Abstract: Observations are presented which show that Li I λ 6708 is detected with an equivalent width of 053 ± 005A in the proper-motion object LP 944-20 (which is also known as BRI 0337-3535) Hα is detected in emission at an equivalent width of 12 ± 05A The detection of Li implies a mass less than 0065 M⊙, making this object a brown dwarf Moreover, the relative weakness of this Li detection (compared to the equivalent widths of 1–2A seen in objects of similar spectral type in the Pleiades) implies that Li has been somewhat depleted This, together with the precisely determined luminosity of LP 944-20, implies a mass between 0056 and 0064 M⊙, and age between 475 and 650 Myr This makes it the first brown dwarf to have a well-constrained mass and age determined

Abstract: We examine the question of why the white dwarfs in dwarf nova systems are found to be rotating much more slowly than expected. We propose a model in which the accreted angular momentum is removed during nova outbursts. We find that the efficiency of transport of angular momentum between the white dwarf and the extended nova envelope is rather low. We show that in using the same efficiency for the core-envelope coupling in giant stars we can explain the initial spins of pulsars.

Abstract: We propose a scenario to form low-mass, single, slow rotating white dwarfs from a solar-like star accompanied by a massive planet, or a brown dwarf, in a relatively close orbit (e.g. HD 89707). Such white dwarfs were recently found by Maxted & Marsh (1998). When the solar-like star ascends the giant branch it captures the planet and the subsequent spiral-in phase expels the envelope of the giant leaving a low-mass he- lium white dwarf remnant. In case the planet evaporizes, or fills its own Roche-lobe, the outcome is a single undermassive white dwarf. The observed distribution of planetary systems supports the applicability of this scenario.

Abstract: Most galaxies that we observe are ancient, containing some stars almost as old as the Universe. But blue compact dwarf galaxies (BCDs) had been thought younger — rare baby galaxies in among the adults. Now that is called into doubt by observations of a nearby BCD, observations sharp enough to resolve old red giant stars. It may be that there are no new galaxies.

Abstract: Using numerical models, we investigate the possibilities of observable effects in the frequency distribution of the normal modes of the pulsating DB-type white dwarfs caused by different evolutionary paths. We find the average period spacing of consecutive radial overtones to be much shorter and the periods of the low k modes to be much longer for DB models resulting from the binary evolution than for those arising from evolution of a single star. Then we compare our models to the GD 358 data and conclude that this star either went through extremely long accretion phase, if it has developed from a binary system, or it is the result of evolution of a single star.

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Abstract: Aside from the many things we have learned about stellar pulsation from the pulsating white dwarfs, seismic study of their interiors feeds back into the theory of stellar evolution. In particular, from pulsating white dwarfs has come a much better understanding of the processes by which white dwarfs are made. In this brief review, I describe what the hot pulsating white dwarfs have taught us about white dwarf interiors, and how this knowledge has tested the theory of stellar evolution. I also discuss some future prospects for continued discoveries in stellar evolution through study of the pulsators. Key wordsi stars: white dwarfs, oscillations, evolution

Abstract: We have begun to look for low-mass stars and brown dwarfs using data from the 2MASS Prototype Cameras. Reported here are some very early, yet very encouraging, results which demonstrate the ease with which 2MASS will uncover missing, very cool members of the solar neighborhood.

Abstract: We performed a search for periodic light variations among low-mass DAO white dwarfs. These objects have theoretically been predicted to exhibit low-order g-mode pulsations due to a potent mechanism in hydrogen-burning shells according to model calculations of Charpinet et al. (1997). No pulsations were detected in any of our nine candidate stars. This can be attributed to the very narrow instability strip predicted or to the fact that pulsationally unstable modes are not necessarily excited, a common phenomenon among pulsating stars. It may also be the case that hydrogen-burning shells in the stars are too small to drive pulsations or not present.

Abstract: Although it a very narrow angle survey, the depth of the HDF results in its sampling a significant volume of the halo of our galaxy Thus it is useful for the purposes of detecting (or placing upper limits on the distribution of) intrinsically faint stars, such as white dwarfs White dwarfs could provide a significant fraction of the total mass of the halo of the Milky Way Constraints on the population of halo white dwarfs from the HDF can directly address this possible partial explanation of the nature of the dark halo of the Milky Way In this review, I illustrate how the HDF can be used to constrain the luminosity function of halo white dwarfs I begin with a brief summary of the observed white dwarf luminosity function (WDLF) of the galactic disk, and show how the HDF serves as a probe of the WDLF for the halo I then review the theoretical background used in interpreting the WDLF in terms of the theory of white dwarf evolution and cooling, and the history of star formation in the galaxy We are then in a position to explore the theoretical WDLF of the disk and then the halo The results of searches for white dwarfs on the HDF can then be examined in terms of the halo white dwarf population

Abstract: Important phenomena are briefly described which have recently been discovered in the Crimean studies of flaring red dwarf stars believed to be the most common type of variable stars in the Galaxy. These phenomena include (i) long-lived radiation from a blueshifted component in the ionized-helium λ 4686 A emission line in the active state of one such star, (ii) a long-lived absorption component in the stellar flare light curves with a lifetime exceeding that of the conventional flare emission, and (iii) solarcycle-like activity periodicity of the star EV Lac, whose mass is only 0.3 solar masses. In theoretical terms, a red dwarf star spot model is constructed which, in contrast to the commonly accepted model, agrees well with the solar spot picture.

Abstract: Optical and HI imaging of gas rich dwarfs, both dwarf irregulars (dI) and blue compact dwarfs (BCD), reveals important clues on how dwarf galaxies evolve and their star formation is regulated. Both types usually show evidence for stellar and gaseous disks. However, their total mass is dominated by dark matter. Gas rich dwarfs form with a range of disk structural properties. These have been arbitrarily separated them into two classes on the basis of central surface brightness. Dwarfs with \mu_{0}(B) <~ 22 mag arcsec^{-2} are usually classified as BCDs, while those faintwards of this limit are usually classified as dIs. Both classes experience bursts of star formation, but with an absolute intensity correlated with the disk surface brightness. Even in BCDs the bursts typically represent only a modest <~ 1 mag enhancement to the B luminosity of the disk. While starbursts are observed to power significant galactic winds, the fractional ISM loss remains modest. Dark matter halos play an important role in determining dwarf galaxy morphology by setting the equilibrium surface brightness of the disk.

Abstract: We present ISOCAM 7 micron and 15 micron observations of 12 nearby white dwarfs, 6 of which have been found to have metals such as Ca, Mg and Fe in their photospheres. Our purpose was to search for an excess of infrared emission above the stellar photospheres. We find that none of the white dwarfs other than G29-38 shows a detectable infrared excess and this places strong constraints on the existence of a dusty disk around these stars. We conclude that ongoing accretion of the interstellar medium seems an unlikely explanation for the existence of metals in the photospheres of cool hydrogen atmosphere white dwarfs. The excess associated with G29-38 is 3.8+/-1.0 mJy and 2.9+/-0.6 mJy at 7 micron and 15 micron respectively. The broadband spectrum of this star strengthens the hypothesis that the infrared excess arises from a disk of particulate matter surrounding the white dwarf rather than from a cool brown dwarf companion.

Abstract: The recent discovery of microlensing of stars in the Large Magellanic Cloud has excited much interest in the nature of the lensing population. Detailed analyses indicate that the mass of these objects ranges from 0.3-0.8 solar masses, suggesting that they might be white dwarfs, the faint remnants of stellar evolution. The confirmation of such an hypothesis would yield profound insights into the early history of our galaxy and the early generations of stars in the universe. Previous attempts have been made to place theoretical constraints on this scenario, but were unduly pessimistic because they relied on inadequate evolutionary models. Here we present the first results from detailed evolutionary models appropriate for the study of white dwarfs of truly cosmological vintage. We find that the commonly held notion that old white dwarfs are red to hold only for helium atmosphere dwarfs and that hydrogen atmosphere dwarfs will be blue, with colours similar to those of the faint point sources found in the Hubble Deep Field. Thus, any direct observational search for the microlensing population should search for faint blue objects rather than faint red ones.

Abstract: A nova outburst is one consequence of the accretion of hydrogen rich material onto a white dwarf in a close binary system. The strong electron degeneracy of a massive white dwarf drives the temperatures in the nuclear burning region to values exceeding 108K under all circumstances. As a result, a major fraction of the CNO nuclei in the envelope are transformed into e{sup +}-decay nuclei, which constrains the nuclear energy generation and yields non-solar CNO isotopic abundance ratios. In addition, the observations demonstrate that white dwarf core material is dredged up into the accreted layers and these nuclei are the catalysts for producing peak rates of energy generation that can exceed 10{sup 16} erg gm{sup -1}s{sup -1}. Observations show that there are two compositional classes of novae, one that occurs on a carbon-oxygen white dwarf and the other that occurs on an oxygen-neon-magnesium white dwarf.

Abstract: We present 21-cm H i line observations of the blue compact dwarf galaxy NGC 1705. Previous optical observations show a strong outflow powered by an ongoing starburst dominating the H ii morphology and kinematics. In contrast, most of the H i lies in a rotating disc. An extraplanar H i spur accounts for ∼8 per cent of the total H i mass, and is possibly associated with the H ii outflow. The inferred mass loss rate out of the core of the galaxy is significant, ∼0.2 − 2 M⊙ yr−1, but does not dominate the H i dynamics. Mass model fits to the rotation curve show that the dark matter (DM) halo is dominant at nearly all radii and has a central density ρ0 ≈ 0.1 M⊙ pc−3: ten times higher than typically found in dwarf irregular galaxies, but similar to the only other mass-modelled blue compact dwarf, NGC 2915. This large difference strongly indicates that there is little evolution between dwarf irregular and blue compact dwarf types. Instead, dominant DM haloes may regulate the morphology of dwarf galaxies by setting the critical surface density for disc star formation. Neither our data nor catalogue searches reveal any likely external trigger to the starburst in NGC 1705.

Abstract: AB S T R AC T The present study is aimed at exploring the effects of hydrogen envelopes on the structure and evolution of lowand intermediate-mass, helium white dwarfs. To this end, we compute the evolution of models of helium white dwarfs with masses ranging from 0.15 to 0.5 M> for low and intermediate effective temperatures. We treat the mass of the hydrogen envelope as a free parameter within the range 10Ð8RMH /MR4A10 . The calculations are carried out by means of a detailed white dwarf evolutionary code in which updated radiative opacities and equations of state for hydrogen and helium plasmas are considered. The energy transport by convection is described by the full spectrum turbulence theory developed by Canuto, Goldman & Mazzitelli, which has no free parameters. We also take into account both convective mixing in the outer layers occurring at low luminosities and the presence of hydrogen burning at the bottom of the hydrogen-rich envelope by means of the inclusion of a detailed network of thermonuclear reaction rates. Our attention is focused mainly on that phase of evolution where finitetemperature effects are particularly significant. In this respect, we find that thick hydrogen envelopes appreciably modify the radii and surface gravity of the nohydrogen models, especially in the case of low-mass configurations. In addition, convective mixing in low-luminosity models with thin hydrogen envelopes leads to objects with helium-dominated outer layers. Finally, we find that the role played by hydrogen burning in these stars is strongly dependent on the mass of the hydrogen envelope. The computations presented here represent the most detailed models of helium white dwarfs with hydrogen envelopes presently available. These models should be particularly valuable for the study of the structure and evolutionary status of the recently detected low-mass white dwarfs in binary systems.

Abstract: We report the discovery, in an Extreme Ultraviolet Explorer (EUVE) short-wavelength spectrum, of an unresolved hot white dwarf companion to the 5th magnitude B5Vp star HR 2875. This is the first time that a non-interacting white dwarf+B star binary has been discovered: previously, the earliest type of star known with a white dwarf companion was Sirius (A1V). As the white dwarf must have evolved from a main-sequence progenitor with a mass greater than that of a B5V star (≯6.0 M⊙), this places a lower limit on the maximum mass for white dwarf progenitors, with important implications for our knowledge of the initial–final mass relation. Assuming a pure-hydrogen atmospheric composition, we constrain the temperature of the white dwarf to be between 39 000 and 49 000 K. We also argue that this degenerate star is likely to have a mass significantly greater than the mean mass for white dwarf stars (≈0.55 M⊙). Finally, we suggest that other bright B stars (e.g. θ Hya) detected in the extreme ultraviolet surveys of the ROSAT Wide Field Camera and EUVE may also be hiding hot white dwarf companions.

Abstract: The accretion of comets onto DA white dwarfs can produce observable metal absorption lines. We show here that comet systems around the progenitor main-sequence star are vulnerable to being lost during asymptotic giant branch mass loss, if the mass loss is sufficiently asymmetric to impart modest linear momentum to the white dwarf. This may have bearing on the frequency of observation of heavy elements in white dwarf stars and on inferences regarding the frequency of comet systems, if the imparted linear velocities of white dwarfs can be estimated.

Abstract: We present observations and an analysis of the X-ray source 1RXS J0832.6–2525 which shows it to be a low field magnetic white dwarf with an unusual high mass. This is the second magnetic white dwarf for which a determination of a spectroscopic mass has been possible, and both stars belong to the growing class of ultramassive white dwarfs (M ≥ 1.1 M⊙).

Abstract: The effect of rotating white dwarf envelopes in determining the structure of nova shells is examined. This is achieved by numerical hydrodynamic simulations of the flows around a binary star system. In previous studies of remnant formation, this rotation has not been included. It is found that the structures formed in the flow are more consistent with observations of nova shells than the previous theoretical studies. The shells produced by the nova become more prolate with increasing white dwarf envelope rotation. Hence the rotation of white dwarf envelopes must be included in any future discussion of remnant formation. A possible method of identifying the dominant process by which mixing of accreted and white dwarf matter takes place is suggested.

Abstract: We present spectroscopy and photometry of GD 448, a detached white dwarf - M dwarf binary with a period of 2.47h. We find that the NaI 8200A feature is composed of narrow emission lines due to irradiation of the M dwarf by the white dwarf within broad absorption lines that are essentially unaffected by heating. Combined with an improved spectroscopic orbit and gravitational red shift measurement from spectra of the H-alpha line, we are able to derive masses for the white dwarf and M dwarf directly (0.41 +/- 0.01 solar masses and 0.096 +/- 0.004 solar masses, respectively). We use a simple model of the CaII emission lines to establish the radius of the M dwarf assuming the emission from its surface to be proportional to the incident flux per unit area from the white dwarf. The radius derived is 0.125 +/- 0.020 solar radii. The M dwarf appears to be a normal main-sequence star in terms of its mass and radius and is less than half the size of its Roche lobe. The thermal timescale of the M dwarf is much longer than the cooling age of the white dwarf so we conclude that the M dwarf was unaffected by the common-envelope phase. The anomalous width of the H-alpha emission from the M dwarf remains to be explained, but the strengh of the line may be due to X-ray heating of the M dwarf due to accretion onto the white dwarf from the M dwarf wind.

Abstract: We obtained Hubble Space Telescope Goddard High-Resolution Spectrograph medium-resolution (G160M grating), phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf nova quiescence, 13 days and 61 days following the end of a narrow outburst. The spectral wavelength ranges were centered on three different line regions: N V (1238 A, 1242 A), Si III (1300 A), and He II (1640 A). All of the quiescent spectra at both epochs are dominated by absorption lines and show no emission features. The Si III and He II absorption-line velocities versus orbital phase trace the orbital motion of the white dwarf, but the N V absorption velocities appear to deviate from the white dwarf motion. We confirm our previously reported low white dwarf rotational velocity, V sin i = 100 km s-1. We obtain a white dwarf orbital velocity semiamplitude K1 = 107 km s-1. Using the γ-velocity of Wade, we obtain an Einstein redshift of 80.4 km s-1 and hence a carbon core white dwarf mass of ~1.1 M☉. We report the first subsolar chemical abundances of C and Si for U Gem with C/H = 0.05 times solar, almost certainly a result of C depletion due to thermonuclear processing. This C depletion is discussed within the framework of a weak thermonuclear runaway, contamination of the secondary during the common envelope phase, and mixing of C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova event. Remarkably, the Teff of the white dwarf 13 days after outburst is only 32,000 K, anomalously cooler than previous early postoutburst measurements. Extensive cooling during an extraordinarily long (210 days) quiescence followed by accretion onto an out-of-equilibrium cooled degenerate could explain the lower Teff.

Abstract: An understanding of the white dwarf stars is central to much of astrophysics, from the structure and evolution of stars to the age and history of the large ensembles of stars that we call galaxies. They are of great potential interest from the standpoint of physics as well, because they offer a chance to study matter under extreme conditions not yet attainable in the laboratory. White dwarfs are the simplest kinds of star that we know of, yet they are still very complicated. There are a number of questions that we must answer in order to understand them and the physics that governs their behaviour. Here I describe how we can use asteroseismology to answer these questions. I give a quick introduction to the field, summarize what has been done, and describe what looks promising for the future.

Abstract: The white dwarf Giclas 29-38 has attracted much attention on account of its large infrared excess and the suggestion that excess might be due to a companion brown dwarf. We observed this object using speckle interferometry at the Keck telescope, obtaining diffraction-limited resolution (55 mas) at the K band, and found it unresolved. Assuming that the entire K-band excess is attributable to a single pointlike companion, we place an upper limit on the binary separation of 30 mas, or 0.42 AU at the star's distance of 14.1 pc. This result, combined with astroseismological data and other images of G29-38, supports the hypothesis that the source of the near-infrared excess is not a cool companion but a dust cloud.

Abstract: The white dwarf luminosity function has proved to be an excellent tool for studying some properties of the Galactic disk, such as its age and the past history of the local star formation rate. The existence of an observational luminosity function for halo white dwarfs could provide valuable information about its age and the time that the star formation rate lasted, and it could also constrain the shape of the allowed initial mass functions. However, the main problem is the scarce number of white dwarfs already identified as halo stars. In this Letter, we show how an artificial intelligence algorithm can be used successfully to classify the population of spectroscopically identified white dwarfs, thus allowing us to identify several potential halo white dwarfs and to improve the significance of its luminosity function.

Abstract: Photometry on the UVI system has been performed on the resolved stellar content of the blue compact dwarf galaxy UGC 6456 using Wide Field Planetary Camera 2 (WFPC2) images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram (CMD) goes to about V = 27.5 and reveals not only a young population of blue main-sequence stars and blue and red supergiants, but also an older evolved population of red giants and a fairly well represented asymptotic giant branch. The distance to the galaxy is estimated from the tip of the red giant branch to be 4.5 Mpc, placing it about 1.5 Mpc farther away than the major members of the M81 Group, with which it is usually associated. The youngest stars are generally associated with H II regions shown on our Hα image and are largely confined to the 745 pc field of our PC images. A comparison of their distribution in the CMD with theoretical isochrones suggests ages from 4 to 10 Myr. The population of older stars is found throughout all WFPC2 camera fields and seems to show an elliptical distribution with an aspect ratio of about 2.4 and an exponential falloff in surface density with distance from a center of symmetry that is not far from the centroid of the youngest stars. Theoretical modeling of the CMD at a metallicity of Z = 0.001 suggests star formation in the age interval 1–2 Gyr, a strong burst in the interval 600–800 Myr, and a lower rate of star formation up to the present. The evidence is compatible with a scenario beginning with the formation of a population of low-metallicity stars, enriching a major residual of prestellar material that subsequently fueled an active episode of star formation. That burst of star formation must have been particularly spectacular and may be related to the activity we now see in the distant blue dwarf galaxies revealed in deep imaging.

Abstract: It has been proposed that the presence of carbon in the mainly helium atmospheres of the DQ white dwarfs is due to convective dredge-up of carbon diffusing outwards from a CO core. The first quantitative study of this process is that of Pelletier at al (1986, hereafter P86), who found that agreement with the observationally determined photospheric C/He ratios required that the mass of the helium-rich layer in the white dwarf progenitor be an order of magnitude less than determined from theoretical calculations of the pre-white dwarf evolution.