C., Benisty, M., Lopez, B., Dominik, C., Gratton, R., Henning, T., Janson, M., Ménard, F., Meyer, M. R., Pinte, C., Sissa, E., Vigan, A., Zurlo, A., Bazzon, A., Buenzli, E., Bonnefoy, M., Brandner, W., Chauvin, G., Cheetham, A., Cudel, M., Desidera, S., Feldt, M., Galicher, R., Kasper, M., Lagrange, A.-M., Lannier, J., Maire, A. L., Mesa, D., Mouillet, D., Peretti, S., Perrot, C., Salter, G., Wildi, F. 09 March 2016 (has links)
Context. The mechanisms governing planet formation are not fully understood. A new era of high-resolution imaging of protoplanetary disks has recently started, thanks to new instruments such as SPHERE, GPI, and ALMA. The planet formation process can now be directly studied by imaging both planetary companions embedded in disks and their e ff ect on disk morphology. Aims. We image disk features that could be potential signs of planet-disk interaction with unprecedented spatial resolution and sensitivity. Two companion candidates have been claimed in the disk around the young Herbig Ae /Be star HD 100546. Thus, this object serves as an excellent target for our investigation of the natal environment of giant planets. Methods. We exploit the power of extreme adaptive optics operating in conjunction with the new high-contrast imager SPHERE to image HD 100546 in scattered light. We obtained the first polarized light observations of this source in the visible (with resolution as fine as 2 AU) and new H and K band total intensity images that we analyzed with the p y n p o i n t package. Results. The disk shows a complex azimuthal morphology, where multiple scattering of photons most likely plays an important role. High brightness contrasts and arm-like structures are ubiquitous in the disk. A double-wing structure (partly due to angular di ff erential imaging processing) resembles a morphology newly observed in inclined disks. Given the cavity size in the visible (11 AU), the CO emission associated to the planet candidate c might arise from within the circumstellar disk. We find an extended emission in the K band at the expected location of b. The surrounding large-scale region is the brightest in scattered light. There is no sign of any disk gap associated to b.
6

The First 40 Million Years of Circumstellar Disk Evolution: The Signature of Terrestrial Planet Formation

Meng, Huan Y. A., Rieke, George H., Su, Kate Y. L., Gáspár, András 07 February 2017 (has links)
We characterize the first 40 Myr of evolution of circumstellar disks through a unified study of the infrared properties of members of young clusters and associations with ages from 2 Myr up to similar to 40 Myr: NGC 1333, NGC 1960, NGC 2232, NGC 2244, NGC 2362, NGC 2547, IC 348, IC 2395, IC 4665, Chamaeleon I, Orion OB1a and OB1b, Taurus, the beta Pictoris Moving Group,. Ophiuchi, and the associations of Argus, Carina, Columba, Scorpius-Centaurus, and Tucana-Horologium. Our work features: (1) a filtering technique to flag noisy backgrounds; (2) a method based on the probability distribution of deflections, P(D), to obtain statistically valid photometry for faint sources; and (3) use of the evolutionary trend of transitional disks to constrain the overall behavior of bright disks. We find that the fraction of disks three or more times brighter than the stellar photospheres at 24 mu m decays relatively slowly initially and then much more rapidly by similar to 10 Myr. However, there is a continuing component until similar to 35 Myr, probably due primarily to massive clouds of debris generated in giant impacts during the oligarchic/chaotic growth phases of terrestrial planets. If the contribution from primordial disks is excluded, the evolution of the incidence of these oligarchic/chaotic debris disks can be described empirically by a log-normal function with the peak at 12-20 Myr, including similar to 13% of the original population, and with a post-peak mean duration of 10-20 Myr.
7

The influence of dust grain porosity on the analysis of debris disc observations

Brunngräber, Robert, Wolf, Sebastian, Kirchschlager, Florian, Ertel, Steve 01 February 2017 (has links)
Debris discs are often modelled assuming compact dust grains, but more and more evidence for the presence of porous grains is found. We aim at quantifying the systematic errors introduced when modelling debris discs composed of porous dust with a disc model assuming spherical, compact grains. We calculate the optical dust properties derived via the fast, but simple effective medium theory. The theoretical lower boundary of the size distribution - the so-called 'blowout size' - is compared in the cases of compact and porous grains. Finally, we simulate observations of hypothetical debris discs with different porosities and feed them into a fitting procedure using only compact grains. The deviations of the results for compact grains from the original model based on porous grains are analysed. We find that the blowout size increases with increasing grain porosity up to a factor of 2. An analytical approximation function for the blowout size as a function of porosity and stellar luminosity is derived. The analysis of the geometrical disc set-up, when constrained by radial profiles, is barely affected by the porosity. However, the determined minimum grain size and the slope of the grain size distribution derived using compact grains are significantly overestimated. Thus, the unexpectedly high ratio of minimum grain size to blowout size found by previous studies using compact grains can be partially described by dust grain porosity, although the effect is not strong enough to completely explain the trend.
8

An optical and near-infrared study of the Type Ia/IIn Supernova PS15si

Kilpatrick, Charles D., Andrews, Jennifer E., Smith, Nathan, Milne, Peter, Rieke, George H., Zheng, WeiKang, Filippenko, Alexei V. 21 November 2016 (has links)
We present optical/near-infrared spectroscopy and photometry of the supernova (SN) PS15si. This object was originally identified as a Type IIn SN, but here we argue that it should be reclassified as a Type Ia SN with narrow hydrogen lines originating from interaction with circumstellar matter (CSM; i.e. SN Ia/IIn or SN Ia-CSM). Based on deep non-detections 27 d before discovery, we infer that this SN was discovered around or slightly before optical maximum, and we estimate the approximate time that it reached R-band maximum based on comparison with other SNe Ia/IIn. In terms of spectral morphology, we find that PS15si can be matched to a range of SN Ia spectral types, although SN 1991T-like SNe Ia provides the most self-consistent match. While this spectral classification agrees with analysis of most other SNe Ia/IIn, we find that the implied CSM-interaction luminosity is too low. We infer that the similarity between PS15si and the hot, overluminous, high-ionization spectrum of SN 1991T is a consequence of a spectrum that originates in ejecta layers that are heated by ultraviolet/X-ray radiation from CSM interaction. In addition, PS15si may have rebrightened over a short time-scale in the B and V bands around 85 d after discovery, perhaps indicating that the SN ejecta are interacting with a local enhancement in CSM produced by clumps or a shell at large radii.
9

POLARIZATION MEASUREMENTS OF HOT DUST STARS AND THE LOCAL INTERSTELLAR MEDIUM

Marshall, J. P., Cotton, D. V., Bott, K., Ertel, S., Kennedy, G. M., Wyatt, M. C., Burgo, C. del, Absil, O., Bailey, J., Kedziora-Chudczer, L. 12 July 2016 (has links)
Debris discs are typically revealed through the presence of excess emission at infrared wavelengths. Most discs exhibit excess at mid- and far-infrared wavelengths, analogous to the solar system's Asteroid and Edgeworth-Kuiper belts. Recently, stars with strong (similar to 1%) excess at near-infrared wavelengths were identified through interferometric measurements. Using the HIgh Precision Polarimetric Instrument, we examined a sub-sample of these hot dust stars (and appropriate controls) at parts-per-million sensitivity in SDSS g' (green) and r' (red) filters for evidence of scattered light. No detection of strongly polarized emission from the hot dust stars is seen. We, therefore, rule out scattered light from a normal debris disk as the origin of this emission. A wavelength-dependent contribution from multiple dust components for hot dust stars is inferred from the dispersion (the difference in polarization angle in red and green) of southern stars. Contributions of 17 ppm (green) and 30 ppm (red) are calculated, with strict 3-sigma upper limits of 76 and 68 ppm, respectively. This suggests weak hot dust excesses consistent with thermal emission, although we cannot rule out contrived scenarios, e.g., dust in a spherical shell or face-on discs. We also report on the nature of the local interstellar medium (ISM), obtained as a byproduct of the control measurements. Highlights include the first measurements of the polarimetric color of the local ISM and the discovery of a southern sky region with a polarization per distance thrice the previous maximum. The data suggest that lambda(max), the wavelength of maximum polarization, is bluer than typical.
10

NULLING DATA REDUCTION AND ON-SKY PERFORMANCE OF THE LARGE BINOCULAR TELESCOPE INTERFEROMETER

Defrère, D., Hinz, P. M., Mennesson, B., Hoffmann, W. F., Millan-Gabet, R., Skemer, A. J., Bailey, V., Danchi, W. C., Downey, E. C., Durney, O., Grenz, P., Hill, J. M., McMahon, T. J., Montoya, M., Spalding, E., Vaz, A., Absil, O., Arbo, P., Bailey, H., Brusa, G., Bryden, G., Esposito, S., Gaspar, A., Haniff, C. A., Kennedy, G. M., Leisenring, J. M., Marion, L., Nowak, M., Pinna, E., Powell, K., Puglisi, A., Rieke, G., Roberge, A., Serabyn, E., Sosa, R., Stapeldfeldt, K., Su, K., Weinberger, A. J., Wyatt, M. C. 14 June 2016 (has links)
The Large Binocular Telescope Interferometer (LBTI) is a versatile instrument designed for high angular resolution and high-contrast infrared imaging (1.5-13 mu m). In this paper, we focus on the mid-infrared (8-13 mu m) nulling mode and present its theory of operation, data reduction, and on-sky performance as of the end of the commissioning phase in 2015 March. With an interferometric baseline of 14.4 m, the LBTI nuller is specifically tuned to resolve the habitable zone of nearby main-sequence stars, where warm exozodiacal dust emission peaks. Measuring the exozodi luminosity function of nearby main-sequence stars is a key milestone to prepare for future exo-Earth direct imaging instruments. Thanks to recent progress in wavefront control and phase stabilization, as well as in data reduction techniques, the LBTI demonstrated in 2015 February a calibrated null accuracy of 0.05% over a 3 hr long observing sequence on the bright nearby A3V star beta Leo. This is equivalent to an exozodiacal disk density of 15-30. zodi for a Sun-like star located at 10 pc, depending on the adopted disk model. This result sets a new record for high-contrast mid-infrared interferometric imaging and opens a new window on the study of planetary systems.

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