  H3+ has been shown (Lenzuni et al. 1991, ApJS, 76, 759) to be the dominant
positive ion, in a zero metallicity gas at low temperature and intermediate to
high density. It therefore affects both the number of free electrons and the
opacity of the gas. The most recent H3+ partition function (Neale and Tennyson,
1995, ApJ, 454, L169) is an order of magnitude larger at 4000 K than all
previous partition functions, implying that H3+ is a more important electron
donor than previously thought.
  Here we present new Rosseland mean opacities for a hydrogen-helium gas of
1000 < T(K) < 9000 and -14 < log10(density [g/cc]) < -2. In the calculation of
these opacities we have made use of the latest collision induced absorption
data as well as the most recent H3+ partition function and line opacity data.
It is shown that these updated and new sources of opacity give rise to a
Rosseland mean opacity for a hydrogen-helium gas which is in general greater
than that calculated in earlier works.
  The new opacity data are then used to model the evolution of low mass
0.15-0.8 Mo zero metallicity stars, from pre-Main Sequence collapse to Main
Sequence turn-off. To investigate the effect of H3+ on the evolution of low
mass zero metallicity stars, we repeat our calculations neglecting H3+ as a
source of electrons and line opacity. We find that H3+ can have an effect on
the structure and evolution of stars of mass ~0.5 Mo or less. A gray atmosphere
is used for the calculation, which is sufficient to demonstrate that H3+
affects the evolution of very low mass stars to a greater degree than
previously believed.
