[vsnet-rvtau 83] Re: U Mon, interchange of primary minima

[Toshihito Ishida <ishida@nhao.go.jp>]

Dear Otero-san, 

Taichi Kato wrote in the message received on Wed, 20 Mar 2002 14:57. 
Subject: [vsnet-campaign-rvtau 2] Re: U Mon, interchange of primary minima
> Re: U Mon, interchange of primary minima
> 
> > I want to ask you about this phenomenon:
> > Does it happen only with minima I or is it possible that there are two
> > consecutive minima II ?
> > Why does it happen?
> 
>    I hope Ishida-san (NHAO), "the master of pulsators", could explain the
> cause of this phenomenon :-)

Sorry for my reply is somewhat late.  I was off from e-mail access 
for a few days.  

But, the master?  Who? :-) 

I think there are many people who knows more than me about pulsators, 
especially concerning observational topics.  And the comment by 
Walker-san below seems to be enough and easily understandable 
explanation, but I will try to make some comments according to 
kind invitation by Kato-san.  

Stan Walker wrote in the message received on on Thu, 21 Mar 2002 14:05. 
> 
> A long time ago I did some UBV measures of U Mon. In this context we made
> some measures of UW Centauri which behaves like an RV Tauri star at maximum
> but often with an apparently triple maximum. It differs greatly from RY
> Sagittarii which has a more regular single-peaked Cepheid-like pulsation at
> maximum (although the rise and fall is rather symmetrical). In discussing
> this with someone - Karen Pollard I think - the comment was made that RV
> Tauri stars are fairly unstable and that the classical descriptions are
> fairly imaginitive. So don't be surprised at erratic behabviour.

Alternating minima is one of the characteristics of the RV Tauri 
stars.  But, we should also remark that there are cycle to cycle 
differences in there light curves.  In some stars, cycle to cycle 
difference is somewhat large,  but in other stars it is not.  
It is also pointed out that there is "irregularity of irregularity" 
in a single star.  Namely, when we observe one star, the star may 
behave almost irregular during some years, and the nearly regular 
light curve will appear after a while.  

Deterministic system with unpredictable behavior (so-called "Chaos") 
is one of the possible cause of such an irregularity.  Photometric 
data and visual observations of some stars are analyzed under 
assumption that it is chaos.  The result seems to suggest that 
the light curve cannot be explained by simplest models with 
single-mode,  but with a few radial pulsation modes.  

So, as Walker-san wrote, it is not surprizing that RV Tauri stars 
indicate irregularity, even if it is difficult to recognize the 
very existence of cycle for a while.  Even from data including such 
irregular epochs, there is possibility that we can derive information 
of these stars, if it is chaos, and observational data is precise, 
homogeneous and long-term enough.  

---
%  Toshihito ISHIDA, Ph. D., Astrophysicist    e-mail: ishida@nhao.go.jp
%  Nishi-Harima Astronomical Observatory, Sayo-cho, Hyogo 679-5313, JAPAN
%  Tel: +81-790-82-3886 ext. 142 Telefax: +81-790-82-3514

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