[vsnet-chat 3042] Re: ETA CARINAE - Two years of observations

[Bish Ishibashi <bish@howdy.gsfc.nasa.gov>]

Howdy all, 

      Harry's measurements show a dispersion of +/- 0.04mag 
in its astronomical magnitude. Considering that the recent
change in astronomical magnitude is only about 0.03 mag. in his 
measurements, I am *not yet* convinced that this is a real 
declining trend. On top of that, the value of B-V seems to 
decline down to 0.65 annually. Now you all know "Occam's Razor":
the simplest explanation for this recent change still appears
to be color-effect due to its high airmass because of its apparent
repeatability in an annual timescale. 

   The point I am making here is the same as the one Stan made: 
[Eta] is a very difficult visual object. If one wants to asertain 
the existence of any type of a trend, the trend has to be 
"statistically significant", i.e., the degree of the change should
be much larger than sysmetic variation of an observer's measurement. 
For that reason I can't say that any recent data I've seen so far 
(including our own with the HST) suggest a strong short-term decline 
in Eta's brightness. 

   Of course, it does NOT say the declining trend has to be 
spurious (Ah, a wonder of statistics), and all the folks down 
under have a chance to prove me wrong by continuing visual 
monitoring of Eta Car as many days, months and years as you 
can persist :-). To say the least, in a few months from now 
we should have a verdict on this business. 

   Cheers and have fun observing, 

   Bish Ishibashi

PS. The recent brightening does not really fit to a so-called 
    "S-Doradous" phase, either. During a normal "SD-phase," 
    bolometric luminosity remains roughly constant but 
    shifts toward longer wavelengths as the photospheric
    radius expands. Davidson et al. (1999) demonstrated
    that the increase in luminosity was much rapid in 
    shorter wavelengths, i.e., Eta was becoming bluer than 
    usual. This is an exact opposite of what we expect
    from an usual eruption of LBV stars. For more, Please
    refer to Davidson et al. (if you do not have an access 
    to ApJ, then ftp colossus.gsfc.nasa.gov, and login as 
    "anonymous" and the usual blah blah to get in, and then 
    do a "cd pub/bish/" & "get preprint4.ps" or "get preprint4.pdf"
    - if pdf, don't forget to type "binary" before download - 
    to obtain the original draft of the paper.) 

PS2. And lastly, Sebastian, Eta Car is tremendously bright 
    in H-alpha emission line; and H-alpha emission is effectively 
    scattered around by the reflection nebula surrounding the star. 
    This fact makes Eta Car very unusually "red" to the human eyes
    while the value of B-V -- remember, a true "V" transmission 
    does not include much of H-alpha emission line -- is not 
    reflecting the peculiar nature.

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