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Davidson, K.

Publications and source records attributed to Davidson, K..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

He II lambda 1640/lambda 4686 and Ly-alpha/H-beta ratios in the extraordinary Seyfert galaxy Markarian 359

The exceptionally narrow-lined Seyfert 1 galaxy Mrk 359 is ideally suited for determining reddening via the He II lambda 1640/lambda 4686 emission-line ratio. Here the validity of He II lines as a reddening indicator for quasars and Seyfert galaxies is reexamined and confirmed in the context of the most complete photoionization models available. In addition, lambda 1640 equivalent width measurements are shown to support previous contentions of substantial differences between observed and intrinsic quasar far-ultraviolet continua. Then IUE satellite and ground-based spectral data are presented, with the surprising result that both the lambda 1640/lambda 4686 and Ly-alpha/H-beta ratios do not differ greatly from case B recombination values in Mrk 359. This object is only slightly reddened, and it shows no evidence for extended, high-optical-depth H(+)/H(0) transition regions in the line-emitting gas. The very narrow lines and lack of deep transition regions and/or reddening may be related. In addition, Mrk 359 has low luminosity compared with most other Seyfert 1 galaxies, measured emission lines suggest an unusually high ionization parameter, and there is a hint of low nitrogen abundance. Further work is needed to examine possible relationships among all these peculiar properties.

Macalpine, G. M.↗
NASA NTRS1982

The remarkable spectrum of some material ejected by Eta Carinae

Some of the outer condensations of the very massive star Eta Carinae have emission-line spectra which convey clues to the nature of the star from which they were ejected. Ground-based spectrograms as well as ultraviolet IUE data on the brightest such condensation is described. Excess nitrogen presumably resulting from the C/N/O cycle implies that the star is at least moderately evolved. The facts seem consistent with recent suggestions that turbulent mixing, combined with mass loss, causes a very massive star like Eta Carinae to evolve quasi-homogeneously.

Davidson, K.↗
NASA NTRS1982

The ultraviolet spectrum of the Crab Nebula

Ultraviolet spectroscopy of the Crab Nebula done by the International Ultraviolet Explorer satellite is described, and an estimate of the carbon abundance is made, noting data reduction to remove spectral defects caused by radiation hits. The important C IV 1549, He II 1640, and semiforbidden C III 1908 emission line intensities were measured and upper limits placed on other ultraviolet features for the brightest filamentary region in the Nebula. The emission lines imply an average ionic abundance ratio n(C+2)/n(O+2) in the range from 0.4 to 1.5 in the observed gaseous condensations. The elemental abundance ratio of carbon to oxygen is probably in the same range. Analysis shows that there is no perceptible excess of carbon due to presupernova nucleosynthesis in the observed region. The large helium abundance, small carbon and oxygen abundances, and presence of a neutron star in the Crab Nebula suggest that the presupernova star had a mass close to eight solar masses.

Davidson, K.↗
NASA NTRS1981

Preliminary report on IUE spectra of the Crab Nebula

The Crab Nebula is marginally observable with the IUE. Observations of the optically brightest filamentary regions, made with IUE in August 1979, show the C IV lambda 1549, He II lambda 1640, and C III lambda 1909 emission lines. The intensities of these lines were compared with the visual wavelength data. It appears that carbon is not overabundant in the Crab; carbon/oxygen is approximately normal and oxygen is slightly scarcer than normal as a fraction of the total mass.

Davidson, K.↗
NASA NTRS1975

Dielectronic recombination and abundances near quasars

Major obstacles to determining certain elemental abundances associated with some quasars are (1) a lack of accurately measured helium line intensities, and (2) uncertainty about the extent to which dielectronic recombination, i.e. C(+3) + e(-) yields C(+2), is suppressed at the relevant gas and radiation densities. Some calculated results are presented in an attempt to reduce the latter obstacle.

Davidson, K.↗
NASA NTRS1974

TV photometry of emission lines in the Crab Nebula

Using an integrating-television method, photometric studies were made of the Crab Nebula in the light of the H beta, forbidden O III, and forbidden O I emission lines. The H beta and forbidden O III fluxes from the whole nebula may be larger than previously estimated. In view of these results, the very difficult problem of finding the mass of the Crab Nebula is discussed.

Davidson, K.↗
NASA NTRS1973

Abundances in the Crab Nebula

The bright visual-wavelength emission lines in spectra of filaments in the Crab Nebula cannot be used to derive unique values for the relative abundances of elements there, even under a simplifying assumption that heating is due entirely to photoionization by the Crab's nonthermal continuum. However, a range of possibilities may be estimated. By mass, helium is more abundant than hydrogen; but the mass fraction of heavier elements may be surprisingly small. Such conclusions are subject to modification if a noticeable fraction of the energy density involves fast particles around 1 MeV.

Davidson, K.↗
NASA NTRS1973

Accretion at a magnetic pole of a neutron star

As accreted material falls to the surface of a magnetized neutron star like Cen X-3 or Her X-1, it is arrested by radiation pressure in such a manner that a hot, dense mound of nearly stationary gas protrudes above each magnetic pole. Energy released above the mound diffuses out as moderately hard X rays; but that released within the mound emerges as soft X rays from the whole surface of the neutron star.

Davidson, K.↗
NASA NTRS1973

Simplified photoionization analysis of quasar emission spectra.

Simplified photoionization calculations, considering quasar emission lines with ultraviolet rest-wavelengths, can be made including only hydrogen, helium, and carbon ions. With plausible assumptions about the helium abundance and ionizing spectral slope, estimates of the (radiation/gas) pressure ratio and carbon abundance are possible. Proportionality relations, showing the dependences of line ratios upon assumed parameters, may be useful in considering differences between individual spectra.

Davidson, K.↗
NASA NTRS1973

Neutron-star accretion in a stellar wind - Model for a pulsed X-ray source.

Description of a relatively simple stellar-wind-driven accretion model that appears to satisfactorily account for many of the observed features of the pulsing X-ray sources Cen X-3 and Her X-1. A rotating magnetized neutron star orbits a more massive slightly evolved star and accretes mass from the stellar wind emanating from that star. Some of the implications of the model are discussed.

Davidson, K.↗
NASA NTRS1973

Models for compact pulsing X-ray sources.

Development of a model of a neutron star (Cen X-3) which releases the infall energy of accreted matter. Some general remarks are made on the possibility of accretion as a source of energy. It is shown that overflow of the Roche lobe is neither necessary as a source of mass nor probable in view of the observed stellar line widths of the two identified X-ray companions. While not excluding the possibility that tidal forces may drive some mass outflow, a number of models are investigated in which the material to be accreted appears in the form of a stellar wind. The behavior of the accreted gas near the accreting object is discussed, as well as the transfer of angular momentum near the accreting object. It is concluded that a relatively simple stellar wind driven accretion model appears to satisfactorily account, in a quantitative but not precise way, for most of the observed features of Cen X-3.

Ostriker, J. P.↗
NASA NTRS1972

Early-type stars and the mean-square electron density in the galactic disk.

An assessment is made of the total supply of ionizing photons from hot stars, within several hundred parsecs of the sun. The resulting photoionized H II regions probably contribute an average emission measure of the order of 5 pc per cm to the 6th power, through the galactic disk; this is smaller than other recent estimates.

Richstone, D. O.↗
NASA NTRS1972

Remarks about X-rays from Eta Carinae.

Two current models for Eta Carinae are discussed. It is shown that the recently observed soft X-ray flux seems to indicate a surprisingly high density for the ambient gas around Eta Carinae, and that some useful constraints on models of this object may result from X-ray observations.

Davidson, K.↗
NASA NTRS1972

Photoionization and the emission-line spectra of quasi-stellar objects.

Detailed photoionization calculations are used to examine the information conveyed by relative emission-line intensities in QSO spectra. It is found that the elemental abundances cannot be estimated with any assurance; but if ?conventional' values are assumed, then certain restrictions may be placed on the ionizing spectra and gas densities. In particular, the ratio between radiation density and gas density is determined. Narrow filaments or sheets of ionized gas seem to be indicated; these may be compressed regions behind shock fronts.

Davidson, K.↗
NASA NTRS1971

On the nature of eta Carinae.

Demonstration that certain details of the spectrum of eta Carinae are consistent with a model in which a compact H II region is photoionized by radiation from a very massive star whose surface temperature is about 30,000 K. The infrared spectrum is presumed to be thermal reemission from dust surrounding the ionized region.

Davidson, K.↗