A Rare Encounter with Very Massive Stars in NGC 3125-A1
  Super star cluster A1 in the nearby starburst galaxy NGC 3125 is
characterized by broad He\ii \lam1640 emission (full width at half maximum,
$FWHM\sim1200$ km s$^{-1}$) of unprecedented strength (equivalent width,
$EW=7.1\pm0.4$ \AA). Previous attempts to characterize the massive star content
in NGC 3125-A1 were hampered by the low resolution of the UV spectrum and the
lack of co-spatial panchromatic data. We obtained far-UV to near-IR
spectroscopy of the two principal emitting regions in the galaxy with the Space
Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS)
onboard the Hubble Space Telescope (\hst). We use these data to study three
clusters in the galaxy, A1, B1, and B2. We derive cluster ages of 3-4 Myr,
intrinsic reddenings of $E(B-V)=0.13$, 0.15, and 0.13, and cluster masses of
$1.7\times10^5$, $1.4\times10^5$, and $1.1\times10^5$ M$_\odot$, respectively.
A1 and B2 show O\vb \lam1371 absorption from massive stars, which is rarely
seen in star-forming galaxies, and have Wolf-Rayet (WR) to O star ratios of
$N(WN5-6)/N(O)=0.23$ and 0.10, respectively. The high $N(WN5-6)/N(O)$ ratio of
A1 cannot be reproduced by models that use a normal IMF and generic WR star
line luminosities. We rule out that the extraordinary He\ii \lam1640 emission
and O\vb \lam1371 absorption of A1 are due to an extremely flat upper IMF
exponent, and suggest that they originate in the winds of very massive
($>120\,M_\odot$) stars. In order to reproduce the properties of peculiar
clusters such as A1, the present grid of stellar evolution tracks implemented
in Starburst99 needs to be extended to masses $>120\,M_\odot$.
