High signal-to-noise optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a detailed analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the subluminous B stars in the theoretical H-R diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-burning cores of ∼0.5 $M(⊙) overlain by very thin layers of hydrogen (≲0.02 M(⊙)). Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into white dwarfs of lower than average mass. The derived distances above the Galactic plane support a scale height for the population of z₀ = 285 pc, consistent with the identification of their progenitor stars as members of the old disk population. Radial velocities of sdB and sdO stars are analyzed to infer their kinematic characteristics. The results for the sdB stars are inconclusive, but for the sdO stars the results also are consistent with the population belonging to the older part of the thin disk.

Dwarf novae are a type of binary star system that consists of a cool red dwarf, as well as a white dwarf with an accretion disk. Occasionally the disk will get significantly brighter for a few days in an event called an outburst. On February 6th, 2002 one such system, CN Orionis, was experiencing such an outburst. The goal of this research was to determine some important characteristics of CN Orionis during its outburst. This thesis will present the light output, temperature, and the area of the accretion disk of CN Orionis. / Department of Physics and Astronomy

Thesis (Ph. D.)--Ohio State University, 1976. / Includes vita. Includes bibliographical references (leaves 74-78).

Thesis--Wisconsin. / Vita. Includes bibliographical references (leaves 118-119).

We analyze the optical counterpart to the ultra-compact high velocity cloud AGC 226067, utilizing imaging taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The color-magnitude diagram of the main body of AGC 226067 reveals an exclusively young stellar population, with an age of similar to 7-50 Myr, and is consistent with a metallicity of [Fe/H] similar to -0.3 as previous work has measured via H II region spectroscopy. Additionally, the color-magnitude diagram is consistent with a distance of D approximate to 17 Mpc, suggesting an association with the Virgo cluster. A secondary stellar system located similar to 1.'6 (similar to 8 kpc) away in projection has a similar stellar population. The lack of an old red giant branch ((sic)5 Gyr) is contrasted with a serendipitously discovered Virgo dwarf in the ACS field of view (Dw J122147+132853), and the total diffuse light from AGC 226067 is consistent with the luminosity function of the resolved similar to 7-50 Myr stellar population. The main body of AGC 226067 has a M-V = -11.3 +/- 0.3, or M-stars = 5.4 +/- 1.3 x 10(4) M-circle dot given the stellar population. We searched 20 deg(2) of imaging data adjacent to AGC. 226067 in the Virgo Cluster, and found two similar stellar systems dominated by a blue stellar population, far from any massive galaxy counterpart-if this population has star-formation properties that are similar to those of. AGC 226067, it implies similar to 0.1Me(circle dot) yr(-1) in Virgo intracluster star formation. Given its unusual stellar population, AGC 226067 is likely a stripped remnant and is plausibly the result of compressed gas from the ram pressure stripped M86 subgroup (similar to 350 kpc away in projection) as it falls into the Virgo Cluster.

No description available.

Dissertation (Ph. D.)--University of South Alabama, 2007. / Vita. Includes bibliographical references (leaves 108-122).

The dynamical histories of three Galactic satellite systems have been investigated. These include the Fornax and Sculptor dwarf spheroidal galaxies, and the globular cluster Omega Centauri. Wide-field CCD photometry was obtained for these objects in two colours, V and I, covering a total sky area of 30 square degrees. These data were used to construct a colour-magnitude diagram for each system, which then provided a filter to select probable member stars. This technique reduced contamination from foreground stars, allowing a comprehensive examination of the poorly-known outer regions of these three Galactic satellites.¶ An initial analysis of the central region of Fornax was made using the deep photometric data provided by Stetson et al. The analysis revealed a shell-like feature located approximately 1.5 core radii southeast from the centre of the dwarf galaxy, with an integrated luminosity of M_V ~ -4. The colour-magnitude data for this shell indicate it to be dominated by stars with an age ~2 Gyr.¶ The complete analysis of Fornax utilised two colour data covering a 3.2 deg times 3.2 deg area on the sky. The colour-magnitude selection technique revealed a second shell-like feature situated 1.3 deg northwest from the Fornax centre, approximately 30' beyond the nominal tidal radius at this position angle. This feature displays an integrated luminosity of M_V ~ -7. The alignment of this second shell is parallel to the original shell, and both are situated on the minor axis. Additionally, a statistical analysis of the extra-tidal region of Fornax revealed two large, faint surface brightness structures located on the minor axis. These structures, combined with the two shells, present strong evidence for shell structure in Fornax. This is the first such structure observed in a dwarf galaxy, and implies that Fornax has experienced a merger event in the recent past.¶ The photometric survey of Sculptor was complete to the depth of the horizontal branch stars. The red horizontal branch stars were found to be significantly more concentrated than the blue horizontal branch stars. An analysis of the distribution of red giant branch-selected stars revealed no significant extra-tidal structure. To further test this result, spectra at far red wavelengths were obtained for over 700 candidate red giant stars over the 10 sq deg region using the 2dF instrument on the Anglo-Australian Telescope. Radial velocities and metallicities for these stars were measured using Ca ii triplet lines, providing additional constraints to select Sculptor members beyond the nominal tidal radius. The distribution of the 179 probable Sculptor members indicated a lack of extra-tidal stars. These results support, at most, a mild level of interaction between this system and the Galaxy. An upper mass limit for extra-tidal material was measured to be 10% of the Sculptor luminous mass.¶ The analysis of Omega Centauri was similar to that of Sculptor. V and I band photometry was obtained to search for the tidal tails proposed by a previous study. As noted by others, dust correction was found to remove these structures. Spectra covering the region 370-450 nm were obtained for approximately 4000 candidate cluster members, and radial velocities were used to distinguish members from field stars. A total of 24 probable members of Omega Cen were found in the extra-tidal regions. Hence, if Omega Cen does possess tidal tails, they comprise at most 1% of the cluster mass.

published_or_final_version / Earth Sciences / Master / Master of Philosophy

An observational program was carried out to investigate the spectrum of Pluto at various points on its lightcurve. Spectrophotometry of Pluto in the wavelength range of 5600 to 10500 Å was obtained on four nights covering lightcurve phases of 0.18, 0.35, 0.49, and 0.98. The four phases included minimum light (0.98) and one near maximum light (0.49). The spectra reveal variations in the adsorption depths of the methane bands at 6200, 7200, 7900, 8400, 8600, 8900, and 10000 Å. The minimum amount of adsorption was found to occur at minimum light. A model for the surface and atmosphere of Pluto was constructed in an attempt to explain the phase variation observed. The model is based upon a previous photometric two-spot model which was constructed to explain the variations in the lightcurve from 1950 to 1982. Two dark circular spots (46° and 28° in radius, both at latitude -23°, separated by 134° in longitude) were used to constrain the surface distribution of methane frost on the surface of Pluto. The reflectance properties of the two terrains were modelled with a theory by B. Hapke (J.G.R., v. 86, p. 3039, 1981) which includes the effects of multiple scattering in the surface frost. The particle size and continuum optical depth of the frost particles were allowed to vary between the dark regions inside the spot boundaries and the brighter regions surrounding the spots. The transmission of the atmosphere was calculated using the Mayer-Goody band model. The model fit to the spectrum required the presence of a frost with particle sizes on the order of 1-20 mm in order to explain the observed phase dependence of the methane bands. Using only the atmosphere and no surface frost implies a variation in column abundance of 30% within three days. From energy balance considerations this variation in column abundance is not possible. By including the absorption of methane frost on the surface a range of model solutions was obtained. This range yields an approximate limit of 5.5 m-amagats to the amount of gas that can be present and still achieve a good fit to the phase variation of the 7200 Å band. If the atmosphere is removed from the model an equally good fit to the 7200 Å band is obtained. A major problem with the model is its failure to reproduce the relative absorption band depths. The gaseous atmospheric calculation on the other hand can fit the spectrum quite well. Possible explanations include a particle size distribution within a given terrain.