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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 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

The accretion of interplanetary dust by Ap and Am stars

Accretion of interplanetary dust and comets by A-type stars is investigated to see whether this process can explain the overabundances of heavy elements in Ap and Am stars. The dust particles spiral in by the Poynting-Robertson effect and evaporate above the star. This process is evaluated by considering the evolution of silicate and graphite particles in the radiation field of Alpha Leo (B7 V). It is found that graphite particles evaporate above 20 stellar radii and silicate dust grains above 50 stellar radii. The evaporated atoms will be quickly ionized. In the case of Am stars the ions will wander off and not reach the photosphere, but in the case of Ap stars they can be trapped by the magnetic field of the star and reach the surface. The magnetosphere of a typical Ap star is modeled, and the fate of the ions is studied. The efficiency of accretion from the magnetosphere is sensitive to the presence of any plasma in the magnetosphere. The comet impact process is found capable of producing the abundance anomalies in Ap as well as Am stars, assuming that the comet impact rate is 0.03/yr, nearly the same as in the solar system.

Kumar, C. Krishna↗
NASA NTRS1994

A search for lithium in Pleiades brown dwarf candidates using the Keck hires echelle

We report Keck Observatory high-resolution echelle spectra of lithium at 670.8 nm in two of the lowest luminosity brown dwarf candidates in the Pleiades. These objects have estimated masses of 0.055 to 0.059 solar mass from their location on a color-magnitude diagram relative to theoretical isochrones. Stellar interior models predict that Li has not burned in them. However, we find no evidence of the Li line, at limits 100 to 1000 times below the initial abundance. This indicates that Li has in fact been depleted, presumably by nuclear processing as occurs in Pleiades stars. Interior models suggest that such large Li depletion occurs only for objects with M greater than 0.09 solar mass at the age of the Pleiades. Thus, it is unlikely that the candidates are brown dwarfs. The brown dwarf candidates present a conflict: either they have masses greater than suggested from their placement on the H-R diagram, or they do have the very low suggested masses but are nonetheless capable of destroying Li, in only 70 Myr. Until this dilemma is resolved, the photometric identification of brown dwarfs will remain difficult. Resolution may reside in higher T(sub eff) derived from optical and IR colors or in lower T(sub eff) in the interior models.

Marcy, Geoffrey W.↗
DOE OSTI2015

CHARACTERIZING THE BROWN DWARF FORMATION CHANNELS FROM THE INITIAL MASS FUNCTION AND BINARY-STAR DYNAMICS

The stellar initial mass function (IMF) is a key property of stellar populations. There is growing evidence that the classical star-formation mechanism by the direct cloud fragmentation process has difficulties reproducing the observed abundance and binary properties of brown dwarfs and very-low-mass stars. In particular, recent analytical derivations of the stellar IMF exhibit a deficit of brown dwarfs compared to observational data. Here we derive the residual mass function of brown dwarfs as an empirical measure of the brown dwarf deficiency in recent star-formation models with respect to observations and show that it is compatible with the substellar part of the Thies-Kroupa IMF and the mass function obtained by numerical simulations. We conclude that the existing models may be further improved by including a substellar correction term that accounts for additional formation channels like disk or filament fragmentation. The term ''peripheral fragmentation'' is introduced here for such additional formation channels. In addition, we present an updated analytical model of stellar and substellar binarity. The resulting binary fraction and the dynamically evolved companion mass-ratio distribution are in good agreement with observational data on stellar and very-low-mass binaries in the Galactic field, in clusters, and in dynamically unprocessed groups of stars if all stars form as binaries with stellar companions. Cautionary notes are given on the proper analysis of mass functions and the companion mass-ratio distribution and the interpretation of the results. The existence of accretion disks around young brown dwarfs does not imply that these form just like stars in direct fragmentation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CONSTRAINING TYPE Ia SUPERNOVA MODELS: SN 2011fe AS A TEST CASE

The nearby supernova SN 2011fe can be observed in unprecedented detail. Therefore, it is an important test case for Type Ia supernova (SN Ia) models, which may bring us closer to understanding the physical nature of these objects. Here, we explore how available and expected future observations of SN 2011fe can be used to constrain SN Ia explosion scenarios. We base our discussion on three-dimensional simulations of a delayed detonation in a Chandrasekhar-mass white dwarf and of a violent merger of two white dwarfs (WDs)-realizations of explosion models appropriate for two of the most widely discussed progenitor channels that may give rise to SNe Ia. Although both models have their shortcomings in reproducing details of the early and near-maximum spectra of SN 2011fe obtained by the Nearby Supernova Factory (SNfactory), the overall match with the observations is reasonable. The level of agreement is slightly better for the merger, in particular around maximum, but a clear preference for one model over the other is still not justified. Observations at late epochs, however, hold promise for discriminating the explosion scenarios in a straightforward way, as a nucleosynthesis effect leads to differences in the {sup 55}Co production. SN 2011fe is close enough to be followed sufficiently long to study this effect.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

Intense extreme ultraviolet emission from the B star Epsilon Canis Majoris

We report the discovery of the brightest nonsolar source of EUV emission: the B2 II star Epsilon Canis Majoris. This source has been detected by the Extreme Ultraviolet Explorer satellite's all-sky photometric survey. It is approximately 30 times brighter at 600 A than the predicted emission from the hot white dwarf star HZ 43, previously believed to be the brightest EUV source. We have fitted a simple B star photospheric model to the observed broadband EUV fluxes to explain this emission. Assuming a stellar temperature of 25,000 K and a gravity (log g) of 3.3, we derive an interstellar hydrogen column density of 1.05 +/- 0.05 x 10 exp 18/sq cm over the 187 pc to the star. This corresponds to a line-of-sight number density of hydrogen, of 0.002/cu cm, which is comparable to values found in the rarefied Local Bubble region which surrounds the sun.

Vallerga, John V.↗
NASA NTRS1977

White-dwarf variability and the rotation of g-modes

The multiperiodic behavior clearly evident after Fourier analysis of many DA white-dwarf light curves is interpreted as arising from outbursts whose timing is regulated by the rotation and oscillation of the star. A model is proposed on the basis of three main assumptions: (1) a broad array of g-mode oscillations is taking place all the time in DA white-dwarf variables; (2) slow rotation permits most of the g-modes to couple and form a small group of nonlinear modes, each characterized by one of the indices of the spherical harmonic functions involved; and (3) the observed brightenings of the star are produced by upwelling of convection on a large horizontal scale when the antinodes of the nonlinear oscillation patterns periodically rotate into alignment and cause a temporary local enhancement of energy per unit volume. This model is used to match precisely almost every strong periodicity observed in the complex light curves of the DA white dwarfs G207-9, G38-29, G29-38, and HL Tau 76. If the model has been applied correctly, the results indicate that all four of these variables are rotating slowly with periods in the range from 250 to 500 sec.

Wolff, C. L.↗
DOE OSTI2014

Recent developments for realistic solar models

The 'solar abundance problem' has triggered a renewed interest in revising the concept of SSM from different perspectives: 1) constituent microphysics: equation of state, nuclear rates, radiative opacities; 2) constituent macrophysics: the physical processes impact the evolution of the Sun and its present-day structure, e.g. dynamical processes induced by rotation, presence of magnetic fields; 3) challenge the hypothesis that the young Sun was chemically homogeneous: the possible interaction of the young Sun with its protoplanetary disk. Here, I briefly review and then present a (personal) view on recent advances and developments on solar modeling, part of them carried out as attempts to solve the solar abundance problem.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Interesting features in the combined Galex and Sloan color diagrams of solar-like galactic populations

We report on intriguing photometric properties of Galactic stars observed in the Galaxy Evolution Explorer (GALEX) satellite's far-UV (FUV) and near-UV (NUV) bandpasses, as well as from the ground-based Sloan Digital Sky Survey (SDSS) and the Kepler Input Catalog. The first property is that the (FUV – NUV) color distribution of stars in the Kepler field consists of two well-separated peaks. A second and more perplexing property is that for stars with spectral types G or later the mean (FUV – NUV) color becomes much bluer, contrary to expectation. Investigating this tendency further, we found in two samples of mid-F through K type stars that 17%-22% of them exhibit FUV excesses relative to their NUV fluxes and spectral types. A correction for FUV incompleteness of the FUV magnitude-limited star sample brings this ratio to 14%-18%. Nearly the same fractions are also discovered among members of the Kepler Eclipsing Binary Catalog and in the published list of Kepler Objects of Interest. These UV-excess ('UVe') colors are confirmed by the negative UV continuum slopes in GALEX spectra of members of the population. The SDSS spectra of some UVe stars exhibit metallic line weakening, especially in the blue. This suggests an enhanced contribution of UV flux relative to photospheric flux of a solar-type single star. We consider the possibility that the UV excesses originate from various types of hot stars, including white dwarf DA and sdB stars, binaries, and strong chromosphere stars that are young or in active binaries. The space density of compact stars is too low to explain the observed frequency of the UVe stars. Our model atmosphere-derived simulations of colors for binaries with main-sequence pairs with a hot secondary demonstrate that the color loci conflict with the observed sequence. As a preferred alternative we are left with the active chromospheres explanation, whether in active close binaries or young single stars, despite the expected paucity of young, chromospherically active stars in the field. We also address a third perplexing color property, namely, the presence of a prominent island of 'UV red' stars surrounded by 'UV blue' stars in the diagnostic (NUV–g), (g – i) color diagram. We find that the subpopulation composing this island is mainly horizontal branch stars. These objects do not exhibit UV excesses and therefore have UV colors typical for their spectral types. This subpopulation appears 'red' in the UV only because the stars' colors are not pulled to the blue by the inclusion of UVe stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

XMM-NEWTON MEASUREMENT OF THE GALACTIC HALO X-RAY EMISSION USING A COMPACT SHADOWING CLOUD

Observations of interstellar clouds that cast shadows in the soft X-ray background can be used to separate the background Galactic halo emission from the local emission due to solar wind charge exchange (SWCX) and/or the Local Bubble (LB). We present an XMM-Newton observation of a shadowing cloud, G225.60–66.40, that is sufficiently compact that the on- and off-shadow spectra can be extracted from a single field of view (unlike previous shadowing observations of the halo with CCD-resolution spectrometers, which consisted of separate on- and off-shadow pointings). We analyzed the spectra using a variety of foreground models: one representing LB emission, and two representing SWCX emission. We found that the resulting halo model parameters (temperature T {sub h} ≈ 2 × 10{sup 6} K, emission measure E{sub h}≈4×10{sup −3} cm{sup −6} pc) were not sensitive to the foreground model used. This is likely due to the relative faintness of the foreground emission in this observation. However, the data do favor the existence of a foreground. The halo parameters derived from this observation are in good agreement with those from previous shadowing observations, and from an XMM-Newton survey of the Galactic halo emission. This supports the conclusion that the latter results are not subject to systematic errors, and can confidently be used to test models of the halo emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TIME SERIES MODELS OF THREE SETS OF RXTE OBSERVATIONS OF 4U 1543-47

The X-ray nova 4U 1543-47 was in a different physical state (low/hard, high/soft, and very high) during the acquisition of each of the three time series analyzed in this paper. Standard time series models of the autoregressive moving average (ARMA) family are fitted to these series. The low/hard data can be adequately modeled by a simple low-order model with fixed coefficients, once the slowly varying mean count rate has been accounted for. The high/soft series requires a higher order model, or an ARMA model with variable coefficients. The very high state is characterized by a succession of 'dips', with roughly equal depths. These seem to appear independently of one another. The underlying stochastic series can again be modeled by an ARMA form, or roughly as the sum of an ARMA series and white noise. The structuring of each model in terms of short-lived aperiodic and 'quasi-periodic' components is discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EVIDENCE FOR ASYMMETRIC DISTRIBUTION OF CIRCUMSTELLAR MATERIAL AROUND TYPE Ia SUPERNOVAE

We study the properties of low-velocity material in the line of sight toward nearby Type Ia supernovae (SNe Ia) that have measured late phase nebular velocity shifts (v{sub neb}), thought to be an environment-independent observable. We have found that the distribution of equivalent widths of narrow blended Na I D1 and D2 and Ca II H and K absorption lines differs significantly between those SNe Ia with negative and positive v{sub neb}, with generally stronger absorption for SNe Ia with v{sub neb} {>=} 0. A similar result had been found previously for the distribution of colors of SNe Ia, which was interpreted as a dependence of the temperature of the ejecta with viewing angle. Our work suggests that (1) a significant part of these differences in color should be attributed to extinction, (2) this extinction is caused by an asymmetric distribution of circumstellar material (CSM), and (3) the CSM absorption is generally stronger on the side of the ejecta opposite to where the ignition occurs. Since it is difficult to explain (3) via any known physical processes that occur before explosion, we argue that the asymmetry of the CSM is originated after explosion by a stronger ionizing flux on the side of the ejecta where ignition occurs, probably due to a stronger shock breakout and/or more exposed radioactive material on one side of the ejecta. This result has important implications for both progenitor and explosion models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ON THE NEUTRINO NON-DETECTION OF GRB 130427A

The recent gamma-ray burst GRB 130427A has an isotropic electromagnetic energy E{sup iso} {approx} 10{sup 54} erg, suggesting an ample supply of target photons for photo-hadronic interactions, which at its low redshift of z {approx} 0.34 would appear to make it a promising candidate for neutrino detection. However, the IceCube collaboration has reported a null result based on a search during the prompt emission phase. We show that this neutrino non-detection can provide valuable information about this gamma-ray burst's (GRB's) key physical parameters such as the emission radius R{sub d} , the bulk Lorentz factor {Gamma}, and the energy fraction converted into cosmic rays {epsilon}{sub p}. The results are discussed both in a model-independent way and in the specific scenarios of an internal shock (IS) model, a baryonic photospheric (BPH) model, and a magnetic photospheric (MPH) model. We find that the constraints are most stringent for the MPH model considered, but the constraints on the IS and the BPH models are fairly modest.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Rotational Splitting and Asteroseismic Modeling of the δ Scuti Star EE Camelopardalis

According to the rotational splitting law of g modes, the frequency spectra of EE Cam can be disentangled only with oscillation modes of ℓ = 0, 1, and 2. Fifteen sets of rotational splits were found, and they contain five sets of ℓ = 1 multiplets and 10 sets of ℓ = 2 multiplets. The rotational period of EE Cam is deduced to be P{sub rot}=1.84{sub −0.05}{sup +0.07} days. When we do model fittings, we use two nonradial oscillation modes (f {sub 11} and f {sub 32}), and the fundamental radial mode f {sub 1}. The fitting results show that χ{sup 2} of the best-fitting model is much smaller than those of other theoretical models. The physical parameters of the best-fitting model are M = 2.04 M {sub ⊙}, Z = 0.028, T {sub eff} = 6433 K, logL/L{sub ⊙}=1.416, R = 4.12 R {sub ⊙}, logg=3.518, and χ{sup 2}=0.00035. Furthermore, we find that f {sub 11} and f {sub 32} are mixed modes, which mainly characterize the features of the helium core. The fundamental radial mode f {sub 1} mainly restricts the features of the stellar envelope. Finally, the acoustic radius τ{sub 0} and the period separation Π{sub 0} are determined to be 5.80 hr and 463.7 s, respectively, and the size of the helium core of EE Cam is estimated to be M {sub He} = 0.181 M {sub ⊙} and R {sub He} = 0.0796 R {sub ⊙}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

PROPERTIES OF BULGELESS DISK GALAXIES. II. STAR FORMATION AS A FUNCTION OF CIRCULAR VELOCITY

We study the relation between the surface density of gas and star formation rate in 20 moderately inclined, bulgeless disk galaxies (Sd-Sdm Hubble types) using CO(1-0) data from the IRAM 30 m telescope, H I emission line data from the VLA/EVLA, H{alpha} data from the MDM Observatory, and polycyclic aromatic hydrocarbon emission data derived from Spitzer IRAC observations. We specifically investigate the efficiency of star formation as a function of circular velocity (v{sub circ}). Previous work found that the vertical dust structure and disk stability of edge-on, bulgeless disk galaxies transition from diffuse dust lanes with large scale heights and gravitationally stable disks at v{sub circ} < 120 km s{sup -1} (M{sub *} {approx}< 10{sup 10} M{sub Sun }) to narrow dust lanes with small scale heights and gravitationally unstable disks at v{sub circ} > 120 km s{sup -1}. We find no transition in star formation efficiency ({Sigma}{sub SFR}/{Sigma}{sub Hi+H{sub 2}}) at v{sub circ} = 120 km s{sup -1} or at any other circular velocity probed by our sample (v{sub circ} = 46-190 km s{sup -1}). Contrary to previous work, we find no transition in disk stability at any circular velocity in our sample. Assuming our sample has the same dust structure transition as the edge-on sample, our results demonstrate that scale height differences in the cold interstellar medium of bulgeless disk galaxies do not significantly affect the molecular fraction or star formation efficiency. This may indicate that star formation is primarily affected by physical processes that act on smaller scales than the dust scale height, which lends support to local star formation models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PARALLAXES AND PROPER MOTIONS OF ULTRACOOL BROWN DWARFS OF SPECTRAL TYPES Y AND LATE T

We present astrometric measurements of 11 nearby ultracool brown dwarfs of spectral types Y and late-T, based on imaging observations from a variety of space-based and ground-based telescopes. These measurements have been used to estimate relative parallaxes and proper motions via maximum likelihood fitting of geometric model curves. To compensate for the modest statistical significance ({approx}< 7) of our parallax measurements we have employed a novel Bayesian procedure for distance estimation which makes use of an a priori distribution of tangential velocities, V {sub tan}, assumed similar to that implied by previous observations of T dwarfs. Our estimated distances are therefore somewhat dependent on that assumption. Nevertheless, the results have yielded distances for five of our eight Y dwarfs and all three T dwarfs. Estimated distances in all cases are {approx}> 3 pc. In addition, we have obtained significant estimates of V {sub tan} for two of the Y dwarfs; both are <100 km s{sup -1}, consistent with membership in the thin disk population. Comparison of absolute magnitudes with model predictions as a function of color shows that the Y dwarfs are significantly redder in J - H than predicted by a cloud-free model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

SUPRATHERMAL ELECTRONS IN THE SOLAR CORONA: CAN NONLOCAL TRANSPORT EXPLAIN HELIOSPHERIC CHARGE STATES?

There have been several ideas proposed to explain how the Sun's corona is heated and how the solar wind is accelerated. Some models assume that open magnetic field lines are heated by Alfvén waves driven by photospheric motions and dissipated after undergoing a turbulent cascade. Other models posit that much of the solar wind's mass and energy is injected via magnetic reconnection from closed coronal loops. The latter idea is motivated by observations of reconnecting jets and also by similarities of ion composition between closed loops and the slow wind. Wave/turbulence models have also succeeded in reproducing observed trends in ion composition signatures versus wind speed. However, the absolute values of the charge-state ratios predicted by those models tended to be too low in comparison with observations. This Letter refines these predictions by taking better account of weak Coulomb collisions for coronal electrons, whose thermodynamic properties determine the ion charge states in the low corona. A perturbative description of nonlocal electron transport is applied to an existing set of wave/turbulence models. The resulting electron velocity distributions in the low corona exhibit mild suprathermal tails characterized by ''kappa'' exponents between 10 and 25. These suprathermal electrons are found to be sufficiently energetic to enhance the charge states of oxygen ions, while maintaining the same relative trend with wind speed that was found when the distribution was assumed to be Maxwellian. The updated wave/turbulence models are in excellent agreement with solar wind ion composition measurements.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SOLAR MODELS WITH NEW LOW METAL ABUNDANCES

In the past decade, the photospheric abundances of the Sun had been revised several times by many observers. The standard solar models constructed with the new low-metal abundances disagree with helioseismic results and detected neutrino fluxes. The solar model problem has puzzled some stellar physicists for more than 10 years. Rotation, enhanced diffusion, convection overshoot, and magnetic fields are used to reconcile the new abundances with helioseismology. The too low helium subsurface abundance in enhanced diffusion models can be improved by the mixing caused by rotation and magnetic fields. The problem of the depth of the convective zone in rotating models can be resolved by convection overshoot. Consequently, the Asplund–Grevesse–Sauval rotation model including overshooting (AGSR) reproduces the seismically inferred sound-speed and density profiles and the convection zone depth as well as the Grevesse and Sauval model computed before. But this model fails to reproduce the surface helium abundance, which is 0.2393 (2.6 σ away from the seismic value), and neutrino fluxes. The magnetic model called AGSM keeps the agreement of the AGSR and improves the prediction of the surface helium abundance. The observed separation ratios r {sub 02} and r {sub 13} are reasonably reproduced by AGSM. Moreover, neutrino fluxes calculated by this model are not far from the detected neutrino fluxes and the predictions of previous works.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1984

Spectral variability in early-type binary X-ray systems

Theoretical models for the ionization of trace elements in a strong stellar wind by a compact binary X-ray source and for the resulting orbital phase dependence of the emergent soft X-ray spectra and the profiles of ultraviolet resonance lines are presented. Model results agree qualitatively with the X-ray and ultraviolet spectra of the system 4U 0900-40/HD 77581 and explain the suppression of the absorption profiles of the Si IV upsilon 1394 and C IV upsilon 1548 lines when the X-ray sources is in front of the star. The model predicts that the absorption profiles of the N V upsilon 1239 and O VI upsilon 1032 lines will be enhanced rather than suppressed during this orbital phase. We predict phase-dependent linear polarization in the resonance lines profiles. Future observations of these phase dependent effects in early-type binary X-ray systems may be used to investigate the dynamics of stellar winds and their interactions with the X-ray source.

Mccray, R.↗