[vsnet-chat 552] Eta Carinae -- 5.52-year periodicity --

[Kazunori Ishibashi <bish@barnegat.gsfc.nasa.gov>]

Hi there,

  I am Kazunori "Bish" Ishibashi, a graduate student at
the University of Minnesota (right now I'm at the Goddard
Space Flight Center for collaboration). I just joined 
in this mailing list and I am not sure how this chat place
really works. So please bear with me if I start rumbling 
on my message...

  A couple of days ago, I was pointed out to a message that 
Dr. Ishida posted regarding on Eta Carinae and its variability, 
and today I saw a Dr. Farrell's reply to his remark. Now that 
I am actively involved with a large collaborative group 
for studying Eta Carinae in multi-waveband observations, allow
me to drop my two cents worth of a comment...(rather a lengthy one at that). 

Toshihito Ishida wrote on Thu, 14 Aug 1997 20:37:23:
>In ApJ vol. 460, L43-L52, 1997, Damineli reported that there exists 

Erratum: The reference is incorrect. The correct one 
is "1996, ApJ, 460, L49." 

>I cannot check the recent light curve of Eta Carinae on vsnet web site, 
>because my computer network environment is poor.  I believe that this 
>star is still not experienced the next brightening from Damineli's letter.  

Let me to clarify what Augusto Damineli really meant 
to tell in his paper. His initial prediction is that 
every 5.52 years "high-excitation events," i.e., 
any events that are associated with high-temperature 
gas or radiation, diminishes. He has reached to this 
conclusion by finding a 5.52-year period in variations
of equivalent widths for high excitation lines (such as
He I 10830 A, [Ne III], etc...). 

Now about his prediction. He predicts that all 
high-excitation lines in Eta Car's spectrum will 
either disappear or weaken. In addition to that, 
he expects the magnitude of Eta Car in H band (it's
in near-Infrared) will reach maximum during the event 
(please refer to Whitelock et al., 1994,MNRAS, 270, 364), 
but did not know by how much. And if my memory serves, 
in the optical photometry his period does not show 
up very clearly (see the counter-arguement to Daminel's 
paper written by van Genderen et al., 1997, A&AS, 318, 81). 
So I wouldn't know what'll happen there. 

Damineli expects this event to happen in January 1, 1998, 
and that's the predicted date when the last time I heard from him. 

>I'm afraid that the mechanism of the major bright-up on 1843 may not be 
>the same as that of the minor one.  However, although they might not the 

I am afraid but no one understands why and how it underwent the "Great
Eruption" in 1843. William J. F. Herschel brought up the "how?" question 
about 150 years ago, and we STILL question the same issue today! So may I say that 
it is quite premature to suggest any minor eruption is a result of a different
mechanism that let Eta Carinae blow up 1 solar mass of its mass...

Fraser Farrell wrote on Mon, 18 Aug 1997 00:05:18:
>How bright is it expected to be?  And for how long? 

Eta Carinae is doing its best for being so obnoxiously 
unpredictable, so I wouldn't know how much brighter it
gets in the H band, but the event itself may last a month 
or longer. In fact, Damineli, Conti, and Lopes (refer as DCL, 1997, 
New Astronomy, 2, 107, and see WWW at 
http://http://vsnet1.elsevier.nl/journals/newast/jnl/articles/S1384107697000080/ )
currently explain the periodicity with a binary model. 
If Eta Carinae is indeed a binary, the minimum state in high-
excitation event (the maximum for the H-band or optical??) 
may last a longer than a month. There will be also an article by Kris Davidson 
commenting on the DCL paper, which will soon appear (pending on its 
acception) in New Astronomy as well. If you want a copy of Kris's paper, 
I can have him send it out electronically for anyone who wants it. 

>Eta Car has been showing small irregular fluctuations between visual magnitudes
>5.4 and 5.8 (on a timescale of days) all this year.  Its behaviour last year was
>similar, although it tended more to the 5.4 than the 5.8 in my opinion.

Fascinating. How often do you do photometry on Eta Carinae? 
It sounds like you do in a daily basis (not right now, I reckon). If so, 
I would like to see your result and see if the optical photometry is
in any way correlated with our X-ray monitoring of Eta Carinae (we generally 
monitor weekly, but in the period of 1997 June 20th ot July 20th, 
we took X-ray data daily). Could you please let me know if this is possible? 

And I searched the log of this mailing list and found that YOU were the one
who sent out that alert about Eta Carinae in last December. If it makes you 
happy, we X-ray astronomers also noted that the hard X-ray flux from Eta began 
its gradual increase at about the same time. Well, coincidence? Who knows? 

>I am wondering if Daminelli's spectral variation mechanism is related to Eta
>Car's recently announced X-ray variablity??

We are working on it; ). But the variability we reported (Ishibashi et al., 
IAUC 6668 and Corcoran et al., IAUC 6701) are not exactly related to the 
5.52-year period (the X-ray period is about 85.5 +/-4.9 days for one reason). 
For the cause of the variability, well, we are working on that, too...

So much to my annoyance, I can't tell you the detail we've found so far...
but you can find out some of what we do in several WWW pages like,

The official WWW for X-ray monitoring of Eta Car by Michael F. Corcoran:

http://lheawww.gsfc.nasa.gov/users/corcoran/eta_car/eta_car.html 

and my own spiffy(?) version of Eta Car WWW page:

http://ast1.spa.umn.edu/bish/resch.html

to see our latest result for X-ray monitoring of Eta Carinae. 

Anyhow, what I really wanted to emphasize here is a NEED FOR a COORDINATED
GROUND BASE MONITORING of ETA CARINAE. So far the STScI refused twice 
our requests for using the HST/STIS to observe Eta Carinae spectroscopically, 
because either they don't think it's important or do not believe that this 
event will happen. This is partly our fault because we did such a poor
PR on this event, and therefore not many astronomers seems to be noticed 
or cared about this. This is really bad, I tell'ya. 

Sooooo, no more poor PR! We will be asking people with a proper instrument 
to monitor this star as much and frequent as they can! Photometry 
may be ok, but spectroscopy near the central star is more important. 
(there is a good reason we don't really care much about the homunculous, 
but that's beyond my intension for this message). Other waveband, like radio, 
IR, should be covered, too, but that's probably too much to ask in this 
mailing list. Please remember, after the next prediction after the one in 
the January of 1999 may occur in the Summer of (no I meant Winter of) 2003, 
when Eta Carinae is probably unobservable from ground.  

So we shouldn't miss this opportunity! Please tell you friend that 
Eta Carinae is getting a hot (I mean cool, in a physical sense) subject!!

Well, sorry about my lengthy blah blah. 

With best regards, 

Bish Ishibashi

Department of Astronomy
University of Minnesota

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