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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 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Cool stars, stellar systems, and the sun; Proceedings of the 7th Cambridge Workshop, Tucson, AZ, Oct. 9-12, 1991

Consideration is given to HST observations of late-type stars, molecular absorption in the UV spectrum of Alpha Ori, EUV emission from late-type stars, Rosat observations of the Pleiades cluster, a deep ROSAT observation of the Hyades cluster, optical spectroscopy detected by EXOSAT, stellar photospheric convection, a structure of the solar X-ray corona, magnetic surface images of the BY Dra Star HD 82558, a Zebra interpretatin of BY Dra stars, optical flares on II Peg, a low-resolution spectroscopic survey of post-T tauri candidates, millimeter and sub-millimeter emission from flare stars, and activity in tidally interacting binaries. Attention is also given to modeling stellar angular momentum evolution, extended 60-micron emission from nearby Mira variables, the PANDORA atmosphere program, the global properties of active regions, oscillations in a stratified atmosphere, lithium abundances in northern RS CVn binaries, a new catalog of cool dwarf stars, the Far UV Spectrograph Explorer, and development of reflecting coronagraphs.

Giampapa, Mark S.↗
NASA NTRS1983

The quiescent chromospheres and transition regions of active dwarf stars - What are we learning from recent observations and models?

Progress in understanding active dwarf stars based on recent IUE, Einstein, and ground-based observations is reviewed. The extent of magnetic field control over nonflare phenomena in active dwarf stars is considered, and the spatial homogeneity and time variability of active dwarf atmospheres is discussed. The possibility that solar like flux tubes can explain enhanced heating in active dwarf stars in examined, and the roles of systematic flows in active dwarf star atmospheres are considered. The relation between heating rates in different layers of active dwarf stars is summarized, and the mechanism of chromosphere and transition region heating in these stars are discussed. The results of one-component and two-component models of active dwarf stars are addressed.

Linsky, J. L.↗
NASA NTRS1992

The standard model and some new directions

A 'standard' model of Active Galactic Nuclei (AGN), based upon a massive black hole surrounded by a thin accretion disk, is defined. It is argued that, although there is good evidence for the presence of black holes and orbiting gas, most of the details of this model are either inadequate or controversial. Magnetic field may be responsible for the confinement of continuum and line-emitting gas, for the dynamical evolution of accretion disks and for the formation of jets. It is further argued that gaseous fuel is supplied in molecular form and that this is responsible for thermal re-radiation, equatorial obscuration and, perhaps, the broad line gas clouds. Stars may also supply gas close to the black hole, especially in low power AGN and they may be observable in discrete orbits as probes of the gravitational field. Recent observations suggest that magnetic field, stars, dusty molecular gas and orientation effects must be essential components of a complete description of AGN. The discovery of quasars with redshifts approaching 5 is an important clue to the mechanism of galaxy formation.

Blandford, R. D.↗
NASA NTRS1988

The evolution of molecular clouds

The problem of the structure and evolution of molecular clouds is reviewed, with particular emphasis given to the relationship with star formation. The basic hypothesis is that magnetic fields are the primary agents for supporting molecular clouds, although damped Alfven waves may play an important role in the direction parallel to the field lines. This picture naturally leads to a conception of 'bimodal star formation'. It is proposed that high-mass stars form from the overall gravitational collapse of a supercritical cloud, whereas low-mass stars form from small individual cores that slowly condense by ambipolar diffusion from a more extended envelope until they pass the brink of graviational instability and begin to collapse dynamically from 'inside-out'. The evidence that the infall stage of protostellar evolution is terminated by the development of a powerful stellar wind is reviewed.

Shu, Frank H.↗
NASA NTRS2019

Cool, Evolved Stars: Results, Challenges, and Promises for the Next Decade

Cool, evolved stars are the main source of chemical enrichment of the interstellar medium (ISM), and understanding their mass loss and structure offers a unique opportunity to study the cycle of matter in the Universe. Pulsation, convection, and other dynamic processes in cool evolved stars create an atmosphere where molecules and dust can form, including those necessary to the formation of life (e.g. Carbon-bearing molecules). Understanding the structure and composition of these stars is thus vital to several aspects of stellar astrophysics, ranging from ISM studies to modeling young galaxies and to exoplanet research. Recent modeling efforts and increasingly precise observations now reveal that our understanding of cool stars photospheric, chromospheric, and atmospheric structures is limited by inadequate knowledge of the dynamic and chemical processes at work. Here we outline promising scientific opportunities for the next decade that can provide essential constraints on stellar photospheres, chromospheres, and circumstellar envelopes (CSE), and tie together analyses of the spectra and interferometric and imaging observations of evolved stars. We identify and discuss the following main opportunities: (1) identify and model the physical processes that must be included in current 1D and 3D atmosphere models of cool, evolved stars; (2) refine our understanding of photospheric, chromospheric, and outer atmospheric regions of cool evolved stars, their properties and parameters, through high-resolution spectroscopic observations, and interferometric observations at high angular resolution; (3) include the neglected role of chromospheric activity in the mass loss process of red giant branch (RGB) and red super giant (RSG) stars and understand the role played by their magnetic fields; (4) identify the important shaping mechanisms for planetary nebulae (PNe) and their relation with the parent Asymptotic Giant Branch (AGB) stars.

Gioia Rau↗
NASA NTRS2000

Singular Isothermal Disks and the Formation of Multiple Stars

A crucial missing ingredient in previous theoretical studies of fragmentation is the inclusion of dynamically important levels of magnetic fields. As a minimal model for a candidate presursor to the formation of binary and multiple stars, we therefore consider the equilibrium configuration of isopedically magnetized, scale-free, singular isothermal disks, without the assumption of axial symmetry. We find that lopsided (M = 1) configurations exist at any dimensionless rotation rate, including zero. Multiple-lobed (M = 2, 3, 4, ...) configurations bifurcate from an underlying axisymmetric sequence at progressively higher dimensionless rates of rotation, but such nonaxisymmetric sequences always terminate in shockwaves before they have a chance to fission into separate bodies. We advance the hypothesis that binary and multiple star-formation from smooth (i.e., not highly turbulent) starting states that are supercritical but in unstable mechanical balance requires the rapid (i.e., dynamical) loss of magnetic flux at some stage of the ensuing gravitational collapse.

Galli, Daniele↗
NASA NTRS2014

Spectral and Timing Nature of the Symbiotic X-Ray Binary 4U 1954+319: The Slowest Rotating Neutron Star in AN X-Ray Binary System

The symbiotic X-ray binary (SyXB) 4U 1954+319 is a rare system hosting a peculiar neutron star (NS) and an M-type optical companion. Its approx. 5.4 hr NS spin period is the longest among all known accretion-powered pulsars and exhibited large (is approx. 7%) fluctuations over 8 yr. A spin trend transition was detected with Swift/BAT around an X-ray brightening in 2012. The source was in quiescent and bright states before and after this outburst based on 60 ks Suzaku observations in 2011 and 2012. The observed continuum is well described by a Comptonized model with the addition of a narrow 6.4 keV Fe-K alpha line during the outburst. Spectral similarities to slowly rotating pulsars in high-mass X-ray binaries, its high pulsed fraction (approx. 60%-80%), and the location in the Corbet diagram favor high B-field (approx. greater than 10(exp12) G) over a weak field as in low-mass X-ray binaries. The observed low X-ray luminosity (10(exp33)-10(exp35) erg s(exp−1)), probable wide orbit, and a slow stellar wind of this SyXB make quasi-spherical accretion in the subsonic settling regime a plausible model. Assuming a approx. 10(exp13) G NS, this scheme can explain the approx. 5.4 hr equilibrium rotation without employing the magnetar-like field (approx. 10(exp16) G) required in the disk accretion case. The timescales of multiple irregular flares (approx. 50 s) can also be attributed to the free-fall time from the Alfv´en shell for a approx. 10(exp13) G field. A physical interpretation of SyXBs beyond the canonical binary classifications is discussed.

accretion disks aEuro" binaries: symbiotic aEuro" ↗
NASA NTRS1994

In-flight scale/distortion calibration of the Hubble Space Telescope fixed-head star trackers

This paper describes an in-flight scale and distortion calibration procedure that has been developed for the Ball Aerospace Systems Division Fixed-Head Star Trackers (FHST's) used on the Hubble Space Telescope (HST). The FHST is a magnetically focused and deflected imaging sensor that is designed to track stars as faint as m(sub v) = 5.7 over an 8 degree by 8 degree field of view. Raw FHST position measurements are accurate to approximately 200 arcseconds, but this can be improved to 10-15 arcseconds by processing the raw measurements through calibration polynomials that correct for flat field, temperature intensity, and magnetic field effects. The coefficients for these polynomials were initially determined using ground test data. On HST the use of three FHST's is an integral part of the preliminary attitude update procedures required before the acquisition of guide stars for science observations. To this end, FHST-based attitude determination having single-axis errors no worse than 22 arcseconds (1 sigma) is required. In early 1991 it became evident that one of the HST FHST's was experiencing a significant change in its optical scale. By mid-1993 the size of this error had grown to a point that, if not corrected, it would correspond to a maximum position error on the order of 100 arcseconds. Subsequent investigations demonstrated that substantial, uncompensated cubic distortion effects had also developed, the maximum contribution to position errors from the cubic terms being on the order of 30 arcseconds. To ensure accurate FHST-based attitude updates, procedures have been developed to redetermine the FHST scale and distortion calibration coefficients based on in-flight data gathered during normal HST operations. These scale and distortion calibrations have proven very effective operationally, and procedures are in place to monitor FHST calibration changes on a continuing basis.

Mccutcheon, R.↗
NASA NTRS2022

Evolution of X-Ray Activity in <25 Myr Old Pre-main Sequence Stars

Measuring the evolution of X-ray emission from pre-main-sequence (PMS) stars gives insight into two issues: the response of magnetic dynamo processes to changes in the interior structure, and the effects of high-energy radiation on protoplanetary disks and primordial planetary atmospheres. We present a sample of 6003 stars with ages 7–25 Myr in 10 nearby open clusters from Chandra X-ray and Gaia-EDR3 surveys. Combined with previous results in large samples of younger (≲5 Myr) stars in MYStIX and SFiNCs star-forming regions, mass-stratified activity-age relations are derived for the early phases of stellar evolution. X-ray luminosity (LX) is constant during the first few Myr, possibly due to the presence of extended X-ray coronas insensitive to temporal changes in stellar size. LX then decays during the 7–25 Myr period, more rapidly as stellar mass increases. This decay is interpreted as decreasing efficiency of the α2 dynamo as radiative cores grow and a solar-type αΩ dynamo emerges. For more massive 3.5–7 M⊙ fully radiative stars, the X-ray emission plummets—indicating the lack of an effective magnetic dynamo. The findings provide improved measurements of high-energy radiation effects on circumstellar material, first for the protoplanetary disk and then for the atmospheres of young planets. The observed X-ray luminosities can be so high that an inner Earth-mass rocky, unmagnetized planet around a solar-mass PMS star might lose its primary and secondary atmospheres within a few (several) million years. PMS X-ray emission may thus have a significant impact on the evolution of early-planetary atmospheres and the conditions promoting the rise of habitability.

Konstantin V Getman↗
NASA NTRS1994

Super-Eddington radiation transfer in soft gamma repeaters

Bursts from soft gamma repeaters (SGRs) have been shown to be super-Eddington by a factor of 1000 and have been persuasively associated with compact objects. Super-Eddington radiation transfer on the surface of a strongly magnetic (greater than or equal to 10(exp 13) G) neutron star is studied and related to the observational constraints on SGRs. In strong magnetic fields, Thompson scattering is suppressed in one polarization state, so super-Eddington fluxes can be radiated while the plasma remains in hydrostatic equilibrium. We discuss a model which offers a somewhat natural explanation for the observation that the energy spectra of bursts with varying intensity are similar. The radiation produced is found to be linearly polarized to one part in 1000 in a direction determined by the local magnetic field, and intensity variations between bursts are understood as a change in the radiating area on the source. The net polarization is inversely correlated with burst intensity. Further, it is shown that for radiation transfer calculations in limit of superstrong magnetic fields, it is sufficient to solve the radiation transfer for the low opacity state rather than the coupled equations for both. With this approximation, standard stellar atmosphere techniques are utilized to calculate the model energy spectrum.

Ulmer, Andrew↗
NASA NTRS2001

A Deep X-Ray Survey of Low Mass PMS Stars in NGC 2264

We have proposed to make a deep X-ray survey of a rotation- and proper-motion selected sample of young (3-15 Myr old), low-mass, pre-main sequence (PMS) stars in the populous star-forming region NGC 2264. These X-ray Multimirror Mission (XMM) data will be combined with an extensive set of rotation data for members of this cluster to allow us, for the first time, to probe the early evolution of magnetic dynamo activity for both fully convective stars and those stars found lower on their Hayashi tracks, which are developing radiative cores. We will use these data to study the interrelationship between rotation, interior structure, and coronal activity as a function of mass and age in the PMS and to define empirical constraints for theoretical models of angular momentum/dynamo evolution.

Patten, Brian M.↗
NASA NTRS1967

Fragmentation.

Fragmentation of clouds of diffuse matter and condensation into stars, discussing gravitational instability and galactic magnetic field effects

Hunter, C.↗
NASA NTRS1974

A new measurement of the Hercules X-1 X-ray pulse profile

Report on measurements of a triple-peaked 1.24-sec pulse profile in a 1-minute rocket-borne exposure to Her X-1, in contrast to the double-peaked profiles expected from models which maximize the X-ray emission at the magnetic equator of an accreting neutron star. The profile exhibits statistically significant energy dependence, with the emission greater than about 12 keV having narrower peaks which appear to lag (by approximately 5% of the pulse period) the corresponding peaks at lower energies. Approximately one-third of the total emission from the source is nonpulsed.

Holt, S. S.↗
NASA NTRS1974

A new measurement of the Her X-1 X-ray pulse profile

A triple peaked 1.24 sec. pulse profile in a 1-minute rocket borne exposure to Her X-1 was measured, in contrast to the doublepeaked profiles expected from models which maximize the X ray emission at the magnetic equator of an accreting neutron star. The profile exhibits statistically significant energy dependence, with the emission approximately greater than 12 keV having narrower peaks which lag (by approximately 5% of the pulse period) the corresponding peaks at lower energies. Approximately one third of the total emission from the source is nonpulsed.

Holt, S. S.↗