The Nova-Like Variable V Sagittae as a Short-Period Eclipsing Binary.
Abstract
The hot "nova-like" irregular variable V Sagittae has been investigated spectroscopically (with the t20-inch and Crossley reflectors, at 48 and 350 A/mm) and photoelectrically, in UB V. The complex light variations have been resolved into three apparently independent activities: (i) a strictly cyclic variation produced by an eclipsing binary of period 0d.5t4t95; (ii) an occasional major and very sudden brightening by as much as 3 mag; (iii) minor fluctuations with a time scale of a few days. When the system is faint, additional small-scale fluctuations with a time scale of about 1 hour ("ffickering") are superimposed on variations (i) and (iii). The spectrum of V Sge contains broad, hazy emission lines of H, He ii, 0 iii, 0 vi, N iv, N v (much as in a WN5 star) on a hot continuum, but also exhibits the unusual feature of sharp !iuorescent lines of O iii at XX 3132, 3444. These latter lines are double, and the two components oscillate (180 out of phase) in the period of the eclipsing binary and with semi-amplitudes of K1 = 320 km/sec, K2 = 85 km/ sec, where component 1 is the star of lesser mass and radius, and of higher surface brightness, that is eclipsed at primary minimum. The hazy 0 vi lines and the absorption reversals in the H and He ii emissions are apparently produced by detached material in the binary system. Analysis of the light-curve and colors indicates that component 1 lies very near its limiting Roche surface while component 2 lies well within its lobe. At the time of a major outburst (activity ii, above), the velocity variation in the 0d51 cycle is masked by an obliteration of the sharp fluorescent 0 iii features and by the strengthening and broadening of nearly all the other emission lines, while the eclipses become very shallow. The colors change when the star brightens, but it is shown that this is due entirely to the differential effect on the UB V colors of the strengthened emission spectrum. The observational data can be interpreted as the explosive ejection from component 1 of a semi-opaque shell of hot material that quickly, at an expansion velocity of 400-500 km/sec, envelops the entire binary system. The small-scale fluctuations of V Sge near minimum light (iii, above) can be interpreted as due to small changes in the effective dimensions of the same star The reddening of V Sge has been estimated as = +040 from a variety of evidence, which value leads to a distance of 2.75 kpc and a total Mv = -1.0 outside eclipse but at a time when there is no activity of category ii, above. The masses of the two stars are estimated to be i = 0.74 and i = 2 8 solar units, and the black-body temperatures as T1 = 44000 and T1 = 22000 K. Both stars lie well below the zero-age main sequence, but when compared to normal dwarfs of the same masses, component 1 is overluminous by about 11 mag. and component 2 by 1-2 mag. Presumably V Sge represents an advanced stage in the evolution of a close binary system.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 1965
- DOI:
- 10.1086/148149
- Bibcode:
- 1965ApJ...141..617H