[vsnet-chat 3032] Re: [vsnet-obs 28038] ETA CARINAE - Two years of observations

["William S G Walker" <astroman@voyager.co.nz>]

Hi Bish and Sebastian,

I read the notes about eta Carinae with interest. As Bish knows, Harry
Williams measures this star in Auckland from time to time using UBV
filters. Your emails encouraged me to reduce his last two measures of May 2
and 24. These seem to show (but to misquote - 'two measures don't make a
decline') a pause in the brightening which has prevailed for some time. The
last dozen or so measures are set out below: JD-2400000, V, B-V, U-B.

It's rather difficult to measure this star using pep and the errors are
probably between 1% and 3%. There's obviously a case for continued
measures. Maybe the 5.5 year cycle is now beginning to make its presence
felt again. But I'd go along with Bish's comments - it's a very difficult
visual object. For what it's worth the colour has reddened by 0.06 in B-V
since 1972 and the U-B colour may have become a little stronger.

Let's all keep looking!

Regards,
Stan

50847.9544,5.442,.642,-.284
50850.9404,5.41,.653,-.275
50856.9378,5.422,.672,-.268
51319.8457,5.218,0.664,-0.312
51322.8565,5.184,0.665,-0.315
51335.8660,5.217,0.672,-0.334
51336.8543,5.222,0.645,-0.318
51348.8614,5.217,0.678,-0.285
51349.8440,5.181,0.665,-0.293
51560.9347,5.125,.695,-.301
51606.8692,5.124,.695,-.332
51612.8989,5.128,.667,-.285
51620.8788,5.103,.673,-.304
51625.9255,5.1,.672,-.329
51627.8502,5.092,.699,-.299
51630.8758,5.096,.668,-.313
51635.8901,5.098,.677,-.321
51666.9385,5.11,.673,-.308
51688.822,5.12,.645,-.379

----------
> From: Bish Ishibashi <bish@howdy.gsfc.nasa.gov>
> To: vsnet-chat@kusastro.kyoto-u.ac.jp; varsao@fullzero.com.ar
> Subject: [vsnet-chat 3029] Re: [vsnet-obs 28038] ETA CARINAE - Two years
of observations
> Date: Monday, June 05, 2000 12:27 AM
> 
> 
> 
> 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                 
> **********************************************************************
> 
>

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