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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 37 records · Page 2
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NASA NTRS1975

Spectroscopic studies of very old hot stars. I - NGC 246 and its exciting star

The planetary nebula NGC 246 and its exciting stars were studied, using an image-tube spectrograph at the Cassegrain focus of a 36-inch telescope. Observations of the central star indicate that it has one of the most highly excited spectra known to optical astronomers. The only lines definitely present in the spectrum are lines of C IV and O VI, although lines of He II are probably present. Since nitrogen lines (in any ionization state) are absent, the stellar spectrum suggests that the star has experienced the triple-alpha process. Nebular observations were used to estimate the atmospheric properties of the exciting star by the Zanstra method. The results are found to correlate well with the spectral appearance of the star. Comparison of the properties of the star and surrounding nebula with Paczynski's evolutionary tracks for planetary nuclei suggests that the star is very near the point of exhaustion of its nuclear fuels.

Heap, S. R.↗
NASA NTRS1984

High Precision Measurement of Stellar Radial Velocity Variations

A prototype instrument for measurement of stellar radial velocity variations to a precision of a few meters per second is discussed. The instrument will be used to study low amplitude stellar non-radial oscillations, to search for binary systems with large mass ratios, and ultimately to search for extrasolar planetary systems. The instrument uses a stable Fabry-Perot etalon, in reflection, to impose a set of fixed reference absorption lines on the stellar spectrum before it enters the coude spectrograph of the McDonald Observatory 2.7-m telescope. The spectrum is recorded on the Octicon detector, which consists of eight Reticon arrays placed end to end. Radial velocity variations of the star are detected by measuring the shift of the stellar lines with respect the artificial Fabry-Perot lines, and correcting for the known motions in the solar system.

Cochran, W. D.↗
NASA NTRS2001

D/H Toward BD+28 4211: First FUSE Results

The atomic deuterium-to-hydrogen abundance ratio has been evaluated for the sight line toward the hot O subdwarf BD+28(sup circ) 4211. High signal-to-noise ratio (S/N is approx. 100) observations covering the wavelength range 905 to 1187 angstroms at a wavelength resolving power of lambda/Delta/lambda at approx. 20,000 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. BD+28(sup circ) 4211 is approx. 00 pc away with a total H I column density of approx. 10(exp 19)/sq cm, much higher than is typically found in the local interstellar medium (ISM). The deuterium column density was measured by analyzing several D I Lyman series transitions (Lyman delta, C, epsilon, eta, theta, iota with curve of growth and profile fitting techniques, after determining which lines were free of interference from other interstellar species and narrow stellar features. The neutral hydrogen column density was measured by an analysis of the Lyman-alpha profile using HST/Space Telescope Imaging Spectrograph (STIS) and Goddard High Resolution Spectrograph (GHRS) spectra. The stellar spectrum of BD+28(sup circ) 4211 was modelled to assist in determining the sensitivity of H I (Ly-alpha) and D I to the continuum placement and to identify stellar transitions. The D I and H I column densities, their uncertainties, and potential sources of systematic error will be presented. This work is based on data obtained for the FUSE Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University. Financial support to U. S. participants has been provided in part by NASA contract NAS5-32985.

Sonneborne, George↗
NASA NTRS1977

Temperature sounding from the absorption spectrum of CO2 at 4.3 microns

A new method is described for obtaining the temperature profile in the stratosphere and lower mesosphere from observations of the absorption spectrum of the high J lines of carbon dioxide at 4.3 microns. This concept is based upon the measurement of the integrated absorption of individual CO2 lines whose strengths depend strongly on temperature and that the absorption of these lines are obtained from measurements of the solar or stellar spectrum through an atmospheric path. The technique involves a rapidly converging iterative process in which the equivalent widths of the individual vibration-rotation lines of CO2 are used. Theoretical calculations are presented for balloon and satellite observations using a model atmosphere. Experimental results are given from spectra obtained with a balloon-borne Fourier interferometer spectrometer in which the sun was observed at low zenith angles. The experimental results are compared to rocketsonde data.

Toth, R. A.↗
NASA NTRS1998

How Hot and Variable are the Coronae of Hertzsprung Gap Giants?

The objective of this final report is to obtain deep PCA measurement of the bright stellar X-ray source Capella to test whether RXTE has sufficient sensitivity to detect changes in temperature and flux of an active late-type "corona." Our conclusion is that--with the present understanding of the background model--relatively soft sources like Capella unfortunately are not productive targets for RXTE. Harder-spectrum stellar sources, like the flaring RS CVns, would be much more suitable.

Ayres, Thomas R.↗
NASA NTRS1993

Overview of the observations of symbiotic stars

The term Symbiotic stars commonly denotes variable stars whose optical spectra simultaneously present a cool absorption spectrum (typically TiO absorption bands) and emission lines of high ionization energy. This term is now used for the category of variable stars with composite spectrum. The main spectral features of these objects are: (1) the presence of the red continuum typical of a cool star, (2) the rich emission line spectrum, and (3) the UV excess, frequently with the Balmer continuum in emission. In addition to the peculiar spectrum, the very irregular photometric and spectroscopic variability is the major feature of the symbiotic stars. Moreover, the light curve is basic to identify the different phases of activity in a symbiotic star. The physical mechanisms that cause the symbiotic phenomenon and its variety are the focus of this paper. An astronomical phenomenon characterized by a composite stellar spectrum with two apparently conflicting features, and large variability has been observed. Our research set out to find the origin of this behavior and, in particular, to identify and measure the physical mechanism(s) responsible for the observed phenomena.

Viotti, Roberto↗
NASA NTRS1990

Line identification studies using traditional techniques and wavelength coincidence statistics

Traditional line identification techniques result in the assignment of individual lines to an atomic or ionic species. These methods may be supplemented by wavelength coincidence statistics (WCS). The strength and weakness of these methods are discussed using spectra of a number of normal and peculiar B and A stars that have been studied independently by both methods. The present results support the overall findings of some earlier studies. WCS would be most useful in a first survey, before traditional methods have been applied. WCS can quickly make a global search for all species and in this way may enable identifications of an unexpected spectrum that could easily be omitted entirely from a traditional study. This is illustrated by O I. WCS is a subject to well known weakness of any statistical technique, for example, a predictable number of spurious results are to be expected. The danger of small number statistics are illustrated. WCS is at its best relative to traditional methods in finding a line-rich atomic species that is only weakly present in a complicated stellar spectrum.

Cowley, Charles R.↗
NASA NTRS1990

Line identification studies using traditional techniques and wavelength coincidence statistics

Traditional line identification techniques result in the assignment of individual lines to an atomic or ionic species. These methods may be supplemented by wavelength coincidence statistics (WCS). The strength and weakness of these methods are discussed using spectra of a number of normal and peculiar B and A stars that have been studied independently by both methods. The present results support the overall findings of some earlier studies. WCS would be most useful in a first survey, before traditional methods have been applied. WCS can quickly make a global search for all species and in this way may enable identifications of an unexpected spectrum that could easily be omitted entirely from a traditional study. This is illustrated by O I. WCS is a subject to well known weakness of any statistical technique, for example, a predictable number of spurious results are to be expected. The danger of small number statistics are illustrated. WCS is at its best relative to traditional methods in finding a line-rich atomic species that is only weakly present in a complicated stellar spectrum.

Cowley, Charles R.↗
NASA NTRS1987

Identification of lines in the satellite ultraviolet - The spectrum of Tau Scorpii

The method of wavelength coincidence statistics (WCS) is applied to the ultraviolet spectra of Tau Sco obtained by the IUE. No identifications of elements heavier than Zn were made. With the exception of Zn IV, all identifications had been previously made in an exhaustive study of Copernicus spectra by Rogerson and Ewell (1985). A comparison of the identifications made in the two studies is in very good agreement for those spectra rich in lines, where the WCS parameters indicate the strong presence of the species. Useful information is added by WCS in cases where species are only weakly, or arguably, present. It is shown that WCS is an important tool in the exploration of any complicated stellar spectrum, and even in the case of a spectrum that has been well studied by traditional methods, it adds important information.

Cowley, Charles R.↗
NASA NTRS1994

Optical, IUE, and ROSAT observations of the eclipsing nova-like variable V347 Puppis (LB 1800)

Using time-resolved optical spectroscopy and UBVRI and high-speed photometry obtained at Mount Stromlo Observatory, Mount John University Observatory, and the South African Astronomical Observatory; International Ultraviolet Explorer (IUE) ultraviolet spectroscopy; and Roentgen Satellite (ROSAT) survey X-ray fluxes, we present a study of the accretion disk, hot spot, and emission line regions in the bright eclipsing nova-like variable V347 Pup (LB 1800). In the optical and UV, V347 Pup is a strong emission line source with a continuum spectrum which is remarkably red for a high-M cataclysmic variable. Consistent with its high inclination, we interpret the continuum spectrum as the superposition of the spectrum of the cool (T(sub eff) approximately 7000 K) outer edge and the hot (T(sub eff) approximately 100,000 K) inner regions of a self-eclipsed accretion disk. For the assumed parameters, the model matches the level and shape of the observed spectrum for an inclination of approximately 88 and a distance of approximately 300 pc. The prominent hump in the optical and UV light curves just before eclipse manifests the presence of the hot spot where the accretion stream strikes the edge of the disk. The wavelength dependence of the amplitude of the hump is best modeled by a spot having an effective temperature of approximately 25,000 K and an area of approximately 3 x 10(exp 18) sq cm if the spot radiates like a blackbody, or an effective temperatue of approximately 14,000 K and an area of approximately 3 x 10(exp 19) sq cm if it radiates with a stellar spectrum. In either case, the hot spot produces only one-tenth of the predicted luminosity for the assumed mass-transfer rate of 10(exp -8) solar mass/yr. Either the hot spot is 'buried' in the edge of the accretion disk, or a significant fraction of its luminosity is radiated away in lines. The difference in azimuth between the peak of the hump and the dynamically expected location of the hot spot suggests that the spot's emitting surface is rotated forward by approximately 36 deg relative to the edge of the disk.

Mauche, Christopher W.↗
NASA NTRS2014

Photosynthesis and Astrobiology: Looking for Life Elsewhere

Photosynthesis produces signs of life we can see from space: the absorbance spectrum of surface photosynthetic pigments and, with oxygenic photosynthesis, atmospheric oxygen. Since the first discovery of a planet in another solar system in 1989, there has been an explosion in the detection of exoplanets (over 1849 as of 7 November 2014) and we are getting ever closer to finding that Goldilocks planet that might harbour life. With telescope observations of these planets, oxygenic photosynthesis has been considered our most robust target 'biosignature' that would not appear on a lifeless planet. Since anoxygenic photosynthetic organisms do not produce unambiguously biogenic gases, there is interest in their pigments serving as spectral indicators of life. But will they look the same as on Earth, can we distinguish them from the abiotic, and what will dominate on another planet? Examples from Earth provide us with the potential to extrapolate some rules for photosynthesis to predict its signature on another planet, but there are yet things we must answer about life here to improve our confidence. In particular, given the combination of the available stellar spectrum and molecular constraints on photon energy use, can we predict the pigment spectral features that will dominate, which reductant will match, and what biogenic gases would result? We take clues from the diversity of anoxygenic photosynthetic metabolisms and three very recent examples of oxygenic photosynthesis utilizing other reaction centre (RC) chlorophylls in addition to chlorophyll a (Chl a).

Pigments↗
NASA NTRS1994

Abundances, planetary nebulae, and stellar evolution

Among Henry Norris Russell's many achievements were his contributions to solar and stellar spectroscopy, in particular, to an analysis of the chemical composition of the solar atmosphere. The question of composition differences between stars was hotly debated; some distinguished astronomers argued that all stars had the solar composition. Some early challenges to this doctrine are described. Determinations of chemical compositions of gaseous nebulae were much more difficult. If we observe the lines of a given chemical element in one ionization stage in a stellar spectrum, we can deduce readily the abundance of that element. No such luxury is available for a planetary or diffuse gaseous nebula. We must measure lines of as many ionization stages as we can. Furthermore, a nebula is an extended object. Often detailed spectroscopy is at hand only for narrow pencil columns taken through the image. Different observers use a variety of apertures. Fortunately it is possible to calculate theoretical spectra for any arbitrary cross section taken through a symmetrical model, so UV, optical, and IR observations all can be compared properly with a prediction. The value of high-resolution spectra obtained with instruments such as the Hamilton Echelle Spectrograph at Lick Observatory is emphasized. Improved fluxes for weak but important transitions are found. Close blends of lines of different ions can be resolved, and checks can be made on predictions of atomic parameters such as Einstein A-values and collision strengths. High spectral resolution data have been obtained and reduced for 22 planetary nebulae of varying size, structure, stellar population membership, dustiness, level of excitation, evolutionary status, and chemical compositions. The promise seems justified that with such extensive, high quality data, additional insights on nebular genesis and late states of stellar evolution can be found. The present survey is confined to nebulae of high surface brightness, but among these are found some engaging objects such as NGC 7027, IC 4997, and NGC 6572.

Aller, Lawrence H.↗
DOE OSTI2023

Deducing neutron star equation of state from telescope spectra with machine-learning-derived likelihoods

The interiors of neutron stars reach densities and temperatures beyond the limits of terrestrial experiments, providing vital laboratories for probing nuclear physics. While the star's interior is not directly observable, its pressure and density determine the star's macroscopic structure which affects the spectra observed in telescopes. The relationship between the observations and the internal state is complex and partially intractable, presenting difficulties for inference. Previous work has focused on the regression from stellar spectra of parameters describing the internal state. We demonstrate a calculation of the full likelihood of the internal state parameters given observations, accomplished by replacing intractable elements with machine learning models trained on samples of simulated stars. Our machine-learning-derived likelihood allows us to perform maximum a posteriori estimation of the parameters of interest, as well as full scans. We demonstrate the technique by inferring stellar mass and radius from an individual stellar spectrum, as well as equation of state parameters from a set of spectra. Our results are more precise than pure regression models, reducing the width of the parameter residuals by 11.8% in the most realistic scenario. The neural networks will be released as a tool for fast simulation of neutron star properties and observed spectra.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Confident detection of doubly ionized thorium in the extreme Ap star CPD-62° 2717

Despite the Universe containing primordial thorium (Th) of sufficient abundance to appear in stellar spectra, detection of Th has to date been tentative and based on just a few weak and blended lines. Here, we present convincing evidence not only for the first Th detection in a magnetic chemically peculiar Ap star but also for the first detection of Th iii in a stellar spectrum. CPD-62° 2717 was initially recognized as a highly magnetized Ap star due to resolved magnetically split lines captured in H-band spectra from the SDSS/APOGEE survey. The star was subsequently pinpointed as extraordinarily peculiar when careful inspection of the H-band line content revealed the presence of five lines of Th iii, none of which are detected in the other ∼1500 APOGEE-observed Ap stars. Follow-up with the VLT + UVES confirmed a similarly peculiar optical spectrum featuring dozens of Th iii lines, among other peculiarities. Unlike past claims of Th detection, and owing to high-resolution observations of the strong (∼8–12 kG) magnetic field of CPD-62° 2717, the detection of Th iii can in this case be supported by matches between the observed and theoretical magnetic splitting patterns. Comparison of CPD-62° 2717 to stars for which Th overabundances have been previously reported (e.g. Przybylski’s Star) indicates that only for CPD-62° 2717 is the Th detection certain. Along with the focus on Th iii, we use time series measurements of the magnetic field modulus to constrain the rotation period of CPD-62° 2717 to ∼4.8 yr, thus establishing it as a new example of a superslowly rotating Ap star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Spectroscopic Fourth-order Coronagraph for the Characterization of Terrestrial Planets at Small Angular Separations from Host Stars

We propose a new approach for high-contrast imaging at the diffraction limit using segmented telescopes in a modest observation bandwidth. This concept, named “spectroscopic fourth-order coronagraphy,” is based on a fourth-order coronagraph with a focal-plane mask that modulates the complex amplitude of the Airy disk along one direction. While coronagraphs applying the complex amplitude mask can achieve the theoretical limit performance for any arbitrary pupils, the focal-plane mask severely limits the bandwidth. Here, focusing on the fact that the focal-plane mask modulates the complex amplitude along one direction, we noticed that the mask can be optimized for each spectral element generated by a spectrograph. We combine the fourth-order coronagraph with two spectrographs to produce a stellar spectrum on the focal plane and reconstruct a white pupil on the Lyot stop. Based on the wave-front analysis of an optical design applying an Offner-type imaging spectrograph, we found that the achievable contrast of this concept is 10{sup −10} at 1.2–1.5 times the diffraction limit over the wavelength range of 650–750 nm for the entrance pupil of the LUVOIR telescope. Thus, this coronagraph concept could bring new habitable planet candidates not only around G- and K-type stars beyond 20–30 pc but also around very nearby M-type stars. This approach potentially promotes the characterization of the atmospheres of nearby terrestrial planets with future on- and off-axis segmented large telescopes.

47 OTHER INSTRUMENTATION↗
NASA NTRS1972

The calculation and publication of a grid of line-blanketed model stellar atmospheres

The luminosity, mass, and elemental abundances, as well as other properties of each star are studied in order to locate them in an evolutionary pattern. A method for determining the flux, gravity, and abundances at the stellar surface is the construction of theoretical stellar atmospheric models that predict the observed energy distribution and detailed stellar spectrum.

Avrett, E. H.↗
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.↗