The Origin of Bright X-Ray Sources in Multiple Stars

Abstract

Luminous X-ray stars are very often found in visual double or multiple stars. Binaries with periods of a few days possess the highest degree of coronal X-ray activity among regular, non-relativistic stars because of their fast, tidally driven rotation. But the orbital periods in visual double stars are too large for any direct interaction between the companions to take place. We suggest that most of the strongest X-ray components in resolved binaries are yetundiscovered short-period binaries, and that a few are merged remnants of such binaries. The omnipresence of shortperiod active stars, e.g., of BY-Dra-type binaries, in multiple systems is explained via the dynamical evolution of triple stars with large mutual inclinations. The dynamical perturbation on the inner pair pumps up the eccentricity in a cyclic manner, a phenomenon known as Kozai cycling. At times of close periapsis, tidal friction reduces the angular momentum of the binary, causing it to shrink. When the orbital period of the inner pair drops to a few days, fast surface rotation of the companions is driven by tidal forces, boosting activity by a few orders of magnitude. If the period drops still further, a merger may take place leaving a rapidly rotating active dwarf with only a distant companion.

Radio Emission from Binary Stars in the AB Doradus Moving Group

“…Considering the radio/X-ray activity correlation (Güdel et al 1993), this perception finds support in other, better reported statistics which relates X-ray activity and binarity, namely, that about 79% of the X-ray emitters are in binary systems (Makarov & Eggleton 2009). Many of these binaries correspond to close binaries, but interestingly, a large fraction of the total X-ray binaries (67%; Makarov & Eggleton 2009) are part of hierarchical triple systems, with the third body in a wider orbit. In this scheme, the presence of the wide companion induces dynamical perturbations to the orbit of the close binary at each periastron passage through the Kozai cycles and tidal friction mechanism (e.g., Mazeh & Shaham 1979;Fabrycky & Tremaine 2007).…”

“…5.3.1, both AB Dor Ba and AB Dor Bb are strong, compact radio emitters, which contributes to the emerging perception that many radio stars reside in double or multiple systems (Melis et al 2013;Azulay et al 2014). Considering the radio/X-ray activity correlation (Güdel et al 1993), this perception finds support in other, better reported statistics which relates X-ray activity and binarity, namely, that about 79% of the X-ray emitters are in binary systems (Makarov & Eggleton 2009). Many of these binaries correspond to close binaries, but interestingly, a large fraction of the total X-ray binaries (67%; Makarov & Eggleton 2009) are part of hierarchical triple systems, with the third body in a wider orbit.…”

“…This appears precisely to be the scenario found in some of the young radio stars in binary systems. Makarov & Eggleton (2009) explained the origin of the high rotation rate of AB Dor A (0.5 days) via Kozai cycling and tidal interaction between a close binary (now merged in AB Dor A) and the 0.090 M ⊙ low-mass companion AB Dor C, orbiting AB Dor A at a mean distance of 2 A.U. with a period of 11.74 yr.…”

Spectroscopic study of early-type multiple stellar systems

“…This is what happens in Kozai cycles, where the interaction of a close binary with a distant third body induces periodic oscillations in the binary eccentricity and inclination, keeping the semiaxis constant (Innanen et al 1997;Ford et al 2000). According to Makarov & Eggleton (2009), the effects are only significant if P out (yr) < ∼ [P in (d)] 1.4 . In the case of NO Pup, the hypothetical perturbing body would be orbiting the binary with P out < 500 d, a out < 2.2 AU.…”

Evolution of Binaries in Dense Stellar Systems

“…Shrinkage of a binary by a combination of Kozai cycling and tidal friction (KCTF; [57,58,59,60]) can be responsible for production of short-period active stars, e.g. BY-Dra-type binaries [61]. KCTF would operate in a cluster if formed triple's Kozai time-scale [62] will be smaller than τ coll .…”