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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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NASA NTRS2020

The CARMENES search for exoplanets around M dwarfs: Two planets on opposite sides of the radius gap transiting the nearby M dwarf LTT 3780

We present the discovery and characterisation of two transiting planets observed by the Transiting Exoplanet Survey Satellite (TESS) orbiting the nearby (d⋆ ≈ 22 pc), bright (J ≈ 9 mag) M3.5 dwarf LTT 3780 (TOI–732). We confirm both planets and their association with LTT 3780 via ground-based photometry and determine their masses using precise radial velocities measured with the CARMENES spectrograph. Precise stellar parameters determined from CARMENES high-resolution spectra confirm that LTT 3780 is a mid-M dwarf with an effective temperature of T(eff) = 3360 ± 51 K, a surface gravity of log g⋆ = 4.81 ± 0.04 (cgs), and an iron abundance of [Fe/H] = 0.09 ± 0.16 dex, with an inferred mass of M⋆ = 0.379 ± 0.016M⊙ and a radius of R⋆ = 0.382 ± 0.012R⊙. The ultra-short-period planet LTT 3780 b (P(b) = 0.77 d) with a radius of 1.35(−0.06,+0.06) R⊕, a mass of 2.34(−0.23,+0.24) M⊕, and a bulk density of 5.24(−0.81,+0.94) g/cu.cm joins the population of Earth-size planets with rocky, terrestrial composition. The outer planet, LTT 3780 c, with an orbital period of 12.25 d, radius of 2.42(−0.10,+0.10) R⊕, mass of 6.29(−0.61,+0.63) M⊕, and mean density of 2.45(−0.37,+0.44) g/cu.cm belongs to the population of dense sub-Neptunes. With the two planets located on opposite sides of the radius gap, this planetary system is an excellent target for testing planetary formation, evolution, and atmospheric models. In particular, LTT 3780 c is an ideal object for atmospheric studies with the James Webb Space Telescope (JWST).

G. Nowak↗

NASA NTRS1989

The effects of Compton scattering in X-ray spectra

The ways in which Compton scattering on free electrons influences the continuous spectrum and temperature structure of a hot stellar atmosphere, where most of its radiation is emitted in the 1-10 keV energy range, are discussed. The numerical results pertain to the continuum spectra of X-ray bursters. The formulation and development of the numerical code, which reproduces angle-averaged relativistic Compton scattering of thermal (unpolarized) X-rays in a plane-parallel atmosphere in hydrostatic and radiative equilibrium, are described.

Madej, Jerzy↗

NASA NTRS1992

Polycyclic aromatic hydrocarbon formation in carbon-rich stellar envelopes

A detailed chemical kinetic scheme is applied to stellar envelope profiles of gas density and temperature profiles in order to study the formation of PAH molecules in carbon-rich stellar outflows. Chemical concentration profiles are calculated for several envelope models by integrating the coupled continuity equations that include spherically expanding flows from an inner boundary at the shock formation radius. The influence of the 'inverse greenhouse' effect experienced by small PAHs is investigated and shown to increase the PAH yield by many orders of magnitude. It is shown that the route through propargyl radicals could be an important channel to produce benzene. PAH formation yields are found to be extremely sensitive to gas density and temperature and are much smaller than values inferred from the observed dust content of late-type carbon-rich stellar envelopes. It is therefore unlikely that aromatic molecules are generated in the stellar outflow itself.

Cherchneff, Isabelle↗

NASA NTRS1981

IUE spectra of the hot close binary V Puppis

Low- and high-dispersion ultraviolet spectra obtained by the IUE of the early B-type close binary V Puppis are reported. The continuum derived from the low-dispersion images is consistent with a solar abundance model of effective temperature 25,000 K and log surface gravity 4.0. The high-dispersion spectra show shallow double lines, representing Doppler-resolved stellar lines, which are broadened as expected for the fast rotational velocities given by Levato (1974). The equivalent widths of the silicon lines, however, do not agree with the temperature derived for the star, while the silicon and carbon absorptions indicate a disordered photosphere in the presence of circumstellar gas. Several interstellar lines are also observed for the first time.

Koch, R. H.↗

NASA NTRS2013

BD-22deg3467, a DAO-type Star Exciting the Nebula Abell 35

Spectral analyses of hot, compact stars with non-local thermodynamical equilibrium (NLTE) model-atmosphere techniques allow the precise determination of photospheric parameters such as the effective temperature (T(sub eff)), the surface gravity (log g), and the chemical composition. The derived photospheric metal abundances are crucial constraints for stellar evolutionary theory. Aims. Previous spectral analyses of the exciting star of the nebula A35, BD-22deg3467, were based on He+C+N+O+Si+Fe models only. For our analysis, we use state-of-the-art fully metal-line blanketed NLTE model atmospheres that consider opacities of 23 elements from hydrogen to nickel. We aim to identify all observed lines in the ultraviolet (UV) spectrum of BD-22deg3467 and to determine the abundances of the respective species precisely. Methods. For the analysis of high-resolution and high signal-to-noise ratio (S/N) far-ultraviolet (FUSE) and UV (HST/STIS) observations, we combined stellar-atmosphere models and interstellar line-absorption models to fully reproduce the entire observed UV spectrum. Results. The best agreement with the UV observation of BD-22deg3467 is achieved at T(sub eff) = 80 +/- 10 kK and log g = 7.2 +/- 0.3. While T(sub eff) of previous analyses is verified, log g is significantly lower. We re-analyzed lines of silicon and iron (1/100 and about solar abundances, respectively) and for the first time in this star identified argon, chromium, manganese, cobalt, and nickel and determined abundances of 12, 70, 35, 150, and 5 times solar, respectively. Our results partially agree with predictions of diffusion models for DA-type white dwarfs. A combination of photospheric and interstellar line-absorption models reproduces more than 90% of the observed absorption features. The stellar mass is M approx. 0.48 Solar Mass. Conclusions. BD.22.3467 may not have been massive enough to ascend the asymptotic giant branch and may have evolved directly from the extended horizontal branch to the white dwarf state. This would explain why it is not surrounded by a planetary nebula. However, the star, ionizes the ambient interstellar matter, mimicking a planetary nebula.

Ziegler, M.↗

NASA NTRS1980

Synthetic line profiles in early-type stellar winds. I - H and He/+/

Statistical equilibrium calculations for the first 10 levels of H and He(+) were carried out for several model atmospheres of a star with an effective temperature of 40,000 deg K and log g equal to 3.5. Atomic level populations and line profiles were computed for a wind model (Klein and Castor, 1978) with a mass-loss rate of about 1.2 x 10 to the -5th solar mass per yr. A comoving frame method was applied and the radiative transfer problem in the expanding atmosphere is treated. Two variants of the Klein-Castor model produce He II at 4686 A in emission with 3203 A in absorption, as is commonly observed in Of spectra.

Kunasz, P. B.↗

NASA NTRS1997

Boron Abundances in A and B-type Stars

Boron abundances in A- and B-type stars may be a successful way to track evolutionary effects in these hot stars. The light elements - Li, Be, and B - are tracers of exposure to temperatures more moderate than those in which the H-burning CN-cycle operates. Thus, any exposure of surface stellar layers to deeper layers will affect these light element abundances. Li and Be are used in this role in investigations of evolutionary processes in cool stars, but are not observable in hotter stars. An investigation of boron, however, is possible through the B II 1362 A resonance line. We have gathered high resolution spectra from the IUE database of A- and B-type stars near 10 solar mass for which nitrogen abundances have been determined. The B II 1362 A line is blended throughout; the temperature range of this program, requiring spectrum syntheses to recover the boron abundances. For no star could we synthesize the 1362 A region using the meteoritic/solar boron abundance of log e (B) = 2.88; a lower boron abundance was necessary which may reflect evolutionary effects (e.g., mass loss or mixing near the main-sequence), the natal composition of the star forming regions, or a systematic error in the analyses (e.g., non-LTE effects). Regardless of the initial boron abundance, and despite the possibility of non-LTE effects, it seems clear that boron is severely depleted in some stars. It may be that the nitrogen and boron abundances are anticorrelated, as would be expected from mixing between the H-burning and outer stellar layers. If, as we suspect, a residue of boron is present in the A-type supergiants, we may exclude a scenario in which mixing occurs continuously between the surface and the deep layers operating the CN-cycle. Further exploitation of the B II 1362 A line as an indicator of the evolutionary status of A- and B-type stars will require a larger stellar sample to be observed with higher signal-to-noise as attainable with the Hubble Space Telescope.

Lambert, David L.↗

NASA NTRS1972

Theoretical effect of various broadening parameters on ultraviolet line profiles.

The relative importance of various damping parameters in UV lines under conditions of normal stellar atmospheres is studied, giving attention to temperatures in the range from 10,000 to 30,000 K. It is found that the damping by electron collisions is always much smaller than either the radiative or the classical damping, except for lines that arise from high-excitation levels and do not therefore develop strong damping wings. A damping constant equal to 10 times the classical value is always much too large when compared to the more realistic (electron + radiative) damping.

Peytremann, E.↗

NASA NTRS2016

The Far-Ultraviolet Spectra of Two Hot PG1159 Stars

PG 1159 stars are hot, hydrogen-deficient (pre-) white dwarfs with atmospheres mainly composed of helium, carbon, and oxygen. The unusual surface chemistry is the result of a late helium-shell flash. Observed element abundances enable us to test stellar evolution models quantitatively with respect to their nucleosynthesis products formed near the helium-burning shell of the progenitor asymptotic giant branch stars. Because of the high effective temperatures (T(sub eff)), abundance determinations require ultraviolet spectroscopy and non-local thermodynamic equilibrium model atmosphere analyses. Up to now, we have presented results for the prototype of this spectral class and two cooler members (T(sub eff) in the range 85,000-140,000 K). Here we report on the results for two even hotter stars (PG 1520+525 and PG 1144+005, both with T(sub eff) = 150,000 K) which are the only two objects in this temperature-gravity region for which useful far-ultraviolet spectra are available, and revisit the prototype star. Previous results on the abundances of some species are confirmed, while results on others (Si, P, S) are revised. In particular, a solar abundance of sulphur is measured in contrast to earlier claims of a strong S deficiency that contradicted stellar evolution models. For the first time, we assess the abundances of Na, Al, andCl with newly constructed non-LTE model atoms. Besides the main constituents (He, C, O), we determine the abundances (or upper limits) of N, F, Ne, Na, Al, Si, P, S, Cl, Ar, and Fe. Generally, good agreement with stellar models is found.

Werner, K.↗

NASA NTRS1977

Does the upper main sequence extend across the whole H-R diagram

The effect of using Carson's (1976) radiative opacities in evolutionary sequences of stellar models has been studied over the mass range from 7 to 60 solar masses. The opacities are very large in the outer part of the envelope and induce such enormous radii for masses greater than about 30 solar masses for a heavy-element fraction of 0.02 or about 20 solar masses for a heavy-element fraction of 0.04 that the evolutionary tracks during the phase of core hydrogen burning extend across the whole H-R diagram. The choice of the Schwarschild or Ledoux criterion for convection makes very little difference for the behavior of the tracks. Evolution through the effective-temperature range (in logarithms) of 3.6 to 4.0 occurs in all cases on a rapid (secular) time scale. Core helium burning takes place exclusively in the red-supergiant configuration for stellar masses exceeding 8 solar masses (heavy-element fraction of 0.02) or 6 solar masses (heavy-element fraction of 0.04). These stellar models seem to be in significantly better agreement with the observed distribution of bright stars on the H-R diagram than are the older models based on the Cox-Stewart opacities. It can be inferred that a large envelope opacity (e.g., Carson's) exists and that substantial mass loss takes place in very massive late-type supergiants.

Stothers, R.↗

NASA NTRS1971

Experiment S013: Ultraviolet astronomical camera

Recordings of the stellar ultraviolet radiation in the wavelength region from 2000 to 4000 angstroms are reported. An analysis of the surface temperatures of the stars, the absorption effects occurring in the atmospheres of the stars, and the absorption effects of the interstellar dust is made from the photographic data obtained. In addition to gathering basic astrophysical data about hot, young stars, the experiment is used to clarify the problems and techniques of uv photography of stars from manned spacecraft.

Henize, K. G.↗

NASA NTRS1973

Acceleration of heavy ions by radiation pressure.

The possible acceleration of heavy ions by the resonance scattering of radiation near bright stellar objects is examined. Planck spectra are assumed. At any radiation temperature, heavy-ion energies are found to be limited by aberration and Doppler effects. Photoionization is found to limit maximum radiation temperatures and energies for each ion. Collisional ionization by ambient electrons and ions may further limit heavy-ion energies. Because of these limitations, the production of the more energetic cosmic rays by this mechanism appears to be very improbable. If supernova radiation temperatures are high enough, they may produce heavy ions with energies up to several hundreds of MeV per nucleon. O and B stars appear to be able to accelerate certain heavy ions to 200 and 80 MeV per nucleon, respectively.

Nakada, M. P.↗

NASA NTRS1981

Theoretical studies of chromospheres and winds in cool stars

Calculated radiative losses from H, H-, Ca II, and Mg II show that cooling for the chromosphere of the supergiant epsilon Gem do not differ greatly from the solar law, although there are differences at approximately 6000K due to ionization effects. With a rough standard law for computation of stellar winds using the Hartmann-MacGregor theory and standard stellar evolutionary calculations, the wind velocities and temperatures in the HR diagram were systematically explored. Results show that cool winds with tempratures 1,000,00K are not possible for log g or = 2. Predicted wind velocities are approximately 1.5 to 2 x larger than observed, particularly for the most luminous cool stars. The ionization balance for the wind of alpha ORI and the hydrogen profile lines for T Tauri stars were computed using the PANDORA computer program.

Dupree, A. K.↗

NASA NTRS1989

Model atmospheres for K and M giants

To aid in the interpretations of observations of K and M giant stars, model atmospheres have been calculated, covering the range in effective temperature between 3000 and 4000 K and in surface gravity-log g between 0.0 and 2.0. These models are computed under the usual assumptions of hydrostatic equilibrium, constancy of total (radiative plus convective) flux, and LTE in plane-parallel, horizontally homogeneous geometry. A significant development in these models is the incorporation of an opacity-sampled treatment of absorption by water vapor, which becomes dominant at lower temperatures. The thermal structure and scaling properties of the models are discussed, as are the effects of certain composition changes expected in the course of stellar evolution.

Brown, Jeffery A.↗

NASA NTRS2012

An In-Depth Study of the Abundance Pattern in the Hot Interstellar Medium in NGC 4649

We present our X-ray imaging spectroscopic analysis of data from deep Suzuku and XMM-Newton Observatory exposures of the Virgo Cluster elliptical galaxy NGC 4649 (M60), focusing on the abundance pattern in the hot interstellar medium (ISM). All measured elements show a radial decline in abundance, with the possible exception of O. We construct steady state solutions to the chemical evolution equations that include infall in addition to stellar mass return and SNIa enrichment, and consider recently published SNIa yields. By adjusting a single model parameter to obtain a match to the global abundance pattern in NGC 4649 we infer that accretion of subsolar metallicity external gas has reduced the overall ISM metallicity and diluted the effectiveness of SNIa to skew the pattern towards low alpha/Fe ratios, and estimate the combination of SNIa rate and level of dilution. Evidently, newly-introduced gas is heated as it is integrated into. and interacts with, the hot gas that is already present. These results indicate a complex flow and enrichment history for NGC 4649, reflecting the continual evolution of elliptical galaxies beyond the formation epoch. The heating and circulation of accreted gas may help reconcile this dynamic history with the mostly passive evolution of elliptical stellar populations. In an appendix we examine the effects of the recent updated atomic database AtomDB in spectral fitting of thermal plasmas with hot ISM temperatures in the elliptical galaxy range.

Lowenstein, Michael↗

NASA NTRS2012

An In-Depth Study of the Abundance Pattern in the Hot Interstellar Medium in NGC 4649

We present our X-ray imaging spectroscopic analysis of data from deep Suzaku and XMM-Newton Observatory exposures of the Virgo Cluster elliptical galaxy NGC 4649 (M60), focusing on the abundance pattern in the hot interstellar medium (ISM). All measured elements show a radial decline in abundance, with the possible exception of Oxygen. We construct steady state solutions to the chemical evolution equations that include infall in addition to stellar mass return and Type Ia supernovae (SNIa) enrichment, and consider recently published SNIa yields. By adjusting a single model parameter to obtain a match to the global abundance pattern in NGC 4649 we infer that introduction of subsolar metallicity external gas has reduced the overall ISM metallicity and diluted the effectiveness of SNIa to skew the pattern towards low alpha/Fe ratios, and estimate the combination of SNIa rate and level of dilution. Evidently, newly-introduced gas is heated as it is integrated into, and interacts with, the hot gas that is already present. These results indicate a complex flow and enrichment history for NGC 4649, reflecting the continual evolution of elliptical galaxies beyond the formation epoch. The heating and circulation of accreted gas may help reconcile this dynamic history with the mostly passive evolution of elliptical stellar populations. In an appendix we examine the effects of the recent updated atomic database AtomDB in spectral fitting of thermal plasmas with hot ISM temperatures in the elliptical galaxy range.

Loewenstein, Michael↗

NASA NTRS1979

Nonequilibrium ionization in solar and stellar winds

Substantial and systematic departures from ionization equilibrium can occur in the solar transition region and corona when mass outflows are present. Modeling calculations illustrate the general characteristics of the ionization balance in such regions. The presence of nonequilibrium conditions suggests a natural explanation for the extended region of EUV line emission that is observed above the solar limb. Comparison with observations of a coronal hole on the disk indicates that outflow may not start until temperatures of about 250,000 K are reached. Additional consequences include a diminution of the density discrepancy between ultraviolet and radio observations of coronal holes, and potential effects on the energy balance in solar and stellar atmospheres undergoing mass loss.

Dupree, A. K.↗

DOE OSTI2026

The 2025 Evaluation of Experimental Thermonuclear Reaction Rates (ETR25)

This work describes the formalism for estimating thermonuclear reaction rates for astrophysical applications, emphasizing modern statistical approaches such as Monte Carlo sampling and Bayesian models. We discuss related topics including the calculation of resonance energies from nuclear Q values, indirect estimates of particle partial widths, and matching of reaction rates at elevated temperatures to statistical model results. We have evaluated available experimental data on cross sections, resonance energies and strengths, partial widths, lifetimes, spin-parities, and spectroscopic factors. Based on these results, we have estimated numerical values of 78 experimental charged-particle thermonuclear reaction rates for target nuclei in the A = 2–40 mass region, for temperatures ranging from 1 MK to 10 GK. For each reaction, three rate values are provided: low, median, and high, corresponding to the 16th, 50th, and 84th percentiles, respectively, of the cumulative reaction rate probability density distribution. Additionally, we present the factor uncertainty of each rate at each temperature grid point. These results enable users to sample the reaction rate probability density in nucleosynthesis calculations, facilitating uncertainty estimates of nuclidic abundances. The rates presented here refer to their laboratory values. For use in stellar model simulations, these values need to be corrected for the effects of thermal excitations of the interacting nuclei. For each reaction, we include graphs that illustrate the fractional contributions to the overall reaction rate along with the associated uncertainty. These visuals are designed to assist both stellar modelers and nuclear experimentalists by identifying the primary sources of rate uncertain=^texttx);ty at specific stellar temperatures. A graphical comparison with earlier Monte Carlo rates is also provided.

Nuclear astrophysics↗