2016 schedule

Fri 12:00-12:30
Theatre 106

Long-term oscillations in the orbital period of RR Cen: third body or Applegate effect?

Jonathan Powles

Variable Stars South, SASER, Canberra Astronomical Society, University of Canberra

RR Cen is an EW-type contact binary that has been continuously observed since 1894. The first calculation of orbital elements and basic modelling were performed by South-African-based Scottish astronomer Alexander Roberts in 1903, and subsequent models had been computed by Knipe (1965), Bookmeyer (1968), King and Hilditch (1984) and Yang, Quian, Zhu et al. (2005) (YQZ). The density of this coverage has provided a good data set for observing changes to the orbital period, and YQZ computed a linear increase in orbital period, which they attributed to mass transfer, coupled with a small 60-year sinusoidal variation in orbital period, which they attributed to a third body in the system. Subsequent observations, including those by VSS members Mark Blackford and Jonathan Powles in 2014-2015, reveal the orbital period increase to itself be part of a sinusoidal oscillation with a longer, 90-year period and much larger amplitude. This paper suggests that a better explanation for such periodic variations would be the Applegate effect - regular oscillations caused by the consequences of the magnetic field cycle of one of the stars - however this theory too has problems in its applicability to a star of such early classification (F0).