Search NASA⌕ Search

Engineering topics

Wallerstein, G.

Publications and source records attributed to Wallerstein, G..

18 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Variable interstellar absorption toward HD 72127A. II - 1981-1988

The present study examines eight new echelle spectra of HD 72127A found between 1981 and 1988 near the Ca II K line, along with four similar spectra near the Na I D lines. In addition to the unrivaled intensity and width of this interstellar K line formed at a distance of not more than 500 pc, which were previously discovered by Thackeray (1974), the spectra show clearly the unique temporal variability of the interstellar absorption along this light path, which was reported in an earlier study by Hobbs et al. (1982). The new results strengthen further the hypothesis that the interstellar absorption toward this star occurs predominantly in the disturbed gas located within the SNR, in which the interstellar grains have been largely destroyed. The variable interstellar lines were found to consist of at least 120 components at the K line and of at least eight generally corresponding components at the D lines. The total column densities summed over all of these interstellar clouds varied irregularly by about 45 percent in five and about 38 percent in ten years, respectively.

Hobbs, L. M.↗
NASA NTRS1986

High-resolution CCD spectra of stars in globular clusters. I - Oxygen in M13

High-resolution (0.3 A) CCD spectra obtained at the 200 in. coude spectrograph have been analyzed for the abundances of O, Sc, Fe, and La in four stars in the globular cluster M13. Fe/H abundance is found to be = -1.6, as found by many other observers of this cluster. For three stars O/Fe abundance is found to be = +0.3 + or - 0.1, which is similar to O/Fe ratios in other globular clusters and metal-poor field stars. For star II-67, no oxygen line is visible at 6300 A and O/Fe abundance is found to be not greater than -0.4 (for a high carbon content) and not greater than -0.7 (for a low carbon content). The latter is more likely to be correct. Two possible explanations of the oxygen deficiency in II-67 are discussed: primordial deficiency, and CNO cycling at or above a temperature of 25,000,000 K.

Leep, E. M.↗
NASA NTRS1986

High resolution CCD spectra of stars in globular clusters. Part 2: Metals and CNO in M71

Palomar coude CCD spectra of resolution 0.3 and 0.6 has been used to redetermine abundances in five stars of the relatively metal rich globular cluster M71. The (Fe/H) value is restricted to the limits of -0.6 to -1.0. The largest source of uncertainty is a systematic difference in f-values between those derived via the Holweger-Muller (1974) solar model and the Bell et al. (1976) solar model. If we use absolute f-values measured by the Oxford group (Blackwell et al. 1982) we find Fe/H to lie in the range of -0.6 to -0.75, i.e., as given by using the Bell et al. solar model. The relative abundances of the light elements, i.e., Na through Ca and probably including Ti show an average excess relative to iron of 0.4 dex. The effect of this difference on metal indices derived from broad- and narrow- band photometry is discussed. For three stars we find O/H = -0.6 using absolute f-values. For CN an analysis of individual rotational lines of the 2-0 band of the red system yields lines in the (C/H,N/H) plane that are consistent with either an original C/Fe = N/Fe = 0 or a modest increase in N relative to C due to CN burning and mixing. A search for C-13N was not successful and an uncertain lower limit of C-12/C-13 near 10 was obtained.

Leep, E. M.↗
NASA NTRS1984

Winds in collision. I - Geometric implications of the emission lines for V 1016 Cyg and HM Sge

Emission line profiles and radial velocity data obtained from spectra in the visual region are presented for V1016 Cyg and HM Sge. The profiles of the forbidden emission lines are found to be inconsistent with spherical symmetry for the hot nebular material. It is argued that part of the nebular material in a detached binary system where both stars have spontaneous winds will naturally assume the shape of a curved, roughly conical, shell. The data can be satisfactorily interpreted in terms of the interaction of a low-velocity wind from a red giant with a high-velocity wind from a white dwarf in a detached binary system. This scheme provides a natural assignment of emission line forming regions to different portions of the interaction shell and the two winds, explaining both the profiles and the velocities of the features.

Willson, L. A.↗
NASA NTRS1984

Winds in collision. II - An analysis of the X-ray emission from the eruptive symbiotic HM Sge

X-ray emissions from HM Sge obtained in 1981 from the HEAO-2 satellite are analyzed and compared quantitatively with observations of HM Sge made in 1980 and of HM Sge, V 1016 Cyg, and RR Tel made in 1979. The change in the X-ray emission from HM Sge between 1979 and 1981 is found to be consistent with the X-ray luminosity and/or temperature of the emitting region declining with an e-folding timescale of the order of one to several decades. Comparison with X-ray data from V 1016 Cyg and RR Tel gives a composite X-ray light curve that is also consistent with such a decline. A comparison of the X-ray observation with spectroscopic information makes it possible to constrain the properties of the X-ray emitting region: the result is consistent with emission from an optically thin region between the two stars in the system where their winds collide head on. It is also shown that the observations are inconsistent with a stellar (blackbody) source, with emission from an accretion disk around a white dwarf or a neutron star, and with emission from a single star wind from either a white dwarf or a neutron star.

Willson, L. A.↗
NASA NTRS1984

High-resolution IUE observations of interstellar absorption lines in the Vela supernova remnant

Ultraviolet spectra of 45 stars in the vicinity of the Vela supernova remnant were recorded by the short-wavelength echelle spectrograph aboard the International Ultraviolet Explorer (IUE). Over one-third of the stars show interstellar absorption lines at large radial velocities (greater than 60 km/s). The mapping of these high-velocity components in the sky suggests the motions are chaotic, rather than from a coherent expansion of the remnant material. In accord with earlier conclusions from Copernicus data, the gas at high velocity exhibits higher than normal ionization and shows substantially less depletion of nonvolatile elements than normal interstellar material at low velocities. Relatively strong lines from neutral carbon in the two excited fine-structure states indicate that the neutral clouds within the remnant have had their pressures enhanced by the passage of the blast wave from the supernova. Also, the remnant seems to show a significant enhancement in the abundances of low-velocity Si IV, C IV, and N V over those found in the general interstellar medium.

Jenkins, E. B.↗
NASA NTRS1982

A brightening of the symbiotic variable SY Muscae

The symbiotic variable SY Muscae has been observed with IUE in September 1980 and June 1981 and in the photographic region in May 1981. The entire ultraviolet spectrum brightened between September and June by about a factor of 5. The spectrum shows high excitation including emission from N v and high electron density, about 10-billion per cu cm as determined from various line ratios in the ultraviolet. The optical spectrum is dominated by permitted lines; even forbidden O III is very weak again indicating high density in the ionized region. The increase in ultraviolet continuum and line emission may be due to enhanced mass transfer from the cool star whose period is 623d and whose maximum was predicted to occur very close to the time of the June 1981 observations. Alternatively the hot star and much of the emitting gas could have been in eclipse in September 1980.

Michalitsianos, A. G.↗
NASA NTRS1982

A new interstellar component in the spectrum of HD 72127A

New high dispersion observations are reported of the very strong, broad interstellar K line of Ca II in the spectrum of HD 72127A, a star located near a filament of the Vela supernova remnant. When compared with similar observations made in 1977, the new data reveal temporal variability of the interstellar absorption, as shown both by the presence of a new, sixth line component and by a 25% increase in the total equivalent width of the K line. All five Ca II components which were seen in both years show very large column-density ratios N(Ca II) /N(Na I) at least equal to 9, probably arising from anomalously large interstellar gas-phase abundances of Ca caused by disruption of interstellar grains. Marked differences in the structure of the K line between the two early-type components of this binary star, which are separated by only 3000 AU, are confirmed.

Hobbs, L. M.↗
NASA NTRS1981

A compressed cloud in the Vela supernova remnant

Strong interstellar absorption lines of C I, arising from the two excited fine-structure levels, are found in IUE observations of HD 72350 (type B4 III). An analysis of the excited-level populations of C I gives local temperature and pressure limits, and auxiliary data on the limit of column density for excited O I and the carbon ionization help to establish that (1) the local temperature is within the limits of 25-100 K, and (2) the pressure/Boltzmann's constant ratio is at least 10 to the 4.3/cu cm K, despite its small size. This high-pressure cloud is discussed in terms of shock compression by the Vela supernova blast wave, along with the relationship of this kind of cloud compression to star formation and to the origin of the characteristic filamentary emission arcs seen in Vela and other supernova remnants

Jenkins, E. B.↗
NASA NTRS1981

Changes in the high state of AM Herculis - A simultaneous X-ray, optical, polarimetric, and spectroscopic study

Simultaneous UBVR photometry, circular polarimetry, and high resolution spectroscopy observations (during May, 1979), coordinated with a monitor proportional counter and a solid state spectrometer, are reported for the cataclysmic variable AM Her, when it was in its normal 'high' state, where the mean visual magnitude was 12.5 mag versus 15 mag for the infrequent 'low' state. The circular polarization was always negative and had the smallest peak to peak amplitude yet reported. The emission lines He II 4686, H-beta, N III 4634, N III 4640-4641, C III 4650-4651, and He I 4713 present at least two components identified with the broad and sharp components of Greenstein at al. (1977). The radial velocity curve of the broad component shows a 50% reduction of amplitude in comparison to their values, and a third very broad component in 4686 is identified. No correlation between the optical photometry and 1-10 keV X-ray intensity was found on flickering time scales. Results of the observations are discussed within the framework of models with accretion onto one or both magnetic poles of the white dwarf.

Crosa, L.↗
NASA NTRS1981

A compressed cloud in the Vela supernova remnant

To elucidate the nature of the interstellar medium in the vicinity of the Vela supernova remnants (SNR) an extensive study with the International Ultraviolet Explorer of interstellar absorption lines toward 35 stars in the vicinity of the Vela SNR was undertaken. Observations of interstellar absorption, in particular of CI, towards one of these stars, HD 72350 (type B4 III), is of particular interest.

Jenkins, E. B.↗
NASA NTRS1980

Interstellar gas in the Gum Nebula

A survey of the interstellar gas near the Gum Nebula by optical observation of 67 stars at Ca II, 42 stars at Na I, and 14 stars in the UV with the Copernicus satellite provided radial velocities and column densities for all resolved absorption components. Velocity dispersions for gas in the Gum Nebula are not significantly larger than in the general interstellar medium; the ionization structure is predominantly that of an H II region with moderately high ionization. Denser, more highly ionized clouds are concentrated toward the Gum Nebula; these clouds do not show the anomalously high ionization observed in the Vela remnant clouds.

Wallerstein, G.↗
NASA NTRS1980

Velocity structure in long period variable star atmospheres

A regression analysis of the dependence of absorption line velocities on wavelength, line strength, excitation potential, and ionization potential is presented. The method determines the region of formation of the absorption lines for a given data and wavelength region. It is concluded that the scatter which is frequently found in velocity measurements of absorption lines in long period variables is probably the result of a shock of moderate amplitude located in or near the reversing layer and that the frequently observed correlation of velocity with excitation and ionization are a result of the velocity gradients produced by this shock in the atmosphere. A simple interpretation of the signs of the coefficients of the regression analysis is presented in terms of preshock, post shock, or across the shock, together with criteria for evaluating the validity of the fit. The amplitude of the reversing layer shock is estimated from an analysis of a series of plates for four long period variable stars along with the most probable stellar velocity for these stars.

Pilachowski, C.↗
NASA NTRS1976

Ultraviolet absorption lines associated with the Vela supernova remnant

Two stars behind the Vela supernova remnant and two stars offset from the remnant have been observed with the UV spectrometer aboard the Copernicus satellite. Over 200 interstellar atomic and molecular absorption features between 1000 and 1400 A have been identified and measured for radial velocity and equivalent width. In many cases, additional information was obtained by studying the detailed shapes of the recorded profiles. Most of the stars show several absorption components, with clouds of the highest radial velocity appearing in the spectra of stars behind the remnant. For each component, column densities were derived using velocity dispersion parameters which yielded the most self-consistent results. Qualitatively, the gas toward the remnant exhibits a number of unusual properties, when compared with normal interstellar material. First, abnormally high radial velocities were evident. Second, the degree of ionization of some elements suggested the existence of ionizing processes significantly more potent than those found in general regions of space. Finally, an investigation of electron densities shows that much of the gas, especially that at high velocity, must exist in the form of relatively thin sheets or filaments. If cosmic abundances prevail, the column densities of high-velocity excited material suggest that H-alpha emission measures could be as large as 100 sq cm/cu pc.

Jenkins, E. B.↗