[vsnet-chat 556] eta Carinae

["STAN WALKER" <astroman@voyager.co.nz>]

I note the interest in eta Carinae. This stars was observed in UBV from
Auckland during the interval 1970 - 1995. During this period it was
brightening at a slow rate - about 0.02 to 0.04 a year. Superimposed on
this were semi-regular brightenings of about 0.3 magnitudes at intervals of
about 1800 days. These rose quite sharply, but faded rather more slowly. 

The reason for looking at this star was a suggestion by visual observers
that it had brightened dramatically. Over the 25 year interval there were
several false alarms of brightening as seen by the visual observers
(usually a group of three or more) but these were always disproved by the
pe measures. These were made using a 31" aperture and always included the
same bright knots in the surrounding nebula. Accuracty was about +/- 0.005
magnitudes.

At its pre-1970 magnitude one of the main comparisons stars was reputed to
be V = 6.4 but this was the eclipsing binary QZ Carinae with an amplitude
of 0.25 in V and a continuously variable light curve. Since then we (mainly
W S G Walker and B F Marino) have compared visual observations from the VSS
RASNZ and pe magnitudes at intervals. At times we wondered if we were
looking at the same star as the visual people!

We concluded that the visual measures confused the situation more than they
helped - eta Carinae is just NOT suited to visual measures. One of the
problems are that most of the comparison stars are blue - O supergiants and
a Wolf-Rayet star. I think that one of the visual comparisons is now an A0
star (5.84) but the other is rather bluer. Thus there are noticeable
extinction effects at low altitude.

Various pe measures have mentioned periods around 5 years, with amplitudes
of about a third of a magnitude - we derived periodicities of about
1750-1850 days from memory - superimposed upon a slow brightening. Our
measures looked like the star was subject to semi-periodic outbursts, hence
the reasonably sharp rise and slower fade. But the one thing we did
conclude was that visual photometry was unreliable - on many occasions we
thought eta was brighter, only to have the pe measures tell us we were
dreaming. 

Like Fraser, I still look at the region every clear night in hope. But so
far no reward! 

Stan Walker

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