This dissertation presents a technique for the identification and classification of late-type stars and for the estimation of M star metallicities. The technique uses broad-band, V and I, CCD images to identify red stars and two intermediate-band CCD images to classify these as carbon or M types. One of the intermediate passbands is centered on a TiO absorption band at 7750Å and the other is centered on a CN absorption band at 8100Å. Color-color plots of V-I versus the intermediate-band index, 77-81, clearly distinguishes carbon from M stars. Observations of both early- and late-type stars were used to define the 77-81 system based upon the intermediate-band filters. The TiO bandstrength deduced from the 77-81 color as a function of V-I color was investigated for field giants and giants in 12 globular clusters. A linear correlation between [Fe/H] and the V-I color at a given TiO bandstrength was found. This correlation can be used to estimate the metallicity of M giants. The stellar population of a field in Baade's Window was examined using this technique. Many late-M stars and no carbon stars were found. The color-color diagram for Baade's Window suggests a range of metallicities for the M giants of [Fe/H] ≈ -0.4 to > +0.2. The stellar population of the Sagittarius Dwarf Irregular galaxy (Sagdig) was examined using the 77-81 system. A method for estimating reddening based upon the color mode of foreground stars was developed for the analysis of the Sagdig data. Sagdig is estimated to be ~ 1.3 megaparsecs distant. Bright blue and red stars in Sagdig are evidence for recent star formation. Carbon stars were identified in Sagdig. They display a bimodal luminosity and color distribution which suggests distinct epochs of star forming activity between 1 and 10 Gyr ago. The spatial distribution of carbon stars and bright red stars in Sagdig shows this galaxy to be much larger than previously thought.

Photometric data on the Kron-Cousins photometric system have been obtained for 118 new late K to middle M-type stars with known distances. These data have been used to obtain absolute red magnitudes, to construct a color-magnitude diagram, and to compute a polynomial function for disk dwarf stars in the color range 1.5 ≤ R-I < 2.0, which can be used to compute absolute red magnitudes to be used for photometric parallaxes. Such photometric parallaxes allow new stellar distance estimations that are essential when modeling the spatial distribution of stars in our Galaxy. This is especially important for M-type stars, as they make up more than half of the mass of the Milky Way. Intermediate-band CaH observations have also been obtained in an ongoing effort to distinguish stellar luminosity classes and populations; R-L and R-I colors are used to identify possible subdwarf stars. A total of seven possible new subdwarfs and three previously known subdwarfs have been identified with this method. / Department of Physics and Astronomy

We present Spitzer/Infrared Array Camera Ch1 and Ch2 monitoring of six brown dwarfs during eight different epochs over the course of 20 months. For four brown dwarfs, we also obtained simulataneous Hubble Space Telescope (HST)/WFC3 G141 grism spectra during two epochs and derived light curves in five narrowband filters. Probing different pressure levels in the atmospheres, the multiwavelength light curves of our six targets all exhibit variations, and the shape of the light curves evolves over the timescale of a rotation period, ranging from 1.4 to 13 hr. We compare the shapes of the light curves and estimate the phase shifts between the light curves observed at different wavelengths by comparing the phase of the primary Fourier components. We use state-of-the-art atmosphere models to determine the flux contribution of different pressure layers to the observed flux in each filter. We find that the light curves that probe higher pressures are similar and in phase, but are offset and often different from the light curves that probe lower pressures. The phase differences between the two groups of light curves suggest that the modulations seen at lower and higher pressures may be introduced by different cloud layers.

The chemical fingerprints of old, metal-poor stars can be used to unravel the events of the newborn Universe and help us understand the properties of the first stars and star clusters. The study of nearby stars to infer properties in the distant past is often referred to as Galactic archaeology. However, the chemical composition of stars cannot be observed directly, but must be inferred by means of spectroscopic modelling. Traditionally, this modelling utilises one-dimensional (1D) stellar atmospheres in hydrostatic and local thermodynamic equilibrium (LTE). Today, we know that departures from LTE (known as NLTE), and differences between 1D model atmospheres and their hydrodynamical three-dimensional (3D) counterparts, become increasingly severe at lower metallicity. The development of NLTE modelling of spectral line formation in 3D atmospheres is still in its infancy, but constitutes a remarkable step forward that has been made possible by parallelised codes and supercomputers. The central theme of this thesis is the application of NLTE analyses to metal-poor stars, to help usher the field of Galactic archaeology forward with important consequences for the nature of the first stellar generations. I present a theoretical NLTE study of aluminium, where I validate the analysis using a set of bright standard stars and provide calculated NLTE effects for a large parameter space. I perform 3D NLTE calculations for the solar spectrum to better constrain the zero-point of the cosmic abundance scale, and find excellent agreement with the meteoritic aluminium abundance. I also present NLTE analyses of metal-poor stars in the globular clusters NGC 6397 and M4. While globular cluster stars were long expected to form from a chemically homogeneous medium, star-to-star abundance variations of light elements indicate multiple epochs of star formation. Massive first-generation stars polluted the interstellar medium from which later generations formed, and I use the observed abundance variations to deduce the properties of the polluting stars. Among the heavier elements, I uncover evolutionary abundance variations that match predictions of stellar evolution models with atomic diffusion. The results indicate that the chemical abundance ratios of unevolved metal-poor stars are affected by gravitational settling, with a bias of the order 25-50 %, increasing towards lower metallicity. This atmospheric depletion mechanism is a probable explanation to why the stellar abundances of lithium fall short of the predictions from standard Big Bang nucleosynthesis. Finally, I apply a 3D NLTE abundance analysis to the red giant SMSS 0313-6708, which is the most iron-deficient star known. The chemical abundance pattern of this star indicates that it formed from gas affected only by Big Bang nucleosynthesis and a single faint supernova. Comparison of the inferred abundance pattern to theoretical predictions leads to constraints on the explosion mechanism and the mass of the metal-free progenitor star.

As the endpoints of stars less massive than roughly eight solar masses, the population of Galactic white dwarfs (WD) contain information about complex stellar evolution processes. Associated pairs of WDs add an extra degree of leverage; both WDs must have formed and evolved together. The work presented in this dissertation uses various populations of double WDs (DWD) to constrain evolution of both single and binary stars. One example is the set of low-mass WDs with unseen WD companions, which are formed through a dynamically-unstable mass loss process called the common envelope. To work toward a quantitative understanding of the common envelope, we develop and apply a Bayesian statistical technique to identify the masses of the unseen WD companions. We provide results which can be compared to evolutionary models and hence a deeper understanding of how binary stars evolve through a common envelope. The statistical technique we develop can be applied to any population of single-line spectroscopic binaries. Binaries widely separated enough that they avoid any significant interaction independently evolve into separate WDs that can be identified in photometric and astrometric surveys. We discuss techniques for finding these objects, known as wide DWDs. We present a catalog of 142 candidate wide DWDs, combining both previously detected systems and systems we identify in our searches in the Sloan Digital Sky Survey. Having been born at the same time, the masses and cooling ages of the WDs in wide DWDs, obtained with our spectroscopic follow-up campaign can be used to constrain the initial-final mass relation, which relates a main sequence star to the mass of the WD into which it will evolve. We develop a novel Bayesian technique to interpret our data and present our resulting constraints on this relation which are particularly strong for initial masses between two and four solar masses. During this process, we identified one wide DWD, HS 2220+2146, that was peculiar since the more massive WD in this system evolved second. We construct an evolutionary formation scenario in which the system began as a hierarchical triple in which the inner binary merged (possibly due to Kozai-Lidov oscillations) forming a post-blue straggler binary. The system then evolved into the DWD we observe today. We further discuss the potential for identifying more wide DWDs, including peculiar systems like HS 2220+2146, in future surveys such as Gaia.

Wolf-Rayet (WR) stars are a late stage in the evolution of massive stars (M ≥ 25 M⊙), characterized by strong stellar winds (dM/dt ~ 10−5 M⊙/yr). Ionizing radiation from the central star heats the expanding outer envelope of material, leading to recombination emission lines of helium, carbon, nitrogen, oxygen, and/or hydrogen in the WR star spectrum. This outflow of material enriches the surrounding ISM, which is further enriched when the WR star likely explodes as a type Ib or Ic supernova. WR stars are also likely progenitors for long soft gamma-ray bursts, and they are excellent tracers of the present sites of massive star formation in our Galaxy. The current Galactic WR star catalog is very incomplete. I discuss three methods of selecting strong WR star candidates from crowded fields in the Galactic plane: image subtraction, narrowband (NB) color, and broadband (BB) color. Using these methods, an extensive near-infrared narrowband survey begun in 2005-2006, and extended by me, has yielded 28% of the known Galactic WR stars to date; I add 59 new WR stars to the total in this thesis. I then compare two recent models of the Galactic population of WR stars, discuss the implications with respect to how many WR stars remain to be found, and use these results to inform an analysis of the remaining 834 strong carbon-rich WC star candidates from the survey. I also provide a listing of these 834 WC star candidates throughout our Galaxy, and map them; a central result of this thesis. Finally, I present selection criteria which may be used to identify [WR] stars (central stars of planetary nebulae which display WR spectral features), and proof of concept observations which led to 7 new confirmed [WC] stars.

In low-mass (≲1.2 M⊙) main-sequence stars, the combination of differential rotation and turbulent flows in the outer convective region generates strong magnetic fields. It has been observed that in these stars, the rotation rate and the strength of the magnetic field decrease over time. This is thought to result from a feedback loop in which magnetized winds carry angular momentum away from the star, braking its rotation and weakening the magnetic dynamo. A well-calibrated age-rotation-activity relation (ARAR) would be particularly valuable for low-mass stars. If we knew the dependence of rotation or magnetic activity on age, a measurement of one of these quantities could be used to determine an accurate age for any isolated field star. Empirical calibrations of the ARAR rely on observations of the co-eval populations of stars in open clusters. In this work, I characterize rotation and magnetic activity, using light curves for the former and X-ray and Hα emission for the latter, in two open clusters of different ages (Alpha Persei, ≈60 Myr, and Messier 37, ≈500 Myr) to analyze the relation between rotation and activity across the low-mass stellar range. I also compare coronal (X-rays) and chromospheric (Hα) activity to understand how magnetic heating varies across stellar atmospheric layers. My results inform models of angular momentum evolution in low-mass stars.

I report on suspected nearby stars with proper motions 1.0 arcsec > μ ≥ 0.5 arcsec/yr in the southern sky (DEC = −90° to 00°). This sample of slow-motion (SLOWMO) stars complements the work of Jao (2004), who reported on faster moving stars with μ ≥ 1.0 arcsec/yr in the entire sky for his doctoral dissertation, and the work of Finch (2007), who uncovered stars moving slower than 0.5 arcsec/yr between declinations −90° and −47°. Characterizations of SLOWMO systems include trigonometric parallaxes, optical and infrared photometry. For stars without trigonometric parallaxes, colors and apparent magnitudes are used to calculate photometric distance estimates and the statistics of this population of stars are analyzed. The SLOWMO sample is comprised of 1906 total stars − 560 estimated to be less than 25 parsecs away, and 245 stars without parallaxes estimated to be within 25 parsecs.

Since the first successful measurements of stellar trigonometric parallax in the 1830s, the study of nearby stars has focused on the highest proper motion stars (mu > 0.18"/yr). Those high proper motion stars have formed the backbone of the last 150 years of study of the Solar Neighborhood and the composition of the Galaxy. Statistically speaking, though, there is a population of stars that will have low proper motions when their space motions have been projected onto the sky. At the same time, over the last twenty years, populations of relatively young stars (less than ~100 Myr), most of them with low proper motions, have been revealed near (<100 >pc) the Sun. This dissertation is the result of two related projects: A photometric search for nearby (<25 >pc) southern-hemisphere M dwarf stars with low proper motions (mu < 0.18"/yr), and a search for nearby (

Supergiant stars of spectral types B and A are characterized by variable and structured winds, as revealed by variability of optical and ultraviolet spectral lines. Non- radial pulsations and magnetically supported loops have been proposed as explanations for these phenomena. The latter hypothesis is tested using a time series of 65 high-resolution (λ/∆λ ∼ 65, 000) circular polarization (Stokes I and V ) spectra of the late B type supergiant Rigel (β Ori, B8 Iae), obtained with the instruments ESPaDOnS and Narval at the Canada-France-Hawaii Telescope and the Bernard Lyot Telescope, respectively. Examination of the unpolarized (Stokes I) spectra using standard spectral analysis tools confirms complex line profile variability during the 5 month period of observations; the high spectral resolution allows the identification of a weak, transient Hα feature similar in behaviour to a High Velocity Absorption event. Analysis of the Stokes V spectra using the cross-correlation technique Least Squares Deconvolution (LSD) yields no evidence of a magnetic field in either LSD Stokes V profiles or longitudinal field measurements, with longitudinal field 1σ error bars of ∼ 12 G for individual observations, and a mean field in the best observed period of 3 ± 2 G. Synthetic LSD profiles fit to the observations using a Monte Carlo approach yield an upper limit on the surface dipolar field strength of Bdip ≤ 50 G for most orientations of the rotational and magnetic axes, lowered to Bdip ≤ 35 G if the mean LSD profile from the most densely time-sampled epoch (with an LSD SNR of ∼80,000) is used. A simple two-spot geometry representing the footpoints of a magnetic loop emerging from the photosphere yields upper limits on the spot magnetic fields of 60–600 G, depending on the filling factor of the spots. Given existing measurements of the mass loss rate and the wind terminal velocity, these results cannot rule out a magnetically confined wind as, for Bdip ≤ 15 G, η∗ ≥ 1. However, the detailed pattern of line profile variability seems inconsistent with the periodic wind modulation characteristic of known magnetic early-type stars, suggesting that magnetic fields do not play a dominant role in Rigel’s variable winds. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2012-04-29 02:10:41.308