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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

The 22 Ne(α,n) 25 Mg reaction - state of the art, astrophysics, and perspectives

One of the most important stellar neutron sources is the 22 Ne(α,n) 25 Mg reaction, which gets activated both during the helium intershell burning in asymptotic giant branch stars and in core helium and shell carbon burning in massive stars. The 22 Ne(α,n) 25 Mg reaction serves as the main neutron producer for the weak s-process and provides a short but strong neutron exposure during the helium flash phase of the main s-process, significantly affecting the abundances at the s-process branch points. The cross section needs to be known at very low energies, as close as possible to the neutron threshold at E α = 562 keV (Q = −478 keV), but both direct and indirect measurements have turned out to be very challenging, leading to significant uncertainties. Here we discuss the current status of the reaction, including recent and upcoming measurements, and provide a discussion on the astrophysical implications as well as an outlook into the near future.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

FLAMES IN TYPE Ia SUPERNOVA: DEFLAGRATION-DETONATION TRANSITION IN THE OXYGEN-BURNING FLAME

The flame in a Type Ia supernova is a conglomerate structure that, depending on density, may involve separate regions of carbon, oxygen, and silicon burning, all propagating in a self-similar, subsonic front. The separation between these three burning regions increases as the density declines until eventually, below about 2 x 10{sup 7} g cm{sup -3}, only carbon burning remains active, the other two burning phases having 'frozen out' on stellar scales. Between 2 and 3 x 10{sup 7} g cm{sup -3}, however, there remains an energetic oxygen-burning region that trails the carbon burning by an amount that is sensitive to the turbulence intensity. As the carbon flame makes a transition to the distributed regime (Karlovitz number {approx}> 10), the characteristic separation between the carbon- and oxygen-burning regions increases dramatically, from a fraction of a meter to many kilometers. The oxygen-rich mixture between the two flames is created at a nearly constant temperature, and turbulence helps to maintain islands of well-mixed isothermal fuel as the temperature increases. The delayed burning of these regions can be supersonic and could initiate a detonation.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2010

Burst Oscillations: Watching Neutron Stars Spin

It is now almost 15 years since the first detection of rotationally modulated emission from X-ray bursting neutron stars, "burst oscillations," This phenomenon enables us to see neutron stars spin, as the X-ray burst flux asymmetrically lights up the surface. It has enabled a new way to probe the neutron star spin frequency distribution, as well as to elucidate the multidimensional nature of nuclear burning on neutron stars. I will review our current observational understanding of the phenomenon, with an eye toward highlighting some of the interesting remaining puzzles, of which there is no shortage.

Strohmayer, Tod↗
DOE OSTI2016

Synthesis of the S-only {sup 122,123,124}TE isotopes and the selective depletion of {sup 123}Te by electron capture process in massive stars

We investigate the production ratios of the naturally occurring s-only nuclides {sup 122−124}Te by solving a nuclear network in the mass number 120⩽A⩽124 region under some conditions representing those in the helium–hydrogen inter-shell in low-mass AGB stars as well as those in the helium burning and the carbon burning phases in massive stars. We show that the electron-capture process on {sup 123}Te (the natural half-life >10{sup 16} years) could proceed decisively fast during the late evolutionary phases of massive stars. We thus re-iterate and re-enforce the decades-old notion that detailed analysis of the relative {sup 122−124}Te abundances in due consideration of the selective {sup 123}Te depletion in massive stars would set a stringent constraint on the s-process modelings. We stress that the relative abundance ratios of {sup 122−124}Te in the solar system become quite difficult to decipher if the s-process in massive stars contributes significantly to the synthesis of nuclides in this intermediate A region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

TOI 694b and TIC 220568520b: Two Low-mass Companions near the Hydrogen-burning Mass Limit Orbiting Sun-like Stars

We report the discovery of TOI 694 b and TIC 220568520 b, two low-mass stellar companions in eccentric orbits around metal-rich Sun-like stars, first detected by the Transiting Exoplanet Survey Satellite (TESS). TOI 694 b has an orbital period of 48.05131 ± 0.00019 days and eccentricity of 0.51946 ± 0.00081, and we derive a mass of 89.0 ± 5.3 M{sub Jup} (0.0849 ± 0.0051 M{sub ⊙}) and radius of 1.111 ± 0.017 R{sub Jup} (0.1142 ± 0.0017 R{sub ⊙}). TIC 220568520 b has an orbital period of 18.55769 ± 0.00039 days and eccentricity of 0.0964 ± 0.0032, and we derive a mass of 107.2 ± 5.2 M{sub Jup} (0.1023 ± 0.0050 M{sub ⊙}) and radius of 1.248 ± 0.018 R{sub Jup} (0.1282 ± 0.0019 R{sub ⊙}). Both binary companions lie close to and above the hydrogen-burning mass threshold that separates brown dwarfs and the lowest-mass stars, with TOI 694 b being 2σ above the canonical mass threshold of 0.075 M{sub ⊙}. The relatively long periods of the systems mean that the magnetic fields of the low-mass companions are not expected to inhibit convection and inflate the radius, which according to one leading theory is common in similar objects residing in short-period tidally synchronized binary systems. Indeed we do not find radius inflation for these two objects when compared to theoretical isochrones. These two new objects add to the short but growing list of low-mass stars with well-measured masses and radii, and highlight the potential of the TESS mission for detecting such rare objects orbiting bright stars.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2016

SU Lyncis, a Hard X-Ray Bright M Giant: Clues Point to a Large Hidden Population of Symbiotic Stars

Symbiotic star surveys have traditionally relied almost exclusively on low resolution optical spectroscopy. However, we can obtain amore reliable estimate of their total Galactic population by using all available signatures of the symbiotic phenomenon. Here we report the discovery of a hard X-ray source, 4PBC J0642.9+5528, in the Swift hard X-ray all-sky survey, and identify it with a poorly studied red giant, SU Lyn, using pointed Swift observations and ground-based optical spectroscopy. The X-ray spectrum, the optical to UV spectrum, and the rapid UV variability of SU Lyn are all consistent with our interpretation that it is a symbiotic star containing an accreting white dwarf. The symbiotic nature of SU Lyn went unnoticed until now, because it does not exhibit emission lines strong enough to be obvious in low resolution spectra. We argue that symbiotic stars without shell-burning have weak emission lines, and that the current lists of symbiotic stars are biased in favour of shell-burning systems. We conclude that the true population of symbiotic stars has been underestimated, potentially by a large factor.

binaries: symbiotic aEuro" stars: individual: SU L↗
DOE OSTI2010

{sup 16}O({alpha},{gamma}){sup 20}Ne S factor: Measurements and R-matrix analysis

The article presents new measurements of low-energy resonances in the {sup 16}O({alpha},{gamma}){sup 20}Ne reaction, which represents the endpoint for the reaction sequence responsible for energy production in stellar helium burning in massive red giant stars. The present data and previous unpublished data are analyzed in the framework of R-matrix theory to derive a reaction rate for the temperature regime of stellar helium burning.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

Effects of the variation of fundamental constants on Pop III stellar evolution

The effect of variations of the fundamental constants on the thermonuclear rate of the triple alpha reaction, {sup 4}He({alpha}{alpha}, {gamma}){sup 12}C, that bridges the gap between {sup 4}He and {sup 12}C is investigated. We have followed the evolution of 15 and 60 M{center_dot} zero metallicity stellar models, up to the end of core helium burning. They are assumed to be representative of the first (Population III) stars. The calculated oxygen carbon abundances resulting from helium burning can then be used to constrain the variation of the fundamental constants.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS2002

Oscillations During Thermonuclear X-ray Bursts: A New Probe of Neutron Stars

Observations of thermonuclear (also called Type 1) X-ray bursts from neutron stars in low mass X-ray binaries (LMXB) with the Rossi X-ray Timing Explorer (RXTE) have revealed large amplitude, high coherence X-ray brightness oscillations with frequencies in the 300 - 600 Hz range. Substantial spectral and timing evidence point to rotational modulation of the X-ray burst flux as the cause of these oscillations, and it is likely that they reveal the spin frequencies of neutron stars in LMXB from which they are detected. Here we review the status of our knowledge of these oscillations and describe how they can be used to constrain the masses and radii of neutron stars as well as the physics of thermonuclear burning on accreting neutron stars.

Strohmayer, Tod E.↗
DOE OSTI2023

Supermassive Dark Star candidates seen by JWST

The first generation of stars in the universe is yet to be observed. There are two leading theories for those objects that mark the beginning of the cosmic dawn: hydrogen burning Population III stars and Dark Stars, made of hydrogen and helium but powered by dark matter heating. The latter can grow to become supermassive (M$_\star$ ~ 10 6 M ⊙ ) and extremely bright (L ~ 109L ⊙ ). We show that each of the following three objects—JADES-GS-z13-0, JADES-GS-z12-0, and JADES-GS-z11-0 (at redshifts z ϵ [11, 14])—are consistent with a Supermassive Dark Star interpretation, thus identifying the first Dark Star candidates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

DEEP MIXING IN EVOLVED STARS. II. INTERPRETING Li ABUNDANCES IN RED GIANT BRANCH AND ASYMPTOTIC GIANT BRANCH STARS

We reanalyze the problem of Li abundances in red giants of nearly solar metallicity. After outlining the problems affecting our knowledge of the Li content in low-mass stars (M {<=} 3 M{sub sun}), we discuss deep-mixing models for the red giant branch stages suitable to account for the observed trends and for the correlated variations of the carbon isotope ratio; we find that Li destruction in these phases is limited to masses below about 2.3 M{sub sun}. Subsequently, we concentrate on the final stages of evolution for both O-rich and C-rich asymptotic giant branch (AGB) stars. Here, the constraints on extra-mixing phenomena previously derived from heavier nuclei (from C to Al), coupled to recent updates in stellar structure models (including both the input physics and the set of reaction rates used), are suitable to account for the observations of Li abundances below A(Li) {identical_to} log {epsilon}(Li) {approx_equal} 1.5 (and sometimes more). Also, their relations with other nucleosynthesis signatures of AGB phases (like the abundance of F, and the C/O and {sup 12}C/{sup 13}C ratios) can be explained. This requires generally moderate efficiencies (M-dot < or approx. 0.3-0.5 x 10{sup -6} M{sub sun} yr{sup -1}) for non-convective mass transport. At such rates, slow extra mixing does not remarkably modify Li abundances in early AGB phases; on the other hand, faster mixing encounters a physical limit in destroying Li, set by the mixing velocity. Beyond this limit, Li starts to be produced; therefore, its destruction on the AGB is modest. Li is then significantly produced by the third dredge up. We also show that effective circulation episodes, while not destroying Li, would easily bring the {sup 12}C/{sup 13}C ratios to equilibrium, contrary to the evidence in most AGB stars, and would burn F beyond the limits shown by C(N) giants. Hence, we do not confirm the common idea that efficient extra mixing drastically reduces the Li content of C stars with respect to K-M giants. This misleading appearance is induced by biases in the data, namely: (1) the difficulty of measuring very low Li abundances in O-rich AGB stars due to the presence of TiO bands and (2) the fact that many, relatively massive (M > 3 M{sub sun}) K- and M-type giants may remain Li-rich, not evolving to the C-rich stages. Efficient extra mixing on the AGB is instead typical of very low masses (M {approx}< 1.5 M{sub sun}). It also characterizes CJ stars, where it produces Li and reduces F and the carbon isotope ratio, as observed in these peculiar objects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Finding the first cosmic explosions. III. Pulsational pair-instability supernovae

Population III supernovae have been the focus of growing attention because of their potential to directly probe the properties of the first stars, particularly the most energetic events that can be seen at the edge of the observable universe. But until now pulsational pair-instability supernovae, in which explosive thermonuclear burning in massive stars fails to unbind them but can eject their outer layers into space, have been overlooked as cosmic beacons at the earliest redshifts. These shells can later collide and, like Type IIn supernovae, produce superluminous events in the UV at high redshifts that could be detected in the near infrared today. We present numerical simulations of a 110 M {sub ☉} pulsational pair-instability explosion done with the Los Alamos radiation hydrodynamics code Radiation Adaptive Grid Eulerian. We find that collisions between consecutive pulsations are visible in the near infrared out to z ∼ 15-20 and can probe the earliest stellar populations at cosmic dawn.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1986

The (32)S/(33)S abundance as a function of galactocentric radius in the Milky Way

Astration of heavy elements by the stars of the Milky Way forms a fossil record which may preserve spacial distribution of the mass function for the stars in the galaxy. Sulfur is among the last common element for which the relative abundance of its various isotopes have yet to be completely measured within our galaxy. Explosive oxygen burning in massive stars is thought to be the process which dominates sulfur production within stars. There models predict that the various isotopes (S-32, S-33, S-34) are formed in relative abundance which depend strongly upon the mass of the parent star. This relative abundance is thought to be unaffected by subsequent stellar procesing since all important sinks of sulfur destroy it without regard for isotopic form. Hence the spacial variation of the mass function (MF) can be studied by measuring the abundance variation of sulfur isotopes in the galaxy provided that the product yields for these isotopes are known accurately as a function of stellar mass.

Greenhouse, M. A.↗
DOE OSTI2010

SIMPLIFIED HYDROSTATIC CARBON BURNING IN WHITE DWARF INTERIORS

We introduce two simplified nuclear networks that can be used in hydrostatic carbon burning reactions occurring in white dwarf interiors. They model the relevant nuclear reactions in carbon-oxygen white dwarfs approaching ignition in Type Ia supernova progenitors, including the effects of the main e {sup -}-captures and {beta}-decays that drive the convective Urca process. They are based on studies of a detailed nuclear network compiled by the authors and are defined by approximate sets of differential equations whose derivations are included in the text. The first network, N1, provides a good first-order estimation of the distribution of ashes and it also provides a simple picture of the main reactions occurring during this phase of evolution. The second network, N2, is a more refined version of N1 and can reproduce the evolution of the main physical properties of the full network to the 5% level. We compare the evolution of the mole fraction of the relevant nuclei, the neutron excess, the photon energy generation, and the neutrino losses between both simplified networks and the detailed reaction network in a fixed temperature and density parcel of gas.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE FINAL FATE OF STARS THAT IGNITE NEON AND OXYGEN OFF-CENTER: ELECTRON CAPTURE OR IRON CORE-COLLAPSE SUPERNOVA?

In the ONeMg cores of 8.8-9.5 M {sub ☉} stars, neon and oxygen burning is ignited off-center. Whether or not the neon-oxygen flame propagates to the center is critical for determining whether these stars undergo Fe core collapse or electron-capture-induced ONeMg core collapse. We present more details of stars that ignite neon and oxygen burning off-center. The neon flame is established in a manner similar to the carbon flame of super-AGB stars, albeit with a narrower flame width. The criteria for establishing a flame can be met if the strict Schwarzschild criterion for convective instability is adopted. Mixing across the interface of the convective shell disrupts the conditions for the propagation of the burning front, and instead the shell burns as a series of inward-moving flashes. While this may not directly affect whether or not the burning will reach the center (as in super-AGB stars), the core is allowed to contract between each shell flash. Reduction of the electron fraction in the shell reduces the Chandrasekhar mass and the center reaches the threshold density for the URCA process to activate and steer the remaining evolution of the core. This highlights the importance of a more accurate treatment of mixing in the stellar interior for yet another important question in stellar astrophysics—determining the properties of stellar evolution and supernova progenitors at the boundary between electron capture supernova and iron core-collapse supernova.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Spectroscopic Comparison of Metal-rich RRab Stars of the Galactic Field with their Metal-poor Counterparts

We investigate atmospheric properties of 35 stable RRab stars that possess the full ranges of period, light amplitude, and metal abundance found in Galactic RR Lyrae stars. Our results are derived from several thousand echelle spectra obtained over several years with the du Pont telescope of Las Campanas Observatory. Radial velocities of metal lines and the H α line were used to construct curves of radial velocity versus pulsation phase. From these we estimated radial velocity amplitudes for metal lines (formed near the photosphere) and H α Doppler cores (formed at small optical depths). We also measured H α emission fluxes when they appear during primary light rises. Spectra shifted to rest wavelengths, binned into small phase intervals, and co-added were used to perform model atmospheric and abundance analyses. The derived metallicities and those of some previous spectroscopic surveys were combined to produce a new calibration of the Layden abundance scale. We then divided our RRab sample into metal-rich (disk) and metal-poor (halo) groups at [Fe/H] = −1.0; the atmospheres of RRab families, so defined, differ with respect to (a) peak strength of H α emission flux, (b) H α radial velocity amplitude, (c) dynamical gravity, (d) stellar radius variation, (e) secondary acceleration during the photometric bump that precedes minimum light, and (f) duration of H α line-doubling. We also detected H α line-doubling during the “bump” in the metal-poor family, but not in the metal-rich one. Although all RRab probably are core helium-burning horizontal branch stars, the metal-rich group appears to be a species sui generis.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Structure of massive main-sequence stars.

Core hydrogen-burning models are discussed together with the associated physical instabilities. A mathematical relation for dynamical instability is presented. Conditions leading to convective instability are analyzed, giving attention to the nature of the convectively unstable, chemically inhomogeneous, zone that is left behind by the retreating convective core. Questions of pulsational instability are also investigated.

Stothers, R.↗