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At least 667 records · Page 37

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NASA NTRS1995

The unusual helium variable AM Canum Venaticorum

The unusual variable star AM CVn has puzzled astronomers for over 40 years. This object, both a photometric and spectroscopic variable, is believed to contain a pair of hydrogen-deficient white dwarfs of extreme mass ratio, transferring material via an accretion disk. We examine the photometric properties of AM CVn, analyzing 289 hours of high-speed photometric data spanning 1976 to 1992. The power spectrum displays significant peaks at 988.7, 1248.8, 1902.5, 2853.8, 3805.2, 4756.5, and 5707.8 microHz (1011.4, 800.8, 525.6, 350.4, 262.8, 210.2, and 175.2 s). We find no detectable power at 951.3 microHz (1051 s), the previously reported main frequency. The 1902.5, 2853.9, and 3805.2 microHz peaks are multiplets, with frequency splitting in each case of 20.77 +/- 0.05 microHz. The 1902.5 microHz seasonal pulse shapes are identical, within measurement noise, and maintain the same amplitude and phase as a function of color. We have determined the dominant frequency to be 1902.50902 +/- 0.00001 microHz with dot P = +1.71 (+/- 0.04) x 10(exp -11) s/s. We discuss the implications of these findings on a model for AM CVn.

Provencal, J. L.↗

NASA NTRS2012

A Study of Planetary Nebulae using the Faint Object Infrared Camera for the SOFIA Telescope

A planetary nebula is formed following an intermediate-mass (1-8 solar M) star's evolution off of the main sequence; it undergoes a phase of mass loss whereby the stellar envelope is ejected and the core is converted into a white dwarf. Planetary nebulae often display complex morphologies such as waists or torii, rings, collimated jet-like outflows, and bipolar symmetry, but exactly how these features form is unclear. To study how the distribution of dust in the interstellar medium affects their morphology, we utilize the Faint Object InfraRed CAmera for the SOFIA Telescope (FORCAST) to obtain well-resolved images of four planetary nebulae--NGC 7027, NGC 6543, M2-9, and the Frosty Leo Nebula--at wavelengths where they radiate most of their energy. We retrieve mid infrared images at wavelengths ranging from 6.3 to 37.1 micron for each of our targets. IDL (Interactive Data Language) is used to perform basic analysis. We select M2-9 to investigate further; analyzing cross sections of the southern lobe reveals a slight limb brightening effect. Modeling the dust distribution within the lobes reveals that the thickness of the lobe walls is higher than anticipated, or rather than surrounding a vacuum surrounds a low density region of tenuous dust. Further analysis of this and other planetary nebulae is needed before drawing more specific conclusions.

infrared sensors↗

NASA NTRS1995

Hydrodynamics of binary coalescence. 2: Polytropes with gamma = 5/3

We present a new numerical study of the equilibrium and stability properties of close binary systems. We use the smoothed-particle hydrodynamics (SPH) technique both to construct accurate equilibrium configurations in three dimensions and to follow their hydrodynamic evolution. We adopt a simple polytropic equation of state p = K(sub rho)(exp gamma) with gamma = 5/3 and K = constant within each star, applicable to low-mass degenerate dwarfs as well as low-mass main-sequence stars. For degenerate configurations, we set the two polytropic constants equal, K = K prime, independent of the mass ratio. For main-sequence stars, we adjust K and K prime so as to obtain a simple mass-radius relation of the form R/R prime = M/M prime, where R prime and M prime are the radius and mass of the secondary. Along a sequence of binary equilibrium configurations for two identical stars, we demonstrate the existence of both secular and dynamical instabilities, confirming directly the results of recent analytic work. We use the SPH method to calculate the nonlinear development of the dynamical instability and to determine the final fate of the system. We find that the two stars merge together into a single, rapidly rotating object in just a few orbital periods. Equilibrium sequences are also constructed for systems containing two nonidentical stars. These sequences terminate at a Roche limit, which we can determine very accurately using SPH. For two low-mass main-sequence stars with mass ratio q approximately less than 0.4 we find that the (synchronized) Roche limit configuration is secularly unstable. We discuss the implications of our results for the evolution of double white-dwarf systems and W Ursae Majoris binaries.

Rasio, Frederic A.↗

NASA NTRS1984

Symbiotic stars

The physical characteristics of symbiotic star systems are discussed, based on a review of recent observational data. A model of a symbiotic star system is presented which illustrates how a cool red-giant star is embedded in a nebula whose atoms are ionized by the energetic radiation from its hot compact companion. UV outbursts from symbiotic systems are explained by two principal models: an accretion-disk-outburst model which describes how material expelled from the tenuous envelope of the red giant forms an inwardly-spiralling disk around the hot companion, and a thermonuclear-outburst model in which the companion is specifically a white dwarf which superheats the material expelled from the red giant to the point where thermonuclear reactions occur and radiation is emitted. It is suspected that the evolutionary course of binary systems is predetermined by the initial mass and angular momentum of the gas cloud within which binary stars are born. Since red giants and Mira variables are thought to be stars with a mass of one or two solar mass, it is believed that the original cloud from which a symbiotic system is formed can consist of no more than a few solar masses of gas.

Kafatos, M.↗

DOE OSTI2022

Ignition of Carbon Burning from Nuclear Fission in Compact Stars

Abstract Type Ia supernovae (SN Ia) are powerful stellar explosions that provide important distance indicators in cosmology. Recently, we proposed a new SN Ia mechanism that involves a nuclear fission chain reaction in an isolated white dwarf (WD). The first solids that form as a WD starts to freeze are actinide rich and potentially support a fission chain reaction. In this Letter, we explore thermonuclear ignition from fission heating. We perform thermal diffusion simulations and find at high densities, above about 7 × 10 8 g cm −3 , that fission heating can ignite carbon burning. This could produce an SN Ia or another kind of astrophysical transient.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1976

White dwarfs, the Galaxy and Dirac's cosmology

The additive and multiplicative versions of Dirac's cosmological hypothesis relating the gravitational constant variation with elapsed time and number of particles populating the universe is invoked to account for the deficiency or absence of white dwarfs fainter than about 0.0001 solar luminosity. An estimate is made of white dwarf luminosity in accordance with the two evolutionary models, and it is conjectured that some old white dwarfs with high space velocities may be on the verge of gravitational collapse. Lack of a special mechanism to produce the vast numbers of black holes or other dead stars accounting for 'missing matter' in the vicinity of the sun and in the galactic halo is noted in Dirac's multiplicative model. Results indicate that either Dirac's theory is untenable, or that radiation and heating are of some unknown nature, or that the process of creation of new matter requires a corresponding input of energy.

Stothers, R.↗

NASA NTRS1980

The Barnes-Evans color-surface brightness relation: A preliminary theoretical interpretation

Model atmosphere calculations are used to assess whether an empirically derived relation between V-R and surface brightness is independent of a variety of stellar paramters, including surface gravity. This relationship is used in a variety of applications, including the determination of the distances of Cepheid variables using a method based on the Beade-Wesselink method. It is concluded that the use of a main sequence relation between V-R color and surface brightness in determining radii of giant stars is subject to systematic errors that are smaller than 10% in the determination of a radius or distance for temperature cooler than 12,000 K. The error in white dwarf radii determined from a main sequence color surface brightness relation is roughly 10%.

Shipman, H. L.↗

NASA NTRS1983

Combined ultraviolet studies of astronomical sources

Ultraviolet spectra of non-radiataive shock waves grain destruction and elemental abundances in interstellar shocks, carbon abundance in M33 and M31 from supernova remnants, determination of the mass function in the large Magellanic cloud, UV spectra of white dwarf pulsars, stellar flares, availability of the double quasar Q 0957 + 56' AB, spectra of late-F dwarfs and their relation to rotation, dynamics of hot gas surrounding hybrid stars, high resolution study of epsilon coronae Austriual, active regions on solar-type dwarfs as a function of rotation rate and age, coordinated chromospheric synoptic observations of selected late-type stars, the two-component atmosphere of Lambda Andromedae, and activity in Hyades giants are discussed.

Dupree, A. K.↗

NASA NTRS1984

Ultraviolet spectroscopy of selected astronomical sources

The birth rates of various kinds of white dwarf progenitors are estimated and the spectra of two very hot O-type white dwarfs are analyzed. The effective temperature of LSE 21 was found to be in excess of 100,000 K. The abundance of iron in the atmospheres of two central stars and numerous very hot O-type subdwarfs was found to be strongly variable. Results obtained by high resolution optical spectroscopy, UBV photometry, and low resolution IUE spectrophotometry for LSS 2018, the central star of the planetary nebulae DS1, are summarized. A planetary nebula surrounding the very hot subdwarf LSS 1362 was discovered. Analysis of results obtained by high resolution optical spectra shows agreement with an effective temperature of 110,000 K derived using low resolution IUE observations.

Drilling, J. S.↗

NASA NTRS1986

DA white dwarf effective temperatures determined from IUE Lyman-alpha profiles

The Lyman-alpha profiles of 12 DA white dwarfs have been obtained with the International Ultraviolet Explorer Satellite. Analysis of these profiles provides an improved, uniform, and relatively bias-free measure of effective temperature for these stars over the range 20,000-60,000 K. Simultaneous estimates of surface gravity yield a mean gravity of log g = 7.96 for the entire sample, with the hottest stars tending to have the lowest gravities. A significant exception to this trend occurs in the case of the gravitation of HZ 43. An important by-product of this work has been the determination of a correction to IUE fluxes over the 1150-1350 A range.

Holberg, J. B.↗

NASA NTRS1987

The ultraviolet variability of AY Ceti

AY Cet is a single-line binary comprised of a spotted G5 III primary and a white dwarf secondary. A series of UV spectra have been obtained with the IUE satellite on five different dates covering a substantial part of the optical cycle of the primary star. No evidence that the continuum or the Ly-alpha absorption line of the secondary star varied is found. There were significant changes in the strengths of the UV emission lines, but the variations were only weakly correlated with either the orbital phase of the binary or the rotational phase of the primary. The UV emission lines were especially strong near maximum visual brightness at a time when the starspot(s) on the primary was least visible. The enhanced line emission is attributed to a flare event on the primary, most likely at a high-latitude site close to the pole of this star. The UV radiative losses of this flare were comparable with those of flares previously observed on the RC CVn variables Lambda And and HR 1099.

Simon, Theodore↗

NASA NTRS1992

Theoretical profiles of Lyman-alpha satellites and application to synthetic spectra of DA white dwarfs

We present new theoretical calculations for the red wing of the Lyman-alpha profile. Close collisions with neutral and ionized hydrogen lead to the formation of the pseudomolecules H-H and H-H(+) with the appearance of satellite features near 1600 and 1400 A. The calculations include multiperturber effects, which are responsible for the formation of H3 and H3 with features near 1950 and 2600 A. The theoretical absorption profiles are included in stellar atmosphere codes and used to predict synthetic spectra for DA white dwarfs of intermediate temperatures (20,000 to 8000 K). These new calculations offer a unique opportunity to determine accurate effective temperatures and surface gravities for the variable ZZ Ceti stars.

Allard, N. F.↗

DOE OSTI2022

Nuclear fission reaction simulations in compact stars

Type-Ia supernovas are powerful stellar explosions that provide important distance indicators in cosmology. Recently, we proposed a new Type-Ia supernova mechanism that involves a nuclear fission chain-reaction in an isolated white dwarf [Phys. Rev. Lett. 126, 131101 (2021)]. Here, in this work, we perform novel reaction network simulations of the actinide-rich first solids in a cooling white dwarf. The network includes neutron-capture and fission reactions on a range of U and Th isotopes with various possible values for 235 U enrichment. We find, for modest 235 U enrichments, neutron capture on 238 U and 232 Th can breed additional fissile nuclei so that a significant fraction of all U and Th nuclei may fission during the chain reaction. Finally, we compute the energy release from the fission chain reaction for various uranium enrichments; a novel result that is a necessary input for thermal diffusion simulations of carbon ignition.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2021

Unlocking the Origins of Ultracool Dwarf Radio Emission

Empirical trends in stellar X-ray and radio luminosities suggest that low-mass ultracool dwarfs (UCDs) should not produce significant radio emission. Defying these expectations, strong nonthermal emission has been observed in a few UCDs in the 1–10 GHz range, with a variable component often attributed to global aurorae and a steady component attributed to other processes, such as gyrosynchrotron emission. While both auroral and gyrosynchrotron emission peak near the critical frequency, only the latter radiation is expected to extend into millimeter wavelengths. We present Atacama Large Millimetre/Submillimeter Array (ALMA) 97.5 GHz and Very Large Array 33 GHz observations of a small survey of 5 UCDs. LP 349-25, LSR J1835+3259, and NLTT 33370 were detected at 97.5 GHz, while LP 423-31 and LP 415-20 resulted in nondetections at 33 GHz. A significant flare was observed in NLTT 33370, which reached a peak flux of 4880 ± 360 μJy, exceeding the quiescent flux by nearly an order of magnitude and lasting 20 s. These ALMA observations show bright 97.5 GHz emission with spectral indices ranging from α = 0.76 to α = −0.29, suggestive of optically thin gyrosynchrotron emission. If such emission traces magnetic reconnection events, then this could have consequences for both UCD magnetic models and the atmospheric stability of planets in orbit around them. Overall, our results provide confirmation that gyrosynchrotron radiation in radio-loud UCDs can remain detectable into the millimeter regime.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

NASA NTRS1987

Reliable samples of quasars and hot stars from a spectrophotometric survey of the U.S. catalogs

The U.S. survey for blue- and ultraviolet-excess starlike objects is reviewed, focusing on the features which have contributed to its accuracy. The spectrophotometric survey is described in terms of the observational setup and procedures. It is suggested that the survey has produced reliably classified samples of quasars and hot evolved stars and that the procedures used in the study provide a means of deriving distance and luminosity information about these objects. Several cumulative number counts and spectra of a DA white dwarf and a quasar with prominent C IV and C III emission are given as examples.

Mitchell, Kenneth J.↗

NASA NTRS1991

Post-asymptotic giant branch nonradial instability strips

Stability analyses are performed for nonradial g(+)-mode pulsations of postasymptotic AGB stellar models to determine the location of their pulsational instability strips in the Hertzsprung-Russell diagram. Stellar models are analyzed that are assumed to have undergone a major mass loss event either near the tip of the AGB or shortly thereafter and, therefore, consist of a 50-percent carbon 50-percent oxygen core. Their mass is 0.6 solar mass, which is in good agreement with the peak of the observed distribution of white dwarf masses. The results are compared both to the observed planetary nebula nuclei variables and the hot pulsating DO variables. An analysis is also presented of the stability of DB white dwarfs, which have much deeper and stronger convection zones, and an instability strip is found between about 18,000 and 26,000 K approximately as observed for the known variable stars.

Stanghellini, Letizia↗

NASA NTRS1990

Dynamic mass exchange in doubly degenerate binaries. I - 0.9 and 1.2 solar mass stars

The dynamic mass exchange process in doubly degenerate binaries was investigated using a three-dimensional numerical simulation of the evolution of a doubly degenerate binary system in which the primary is a 1.2-solar-mass white dwarf and the Roche lobe filling secondary is a 0.9-solar-mass dwarf. The results show that, in a little more than two orbital periods, the secondary is completely destroyed and transformed into a thick disk orbiting about the primary. Since only a very small fraction of the mass (0.0063 solar mass) escapes the system, the evolution of the binary results in the formation of a massive object. This object is composed of three parts, the initial white dwarf primary, a very hot pressure-supported spherical envelope, and a rotationally supported outer disk. The evolution of the system can be understood in terms of a simple analytical model where it is shown that the angular momentum carried by the mass during the transfer and stored in the disk determines the evolution of the system.

Benz, W.↗

DOE OSTI2022

The whisper of a whimper of a bang: 2400 d of the Type Ia SN 2011fe reveals the decay of 55Fe

ABSTRACT We analyse new multifilter Hubble Space Telescope (HST) photometry of the normal Type Ia supernova (SN Ia) 2011fe out to ≈2400 d after maximum light, the latest observations to date of a SN Ia. We model the pseudo-bolometric light curve with a simple radioactive decay model and find energy input from both 57Co and 55Fe are needed to power the late-time luminosity. This is the first detection of 55Fe in a SN Ia. We consider potential sources of contamination such as a surviving companion star or delaying the deposition time-scale for 56Co positrons but these scenarios are ultimately disfavored. The relative isotopic abundances place direct constraints on the burning conditions experienced by the white dwarf (WD). Additionally, we place a conservative upper limit of <10−3 M⊙ on the synthesized mass of 44Ti. Only two classes of explosion models are currently consistent with all observations of SN 2011fe: (1) the delayed detonation of a low-ρc, near-MCh (1.2–1.3 M⊙) WD, or (2) a sub-MCh (1.0–1.1 M⊙) WD experiencing a thin-shell double detonation.

Astronomy & Astrophysics↗