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Evans, A.

Publications and source records attributed to Evans, A..

26 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Multiwavelength observations of RV Tauri stars. 4: SX Centauri

We present ultraviolet, optical and infrared observations of the RV Tauri star SX Cen. From a fit of model atmospheres to the data we conclude that Z/Z solar = 0.033, that SX Cen has an extended atmosphere, and that the spectral types implied by the ultraviolet data close to deep minimum are consistent with those implied from optical spectra. Unlike AC Her, there seems to be no silicate component in the circumstellar dust shell, and the star pulsates in purely radial modes. The implication is that, despite their many similarities, SX Cen seems to be at a significantly earlier phase of post-AGB evolution than AC Her. Additional data, particularly in the infrared, are necessary to confirm this conclusion.

Shenton, M.↗
NASA NTRS1993

The reddening and variability of XX Ophiuchi

We present polarization data on the XX Oph system which suggest that the interstellar extinction to this object has been overestimated in the past: our data imply A(V) = 1.6 mag. Our photometry and infrared spectroscopy suggest a spectral class of M7III for the late component, and a BOV companion that ionizes the wind of the cool component. XX Oph seems more like a Zeta Aur/VV Cep system than a 'symbiotic object'. The photometric variability of XX Oph seems to have a number of causes, ranging from shell-type variability in the U band to variations in the M component in the infrared.

Evans, A.↗
NASA NTRS1992

An observational case against nova hibernation

We use WHT spectroscopy and imaging to show that nova Vul 1670 (= CK Vul) has been incorrectly identified, and thus its luminosity cannot be used as evidence that novae fade into a 'hibernation' phase within 300 yr of their outbursts. INT spectroscopy is used to correct the magnitude of nova Sge 1783 (= WY Sge) for inclination, this result also implying that novae do not fade significantly. We therefore suggest that, while novae decline in the first 60 yr after outburst, thereafter their luminosity remains constant, and they never undergo a 'hibernation' phase. We show that this idea is consistent with the space density of novae and novalike variables, the outburst interval of SS Cyg and the current luminosities of old novae.

Naylor, T.↗
NASA NTRS1989

Atomic environments in iron meteorites using EXAFS

Extended x ray absorption fine structure (EXAFS) is observed as a modulation on the high energy side of an x ray absorption edge. It occurs when the photo-ejected electron wave is scattered by neighboring atoms in a solid, and interference occurs between the outgoing and scattered waves. The result is that the absorption spectrum carries a signature that is characteristic of the identity and disposition of scattering atoms around the absorbing atom. Therefore, it may be shown that the Fourier transform of the normalized EXAFS can provide detailed information about the immediate environment of specific atoms in a solid and is ideally suited to the study of cosmic dusts. A study of cosmic dust was initiated using EXAFS and other techniques. The simplest type of cosmic material, namely iron meteorites, was investigated.

Cressey, G.↗
NASA NTRS1985

Search for scattered X-ray halos around variable sources - The X-ray halo of Cygnus X-1

The results of a program to search for the presence of halos due to X-ray scattering by interstellar grains in the line-of-sight to variable X-ray sources are reported. As part of this program, four Einstein HRI images of Cyg X-1 were examined. The analysis technique exploits the intrinsic aperiodic variability of this source to map the point response function of the optics. Any scattered halo present will not reflect short-term central source time variability, since such variability is smoothed by differential time delays of order days. Thus, a residual, nonvariable component to the surface brightness distribution (comprising about 12 percent or more of the source flux) is interpreted as a scattered halo. The Cyg X-1 halo is consistent with those of other sources found in previous studies using different techniques. Comparison is also made with a scattering model and, despite uncertainties in source spectrum and distance, reasonable agreement with the observatons is found using a standard interstellar grain model. The potential of X-ray scattering as a probe of the properties of interstellar grains is demonstrated.

Bode, M. F.↗
NASA NTRS1985

The X-ray halo of Cygnus X-1

Four Einstein HRI images of Cygnus X-1 were examined for the presence of a halo due to scattering of X-rays by interstellar grains. The analysis technique exploits the intrinsic aperiodic variability of the source to map the point response function of the optics. A residual, nonvariable component to the surface brightness distribution (comprising approximately more than 12 percent of the source flux) is interpreted as a scattered halo. The halo flux does not reflect the short term time variability of the central source as it is smoothed by differential time delays of order days. The Cygnus X-1 halo is consistent with those of other sources derived in previous studies using different techniques. Comparison is made with a scattering model, and the sensitivity of the halo flux to maximal grain size is demonstrated.

Bode, M. F.↗