Abstract: The infrared excess around the white dwarf G29-38 can be explained by emission from an opaque flat ring of dust with an inner radius of 0.14 R☉ and an outer radius of less than 1 R☉. This ring lies within the Roche region of the white dwarf where an asteroid could have been tidally destroyed, producing a system reminiscent of Saturn's rings. Accretion onto the white dwarf from this circumstellar dust can explain the observed calcium abundance in the atmosphere of G29-38. Either as a bombardment by a series of asteroids or because of one large disruption, the total amount of matter accreted onto the white dwarf may have been ~4 × 1024 g, comparable to the total mass of asteroids in the solar system, or, equivalently, about 1% of the mass in the asteroid belt around the main-sequence star ζ Lep.

Abstract: We report Keck telescope HIRES echelle observations of DA white dwarfs in a continuation of an extensive search for metals. These spectra are supplemented with new JHK magnitudes that are used to determine improved atmospheric parameters. Of the DA white dwarfs not in binary or common proper motion systems, about 25% show Ca II lines. For these, Ca abundances are determined from comparison with theoretical equivalent widths from model atmosphere calculations; in a few cases we also obtain Mg, Fe, Si, and Al abundances. If Ca is not observed, we generally determine very stringent upper limits. We compare the data to predictions of previously published models involving the accretion/diffusion of interstellar matter and of comets. The derived abundances are not obviously compatible with the predictions of either model, which up to now could only be tested with traces of metals in helium-rich white dwarfs. By modifying certain assumptions in the published interstellar accretion model we are able to match the distribution of the elements in the white dwarf atmospheres, but, even so, tests of other expectations from this scenario are less successful. Because comet accretion appears unlikely to be the primary cause of the DAZ phenomenon, the data suggest that no more than about 20% of F-type main-sequence stars are accompanied by Oort-like comet clouds. This represents the first observational estimate of this fraction. A plausible alternative to the accretion of cometary or interstellar matter is disruption and accretion of asteroidal material, a model first suggested in 1990 to explain excess near-infrared emission from the DAZ G29-38. An asteroidal debris model to account for the general DAZ phenomenon does not presently disagree with the HIRES data, but neither is there any compelling evidence in support of such a model. The HIRES data indicate that in close red dwarf/white dwarf binaries not known to be cataclysmic variables there is, nonetheless, significant mass transfer, perhaps in the form of a wind flowing off the red dwarf. As a by-product we find from the kinematics of GD 165 a likely age of more than 2 Gyr for its probable brown dwarf companion GD 165B.

Abstract: We define an enhanced spectral classification scheme for M dwarf stars and use it to derive spectral classification of 104 northern stars with proper motions larger than 05 yr-1 that we discovered in a survey of high proper motion stars at low Galactic latitudes. The final tally is as follows: 54 M dwarfs, 25 sdK and sdM subdwarfs, 14 esdK and esdM extreme subdwarfs, and 11 DA and DC white dwarfs. Among the most interesting cases, we find one star to be the coolest subdwarf ever reported (LSR 2036+5059, with spectral type sdM7.5), a new M9.0 dwarf only about 6 pc distant (LSR 1835+3259), and a new M6.5 dwarf only 7 pc from the Sun (LSR 2124+4003). Spectroscopic distances suggests that 27 of the M dwarfs, three of the white dwarfs, and one of the subdwarfs (LSR 2036+5059) are within 25 pc of the Sun, making them excellent candidates for inclusion in the solar neighborhood census. Estimated sky-projected velocities suggest that most of our subdwarfs and extreme subdwarfs have halo kinematics. We find that several white dwarfs and non–metal-poor M dwarfs also have kinematics consistent with the halo, and we briefly discuss their possible origin.

Abstract: Our previous theoretical study of the impact of an accreting envelope on the thermal state of an underlying white dwarf (WD) has yielded equilibrium core temperatures, classical nova ignition masses, and thermal luminosities for WDs accreting at time-averaged rates of = 10-11 to 10-8 M☉ yr-1. These values are appropriate to WDs in cataclysmic variables (CVs) of Porb 7 hr, many of which accrete sporadically as dwarf novae. Approximately 30 nonmagnetic dwarf novae have been observed in quiescence, when the accretion rate is low enough for spectral detection of the WD photosphere and a measurement of Teff. We use our theoretical work to translate the measured Teff values into local time-averaged accretion rates, confirming the factor of 10 drop in predicted for CVs as they transit the period gap. For dwarf novae below the period gap, we show that if is that given by gravitational radiation losses alone, then the WD masses are greater than 0.8 M☉. An alternative conclusion is that the masses are closer to 0.6 M☉ and is 3-4 times larger than that expected from gravitational radiation losses. In either case, it is very plausible that a subset of CVs with Porb < 2 hr will have Teff values low enough for them to become nonradial pulsators, as discovered by van Zyl and collaborators for GW Lib.

Abstract: A reliable age estimation for the field brown dwarf LP 944-20 is accomplished through the analysis of its kinematic properties. The space velocities of this star strongly suggest its membership in the so-called Castor moving group. LP 944-20 can be sensibly assumed to have the group's age, which is estimated to be ∼320 Myr, and metal content, which is found to be roughly solar. With these new constrains and the available photometry and lithium abundance, current brown dwarf models are put to a test. Using the IR magnitudes and the lithium diagnostics, the models are able to provide a reasonable description of the brown dwarf's properties (to within a few sigma) but yield an age which is roughly 50% larger than our estimate. Possible reasons for this discrepancy are discussed.

Abstract: Due to a new global analysis method, it is now possible to measure the internal composition of pulsating white dwarf stars, even with relatively simple theoretical models. The precise internal mixture of carbon and oxygen is the largest single source of uncertainty in ages derived from white dwarf cosmochronometry, and contains information about the rate of the astrophysically important, but experimentally uncertain, ^12C(alpha,gamma)^16O nuclear reaction. Recent determinations of the internal composition and structure of two helium-atmosphere variable (DBV) white dwarf stars, GD 358 and CBS 114, initially led to conflicting implied rates for the ^12C(alpha,gamma)^16O reaction. If both stars were formed through single-star evolution, then the initial analyses of their pulsation frequencies must have differed in some systematic way. I present improved fits to the two sets of pulsation data, resolving the tension between the initial results and leading to a value for the ^12C(alpha,gamma)^16O reaction rate that is consistent with recent laboratory measurements.

Abstract: In Galactic open clusters, there is an apparent paucity of white dwarfs compared with the number expected assuming a reasonable initial mass function and assuming that main-sequence stars with an initial mass of less than ~8 M☉ become white dwarfs We suggest that this lack of white dwarfs is due at least in part to dynamical processes Nonspherically symmetric mass loss during the post-main-sequence evolution would lead to an isotropic recoil speed of a few kilometers per second for the white dwarf remnant This recoil speed can cause a substantial fraction of the white dwarfs formed in a cluster to leave the system We investigate this dynamical process by carrying out high-precision N-body simulations of intermediate-mass open clusters, where we apply an isotropic recoil speed to the white dwarf remnants Our models suggest that almost all white dwarfs would be lost from the cluster if the average recoil speed were to exceed twice the velocity dispersion of the cluster

Abstract: White dwarfs experience a thermal renaissance when they receive mass from a stellar companion in a binary. For accretion rates >~ 10^-10 Msun/yr, and that for below this the particular author's assumption concerning T_c, which we calculate consistently, is a determining factor. Initial comparisons of our CN ignition masses with measured ejected masses find reasonable agreement and point to ejection of material comparable to that accreted.

Abstract: White dwarfs rotate. The angular momentum in single white dwarfs must originate early in the life of the star, but also must be modified (and perhaps severely modified) during the many stages of evolution between birth as a main--sequence star and final appearance as a white dwarf. Observational constraints on the rotation of single white dwarf stars come from traditional spectroscopy and from asteroseismology, with the latter providing hints of angular velocity with depth. Results of these observational determinations, that white dwarfs rotate with periods ranging from hours to days (or longer), tells us that the processes by which angular momentum is deposited and/or drained from the cores of AGB stars are complex. Still, one can place strong limits on these processes by considering relatively simple limiting cases for angular momentum evolution in prior stages, and on subsequent angular momentum evolution in the white dwarfs. These limiting-case constraints will be reviewed in the context of the available observations.

Abstract: The aim of this thesis is to revisit the properties of white dwarf stars in the Solar neighbourhood (distance > 100 pc), in particular their magnetic fields, the occurrence of binarity and their space density. This thesis presents observations and analysis of a sample of white dwarfs from the southern hemisphere. Over 80 objects were observed spectroscopically, and 65 of these were also observed with a spectropolarimeter. Many of the white dwarfs observed belong to the Solar neighbourhood, and can be used to study the star formation and evolution in this region. Our spectropolarimetric measurements helped constrain the fraction of magnetic white dwarfs in the Solar neighbourhood. Combining data from different surveys, I found a higher fraction of these objects in the relatively old local population than in other younger selections such as the Palomar-Green survey which suggests magnetic field evolution in white dwarfs, or different sets of progenitors. The progenitors of magnetic white dwarfs have been assumed to be Ap and Bp stars, however I find that the properties and number of Ap and Bp stars would only explain white dwarfs with magnetic fields larger than 100 MG. The number of known white dwarfs is believed to be complete to about 13 pc, however the sample is certainly incomplete to 20 pc from the Sun. To identify new white dwarfs in the Solar neighbourhood, some possibly magnetic or in binaries, numerous candidate white dwarfs from the Revised NLTT catalogue have been observed, which resulted in the discovery of 13 new white dwarfs, with 4 of these having a distance that places them within 20 pc of the Sun. The candidates were selected using a V - J reduced-proper-motion diagram and optical-infrared diagram. A total of 417 white dwarf candidates were selected, 200 of these have already been spectroscopically confirmed as white dwarfs. Spectroscopic confirmation is required for the remaining 217 candidates, many of these are likely to belong to the Solar neighbourhood. Four close binaries consisting of a white dwarf and a cool companion were also observed, for which atmospheric and orbital parameters were obtained. The photometry for two of these binary systems, BPM 71214 and EC 13471-1258 shows that the secondary stars are filling their Roche lobes, and combined with their orbital parameters, these systems are very good candidates for hibernating novae. The time of their previous interaction or the extent of this interaction are unknown. The two other binary systems, BPM 6502 and EUVE J0720-31.7 are post-common envelope binaries. BPM 6502 is not expected to interact within a Hubble time, however EUVE J0720-31 is expected to become a cataclysmic variable within a Hubble time. The atmospheric parameters of the white dwarfs were determined using model atmosphere codes which were modified for the present study to include convective energy transfer, self-broadening and Lyman satellite features.

Abstract: Understanding how galaxies formed their first stars is a vital cosmological question, but the study of high-redshift objects, caught in the act of forming their first stars, is difficult. Here we argue that two extremely low-metallicity Blue Compact Dwarf galaxies (BCDs), IZw18 and SBS0335-052, could be local templates for primordial star formation, since both lack evolved ($> $1 Gyr) stellar populations; but they form stars differently.

Abstract: The Far Ultraviolet Spectroscopic Explorer (FUSE) has opened a new spectral window that permits the study of important ions such as OVI, PV, and SVI in FUV spectra of hot white dwarfs. The detection of the OVI 1031.912 A and 1037.613 A absorption lines in the FUSE spectra of Lan 23, GD 984, RE J1032+532, and RE J2156-543 came as a surprise, as these stars have effective temperatures ranging from 45,000 K to 60,000 K, which are on the cold side regarding the detection of the high ionization OVI ion. Even more puzzling, given the strength of the OVI lines, no OIV and OV lines are detected in the STIS high resolution spectra of RE J1032+532 and RE J2156-543, and no oxygen photoionization edges are observed in the EUVE data of GD 984, RE J1032+532 and RE J2156-543. In a combined analysis of FUSE, STIS, and EUVE data, we came to the conclusion that the oxygen abundance in the atmospheres of these stars is stratified.

Abstract: This paper reports on several observational programs with the Hubble Space Telescope (HST) that focus on spectroscopy and direct imaging of white dwarfs (WDs) in binary systems. I have been fortunate to have the collaboration on these projects of the following astronomers: M. S. O’Brien, E. M. Sion, M. A. Barstow, M. R. Burleigh, J. B. Holberg, E. Nelan, R. Gilliland, T. Girard, D. L. Pollacco, and R. F. Webbink.

Abstract: The orbital elements of extreme mass ratio binaries will be modified by interactions with surrounding circumstellar disks. For brown dwarf companions to Solar-type stars the resulting orbital migration is sufficient to drive short period systems to merger, creating a brown dwarf desert at small separations. We highlight the similarities and the differences between the migration of brown dwarfs and massive extrasolar planets, and discuss how observations can test a migration model for the brown dwarf desert.

Abstract: We present the results of a Galactic model able to describe the observational behavior of the various stellar components in terms of suitable assumptions on their evolutionary status. The theoretical framework allow us to predict the expected distribution of the Galactic White Dwarfs (WDs) population. Results appear in rather good agreement with recent estimates of the local WD luminosity function.

Abstract: Here we show striking similarities between the emission lines of two accreting white dwarfs with jets (CH Cyg and MWC 560) and the active galactic nuclei (AGN), in spite of the mass difference (a typical black hole in Seyfert galaxies has a mass ∼ 106 – 109 M ⊙ and the accreting white dwarfs ∼ 1 M ⊙). Practically almost every emission line visible in the spectrum of the quasar I Zw 1 (widely used as template for AGN spectra) has corresponding features in the spectra of CH Cyg and MWC 560. An obvious similarity is visible also between the UV spectra.

Abstract: Based on 1D radiation-hydrodynamics simulations along the upper AGB we followed the evolution of AGB wind envelopes towards the white dwarf domain. Taking into account photoionization and wind interaction in a realistic manner, the hydrodynamics of the post-AGB phase leads, if coupled to an evolving 0.6 M⊙ central star model, to morphologies very similar to what is observed for spherical/elliptical planetary nebulae.

Abstract: Only a small fraction of white dwarfs shows evidence for elements other than hydrogen and helium in their spectra. If metals are observed in the atmospheres of cool white dwarfs, they must have come from outside the star, most probably from the interstellar medium, because the radiation field is not strong enough to compete with gravity, and heavy elements are expected to sink down out of the atmosphere on time scales, which are short compared to evolutionary time scales. The observed metal abundances in helium-rich white dwarfs are in reasonable agreement with accretion in solar element proportions (Dupuis et al. 1992, 1993a, b). However, the observed abundances of hydrogen are much too low to be compatible with the accretion rates inferred from metal accretion. Wesemael & Truran (1982) have proposed that metals are accreted in the form of grains onto a slowly rotating, weakly magnetized (103 G-105 G) white dwarf, whereas ionized hydrogen is repelled at the Alfven radius (propeller mechanism). However, we found no indications of magnetic fields in our circular polarization spectra of GD 40 and L745-46A, taken with the VLT-UT1 and PMOS (dispersion lA/pix) and exposure times of 3.3 hours and 2.7 hours, respectively.

Abstract: Our recent theoretical work (Townsley and Bildsten 2002) on the thermal state of white dwarfs (WDs) in low mass transfer rate binaries allows us to predict the broadband colors of the binary from those of the WD and companion when the disk is dim. The results based on standard CV evolution are presented here. These will aid the discovery of such objects in field surveys and proper-motion selected globular cluster surveys with HST; especially for the largely unexplored post period minimum Cataclysmic Variables (CVs) with the lowest accretion rates and degenerate companions. We have also calculated the fraction of time that the WD resides in the ZZ Ceti instability strip thus clarifying that we expect many accreting WDs to exhibit non-radial oscillations. The study of these will provide new insights into the rotational and thermal structure of an actively accreting WD.

Abstract: The possibility of a gaseous halo stream which was stripped from the Sagittarius dwarf galaxy is presented. The total mass of the neutral hydrogen along the orbit of the Sgr dwarf in the direction of the Galactic anticenter is 4-10 x 10^6 Msun (at 36 kpc, the distance to the stellar debris in this region). Both the stellar and gaseous components have negative velocities in this part of the sky, but the gaseous component extends to higher negative velocities. We suggest this gaseous stream was stripped from the main body of the dwarf 0.2 - 0.3 Gyr ago during its current orbit after a passage through a diffuse edge of the Galactic disk with a density > 10^{-4} cm^{-3}. The gas would then represent the dwarf's last source of star formation fuel and explains how the galaxy was forming stars 0.5-2 Gyr ago.

Abstract: I report on the recent discoveries of the Red Giant Branch Bump(s) in many dwarf Spheroidal galaxies of the Local Group The perspectives for the use of this new observational feature to obtain constraints on the Star Formation History and Age-Metallicity Relation of nearby galaxies are shortly explored and reviewed

Abstract: We have been studying the pulsation periods for the DAV variable white dwarf star G117-B15A for the last 27 years, showing it is an extremely stable pulsator and measuring the rate of pulsation period, directly related to the cooling of the white dwarf. On the other hand, the pulsating DBV GD358 shows changes in amplitudes and frequencies on every timescale we studied it since 1990. The phases also change at least as fast as a monthly timescale.

Abstract: We present results from the most complex hydrodynamical star formation calculation performed to date. It follows the collapse and fragmentation of a large-scale turbulent molecular cloud to form dozens of stars and brown dwarfs. It resolves all fragmentation down to the opacity limit, binary stars with separations as small as 1 AU, and circumstellar disks with radii down to ≈ 10 AU. In this proceedings, we examine the formation mechanism of the brown dwarfs and compare the initial mass function and the properties of the brown dwarfs with observations.

Abstract: We present the results of a study of white dwarfs (WD) in wide binaries that have comparable space motions to the Oppenheimer et al. (2001), (hereafter OHDHS) WDs. These WDs have M dwarf (dM) companions from which we determine the full space motions of the WDs.

Abstract: We present deep Hα+[NII] narrowband imaging of FCC046 and FCC207, two dwarf ellipticals (dEs) in the Fornax Cluster. Although one does not expect much of an interstellar medium (ISM) to be present in dEs, FCC207 shows a central emission region, whereas FCC046 also contains fainter emission regions. The central emission can be explained as photo-ionization by post-AGB stars. Some of the emission clouds in FCC046 are resolved and have diameters of the order of 50 – 150 pc and Ha luminosities of 1030 W, comparable to supernova remnants (SNRs), pointing to recent star formation. Dwarfs like FCC046 could be the evolutionary link between normal, quiescent dEs and more vigorously starforming blue compact dwarfs (BCDs).