Engineering topics

Boehm-Vitense, E.

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

At least 19 records

NASA NTRS1984

The ultraviolet spectra of the O and B stars in the young galactic cluster NGC 6530

The UV spectra between 1200 and 3000 A of stars in the young galactic cluster NGC 6530 and the surrounding association are studied. From the UBV colors and empirical as well as theoretical calibrations, the T(eff) and L for those stars which follow a sequence in the H-R diagram corresponding to the main sequence are determined. From a comparison with theoretical evolutionary tracks, the age of the cluster is estimated to be 5 + or - 2 x 10 to the 6th yr, with a very small scatter for the different stars. The UV extinction is determined for the stars from a comparison of theoretical model energy distributions for the stellar T(eff)s and the observed energy distributions. The stellar wind lines are studied, and strong stellar winds are found for bolometric magnetidues less than -8.

Boehm-Vitense, E.↗

NASA NTRS1984

Search for white dwarf companions of cool stars with peculiar element abundances

A search for a white dwarf companions of cool stars with peculiar element abundances was undertaken. One additional star the xi Cet, was found with a white dwarf companion. It was found that HR 1016, 56Uma, 16 Ser, have high excitation emission lines which indicate a high temperature object in the system. It is suggested that since these indications for high temperature companions were seen for all nearby Ba stars, it is highly probable that all Ba stars have white dwarf companions, and that the peculiar element abundances seen in the Ba stars are due to mass transfer. Observations, arguments and conclusions are presented. White dwarf companions were not found. Together with the Li and Be abundances and the chromospheric emission line spectra in these stars were studied. No white dwarf companions were seen for subgiant CH stars.

Boehm-Vitense, E.↗

NASA NTRS1984

Ultraviolet studies of O and B stars in the LMC cluster NGC 2100, the SMC cluster NGC 330 and the Galactic cluster NGC 6530

High-resolution and low-resolution IUE spectra of O and B stars in the LMC cluster NGC 2100, the SMC cluster NGC 330, and the young Galactic cluster NGC 6530 are investigated. Temperatures and luminosities are determined. In the LMC and SMC clusters, the most luminous stars are evolved stars on the horizontal supergiant branch, while in NGC 6530 the stars are all still on the main sequence. Extinction laws were determined. They confirm the known differences between LMC and Galactic extinctions. No mass loss was detected for the evolved B stars in the LMC and SMC clusters, while the high-luminosity stars in NGC 6530 show P Cygni profiles.

Boehm-Vitense, E.↗

NASA NTRS1982

The interstellar absorption-line spectrum of Mu Ophiuchi

UV interstellar lines have been measured on high-resolution, long- and short-wavelength IUE spectra of the B8 V star Mu Oph. Column densities for the observed atoms and ions have been determined as well as turbulent velocities. The interstellar spectrum of Mu Oph is similar to the ones for Rho Oph and Zeta Oph. The ionization equilibria of several elements give consistent limits for the electron density. The C I line arising from different fine-structure levels are studied to yield estimates on the physical conditions in the cloud. Relative depletion of elements in the cloud seen in the interstellar spectrum of Mu Oph follows the same pattern as seen in the interstellar spectra of Zeta Oph and six other stars in the Rho Oph cloud complex.

Cardelli, J.↗

NASA NTRS1982

The ultraviolet continuous and emission-line spectra of the Herbig-Haro objects HH 2 and HH 1

Recent studies of the continuous spectrum of Herbig-Haro (HH) objects at optical and near-infrared wavelengths and the observation of continuous radiation in the ultraviolet have shown an unexpectedly steep increase of the flux toward shorter wavelengths. The present investigation provides the results of ultraviolet observations of HH 2. The obtained data are compared with the HH 1 data. It is found that HH 2 has an ultraviolet continuous and emission-line spectrum which is similar to that of HH 1. The UV line spectrum of HH 2H indicates an even somewhat larger ionization than does the HH 1 spectrum. As in HH1, the UV emission-line spectrum shows a much higher degree of ionization than that derived from the optical spectrum. Consequently, the same difficulty arises as in the case of HH 1. The complete UV plus optical spectrum cannot be explained by a single plane-parallel shock-wave model.

Boehm-Vitense, E.↗

NASA NTRS1982

The Mg II resonance line emission at 2800 A in stars with different rotational velocities and different metal abundances

In many investigations, a study of the Mg II 2800 A emission was conducted in order to derive more information about the correlation of chromospheric emission with the properties of the convection zone. It has been suggested by Ayres and Linsky (1980) that there is a positive correlation between rotation and chromospheric emission at least for binary stars. The present investigation is concerned with such a correlation between rotation and chromospheric emission. Since large differences in rotational velocities are observed for F stars for which also strong chromospheric emission is observable, this particular study deals mainly with F stars. On the basis of observations of the Mg II emission in F and G stars, it is found that the emission is abnormally strong in close binary F stars. This may be explained by the generation of a large differential rotation due to tidal effects which may give very efficient dynamo generation of a surface magnetic field.

Boehm-Vitense, E.↗

NASA NTRS1982

Effective temperatures of A and F stars

Effective temperatures of late A and early F stars are determined from the observed fluxes in the visual at 1900 A and 1420 A. The observed ratios are compared with those calculated by Kurucz (1979). A correction of the theoretical fluxes at 1900 A brings the effective temperatures obtained from different ratios into reasonable agreement. The effective temperatures determined in this way for late A stars agree well with those obtained from the optical region. For F stars, however, the effective temperatures obtained from the UV are found to be higher than those obtained from the optical region if radiative equilibrium models are used for the comparison. It is thought that this discrepancy may derive from the effects of temperature, pressure, and absorption coefficient inhomogeneities caused by convection.

Boehm-Vitense, E.↗

NASA NTRS1982

Suggested UV spectral classification criteria for A stars

Criteria for determining effective temperature and electron density independently of abundances, for A, late B, and F stars are given. Comparison of IUE data with standard classification techniques, shows that for late B and A stars, temperature classification by UV spectra is more sensitive than classification using visual spectra. The long wavelength wing of the Lyman is recommended for these stars. For A and F stars, the ratio of the Mg (2580 A) and Mg+ (2800A) resonance lines gives a good temperature/electron density (ne) criterion which is nearly independent of metal abundance (Z). The SiI discontinuities give an excellent temperature classification for A and F stars, but it is dependent on Z and ne. If the absolute intensities of either of the Mg lines are added to the criteria, three classification criteria for a three dimensional classification, according to temperature, ne and Z, are obtained.

Boehm-Vitense, E.↗

NASA NTRS1982

Ultraviolet spectra of Herbig-Haro objects and of the environment of the Cohen-Schwartz star

Observations in both spectral regions were used in order to determine the continuous energy distribution and the emission line fluxes for H-H 2. The continuous spectrum is similar to that in H-H 1 and F lambda increases rapidly towards shorter wavelengths. This statement is found to be qualitatively correct for all obvious choices of the ultraviolet extinction curve if we use the E(B-V) value determined by the use of the S II method. The origin of the continuum remains enigmatic. The emission line spectrum of H-H 2 shows an even somewhat higher degree of ionization than the spectrum of H-H 1, indicating an even larger discrepancy between ionization information from optical data and that contained in the UV spectra. The immediate environment of the Cohen-Schwartz star emits a continuous spectrum similar to that of a H-H-object but increasing even more steeply towards shorter wavelengths.

Boehm, K. H.↗

NASA NTRS1982

Chromospheric, transition layer and coronal emission of metal deficient stars

It is shown that while MgII k line emission decreases for metal deficient stars, the Ly alpha emission increases. The sum of chromospheric hydrogen and metallic emission appears to be independent of metal abundances. The total chromospheric energy loss is estimated to be 0.0004 F sub bol. The chromospheric energy input does not seem to decrease for increasing age. The transition layer emission is reduced for metal deficient stars, but it is not known whether the reduction is larger than can be explained by curve of growth effects only. Coronal X-ray emission was measured for 4 metal deficient stars. Within a 12 limit it could still be consistent with the emission of solar abundance stars.

Boehm-Vitense, E.↗

NASA NTRS1982

Chromospheric, transition layer and X-ray emission for stars with different rotational velocities

In agreement with previous findings for the MgII k line emission in F stars an increase of Lya and transition layer emission with increasing V sub r sin i, if v sub r sin i greater than 30 km/sec. was not found. For V sub r sin i 30 km/sec., the measured line intensities are consistent with an increase in emission with increasing V sub r sin i. Such a relation between emission and rotation for single stars is also in agreement with X-ray observations. For the young F stars in the Hyades we find generally enhanced emission independently of rotational velocities. The enhancement is most pronounced for low excitation lines.

Boehm-Vitense, E.↗

NASA NTRS1981

The ultraviolet energy distributions of late A stars

Observed late A star energy distributions for the wavelength range 1400-2500 A are compared. No difference is found between energy distributions of Am stars and those of normal slowly rotating A stars. The fluxes of rapidly rotating stars, however, appear to be increased for wavelengths smaller than 1530 A; this cannot be understood as an effect of pole heating or reduced gravity. In addition, the comparison of the UV energy distributions with model atmosphere energy distributions of Kurucz indicates some problems with the theoretical Si I absorption edges at 1530 A.

Boehm-Vitense, E.↗

NASA NTRS1981

Interstellar absorption in the Mg II resonance line k2 and h2 emissions

High-resolution (0.2 A) IUE spectra for the long wavelength range (1800-3000 A) have been studied. It is shown that narrow interstellar Mg II lines are seen in the center of the k2 and h2 emissions from nearby stars with large rotational velocities. For all observed stars, the radial velocity of the central k3 absorption component in the rest system of the star is strongly correlated with the mirror image of the radial velocity of the stars; this shows that a major fraction if not all of the k3 absorption is due to interstellar absorption in the solar neighborhood. The violet to red asymmetry of the k2 emission also correlates with the radial velocities of the star; this shows that the shift of k3 is due to the velocity shift of the local interstellar cloud with respect to the star.

Boehm-Vitense, E.↗

NASA NTRS1981

Outlook for ultraviolet astronomy

A brief overview of galactic and extragalactic research is given with emphasis on the problems of temperature determination, chemical abundance determination, and the question about the energy sources for the high temperature regions. Stellar astronomy, stellar winds, and the interstellar medium are among the topics covered.

Boehm-Vitense, E.↗

NASA NTRS1981

The chromospheric and transition layer emission of stars with different metal abundances

Preliminary results on observations of chromospheric and transition layer emission of stars with different metal abundances are reported. Metal deficient stars generally show reduced emission in the Mg II resonance lines and also in the other chromospheric and transition layer emission lines. This is interpreted as showing that energy fluxes other than acoustic fluxes must at least be coresponsible for the coronal and transition layer heating.

Boehm-Vitense, E.↗

NASA NTRS1981

The white dwarf companion of the B a 2 star zeta Cap

The Ba II star zeta Cap has a white dwarf companion. Its T (sub eff) is determined to be 22000 K, its mass is approximately one solar mass. The importance of this finding for the explanation of abundance peculiarities is discussed.

Boehm-Vitense, E.↗

NASA NTRS1981

Far-ultraviolet energy distributions of the metal-poor A stars HD 109995 and HD 161817

Low-resolution IUE spectra at wavelengths between 1300 and 3400 A of the metal-poor stars HD 109995 (A1p) and HD 161817 (A4p) have been compared with model-atmosphere energy distributions computed by Kurucz (1979). Good overall agreement is found. Effective temperatures, metal abundances, and angular diameters could be determined. Assuming an absolute visual magnitude of 0.7, the previously determined gravity log = 3 yields masses of 0.5 solar masses for both stars. It is found that the theoretical UBV colors calculated earlier agree reaonably well with the ones observed for these stars.

Boehm-Vitense, E.↗

NASA NTRS1980

The boundary line in the H-R diagram for stellar chromospheres and the theory of convection

Ultraviolet emission-line spectra of A, F, and early G stars have been observed with the International Ultraviolet Explorer. For supergiants, classical chromospheric and transition-layer emission is seen only on the red side of the Cepheid instability strip. For luminosity classes III-V, chromospheric emission can be detected for spectral types F2 and later. For none of the A stars was normal chromospheric emission detected, regardless of their rotational velocities or peculiarities (i.e., Am or Ap).

Boehm-Vitense, E.↗