  Numerous theoretical models have long proposed that a strong He II 1640
emission line is the most prominent and unique feature of massive Population
III (Pop III) stars in high redshift galaxies. The He II 1640 line strength can
constrain the mass and IMF of Pop III stars. We use F132N narrowband filter on
the Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3) to look for
strong He II lambda 1640 emission in the galaxy BDF-521 at z=7.01, one of the
most distant spectroscopically-confirmed galaxies to date. Using deep F132N
narrowband imaging, together with our broadband imaging with F125W and F160W
filters, we do not detect He II emission from this galaxy, but place a 2-sigma
upper limit on the flux of 5.3x10^-19 ergs s^-1 cm^-2. This measurement
corresponds to a 2-sigma upper limit on the Pop III star formation rate
(SFR_PopIII) of ~ 0.2 M_solar yr^-1, assuming a Salpeter IMF with 50< M/M_solar
< 1000. From the high signal-to-noise broadband measurements in F125W and
F160W, we fit the UV continuum for BDF-521. The spectral flux density is ~ 3.6x
10^-11 lambda^-2.32 ergs s^-1 cm^-2 A^-1, which corresponds to an overall
unobscured SFR of ~ 5 M_solar yr^-1. Our upper limit on SFR_PopIII suggests
that massive Pop III stars represent < 4% of the total star formation. Further,
the HST high resolution imaging suggests that BDF-521 is an extremely compact
galaxy, with a half-light radius of 0.6 kpc.
