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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

On the accuracy of CO line positions for high resolution IR stellar spectroscopy

The paper demonstrates the high accuracy of line positions derived from improved sets of Dunham coefficients for the four more abundant isotopic species of carbon monoxide - (C-12)(O-16), (C-13)(O-16), (C-12)(O-18), and (C-12)(O-17) - which are present in the sun and in cool stellar atmospheres. These new spectroscopic constants make it possible to predict very accurate positions of CO lines at any J-values, especially at very high rotational excitation (up to J around 135). Earlier proposed identifications of CO lines at large J-values are checked, and some incorrect identifications in sunspot spectra are found. The present accurate line positions are also compared with predictions from other available sets of molecular constants. It is concluded that the present improved sets of molecular constants are the most appropriate to all problems of high-resolution stellar and solar spectroscopy at any J- and v-values, particularly for synthetic spectra of cool stars.

Sauval, A. J.↗
NASA NTRS1994

On the determination of the number of O stars in H II regions and starburst galaxies

The hot star population in H II regions, H II galaxies, and starburst galaxies is often described in terms of the number of 'equivalent' O stars of a single representative subtype and luminosity class needed to produce the ionizing luminosity deduced from the nebular recombination lines in the optical spectra. In this paper we define conversion factors eta(sub 0), eta(sub 1), and zeta(sub 5000) with which the total number of O V stars and their flux contribution at 5000 A can be derived from the number of these 'equivalent' stars. These quantities depend primarily on three parameters: the slope and upper mass limit of the stellar mass function and the metallicity of the region. Using the latest stellar atmosphere and evolution models, we calculate eta(sub 0), eta(sub 1), and zeta(sub 5000) for a large number of values of these parameters. The results are presented in tabular as well as graphical form. We apply our results to two H II regions for which the hot star population are known and find that the predicted numbers of O stars agree well the observed counts. In addition, we describe a method by which the values of eta(sub 0) and eta(sub 1) and the observed emission-line fluxes can be used to place constraints on the allowed values of the slope and upper mass limit of the stellar mass function in a region.

Vacca, William D.↗
NASA NTRS1987

The delta upsilon = 1 fundamental bands of carbon monoxide in late-type stars

A program was begun to develop the fundamental vibration-rotation bands of CO near 4.7 micron, as a tool to investigate the properties of late type stellar atmospheres. The strong delta v = 1 lines are sensitive probes of the thermal structure of the outer layers, which can be strongly affected by the CO, itself: cooling in the fundamental lines can depress the surface temperature to low values that are in sharp contrast with those derived from chromospheric diagnostics like Ca II K and Mg II k. Cool atmospheric models are being constructed to reproduce observed high resolution CO spectra of several late-type stars. The sensitivity is illustrated of CO LTE spectra to variations in T sub eff, log g, (Fe/H), and isotopic abundances of C and O for a grid of such models. In parallel, the assumptions of LTE line formation are tested using newly available cross sections for translational-to-vibrational energy exchange in collisions of atomic H on CO. In addition, spectra of CO and Ca II K calculated from semi-empirical UV based hot chromospheric models are used to estimate geometrical filling factors for the CO dominated and chromospheric regions on the stellar surface, in the spirit of Ayres' thermal bifurcation scenario.

Wiedemann, G.↗
NASA NTRS1994

Non-LTE model atmosphere analysis of Nova Cygni 1992

We use spherically symmetric non-local thermodynamic equilibrium (non-LTE), line-blanketed, expanding model atmospheres to analyze the International Ultraviolet Explorer (IUE) and optical spectra of Nova Cygni 1992 during the early phases of its outburst. We find that the first IUE spectrum obtained just after discovery on 1992 February 20, is best reproduced by a model atmosphere with a steep density gradient and homologous expansion, whereas the IUE and optical spectra obtained on February 24 show an extended, optically thick, wind structure. Therefore, we distinguish two phases of the early evolution of the nova photosphere: the initial, rapid, 'fireball' phase and the subsequent, much longer, optically thick 'wind' phase. The importance of line-blanketing in nova spectra is demonstrated. Our preliminary abundance analysis implies that hydrogen is depeleted in the ejecta, corresponding to abundance enhancements of Fe by a factor of approximately 2 and of CNO by more than a factor of 10 when compared to solar abundances. The synthetic spectra reproduce both the observed pseudo-continua as well as most of the observed features from the UV to the optical spectral range and demonstrate the importance of obtaining nearly simultaneous UV and optical spectra for performing accurate analyses of expanding stellar atmospheres (for both novae and supernovae).

Hauschildt, P. H.↗
DOE OSTI2026

Evidence of Free-Bound Transitions in Warm Dense Matter and Their Impact on Equation-of-State Measurements

Warm dense matter (WDM) is now routinely created and probed in laboratories around the world, providing unprecedented insights into conditions achieved in stellar atmospheres, planetary interiors, and inertial confinement fusion experiments. However, the interpretation of these experiments is often filtered through models with systematic errors that are difficult to quantify. Due to the simultaneous presence of quantum degeneracy and thermal excitation, transitions in which free electrons are de-excited into thermally unoccupied bound states transferring momentum and energy to a scattered x-ray photon become viable. Here we show that such free-bound transitions are a particular feature of WDM and vanish in the limits of cold and hot temperatures. The inclusion of these transitions into the analysis of recent X-ray Thomson Scattering experiments on WDM at the National Ignition Facility and the Linac Coherent Light Source is required to obtain a physically consistent temperature from the Chihara decomposition. This interpretation is corroborated by agreement with a recently developed model-free thermometry technique and presents an important step for precisely characterizing and understanding the complex WDM state of matter.

Boehme, M [HZDR Dresden]↗
DOE OSTI2026

RadAI

A physics-informed neural network for learning the steady-state, one-dimensional radiative transfer equation in plane-parallel geometries for stellar atmosphere applications.

Ristić, Marko [LANL]↗
DOE OSTI2025

The DECam MAGIC Survey: Spectroscopic Follow-up of the Most Metal-poor Stars in the Distant Milky Way Halo *

In this work, we present high-resolution spectroscopic observations for six metal-poor stars with [Fe/H] < –3 (including one with [Fe/H] < –4), selected using narrowband Ca ii HK photometry from the DECam MAGIC Survey. The spectroscopic data confirm the accuracy of the photometric metallicities and allow for the determination of chemical abundances for 16 elements, from carbon to barium. The program stars have chemical abundances consistent with the [Fe/H] < –3 range. A kinematic/dynamical analysis suggests that all program stars belong to the distant Milky Way halo population (heliocentric distances 35 < d helio /kpc ≲ 55), including three with high-energy orbits that might have been associated with the Magellanic system and one, J0026−5445, having parameters consistent with being a member of the Sagittarius stream. The remaining two stars show kinematics consistent with the Gaia-Sausage/Enceladus dwarf galaxy merger. J0433−5548, with [Fe/H] = –4.12, is a carbon-enhanced ultra metal-poor star, with [C/Fe] = +1.73. This star is believed to be a bona fide second-generation star, and its chemical abundance pattern was compared with yields from metal-free supernova models. Results suggest that J0433−5548 could have been formed from a gas cloud enriched by a single supernova explosion from an ∼11 M ⊙ star in the early Universe. The successful identification of such objects demonstrates the reliability of photometric metallicity estimates, which can be used for target selection and statistical studies of faint targets in the Milky Way and its satellite population. These discoveries illustrate the power of measuring chemical abundances of metal-poor Milky Way halo stars to learn more about early galaxy formation and evolution.

CEMP stars↗
DOE OSTI2025

The DESI Y1 RR Lyrae catalog I: Empirical modeling of the cyclic variation of spectroscopic properties and a chemodynamical analysis of the outer halo

We present the catalog of RR Lyrae stars observed in the first year of operations of the Dark Energy Spectroscopic Instrument (DESI) survey. This catalog contains 6,240 RR Lyrae stars out to $\sim100$ kpc from the Galactic center and over 12,000 individual epochs with homogeneously-derived stellar atmospheric parameters. We introduce a novel methodology to model the cyclical variation of the spectroscopic properties of RR Lyrae from single-epoch measurements. We employ this method to infer the radial velocity and effective temperature variation of fundamental mode and first-overtone RR Lyrae stars and to determine their systemic velocities and mean temperatures. For fundamental mode pulsators, we obtain radial velocity curves with amplitudes of $\sim$30--50 km s$^{-1}$ and effective temperature curves with 300--1,000 K variations, whereas for first-overtone pulsators these amplitudes are $\sim20$ km s$^{-1}$ and $\sim 600$ K, respectively. We use our sample to study the metallicity distribution of the halo and its dependence on Galactocentric distance ($R_{\rm GC}$). Using a radius-dependent mixture model, we split the data into chemodynamically distinct components and find that our inner halo sample ($R_{\rm GC}\lesssim50$ kpc) is predominantly composed of stars with [Fe/H] $\sim-1.5$ dex and largely radial orbits (with an anisotropy parameter $\beta\sim0.94$), that we associate with the Gaia-Sausage-Enceladus merger event. Stars in the outer halo field exhibit a broader and more metal-poor [Fe/H] distribution with more circular orbits ($\beta\sim0.39$). The metallicity gradient of the metal-rich and the metal-poor components is found to be $0.005$ and $0.010$ dex kpc$^{-1}$, respectively. Our catalog highlights DESI's tantalizing potential for studying the Milky Way and the pulsation properties of RR Lyrae stars in the era of large spectroscopic surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1966

The opacity of hot water vapor.

Hot water vapor opacity effects on dispersion spectra of red giant stars, constructing stellar atmospheric models

Auman, J. R., Jr.↗
NASA NTRS1968

Line blanketing in stars.

Scaled solar and gray-body temperature distribution models compared to determine stellar atmosphere

Swamy, K. S. K.↗
NASA NTRS1969

On the composition of circumstellar grains.

Circumstellar grain compositions of stars with various O to C abundance ratios, calculating molecular equilibrium of condensates and gaseous compounds in stellar atmospheres

Gilman, R. C.↗