[vsnet-chat 4737] Re: Eta Carinae

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

Hi Sebastian and all, 


Sorry for the delay responding to your query. 
I had been quite *busy* for many reasons...


First, our HST/STIS team cannot refute if the brightening has started 
again in May 2001, though we have an opportunity to verify the degree
of change in far ultraviolet flux (lambda < 1200 AA) since March 2001. 
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!]  


By the way, you said that the degree of the recent brightening is about 
0.3 astronomical magnitudes or more. That number - 0.3 mag - reminds me 
of a nice conversation I had with Chris Sterken at a meeting held in 
Montana about 3 years ago. He was presenting the visual light curve of
Eta Carinae measured by Albert Jones. What fascinated me was to learn 
that 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. Well, as naive as I was then, I told Chris 
that I was amazed to know that human eyes could measure the visual 
magnitude of a star so ACCURATELY. Chris immediately corrected that 
"these measurements are NOT ACCURATE; but they are taken CONSISTENTLY." 
Oops. Obviously it's a slip of tongue, but the lesson was learned. (*)
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. 


(*) by the way, the reverse is also true. Accurate measurements don't 
    always mean they are taken consistently. THIS is a lesson that
    a few professional astronomers sometimes forget. 


Ok this is getting too lengthy as usual. Let me wrap up by answering 
the last query about the nomenclature on S Dor phase. I read the IBVS
article by Chris Sterken (#5000) carefully and realized that he redefines
the meaning of "S Dor phase" as, I quote, "large-amplitude long-term 
variability in light and colour ranging over magnitudes on time scales 
of years." Note that it doesn't say anything about how colour should
change as a star brightens or dims under this definition. Since Chris 
Sterken does define what he means by "S Dor Phase", technically he could
use the phrase "S Dor phase" to express the recent variations seen in 
Eta Carinae.   

HOWEVER, make no mistake about this. Going way back to the day of Thackeray, 
spectroscopic observations had shown that the spectrum of S Dor appeared unusually 
cooler (or redder) during its maximum phase. This remark first appears in the 
article by Thackeray, A. D., 1965, MNRAS, 129, pg 169. Later on 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. Alas, I expressed in 
my earlier message about my uneasiness of your use of the phrase "S Dor phase" 
in VSNET. 

Sebastian, to be blunt, I don't really care if you continue to use the phrase
"S Dor" so as long as we all are clear about the changes in true physical 
conditions that the term refers to. Not a name or classification is what 
matters to me most; physics does. 


Cheers, 

Bish Ishibashi


PS. Totally off the topic but...
    If any of you ever get an opportunity to go see a space shuttle launch 
    at Kennedy Space Center, FL, make an extra effort to do it! It's a very 
    rewarding experience. 

-- 
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   Kazunori "Bish" Ishibashi(NRC/NAS)  E: bish@howdy.gsfc.nasa.gov   
   NASA/GSFC Code 681                  W: 1-301-286-8904
   Greenbelt, MD 20771                 F: 1-301-286-1752
   http://hires.gsfc.nasa.gov/~bish/   
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