September 18, 2019

14.9.9:U Scorpii:A Nova in Waiting?

(Image credit & copyright courtesy of Mark A. Garlick; used by permission.

Please do not use this image in any way whatsoever without first contacting the artist).

Recurrent novae are among the rarest of beasts. While one-off galactic nova come and go throughout the year, recurrent novae are among those very few stars that have been known to exhibit nova-like flares multiple times. This week, I turn your attention towards U Scorpii, a known recurrent nova in the head of the constellation Scorpius. As the bright Moon is currently out of the evening sky, now and next month is the time to peek at this unique star before it slides behind the Sun. First discovered in 1863 by English astronomer N.R. Pogson during an outburst, U Scorpii stands as one of the fastest recurrent nova known, historically rising from sub- 15th magnitude obscurity to 8th to 9th magnitude in under 24 hours! The decline is also equally abrupt; generally, U Scorpii has dipped back down below 10th magnitude a week after outburst. The position is about 8 ˝ degrees north of Antares, in a very rich star field along the galactic plane. Check out AAVSO’s observing campaign site for star charts. Previous recorded outbursts have been in 1979, 1987, and 1999 and at a pattern of about 10 years (plus or minus 2), U Scorpii should be ready to pop! Amateurs have always been the first to nab this rare event, and for the first time, Hubble and the Spitzer space telescope will be ready to join in the action. If you have a telescope or decent pair of binocs, be sure to familiarize yourself with this region; you might be the first to bag a rare event!

The astro term for the week is, you guessed it; recurrent nova. Only ten of these bizarre beasts are known to have rates less than the span of the average human lifetime, and thus be definitely identified. U Scorpii is a member of a very exclusive club of variables, one that includes T Pyxidis, which is long overdue to burst! In a recurrent nova, two stars are in orbit about each other; a normal main sequence star which is feeding accretion material to a hungry companion white dwarf, which layers on the in falling hydrogen until a runaway fusion reaction occurs. The star then “flares up” radically, briefly burning the thin shell hydrogen layer off into space. There is some thought that all cataclysmic novae are perhaps recurrent; its just the geometry and relative masses of each binary situation that determine the rate of outburst.

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