No description available.

The potential for luminosity functions (LFs) of post-turnoff stars to constrain basic cluster parameters such as age, metallicity, and helium abundance is examined in this dissertation. A review of the published LFs for the globular cluster (GC) M92 suggests that the morphology of the transition from the main sequence to the red giant branch (RGB) is sensitive to these parameters. In particular, a small bump in this region may provide an important age discriminant for GCs. A significant deficiency in the number of stars over a 2 mag interval, just below the turnoff, remains unexplained. A method of interpolating isochrones and LFs accurately from evolutionary sequences, from the lower main sequence to the RGB tip, is discussed. The interpolation scheme is based on primary interpolation points which are identified by the behaviour of the derivative [special characters omitted] along an evolutionary sequence. New BV CCD observations, calibrated with Landolt and Graham standard stars, for the old open cluster NGC 2243 and for the bright stars in the GCs NGC 288 and NGC 7099 are presented. The colour magnitude diagram (CMD) of NGC 2243 contains a strong binary star component. Comparisons with the fiducial sequences of the GC 47 Tuc (Hesser et al. 1987) indicate that the two clusters have similar abundances, while comparisons with the new oxygen-enhanced isochrones (Bergbusch & VandenBerg 1992) suggest that NGC 2243 has an age of 4-5 Gyr, and a metallicity [special characters omitted]. The morphology of both the CMD and the LF through the turnoff region cannot be attributed to the merging of the binary and single star sequences, but convective overshooting works in the correct sense to account for the differences between the isochrones and the CMD. For NGC 288 and NGC 7099, excellent overall consistency among the Zero-Age Horizontal Branch, isochrone, and LF fits is obtained for cluster ages of 14-16 Gyr. The manifestation of the transition bump in NGC 288's LF provides a particularly strong constraint on the age, since this feature becomes more prominent as the metallicity increases. R-method helium abundance estimates give Y ≈ 0.23 for NGC 288 and Y ≈ 0.31 for NGC 7099. The 2nd parameter problem is discussed in light of these results. The RGB bump, present in canonical LFs, is only weakly identified in the cumulative LF (CLF) of NGC 288, and may not be present at all in NGC 7099's CLF. However, the brightest RGB stars in both clusters are found within ≈ 0.2 mag of the RGB tip predicted by the oxygen-enhanced models. / Graduate

Luminosity-velocity diagrams for 12 spiral galaxies in the Virgo cluster are presented. Optical rotation curves obtained for the innermost portions of eight galaxies, complemented with velocity data from the literature, are coupled with luminosity growth curves to investigate the distance indication capabilities of the initial linear branch (ILB) feature and to delve into the physical basis for the T-F relation. Luminosity growth curves are obtained from Gunn r CCD images. The ILB feature is found to have a substantially larger dispersion in slope (~0.9) (and consequently, zero point) than previously thought. Plotting the magnitude and velocity of the final point in the ILB for all the galaxies in our sample yields a tight correlation (essentially an "inner T-F relation"), with the caveat that two galaxies are rejected from the fit (one is foreground, the other is a member of a binary pair). Ramifications of this relation are briefly discussed. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

We present a detailed analysis of the white dwarf luminosity functions derived from the local 40 pc sample and the deep proper motion catalog of Munn et al. Many previous studies have ignored the contribution of thick disk white dwarfs to the Galactic disk luminosity function, which results in an erroneous age measurement. We demonstrate that the ratio of thick/thin disk white dwarfs is roughly 20% in the local sample. Simultaneously fitting for both disk components, we derive ages of 6.8-7.0 Gyr for the thin disk and 8.7 +/- 0.1 Gyr for the thick disk from the local 40 pc sample. Similarly, we derive ages of 7.4-8.2 Gyr for the thin disk and 9.5-9.9 Gyr for the thick disk from the deep proper motion catalog, which shows no evidence of a deviation from a constant star formation rate in the past 2.5 Gyr. We constrain the time difference between the onset of star formation in the thin disk and the thick disk to be 1.6(-0.4)(+0.3) Gyr. The faint end of the luminosity function for the halo white dwarfs is less constrained, resulting in an age estimate of 12.5(-3.4)(+1.4) Gyr for the Galactic inner halo. This is the first time that ages for all three major components of the Galaxy have been obtained from a sample of field white dwarfs that is large enough to contain significant numbers of disk and halo objects. The resultant ages agree reasonably well with the age estimates for the oldest open and globular clusters.

A catalog of 8472 white dwarf (WD) candidates is presented, selected using reduced proper motions from the deep proper motion catalog of Munn et al. Candidates are selected in the magnitude range 16 < r < 21.5 over 980 square degrees, and 16 < r < 21.3 over an additional 1276 square degrees, within the Sloan Digital Sky Survey (SDSS) imaging footprint. Distances, bolometric luminosities, and atmospheric compositions are derived by fitting SDSS ugriz photometry to pure hydrogen and helium model atmospheres (assuming surface gravities log g = 8). The disk white dwarf luminosity function (WDLF) is constructed using a sample of 2839 stars with 5.5 < M-bol < 17, with statistically significant numbers of stars cooler than the turnover in the luminosity function. The WDLF for the halo is also constructed, using a sample of 135 halo WDs with 5 < M-bol < 16. We find space densities of disk and halo WDs in the solar neighborhood of 5.5 +/- 0.1 x 10(-3) pc(-3) and 3.5 +/- 0.7 x 10(-5) pc(-3), respectively. We resolve the bump in the disk WDLF due to the onset of fully convective envelopes in WDs, and see indications of it in the halo WDLF as well.

White dwarfs are the final remnants of low- and intermediate-mass stars. About 95% of main- sequence stars will end their evolutionary pathways as white dwarfs and, hence, the study of the white dwarf population provides details about the late stages of the life of the vast majority of stars.Since white dwarfs are long-lived objects, they also constitute useful objects to study the structure and evolution of our Galaxy. For instance, the initial-final mass relationship, which connects the final mass of a white dwarf with the initial mass of its main-sequence progenitor, is of paramount importance for different aspects in modern astrophysics. This function is used for determining the ages of globular clusters and their distances, for studying the chemical evolution of galaxies, and also to understand the properties of the Galactic population of white dwarfs. Despite its relevance, this relationship is still relatively poorly constrained.The main aim of this thesis is the study of the initial-final mass relationship. For such purpose we have used two different approaches. From an observational perspective, the statistical significance of the current initial final mass relationship can be improved by performing spectroscopic observations of white dwarfs for which some important parameters are available. Since this approach involves the use of theoretical stellar evolutionary tracks the resulting initial-final mass relationship is, in fact, semi-empirical. In this thesis we present a promising method which consists in using common proper motion pairs comprised of a white dwarf and a FGK star. It is sound to assume that the members of the system were born simultaneously and with the same chemical composition. Moreover, these stars are well separated and it can be considered that they have evolved as isolated stars, since mass exchange between them is unlikely. Thus, a careful analysis of the observational data of both members of each pair allows us to derive the initial and final masses of the white dwarf components, something which is totally impossible when white dwarfs are isolated. Considering the new data that we have obtained with this work and the observational data currently used to define the initial-final mass relationship we have carried out a revision of this relationship, giving some clues on its dependence on different parameters, especially on metallicity.The second approach to improve the initial-final mass relationship involves an indirect measurement, which has been carried out by studying its influence on one of the powerful tools related to the white dwarf population, the white dwarf luminosity function. We have computed a series of luminosity functions using different theoretical initial-final mass relationships, and also, considering the semi-empirical relation derived in this thesis. We have compared these computations with the available observational data in order to evaluate the validity of each of these relations.In order to increase the statistical significance of the white dwarf luminosity function and to improve the initial-final mass relationship it is necessary to extend the amount of accurate and reliable observational data. For this reason part of the thesis is devoted to the Alhambra Survey, which is a good example of the new deep surveys currently under development. These observational projects will detect thousands of new white dwarfs, some of them belonging to common proper motion pairs, which could be eventually used to extend our analysis. Thus, we have performed an exhaustive study to optimize the identification procedure of the white dwarf candidates which will be eventually detected by the Alhambra survey.

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995. / Includes bibliographical references (leaves 106-112). / Microfiche. / x, 112 leaves, bound ill. (some col.) 29 cm

abstract: Numerical simulations are very helpful in understanding the physics of the formation of structure and galaxies. However, it is sometimes difficult to interpret model data with respect to observations, partly due to the difficulties and background noise inherent to observation. The goal, here, is to attempt to bridge this gap between simulation and observation by rendering the model output in image format which is then processed by tools commonly used in observational astronomy. Images are synthesized in various filters by folding the output of cosmological simulations of gasdynamics with star-formation and dark matter with the Bruzual- Charlot stellar population synthesis models. A variation of the Virgo-Gadget numerical simulation code is used with the hybrid gas and stellar formation models of Springel and Hernquist (2003). Outputs taken at various redshifts are stacked to create a synthetic view of the simulated star clusters. Source Extractor (SExtractor) is used to find groupings of stellar populations which are considered as galaxies or galaxy building blocks and photometry used to estimate the rest frame luminosities and distribution functions. With further refinements, this is expected to provide support for missions such as JWST, as well as to probe what additional physics are needed to model the data. The results show good agreement in many respects with observed properties of the galaxy luminosity function (LF) over a wide range of high redshifts. In particular, the slope (alpha) when fitted to the standard Schechter function shows excellent agreement both in value and evolution with redshift, when compared with observation. Discrepancies of other properties with observation are seen to be a result of limitations of the simulation and additional feedback mechanisms which are needed. / Dissertation/Thesis / Ph.D. Physics 2012

I analyse an x-ray selected sample of 128 late-type (F-M) stars. These stars were identified as optical counterparts to serendipitous x-ray detections made by the Einstein Observatory Extended Medium Sensitivity Survey. Once identified as x-ray sources, the stars were reobserved with an extensive program of optical observations consisting of high- and low-resolution spectroscopy and photometry. Spectral types, luminosity classes, absolute magnitudes, distances, x-ray luminosities, projected rotation rates (v sin i), radial velocities, and binary status have been determined for the sample. I find that Lₓ is correlated with v sin i for single stars. However, Lₓ does not correlate with Ω sin i, which leads me to believe that the correlation seen with v sin i is actually a correlation with radius. Indeed, Lₓ correlates strongly with radius (color, mass) for main sequence stars. This result provides a plausibility argument for rotational saturation in the coronae of late-type stars. Since this sample is flux limited, I use sky coverage and sensitivity information from the Einstein Observatory to calculate the bright end of the x-ray luminosity function for late-type stars. It appears that previously calculated luminosity functions from optically selected samples have underestimated the number of x-ray bright F and G dwarfs. I have also discovered 8 previously uncatalogued M dwarfs within 25 pc of the sun. My sample includes only M dwarfs of spectral type M5 and earlier, 93% of which are "emission" stars (i.e. type Me V), as well as two pre-main sequence M stars. Arguments involving kinematics and stellar rotational velocities are used to estimate the age of these x-ray "bright" M dwarfs; they appear to be quite young (≤ 1-3 x 10⁹ yrs). Since the local space density of x-ray "bright" M dwarfs increases with mass, I infer a longer activity timescale for lower masses. M dwarfs later than M5 lie below the sample's x-ray sensitivity limit. An upper limit of log Lₓ = 27.45 is put on their coronal emission. I also present H(α) and Ca II K line fluxes for most members of the M dwarf sample and show that the H(α) and Ca II K luminosities do indeed correlate with Lₓ. However, these chromospheric luminosities are weaker functions of rotation than Lₓ and may, in fact, represent saturated levels of activity. My results are consistent with the hypothesis that the chromosphere is heated by x-rays from the overlying corona. Finally, I discuss two unusual members of the sample which are attractive candidates for the recently proposed class of FK Comae stars.

We present a robust measurement of the rest-frame UV luminosity function (LF) and its evolution during the peak epoch of cosmic star formation at 1 < z < 3. We use our deep near-ultraviolet imaging from WFC3/UVIS on the Hubble Space Telescope and existing Advanced Camera for Surveys (ACS)/WFC and WFC3/IR imaging of three lensing galaxy clusters, Abell 2744 and MACS J0717 from the Hubble Frontier Field survey and Abell 1689. Combining deep UV imaging and high magnification from strong gravitational lensing, we use photometric redshifts to identify 780 ultra-faint galaxies with M-UV < -12.5 AB mag at 1 < z < 3. From these samples, we identified five new, faint, multiply imaged systems in A1689. We run a Monte Carlo simulation to estimate the completeness correction and effective volume for each cluster using the latest published lensing models. We compute the rest-frame UV LF and find the best-fit faint-end slopes of alpha = -1.56 +/- 0.04, alpha = -1.72 +/- 0.04, and alpha = -1.94 +/- 0.06 at 1.0 < z < 1.6, 1.6 < z < 2.2, and 2.2 < z < 3.0, respectively. Our results demonstrate that the UV LF becomes steeper from z similar to 1.3 to z similar to 2.6 with no sign of a turnover down to MUV = -14 AB mag. We further derive the UV LFs using the Lyman break "dropout" selection and confirm the robustness of our conclusions against different selection methodologies. Because the sample sizes are so large and extend to such faint luminosities, the statistical uncertainties are quite small, and systematic uncertainties (due to the assumed size distribution, for example) likely dominate. If we restrict our analysis to galaxies and volumes above >50% completeness in order to minimize these systematics, we still find that the faint-end slope is steep and getting steeper with redshift, though with slightly shallower (less negative) values (alpha = -1.55 +/- 0.06, -1.69 +/- 0.07, and -1.79 +/- 0.08 for z similar to 1.3, 1.9, and 2.6, respectively). Finally, we conclude that the faint star-forming galaxies with UV magnitudes of -18.5 < M-UV < -12.5 covered in this study produce the majority (55%-60%) of the unobscured UV luminosity density at 1 < z < 3.