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1
Kepler, S. O., Alejandra Daniela Romero, Ingrid Pelisoli, and Gustavo Ourique. "White Dwarf Stars." International Journal of Modern Physics: Conference Series 45 (January 2017): 1760023. http://dx.doi.org/10.1142/s2010194517600230.
Повний текст джерелаАнотація:
White dwarf stars are the final stage of most stars, born single or in multiple systems. We discuss the identification, magnetic fields, and mass distribution for white dwarfs detected from spectra obtained by the Sloan Digital Sky Survey up to Data Release 13 in 2016, which lead to the increase in the number of spectroscopically identified white dwarf stars from 5[Formula: see text]000 to 39[Formula: see text]000. This number includes only white dwarf stars with [Formula: see text], i.e., excluding the Extremely Low Mass white dwarfs, which are necessarily the byproduct of stellar interaction.
2
Shipman, H. L. "Variable white dwarfs." Proceedings of the International Astronomical Union 15, S357 (October 2019): 107–9. http://dx.doi.org/10.1017/s1743921320001386.
Повний текст джерелаАнотація:
AbstractAsteroseismology of white dwarf stars has led to a number of interesting results pertaining to the long term evolution and present state of white dwarf interiors. I will review recent results and will give a not necessarily comprehensive view of the prospects for further progress in this area. Two – but only two white dwarf stars - have shown the expected cooling as they age. Careful observations of a few white dwarfs with rich pulsational properties reveal interior compositions as well as the thickness of their surface layers. A few very well observed stars have revealed changes in their pulsational spectra which we don’t understand yet.
3
Kepler, S. O., Ingrid Pelisoli, Detlev Koester, Nicole Reindl, Stephan Geier, Alejandra D. Romero, Gustavo Ourique, Cristiane de Paula Oliveira, and Larissa A. Amaral. "White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14." Monthly Notices of the Royal Astronomical Society 486, no. 2 (April 8, 2019): 2169–83. http://dx.doi.org/10.1093/mnras/stz960.
Повний текст джерелаАнотація:
ABSTRACT 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 15 793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below $T_\mathrm{eff}\simeq 10\, 000$ K; most are likely low-mass metal-poor main-sequence stars, but some could be the result of interacting binary evolution.
4
Hogg, M. A., S. L. Casewell, G. A. Wynn, E. S. Longstaff, I. P. Braker, M. R. Burleigh, R. H. Tilbrook, et al. "Confirming new white dwarf-ultracool dwarf binary candidates." Monthly Notices of the Royal Astronomical Society 498, no. 1 (August 10, 2020): 12–24. http://dx.doi.org/10.1093/mnras/staa2233.
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ABSTRACT We present the results of a study to discover prospective new white dwarf-L dwarf binaries as identified by their near-infrared excesses in the UKIDSS catalogue. We obtained optical spectra to validate the white dwarf nature for 22 of the candidate primary stars, confirming ten as white dwarfs and determining their effective temperatures and gravities. For all 10 white dwarfs, we determined that the near-infrared excess was indeed indicative of a cool companion. Six of these are suggestive of late M dwarf companions, and three are candidate L dwarf companions, with one straddling the M−L boundary. We also present near-infrared spectra of eight additional candidate white dwarf-ultracool dwarf binaries, where the white dwarf primary had been previously confirmed. These spectra indicate one candidate at the M−L boundary, three potential L dwarf companions, and one suspected M dwarf, which showed photometric variability on a ∼6 h period, suggesting the system may be close. Radial velocity follow-up is required to confirm whether these systems are close, or widely separated.
5
Kepler, S. O., Detlev Koester, Ingrid Pelisoli, Alejandra D. Romero, and Gustavo Ourique. "White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 16." Monthly Notices of the Royal Astronomical Society 507, no. 3 (August 24, 2021): 4646–60. http://dx.doi.org/10.1093/mnras/stab2411.
Повний текст джерелаАнотація:
ABSTRACT White dwarfs are the end state of the evolution of more than 97 per cent of all stars, and therefore carry information on the structure and evolution of the Galaxy through their luminosity function and initial-to-final mass relation. Examining the new spectra of all white or blue stars in the Sloan Digital Sky Survey Data Release 16, we report the spectral classification of 2410 stars, down to our identification cut-off of signal-to-noise ratio equal to three. We newly identify 1404 DAs, 189 DZs, 103 DCs, 12 DBs, and nine CVs. The remaining objects are a mix of carbon or L stars (dC/L), narrow-lined hydrogen-dominated stars (sdA), dwarf F stars, and P Cyg objects. As white dwarf stars were not targeted by SDSS DR16, the number of new discoveries is much smaller than in previous releases. We also report atmospheric parameters and masses for a subset consisting of 555 new DAs, 10 new DBs, and 85 DZs for spectra with signal-to-noise ratio larger than 10.
6
Solheim, J. E. "Interacting Binary White Dwarf Stars." Symposium - International Astronomical Union 151 (1992): 461–64. http://dx.doi.org/10.1017/s0074180900122727.
Повний текст джерелаАнотація:
This group of stars consists of 4 systems, also called helium cataclysmics. Three of them show photometric variations and have been studied by the Whole Earth Telescope (WET), which have revealed multiperiodic light curves showing the signature of g-mode non-radial pulsations on the accreting star. The combination of accretion and g-mode pulsations gives a unique opportunity to test models for the accreator's structural changes in response to accretion. IUE-spectra provide additional physical parameters.
7
Finch, Nicolle L., S. P. Preval, M. A. Barstow, S. L. Casewell, T. Ayres, B. Welsh, M. Bainbridge та N. Reindl. "What can ISM and non-photospheric highly ionised lines in white dwarf spectra reveal about the β CMa tunnel?" Proceedings of the International Astronomical Union 15, S357 (жовтень 2019): 220–24. http://dx.doi.org/10.1017/s1743921320001374.
Повний текст джерелаАнотація:
AbstractWhite dwarfs are useful objects with which to study the local interstellar medium (ISM). High ionisation state absorption features that cannot be attributed to the photosphere or the ISM have been observed along the line-of-sight to a number of white dwarf stars. Suggested origins of these lines include ionisation from past supernovae, stellar winds, circumstellar disks, photoionisation from nearby hot stars or also from the white dwarf itself. In this study we consider the origin of these non-photospheric highly ionised lines in two stars towards a rarefied region of the galaxy known as the extended β CMa Tunnel. We present preliminary results from our analysis of the first of these two stars.
8
Fontaine, G., P. Brassard, P. Bergeron, and F. Wesemael. "White Dwarf Seismology at the Université de Montréal." International Astronomical Union Colloquium 139 (1993): 120. http://dx.doi.org/10.1017/s0252921100117063.
Повний текст джерелаАнотація:
Over the last several years, we have developed a comprehensive program aimed at better understanding the properties of pulsating DA white dwarfs (or ZZ Ceti stars). These stars are nonradial pulsators of the g-type, and their study can lead to inferences about their internal structure. For instance, the period spectrum of a white dwarf is most sensitive to its vertical chemical stratification, and one of the major goals of white dwarf seismology is to determine the thickness of the hydrogen layer that sits on top of a star. This can be done, in principle, by comparing in detail theoretical period spectra with the periods of the observed excited modes. Likewise, because the cooling rate of a white dwarf is very sensitive to the specific heat of its core material (and hence to its composition), it is possible to infer the core composition through measurements and interpretations of rates of period change in a pulsator.
9
Allard, N. F., J. F. Kielkopf, S. Blouin, P. Dufour, F. X. Gadéa, T. Leininger, and G. Guillon. "Line shapes of the magnesium resonance lines in cool DZ white dwarf atmospheres." Astronomy & Astrophysics 619 (November 2018): A152. http://dx.doi.org/10.1051/0004-6361/201834067.
Повний текст джерелаАнотація:
Context. Line shapes of the magnesium resonance lines in white dwarf spectra are determined by the properties of magnesium atoms and the structure of the white dwarf atmosphere. Through their blanketing effect, these lines have a dominant influence on the model structure and thus on the determination from the spectra of other physical parameters that describe the stellar atmosphere and elemental abundances. Aims. In continuation of previous work on Mg+He lines in the UV, we present theoretical profiles of the resonance line of neutral Mg perturbed by He at the extreme density conditions found in the cool largely transparent atmosphere of DZ white dwarfs. Methods. We accurately determined the broadening of Mg by He in a unified theory of collisional line profiles using ab initio calculations of MgHe potential energies and transition matrix elements among the singlet electronic states that are involved for the observable spectral lines. Results. We computed the shapes and line parameters of the Mg lines and studied their dependence on helium densities and temperatures. We present results over the full range of temperatures from 4000 to 12 000 K needed for input to stellar spectra models. Atmosphere models were constructed for a range of effective temperatures and surface gravities typical for cool DZ white dwarfs. We present synthetic spectra tracing the behavior of the Mg resonance line profiles under the low temperatures and high gas pressures prevalent in these atmospheres. Conclusions. The determination of accurate opacity data of magnesium resonance lines together with an improved atmosphere model code lead to a good fit of cool DZ white dwarf stars. The broadening of spectral lines by helium needs to be understood to accurately determine the H/He and Mg/He abundance ratio in DZ white dwarf atmospheres. We emphasize that no free potential parameters or ad hoc adjustments were used to calculate the line profiles.
10
Majlinger, Zlatko, Milan S. Dimitrijević, and Vladimir A. Srećković. "Stark broadening of Co ii spectral lines in hot stars and white dwarf spectra." Monthly Notices of the Royal Astronomical Society 496, no. 4 (July 9, 2020): 5584–90. http://dx.doi.org/10.1093/mnras/staa1947.
Повний текст джерелаАнотація:
ABSTRACT Stark full widths at half-maximum for 46 Co ii multiplets have been calculated using modified semi-empirical method. The obtained results have been used to demonstrate the importance of Stark broadening mechanism in DA and DB white dwarf and A star atmospheres. With the obtained results we also test possibility of using some approximate methods of Stark width calculations developed on the basis of regularities and systematic trends.
11
Aloui, Rihab, Haykel Elabidi, and Sylvie Sahal-Bréchot. "Sr v–vi line widths in hot white dwarf atmospheres." Monthly Notices of the Royal Astronomical Society 512, no. 2 (February 22, 2022): 1598–607. http://dx.doi.org/10.1093/mnras/stac405.
Повний текст джерелаАнотація:
ABSTRACT Missing Stark widths for 37 spectral lines of strontium ions (17 Sr v lines and 20 Sr vi lines) have been calculated using a quantum-mechanical method. Twenty-three spectral lines of Sr v have been recently discovered, for the first time, in the ultraviolet spectrum of the hot white dwarf RE 0503−289. This recent discovery prompts us to calculate the Stark widths of the new lines. These calculations can fill the lack of the data base STARK-B and can be used to investigate the observed spectra in such stars. To perform the line broadening calculations, preliminary structure and collision calculations have been carried out using the sequence of the University College London codes (superstructure, distorted wave, and jajom). Results for the 37 lines are provided for different electron temperatures and at density Ne = 1017 cm−3. These results will enter the STARK-B data base, which is a node of the Virtual Atomic and Molecular Data Center. We hope that the obtained results will be useful for the non-local thermodynamic equilibrium modelling of stellar atmospheres.
12
Werner, Klaus. "Central stars of planetary nebulae: The white dwarf connection." Proceedings of the International Astronomical Union 7, S283 (July 2011): 196–203. http://dx.doi.org/10.1017/s1743921312010952.
Повний текст джерелаАнотація:
AbstractThis paper is focused on the transition phase between central stars and white dwarfs, i.e. objects in the effective temperature range 100 000 – 200 000 K. We confine our review to hydrogen-deficient stars because the common H-rich objects are subject of the paper by Ziegler et al. in these proceedings. We address the claimed iron-deficiency in PG1159 stars and [WC] central stars. The discovery of new Ne vii and Ne viii lines in PG1159 stars suggests that the identification of O vii and O viii lines that are used for spectral classification of [WCE] stars is wrong. We then present evidence for two distinct post-AGB evolutionary sequences for H-deficient stars based on abundance analyses of the He-dominated O(He) stars and the hot DO white dwarf KPD 0005+5106. Finally, we report on evidence for an H-deficient post-super AGB evolution sequence represented by the hottest known, carbon/oxygen-atmosphere white dwarf H 1504+65 and the recently discovered carbon-atmosphere “hot DQ” white dwarfs.
13
Miller, David R., Ilaria Caiazzo, Jeremy Heyl, Harvey B. Richer, and Pier-Emmanuel Tremblay. "The Ultramassive White Dwarfs of the Alpha Persei Cluster." Astrophysical Journal Letters 926, no. 2 (February 1, 2022): L24. http://dx.doi.org/10.3847/2041-8213/ac50a5.
Повний текст джерелаАнотація:
Abstract We searched through the entire Gaia EDR3 candidate white dwarf catalog for stars with proper motions and positions that are consistent with them having escaped from the Alpha Persei cluster within the past 81 Myr, the age of the cluster. In this search we found five candidate white dwarf escapees from Alpha Persei and obtained spectra for all of them. We confirm that three are massive white dwarfs sufficiently young to have originated in the cluster. All these are more massive than any white dwarf previously associated with a cluster using Gaia astrometry, and possess some of the most massive progenitors. In particular, the white dwarf Gaia EDR3 4395978097863572, which lies within 25 pc of the cluster center, has a mass of about 1.20 solar masses and evolved from an 8.5 solar-mass star, pushing the upper limit for white dwarf formation from a single massive star, while still leaving a substantial gap between the resulting white dwarf mass and the Chandrasekhar mass.
14
Rohrmann, R. D., A. M. Serenelli, L. G. Althaus, and O. G. Benvenuto. "Improved synthetic spectra of helium-core white dwarf stars." Monthly Notices of the Royal Astronomical Society 335, no. 2 (September 2002): 499–511. http://dx.doi.org/10.1046/j.1365-8711.2002.05644.x.
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15
Wesemael, F., J. L. Greenstein, James Liebert, R. Lamontagne, G. Fontaine, P. Bergeron, and J. W. Glaspey. "An atlas of optical spectra of white-dwarf stars." Publications of the Astronomical Society of the Pacific 105 (July 1993): 761. http://dx.doi.org/10.1086/133228.
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Kepler, S. O., B. G. Castanheira, A. F. M. Costa, and D. Koester. "Gemini spectra of 12 000 K white dwarf stars." Monthly Notices of the Royal Astronomical Society 372, no. 4 (November 11, 2006): 1799–803. http://dx.doi.org/10.1111/j.1365-2966.2006.10992.x.
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17
Chayer, P., G. Fontaine, and F. Wesemael. "Hervy Element Abundances Predicted by Radiative Support Theory in the Atmospheres of Hot White Dwarfs." International Astronomical Union Colloquium 114 (1989): 253–57. http://dx.doi.org/10.1017/s0252921100099656.
Повний текст джерелаАнотація:
The surface composition of a white dwarf evolves as a result of the interaction of several mechanisms, the most important of which being gravitational settling. In the early phases of the evolution, theory shows that selective radiative levitation can occasionally defeat settling and, thus, prevent the formation of a pristine pure hydrogen (helium) atmospheric layer in a hot DA (non-DA) white dwarf (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979). The exciting discovery of sharp metallic features in the ultraviolet spectra of several hot DA and non-DA stars alike resulting from the work of several investigators has provided the essential motivation for further theoretical investigations of radiative levitation in the atmospheres of white dwarfs. Additionnal impetus comes from the continuing investigations of hot DA white dwarfs carried out by Bruhweiler and Kondo which have already revealed a most interesting observational pattern of heavy elements in these stars (Bruhweiler 1985). Moreover the recent availability of theoretical equivalent widths of selected astrophysically important ultraviolet metal lines in hot DA white dwarfs (Henry, Shipman, and Wesemael 1985) makes a comparison between theory and observations -in at least this type of stars- a timely and useful exercise.
18
Reindl, Nicole, Veronika Schaffenroth, Semih Filiz, Stephan Geier, Ingrid Pelisoli, and Souza Oliveira Kepler. "Mysterious, variable, and extremely hot: White dwarfs showing ultra-high excitation lines." Astronomy & Astrophysics 647 (March 2021): A184. http://dx.doi.org/10.1051/0004-6361/202140289.
Повний текст джерелаАнотація:
Context. About 10% of all stars exhibit absorption lines of ultra-highly excited (UHE) metals (e.g., O VIII) in their optical spectra when entering the white dwarf cooling sequence. This is something that has never been observed in any other astrophysical object, and poses a decades-long mystery in our understanding of the late stages of stellar evolution. The recent discovery of a UHE white dwarf that is both spectroscopically and photometrically variable led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere. Aims. We aim to gain a better understanding of these mysterious objects by studying the photometric variability of the whole population of UHE white dwarfs, and white dwarfs showing only the He II line problem, as both phenomena are believed to be connected. Methods. We investigate (multi-band) light curves from several ground- and space-based surveys of all 16 currently known UHE white dwarfs (including one newly discovered) and eight white dwarfs that show only the He II line problem. Results. We find that 75−13+8% of the UHE white dwarfs, and 75−19+9% of the He II line problem white dwarfs are significantly photometrically variable, with periods ranging from 0.22 d to 2.93 d and amplitudes from a few tenths to a few hundredths of a magnitude. The high variability rate is in stark contrast to the variability rate amongst normal hot white dwarfs (we find 9−2+4%), marking UHE and He II line problem white dwarfs as a new class of variable stars. The period distribution of our sample agrees with both the orbital period distribution of post-common-envelope binaries and the rotational period distribution of magnetic white dwarfs if we assume that the objects in our sample will spin-up as a consequence of further contraction. Conclusions. We find further evidence that UHE and He II line problem white dwarfs are indeed related, as concluded from their overlap in the Gaia HRD, similar photometric variability rates, light-curve shapes and amplitudes, and period distributions. The lack of increasing photometric amplitudes towards longer wavelengths, as well as the nondetection of optical emission lines arising from the highly irradiated face of a hypothetical secondary in the optical spectra of our stars, makes it seem unlikely that an irradiated late-type companion is the origin of the photometric variability. Instead, we believe that spots on the surfaces of these stars and/or geometrical effects of circumstellar material might be responsible.
19
Fontaine, G., and F. Wesemael. "Successes and Challenges of the Theory of White Dwarf Spectral Evolution." Symposium - International Astronomical Union 145 (1991): 421–34. http://dx.doi.org/10.1017/s0074180900227551.
Повний текст джерелаАнотація:
Of all stars in the Hertzprung-Russell diagram, white dwarfs are those for which the clues on past evolution given by photospheric abundances are probably the hardest to decipher. This is because the cooling phase of white dwarfs, a relatively uneventful phase from an evolutionary point of view, is, in contrast, a most active phase for the evolution of the chemical composition of the envelope. Indeed, it is now well established that the often puzzling variety of surface abundances observed in white dwarf stars can be traced to the simultaneous operation, in the outer layers of these stars, of a variety of physical processes which will also erase the abundances present in the photosphere at the onset of cooling.Downward element diffusion in the intense gravitational field of the degenerate star is perhaps the mechanism which is the most closely identified with white dwarf stars. However, convective mixing, ordinary diffusion, radiative forces, winds, and accretion from the interstellar medium all are equally important processes which, at times, compete efficiently with the rapid element segregation expected in those stars. The various regions, along the cooling sequence of white dwarfs, where individual processes are expected to operate, have been summarized by Fontaine and Wesemael (1987). We illustrate here various combinations of these mechanisms which have been found in white dwarfs, and show how their competition affects the observed abundance patterns. The unity underlying these cases stems from the fact that, in many cases, progress in investigating these complicated situations has come only through the combination of evolutionary calculations with new and powerful numeriques techniques which have been developed at Montréal (Pelletier 1986; Pelletier, Fontaine, and Wesemael 1989).
20
Wilson, Thomas G., Jay Farihi, Boris T. Gänsicke, and Andrew Swan. "The unbiased frequency of planetary signatures around single and binary white dwarfs using Spitzer and Hubble." Monthly Notices of the Royal Astronomical Society 487, no. 1 (April 13, 2019): 133–46. http://dx.doi.org/10.1093/mnras/stz1050.
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Abstract This paper presents combined Spitzer IRAC and Hubble COS results for a double-blind survey of 195 single and 22 wide binary white dwarfs for infrared excesses and atmospheric metals. The selection criteria include cooling ages in the range 9 to 300 Myr, and hydrogen-rich atmospheres so that the presence of atmospheric metals can be confidently linked to ongoing accretion from a circumstellar disc. The entire sample has infrared photometry, whereas 168 targets have corresponding ultraviolet spectra. Three stars with infrared excesses due to debris discs are recovered, yielding a nominal frequency of $1.5_{-0.5}^{+1.5}$ per cent, while in stark contrast, the fraction of stars with atmospheric metals is 45 ± 4 per cent. Thus, only one out of 30 polluted white dwarfs exhibits an infrared excess at 3–4 $\mu$m in IRAC photometry, which reinforces the fact that atmospheric metal pollution is the most sensitive tracer of white dwarf planetary systems. The corresponding fraction of infrared excesses around white dwarfs with wide binary companions is consistent with zero, using both the infrared survey data and an independent assessment of potential binarity for well-established dusty and polluted stars. In contrast, the frequency of atmospheric pollution among the targets in wide binaries is indistinct from apparently single stars, and moreover the multiplicity of polluted white dwarfs in a complete and volume-limited sample is the same as for field stars. Therefore, it appears that the delivery of planetesimal material on to white dwarfs is ultimately not driven by stellar companions, but by the dynamics of planetary bodies.
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Castanheira, B. G., S. O. Kepler, G. Handler, and D. Koester. "Analysis of IUE spectra of helium-rich white dwarf stars." Astronomy & Astrophysics 450, no. 1 (April 2006): 331–37. http://dx.doi.org/10.1051/0004-6361:20054221.
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Hollands, Mark A., Boris T. Gänsicke, Detlev Koester, Vadim Alekseev, and Emma L. Herbert. "Chemistry of the oldest white dwarf planetary systems." Proceedings of the International Astronomical Union 13, S332 (March 2017): 202–9. http://dx.doi.org/10.1017/s1743921317006779.
Повний текст джерелаАнотація:
AbstractAlmost all stars in the Milky Way, including the Sun, will end their lives as white dwarfs. Their relatively peaceful transition off of the main sequence implies that most of their planetary systems will survive engulfment during the deaths of their host stars. These remnant planetary systems remain detectable for many Gyr through the occasional metal-contamination of the white dwarf photospheres by tidally disrupted planetesimals. Spectral analysis of these “metal-polluted” white dwarfs therefore provides a direct method for measuring the chemical compositions of extrasolar material. Here we present our sample of 230 cool white dwarfs with metal-rich photospheres, explore the diverse range of compositions of the accreted matter, and discuss two extreme systems which have respectively accreted planetesimals consistent with crust-like and core-like planetary material.
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Sánchez Arias, Julieta P., Alejandra D. Romero, Alejandro H. Córsico, Ingrid Pelisoli, Victoria Antoci, S. O. Kepler, Leandro G. Althaus та Mariela A. Corti. "Comparing the asteroseismic properties of pulsating pre-extremely low mass white dwarf and δ Scuti stars". Astronomy & Astrophysics 616 (серпень 2018): A80. http://dx.doi.org/10.1051/0004-6361/201731808.
Повний текст джерелаАнотація:
Context. Pulsating extremely low-mass pre-white dwarf stars (pre-ELMV), with masses between ~0.15 M⊙ and ~0.30 M⊙, constitute a new class of variable stars showing g- and possibly p-mode pulsations with periods between 320 and 6000 s (frequencies between 14.4 and 270 c/d), driven by the κ mechanism operating in the second He ionization zone. On the other hand, main sequence δ Scuti stars, with masses between 1.2 and 2.5 M⊙, pulsate in low-order g and p modes with periods in the range [700–28 800] s (frequencies in the range [3–123] c/d), driven by the κ mechanism operating in the He II ionization zone and the turbulent pressure acting in the HI ionization layer. Interestingly enough, the instability strips of pre-ELM white dwarf and δ Scuti stars nearly overlap in the Teff vs. log g diagram, leading to a degeneracy when spectroscopy is the only tool to classify the stars and pulsation periods only are considered. Aims. Pre-ELM white dwarf and δ Scuti stars are in very different stages of evolution and therefore their internal structure is very distinct. This is mirrored in their pulsational behavior, thus employing asteroseismology should allow us to distinguish between these groups of stars despite their similar atmospheric parameters. Methods. We have employed adiabatic and non-adiabatic pulsation spectra for models of pre-ELM white dwarfs and δ Scuti stars, and compare their pulsation periods, period spacings, and rates of period change. Results. Unsurprisingly, we found substantial differences in the period spacing of δ Scuti and pre-ELM white dwarf models. Even when the same period range is observed in both classes of pulsating stars, the modes have distinctive signature in the period spacing and period difference values. For instance, the mean period difference of p-modes of consecutive radial orders for δ Scuti model are at least four times longer than the mean period spacing for the pre-ELM white dwarf model in the period range [2000–4600] s (frequency range [18.78–43.6] c/d). In addition, the rate of period change is two orders of magnitudes larger for the pre-ELM white dwarfs compared to δ Scuti stars. In addition, we also report the discovery of a new variable star, SDSSJ075738.94+144827.50, located in the region of the Teff versus log g diagram where these two kind of stars coexist. Conclusions.The characteristic spacing between modes of consecutive radial orders (p as well as g modes) and the large differences found in the rates of period change for δ Scuti and pre-ELM white dwarf stars suggest that asteroseismology can be employed to discriminate between these two groups of variable stars. Furthermore, we found that SDSSJ075738.94+144827.50 exhibits a period difference between p modes characteristic of a δ Sct star, assuming consecutive radial order for the observed periods.
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Brassard, P., G. Fontaine, F. Vesemael, and S. D. Kawaler. "Are Pulsations a Useful Probe of the Structure of the Outer Layers of White Dwarfs ?" International Astronomical Union Colloquium 114 (1989): 263–68. http://dx.doi.org/10.1017/s025292110009967x.
Повний текст джерелаАнотація:
The most fundamental aspect of white dwarf seismology is the determination of the gravity-mode (g-mode) period structures of models of isolated pulsating white dwarfs. These stars show multiperiodic luminosity variations which result from the superposition of excited pulsation modes. Among the many oscillation modes available in the very rich nonradial g-mode spectra of white dwarfs, the observed modes are selectively chosen by a filtering mechanism. Although the period evolution is strongly tied to the core temperature evolution in a white dwarf, the period structure remains largely specified by the mechanical properties of the star. The most basic structural feature of a white dwarf is its highly degenerate interior, which leads to nearly isothermal and nearly isentropic stratifications in the core region containing more than 99% of the mass of the star. In particular, because the density gradient is almost adiabatic throughout the interior of a white dwarf, the Brünt-Väisälä frequency (see below) is very small there and low-order g-modes cannot propagate. As a result, g-modes are essentially envelope modes in white dwarfs, with large amplitudes occuring only in the non-degenerate outer layers. One can thus expect that g-modes in white dwarfs are extremely sensitive to envelope properties such as compositional stratification and partial ionization mechanisms. Compositional stratification is, in fact, the second structural feature of a white dwarf model which has strong effects on the period structure. Indeed, trapped modes result when a resonance or near-resonance occurs between the local g-mode radial wavelength and the thickness of one of the composition layers. This results in a period structure which strongly bears the signature of compositional stratification in the outer layers. Thus, it has been widely accepted that white dwarf pulsations probe primarily the outer layers of these stars. This point of view has been borne out by detailed pulsation calculations carried out by several independant groups (see Winget 1987 and references therein).
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Lew, Ben W. P., Dániel Apai, Yifan Zhou, Mark Marley, L. C. Mayorga, Xianyu Tan, Vivien Parmentier, Sarah Casewell, and Siyi Xu (许偲艺). "Mapping the Pressure-dependent Day–Night Temperature Contrast of a Strongly Irradiated Atmosphere with HST Spectroscopic Phase Curve." Astronomical Journal 163, no. 1 (December 13, 2021): 8. http://dx.doi.org/10.3847/1538-3881/ac3001.
Повний текст джерелаАнотація:
Abstract Many brown dwarfs are on ultrashort-period and tidally locked orbits around white dwarf hosts. Because of these small orbital separations, the brown dwarfs are irradiated at levels similar to hot Jupiters. Yet, they are easier to observe than hot Jupiters because white dwarfs are fainter than main-sequence stars at near-infrared wavelengths. Irradiated brown dwarfs are, therefore, ideal hot Jupiter analogs for studying the atmospheric response under strong irradiation and fast rotation. We present the 1.1–1.67 μm spectroscopic phase curve of the irradiated brown dwarf (SDSS1411-B) in the SDSS J141126.20 + 200911.1 brown dwarf–white dwarf binary with the near-infrared G141 grism of the Hubble Space Telescope Wide Field Camera 3. SDSS1411-B is a 50M Jup brown dwarf with an irradiation temperature of 1300 K and has an orbital period of 2.02864 hr. Our best-fit model suggests a phase-curve amplitude of 1.4% and places an upper limit of 11° for the phase offset from the secondary eclipse. After fitting the white dwarf spectrum, we extract the phase-resolved brown dwarf emission spectra. We report a highly wavelength-dependent day–night spectral variation, with a water-band flux variation of about 360% ± 70% and a comparatively small J-band flux variation of 37% ± 2%. By combining the atmospheric modeling results and the day–night brightness temperature variations, we derive a pressure-dependent temperature contrast. We discuss the difference in the spectral features of SDSS1411-B and hot Jupiter WASP-43b, as well as the lower-than-predicted day–night temperature contrast of J4111-BD. Our study provides the high-precision observational constraints on the atmospheric structures of an irradiated brown dwarf at different orbital phases.
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Hernandez, M. S., M. R. Schreiber, S. G. Parsons, B. T. Gänsicke, F. Lagos, R. Raddi, O. Toloza, et al. "The White Dwarf Binary Pathways Survey – IV. Three close white dwarf binaries with G-type secondary stars." Monthly Notices of the Royal Astronomical Society 501, no. 2 (December 14, 2020): 1677–89. http://dx.doi.org/10.1093/mnras/staa3815.
Повний текст джерелаАнотація:
ABSTRACT Constraints from surveys of post-common envelope binaries (PCEBs) consisting of a white dwarf plus an M-dwarf companion have led to significant progress in our understanding of the formation of close white dwarf binary stars with low-mass companions. The white dwarf binary pathways project aims at extending these previous surveys to larger secondary masses, i.e. secondary stars of spectral-type AFGK. Here, we present the discovery and observational characterization of three PCEBs with G-type secondary stars and orbital periods between 1.2 and 2.5 d. Using our own tools as well as MESA, we estimate the evolutionary history of the binary stars and predict their future. We find a large range of possible evolutionary histories for all three systems and identify no indications for differences in common envelope evolution compared to PCEBs with lower mass secondary stars. Despite their similarities in orbital period and secondary spectral type, we estimate that the future of the three systems is very different: TYC 4962-1205-1 is a progenitor of a cataclysmic variable system with an evolved donor star, TYC 4700-815-1 will run into dynamically unstable mass transfer that will cause the two stars to merge, and TYC 1380-957-1 may appear as supersoft source before becoming a rather typical cataclysmic variable star.
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Kenyon, Scott J. "Symbiotic Binary Stars." Symposium - International Astronomical Union 151 (1992): 137–46. http://dx.doi.org/10.1017/s0074180900122132.
Повний текст джерелаАнотація:
This paper briefly reviews the physical properties of symbiotic stars: long-period interacting binaries composed of a red giant primary star and a hot companion. Two types of binaries produce symbiotic optical spectra: semi-detached systems with a main sequence secondary and detached systems with a white dwarf secondary. Semi-detached symbiotics resemble cataclysmic variables and Algol binaries, but on a much larger scale, and undergo dwarf nova-like eruptions. Wind accretion powers detached systems; occasional thermonuclear runaways produce symbiotic novae - distant cousins of classical novae.
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Wang, Kun, Péter Németh, Yangping Luo, Xiaodian Chen, Qingquan Jiang, and Xingmei Cao. "Extremely Low-mass White Dwarf Stars Observed in Gaia DR2 and LAMOST DR8." Astrophysical Journal 936, no. 1 (August 25, 2022): 5. http://dx.doi.org/10.3847/1538-4357/ac847c.
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Abstract We present the first results from our ongoing project to study extremely low-mass (ELM) white dwarfs (WDs) (M ≤ 0.3M ☉) with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectra. Based on the LAMOST DR8 spectral database, we analyzed 136 ELM WD candidates selected from Gaia DR2 data and 12 known objects previously identified by the ELM Survey. The atmospheric parameters and radial velocities of these stars were obtained by fitting the LAMOST low-resolution spectra. After comparing the atmospheric parameters of the 12 known objects from this work to the results reported by the ELM Survey, we demonstrated the potential of LAMOST spectra in probing into the nature of ELM WDs. Based on the atmospheric parameters and Gaia EDR3 data, we identified 21 new high-probability ELM WDs with masses M ≤ 0.3M ☉ and parallax estimates that agree to within a factor of 3. Two of them, J0338+4134 and J1129+4715, show significant radial velocity variability and are very likely to be binary systems containing at least one ELM WD.
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Escorza, A., D. Karinkuzhi, A. Jorissen, L. Siess, H. Van Winckel, D. Pourbaix, C. Johnston, et al. "Barium and related stars, and their white-dwarf companions." Astronomy & Astrophysics 626 (June 2019): A128. http://dx.doi.org/10.1051/0004-6361/201935390.
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Barium (Ba) dwarfs and CH subgiants are the less evolved analogues of Ba and CH giants. They are F- to G-type main-sequence stars polluted with heavy elements by their binary companions when the companion was on the asymptotic giant branch (AGB). This companion is now a white dwarf that in most cases cannot be directly detected. We present a large systematic study of 60 objects classified as Ba dwarfs or CH subgiants. Combining radial-velocity measurements from HERMES and SALT high-resolution spectra with radial-velocity data from CORAVEL and CORALIE, we determine the orbital parameters of 27 systems. We also derive their masses by comparing their location in the Hertzsprung–Russell diagram with evolutionary models. We confirm that Ba dwarfs and CH subgiants are not at different evolutionary stages, and that they have similar metallicities, despite their different names. Additionally, Ba giants appear significantly more massive than their main-sequence analogues. This is likely due to observational biases against the detection of hotter main-sequence post-mass-transfer objects. Combining our spectroscopic orbits with the HIPPARCOS astrometric data, we derive the orbital inclination and the mass of the WD companion for four systems. Since this cannot be done for all systems in our sample yet (but should be possible with upcoming Gaia data releases), we also analyse the mass-function distribution of our binaries. We can model this distribution with very narrow mass distributions for the two components and random orbital orientations on the sky. Finally, based on BINSTAR evolutionary models, we suggest that the orbital evolution of low-mass Ba systems can be affected by a second phase of interactions along the red giant branch of the Ba star, which impact the eccentricities and periods of the giants.
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Price, C. W., and H. L. Shipman. "Balmer-line spectra of white dwarf stars with chemically stratified atmospheres." Astrophysical Journal 295 (August 1985): 561. http://dx.doi.org/10.1086/163399.
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Allard, Nicole F., John F. Kielkopf, Siyi Xu, Grégoire Guillon, Bilel Mehnen, Roberto Linguerri, Muneerah Mogren Al Mogren, Majdi Hochlaf та Ivan Hubeny. "H–He collision-induced satellite in the Lyman α profile of DBA white dwarf stars". Monthly Notices of the Royal Astronomical Society 494, № 1 (13 березня 2020): 868–75. http://dx.doi.org/10.1093/mnras/staa707.
Повний текст джерелаАнотація:
ABSTRACT The spectra of helium-dominated white dwarf stars with hydrogen in their atmosphere present a distinctive broad feature centred around 1160 Å in the blue wing of the Lyman α line. It is extremely apparent in WD 1425+540 recently observed with Hubble Space Telescope(HST) Cosmic Origins Spectrograph (COS). With new theoretical line profiles based on ab initio atomic interaction potentials we show that this feature is a signature of a collision-induced satellite due to an asymptotically forbidden transition. This quasi-molecular spectral satellite is crucial to understanding the asymmetrical shape of Lyman α seen in this and other white dwarf spectra. Our previous work predicting this absorption feature was limited by molecular potentials that were not adequate to follow the atomic interactions with spectroscopic precision to the asymptotic limit of large separation. A new set of potential energy curves and electronic dipole transition moments for the lowest electronic states of the H–He system were developed to account accurately for the behaviour of the atomic interactions at all distances, from the chemical regime within 1 Å out to where the radiating H atoms are not significantly perturbed by their neighbours. We use a general unified theory of collision-broadened atomic spectral lines to describe a rigorous treatment of hydrogen Lyman α with these potentials and present a new study of its broadening by radiative collisions of hydrogen and neutral helium. These results enable ab initio modelling of radiative transport in DBA white dwarf atmospheres.
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Vauclair, Gérard. "Diffusion and Metal Abundances in Hot White Dwarfs Gerard Vauclair." International Astronomical Union Colloquium 114 (1989): 176–87. http://dx.doi.org/10.1017/s0252921100099541.
Повний текст джерелаАнотація:
While the efficiency of gravitational settling to produce chemically pure atmospheres in white dwarf stars was outlined for the first time 30 years ago (Schatzman 1958), the competing role of the radiation flux in the hot white dwarfs was considered only 10 years ago (Fontaine and Michaud 1979; Vauclair, Vauclair and Greenstein 1979). At that time, there was more motivation to understand how metals could reappear in the long lived cool non DA white dwarfs, where diffusion time scales are shorter by orders of magnitude than evolutionary time scales. Various processes were invoked to help restore some metal content in the white dwarf atmospheres: convection mixing and dredge up, accretion of interstellar matter. In cool white dwarfs, the radiative acceleration is negligeable in the diffusion process; this is not the case at the hot end of the sequence where radiation may balance gravity. The short lived hot white dwarfs just started to become exciting with the contemporary discoveries that i) some show metallic lines in their spectra, both hydrogen rich and hydrogen poor; ii) some of these are pulsating. In the following years, the number of hot white dwarfs revealing trace abundance of metals has increased, mainly owing to IUE observations.
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Kawaler, Steven D. "Probing the Extraordinary Ends of Ordinary Stars: White Dwarf Seismology with the Whole Earth Telescope." International Astronomical Union Colloquium 155 (1995): 81–91. http://dx.doi.org/10.1017/s0252921100036800.
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AbstractDuring the final evolution of most stars, they shed their outer skin and expose their core of the hot ashes of nuclear burning. As these hot and very dense cores cool into white dwarf stars, they go through episodes of multiperiodic, nonradial g-mode pulsation. The tools of stellar seismology allow us to use the pulsation spectra as powerful probes into the deep interiors of these stars. Progress in white dwarf seismology has required significant international cooperation, since another consequence of the complex pulsations of these stars is decoding the true pulsation frequencies requires a coordinated global effort involving high-speed photometric observations. Through one such effort, the Whole Earth Telescope project, we have located subsurface composition changes, detected differential rotation and magnetic fields, and measured fundamental quantities such as stellar mass, luminosity, and distance to extraordinary accuracy.
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Handler, G., M. A. Wood, and A. Nitta. "Towards Ensemble Asteroseismology of the Pulsating DB White Dwarf Stars." International Astronomical Union Colloquium 185 (2002): 608–9. http://dx.doi.org/10.1017/s0252921100017231.
Повний текст джерелаАнотація:
The origin of the helium-atmosphere DB white dwarfs is still a matter of debate. In particular, the question is unresolved whether binary evolution produces a significant number of DBs. The pulsating DB white dwarfs (DBV stars) offer a complementary insight into this problem through asteroseismology; DBs descending from binaries will have different interior structures than DBs originating from single stars (Nitta & Winget, 1998).GD 358 is by far the best-observed pulsating DBV star, and the only one for which asteroseismology has been performed to date. This star’s structure has been shown to be inconsistent with an origin from binary evolution (Nitta & Winget, 1998), but most of the other DBVs are relatively poorly studied.We therefore analysed archival data on all DBVs and obtained new measurements of stars with very little data available (Table 1), firstly to identify suitable targets for asteroseismological investigations and secondly to examine the pulsation spectra of the DBVs as a group, following the works of Clemens (1994) and Kleinman (1995) on the pulsating DA white dwarfs. Our study also produced new seismological results on individual stars and promising targets for future Whole Earth Telescope (WET, Nather et al., 1990) runs.
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Gänsicke, Boris T., Pablo Rodríguez-Gil, Nicola P. Gentile Fusillo, Keith Inight, Matthias R. Schreiber, Anna F. Pala, and Pier-Emmanuel Tremblay. "Single magnetic white dwarfs with Balmer emission lines: a small class with consistent physical characteristics as possible signposts for close-in planetary companions." Monthly Notices of the Royal Astronomical Society 499, no. 2 (September 29, 2020): 2564–74. http://dx.doi.org/10.1093/mnras/staa2969.
Повний текст джерелаАнотація:
ABSTRACT We report the identification of SDSS J121929.45+471522.8 as the third apparently isolated magnetic (B ≃ 18.5 ± 1.0 MG) white dwarf exhibiting Zeeman-split Balmer emission lines. The star shows coherent variability at optical wavelengths with an amplitude of ≃0.03 mag and a period of 15.26 h, which we interpret as the spin period of the white dwarf. Modelling the spectral energy distribution and Gaia parallax, we derive a white dwarf temperature of 7500 ± 148 K, a mass of 0.649 ± 0.022 M⊙, and a cooling age of 1.5 ± 0.1 Gyr, as well as an upper limit on the temperature of a sub-stellar or giant planet companion of ≃250 K. The physical properties of this white dwarf match very closely those of the other two magnetic white dwarfs showing Balmer emission lines: GD356 and SDSS J125230.93−023417.7. We argue that, considering the growing evidence for planets and planetesimals on close orbits around white dwarfs, the unipolar inductor model provides a plausible scenario to explain the characteristics of this small class of stars. The tight clustering of the three stars in cooling age suggests a common mechanism switching the unipolar inductor on and off. Whereas Lorentz drift naturally limits the lifetime of the inductor phase, the relatively late onset of the line emission along the white dwarf cooling sequence remains unexplained.
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Berdyugin, Andrei V., Vilppu Piirola, Stefano Bagnulo, John D. Landstreet, and Svetlana V. Berdyugina. "Highly sensitive search for magnetic fields in white dwarfs using broad-band circular polarimetry." Astronomy & Astrophysics 657 (January 2022): A105. http://dx.doi.org/10.1051/0004-6361/202142173.
Повний текст джерелаАнотація:
Circular polarisation measurements of white dwarfs of various ages and spectral types are useful to understand the origin and evolution of the magnetic field in degenerate stars. In the latest stages of white dwarf evolution, when stars are so cool that spectral lines are no longer formed in the normal H- or He-dominated atmospheres, magnetic fields can be probed only by means of circular polarimetry of the continuum. The study of the fields of featureless DC white dwarfs may reveal whether Ohmic decay acts on magnetic white dwarfs, or if magnetic fields continue to be generated even several billion years after white dwarf formation. Compared to spectropolarimetry, broad-band circular polarisation measurements have the advantage of reaching a higher accuracy in the continuum, with the potential of detecting magnetic fields as weak as a fraction of a MG in DC stars, if the telescope size is adequate for the star’s magnitude. Here we present the results of a first (short) observing campaign with the DIPol-UF polarimeter, which we have used to measure broad-band circular polarisation of white dwarfs. Our observing run was in part aimed to fully characterise the instrument, and in part to study the relationship between magnetic field strength (when known from spectropolarimetry) and circular polarisation of the continuum. We also observed a small number of previously unexplored DC white dwarfs, and we present the discovery of two new magnetic white dwarfs of spectral class DC, probably the first discovery of this kind made with broad-band circular polarimetric techniques since the late 1970s. We also discuss the characteristics of our instrument, and predict the level of polarimetric accuracy that may be reached as a function of stellar magnitude, exposure time, and telescope size.
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Werner, K., T. Rauch, and J. W. Kruk. "Metal abundances in hot white dwarfs with signatures of a superionized wind." Astronomy & Astrophysics 609 (January 2018): A107. http://dx.doi.org/10.1051/0004-6361/201731740.
Повний текст джерелаАнотація:
About a dozen hot white dwarfs with effective temperatures Teff = 65 000−120 000 K exhibit unusual absorption features in their optical spectra. These objects were tentatively identified as Rydberg lines of ultra-high excited metals in ionization stages v–x, indicating line formation in a dense environment with temperatures near 106 K. Since some features show blueward extensions, it was argued that they stem from a superionized wind. A unique assignment of the lines to particular elements is not possible, although they probably stem from C, N, O, and Ne. To further investigate this phenomenon, we analyzed the ultraviolet spectra available from only three stars of this group; that is, two helium-rich white dwarfs, HE 0504–2408 and HS 0713+3958 with spectral type DO, and a hydrogen-rich white dwarf, HS 2115+1148 with spectral type DAO. We identified light metals (C, N, O, Si, P, and S) with generally subsolar abundances and heavy elements from the iron group (Cr, Mn, Fe, Co, Ni) with solar or oversolar abundance. The abundance patterns are not unusual for hot WDs and can be interpreted as the result of gravitational settling and radiative levitation of elements. As to the origin of the ultra-high ionized metals lines, we discuss the possible presence of a multicomponent radiatively driven wind that is frictionally heated.
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Blouin, Simon, and Patrick Dufour. "The spectral evolution of cool white dwarfs." Proceedings of the International Astronomical Union 15, S357 (October 2019): 166–69. http://dx.doi.org/10.1017/s1743921320000277.
Повний текст джерелаАнотація:
AbstractEmpirically characterizing the spectral evolution of cool white dwarfs is a prerequisite to understanding the physical processes that shape the evolution of these old objects. However, the high photospheric densities of cool helium-rich atmospheres seriously complicate the study of those stars. We have recently developed an updated atmosphere code that is appropriate for high densities and that can model any cool white dwarf (including DZs and DQpecs). Here, we present recent advances in our understanding of the spectral evolution of cool white dwarfs that were made possible thanks to these improved models. We discuss in particular the evolution of the hydrogen-rich to helium-rich ratio at low effective temperatures as well as the DQ→ DQpec transition.
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Kuuttila, J., and M. Gilfanov. "Optical emission-line spectra of symbiotic binaries." Monthly Notices of the Royal Astronomical Society 507, no. 1 (August 18, 2021): 594–607. http://dx.doi.org/10.1093/mnras/stab2025.
Повний текст джерелаАнотація:
ABSTRACT Symbiotic stars are long-period interacting binaries where the compact object, most commonly a white dwarf, is embedded in the dense stellar wind of an evolved companion star. Ultraviolet and soft X-ray emission of the accretion disc and the nuclear-burning white dwarf plays a major role in shaping the ionization balance of the surrounding wind material, giving rise to the rich line emission. In this paper, we employ two-dimensional photoionization calculations based on the cloudy code to study the ionization state of the circumbinary material in symbiotic systems and to predict their emission-line spectra. Our simulations are parametrized via the orbital parameters of the binary and the wind mass-loss rate of the donor star, while the mass accretion rate, temperature and luminosity of the white dwarf are computed self-consistently. We explore the parameter space of symbiotic binaries and compute luminosities of various astrophysically important emission lines. The line ratios are compared with traditional diagnostic diagrams used to distinguish symbiotic binaries from other types of sources, and it is shown how the binary system parameters shape these diagrams. In the significant part of the parameter space, the wind material is nearly fully ionized, except for the ‘shadow’ behind the donor star, so the white dwarf emission is typically freely escaping the system.
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Holberg, J. B., M. A. Barstow, and M. R. Burleigh. "An Archive of IUE Low‐Dispersion Spectra of the White Dwarf Stars." Astrophysical Journal Supplement Series 147, no. 1 (July 2003): 145–66. http://dx.doi.org/10.1086/374886.
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Bell, K. J., I. Pelisoli, S. O. Kepler, W. R. Brown, D. E. Winget, K. I. Winget, Z. Vanderbosch, et al. "The McDonald Observatory search for pulsating sdA stars." Astronomy & Astrophysics 617 (September 2018): A6. http://dx.doi.org/10.1051/0004-6361/201833279.
Повний текст джерелаАнотація:
Context. The nature of the recently identified “sdA” spectroscopic class of stars is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravity values between main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors or blue stragglers in the halo. Aims. Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by sdA stars would contribute substantially to our developing understanding of this class. Methods. We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from the McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signatures from four new pulsators. Results. Out of 23 sdA stars observed, we clearly detect stellar pulsations in 7. Dominant pulsation periods range from 4.6 min to 12.3 h, with most on timescales of approximately one hour. We argue specific classifications for some of the new variables, identifying both compact and likely main sequence dwarf pulsators, along with a candidate low-mass RR Lyrae star. Conclusions. With dominant pulsation periods spanning orders of magnitude, the pulsational evidence supports the emerging narrative that the sdA class consists of multiple stellar populations. Since multiple types of sdA exhibit stellar pulsations, follow-up asteroseismic analysis can be used to probe the precise evolutionary natures and stellar structures of these individual subpopulations.
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Werner, Klaus, Nicole Reindl, Stephan Geier, and Max Pritzkuleit. "Discovery of hot subdwarfs covered with helium-burning ash." Monthly Notices of the Royal Astronomical Society: Letters 511, no. 1 (February 11, 2022): L66—L71. http://dx.doi.org/10.1093/mnrasl/slac005.
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Abstract Helium-rich subdwarf O stars (sdOs) are hot compact stars in a pre-white dwarf evolutionary state. Most of them have effective temperatures and surface gravities in the range Teff = 40 000–50 000 K and log g = 5.5–6.0. Their atmospheres are helium dominated. If present at all, C, N, and O are trace elements. The abundance patterns are explained in terms of nucleosynthesis during single star evolution (late helium core flash) or a binary He-core white dwarf merger. Here we announce the discovery of two hot hydrogen-deficient sdOs (PG1654+322 and PG1528+025) that exhibit unusually strong carbon and oxygen lines. A non-LTE model atmosphere analysis of spectra obtained with the Large Binocular Telescope and by the LAMOST survey reveals astonishingly high abundances of C ($\approx 20{{\ \rm per\ cent}}$) and O ($\approx 20{{\ \rm per\ cent}}$) and that the two stars are located close to the helium main sequence. Both establish a new spectroscopic class of hot H-deficient subdwarfs (CO-sdO) and can be identified as the remnants of a He-core white dwarf that accreted matter of a merging low-mass CO-core white dwarf. We conclude that the CO-sdOs represent an alternative evolutionary channel creating PG1159 stars besides the evolution of single stars that experience a late helium-shell flash.
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Bédard, A., P. Brassard, P. Bergeron, and S. Blouin. "On the Spectral Evolution of Hot White Dwarf Stars. II. Time-dependent Simulations of Element Transport in Evolving White Dwarfs with STELUM." Astrophysical Journal 927, no. 1 (March 1, 2022): 128. http://dx.doi.org/10.3847/1538-4357/ac4497.
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Abstract White dwarf stars are subject to various element transport mechanisms that can cause their surface composition to change radically as they cool, a phenomenon known as spectral evolution. In this paper, we undertake a comprehensive theoretical investigation of the spectral evolution of white dwarfs. First, we introduce STELUM, a new implementation of the stellar evolutionary code developed at the Université de Montréal. We provide a thorough description of the physical content and numerical techniques of the code, covering the treatment of both stellar evolution and chemical transport. Then, we present two state-of-the-art numerical simulations of element transport in evolving white dwarfs. Atomic diffusion, convective mixing, and mass loss are considered simultaneously as time-dependent diffusive processes and are fully coupled to the cooling. We first model the PG 1159−DO−DB−DQ evolutionary channel: a helium-, carbon-, and oxygen-rich PG 1159 star transforms into a pure-helium DB white dwarf due to gravitational settling and then into a helium-dominated, carbon-polluted DQ white dwarf through convective dredge-up. We also compute for the first time the full DO−DA−DC evolutionary channel: a helium-rich DO white dwarf harboring residual hydrogen becomes a pure-hydrogen DA star through the float-up process and then a helium-dominated, hydrogen-bearing DC star due to convective mixing. We demonstrate that our results are in excellent agreement with available empirical constraints. In particular, our DO−DA−DC simulation perfectly reproduces the lower branch of the bifurcation observed in the Gaia color–magnitude diagram, which can therefore be interpreted as a signature of spectral evolution.
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Moehler, S., W. B. Landsman, T. Lanz, and M. M. Miller Bertolami. "Hot UV-bright stars of galactic globular clusters." Astronomy & Astrophysics 627 (June 27, 2019): A34. http://dx.doi.org/10.1051/0004-6361/201935694.
Повний текст джерелаАнотація:
Context. We have performed a census of the UV-bright population in 78 globular clusters using wide-field UV telescopes. This population includes a variety of phases of post-horizontal branch (HB) evolution, including hot post-asymptotic giant branch (AGB) stars, and post-early AGB stars. There are indications that old stellar systems like globular clusters produce fewer post-(early) AGB stars than currently predicted by evolutionary models, but observations are still scarce. Aims. We wish to derive effective temperatures, surface gravities, and helium abundances of the luminous hot UV-bright stars in these clusters to determine their evolutionary status and compare the observed numbers to predictions from evolutionary theory. Methods. We obtained FORS2 spectroscopy of eleven of these UV-selected objects (covering a range of −2.3 < [Fe/H] < −1.0), which we (re-)analysed together with previously observed data. We used model atmospheres of different metallicities, including super-solar ones. Where possible, we verified our atmospheric parameters using UV spectrophotometry and searched for metal lines in the optical spectra. We calculated evolutionary sequences for four metallicity regimes and used them together with information about the HB morphology of the globular clusters to estimate the expected numbers of post-AGB stars. Results. We find that metal-rich model spectra are required to analyse stars hotter than 40 000 K. Seven of the eleven new luminous UV-bright stars are post-AGB or post-early AGB stars, while two are evolving away from the HB, one is a foreground white dwarf, and another is a white dwarf merger. Taking into account published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low. Conclusions. Spectroscopy is clearly required to identify the evolutionary status of hot UV-bright stars. For hotter stars, metal-rich model spectra are required to reproduce their optical and UV spectra, which may affect the flux contribution of hot post-AGB stars to the UV spectra of evolved populations. While the observed numbers of post-AGB and post-early AGB stars roughly agree with the predictions, our current comparison is affected by low number statistics.
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Jorissen, A., H. M. J. Boffin, D. Karinkuzhi, S. Van Eck, A. Escorza, S. Shetye, and H. Van Winckel. "Barium and related stars, and their white-dwarf companions." Astronomy & Astrophysics 626 (June 2019): A127. http://dx.doi.org/10.1051/0004-6361/201834630.
Повний текст джерелаАнотація:
Context. Barium and S stars without technetium are red giants and are suspected of being members of binary systems due to their overabundances in heavy elements. These elements are produced by the s-process of nucleosynthesis, despite the stars not being evolved enough to be able to activate the s-process in their interiors. A companion formerly on the asymptotic giant branch (now a white dwarf) is supposed to be responsible for the barium- and S-star enrichment in s-process elements through mass transfer. Aims. This paper provides both long-period and revised orbits for barium and S stars, adding to previously published orbits. The sample of barium stars with strong anomalies (i.e., those classified as Ba3, Ba4, or Ba5 in the Warner scale) comprises all known stars of that kind, and in that sense forms a complete sample that allows us to investigate several orbital properties of these post-mass-transfer binaries in an unbiased way. Methods. Orbital elements are derived from radial velocities collected from a long-term radial-velocity monitoring campaign performed with the HERMES spectrograph mounted on the Mercator 1.2 m telescope. These new measurements were combined with older, CORAVEL measurements. With the aim of investigating possible correlations between orbital properties and abundances, we also collected a set of abundances for barium stars with orbital elements that is as homogeneous as possible. When unavailable in the literature, abundances were derived from high-resolution HERMES spectra. Results. We find orbital motion for all barium and extrinsic S stars monitored (except for the mild barium star HD 95345). We obtain the longest period known so far for a spectroscopic binary involving an S star, namely 57 Peg with a period of the order of 100−500 yr. We present the mass distribution for the barium stars, which ranges from 1 to 3 M⊙, with a tail extending up to 5 M⊙ in the case of mild barium stars. This high-mass tail is mostly comprised of high-metallicity objects ([Fe/H] ≥ −0.1). The distribution of the companion masses was extracted from the barium-star mass distribution combined with the finding that Q ≡ f(MBa,MWD)/sin3 i = MWD3/(MBa + MWD)2 is peaked at 0.057 ± 0.009 and 0.036 ± 0.027 M⊙ for strong and mild barium stars, respectively (f(MBa, MWD) is the mass function obtained from the orbital elements of spectroscopic binaries with one observable spectrum). Mass functions are compatible with WD companions whose masses range from 0.5 to 1 M⊙. Strong barium stars have a tendency to be found in systems with shorter periods than mild barium stars, although this correlation is rather lose, with metallicity and WD mass also playing a role. Using the initial–final mass relationship established for field WDs, we derived the distribution of the mass ratio q′=MAGB, ini/MBa (where MAGB, ini is the WD progenitor initial mass, i.e., the mass of the former primary component of the system) which is a proxy for the initial mass ratio (the less mass the barium star has accreted, the better the proxy). It appears that the distribution of q′ is highly nonuniform, and significantly different for mild and strong barium stars, the latter being characterized by values mostly in excess of 1.4, whereas mild barium stars occupy the range 1−1.4. Conclusions. The orbital properties presented in this paper pave the way for a comparison with binary-evolution and nucleosynthesis models, which should account for the various significant correlations found between abundances and dynamical parameters (e.g. between MBa on one hand and MWD, [Fe/H], and [s/Fe] on the other hand, between q′ and [s/Fe], between P and e, and between P and [s/Fe] altogether).
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Wunner, G., F. Geyer, and H. Ruder. "Atomic Data Relevant to Line Formation in Strongly Magnetized White Dwarf Stars." International Astronomical Union Colloquium 93 (1987): 595–99. http://dx.doi.org/10.1017/s025292110010538x.
Повний текст джерелаАнотація:
AbstractThe discovery of magnetic field strengths in the range of 500 million Gauss in the objects Grw+70°8247 and PG1031+224 has given enormous impetus to investigations of magnetic white dwarf stars. We have determined intensities of Balmer transitions for B > 108 Gauss as a function of field and find strong oscillations of the transitions strengths, which are of importance to the quantitative analysis of the observed spectra and the interpretation of polarimetric measurements in these objects.
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Craig, N., A. Fruscione, J. Dupuis, M. Mathioudakis, J. J. Drake, M. Abbott, C. Christian, R. Green, T. Boroson, and S. B. Howell. "TheEUVEOptical Identification Campaign II: Late-Type and White Dwarf Stars." International Astronomical Union Colloquium 152 (1996): 491–96. http://dx.doi.org/10.1017/s0252921100036447.
Повний текст джерелаАнотація:
We present optical identifications of nine previously unidentified extreme ultraviolet (EUV) sources discovered during theExtreme Ultraviolet Explorer(EUVE) satellite surveys. The all-sky survey detected four of the sources and the more sensitive deep survey detected the other five sources. Three of the four all-sky survey sources, EUVE_J1918+59.9, EUVE_J2249+58.5, and EUVE_J2329+41.4, are listed in present catalogs as having possible associations with optical counterparts but without spectral class. The first two of these sources are hot DA white dwarfs showing an optical spectrum with broad Balmer lines. The source EUVE_J2329+41.4 is listed as having a possible association with an unclassified M star. We show that a pair of dMe stars are actually optical counterparts located within the error circle of theEUVEsource position. The EUVE_J2114+503 remains unidentified even though all the possible candidates have been studied. Based on the count rates we predict a fainter white dwarf or a cataclysmic variable counterpart for this candidate. All five sources discovered with theEUVEdeep survey, EUVE_J0318+184, EUVE_J0419+217, EUVE_J2053−175, EUVE_J2056−171 and EUVE_J2233−096, have been identified as late-type stars. The spectral classes, distances, visual magnitudes, and estimated hydrogen column densities for theseEUVEsources are presented.
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Lam, M. C., N. C. Hambly, N. Lodieu, S. Blouin, E. J. Harvey, R. J. Smith, M. C. Gálvez-Ortiz, and Z. H. Zhang. "First discovery of an ultra-cool white dwarf benchmark in common proper motion with an M dwarf." Monthly Notices of the Royal Astronomical Society 493, no. 4 (February 27, 2020): 6001–10. http://dx.doi.org/10.1093/mnras/staa584.
Повний текст джерелаАнотація:
ABSTRACT Ultra-cool white dwarfs are among the oldest stellar remnants in the Universe. Their efficient gravitational settling and low effective temperatures are responsible for the smooth spectra they exhibit. For that reason, it is not possible to derive their radial velocities or to find the chemistry of the progenitors. The best that can be done is to infer such properties from associated sources, which are coeval. The simplest form of such a system is a common proper motion pair where one star is an evolved degenerate and the other a main-sequence star. In this work, we present the discovery of the first of such a system, the M dwarf LHS 6328 and the ultra-cool white dwarf PSO J1801+625, from the Pan-STARRS 1 3π survey and the Gaia Data Release 2. Follow-up spectra were collected covering a usable wavelength range of 3750–24 500 Å. Their spectra show that the white dwarf has an effective temperature of 3550 K and surface gravity of log g = 7.45 ± 0.13 or log g = 7.49 ± 0.13 for a CO or He core, respectively, when compared against synthetic spectra of ultra-cool white dwarf atmosphere models. The system has slightly subsolar metallicity with −0.25 < [Fe/H] < 0.0, and a spatial velocity of (U, V, W) = (−114.26 ± 0.24, 222.94 ± 0.60, 10.25 ± 0.34) km s−1, the first radial velocity and metallicity measurements of an ultra-cool white dwarf. This makes it the first and only benchmark of its kind to date.
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Pelletier, C., G. Fontaine, and F. Wesemael. "Finite Element Analysis of Diffusion Processes in White Dwarfs." International Astronomical Union Colloquium 114 (1989): 249–52. http://dx.doi.org/10.1017/s0252921100099644.
Повний текст джерелаАнотація:
The spectral evolution of white dwarfs is governed by diffusion processes which enter into competition with mechanisms such as mass loss, convective mixing, and accretion from the interstellar medium in various phases of the evolution. Until recently, our theoretical understanding of the chemical evolution of these stars has been limited by the very severe numerical difficulties which plague a time-dependent description of the problem. Indeed, diffusion problems in white dwarf interiors and envelopes are particularly demanding from a computational standpoint: they involve relative chemical abundances spanning many orders of magnitude, time integration length of a few billion years, and many physical processes operating with greatly different time constants. We have already introduced in the field a robust numerical technique based on an implicit finite difference scheme designed for nonlinear two-point boundary value problems (Pelletier 1986). This method has been used to investigate a number of problems related to the spectral evolution of white dwarfs (Pelletier 1986; Pelletier et al. 1986; Dupuis et al. 1987). As requirements for further progress in the field become more exacting and in the interest of improving the efficiency, we have sought to develop even more powerful numerical techniques. We briefly introduce here an efficient computational approach to diffusion problems in white dwarfs based on a Galerkin finite element method to solve the convective-diffusion equation in an evolving white dwarf model. As an illustrative example, we discuss some sample results of a detailed investigation of the problem of chemical sedimentation (H, He, and C) in the envelopes of hot white dwarfs and the formation of DA stars.
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Greenstein, Jesse L. "White Dwarfs as Observed at High Signal to Noise." Symposium - International Astronomical Union 132 (1988): 175–83. http://dx.doi.org/10.1017/s0074180900034987.
Повний текст джерелаАнотація:
The goal is largely historical, 30 years of instrumental progress in a difficult new field, faint white dwarfs, and some results. High signal-to-noise spectrophotometry at 40–160 Å resolution revealed the separation between hydrogen- and helium-rich atmospheres, and provided a temperature scale from models. The white-dwarf color-luminosity relation proved narrow. Their simple spectra made brute-force averaging possible to 14th magnitude at good photographic resolution. Features as shallow as 5% and 200 Å wide included C2, but in magnetic stars some strong absorptions remain unidentified. Metals are deficient, gravitational diffusion setting the surface composition. The Palomar double CCD spectrograph can now give S/N ≈ 100 to 17m. Some polarized white dwarfs have Zeeman triplets in magnetic fields near 20 megagauss. In one, Zeeman components are shifted up to 2000 Å at 300 megagauss. Rotation is small in all white dwarfs, angular momentum mostly lost. Non-LTE cores of Hα, Hβ exist and permit improved gravitational redshifts. An evolutionary phenomenon is progressive steepening of the Balmer decrement below 7000 K, cool atmospheres being helium-dominated.