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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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Limit on overshooting from the convective core in upper main-sequence stars

Overshooting of convective elements far beyond the classical Schwarzschild convective core boundary in theoretical models of massive ZAMS stars is found to lead to significant reductions of stellar luminosity and radius, provided that the temperature gradient in the overshoot region can be approximated by the adiabatic gradient. Comparison of these theoretical models with binary star data for the range of 5-15 solar masses indicates that the ratio of overshoot distance to pressure scale height is probably less than 1.5. This result apparently excludes any overshooting theories that predict very extended adiabatic convective cores.

Stothers, Richard B.↗
NASA NTRS1991

Neutrino-heated stars and broad-line emission from active galactic nuclei

Nonthermal radiation from active galactic nuclei indicates the presence of highly relativistic particles. The interaction of these high-energy particles with matter and photons gives rise to a flux of high-energy neutrinos. In this paper, the influence of the expected high neutrino fluxes on the structure and evolution of single, main-sequence stars is investigated. Sequences of models of neutrino-heated stars in thermal equilibrium are presented for masses 0.25, 0.5, 0.8, and 1.0 solar mass. In addition, a set of evolutionary sequences for mass 0.5 solar mass have been computed for different assumed values for the incident neutrino energy flux. It is found that winds driven by the heating due to high-energy particles and hard electromagnetic radiation of the outer layers of neutrino-bloated stars may satisfy the requirements of the model of Kazanas (1989) for the broad-line emission clouds in active galactic nuclei.

Macdonald, James↗
DOE OSTI2016

THE FIRST CIRCUMBINARY PLANET FOUND BY MICROLENSING: OGLE-2007-BLG-349L(AB)c

We present the analysis of the first circumbinary planet microlensing event, OGLE-2007-BLG-349. This event has a strong planetary signal that is best fit with a mass ratio of q ≈ 3.4 × 10{sup −4}, but there is an additional signal due to an additional lens mass, either another planet or another star. We find acceptable light-curve fits with two classes of models: two-planet models (with a single host star) and circumbinary planet models. The light curve also reveals a significant microlensing parallax effect, which constrains the mass of the lens system to be M {sub L} ≈ 0.7 M{sub ⊙}. Hubble Space Telescope (HST) images resolve the lens and source stars from their neighbors and indicate excess flux due to the star(s) in the lens system. This is consistent with the predicted flux from the circumbinary models, where the lens mass is shared between two stars, but there is not enough flux to be consistent with the two-planet, one-star models. So, only the circumbinary models are consistent with the HST data. They indicate a planet of mass m {sub c} = 80 ± 13 M{sub ⊕}, orbiting a pair of M dwarfs with masses of M {sub A} = 0.41 ± 0.07 and M {sub B} = 0.30 ± 0.07, which makes this the lowest-mass circumbinary planet system known. The ratio of the separation between the planet and the center of mass to the separation of the two stars is ∼40, so unlike most of the circumbinary planets found by Kepler, the planet does not orbit near the stability limit.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2016

The First Circumbinary Planet Found by Microlensing: OGLE-2007-BLG-349L(AB)c

We present the analysis of the first circumbinary planet microlensing event, OGLE-2007-BLG-349. This event has a strong planetary signal that is best fit with a mass ratio of q approx. = 3.4×10(exp −4), but there is an additional signal due to an additional lens mass, either another planet or another star. We find acceptable light-curve fits with two classes of models: two-planet models (with a single host star) and circumbinary planet models. The light curve also reveals a significant microlensing parallax effect, which constrains the mass of the lens system to be M(sub L) approx. = 0.7 Stellar Mass. Hubble Space Telescope (HST) images resolve the lens and source stars from their neighbors and indicate excess flux due to the star(s) in the lens system. This is consistent with the predicted flux from the circumbinary models, where the lens mass is shared between two stars, but there is not enough flux to be consistent with the two-planet, one-star models. So, only the circumbinary models are consistent with the HST data. They indicate a planet of mass m(sub c) = 80 +/- 13 Stellar Mass, orbiting a pair of M dwarfs with masses of M(sub A) = 0.41+/- 0.07 and M(sub B) = 0.30 +/- 0.07, which makes this the lowest-mass circumbinary planet system known. The ratio of the separation between the planet and the center of mass to the separation of the two stars is approx.40, so unlike most of the circumbinary planets found by Kepler, the planet does not orbit near the stability limit.

OGLE-2007-BLG-349↗
DOE OSTI2018

Simultaneous Iron and Nickel Isotopic Analyses of Presolar Silicon Carbide Grains.

Aside from recording stellar nucleosynthesis, a few elements in presolar grains can also provide insights into the galactic chemical evolution (GCE) of nuclides. We have studied the carbon, silicon, iron, and nickel isotopic compositions of presolar silicon carbide (SiC) grains from asymptotic giant branch (AGB) stars to better understand GCE. Since only the neutron-rich nuclides in these grains have been heavily influenced by the parent star, the neutron-poor nuclides serve as GCE proxies. Using CHILI, a new resonance ionizationmass spectrometry (RIMS) instrument, we measured 74 presolar SiC grains for all iron and nickel isotopes. With the CHARISMA instrument, 13 presolar SiC grains were analyzed for iron isotopes. All grains were also measured by NanoSIMS for their carbon and silicon isotopic compositions. A comparison of the measured neutron-rich isotopes with models for AGB star nucleosynthesis shows that our measurements are consistent with AGB star predictions for low-mass stars between half-solar and solar metallicity. Furthermore, our measurements give an indication on the Ne-22(alpha, n)Mg-25 reaction rate. In terms of GCE, we find that the GCE-dominated iron and nickel isotope ratios, Fe-54/Fe-56 and Ni-60/Ni-58, correlate with their GCE-dominated counterpart in silicon, Si-29/Si-28. The measured GCE trends include the Solar System composition, showing that the Solar System is not a special case. However, as seen in silicon and titanium, many presolar SiC grains are more evolved for iron and nickel than the Solar System. This confirms prior findings and agrees with observations of large stellar samples that a simple age-metallicity relationship for GCE cannot explain the composition of the solar neighborhood. (C) 2017 Elsevier Ltd. All rights reserved.

galactic chemical evolution↗
NASA NTRS1990

High accuracy optical rate sensor

Optical rate sensors, in particular CCD arrays, will be used on Space Station Freedom to track stars in order to provide inertial attitude reference. An algorithm to provide attitude rate information by directly manipulating the sensor pixel intensity output is presented. The star image produced by a sensor in the laboratory is modeled. Simulated, moving star images are generated, and the algorithm is applied to this data for a star moving at a constant rate. The algorithm produces accurate derived rate of the above data. A step rate change requires two frames for the output of the algorithm to accurately reflect the new rate. When zero mean Gaussian noise with a standard deviation of 5 is added to the simulated data of a star image moving at a constant rate, the algorithm derives the rate with an error of 1.9 percent at a rate of 1.28 pixels per frame.

Uhde-Lacovara, J.↗
NASA NTRS1990

Modeling the ultraviolet photospheric spectrum of cool giant stars. I - Arcturus. II - Gamma Crucis

The UV photospheric spectra of cool giants are modeled analytically to facilitate the identification of UV spectral features and aid in the measurement of actual chromospheric flux at different wavelengths. Atomic line data derived experimentally and semiempirically are used with LTE model atmospheres by means of the SYNTHE FORTRAN code to generate the synthetic spectra. Surface flux spectra are determined and fitted to the absorption features of observed IUE high resolution spectra, and the modeled results are compared to the observed spectra of the cool giants Arcturus and the cooler Gamma Crucis. It is shown that the synthetic spectra can be produced for the stars, and chromospheric emission lines are thought to account for significant discrepancies between observed and calculated values. Two new fluorescence mechanisms are identified by modeling the ultraviolet photospheric spectrum of Gamma Crucis.

Carpenter, Kenneth G.↗
DOE OSTI2015

Testing model atmospheres for young very-low-mass stars and brown dwarfs in the infrared: Evidence for significantly underestimated dust opacities

We test state-of-the-art model atmospheres for young very-low-mass stars and brown dwarfs in the infrared, by comparing the predicted synthetic photometry over 1.2–24 μm to the observed photometry of M-type spectral templates in star-forming regions. We find that (1) in both early and late young M types, the model atmospheres imply effective temperatures (T{sub eff}) several hundred Kelvin lower than predicted by the standard pre-main sequence (PMS) spectral type–T{sub eff} conversion scale (based on theoretical evolutionary models). It is only in the mid-M types that the two temperature estimates agree. (2) The T{sub eff} discrepancy in the early M types (corresponding to stellar masses ≳0.4 M{sub ⊙} at ages of a few Myr) probably arises from remaining uncertainties in the treatment of atmospheric convection within the atmospheric models, whereas in the late M types it is likely due to an underestimation of dust opacity. (3) The empirical and model-atmosphere J-band bolometric corrections are both roughly flat, and similar to each other, over the M-type T{sub eff} range. Thus the model atmospheres yield reasonably accurate bolometric luminosities (L{sub bol}), but lead to underestimations of mass and age relative to evolutionary expectations (especially in the late M types) due to lower T{sub eff}. We demonstrate this for a large sample of young Cha I and Taurus sources. (4) The trends in the atmospheric model J−K{sub s} colors, and their deviations from the data, are similar at PMS and main sequence ages, suggesting that the model dust opacity errors we postulate here for young ages also apply at field ages.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1980

Mercury in a thin layer in HgMn stars - A test of a diffusion model

Lines of the first three states of ionization of mercury have been observed in Mu Leporis and Chi Lupi using the Copernicus satellite. Lines of Hg II and Hg III have been observed in Alpha Andromedae. There appears to be an absorption feature at every wavelength where there is expected to be a mercury line. The presence of all three states of ionization is likely in Mu Lep and Chi Lup. The relative equivalent widths of the lines of the various states of ionization do not depend on the effective temperature of the stars, in contradiction to what is expected if mercury were uniformly distributed in the atmosphere. It is, however, expected if mercury has been concentrated, by diffusion, in a thin layer, where the radiative forces just equal the gravitational forces on mercury. That mercury should be so concentrated is also required by the explanation of the mercury isotope anomaly proposed by Michaud, Reeves, and Charland.

Megessier, C.↗
DOE OSTI2019

Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins

We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19) and Z (18), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. Here direct comparison of the Y and Z grain data with data for mainstream grains from AGB stars of close-to-solar metallicity demonstrates that the three types of grains have indistinguishable Mo isotopic compositions. We show that the Mo isotope data can be used to constrain the maximum stellar temperatures ( T MAX ) during thermal pulses in AGB stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with the grain data for Mo isotopes points to an origin from low-mass (~1.5–3 M ⊙ ) rather than intermediate-mass (>3–~9 M ⊙ ) AGB stars. Because of the low efficiency of 22 Ne(α, n ) 25 Mg at the low T MAX values attained in low-mass AGB stars, model calculations cannot explain the large 30 Si excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1980

Line formation in winds with enhanced equatorial mass-loss rates and its application to the Wolf-Rayet star HD 50896

A model having a spherically symmetric velocity distribution with a higher density at the equatorial region was developed to simulate the UV spectrum of the Wolf-Rayet star HD 50896. The spectrum showed P Cygni-shaped profiles whose emissions are stronger than expected in a spherically symmetric stellar wind. The model was studied varying the inclination angle of the star-wind system and the polar to equatorial density ratios; it was shown that HD 50896 could possess a nonspherically symmetric wind and that its symmetry axis is inclined between 60 and 90 deg. It is possible that the velocity distribution of the wind could include an inner constant velocity plateau beyond which the wind accelerates to its terminal velocity as indicated by infrared continuum investigations.

Rumpl, W. M.↗
NASA NTRS1992

Non-LTE, line-blanketed model atmospheres for late O- and early B-type stars

The use of non-LTE line-blanketed model atmospheres to analyze the spectra of hot stars is reported. The stars analyzed are members of clusters and associations, have spectral types in the range O9-B2 and luminosity classes in the range III-IV, have slow to moderate rotation, and are photometrically constant. Sampled line opacities of iron-group elements were incorporated in the radiative transfer solution; solar abundances were assumed. Good to excellent agreement is obtained between the computed profiles and essentially all the line profiles used to fix the model, and reliable stellar parameters are derived. The synthetic M II 5581 equivalent widths agree well with the observed ones at the low end of the temperature range studied, but, above 25,000 K, the synthetic line is generally stronger than the observed line. The behavior of the observed equivalent widths of N II, N III, C II and C III lines as a function of Teff is studied. Most of the lines show much scatter, with no consistent trend that could indicate abundance differences from star to star.

Grigsby, James A.↗
NASA NTRS1991

Dynamical capture of field stars by interstellar clouds

Evidence suggesting the existence of old stars in young galactic clusters, has motivated an investigation of a model in which such stars are captured from the field during the gas cloud collapse phase of star formation. A hyperbolic field star cannot be gravitationally captured by a static cloud since the star's total energy is conserved. However, if the cloud's potential is an explicit function of time, the energy of a penetrating star can be decreased. As a first step in modeling the dynamical capture of field stars, capture by freely falling, homogeneous spherical cloud is considered. This simplified model is solved, and a prediction is obtained for number of captured field stars in terms of the cloud parameters and dispersion velocities of the field stars.

Whitman, P. G.↗
DOE OSTI2015

THE HYDRODYNAMICAL MODELS OF THE COMETARY COMPACT H ii REGION

We have developed a full numerical method to study the gas dynamics of cometary ultracompact H ii regions, and associated photodissociation regions (PDRs). The bow-shock and champagne-flow models with a 40.9/21.9 M{sub ⊙} star are simulated. In the bow-shock models, the massive star is assumed to move through dense (n = 8000 cm{sup −3}) molecular material with a stellar velocity of 15 km s{sup −1}. In the champagne-flow models, an exponential distribution of density with a scale height of 0.2 pc is assumed. The profiles of the [Ne ii] 12.81 μm and H{sub 2} S(2) lines from the ionized regions and PDRs are compared for two sets of models. In champagne-flow models, emission lines from the ionized gas clearly show the effect of acceleration along the direction toward the tail due to the density gradient. The kinematics of the molecular gas inside the dense shell are mainly due to the expansion of the H ii region. However, in bow-shock models the ionized gas mainly moves in the same direction as the stellar motion. The kinematics of the molecular gas inside the dense shell simply reflects the motion of the dense shell with respect to the star. These differences can be used to distinguish two sets of models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

EVOLUTION OF THERMALLY PULSING ASYMPTOTIC GIANT BRANCH STARS. V. CONSTRAINING THE MASS LOSS AND LIFETIMES OF INTERMEDIATE-MASS, LOW-METALLICITY AGB STARS

Thermally pulsing asymptotic giant branch (TP-AGB) stars are relatively short lived (less than a few Myr), yet their cool effective temperatures, high luminosities, efficient mass loss, and dust production can dramatically affect the chemical enrichment histories and the spectral energy distributions of their host galaxies. The ability to accurately model TP-AGB stars is critical to the interpretation of the integrated light of distant galaxies, especially in redder wavelengths. We continue previous efforts to constrain the evolution and lifetimes of TP-AGB stars by modeling their underlying stellar populations. Using Hubble Space Telescope (HST) optical and near-infrared photometry taken of 12 fields of 10 nearby galaxies imaged via the Advanced Camera for Surveys Nearby Galaxy Survey Treasury and the near-infrared HST/SNAP follow-up campaign, we compare the model and observed TP-AGB luminosity functions as well as the ratio of TP-AGB to red giant branch stars. We confirm the best-fitting mass-loss prescription, introduced by Rosenfield et al., in which two different wind regimes are active during the TP-AGB, significantly improves models of many galaxies that show evidence of recent star formation. This study extends previous efforts to constrain TP-AGB lifetimes to metallicities ranging −1.59 ≲ [Fe/H] ≲ −0.56 and initial TP-AGB masses up to ∼4 M {sub ⊙}, which include TP-AGB stars that undergo hot-bottom burning.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

RADIAL STELLAR PULSATION AND THREE-DIMENSIONAL CONVECTION. IV. FULL AMPLITUDE THREE-DIMENSIONAL SOLUTIONS

Three-dimensional hydrodynamic simulations of full amplitude RR Lyrae stars have been computed for several models across the instability strip. The three-dimensional nature of the calculations allows convection to be treated without reference to a phenomenological approach such as the local mixing length theory. Specifically, the time-dependent interaction of large-scale eddies and radial pulsation is controlled by conservation laws, while the effects of smaller convective eddies are simulated by an eddy viscosity model. The light amplitudes for these calculations are quite similar to those of our previous two-dimensional calculations in the middle of the instability strip, but somewhat lower near the red edge, the fundamental blue edge, and for the one first overtone model we computed. The time-dependent interaction between the radial pulsation and the convective energy transport is essentially the same in three dimensions as it is in two dimensions. There are some differences between the light curves of the two- and three-dimensional simulations, particularly during decreasing light. Reasons for the differences, both numerical and physical, are explored.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2012

Chandra Phase-Resolved X-ray Spectroscopy of the Crab Pulsar II

We present a new study of the X-ray spectral properties of the Crab Pulsar. The superb angular resolution of the Chandra X-ray Observatory enables distinguishing the pulsar from the surrounding nebulosity. Analysis of the spectrum as a function of pulse phase allows the least-biased measure of interstellar X-ray extinction due primarily to photoelectric absorption and secondarily to scattering by dust grains in the direction of the Crab Nebula. We modify previous findings that the line-of-sight to the Crab is under-abundant in oxygen and provide measurements with improved accuracy and less bias. Using the abundances and cross sections from Wilms, Allen & McCray (2000) we find [O/H] = (5.28+\-0.28) x 10(exp -4) (4.9 x 10(exp -4) is solar abundance). \rVe also measure for the first time the impact of scattering of flux out of the image by interstellar grains. \rYe find T(sub scat) = 0.147+/-0.043. Analysis of the spectrum as a function of pulse phase also measures the X-ray spectral index even at pulse minimum - albeit with increasing statistical uncertainty. The spectral variations are, by and large, consistent with a sinusoidal variation. The only significant variation from the sinusoid occurs over the same phase range as some rather abrupt behavior in the optical polarization magnitude and position angle. We compare these spectral variations to those observed in Gamma-rays and conclude that our measurements are both a challenge and a guide to future modeling and will thus eventually help us understand pair cascade processes in pulsar magnetospheres. The data were also used to set new. and less biased, upper limits to the surface temperature of the neutron star for different models of the neutron star atmosphere.

Weisskopf, Martin C.↗