Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2724p&link_type=abstract
American Astronomical Society, DPS meeting #42, #27.24; Bulletin of the American Astronomical Society, Vol. 42, p.1064
Astronomy and Astrophysics
Astronomy
Scientific paper
We show that the observed 170 day period in the light curve of 4U 1820-30 follows naturally from the assumption that there is a third body in the system with a large inclination to the binary orbit, and that the system is librating around the stable fixed point in the Kozai resonance. Mass transfer drives the system toward this fixed point; we calculate, both analytically and via numerical integrations, that the period of libration is of order 170 days. We show both analytically and numerically that the system will not be trapped in libration if the semimajor axis is decreasing because the action of the resonance separatrix decreases with decreasing binary semi-major axis. The finding that the 170 day period in 4U 1820-30 results from libration in the Kozai resonance implies that on the secular time scale, and contrary to observations, the orbital period of the inner binary is increasing with time; if the apparent decrease in orbital period is real, and continues for dynamically significant times, the Kozai resonance will be broken, indicating that the current configuration is unstable, a situation we find untenable. The implication that the mass transfer has to win over tidal dissipation (resulting in semimajor axis expansion) allows us to give a lower limit on Q 108. We conclude that the observed negative period derivative is either incorrect or not intrinsic to the system if 4U 1820-30 is a hierarchical triple.
Murray Nathan
Prodan Snezana
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