arXiv subjects

S. O. Kepler

Publications and source records attributed to S. O. Kepler.

At least 55 records · Page 3Linked to original sources

arXiv2020

$\textit{Gaia}$ white dwarfs within 40 pc I: spectroscopic observations of new candidates

We present a spectroscopic survey of 230 white dwarf candidates within 40 pc of the Sun from the William Herschel Telescope and Gran Telescopio Canarias. All candidates were selected from $\textit{Gaia}$ Data Release 2 (DR2) and in almost all cases had no prior spectroscopic classifications. We find a total of 191 confirmed white dwarfs and 39 main-sequence star contaminants. The majority of stellar remnants in the sample are relatively cool ($\langle T_{\rm eff} \rangle$ = 6200 K), showing either hydrogen Balmer lines or a featureless spectrum, corresponding to 89 DA and 76 DC white dwarfs, respectively. We also recover two DBA white dwarfs and 9--10 magnetic remnants. We find two carbon-bearing DQ stars and 14 new metal-rich white dwarfs. This includes the possible detection of the first ultra-cool white dwarf with metal lines. We describe three DZ stars for which we find at least four different metal species, including one which is strongly Fe- and Ni-rich, indicative of the accretion of a planetesimal with core-Earth composition. We find one extremely massive (1.31 $\pm$ 0.01 M$_{\odot}$) DA white dwarf showing weak Balmer lines, possibly indicating stellar magnetism. Another white dwarf shows strong Balmer line emission but no infrared excess, suggesting a low-mass sub-stellar companion. High spectroscopic completeness ($>$99%) has now been reached for $\textit{Gaia}$ DR2 sources within 40 pc sample, in the northern hemisphere ($δ>$ 0 deg) and located on the white dwarf cooling track in the Hertzsprung-Russell diagram. A statistical study of the full northern sample is presented in a companion paper.

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arXiv2020

SDSS J124043.01+671034.68: The partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?

The white dwarf SDSS J124043.01+671034.68 (SDSS J1240+6710) was previously found to have an oxygen-dominated atmosphere with significant traces of neon, magnesium, and silicon. A possible origin via a violent late thermal pulse or binary interactions have been suggested to explain this very unusual photospheric composition. We report the additional detection of carbon, sodium, and aluminium in far-ultraviolet and optical follow-up spectroscopy. No iron-group elements are detected, with tight upper limits on iron, cobalt and nickel, suggesting that the star underwent partial oxygen burning, but failed to ignite silicon burning. Modelling the spectral energy distribution and adopting the distance based on the Gaia parallax, we infer a low white dwarf mass, M(wd)=0.41+/-0.05Msun. The large space velocity of SDSS J1240+6710, computed from the Gaia proper motion and its radial velocity, is compatible with a Galactic rest-frame velocity of ~250km/s in the opposite direction with respect to the Galactic rotation, strongly supporting a binary origin of this star. We discuss the properties of SDSS J1240+6710 in the context of the recently identified survivors of thermonuclear supernovae, the D6 and LP 40-365 stars, and conclude that it is unlikely related to either of those two groups. We tentatively suggest that SDSS J1240+6710 is the partially burned remnant of a low-mass white dwarf that underwent a thermonuclear event.

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arXiv2020

Pulsation in the white dwarf HE 1017-1352: confirmation of the class of hot DAV stars

We report the detection of periodic variations on the T_eff ~32 000 K DA white dwarf star HE 1017-1352. We obtained time series photometry using the 4.1 m SOAR telescope on three separate nights for a total of 16.8 h. From the frequency analysis we found four periods of 605 s, 556 s, 508 s and 869 s with significant amplitudes above the 1/1000 false alarm probability detection limit. The detected modes are compatible with low harmonic degree g-mode non-radial pulsations with radial order higher than ~ 9. This detection confirms the pulsation nature of HE 1017-1352 and thus the existence of the new pulsating class of hot DA white dwarf stars. In addition, we detect a long period of 1.52 h, compatible with a rotation period of DA white dwarf stars.

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arXiv2020

New white dwarf envelope models and diffusion. Application to DQ white dwarfs

Recent studies of the atmospheres of carbon-rich (DQ) white dwarfs have demonstrated the existence of two different populations that are distinguished by the temperature range, but more importantly, by the extremely high masses of the hotter group. The classical DQ below 10000 K are well understood as the result of dredge-up of carbon by the expanding helium convection zone. The high-mass group poses several problems regarding their origin and also an unexpected correlation of effective temperature with mass. We propose to study the envelopes of these objects to determine the total hydrogen and helium masses as possible clues to their evolution. We developed new codes for envelope integration and diffusive equilibrium that are adapted to the unusual chemical composition, which is not necessarily dominated by hydrogen and helium. Using the new results for the atmospheric parameters, in particular, the masses obtained using Gaia parallaxes, we confirm that the narrow sequence of carbon abundances with Teff in the cool classical DQ is indeed caused by an almost constant helium to total mass fraction, as found in earlier studies. This mass fraction is smaller than predicted by stellar evolution calculations. For the warm DQ above 10000 K, which are thought to originate from double white dwarf mergers, we obtain extremely low hydrogen and helium masses. The correlation of mass with Teff remains unexplained, but another possible correlation of helium layer masses with Teff as well as the gravitational redshifts casts doubt on the reality of both and suggests possible shortcomings of current models.

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arXiv2020

Evidence of Spectral Evolution on the white dwarf sample from the Gaia Mission

Since the Gaia data release 2, several works were published describing a bifurcation in the observed white dwarf colour$-$magnitude diagram for $\mbox{$G_{\mathrm{BP}}$}-\mbox{$G_{\mathrm{RP}}$} > 0$. Some possible explanations in the literature include the existence of a double population with different initial mass function or two distinct populations, one formed by hydrogen$-$ and one formed by helium$-$envelope white dwarfs. We propose instead spectral evolution to explain the bifurcation. From a population synthesis approach, we find that the spectral evolution occurs for effective temperature below ${\simeq}11\,000\,\mathrm{K}$ and masses mainly between $0.64\,\mathrm{M}_\odot$ and $0.74\,\mathrm{M}_\odot$, which correspond to around $16$ per cent of all DA white dwarfs. We also find the Gaia white dwarf colour-magnitude diagram indicates a star formation history that decreases abruptly for objects younger than $1.4\,\mathrm{Gyr}$ and a top-heavy initial mass function for the white dwarf progenitors.

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arXiv2019

On the existence of warm H-rich pulsating white dwarfs

The possible existence of warm ($T_{\rm eff}\sim19\,000$ K) pulsating DA white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical studies more than 30 yr ago. However, to date, no pulsating warm DA WD has been discovered. We re-examine the pulsational predictions for such WDs on the basis of new full evolutionary sequences. We analyze all the warm DAs observed by TESS satellite up to Sector 9 in order to search for the possible pulsational signal. We compute WD evolutionary sequences with H content in the range $-14.5 \lesssim \log(M_{\rm H}/M_{\star}) \lesssim -10$, appropriate for the study of warm DA WDs. We use a new full-implicit treatment of time-dependent element diffusion. Non-adiabatic pulsations were computed in the effective temperature range of $30\,000-10\,000$ K, focusing on $\ell= 1$ $g$ modes with periods in the range $50-1500$ s. We find that extended He/H transition zones inhibit the excitation of $g$ modes due to partial ionization of He below the H envelope, and only in the case that the H/He transition is assumed much more abrupt, models do exhibit pulsational instability. In this case, instabilities are found only in WD models with H envelopes in the range of $-14.5 \lesssim \log(M_{\rm H}/M_{\star}) \lesssim -10$ and at effective temperatures higher than those typical of ZZ Ceti stars, in agreement with previous studies. None of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Our study suggests that the non-detection of pulsating warm DAs, if WDs with very thin H envelopes do exist, could be attributed to the presence of a smooth and extended H/He transition zone. This could be considered as an indirect proof that element diffusion indeed operates in the interior of WDs.

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arXiv2019

Ground based observation of ZZ Ceti stars and the discovery of four new variables

We perform ground based photometric observations of 22 DA white dwarf stars, 10 already known ZZ Cetis and 12 candidates with atmospheric parameters inside the classical instability strip. We report on the discovery of four new variable DA white dwarf stars. Two objects are near the middle of the instability strip, SDSS J082804.63+094956.6 and SDSS J094929.09+101918.8, and two red edge pulsators, GD 195 and L495-82. In addition, we classified four objects as possible variables, since evidence of variability was detected in the light curve, but the S/N was not sufficient to establish a definite detection. Follow-up observations were performed for 10 know ZZ Ceti stars to verify period stability and search for new periodicities. For each confirmed variable, we perform a detailed asteroseismological fit and compare the structural parameters obtained from the best fit models with those obtained from spectroscopy and photometry from Gaia. Finally we present a study of the asteroseismological properties of a sample of 91 ZZ Ceti stars.

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arXiv2019

Carbon-rich (DQ) white dwarfs in the Sloan Digital Sky Survey

Among the spectroscopically identified white dwarfs, a fraction smaller than 2% have spectra dominated by carbon lines, mainly molecular C2, but also in a smaller group by CI and CII lines. These are together called DQ white dwarfs. We want to derive atmospheric parameters Teff,log g, and carbon abundances for a large sample of these stars and discuss implications for their spectral evolution. Sloan Digital Sky Survey spectra and ugriz photometry were used, together with GAIA Data Release 2 parallaxes and G band photometry. These were fitted to synthetic spectra and theoretical photometry derived from model atmospheres. We found that the DQs hotter than Teff ~10000 K have masses ~0.4 Msun larger than the cooler ones, which have masses typical for the majority of white dwarfs, ~0.6 Msun. A significant fraction of the hotter objects with Teff > 14500 K have atmospheres dominated by carbon.

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arXiv2019

Pulsating white dwarfs: new insights

White dwarf stars constitute the final evolutionary stage for more than 95 per cent of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components ---including the star formation history of the Milky Way. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Also, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model. The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has allowed us to make meaningful comparison of evolutionary models with observations. While some information like surface chemical composition, temperature and gravity of isolated white dwarfs can be inferred from spectroscopy, and the total mass and radius can be derived as well when they are in binaries, the internal structure of these compact stars can be unveiled only by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable stars and the periods predicted by appropriate theoretical models. The asteroseismological techniques allow us to infer details of the internal chemical stratification, the total mass, and even the stellar rotation profile. In this review, we first revise the evolutionary channels currently accepted that lead to the formation of white-dwarf stars, and then, we give a detailed account of the different sub-types of pulsating white dwarfs known so far, emphasizing the recent observational and theoretical advancements in the study of these fascinating variable stars.

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arXiv2019

White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14

White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with $3σ$ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and 311 cataclysmic variables. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs), hot subdwarfs (sdBs and sdOs), and estimate photometric Teff for white dwarf stars with continuum spectra (DCs). We find 15793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below Teff= 10000 K; most are likely low-mass metal-poor main sequence stars, but some could be the result of interacting binary evolution.

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arXiv2019

The white dwarf mass-radius relation and its dependence on the hydrogen envelope

We present a study of the dependence of the mass-radius relation for DA white dwarf stars on the hydrogen envelope mass and the impact on the value of log g, and thus the determination of the stellar mass. We employ a set of full evolutionary carbon-oxygen core white dwarf sequences with white dwarf mass between 0.493 and 1.05 Msun. Computations of the pre-white dwarf evolution uncovers an intrinsic dependence of the maximum mass of the hydrogen envelope with stellar mass, i.e., it decreases when the total mass increases. We find that a reduction of the hydrogen envelope mass can lead to a reduction in the radius of the model of up to ~12%. This translates directly into an increase in log g for a fixed stellar mass, that can reach up to 0.11 dex, mainly overestimating the determinations of stellar mass from atmospheric parameters. Finally, we find a good agreement between the results from the theoretical mass-radius relation and observations from white dwarfs in binary systems. In particular, we find a thin hydrogen mass of MH ~ 2 10^-8 Msun, for 40 Eridani B, in agreement with previous determinations. For Sirius B, the spectroscopic mass is 4.3% lower than the dynamical mass. However, the values of mass and radius from gravitational redshift observations are compatible with the theoretical mass-radius relation for a thick hydrogen envelope of MH = 2 10^-6 Msun.

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arXiv2019

Phase Transitions in Neutron Stars

In this paper we review the most common descriptions for the first order phase transition to deconfined quark matter in the core of neutron stars. We also present a new description of these phase transitions in the core of proto-neutron stars, in which more constraints are enforced so as to include trapped neutrinos. Finally, we calculate the emission of gravitational waves associated with deconfinement phase transitions, discuss the possibility of their detection, and how this would provide information about the equation of state of dense matter.

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arXiv2018

The sdA problem - III. New extremely low-mass white dwarfs and their precursors from Gaia astrometry

The physical nature of the sdA stars---cool hydrogen-rich objects with spectroscopic surface gravities intermediate between main sequence and canonical mass white dwarfs---has been elusive since they were found in Sloan Digital Sky Survey Data Release 12 spectra. The population is likely dominated by metal-poor A/F stars in the halo with overestimated surface gravities, with a small contribution of extremely low-mass white dwarfs and their precursors, i.e., ELMs and pre-ELMs. In this work, we seek to identify (pre-)ELMs with radii smaller than is possible for main sequence stars, allowing even for very low metallicity. We analyse 3891 sdAs previously identified in the Sloan Digital Sky Survey using Gaia DR2 data. Our Monte Carlo analysis supports that 90 of these are inconsistent with the main sequence. 37 lie close to or within the canonical white dwarf cooling sequence, while the remaining 53 lie between the canonical white dwarfs and main sequence, which we interpret as likely (pre-)ELMs given their spectral class. Of these, 30 pass more conservative criteria that allow for higher systematic uncertainties on the parallax, as well as an approximate treatment of extinction. Our identifications increase the number of known (pre-)ELMs by up to 50 per cent, demonstrating how Gaia astrometry can reveal members of the compact (pre-)ELM subpopulation of the sdA spectral class.

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arXiv2018

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences VI. Thin H-envelope sequences and asteroseismology of ELMV stars revisited

Some low-mass white-dwarf (LMWD) stars with H atmospheres show long-period g-mode pulsations, comprising the class of pulsating WDs called extremely low-mass variable (ELMV) stars. It is generally believed that these stars have thick H envelopes. However, from stellar evolution considerations, the existence of LMWDs with thin H envelopes is also possible. We present a thorough asteroseismological analysis of ELMV stars based on a complete set of fully evolutionary models that represents low-mass He-core WD stars harboring a range of H envelope thicknesses. Although there are currently nine ELMVs, here we only focus on those that exhibit more than three periods and whose periods do not show significant uncertainties. We considered g-mode adiabatic pulsation periods for low-mass He-core WD models with $M_*$ in the range [0.1554-0.4352]$M_{\odot}$, $T_{\rm eff}$ in the range [6000-10000]K, and H envelope thicknesses in the range -5.8<log($M_{\rm H}/M_*$)<-1.7. We explore the effects of employing different H-envelope thicknesses on the adiabatic pulsation properties of low-mass He-core WD models, and perform period-to-period fits to ELMVs to search for a representative model. We found that the mode-trapping effects of g modes depend sensitively on $M_{\rm H}$, with the trapping cycle and trapping amplitude larger for thinner H envelopes. Also, the asymptotic period spacing is longer for thinner H envelopes. Finally, we found asteroseismological models (when possible) characterized by canonical (thick) and by thin H envelope, with $T_{\rm eff}$ and $M_*$ in agreement with the spectroscopic determinations. The fact that we have found asteroseismological solutions with H envelopes thinner than canonical gives a clue of the possible scenario of formation of these stars. Indeed, in the light of our results, some of these stars could have been formed by binary evolution through unstable mass loss.

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arXiv2018

A Study of Cool White Dwarfs in the Sloan Digital Sky Survey Data Release 12

In this work we study white dwarfs where $30\,000\,\text{K} {>} \mathrm{T}_{\rm{eff}} {>} 5\,000\,\text{K}$ to compare the differences in the cooling of DAs and non-DAs and their formation channels. Our final sample is composed by nearly $13\,000$ DAs and more than $3\,000$ non-DAs that are simultaneously in the SDSS DR12 spectroscopic database and in the \textit{Gaia} survey DR2. We present the mass distribution for DAs, DBs and DCs, where it is found that the DCs are ${\sim}0.15\,\mathrm{M}_\odot$ more massive than DAs and DBs on average. Also we present the photometric effective temperature distribution for each spectral type and the distance distribution for DAs and non-DAs. In addition, we study the ratio of non-DAs to DAs as a function of effective temperature. We find that this ratio is around ${\sim}0.075$ for effective temperature above ${\sim}22\,000\,\text{K}$ and increases by a factor of five for effective temperature cooler than $15\,000\,\text{K}$. If we assume that the increase of non-DA stars between ${\sim}22\,000\,\text{K}$ to ${\sim}15\,000\,\text{K}$ is due to convective dilution, $14{\pm}3$ per cent of the DAs should turn into non-DAs to explain the observed ratio. Our determination of the mass distribution of DCs also agrees with the theory that convective dilution and mixing are more likely to occur in massive white dwarfs, which supports evolutionary models and observations suggesting that higher mass white dwarfs have thinner hydrogen layers.

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arXiv2018

New full evolutionary sequences of H and He atmosphere massive white dwarf stars using MESA

We explore the evolution of hydrogen-rich and hydrogen-deficient white dwarf stars with masses between 1.012 and 1.307 Msun, and initial metallicity of Z=0.02. These sequences are the result of main sequence stars with masses between 8.8 and 11.8 Msun. The simulations were performed with MESA, starting at the zero-age main sequence, through thermally pulsing and mass-loss phases, ending at the white dwarfs cooling sequence. We present reliable chemical profiles for the whole mass range considered, covering the different expected central compositions, i.e. C/O, O/Ne and Ne/O/Mg, and its dependence with the stellar mass. In addition, we present detailed chemical profiles of hybrid C/O-O/Ne core white dwarfs, found in the mass range between 1.024 and 1.15 Msun. We present the initial-to-final mass relation, mass-radius relation, and cooling times considering the effects of atmosphere and core composition.

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arXiv2018

The impact of pre-white dwarf evolution on the pulsational properties and asteroseismological inferences of ZZ Ceti stars

ZZ Ceti stars are pulsating white dwarfs with a carbon-oxygen core (or possibly ONe for the most massive stars) build up during the core helium burning (CHeB) and thermally pulsing Asymptotic Giant Branch (TP-AGB) phases. Through the interpretation of their pulsation periods by means of asteroseismology, details about their origin and evolution can be inferred. The whole pulsation spectrum exhibited by ZZ Ceti stars strongly depend on the inner chemical structure. At present, there are several processes affecting the chemical profiles that are still not accurately determined. We present a study of the impact of current uncertainties in the evolution of white dwarf progenitor on the expected pulsation properties and on the stellar parameters inferred from asteroseismological fits of ZZ Ceti stars. Our analysis is based on a set of carbon-oxygen core white dwarf models that are derived from full evolutionary computations from the ZAMS to the ZZ Ceti domain. We considered models in which we varied the number of thermal pulses, the amount of overshooting, and the carbon-alpha reaction rate within their uncertainties. We explore the impact of these major uncertainties in prior evolution on the chemical structure and expected pulsation spectrum. We find that these uncertainties yield significant changes in the g-mode pulsation periods being those found during the TP-AGB phase the most relevant for the pulsational properties and the asteroseismological derived stellar parameters of ZZ Ceti stars. We conclude that the uncertainties in the white dwarf progenitor evolution should be taken into account in detailed asteroseismological analyses of these pulsating stars.

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arXiv2018

Evolution and Asteroseismology of Pulsating Low-Mass White Dwarfs

Many low-mass white dwarfs are being discovered in the field of our galaxy and some of them exhibit $g$-mode pulsations, comprising the extremely low-mass variable (ELMV) stars class. Despite it is generally believed that these stars are characterized by thick H envelopes, from stellar evolution considerations, the existence of low-mass WDs with thin H envelopes is also possible. We have performed detailed asteroseismological fits to all the known ELMVs to search for a representative model by employing a set of fully evolutionary models that are representative of low-mass He-core white dwarf stars with a range of stellar masses $[0.1554-0.4352]\ M_{\odot}$, effective temperatures $[6000-10000]\ $K, and also with a range of H envelope thicknesses $-5.8 \lesssim \log(M_{\rm H}/M_{\star}) \lesssim -1.7$, hence expanding the space of parameters. We found that some of the stars under analysis are characterized by thick H envelopes, but others are better represented by models with thin H envelope.

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