Ages of White Dwarf-Red Subdwarf Systems

We provide the first age estimates for two recently discovered white dwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These unusual systems provide a new opportunity for linking the reliable age estimates for the white dwarfs to the (measurable) metallicities of the red subdwarfs. We have obtained precise photometry in the $V_{J}R_{KC}I_{KC}JH$ bands and spectroscopy covering from 6000\AA to 9000\AA for the two new systems, as well as for a comparison white dwarf-main sequence red dwarf system, GJ 283 AB. Using model grids available in the literature, we estimate the cooling age as well as temperature, surface gravity, mass, progenitor mass and {\it total} lifetimes of the white dwarfs. The results indicate that the two new systems are probably ancient thick disk objects with ages of at least 6-9 Gyr. We also conduct searches of red dwarf and white dwarf compendia from SDSS data and the L{\'e}pine Shara Proper Motion (LSPM) catalog for additional common proper motion white dwarf-red subdwarf systems. Only seven new candidate systems are found, which indicates the rarity of these systems.Comment: accepted for publication in Ap

An Ultraluminous Supersoft X-ray Source in M81: An Intermediate-Mass Black Hole?

Ultraluminous supersoft X-ray sources (ULSSS) exhibit supersoft spectra with blackbody temperatures of 50-100 eV and bolometric luminosities above $10^{39}$ erg s$^{-1}$, and are possibly intermediate mass black holes (IMBHs) of $\ge10^3 M_\odot$ or massive white dwarfs that are progenitors of type Ia supernovae. In this letter we report our optical studies of such a source in M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a point-like object, the spectral energy distribution of which reveals a blue component in addition to the companion of an AGB star. The blue component is consistent with the power-law as expected from the geometrically-thin accretion disk around an IMBH accretor, but inconsistent with the power-law as expected from the X-ray irradiated flared accretion disk around a white dwarf accretor. This result is strong evidence that M81-ULS1 is an IMBH instead of a white dwarf.Comment: 12 pages, 1 table, 3 figure

Nonrenal regulation of EPO synthesis

Erythropoietin (EPO) is a circulating glycoprotein hormone whose principal function is thought to be red blood cell production. It is a classic example of a hypoxia-inducible gene, and studies of the induction of EPO synthesis by low oxygen led to the discovery of a widespread system of hypoxia-inducible transcription factors. Tissue-specific expression of the EPO gene is tightly controlled, and in the adult organism the kidney produces around 90% of systemic EPO. Before birth, the liver is the main site of EPO production; factors contributing to the liver-to-kidney switch are still elusive, but may provide clues to the tissue-specificity of EPO gene expression. EPO has also been detected in non-erythropoietic tissues such as the brain, where it is suggested to exert local protective effects. Apart from classical ways of regulating renal EPO during hypoxia and anemia, novel pathways have been discovered that demonstrate that other organ systems in the adult might not only be important for the production of EPO but also for modulating the hypoxic EPO response. Knowledge of the molecular bases of these non-renal pathways will eventually help to develop pharmacological strategies to induce endogenous EPO production when the main source, the kidney, is significantly impaired. This review will provide an overview of the molecular aspects of EPO gene regulation by hypoxia-inducible transcription factors and of the tissue-specific regulation of EPO production in adult mammals. Insights into the biology of EPO production in genetically modified animals, with an emphasis on recent advances in the understanding of non-renal EPO regulation, will be discussed

Towards an Empirical Determination of the ZZ Ceti Instability Strip

We present atmospheric parameters for a large sample of DA white dwarfs that are known to be photometrically constant. For each star, we determine the effective temperature and surface gravity by comparing high signal-to-noise ratio optical spectra to the predictions of detailed model atmosphere calculations. We also report the successful prediction and detection of photometric variability in G232-38 based on similar Teff and log g determinations. The atmospheric parameters derived for this sample of constant stars as well as those for the known sample of bright ZZ Ceti stars (now boosted to a total of 39) have been obtained in a highly homogeneous way. We combine them to study the empirical red and blue edges as well as the purity of the ZZ Ceti instability strip. We find that the red edge is rather well constrained whereas there exists a rather large range of possibilities for the slope of the blue edge. Furthermore, the ZZ Ceti instability strip that results from our analysis contains no nonvariable white dwarfs. Our sample of constant stars is part of a much broader spectroscopic survey of bright (V < 17) DA white dwarfs, which we have recently undertaken. We also present here some preliminary results of this survey. Finally, we revisit the analysis by Mukadam et al. of the variable and nonvariable DA stars uncovered as part of the Sloan Digital Sky Survey. Their erroneous conclusion of an instability strip containing several nonvariable stars is traced back to the low signal-to-noise ratio spectroscopic observations used in that survey.Comment: 43 pages, 2 tables, 14 figures, accepted for publication in the Astrophysical Journa

The Extent and Cause of the Pre-White Dwarf Instability Strip

One of the least understood aspects of white dwarf evolution is the process by which they are formed. We are aided, however, by the fact that many H- and He-deficient pre-white dwarfs (PWDs) are multiperiodic g-mode pulsators. Pulsations in PWDs provide a unique opportunity to probe their interiors, which are otherwise inaccesible to direct observation. Until now, however, the nature of the pulsation mechanism, the precise boundaries of the instability strip, and the mass distribution of the PWDs were complete mysteries. These problems must be addressed before we can apply knowledge of pulsating PWDs to improve understanding of white dwarf formation. This paper lays the groundwork for future theoretical investigations of these stars. In recent years, Whole Earth Telescope observations led to determination of mass and luminosity for the majority of the (non-central star) PWD pulsators. With these observations, we identify the common properties and trends PWDs exhibit as a class. We find that pulsators of low mass have higher luminosity, suggesting the range of instability is highly mass-dependent. The observed trend of decreasing periods with decreasing luminosity matches a decrease in the maximum (standing-wave) g-mode period across the instability strip. We show that the red edge can be caused by the lengthening of the driving timescale beyond the maximum sustainable period. This result is general for ionization-based driving mechanisms, and it explains the mass-dependence of the red edge. The observed form of the mass-dependence provides a vital starting point for future theoretical investigations of the driving mechanism. We also show that the blue edge probably remains undetected because of selection effects arising from rapid evolution.Comment: 40 pages, 6 figures, accepted by ApJ Oct 27, 199

The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey

Spectrophotometric observations at high signal-to-noise ratio were obtained of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey. Fits of observed Balmer lines to synthetic spectra calculated from pure-hydrogen model atmospheres were used to obtain robust values of Teff, log g, masses, radii, and cooling ages. The luminosity function of the sample, weighted by 1/Vmax, was obtained and compared with other determinations. The mass distribution of the white dwarfs is derived, after important corrections for the radii of the white dwarfs in this magnitude-limited survey and for the cooling time scales. The formation rate of DA white dwarfs from the PG is estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from a theoretical study of the white dwarf formation rate for single stars indicates that >80% of the high mass component requires a different origin, presumably mergers of lower mass double degenerate stars. In order to estimate the recent formation rate of all white dwarfs in the local Galactic disk, corrections for incompleteness of the PG, addition of the DB-DO white dwarfs, and allowance for stars hidden by luminous binary companions had to be applied to enhance the rate. An overall formation rate of white dwarfs recently in the local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two recent studies of samples of nearby Galactic planetary nebulae lead to estimates around twice as high. Difficulties in reconciling these determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen

The Masses of Population II White Dwarfs

Globular star clusters are among the first stellar populations to have formed in the Milky Way, and thus only a small sliver of their initial spectrum of stellar types are still burning hydrogen on the main-sequence today. Almost all of the stars born with more mass than 0.8 M_sun have evolved to form the white dwarf cooling sequence of these systems, and the distribution and properties of these remnants uniquely holds clues related to the nature of the now evolved progenitor stars. With ultra-deep HST imaging observations, rich white dwarf populations of four nearby Milky Way globular clusters have recently been uncovered, and are found to extend an impressive 5 - 8 magnitudes in the faint-blue region of the H-R diagram. In this paper, we characterize the properties of these population II remnants by presenting the first direct mass measurements of individual white dwarfs near the tip of the cooling sequence in the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS multiobject spectroscopic observations, our results indicate that 0.8 M_sun population II main-sequence stars evolving today form 0.53 +/- 0.01 M_sun white dwarfs. We discuss the implications of this result as it relates to our understanding of stellar structure and evolution of population II stars and for the age of the Galactic halo, as measured with white dwarf cooling theory.Comment: Accepted for Publication in Astrophys. J. on Aug. 05th, 2009. 19 pages including 9 figures and 2 tables (journal format

Patterns in island endemic forest-dependent bird research: the Caribbean as a case-study

From Springer Nature via Jisc Publications RouterHistory: received 2018-10-25, rev-recd 2019-03-19, registration 2019-04-26, accepted 2019-04-26, online 2019-05-04, epub 2019-05-04, ppub 2019-06Publication status: PublishedAbstract: Unequal patterns in research effort can result in inaccurate assessments of species extinction risk or ineffective management. A group of notable conservation concern are tropical island endemic birds, many of which are also forest-dependent, which increases their vulnerability to extinction. Yet, island bird species have received limited research attention compared to their continental congeners, despite this taxon being globally regarded as well-studied. We used the insular Caribbean, a globally important endemism hotspot with high rates of deforestation, to explore research bias of island and regional endemic forest-dependent birds. A review of the published literature (n = 992) found no significant increase in the number of studies over the search period. Research effort was significantly higher among species with threatened status, long generation time, wide habitat breadth and low to intermediate elevational distributions. Among family groups, the Psittacidae received the highest research effort, while the Cuculidae were the most underrepresented family (30-fold higher and six-fold less than expected, respectively). We found geographic biases in effort, with Jamaica having six-fold less and Puerto Rico eight times more research than expected for their level of endemism. These patterns likely reflect individual interests and limited capacity and funding, typical of Small Island Developing States. With over 50% of species in this review having declining population trends, we recommend prioritizing research that emphasises conservation- and management-relevant data across underrepresented families and islands, by fostering greater collaboration between researchers, practitioners and the existing local amateur ornithological community

From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main Sequence Stellar Evolution in Open Clusters

I explore the current ability of both white dwarf cooling theory and main sequence stellar evolution theory to accurately determine stellar population ages by comparing ages derived using both techniques for open clusters ranging from 0.1 to 4 Gyr. I find good agreement between white dwarf and main sequence evolutionary ages over the entire age range currently available for study. I also find that directly comparing main sequence turn-off ages to white dwarf ages is only weakly sensitive to realistic levels of errors in cluster distance, metallicity, and reddening. Additional detailed comparisons between white dwarf and main sequence ages have tremendous potential to refine and calibrate both of these important clocks, and I present new simulations of promising open cluster targets. The most demanding requirement for these white dwarf studies are very deep (V > 25-28) cluster observations made necessary by the faintness of the oldest white dwarfs.Comment: 25 pages, incl. 10 figures, ApJ accepted for April, 200

The Early Palomar Program (1950-1955) for the Discovery of Classical Novae in M81: Analysis of the Spatial Distribution, Magnitude Distribution, and Distance Suggestion

Data obtained in the 1950-1955 Palomar campaign for the discovery of classical novae in M81 are set out in detail. Positions and apparent B magnitudes are listed for the 23 novae that were found. There is modest evidence that the spatial distribution of the novae does not track the B brightness distribution of either the total light or the light beyond an isophotal radius that is 70\arcsec from the center of M81. The nova distribution is more extended than the aforementioned light, with a significant fraction of the sample appearing in the outer disk/spiral arm region. We suggest that many (perhaps a majority) of the M81 novae that are observed at any given epoch (compared with say $10^{10}$ years ago) are daughters of Population I interacting binaries. The conclusion that the present day novae are drawn from two population groups, one from low mass white dwarf secondaries of close binaries identified with the bulge/thick disk population, and the other from massive white dwarf secondaries identified with the outer thin disk/spiral arm population, is discussed. We conclude that the M81 data are consistent with the two population division as argued previously from (1) the observational studies on other grounds by Della Valle et al. (1992, 1994), Della Valle & Livio (1998), and Shafter et al. (1996) of nearby galaxies, (2) the Hatano et al. (1997a,b) Monte Carlo simulations of novae in M31 and in the Galaxy, and (3) the Yungelson et al. (1997) population synthesis modeling of nova binaries. Two different methods of using M81 novae as distance indicators give a nova distance modulus for M81 as $(m-M)_0 = 27.75$, consistent with the Cepheid modulus that is the same value.Comment: 24 pages, 7 figures, accepted to PAS