AT LAST—A V777 HER PULSATOR IN THE
                    <i>KEPLER</i>
                    FIELD

Østensen, R. H.; Bloemen, S.; Vučković, M.; Aerts, C.; Oreiro, R.; Kinemuchi, Karen; Still, M.; Koester, D.

doi:10.1088/2041-8205/736/2/l39

Cited by 45 publications

(27 citation statements)

References 33 publications

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“…1. Our fit gives values of T eff = 28, 480 K and log g = 7.89, discrepant in temperature with the original estimates from [1] (T eff = 24.950 K and log g = 7.91), but consistent with the expected period-luminosity relationship, given that the observed pulsation periods are the shortest ever found in a pulsating DB star. These estimates make that star the hottest star of the V777 Her type.…”

Section: Constraints From Spectroscopycontrasting

confidence: 74%

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The case of theKeplerDBAV star J1929+4447

Brassard

Fontaine

2013

EPJ Web of Conferences

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Abstract.A first pulsating white dwarf has been discovered recently in the Kepler field of view [1]. This star, catalogued as GALEX J192904.6+444708 or KIC 8626021, is a He-atmosphere white dwarf of the V777 Her (DBV) type. It appears to be the hottest of its class and, as such, has the potential to be a key object in our understanding of the DB gap problem. We present here a seismological analysis of this star and a look at his previous history. CONSTRAINTS FROM SPECTROSCOPYA good S/N ≈ 50 optical spectrum of the rather faint target (K p = 18.42) was kindly obtained by our collaborator Betsy Green at the Steward Observatory. The resulting fit of our detailed analysis of that spectrum is shown in our Fig. 1. Our fit gives values of T eff = 28, 480 K and log g = 7.89, discrepant in temperature with the original estimates from [1] (T eff = 24.950 K and log g = 7.91), but consistent with the expected period-luminosity relationship, given that the observed pulsation periods are the shortest ever found in a pulsating DB star. These estimates make that star the hottest star of the V777 Her type. However, our detection of a trace of hydrogen makes it formally a DBAV star. WHAT DIRECT ASTEROSEISMOLOGICAL METHODS TELL USWe use our standard direct method that consists of searching in parameter space the model with pulsation periods that best fit the observed periods. We define such a model as one that minimizes the goodness-of-fit function given by 2 = N obs i=1 (P obs − P th ), where P obs is an observed period and P th is the corresponding theoretical period obtained from parametrized static models. Our results (see Fig. 2 and Fig. 3) are presented as T eff vs. log g maps, where the obtained 2 values for each given T eff − log g pair needed to build the map have been optimized independently for the other structural parameters. Such computations are really demanding in terms of computing power: over 60 million stellar models have already been computed during the course of this ongoing investigation. Fortunately, our group has access to a dedicated cluster of 320 processor nodes. We have first taken a look at two hypotheses about the white dwarf structure: A canonical DB white dwarf or a double-layered structure arising from a PG1159 star progenitor.In the first case, we have looked at the simplest possible structure for our white dwarf model: An onion-like structure composed of a C/O core wrapped by an He layer. Figure 2 gives the results for the entire DBV range of possible values for T eff and log g. All points (2501) used to produce the map have a

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Section: Constraints From Spectroscopycontrasting

confidence: 74%

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confidence: 99%

The case of theKeplerDBAV star J1929+4447

Brassard

Fontaine

2013

EPJ Web of Conferences

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“…17. Hence, the true effective temperature of KIC08626021 has to be much higher than the estimate provided 8 , which is more characteristic of the red edge of the V777 Her instability strip 42 . The need for a better optical spectrum of KIC08626021 was subsequently emphasized 37 because of the apparent conflict between the initial spectroscopic estimate of the effective temperature and the seismic model, which indicated a value of T eff ≈ 29,200 K.…”

Section: Methodsmentioning

confidence: 85%

“…It is also appropriate to compare our seismic results with those obtained in previous (adiabatic) studies dedicated to KIC08626021. Following the report of the discovery of five distinct pulsation modes in this star on the basis of one month of Kepler photometry 8 , three different groups presented preliminary seismic analyses. The first study 37 made use of parameterized evolutionary models computed with the white dwarf evolution code (WDEC).…”

Section: Methodsmentioning

confidence: 99%

A large oxygen-dominated core from the seismic cartography of a pulsating white dwarf

Giammichele

Charpinet

Fontaine

et al. 2018

Nature

120

177

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White-dwarf stars are the end product of stellar evolution for most stars in the Universe 1 . Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution 2,3 . Moreover, they are important chronometers for dating galactic stellar populations, and their mergers with other white dwarfs now appear to be responsible for producing the type Ia supernovae that are used as standard cosmological candles 4 . However, the internal structure of white-dwarf stars-in particular their oxygen content and the stratification of their cores-is still poorly known, because of remaining uncertainties in the physics involved in stellar modelling codes 5,6 . Here we report a measurement of the radial chemical stratification (of oxygen, carbon and helium) in the hydrogendeficient white-dwarf star KIC08626021 (J192904.6+444708), independently of stellar-evolution calculations. We use archival data 7,8 coupled with asteroseismic sounding techniques 9,10 to determine the internal constitution of this star. We find that the oxygen content and extent of its core exceed the predictions of existing models of stellar evolution. The central homogeneous core has a mass of 0.45 solar masses, and is composed of about 86 per cent oxygen by mass. These values are respectively 40 per cent and 15 per cent greater than those expected from typical white-dwarf models. These findings challenge present theories of stellar evolution and their constitutive physics, and open up an avenue for calibrating white-dwarf cosmochronology 11 .Hydrogen-deficient DB-type white dwarfs are dying stars that have effective temperatures ranging from about 12,000 K to upwards of 35,000 K (ref. 12). They have undergone a late final flash as postasymptotic-giant-branch (post-AGB) stars, which freed them from their thin remaining hydrogen envelope [13][14][15] . Their deeper chemical stratification-as in any typical white dwarf-is otherwise determined by the rate of the nuclear reaction 12 C(α ,γ ) 16 O, which synthesizes oxygen from the ashes of the core-helium-burning phase, by means of mixing processes associated with convection and rotation as well as the number of thermal pulses that occur in evolved AGB stars 16 . The descendant DB white dwarfs must bear the signature of such processes in their core, as well as the imprint of the still-ongoing settling of carbon and oxygen in the surrounding helium-rich envelope.KIC08626021 (GALEX J192904.6+ 444708) is the first pulsating DB white dwarf (also known as the V777 Her class of variable stars) to be moni tored extensively by the Kepler spacecraft for its pulsation properties 8 . This star shows an oscillation spectrum composed of non-radial g-modes, which are potentially sensitive to the deep interior of the star 17 . Using an analysis of 23 months of Kepler highprecision photo metric data 18 , we exploit the eight well secured independent modes, which have periods ranging from 143.2 s to 376.1 s (Fig. 1, Extended Data Table 1 and Methods). These relatively short pulsation periods are-accordin...

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“…However, up to now, only one V777 Her and three ZZ Ceti stars were monitored from space (Greiss et al 2014;Hermes et al 2011;Østensen et al 2011;Hermes et al 2014). This means that exploring their pulsational properties and thus studying their interiors is still mainly based on ground-based observations.…”

Section: Discussionmentioning

confidence: 99%

Revealing the pulsational properties of the V777 Herculis star KUV 05134+2605 by its long-term monitoring

Bognár

Paparó

Córsico

et al. 2014

A&A

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Context. KUV 05134+2605 is one of the 21 pulsating DB white dwarfs (V777 Her or DBV variables) known so far. The detailed investigation of the short-period and low-amplitude pulsations of these relatively faint targets requires considerable observational efforts from the ground, long-term single-site or multi-site observations. The observed amplitudes of excited modes undergo short-term variations in many cases, which makes determining pulsation modes difficult. Aims. We aim to determine the pulsation frequencies of KUV 05134+2605, find regularities between the frequency and period components, and perform an asteroseismic investigation for the first time. Methods. We re-analysed the published data and collected new measurements. We compared the frequency content of the different datasets from the different epochs and performed various tests to check the reliability of the frequency determinations. The mean period spacings were investigated with linear fits to the observed periods, Kolmogorov-Smirnov and inverse variance significance tests, and with a Fourier analysis of different period sets, including a Monte Carlo test that simulated the effect of alias ambiguities. We employed fully evolutionary DB white dwarf models for the asteroseismic investigations. Results. We identified 22 frequencies between 1280 and 2530 μHz. These form 12 groups, which suggests at least 12 possible frequencies for the asteroseismic investigations. Thanks to the extended observations, KUV 05134+2605 joined the group of rich white dwarf pulsators. We identified one triplet and at least one doublet with a ≈9 μHz frequency separation, from which we derived a stellar rotation period of 0.6 d. We determined the mean period spacings of ≈31 s and 18 s for the modes we propose as dipole and quadrupole. We found an excellent agreement between the stellar mass derived from the = 1 period spacing and the period-to-period fits, all providing M * = 0.84−0.85 M solutions. Our study suggests that KUV 05134+2605 is the most massive amongst the known V777 Her stars.

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AT LAST—A V777 HER PULSATOR IN THE KEPLER FIELD

Cited by 45 publications

References 33 publications

The case of theKeplerDBAV star J1929+4447

The case of theKeplerDBAV star J1929+4447

A large oxygen-dominated core from the seismic cartography of a pulsating white dwarf

Revealing the pulsational properties of the V777 Herculis star KUV 05134+2605 by its long-term monitoring

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