ZZ Ceti Stars Research Articles

Modelling compact pulsators

We currently know of five distinct classes of compact pulsators, loosely defined as oscillating stars with a surface gravity above log g = 5. Three of these fall into the white dwarf regime (GW Vir, V777 Her and ZZ Ceti stars), while the other two are identified with hot B subdwarfs (EC 14026 and PG 1716 stars). In all cases, the instabilities are thought to be associated with the partial ionisation of the envelope constituents. We discuss, for each type of pulsator, our current theoretical understanding of the observed instability strips as well as the details of the driving mechanism at work. In terms of attempts at a quantitative exploitation of the observed period spectra through asteroseismology, we focus on the particularly successful case of the rapidly pulsating subdwarf B (EC 14026) stars. So far, we have been able to constrain the fundamental stellar parameters for 12 targets. This is sufficient for first comparisons with different evolutionary scenarios, and will likely play a crucial part in an eventual understanding of the formation of these objects.

Pulsations of the Low-Mass ZZ Ceti Star HS 1824+6000

ABSTRACTMeasuring g-mode pulsations of isolated white dwarfs can reveal their interior properties to high precision. With a spectroscopic mass of ≈0.51 M⊙ (log g = 7.82), the DAV white dwarf HS 1824+6000 is near the transition between carbon/oxygen-core and helium-core white dwarfs, motivating our photometric search for additional pulsations from the Palomar 60-inch telescope. We confirmed (with much greater precision) the three frequencies: 2.751190 ± 0.000010 mHz (363.479 sec), 3.116709 ± 0.000006 mHz (320.851 sec), 3.495113 ± 0.000009 mHz (286.114 sec), previously found by B. Voss and collaborators, and found an additional pulsation at 4.443120 ± 0.000012 mHz (225.067 sec). These observed frequencies are similar to those found in other ZZ Ceti white dwarfs of comparable mass (e.g. log g < 8). We hope that future observations of much lower-mass ZZ Ceti stars (< 0.4 M⊙) will reveal pulsational differences attributable to a hydrogen covered helium core.

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 pulsating white dwarfs with hydrogen-dominated atmospheres, the ZZ Ceti stars. We also compared the computed modes with the observed ones for five ZZ Ceti stars that are a representative sample of the whole class of pulsators. We describe our new approach for seismological studies, using the relative observed amplitudes to give weights for the periods in the fit and the external mass and temperature determinations as a guide. Our seismological study is clear evidence that seismology is indeed a powerful tool in the study of stellar structure and evolution.

Spectroscopic evidence of velocity fields at the surface of ZZ Ceti stars

Aims. High resolution spectra of DA and DAZ white dwarfs show peculiar line profiles for objects in or near the ZZ Ceti instability strip. The lines are broader and shallower than in slightly hotter or cooler objects. Rotational broadening could produce a similar effect but several of the objects are known to be very slow rotators. We therefore study here whether these profiles could be caused by velocity fields on the surface originating from the pulsation of the stars. Methods. Theoretical line profiles are simulated by dividing the projected stellar disk into many small area elements. The intensity at each point in the direction of the observer is calculated from accurate theoretical atmosphere models, and the contributions are summed up, taking into account Doppler shifts from the velocity field. Results. We obtain predicted light curves, variation of the average line-of-sight velocity, of the distribution of radial velocity over the visible surface, and of spectral line profiles for the Ca II K resonance line. Conclusions. The predicted line profiles are very similar to those observed. It is thus very plausible that indeed the pulsations and associated velocity fields are at the origin of these peculiarities.

White Dwarfs as Astroparticle Physics Laboratories

White dwarfs are well studied objects. The relative simplic- ity of their physics allows to obtain very detailed models which can be ultimately compared with their observed properties. Among them there is a specific class of stars, the ZZ–Ceti stars that are character- ized by the extreme stability of their periods of pulsation. The rate of change of the period is closely related to the characteristic cooling time of the star, which can be accurately computed. This property not only allows to directly test the evolution of white dwarfs but also to constrain parameters of any new physical theory able to perturb the cooling regime. This technique has been successfuly aplied to the case of axions and can be used to constrain the properties of other theoreti- cal particles. The program we propose here consists in using the ability of the Antartic plateau to perform long time and non–interrupted ob- servations to establish the seismological properties of a well defined set of variable white dwarfs (and other stable pulsators).

MONET, HET and SALT and asteroseismological observations and theory in Göttingen

Inhalt (u.a.): M. BREGER: Delta Scuti stars: Observational aspects; M.S. Cunha: Theory of rapidly oscillating Ap stars; M. SACHKOV et al.: Vertical structure of pulsations in roAp stars; G. HOUDEK: Theoretical asteroseismology of solar-like oscillations; B.L. POPIELSKI: Core modes as a seismic probe of mixing beyond the convective core; M. GODART: Temperature gradients in the core overshooting region; W.A. DZIEMBOWSKI – J. DASZYŃSKA-DASZKIEWICZ – A.A. PAMYATNYKH: Interpretation of the Be star HD 163868 oscillation spectrum based on the MOST observations; D. KILKENNY: Pulsating Hot Subdwarfs – An Observational Review; M.H. MONTGOMERY: Mapping Convection using Pulsating White Dwarf Stars; M. ROTH: The Network Activities in HELAS; I. KUDZEJ et al.: A New Slovak Observatory 500 km from Vienna; W.W. WEISS: Microsatellites; G. KOPACKI – S. FRANDSEN: RR Lyrae stars in M4

High-resolution ultraviolet spectroscopy of PG 1159-035 with HST and FUSE

Context. PG 1159-035 is the prototype of the PG 1159 spectral class which consists of extremely hot hydrogen-deficient (pre-) white dwarfs. It is also the prototype of the GW Vir variables, which are non-radial g-mode pulsators. The study of PG 1159 stars reveals insight into stellar evolution and nucleosynthesis during AGB and post-AGB phases. Aims. We perform a quantitative spectral analysis of PG 1159-035 focusing on the abundance determination of trace elements. Methods. We have taken high-resolution ultraviolet spectra of PG 1159-035 with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. They are analysed with non-LTE line blanketed model atmospheres. Results. We confirm the high effective temperature with high precision (T eff = 140000 ± 5000 K) and the surface gravity of log g = 7. For the first time we assess the abundances of silicon, phosphorus, sulfur, and iron. Silicon is about solar. For phosphorus we find an upper limit of solar abundance. A surprisingly strong depletion of sulfur (2% solar) is discovered. Iron is not detected, suggesting an upper limit of 30% solar. This coincides with the Fe deficiency found in other PG 1159 stars. We redetermine the nitrogen abundance and find it to be lower by one dex compared to previous analyses. Conclusions. The sulfur depletion is in contradiction with current models of AGB star intershell nucleosynthesis. The iron deficiency confirms similar results for other PG 1159 stars and is explained by the conversion of iron into heavier elements by n-capture in the s-processing environment of the precursor AGB star. However, the extent of the iron depletion is stronger than predicted by evolutionary models. The relatively low nitrogen abundance compared to other pulsating PG 1159 stars weakens the role of nitrogen as a distinctive feature of pulsators and non-pulsators in the GW Vir instability strip.

New nonadiabatic pulsation computations on full PG 1159 evolutionary models: the theoretical GW Virginis instability strip revisited

Aims. We reexamine the theoretical instability domain of pulsating PG 1159 stars (GW Vir variables). Methods. We performed an extensive g-mode stability analysis on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M ⊙ , for which the complete evolutionary stages of their progenitors from the ZAMS, through the thermally pulsing AGB and born-again phases to the domain of the PG 1159 stars have been considered. Results. We found that pulsations in PG 1159 stars are excited by the κ-mechanism due to partial ionization of carbon and oxygen, and that no composition gradients are needed between the surface layers and the driving region, much in agreement with previous studies. We show, for the first time, the existence of a red edge of the instability strip at high luminosities. We found that all of the GW Vir stars lay within our theoretical instability strip. Our results suggest a qualitative good agreement between the observed and the predicted ranges of unstable periods of individual stars. Finally, we found that generally the seismic masses (derived from the period spacing) of GW Vir stars are somewhat different from the masses suggested by evolutionary tracks coupled with spectroscopy. Improvements in the evolution during the thermally pulsing AGB phase and/or during the core helium burning stage and early AGB could help to alleviate the persisting discrepancies.

Mapping the ZZ Ceti Instability Strip: Discovery of Six New Pulsators

As part of an ongoing program to better map the empirical instability strip of pulsating ZZ Ceti white dwarfs, we present a brief progress report based on our last observing season. We discuss here high-speed photometric measurements for six new pulsators. These stars were selected on the basis of preliminary measurements of their effective temperatures and surface gravities, which placed them inside or near the known ZZ Ceti instability strip. We also report detection limits for a number of DA white dwarfs that showed no sign of variability. Finally, we revisit the ZZ Ceti star G232-38, for which we obtained improved high-speed photometry.

The ZZ Ceti star G 185-32: new insight from asteroseismology

One of the brightest pulsating DA white dwarfs (ZZ Ceti stars), G 185-32, shows 19 periods in its power spectrum, from 71 s to 651 s. Its short 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 with wavelength, in contrast with what the linear theory predicts for $\ell=1$ or $\ell=2$ non-radial g -modes. The mass and T eff of G 185-32 are well constrained by spectroscopic observations and parallax measurement. The aim of the present paper is to take advantage of this rich observational background to derive the fundamental parameters of the star from asteroseismology. We computed a grid of realistic models for G 185-32 and the periods of their adiabatic $\ell=1$ and $\ell=2$ non-radial g -modes. We determined the model which fits best the observations. We identify 14 periods as real modes and determine their $\ell$, k , and m values. We show that the remaining 5 periods are linear combinations.We find that G 185-32 oscillates mostly according to $\ell=2$ modes. We present a new hypothesis to account for the peculiar behaviour of the 141.9 s mode's amplitude and suggest that it is a true mode interfering with higher $\ell$ degree modes, involving a possible triplet resonance. We determine the main parameters of G 185-32: its total mass $M_{*}=0.638$ ( ± 0.007) $M_{\odot}$, its hydrogen mass fraction $M_{\rm H}= 1.70$ ( ± 0.10) $\times~10^{-4}~M_{*}$, which implies a helium mass fraction of ≈ $10^{-2}~M{*}$, its $T_{\rm eff}= 12\,280$ ( ± )80 K, and estimate its rotation period to be 14.5 h.

Discovery of seven ZZ Ceti stars using a new photometric selection method

We report the detection of non-radial g-mode oscillations in seven relatively bright (14.4 < B < 16.5) DA white dwarf stars from time-series photometry collected at the Nordic Optical Telescope (NOT) at Roque de Los Muchachos Observatory, Spain. The candidate variable stars were selected from preparatory photometric observations of objects from the Hamburg Quasar Survey (HQS) and from spectroscopy of stars from the Villanova (WD) catalogue. The selection of ZZ Ceti candidate stars from photometric data proved nearly as successful as the more common spectroscopic selection method. The effective temperatures that we derive from both methods are consistent with a pure ZZ Ceti instability strip.

Ensemble Characteristics of the ZZ Ceti Stars

We present the observed pulsation spectra of all known non-interacting ZZ Ceti stars (hydrogen atmosphere white dwarf variables; DAVs) and examine changes in their pulsation properties across the instability strip. We confirm the well established trend of increasing pulsation period with decreasing effective temperature across the ZZ Ceti instability strip. We do not find a dramatic order of magnitude increase in the number of observed independent modes in ZZ Ceti stars, traversing from the hot to the cool edge of the instability strip; we find that the cool DAVs have one more mode on average compared to the hot DAVs. We confirm the initial increase in pulsation amplitude at the blue edge, and find strong evidence of a decline in amplitude prior to the red edge. We present the first observational evidence that ZZ Ceti stars lose pulsation energy just before pulsations shut down at the empirical red edge 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 observed during two Whole Earth Telescope campaigns. It was a second priority target during the XCOV13 campaign in 1996 and the first priority one during the XCOV18 campaign in 1999. The 1999 campaign reached 66.5% duty cycle. With a total duration of 18 days, the frequency resolution achieved is 0.68 μHz. With such a frequency resolution, we were able to find as many as 78 significant frequencies in the power spectrum, of which 34 are independent frequencies after removal of all linear combinations. In taking into account other frequencies present during the 1996 WET campaign and those present in earlier data, which do not show up in the 1999 data set, we find a total of 43 independent frequencies. This makes HL Tau 76 the richest ZZ Ceti star in terms of number of observed pulsation modes. We use those pulsation frequencies to 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 s and 1390 s. We propose an identification for 39 of those 43 frequencies in terms of and non-radial g-modes split by rotation. We derive an average rotation period of 2.2 days. The period distribution of HL Tau 76 is best reproduced if the star has a moderately “thick” hydrogen mass fraction log . The results presented in this paper constitute a starting point for a detailed comparison of the observed periods with the periods calculated for models 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 study of the cool ZZ Ceti star HL Tau 76. A grid of models has been computed in a parameter space covering the range of log g and Teff, formerly determined by spectroscopy, and a large range of hydrogen mass fraction. The adiabatic non-radial oscillations for all the models have been computed for the modes of degrees and . An algorithm based on a test was applied to evaluate the quality of the fit between observed and theoretical periods. This method resulted in selecting a best fitting model for which the average relative matching of the periods is 0.7%. Then, a detailed comparison between the observed and the computed periods for the and modes of the best fitting model was achieved in order to identify as many observed modes as possible. To perform this identification we used the calculated periods for which we applied the rotational splitting as deduced from the observations. Through this process we identify the 36 independent modes observed in HL Tau 76. The best fitting model for HL Tau 76 is well constrained due to the large number of oscillations detected in this ZZ Ceti star. The main stellar parameters of HL Tau 76 derived from this analysis are: the total mass , the hydrogen mass fraction qH, estimated as thick as . The helium mass fraction consistent with qH must be . The method is not sensitive to Teff variations in the narrow domain of temperature derived from spectroscopy for HL Tau 76. The best adjustment is found however for K. The other derived stellar parameters are the luminosity () and the radius (). We note some trends in the fit of the observed periods with the computed ones which suggest that the rotational splitting could be non-uniform and that the large amplitude modes might contain information on the convection-driven excitation mechanism.

Ensemble Characteristics of the ZZ Ceti Stars

Ensemble Characteristics of the ZZ Ceti Stars

Mode Identification from Combination Frequency Amplitudes in ZZ Ceti Stars

The light curves of variable DA stars are usually multiperiodic and nonsinusoidal, so that their Fourier transforms show peaks at eigenfrequencies of the pulsation modes and at sums and differences of these frequencies. These combination frequencies provide extra information about the pulsations, both physical and geometrical, that is lost unless they are analyzed. Several theories provide a context for this analysis by predicting combination frequency amplitudes. In these theories, the combination frequencies arise from nonlinear mixing of oscillation modes in the outer layers of the white dwarf, so their analysis cannot yield direct information on the global structure of the star as eigenmodes provide. However, their sensitivity to mode geometry does make them a useful tool for identifying the spherical degree of the modes that mix to produce them. In this paper we analyze data from eight hot, low-amplitude DAV white dwarfs and measure the amplitudes of combination frequencies present. By comparing these amplitudes to the predictions of the theory of Goldreich & Wu, we have verified that the theory is crudely consistent with the measurements. We have also investigated to what extent the combination frequencies can be used to measure the spherical degree (?) of the modes that produce them. We find that modes with ? > 2 are easily identifiable as high ? based on their combination frequencies alone. Distinguishing between ? = 1 and 2 is also possible using harmonics. These results will be useful for conducting seismological analysis of large ensembles of ZZ Ceti stars, such as those being discovered using the Sloan Digital Sky Survey. Because this method relies only on photometry at optical wavelengths, it can be applied to faint stars using 4 m class telescopes.

Rotation velocities of white dwarfs determined from the Ca II K line

Department of Physics & Astronomy and Center for Astrobiology, UCLA, Los Angeles, CA 90095-1562, USAAbstract. We determine projected rotation velocities v sin i in DAZ white dwarfs, for the first time using therotational broadening of the CaII K line. The results confirm previous findings that white dwarfs are very slowrotators, and set even more stringent upper limits of typically less than 10 km/s. The few exceptions include 3stars known or suspected to be variable ZZ Ceti stars, where the line broadening is very likely not due to rotation.The results demonstrate that the angular momentum of the core cannot be preserved completely between mainsequence and final stage.Key words. stars: white dwarfs – stars: rotation

The formation of DA white dwarfs with thin hydrogen envelopes

We study the formation and evolution of DA white dwarfs, the progenitors of which have experienced a late thermal pulse (LTP) shortly after the departure from the thermally pulsing AGB. To this end, we compute the complete evolution of an initially 2.7 Mo star all the way from the zero-age main sequence to the white dwarf stage. We find that most of the original H-rich material of the post-AGB remnant is burnt during the post-LTP evolution, with the result that, at entering its white dwarf cooling track, the remaining H envelope becomes 10^-6 Mo in agreement with asteroseismological inferences for some ZZ Ceti stars.

Toward an Empirical Determination of the ZZ Ceti Instability Strip

We present atmospheric parameters for a large sample of DA white dwarfs that are known to be photometrically constant. For each star, we determine the effective temperature and surface gravity by comparing high signal-to-noise ratio optical spectra to the predictions of detailed model atmosphere calculations. We also report the successful prediction and detection of photometric variability in G232-38 based on similar Teff and log g determinations. The atmospheric parameters derived for this sample of constant stars, as well as those for the known sample of bright ZZ Ceti stars (now boosted to a total of 39), have been obtained in a highly homogeneous way. We combine them to study the empirical red and blue edges, as well as the purity of the ZZ Ceti instability strip. We find that the red edge is rather well constrained, whereas there exists a rather large range of possibilities for the slope of the blue edge. Furthermore, the ZZ Ceti instability strip that results from our analysis contains no nonvariable white dwarfs. Our sample of constant stars is part of a much broader spectroscopic survey of bright (V < 17) DA white dwarfs, which we have recently undertaken. We also present here some preliminary results of this survey. Finally, we revisit the analysis by Mukadam et al. of the variable and nonvariable DA stars uncovered as part of the Sloan Digital Sky Survey. Their erroneous conclusion of an instability strip containing several nonvariable stars is traced back to the low signal-to-noise ratio spectroscopic observations used in that survey.

Rotation velocities of white dwarfs

The sharp Hα NLTE line cores of hydrogen-rich (DA) white dwarfs allow their projected rotational velocities to be determined. High resolution optical spectra of 22 stars obtained with the Keck I telescope are matched by synthetic spectra computed from a grid of NLTE model atmospheres. In this paper, the third in a series on white dwarf rotation, we concentrate preferentially on DA white dwarfs with convective atmospheres, i.e. with T e f f < 14000 K. Previous analyses found DA white dwarfs hotter than 14000 K to be very slow rotators and rarely show any spectroscopically detectable rotation. For 19 of our programme stars we were able to derive projected rotational velocities or upper limits. No rotation could be detected for seven stars in our sample. However twelve stars show significant line broadening. In the case of the ZZ Ceti star G 117-B15A, the observed Hα line profile cannot be matched by a rotationally broadened profile as its line core is too narrow. Combining our results with those from two similar studies, we have obtained information on the rotation or other line broadening mechanisms (such as caused by magnetic fields) of 56 DA white dwarfs. The fraction of rotating DA white dwarfs whose line profiles can be matched for a vanishing projected rotation velocity is high for hot white dwarfs with radiative atmospheres (25 out of 28). Amongst the cool white dwarfs with presumedly convective atmospheres, only for a few stars (8 out of 22) has no additional line broadening to be invoked to explain their observed Hα line profiles. We conjecture that the physics of Hα line formation in convective DA while dwarf atmospheres is not yet sufficiently well understood and additional observational and theoretical efforts have to be made.