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Bohm-Vitense, E.

Publications and source records attributed to Bohm-Vitense, E..

15 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Problems with the Baade-Wesselink method

The discrepancy noted in radii obtained by the Baade-Wesselink method when different colors are used to determine the effective temperatures is explored. The discrepancy is found to be due to an inconsistency in the applied temperature-color calibrations. The assumption of the maximum likelihood method that beta (the effective temperature + 0.1 times the bolometric correction) is a linear function of the color is valid for the B-V and V-I colors, but not for the V-R colors. It is suggested that the errors introduced by the nonlinearity in the relation between beta and the V-R colors will produce radii which are too large. The radii derived from the V-B colors appear to be too small.

Bohm-Vitense, E.↗
NASA NTRS1986

The Cepheid mass problem and Cepheid binaries

Existing mass determinations for Cepheids with different periods are examined. Wesselink masses are independent of the adopted distance scale. For short periods (less than 6 days) they follow the sequence of evolutionary masses. For periods longer than 10 days they are lower by up to a factor of 2. The lower mass branch joins up with the bump masses. The new pulsational masses agree with the Wesselink masses for periods longer than 6 days. Cepheid masses determined by means of their giant companions also agree with the Wesselink masses and the new pulsational masses. While the error bars are large, the derived dynamical masses determined for S Mus and V636 Sco also agree with the low Wesselink and giant companion masses.

Bohm-Vitense, E.↗
NASA NTRS1985

Cepheid distances from blue main-sequence companions

The absolute visual magnitudes of main-sequence Cepheid companions are here determined from their effective temperatures. The latter are obtained by comparing the measured relative energy distributions with model atmosphere energy distributions. Assuming an average Galactic extinction law, it is found that the distance moduli for the Cepheids should be smaller by Delta (mv-Mv) = -0.5 as compared to the Sandage-Tammann relation.

Bohm-Vitense, E.↗
NASA NTRS1985

Blue companions of Cepheids

Twenty-one Cepheids, known or suspected to have blue companions, were studied with the International Ultraviolet Explorer satellite. For 13 of them, companions were indeed seen, though they were generally fainter in the UV than expected. For four Population I Cepheids, the suspected companions were not seen. For none of the Population II Cepheids could a companion be detected. The effective temperatures and luminosities of the companions which could be observed are discussed, and the positions of Cepheids and companions in the T(eff) luminosity diagrams are compared with positions expected from stellar evolution calculations.

Bohm-Vitense, E.↗
NASA NTRS1985

Detection of a compact companion of the mild barium star Xi-1 Ceti

In the present paper, the observation of a white dwarf companion of the mild Ba star Xi-1 Ceti (= 65 Ceti = HR 649 = HD 13611) is reported, taking into account also the properties of the mild Ba star and of its companion. The UV spectrum of Xi-1 Ceti is discussed along with an interpretation of this spectrum. Attention is given to the effective temperature of the companion, the absorption bands in the spectrum, the radius and mass of the Xi-1 Ceti companion, and questions regarding the obscuration of the companion by the atmosphere of the Ba star. It is found that the overall energy distribution of the Xi-1 Ceti companion can best be matched with a 14,000 K DA white dwarf of log g = 8 or less. However, the absolute intensity is too small and would require a radius too small and a mass too large for such a gravity.

Bohm-Vitense, E.↗
NASA NTRS1985

The unexpected ultraviolet variability of Herbig-Haro object 1

The line fluxes of the C IV 1550 A and semiforbidden C III 1909 A emission lines in HH 1 are noted to have monotonically decreased by factors of 4-6 between 1979 and 1983, despite a lack of indications of optical range changes. The thickness of the shocked layers and preshock density can be estimated on the basis of these rapid changes, with results suggesting a clumpy medium that leads to truncated shock waves with column densities insufficient for the development of complete recombination zones.

Brugel, E. W.↗
NASA NTRS1985

Ultraviolet studies of stars in the populous cluster NGC 2100 in the Large Magellanic Cloud

The ultraviolet energy distributions of the B stars in the LMC cluster NGC 2100 have been studied. The ultraviolet extinction law was studied for the LMC cluster. The T(eff)-luminosity diagrams were determined. The brightest stars in the cluster are evolved stars on the 'horizontal' supergiant branch. No P Cygni profiles indicating mass loss were recognized for these supergiants.

Bohm-Vitense, E.↗
NASA NTRS1984

Ultraviolet analysis of the peculiar supergiant HD 112374 = HR 4912

The ultraviolet energy distribution of the metal-poor supergiant HD 112374 is analyzed based on observations from the International Ultraviolet Explorer (IUE) satellite for the region between 1200 and 2000 A. A discontinuity was found in the UV spectra at 2600 A which confirmed the low-abundance of heavy elements found by Luck et al. (1983). Values for effective temperature and log g in HD112374 were consistent with the star being a very luminous Population II semi-regular variable. The full observational results are presented in a table.

Bohm-Vitense, E.↗
NASA NTRS1984

The problem of the barium stars

Ultraviolet observations of barium stars and other cool stars with peculiar element abundances are reported. Those observations attempted to find hot white dwarf companions. Among six real barium stars studied, only Zeta Cap was found to have a white dwarf companion. Among seven mild, or marginal, barium stars studied, at least three were found to have hot subluminous companions. It is likely that all of them have white dwarf companions.

Bohm-Vitense, E.↗
NASA NTRS1984

The ultraviolet spectrum of a strongly reddened, high-excitation Herbig-Haro object

Short- and long-wavelength IUE spectra of the rather strongly reddened, high radial velocity Herbig-Haro object HH 32A have been obtained. The object is well known for its large positive radial velocity and for its unusually large velocity dispersion. In consequence of the large ultraviolet extinction, the spectra are faint. The semiforbidden C III 1909 and Mg II 2800 emission lines are, however, clearly detected, and their fluxes can be measured with reasonable accuracy. C IV 1550 and Si III 1892 are very probably present. The continuum can be detected convincingly in the wavelength range 1500-1900 A. In other regions it is just barely detectable. The continuum has an energy distribution which is probably very similar to that of HH 1 and HH 2H. If an average galactic extinction curve is used, the line fluxes for semiforbidden C III 1909 and C IV 1550 indicate a shock velocity of at least 140 km/s which is higher than for HH 1 or HH 2. If a Theta Ori-type extinction curve is used instead, ultraviolet line ratios are obtained which are rather similar to those in HH 1 and HH 2.

Bohm, K. H.↗
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 NTRS1983

Einstein observations of three classical Cepheids

We have looked for X-ray emission from the classical Cepheids delta Cep, beta Dor, and zeta Gem during phases when the latter two stars show emission in low excitation chromospheric lines. No X-ray flux was detected except possibly from zeta Gem at phase 0.26. Derived upper limits are in line with emission flux or upper limits obtained for other F and G supergiants.

Bohm-Vitense, E.↗
NASA NTRS1981

The ultraviolet spectrum of Herbig-Haro Object 1

Continuum and emission line fluxes for HH 1 in the range of 1200-1300 A have been determined using IUE spectra, and it is found that the luminosity of HH 1 in this wavelength interval is slightly larger than 1 solar luminosity, or 20 times greater than for the wavelength range 3200-11,000 A. If averaged over intervals of 100 A, it is shown that the UV continuum rises almost monotonically toward shorter wavelengths over the entire interval 1300-3000 A. The UV continuum energy distribution cannot be explained by a T Tauri stellar continuum that is scattered by dust. Emission line identifications and flux measurements strongly confirm the statement that the high-ionization emission lines are too strong to be explained by shock-wave models derived from the emission line spectrum in the interval 3700-11,000 A. A possible explanation for this fact is discussed.

Boehm, K. H.↗