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At least 181 records · Page 10

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NASA NTRS1976

Properties of Ap stars in the ultraviolet

A review is presented of the current state of knowledge concerning the emitted flux distributions of Ap stars and their ultraviolet photometric variability especially as it relates to photometric and spectrum variability observed in the visible. A table of Ap stars observed in the ultraviolet from space vehicles is presented along with a graph which shows the observed flux distributions, normalized to unity flux at 3320 A, for the Si star theta Aur and for the normal stars 134 Tau and gamma UMa. Attention is given to ultraviolet color indices and ultraviolet light curves for variable Ap stars.

Leckrone, D. S.↗

NASA NTRS1987

On the origin of the 40-120 micron emission of galaxy disks: A comparison with H-alpha fluxes

A comparison of 40 to 120 micron Infrared Astronomy Satellite (IRAS) fluxes with published H alpha and UBV photometry shows that the far infrared emission of galaxy disks consists of at least two components: a warm one associated with OB stars in HII-regions and young star-forming complexes, and a cooler one from dust in the diffuse, neutral interstellar medium, heated by the more general interstellar radiation field of the old disk population (a cirrus-like component). Most spiral galaxies are dominated by emission from the cooler component in this model. A significant fraction of the power for the cool component must originate with non-ionizing stars. For a normal spiral disk there is a substantial uncertainty in a star formation rate derived using either the H alpha or the far infrared luminosity.

Persson, Carol J. Lonsdale↗

NASA NTRS1973

A study of B 6 stars.

Because detailed information about the spectra of normal late B type stars is deficient, a study of the spectrum of zeta-Draconis, beta-Sextantis, and alpha-Leonis was undertaken to fill this lack and to provide data for comparison with observations of Bp stars, many of which are of late B spectral type. The study was made from high dispersion spectrograms which cover the spectral region 3100 to 6700 A, and from OAO-II spectral scans covering the spectral region 1100 to 3600 A. Profiles, equivalent widths, and central intensities of many lines in the spectrum of zeta-Draconis and of the major lines of beta-Sextantis are presented. The star alpha-Leonis rotates too rapidly for these measurements to be made from high resolution spectrograms. The star beta-Sextantis is the most massive and hottest of the three stars studied.

Underhill, A. B.↗

NASA NTRS1979

Ultraviolet photometry from the Orbiting Astronomical Observatory. XXXIII - The symbiotic star AG Pegasi

Ultraviolet broadband photometry obtained with the Wisconsin Experiment Package on OAO 2 is presented for the symbiotic binary star AG Peg. The hot component of the binary is found to be a luminous ultraviolet source, with an energy distribution consistent with its WN6 optical spectral type. Total luminosities of 1000 and 17,000 suns are found for the hot star by assuming, respectively, that the giant primary of AG Peg is a normal M3 III star and that it fills its Roche lobe. The eruptive behavior of AG Peg is shown to require the higher luminosity, and the activity in AG Peg is discussed in terms of a very slow novalike nuclear-powered event.

Gallagher, J. S.↗

NASA NTRS1973

Ultraviolet observations of peculiar B and A stars from OAO-2

Plotted photometric data in two far-ultraviolet bandpasses at 1550 and 1430 A for both normal and peculiar stars are compared for flux distribution. It is shown that the ultraviolet flux distribution of an A sub p or B sub p star is consistent with its spectral class and is notably inconsistent with its UBV colors. It is assumed that the helium line strength suppression mechanism is related to an abnormal atmospheric structure in the peculiar stars.

Leckrone, D. S.↗

NASA NTRS2003

Finding the Onset of Convection in Main Sequence Stars

The primary goal of the work performed under this grant was to locate, if possible, the onset of subphotospheric convection zones in normal main sequence stars by using the presence of emission in high temperature lines in far ultraviolet spectra from the FUSE spacecraft as a proxy for convection. The change in stellar structure represented by this boundary between radiative and convective stars has always been difficult to find by other empirical means. A search was conducted through observations of a sample of A-type stars, which were somewhat hotter and more massive than the Sun, and which were carefully chosen to bridge the theoretically expected radiative/convective boundary line along the main sequence.

Simon, Theodore↗

NASA NTRS1980

Attitude determination of Galileo spacecraft from star data

An entirely autonomous attitude determination algorithm has been developed for the dual spin Galileo spacecraft in its mission to Jupiter. A batch mode process is established which identifies three stars within the scanner's field-of-view based on the criteria of intensity and geometry. This is followed by a continuous star acquisition procedure which provides star transit times and a spacecraft spin rate estimate. A least-squares estimator then sequentially determines the spacecraft's attitude from successive star crossings by minimizing an error derived using the necessary condition of star and scanner slit normal orthogonality. Simulation results are presented, showing successful star identification and attitude convergence in the presence of nutation and star transit time uncertainty.

Wong, E. C.↗

NASA NTRS1979

X-ray emission from an Ap star /Phi Herculis/ and a late B star /Pi Ceti/

Using the HEAO 1 soft X-ray sky survey, a search was conducted for X-ray emission from 18 stars in the spectral range B5-A7. The detection of 0.25 keV X-ray sources consistent with the positions of Pi Ceti, a normal B7 V star, and Phi Herculis, a classic Ap star was reported. The detection of these stars argues for large mass motions in the upper layers of stars in this spectral range, and argues against radiative diffusion as the source of abundance anomalies in Ap stars.

Cash, W.↗

NASA NTRS1977

Copernicus spectra and infrared photometry of 42 Orionis

The Orion sword star 42 Ori is embedded in a nebula north of and separated from the Orion nebula. The B1 V star is probably normal. Other members of the multiple remain poorly defined, and the nebula may exhibit some peculiarities that may depend on them. Copernicus ultraviolet spectra of the star are described here, especially in the form of tables of wavelength identifications. The properties of the interstellar material in the line of sight are also discussed. Infrared photometry is presented which suggests that the ratio of total to selective extinction ranges from 3 to 3.5 for the interstellar matter in the direction of 42 Ori. The IR photometry provides no evidence for companion stellar or circumstellar components.

Johnson, H. M.↗

NASA NTRS2022

Elevated Hot Gas and High-mass X-Ray Binary Emission in Low-metallicity Galaxies: Implications for Nebular Ionization and Intergalactic Medium Heating in the Early Universe

High-energy emission associated with star formation has been proposed as a significant source of interstellar medium (ISM) ionization in low-metallicity starbursts and an important contributor to the heating of the intergalactic medium (IGM) in the high-redshift (z ≳ 8) universe. Using Chandra observations of a sample of 30 galaxies at D ≈ 200–450 Mpc that have high specific star formation rates of 3–9 Gyr^(−1) and metallicities near Z ≈ 0.3Z_⊙, we provide new measurements of the average 0.5–8 keV spectral shape and normalization per unit star formation rate (SFR). We model the sample-combined X-ray spectrum as a combination of hot gas and high-mass X-ray binary (HMXB) populations and constrain their relative contributions. We derive scaling relations of log L^(HMXB)_(0.5-8keV) / SFR = 40.19 ± 0.06 and log L^(gas)_(0.5-2keV) / SFR = 39.58 ^(+0.17)_(-0.28), significantly elevated compared to local relations. The HMXB scaling is also somewhat higher than L^(HMXB)_(0.5-8keV) -SFR-Z relations presented in the literature, potentially due to our galaxies having relatively low HMXB obscuration and young and X-ray luminous stellar populations. The elevation of the hot gas scaling relation is at the level expected for diminished attenuation due to a reduction of metals; however, we cannot conclude that an L^(gas)_(0.5-2keV) -SFR-Z relation is driven solely by changes in ISM metal content. Finally, we present SFR-scaled spectral models (both emergent and intrinsic) that span the X-ray-to-IR band, providing new benchmarks for studies of the impact of ISM ionization and IGM heating in the early universe.

high-mass x-ray binary stars↗

NASA NTRS1985

Ultraviolet interstellar absorption toward stars in the Small Magellanic Cloud. IV - Highly ionized gas associated with the Small Magellanic Cloud

High-dispersion International Ultraviolet Explorer satellite spectra of seven stars in the Small Magellanic Cloud (SMC) are examined to study the properties of interstellar C IV and Si IV absorption in the SMC. Absorption by C IV or Si IV or both is found near 160 km/s for all the stars. The velocity and the relative C IV and Si IV strengths suggest UV-photoionized nebular gas as the origin of this absorption. In addition, the stars show absorption by C IV and, sometimes, Si IV in the velocity range 100-130 km/s. This velocity is 30-60 km/s more negative than that expected for normal nebular gas, and the relative C IV and Si IV strengths indicate an ionization source other than stellar UV photoionization by normal Population I stars. Possible global origins are considered for this absorption, including a hot phase of the SMC interstellar medium and a circum-SMC distribution of highly ionized gas. The only detection of interstellar N V toward a SMC star is for HD 5980. The line is broad, possibly complex, and spans the velocity range of the nebular absorption and the 100-130 km/s absorption.

Fitzpatrick, E. L.↗

DOE OSTI2022

RRab variables with identical light-curve shapes at different pulsation periods

In this paper, we report on the detection of RRab stars with quasi-identical-shape light curves but period differences as large as 0.05–0.21 d using the Galactic bulge data of the OGLE-IV survey. We have examined stars with shorter periods than the Oosterhoff I ridge of the bulge. These stars generally have smaller amplitudes and larger Fourier phase-differences than the typical bulge RRab stars have at the same period. Many of these ‘anomalous’ stars have good-quality light curves without any sign of the Blazhko modulation. Examining their Fourier parameters revealed that several of these stars show very similar light curve to the typical bulge RR Lyrae. We found hundreds of quasi-identical-shape light-curve pairs with different periods between the ‘anomalous’- and the ‘normal’-position RRab stars based on the OGLE I-band data. The OGLE V-band, and the archive VVV and MACHO surveys K s -, b- and r -band data of these stars were also checked for light-curve-shape similarity. Finally, 149 pairs with identical-shape light curves in each available photometric band were identified. Calculating the physical properties of the variables using empirical formulae, on average, −0.5 dex, −0.13 mag, 0.67, and 165 K differences between the [Fe/H], M V , R/R ⊙ , and T eff values of the members of the pairs are derived, being the short-period stars less metal-poor, fainter, smaller, and hotter than the long-period variables. To explain the existence of variables with different physical properties and pulsation periods but with identical-shape light curves is a challenging task for modelling.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1974

On the nature of BD-10 deg 4662

Attention was first called to BD-10 deg 4662 (now assigned variable-star designation FK Ser) by Stienon (1971), who reported a temporary brightening in the blue on a Case objective-prism plate, accompanied by Balmer emission and a strong ultraviolet continuum. In its normal state the star appeared to be an ordinary late K-type object without emission features. Infrared observations show that BD-10 deg 4662 has excesses in the H - K and K - L colors similar to ordinary T Tauri stars. Emission at Ca II H and K confirms the close relationship to that group of objects.

Zappala, R. R.↗

NASA NTRS1988

HD 38452 - J. R. Hind's star that changed colour

In 1851, John Russell Hind announced that a star previously observed by him to be very red had become bluish white in color. It is shown that this star, HD 38451, is a ninth magnitude shell star which presumably was ejecting a shell when Hind first observed it. From high dispersion coude spectra, low dispersion IUE spectra, and ground-based photometry, HD 38451 is found to be a normal A21V shell star. Its current values of E(B-V) of about 0.14 is probably caused by interstellar rather than circumstellar reddening. There remains a problem to reconcile the large amount of reddening present when Hind first observed the star with its evidently small diminution in visual brightness at that time.

Warner, Brian↗

NASA NTRS1973

Many-particle theory of nuclear systems with application to neutron star matter

The research is reported concerning energy-density relation for the normal state of neutron star matter, and the effects of superfluidity and polarization on neutron star matter. Considering constraints on variation, and the theory of quantum fluids, three methods for calculating the energy-density range are presented. The effects of polarization on neutron star structure, and polarization effects on condensation and superfluid-state energy are discussed.

Chakkalakal, D. A.↗

NASA NTRS1990

The Wolf-Rayet star population in the most massive giant H II regions of M33

Narrow-band images of NGC 604, NGC 595, and NGC 592, the most massive giant H II regions (GHRs) in M33 have been obtained, in order to study their Wolf-Rayet content. These images reveal the presence of nine candidates in NGC 604 (seven WN, two WC), 10 in NGC 595 (nine WN, one WC), and two in NGC 592 (two WN). Precise positions and estimated magnitudes are given for the candidates, half of which have so far been confirmed spectroscopically as genuine W-R stars. The flux in the emission lines of all candidates is comparable to that of normal Galactic W-R stars of similar subtype. A few of the putative superluminous W-R stars are shown to be close visual double or multiple stars; their newly estimated luminosities are now more compatible with those of normal W-R stars. NGC 595 seems to be overabundant in W-R stars for its mass compared to other GHRs, while NGC 604 is normal. Factors influencing the W-R/O number ratio in GHRs are discussed: metallicity and age appear to be the most important.

Drissen, Laurent↗

NASA NTRS1982

Observations of normal main-sequence and giant B stars

When interpreting the continuous and line spectra of B stars, it is helpful to think in terms of a model consisting of a photosphere and a mantle which is the outer part of the atmosphere where the effects of nonradiative heating are seen. A survey of the spectra of these stars shows that conditions in the photosphere determine most of what is seen, and in the case of most B stars, the presence of the mantle can be detected only by a special effort. The shape of the visible continuum spectrum and the shape and absolute value of the UV continuous spectrum as determined from low resolution spectra are discussed. Effective temperature for B stars in the main sequence, including corrections for interstellar extinction and bolometric corrections are explored. The major constituents of B-type spectra, variation of the strength of line along the main sequence band, the UV spectra, UV line blocking, intrinsic colors, and variations in light and spectra are also examined.

Source record↗

NASA NTRS1986

UV observations of blue stragglers and population 2 K dwarfs

Blue stragglers are stars, found usually in either open or globular clusters, that appear to lie on the main sequence, but are brighter and bluer than the cluster turn-off. Currently, two rival models are invoked to explain this apparently pathological behavior: internal mixing (so that fresh fuel is brought into the stellar core); and mass transfer (by which a normal main sequence star acquires mass from an evolving nearby companion and so moves up the main sequence). The latter model predicts that in the absence of complete mass transfer (i.e., coalescence), blue stragglers should be binary systems with the fainter star in a post-main sequence evolutionary state. It is important to ascertain the cause of this phenomenon since stellar evolution models of main sequence stars play such a vital role in astronomy. If mass transfer is involved, one may easily exclude binaries from age determinations of clusters, but if mixing is the cause, our age determinations will be much less accurate unless we can determine whether all stars or only some mix, and what causes the mixing to occur at all.

Carney, B. W.↗