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At least 469 records · Page 26
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NASA NTRS1982

Emission mechanisms of the March 5th 1979 gamma ray transient

This paper reviews the spectral and temporal constraints on the emission mechanisms of the initial burst and subsequent pulses of the March 5, 1979 gamma-transient. Interpretation of the hard tail above 300 keV as due to inverse comptonization, when combined with the synchrotron-pair annihilation model of Ramaty et al. (1981), makes it possible to derive from first principles the intrinsic luminosity of the source. This is in general agreement with that expected from N49 in LMC. The pulsed emissions have characteristics difficult to explain in terms of conventional models.

Liang, E. P. T.↗
NASA NTRS1981

The circumstellar envelopes of F- and G-type supergiants in the Large Magellanic Cloud

The outer atmospheres of four F- and G-type supergiants in the LMC are compared with those of their Milky Way counterparts by means of 2.5 and 5.1 A/mm high dispersion Echelle spectra. Na I D line doubling indicates extensive circumstellar envelopes, mass loss rates greater than 0.00001 solar masses/year, and outflow velocities of 10-60 km/sec. The Ca II H and K lines yield new data on extragalactic star chromospheres.

Hagen, W.↗
NASA NTRS1983

Dust in galaxies

The manifestations of dust in the Galaxy, in normal galaxies, active nuclei galaxies (ANGs), and in QSOs are discussed. Findings on the composition, abundance, size distribution, and global properties of the Galaxy's dust obtained with absorption and emission studies are reviewed. The properties of dust in other galaxies, the variation of those properties among galaxies, and the effect of the dust on the appearance of galaxies are considered, discussing the LMC, M51, and M82 as examples. Evidence for the existence of dust in the nuclei of ANGs and in QSOs is examined with regard to reddening, dust emission and absorption, and polarization. The question of the existence of intergalactic dust is briefly addressed.

Stein, W. A.↗
NASA NTRS1983

Photometry of resolved galaxies. II - Sextans A

Luminosity functions and color-magnitude diagrams are given for 652 stars in the Sextans A dwarf irregular galaxy, and they are compared with other Local Group galaxies. The brightest red and blue stars presently determined are in agreement with those of Sandage and Carlson (1982), and a group of red stars is identified which may be analogous to the bright carbon star population recently found in the Magellanic Clouds. While the similarity of Sextans A's luminosity function with those of the LMC and the Milky Way is in keeping with a universal luminosity function, stellar mass function and birthrate resemble those of the SMC. Per unit gas mass, Sextans A has a birthrate almost identical to the SMC, suggesting that the mean age of stars in Sextans A is significantly less than of those in the Magellanic Clouds.

Hoessel, J. G.↗
NASA NTRS1983

Observations of two peculiar emission objects in the Large Magellanic Cloud

Ultraviolet and visual wavelength spectra were obtained of two peculiar emission objects, Henize S63 and Sanduleak's star in the Large Magellanic Cloud. Previously not observed in the near- or far-ultraviolet, both objects exhibit strong permitted and semiforbidden line emissions. Estimates based on the absolute continuum flux of the hot companion star in Hen S63 indicate that it rivals the luminosity of the carbon star primary. The emission-line profile structure in both objects does not suggest Wolf-Rayet type emission. Carbon in Sanduleak's star (LMC anonymous) is conspicuously absent, while N V, semiforbidden N IV, and semiforbidden N III dominate the UV emission-line spectrum. Nitrogen is overabundant with respect to carbon and oxygen in both objects. The large overabundance of nitrogen in Sanduleak's star suggests evidence for CNO processes material similar to that seen in Nu Car.

Kafatos, M.↗
NASA NTRS1983

The kinematics and structure of N49

The present investigation of N49, which is located in the Large Magellanic Cloud (LMC), makes use of an analysis of optical emission-line profiles. Echelle spectrograph observations indicate the large-scale kinematic and structural properties of N49. It is found that this supernova remnant (SNR) is evolving in a clumpy interstellar medium (ISM), probably as many other remnants are. The very high resolution spectra of this remnant show three kinds of spectral feature, called broad bands, narrow bands, and spikes. It is shown that N49 can be regarded as the product of a supernova explosion in a heterogeneous interstellar medium. The heterogeneity is indicated in low-speed and high-speed systems of shocked material in N49.

Shull, P., Jr.↗
NASA NTRS1984

IUE and ground-based observations of the Hubble-Sandage variables in M31 and M33

Ultraviolet spectra were obtained from the International Ultraviolet Explorer for the brightest Hubble-Sandage (H-S) variables in M31 and M33. The ultraviolet fluxes were then used in combination with ground-based visual and infrared photometry to determine the energy distributions, luminosities, and temperatures of these stars. When corrected for interstellar extinction, the integrated energy distributions yield the total luminosities and blackbody temperatures of the H-S variables. The resulting bolometric magnitudes and temperatures confirm that these peculiar stars are indeed very luminous, hot stars. They occupy the same regions of the bolometric magnitude vs temperature diagram as Eta Car and P Cyg in the Galaxy and S Dor in the LMC.

Blaha, C.↗
NASA NTRS1983

Infrared properties of extragalactic H II complexes

Infrared observations of relatively isolated giant H II complexes in the LMC and the spiral arms of M33 and M101 are discussed, and complexes which extend over several kiloparsecs near the galactic centers are examined. The 30 Doradus nebula is emphasized, showing the variation of the 50-100 micron color temperature with position in that object. Ten-micron scans through the nucleus of NGC 1097 are compared to a visual photograph, showing the enhanced infrared emission which occurs at the nucleus and at the ring of H II regions about 1 kpc from the nucleus.

Telesco, C. M.↗
NASA NTRS1983

X-ray emission from normal galaxies

The results of Einstein Observatory studies of X-ray emission from normal galaxies, including the LMC and SMC, M31, M33, M101, NGC 247, M81 and M100, and N253 are surveyed. The X-ray luminosity of normal galaxies is proportional to their optical luminosity, revealing no strong dependence on galaxy type. The number of individual sources detected are comparable to the number of sources expected on mass considerations. There are substantial numbers of X-ray sources in the Magellanic Clouds with luminosities in the range 10 to the 35th-36th ergs/s, lower than most X-ray binaries but higher than known uncollapsed stellar systems. About seven X-ray sources with luminosities of at least 10 to the 39th ergs/s in the 0.5-3.0 keV band have been found in the arms of nearby spiral galaxies.

Long, K. S.↗
NASA NTRS1984

Interpretation of the number versus diameter distribution for supernova remnants in the Large Magellanic Cloud

An examination is conducted of the cumulative number versus diameter relation for an X-ray selected sample of supernova remnants in the Large Magellanic Cloud in an attempt to understand the evolutionary state of these objects. Previous studies have suggested that the observed linear N(D) relation requires the remnants in the cloud to be freely expanding. Detailed calculations have been carried out to determine the effect of a luminosity threshold on the observed distribution and it is shown that the observations can be fitted by remnants which are in the adiabatic or later stages of evolution. The implications of the results for the supernova creation rate in the LMC are discussed.

Hughes, J. P.↗
NASA NTRS1983

Heating of the interstellar medium by supernova remnants

Models for the mechanisms active during supernova (SN) heating of surrounding regions are examined. SNs heat both the matrix material and material in the nearby interstellar medium, and also accelerate cosmic rays with shock waves. Fountain and wind models vary according to the SN power and mass acquisition rate. Additionally, a warm intercloud medium may be fed by returning fountain material and stellar mass loss. The thermal wind temperature varies with the number of solar massess heated, although remnants have been observed to vanish in the LMC when more than 100 solar masses were heated. The possibility that the solar system is inside a remnant that produces the observed soft X-ray background is considered.

Cox, D. P.↗
NASA NTRS1985

High energy neutrino astronomy with MACRO

A large area underground detector with accurate muon tracking and directionality can be used for the search of extraterrestrial sources of high energy neutrinos. The sensitivity of the MACRO detector to possible sources of neutrinos was evaluated with a Monte-Carlo simulation of the neutrino interaction in the rock and and of the detection in the real apparatus. Two categories of possible neutrino sources are discussed in comparison with the detector sensitivity. Promising candidate objects for this search appear to be the two binary X-ray sources in the southern key Vela X1 and LMC X4, which are known to emit gamma rays up to the 10,000 TeV region.

Auriemma, G.↗
NASA NTRS1984

IUE observations of the clusters of the Magellanic Clouds

IUE observations of 17 clusters in the Large Magellanic Cloud are presented. The derived ultraviolet colors become progressively redder as the cluster's classification in the scheme of Searle, Wilkinson, and Bagnuolo increases. With the aid of theoretical models of the integrated light of coeval clusters of varying ages, it is shown that the SWB classification is one of increasing age accompanied by decreasing metallicity toward later SWB classes. The behavior of the UV absorption line features in the integrated light of the LMC clusters supports the contention.

Cohen, J. G.↗
NASA NTRS1983

Corequake and shock heating model of the 5 March 1979 gamma-ray burst

Ramaty et al. (1980) have proposed a model of a neutron-star corequake and subsequent shock heating of the atmosphere to account for the gamma-ray burst of March 5, 1979. This model is elaborated by examining the overall energetics and characteristics of the radiation-dominated gas shocks under the assumption of thermodynamic equilibrium, taking into account the electron-positron pair production behind the shock. Using values for the density typical of those expected for neutron-star crusts (100-10,000 g/cu cm) and shock velocities characteristic of the sound speed (40,000-100,000), shock luminosities are obtained comparable to those required if the burst originated in the LMC. In addition, the fraction of energy deposited in electron-positron pairs is in good agreement with observation. Uneven shock heating between the polar and equatorial regions of the neutron star due to the presence of a dipole magnetic field, coupled with a rotation rate of 8 s, can also naturally account for the pulsating phase of the burst.

Ellison, D. C.↗
NASA NTRS1984

X-ray surveys of the Magellanic Clouds

The proximity, well-determined distance, and low foregound obscuration of the Magellanic Clouds makes them an ideal target for the study of galactic X-ray source populations. Steady progress since the initial detection of X-rays from the LMC in 1968 culminated in the recent Einstein Observatory surveys from which over three dozen supernova remnants and about 10 compact binaries have been identified. This review records the 15-year history of Magellanic Cloud X-ray research, summarizes current knowledge, and offers a brief prospectus of what the next 15 years may hold.

Helfand, D. J.↗
NASA NTRS1985

Stellar winds from hot stars in the Magellanic Clouds

O and B stars in the LMC and SMC have been observed with the IUE in order to compare their winds and mass loss rates with those of their galactic counterparts. It is found that while the wind terminal velocities are generally about 20 percent lower than in galactic stars of the same spectral type, it is not clear that the mass loss rates are significantly lower. This conclusion differs from the conclusions of other studies, and the reasons are discussed. The most important reason is that a less luminous set of stars with new spectral types determined from slit spectra is being examined. The discussion of stellar winds rests heavily on a comparison of relevant wind parameters observed in O stars using both high and low dispersion IUE spectra.

Garmany, C. D.↗
NASA NTRS1985

Ultraviolet interstellar absorption toward stars in the Small Magellanic Cloud. III - The structure and kinematics of the Small Magellanic Cloud

The structural and kinematic properties of the Small Magellanic Cloud (SMC) were studied by combining IUE satellite observations of individual bright stars (Sk 159 and HD 5980) with existing optical and radio data. It is shown that the SMC has a complex structure due to gravitational interaction with the Milky Way and the Large Magellanic Cloud (LMC). Stars observed in the southwest regions of the SMC were associated with the 134 km/s H I complex. H II regions in the SMC were distributed according to a rotational velocity of 167 km/s. All IUE high-dispersion targets exhibit ultraviolet absorption near 100 km/s and 200 km/s. It is shown that the absorption arises in the widespread H I complexes at velocities similar to the H II regions. Some possible explanations for the origin of H II and H I regions in the SMC are discussed. The locations of the nine high-dispersion IUE targets in the SMC are given.

Fitzpatrick, E. L.↗
NASA NTRS1986

Two supergiants in the Large Magellanic Cloud with thick dust shells

Ground-based observations from 0.6 to 20 microns have identified two luminous, evolved stars surrounded by thick dust shells among the Magellanic Cloud sources detected by the Infrared Astronomical Satellite. Their energy distributions resemble those of typical Galactic OH/IR stars, but they have bolometric magnitudes brighter than -9 and the small-amplitude variability of supergiants. One star, IRAS 04553 - 6825, has a special type of M7.5 and a dust shell which absorbs and reradiates roughly 75 percent of the star's luminosity; its radial velocity confirms its LMC membership. The second star, IRAS 05346 - 6949, has an even thicker dust shell, and the central star is not observable.

Elias, J. H.↗