Space Density Of White Dwarfs Research Articles

Selection Functions in Astronomical Data Modeling, with the Space Density of White Dwarfs as a Worked Example

Abstract Statistical studies of cataloged object properties are central to astrophysics. But one cannot model those objects’ population properties without the sample’s selection function, the quantitative understanding of which objects could have ended up in such a catalog. As didactic introductions to this topic are scarce in the astrophysical literature, we provide one here, addressing the following questions: What is a selection function? On what arguments q should it depend? Over what domain must a selection function be defined? What simplifying approximations can be made? And, how is a selection function used in “modeling”? We argue that volume-complete samples, limited by the faintest objects, reflect a highly suboptimal selection function, needlessly reducing the number of bright and usually rare sample members. We illustrate these points by a worked example: github.com/gaia-unlimited/WD-selection-function, deriving the space density of white dwarfs (WDs) in the Galactic neighborhood as a function of their luminosity and color, Φ0(M G , (B − R)) in [mag−2 pc−3]. We construct a sample  of 105 presumed WDs through straightforward selection cuts on the Gaia EDR3 catalog in magnitude, color, and parallax, q = (G, (B − R), ϖ). We then combine a simple model for Φ0 with this selection function’s S  ( q ) effective survey volume to estimate Φ0(M G , (B − R)) precisely and robustly against the detailed choices for S  ( q ) . This resulting WD luminosity–color function Φ0(M G , (B − R)) differs dramatically from the initial number density distribution in the luminosity−color plane: by orders of magnitude in density and by four magnitudes in density peak location.

Selection Functions in Astronomical Data Modeling, with the Space Density of White Dwarfs as a Worked Example

Selection Functions in Astronomical Data Modeling, with the Space Density of White Dwarfs as a Worked Example

A white dwarf catalogue from Gaia-DR2 and the Virtual Observatory

We present a catalogue of 73,221 white dwarf candidates extracted from the astrometric and photometric data of the recently published Gaia DR2 catalogue. White dwarfs were selected from the Gaia Hertzsprung-Russell diagram with the aid of the most updated population synthesis simulator. Our analysis shows that Gaia has virtually identified all white dwarfs within 100 pc from the Sun. Hence, our sub-population of 8,555 white dwarfs within this distance limit and the colour range considered, $-\,0.52<(G_{\rm BP}-G_{\rm RP})<0.80$, is the largest and most complete volume-limited sample of such objects to date. From this sub-sample we identified 8,343 CO-core and 212 ONe-core white dwarf candidates and derived a white dwarf space density of $4.9\pm0.4\times10^{-3}\,{\rm pc^{-3}}$. A bifurcation in the Hertzsprung-Russell diagram for these sources, which our models do not predict, is clearly visible. We used the Virtual Observatory tool VOSA to derive effective temperatures and luminosities for our sources by fitting their spectral energy distributions, that we built from the UV to the NIR using publicly available photometry through the Virtual Observatory. From these parameters, we derived the white dwarf radii. Interpolating the radii and effective temperatures in hydrogen-rich white dwarf cooling sequences, we derived the surface gravities and masses. The Gaia 100 pc white dwarf population is clearly dominated by cool ($\sim$ 8,000 K) objects and reveals a significant population of massive ($M \sim 0.8 M_{\odot}$) white dwarfs, of which no more than $\sim$ $30-40 \%$ can be attributed to hydrogen-deficient atmospheres, and whose origin remains uncertain.

The 25 parsec local white dwarf population

We have extended our detailed survey of the local white dwarf population from 20 pc to 25 pc, effectively doubling the sample volume, which now includes 232 stars. In the process new stars within 20 pc have been added, a more uniform set of distance estimates as well as improved spectral and binary classifications are available. The present 25 pc sample is estimated to be about 68% complete (the corresponding 20 pc sample is now 86\% complete). The space density of white dwarfs is unchanged at 4.8 \pm 0.5 x 10^{-3} pc^{-3}. This new study includes a white dwarf mass distribution and luminosity function based on the 232 stars in the 25 pc sample. We find a significant excess of single stars over systems containing one or more companions (74\% vs 26\%). This suggests mechanisms that result in the loss of companions during binary system evolution. In addition this updated sample exhibits a pronounced deficiency of nearby Sirius-Like systems. Eleven such systems were found within the 20 pc volume vs, only one additional system found in the volume between 20 pc and 25 pc. An estimate of white dwarf birth rates during the last 8 Gyr is derived from individual remnant cooling ages. A discussion of likely ways new members of the local sample may be found is provided.

MOVING OBJECTS IN THE HUBBLE ULTRA DEEP FIELD

We identify proper motion objects in the Hubble Ultra Deep Field (UDF) using the optical data from the original UDF program in 2004 and the near-infrared data from the 128-orbit UDF 2012 campaign. There are 12 sources brighter than I=27 mag that display >3sigma significant proper motions. We do not find any proper motion objects fainter than this magnitude limit. Combining optical and near-infrared photometry, we model the spectral energy distribution of each point-source using stellar templates and state-of-the-art white dwarf models. For I<27 mag, we identify 23 stars with K0-M6 spectral types and two faint blue objects that are clearly old, thick disk white dwarfs. We measure a thick disk white dwarf space density of 0.1-1.7 E-3 per cubic parsec from these two objects. There are no halo white dwarfs in the UDF down to I=27 mag. Combining the Hubble Deep Field North, South, and the UDF data, we do not see any evidence for dark matter in the form of faint halo white dwarfs, and the observed population of white dwarfs can be explained with the standard Galactic models.

A determination of the space density and birth rate of hydrogen-line (DA) white dwarfs in the Galactic plane, based on the UVEX survey

We present a determination of the average space density and birth rate of hydrogen-line (DA) white dwarfs within a radius of 1 kpc around the Sun, based on an observational sample of 360 candidate white dwarfs with g<19.5 and (g-r)<0.4, selected from the UV-excess Survey of the Northern Galactic Plane (UVEX), in combination with a theoretical white dwarf population that has been constructed to simulate the observations, including the effects of reddening and observational selection effects. The main uncertainty in the derivation of the white dwarf space density and current birth rate lies in the absolute photometric calibration and the photometric scatter of the observational data, which influences the classification method on colours, the completeness and the pollution. Corrections for these effects are applied. We derive an average space density of hydrogen-line (DA) white dwarfs with T_eff > 10,000K (M_V<12.2) of (3.8 +/- 1.1) x 1e-4 pc^-3, and an average DA white dwarf birth rate over the last 7e7 years of (5.4 + 1.5) x 1e-13 pc^-3 yr^-1. Additionally, we show that many estimates of the white dwarf space density from different studies are consistent with each other, and with our determination here.

THE WHITE DWARFS WITHIN 20 PARSECS OF THE SUN: KINEMATICS AND STATISTICS

We present the kinematical properties, distribution of spectroscopic subtypes, stellar population subcomponents of the white dwarfs within 20 pc of the sun. We find no convincing evidence of halo white dwarfs in the total 20 pc sample of 129 white dwarfs nor is there convincing evidence of genuine thick disk subcomponent members within 20 parsecs. Virtually the entire 20 pc sample likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc sample is 1.6, a manifestation of deepening envelope convection which transforms DA stars with sufficiently thin H surface layers into non-DAs. The addition of 5 new stars to the 20 pc sample yields a revised local space density of white dwarfs of $4.9\pm0.5 \times 10^{-3}$ M$_{\sun}$/yr and a corresponding mass density of $3.3\pm0.3 \times 10^{-3}$ M$_{\sun}$/pc$^{3}$. We find that at least 15% of the white dwarfs within 20 parsecs of the sun (the DAZ and DZ stars) have photospheric metals that possibly originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, this suggests the possibility that the same percentage have planets or asteroid-like bodies orbiting them.

Trigonometric parallaxes of high velocity halo white dwarf candidates

The status of 38 halo white dwarf candidates identified by Oppenheimer et al. (2001) has been intensively discussed by various authors. In analyses undertaken to date, trigonometric parallaxes are crucial missing data. Distance measurements are mandatory to kinematically segregate halo object from disk objects and hence enable a more reliable estimate of the local density of halo dark matter residing in such objects. We present trigonometric parallax measurements for 15 candidate halo white dwarfs (WDs) selected from the Oppenheimer et al. (2001) list. We observed the stars using the ESO 1.56-m Danish Telescope and ESO 2.2-m telescope from August 2001 to July 2004. Parallaxes with accuracies of 1--2 mas were determined yielding relative errors on distances of $\sim5$% for 6 objects, $\sim12$% for 3 objects, and $\sim20$% for two more objects. Four stars appear to be too distant (probably farther than 100 pc) to have measurable parallaxes in our observations. Distances, absolute magnitudes and revised space velocities were derived for the 15 halo WDs from the Oppenheimer et al. (2001) list. Halo membership is confirmed unambiguously for 6 objects while 5 objects may be thick disk members and 4 objects are too distant to draw any conclusion based solely on kinematics. Comparing our trigonometric parallaxes with photometric parallaxes used in previous work reveals an overestimation of distance as derived from photometric techniques. This new data set can be used to revise the halo white dwarf space density, and that analysis will be presented in a subsequent publication.

Low frequency gravitational waves from black hole MACHO binaries

The intensity of low frequency gravitational waves from black hole MACHO binaries is studied. First we estimate the gravitational wave background produced by black hole MACHO binaries in the Milky Way halo as well as the cosmological gravitational wave background produced by the extragalactic black hole MACHO binaries. It is found that the cosmological gravitational wave background due to black hole MACHO binaries is larger than the halo background unless an extreme model of the halo is assumed, while it is smaller than the background due to close white dwarf binaries at $\nu_{gw} \siml 10^{-2.5}$ Hz if the actual space density of white dwarfs is maximal. This cosmological background due to black hole MACHO binaries is well below the observational constraints from the pulsar timing, quasar proper motions and so on. We find that one year observation by LISA will be able to detect gravitational waves from at least several hundreds of nearby independent black hole MACHO binaries whose amplitudes exceed these backgrounds. This suggests that LISA will be able to pin down various properties of primordial black hole MACHOs together with the results of LIGO-VIRGO-TAMA-GEO network. Furthermore, it may be possible to draw a map of the mass distribution of our halo, since LISA can determine the position and the distance to individual sources consisted of black hole MACHO binaries. Therefore, LISA may open a new field of the gravitational wave astronomy.

A study of the white dwarf luminosity function

view Abstract Citations (116) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Study of the White Dwarf Luminosity Function Iben, Icko, Jr. ; Laughlin, Gregory Abstract The dependence of white dwarf luminosity functions on the assumed age of the Galactic disk is examined, taking into account the relationship between white dwarf mass and progenitor mass, the progenitor lifetime, the mass dependence of cooling curves, and possible variations in the birthrate function in the past. At low luminosities, the resultant distribution in number versus luminosity depends very strongly not only on the shape of the cooling curves for different masses, but also on the spectrum of white dwarf masses that contribute to the distribution. For an age of about 9 Gyr, it is suggested that at luminosities only a quarter of a magnitude fainter than the dimmest white dwarfs yet found, the space density of white dwarfs is larger by a factor of 10 than the space density which the Winget et al. (1987) luminosity function suggests. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/167496 Bibcode: 1989ApJ...341..312I Keywords: Galactic Structure; Star Formation; Stellar Evolution; Stellar Luminosity; White Dwarf Stars; Chronology; Polynomials; Astrophysics; LUMINOSITY FUNCTION; STARS: EVOLUTION; STARS: STELLAR STATISTICS; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (1)

The Luminosity Function of White Dwarfs in the Local Disk and Halo

The luminosity function (LF) and total space density of white dwarfs in the solar neighborhood contain important information about the star formation history of the stellar population, and provide an independent method of measuring its age. The first empirical estimates of the LF for degenerate stars were those of Weidemann (1967), Kovetz and Shaviv (1976) and Sion and Liebert (1977). The follow-up investigations made possible by the huge Luyten Palomar proper motion surveys, however, added many more faint white dwarfs to the known sample. While the number of known cool white dwarfs grew to nearly one hundred, these did not include any that were much fainter intrinsically than the coolest degenerates found from the early Luyten, van Biesbroeck and Eggen-Greenstein lists.

The KPD survey for galactic plane ultraviolet-excess objects Space densities of white dwarfs and subdwarfs

view Abstract Citations (125) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The KPD Survey for Galactic Plane Ultraviolet-Excess Objects: Space Densities of White Dwarfs and Subdwarfs Downes, R. A. Abstract A survey for ultraviolet-excess objects has been performed for over about 1000 sq deg of the Galactic plane. Photographic blue and ultraviolet plates were obtained to select ultraviolet-excess candidates, and spectroscopic observations were used to determine the nature of those objects. Over 70 white dwarfs and subdwarfs were identified from the survey plates, and these objects were analyzed statistically to determine scale heights and space densities. The sample was complete to B = 15.3 mag, with the space density of white dwarfs for Mv of less than 12.25 equal to 0.72 + or 0.25/1000 cu pc, slightly higher than that from the Palomar-Green survey. The average absolute magnitude determined for both the subdwarf O and the subdwarf B stars was considerably fainter than previous determinations, while the scale height for the subdwarfs was lower than earlier suggested values. The space density for the subdwarf O stars is about 7 x 10 to the -7th/cu pc, while the value for the subdwarf B stars is about 2 x 10 to the -6th/cu pc, both values considerably higher than previous estimates. The space densities can be used to place evolutionary constraints on these objects. Publication: The Astrophysical Journal Supplement Series Pub Date: July 1986 DOI: 10.1086/191124 Bibcode: 1986ApJS...61..569D Keywords: Blue Stars; Subdwarf Stars; Ultraviolet Astronomy; White Dwarf Stars; Interstellar Extinction; Photographic Plates; Stellar Magnitude; Astrophysics; STARS: FAINT BLUE; STARS: STELLAR STATISTICS; STARS: SUBDWARFS; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (94) CDS (2) Related Materials (1) Catalog: 2009yCat..20610569D

An observation of the diffuse soft X-ray/extreme-ultraviolet background

Results are reported for observations of diffuse radiation in the spectral bands from 114 to 150 A and from 44 to 120 A, which were made with rocket-borne proportional counters in areas of the sky near galactic coordinates 150 deg longitude, -15 deg latitude and 189 deg longitude, 3 deg latitude. The observed fluxes are 0.125 photon per sq cm/s/sr/eV at 180 eV, which is consistent with previous measurements, and 3.2 photons per sq cm/s/sr/eV at 100 eV, which indicates a steeply rising spectrum. It is noted that these fluxes are significantly smaller than those detected by Yentis et al. (1972), and the differences are attributed to either a terrestrial origin of the fluxes or the direction of observation. Constraints are placed on the parameters of simple emission models of celestial origin, and a rigorous upper limit is determined for the space density of white dwarfs at a blackbody temperature of at least 100,000 K.