Engineering topics

Dobias, J. J.

Publications and source records attributed to Dobias, J. J..

7 records

NASA NTRS1994

Observations of changes in the bolometric contrast of sunspots

Rapid changes in the total solar irradiance from space borne sensors are largely due to the passage of large sunspots across the disk. The effect of sunspots has often been modeled, using ground-based observations, by the use of a sunspot index such as the PSI, which assumes that all sunspots have the same thermal structure, which remains constant with time. In this paper, we report on photometric observations of sunspot groups that show significant differences in their mean bolometric contrast ( up to a factor of 2) and some of which show cooling or warming during their disk transit. Most of these changes can be ascribed to the changing ratio of umbral-to-prenumbral area. By measuring the mean temperature or bolometric contrast, together with corrected (hemispherical) areas, we can determine the instantaneous solar luminosity fluctuation and its diurnal change due to individual sunspot groups. These results show that the use of solar indices based on estimates of sunspot area and fixed sunspot contrast, such as the photometric sunspot index, do not remove all of the significant sunspot effects from satellite measurements of the total solar irradiance.

Chapman, G. A.↗

NASA NTRS1985

A redetermination of the luminosity, distance, and reddening of Beta Lyrae

The most likely distance to the Beta Lyrae system is redetermined to lie at 370 pc, corresponding to a distance modulus of 7.8 mag, while the correct modulus value probably lies in the 7.7-8.0 mag range, corresponding to a distance range between 345 and 400 pc. On the basis of Wilson's (1974) model, the phase-averaged absolute visual magnitude of the primary component is -4.1 mag; this star is therefore a very luminous giant, classifiable as B8.5 or B9, with a luminosity class of II-Ib. The present data are based on optical and IUE scans of the brightest visual companion to Beta Lyrae, HD 174664, together with an analysis of the H line profiles in its spectrum.

Dobias, J. J.↗

NASA NTRS1985

IUE and optical spectral scans of U Sagittae - An analysis and comparison with U Cephei

Recent optical and ultraviolet observations of U Sge are presented, and the results are compared with observations of U Cephei. The observations were carried out with: the International Ultraviolet Explorer (IUE) satellite; with the ITS scanner of the Lick Observatory; and with a Varo image-tube camera attached to a coude spectrograph. Comparison with data for U Ceph showed that U Sge is of the B7.5 V spectral type with an effective temperature of 12,250 + or 250 K, and a surface gravity of 3.9 + or - 0.1 log g. The secondary component of U Sge appears to be of the G4 III-IV spectral type. The best estimate for the distance of U Sge was 295 + or - 20 pc. Emission lines of the W Serpentis type were found in the IUE spectrum for U Sge. Although the observed fluxes for the lines were low, the powers were half as strong as those observed in U Cep. Selected emission lines of Fe II, Si II, and Si III were also found assuming solar abundances. A complete summary of the IUE and optical data is provided in a series of graphs and tables.

Dobias, J. J.↗

NASA NTRS1983

RW Tauri as a weak W Serpentis star

Observations of the short-period eclipsing binary RW Tau made with the LWR and SWP cameras of the IUE satellite and with the ITS scanner at the 3-m Shane telescope of the Lick Observatory in August and October, 1982, are reported and analyzed. At total eclipse, weak excess continuous radiation in the LWR range and emission lines at Si IV (1), C IV (1), Al III (1), Fe III (34), and Mg II (1) in the SWP range were detected. These findings are similar to those for W Ser-type binaries such as U Cep. The parameters of the system are estimated using a bilinearly interpolated atmospheric model of the type developed by Kurucz (1979): primary-star T(eff) = 11,750 K, log g = 4.2, cool-component spectral type = K0 III, E(B-V) color excess = 0.14 mag, system distance = 270 pc. The implications of these findings for proposed models of the W Ser binaries are discussed, and the observation of a possible distant optical companion star 45 arcsec from RW Tau is reported.

Plavec, M. J.↗

NASA NTRS1982

Model fitting of the Algol binaries

Energy distribution of the component stars of the Algol type semidetached binaries were studied by combining IUE low dispersion spectra with optical scans made with the same resolution. In most cases the flux distributions are matched by Kuruz model atmospheres with normal solar composition. Deviations from normal atmospheres which indicate a higher level of activity and interaction in the binaries are examined. Eclipse observations in U Cep and V356 Sgr reveal emission lines of the W serpentis type.

Dobias, J. J.↗

NASA NTRS1982

continuum and emission lines in eta Lyrae

Integral photometric characteristics of beta Lyrae were redetermined using its brightest visual companion, the star HD 174664. It was attempted to separate the fluxes coming from the two components. The flux distributions are badly contaminated by the radiation of the circumstellar hydrogen. Peculiarities seem to be present in both components; in particular, the secondary star cannot be represented by an atmosphere with a unique effective temperature, and best represented by a thick disk.

Plavec, M. J.↗

NASA NTRS1982

Spectral energy distribution /119 - 685 nm/ in 16 shell stars and a tentative model for accreting Be stars

IUE low-dispersion spectra and spectral scans made with the Lick Observatory IDS scanners have been combined for 16 shell stars. Eleven objects can be represented by Kurucz (1979) model atmospheres, although some of them display strong shell-type line spectra. Five among them are known binaries. The six remaining objects display complex spectra. A model involving continuum and line radiation from a hydrogen cloud surrounding the accreting component is proposed. A generalization of this model with optically thick segments of the cloud promises to explain even more exotic objects such as beta Lyrae, W Serpentis and possibly epsilon Aurigae.

Plavec, M. J.↗