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Results for “STAR EVOLUTION”

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 811 records · Page 45
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NASA NTRS1990

Long-term stability of a Fabry-Perot interferometer used for measurement of stellar Doppler shift

The use of a fiber-optic-link CCD-detector Fabry-Perot interferometer (McMillan et al., 1985, 1986, and 1988) to obtain high-accuracy measurements of stellar Doppler shifts at KPNO is described in detail and illustrated with sample data. Particular attention is given to accuracy requirements and techniques for reducing errors, resolution (orders of 50 mA at wavelength 4300 A are separated by 640 mA), CCD sensitivity, observing and data-processing operations, and the control of environmental conditions. Standard-deviation data and statistics on seven solar-type stars are presented in tables, and the time evolution of the radial velocity of Beta Com is shown in a graph.

Mcmillan, R. S.↗
NASA NTRS1991

Brown dwarfs in young stellar clusters

The present calculations of the early evolution of brown dwarfs and very low mass stars (LMSs) yield isochrones spanning 0.01-0.2 solar masses for ages in the 1 to 300 million year range. Since the brown dwarfs remain sharply segregated in T(eff) from LMSs for ages of less than 100 million years, it follows that for coeval populations of known age, a domain exists in the H-R diagram in which only brown dwarfs exist. These theoretical results are compared with recent observations of the Pleiades brown dwarf candidates, using two new sets of color-T(eff) transformations. Both sets yield consistent interpretations.

Stringfellow, Guy S.↗
NASA NTRS1991

The evolution of young stellar object disks and their environment

The main efforts were directed towards determining the frequency of disk occurrence and the timescales for disk evolution for solar-type and intermediate mass stars. The results of the investigation showed that optically thick disks are accretion disks. The projected accomplishments are also discussed.

Strom, Stephen E.↗
NASA NTRS1992

Ultraviolet properties of individual hot stars in globular cluster cores. 1: NGC 1904 (M 79)

As part of an observing program using the International Ultraviolet Explorer (IUE) satellite to investigate the ultraviolet properties of stars found within the cores of galactic globular clusters with blue horizontal branches (HBs), we obtained three spectra of the cluster NGC 1904 (M 79). All three were long integration-time, short-wavelength (SWP) spectra obtained at the so called 'center of light' and all three showed evidence of sources within the IUE large aperture (21.4 in. by 10 in.). In this paper we shall describe the analysis of these spectra and present evidence that the UV sources represent individual hot stars in the post-HB stage of evolution.

Altner, Bruce↗
NASA NTRS1993

Population II Li-6 as a probe of nucleosynthesis and stellar structure and evolution

We discuss the importance of Population II Li-6 as a diagnostic for models of primordial nucleosynthesis, cosmic-ray nucleosyntheses in the early Galaxy, and the structure and evolution of metal-poor solar-type stars. The observation of Li-6 in the subdwarf HD 84937 is shown to be consistent with the existing Population II LiBeB data within the context of a simple three-component model: (1) standard big bang nucleosynthesis, (2) Population II cosmic-ray nucleosynthesis, (3) standard (nonrotating) stellar LiBeB depletion. If this interpretation is correct, we predict a potentially detectable boron abundance for this star: about 2 x 10 exp -12. Subsequent Population II LiBeB observations, and in particular further observations of Population II Li-6, are shown to be crucial to our understanding of the primordial and early galactic creation and destruction mechanisms for light elements.

Steigman, Gary↗
NASA NTRS1994

Mass loss during the RR Lyrae phase of the horizontal branch: Mass dispersion on the horizontal branch and RR Lyrae period changes

Mass loss on the horizontal branch has been invoked in the literature to explain such phenomena as the color (mass) dispersion of the horizontal branch and the observed distribution of period changes in RR Lyrae stars. To test these claims, the Yale stellar evolution code was used to evolve horizontal branch models of masses 0.64, 0.66, 0.68, 0.70, and 0.72 solar mass with Z of 0.001, core mass of 0.4893, main-sequence helium abundance of 0.23, and constant mass loss rates of 0, 10(exp -10), 5 x 10(exp -10), and 10(exp -9) solar mass/yr. Mass loss was assumed to occur only in the instability strip, where a mechanism is most likely to exist. Synthetic horizontal branches, constructed from the models, show that mass loss on the horizontal branch cannot produce the observed color dispersion even for the highest mass-loss rate of 10(exp -9) solar mass/yr. Mass loss is unlikely to occur at a higher rate without significant effects on the horizontal branch morphology, which would destroy the good agreement between standard synthetic models without mass loss and observed horizontal branches. Periods and period changes were calculated for all models. The period changes are not significantly larger for models with mass loss. The effect of mass loss in clusters of other metallicities is discussed.

Koopmann, Rebecca A.↗
NASA NTRS1997

Persistent Sub-Yearly Chromospheric Variations in Lower Main-Sequence Stars: Tau Booe and alpha Com

The recent discoveries of extrasolar planetary systems around lower main-sequence stars such as tau Booe (HD 120136) has prompted further investigation into their stellar activity. A cursory analysis of tau Booe for cyclic chromospheric activity, based on its 30-yr record of Ca 2 H and K fluxes obtained as part of the HK Project from Mount Wilson Observatory, finds an intermediate, sub-yearly period (approximately 117 d) in chromospheric activity in addition to, and separate from, both its rotation (3.3 d) and long-term variability. As a persistent subyearly period in surface magnetic activity is unprecedented, we investigate this apparent anomaly further by examining chromospheric activity levels of other stars with similar mass, searching for variability in chromospheric activity with periods of less than one year, but longer than measured or predicted rotation. An examination of the time series of 40 mid-to-late F dwarfs yielded one other star for further analysis: alpha Com (HD 114378, P approximately 132 d). The variations for these two stars were checked for persistence and coherence. Based on these determinations, we eliminate the possibilities of rotation, long-term activity cycle, and the evolution of active regions as the cause of this variation in both stars. In particular, for tau Booe we infer that the phenomenon may be chromospheric in origin; however, beyond this, it is difficult to identify anything further regarding the cause of the activity variations, or even whether the observed modulation in the two stars have the same origin.

Maulik, Davesh↗
NASA NTRS1997

Issues concerning the Orion gamma ray line observations: Line splitting and LiBeB origin

The phenomenon of broad gamma ray line splitting is discussed, together with a scenario for the suppression of the blue wings of these broad lines due to the geometry of the accelerated particle interaction region. The broad and narrow gamma ray line emissions are compared taking into account the line splitting effect. It is concluded that the observed gamma ray lines from Orion are most likely broad, implying that the low energy cosmic rays which produce this line emission consist mostly of C and heavier ions. The suppression of the proton and alpha particle abundances requires acceleration conditions such as the acceleration of the ejecta of the supernovae before mixing with the interstellar medium. Similar conditions are implied by observations of the B and Be in low metallicity stars formed during the first Gyr of galactic evolution.

Ramaty, R.↗
NASA NTRS1996

The Long and the Short of Gamma-Ray Bursts

We report evidence from the Third BATSE Gamma-Ray Burst Catalog that long (T(sub 90) greater than 10 s) and short (T(sub 90) less than 10 s) gamma-ray bursts (GRBS) represent distinct source populations. Their spatial distributions are significantly different, with long bursts having = 0.282+/- 0.014 but short bursts having = 0.385 +/- 0.019, differing by 0.103 +/- 0.024, significant at the 4.3 or level. This implies different spatial origin and physical processes for long and short bursts. Long bursts may be explained by accretion-induced collapse. Short bursts require another mechanism, for which we suggest neutron star collisions. These are capable of producing neutrino bursts as short as a few milliseconds, consistent with the shortest observed timescales in GRBS. We briefly investigate the parameters of clusters in which neutron star collisions may occur, and discuss the nuclear evolution of expelled and accelerated matter.

Katz, J. I.↗
NASA NTRS2002

Large Format Si:As IBC Array Performance for NGST and Future IR Space Telescope Applications

A mid-IR (5-30micrometer) instrument aboard a cryogenic space telescope can have an enormous impact in resolving key questions in astronomy and cosmology. A space platform's greatly reduced thermal backgrounds (compared to airborne or ground-based platforms), allow for more sensitive observations of dusty young galaxies at high redshifts, star formation of solar-type stars in the local universe, and formation and evolution of planetary disks and systems. The previous generation's largest, in sensitive IR detectors at these wavelengths are 256x256 pixel Si:As Impurity Band Conduction (IBC) devices built by Raytheon Infrared Operations (RIO) for the Space Infrared Telescope Facility/Infrared Array Camera (SIRTF)/(IRAC) instrument. RIO has successfully enhanced these devices, increasing the pixel count by a factor of 16 while matching or exceeding SIRTF/IRAC device performance. NASA-ARC in collaboration with RIO has tested the first high performance large format (1024x 1024) Si:As IBC arrays for low background applications, such as for the middle instrument on Next Generation Space Telescope (NGST) and future IR Explorer missions. These hybrid devices consist of radiation hard SIRTF/IRAC-type Si:As IBC material mated to a readout multiplexer that has been specially processed for operation at low cryogenic temperatures (below 10K), yielding high device sensitivity over a wavelength range of 5-28 micrometers. We present laboratory testing results from these benchmark, devices. Continued development in this technology is essential for conducting large-area surveys of the local and early universe through observation and for complementing future missions such as NGST, Terrestrial Planet Finder (TPF), and Focal Plane Instruments and Requirement Science Team (FIRST).

Ennico, Kimberly↗
NASA NTRS2002

Far Ultraviolet Astronomy

The Far Ultraviolet Spectroscopic Explorer (FUSE) is studying a wide range of astronomical problems in the 905-1187 Angstrom wavelength region through the use of high resolution spectroscopy. The FUSE bandpass forms a nearly optimal complement to the spectral coverage provided by the Hubble Space Telescope (HST), which extends down to approximately 1170 Angstroms. The photoionization threshold of atomic hydrogen (911 Angstroms) sets a natural short-wavelength limit for the FUV. FUSE was launched in June 1999 from Cape Canaveral, Florida, on a Delta II rocket into a 768 km circular orbit. Scientific observations started later that year. This spectral region is extremely rich in spectral diagnostics of astrophysical gases over a wide range of temperatures (100 K to over 10 million K). Important strong spectral lines in this wavelength range include those of neutral hydrogen, deuterium, nitrogen, oxygen, and argon (H I, D I, N I, O I, and Ar I), molecular hydrogen (H2), five-times ionized oxygen (O VI), and several ionization states of sulfur (S III - S VI). These elements are essential for understanding the origin and evolution of the chemical elements, the formation of stars and our Solar System, and the structure of galaxies, including our Milky Way. FUSE is one of NASA's Explorer missions and a cooperative project of NASA and the space agencies of Canada and France. These missions are smaller, more scientifically focused missions than the larger observatories, like Hubble and Chandra. FUSE was designed, built and operated for NASA by the Department of Physics and Astronomy at Johns Hopkins University. Hundreds of astronomers world-wide are using FUSE for a wide range of scientific research. Some of the important scientific discoveries from the first two years of the mission are described.

Sonneborn, George↗
NASA NTRS2004

Extrasolar Planet Inferometric Survey (EPIcS)

The discovery of the nature of the solar system was a crowning achievement of Renaissance science. The quest to evaluate the properties of extrasolar planetary systems is central to both the intellectual understanding of our origins and the cultural understanding of humanity's place in the Universe; thus it is appropriate that the goals and objectives of NASA's breakthrough Origins program emphasize the study of planetary systems, with a focus on the search for habitable planets. We propose an ambitious research program that will use SIM - the first major mission of the Origins program - to explore planetary systems in our Galactic neighborhood. Our program is a novel two-tiered SIM survey of nearby stars that exploits the capabilities of SIM to achieve two scientific objectives: (i) to identify Earth-like planets in habitable regions around nearby Sunlike stars: and (ii) to explore the nature and evolution of planetary systems in their full variety. The first of these objectives was recently recommended by the Astronomy and Astrophysics Survey Committee (the McKee-Taylor Committee) as a prerequisite for the development of the Terrestrial Planet Finder mission later in the decade. Our program combines this two-part survey with preparatory and contemporaneous research designed to maximize the scientific return from the limited and thus precious observing resources of SIM.

Shao, Michael↗
NASA NTRS2007

The Galaxy Evolution Explorer (GALEX)

The Galaxy Evolution Explorer was designed to detect and quantify star formation in a wide variety of environments. I report new scientific results, based on GALEX observations, such as star formation in tidal tails, extended disks, intra-group and Intra-cluster, and induced by outflows from active galactic nuclei.

Neff, Susan↗
NASA NTRS2008

Overview of the James Webb Space Telescope

The James Webb Space Telescope (JWST) is a large aperture (6.5 meter), cryogenic space telescope with a suite of near and mid-infrared instruments covering the wavelength range of 0.6 micrometers to 28 micrometers. JWST's primary science goal is to detect and characterize the first galaxies. It will also study the assembly of galaxies, star formation and protoplanetary systems, and formation of evolution of planetary systems. We will review JWST's science goals and discuss recent progress in the design of JWST's observatory architecture following the completion of a key milestone, the Mission Preliminary Design Review (PDR). In particular we will discuss the status of JWST's optical system, progress in the primary mirror fabrication program, and the status of key observatory elements such as the sunshield. We will also address the current projected scientific performance of the observatory in the context of its science goals.

Clampin, Mark↗
NASA NTRS2008

Origin of the Universe: From the First Stars to Planets with JWST

The James Webb Space Telescope (JWST) is a large aperture (6.5 meter), cryogenic space telescope with a suite of near and mid-infrared instruments. JWST's primary science goal is to detect and characterize the first galaxies. It will also study the assembly of galaxies, star formation, protoplanetary systems, and the formation of evolution of planetary systems. We will review the motivations for JWST's science goals in the context of recent Hubble Space Telescope, and Spitzer Space Telescope observations and review the status of the JWST Observatory.

Clampin, Mark↗
NASA NTRS2012

Overview of the James Webb Space Telescope

The James Webb Space Telescope (JWST) is a large aperture (6.5 meter), cryogenic space telescope with a suite of near and mid-infrared instruments covering the wavelength range of 0.6 microns to 28 microns. JWST's primary science goal is to detect and characterize the first galaxies. It will also study the assembly of galaxies, star formation, protoplanetary systems, and the formation of evolution of planetary systems. We will review recent progress in the design of JWST' s observatory architecture leading up to JWST's recent Mission Preliminary Design Review (PDR). In particular, we will discuss the status of JWST's optical system, the current status of the telescope mirror fabrication effort, the final design of the observatory sunshield and the plans for integration and test. In this context, we will discuss the expected scientific performance of the observatory.

Clampin, Mark↗
NASA NTRSDate not supplied

The LHS 1678 System: Two Small Planets and a Likely Brown Dwarf Orbiting a Nearby M Dwarf in Unconventional Circumstances

We present the LHS 1678 (TOI-696) exoplanet system: two nearly Earth-sized transiting planets detected by TESS and a likely brown dwarf orbiting a bright M2 dwarf at 19.9 pc. The ultra-short-period LHS 1678 b (0.70 Earth radii, 0.9-day orbit) is a captivating target for emission spectroscopy observations with the JWST. LHS 1678 c (0.98 Earth radii, 3.7- day orbit) is in the Venus-zone and may be Venus density: a promising target for greenhouse effect studies. Both planets are favorable targets for EPRV mass measurements and for JWST transmission spectroscopy observations to study their atmospheres. The substellar companion, detected via CTIO/SMARTS 0.9m astrometry, is on a decades-long orbit and may someday eclipse the host star, revealing a rare system architecture in which more and less massive objects orbit in the same plane. There is also a candidate third planet detected in TESS multi-cycle data in near 4:3 resonance with LHS 1678 c. The host star is associated with an observed gap in the HR diagram tied to a change in M dwarf energy transport mechanisms. The effect of the associated stellar astrophysics on exoplanet evolution is currently unknown. In aggregate, LHS 1678 an exciting playground for comparative exoplanet science and understanding the formation and evolution of small, short-period exoplanets orbiting low-mass stars.

Michele L Silverstein↗
NASA NTRS1987

On the evolution of tidal capture X-ray binaries - 4U 2127+12 (M15) to 4U 1820-30 (NGC 6624)

A new evolutionary scenario for X-ray binaries in globular clusters, which begins with a tidal capture of a main-sequence star by a neutron star and ends with a white dwarf-neutron star system, is presented. For tidal captures of main-sequence stars into orbits too wide to begin mass transfer immediately, the subsequent evolution of the secondary can lead to a common envelope binary similar to what the 9 hr X-ray binary 4U 2127+12 in M15 is suspected to be. If the common envelope is thick enough, it may cause the neutron star and the white dwarf core of the secondary to spiral in, producing a detached white dwarf-neutron star system. Subsequently, gravitational radiation losses may evolve this into the configuration seen in the 11 minute X-ray binary 4U 1820-30 in NGC 6624. This model appears more likely on statistical grounds than formation by collision of a neutron star and a red giant. In some circumstances, the latter process may result in unstable mass transfer, which would result in coalescence rather than a binary system like 4U 1820-30.

Bailyn, Charles D.↗