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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Rapid decreasing in the orbital period of the detached white dwarf–main sequence binary SDSS J143547.87+373338.5

SDSS J143547.87+373338.5 is a detached eclipsing binary that contains a white dwarf with a mass of 0.5 M⊙ and a fully convective star with a mass of 0.21 M⊙. The eclipsing binary was monitored photometrically from 2009 March 24 to 2015 April 10, by using two 2.4-m telescopes in China and in Thailand. The changes in the orbital period are analyzed based on eight newly determined eclipse times together with those compiled from the literature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The ELM Survey. VII. Orbital properties of low-mass white dwarf binaries

We present the discovery of 15 extremely low-mass (5<logg<7) white dwarf (WD) candidates, 9 of which are in ultra-compact double-degenerate binaries. Our targeted extremely low-mass Survey sample now includes 76 binaries. The sample has a lognormal distribution of orbital periods with a median period of 5.4 hr. The velocity amplitudes imply that the binary companions have a normal distribution of mass with 0.76 M{sub ⊙} mean and 0.25 M{sub ⊙} dispersion. Thus extremely low-mass WDs are found in binaries with a typical mass ratio of 1:4. Statistically speaking, 95% of the WD binaries have a total mass below the Chandrasekhar mass, and thus are not type Ia supernova progenitors. Yet half of the observed binaries will merge in less than 6 Gyr due to gravitational wave radiation; probable outcomes include single massive WDs and stable mass transfer AM CVn binaries.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

OPTICAL THERMONUCLEAR TRANSIENTS FROM TIDAL COMPRESSION OF WHITE DWARFS AS TRACERS OF THE LOW END OF THE MASSIVE BLACK HOLE MASS FUNCTION

In this paper, we model the observable signatures of tidal disruptions of white dwarf (WD) stars using massive black holes (MBHs) of moderate mass, ≈10{sup 3}–10{sup 5} M{sub ⊙}. When the WD passes deep enough within the MBH’s tidal field, these signatures include thermonuclear transients from burning during maximum compression. We combine a hydrodynamic simulation that includes nuclear burning of the disruption of a 0.6 M{sub ⊙} C/O WD with a Monte Carlo radiative transfer calculation to synthesize the properties of a representative transient. The transient’s emission emerges in the optical, with light curves and spectra reminiscent of Type I supernovae. The properties are strongly viewing angle dependent, and key spectral signatures are ≈10,000 km s{sup −1} doppler shifts, due to the orbital motion of the unbound ejecta. Disruptions of He WDs likely produce large quantities of intermediate-mass elements, offering a possible production mechanism for Ca-rich transients. Accompanying multi-wavelength transients are fueled by accretion and arise from the nascent accretion disk and relativistic jet. If MBHs of moderate mass exist with number densities similar to those of supermassive BHs, both high-energy wide-field monitors and upcoming optical surveys should detect tens to hundreds of WD tidal disruptions per year. The current best strategy for their detection may therefore be deep optical follow-up of high-energy transients of unusually long duration. The detection rate or the nondetection of these transients by current and upcoming surveys can thus be used to place meaningful constraints on the extrapolation of the MBH mass function to moderate masses.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

GROWING WHITE DWARFS TO THE CHANDRASEKHAR LIMIT: THE PARAMETER SPACE OF THE SINGLE DEGENERATE SN Ia CHANNEL

Can a white dwarf (WD), accreting hydrogen-rich matter from a non-degenerate companion star, ever exceed the Chandrasekhar mass and explode as a SN Ia? We explore the range of accretion rates that allow a WD to secularly grow in mass, and derive limits on the accretion rate and on the initial mass that will allow it to reach 1.4M{sub ⊙}—the Chandrasekhar mass. We follow the evolution through a long series of hydrogen flashes, during which a thick helium shell accumulates. This determines the effective helium mass accretion rate for long-term, self-consistent evolutionary runs with helium flashes. We find that net mass accumulation always occurs despite helium flashes. Although the amount of mass lost during the first few helium shell flashes is a significant fraction of that accumulated prior to the flash, that fraction decreases with repeated helium shell flashes. Eventually no mass is ejected at all during subsequent flashes. This unexpected result occurs because of continual heating of the WD interior by the helium shell flashes near its surface. The effect of heating is to lower the electron degeneracy throughout the WD, especially in the outer layers. This key result yields helium burning that is quasi-steady state, instead of explosive. We thus find a remarkably large parameter space within which long-term, self-consistent simulations show that a WD can grow in mass and reach the Chandrasekhar limit, despite its helium flashes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE CRITICAL MASS RATIO OF DOUBLE WHITE DWARF BINARIES FOR VIOLENT MERGER-INDUCED TYPE IA SUPERNOVA EXPLOSIONS

Mergers of carbon–oxygen (CO) white dwarfs (WDs) are considered to be one of the potential progenitors of type Ia supernovae (SNe Ia). Recent hydrodynamical simulations showed that the less massive (secondary) WD violently accretes onto the more massive (primary) one, carbon detonation occurs, the detonation wave propagates through the primary, and the primary finally explodes as a sub-Chandrasekhar mass SN Ia. Such an explosion mechanism is called the violent merger scenario. Based on the smoothed particle hydrodynamics simulations of merging CO WDs, we derived a critical mass ratio (q{sub cr}) leading to the violent merger scenario that is more stringent than previous results. We conclude that this difference mainly comes from the differences in the initial condition of whether or not the WDs are synchronously spinning. Using our new results, we estimated the brightness distribution of SNe Ia in the violent merger scenario and compared it with previous studies. We found that our new q{sub cr} does not significantly affect the brightness distribution. We present the direct outcome immediately following CO WD mergers for various primary masses and mass ratios. We also discussed the final fate of the central system of the bipolar planetary nebula Henize 2-428, which was recently suggested to be a double CO WD system whose total mass exceeds the Chandrasekhar-limiting mass, merging within the Hubble time. Even considering the uncertainties in the proposed binary parameters, we concluded that the final fate of this system is almost certainly a sub-Chandrasekhar mass SN Ia in the violent merger scenario.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SUPERNOVAE FROM DIRECT COLLISIONS OF WHITE DWARFS AND THE ROLE OF HELIUM SHELL IGNITION

Models for supernovae (SNe) arising from thermonuclear explosions of white dwarfs (WDs) have been studied extensively over the past few decades, mostly focusing on the single-degenerate (accretion of material by a WD) and double-degenerate (WD–WD merger) scenarios. In recent years it was suggested that direct WD–WD collisions provide an additional channel for such explosions. Here we extend the studies of such explosions and explore the role of helium shells in affecting the thermonuclear explosions. We study the impact of both low-mass (∼0.01 M {sub ⊙}) and high-mass (≥0.1 M {sub ⊙}) helium shells. We find that detonation of the massive helium layers precedes the detonation of the WD carbon–oxygen (CO) bulk during the collision and can change the evolution of the explosion and the outcomes for the cases of high-mass He shells. In particular, the He shell detonation propagates on the WD surface and inefficiently burns material prior to the CO detonation that later follows in the central parts of the WD. Such evolution leads to greater production of intermediate elements, producing larger yields of {sup 44}Ti and {sup 48}Cr relative to the pure CO–CO WD collisions. Collisions of WDs with a low-mass He shell do not give rise to helium detonation, but helium burning does precede the CO bulk detonation. Such collisions eject at high velocity a low mass of burned material enriched with intermediate elements and produce smaller changes in the overall explosion outcomes. The various effects arising from the contribution of low-/high-mass He layers change the kinematics and the morphological structure of collision-induced SNe and may thereby provide unique observational signatures for such SNe and play a role in the chemical enrichment of galaxies and the production of intermediate elements and positrons from their longer-term decay.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Most double degenerate low-mass white dwarf binaries merge

We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM; <0.3 M {sub ⊙}) white dwarfs (WDs) in the Galaxy. Such WDs are detectable for timescales of 0.1–1 Gyr in the ELM Survey; the binaries they reside in have gravitational wave merger times of 0.001–100 Gyr. To explain the observed distribution requires that most ELM WD binary progenitors detach from the common envelope phase with <1 hr orbital periods. We calculate the local space density of ELM WD binaries and estimate a merger rate of 3 × 10{sup −3} yr{sup −1} over the entire disk of the Milky Way; the merger rate in the halo is 10 times smaller. The ELM WD binary merger rate exceeds by a factor of 40 the formation rate of stable mass transfer AM CVn binaries, marginally exceeds the rate of underluminous supernovae, and is identical to the formation rate of R CrB stars. On this basis, we conclude that ELM WD binaries can be the progenitors of all observed AM CVn and possibly underluminous supernovae; however, the majority of He+CO WD binaries go through unstable mass transfer and merge, e.g., into single massive ∼1 M {sub ⊙} WDs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

MONTE CARLO POPULATION SYNTHESIS OF POST-COMMON-ENVELOPE WHITE DWARF BINARIES AND TYPE Ia SUPERNOVA RATE

Binary population synthesis (BPS) studies provide a comprehensive way to understand the evolution of binaries and their end products. Close white dwarf (WD) binaries have crucial characteristics for examining the influence of unresolved physical parameters on binary evolution. In this paper, we perform Monte Carlo BPS simulations, investigating the population of WD/main-sequence (WD/MS) binaries and double WD binaries using a publicly available binary star evolution code under 37 different assumptions for key physical processes and binary initial conditions. We considered different combinations of the binding energy parameter ( λ {sub g}: considering gravitational energy only; λ {sub b}: considering both gravitational energy and internal energy; and λ {sub e}: considering gravitational energy, internal energy, and entropy of the envelope, with values derived from the MESA code), CE efficiency, critical mass ratio, initial primary mass function, and metallicity. We find that a larger number of post-CE WD/MS binaries in tight orbits are formed when the binding energy parameters are set by λ {sub e} than in those cases where other prescriptions are adopted. We also determine the effects of the other input parameters on the orbital periods and mass distributions of post-CE WD/MS binaries. As they contain at least one CO WD, double WD systems that evolved from WD/MS binaries may explode as type Ia supernovae (SNe Ia) via merging. In this work, we also investigate the frequency of two WD mergers and compare it to the SNe Ia rate. The calculated Galactic SNe Ia rate with λ = λ {sub e} is comparable to the observed SNe Ia rate, ∼8.2 × 10{sup 5} yr{sup 1} – ∼4 × 10{sup 3} yr{sup 1} depending on the other BPS parameters, if a DD system does not require a mass ratio higher than ∼0.8 to become an SNe Ia. On the other hand, a violent merger scenario, which requires the combined mass of two CO WDs ≥ 1.6 M {sub ⊙} and a mass ratio >0.8, results in a much lower SNe Ia rate than is observed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

CAN HELIUM ENVELOPES CHANGE THE OUTCOME OF DIRECT WHITE DWARF COLLISIONS?

A pivotal feature for the viability of white dwarf (WD) collisions as SN Ia progenitors is that a significant fraction of the mass is highly compressed to the densities required for efficient {sup 56}Ni production before the ignition of the detonation wave. Previous studies have employed model WDs composed of carbon–oxygen (CO), whereas WDs typically have a non-negligible helium envelope. Given that helium is more susceptible to explosive burning than CO under the conditions of WD collision, a legitimate concern is whether or not early time He detonation ignition can lead to early time CO detonation, drastically reducing {sup 56}Ni synthesis. We investigate the role of He in determining the fate of WD collisions by performing a series of two-dimensional hydrodynamics calculations. We find that a necessary condition for non-trivial reduction of the CO ignition time is that the He detonation birthed in the contact region successfully propagates into the unshocked shell. We determine the minimal He shell mass as a function of the total WD mass that upholds this condition. Although we utilize a simplified reaction network similar to those used in previous studies, our findings are in good agreement with detailed investigations concerning the impact of network size on He shell detonations. This allows us to extend our results to the case with more realistic burning physics. Based on the comparison of these findings against evolutionary calculations of WD compositions, we conclude that most, if not all, WD collisions will not be drastically impacted by their intrinsic He components.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

TIME-VARIABLE ALUMINUM ABSORPTION IN THE POLAR AR URSAE MAJORIS, AND AN UPDATED ESTIMATE FOR THE MASS OF THE WHITE DWARF

We present spectra of the extreme polar AR Ursae Majoris (AR UMa), which display a clear Al i absorption doublet, alongside spectra taken less than a year earlier in which that feature is not present. Re-examination of earlier SDSS spectra indicates that the Al i absorption doublet was also present ≈8 years before our first non-detection. We conclude that this absorbing material is unlikely to be on the surface of either the white dwarf (WD) or the donor star. We suggest that this Al i absorption feature arises in circumstellar material, perhaps produced by the evaporation of asteroids as they approach the hot WD. The presence of any remaining reservoir of rocky material in AR UMa might help to constrain the prior evolution of this unusual binary system. We also apply spectral decomposition to find the stellar parameters of the M dwarf companion, and attempt to dynamically measure the mass of the WD in AR UMa by considering both the radial velocity curves of the H {sub β} emission line and the Na i absorption line. Thereby we infer a mass range for the WD in AR UMa of 0.91 M {sub ⊙} < M {sub WD} < 1.24 M {sub ⊙}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Microarcsecond VLBI pulsar astrometry with PSRπ. I. Two binary millisecond pulsars with white dwarf companions

Model-independent distance constraints to binary millisecond pulsars (MSPs) are of great value to both the timing observations of the radio pulsars and multiwavelength observations of their companion stars. Astrometry using very long baseline interferometry (VLBI) can be employed to provide these model-independent distances with very high precision via the detection of annual geometric parallax. Using the Very Long Baseline Array, we have observed two binary MSPs, PSR J1022+1001 and J2145–0750, over a two-year period and measured their distances to be 700{sub −10}{sup +14} pc and 613{sub −14}{sup +16} pc respectively. We use the well-calibrated distance in conjunction with revised analysis of optical photometry to tightly constrain the nature of their massive (M∼0.85 M{sub ⊙}) white dwarf companions. Finally, we show that several measurements of the parallax and proper motion of PSR J1022+1001 and PSR J2145–0750 obtained by pulsar timing array projects are incorrect, differing from the more precise VLBI values by up to 5σ. We investigate possible causes for the discrepancy, and find that imperfect modeling of the solar wind is a likely candidate for the errors in the timing model given the low ecliptic latitude of these two pulsars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

An eccentric binary millisecond pulsar with a helium white dwarf companion in the galactic field

Low-mass white dwarfs (LMWDs) are believed to be exclusive products of binary evolution, as the universe is not old enough to produce them from single stars. Because of the strong tidal forces operating during the binary interaction phase, the remnant systems observed today are expected to have negligible eccentricities. Here, we report on the first unambiguous identification of an LMWD in an eccentric (e = 0.13) orbit around the millisecond pulsar PSR J2234+0511, which directly contradicts this picture. We use our spectra and radio-timing solution (derived elsewhere) to infer the WD temperature (T{sub eff}=8600±190 K), and peculiar systemic velocity relative to the local standard of rest (≃31 km s{sup −1}). We also place model-independent constraints on the WD radius (R{sub WD}=0.024{sub −0.002}{sup +0.004} R{sub ⊙}) and surface gravity (log g=7.11{sub −0.16}{sup +0.08} dex). The WD and kinematic properties are consistent with the expectations for low-mass X-ray binary evolution and disfavor a dynamic three-body formation channel. In the case of the high eccentricity being the result of a spontaneous phase transition, we infer a mass of ∼1.60 M {sub ⊙} for the pulsar progenitor, which is too low for the quark-nova mechanism proposed by Jiang et al., and too high for the scenario of Freire and Tauris, in which a WD collapses into a neutron star via a rotationally delayed accretion-induced collapse. We find that eccentricity pumping via interaction with a circumbinary disk is consistent with our inferred parameters. Finally, we report tentative evidence for pulsations that, if confirmed, would transform the star into an unprecedented laboratory for WD physics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Type Ia Supernova Explosions From Hybrid Carbon–Oxygen–Neon White Dwarf Progenitors

Motivated by recent results in stellar evolution that predict the existence of hybrid white dwarf (WD) stars with a C–O core inside an O–Ne shell, we simulate thermonuclear (Type Ia) supernovae from these hybrid progenitors. We use the FLASH code to perform multidimensional simulations in the deflagration-to-detonation transition (DDT) explosion paradigm. Our hybrid progenitor models were produced with the MESA stellar evolution code and include the effects of the Urca process, and we map the progenitor model to the FLASH grid. We performed a suite of DDT simulations over a range of ignition conditions consistent with the progenitor's thermal and convective structure assuming multiple ignition points. To compare the results from these hybrid WD stars to previous results from C–O WDs, we construct a set of C–O WD models with similar properties and similarly simulate a suite of explosions. We find that despite significant variability within each suite, trends distinguishing the explosions are apparent in their ${}^{56}\mathrm{Ni}$ yields and the kinetic properties of the ejecta. We compare our results with other recent work that studies explosions from these hybrid progenitors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

High-energy neutrino emission from white dwarf mergers

The merger of two white dwarfs is expected to result in a central fast-rotating core surrounded by a debris disk, in which magnetorotational instabilities give rise to a hot magnetized corona and a magnetized outflow. The dissipation of magnetic energy via reconnection could lead to the acceleration of cosmic-rays (CRs) in the expanding material, which would result in high energy neutrinos. We discuss the possibility of using these neutrino signals as probes of the outflow dynamics, magnetic energy dissipation rate, and CR acceleration efficiency. Importantly, the accompanying high-energy gamma-rays are absorbed within these sources because of the large optical depth, so these neutrino sources can be regarded as hidden cosmic-ray accelerators that are consistent with the non-detection of gamma-rays with Fermi-LAT. While the CR generation rate is highly uncertain, if it reaches ∼10{sup 45} erg Mpc{sup −3} yr{sup −1}, the diffuse neutrino flux could contribute a substantial fraction of the IceCube observations. We also evaluate the prospect of observing individual merger events, which provides a means for testing such sources in the future.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Solar Neighborhood. XXXIX. Parallax Results from the CTIOPI and NOFS Programs: 50 New Members of the 25 parsec White Dwarf Sample

We present 114 trigonometric parallaxes for 107 nearby white dwarf (WD) systems from both the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI) and the U. S. Naval Observatory Flagstaff Station (NOFS) parallax programs. Of these, 76 parallaxes for 69 systems were measured by the CTIOPI program and 38 parallaxes for as many systems were measured by the NOFS program. A total of 50 systems are confirmed to be within the 25-pc horizon of interest. Coupled with a spectroscopic confirmation of a common proper-motion companion to a Hipparcos star within 25 pc as well as confirmation parallax determinations for two WD systems included in the recently released Tycho Gaia Astrometric Solution catalog, we add 53 new systems to the 25-pc WD sample—a 42% increase. Our sample presented here includes four strong candidate halo systems, a new metal-rich DAZ WD, a confirmation of a recently discovered nearby short-period ( P = 2.85 hr) double degenerate, a WD with a new astrometric perturbation (long period, unconstrained with our data), and a new triple system where the WD companion main-sequence star has an astrometric perturbation ( P ∼ 1.6 year).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Hubble COS Spectroscopy of the Dwarf Nova CW Mon: The White Dwarf in Quiescence?

We present a synthetic spectral analysis of the HST COS spectrum of the U Geminorum-type dwarf nova CW Mon, taken during quiescence as part of our COS survey of accreting white dwarfs (WDs) in Cataclysmic Variables. We use a synthetic photosphere and optically thick accretion disk spectra to model the COS spectrum as well as archival IUE spectra obtained decades ago, when the system was in an even deeper quiescent state. Assuming a reddening of E(B−V) = 0.06, an inclination of 60° (CW Mon has eclipses of the accretion disk), and a WD mass of 0.8 M{sub ⊙}, our results indicate the presence of a 22–27,000 K WD and a low mass accretion rate M-dot ≲10{sup −10} M{sub ⊙} yr{sup −1}, for a derived distance of ∼200 to ∼300 pc.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

New Grids of Pure-hydrogen White Dwarf NLTE Model Atmospheres and the HST/STIS Flux Calibration

Nonlocal thermodynamic equilibrium (NLTE) calculations of hot white dwarf (WD) model atmospheres are the cornerstone of modern flux calibrations for the Hubble Space Telescope and for the CALSPEC database. These theoretical spectral energy distributions (SEDs) provide the relative flux versus wavelength, and only the absolute flux level remains to be set by reconciling the measured absolute flux of Vega in the visible with the Midcourse Space Experiment values for Sirius in the mid-IR. The most recent SEDs calculated by the tlusty and tmap NLTE model atmosphere codes for the primary WDs G191-B2B, GD 153, and GD 71 show improved agreement to 1% from 1500 Å to 30 μm, in comparison to the previous 1% consistency only from 2000 Å to 5 μm. These new NLTE models of hot WDs now provide consistent flux standards from the far UV to the mid-IR.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Estimating Atmospheric Parameters of DA White Dwarf Stars with Deep Learning

The determination of atmospheric parameters of white dwarf stars (WDs) is crucial for researches on them. Traditional methodology is to fit the model spectra to observed absorption lines and report the parameters with the lowest χ 2 error, which strongly relies on theoretical models that are not always publicly accessible. In this work, we construct a deep learning network to model-independently estimate T eff and log g of DA stars (DAs), corresponding to WDs with hydrogen-dominated atmospheres. The network is directly trained and tested on the normalized flux pixels of full optical wavelength range of DAs spectroscopically confirmed in the Sloan Digital Sky Survey. Experiments in test yield that the rms error for T eff and log g approaches 900 K and 0.1 dex, respectively. This technique is applicable for those DAs with T eff from 5000 to 40,000 K and log g from 7.0 to 9.0 dex. Furthermore, the applicability of this method is verified for the spectra with degraded resolution of ~200. So it is also practical for the analysis of DAs that will be detected by the Chinese Space Station Telescope.

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