An isochrone population function ( IPF ) gives the relative distribution of stars along an isochrone .
IPFs contain the information needed to calculate both luminosity functions and color functions , and they provide a straightforward way of generating synthetic stellar populations .
An improved algorithm for interpolating isochrones and isochrone population functions , based on the scheme introduced by Bergbusch & VandenBerg ( 1992 , \apjs , 81 , 163 ) , is described .
Software has been developed to permit such interpolations for any age encompassed by an input grid of stellar evolutionary tracks .
Our first application of this software is to the models presented in this series of papers for 17 [ Fe/H ] values between -2.31 and and -0.3 , with three choices of [ \alpha /Fe ] at each iron abundance ( specifically , 0.0 , 0.3 , and 0.6 ) .
[ These models do not treat gravitational settling or radiative acceleration processes , but otherwise they are based on up-to-date physics .
Additional grids will be added to this data base as they are completed . ]
The computer programs ( written in FORTRAN 77 ) and the grids of evolutionary tracks which are presently available for processing by these codes into isochrones and IPFs are freely available to interested users .
In addition , we add to the evidence presented in previous papers in this series in support of the { T _ { eff } } and color scales of our models .
In particular , the temperatures derived by Gratton et al .
( 1996 , A & A , 314 , 191 ) for local Population II subdwarfs with accurate ( Hipparcos ) parallaxes are shown to be in excellent agreement with those predicted for them , when the Gratton et al .
[ Fe/H ] scale is also assumed .
Interestingly , the locus defined by local subdwarfs and subgiants on the ( M _ { V } , \log T _ { eff } ) plane and the morphologies of globular cluster C-M diagrams are well matched by the present models , despite the neglect of diffusion — which suggests that some other process ( es ) must be at play to limit the expected effects of gravitational settling on predicted temperatures .
The three field halo subgiants in our sample all appear to have ages \mathrel { \hbox { \raise 2.58 pt \hbox { $ > $ } \kern - 7.31 pt \lower 2.15 pt \hbox { $ \sim$ } } } 15 Gyr , which is favored for the Galaxy ’ s most metal-poor globular clusters ( GCs ) as well .
( The settling of helium and heavy elements in the central regions of stars is expected to cause about a 10 % reduction in these age estimates : this effect should persist even if some process , such as turbulence at the base of the convective envelope , counteracts diffusion in the surface layers . )
Furthermore , our isochrones accurately reproduce the Da Costa & Armandroff ( 1990 , AJ , 100 , 162 ) red-giant branch fiducials for M 15 , NGC 6752 , NGC 1851 , and 47 Tuc on the [ M _ { I } , ( V - I ) _ { 0 } ] -diagram .
However , our models fail to predict the observed luminosities of the red-giant bump by \approx 0.25 mag : this could be an indication that there is some amount of inward overshooting of convective envelopes in red giants .
For consistency reasons , the Zinn & West ( 1984 , ApJS , 55 , 45 ) metallicities for intermediate metal-poor GCs ( -1.8 \mathrel { \hbox { \raise 2.58 pt \hbox { $ > $ } \kern - 7.31 pt \lower 2.15 pt \hbox { $ \sim% $ } } } [ Fe/H ] \mathrel { \hbox { \raise 2.58 pt \hbox { $ > $ } \kern - 7.31 pt \lower 2.15 pt \hbox { $ \sim$ } } } % -1.1 ) seem to be preferred over recent spectroscopic results ( based on the brightest cluster giants ) , suggesting that there is an inconsistency between current subdwarf and GC [ Fe/H ] scales .