[vsnet-chat 560] Re: eta Carinae

[Fraser Farrell <fraserf@dove.net.au>]

G'day all,

Stan Walker wrote:
   >I'm a little puzzled by the comments that eta Carinae changes by 0.4
   >magnitudes on a timescale of days. At a distance of 2000 parsecs or
   >so the object we see visually (about 10" in diameter?) is about 20,
   >000 AUs across which gives a light travel time of over a hundred days.

I think much of the light we see is coming "direct", thanks to forward
scattering.

The latest edition of Hartung's quotes 3200 parsecs and a diameter of 15" for
the Homunculus.  This yields 48,000 AUs and it's still expanding at over 200 AU
per year!

However, the Homunculus is obviously non-spherical (I see it about 14"x10" in my
15cm) and there is a noticeably brighter centre about 3-4" diameter. On nights
of perfect seeing, some mottling is evident at high magnifications. The colours
(to my eye) are a rusty red-orange grading to a pale orange in the centre.  Very
rarely, this bright core contains a central star - which has always been a
peculiar greenish colour (contrast effect) when I see it.

Readers will be familiar with the "bright patch" that replaces the Sun or Moon
when cirrostratus cloud or dense haze covers them.  Eta Car looks similar at
high magnifications.


   >Presuming  the Homunculus is more or less spherical there is
   >a light travel lag of several weeks between the edge and the centre
   >as we see it. So any central event which causes the visible object
   >to brighten by 40% should be seen initially as a bright central
   >spot at least twice the brightness of the edges. As the centre
   >dimmed the edges would then brighten to appear ringlike. Noone
   >reports anything like this - or have I missed it? Or is my thinking
   >muddled?

A light-echo effect?  Most of us would now be observing Eta Car at low to
moderate magnification (to simplify location of the comparison stars) and would
see Eta as a "star" within a tiny elliptical orange blob.  The image would not
be big enough to easily notice a ring; and the centre is always brighter anyway.

I don't know how easy the light-echo would be to detect.  Powdered silicates
reflect only 5-10% of incident light; and the bright patch centred in the
Homunculus shows that most of the light is forward-scattered.  Therefore very
little is sent "sideways" to Earth from a light pulse. Maybe try a series of
high-magnification CCD images??

Stan other post makes a good point about the colours of the available comparison
stars.  There are few red stars of a suitable magnitude nearby, and most of
those are actual or suspected variables.  Eta Car is not a star I recommend for
beginners.  Even its experienced observers disagree - in a systematic way - by
0.2-0.3 mags.

My comments a few days ago related to _my_ observations.  I don't have to worry
about differential extinction at low altitude on this star, because the
neighbour's trees block the southern horizon quite effectively!

I have done a period analysis of our Eta Car database.  After correcting for the
systematic differences between the six observers, and discarding the doubtful
estimates, I was left with about 270 estimates from ~JD 2448900 to now.  I
tested for periods from 0 to 1700 days; the data time span wasn't quite long
enough to test the 5.52-year periodicy.

The PDM analysis showed a weak 360-370 day period.  Discrete Fourier showed weak
peaks at 169 and 370 days - probably "year" aliases - and hinted at a weak peak
in the 1700-1800 day range.  In particular, there was no periodicy corresponding
to the ~85 day X-ray variability, *but* most of the X-ray maxima happened on
cloudy nights here.  All six observers tend to get clouded out together!



cheers,
Fraser Farrell

http://vsnet.dove.net.au/~fraserf/   email: fraserf@dove.net.au
traditional: PO Box 332, Christies Beach, SA  5165, Australia

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