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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Producing the deuteron in stars: anthropic limits on fundamental constants

Stellar nucleosynthesis proceeds via the deuteron (D), but only a small change in the fundamental constants of nature is required to unbind it. Here, we investigate the effect of altering the binding energy of the deuteron on proton burning in stars. We find that the most definitive boundary in parameter space that divides probably life-permitting universes from probably life-prohibiting ones is between a bound and unbound deuteron. Due to neutrino losses, a ball of gas will undergo rapid cooling or stabilization by electron degeneracy pressure before it can form a stable, nuclear reaction-sustaining star. We also consider a less-bound deuteron, which changes the energetics of the pp and pep reactions. The transition to endothermic pp and pep reactions, and the resulting beta-decay instability of the deuteron, do not seem to present catastrophic problems for life.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The AGB star nucleosynthesis in the light of the recent {sup 17}O(p,α){sup 14}N and {sup 18}O(p,α){sup 15}N reaction rate determinations

Presolar grains form in the cold and dusty envelopes of Asymptotic Giant Branch (AGB) stars. These solides, once that have been ejected by stellar winds, come to us as inclusions in meteorites providing invaluable benchmarks and constraints for our knowledge of low temeperature H-burning in stars. The Trojan Horse Method (THM) has been used to investigate the low-energy cross sections of the {sup 17}O(p,α){sup 14}N and {sup 18}O(p,α){sup 15}N reactions. Moreover, the strength of the 65 keV resonance in the {sup 17}O(p,α){sup 14}N reaction, measured by means of the THM, has been used to renormalize the corresponding resonance strength in the {sup 17}O+p radiative capture channel. The new estimates of the reaction rates have been introduced into calculations of AGB star nucleosynthesis and the results have been compared with geochemical analysis of 'presolar' grains to determine their impact on astrophysical environments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

New Study of the Astrophysical Reaction {sup 12}C({alpha},{gamma}){sup 16}O via the {sup 12}C({sup 7}Li,t){sup 16}O Transfer Reaction

The {sup 12}C({alpha},{gamma}){sup 16}O reaction plays an important role in helium burning in massive stars and their evolution. However, despite many experimental studies, the low-energy cross section of {sup 12}C({alpha},{gamma}){sup 16}O still highly uncertain. The extrapolation of the measured cross sections to stellar energies (E{approx}300 keV) is made difficult by the presence of the two sub-threshold states at 6.92 and 7.12 MeV of {sup 16}O. In order to further investigate the contribution of these two-subthreshold resonances to the {sup 12}C({alpha},{gamma}){sup 16}O cross section, we performed a new determination of the {alpha}-reduced widths via a measurement of the transfer reaction {sup 12}C({sup 7}Li,t){sup 16}O at two incident energies, 34 and 28 MeV. The measured and calculated differential cross sections will be presented as well as the obtained spectroscopic factors and the {alpha}-reduced widths.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Direct Total Cross Section Measurement of the 16O(α,γ)20Ne Reaction at Ec.m. = 2.26 MeV

In stellar helium burning, (16)O represents the endpoint of the helium-burning sequence due to the low rate of (16)O(alpha,gamma)(20)Ne. We present a new direct measurement of the total capture reaction rate of (16)O(alpha,gamma)(20)Ne at E(c.m.) = 2.26MeV employing the DRAGON recoil separator. For the first time, the total S factor and its contributing direct capture transitions could be determined in one experiment.

Hager, Ulrike↗
NASA NTRS1985

Evolution of massive stars in very young clusters and associations

Statistics concerning the stellar content of young galactic clusters and associations which show well defined main sequence turnups have been analyzed in order to derive information about stellar evolution in high-mass galaxies. The analytical approach is semiempirical and uses natural spectroscopic groups of stars on the H-R diagram together with the stars' apparent magnitudes. The new approach does not depend on absolute luminosities and requires only the most basic elements of stellar evolution theory. The following conclusions are offered on the basis of the statistical analysis: (1) O-tupe main-sequence stars evolve to a spectral type of B1 during core hydrogen burning; (2) most O-type blue stragglers are newly formed massive stars burning core hydrogen; (3) supergiants lying redward of the main-sequence turnup are burning core helium; and most Wolf-Rayet stars are burning core helium and originally had masses greater than 30-40 solar mass. The statistics of the natural spectroscopic stars in young galactic clusters and associations are given in a table.

Stothers, R. B.↗
NASA NTRS2021

Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit

We report the discovery of five transiting companions near the hydrogen-burning mass limit in close orbits around main sequence stars originally identified by the Transiting Exoplanet Survey Satellite (TESS) as TESS objects of interest (TOIs): TOI-148, TOI-587, TOI-681, TOI-746, and TOI-1213. Using TESS and ground-based photometry as well as radial velocities from the CORALIE, CHIRON, TRES, and FEROS spectrographs, we found the companions have orbital periods between 4.8 and 27.2 days, masses between 77 and 98 MJup , and radii between 0.81 and 1.66 RJup . These targets have masses near the uncertain lower limit of hydrogen core fusion (~73-96 MJup ), which separates brown dwarfs and low-mass stars. We constrained young ages for TOI-587 (0.2 ± 0.1 Gyr) and TOI-681 (0.17 ± 0.03 Gyr) and found them to have relatively larger radii compared to other transiting companions of a similar mass. Conversely we estimated older ages for TOI-148 and TOI-746 and found them to have relatively smaller companion radii. With an effective temperature of 9800 ± 200 K, TOI-587 is the hottest known main-sequence star to host a transiting brown dwarf or very low-mass star. We found evidence of spin-orbit synchronization for TOI-148 and TOI-746 as well as tidal circularization for TOI-148. These companions add to the population of brown dwarfs and very low-mass stars with well measured parameters ideal to test formation models of these rare objects, the origin of the brown dwarf desert, and the distinction between brown dwarfs and hydrogen-burning main sequence stars.

Nolan Grieves↗
DOE OSTI2011

The {sup 14}N(p,{gamma}){sup 15}O reaction studied with a composite germanium detector

The rate of the carbon-nitrogen-oxygen (CNO) cycle of hydrogen burning is controlled by the {sup 14}N(p,{gamma}){sup 15}O reaction. The reaction proceeds by capture to the ground states and several excited states in {sup 15}O. In order to obtain a reliable extrapolation of the excitation curve to astrophysical energy, fits in the R-matrix framework are needed. In an energy range that sensitively tests such fits, new cross-section data are reported here for the four major transitions in the {sup 14}N(p,{gamma}){sup 15}O reaction. The experiment has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) 400-kV accelerator placed deep underground in the Gran Sasso facility in Italy. Using a composite germanium detector, summing corrections have been considerably reduced with respect to previous studies. The cross sections for capture to the ground state and to the 5181, 6172, and 6792 keV excited states in {sup 15}O have been determined at 359, 380, and 399 keV beam energy. In addition, the branching ratios for the decay of the 278-keV resonance have been remeasured.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Identification of analog states in the T=1/2 A=27 mirror system from low excitation energies to the region of hydrogen burning in the {sup 26}Al{sup g,m}(p,{gamma}){sup 27}Si reactions

The reactions {sup 26}Al{sup g}(p, {gamma}){sup 27}Si and {sup 26}Al{sup m}(p, {gamma}){sup 27}Si are important for influencing the galactic abundance of the cosmic {gamma}-ray emitter {sup 26}Al{sup g} and for the excess abundance of {sup 26}Mg found in presolar grains, respectively. Precise excitation energies and spin assignments of states from the ground state to the region of astrophysical interest in {sup 27}Si, including the identification and pairing of key astrophysical resonances with analog states in the mirror nucleus {sup 27}Al, are reported using {gamma} rays observed in the {sup 12}C + {sup 16}O fusion reaction. The detailed evolution of Coulomb energy differences between the states in {sup 27}Si and {sup 27}Al is explored, including the region above the astrophysical reaction thresholds.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

SUPERNOVA EJECTA IN THE YOUNGEST GALACTIC SUPERNOVA REMNANT G1.9+0.3

G1.9+0.3 is the youngest known Galactic supernova remnant (SNR), with an estimated supernova (SN) explosion date of {approx}1900, and most likely located near the Galactic center. Only the outermost ejecta layers with free-expansion velocities {approx}>18,000 km s{sup -1} have been shocked so far in this dynamically young, likely Type Ia SNR. A long (980 ks) Chandra observation in 2011 allowed spatially resolved spectroscopy of heavy-element ejecta. We denoised Chandra data with the spatio-spectral method of Krishnamurthy et al., and used a wavelet-based technique to spatially localize thermal emission produced by intermediate-mass elements (IMEs; Si and S) and iron. The spatial distribution of both IMEs and Fe is extremely asymmetric, with the strongest ejecta emission in the northern rim. Fe K{alpha} emission is particularly prominent there, and fits with thermal models indicate strongly oversolar Fe abundances. In a localized, outlying region in the northern rim, IMEs are less abundant than Fe, indicating that undiluted Fe-group elements (including {sup 56}Ni) with velocities >18,000 km s{sup -1} were ejected by this SN. However, in the inner west rim, we find Si- and S-rich ejecta without any traces of Fe, so high-velocity products of O-burning were also ejected. G1.9+0.3 appears similar to energetic Type Ia SNe such as SN 2010jn where iron-group elements at such high free-expansion velocities have been recently detected. The pronounced asymmetry in the ejecta distribution and abundance inhomogeneities are best explained by a strongly asymmetric SN explosion, similar to those produced in some recent three-dimensional delayed-detonation Type Ia models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Properties of {sup 20}Na, {sup 24}Al, {sup 28}P, {sup 32}Cl, and {sup 36}K for studies of explosive hydrogen burning

The radiative proton-capture reactions {sup 19}Ne(p,{gamma}){sup 20}Na, {sup 23}Mg(p,{gamma}){sup 24}Al, {sup 27}Si(p,{gamma}){sup 28}P, {sup 31}S(p,{gamma}){sup 32}Cl, and {sup 35}Ar(p,{gamma}){sup 36}K potentially influence energy generation and/or nucleosynthesis during explosive hydrogen burning in classical novae and/or type I x-ray bursts. The thermonuclear rates of these reactions are dependent on resonance energies E{sub r}=E{sub x}-Q and strengths {omega}{gamma}. The {sup 20}Ne({sup 3}He,t){sup 20}Na, {sup 24}Mg({sup 3}He,t){sup 24}Al, {sup 28}Si({sup 3}He,t){sup 28}P, {sup 32}S({sup 3}He,t){sup 32}Cl, and {sup 36}Ar({sup 3}He,t){sup 36}K reactions have been measured using a 32-MeV, {sup 3}He{sup 2+} beam; ion-implanted carbon-foil targets developed at the University of Washington; and the Munich Q3D magnetic spectrograph. This experiment has already yielded precision mass measurements of {sup 20}Na, {sup 24}Al, {sup 28}P, and {sup 32}Cl [C. Wrede et al., Phys. Rev. C 81, 055503 (2010)], which are used presently to constrain the corresponding (p,{gamma}) reaction Q values. The new {sup 24}Al and {sup 28}P masses resolve a discrepancy in the energy of the lowest-energy resonance in the {sup 23}Mg(p,{gamma}){sup 24}Al reaction and better constrain a direct measurement of its strength. Excitation energies in {sup 32}Cl and {sup 36}K have also been measured. An important new proton-unbound level has been found at E{sub x}=2196.9(7) keV in {sup 36}K and the uncertainties in {sup 36}K excitation energies have been reduced by over an order of magnitude. Using the new data on {sup 36}K, the A=36, T=1 triplets have been reassigned. The thermonuclear {sup 35}Ar(p,{gamma}){sup 36}K reaction rate is found to be much higher than a commonly adopted rate and this could affect energy generation in type I x-ray bursts.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2012

ASYMMETRY AND THE NUCLEOSYNTHETIC SIGNATURE OF NEARLY EDGE-LIT DETONATION IN WHITE DWARF CORES

Most of the leading explosion scenarios for Type Ia supernovae involve the nuclear incineration of a white dwarf star through a detonation wave. Several scenarios have been proposed as to how this detonation may actually occur, but the exact mechanism and environment in which it takes place remain unknown. We explore the effects of an off-center initiated detonation on the spatial distribution of the nucleosynthetic yield products in a toy model-a pre-expanded near Chandrasekhar-mass white dwarf. We find that a single-point near edge-lit detonation results in asymmetries in the density and thermal profiles, notably the expansion timescale, throughout the supernova ejecta. We demonstrate that this asymmetry of the thermodynamic trajectories should be common to off-center detonations where a small amount of the star is burned prior to detonation. The sensitivity of the yields on the expansion timescale results in an asymmetric distribution of the elements synthesized as reaction products. We tabulate the shift in the center of mass of the various elements produced in our model supernova and find an odd-even pattern for elements past silicon. Our calculations show that off-center single-point detonations in carbon-oxygen white dwarfs are marked by significant composition asymmetries in their remnants which bear potentially observable signatures in both velocity and coordinate space, including an elemental nickel mass fraction that varies by a factor of 2-3 from one side of the remnant to the other.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ASYMMETRIC EJECTA DISTRIBUTION IN SN 1006

We present the results from deep X-ray observations ({approx}400 ks in total) of SN 1006 with Suzaku. The thermal spectrum from the entire supernova remnant (SNR) exhibits prominent emission lines of O, Ne, Mg, Si, S, Ar, Ca, and Fe. The observed abundance pattern in the ejecta components is in good agreement with that predicted by a standard model of Type Ia supernovae (SNe). The spatially resolved analysis reveals that the distribution of the O-burning and incomplete Si-burning products (Si, S, and Ar) is asymmetric, while that of the C-burning products (O, Ne, and Mg) is relatively uniform in the SNR interior. The peak position of the former is clearly shifted by 5' ({approx}3.2 pc at the distance of 2.2 kpc) to the southeast (SE) from the SNR's geometric center. Using the SNR age of {approx}1000 yr, we constrain that the velocity asymmetry (in projection) of the ejecta is {approx}3100 km s{sup -1}. The Fe abundance is also significantly higher in the SE region than in the northwest. Given that the non-uniformity is observed only in the heavier elements (Si through Fe), we argue that SN 1006 originates from an asymmetric explosion, as is expected from recent multidimensional simulations of Type Ia SNe, although we cannot eliminate the possibility that inhomogeneous ambient medium had induced the apparent non-uniformity. Possible evidence for the Cr-K-shell line and line broadening in the Fe-K-shell emission is also found.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Measurement of the {beta}-p emission of {sup 20}Mg and the breakout from the hot CNO cycle

In the astrophysically important reaction {sup 19}Ne(p,{gamma}){sup 20}Na, the rate is dominated by a single key resonance at 450 keV above the proton-emission threshold in {sup 20}Na. Throughout the last few decades many experiments have been performed aimed at finding the identity of this state. Despite this, the spin-parity of the key resonant state is still up for debate. The present paper describes a new experiment studying the {beta}-delayed proton decay of {sup 20}Mg aimed at solving this issue.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

NuGrid stellar data set – II. Stellar yields from H to Bi for stellar models with MZAMS = 1–25 M⊙ and Z = 0.0001–0.02

We provide here a significant extension of the NuGrid Set 1 models in mass coverage and towards lower metallicity, adopting the same physics assumptions. The combined data set now includes the initial masses M ZAMS /M ⊙ = 1, 1.65, 2, 3, 4, 5, 6, 7, 12, 15, 20, 25 for Z = 0.02, 0.01, 0.006, 0.001, 0.0001 with α-enhanced composition for the lowest three metallicities. These models are computed with the MESA stellar evolution code and are evolved up to the AGB, the white dwarf stage, or until core collapse. The nucleosynthesis was calculated for all isotopes in post-processing with the NuGrid MPPNP code. Explosive nucleosynthesis is based on semi-analytic 1D shock models. Metallicity-dependent mass-loss, convective boundary mixing in low- and intermediate-mass models and H and He core burning massive star models are included. Convective O-C shell mergers in some stellar models lead to the strong production of odd-Z elements P, Cl, K, and Sc. In AGB models with hot dredge-up, the convective boundary mixing efficiency is reduced to accommodate for its energetic feedback. In both low-mass and massive star models at the lowest metallicity, H-ingestion events are observed and lead to i-process nucleosynthesis and substantial 15 N production. Finally, complete yield data tables, derived data products and online analytic data access are provided.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Constraining the S factor of {sup 15}N(p,{gamma}){sup 16}O at astrophysical energies

The {sup 15}N(p,{gamma}){sup 16}O reaction represents a breakout reaction linking the first and second cycles of the CNO cycles redistributing the carbon and nitrogen abundances into the oxygen range. The reaction is dominated by two broad resonances, at E{sub p}=338 and 1028 keV, and a direct capture contribution to the ground state of {sup 16}O. Interference effects between these contributions both in the low-energy region (E{sub p}<338 keV) and between the two resonances (338<E{sub p}<1028 keV) can dramatically affect the extrapolation to energies of astrophysical interest. To facilitate a reliable extrapolation, the {sup 15}N(p,{gamma}){sup 16}O reaction has been remeasured covering the energy range from E{sub p}=1800 keV down to 130 keV. The results have been analyzed in the framework of a multilevel R-matrix theory and an S(0) value of 39.6 keV b has been found.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Multigroup Radiation Transport in Supernova Light Curve Calculations

The supernova associated with long duration gamma0ray bursts are always found to be Type Ic supernova (SN). Type Ic SN are observed to have extremely small amounts of He in their spectra. Astrophysicists have been exploring ways to either hide the He in the observations or eject the He prior to the SN. Recent calculations by Patrick Young using new mixing algorithms suggest that many massive stars may burn the He layer to O during its evolution and therefore naturally account for the lack of He in the spectra of the SN. The LANL supernova light curve codes are being used to generate simulated observations of these new models.

79 ASTRONOMY AND ASTROPHYSICS↗