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Kappa Pavonis - a star story

Posted: Tue Jan 10, 2017 9:53 pm
by RMSteele
It was unbearably hot outside in Adelaide. We found him hanging in the sombre Art Gallery of South Australia. There we were, my wife and I, facing this pulseless Yorkshireman called Richard or Rychard if you prefer the old spelling. He was four centuries and half a sphere from his home in Ribston, Yorkshire.
Unbeknown to the eclectic picture-hangers of Adelaide, Richard Goodricke was an ancestral relative of John Goodricke. John was the earlier of two interesting, though unrelated, deaf Yorkshire astrophysicists. Fred Hoyle is the other one in case you wondered. It was in eighteenth century York that John Goodricke proposed that Algol was an eclipsing binary star and he drew attention to the light pulsations of Delta Cephei, prototype star of the Cepheid variables. Five hundred years from South Australia, one such star was beating in a peacock’s breast.
A century ago it was assumed that the Cepheids comprised a homogenous class of variable stars, obedient to a convenient period/luminosity rule that provided astronomers with a consistent tool for measuring distances in the cosmos. But it was later realised (by Baade) that the Cepheids consisted of two distinct luminosity classes. Type I are bright, young, massive stars inhabiting the flat disc of the Galaxy. Type II are less luminous, older and much less massive stars spread through the galactic halo and globular clusters.
Kappa in Pavo the peacock is a type II Cepheid. It is reckoned to be a W Virginis class variable. Like the incongruous portrait however, Kappa Pavonis is a bit of an oddity in its setting. It is the brightest (apparent) W Virginid star and the only one that is visible to the unaided eye in an unpolluted sky, varying between visual magnitudes 3.9 and 4.8. W Virginids have periods ranging between 10 and 20 days. Kappa’s period is usually accepted as about 9.1 days. It has been suggested that it is intrinsically brighter than it ought to be. There are such luminous W Virginids in some globular clusters and Kappa might be of that sort but, as mentioned, if it is, it is hanging in the wrong galactic gallery, for it is not a cluster member.
A puzzle for me was the difficulty I experienced in ferreting out more than a few generalities about Kappa Pavonis. No predictions, no classical light curves could I find on t’internet so I decided to ask Kappa to sit for its portrait. Back in Melbourne I made 28 estimates of Kappa Pavonis between 27 September 2016 and 3 December 2016. This small set of observations yielded a 9 day cycle with indications of a 4 day rise to maximum and a 5 day fall to minimum. I observed a peak brightness at visual magnitude 3.9 on 27/9, 14/10, 10/11, 19/11 and 29/11 (day/month). The faintest estimate was visual magnitude 4.7 on 16/11. I hope that Kappa will sit for me again when I next stalk the Peacock.
And Rychard? Requiescat in corde pavonis.