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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 1,207 records · Page 67
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NASA NTRS1986

Crystallization sequences of Ca-Al-rich inclusions from Allende - The effects of cooling rate and maximum temperature

The crystallization sequences, mineral chemistries, and textures resulting from the cooling in air from 1275-1580 C to below 100 C at rates between 0.5 and 100 c/hr of an average Type B Ca-Al-rich inclusion composition are investigated. Comparison of experimental results with petrographic observations of Type B CaIs indicates that most inclusions were partially melted and then cooled at rates of the order of a few tenths to tens of degrees per hour, with maximum temperatures of about 1400 C suggested for intermediate Type B Allende inclusions. Data are consistent with the drag heating of particles falling though nebular gas, heating in nebular shock fronts, or other thermal heterogeneities in the early nebula allowing time scales for heating of CAIs much shorter than those for the nebular cloud as a whole.

Stolper, E.↗
NASA NTRS1986

Primordial stellar evolution - The pre-main-sequence phase

The quasi-static contraction of primordial stars composed of pure hydrogen and helium gas is studied by following numerically the evolution of a star of five solar masses from the end of protostellar accretion to the onset of hydrogen burning. Although the protostellar core of this mass is radiatively stable and undergoing nonhomologous contraction, its large surface area and luminosity force the star to a partially convective, homologously contracting state within only 100 yr. Deuterium later ignites at an off-center temperature maximum but fails to produce interior convection. The star follows a conventional premain sequence track in the HR diagram, reaching the ZAMS after 1.2 million yr, with a luminosity of 880 solar luminosities and a radius of 1.2 solar radii.

Stahler, S. W.↗
NASA NTRS1987

The naked T Tauri stars - The low-mass pre-main sequence unveiled

The search for low-mass premain-sequence (PMS) stars associated with X-ray sources in regions of star formation is discussed. The survey to date has revealed at least 30 low-mass PMS stars in the Tau-Aur region, and a comparable number in Oph. These stars are the naked T Tau stars, unveiled versions of the well-known classical T Tau stars. The properties of these newly discovered PMS stars and their relation to the classical T Tau stars are discussed, and it is concluded that the naked T Tau stars are the true low-mass PMS stars, and that the observable characteristics defining the classical T Tau stars are due to the interaction of an underlying, fairly normal star with a dominant circumstellar environment. The impact the naked T Tau stars are likely to have on models of the PMS evolution of low-mass stars is considered.

Walter, Frederick M.↗
NASA NTRS1987

Ultraviolet spectral morphology of the O stars. IV - The OB supergiant sequence

An atlas of 25 O3-B8 supergiant spectra in the wavelength ranges 1320-1580 A and 1620-1880 A is presented, based on high-resolution data from the IUE archives. The remarkably detailed relationship between the stellar-wind profiles and the optical spectral classifications throughout this sequence is emphasized. For instance, the (Si IV)/(C IV) ratio reverses between O4 and O6.5; and the B0, B0.5, and B0.7 Ia wind characteristics are each qualitatively unique and distinct from one another. The systematic behavior of nine stellar-wind features with ionization potentials ranging from 114 to 19 eV is summarized as a function of advancing spectral type.

Walborn, Nolan R.↗
NASA NTRS1987

X-ray sources in regions of star formation. II - The pre-main-sequence G star HDE 283572

This paper reports the detection of HDE 283572, a ninth-magnitude G star 8 arcmin south of RY Tau, as a bright X-ray source. The observations reveal this object to be a fairly massive (about 2 solar masses) pre-main-sequence star associated with the Taurus-Auriga star formation complex. It exhibits few of the characteristics of the classical T Tauri stars and is a good example of a 'naked' T Tauri star. The star is a mid-G subgiant, of about three solar radii and rotates with a period of 1.5 d. The coronal and chromospheric surface fluxes are similar to those of the most active late type stars (excluding T Tauri stars). The X-ray and UV lines most likely arise in different atmospheric structures. Radiative losses are some 1000 times the quiet solar value and compare favorably with those of T Tauri stars.

Walter, F. M.↗
NASA NTRS1987

Low-mass, pre-main-sequence stars - The infrared emission from circumstellar dust

Models for the dust shells around the low-mass, premain sequence stars IRS 1 and IRS 2 embedded in the Serpens molecular cloud are calculated which accurately fit the stars' observed near-IR fluxes. The far-IR fluxes are predicted, and the model parameters are adjusted to provide a best fit to the observed near-IR spectra. Best fit models are obtained for protostar masses of 2.97 and 2.14 solar masses, an accretion rate of about 10 to the -7th solar masses/yr, and inner shell radii of 9.6 x 10 to the 12th and 4.6 x 10 to the 12th cm, respectively, for IRS 1 and 2. The calculated spectra of the dust shells alone peak at about 2 microns and have no other broad continuum maxima longward of 2-3 microns. The empirical technique of Churchwell and Koornneef (1986) for estimating the mean temperature, radius, and luminosity of circumstellar dust shells by using H and K fluxes gives values in reasonably good agreement with the model calculations.

Wolfire, Mark G.↗
NASA NTRS1987

The evolution of red clump stars - Theoretical sequences

Two grids of horizontal-branch evolutionary sequences are presented which extend the grid of Sweigart and Gross (1976) to higher masses and higher metallicities. These two grids were constructed specifically for the study of red clump stars observed in intermediate-age star clusters in the Magellanic Clouds and in the Galaxy. An additional evolutionary track, with a thick hydrogen-burning shell which simulates the effect of a modest amount of mixing from the helium core into the shell at the time of the helium-core flash, is also presented. It is found that a small amount of mixing does not alter in a significant way the standard course of horizontal-branch evolution.

Seidel, E.↗
NASA NTRS1987

Soft X-ray observations of pre-main sequence stars in the chamaeleon dark cloud

Einstein IPC observations of the nearby Chamaeleon I star forming cloud show 22 well-resolved soft X-ray sources in a 1x2 deg region. Twelve are associated with H-alpha emission line pre-main sequence (PMS) stars, and four with optically selected PMS stars. Several X-ray sources have two or more PMS stars in their error circles. Optical spectra were obtained at CTIO of possible stellar counterparts of the remaining X-ray sources. They reveal 5 probable new cloud members, K7-MO stars with weak or absent emission lines. These naked X-ray selected PMS stars are similar to those found in the Taurus-Auriga cloud. The spatial distributions and H-R diagrams of the X-ray and optically selected PMS stars in the cloud are very similar. Luminosity functions indicate the Chamaeleon stars are on average approximately 5 times more X-ray luminous than Pleiad dwarfs. A significant correlation between L sub x and optical magnitude suggests this trend may continue within the PMS phase of stellar evolution. The relation of increasing X-ray luminosity with decreasing stellar ages is thus extended to stellar ages as young as 1 million years.

Feigelson, Eric D.↗
NASA NTRS1987

Flap-lag equations of motion of rigid, articulated rotor blades with three hinge sequences

A derivation of coupled flap-lag equations of motion for a rigid articulated rotor with hinge springs and viscous dampers is reported. Three different flapping-lag-pitch hinge sequences are considered and the Lagrange method is used to derive the equations. The effects of the complete six degrees-of-freedom aircraft motions are included and all the inertia dynamic terms are retained; no small-angle assumptions are used in the development. Comparisons of the results with those available in the literature are made. Sources of terms missing in previous analyses, especially those of the inertia dynamics, are identified.

Chen, Robert T. N.↗
NASA NTRS1987

An X-ray survey for pre-main-sequence stars in the Taurus-Auriga and Perseus molecular cloud complexes

Seventy-five fields from the Einstein X-Ray Observatory IPC detector between 3h and 5h right ascension and 15 deg and 35 deg declination are examined to search for X-ray-emitting low-mass pre-main-sequence (PMS) stars. Six such stars were previously found; they appear to be similar to T Tauri stars but without dense circumstellar envelopes or winds. Finding charts are presented for 59 X-ray sources that may be PMS stars. While some are likely to be spurious X-ray sources, chance coincidences with unrelated stars, or non-PMS stellar sources such as RS CVn type binaries, approximately half are probably X-ray-selected PMS stars.

Feigelson, E. D.↗
NASA NTRS1987

Pre-main sequence stars

Observations of premain-sequence (PMS) stars obtained with the IUE satellite since its launch in 1978 are reviewed. The major types of PMS stars are listed and briefly characterized: T Tau stars, Herbig Ae stars, FU Ori objects, weak-emission PMS stars, and Herbig-Haro objects. The IUE data for a catalog of PMS objects are then presented in a table, graphs, and sample spectra and described in detail. Topics addressed include general spectral profiles, the Mg II h and k lines, the FUV emission lines, the UV continuum, and the impact of the IUE on astrophysical models of PMS stars. The need for observations with higher spectral resolution and much greater sensitivity is indicated, and the ability of the Hubble Space Telescope High-Resolution Spectrograph and the proposed NASA/ESA Lyman FUV Spectroscopic Explorer to meet these requirements is considered.

Imhoff, Catherine L.↗
NASA NTRS1988

1s2 1S-1s np 1P transitions of the helium isoelectronic sequence members up to z = 30

The radial integrals required to calculate oscillator strengths for the helium sequence ions have been obtained using a one-electron hydrogenic model. The present approach retains two different values of the effective charge parameter, one for the initial state, and one for the final state. The model is shown to be capable of reasonably reproducing the existing dipole oscillator strength values with minimal computational effort. Dipole oscillator strengths for certain ions are presented for the first time.

Khan, F.↗
NASA NTRS1988

A search for molecular outflows toward the pre-main-sequence objects

A search for molecular outflows has been conducted toward a sample of 71 young stellar objects dominated by optically selected pre-main-sequence stars. Molecular outflows ranging in size from 0.07 to about 5 pc, with expansion velocities from 6 to 60 km/s, have been detected toward 20 of these objects and in an additional six infrared objects not included in the original survey. The apparent ages of the outflows are from 1000 to 500,000 yr; about half of the observed outflows are bipolar, with the rest showing a wide variety of morphologies. It is found that outflows driven by low-luminosity objects tend to be more bipolar than those driven by high-luminosity objects. The implications of the results of the study are discussed.

Levreault, Russell M.↗
NASA NTRS1988

Analysis of C II resonance lines in some main sequence early-type stars

IUE data are used to investigate C II resonance lines at 1335 A in eight main-sequence stars of spectral types from A0 to B3, and both LTE and non-LTE line profiles have been computed. In stars with low rotational velocities (such as Vega, Pi Cet, and Tau Her), logarithmic carbon abundances log N(C/H) of -3.55 to -3.45 are obtained for the non-LTE case. The LTE analysis reveals lower carbon abundances by about 0.1 dex. Significant differences among the fast rotating stars are pointed out.

Cugier, H.↗
NASA NTRS1988

Molecular outflows and mass loss in the pre-main-sequence stars

Molecular outflows are used here to probe mass loss in premain sequence (PMS) stars. Mass-loss rates are determined for 26 objects ranging in luminosity from four to about 100,000 solar, in mass from 0.5 to 30 solar, and in age from about 10,000 to about a million years. The derived mass-loss rates range from 9 x 10 to the -9th to 9 x 10 to the -4th solar mass/yr, with a typical value of 3 x 10 to the -7th solar mass/yr. PMS objects showing mass loss fall in a clearly demarcated region of the H-R diagram. The mass loss is proportional to bolometric luminosity to the 0.6 power and to stellar mass to the 1.8 power. The implications of these findings for the nature of the PMS mass-loss mechanism, for self-regulated low-mass star formation, and for planetary formation are discussed.

Levreault, Russell M.↗
NASA NTRS1989

Artificial Intelligence Controls Tape-Recording Sequence

Developmental expert-system computer program intended to schedule recording of large amounts of data on limited amount of magnetic tape. Schedules recording using two sets of rules. First set incorporates knowledge of locations for recording of new data. Second set incorporates knowledge about issuing commands to recorder. Designed primarily for use on Voyager Spacecraft, also applicable to planning and sequencing in industry.

Schwuttke, Ursula M.↗
NASA NTRS1988

Petrology and tectonic development of supracrustal sequence of Kerala Khondalite Belt, Southern India

The granulite terrain of southern India, of which the Kerala Khondalite belt (KKB) is a part, is unique in exposing crustal sections with arrested charnockite growth in different stages of transformation and in varied lithological association. The KKB with rocks of surficial origin and incipient charnockite development, poses several problems relating to the tectonics of burial of vast area and mechanisms involved in expelling initial H2O (causes of dryness) for granulite facies metamorphism. It is possible to infer the following sequence of events based on the field and laboratory studies: (1) derivation of protoliths of KKB from granitic uplands and deposition in fault bounded basin (cratonic rift); (2) subhorizontal deep burial of sediments; (3) intense deformation of infra and supracrustal rocks; (4) early granulite facies metamorphism predating F sub 2 - loss of primary structure in sediments and formation of charnockites from amphibole bearing gneisses and khondalites from pelites; (5) migmatisation and deformation of metasediments and gneisses; (6) second event of charnockite formation probably aided by internal CO2 build-up; and (7) isothermal uplift, entrapment of late CO2 and mixed CO2-H2O fluids, formation of second generation cordierites and cordierite symplectites.

Kumar, G. R. Ravindra↗
NASA NTRS1989

A VLSI chip set for real time vector quantization of image sequences

The architecture and implementation of a VLSI chip set that vector quantizes (VQ) image sequences in real time is described. The chip set forms a programmable Single-Instruction, Multiple-Data (SIMD) machine which can implement various vector quantization encoding structures. Its VQ codebook may contain unlimited number of codevectors, N, having dimension up to K = 64. Under a weighted least squared error criterion, the engine locates at video rates the best code vector in full-searched or large tree searched VQ codebooks. The ability to manipulate tree structured codebooks, coupled with parallelism and pipelining, permits searches in as short as O (log N) cycles. A full codebook search results in O(N) performance, compared to O(KN) for a Single-Instruction, Single-Data (SISD) machine. With this VLSI chip set, an entire video code can be built on a single board that permits realtime experimentation with very large codebooks.

Baker, Richard L.↗