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The cool ZZ Ceti star PG 2303+243: observations and analysis of variability in 2004
PG 2303+243 is a cool DA variable (also called ZZ Ceti) star with a richpulsation spectrum and variable amplitudes. A mini-campaign involvingsix observatories yielded time-resolved photometric measurements of PG2303+243 during the period 2004 September 5-20. A duty cycle of 35 percent was achieved. We detected 24 possible independent frequencies,their amplitudes and phases for future mode identification. We confirmthe occurrence of short-term amplitude and frequency variations. Ouranalysis suggests an l= 1 rotational splitting around 8.4 ?Hz,implying a rotation period of 16.5 h.
| The inverse problem of the theory of degenerate dwarfs
Based on the radii and masses of degenerate dwarfs derived fromHIPPARCOS and other observations, we estimate the microscopic parametersof a Chandrasekhar model (the relativistic parameter at the stellarcenter x 0, and the chemical-composition parameterµ e = A/Z, where A is the mass number and Z is thenuclear charge). We have obtained analytical expressions for themacroscopic characteristics (mass, radius, energy) as functions of x0 and µ e . From the calculated dependenceof the energy on these parameters, we have found constraints on therange of variability of x 0, which are in good agreement withthe observed radius distribution of dwarfs. The critical value of x0 at which stability breaks down due to general-relativisitceffects is found more accurately than previously. We propose ageneralized model with an inhomogeneous (coordinate-dependent) chemicalcomposition, with µ e = µ e ( r).
| Lyman ? wing absorption in cool white dwarf stars
Kowalski & Saumon identified the missing absorption mechanism in theobserved spectra of cool white dwarf stars as the Lyman ? red wingformed by the collisions between atomic and molecular hydrogen andsuccessfully explained entire spectra of many cool DA-type white dwarfs.Owing to the important astrophysical implications of this issue, wepresent here an independent assessment of the process. For this purpose,we compute free-free quasi-molecular absorption in Lyman ? due tocollisions with H and H2 within the one-perturber,quasi-static approximation. Line cross-sections are obtained usingtheoretical molecular potentials to describe the interaction between theradiating atom and the perturber. The variation in the electric dipoletransition moment with the interparticle distance is also considered.Six and two allowed electric dipole transitions due to H-H andH-H2 collisions, respectively, are taken into account. Thenew theoretical Lyman ? line profiles are then incorporated in ourstellar atmosphere program for the computation of synthetic spectra andcolours of DA-type white dwarfs. Illustrative model atmospheres andspectral energy distributions are computed, which show that Lyman? broadening by atoms and molecules has a significant effect onthe white dwarf atmosphere models. The inclusion of thiscollision-induced opacity significantly reddens spectral energydistributions and affects the broad-band colour indices for modelatmospheres with Teff < 5000 K. These results confirmthose previously obtained by Kowalski & Saumon. Our study points outthe need for reliable evaluations of H3 potential energysurfaces covering a large region of nuclear configurations, in order toobtain a better description of H-H2 collisions and a moreaccurate evaluation of their influence on the spectrum of cool whitedwarfs.
| Identification of blue high proper motion objects in the Tycho-2 and 2MASS catalogues using Virtual Observatory tools
Aims: With available Virtual Observatory tools, we looked for newbright blue high proper motion objects in the entire sky: white dwarfs,hot subdwarfs, runaway OB stars, and early-type stars in nearby youngmoving groups. Methods: We performed an all-sky cross-matchbetween the optical Tycho-2 and near-infrared 2MASS catalogues withAladin, and selected objects with proper motions ? > 50 masyr-1 and colours VT - Ks < -0.5 magwith TOPCAT. We also collected multi-wavelength photometry, constructedthe spectral energy distributions and estimated effective temperaturesfrom fits to atmospheric models with VOSA for the most interestingtargets. Results: We assembled a sample of 32 bright blue highproper motion objects, including ten sdO/B subdwarfs, nine DA whitedwarfs, five young early-type stars (two of which are runaway stars),two blue horizontal branch stars, one star with poor information, andfive objects reported for the first time in this work. These last fiveobjects have magnitudes BT ? 11.0-11.6 mag, effectivetemperatures Teff ? 24 000-30 000 K, and are located inthe region of known white dwarfs and hot subdwarfs in a reduced propermotion-colour diagram. We confirmed the hot subdwarf nature of one ofthe new objects, Albus 5, with public far-ultraviolet spectroscopic dataobtained with FUSE.
| Evolutionary and pulsational properties of white dwarf stars
White dwarf stars are the final evolutionary stage of the vast majorityof stars, including our Sun. Since the coolest white dwarfs are very oldobjects, the present population of white dwarfs contains a wealth ofinformation on the evolution of stars from birth to death, and on thestar formation rate throughout the history of our Galaxy. Thus, thestudy of white dwarfs has potential applications in different fields ofastrophysics. In particular, white dwarfs can be used as independentreliable cosmic clocks, and can also provide valuable information aboutthe fundamental parameters of a wide variety of stellar populations,such as our Galaxy and open and globular clusters. In addition, the highdensities and temperatures characterizing white dwarfs allow these starsto be used as cosmic laboratories for studying physical processes underextreme conditions that cannot be achieved in terrestrial laboratories.Last but not least, since many white dwarf stars undergo pulsationalinstabilities, the study of their properties constitutes a powerful toolfor applications beyond stellar astrophysics. In particular, whitedwarfs can be used to constrain fundamental properties of elementaryparticles such as axions and neutrinos and to study problems related tothe variation of fundamental constants. These potential applications ofwhite dwarfs have led to renewed interest in the calculation of verydetailed evolutionary and pulsational models for these stars. In thiswork, we review the essentials of the physics of white dwarf stars. Weenumerate the reasons that make these stars excellent chronometers, andwe describe why white dwarfs provide tools for a wide variety ofapplications. Special emphasis is placed on the physical processes thatlead to the formation of white dwarfs as well as on the different energysources and processes responsible for chemical abundance changes thatoccur along their evolution. Moreover, in the course of their lives,white dwarfs cross different pulsational instability strips. Theexistence of these instability strips provides astronomers with a uniqueopportunity to peer into their internal structure that would otherwiseremain hidden from observers. We will show that this allows one tomeasure stellar masses with unprecedented precision and to infer theirenvelope thicknesses, to probe the core chemical stratification, and todetect rotation rates and magnetic fields. Consequently, in this work,we also review the pulsational properties of white dwarfs and the mostrecent applications of white dwarf asteroseismology.
| New Chemical Profiles for the Asteroseismology of ZZ Ceti Stars
We compute new chemical profiles for the core and envelope of whitedwarfs appropriate for pulsational studies of ZZ Ceti stars. Theseprofiles are extracted from the complete evolution of progenitor stars,evolved through the main sequence and the thermally pulsing asymptoticgiant branch (AGB) stages, and from time-dependent element diffusionduring white dwarf evolution. We discuss the importance of theinitial-final mass relationship for the white dwarf carbon-oxygencomposition. In particular, we find that the central oxygen abundancemay be underestimated by about 15% if the white dwarf mass is assumed tobe the hydrogen-free core mass before the first thermal pulse. We alsodiscuss the importance for the chemical profiles expected in theoutermost layers of ZZ Ceti stars of the computation of the thermallypulsing AGB phase and of the phase in which element diffusion isrelevant. We find a strong dependence of the outer layer chemicalstratification on the stellar mass. In particular, in the less massivemodels, the double-layered structure in the helium layer built up duringthe thermally pulsing AGB phase is not removed by diffusion by the timethe ZZ Ceti stage is reached. Finally, we perform adiabatic pulsationcalculations and discuss the implications of our new chemical profilesfor the pulsational properties of ZZ Ceti stars. We find that the wholeg-mode period spectrum and the mode-trapping properties of thesepulsating white dwarfs as derived from our new chemical profiles aresubstantially different from those based on chemical profiles widelyused in existing asteroseismological studies. Thus, we expect theasteroseismological models derived from our chemical profiles to besignificantly different from those found thus far.
| Multi-site Observations of Pulsation in the Accreting White Dwarf SDSS J161033.64-010223.3 (V386 Ser)
Non-radial pulsations in the primary white dwarfs of cataclysmicvariables can now potentially allow us to explore the stellar interiorof these accretors using stellar seismology. In this context, weconducted a multi-site campaign on the accreting pulsator SDSSJ161033.64-010223.3 (V386 Ser) using seven observatories located aroundthe world in 2007 May over a duration of 11 days. We report the best-fitperiodicities here, which were also previously observed in 2004,suggesting their underlying stability. Although we did not uncover asufficient number of independent pulsation modes for a uniqueseismological fit, our campaign revealed that the dominant pulsationmode at 609 s is an evenly spaced triplet. The even nature of thetriplet is suggestive of rotational splitting, implying an enigmaticrotation period of about 4.8 days. There are two viable alternativesassuming the triplet is real: either the period of 4.8 days isrepresentative of the rotation period of the entire star withimplications for the angular momentum evolution of these systems, or itis perhaps an indication of differential rotation with a fast rotatingexterior and slow rotation deeper in the star. Investigating thepossibility that a changing period could mimic a triplet suggests thatthis scenario is improbable, but not impossible. Using time-seriesspectra acquired in 2009 May, we determine the orbital period of SDSSJ161033.64-010223.3 to be 83.8 ± 2.9 minutes. Three of theobserved photometric frequencies from our 2007 May campaign appear to belinear combinations of the 609 s pulsation mode with the first harmonicof the orbital period at 41.5 minutes. This is the first discovery of alinear combination between non-radial pulsation and orbital motion for avariable white dwarf.
| The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopicsubtypes, and stellar population subcomponents of the white dwarfswithin 20 pc of the Sun. We find no convincing evidence of halo whitedwarfs in the total 20 pc sample of 129 white dwarfs nor is thereconvincing evidence of genuine thick disk subcomponent members within 20parsecs. Virtually, the entire 20 pc sample likely belongs to the thindisk. The total DA to non-DA ratio of the 20 pc sample is 1.6, amanifestation of deepening envelope convection which transforms DA starswith sufficiently thin H surface layers into non-DAs. The addition offive new stars to the 20 pc sample yields a revised local space densityof white dwarfs of (4.9 ± 0.5) × 10–3pc–3 and a corresponding mass density of (3.3 ±0.3) × 10–3 M sunpc–3. We find that at least 15% of the white dwarfswithin 20 parsecs of the Sun (the DAZ and DZ stars) have photosphericmetals that possibly originate from accretion of circumstellar material(debris disks) around them. If this interpretation is correct, thissuggests the possibility that the same percentage have planets orasteroid-like bodies orbiting them.
| Seismological studies of ZZ Ceti stars - II. Application to the ZZ Ceti class
We used the detected pulsation modes and adiabatic pulsation models todo seismology of the class of ZZ Ceti stars and measure the H layer massfor 83 stars. We found the surface hydrogen layer to be within the range10-9.5 >= MH/M* >=10-4, with an average of MH/M* =10-6.3, which is thinner than the predicted value ofMH/M* = 10-4, indicating that the starslose more mass during their evolution than previously expected. Theseresults are preliminary and do not include the possible effects ofrealistic C/O profiles on the fits.
| Catalogue of averaged stellar effective magnetic fields - II. Re-discussion of chemically peculiar A and B stars
This paper presents a catalogue and the method of determining averagedquadratic effective magnetic fields for 1212main-sequence and giant stars, and 11 white dwarf stars. The catalogueincludes stars that are members of several open clusters. We havecompiled measurements of the longitudinal magnetic field for thosestars, which were scattered in the existing literature. A new parameter,magnetization (MA), has been defined, and we present values of MA forstars of various spectral classes. Our sample includes a subset of 610chemically peculiar early-type stars. We confirm the conclusion of ourprevious study that the number distribution of all chemically peculiarstars versus the averaged magnetic field strength is described by adecreasing exponential function. Relations of this type also hold forstars of all the analysed subclasses of chemical peculiarity.Magnetization tends to correlate with the effective temperature only athigh MA, for He-weak and He-rich stars.
| The Pulsating White Dwarf Stars
We present a summary of what is currently known about the three distinctfamilies of isolated pulsating white dwarfs. These are the GW Vir stars(He/C/O-atmosphere stars with Teff ? 120,000 K), theV777 Her stars (He-atmosphere, Teff ? 25,000 K), and the ZZ Cetistars (H-atmosphere, Teff ? 12,000 K), all showingmultiperiodic luminosity variations caused by low-order and low-degreeg-mode instabilities. We also provide, in an Appendix, a very briefoverview of the newly found evidence in favor of the existence of afourth category of oscillating white dwarfs bearing strong similaritieswith these families of pulsators. We begin our survey with a shorthistorical introduction, followed by a general discussion of pulsatingwhite dwarfs as compact pulsators. We then discuss the class propertiesof these objects, including an updated census. We next focus on theinstability domains for each family of pulsators in the log g -Teff diagram, and present their time-averaged properties inmore detail. This is followed by a section on excitation physics, i.e.,the causes of the pulsational instabilities, with emphasis on the commonproperties of the different types of pulsator. We then discuss thetime-dependent properties of the pulsating white dwarfs featuring, amongother things, a brief "picture tour" across the ZZ Ceti instabilitystrip. We next review the methods used to infer or constrain the angulargeometry of a pulsation mode in a white dwarf. These include multicolorphotometry and time-resolved spectroscopy, the exploitation of thenonlinear features in the observed light curves, and rotationalsplitting. We also consider basic adiabatic asteroseismology startingwith a discussion of the reaction of the period spectrum to variationsof model parameters. We next review the various asteroseismologicalinferences that have so far been claimed for white dwarfs. We alsodiscuss the potential of exploiting the rates of period change. Wefinally provide some concluding remarks, including a list with severalsuggestions for future progress in the field.
| A Far Ultraviolet Archival Study of Cataclysmic Variables. I. FUSE and HST STIS Spectra of the Exposed White Dwarf in Dwarf Nova Systems
We present a synthetic spectral analysis of Far UltravioletSpectroscopic Explorer (FUSE) and Hubble Space Telescope Space TelescopeImaging Spectrograph (HST STIS) spectra of five dwarf novae above andbelow the period gap during quiescence. We use our synthetic spectralcode, including options for the treatment of the hydrogenquasi-molecular satellite lines (for low-temperature stellaratmospheres), non-LTE (NLTE) approximation (for high-temperature stellaratmospheres), and for one system (RU Peg) we model the interstellarmedium (ISM) molecular and atomic hydrogen lines. In all the systemspresented here the FUV flux continuum is due to the white dwarf (WD).These spectra also exhibit some broad emission lines. In this work weconfirm some of the previous FUV analysis results, but we also presentnew results. For four systems we combine the FUSE and STIS spectra tocover a larger wavelength range and to improve the spectral fit. Thiswork is part of our broader HST archival research program, in which weaim to provide accurate system parameters for cataclysmic variablesabove and below the period gap by combining FUSE and HST FUV spectra.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| a New Look at the Local White Dwarf Population
We have conducted a detailed new survey of the local population of whitedwarfs lying within 20 pc of the Sun. A new revised catalog of localwhite dwarfs containing 122 entries (126 individual degenerate stars) ispresented. This list contains 27 white dwarfs not included in a previouslist from 2002, as well as new and recently published trigonometricparallaxes. In several cases new members of the local white dwarfpopulation have come to light through accurate photometric distanceestimates. In addition, a suspected new double degenerate system (WD0423+120) has been identified. The 20 pc sample is currently estimatedto be 80% complete. Using a variety of recent spectroscopic,photometric, and trigonometric distance determinations, we re-compute aspace density of 4.8 ± 0.5 × 10 3 pc3 corresponding to a mass density of 3.2 ± 0.3 ×10 3 M sun pc 3 from the completeportion of the sample within 13 pc. We find an overall mean mass for thelocal white dwarfs of 0.665 M sun, a value larger than mostother non-volume-limited estimates. Although the sample is small, wefind no evidence of a correlation between mass and temperature in whichwhite dwarfs below 13,000 K are systematically more massive than thoseabove this temperature. Within 20 pc 25% of the white dwarfs are inbinary systems (including double degenerate systems). Approximately 6%are double degenerates and 6.5% are Sirius-like systems. The fraction ofmagnetic white dwarfs in the local population is found to be 13%.
| Seismological studies of ZZ Ceti stars - I. The model grid and the application to individual stars
We calculate and explore an extensive adiabatic model grid for pulsatingwhite dwarfs with hydrogen-dominated atmospheres, the ZZ Ceti stars. Wealso compared the computed modes with the observed ones for five ZZ Cetistars that are a representative sample of the whole class of pulsators.We describe our new approach for seismological studies, using therelative observed amplitudes to give weights for the periods in the fitand the external mass and temperature determinations as a guide. Ourseismological study is clear evidence that seismology is indeed apowerful tool in the study of stellar structure and evolution.
| The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing
We determine the ratio of helium- to hydrogen-atmosphere white dwarfstars as a function of effective temperature from a model atmosphereanalysis of the infrared photometric data from the Two Micron All SkySurvey combined with available visual magnitudes. Our study surpassesany previous analysis of this kind, both in terms of the accuracy of theTeff determinations and the size of the sample. We observethat the ratio of helium- to hydrogen-atmosphere white dwarfs increasesgradually from a constant value of ~0.25 between Teff=15,000and 10,000 K to a value twice as large in the range 10,000K>Teff>8000 K, suggesting that convective mixing, whichoccurs when the bottom of the hydrogen convection zone reaches theunderlying convective helium envelope, is responsible for this gradualtransition. The comparison of our results with an approximate model usedto describe the outcome of this convective mixing process implieshydrogen mass layers in the rangeMH/Mtot=10-10 to 10-8 forabout 15% of the DA stars that survived the DA-to-DB transition nearTeff~30,000 K, the remainder having presumably more massivelayers above MH/Mtot~10-6.
| A Comparative Study of Optical and Ultraviolet Effective Temperatures for DA White Dwarfs from the IUE Archive
We present a comparative study of effective temperatures determined fromthe hydrogen Balmer lines and from the UV energy distribution for 140 DAwhite dwarfs drawn from the International Ultraviolet Explorer (IUE)archive. Our results indicate that the optical and UV temperatures ofthe majority of stars below Teff~40,000 K and within ~75 pcare in fairly good agreement given the uncertainties. At highertemperatures and/or larger distances, however, significant discrepanciesare observed. Several mechanisms are investigated to account for thesediscrepancies, including the effect of interstellar reddening, thepresence of metals in the photosphere, and the existence of unresolvedbinary white dwarfs. The results of our analysis reveal thatwavelength-dependent extinction is the most natural explanation for theobserved temperature differences. We also attempt to predict thedifferences in optical and UV temperatures expected from unresolveddegenerate binaries by performing an exhaustive simulation of compositemodel spectra. In light of these simulations, we then discuss some knowndouble degenerates and identify new binary candidates by restricting ouranalysis to stars located within 75 pc, where the effect of interstellarreddening is significantly reduced.
| Albus 1: A Very Bright White Dwarf Candidate
We have serendipitously discovered a previously unknown, bright source(BT=11.75+/-0.07 mag) with a very blueVT-Ks color, which we have named Albus 1. Aphotometric and astrometric study using Virtual Observatory tools hasshown that it possesses an appreciable proper motion and magnitudes andcolors very similar to those of the well-known white dwarf G191-B2B. Weconsider Albus 1 as a DA-type white dwarf located at about 40 pc. If itsnature is confirmed, Albus 1 would be the sixth brightest isolated whitedwarf in the sky, which would make it an excellent spectrophotometricstandard.
| A Spitzer White Dwarf Infrared Survey
We present mid-infrared photometry of 124 white dwarf stars with theSpitzer Space Telescope. Objects were observed simultaneously at 4.5 and8.0 ?m with sensitivities better than 0.1 mJy. This data set can beused to test models of white dwarf atmospheres in a new wavelengthregime, as well as to search for planetary companions and debris disks.
| The mass and luminosity functions and the formation rate of DA white dwarfs in the Sloan Digital Sky Survey
Aims.The SDSS Data Release 1 includes 1833 DA white dwarfs (WDs) andforms the largest homogeneous sample of WDs. This sample provides thebest opportunity to study the statistical properties of WDs. Methods:We adopt a recently established theoretical model to calculate the massand distance of each WD using the observational data. Then we adopt abin-correction method to correct for selection effects and use the 1/Vweight-factor method to calculate the luminosity function, thecontinuous mass function and the formation rate of these WDs. Results:The SDSS DA WD sample is incomplete and suffers seriously from selectioneffects. After corrections for the selection effects, only 531 WDsremain. From this final sample we derive the most up-to-date luminosityfunction and mass function, in which we find a broad peak of WD massescentered around 0.58 Mȯ. The DA WD space density iscalculated as 8.81×10-5 pc-3 and theformation rate is 2.579×10-13 pc-3yr-1. Conclusions: The statistical properties of the SDSS DAWD sample are generally in good agreement with previous observationaland theoretical studies, and provide us information on the formation andevolution of WDs. However, a larger and more complete all-sky WD sampleis still needed to explain some subtle disagreements and unresolvedissues.Full Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/627
| Infrared Photometric Analysis of White Dwarfs from the Two Micron All Sky Survey and the Spitzer Space Telescope
We review the available near- and mid-infrared photometry for whitedwarfs obtained from the Two Micron All Sky Survey (2MASS) and by theSpitzer Space Telescope. Both data sets have recently been used to seekwhite dwarfs with infrared excesses due to the presence of unresolvedcompanions or circumstellar disks, and also to derive the atmosphericparameters of cool white dwarfs. We first attempt to evaluate thereliability of the 2MASS photometry by comparing it with an independentset of published JHK CIT magnitudes for 160 cool white dwarf stars, andalso by comparing the data with the predictions of detailed modelatmosphere calculations. The possibility of using 2MASS to identifyunresolved M dwarf companions or circumstellar disks is then discussed.We also revisit the analysis of 46 binary candidates from Wachter et al.using the synthetic flux method and confirm the large near-infraredexcesses in most objects. We perform a similar analysis by fittingSpitzer 4.5 and 8 ?m photometric observations of white dwarfs withour grid of model atmospheres, and demonstrate the reliability of boththe Spitzer data and the theoretical calculations up to 8 ?m.Finally, we search for massive disks resulting from the merger of twowhite dwarfs in a 2MASS sample composed of 57 massive degenerates, andshow that massive disks are uncommon in such stars.
| Towards a pure ZZ Ceti instability strip
Context: We have observed again two stars inside the ZZ Ceti instabilitystrip that were previously classified as not-observed-to-vary (NOV) byMukadam et al. (2004) and found them to be low-amplitude variables. Someevidence points to a pure ZZ Ceti instability strip; other evidencecontests it. Aims: The two stars previously classified as NOVhave Sloan Digital Sky Survey (SDSS) spectroscopic effectivetemperatures that place them inside the ZZ Ceti instability strip, andthey were "contaminating" the strip as constant stars, which couldindicate that the instability strip was no longer a simple evolutionarystage. A pure instability strip indicates that pulsation is a normalphase which all DAs must go through. Methods: We used effectivetemperatures derived from SDSS optical spectra by comparing them withmodel atmospheres to look for pulsators through time-resolved photometryand stars previously classified as NOV. Results: Our new resultsindicate, but do not prove, a pure instability strip, because there arestill other NOV stars that need to be observed again. Additionally, wehave discovered five other ZZ Ceti stars based on their effectivetemperatures.Partially based on observations at Observatório do Pico dosDias/LNA; the Southern Astrophysical Research telescope, a collaborationbetween CNPq-Brazil, NOAO, UNC, and MSU; and McDonald Observatory of TheUniversity of Texas at Austin.
| A Search for Kilogauss Magnetic Fields in White Dwarfs and Hot Subdwarf Stars
We present new results of a survey for weak magnetic fields among DAwhite dwarfs, including some brighter hot subdwarf stars. We havedetected variable circular polarization in the H? line of the hotsubdwarf star Feige 34 (spectroscopic type: sdO). From these data, weestimate that the longitudinal magnetic field of this star varies from-1.1+/-3.2 to +9.6+/-2.6 kG, with a mean of about +5 kG and a periodlonger than 2 hr. In this study, we also confirm the magnetic nature ofwhite dwarf WD 1105-048, found earlier in a study by Aznar Cuadrado andcoworkers, and present upper limits of kilogauss longitudinal magneticfields of the five brightest DA white dwarfs. Our data support thefinding of Aznar Cuadrado and coworkers that ~25% of white dwarfs havekilogauss magnetic fields. This frequency also confirms results of earlyestimates obtained using the magnetic field function of white dwarfs(Fabrika & Valyavin).
| The ZZ Ceti star G 185-32: new insight from asteroseismology
One of the brightest pulsating DA white dwarfs (ZZ Ceti stars), G185-32, shows 19 periods in its power spectrum, from 71 s to 651 s. Itsshort periods are among the shortest ones observed in a ZZ Ceti star.The one at 141.9 s shows an amplitude which does not vary withwavelength, in contrast with what the linear theory predicts for ?=1or ?=2 non-radial g-modes. The mass and T_eff of G 185-32 are wellconstrained by spectroscopic observations and parallax measurement. Theaim of the present paper is to take advantage of this rich observationalbackground to derive the fundamental parameters of the star fromasteroseismology. We computed a grid of realistic models for G 185-32and the periods of their adiabatic ?=1 and ?=2 non-radialg-modes. We determined the model which fits best the observations. Weidentify 14 periods as real modes and determine their ?, k, and mvalues. We show that the remaining 5 periods are linear combinations.Wefind that G 185-32 oscillates mostly according to ?=2 modes. Wepresent a new hypothesis to account for the peculiar behaviour of the141.9 s mode's amplitude and suggest that it is a true mode interferingwith higher ? degree modes, involving a possible triplet resonance.We determine the main parameters of G 185-32: its total massM*=0.638 (±0.007) M&sun;, its hydrogen massfraction M_H= 1.70 (±0.10) × 10-4 M*,which implies a helium mass fraction of ?10-2 M{*}, itsT_eff= 12 280 (±)80 K, and estimate its rotation period to be14.5 h.
| Ensemble Characteristics of the ZZ Ceti Stars
We present the observed pulsation spectra of all known noninteracting ZZCeti stars (hydrogen atmosphere white dwarf variables [DAVs]) andexamine changes in their pulsation properties across the instabilitystrip. We confirm the well-established trend of increasing pulsationperiod with decreasing effective temperature across the ZZ Cetiinstability strip. We do not find a dramatic order-of-magnitude increasein the number of observed independent modes in ZZ Ceti stars, traversingfrom the hot to the cool edge of the instability strip; we find that thecool DAVs have one more mode on average than the hot DAVs. We confirmthe initial increase in pulsation amplitude at the blue edge and findstrong evidence of a decline in amplitude prior to the red edge. Wepresent the first observational evidence that ZZ Ceti stars losepulsation energy just before pulsations shut down at the empirical rededge of the instability strip.
| Whole Earth telescope observations of the ZZ Ceti star HL Tau 76
This paper analyses the Whole Earth Telescope observations of HL Tau 76,the first discovered pulsating DA white dwarf. The star was observedduring two Whole Earth Telescope campaigns. It was a second prioritytarget during the XCOV13 campaign in 1996 and the first priority oneduring the XCOV18 campaign in 1999. The 1999 campaign reached 66.5% dutycycle. With a total duration of 18 days, the frequency resolutionachieved is 0.68 ?Hz. With such a frequency resolution, we were ableto find as many as 78 significant frequencies in the power spectrum, ofwhich 34 are independent frequencies after removal of all linearcombinations. In taking into account other frequencies present duringthe 1996 WET campaign and those present in earlier data, which do notshow up in the 1999 data set, we find a total of 43 independentfrequencies. This makes HL Tau 76 the richest ZZ Ceti star in terms ofnumber of observed pulsation modes. We use those pulsation frequenciesto determine as much as possible of the internal structure of HL Tau 76.The pulsations in HL Tau 76 cover a wide range of periods between 380 sand 1390 s. We propose an identification for 39 of those 43 frequenciesin terms of ?=1 and ?=2 non-radial g-modes split by rotation. Wederive an average rotation period of 2.2 days. The period distributionof HL Tau 76 is best reproduced if the star has a moderately "thick"hydrogen mass fraction log qH ? -7.0. The resultspresented in this paper constitute a starting point for a detailedcomparison of the observed periods with the periods calculated formodels as representative as possible of HL Tau 76.
| Asteroseismological constraints on the structure of the ZZ Ceti star HL Tau 76
This paper reports the results derived from an asteroseismological studyof the cool ZZ Ceti star HL Tau 76. A grid of models has been computedin a parameter space covering the range of log g and Teff,formerly determined by spectroscopy, and a large range of hydrogen massfraction. The adiabatic non-radial oscillations for all the models havebeen computed for the modes of degrees ?=1 and ?=2. An algorithmbased on a ? 2 test was applied to evaluate the qualityof the fit between observed and theoretical periods. This methodresulted in selecting a best fitting model for which the averagerelative matching of the periods is 0.7%. Then, a detailed comparisonbetween the observed and the computed periods for the ?=1 and?=2 modes of the best fitting model was achieved in order toidentify as many observed modes as possible. To perform thisidentification we used the calculated periods for which we applied therotational splitting as deduced from the observations. Through thisprocess we identify the 36 independent modes observed in HL Tau 76. Thebest fitting model for HL Tau 76 is well constrained due to the largenumber of oscillations detected in this ZZ Ceti star. The main stellarparameters of HL Tau 76 derived from this analysis are: the total massMstar=0.575 ±0.005 M&sun;, the hydrogenmass fraction qH, estimated as thick as 2.35±0.10× 10-4. The helium mass fraction consistentwith qH must be qHe=~1× 10-2. Themethod is not sensitive to Teff variations in the narrowdomain of temperature derived from spectroscopy for HL Tau 76. The bestadjustment is found however for Teff= 11 375 K. The otherderived stellar parameters are the luminosity (L/L&sun;=0.00389) and the radius (R/R&sun;= 0.0162). We note sometrends in the fit of the observed periods with the computed ones whichsuggest that the rotational splitting could be non-uniform and that thelarge amplitude modes might contain information on the convection-drivenexcitation mechanism.
| Preliminary seismology of the DA white dwarf G 185-32 .
G 185-32 is a pulsating hydrogen atmosphere white dwarf with arelatively rich pulsation spectrum of 14 modes identified from WholeEarth Telescope data. Castanheira et al. (2004) provide some suggestedmode identifications, along with a recent Teff and log gvalue of 11,960 K and 8.02± 0.04. We present our preliminaryseismological results based on evolutionary white dwarf models thatmatch the observed Teff and log g value. We are able to fitmost of the observed periods to within about 1&%slash; in period(1-3 s) using models having masses of 0.60 to 0.65 M&sun;.The best fitting hydrogen layer mass is either about 10-4M&sun; if the 72.5 s mode is ? =2 or about10-6 M&sun; if the 72.5 s mode is ? =4.
| Low-Luminosity Companions to White Dwarfs
This paper presents results of a near-infrared imaging survey forlow-mass stellar and substellar companions to white dwarfs. A wide-fieldproper-motion survey of 261 white dwarfs was capable of directlydetecting companions at orbital separations between ~100 and 5000 AUwith masses as low as 0.05 Msolar, while a deep near-fieldsearch of 86 white dwarfs was capable of directly detecting companionsat separations between ~50 and 1100 AU with masses as low as 0.02Msolar. Additionally, all white dwarf targets were examinedfor near-infrared excess emission, a technique capable of detectingcompanions at arbitrarily close separations down to masses of 0.05Msolar. No brown dwarf candidates were detected, whichimplies a brown dwarf companion fraction of <0.5% for white dwarfs.In contrast, the stellar companion fraction of white dwarfs as measuredby this survey is 22%, uncorrected for bias. Moreover, most of the knownand suspected stellar companions to white dwarfs are low-mass starswhose masses are only slightly greater than the masses of brown dwarfs.Twenty previously unknown stellar companions were detected, five ofwhich are confirmed or likely white dwarfs themselves, while 15 areconfirmed or likely low-mass stars. Similar to the distribution of coolfield dwarfs as a function of spectral type, the number of coolunevolved dwarf companions peaks at mid-M type. Based on the presentwork, relative to this peak, field L dwarfs appear to be roughly 2-3times more abundant than companion L dwarfs. Additionally, there is noevidence that the initial companion masses have been altered bypost-main-sequence binary interactions.
| Mode Identification from Combination Frequency Amplitudes in ZZ Ceti Stars
The light curves of variable DA stars are usually multiperiodic andnonsinusoidal, so that their Fourier transforms show peaks ateigenfrequencies of the pulsation modes and at sums and differences ofthese frequencies. These combination frequencies provide extrainformation about the pulsations, both physical and geometrical, that islost unless they are analyzed. Several theories provide a context forthis analysis by predicting combination frequency amplitudes. In thesetheories, the combination frequencies arise from nonlinear mixing ofoscillation modes in the outer layers of the white dwarf, so theiranalysis cannot yield direct information on the global structure of thestar as eigenmodes provide. However, their sensitivity to mode geometrydoes make them a useful tool for identifying the spherical degree of themodes that mix to produce them. In this paper we analyze data from eighthot, low-amplitude DAV white dwarfs and measure the amplitudes ofcombination frequencies present. By comparing these amplitudes to thepredictions of the theory of Goldreich & Wu, we have verified thatthe theory is crudely consistent with the measurements. We have alsoinvestigated to what extent the combination frequencies can be used tomeasure the spherical degree (l) of the modes that produce them. We findthat modes with l>2 are easily identifiable as high l based on theircombination frequencies alone. Distinguishing between l=1 and 2 is alsopossible using harmonics. These results will be useful for conductingseismological analysis of large ensembles of ZZ Ceti stars, such asthose being discovered using the Sloan Digital Sky Survey. Because thismethod relies only on photometry at optical wavelengths, it can beapplied to faint stars using 4 m class telescopes.
| Measuring the Evolution of the Most Stable Optical Clock G 117-B15A
We report our measurement of the rate of change of period with time(P?) for the 215 s periodicity in the pulsating white dwarf G117-B15A, the most stable optical clock known. After 31 years ofobservations, we have finally obtained a 4 ? measurementP?observed=(4.27+/-0.80)×10-15 ss-1. Taking into account the proper-motion effect ofP?proper=(7.0+/-2.0)×10-16 ss-1, we obtain a rate of change of period with time ofP?=(3.57+/-0.82)×10-15 s s-1. This valueis consistent with the cooling rate in our white dwarf models only forcores of C or C/O. With the refinement of the models, the observed rateof period change can be used to accurately measure the ratio of C/O inthe core of the white dwarf.
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