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All sourcesNASA NTRSDOE OSTI
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↗
NASA NTRS1981

IUE observations and interpretation of the symbiotic star RW Hya

The IUE observations of the high excitation symbiotic star RW Hya (gM2 + pec) are discussed. Analysis of the intense UV continuum observed between 1100 A to 2000 A suggests this star is a binary system in which the secondary is identified as a hot subdwarf with T sub eff being approximately 100,000 K. A distance to the system of 1000 pc is deduced. The UV spectrum consists of mainly semiforbidden and allowed transition lines of which the CIV (1548 A, 1550 A) emission lines are particularly strong, and UV continuum at both shorter and longer wavelengths. Strong forbidden lines seem to be absent suggesting the presence of a nebula of high densities. Tidal interaction between the red giant primary and the hot subdwarf is suggested as a likely means to form the observed nebula. RW Hya is suggested as a possible source of soft X-ray emission from material accreting onto the surface of the hot subdwarf. Detection of such emission with HEAO-B would give information if this accretion is taking place via Roche lobe overlow or via capture from a stellar wind emitted by the primary. A general discussion of elemental and ionic abundances in the nebula is also presented.

Kafatos, M.↗
NASA NTRS1993

Starburst ages in HII galaxies

The age of the starbursts of a few H2 galaxies is derived from optical photometry and compared with previous results from radio continuum spectra. H2 galaxies are gas rich and characterized by the dominance of giant H2 regions, blue stellar colors, high surface brightness, and narrow emission lines. The presently observed high abundances of bright young stars must be the result of a recent -or even ongoing- starburst. The question, whether the observed starburst is the first one or if it is a recurrent phenomenon, is still open. If the starburst is the first one, H2 galaxies can be interpreted as being among the most unevolved galaxies known - their star formation characteristics possibly paralleling that of normal galaxies early in their evolution. A sample of five H2 galaxies was observed with B, R, and I broadband filters. They have been observed previously at several radio continuum frequencies. The photometric data are given. The results of the photometry and a comparison with radio continuum spectra are given.

Deeg, Hans-Joerg↗
NASA NTRS2008

Heavy Elements and Cool Stars

We report on progress in the analysis of high-resolution near-IR spectra of alpha Orionis (M2 Iab) and other cool, luminous stars. Using synthetic spectrum techniques, we search for atomic absorption lines in the stellar spectra and evaluate the available line parameter data for use in our abundance analyses. Our study concentrates on the post iron-group elements copper through zirconium as a means of investigating the slow neutron-capture process of nucleosynthesis in massive stars and the mechanisms that transport recently processed material up into the photospheric region. We discuss problems with the atomic data and model atmospheres that need to be addressed before theoretically derived elemental abundances from pre-supernova nucleosynthesis calculations can be tested by comparison with abundances determined from observations of cool, massive stars.

Wahlgren, Glenn M.↗
NASA NTRS1986

The ionization structure of the Orion Nebula - Infrared line observations and models

Observations of the forbidden O III 52 and 88 microns lines and the forbidden N III 57 microns line have been made at six positions and the forbidden Ne III 36 microns line at four positions in the Orion Nebula to probe its ionization structure. The wavelength of the forbidden Ne III line was measured to be 36.009-36.017 microns. Electron densities and abundance ratios of N(++)/O(++) have been calculated and compared to other radio and optical observations. Detailed one-component and two-component (bar plus halo) spherical models were calculated for exciting stars with effective temperatures of 37,000-40,000 K and log g = 4.0 and 4.5. Both the new IR observations and the visible line measurements of oxygen and nitrogen require Teff of no more than 37,000 K. However, the doubly ionized neon requires a model with Teff of at least 39,000 K, which is more consistent with that inferred from the radio flux or spectral type. These differences in Teff are not due to effects of dust on the stellar radiation field but are probably due to inaccuracies in the assumed stellar spectrum. Neon and nitrogen are approximately solar, but oxygen is half-solar in abundance. From the IR O(++) lines, it is concluded that the ionization bar results from an increase in column depth rather than from a local density enhancement.

Simpson, J. P.↗
DOE OSTI2025

Cosmological Simulations of Stellar Halos with Gaia Sausage–Enceladus Analogs: Two Sausages, One Bun?

Observations of the Milky Way’s stellar halo find that it is predominantly comprised of a radially biased population of stars, dubbed the Gaia Sausage–Enceladus, or GSE. These stars are thought to be debris from dwarf galaxy accretion early in the Milky Way’s history. Though typically considered to be from a single merger, it is possible that the GSE debris has multiple sources. To investigate this possibility, we use the IllustrisTNG50 simulation to identify stellar accretion histories in 98 Milky Way analogs—the largest sample for which such an identification has been performed—and find GSE-like debris in 32, with two-merger GSEs accounting for a third of these cases. Distinguishing single-merger GSEs from two-merger GSEs is difficult in common kinematic spaces, but differences are more evident through chemical abundances and star formation histories. This is because single-merger GSEs are typically accreted more recently than the galaxies in two-merger GSEs: the median infall times (with 16th and 84th percentiles) are $5.{9}_{-2.0}^{+3.3}$ and $10.{7}_{-3.7}^{+1.2}$ Gyr ago for single- and two-merger scenarios, respectively. The systematic shifts in abundances and ages that occur as a result suggest that efforts in modeling these aspects of the stellar halo prove ever-important in understanding its assembly.

galactic archaeology↗
DOE OSTI2020

Detection of young (≤20 Myr) stellar populations in apparently quenched low-mass galaxies using red spectral line indices

ABSTRACT We report on the detection of a small contribution (around and below 1 per cent in mass) from young stellar components with ages ≤20 Myr in low-mass galaxies purposely selected from the MaNGA survey to be already-quenched systems. Among the sample of 28 galaxies, 8 of them show signatures of having suffered a very recent burst of star formation. The detection has been done through the analysis of line-strength indices in the red spectral range [5700,8800] Å. The increasing contribution of red supergiants to this red regime is responsible for a deviation of the index measurements with respect to their position within the model grids in the standard spectral range [3600,5700] Å. We demonstrate that a combination of red indices, as well as a qualitative assessment of the mean luminosity-weighted underlying stellar population, is required in order to distinguish between a true superyoung population and other possible causes of this deviation, such as abundance ratio variations. Our result implies that many presumably quenched low-mass galaxies actually contain gas that is triggering some level of star formation. They have, therefore, either accreted external gas, internally recycled enough gas from stellar evolution to trigger new star formation, or they kept a gas reservoir after the harassment or stripping process that quenched them in the first place. Internal processes are favoured since we find no particular trends between our non-quenched galaxies and their environment, although more work is needed to fully discard an external influence.

de Lorenzo-Cáceres, A.↗
DOE OSTI2022

The K2 Galactic Archaeology Program Data Release 3: Age-abundance Patterns in C1–C8 and C10–C18

We present the third and final data release of the K2 Galactic Archaeology Program (K2 GAP) for Campaigns C1–C8 and C10–C18. We provide asteroseismic radius and mass coefficients, κ R and κ M , for ~19,000 red giant stars, which translate directly to radius and mass given a temperature. As such, K2 GAP DR3 represents the largest asteroseismic sample in the literature to date. K2 GAP DR3 stellar parameters are calibrated to be on an absolute parallactic scale based on Gaia DR2, with red giant branch and red clump evolutionary state classifications provided via a machine-learning approach. Combining these stellar parameters with GALAH DR3 spectroscopy, we determine asteroseismic ages with precisions of ~20%–30% and compare age-abundance relations to Galactic chemical evolution models among both low- and high-α populations for α, light, iron-peak, and neutron-capture elements. We confirm recent indications in the literature of both increased Ba production at late Galactic times as well as significant contributions to r-process enrichment from prompt sources associated with, e.g., core-collapse supernovae. With an eye toward other Galactic archeology applications, we characterize K2 GAP DR3 uncertainties and completeness using injection tests, suggesting that K2 GAP DR3 is largely unbiased in mass/age, with uncertainties of 2.9% (stat.) ± 0.1% (syst.) and 6.7% (stat.) ± 0.3% (syst.) in κ R and κ M for red giant branch stars and 4.7% (stat.) ± 0.3% (syst.) and 11% (stat.) ± 0.9% (syst.) for red clump stars. We also identify percent-level asteroseismic systematics, which are likely related to the time baseline of the underlying data, and which therefore should be considered in TESS asteroseismic analysis.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1985

Departures from LTE Populations Versus Anomalous Abundances: the Effects of Heating

The deposit of nonradiative heat and momentum in the mantle of a hot star affects the interpretation of the stellar spectrum in two ways. First, a superheated and moving plasma should be considered when doing the analysis, and second, a model atom which is appropriate for the physical state of the line forming regions. Some examples are presented for H and He showing how the changes in the electron temperature affect the solution of the equations of statistical equilibrium. The observed spectra of the Wolf-Raynet stars HD 191765, HD 192103, and HD 192163 are compatible with a normal H/He abundance ratio.

Underhill, A. B.↗
DOE OSTI2024

Dynamics of dark matter misalignment through the Higgs portal

A light singlet scalar field feebly coupled through the super-renormalizable Higgs portal provides a minimal and well-motivated realization of ultra-light bosonic dark matter. We study the cosmological production of dark matter in this model by elucidating the dynamics of two sources of scalar field misalignment generated during the radiation era. For large scalar masses (above $\mathcal{O}$ (10 −3 eV)), dark matter is produced through thermal misalignment, by which the scalar field is driven towards large field values as a result of the finite-temperature effective potential. The dominance of thermal misalignment in this mass range leads to a sharp relic abundance prediction which is, to a significant extent, insensitive to the initial conditions of the scalar field. On the other hand, for low mass scalars (below $\mathcal{O}$ (10 −5 eV)), dark matter is produced via VEV misalignment, which is caused by the induced scalar field vacuum expectation value triggered by the electroweak phase transition. We show that the relic abundance in this low mass range is sensitive to the scalar field initial conditions. In the intermediate mass range, the relic abundance is a consequence of a competition between thermal misalignment and VEV misalignment, which can potentially lead to novel forced resonance effects which cause a recurring enhancement and suppression in the late time oscillation amplitude as a function of the scalar mass. We compare our relic abundance predictions with constraints and projections from equivalence principle and inverse square law tests, stellar cooling, resonant molecular absorption, and observations of extra-galactic background light and diffuse X-ray backgrounds. New experimental ideas are needed to probe most of the cosmologically motivated regions of parameter space.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2026

Massive black holes seeded by dark matter: Implications for little red dots and gravitational wave signatures

Observations of supermassive black holes (SMBHs) at high redshifts challenge standard seeding scenarios. We examine a dissipative self-interacting dark matter (dSIDM) model in which gravothermal collapse leads to the formation of massive BH seeds ab initio. We utilize a semianalytical framework to predict properties of the dSIDM-seeded SMBH population. Billion solar mass quasars are reproduced along with low-mass faint active galactic nuclei (known as little red dots) with SMBH-to-galaxy stellar mass ratios consistent with recent James Webb Space Telescope observations. To match the abundance of the observed bright quasars, a subpercent-level duty cycle is suggested, implying a large population of dormant SMBHs. The gravitational wave (GW) signals from mergers of these massive SMBHs can be detected by LISA while remaining within the NANOGrav constraints on the GW background. These results provide testable signatures of DM-driven SMBH formation, offering a pathway to probe hidden-sector physics through SMBH and GW observables.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Lithium in T Coronae Borealis

T Coronae Borealis is a recurrent, symbiotic nova system currently in quiescence between its periodic ≈80 yr cycle of eruptions. Observations during inter-outburst epochs provide an opportunity to study properties of the accretion disk and the M red giant. Here we present new irradiated (blackbody veiling) models, incorporating modern molecular opacities and line lists, of spectra derived from high-resolution (22,000 ≲ R ≲ 120,000) optical echelle observations obtained at two epochs, one prior to and one post the 2015 rebrightening event at similar spectroscopic system phase. We find a lithium abundance in the secondary at both epochs to be comparable. The non-irradiated (classical) model atmospheres yield a lithium abundance, A(Li) = 1.3 ± 0.1. The irradiated model (veiled) atmospheres, which are likely a better representation of the system in which the white dwarf and accretion disk illuminate the red giant, give A(Li) = 2.4 ± 0.1.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1974

Molecular column densities in selected model atmospheres

From an examination of predicted column densities, the following conclusions were drawn: (1) The SiO ought to be visible in carbon stars which were generated from triple alpha burning, but absent from carbon stars generated from the CNO bi-cycle. (2) Variation in the observed relative strengths of TiO and ZrO is indicative of real differences in the ratio Ti/Zr. (3) The TiO/ZrO ratio shows a small variation as C/O and effective temperature is changed. (4) Column density of silicon dicarbide (SiC2) is sensitive to abundance, temperature, and gravity; hence all relationships between the strength of SiC2 and other stellar parameters will show appreciable scatter. There is however, a substantial luminosity effect present in the SiC2 column densities. (5) Unexpectedly, SiC2 is anti-correlated with C2. (6) The presence of SiC2 in a carbon star eliminates the possibility of these stars having temperatures greater than or equal to 3000 K, or being produced through the CNO bi-cycle.

Johnson, H. R.↗
NASA NTRS1974

Solar wind three; Proceedings of the Third Conference, Pacific Grove, Calif., March 25-29, 1974

Papers are presented relating to the study of solar abundances, the evolution of the solar wind, solar corona dynamics, solar wind interactions, and stellar winds. Some of the topics covered include the nuclear composition of cosmic rays, the solar wind as deduced from lunar samples, spectral characteristics of flares, numerical simulation of interplanetary shock ensembles, radial gradient of solar wind velocity from 1 to 5 AU, the enhancement of solar wind fluctuations with scale size near the proton gyroradius, solar wind-Venus interaction observed from magnetic field experiment on Mariner 10, and binary stellar winds. Individual items are announced in this issue.

Russell, C. T.↗
NASA NTRS1981

Outlook for ultraviolet astronomy

A brief overview of galactic and extragalactic research is given with emphasis on the problems of temperature determination, chemical abundance determination, and the question about the energy sources for the high temperature regions. Stellar astronomy, stellar winds, and the interstellar medium are among the topics covered.

Boehm-Vitense, E.↗
NASA NTRS1986

Black holes and local dark matter

Two independent constraints are placed on the amount of dark matter in black holes contained in the galactic disk. First, gas accretion by black holes leads to X-ray emission which cannot exceed the observed soft X-ray background. Second, metals produced in stellar processes that lead to black hole formation cannot exceed the observed disk metal abundance. Based on these constraints, it appears unlikely that the missing disk mass could be contained in black holes. A consequence of this conclusion is that at least two different types of dark matter are needed to solve the various missing mass problems.

Hegyi, D. J.↗
NASA NTRS1993

Population II Li-6 as a probe of nucleosynthesis and stellar structure and evolution

We discuss the importance of Population II Li-6 as a diagnostic for models of primordial nucleosynthesis, cosmic-ray nucleosyntheses in the early Galaxy, and the structure and evolution of metal-poor solar-type stars. The observation of Li-6 in the subdwarf HD 84937 is shown to be consistent with the existing Population II LiBeB data within the context of a simple three-component model: (1) standard big bang nucleosynthesis, (2) Population II cosmic-ray nucleosynthesis, (3) standard (nonrotating) stellar LiBeB depletion. If this interpretation is correct, we predict a potentially detectable boron abundance for this star: about 2 x 10 exp -12. Subsequent Population II LiBeB observations, and in particular further observations of Population II Li-6, are shown to be crucial to our understanding of the primordial and early galactic creation and destruction mechanisms for light elements.

Steigman, Gary↗
NASA NTRS2005

Coronal Diagnostics of Intermediate Activity Star XI Boo A

The analysis of Xi Boo A proved difficult to adapt to our line-by-line approach because of the strong wings of the RGS instrumental profile, as has been detailed in earlier reports. While progress was also delayed because of problems in using SAS v4, we succeeded in the past year or so to bring the analysis to conclusion. Abundances have been derived using both EPIC and RGS data, confirming earlier EUVE findings of a mild solar-like FIP effect, though with some evidence of a turn-up in abundances of elements with higher FIP. Plasma densities appear normal for a moderately active stellar corona. Xi Boo A nicely bridges the gap between the very active stars and stars like the Sun, and it indeed does appear that these are the stars in which the solar-like FIP effects begins to change to the "inverse FIP" type of effect seen in the very active stars. Probing this divide was the main goal of the proposal. These results are in the process of being prepared for publication, though we have not decided the target journal as yet.

Drake, Jeremy↗