Halo White Dwarfs Research Articles

Search for cool white dwarfs with GSC2

Microlensing experiments have suggested that a significant part of the dark halo of the Milky Way could be composed of matter in the form of massive compact halo objects (MACHOs). Cool ancient white dwarfs (WDs) are the natural candidates. Here we present a new survey of halo WDs and evaluate the local space density using an accurate kinematic analysis. A comparison to a revaluation of the Oppenheimer et al. result is also provided. the local space density estimated for the two independent samples is about ~ 10–5M⊙ pc–3.

Simulating a white dwarf dominated Galactic halo

Observational evidence has suggested the possibility of a Galactic halo dominated by white dwarfs (WDs). While debate continues concerning the interpretation of this evidence, it is clear that an initial mass function (IMF) biased heavily toward WD precursors (1≲m/M⊙≲ 8), at least in the early Universe, would be necessary in generating such a halo. Within the framework of homogeneous, closed-box models of Galaxy formation, such biased IMFs lead to an unavoidable overproduction of carbon and nitrogen relative to oxygen (as measured against the abundance patterns in the oldest stars of the Milky Way). Using a three-dimensional Tree N-body smoothed particle hydrodynamics code, we study the dynamics and chemical evolution of a galaxy with different IMFs. Both invariant and metallicity-dependent IMFs are considered. Our variable IMF model invokes a WD-precursor-dominated IMF for metallicities less than 5 per cent solar (primarily the Galactic halo), and the canonical Salpeter IMF otherwise (primarily the disc). Halo WD density distributions and C,N/O abundance patterns are presented. While Galactic haloes composed of ≳5 per cent (by mass) of WDs are not supported by our simulations, mass fractions of ∼1–2 per cent cannot be ruled out. This conclusion is consistent with present-day observational constraints.

Using self-organizing maps to identify potential halo white dwarfs

We present the results of an unsupervised classification of the disk and halo white dwarf populations in the solar neighborhood. The classification is done by merging the results of detailed Monte Carlo (MC) simulations, which reproduce very well the characteristics of the white dwarf populations in the solar neighborhood, with a catalogue of real stars. The resulting composite catalogue is analyzed using a competitive learning algorithm. In particular we have used the so-called self-organized map. The MC simulated stars are used as tracers and help in identifying the resulting clusters. The results of such an strategy turn out to be quite satisfactory, suggesting that this approach can provide an useful framework for analyzing large databases of white dwarfs with well determined kinematical, spatial and photometric properties once they become available in the next decade. Moreover, the results are of astrophysical interest as well, since a straightforward interpretation of several recent astronomical observations, like the detected microlensing events in the direction of the Magellanic Clouds, the possible detection of high proper motion white dwarfs in the Hubble Deep Field and the discovery of high velocity white dwarfs in the solar neighborhood, suggests that a fraction of the baryonic dark matter component of our galaxy could be in the form of old and dim halo white dwarfs.

A Critical Examination of Halo White Dwarf Candidates

A detailed analysis of halo white dwarf candidates is presented, which is based on model atmosphere fits to observed energy distributions built from photoelectric or photographic magnitudes. Most of the candidates identified in reduced proper motion diagrams are shown to be too warm (Teff > 5000 K) and most likely too young to be members of the Galactic halo, while the tangential velocities of the cooler and thus older white dwarfs are shown to be entirely consistent with the disk population. The results suggest that some white dwarf stars born in the young disk may have high velocities with respect to the local standard of rest. Such objects could represent the remnants of donor stars from close mass-transfer binaries that produced Type Ia supernovae via the single-degenerate channel or other scenarios suggested in the literature. Ongoing surveys that rely solely on reduced proper motion diagrams are likely to identify more of these high-velocity young degenerates, rather than to unveil the old white dwarf population of the Galactic halo. The importance of infrared photometry for studying extremely cool white dwarfs is also emphasized.

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.

Dynamical limits on galactic winds, halo MACHOs and intergalactic globular clusters

We argue that any violent galactic winds following early epoch of star bursts would significantly weaken the potentials of galaxies, and leave lasting signatures such as a lowered dark halo density and preferentially radial/escaping orbits for halo tracers such as globular clusters. A galaxy is disintegrated if more than half of its dynamical mass is blown off. The presence of dense halos in galaxies and the absence of intergalactic/escaping globulars should imply an upper limit on the amount of baryons lost in galactic winds around 4% of the total mass of the galaxy. This translates to limits on the baryons participating the early star bursts and baryons locked in stellar remnents, such as white dwarfs. The amount of halo white dwarfs claimed in recent proper motion searches and microlensing observations in the Galactic halo are too high to be consistent with our dynamical upper limits. Similar arguments also imply upper limits for the amount of neutron stars and stellar black holes, in galaxy halos. Nevertheless, a milder outflow is desirable, especially in dwarf galaxies, both for lowering their cold dark matter central density and for injecting metals to the intergalactic medium.

Wide Binary Systems and the Nature of High-Velocity White Dwarfs

We present measured radial velocities and complete space motions for 116 white dwarf stars with M dwarf companions. Thirteen pairs have halo-like velocities. According to a recent study by Oppenheimer et al., all these high-velocity white dwarfs should be considered part of the dark matter heavy halo of the Galaxy, based on their kinematics. Based on the near-solar abundance levels of the M dwarf companions, we conclude that 12 of our 13 high-velocity white dwarfs are actually part of the high-velocity tail of the thick disk, rather than the dark matter halo of the Galaxy, in agreement with the results of a recent study of 514 M dwarfs performed by Reid, Sahu, & Hawley. We find only one potential stellar halo white dwarf (LP 164-52) in our sample. The M dwarf companion of LP 164-52 is a metal-poor, intermediate subdwarf with high-velocity UVW-space motions. In view of the similarity to our sample, we conclude that the majority of the dark matter halo white dwarfs identified by Oppenheimer et al. are most likely to be members of the thick disk, and hence their contribution to the dark matter content of the halo is very much overestimated. Our results suggest that the assignment of population membership solely on incomplete kinematical information is not definitive and that a more robust examination of suspected halo white dwarfs must be performed.

Collision-induced absorption coefficients of H2pairs at temperatures from 60 K to 1000 K

New collision-induced absorption data of H2 pairs at temperatures between 400 K and 1000 K are presented here for the first time. They are based on a careful interpolation and extension of the existing 'low' temperature data, which were suited for use in planetary atmospheric modelling. The new data are aimed to help in modelling the coolest halo white dwarfs, the coolest M dwarfs, the L- and T- type dwarf stars, as well as brown dwarfs and hot jupiters. In addition, for the convenience of the future users, the already existing low temperature (60 K to 400 K) data are provided, in a consistent and user friendly tabular form. The data above 1000 K, which already existed, form a continuous sequence with the new data presented here. The total data set from 60 K to 7000 K and frequencies from 0 cm 1 to17 000 cm 1 (computed for every 10 cm 1 ) is now located at www.astro.ku.dk/aborysow/programs.

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.

EROS 2 proper motion survey: Constraints on the halo white dwarfs

We are conducting a 377 2 proper motion survey in the V and I bands in order to determine the cool white dwarf contribution to the Galactic dark matter. Using the 250 2 for which we possess three epochs, and applying selection criteria designed to isolate halo-type objects, we nd no candidates in a 5500 pc 3 eective volume for old, fast MV = 17 white dwarfs. We check the detection eciency by cross-matching our catalogue with Luyten's NLTT catalogue. The halo white dwarf contribution cannot exceed 5% (95% C.L.) for objects with MV =1 7 and 1 V I 1:5. The same conclusion applies to a 14 Gyr halo composed of white dwarfs with hydrogen atmosphere, as modeled by Chabrier.

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.

On the Possible Sources of D/H Dispersion at High Redshift

Recent observations suggest the existence of a white dwarf population in the Galactic halo, while others suggest that deuterium has been astrated in systems at high redshift and low metallicity. We propose that these observations could be signatures of an early population of intermediate-mass stars. Such a population requires a Population III initial mass function different from that of the solar neighborhood, as perhaps also suggested by the observed cosmic infrared background. Also, to avoid overproduction of C and N, it is required that the Z=0 yields of these stars have low ($\sim 10^{-3}$ solar) abundances as suggested by some recent calculations. Under these assumptions, we present a model which reproduces the observed D vs Si trend, and predicts a high cosmic Type Ia supernova rate, while producing a white dwarf population that accounts for only $\sim$ 1.5% of the dark halo. This scenario can be tested by observations of the cosmic supernova rate, and by confirmation and further studies of the putative white dwarf halo population.

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.

Russell Lecture: Dark Star Formation and Cooling Instability

Optically thin cooling gas at most temperatures above 30 K will make condensations by pressure, pushing material into cool, dense regions. This works without gravity. Cooling condensations will flatten and become planar/similarity solutions. Most star formation may start from cooling condensations, where gravity is only important in the later stages. The idea that some of the dark matter could be pristine white dwarfs that condensed slowly onto planetary-sized seeds without firing nuclear reactions is found lacking. However, recent observations indicate 50 times more halo white dwarfs than have previously been acknowledged, enough to make the halo fraction observed as MACHOs. A cosmological census shows that only 1% of the mass of the universe is of known constitution.

The Halo White Dwarf WD 0346+246 Revisited

The extremely helium-rich atmospheric composition determined for the halo white dwarf WD 0346+246 is reexamined. This solution is shown to be improbable from an astrophysical point of view when accretion of hydrogen and metals from the interstellar medium is taken into account. An alternate solution is proposed in which hydrogen and helium are present in the atmospheric regions in equal amounts. The best fit, at Teff = 3780 K, log g = 8.34, and N(He)/N(H) = 1.3, is achieved by including in the model calculations a bound-free opacity from the Lyman edge associated with the so-called dissolved atomic levels of the hydrogen atom, or pseudocontinuum opacity.

Physics of mirror photons

The physics of kinetic mixing between ordinary and mirror photons is discussed. An important role is played by four linear combinations we dub the physical photon, the sterile photon, the physical mirror photon, and the sterile mirror photon. Because of the mass degeneracy between the two gauge bosons, quantum coherence effects are important. The physical photon becomes a certain coherent superposition of the bare ordinary photon and the bare mirror photon. Similarly, the physical mirror photon is another, but not orthogonal, coherent superposition. We discuss the physics of the interaction between physical mirror photons and ordinary matter. Observational signatures for some hybrid ordinary/mirror binary astrophysical systems are qualitatively discussed. We show that a small amount of ordinary matter at the center of a mirror star may make the mirror star observable. We speculate that the recently reported halo white dwarfs might actually be mirror halo stars.

Infrared Star-Count Models and Their Application to the Subaru Deep Field

We construct infrared star count models of the Galaxy applicable at faint magnitudes. In addition to dwarfs and subdwarfs, we also take into account L dwarfs, T dwarfs, and white dwarfs of both disk and halo. As a test case of the models, we analyze the infrared imaging data at J and K' obtained during the Subaru Deep Field Survey to study stellar objects. Infrared star count models we have obtained predict that the Next Generation Space Telescope will primarily see T dwarfs, M subdwarfs, and old halo white dwarfs at faint magnitudes.

Low-Frequency Gravitational Waves from White Dwarf MACHO Binaries

The possibility that Galactic halo MACHOs are white dwarfs has recently attracted much attention. Using the known properties of white dwarf binaries in the Galactic disk as a model, we estimate the possible contribution of halo white dwarf binaries to the low-frequency (10^{-5} Hz} < f < 10^{-1}Hz) gravitational wave background. Assuming the fraction of white dwarfs in binaries is the same in the halo as in the disk, we find the confusion background from halo white dwarf binaries could be five times stronger than the expected contribution from Galactic disk binaries, dominating the response of the proposed space based interferometer LISA. Low-frequency gravitational wave observations will be the key to discovering the nature of the dark MACHO binary population.

Discovery of High Proper-Motion Ancient White Dwarfs: Nearby Massive Compact Halo Objects?

We present the discovery and spectroscopic identification of two very high proper-motion ancient white dwarf stars, found in a systematic proper-motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool halo white dwarf WD 0346+246. Canonical stellar halo models predict a white dwarf volume density that is 2 orders of magnitude less than the rho approximately 7x10-4 M middle dot in circle pc-3 inferred from this survey. With the caveat that the sample size is very small, it appears that a significant fraction, approximately 10%, of the local dark matter halo is in the form of very old, cool, white dwarfs.

Identification of Cool White Dwarfs in the Sloan Digital Sky Survey

AbstractWe are searching for the coolest white dwarf stars in the galactic disk and halo. The Sloan survey, in due course, will identify an enormous number of new white dwarf stars which will better define the white dwarf luminosity function—an important tool for understanding the age and history of the stellar population of the galaxy. The broadband filter data obtained in the digital photometry phase of the survey will not permit identification of the most interesting of these, the coolest white dwarf stars. This is because the cool main sequence and subdwarf stars become indistinguishable from the white dwarfs in the various colorcolor diagrams. We have interference filters designed to separate out these classes of objects. We have obtained photometry of test fields to complement the Sloan data and identify the population of cool white dwarf stars. These data will ultimately resolve the controversies, based for the most part on small-number statistics, of the location of the turndown in the white dwarf luminosity function for the disk. If the halo is significantly older than the disk, we will find a second peak in the white dwarf luminosity function, at lower luminosities than the disk turndown. Our data will provide the first meaningful constraints on the location of the turndown in the halo white dwarf luminosity function.