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

Thesis (Ph.D.) - University of St Andrews, August 2008.

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.

Modern wide-field imaging transient search projects led to the discovery of a new class of rare, over-luminous stellar explosions. Events like SN 2005ap (Quimby et al. 2007a), SN 2006gy (Quimby 2006; Smith et al. 2007), SN 2006tf (Quimby, Castro \& Mondol 2007; Quimby et al. 2007b; Smith et al. 2008), SN 2008am (Chatzopoulos et al. 2010), SN 2008es (Yuan et al. 2008; Gezari et al. 2008; Miller et al. 2008) SN 2008fz (Drake et al. 2009) and SN 2008iy (Miller et al. 2010) introduced us new evidence about stellar death, since traditional ideas about the mechanisms that can power these phenomena are found to be inadequate to explain the observed properties. The Texas Supernova Search Project (TSS; Quimby et al. 2005) and its successor, the ROTSE-Supernova Verification Project (RSVP; Yuan et al. 2007) discovered most of the above mentioned over-luminous supernovae (OLSNe) over the past five years of their operation. The advantage of this project is that it is essentially free of selection bias or the limits of a targeted search; the automated wide field (~3 square degrees) ROTSE-III telescopes (Akerlof et al. 2003), scan the whole sky blindly, looking for transients down to unfiltered magnitude ~19 mag and they do not focus on pre-selected galaxies. An estimated rate for these exceptionally luminous supernovae is ~ 2.6 10^{-7} events/Mpc^{3}/yr (Quimby et al. 2009a). Current and future transient search projects such as the SDSS-II Supernova Survey (Frieman et al. 2008),the Palomar Transient Factory (PTF; Law et al. 2009), SkyMapper (Schmidt et al. 2005), PanSTARRS (Chambers 2006) and Large Synoptic Survey Telescope (Tyson \& LSST collaboration 2002) are expected to increase the number of rare over-luminous (or, under-luminous) supernove and to discover new, unusual transients. / text

Thesis (Ph. D.)--University of Hawaii at Manoa, 2003. / Includes bibliographical references.

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.

Circumstellar disks are critical structures in the star and planet formation processes, as they provide a conduit to channel material onto the central object and supply a reservoir of dust and gas to form planets. This thesis focuses on the far-infrared, and sub-millimeter observations of circumstellar disks at two key evolutionary phases; pri- mordial proto-planetary disks, and evolved debris disks – remnants of a system that has undergone a degree of planet formation. Four individual studies of circumstellar disks are presented in this thesis. The results of a 97% complete census of far-infrared emission measured with the Herschel Space Observatory, targeting stars of spectral types M4 and later, in the Taurus molecular cloud are presented. This census is the first large-scale survey sensitive to emission across the stellar and sub-stellar boundary. Results from an initial test grid of model spectral energy distributions, generated with the radiative transfer code MCFOST, show that 73% of the observed Class II population are constrained by canonical disks that are viewed from face-on to edge-on inclinations. Sub-millimeter observations with the Caltech Sub-millimeter Observatory are presented for an association of young T-Tauri stars in the Aquila star-forming region. The results of disk frequency and disk mass of this complete census are investigated in this extremely low stellar density environment. A sub-millimeter investigation for two populations of candidate debris disk; warm and cold excess disks is presented. None of the candidate disks were detected in the sub-millimeter despite upper-limits below that expected, based on blackbody model fits to excesses at shorter wavelengths. Several scenarios are investigated in order to identify the null detection rate, such as stellar multiplicity and background-source contamination. Finally, a partially resolved sub-millimeter map of the debris disk around the HR 8799 multiple planetary system is presented. The planet formation history of the system is investigated through the witnessed morphology of the emission.

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.

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.

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.