[MODEST] summer plans?

[James Lombardi]

Hello all,

Piet and I had an email exchange in which I summarized what Paul, Scott and 
I are currently working on here at Vassar.  Piet suggested I send a similar 
summary to the list, so here it is!  Hopefully some of you will give 
feedback, or email everyone a similar summary of your planned work.  This 
would help foster collaborations among us and keep the momentum going now 
that the conference is over.

Paul has been working on a module that decides the outcome of a 
collision.  If the stars don't merge, his module will give the new masses 
and orbital elements, or says that the parents were destroyed.  Basically 
we imagine the stars as spheroids, and integrate to follow their motion by 
considering
just a few forces: point mass gravity, a tidal correction, a repulsive 
pressure force between stars when they're in contact, and an effective drag 
force from momentum exchange while they touch.  If they do merge, his 
module will use Make Me A Star to generate a remnant model.  The centrally 
attractive component of the tidal correction scales like 1/r^7 (this 
dependence can be argued analytically), with a coefficient that we 
calculate from the structure of the stars.  So there is a simple term that 
stellar dynamicists can easily add to point mass gravity to model close 
encounters slightly better.

Scott has become our local expert on Arnett's stellar evolution code, 
Tycho.  He's successfully evolved stars with masses in the range from ~1 
M_sun to ~ 75 M_sun.  Simon Portegies Zwart has kindly sent us the masses, 
age, orbitals elements, etc. of stars in a collision sequence that occurs 
in one of his 12k cluster simulations.  We are now using SPH and parent 
stars generated by Tycho to model the first collision in that sequence.

Not surprisingly, I think it will be tricky to get Tycho (or any stellar 
evolution code) to evolve remnant models.  We haven't actually tried this 
yet, but our (limited) experience suggests that for Tycho to run without 
human intervention requires that the starting model be scaled by < 20% from 
some other model that works nicely.  Once Tycho makes it through the 
thermal relaxation, it runs great!  I have the idea to write my own very 
simple code that does only thermal relaxation... it need not be concerned 
with nuclear burning, detailed envelope structure or even convection 
(provided the entropy profile is steep). Such a code would relax a remnant 
enough to make it palatable to a full scale evolution code.    Dina & Attay 
and Onno, would you be willing to "report" on what problems or luck you've 
had with the first attempts at evolving non-canonical stars?  If other 
stellar evolutionists are interested, I can send models of merger products 
(or the MMAS package for doing so can be downloaded from my web pages).

It would be nice to see a fair number of messages posted to the list, as in 
the days following the conference.  Feedback or summaries of your planned 
work would help us all work as efficiently and effectively as possible!

I hope all is well.

Cheers,
Jamie
*********************************************************************
  James Lombardi                               Phone:  (845) 437-7081
  Assistant Professor of Physics                 Fax:  (845) 437-5995
  Department of Physics and Astronomy
  Vassar College
  124 Raymond Ave.
  Mail Drop 562
  Poughkeepsie, NY 12604-0562    http://faculty.vassar.edu/~lombardi/
*********************************************************************