A Semi-Detached Binary with a Spotted Primary Star

Figure 1: Artist’s impression of what “twin Suns” might look like from the surface of a circumbinary planet.

Photometric observations of NR Pegasi by A. Erdem et al. (2014) in 2007 and 2008 show that it is a highly active semi-detached binary star system. The primary and secondary stars of the binary system are estimated to be 1.60 ± 0.03 and 0.57 ± 0.02 times the Sun’s mass, 3.35 ± 0.07 and 3.55 ± 0.08 times the Sun’s radius, and 9.10 ± 1.78 and 3.47 ± 0.93 times the Sun’s luminosity, respectively. Both the primary and secondary stars also have respective surface temperatures of 5485 ± 200 K and 4186 ± 241 K. The binary system has an orbital period of 3.3978 days. NR Pegasi is an eclipsing binary system where the primary star blocks part of the secondary star and vice versa during each orbit. As a result, the observed brightness of NR Pegasi varies with time.

Light curves of NR Pegasi obtained in 2007 and 2008 show large asymmetries, and variations could be seen in the light curves from night to night. For example, the depth of the primary minimum (i.e. the secondary star passing in front of and blocking part of the primary star) in the 2007 light curves is shallower than that of the 2008 light curves. These peculiar variations are believed to be caused by the presence of large dark sports on the primary star. The dark spots are not literally dark, but rather, they are spots where the temperatures are somewhat lower than on the rest of the star. Models of NR Pegasi show that the presence of 2 large dark spots (2008 light curves) or 4 large dark spots (2007 light curve) on the primary star could account for the peculiarities in the light curves.

The less massive and cooler secondary star fills its entire Roche lobe, while the more massive and hotter primary star fills 64 percent of its Roche lobe. Basically, the Roche lobe of a star is the region around the star where its own gravity is stronger than that of its companion. NR Pegasi is a semi-detached binary system because one star fills its Roche lobe while the other does not. As a result, gas from the Roche-lobe-filling secondary star (i.e. donor star) is transferred to the primary star (i.e. accreting star). Both stars of NR Pegasi have evolved to the point where they are beginning to exhaust hydrogen in their cores, placing them near the terminal age main sequence phase of their evolution. However, the less massive secondary star appears to be significantly over-sized and over-luminous in comparison to theoretical evolutionary models.

Figure 2: Best fits to the 2007 light curves of the NR Pegasi binary system. A. Erdem et al. (2014).

 Figure 3: Best fits to the 2008 light curves of the NR Pegasi binary system. A. Erdem et al. (2014).

Figure 4: (a) Four spots and (b) two spots for the 2007 and 2008 light curves, respectively. A. Erdem et al. (2014).

Figure 5: Roche lobe geometry of the NR Pegasi binary system. A. Erdem et al. (2014).

Reference:

A. Erdem et al., “NR Peg: A new highly active semi-detached binary”, New Astronomy, Volume 33, November 2014, Pages 38-43.