Hot DB Research Articles

The chemical structure of the hot pulsating DB white dwarf KIC 08626021 from asteroseismology

AbstractGiammichele et al. (2018) proposed a full determination, largely independent of evolution calculations, of the chemical composition and stratification inside the hot pulsating DB white dwarf KIC 08626021. However, Timmes et al. (2018) pointed out that neglecting the effects of neutrino cooling, such as in the static models used in Giammichele et al. study, could impact significantly the derived seismic solution and compromise conclusions drawn upon it. Here we present a reanalysis of KIC 08626021, using improved static models which now incorporate more realistic luminosity profiles that reflect the still significant energy losses induced by neutrino emission mechanisms in hot DB white dwarfs. We show that this effect has only a limited impact on the derived seismic model properties and, more importantly, that all the conclusions brought by Giammichele et al. (2018) remain entirely valid.

Improved seismic model of the pulsating DB white dwarf KIC 08626021 corrected from the effects of neutrino cooling

Asteroseismology is a powerful tool to unravel the chemical composition and stratification inside white dwarfs, as recently achieved by Giammichele et al. (2018, Nature, 554, 73) for the pulsating DB star KIC 08626021. However, Timmes et al. (2018, ApJ, 867, L30) pointed out that neglecting the effects of neutrino cooling, such as in the models used in Giammichele et al. study, could significantly impact the derived seismic solution and compromise conclusions drawn upon it. In this context, we perform a complete reevaluation of the seismic solution uncovered for KIC 08626021, using improved static models which incorporate more realistic luminosity profiles that reflect the still significant energy losses induced by neutrino emission mechanisms in hot DB white dwarfs. We find that including (or neglecting) neutrino cooling for the specific case of KIC 08626021 induces frequency differences of ∼35 μHz on average (with variations up to ∼84 μHz) for the relevant g-modes, that is, similar to the frequency shifts estimated in Timmes et al. study. However, we show that the propagation of these variations into the derived seismic model properties remain limited and mainly trigger changes of the C/O and C/He composition ratio in the intermediate layers of the seismic model, while other important parameters are only slightly affected. In particular, the derived central oxygen mass fraction and extent of the homogeneous inner part of the core are essentially unchanged. Hence, as found by Timmes et al., seismic investigations of hot pulsating DB white dwarfs that rely on parameterized static models should include the non-negligible effects of neutrino cooling to provide more accurate solutions, but all the important conclusions brought by Giammichele et al. from the analysis of KIC 08626021 remain entirely valid.

Seven-Period Asteroseismic Fit of KIC 8626021

We present a new, better-constrained asteroseismic analysis of the helium-atmosphere (DB) white dwarf discovered in the field of view of the original Kepler mission. Observations obtained over the course of two years yield at least seven independent modes, two more than were found in the discovery paper for the object. With several triplets and doublets, we are able to fix the l and m identification of several modes before performing the fitting, greatly reducing the number of assumptions we must make about mode identification. We find a very thin helium layer for this relative hot DB, which adds evidence to the hypothesis that helium diffuses outward during DB cooling.

SEVEN-PERIOD ASTEROSEISMIC FIT OF THEKEPLERDBV

We present a new, better-constrained asteroseismic analysis of the helium-atmosphere (DB) white dwarf discovered in the field of view of the original Kepler mission. Observations obtained over the course of 2 yr yield at least seven independent modes, two more than were found in the discovery paper for the object. With several triplets and doublets, we are able to fix the l and m identification of several modes before performing the fitting, greatly reducing the number of assumptions we must make about mode identification. We find a very thin helium layer for this relatively hot DB, which adds evidence to the hypothesis that helium diffuses outward during DB cooling. At least a few of the modes appear to be stable on evolutionary timescales and could allow us to obtain a measurement of the rate of cooling with monitoring of the star over the course of the next few years with ground-based follow-up.

Atmospheric parameters and carbon abundance for hot DB white dwarfs

Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near effective temperatures of 25000K are extremely difficult to determine from optical spectroscopy. This is particularly unfortunate, because this is the range of variable DBV or V777 Her stars. Accurate atmospheric parameters are needed to help or confirm the asteroseismic analysis of these objects. Another important aspect is the new class of white dwarfs - the hot DQ - detected by Dufour et al. (2007), with spectra dominated by carbon lines. The analysis shows that their atmospheres are pure carbon. The origin of these stars is not yet understood, but they may have an evolutionary link with the hotter DBs as studied here. Our aim is to determine accurate atmospheric parameters and element abundances and study the implications for the evolution white dwarfs of spectral classes DB and hot DQ. High resolution UV spectra of five DBs are studied with model atmospheres. We determine stellar parameters and abundances or upper limits of C and Si. These objects are compared with cooler DBs below 20000K. We find photospheric C and no other heavy elements - with extremely high limits on the C/Si ratio - in two of the five hot DBs. We compare various explanations for this unusual composition, which have been proposed in the literature: accretion of interstellar or circumstellar matter, radiative levitation, carbon dredge-up from deeper interior below the helium layer, and a residual stellar wind. None of these explanations is completely satisfactory, and the problem of the origin of the hot DQ remains an open question.

MULTIWAVELENGTH OBSERVATIONS OF THE HOT DB STAR PG 0112+104

We present a comprehensive multiwavelength analysis of the hot DB white dwarf PG 0112+104. Our analysis relies on newly-acquired FUSE observations, on medium-resolution FOS and GHRS data, on archival high-resolution GHRS observations, on optical spectrophotometry both in the blue and around Halpha, as well as on time-resolved photometry. From the optical data, we derive a self-consistent effective temperature of 31,300+-500 K, a surface gravity of log g = 7.8 +- 0.1 (M=0.52 Msun), and a hydrogen abundance of log N(H)/N(He) < -4.0. The FUSE spectra reveal the presence of CII and CIII lines that complement the previous detection of CII transitions with the GHRS. The improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 +- 0.23. No photospheric features associated with other heavy elements are detected. We reconsider the role of PG 0112+104 in the definition of the blue edge of the V777 Her instability strip in light of our high-speed photometry, and contrast our results with those of previous observations carried out at the McDonald Observatory.

FUSEObservations of Heavy Elements in the Photospheres of Cool DB White Dwarfs

We present a comprehensive analysis of the far-ultraviolet spectra offive DB white dwarfs spanning the effective temperature range between 14,700 and 20,800 K. The FUSE line analysis shows that carbon features, previously observed in several hot DB stars at or above 22,000 K, are present in the two coolest (GD 408 and GD 378) and in the hottest (G270� 124) target. The observed carbon abundances range from log N(C)/N(He) �� 6: 9t o �� 8:8. In addition, fourofthefiveobjectsdisplayphotosphericlinesof silicon. Otherelementssuchasoxygen,iron,andsulfur are also observed in some objects. The variations of the abundances of heavy elements as a function of effective temperature in DB stars are discussed in terms of a competition between a stellar wind, gravitational settling, accretion from interstellar (and circumstellar) matter, and convective dredge-up. Subject headingg ultraviolet: stars — white dwarfs

Is There Evidence for Rotation or Horizontal Motions in Hot DB White Dwarfs?

In a recent analysis of Goddard High Resolution Spectrograph observations of three hot DB white dwarfs, Provencal et al. suggested that the C II λ1335 doublet in two such objects is abnormally broadened. This was interpreted as the signature of rotation, or perhaps of pulsation-related horizontal motions, at a velocity of v ~ 60 km s-1. Our reanalysis of these observations shows that a self-consistent analysis of the observed C II profiles is possible without calling on additional broadening mechanisms. There is thus currently no evidence for significant rotation velocities or horizontal motions in hot DB white dwarfs.

LB 8827: A Variable, Magnetic DBA White Dwarf

We present accurate CCD spectropolarimetry and spectra that show that the hot DB star LB 8827 (PG 0853+164) is a weakly magnetic, variable DBA white dwarf. We confirm Putney's detection of circular dichroism in the He I lines, but on three occasions we measure a weaker field strength and opposite sign of circular polarization, Be 100 kG. The Balmer line strengths appear to vary with time, indicative of changes in the apparent H/He abundance ratio or irregularities over the stellar surface. Fits to the observed energy distribution and to the optical He I line spectrum with model atmospheres yield effective temperatures in the 21,200-27,700 K range, with lower values favored. This suggests that LB 8827 falls within or near the instability strip of the V777 Herculis stars. High time resolution photometric observations presented here rule out white-light brightness variations with amplitudes larger than 1.2 mmag in the period window 20 s to 2 hr. The possibility that the magnetic field might tend to suppress pulsations is discussed.

Carbon and Hydrogen in Hot DB White Dwarfs

The roles played by carbon and hydrogen in helium atmosphere (DB) white dwarf evolution is not well understood. We present GHRS observations (centered at 1215 and 1335 A) of the hot helium white dwarfs PG 0112+104 (≈30,000 K) and GD 190 (≈23,000 K) and EUVE observations of the well-studied DB GD 358 (≈27,000 K) and revisit pre-COSTAR observations of GD 358. We detect Lyα and carbon absorption features in all three of our hot DB white dwarfs. Using the non-LTE model atmosphere code TLUSTY, we calculate carbon and hydrogen surface abundances and mass fractions. We discuss the implications of our analysis for two aspects of DB evolution. First, we find hydrogen surface abundances corresponding to mass fractions over an order of magnitude too low to explain the observed lack of helium atmosphere white dwarfs between 45,000 and 30,000 K unless we allow extensive mixing in DB atmospheres. Second, the photospheric carbon in all three objects cannot be explained by interstellar accretion or conventional convective dredge-up. We present evidence for substantial horizontal motions in two of our objects that may explain both the presence of carbon and the lower than predicted hydrogen surface abundance.

Spectroscopic Studies of DB White Dwarfs: The Instability Strip of the Pulsating DB (V777 Herculis) Stars

We have secured optical spectra for the eight currently known variable DB, or V777 Her, stars. With the help of a new generation of synthetic spectra, spectroscopic effective temperatures are derived for these objects, as well as for 15 other DB or DBA stars above 20,000 K. We find that the location of the boundaries of the instability strip is sensitive to the atmospheric hydrogen abundance assumed for DB stars: the strip covers the range 22,400-27,800 K if atmospheres of pure helium are used and the range 21,800-24,700 K if undetectable traces of hydrogen are allowed for in the DB models. These determinations provide independent constraints for current seismological analyses of the V777 Her stars. More sensitive searches for weak hydrogen features in hot DB stars should help decide between the two temperature scales.

Temperatures for hot and pulsating DB white dwarfs obtained with the IUE Observatory

view Abstract Citations (98) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Temperatures for Hot and Pulsating DB White Dwarfs Obtained with the IUE Observatory Liebert, James ; Wesemael, F. ; Hansen, C. J. ; Fontaine, G. ; Shipman, Harry L. ; Sion, Edward M. ; Winget, D. E. ; Green, Richard F. Abstract Ultraviolet energy distributions are presented for 12 hot, helium-atmosphere DB white dwarfs, including the four known pulsating stars which define an empirical DB instability strip. Temperatures are derived exclusively from fits to the ultraviolet energy distributions, especially in the λλ1200 - 2000 region which is most sensitive to temperature. Comparison is made between fits using various atmospheric models and with temperatures derived from optical observations. Uncertainties due to the model atmospheres, the IUE observations, and interstellar reddening are discussed. Publication: The Astrophysical Journal Pub Date: October 1986 DOI: 10.1086/164595 Bibcode: 1986ApJ...309..241L Keywords: Hot Stars; Stellar Oscillations; Stellar Spectrophotometry; Stellar Temperature; Ultraviolet Spectra; White Dwarf Stars; Iue; Spectral Energy Distribution; Stellar Atmospheres; Astrophysics; SPECTROPHOTOMETRY; STARS: WHITE DWARFS; ULTRAVIOLET: SPECTRA full text sources ADS | data products SIMBAD (19) MAST (1) INES (1)

Einstein Observations of hot DB white dwarfs

view Abstract Citations (21) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Einstein observations of hot DB white dwarfs. Fontaine, G. ; Montmerle, T. ; Michaud, G. Abstract The observations of 10 hot DB white dwarfs obtained with the IPC instrument aboard the Einstein Observatory are reported. No flux was detected in nine objects yielding upper limits for the soft X-ray luminosity that range from 2 x 10 to the 28th to 8 x 10 to the 28th ergs/s in the 0.1-5 keV bandwidth. A possible detection, at the 2.2 sigma level, was obtained for GD 205 (WD 1709+23) which corresponds to a luminosity of 5.2-11.6 x 10 to the 28th ergs/s. For these helium-rich white dwarfs, the present results remain compatible with the models which require the presence of a corona causing hydrogen-rich stellar winds that may explain the strong hydrogen deficiency of these objects. Publication: The Astrophysical Journal Pub Date: June 1982 DOI: 10.1086/160024 Bibcode: 1982ApJ...257..695F Keywords: B Stars; Hot Stars; Stellar Coronas; Stellar Winds; White Dwarf Stars; X Ray Sources; Data Reduction; Heao 2; Helium; Hydrogen; Stellar Luminosity; Stellar Spectrophotometry; Astronomy full text sources ADS |

Detection of a He I 4517 A absorption feature in the DB white dwarf D 190

The spectrum of GD 190 (EG 193), a strong-lined DB white dwarf, is shown to contain an absorption feature at 4517 A in the redward wing of the He I lambda 4471 line. This feature, which is believed to be due to the forbidden 2 /sup 3/P--4 /sup 3/P transition in He I, has not been detected previously in DB white dwarfs. While its presence is predicted from model atmosphere calculations, agreement between theory and observation remains qualitative. It is possible that the lambda 4517 feature may provide a valuable gravity discriminant at least for hot DB white dwarfs. (auth)