The massive evolved star. Carinae is the most luminous star in the Milky Way and has the highest steady wind mass-loss rate of any known star. Radiative transfer models of the spectrum by Hillier et al. predict that Ha is mostly emitted in regions of the wind at radii of 6-60 au from the star (2.5-25 mas at 2.35 kpc). We present diffraction-limited images (FWHM similar to 25 mas) with Magellan adaptive optics in two epochs, showing that. Carinae consistently appears similar to 2.5-3 mas wider in Ha emission compared to the adjacent 643 nm continuum. This implies that the H alpha line-forming region may have a characteristic emitting radius of 12 mas or similar to 30 au, in very good agreement with the Hillier stellar-wind model. This provides direct confirmation that the physical wind parameters of that model are roughly correct, including the mass-loss rate of M= 10(-3)M(circle dot) yr(-1), plus the clumping factor, and the terminal velocity. Comparison of the Ha images (ellipticity and PA) to the continuum images reveals no significant asymmetries at H alpha. Hence, any asymmetry induced by a companion or by the primary's rotation do not strongly influence the global H alpha emission in the outer wind.

We report on high angular resolution observations of AFGL 2136 at 50 and 100 μm from NASA's Kuiper Airborne Observatory. Our data consist of diffraction-limited scans in two orthogonal directions as well as photometry. The far-infrared (FIR) emission is very compact with an unresolved core and also exhibits low surface brightness wings that extend out to a radius of order 70″ at 100 μm. We have attempted to fit our observations and the photometry and size data at other wavelengths with simple, dust envelope models in order to place limits on the quantity and distribution of dust around AFGL 2136. Spherically symmetric models cannot fit the data, but we show that a simple approximation to a toroidal dust distribution can fit the energy distribution and size data rather well. The successful models imply a density gradient in the cloud of order ρ ∝ r-1.5 and optical depth at 100 μm of order unity.

Mira and semi-regular variable stars have been studied for centuries but continue to be enigmatic. One unsolved mystery is the presence of polarization from these stars. In particular, we present 40 years of polarization measurements for the prototype o Ceti and V CVn and find very different phenomena for each star. The polarization fraction and position angle for Mira is found to be small and highly variable. On the other hand, the polarization fraction for V CVn is large and variable, from 2-7%, and its position angle is approximately constant, suggesting a long-term asymmetric structure. We suggest a number of potential scenarios to explain these observations.

Both pulsation and mass loss are commonly observed in stars and are important ingredients for understanding stellar evolution and structure, especially for massive stars. There is a growing body of evidence that pulsation can also drive and enhance mass loss in massive stars and that pulsation-driven mass loss is important for stellar evolution. In this review, I will discuss recent advances in understanding pulsation-driven mass loss in massive main-sequence stars, classical Cepheids and red supergiants and present some challenges remaining.

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Monte Carlo radiation transfer provides an efficient modelling tool for probing the dusty local environment of young stars. Within this thesis, such theoretical models are used to study the disk structure of objects across the mass spectrum - young low mass Brown Dwarfs, solar mass T-Tauri stars, intermediate mass Herbig Ae stars, and candidate B-stars with massive disks. A Monte Carlo radiation transfer code is used to model images and photometric data in the UV - mm wavelength range. These models demonstrate how modelling techniques have been updated in an attempt to reduce the number of unknown parameters and extend the diversity of objects that can be studied.

Stars of all ages and evolutionary stages are seen to be surrounded by discs of matieral. during the formation of a stellar system the stars are orbited by a massive protoplanetary disc composed of interstellar gas and dust, in which planet formation occurs. Betewwen 1 and 10 Myr the protoplanetary disc disperses, leaving behind the newly formed system of planets and smaller bodies. The remaining material which has not formed into planets is referred to as a debris disc. Even though the interstellar dust grains from the protoplanetary disc have long been removed from the system, debris discs can contain large quantities of dust due to collisions between larger bodies and cometary activity. such dust can be detected by its thermal emission. This thesis focuses on observational studies at far-infrared and sub-millimetre wavelengths of debris discs and the late stages of protoplanetary disc evolution. An overview of surveys for debris discs performed to date is presented, highlighting the limitations and statistical biases. the motivation, design and sample selection for two large surveys for debris discs around nearby stars, with the Hershel space observatory and the SCUBA-2 sub-millimetre camera on the James Clerk Maxwell Telescope, are described. The combination of a uniform obstevational strategy, longer wavelengths than previous surveys, and a large, clearly chosen sample - unbiased by stellar properties - will allow robust statistical conclusions of how the incidence and properties of debris discs depend on system parameters such as stellar mass, age, metallicity, binarity and the presence of planets. As a precursor to the Hershel and SCUBA-2 surveys, a volume-limited ample of 130 A type star systems was surveyed using observations at 24 and 70 μm, which were required to determine the presence of emission from dust, were predicted by fitting model flux distributions to optical and near-infrared photometry. Debris discs were detected around 46 systems, 12 of which including the system with the largest dust mass - are new discoveries. This survey adds to the results of previous studies which show that debris disc incidence is not correleated with host star metallicity despite the wll known giant planet - metallicity correlation, This is in accordance with what is predicted from the core accretion theory of planet formation. The most signigicant result from this survey is that, contrary to results reported in a previous work, debris discs are oberall less common around binary stars. Further investigation shows that systems with separations of ~3-150 AU are especially deficient of debris, while closer binaries and the primaries of wider binaries show debris detection rates consisten with those for single stars. A sample of circumstellar discs around 29 young stellar systems with ages of 5-30 Myr were observed with the LABOCA sub-millimetre instrument on the APEX telescope at 870μm, to provide disc masses or mass upper limits in support of a large Hershel programme. These targets included the η Chamaeleontis cluster and four bright Herbig Ae/Be stars which have not previously been observed at this wavelength. All but the Herbig Ae/Be stars were not detected, and 3σ dust mass upper limits of ~ 0.1-3 M are determined, with corresponding total disc masses of ~0.03-1Mjup. These mass limits indicate that there is insufficient remaining material in these discs to form gas giant planets, and add to the prevailing view that protoplanetary discs typically disperse within 10 Myr and that gas giant planet formation must be completed before this time. A search for cold dust emimission from two of the Solar System's nearest neighbours - α Centauri AB and ε Indi - was also performed with LABOCA. In both cases no debris disc emission was detected. A bright resolved feature was detected near α Centauri AB, nowever, follow-up observations at a second epoch, two years after the initial observations, showed that the feature is not co-moving with the stars. It is argued that the feature is most likely a pre-stellar core. The stars α Centauri A and B are detected, which is one of only very few detections of main sequence stellar photospheres at sub-millimetre wavelengths.

We present a new velocity-resolved survey of 2.9 mu m spectra of hot H2O and OH gas emission from protoplanetary disks, obtained with the Cryogenic Infrared Echelle Spectrometer at the VLT (R similar to 96,000). With the addition of archival Spitzer-IRS spectra, this is the most comprehensive spectral data set of water vapor emission from disks ever assembled. We provide line fluxes at 2.9-33 mu m that probe from the dust sublimation radius at similar to 0.05 au out to the region of the water snow line. With a combined data set for 55 disks, we find a new correlation between H2O line fluxes and the radius of CO gas emission, as measured in velocity-resolved 4.7 mu m spectra (R-co), which probes molecular gaps in inner disks. We find that H2O emission disappears from 2.9 mu m (hotter water) to 33 mu m (colder water) as R-co increases and expands out to the snow line radius. These results suggest that the infrared water spectrum is a tracer of inside-out water depletion within the snow line. It also helps clarify an unsolved discrepancy between water observations and models by finding that disks around stars of M-star > 1.5M(circle dot) generally have inner gaps with depleted molecular gas content. We measure radial trends in H2O, OH, and CO line fluxes that can be used as benchmarks for models to study the chemical composition and evolution of planet-forming disk regions at 0.05-20 au. We propose that JWST spectroscopy of molecular-gas may be used as a probe of inner disk gas depletion, complementary to the larger gaps and holes detected by direct imaging and by ALMA.

We present new deep UBVRI images and high-resolution multi-object optical spectroscopy of the young (similar to 6-10 Myr old), relatively nearby (800 pc) open cluster IC 2395. We identify nearly 300 cluster members and use the photometry to estimate their spectral types, which extend from early B to middle M. We also present an infrared imaging survey of the central region using the IRAC and MIPS instruments on board the Spitzer Space Telescope, covering the wavelength range from 3.6 to 24 mu m. Our infrared observations allow us to detect dust in circumstellar disks originating over a typical range of radii from similar to 0.1 to similar to 10 au from the central star. We identify 18 Class II, 8 transitional disk, and 23 debris disk candidates, respectively, 6.5%, 2.9%, and 8.3% of the cluster members with appropriate data. We apply the same criteria for transitional disk identification to 19 other stellar clusters and associations spanning ages from similar to 1 to similar to 18 Myr. We find that the number of disks in the transitional phase as a fraction of the total with strong 24 mu m excesses ([8] - [24]. 1.5) increases from (8.4. +/- 1.3)% at similar to 3 Myr to (46. +/- 5)% at similar to 10 Myr. Alternative definitions of transitional disks will yield different percentages but should show the same trend.

We present multi-epoch mid-infrared (IR) photometry and the optical discovery observations of the "impostor" supernova (SN) 2010da in NGC. 300 using new and archival Spitzer Space Telescope images and ground-based observatories. The mid-infrared counterpart of SN. 2010da was detected as Spitzer Infrared Intensive Transient Survey (SPIRITS). 14bme in the SPIRITS, an ongoing systematic search for IR transients. Before erupting on 2010 May 24, the SN. 2010da progenitor exhibited a constant mid-IR flux at 3.6 and only a slight similar to 10% decrease at 4.5 mu m between 2003 November and 2007 December. A sharp increase in the 3.6 mu m flux followed by a rapid decrease measured similar to 150 days before and similar to 80 days after the initial outburst, respectively, reveal a mid-IR counterpart to the coincident optical and high luminosity X-ray outbursts. At late times, after the outburst (similar to 2000 days), the 3.6 and 4.5 mu m emission increased to over a factor of two. times the progenitor flux and is currently observed (as of 2016 Feb) to be fading, but still above the progenitor flux. We attribute the re-brightening mid-IR emission to continued dust production and increasing luminosity of the surviving system associated with SN. 2010da. We analyze the evolution of the dust temperature (T-d similar to 700-1000 K), mass (Md similar to 0.5-3.8 x. 10(-7) M circle dot), luminosity (L-IR similar to 1.3-3.5 x 10(4) L circle dot), and the equilibrium temperature radius (R-eq similar to 6.4-12.2 au) in order to resolve the nature of SN. 2010da. We address the leading interpretation of SN. 2010da as an eruption from a luminous blue variable high-mass X-ray binary (HMXB) system. We propose that SN. 2010da is instead a supergiant (sg)B[e]-HMXB based on similar luminosities and dust masses exhibited by two other known sgB[e]-HMXB systems. Additionally, the SN. 2010da progenitor occupies a similar region on a mid-IR color-magnitude diagram (CMD) with known sgB[e] stars in the Large Magellanic Cloud. The lower limit estimated for the orbital eccentricity of the sgB[e]-HMXB (e > 0.82) from X-ray luminosity measurements is high compared to known sgHMXBs and supports the claim that SN. 2010da may be associated with a newly formed HMXB system.