Abstract: The results of UBV and Hα imaging of a large sample of isolated dwarf irregular galaxies are interpreted in the context of composite stellar population models. The observed optical colors are best fitted by composite stellar populations that have had approximately constant star formation rates for at least 10 Gyr. The galaxies span a range of central surface brightness, from 20.5 to 25.0 mag arcsec-2; there is no correlation between surface brightness and star formation history. Although the current star formation rates are low, it is possible to reproduce the observed luminosities without a major starburst episode. The derived gas depletion timescales are long, typically ~20 Gyr. These results indicate that dwarf irregular galaxies (dI's) will be able to continue with their slow, but constant, star formation activity for at least another Hubble time. The sample of isolated dI's is compared with a sample of starbursting dwarf galaxies taken from the literature. The starbursting dwarf galaxies have many similar properties; the main difference between these two types of gas-rich dwarf galaxies is that the current star formation is concentrated in the center of the starbursting systems, while it is much more distributed in the quiescent dI's. This results in pronounced color gradients for the starbursting dwarf galaxies, while the majority of the quiescent dwarf irregular galaxies have minor or nonexistent color gradients. Thus, the combination of low current star formation rates, blue colors, and the lack of significant color gradients indicates that star formation percolates slowly across the disks of normal dwarf galaxies in a quasi-continuous manner.

Abstract: Brown dwarfs are sometimes referred to as failed stars because they emit extremely little radiation. Astronomers have now found a brown dwarf that 'whistles' strongly at radio wavelengths.

Abstract: The white dwarf binaries considered in this chapter are better known as cataclysmic variables (CVs) and are interacting binaries in that the white dwarf is accreting material from its (usually) cool, late-type companion star in a short (of the order of hours) orbital period They are one of the few classes of object considered in this book that were actually known and observed prior to the twentieth century The novae become naked-eye objeets (eg Nova Cyg 1975 which, at a peak of 2nd magnitude, completely transformed the appearance of Cygnus for a few weeks in the summer of 1975) and hence have been observed throughout human his-lory, and dwarf novae were first reported in the middle of Ihe nineteenth Century (Hind 1856) It was the non-periodic but continuous eruptions displayed by dwarf novae (such as U Gern and SS Cyg) that led to their cataelysmic designation, a term now widely applied to all interacting binaries where white dwarfs are accreting However, their physical nature was not understood until the pioneering spectroscopic studies of Kraft in the late 1950s which revealed their binary signature CVs are of considerable importance for astronomy in general because of the significance of aecretion processes on virtually all scales, from star and planetary formation (the proto-star aecretes material from its surrounding molecular cloud) to aecretion onto supermassive black holes in the cen-tres of active galactic nuclei

Abstract: We present a detailed calculation of the evolution of low-mass helium white dwarfs. These white dwarfs are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with masses between 0.001 to 0.06 M⊙. We found that the majority of our computed models experience one or two hydrogen shell flashes. The duration of the flashes is between a few ×106 y to a few ×107 y. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. The presence of low-mass helium white dwarf secondaries in millisecond pulsar binaries allows to determine the age of the systems independently of the rotational history of the pulsars. The same method may be applied to double degenerate systems. We discuss the cooling history of the low-mass, helium core white dwarfs in short orbital period millisecond pulsars.

Abstract: Optical and HI imaging of both dwarf irregular (dI) and Blue Compact Dwarf (BCD) galaxies reveal important clues on how dwarf galaxies evolve and their star formation is regulated. Both 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 fainter than 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 deep VLT/FORS1 observations of the two distant, isolated Local Group dwarfs Phoenix and Antlia. Our results provide further evidence for the presence of old stars in these star-forming dwarf galaxies. Old stellar populations are known in all of the Local Group dwarf spheroidal galaxies and in some dwarf irregulars, implying that dwarf galaxies started forming stars at a sharply defined early epoch irrespective of their subsequent star formation histories (e.g., Held et al., 2000; Saviane et al., 2000; and references therein). The new color-magnitude diagrams of Phoenix confirm the presence of a spatially extended blue HB population, indicating a conspicuous old component (Held et al.,1999; Martinez-Delgado et al.,1999). A preliminary analysis of stellar variability has led to the discovery of several tens RR Lyrae variables, which can provide clue information on the earliest star formation episode (see, e.g., Siegel and Majewski, 2000). The young main sequence extends down to the limit of our photometry (V = 25.5 mag), which suggests that Phoenix underwent nearly continuous star formation in the last 2 Gyr. Our deep color-magnitude diagrams of Antlia have been used to investigate the gradient in the stellar populations of this dwarf irregular/spheroidal galaxy. While the young stars appear to be concentrated in a round central region (Aparicio et al., 1997; Sarajedini et al., 1997), the spatial distribution of the red giant stars defines an extended flattened halo (or disk) 2–3 kpc across.

Abstract: The determination of molecular gas masses in star forming dwarf irregular galaxies is crucial to assess the star formation process in these objects. But the derivation of the molecular gas content of dwarf galaxies has been a long-standing problem. CO, as the only practical tracer of cold molecular gas, has been (and to some extent still is) notoriously diffucult to detect. Yet, star formation clearly takes place in many dwarf irregulars. This conference contribution contrasts a number of methods commonly used to derive the molecular gas contents of dwarf galaxies based on CO observations: Procedures based on the virial theorem and those relying on radiative transfer arguments. It is shown that both classes of methods have serious drawbacks. Still, examples show that there seem to be real differences in the `correct' conversion factor both between and within star forming dwarf irregular galaxies.

Abstract: Over a dozen T dwarfs, brown dwarfs that exhibit methane absorption features at 1.6 and 2.2 $\micron$, have been discovered by the 2MASS survey. We discuss how the search for these objects has been made, point out some of the limitations of using near-infrared data to find the warmest T dwarfs, and highlight a few of the more interesting T dwarfs that have been identified in 2MASS data.

Abstract: We present the mass distribution, gravitational redshifts, radial velocities, and space motions of white dwarf stars in common proper motion binary systems. The mass distribution we derive for the 41 DA white dwarfs in this study has a mean of 0.68 ± 0.04 M⊙. This distribution has a slightly higher mean and larger dispersion than most previous white dwarf studies. We hypothesize that this is due to a higher fraction of cool (average Teff ~ 10,000 K), hence old, white dwarfs in our sample. Our results indicate that samples made up of predominantly cool, old white dwarf stars tend to have a bimodal distribution with a second mass peak at ~1.0 M⊙, which skews the mean toward a higher mass. Both the mean and individual white dwarf masses we report here are in better agreement with those determined from model atmosphere spectroscopic fits to line profiles than with most previous gravitational redshift studies of cool white dwarfs. Our results indicate that measurement biases and weak geocoronal emission lines in the observed spectra may have affected previous gravitational redshift measurements. These have been minimized in our study. We present measurements for some previously unobserved white dwarfs, as well as independent new measurements for some that have been reported in the literature. A list of complete space motions for 50 wide binary white dwarfs is presented, derived from radial velocity measurements of their nondegenerate companions. We find that the UVW space motions and dispersions of the common proper motion binaries that contain white dwarf components are consistent with those of old, metal-poor disk stars.

Abstract: We report the discovery of a probable L1 companion to the nearby K2 dwarf GJ 1048 using the Two Micron All-Sky Survey (2MASS).

Abstract: The aim of this work is to investigate the effect of element diffusion on the evolution of helium white dwarfs. To this end, we couple the multicomponent flow equations that describe gravitational settling, chemical and thermal diffusion to an evolutionary code. We compute the evolution of a set of helium white dwarf models with masses ranging from 0.169 to 0.406 M⊙. In particular, several low-mass white dwarfs have been found in binary systems as companion to millisecond pulsars. In these systems, pulsar emission is activated by mass transfer episodes so that, if we place the zero-age point at the end of such mass transfer, then the pulsar and the white dwarf ages should be equal. Interestingly enough, available models of helium white dwarfs neglect element diffusion. Using such models, good agreement has been found between the ages of the components of the PSR J1012+5307 system. However, recent observations of the PSR B1855+09 system cast doubts on the correctness of such models, which predict a white dwarf age twice as long as the spin-down age of the pulsar. In this work, we find that element diffusion induces thermonuclear hydrogen shell flashes for models in the mass interval 0.18≲M/M⊙ ≲ 0.41. We show, in particular, that the occurrence of these diffusion-induced flashes eventually leads to white dwarf models with hydrogen envelope masses too small to support any further nuclear burning, thus implying much shorter cooling ages than in the case when diffusion is neglected. In particular, excellent agreement is found between the ages of PSR B1855+09 system components, solving the age discrepancy from first principles.

Abstract: After summarizing the characteristics of different types of dwarf galaxies I briefly review our current state of knowledge of dwarf galaxy evolution in the Local Group, for which we now have a fairly detailed although by no means comprehensive picture. All Local Group dwarfs studied to date contain an old population, though its fraction varies considerably. The majority of the dwarf companions of the Milky Way shows evidence for a common epoch of ancient star formation. Spatial variations in star formation are frequently observed in many dwarf galaxies in the Local Group and beyond. These spatial variations range from a seemingly stochastic distribution of star-forming regions in gas-rich, high-mass dwarfs to radial gradients in low-mass dwarfs. The global mode of star formation may be either continuous with amplitude variations or episodic. High-mass dwarf galaxies tend to form stars over a Hubble time, whereas low-mass dwarfs eventually cease to form stars, possibly aided by environmental effects. Much less is known about the content and properties of dwarf galaxies in the Local Volume, which we are trying to remedy through a large observational effort. Dwarf galaxies in the Local Volume follow a similar trend with absolute magnitude, mean metallicity, and central surface brightness as the Local Group dwarfs do, and appear to be subject to morphological segregation.

Abstract: We report here on time-resolved photometric observations of the superoutburst of a dwarf nova, RZ Leo, which occurred during 2000 December–2001 January. We reveal the following characteristics of the outburst based on our observations: long duration, large amplitude, and the existence of two types of superhumps — “early superhumps” and “normal superhumps”. In addition to the extremely low frequency of the outbursts, the characteristics of the outburst strongly indicate that RZ Leo is the fifth member of the WZ Sge-type dwarf novae. The orbital period of RZ Leo is 0.07651 d, which is remarkably longer than the “canonical” period of other WZ Sge stars (∼ 0.058 d). The longer period indicates a non-degenerate secondary of RZ Leo, which suggests that the idea of a low quiescence viscosity due to a degenerate brown dwarf secondary may not be correct. The present discovery of RZ Leo as being a WZ Sge star provides the first clear indication that the decay of magnetic turbulence in accretion disks of quiescent WZ Sge stars plays an important role in producing the proposed low quiescent viscosity.

Abstract: The 33 s pulsing component observed from AE Aqr is frequently assumed to be the result of accretion onto the surface of a rotating white dwarf. The validity of this assumption is discussed in the present paper. I show that under the conditions of interest the white dwarf is in the state of supersonic propeller and the eciency of plasma penetration into its magnetosphere is < 0:1%. This is too small to explain the observed luminosity of the pulsing component. Moreover, I nd that for the currently established value of the angle between the magnetic and rotational axes of the white dwarf, the material entering the magnetosphere at the boundary can never reach its surface by flowing along the magnetic eld lines. I conclude that the assumption about the direct accretion onto the surface of the white dwarf in AE Aqr contradicts the observational data obtained in the optical/UV and thus cannot be accepted.

Abstract: Uses three field L and T dwarfs that were discovered to be wide companions to known stars by the Two Micron All-Sky Survey to derive a preliminary brown dwarf companion frequency.

Abstract: We present a high‐gravity, solar composition model atmosphere and model accretion disk study of the U Gem type dwarf nova UU Aql. We have identified the far‐UV signature of the underlying white dwarf for the first time. Our best‐fit model atmosphere to the observed far‐UV spectrum in quiescence is 27,000 ± 1000 K. The solar abundance high‐gravity photosphere provides a consistent explanation for the sharp absorption lines due to metals in the white dwarf's atmosphere. This interpretation is also consistent with the predicted 20,000–30,000 K small, hot, far‐UV source (smaller than an accretion disk) proposed by Patterson & Raymond. Model accretion disk fits do not account for the sharp absorption lines and continuum slope. The best‐fit accretion disk corresponds to Mwd = 0.8 M⊙, i = 60°, and M ˙ = 10-9.5 M⊙ yr−1. Optically thick disk models at accretion rates lower than 1 × 10-9.5 M⊙ yr−1 are ruled out because of marked flux deficiency shortward of 1400 A. Theoretical arguments are presented that rule out accretion rates as high as 1 × 10-9.5 M⊙ yr−1 during dwarf nova quiescence. While we can rule out the quiescent disk as being a significant UV flux contributor, we believe the heated white dwarf accounts for the majority of the far‐UV flux. The white dwarf is only the fifth accretor among U Gem systems with a known surface temperature. The temperatures of all five accretors lie between 25,000 and 35,000 K.

Abstract: Recent spectroscopic and morphological observational studies of galaxies around NGC 1399 in the Fornax Cluster have discovered several ultracompact dwarf galaxies with intrinsic sizes of similar to 100 pc and absolute B-band magnitudes ranging from -13 to -11 mag. In order to elucidate the origin of these enigmatic objects, we perform numerical simulations on the dynamical evolution of nucleated dwarf galaxies orbiting NGC 1399 and suffering from its strong tidal gravitational field. Adopting a plausible scaling relation for dwarf galaxies, we find that the outer stellar components of a nucleated dwarf are totally removed. This is due to them being tidally stripped over the course of several passages past the central region of NGC 1399. The nucleus, however, manages to survive. We also find that the size and luminosity of the remnant are similar to those observed for ultracompact dwarf galaxies, if the simulated precursor nucleated dwarf has a mass of similar to 10(8) M.. These results suggest that ultracompact dwarf galaxies could have previously been more luminous dwarf spheroidal or elliptical galaxies with rather compact nuclei.

Abstract: Early data taken during commissioning of the Sloan Digital Sky Survey (SDSS) have resulted in the discovery of a very cool white dwarf. It appears to have stronger collision-induced absorption from molecular hydrogen than any other known white dwarf, suggesting it has a cooler temperature than any other. While its distance is presently unknown, it has a surprisingly small proper motion, making it unlikely to be a halo star. An analysis of white dwarf cooling times suggests that this object may be a low-mass star with a helium core. The SDSS imaging and spectroscopy also recovered LHS 3250, the coolest previously known white dwarf, indicating that the SDSS will be an effective tool for identifying these extreme objects.

Abstract: We have obtained near-infrared spectra of three L dwarfs discovered by 2MASS, 1506+13 (L3), 1507-16 (L5), and 0920+35 (L6.5). From the comparison of the H and K band spectra of these L dwarfs, we have found the presence of methane absorption in 0920+35. This implies that detectable methane absorption in the H and K bands, usually considered the signature of a T dwarf, can be present in objects classified optically as late L. Methane detection in L dwarfs is consistent with the presence of a dust layer deep in the atmosphere as the unified model of Tsuji suggests.

Abstract: A detailed photometric and spectroscopic analysis of cool (Teff 12,000 K) white dwarf stars is presented. The sample has been drawn from the Yale Parallax Catalog and from a proper motion survey in the southern hemisphere. Optical BVRI and infrared JHK photometry, as well as spectroscopy at Hα, have been secured for a sample of 152 white dwarfs. The discovery of seven new DA white dwarfs, two new DQ white dwarfs, one new magnetic white dwarf, and three weak magnetic white dwarf candidates, is reported. Our sample also identifies 19 known or suspected double degenerates. The photometric energy distributions, the Hα line profiles, and the trigonometric parallax measurements are combined and compared against the predictions of model atmosphere calculations to determine the effective temperature and the radius of each object in the sample and also to constrain the atmospheric composition. New evolutionary sequences with carbon/oxygen cores with thin and thick hydrogen layers are used to derive stellar masses and ages. The results are used to improve our understanding of the chemical evolution of cool white dwarfs. We confirm the existence of a range in effective temperature between ~5000 and 6000 K where almost all white dwarfs have hydrogen-rich atmospheres. Our sample shows little evidence for mixed H/He white dwarfs, with the exception of two helium-rich DA stars, and four (possibly five) C2H white dwarfs which have been interpreted as having mixed H/He/C atmospheres. The observed sequence of DQ stars is found to terminate abruptly near 6500 K, below which they are believed to turn into C2H stars. True DC stars slightly above this temperature are found to exhibit hydrogen-like energy distributions despite the lack of Hα absorption features. The mean mass of our complete sample is 0.65 M☉ with a dispersion of σ ~ 0.20 M☉. Attempts to interpret the chemical evolution of cool white dwarfs show the problem to be complex. Convective mixing is called upon to account for the increase of the non-DA to DA ratio below 12,000 K, as well as the reappearance of helium-rich stars below ~5000 K. The possible presence of helium in cool DA stars, the existence of the non-DA gap, and the nature of the peculiar DC stars are also explained in terms of convective mixing, although our understanding of how this mechanism works needs to be revised in order to account for these observations. Given this chemical evolution uncertainty, it is not clear whether thick or thin hydrogen layer models should be used to determine cooling ages. The oldest object in our sample is ~7.9 Gyr or ~9.7 Gyr old depending on whether thin or thick hydrogen layer models are used, respectively.

Abstract: The problem of detecting Jovian-sized planets orbiting white dwarf stars is considered. Significant IR excesses result from warm Jupiters orbiting a white dwarf of Teff = 10,000 K at a distance of ~103 white dwarf radii (corresponding to ~102 Jupiter radii or a few tenths of an AU) with an orbital period of ~100 days. Such a planet will have a 10 μm flux density at its Wien peak that is comparable to the emission of the white dwarf at that wavelength. Although the white dwarf is much hotter than the planet, the planet will have peak brightness at the IR, well into the Rayleigh-Jeans tail of the white dwarf; plus, Jovians are about 10 times larger than white dwarfs, so there is a substantial gain in the planet-to-star brightness contrast as compared to planets around main-sequence stars. In the solar neighborhood, there are 51 white dwarf stars within 13 pc of the Sun. At 10 pc, the IR flux density of "warm" Jupiters (a few hundred kelvins) will fall in the range 10-100 μJy, which should be observable with SIRTF.

Abstract: White dwarfs are the final remnants of low and intermediate mass stars. Their evolution is essentially a cooling process that lasts for ~ 10 Gyr and allows to obtain information about the age of the Galaxy as well as about the past stellar formation rate in the solar neighborhood. We show in this paper the state of the art of the white dwarf cooling theory and the uncertainties still remaining. We also provide some applications of the theory of cooling white dwarfs to escrutiny the past history of the Milky Way. These applications range from an independent derivation of the past history of the local star formation rate to a determination of the properties of the halo.

Abstract: 0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 M( and follow their evolution from the end of mass-loss episodes, during their pre-white dwarf evolution, down to very low surface luminosities. We find that when the effective temperature decreases below 4000 K, the emergent spectrum of these stars becomes bluer within time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour ‐ colour and in the colour ‐ magnitude diagrams and find that helium-core white dwarfs with masses ranging from ,0.18 to 0.3 M( can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than MV < 16:5. In view of these results, many low-mass helium white dwarfs could have had enough time to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.