[vsnet-chat 1809] Re: Yellow Alert from Eta Carinae

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

Greetings,

The recent discussion about eta Carinae has caused me to try to reconstruct
the Auckland Observatory's database on this star. We began measures in 1971
and continued through to mid-1987 after which observations became quite
sporadic. Unfortunately the computerised records were lost between 1990 and
1995 and I've had to search the handwritten summaries or even the hard
copies of the reduction printouts. To date I have a light curve from 1971
to 1982, but observations after this will need to be rereduced from the
original observing sheets.

I'm not up with the present behaviour of this star but we concluded that it
had a quasi-periodicity of about 1800 days (see Marino et al 1979 IAU Coll
46). Later observations show a rise, similar to that which is now causing
excitement, in 1980-81. In 1980 August it was at V = 5.96, by 1981 May it
had peaked at V = 5.70. By the following July - 1982 - it had dropped to
around V = 6.0 (unreduced data at the moment). The measures were reasonably
reliable, using a 50cm Zeiss reflector and an EMI 9502 pm tube and
calibration against the E Region stars measured by Cousins et al. A 30"
aperture was used and the three bright knots in the Homunculus were always
included. But there was bound to have been some shifting of the exact
region measured and it is now suggested that our comparison star is
slightly variable. The checks, however, usually matched to within 2-3% so
any problems there would have been minor.

Some criticism was made of the visual measures in a paper by Feinstein and
Maracco (A&A 30,271 - 1974) in which they attributed a decline in the
reliability of results to an influx of new observers in the 1960s. It so
happened that eta was brightening from 6.6 to 6.1 during this interval and
the sequence at this level had some bad errors. The most convenient
comparison was the six day eclipsing binary QZ Carinae and the magnitudes
of two other stars were badly in error. So probably the visual estimates of
this star now that the sequence has been corrected are no better or worse
than any others. 

I'll try to rework the 1982 to 1997 observations over the next few days and
post the completed datafile to the Auckland Observatory's web page. Our
measures seem to indicate that the brightening which began around 1940 is
now very slow. But superimposed on this are some type of outburst event
with a period of 4.5 to 5 years.

Regards,
Stan Walker

----------
> From: Bish Ishibashi <bish@astro.spa.umn.edu>
> To: vsnet-chat@kusastro.kyoto-u.ac.jp
> Cc: bish@slonce.spa.umn.edu
> Subject: [vsnet-chat 1804] Yellow Alert from Eta Carinae 
> Date: Tuesday, April 20, 1999 11:27 PM
> 
> 
> This is not so much for an alert-type letter, so allow me to post
> it here as a vschat material. 
> 
> As we noted in IAUC 7146, Eta Carinae has been increasing its 
> brightness at a unusual rate in the optical wavebands. 
> In a past few years, as reported by van Genderen et al.
> (Astr. Astrophysics, 343, 847, 1999), the rate of increase 
> in the Johnson V magnitude is about -0.05 to 0.1 mag per 
> year; photometry done at CTIO and SAAO on 17 Apr 1999
> by several groups (Humphreys at UMN, Hamuy at UA, Berdnikov
> and Whitelock at SAAO, and Metcalfe UT Austin) shows the larger  
> increase and gives an estimated V magnitude of 5.25, indicating
> nearly the rate of change of a -0.3 mag per year since the end of
> 1997. This is by far the largest, and the most rapid brightening
> of Eta Car in the recent history of this star. As Kris Davidson
> likes to describe it, "the last time it was this bright, Jubal Early's
> confederate army was raiding not far from the present site of Goddard
> Space Flight Center, Maryland..." 
> 
> (Also many thanks to Danie Overbeek at SA for checking up the star
> for us in a short notice!)
> 
> I understand, from Frasier's comment for a long ago, that Eta Carinae 
> is a difficult star to observe for novice amateurs because of many 
> reasons (1., the "homunculus" nebula is bright, and 2., the spectrum 
> is strongly contaminated by EMISSION lines and appear reddish due to 
> a strong H-Alpha emission line formed in the wind, perhaps, and etc). 
> But, we still like to encourage any amateurs or professionals to 
> "keep an eye on" Eta Carinae as much as prudent. Even though this is
> not a SUDDEN event like supernovae or novae or the rate of increase
> is not terribly alarming, something *unusual* apparently is happening
> to the star once again. And, the more we have people looking at the star,

> the less we are likely to miss something more dramatic...
> 
> Anyhow, I hope this letter encourages many observers to take a 
> quick look on this star. In a meantime, X-ray monitoring of Eta Carinae, 
> with M. F. Corcoran as a P.I., will continue to observe this star
> in hard X-ray radiation throughout 1999, and hopefully a year after that.

> Any coordinated observations are encouraged (please contact me for 
> more details). 
> 
> Cheers, 
> 
> Bish Ishibashi
> University of Minnesota 
> 
> PS. I am actually an X-ray astronomer -- so pardon me if I didn't  
>     get/write the details correctly. I am not used to talking UVBRI
>     -- I understand "eV" better...
> 
> 
> 
> 
>

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