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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

An infrared study of the Ophiuchus dark cloud

A near-infrared survey has been conducted of nearly 18 square degrees of the Ophiuchus dark cloud complex. Additional observations have been made of selected objects found in this survey and of the brighter objects found by Grasdalen et al. and by Vrba et al. in this region in order to identify and study the young stars associated with the cloud. These observations show that very recent star formation has been largely restricted to a small region at the center of the dark cloud complex, no more than a few parsecs in extent. Most of the young stars do not appear to be main-sequence stars. At least three of these objects appear to be surrounded by infrared reflection nebulae. Many of the objects studied are background K and M giants which can be used to determine the near-infrared extinction due to the cloud.

Elias, J. H.↗
NASA NTRS1986

Radio emission from binary stars

This paper reviews the radio emission from binary star systems - the emission processes that occur, the characteristics of the binary systems inferred from the radio observations, and the reasons for the activity. Several classes of binary stars are described including those with two main sequence stars, those with one normal star and a white dwarf, and those containing a neutron star or a black hole.

Dulk, G. A.↗
NASA NTRS1988

IUE observations of X-ray-selected stars

UV spectra for four X-ray-selected bright (V less than 10) late-type (F and G) stars have been obtained in both the long- and short-wavelength cameras of the IUE satellite. The stars were selected because they had the largest ratios of X-ray to optical luminosity for their spectral type, with known binaries being eliminated from the sample in order to determine if other mechanisms for extreme X-ray emission are present. Three of the four stars in the sample have since been observed to be binaries with radial-velocity variations consistent with periods of a few days. The fourth star, HD 142361 is a pre-main-sequence star based on the detection of Li I at a wavelength of 6707 A in absorption.

Bergoffen, Martin J.↗
NASA NTRS1990

The space distribution of AGB stars

The AGB stars can be classified into three main types on the basis of their atmospheric composition: carbon-rich, oxygen-rich, and S-type. The carbon-rich stars typically have 1.5-solar-mass main sequence stars as progenitors. There are about 40 of these stars per sq kpc projected onto the plane of the Milky Way galaxy in the neighborhood of the sun with an exponential scale height above the galactic disk of about 200 pc. Contrary to the general distribution of mass, there is no decrease with galactocentric radius of the surface density of these stars for at least 3 kpc beyond the solar circle. The S-type stars appear to have the same spatial distribution as the carbon stars; there are about 1/3 as many 'pure' S stars as there are luminous carbon stars. One major class of oxygen-rich AGB stars is the Mira variables. There are approximately between 1 and 2 times as many of these stars per sq kpc projected onto the galactic plane of the Milky Way as there are carbon stars.

Jura, M.↗
NASA NTRS1992

Production of recycled pulsars in globular clusters via two-body tidal capture

The numbers of binary systems containing a neutron star that are formed in the cores of globular clusters via two-body tidal capture are investigated. The subsequent evolution of the resulting binary systems is examined. Monte Carlo techniques are employed to trace the histories of many neutron stars in the core of a model globular cluster. Various scenarios for mass stripping during capture and orbital circularization, and the effects of magnetic braking on the circularized orbits are explored. Calculations are carried out for two model clusters: 47 Tuc and Omega Cen. Distributions of orbital periods for newly formed binaries containing a neutron star in orbit with either a main-sequence star or a giant are presented. Each binary is followed until contact between the captured star and the neutron star is established. It is argued that these systems should be regarded as potential progenitors of 'recycled' pulsars.

Di Stefano, R↗
NASA NTRS1995

Coronal gas in the Galactic halo: ORFEUS observations of NGC 346 #1

We present high resolution (lambda/delta lambda = 3000) observations of the Small Magellanic Cloud (SMC) star NGC 346 #1 near the O VI resonance doublet (1031.9, 1037.6 A). The data were collected with the Berkeley extreme/far ultraviolet (EUV/FUV) spectrometer in the ORFEUS telescope aboard the space platform Astro-SPAS during the missiom of space shuttle Discovery flown in September 1993. The O VI features shows broad absorption centered near nu(sub LSR) is approximately equal to +140 km/sec. This is almost certainly associated with the OB cluster NGC 346 itself, which also shows deep absorption at Si IV and C IV at comparable velocities. An absorption feature corresponding to O VI near nu(sub LSR) = -230 km/sec is present in our data, but this feature is caused by contamination from a second star, probably an early B-type main sequence star at lower standard of rest (LSR) velocity, which is also present in the 20 min circular aperture. Between these velocity extrema lies one broad feature (b is approximately equal 70 km/sec) or multiple narrower features, with a combined column density of about 1.4 x 10(exp 14)/sq cm. Assuming an exponential distribution of O VI with the in-plane density of Jenkins, we infer a scale height, H(sub 0), of no more than approximately equal 1.2 kpc toward NGC 346 #1. Our O VI column density is about a factor of 4 below the predictions of cooling fountain models, but our low scale height is consistent with more recent predictions.

Hurwitz, M.↗
NASA NTRS1999

The SOHO-Stellar Connection

I discusses practical aspects of the so-called "solar-stellar" connection; namely, the fundamental principles, the tools at the disposal of the stellar astronomer, and a few recent examples of the connection in action. I provide an overall evolutionary context for coronal activity, calling attention to the very different circumstances of low mass main sequence stars like the Sun, which are active mainly early in their lives; compared with more massive stars, whose coronally active phase occurs near the end of their lives, during their brief incursion into the cool half of the Hertzsprung-Russell diagram as yellow and then red giants. On the instrumental slide, I concentrate primarily on spectroscopy, in the ultraviolet and X-ray bands where coronae leave their most obvious signatures. I present an early glimpse of the type of moderate resolution spectra we can expect from the recently launched Chandra observatory, and contemporaneous HST STIS high-resolution UV measurements of the CXO calibration star Capella (alpha Aur; G8 III + G1 III). I compare STIS spectra of solar-type dwarfs-zeta Dor (F7 V), an active coronal source; and alpha Cen A (G2 V), a near twin of the Sun-to a trace obtained with the SOHO SUMER imaging UV spectrometer. I also compare STIS line profiles of the active coronal dwarf to the corresponding features in the mixed-activity "hybrid-chromosphere" bright giant alpha TrA (K2 II) and the archetype "noncoronal" red giant Arcturus (alpha Boo; K2 III). The latter shows dramatic evidence for a "cool absorber" in its outer atmosphere that is extinguishing the "hot lines" (like Si IV lambda1393 and N V lambda1238) below about 1500 A, probably through absorption in the Si I lambda1525 and C I lambda1240 photoionization continua. The disappearance of coronae across the "Linsky-Haisch" dividing line near K1 III thus apparently is promoted by a dramatic overturning in the outer atmospheric structure, namely the coronae of the red giants seem to lie beneath their extended chromospheres, rather than outside as in the Sun. I then discuss an intriguing long-slit STIS low-resolution observation of an X-ray active late-A dwarf in the nearby Hyades cluster: the spatially resolved UV spectroscopy clearly shows that a previously unseen close companion (a dKe or dMe) likely is responsible for the coronal activity, rather than some unexplained departure of the A dwarf from its expected state of X-ray dormancy. Finally, I describe early results from a joint observing campaign involving SOHO SUMER, TRACE, and the Kitt Peak Infrared Imaging Spectrometer, conducted May 1999, to explore the dynamics of the quiet solar atmosphere through the key "magnetic transition zone" that separates the kinetically dominated deep photosphere from the magnetically dominated coronal regime. Linking spatially and temporally resolved solar properties to aspects of the averaged lineshapes (for example: widths, asymmetries, intensity ratios, and Doppler shifts) is a crucial step in carrying any physical wisdom we develop in the solar setting to the distant stars.

Ayres, Thomas R.↗
NASA NTRS2014

Relating Jet Structure to Photometric Variability: the Herbig Ae Star HD 163296

Herbig Ae/Be stars are intermediate-mass pre-main sequence stars surrounded by circumstellar dust disks. Some are observed to produce jets, whose appearance as a sequence of shock fronts (knots) suggests a past episodic outflow variability. This "jet fossil record" can be used to reconstruct the outflow history. We present the first optical to near-infrared (NIR) spectra of the jet from the Herbig Ae star HD 163296, obtained with VLT/X-shooter. We determine the physical conditions in the knots and also their kinematic "launch epochs". Knots are formed simultaneously on either side of the disk, with a regular interval of ∼16 yr. The velocity dispersion versus jet velocity and the energy input are comparable between both lobes. However, the mass-loss rate, velocity, and shock conditions are asymmetric. We find ˙M(jet)/ ˙M(acc) is approximately 0.01−0.1, which is consistent with magneto-centrifugal jet launching models. No evidence of any dust is found in the high-velocity jet, suggesting a launch region within the sublimation radius (greater than 0.5 au). The jet inclination measured from proper motions and radial velocities confirms that it is perpendicular to the disk. A tentative relation is found between the structure of the jet and the photometric variability of the central source. Episodes of NIR brightening were previously detected and attributed to a dusty disk wind. We report for the first time significant optical fadings lasting from a few days up to a year, coinciding with the NIR brightenings. These are very likely caused by dust lifted high above the disk plane, and this supports the disk wind scenario. The disk wind is launched at a larger radius than the high-velocity atomic jet, although their outflow variability may have a common origin. No significant relation between outflow and accretion variability could be established. Our findings confirm that this source undergoes periodic ejection events, which may be coupled with dust ejections above the disk plane.

kinematic launch epochs↗
DOE OSTI2021

A Deep Study of an Intermediate-age Open Cluster SAI 35 (Juchert 20) Using Ground-based Imaging and Gaia EDR3 Astrometry

We present a CCD UBVI photometric study of poorly studied intermediate-age open cluster SAI 35 (Juchert 20) for the first time. To accomplish this study, we also used LAMOST DR5, Two Micron All Sky Survey, and Gaia EDR3 databases. We identified 214 most probable cluster members with membership probability higher than 50%. The mean proper motion of the cluster is found as μ{sub α}cos δ=1.10±0.01 and μ {sub δ} = −1.66 ± 0.01 mas yr{sup −1}. We find the normal interstellar extinction law using the various two-color diagrams. The age, distance, reddening, and radial velocity of the cluster are estimated to be 360 ± 40 Myr, 2.9 ± 0.15 kpc, 0.72 ± 0.05 mag, and −91.62 ± 6.39 km s{sup −1}, respectively. The overall mass function slope for main-sequence stars is found to be 1.49 ± 0.16 within the mass range 1.1–3.1 M {sub ⊙}, which is in agreement with Salpeter’s value within uncertainty. The present study demonstrates that SAI 35 is dynamically relaxed. Galactic orbital parameters are determined using Galactic potential models. We found that this object follows a circular path around the Galactic center.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1995

X-ray emitting T Tauri stars in the L1551 cloud

Low mass pre-main sequence stars in the nearby Lynds 1551 star forming cloud are studied with the ROSAT and ASCA X-ray satellites. An 8 ksec ROSAT image reveals 38 sources including 7 well-known T Tauri stars, 2 likely new weak-lined T Tauri stars, 5 potential new weak-lined T Tauri stars, one is a young B9 star, and the remaining sources are unrelated to the cloud or poorly identified. A 40 ksec ASCA image of the cloud detects seven of the ROSAT sources. Spectral fitting of the brighter X-ray emitting stars suggests the emission is produced in either a multi-temperature plasma, with temperatures near 0.2 and 1 keV, or a single-temperature plasma with low metal abundances. XZ Tau, a young classical T Tauri star, is much stronger in ASCA than ROSAT observations showing a harder (1.5-2.0 kev) component. Timing analysis reveals all but one of the T Tauri stars are variable on timescales ranging from one hour to a year. A powerful flare, emitting 3 x 10(exp 34) ergs within a 40 minute rise and fall, was observed by ASCA on the weak-lined T Tauri star V826 Tau. The event was preceded and followed by constant quiescent X-ray emission. The extreme classical T Tauri star XZ Tau was also caught during both high and low states, varying by a factor of 15 between the ASCA and ROSAT observations. Neither of the luminous infrared embedded protostars L1551-IRS 5 or L1551NE were detected by ROSAT or ASCA.

Koyama, Katsuji↗
NASA NTRS2004

The Mineralogy and Grain Properties of the Disk Surfaces in Three Herbig Ae/Be Stars

The ubiquity of accretion disks around pre-main sequence and young main sequence stars having the potential to form planetary systems is now well established. However, unknown is an accurate estimate of the fraction of single stars with disks that have produced planetary systems. Theoretical models of particle aggregation show that if particles can grow from submicron to mm to cm in size, then the formation of planetesimals is possible in the time before the disk dissipates. The problem remains to understand how grains condense from nebular gases, and how relic interstellar grains survive and are modified by their transport in the disk. If grains are lofted above the disk photosphere by processes such as winds, turbulent convection, or changes in vertical structure, the evolution of dust can be investigated by observing the properties of the small (less than or = 1 micron) grains in the optically thin disk surface layer or atmosphere.

Harker, D. E.↗
NASA NTRS1983

Observations of suspected low-mass post-T Tauri stars and their evolutionary status

The results of a study of five X-ray discovered weak emission pre-main-sequence stars in the Taurus-Auriga star formation complex are presented. All are of spectral type K7-M0, and about 1-2 mag above the main sequence. One is a double-lined spectroscopic binary, the first spectroscopic binary PMS star to be confirmed. The ages, masses, and radii of these stars as determined by photometry and spectroscopy are discussed. The difference in emission strength between these and the T Tauri stars is investigated, and it is concluded that these 'post-T Tauri' stars do indeed appear more evolved than the T Tauri stars, although there is no evidence of any significant difference in ages.

Mundt, R.↗
NASA NTRS1989

Additional measurements of pre-main-sequence stellar rotation

New rotational-velocity measurements for pre-main-sequence stars in the Taurus-Auriga molecular cloud are reported. Rotational velocities or upper limits of 10 km/s are now available for 90 percent of the T Tauri stars with V less than 14.7 in the catalog of Cohen and Kuhi. Measurements of 'continuum emission' stars, thought to be accreting high-angular-momentum material from a circumstellar disk, show that these objects are not especially rapid rotators. The results confirm earlier findings that angular-momentum loss proceeds very efficiently in the earliest stages of star formation, and suggest that stars older than about one million yr contract to the main sequence at nearly constant angular momentum. The slow rotation of T Tauri stars probably requires substantial angular-momentum loss via a magnetically coupled wind.

Hartmann, L.↗
NASA NTRS1992

Galactic OB associations in the northern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg

The literature on all OB associations was reviewed, and their IRAS point source content was studied, between galactic longitude 55 and 150 deg. Only one third of the 24 associations listed by Ruprecht et al. (1981) have been the subject of individual studies designed to identify the brightest stars. Distances to all of these were recomputed using the method of cluster fitting of the B main sequence stars, which makes it poossible to reexamine the absolute magnitude calibration of the O stars, as well as for the red supergiant candidate stars. Also examined was the composite HR diagram for these associations. Associations with the best defined main sequences, which also tend to contain very young clusters, referred to here as OB clusters, have extremely few evolved B and A or red supergiants. Associations with poorly defined main sequences and few OB clusters have many more evolved stars. They also show an effect in the upper HR diagram referred to as a ledge by Fitzpatrick and Garmany (1990) in similar data for the Large Magellanic Cloud. It is suggested that the differences in the associations are not just observational selection effects but represent real differences in age and formation history.

Garmany, C. D.↗
NASA NTRS1981

The smothered coronae of T Tauri stars

Einstein IPC observations of 10 fields containing pre-main-sequence stars are presented. The detection of significant soft X-ray flux from eight cataloged T Tauri stars plus the optical identification of two new T Tauri-like stars associated with X-ray sources are reported. It is demonstrated than an inverse correlation between the H-alpha equivalent width and the detection of X-ray flux exists. This is interpreted in terms of a model wherein the X-ray emission arises in a corona similar to that of active late-type stars, which is small compared to the extent of the circumstellar envelope. It is postulated that the lack of correlation of X-ray flux (or H-alpha) with A(v) is due to a lack of grains in the envelopes of the more active T Tauri stars.

Walter, F. M.↗
NASA NTRS1990

On the origin of metal homogeneities in globular clusters

Various transport processes which may have affected the chemical homogeneity in protocluster clouds are examined. It is shown that the characteristic diffusion time scale associated with collisions between grains and gas atoms is considerably longer than that on which star formation is expected to occur. Collisions between large grains and gas atoms lead to mass segregation and metallicity gradients on a time scale comparable to the crossing time of the clusters in the Galaxy. One possible mechanism for inducing and maintaining chemical homogeneity is turbulent diffusion in the clouds. The mixing time scale required in this case is comparable to several internal dynamical time scales, longer than the evolutionary time scale of the most massive stars, and shorter than the Galactic orbital time scale of the clouds. Thus, metals in presently observed stars probably did not originate from upper main-sequence stars of a coeval generation.

Murray, Stephen D.↗