[vsnet-chat 3029] Re: [vsnet-obs 28038] ETA CARINAE - Two years of observations

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

Howdy all, 

   As this is not a report for observations, I've posted 
my comments on [vsnet-obs 28038] to [vs-chat], instead. 


Sebasti=E1n Otero wrote on Mon, 5 Jun 2000 01:20:03 -0300
>In spite of all the reported variations I have never noticed a change =
>beyond the 0.3 magnitude range. However, until February, the star was =
>always in a slow rise, probably due to the dispersion of the envelope =
>ejected during the big explosion. But this trend stopped. Or at least =
>some kind of variability is starting to show up more clearly. The star =
>is -sure enough- fainter than  at the end of 1999. Maybe it is a prelude =
>for something interesting in the months to come. Who knows?


   First I'd like to direct those who are interested in 
eta Carinae to a published journal on the recent brightening:

	 Davidson et al. 1999,ApJ,118,1777-1783,
	 "An Unusual Brightening of eta Carinae"

which makes an argument that the recent brightening may not
be explained merely by the expansion of the circumstellar 
gas. Instead we argue that the brightening is due to the 
increase in intrinsic brightness of the star itself. That said, 
the progressive brightening seen in Sebastin's lightcurve appears 
consistent with what we've documented as "an unusual brightening" 
in the paper. 

   Second, however, I am not so sure about this recent "decline" 
in brightness based on Sebastin's lightcurve. I just ran a crude
discrete Fourier transform (see Scargle 1989,ApJ,343,874-887) to 
the datasets in [vsnet-obs 28038], and saw a modest power around 
Period ~ 350 days, or say, ~ one year, that doesn't show up at all 
in the window function. Knowing that eta Car is low in the altitude
around this time of the year, I'd make sure with other amatuers'
data that this change is not due to color-effect that arises at 
the high-airmasses. See A. Jones's paper in conference proceedings 
of "Eta Carinae at the Millennium" (ASP conf. Vol 179, 1999) at 
pages 209 -- 210 for this exact topic. 

   Third and lastly, I'll comment on why I am fascinated by this 
report. Since the publication (see above), we had other observations
with the HST/STIS instrument (four more to be exact) on March 2000. 
Each time we obtained an image of the central star with 0.05" 
resolution for the result of target acquisition/peakup, which we used
to do photometry of the star. The results were a small decline 
in the RATE of brightening (should not to be mistaken with the decline 
in brightness) compared with the brightening trend derived from 
the earlier datapoints. So our naive consensus was that the lightcurve
of the star (+ the nebula) was beginning to turn over. Now you can 
see why the report on eta Car caught my attention when it came up 
at vsnet!  In addition to that, there is this "5.5yr periodicity" of 
eta Carinae that reached the phase = 0 about 2.5 years ago (1998.0). 
The nature of periodicity is still unproven (I say this loud and clear), 
but I've been keen on learning if anything peculiar happens to the star 
at Phase=0.5, or around September 2000. 


I am still kind of hoping that the reports and this comment 
would trigger more communique from other observers down under. 
Especially it's really important to confirm or disprove if this
claimed decline is real or not. We professionals are a hapless
bunch when it comes to daily or weekly monitoring. And there, 
you gentlemen can make the significant difference. 


Cheers and thanks for all of your great works!

Bish 

PS. I'd like to note that our group still have one orbit worth
    of the HST time this year. We'd be damned if the star were truly 
    declining in its brightness AND we'd missed it by choosing not to 
    observe the star around September 2000 [right now, we're planning
    to use it for observing some nebular blobs of Eta that I don't
    really care that much]. IF the decline is real, then I have 
    a good reason to argue that the last orbit should be used 
    to observe the star as well.  

-- 
**********************************************************************
   Kazunori "Bish" Ishibashi(NRC/NAS)  E: bish@howdy.gsfc.nasa.gov   
   NASA/GSFC Code 681                  
   Greenbelt, MD 20771                 
**********************************************************************

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp