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Molnar, M. R.

Publications and source records attributed to Molnar, M. R..

13 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

OAO 2 observations of the Alpha Persei cluster

UV photometry of the Alpha Persei cluster obtained with the OAO 2 satellite is analyzed. The data are used together with ground-based coude spectra to examine 20 stars ranging in spectral type from B3 to A3 for rotation effects as predicted by the models of Collins (1974) and of Collins and Sonneborn (1977). Values of v sin i are estimated on the recalibrated system of Slettebak et al. (1975), and an H-R diagram is plotted for the program stars. The distribution of the stars on the diagram is compared with the ZAMS for a chemical composition of X = 0.7 and Y = 0.02 as well as with two isochrones for logarithmic nuclear ages (in years) of 7.5 and 7.7. It is found that there is a seemingly narrow band of early-type stars lying a magnitude or so above the ZAMS on the plot, while the later types form a looser collection straddling the ZAMS. The data for Alpha Persei itself yield an upper limit of 7.4 to the logarithmic age. The analysis for rotation effects reveals one late-type star that does not conform to the model predictions and one 'weak helium' star. Evidence is discussed that these stars are not pole-on rapid rotators. It is concluded that there is a photometric effect for stars earlier than about B6 whereby these stars are displaced redward in color-magnitude diagrams.

Molnar, M. R.↗
NASA NTRS1977

Copernicus observations of the Ap star Epsilon Ursae Majoris

Spectral scans of the Ap star Epsilon UMa made with the Copernicus satellite show strong line blanketing from profuse Cr II and Fe II lines. In the spectral region covering 1900 to 3000 A, about 500 lines are present which suppress the apparent continuum by at least 15-30%. An accurate line-identification list is compiled showing Eu II present in addition to Mn II and Ni II. The identification of Eu II, however, rests on very stringent identification limits for Fe II. If these are relaxed, the existence of Eu II is dubious. There are no broad features in this spectral region which would suggest strong photoionization discontinuities by metals, but one feature near 2137 A might contain the photoionization edge due to Cr I 5S lying 0.94 eV above the ground level. However, a significant correlation between the line-blanketing strength and the amplitude of the OAO-2 ultraviolet light curves was found such that both monotonically increase in the same proportion toward shorter wavelengths. This gives additional strength to the suggestion that variations in the metal line-blanketing cause the observed photometric variations.

Mallama, A. D.↗
NASA NTRS1976

The ultraviolet variations of iota Cas

The Ap variable star iota Cas was observed with the photometers on OAO-2 covering the spectral range 1430-4250 A. The ultraviolet light curves show a double wave with primary minimum and maximum at phase ? 0.00 and 0.35, respectively. Secondary minimum light is at phase ? 0.65 with secondary maximum at phase ? 0.85. The light curves longward of 3150 A vary in opposition to those shortward of this 'null region'. Ground-based coude spectra show that the Fe II and Cr II line strengths have a double-wave variation such that maximum strength occurs at minimum ultraviolet light. We suggest that the strong ultraviolet opacities due to photoionization and line blanketing by these metals may cause the observed photometric variations. We have also constructed an oblique-rotator model which shows iron and chromium lying in a great circle band rather than in circular spots.

Molnar, M. R.↗
NASA NTRS1975

Ultraviolet variations of epsilon UMa

OAO-2 spectrometer observations of the Ap variable epsilon UMa indicate that the photometric variations are due to variable ultraviolet absorption from apparently overabundant metals. These data also point out the presence of a secondary maximum of the Fe-group elements, notably Cr, that has not been reported.

Molnar, M. R.↗
NASA NTRS1974

OAO-2 observations of the helium spectrum variable a Centauri

Ultraviolet photometry and spectrometer data are presented for a Cen, which exhibits large variations in the strength of its He I lines. The photometry indicates that the star becomes brighter at the helium-weak phase by 0.05 mag throughout the ultraviolet and visual region. A model atmosphere analysis indicates that the photometric variations are not due to flux redistribution from back-warming effects, but can be interpreted as due to the rotation of a distorted stellar disk in the shape of a prolate spheroid.

Molnar, M. R.↗
NASA NTRS1973

Ultraviolet photometry from the Orbiting Astronomical Observatory. VII alpha squared Canum Venaticorum.

Far-ultraviolet photometry by OAO-2 is presented for alpha squared CVn covering the entire 5.47-day period of this magnetic Ap variable. The light curves ranging from 1330 to 3320 A indicate that there are two important sources of back warming from far-ultraviolet absorption. Strong line blanketing by the rare earths redistributes flux longward of the Balmer discontinuity, causing the major observed photometric variations. In addition, a second source, which may be due to a combination of continuous opacities and line blanketing from the iron-peak and rare-earth groups below 1600 A, apparently redistributes flux into the region of the Balmer discontinuity. It is also found that ground-based determinations of the effective temperature are overestimated due to the flux redistribution from far-ultraviolet absorption. The effective temperature of alpha squared CVn derived from OAO and ground-based absolute fluxes is found to be 11,200 K, which remains constant over the cycle.

Molnar, M. R.↗
NASA NTRS1972

Mariner 9 ultraviolet spectrometer experiment - Stellar observations.

Photoelectric spectra have been obtained for a number of early-type stars in the 1100- to 2000-A region with the Mariner 9 UV spectrometer. The resonance lines of H I, Si IV, and C IV are easily identified, as are features due to C II, C III, Si III, Fe II N IV. The absolute energy distribution derived from the data lie about 20% below those of OAO-2 in the 1200- to 2000-A region.

Lillie, C. F.↗
NASA NTRS1972

Metal line blanketing and opacity in the ultraviolet of alpha 2 Canum Venaticorum

Ultraviolet photometry by OAO-2 was made of alpha 2 CVn covering the entire 5.5d period of this magnetic Ap variable. The light curves ranging from 1330 A to 3320 A indicate the dominant role of rare-earth line-blanketing in redistributing flux. In a broad depression of the continuum covering 2300-2600 A, scanner observations possibly identify strong lines of Eu III as major contributors to this feature. At maximum intensity of the rare-earth lines, the ultraviolet continuum shortward of 2900 A is greatly diminished while the longer wavelength regions into the visual become brighter. In addition, there is evidence that the hydrogen line opacity is variable and the photoionization edge of Si I at 1680 A is identified.

Molnar, M. R.↗
NASA NTRS1972

Ultraviolet observations of weak-helium stars

Ultraviolet filter photometry aboard OAO-2 has been carried out for the weak helium stars 3 Cen A, 3 Sco, HD 144334, HD 21699, HD 144844, alpha Scl, HR 8535, HR 8770 and HD 144661. The flux distribution is compared with that of B3 to B8 main sequence and giant stars in terms of color-color diagrams. When allowance is made for line blocking shortward of 2800 A that most of the weak helium stars have normal fluxes in good agreement with their ground-based colors. The star alpha Scl is a light and spectrum variable. The strengthening of Ti II and Sr II lines around 2400 A appears to redistribute flux longward of the Balmer discontinuity. The flux distribution indicates an effective temperature about 1500 K higher than previously determined.

Bernacca, P. L.↗