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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

γ DORADUS PULSATIONS IN THE ECLIPSING BINARY STAR KIC 6048106

We present the Kepler photometry of KIC 6048106, which is exhibiting the O’Connell effect and multiperiodic pulsations. Including a starspot on either of the components, light-curve synthesis indicates that this system is a semi-detached Algol with a mass ratio of 0.211, an orbital inclination of 73.°9, and a large temperature difference of 2534 K. To examine in detail both the spot variations and pulsations, we separately analyzed the Kepler time-series data at the interval of an orbital period in an iterative way. The results reveal that the variable asymmetries of the light maxima can be interpreted as the changes with time of a magnetic cool spot on the secondary component. Multiple frequency analyses were performed in the outside-eclipse light residuals after removal of the binarity effects from the observed Kepler data. We detected 30 frequencies with signal to noise amplitude ratios larger than 4.0, of which six ( f {sub 2}– f {sub 6} and f {sub 10}) can be identified as high-order (17 ≤ n ≤ 25) low-degree ( ℓ = 2) gravity-mode pulsations that were stable during the observing run of 200 days. In contrast, the other frequencies may be harmonic and combination terms. For the six frequencies, the pulsation periods and pulsation constants are in the ranges of 0.352–0.506 days and 0.232–0.333 days, respectively. These values and the position on the Hertzsprung–Russell diagram demonstrate that the primary star is a γ Dor variable. The evolutionary status and the pulsation nature of KIC 6048106 are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Solar Neighborhood. XXXX. Parallax Results from the CTIOPI 0.9 m Program: New Young Stars Near the Sun

As a step toward completing and characterizing the census of the solar neighborhood, we present astrometric, photometric, and spectroscopic observations of 32 systems observed with the Cerro Tololo Inter-American Observatory 0.9 m and 1.5 m telescopes. Astrometry from the 0.9 m indicates that among the 17 systems that had no previous published trigonometric parallaxes, 14 are within 25 pc. In the full sample, nine systems have proper motions larger than 0.″5 yr{sup −1}, including 2MASS J02511490-0352459, which exceeds 2.″0 yr{sup −1}. VRI photometry from the 0.9 m and optical spectra from the 1.5 m indicate that the targets have V = 11–22 mag and spectral types M3.0V–L3.0V. For 2MASS J23062928-0502285 (TRAPPIST-1), we present updated astrometry and photometric variability based on over 12 years of observations. Of the nine binaries in the sample, two promise mass determinations in the next decade: LHS 6167AB, an M4.5V system for which we present an accurate parallax placing the binary at 9.7 pc, and 2MASS J23515048-2537367AB, an M8.5V system at 21.1 pc for which we present the first evidence of an unseen, low-mass companion. Most importantly, Na i and K i gravity indicators, H α measurements, long-term photometric variability, locations on the H-R diagram, and kinematic assessments indicate that as many as 13 of the systems are young, including candidate members of young moving groups, with ages less than ∼120 Myr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

The Globular Cluster NGC 6402 (M14). II. Variable Stars

We present time-series BVI photometry for the Galactic globular cluster NGC 6402 (M14). The data consist of ∼137 images per filter, obtained using the 0.9 and 1.0 m SMARTS telescopes at the Cerro Tololo Inter-American Observatory. The images were obtained during two observing runs in 2006–2007. The image-subtraction package ISIS, along with DAOPHOT II/ALLFRAME, was used to perform crowded-field photometry and search for variable stars. We identified 130 variables, eight of which are new discoveries. The variable star population is comprised of 56 ab-type RR Lyrae stars, 54 c-type RR Lyrae, 6 type II Cepheids, 1 W UMa star, 1 detached eclipsing binary, and 12 long-period variables. We provide Fourier decomposition parameters for the RR Lyrae, and discuss the physical parameters and photometric metallicity derived therefrom. The M14 distance modulus is also discussed, based on different approaches for the calibration of the absolute magnitudes of RR Lyrae stars. The possible presence of second-overtone RR Lyrae in M14 is critically addressed, with our results arguing against this possibility. By considering all of the RR Lyrae stars as members of the cluster, we derive 〈P{sub ab}〉=0.589 days. This, together with the position of the RR Lyrae stars of both Bailey types in the period–amplitude diagram, suggests an Oosterhoff-intermediate classification for the cluster. Such an intermediate Oosterhoff type is much more commonly found in nearby extragalactic systems, and we critically discuss several other possible indications that may point to an extragalactic origin for this cluster.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Atomic Absorption Line Diagnostics for the Physical Properties of Red Supergiants

Red supergiants (RSGs) are evolved massive stars that represent extremes, in both their physical sizes and their cool temperatures, of the massive star population. The effective temperature (T {sub eff}) is the most critical physical property needed to place an RSG on the Hertzsprung–Russell Diagram, due to the stars’ cool temperatures and resulting large bolometric corrections. Several recent papers have examined the potential utility of atomic line equivalent widths (EWs) in cool supergiant (CSG) spectra for determining T {sub eff} and other physical properties and found strong correlations between Ti i and Fe i spectral features and T {sub eff} in earlier-type CSGs (G and early K) but poor correlations in M-type stars, a spectral subtype that makes up a significant fraction of RSGs. We have extended this work by measuring the EWs of Ti, Fe, and Ca lines in late K- and M-type RSGs in the Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud, and compared these results to the predictions of the theoretical stellar LTE atmosphere models (MARCS) stellar atmosphere models. Our analyses show a poor correlation between T {sub eff} and the Fe i and Ti i lines in our observations (at odds with strong correlations predicted by stellar atmosphere models), but do find statistically significant correlations between T {sub eff} and the Ca ii triplet (CaT) features of Milky Way RSGs, suggesting that this could be a potential diagnostic tool for determining T {sub eff} in M-type supergiants. We also examine correlations between these spectral features and other physical properties of RSGs (including metallicity, surface gravity, and bolometric magnitude), and consider the underlying physics driving the evolution of atomic line spectra in RSGs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The star formation histories of local group dwarf galaxies. III. Characterizing quenching in low-mass galaxies

We explore the quenching of low-mass galaxies (10{sup 4} ≲ M{sub ⋆} ≲ 10{sup 8} M{sub ⊙}) as a function of lookback time using the star formation histories (SFHs) of 38 Local Group dwarf galaxies. The SFHs were derived by analyzing color–magnitude diagrams of resolved stellar populations in archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) lower-mass galaxies quench earlier than higher-mass galaxies; (2) inside of R{sub virial} there is no correlation between a satellite’s current proximity to a massive host and its quenching epoch; and (3) there are hints of systematic differences in the quenching times of M31 and Milky Way (MW) satellites, although the sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive conclusions. Combined with results from the literature, we qualitatively consider the redshift evolution (z = 0–1) of the quenched galaxy fraction over ∼7 dex in stellar mass (10{sup 4} ≲ M{sub ⋆} ≲ 10{sup 11.5} M{sub ⊙}). The quenched fraction of all galaxies generally increases toward the present, with both the lowest and highest-mass systems exhibiting the largest quenched fractions at all redshifts. In contrast, galaxies between M{sub ⋆} ∼ 10{sup 8}–10{sup 10} M{sub ⊙} have the lowest quenched fractions. We suggest that such intermediate-mass galaxies are the least efficient at quenching. Finally, we compare our quenching times with predictions for infall times for low-mass galaxies associated with the MW. We find that some of the lowest-mass satellites (e.g., CVn II, Leo IV) may have been quenched before infall, while higher-mass satellites (e.g., Leo I, Fornax) typically quench ∼1–4 Gyr after infall.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Dark branches of immortal stars at the Galactic Center

We show that stars in the inner parsec of the Milky Way can be significantly affected by dark matter annihilation, producing population-level effects that are visible in a Hertzsprung-Russell (HR) diagram. We establish the dark HR diagram, where stars lie on a new stable dark main sequence with similar luminosities, but lower temperatures, than the standard main sequence. The dark matter density in these stars continuously replenishes, granting these stars immortality and solving multiple stellar anomalies. Upcoming telescopes could detect the dark main sequence, offering a new dark matter discovery avenue.

astrophysical simulations↗
DOE OSTI2017

Multiple Stellar Populations of Globular Clusters from Homogeneous Ca-CN Photometry. II. M5 (NGC 5904) and a New Filter System

Using our ingeniously designed new filter systems, we investigate multiple stellar populations of the red giant branch (RGB) and the asymptotic giant branch (AGB) in the globular cluster (GC) M5. Our results are the following. (1) Our cn{sub JWL} index accurately traces nitrogen abundances in M5, while other color indices fail to do so. (2) We find bimodal CN distributions in both RGB and AGB sequences, with number ratios between CN-weak (CN-w) and CN-strong (CN-s) of n(CN-w):n(CN-s) = 29:71 (±2) and 21:79 (±7), respectively. (3) We also find a bimodal photometric [N/Fe] distribution for M5 RGB stars. (4) Our cn{sub JWL}–[O/Fe] and cn{sub JWL}–[Na/Fe] relations show clear discontinuities between the two RGB populations. (5) Although small, the RGB bump of CN-s is slightly brighter, ΔV{sub bump} = 0.07 ± 0.04 mag. If real, the difference in the helium abundance becomes ΔY = 0.028 ± 0.016, in the sense that CN-s is more helium enhanced. (6) Very similar radial but different spatial distributions with comparable center positions are found for the two RGB populations. The CN-s RGB and AGB stars are more elongated along the NW–SE direction. (7) The CN-s population shows a substantial net projected rotation, while that of the CN-w population is nil. (8) Our results confirm the deficiency of CN-w AGB stars previously noted by others. We show that it is most likely due to stochastic truncation in the outer part of the cluster. Finally, we discuss the formation scenario of M5.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1982

The evolution of massive stars

The hypotheses underlying theoretical studies of the evolution of massive model stars with and without mass loss are summarized. The evolutionary tracks followed by the models across theoretical Hertzsprung-Russell (HR) diagrams are compared with the observed distribution of B stars in an HR diagram. The pulsational properties of models of massive star are also described.

Source record↗
NASA NTRS1994

Spectropolarimetry of hot, luminous stars

I review polarimetric observations of presumably single, hot luminous stars. The stellar types discussed are OB stars. B(e) supergiants, Luminous Blue Variables (LBV), Wolf-Rayet (W-R) stars, and type II supernovae (SN). It is shown that variable, intrinsic polarization is a common phenomenon in that part of the Hertzsprung-Russell (HR) diagram which these stars occupy. However, much observational work remains to be done before we can answer the most basic, statistical questions about the polarimetric properties of different groups of hot, luminous stars. Insight into the diagnostic power of polarization observations has been gained, but cannot be exploited without detailed models. Thus, while polarimetric observations do tell us that the mass-loss processes of all types of massive stars are time-dependent and anisotropic, the significance that this might have for the accuracy of their stellar parameters and evolutionary paths remains elusive.

Schulte-Ladbeck, Regina E.↗
DOE OSTI2019

Evolving R Coronae Borealis stars with mesa

The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich supergiants. They undergo extreme, irregular declines in brightness of many magnitudes due to the formation of thick clouds of carbon dust. It is thought that RCB stars result from the mergers of CO/He white dwarf (WD) binaries. We constructed post-merger spherically symmetric models computed with the mesa code, and then followed the evolution into the region of the Hertzsprung-Russell (H-R) diagram where the RCB stars are located. We also investigated nucleosynthesis in the dynamically accreting material of CO/He WD mergers which may provide a suitable environment for significant production of 18 O and the very low 16 O/ 18 O values observed. We have also discovered that the N abundance depends sensitively on the peak temperature in the He-burning shell. Our mesa modelling consists of engineering the star by adding He-WD material to an initial CO-WD model, and then following the post-merger evolution using a nuclear-reaction network to match the observed RCB abundances as it expands and cools to become an RCB star. These new models are more physical because they include rotation, mixing, mass-loss, and nucleosynthesis within mesa . We follow the later evolution beyond the RCB phase to determine the stars’ likely lifetimes. The relative numbers of known RCB and extreme helium stars correspond well to the lifetimes predicted from the mesa models. In addition, most of computed abundances agree very well with the observed range of abundances for the RCB class.

Astronomy & Astrophysics↗
NASA NTRS1994

More missing stellar opacity?

Observational data for Population I stars have shown that blue loops on the Hertzsprung-Russell (H-R) diagram form for stellar masses as low as approximately 4 solar mass. However, current state-of-the-art stellar models, unlike the older ones that were based on smaller opacities, fail to loop out of the red-giant region during core helium burning for masses less than 7 solar mass. A possible explanation is that the currently used Livermore opacities need to be further increased, by at least 70%, at temperatures characteristic of the base of the outer convection zone, around 1 x 10(exp 6) K. Indeed, no other suggested remedy seems to yield a blue loop at the lowest observed loop luminosities.

Stothers, Richard B.↗
NASA NTRS1994

Atmospheric parameters of field subdwarf B stars

High signal-to-noise ratio optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet-excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a detailed analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the subluminous B stars in the theoroetical Hertzsprung-Russell (H-R) diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-burning cores of approximately 0.5 solar mass overlain by very thin layers of hydrogen (approximately less than 0.02 solar mass). Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into white dwarfs of lower than average mass.

Saffer, Rex A.↗
NASA NTRS1995

On the origin of 'dividing lines' for late-type giants and supergiants

We show how a change in the nature of the stellar dyanmo can lead to a transition in the topological character of stellar magnetic fields of evolved stars, from being mainly closed on the blueward side of the giant tracks in the Hertzsprung-Russell (H-R) diagram to being mainly open on their redward side. If such a topological transition occurs, then these stars naturally segregate into two classes: those having hot coronae on the blueward side, and those having massive cool winds on the redward side, thus leading naturally to the so-called dividing lines.

Rosner, R.↗
NASA NTRS1993

ROSAT observations of the Chamaeleon star forming cloud

Two-soft X-ray images of the Chamaeleon I star forming cloud obtained with the ROSAT Position Sensitive Proportional Counter (PSPC) are presented. Seventy reliable, and perhaps 19 additional, X-ray sources are found. Up to Ninety percent of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Twenty to 35 are probably previously unrecognized 'weak' T Tauri (WTT) stars. T Tauri X-ray luminosities range from log bar-L(sub x) less than 28.8 to 31.1 erg/s, or 10(exp 2) - 10(exp 4) times solar levels, with mean value of log bar-L(sub x) = 10(exp 29.2) erg/s in the 0.2-2.5 keV band. The X-ray luminosities of well-studied Chamaeleon cloud members are correlated with a complex of four stellar properties: effective temperature, mass, radius and bolometric luminosity. The spatial distribution, Hertzsprung-Russell (H-R) diagram locations of the stars indicate WTT and CTT are coeval. The total premain sequence population of the cloud is likely to be greater than 100 stars, with WTT stars outnumbering 'classical' T Tauri (CTT) stars by approximately 2:1.

Feigelson, E. D.↗
NASA NTRS1998

On Wave Processes in the Solar Atmosphere

This grant was awarded by NASA/MSFC to The University of Alabama in Huntsville (UAH) to investigate the physical processes responsible for heating and wind acceleration in the solar atmosphere, and to construct theoretical, self-consistent and time-dependent solar wind models based on the momentum deposition by finite amplitude and nonlinear Alfven waves. In summary, there are three main goals of the proposed research: (1) Calculate the wave energy spectra and wave energy fluxes carried by magnetic non- magnetic waves. (2) Find out which mechanism dominates in supplying the wave energy to different parts of the solar atmosphere. (3) Use the results obtained in (1) and (2) to construct theoretical, self-consistent and time- dependent models of the solar wind. We have completed the first goal by calculating the amount of non-radiative energy generated in the solar convection zone as acoustic waves and as magnetic tube waves. To calculate the amount of wave energy carried by acoustic waves, we have used the Lighthill-Stein theory for sound generation modified by Musielak, Rosner, Stein & Ulmschneider (1994). The acoustic wave energy fluxes for stars located in different regions of the Hertzsprung-Russell (H-R) diagram have also been computed. The wave energy fluxes carried by longitudinal and transverse waves along magnetic flux tubes have been calculated by using both analytical and numerical methods. Our analytical approach is based a theory developed by Musielak, Rosner & Ulmschnelder and Musielak, Rosner, Gall & Ulmschneider, which allows computing the wave energy fluxes for linear tube waves. A numerical approach has been developed by Huang, Musielak & Ulmschneider and Ulmschneider & Musielak to compute the energy fluxes for nonlinear tube waves. Both methods have been used to calculate the wave energy fluxes for stars located in different regions of the HR diagram (Musielak, Rosner & Ulmschneider 1998; Ulmschneider, Musielak & Fawzy 1998). Having obtained the wave energy fluxes for acoustic and magnetic tube waves, we have investigated the behavior of these waves in the solar and stellar atmospheres. The results of our extensive studies have been published in many papers and presented at numerous scientific meetings. In these studies we have investigated different aspects of propagation of acoustic and magnetic waves, the efficiency of energy transfer along magnetic structures in the solar atmosphere, and behavior of Alfven waves in stgeady and expanding solar and stellar atmospheres. Recently, we have used some of these results to construct first purely theoretical, two component and time-dependent models of solar and stellar chromospheres. Finally, to address the third goal, we have constructed first fully theoretical, self-consistent and time dependent wind models based on the momentum deposition by non-linear Alfven waves. The full set of single-fluid MHD equations with the background flow has been solved by using a modified version of the ZEUS MHD code. The constructed wind models are radially symmetric with the magnetic field decreasing radially and the initial outflow is described by the standard Parker wind solution. In contrast to previous studies, no assumptions regarding wave linearity, wave damping, and wave-flow interaction are made; the models thus naturally account for the backreaction of the wind on the waves as well as for the nonlinear interaction between different types of MHD waves. The models have been used to explain the origin of fast speed streams in solar coronal holes. The obtained results clearly demonstrate that the momentum deposition by Alfven waves in the solar wind can be sufficient to explain the origin of fast stream components of the solar wind. The range of wave amplitudes required to obtain the desired results seems to be in good agreement with recent observations.

Musielak, Z. E.↗