Browsing by Subject "white dwarfs"

Now showing 1 - 20 of 85

A 12 Minute Orbital Period Detached White Dwarf Eclipsing Binary
(2011-08) Brown, Warren R.; Kilic, Mukremin; Hermes, J. J.; Prieto, Carlos Allende; Kenyon, Scott J.; Winget, D. E.; Hermes, J. J.
We have discovered a detached pair of white dwarfs (WDs) with a 12.75 minute orbital period and a 1315 km s(-1) radial velocity amplitude. We measure the full orbital parameters of the system using its light curve, which shows ellipsoidal variations, Doppler boosting, and primary and secondary eclipses. The primary is a 0.25 M-circle dot tidally distorted helium WD, only the second tidally distorted WD known. The unseen secondary is a 0.55 M-circle dot carbon-oxygen WD. The two WDs will come into contact in 0.9 Myr due to loss of energy and angular momentum via gravitational wave radiation. Upon contact the systems may merge (yielding a rapidly spinning massive WD), form a stable interacting binary, or possibly explode as an underluminous Type Ia supernova. The system currently has a gravitational wave strain of 10(-22), about 10,000 times larger than the Hulse-Taylor pulsar; this system would be detected by the proposed Laser Interferometer Space Antenna gravitational wave mission in the first week of operation. This system's rapid change in orbital period will provide a fundamental test of general relativity.
25 years of using the spectroscopic method: where do we stand?
(2018-07) Bergeron, P.; Genest-Beaulieu, C.; Coutu, S.; Blouin, S.; Bédard, A.; Cloutier, S.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
The spectroscopic technique, which consists in fitting high signal-to-noise optical spectra with the predictions of state-of-the-art model atmospheres, has routinely been used for over 25 years to measure the atmospheric parameters of DA white dwarfs, and DB stars as well. We review the technique, the progress we have made, and more importantly, we attempt to assess its validity and also try to identify some of the current challenges in the physics of white dwarf atmospheres related to the use of the spectroscopic technique.
An Analysis of the Circumstellar Absorption of WD 1145+017
(2018-07) Fortin-Archambault, M.; Dufour, P.; Xu, S.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
WD 1145+017 is a white dwarf that has an actively disintegrating object in orbit and also uniquely displays circumstellar gas absorption. We present a preliminary analysis of the circumstellar absorption features present in the ultraviolet and optical spectra of WD 1145+017 using an eccentric precessing gas disk model similar to Cauley et al. (2018) who tried to reproduce the periodic changes in the velocity shift for 3 circumstellar lines. Our goal is be able to perform a similar analysis, but for the whole ultraviolet and optical spectra using a full radiative transfer model which considers both dynamical and chemical properties.
Are C-60 Molecules Detectable In Circumstellar Shells Of R Coronae Borealis Stars?
(2011-03) Garcia-Hernandez, D. Anibal; Rao, N. Kameswara; Lambert, David L.; Rao, N. Kameswara; Lambert, David L.
The hydrogen-poor, helium-rich, and carbon-rich character of the gas around R Coronae Borealis (RCB) stars has been suggested to be a site for formation of C-60 molecules. This suggestion is not supported by observations reported here showing that infrared transitions of C-60 are not seen in a large sample of RCB stars observed with the Infrared Spectrograph on the Spitzer Space Telescope. The infrared C-60 transitions are seen, however, in emission and blended with polycyclic aromatic hydrocarbon (PAH) features in spectra of DY Cen and possibly also of V854 Cen, the two least hydrogen-deficient (hydrogen deficiency of only similar to 10-100) RCB stars. The speculation is offered that C-60 (and the PAHs) in the moderately H-deficient circumstellar envelopes may be formed by the decomposition of hydrogenated amorphous carbon but fullerene formation is inefficient in the highly H-deficient environments of most RCBs.
Asteroseismological Study on PG 0112+104 with Pure and Screened Coulomb Potential
(2018-07) Chen, Y.H.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
With 78.7 days of observations by Kepler space telescope, Hermes et al. (2017) made a detailed mode identification on a hot DBV star PG 0112+104. In total, 11 likely m = 0 components with 5 triplets and 3 quintuplets were identified. Those modes can be used to constrain fitting models. A grid of main sequence stars are evolved to be white dwarfs by MESA. Being results of thermal nuclear burning, the core compositions are taken out and added into WDEC to evolve grids of DBV stars. The element diffusion effect adopting pure Coulomb potential and screened Coulomb potential are added into WDEC. Two grids of DBV star models are evolved with the two Coulomb potential scenarios. The eigenfrequencies are calculated and used to fit the 11 observed modes. Two preferredmodels with pure and screened Coulomb potential scenarios are selected. They are consistent with previous spectral and asteroseismological results. The root-mean-square residual is improved by 27%. The differences for the two scenarios are obvious at the C/O-He interface area. Long-period modes are sensitive to this area, but most of the observed modes are short-period modes. DBV stars with lots of long-period modes should be found in future work to study the effect.
Astrometry With The Hubble Space Telescope: Trigonometric Parallaxes Of Planetary Nebula Nuclei NGC 6853, NGC 7293, Abell 31, And Deht 5
(2009-12) Benedict, G. Fritz; McArthur, Barbara E.; Napiwotzki, Ralph; Harrison, Thomas E.; Harris, Hugh C.; Nelan, Edmund; Bond, Howard E.; Patterson, Richard J.; Ciardullo, Robin; Benedict, G. Fritz; McArthur, Barbara E.
We present absolute parallaxes and relative proper motions for the central stars of the planetary nebulae NGC 6853 (The Dumbbell), NGC 7293 (The Helix), Abell 31, and DeHt 5. This paper details our reduction and analysis using DeHt 5 as an example. We obtain these planetary nebula nuclei (PNNi) parallaxes with astrometric data from Fine Guidance Sensors FGS 1r and FGS 3, white-light interferometers on the Hubble Space Telescope. Proper motions, spectral classifications and VJHKT(2)M and DDO51 photometry of the stars comprising the astrometric reference frames provide spectrophotometric estimates of reference star absolute parallaxes. Introducing these into our model as observations with error, we determine absolute parallaxes for each PNN. Weighted averaging with previous independent parallax measurements yields an average parallax precision, sigma(pi)/pi = 5%. Derived distances are: d(NGC6853) = 405(-25)(+28) pc, d(NGC7293) = 216(-12)(+14) pc, d(Abell 31) = 621(-70)(+91) pc, and d(DeHt 5) = 345(-17)(+19) pc. These PNNi distances are all smaller than previously derived from spectroscopic analyses of the central stars. To obtain absolute magnitudes from these distances requires estimates of interstellar extinction. We average extinction measurements culled from the literature, from reddening based on PNNi intrinsic colors derived from model SEDs, and an assumption that each PNN experiences the same rate of extinction as a function of distance as do the reference stars nearest (in angular separation) to each central star. We also apply Lutz-Kelker bias corrections. The absolute magnitudes and effective temperatures permit estimates of PNNi radii through both the Stefan-Boltzmann relation and Eddington fluxes. Comparing absolute magnitudes with post-AGB models provides mass estimates. Masses cluster around 0.57 M-circle dot, close to the peak of the white dwarf mass distribution. Adding a few more PNNi with well-determined distances and masses, we compare all the PNNi with cooler white dwarfs of similar mass, and confirm, as expected, that PNNi have larger radii than white dwarfs that have reached their final cooling tracks.
Blue Large-Amplitude Pulsators (BLAPs): Possible Origin, Evolutionary Status, and Nature of their Pulsations
(2018-07) Corsico, A.H.; Romero, A.D.; Althaus, L.G.; Pelisoli, I.; Kepler, S.O.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
The Blue Large-Amplitude Pulsators (BLAPs) constitute a new class of pulsating stars. They are hot stars with effective temperatures of Teff ~ 30000 K and surface gravities of log g ~ 4.9, that pulsate with periods in the range 20 - 40 min. In Romero et al. (2018), we proposed that BLAPs are hot low-mass He-core pre-white dwarf (WD) stars that pulsate either in high-order non-radial g(gravity) modes or low-order radial modes, including the fundamental radial mode. The theoretical modes with periods in the observed range are unstable due to the k mechanism associated with the Z bump in the opacity at log T ~ 5.25. In this work, we extend the study of Romero et al. (2018) by assessing the rate of period changes of nonradial g modes and radial modes and comparing them with the values measured for BLAPs, in an attempt to validate the proposed evolutionary scenario, and to discern whether the observed modes are high-order g modes or radial modes.
Chandra Grating Spectroscopy Of Three Hot White Dwarfs
(2012-10) Adamczak, J.; Werner, K.; Rauch, T.; Schuh, S.; Drake, J. J.; Kruk, J. W.; Adamczak, J.
Context. High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG1159 (PG1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD246). Aims. The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods. The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results. No metals could be identified in LB1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG1520+525 constrains the effective temperature to T-eff = 150 000 +/- 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GWVir class (PG 1159-035) defines the location of the blue edge of the GWVir instability region. The result is in accordance with predictions from nonadiabatic stellar pulsation models. Such models are therefore reliable tools to investigate the interior structure of GW Vir variables. Conclusions. Our soft X-ray study reveals that the understanding of metal abundances in hot DA white dwarf atmospheres is still incomplete. On the other hand, model atmospheres of hydrogen-deficient PG 1159-type stars are reliable and reproduce well the observed spectra from soft X-ray to optical wavelengths.
Cno Abundances Of Hydrogen-Deficient Carbon And R Coronae Borealis Stars: A View Of The Nucleosynthesis In A White Dwarf Merger
(2009-05) Garcia-Hernandez, D. Anibal; Hinkle, Ken. H.; Lambert, David L.; Eriksson, Kiell; Lambert, David L.
We present high-resolution (R similar to 50,000) observations of near-IR transitions of CO and CN of the five known hydrogen-deficient carbon (HdC) stars and four R Coronae Borealis (RCB) stars. We perform an abundance analysis of these stars by using spectrum synthesis and state-of-the-art MARCS model atmospheres for cool hydrogen-deficient stars. Our analysis confirms reports by Clayton and colleagues that those HdC stars exhibiting CO lines in their spectrum and the cool RCB star SAps are strongly enriched in (18)O(with (16)O/(18)Oratios ranging from 0.3 to 16). Nitrogen and carbon are in the form of (14)N and (12)C, respectively. Elemental abundances for CNO are obtained from C I, Ci2, CN, and CO lines. Difficulties in deriving the carbon abundance are discussed. Abundances of Na from Na I lines and S from S I lines are obtained. Elemental and isotopic CNO abundances suggest that HdC and RCB stars may be related objects, and that they probably formed from a merger of an He white dwarf with a C-O white dwarf.
Constraining Low-Mass White Dwarf Binaries from Ellipsoidal Variations
(2018-07) Bell, K.J.; Hermes, J. J.; Kuszlewicz, J.S.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
Stars are stretched by tidal interactions in tight binaries, and changes to their projected areas introduce photometric variations twice per orbit. Hermes et al. (2014) utilized measurements of these ellipsoidal variations to constrain the radii of lowmass white dwarfs in eight single-lined spectroscopic binaries. We refine this method here, using Monte Carlo simulations to improve constraints on many orbital and stellar properties of binary systems that exhibit ellipsoidal variations. We analyze the recently discovered tidally distorted white dwarf binary system SDSS J1054-2121 in detail, and also revisit the Hermes et al. (2014) sample. Disagreements in some cases between the observations, ellipsoidal variation model, and Gaia radius constraints suggest that extrinsic errors are present, likely in the surface gravities determined through model atmosphere fits to stellar spectra.
Constraining the Milky Way potential with Double White Dwarfs
(2018-07) Korol, V.; Rossi, E.M.; Barausse, E.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
The upcoming LISA mission is the only experiment that will allow us to study the Milky Way’s structure using gravitational wave signals from Galactic double white dwarfs (DWDs). The total number of expected detections exceeds 105. Furthermore, up to a hundred DWDs can be simultaneously detected in both gravitational and optical radiation (e.g. with Gaia and LSST as eclipsing), making DWDs ideal sources for performing a multi-messenger tomography of the Galaxy. We show that LISA will detect DWDs everywhere, mapping also the opposite side of the Galaxy. This complete coverage will : (1) provide precise and unbiased constraints on the scale radii of the Milky Way’s bulge and disc, and (2) allow us to compute the rotation curve and derive competitive estimates for the bulge and disc masses, when combining gravitational wave and optical observations.
Constraining the Surface Inhomogeneity and Settling Times of Metals on Accreting White Dwarfs
(2008-10) Montgomery, M. H.; Thompson, S. E.; von Hippel, Ted; Montgomery, M. H.; von Hippel, T.
Due to the short settling times of metals in DA white dwarf atmospheres, any white dwarfs with photospheric metals must be actively accreting. It is therefore natural to expect that the metals may not be deposited uniformly on the surface of the star. We present calculations showing how the temperature variations associated with white dwarf pulsations lead to an observable diagnostic of the surface metal distribution, and we show what constraints current data sets are able to provide. We also investigate the effect that time-variable accretion has on the metal abundances of different species, and we show how this can lead to constraints on the gravitational settling times.
Convection: A Seismological Perspective
(2007-11) Montgomery, M. H.; Montgomery, M.H.
I present an overview of the close relationship which convection and stellar pulsation share. This includes the driving, damping, and modulation of oscillations in stars such as the Sun, other solar-type stars, the >classical> pulsators (e.g., Cepheids, RR Lyrae), as well as more recent results from the realm of pulsating white dwarf stars.
Creating White Dwarf Photospheres In The Laboratory
(2010-08) Falcon, R. E.; Rochau, G. A.; Bailey, J. E.; Ellis, J. L.; Montgomery, M. H.; Winget, D. E.; Gomez, M. R.; Leeper, R. J.; Falcon, Ross E.; Winget, D. E.; Montgomery, M. H.
We present a preliminary report from the laboratory astrophysics experiments to create macroscopic (similar to 19 cm(3)) hydrogen-plasmas with white dwarf (WD) photospheric conditions (i.e., temperature, electron density). These experiments, performed at the Z Pulsed Power Facility at Sandia National Laboratories, will serve as benchmarks for fundamental atomic line profile measurements in emission and absorption; they are targeted to address the discrepancy between theory and observation of WD photospheres - cooler photospheres in particular.
A Dark Spot On A Massive White Dwarf
(2015-12) Kilic, Mukremin; Gianninas, Alexandros; Bell, Keaton J.; Curd, Brandon; Brown, Warren R.; Hermes, J. J.; Dufour, P.; Wisniewski, J. Patrick; Winget, D. E.; Winget, K. I.; Bell, Keaton J.; Winget, D. E.; Winget, K. I.
We present the serendipitous discovery of eclipse-like events around the massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We selected J1529+2928 for time-series photometry based on its spectroscopic temperature and surface gravity, which place it near the ZZ Ceti instability strip. Instead of pulsations, we detect photometric dips from this white dwarf every 38 minutes. Follow-up optical spectroscopy observations with Gemini reveal no significant radial velocity variations, ruling out stellar and brown dwarf companions. A disintegrating planet around this white dwarf cannot explain the observed light curves in different filters. Given the short period, the source of the photometric dips must be a dark spot that comes into view every 38 minutes due to the rotation of the white dwarf. Our optical spectroscopy does not show any evidence of Zeeman splitting of the Balmer lines, limiting the magnetic field strength to B < 70 kG. Since up to 15% of white dwarfs display kG magnetic fields, such eclipse-like events should be common around white dwarfs. We discuss the potential implications of this discovery on transient surveys targeting white dwarfs, like the K2 mission and the Large Synoptic Survey Telescope.
A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan DIGITal Sky Survey
(2010-09) Kilic, Mukremin; Leggett, S. K.; Tremblay, P. E.; von Hippel, Ted; Bergeron, P.; Harris, Hugh C.; Munn, Jeffrey A.; Williams, Kurtis A.; Gates, Evalyn; Farihi, J.; Williams, Kurtis A.
We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs (WDs) in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNOB astrometry and a dozen previously known ultracool WD candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool WDs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope data sets) is possible using a reduced proper motion diagram and a tangential velocity cut-off (depending on the proper motion accuracy) of 30 km s(-1). Our near-infrared observations reveal eight new stars with significant absorption. We use the optical and near-infrared photometry to perform a detailed model atmosphere analysis. More than 80% of the stars in our sample are consistent with either pure hydrogen or pure helium atmospheres. However, the eight stars with significant infrared absorption and the majority of the previously known ultracool WD candidates are best explained with mixed hydrogen and helium atmosphere models. The age distribution of our sample is consistent with a Galactic disk age of 8 Gyr. A few ultracool WDs may be as old as 12-13 Gyr, but our models have problems matching the spectral energy distributions of these objects. There are only two halo WD candidates in our sample. However, trigonometric parallax observations are required for accurate mass and age determinations and to confirm their membership in the halo.
The difficulty of finding double-degenerate progenitors of type Ia supernovae
(2018-07) Rebassa-Manserga, A.; Toonen, S.; Korol, V.; Torres, S.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
The double-degenerate progenitors of type Ia supernovae (SNIa) are double white dwarf binaries with a total mass near the Chandrasekhar limit and with merger timescales smaller than the Hubble time. The fact that we have not yet observationally confirmed the existence of these objects arises the question of whether the double-degenerate channel is a viable path for producing SNIa. In this contribution we show that this lack of detections is due to observational effects and that the probability of finding double white dwarf SNIa progenitors in the Galaxy is (2.1 ± 1.0) × 10 −5.
Discovery Of A New AM CVn System With The Kepler Satellite
(2011-01) Fontaine, G.; Brassard, P.; Green, E. M.; Charpinet, S.; Dufour, Patrick; Hubeny, I.; Steeghs, D.; Aerts, C.; Randall, S. K.; Bergeron, P.; Guvenen, B.; O'Malley, C. J.; Van Grootel, V.; Ostensen, R. H.; Bloemen, S.; Silvotti, R.; Howell, Steve B.; Baran, A.; Kepler, S. O.; Marsh, T. R.; Montgomery, Michael H.; Oreiro, R.; Provencal, J.; Telting, J.; Winget, D. E.; Zima, W.; Christensen-Dalsgaard, J.; Kjeldsen, H.; Montgomery, Michael H.
We report the discovery of a new AM CVn system on the basis of broadband photometry obtained with the Kepler satellite supplemented by ground-based optical spectroscopy. Initially retained on Kepler target lists as a potential compact pulsator, the blue object SDSS J190817.07+394036.4 (KIC 004547333) has turned out to be a high-StateAM CVn star showing the He-dominated spectrum of its accretion disk significantly reddened by interstellar absorption. We constructed new grids of NLTE synthetic spectra for accretion disks in order to analyze our spectroscopic observations. From this analysis, we infer preliminary estimates of the rate of mass transfer, the inclination angle of the disk, and the distance to the system. The AM CVn nature of the system is also evident in the Kepler light curve, from which we extracted 11 secure periodicities. The luminosity variations are dominated by a basic periodicity of 938.507 s, likely to correspond to a superhump modulation. The light curve folded on the period of 938.507 s exhibits a pulse shape that is very similar to the superhump wavefront seen in AM CVn itself, which is a high-Statesystem and the prototype of the class. Our Fourier analysis also suggests the likely presence of a quasi-periodic oscillation similar to those already observed in some high-StateAM CVn systems. Furthermore, some very low-frequency, low-amplitude aperiodic photometric activity is likely present, which is in line with what is expected in accreting binary systems. Inspired by previous work, we further looked for and found some intriguing numerical relationships between the 11 secure detected frequencies, in the sense that we can account for all of them in terms of only three basic clocks. This is further evidence in favor of the AM CVn nature of the system.
Discovery of A ZZ Ceti in the Kepler Mission Field
(2011-11) Hermes, J. J.; Mullally, Fergal; Ostensen, R. H.; Williams, Kurtis A.; Telting, John; Southworth, John; Bloemen, S.; Howell, Steve B.; Everett, Mark; Winget, D. E.; Hermes, J. J.; Winget, D. E.
We report the discovery of the first identified pulsating DA white dwarf, WD J1916+3938 (Kepler ID 4552982), in the field of the Kepler mission. This ZZ Ceti star was first identified through ground-based, time-series photometry, and follow-up spectroscopy confirms that it is a hydrogen-atmosphere white dwarf with T(eff) = 11,129 +/- 115 K and log g = 8.34 +/- 0.06, placing it within the empirical ZZ Ceti instability strip. The object shows up to 0.5% amplitude variability at several periods between 800 and 1450 s. Extended Kepler observations of WD J1916+3938 could yield the best light curve, to date, of any pulsating white dwarf, allowing us to directly study the interior of an evolved object representative of the fate of the majority of stars in our Galaxy.
Discovery of an Ultramassive Pulsating White Dwarf
(2013-07) Hermes, J. J.; Kepler, S. O.; Castanheira, Barbara G.; Gianninas, A.; Winget, D. E.; Montgomery, M. H.; Brown, Warren R.; Harrold, Samuel T.; Hermes, J. J.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, M. H.; Harrold, Samuel T.
We announce the discovery of the most massive pulsating hydrogen-atmosphere white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log g = 9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03M(circle dot). Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains Type Ia supernovae progenitor systems.