Disk White Dwarfs Research Articles

High-proper-motion white dwarfs and halo dark matter

The interpretation of the old, cool white dwarfs recently found by Oppenheimer et al. is still controversial. Whereas these authors claim that they have finally found the elusive ancient-halo white dwarf population that contributes significantly to the mass budget of the galactic halo, there have been several other contributions that argue that these white dwarfs are not genuine halo members but, instead, thick-disc stars. We show here that the interpretation of this sample is based on the adopted distances, which are obtained from a colour–magnitude calibration, and we demonstrate that when the correct distances are used a sizeable fraction of these putative halo white dwarfs belong indeed to the disc population. We also perform a maximum likelihood analysis of the remaining set of white dwarfs and we find that they most probably belong to the thick-disc population. However, another possible explanation is that this sample of white dwarfs has been drawn from a 1:1 mixture of the halo and disc white dwarf populations.

The White Dwarf Cooling Sequence of the Globular Cluster Messier 4

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I = 28. The white dwarf luminosity function rises sharply for I >25.5, consistent with the behaviour expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 magnitudes fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models (Hansen 1999) to fit each luminosity function yields ages of 7.3 +/- 1.5 Gyr for the disk and 12.7 +/- 0.7 Gyr for M4 (2-sigma statistical errors).

A Proper‐Motion Survey for White Dwarfs with the Wide Field Planetary Camera 2

We have performed a search for halo white dwarfs as high proper motion objects in a second-epoch Wide Field Planetary Camera 2 image of the Groth-Westphal strip. The survey covers 74.8 arcmin2 and is complete to V ~ 26.5. We identify 24 high proper motion objects with μ > 0014 yr-1. Five of these high proper motion objects are identified as strong white dwarf candidates on the basis of their position in a reduced proper motion diagram. We also identify two marginal candidates whose photometric errors place them within ~1 σ of the white dwarf region of the reduced proper motion diagram. We create a model of the Milky Way thin disk, thick disk, and stellar halo and find that this sample of white dwarfs is clearly an excess above the ≤2 detections expected from these known stellar populations. The origin of the excess signal is less clear. Possibly, the excess cannot be explained without invoking a fourth galactic component: a white dwarf dark halo. Previous work of this nature has separated white dwarf samples into various galactic components based on kinematics; distances, and thus velocities, are unavailable for a sample this faint. Therefore, we present a statistical separation of our sample into the four components and estimate the corresponding local white dwarf densities using only the directly observable variables, V, (V-I), and μ. For all Galactic models explored, our five white dwarf sample separates into about three disk white dwarfs and two halo white dwarfs. However, the further subdivision into the thin and thick disk and the stellar and dark halo, and the subsequent calculation of the local densities, are sensitive to the input parameters of our model for each Galactic component. Using the lowest mean mass model for the dark halo and the five white dwarf sample, we find n0, thin disk = 2.4 × 10-2 pc-3, n0, thick disk = 0.0+7.6 × 10-4 pc-3, n0, stellar halo = 0.0+7.7 × 10-5 pc-3, and n0, dark halo = 1.0 × 10-3 pc-3. This implies a 7% white dwarf halo and 6 times the canonical value for the thin disk white dwarf density (at marginal statistical significance), but possible systematic errors due to uncertainty in the model parameters likely dominate these statistical error bars. The white dwarf halo can be reduced to ~1.5% of the halo dark matter by changing the initial mass function slightly. The local thin disk white dwarf density in our solution can be made consistent with the canonical value by assuming a larger thin disk scale height of 500 pc.

Absolute Proper Motions toB ∼ 22.5. IV. Faint, Low‐Velocity White Dwarfs and the White Dwarf Population Density Law

The reduced proper-motion diagram (RPMD) for a complete sample of 819 faint (B ≤ 22.5) stars with high-accuracy proper motions (σμ ~ 1 mas yr-1) in an area of 0.3 deg2 in the north Galactic pole field SA 57 is investigated. Eight stars with very large reduced proper motions are identified as faint white dwarf candidates. On the basis of larger than 6 σ measured proper motions and the lack of photometric variability over a 20 yr baseline, we discriminate these white dwarf candidates from the several times more numerous quasi-stellar objects (QSOs), which can potentially occupy a similar location in the RPMD. For comparison, less than 4 σ proper motions and photometric variability are found in all but one of 35 spectroscopically confirmed QSOs in the same field. While spectroscopic confirmation of their status as white dwarfs is a necessary, but difficult, outstanding task, we discuss the implausibility that these stars could be any kind of survey contaminant. High-quality proper motions lend confidence in our ability to separate white dwarfs from subdwarfs in the RPMD. If bona fide white dwarfs, the eight candidates found here represent a portion of the white dwarf population that hitherto has remained uninvestigated by previous surveys by virtue of the faint magnitudes and low proper motions of the stars. This faint, low-velocity sample represents an increase in the white dwarf sky surface density to B = 22.5 by an order of magnitude than that found in the previously most complete surveys to this depth. However, because the majority of the stars discovered here are at projected distances of more than a disk scale height above the Galactic midplane, their existence does not affect significantly the typical estimates of the local white dwarf density. On the other hand, as distant white dwarf candidates with low, typically thin-disk-like transverse velocities (<40 km s-1), the newly discovered stars suggest a disk white dwarf scale height larger than the values of 250-350 pc typically assumed in assessments of the local white dwarf density (and thought to characterize the Galactic old thin disk in stellar population models). Both a ⟨V/Vmax⟩ and a more complex maximum-likelihood analysis of the spatial distribution of our likely thin-disk white dwarfs yield scale heights of 400-600 pc while at the same time giving a reasonable match to the local white dwarf volume density found in other surveys (although this good match is a result of the dominance of the one relatively nearby white dwarf in the 1/Vmax density calculation). A high scale height persists even if the relatively small sample is pruned of any potential thick-disk or halo white dwarfs. While our work is not optimized toward the study of halo white dwarfs as potential MACHO objects, our results do have interesting implications for this hypothesis. We can place some direct constraints (albeit weak ones) on the contribution of halo white dwarfs to the dark matter of the Galaxy. Moreover, the elevated scale height that we measure for the thin disk could alter the interpretation of microlensing results to the extent of making white dwarfs untenable as the dominant MACHO contributor.

The impact of a merger episode in the galactic disc white dwarf population

In this paper we analyse the consequences in the white dwarf population of a hypothetical merger episode in our Galactic disc. We have studied several different merging scenarios with our Monte Carlo simulator. For each one of these scenarios we have derived the main characteristics of the resulting white dwarf population and we have compared them with the available observational data, namely the white dwarf luminosity function and the kinematic properties of the white dwarf population. Our results indicate that very recent (less than ∼6 Gyr ago) and massive (∼16 per cent of the mass of our Galaxy) merger episodes are quite unlikely in view of the available kinematical properties of the disc white dwarf population. Smaller merger episodes (of the order of ∼4 per cent of the mass of our Galaxy) are, however, compatible with our current knowledge of those kinematical properties. Finally, we prove that the white dwarf luminosity function is quite insensitive to such a merger episode.

On high proper motion white dwarfs from photographic surveys

The interpretation of high proper motion white dwarfs detected by Oppenheimer et al (2001) was the start of a lively controversy. While the discoverers identify a large fraction of their findings as dark halo members, others interpret the same sample as essentially made of disc and/or thick disc stars. We use the comprehensive description of Galactic stellar populations provided by the Besancon model to produce a realistic simulation of Oppenheimer et al. data, including all observational selections and calibration biases. The conclusion is unambiguous: Thick disc white dwarfs resulting from ordinary hypotheses on the local density and kinematics are sufficient to explain the observed objects, there is no need for halo white dwarfs. This conclusion is robust to reasonable changes in model ingredients. The main cause of the misinterpretation seems to be that the velocity distribution of a proper motion selected star sample is severely biased in favour of high velocities. This has been neglected in previous analyses. Obviously this does not prove that no such objects like halo white dwarfs can exist, but Oppenheimer et al. observations drive their possible contribution in the dark matter halo down to an extremely low fraction.

The Luminosity Function of Magnitude and Proper‐Motion–selected Samples: The Case of White Dwarfs

The luminosity function of white dwarfs is a powerful tool for studies of the evolution and formation of the Milky Way. The (theoretical) white dwarf cooling sequence provides a useful indicator of the evolutionary timescales involved in the chronometry and star formation history of the galactic disk; therefore, intrinsically faint (and old) white dwarfs in the immediate solar neighborhood can be used to determine an upper limit for the age of the galactic disk. Most determinations of the white dwarf luminosity function have relied on the use of Schmidt's 1/Vmax (Schmidt) method for magnitude- and proper-motion-selected samples, the behavior of which has been demonstrated to follow a minimum variance maximum-likelihood pattern for large samples. Additionally, recent numerical simulations have also demonstrated that the 1/Vmax provides a reliable estimator of the true LF, even in the case of small samples (Wood & Oswalt; Garcia-Berro et al.). However, the conclusions from all these previous studies have been based on noise-free data, where errors in the derived LF have been either assumed to follow a Poisson distribution (valid only for large samples) or where other simple estimates for the uncertainties have been used. In this paper we examine the faint-end (MV > +14) behavior of the disk white dwarf luminosity function using the 1/Vmax method but fully including the effects of realistic observational errors in the derived luminosity function. We employ a Monte Carlo approach to produce many different realizations of the luminosity function from a given data set with prespecified and reasonable errors in apparent magnitude, proper motions, parallaxes and bolometric corrections. These realizations allow us to compute both a mean and an expected range in the luminosity function that is compatible with the observational errors. We find that current state-of-the-art observational errors, mostly in the bolometric corrections and trigonometric parallaxes, play a major role in obliterating (real or artificial) small-scale fluctuations in the luminosity function. We also find that a better estimator of the true luminosity function seems to be the median oversimulations, rather than the mean. When using the latter, an age of 10 Gyr or older can not be ruled out from the sample of Leggett, Ruiz, & Bergeron.

A Very Low Luminosity, Very Cool, DC White Dwarf

The star LHS 3250 is found to be a white dwarf at a distance of 30 pc. Its absolute magnitudes (M_V = 15.72; M_bol = 16.2) put it among the least-luminous white dwarfs known. Its optical spectrum shows no features, indicating it has a DC classification, and it shows no detectable polarization, indicating it does not have a very strong magnetic field. However, its broadband colors show it to have a unique spectral energy distribution, and it stands out from all other stars in BVI and other broadband photometric surveys. We discuss these properties, and conclude that LHS 3250 must be an extremely cool white dwarf with strong collision-induced absorption at red-infrared wavelengths from molecular hydrogen, in accord with models for very cool white dwarf atmospheres. If so, it is the first such star known, and the first star to provide observational evidence supporting these models. It suggests that other very cool white dwarfs, both halo white dwarfs and the oldest disk white dwarfs, also may have colors affected by similar absorption. The atmospheric composition of LHS 3250 is not known, and therefore its temperature is poorly determined. It may be a helium-core star with a mass 0.3-0.45 M_solar and a product of mass-transfer in a close binary system. However, until its temperature is better known, its mass and age remain uncertain.

A survey of unresolved objects in the Hubble Deep Field South

We present a survey of unresolved objects in the Hubble Deep Field South. We find 39 such objects with V 606>27.3. Of these, 29 are candidate main-sequence stars. Their number counts are consistent with no extended stellar halo around the Galaxy, in agreement with analyses of the Hubble Deep Field North. We identify 6 objects which have colours consistent with either halo white dwarfs or unresolved star formation regions. We argue that they are more likely to be the latter. We find one possible disk white dwarf and one candidate quasar.

Monte Carlo simulations of the disc white dwarf population

In order to understand the dynamical and chemical evolution of our Galaxy it is of fundamental importance to study the local neighbourhood. White dwarf stars are ideal candidates to probe the history of the solar neighbourhood, since these `fossil' stars have very long evolutionary time-scales and, at the same time, their evolution is relatively well understood. In fact, the white dwarf luminosity function has been used for this purpose by several authors. However, a long-standing problem arises from the relatively poor statistics of the samples, especially at low luminosities. In this paper we assess the statistical reliability of the white dwarf luminosity function by using a Monte Carlo approach.

Contribution of White Dwarfs to Cluster Masses

I have undertaken a literature search through 1997 July 31 of white dwarfs (WDs) in open and globular clusters. I have tried to make a careful evaluation in each case of the likelihood that the object is a WD and that it is a cluster member. The results are presented for 13 open clusters and 11 globular clusters. Currently there are 36 single WDs and five WDs in binaries known among the open clusters, and 340 single WDs and 11 WDs in binaries known among the globular clusters. From these data, I have calculated WD mass fractions for four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one globular cluster (NGC 6121). I develop a simple model of cluster evolution that incorporates stellar evolution but not dynamical evolution to interpret the WD mass fractions. I augment the results of my simple model by turning to sophisticated N-body simulations incorporating stellar evolution. I find that even though these clusters undergo a range of degrees of kinematic evolution, from moderate (the Pleiades, NGC 2168, and NGC 3532) to strong (the Hyades and NGC 6121), the WD mass fraction is relatively insensitive to kinematic evolution and little changed from a model incorporating only stellar evolution with a Salpeter-like initial mass function. By comparing the cluster mass functions with that of the Galactic disk, and incorporating plausibility arguments for the mass function of the Galactic halo, I estimate the WD mass fraction in these two field populations. I assume the Galactic disk is ~10 Gyr old and that the Galactic halo is ~12 Gyr old, although the WD mass fraction is insensitive to age within this regime. I find that the Galactic halo should contain from 8%–9% (α = -2.35) to perhaps as much as 15%–17% (α = -2.0) of its stellar mass in the form of WDs. The Galactic disk WD mass fraction should be 6% to 7% (for a median stellar age of 5 to 7 Gyr and α = -2.35), consistent with the empirical estimates of 3% to 7%.

Cool white dwarfs: cooling theory and galactic implications

The understanding of the physics of cool white dwarfs (WD) bears important consequences for Galactic evolution and cosmological implications. The observed cutoff in the disk WD luminosity function (WDLF) yields the determination of the age of the Galactic disk, as suggested initially by Winget et aI. (1987). The recent microlensing observations toward the LMC (Alcock et al., 1996) suggest that WDs might provide a substantial fraction of the halo dark matter (Chabrier, Segretain &amp; Méra, 1996; Adams &amp; Laughlin, 1996). The correct analysis of these applications implies a correct WD cooling theory and reliable photometric predictions, which in turn require accurate interior and atmosphere models. Important improvement in this latter domain has been accomplished recently by Bergeron, Saumon &amp; Wesemael (1995) and Bergeron, Wesemael &amp; Beauchamp (1995), which yields the determination of photometric color indices and bolometric corrections down to 4000 K (see Leggett, these proceedings). In this paper, we review the most recent improvement in WD interior and cooling theory.

Star counts in the Hubble Deep Field: constraining galactic structure models

Stellar sources are identified in the Hubble Deep Field, and accurate colours and magnitudes are presented. The predictions of a Galactic starcounts model are compared with the faint stellar counts in this field. The model reproduces the observations very well in the magnitude range 21.0 < V < 26.4, while it overpredicts the counts by a factor of four in the range 26.4 < V < 30.0. The luminosity function for halo objects must be a factor of two smaller than that predicted by an extrapolation of the solar-neighborhood luminosity function for disc stars (with 95% confidence level). This result, seen before in deep Hubble Space Telescope images of globular clusters, is therefore confirmed for the halo field population. The possible nature of a group of faint-blue objects is also investigated, concluding that they are most likely non-stellar. The possibility that they are QSOs is ruled out. If we insist upon their stellar nature, they would be halo white dwarfs, with either a very steep halo white dwarf luminosity function for Mv > +11.0, or a stellar density 0.4 times that of the disc white dwarfs in the solar-neighborhood.

The white dwarf luminosity function - A possible probe of the galactic halo

view Abstract Citations (45) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The White Dwarf Luminosity Function: A Possible Probe of the Galactic Halo Tamanaha, Christopher M. ; Silk, Joseph ; Wood, M. A. ; Winget, D. E. Abstract The dynamically inferred dark halo mass density, amounting to above 0.01 solar masses/cu pc at the sun's Galactocentric radius, can be composed of faint white dwarfs provided that the halo formed in a sufficiently early burst of star formation. The model is constrained by the observed disk white dwarf luminosity function which falls off below log (L/solar L) = -4.4, due to the onset of star formation in the disk. By using a narrow range for the initial mass function and an exponentially decaying halo star formation rate with an e-folding time equal to the free-fall time, all the halo dark matter is allowed to be in cool white dwarfs which lie beyond the falloff in the disk luminosity function. Although it is unlikely that all the dark matter is in these dim white dwarfs, a definite signature in the low-luminosity end of the white dwarf luminosity function is predicted even if they comprise only 1 percent of the dark matter. Current CCD surveys should answer the question of the existence of this population within the next few years. Publication: The Astrophysical Journal Pub Date: July 1990 DOI: 10.1086/168971 Bibcode: 1990ApJ...358..164T Keywords: Halos; Milky Way Galaxy; Star Formation; Stellar Luminosity; White Dwarf Stars; Dark Matter; Star Distribution; Stellar Models; Astrophysics; GALAXIES: STELLAR CONTENT; GALAXIES: THE GALAXY; LUMINOSITY FUNCTION; STARS: FORMATION; STARS: WHITE DWARFS full text sources ADS |