Amendments on Y^2 isochrones
			(Version 2)

			May 14, 2003
		

We now release the version 2 Y^2 isochrones (YYiso_v2.tar.gz)
with an improved interpolator (YYmix2.f) as part of our Y^2 
isochrone-Overshoot (OS) Project.

Our isochrones are far from perfect. The previous version released 
through Yi et al. (2001) and Kim et al. (2002) had some problems. 

One problem is regarding the performance of our interpolator 
(released along with the isochrones), YYmix.f. It is a convenient 
tool, generating isochrones for any age, metallicity, and 
alpha-enhancement within the provided range. But occasionally, it 
produces incorrect isochrones in terms of shape (in particular at 
1-3Gyr range) and/or of mass sequence (mass does not increase 
monotinically) which usually happens at small ages. Such problems 
arise mainly for two reasons.
1. Nrow in isochrone tables is not strictly EEP (equal-evolutionary 
	point) and is not a perfect interpolation parameter. 
	Yet it was used so in YYmix.f.
2. Between 1 and 3Gyr, there is discontinuity in Overshoot treatment
	as explained in Yi et al. (2001).


We now release a new version of isochrones that are largely free from
those problems. This version has now finer (41) age steps that greatly
improves the capability of the interpolator. The new interpolator
(YYmix2.f), although not perfect, now works much better than 
the paper-released YYmix.f and a later revised code mYYmix.f. 
The new interpolator still occasionally fails to produce good 
isochrones for ages around 1-2Gyr. But now with the finer age grids
(i.e., 0.2gyr step at 1-2gyr) interpolation is hardly necessary.

Besides, we now adopt a ramping function for OVERSHOOT. Unlike in the
previous version, the new version is based on stellar models with
a gradually increasing value of OS. Here is the prescription. We
first find M_crit (mass above which convective core develops) for
each chemical composition (note that it is very sensitive to chemical
composition - see Yi et al. 2001) and then adopt the following scheme.

	Mass		       OS(in H_p)
	----------------------------------------
			Previous	Version2
	=< M_crit	0.0		0.0
	M_crit		0.2		0.05
	M_crit+0.1	0.2		0.10
	M_crit+0.2	0.2		0.15
	>= M_crit+0.3	0.2		0.20

With this OS-ramping the transition in the shape and other properties
of isochrones is much smoother than before, helping the interpolator
perform better as well.

There are other outstanding problems that still need attentions.
For example, because we didn't pay too much attention to the lower 
main-sequence, our isochrones may not be very impressive for fitting
lower MS CMDs. Our isochrones do not come with post-RGB tracks either.
It is because we are first trying to find the standard values of
Mcore and Yenv for HB model construction, which can be known only
after rigorous tests of MS-RGB models. So please be patient with us.

We apologise the users for any confusion/imperfection caused by the
use of our isochrones. We constantly work to improve them. And please
visit out WEB sites occasionally to find any major improvements.
We thank all who apprecaite our isochrones and who give feedbacks to us.

Sincerely,

Pierre		Demarque
Yong-Cheol	Kim
Sukyoung Ken 	Yi