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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Flexible Stellarator Physics Facility

We propose to build a Flexible Stellarator Physics Facility to explore promising regions of the vast parameter space of disruption-free stellarator solutions for Fusion Pilot Plants (FPPs). The FESAC Long Range Plan recognized the quasi-symmetric stellarator as “the leading US approach to developing disruption-free, low-recirculating-power fusion configurations”. To deliver the ambitious Decadal Vision for Commercial Fusion Energy, we must establish a persuasive stellarator program in parallel to the tokamak one: the stellarator will prove to be a better path to a reactor if theoretical predictions are confirmed and novel optimization techniques and strategies work as desired. Since the release of the Long Range Plan in 2020, stellarators have arguably made the most significant advances of all fusion concepts. Groundbreaking results from W7-X demonstrated low neoclassical transport and the successful operation of the island divertor. Advances in theory and modeling now allow us to minimize turbulent transport, to achieve equilibria with precise quasisymmetry, to reduce neoclassical transport and fast ion loss to levels far below what has been previously achieved, and to minimize the effect of coil manufacturing errors. If realized, these advances will lead to cost-effective stellarator designs with confinement comparable to tokamaks but without the fundamental challenges of disruptions and current drive.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2005

Book Review: A Concise History of Solar and Stellar Physics

There is no doubt that the awareness of the often long history and its principal players of a scientific specialty is disappearing among present-day researchers. The reason is the inexorable rise of specialization, in which scientists are expected to keep pace with publications in their own field, not to mention the inevitable round of writing grant proposals and teaching and other mundane responsibilities. The authors of this small book had the intention of rectifying this for solar and stellar physics, disciplines which are still broad enough to embrace fields as diverse as nuclear fusion, magnetohydrodynamics, and the dynamic theory of gas spheres. They take the reader on a journey from ancient Greek and middle Eastern astronomy to the late 1990s, one which has an emphasis very much on a theoretical point of view. For the authors, it is the ideas that are central, not the observations.

Phillips, Kenneth J. H.↗
NASA NTRS2005

Book Review: A Concise History of Solar and Stellar Physics

There is no doubt that the awareness of the often long history and its principal players of a scientific specialty is disappearing among present-day researchers. The reason is the inexorable rise of specialization, in which scientists are expected to keep pace with publications in their own field, not to mention the inevitable round of writing grant proposals and teaching and other mundane responsibilities. The authors of this small book had the intention of rectifying this for solar and stellar physics, disciplines which are still broad enough to embrace fields as diverse as nuclear fusion, magnetohydrodynamics, and the dynamic theory of gas spheres. They take the read on a journey from ancient Greek and middle Eastern astronomy to the late 1990s, one which has an emphasis very much on a theoretical point of view. For the authors, it is the ideas that are central, not the observations.

Phillips, Kenneth J. H.↗
DOE OSTI2025

Exogeological inferences from white dwarf pollutants: the impact of stellar physics

Many white dwarfs have accreted material from their own planetary systems. These objects can be used to infer the composition of exoplanetary material and identify evidence for key geological processes. However, the white dwarf atmospheric physics distorts the inferred material composition away from the true composition, mainly through differential atomic diffusion of the accreted metals. Correcting for this effect is essential, but is dependent on various physical assumptions associated with the white dwarf itself. We first focus on the effect of assumptions related to convective overshoot and thermohaline mixing on the atomic diffusion time-scales. For white dwarfs with H-dominated atmospheres between 12000 K and 18000 K⁠, we find that including a complete treatment of convective overshoot decreases the inferred Fe and O abundances in accreted material. For these white dwarfs, we also find that including thermohaline mixing decreases Fe and O abundances. For He-dominated systems, the effect of convective overshoot is comparatively minor. We then explore the overall effect of other physical assumptions by comparing publicly available grids of diffusion time-scales. We find that the choice of model grid can have a large impact for white dwarfs with He-dominated atmospheres, notably on the inferred core to mantle ratio of accreted material. We identify several systems for which the geological interpretation is robust against these systematics. We also present a ‘discrepancy metric’ which can be used to estimate the potential impact of changing the stellar physics without requiring detailed modelling.

Astronomy and AstroPhysics↗
NASA NTRS1985

The future of solar physics

Outstanding problems for the future of solar physics and stellar physics are examined. The physics of stellar interiors has been called into serious question by the very low measured neutrino flux from the sun. The Ga-71 neutrino detection experiment is the next step in unravelling this mystery. The new methods of helioseismology, for probing the interior of the sun, have already found the primordial rapid rotation of the central core. The forthcoming worldwide helioseismology observing network will permit fuller exploitation of the method, promising to provide the first direct sounding of the interior of a star, hitherto known to us only through theoretical inference and the discrepant neutrino emission. An essential step in developing the physics of stellar activity will be the Solar Optical Telescope (presently planned by NASA to be launched early in the next decade) to permit a 'microscopic' examination of the surface of the sun to study the source of the action. The activity and X-ray emission of other stars depend on much the same effects, so that the study of the sun is essential to determining the significance of the X-ray emission from other stars.

Parker, E. N.↗
DOE OSTI2024

Nature versus nurture: distinguishing effects from stellar processing and chemical evolution on carbon and nitrogen in red giant stars

ABSTRACT The surface [C/N] ratios of evolved giants are strongly affected by the first dredge-up (FDU) of nuclear-processed material from stellar cores. C and N also have distinct nucleosynthetic origins and serve as diagnostics of mixing and mass-loss. We use subgiants to find strong trends in the birth [C/N] with [Fe/H], which differ between the low-α and high-α populations. We demonstrate that these birth trends have a strong impact on the surface abundances after the FDU. This effect is neglected in current stellar models, which use solar-scaled C and N. We map out the FDU as a function of evolutionary state, mass, and composition using a large and precisely measured asteroseismic data set in first-ascent red giant branch (RGB) and core He-burning, or red clump (RC), stars. We describe the domains where [C/N] is a useful mass diagnostic and find that the RC complements the RGB and extends the range of validity to higher mass. We find evidence for extra mixing on the RGB below [Fe/H] = −0.4, matching literature results, for high-α giants, but there is no clear evidence of mixing in the low-α giants. The predicted signal of mass-loss is weak and difficult to detect in our sample. We discuss implications for stellar physics and stellar population applications.

Roberts, John D. (ORCID:0000000228545796)↗
NASA NTRS1985

On stellar X-ray emission

Stellar X-ray astronomy represents an entirely new astronomical discipline which has emerged during the past five years. It lies at the crossroads of solar physics, stellar physics, and general astrophysics. The present review is concerned with the main physical problems which arise in connection with a study of the stellar X-ray data. A central issue is the extent to which the extrapolation from solar physics is justified and the definition (if possible) of the limits to such extrapolation. The observational properties of X-ray emission from stars are considered along with the solar analogy and the modeling of X-ray emission from late-type stars, the modeling of X-ray emission from early-type stars, the physics of stellar X-ray emission, stellar X-ray emission in the more general astrophysical context, and future prospects.

Rosner, R.↗
NASA NTRS1976

Skylab results - Review and outlook

In the history of space flight, Project Skylab marks the transition from short manned flights to long-time manned orbital operations, from single-purpose spacecraft to multipurpose space stations, and from a time of space exploration to a time of space utilization. In the present paper, some of the Skylab scientific programs in the areas of solar physics, stellar astronomy, space physics, earth observations, life sciences, and materials sciences are reviewed. Visible, UV, and X-ray observations showed the sun's surface to be far more active and more differentiated than was earlier assumed. The unexpected apparition of the comet Kohoutek provided valuable information on the composition of cometary nuclei and tails. Pictures taken with earth-looking cameras have provided insight into specific data analysis and assessment problems. Results of the materials science program offer much promise for future work.

Stuhlinger, E.↗
NASA NTRS1980

A suggestion concerning the generation of the physical state of stellar mantles

The physical state of the winds of B supergiants varies from one of a high level of ionization and rapid outflow to one of relatively low ionization and a moderate rate of outflow as one goes from type B0 to type A0. It is proposed here that this is the result of magnetodynamic energy being released in the mantles of these stars, much as occurs in the sun. Data are presented to show that this idea is plausible. It is noted that the release of magnetodynamic energy as the result of interaction between magnetic fields and differential motion in an inhomogeneous structure is probably the cause of the heating in all outer stellar atmospheres (mantles). In the late-type stars, the material in the mantle must first be ionized by the deposition of energy from acoustic waves; in the early-type stars, the material is already ionized at least once in the photosphere, as a result of the normal transfer of radiative energy, so no priming process is necessary for these stars.

Underhill, A. B.↗
DOE OSTI2024

The Compelling Need for a Mid-Scale Stellarator Facility

In the pursuit of the goal of commercial fusion as an abundant and safe source of energy, the stellarator is a leading concept with compelling attractiveness and demonstrated performance. In this white paper we, as a community of US researchers from Universities, National Laboratories, and Private Industry, involved in studying the stellarator concept, lay out the programmatic and technical motivation for a new mid-size stellarator research facility in the US. This contribution is complementary to several other whitepapers authored by members of our community which address different mid-scale stellarator aspects. A community based technical facility proposal has been prepared by F. Parra, et al: Flexible Stellarator Physics Facility. Two private stellarator companies have submitted proposals supporting the development of a mid-scale stellarator: Thea Energy (C.P.S. Swanson, et al.), Type One Energy (W. Guttenfelder, et al.).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1980

Absorption-line spectra of quasi-stellar objects - Physical properties of absorbing gas

The optical absorption lines of quasars are examined, and implications of the lines for the physical properties of the absorbing gas are discussed. Consideration is given to the characteristics of broad absorption lines observed in such quasars as PHL 5200 and Q 1246-057 and of the more prevalent extremely narrow lines, with discussion of the presumed physical conditions in the absorbing gas and the distribution of absorption-line redshifts with respect to emission-line redshifts. It is pointed out that absorption features become more prevalent in quasar spectra with emission redshifts greater than 2.3, and results of studies of column densities and abundances and of the presence of absorption in radio compact quasars are indicated. Research programs currently under way are then described, including a search for the distribution of absorption lines in a fixed wavelength range less than the Lyman alpha emission wavelength, study of low absorption-redshift systems, a study of individual objects such as the quasar pair 1548+114a and b, and various theoretical investigations.

Burbidge, E. M.↗
NASA NTRS2003

Structure and Dynamics of the Quiet Solar Chromosphere

For the meeting of the AAS/SPD in Albuquerque, NM, I organized a Topical Session of the AAS on Structure and Dynamics of Chromospheres. The grant support was used to bring to the US two of the speakers from abroad. I had invited them for presentations at the Session: Dr. Klaus Wilhelm, the former PI of the SUMER instrument on SOHO, from the Max-Planck Institut in Lindau, Germany, and Dr. Sirajul Hasan, from the Indian Institute of Astrophysics in Bangalore, India. Both speakers preceded their trip to the AAS meeting with a stay at the Smithsonian Astrophysical Observatory, where they interacted with members of the Solar and Stellar Physics division. The highlights of the visits were the talks at the AAS/SPD meeting, in which six invited speakers told the audience of astronomers about current problems in solar physics and their relation to stellar problems. An important result of the visits is a paper by Dr. Wilhelm and me on 'Observations of the upper solar chromosphere with SUMER on SOHO', which has been submitted to Astronomy and Astrophysics for publication.

Kalkofen, Wolfgang↗
DOE OSTI2021

The Swan: Data-driven Inference of Stellar Surface Gravities for Cool Stars from Photometric Light Curves

Stellar light curves are well known to encode physical stellar properties. Precise, automated, and computationally inexpensive methods to derive physical parameters from light curves are needed to cope with the large influx of these data from space-based missions such as Kepler and TESS. Here we present a new methodology that we call “The Swan,” a fast, generalizable, and effective approach for deriving stellar surface gravity (logg) for main-sequence, subgiant, and red giant stars from Kepler light curves using local linear regression on the full frequency content of Kepler long-cadence power spectra. With this inexpensive data-driven approach, we recover logg to a precision of ~0.02 dex for 13,822 stars with seismic logg values between 0.2 and 4.4 dex and ~0.11 dex for 4646 stars with Gaia-derived logg values between 2.3 and 4.6 dex. We further develop a signal-to-noise metric and find that granulation is difficult to detect in many cool main-sequence stars (T {sub eff} ≲ 5500 K), in particular K dwarfs. By combining our logg measurements with Gaia radii, we derive empirical masses for 4646 subgiant and main-sequence stars with a median precision of ~7%. Finally, we demonstrate that our method can be used to recover logg to a similar mean absolute deviation precision for a TESS baseline of 27 days. Our methodology can be readily applied to photometric time series observations to infer stellar surface gravities to high precision across evolutionary states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Asteroseismic modelling of solar-type stars: a deeper look at the treatment of initial helium abundance

ABSTRACT Detailed understanding of stellar physics is essential towards a robust determination of stellar properties (e.g. radius, mass, and age). Among the vital input physics used in the modelling of solar-type stars which remain poorly constrained, is the initial helium abundance. To this end, when constructing stellar model grids, the initial helium abundance is estimated either (i) by using the semi-empirical helium-to-heavy element enrichment ratio, (ΔY/ΔZ), anchored to the standard big bang nucleosynthesis value, or (ii) by setting the initial helium abundance as a free variable. Adopting 35 low-mass, solar-type stars with multiyear Kepler photometry from the asteroseismic ‘LEGACY’ sample, we explore the systematic uncertainties on the inferred stellar parameters (i.e. radius, mass, and age) arising from the treatment of the initial helium abundance in stellar model grids. The stellar masses and radii derived from grids with free initial helium abundance are lower compared to those from grids based on a fixed ΔY/ΔZ ratio. We find the systematic uncertainties on mean density, radius, mass, and age arising from grids which employ a fixed value of ΔY/ΔZ and those with free initial helium abundance to be ∼ 0.9 per cent, ∼ 2 per cent, ∼ 5 per cent, and ∼ 29 per cent, respectively. We report that the systematic uncertainties on the inferred masses and radii arising from the treatment of initial helium abundance in stellar grids lie within the expected accuracy limits of ESA’s PLATO, although this is not the case for the age.

Nsamba, Benard↗
DOE OSTI2024

Determining Stellar Elemental Abundances from DESI Spectra with the Data-driven Payne

Abstract Stellar abundances for a large number of stars provide key information for the study of Galactic formation history. Large spectroscopic surveys such as the Dark Energy Spectroscopic Instrument (DESI) and LAMOST take median-to-low-resolution (R≲ 5000) spectra in the full optical wavelength range for millions of stars. However, the line-blending effect in these spectra causes great challenges for elemental abundance determination. Here we employDD-Payne, a data-driven method regularized by differential spectra from stellar physical models, to the DESI early data release spectra for stellar abundance determination. Our implementation delivers 15 labels, including effective temperatureT eff , surface gravity log g , microturbulence velocityv mic , and the abundances for 12 individual elements, namely C, N, O, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, and Ni. Given a spectral signal-to-noise ratio of 100 per pixel, the internal precisions of the label estimates are about 20 K forT eff , 0.05 dex for log g , and 0.05 dex for most elemental abundances. These results agree with the theoretical limits from the Crámer–Rao bound calculation within a factor of 2. The majority of the accreted halo stars contributed by the Gaia–Enceladus–Sausage are discernible from the disk and in situ halo populations in the resultant [Mg/Fe]–[Fe/H] and [Al/Fe]–[Fe/H] abundance spaces. We also provide distance and orbital parameters for the sample stars, which spread over a distance out to ∼100 kpc. The DESI sample has a significantly higher fraction of distant (or metal-poor) stars than the other existing spectroscopic surveys, making it a powerful data set for studying the Galactic outskirts. The catalog is publicly available.

Astronomy & Astrophysics↗