[vsnet-chat 4744] Re: Eta Carinae

["Sebastian Otero" <varsao@fullzero.com.ar>]

> the light curve exhibited a yearly variation with an amplitude of
> roughly 0.3 astronomical magnitudes. Basically the annual change in
> atmospheric extinction (color-effect that occurs at high air-masses) gave
> rise to the yearly variation.
> Anyhow, here comes my question to Sebastian: how sure are you that
> you are not suffering from the same color-effect? The same question
> would go to the bandwagon of people who do digital or analogue photometry.

Hi, Bish and all:
                        I've also found some variability with an annual
period in my data...
However, the star hadn't been that faint (V= 5.5) since the middle of 1998.
And the recent brightening has been sharper than any other brightening I
have recorded recently although the star is still rather high in my sky.
In my opinion, a colour-effect would affect comparison stars also, and as
Stan commented, 0.3 magnitudes is too much of a difference. My sequence is
not so bad. (a 5.08 orange star and 5.38 and 5.47 microvariables of white
and yellow color)
These are the magnitudes I obtained since the beginning of the brightnening.
Note that it seems to have stopped and if this is only being caused by
atmospheric extinction, this is very strange.

2034.460 5.43  May 4.960 UT
2039.610 5.43
2042.485 5.38
2048.456 5.33
2052.463 5.33
2057.460 5.34
2058.595 5.33
2067.422 5.31
2081.431 5.25
2088.440 5.25
2115.415 5.14
2118.443 5.15
2124.428 5.11  August 2.928 UT
2129.427 5.14
2130.435 5.14
2141.439 5.16

Magnitudes are, as Bish said, "consistent" rather than accurate. With eta
Car things are more difficult than with other stars. External errors might
be higher than 0.2 mag. and internal errors are probably in the 0.1 mag.
level.
My observations were 0.1 - 0.15 mag. fainter than the PEP(V) values obtained
by H. Williams and S. Walker.
So if we apply this correction, a 5.0 magnitude (as reported by the other
observers from LIADA) is a good mean value.
I think it is very important to see if during the days to come, the star
continues to fade. This would prove that atmosphecric extinction can't play
a role that important.
Although if the star decides to brighten we could blame airmasses and
actually it wouldn't prove a thing !! ;-)).....so...


> Our next visit to Eta Car with the HST/STIS happens in between September
> through November 2001 also. So stay tuned. [And while waiting, please
> give your best shot to observe it as much as you can!]


........we'll be anxiously waiting these measures!!



> HOWEVER, make no mistake about this. Going way back to the day of
Thackeray,
> a class of
> variable stars which show the same trend -- the brighter, the redder -- is
> referred as S Dor variables. So naturally the word "S Dor" shouldn't
really
> be used as part of the phrase that refers to a spectroscopic variation
that shows
> a bluer (hotter) spectrum during the brightening phase

Thanks, Bish. You have been very clear.
Has the HST/STIS made measurements of other S Dor stars?
Specially HR Carinae, coincidentally this star is NOW brighter than ever
before in history with a visual magnitude of 6.75 and rising slowly month to
month with slight variations.
Is this star showing typical SDOR behaviour?

Regards,
Sebastian




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