[vsnet-chat 1806] Re: Yellow Alert from Eta Carinae

[Bish Ishibashi <bish@astro.spa.umn.edu>]

Howdy all, 

On Thu, 22 Apr 1999 12:31:06 +0930, Fraser Farrell wrote:
>I've been getting visual magnitudes of 5.1-5.2 for the last couple of 
>months.  To continue the historical context, last time Eta Car was 
>this bright, my place was unexplored territory inhabited by the 
>Ngarrindijeri aboriginies, gold had just been discovered in the 
>colony of New South Wales, and John Tebbutt was still basking in the 
>fame of discovering the Great Comet of 1831.

First, my apology to mis-spell your name.
And I'll make sure to quote this historical remark some day...: ). 

>I think, now that it has brightened, it is an easier target. The 
>homunculus is contributing relatively less light than before.  On 
>moonless nights Eta Car is now visible to the naked eye, and in 7x50 
>binoculars it has a definite orange-red tint.  

One caveat: from the images of AQU/PEAK(*)(acquisition and peakup) for 
the STIS observations, we can tell the surrounding nebula immediately 
outside Eta Car is slowly getting brighter as well. So it is not surprising 
if the entire homunculus becomes brighter eventually (it all depends on how 
electron scattering of photons takes place inside the nebula). So getting a 
right magnitude for Eta Car itself may be proven difficult still...but I hope 
that doesn't stop many observers to "keep an eye" on it. 

>I presume that the brightening is due to the thinning of the surrounding 
>dust shell(s) as they expand.  However, I'm eagerly awaiting the day 
>when I need to wear sunglasses to observe Eta Car, and use Venus as a 
>"comparison star" ;-)


Not entirely sure if the thinning of the dust (or reduction of Av) is the
main cause. It is also conceivable that the star has increased its visual 
brightness as well. That's still pending more spectral analyses. Right now
all I can say is, "isn't this one of the great stars to check our understanding 
of stellar physics or what?" 

Cheers, 

Bish Ishibashi
Univeristy of Minnesota

(*) For those who are not familiar with the detail of the Space Telescope
    Imaging Spectrograph (STIS): during a target acquisition -- "peakup" --  
    sequence, the STIS uses a subarray of its CCD (~0.05 or 0.1" per pixel, 
    which is almost as good as the famous WFPC camera) to capture a image of the 
    target and make sure that the telescope acquired the target at its aperture. 
    Not many people pay attention to this datum since making an assumption that 
    the attitude control (or pointing) on the HST always works without failure...

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