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At least 325 records · Page 18
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NASA NTRS1983

Interacting binary stars - Freaks or rosetta stones

Attention is given to semidetached binaries of the Algol type and to related interacting systems, such as Beta Lyrae and the W Serpentis stars. A brief description is given of observational problems. The basic properties of the Algol variables are satisfactorily explained by means of evolutionary models that assume considerable mass transfer between the components. It is pointed out that ultraviolet spectra obtained with the IUE satellite corroborate the view that the present subgiant secondary component was initially the more massive star and that it has been stripped of a large fraction of its mass so that the deep layers affected by CNO processing are now denuded. It is believed that the present-day 'classical' Algols must have undergone a phase of rapid mass transfer in the past and that Beta Lyrae and the W Serpentis stars are probably closer to that stage than ordinary Algols. Around the accreting star a complex structure is formed, and an induced stellar wind may blow a large part of the transferred matter out of the system.

Plavec, M. J.↗
NASA NTRS1984

Sulfur dioxide - Episodic injection shows evidence for active Venus volcanism

Pioneer Venus ultraviolet spectra from the first 5 years of operation show a decline (by more than a factor of 10) in sulfur dioxide abundance at the cloud tops and in the amount of submicron haze above the clouds. At the time of the Pioneer Venus encounter, the values for both parameters greatly exceeded earlier upper limits. However, Venus had a similar appearance in the late 1950's, implying the episodic injection of sulfur dioxide possibly caused by episodic volcanism. The amount of haze in the Venus middle atmosphere is about ten times that found in earth's stratosphere after the most recent major volcanic eruptions, and the thermal energy required for this injection on Venus is greater by about an order of magnitude than the largest of these recent earth eruptions and about as large as the Krakatoa eruption of 1883. The episodic behavior of sulfur dioxide implies that steady-state models of the chemistry and dynamics of cloud-top regions may be of limited use.

Esposito, L. W.↗
NASA NTRS1984

IUE observations of weak G-band stars

High- and low-resolution ultraviolet spectra of several weak G-band giants have been obtained with the IUE satellite, to derive Be abundances and to search for degenerate companions. The abundances of Be in these stars are about log epsilon (Be) approximately -0.5. These values are in good agreement with the Be abundances of Hyades giants, and are consistent with standard theories of post-main-sequence Be depletion in stars. However, the Be abundances do not fit with the large Li abundances in weak G-band stars. Post-main-sequence production of Li or element segregation in main-sequence stars may explain the high Li abundance in weak G-band stars. No direct evidence is seen for the presence of hot subluminous companions. The peculiar abundances in these stars probably are not due to binary mass transfer.

Parthasarathy, M.↗
NASA NTRS1984

Evolution of the ultraviolet coma of comet Austin (1982g.)

Ultraviolet spectra of comet Austin (1982) obtained in July 1982 at heliocentric distances ranging from 1.10 to 0.81 AU preperihelion with the IUE reveal that the comet is very similar in chemical composition and appearance to comet Bradfield (1979X). In addition, the derived H2O production rate is found to vary with heliocentric distance as r to the -3.6, similar to the r to the -3.7 behavior found for comet Bradfield. It is pointed out, however, that the limited sample rate precludes the observation of the short-term variations which could be produced by a rotating nonuniform cometary nucleus.

Feldman, P. D.↗
NASA NTRS1984

Variable, optically thick, hot plasma observed in interacting binaries

Recent International Ultraviolet Explorer (IUE) observations of two interacting binaries, R Arae and HD 207739 are reported. The ultraviolet spectra indicate the presence of optically-thick, variable hot plasma in those binary systems. These two binaries may belong to a class of binaries that are currently undergoing a rarely observed and probably short-lived phase in their evolution. Their properties are compared with those of two other interacting binaries, U Cephei and Beta Lyrae.

Kondo, Y.↗
NASA NTRS1984

IUE spectroscopy of the degenerate components in the Hyades close binaries V471 Tauri and HZ9

Effective temperatures and gravities have been determined for the degenerate components in the Hyades close binaries V471 Tauri and HZ9 from model atmosphere analysis of IUE ultraviolet spectra. For V471 Tauri, T(e) = 35,000 K and log g is about 8. For HZ9, T(e) = 20,000 K and log g = 8. Both systems contain DA2 white dwarfs. No photospheric metal lines were detected at low IUE resolution in either DA2 star. The 1400 A and 1600 A features are absent in the HZ9 degenerate, despsite being within the temperature range of some other DA stars with the 1400 A absorption feature.

Guinan, E. F.↗
NASA NTRS1984

High-resolution, far-ultraviolet study of Beta Draconis (G2 Ib-II) - Transition region structure and energy balance

High-resolution far ultraviolet spectra of the star Beta Draconis have been obtained with the IUE satellite. The observations and emission line data from the spectra are presented, the interpretation of the emission line widths and shifts is discussed, and the implications are given in terms of atmospheric properties. The emission measure distribution is derived, and density diagnostics involving both line ratios and line opacity arguments is investigated. The methods for calculating spherically symmetric models of the atmospheric structure are outlined, and several such models are presented. The extension of these models to log T(e) greater than 5.3 using the observed X-ray flux is addressed, the energy balance of an 'optimum' model is investigated, and possible models of energy transport and deposition are discussed.

Brown, A.↗
NASA NTRS1984

Feii and Mgii in the Nearby Interstellar Medium

Ultraviolet spectra with 15 km/s resolution of bright stars were searched for FeII and MgII interstellar lines. Equivalent widths of absorption for 8 stars are presented and average line-of-sight densities n(FeII) = 4 x 10(-8) and n(MgII) = 2 x 10(-7)/cubic cm are calculated. This represents approximately n(H) = 0.03/cuib cm.

Deboer, K. S.↗
NASA NTRS1984

Observations of eight close binaries with the Einstein Observatory

Eight close binaries were observed with the Imaging Proportional Counter aboard the Einstein Observatory. The binaries R Ara, Zeta Aur, RZ Cas, 31 Cyg, Delta Lib, U Sge, RZ Sct, and RW Tau all show strong indications of active mass flow in their ground-based and/or ultraviolet spectra. The systems R Ara, RZ Cas, 31 Cyg, Delta Lib, and U Sge were detected as X-ray sources while only upper limits on the X-ray luminosities of Zeta Aur, RZ Sct, and RW Tau could be determined. Nondetection of the latter three is probably attributable to their distance. With the exception of 31 Cyg which has an X-ray luminosity of about 10 to the 32nd erg/s, the detected sources have X-ray luminosities in the range 10 to the 30th-31st erg/s. These luminosities are approximately equal to those observed for single stars of similar spectral types. It appears that the X-ray luminosity of 31 Cyg is at least 60 times higher than that of Zeta Aur.

Mccluskey, G. E., Jr.↗
NASA NTRS1985

Ultraviolet spectroscopic observations of HD 77581(Vela X-1 = 4U 0900-40)

Ultraviolet spectra of HD 77581 obtained with the IUE satellite in December 1982 and January 1983 are analyzed. The effective temperature is found to be 25,000 + or - 1000 K by comparing its photospheric absorption lines with those in four well-studied B0-B1 supergiants. The distance of 1.9 + or - 0.2 kpc is derived from its luminosity log L/solar-L = +5.53, which is obtained from the effective temperature and from its radius. Circumstellar matter is detected not only in the C IV and Si IV lines but also in Al III, Fe III, C II, and Mg II lines. Profiles of the C IV and Si IV lines show a similar dependence on the orbital phase as in 1978. Two distinct sharp components are found in the shortward-shifted absorptions of the Al III and Fe III lines. Once component is present only in the second half of the orbital period, and its origin may be a 'trailing wake' behind the neutron star. The other component is seen all over the period, and it can be interpreted as originating in a cool expanding region of gas in the orbital plane.

Sadakane, K.↗
NASA NTRS1985

Time-resolved UV spectroscopy of BE Ursae Majoris

Far-ultraviolet spectra have been obtained of the eclipsing, hot subdwarf binary, BE UMa, with phase coverage in the 2.3-day orbital period concentrated near minimum light. No clearly phase-related spectra changes were found, but the UV continuum appeared to be in a highly variable state, with changes of a factor of two occurring on a time scale of hours. The hot star in this system may be one of the few known PG1159-035 variables, which show nonradial pulsations. If so, this is the first such star known in a binary system and thus should be important for determining the physical properties of these stars.

Hutchings, J. B.↗
NASA NTRS1985

Stratification of the extended atmosphere of the Wolf-Rayet component of V444 Cygni

Ultraviolet spectra covering both eclipses of V444 Cygni have been obtained. Longward of 1570 angstroms, the continuum brightness showed essentially the same variation seen in the optical (at 4244 angstroms). Striking atmospheric eclipses of up to 0.7 mag in depth were detected, however, in individual strong lines as well as over large ranges of the continuum shortward of 1500 angstroms. IUE data are used to estimate the degree of interstellar extinction and terminal expansion velocity of the system and to study the phase dependence of the continuum and line absorptions.

Eaton, J. A.↗
NASA NTRS1985

Strong Lyman-alpha emission in three distant radio galaxies

Very strong, narrow Lyman alpha emission (equivalent width approximately 1000 angstroms) has been discovered in the redshifted ultraviolet spectra of three radio galaxies having z between 1.62 and 1.82. The three are 3C 256, 3C 239, and 3C 241. It is noted that Lyman alpha will probably be a very useful redshift and classification determinant in future spectroscopic surveys of active galaxies at still greater distances.

Spinrad, H.↗
NASA NTRS1985

Mass loss from solar-type stars

The present picture of mass loss from solar-type (low-mass) stars is described, with special emphasis on winds from pre-main-sequence stars. Attention is given to winds from T Tauri stars and to angular momentum loss. Prospects are good for further advances in our understanding of the powerful mass loss observed from young stars; ultraviolet spectra obtainable with the Space Telescope should provide better estimates of mass loss rates and a clearer picture of physical conditions in the envelopes of these stars. To understand the mass ejection from old, slowly rotating main-sequence stars, we will have to study the sun.

Hartmann, L.↗
NASA NTRS1986

Combined ultraviolet studies of astronomical sources

Topics addressed include: Cygnus Loop; P Cygni profiles in dwarf novae; YY Gem; nova shells; HZ Herculis; activity cycles in cluster giants; Alpha Ori; metal deficient giant stars; ultraviolet spectra of symbiotic stars detected by the Very Large Array; time variability in symbiotic stars; blue galaxies; and quasistellar objects with X-ray spectra.

Baliunas, S. L.↗
NASA NTRS1986

Evidence of large-scale structures in the atmosphere of the active K-dwarf component of V471 Tauri

Contemporaneous IUE ultraviolet spectra and visible photoelectric data of the eclipsing binary V471 Tauri (K2V + DA) between 1979 and 1985 was analyzed. The combined data detail the three-dimensional structure of atmospheric loops and their associated starspots on the K dwarf. The distribution of starspot regions on the surface of the K star was inferred from the visible photometry. When spots are located near the limb of the K dwarf prior to and shortly after the total eclipse of the white dwarf, absorption lines such as C II, C III, c IV, and Si IV appear superimposed on the continuum of the white dwarf. These absorption lines are likely caused by cool coronal loops overlying the spots in the atmosphere of the K dwarf. The loops can extend nearly one stellar radius above the surface of the K2V star.

Guinan, E. F.↗
NASA NTRS1986

On the terminal velocities of winds in central stars of planetary nebulae

The theory of radiatively driven stellar winds is applied to the central stars of planetary nebulae, and the predicted relation between escape velocity and terminal velocity of the wind is assessed. Ultraviolet spectra obtained with IUE indicate that the terminal velocities of winds from planetary nuclei, which range from 600 to 3600 km/sec, are strongly correlated with stellar temperature. The theory of radiative winds predicts that the terminal velocity of the wind = T(1.2), the constant of proportionality being a function of stellar mass and line-force parameter, alpha. Given a mass of 0.60 solar mass for central stars with winds, the line-force parameter alpha = 0.70, a value higher than Abbott's predictions, alpha = 0.61 (1982).

Heap, Sara R.↗
NASA NTRS1986

Neutral oxygen and sulfur densities in the Io torus

Ultraviolet spectra (1150-1750 A) of the Io torus, obtained using a rocket-borne telescope and spectrometer in May 1981, show emissions of neutral sulfur and neutral oxygen, as well as ionic emissions of S II, S III, and S IV. The emissions are observed almost 180 deg from the location of the satellite Io and imply the existence of an extended neutral atomic cloud associated with the torus. In the present work, neutral densities and ion mixing ratios are derived by modeling the observations using torus electron temperature and density profiles proportional to those at the time of the Voyager 1 encounter. The model includes the effects of photoexcitation, which can be considerable if the neutral cloud is large enough. At the time of the observation, substantial densities of neutral oxygen and neutral sulfur are found in regions of the torus far from Io. These lower limits on the neutral densities are compared to theoretical models of the neutral source, and the ion mixing ratios are compared to IUE observations of the torus obtained over a period of 5 yr. Arguments that tend to rule out Europa as the source of the neutral emission are also presented.

Skinner, T. E.↗