Engineering topics

Bohm-Vitense, Erika

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

14 records

NASA NTRS1994

The lithium content and other properties of F2-G5 giants in the Hertzsprung Gap

As stars of 2-5 solar mass evolve across the Hertzsprung Gap they should first deplete their surface lithium by convective dilution and then, when convection penetrates deeper, begin to bring CN processed material to their surfaces. To investigate this process we have observed 52 giants, 25 of which have known C/N ratios, for their Li abundances. After eliminating four stars that may actually be dwarfs and including the two components of Capella analyzed by Pilachowski and Sowell we have compared our (Li/Fe) ratios with models of Swenson. For stars showing v sin i greater than 50 km/s we find (Li/Fe) to be uneffected by mixing for B - V less than 0.7 as predicted. For stars cooler than B - V = 0.7 both v sin i and (Li/Fe) drop to smaller values. For the sharp lined stars (v sin i less than 50 km/s) we find a drop in Li between B - V = 0.45 and 0.60 which cannot be understood in terms of dilution by convection. Various possible causes of such an early depletion or dilution of surface Li are discussed including diffusion at the base of the convection zone, mass loss possibly enhanced by pulsation, and magnetic activity as in the magnetic A and B type stars. The models of Richer & Michaud (1993) with diffusion point toward a satisfactory solution. A few giants with low v sin i values stand out with much higher than expected (Li/Fe) values despite their cool effective temperatures. We do not understand why those stars have not depleted their lithium as have most giants of similar color. The correlation of (N/C) with (Li/Fe) follows expectations in so far as almost all stars with enhanced (N/C) have depleted their Li as well.

Wallerstein, George↗

NASA NTRS1994

The two period-luminosity relations for population I Cepheids

We summarize the evidence that most Population I Cepheids with periods less than approximately 8 days pulsate in the first overtone mode. Fundamental model and first overtone pulsators must follow different period-luminosity (P - L) relations. We demonstrate these different relations for different stellar systems, especially for the calibrating Cepheids in clusters and for Cepheids in the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), M31, and IC 4182.

Bohm-Vitense, Erika↗

NASA NTRS1994

Emission lines in the long-period Cepheid l Carinae

For the Cepheid l Carinae, with a pulsation period of 35.5 days, we have studied the emission-line fluxes as a function of pulsational phase in order to find out whether we see chromospheric and transition-layer emission due to an outward-moving shock. All emission lines show a steep increase in flux shortly before maximum light, suggestive of a shock moving through the surface layers. The large ratio of C IV to C II line fluxes shows that these are not transition-layer lines. During maximum light the large ratio of the large ratio of the C IV to C II line fluxes also suggests that we see emission from a shock with velocities greater than 100 km/s such that C IV emission can be excited. With such velocities mass outflow appears possible. The variations seen in the MG II line profiles show that there is an external absorption over a broad velocity band independent of the pulsation phase. We attribute this absorption to a circumstellar 'shell.' This 'shell' appears to be seen also as spatially extended emission in the O I line at 1300 A, which is probably excited by resonance with Ly beta.

Bohm-Vitense, Erika↗

NASA NTRS1991

Heating mechanism(s) for transition layers in giants

The emission-line fluxes of lines originating in the lower parts of the transition layers between stellar chromospheres and coronas are studied. Simon and Drake (1989) suspect different heating mechanisms for 'hot' and cool stars. Changes in the flux ratios for the C IV to C II emission lines support this suspicion. Large C IV/C II line flux ratios appear to be indicative of magnetically controlled heating. A correlation between excess continuum flux around 1950 A and C II emission-line fluxes are confirmed for the cooler giants (late F and cooler). Excess continuum flux correlates positively with large C IV/C II line flux ratio. The excess continuum flux corresponds to an increase in temperature by several hundred degrees in layers with a mean optical depth of about 0.03. For chromospherically active stars these layers experience a mechanical flux deposition of the order of 1 percent of the total radiative flux. This flux is tentatively identified as an MHD wave flux similar to Alfven waves.

Bohm-Vitense, Erika↗

NASA NTRS1990

Ultraviolet gas absorption and dust extinction toward M8

Interstellar absorption lines are analyzed using high-resolution IUE spectra of 11 stars in the young cluster NGC 6530 located in the M8 region. High-velocity clouds at -35 km/s and -60 km/s are seen toward all cluster stars. The components arise in gases that are part of large interstellar bubbles centered on the cluster and driven by stellar winds of the most luminous members. Absorption lines of species of different ionization states are separated in velocity. The velocity stratification is best explained as a 'champagne' flow of ionized gas away from the cluster. The C IV/Si IV ratios toward the hotter cluster members are consistent with simple photoionization models if the gas-phase C/Si ratio is increased by preferential accretion onto dust grains. High ion column densities in the central cluster decline with distance from W93, suggesting that radiation from a hot source near W93 has photoionized gas in the central cluster.

Boggs, Don↗

NASA NTRS1990

The dynamical mass of S Muscae

The amplitudes and phase relations of pulsational velocities of the Cepheid S Muscae, measured for lines originating at different atmospheric depths, are investigated. A mass M(A) = 4.4 + or - 0.5 solar masses is found for the Cepheid; however, the Cepheid is the evolved star and should have a mass larger than that of its companion. If the measured velocity differences are too low by 2.5 km/s on the average, then the mass ratio M(B)/M(A) may be decreased to 0.94, giving an estimated upper limit to the Cepheid mass of 6.1 solar masses, consistent with the Cepheid being the more evolved star. Considering all the uncertainties, it is concluded that the mass of the Cepheid is between 4.6 and 6.1 solar masses.

Bohm-Vitense, Erika↗

NASA NTRS1989

Ultraviolet extinction differences within NGC 6530

Ultraviolet extinction curves are presented for 14 stars in the very young cluster NGC 6530. The variation of the short-wavelength extinction is consistent with that expected based on random errors alone, although the weakest extinction is found only toward the cluster center. Variations significantly larger than the measurement uncertainty are found in the area, width, and central peak position of the 2200 A extinction bump. These parameters are largest in the cluster center and decrease systematically outwards. Physical processes capable of modifying the grains in the H II region containing the cluster are discussed.

Boggs, Don↗

NASA NTRS1989

Effects of SN 1987A on the interstellar medium around it

It is shown that the interstellar spectrum from SN 1987A in the LMC shows a rather strong component in C IV and Si IV, and possibly a weak component in N V, at a velocity of about 300 + or - 10 Km/s. In the spectra of the neighboring stars HD38268 and R123, a component at this velocity is only seen in low-ionization lines, not in the C IV, or N V lines. The possibility is studied that SN 1987A ionized a measurable fraction of this 300 Km/s cloud within less than one day since the Si IV, C IV, and possibly, N V interstellar lines are seen only about one day after the first light from the supernova explosion was detected. This may be the case, but only if the supernova temperature at outburst was larger than 50,000 K, depending on the density assumed for the interstellar cloud. Also, H and He must have been preionized once.

Bohm-Vitense, Erika↗

NASA NTRS1988

ISM chemical abundances in two intermediate-velocity clouds in the line of sight to SN 1987A

The earliest IUE high-resolution spectra of SN 1987A have been studied and reveal the presence of several clouds in the line of sight to the LMC. In particular, there are two clouds with radial velocities of about 130 km/s and about 180 km/s. These clouds' velocities are between those of Galactic clouds at 0-80 km/s and those of LMC gas at about 270 km/s. Chemical-abundance determinations may help to determine the origin and location of these clouds. Curve-of-growth analysis and 21-cm observations show that they may be underabundant in heavy elements by about a factor of 2 as compared to solar abundances. No depletion indicative of grain formation can be seen.

Morgan, Siobahn↗

NASA NTRS1988

Are most short-period Cepheids overtone pulsators?

Data given and analyzed recently by Coulson et al. (1986) and Gieren (1982) provide a consistent set of Wesselink radii and Wesselink masses, for which the internal error bars appear to be small enough that a quantitative discussion appears to be warranted. When it is assumed that all Cepheids pulsate in the fundamental mode, a steep discontinuity in mass is found for periods around 6.5 days. When short-period Cepheids are interpreted as first-overtone pulsators as required theoretically (Christy, 1966; Stobie, 1969), the discontinuity disappears and the mass-period relation passes through the bump and beat masses for Cepheids.

Bohm-Vitense, Erika↗

NASA NTRS1987

Theory of transition-layer emission measures and coronae

The basic equations describing the energy equilibria, the conductive heat flux, and the temperature stratifications for stellar transition layers and coronae with 'open' field lines are summarized. The temperature dependence of the emission measures for lines originating in different temperature regions of the transition zone is determined. It is found that the stellar transition regions consist of two basically different parts: the lower part where mechnical energy input is balanced by the radiative losses, and the upper part where the mechanical energy input is balanced by the divergence of the conductive flux and radiative losses. In the lower part, the temperature stratification is determined by an equilibrium between mechanical flux input and radiative energy losses. The coronal temperatures increase with increasing mechanical flux and damping length in the upper transition zone.

Bohm-Vitense, Erika↗

NASA NTRS1987

A nonspherically symmetric model for the peculiar A star Alpha-2

Observations show that in the optical region the peculiar A star Alpha-2 CVn has a flatter energy distribution during maximum light than during minimum light. This indicates that during maximum light a lower-temperature region, but necessarily larger surface area, than during minimum light is seen. This suggests a nonspherically symmetric star, which is oblate with respect to the magnetic axis and which is cooler at the magnetic poles than at the magnetic equator. The light variations of such nonspherically symmetric oblique rotator models have been studied. It is found that, for an oblate ellipsoid with an axial ratio of 0.92 and a temperature difference of about 1000 K between the poles and the equator, the overall variations of the optical and the ultraviolet energy distributions can be well understood.

Bohm-Vitense, Erika↗

NASA NTRS1987

Intrinsically variable stars

The characteristics of intrinsically variable stars are examined, reviewing the results of observations obtained with the IUE satellite since its launch in 1978. Selected data on both medium-spectral-class pulsating stars (Delta Cep stars, W Vir stars, and related groups) and late-type variables (M, S, and C giants and supergiants) are presented in spectra, graphs, and tables and described in detail. Topics addressed include the calibration of the the period-luminosity relation, Cepheid distance determination, checking stellar evolution theory by the giant companions of Cepheids, Cepheid masses, the importance of the hydrogen convection zone in Cepheids, temperature and abundance estimates for Population II pulsating stars, mass loss in Population II Cepheids, SWP and LWP images of cold giants and supergiants, temporal variations in the UV lines of cold stars, C-rich cold stars, and cold stars with highly ionized emission lines.

Bohm-Vitense, Erika↗

NASA NTRS1987

A possible solution to the Cepheid mass problem?

With new, smaller distances of the Cepheids, as determined recently by Schmidt (1984) and by Bohm-Vitense (1985), smaller pulsational masses are obtained than previously. Giant companions of Cepheids show that the luminosities of the Cepheids are too large in comparison with those of the giants. If increased mixing is responsible for this, then an increase in the luminosity of the Cepheids of a given mass by approximately a factor of 4 (as compared to conventional evolution calculations) is expected. Taking into account both of these effects good agreement is found between the corrected evolutionary masses, the pulsational masses, the dynamical masses, the giant companion masses, and the Wesselink masses. The bump masses are only slightly smaller than the other masses.

Bohm-Vitense, Erika↗