Ultraviolet spectra of extreme nearby star-forming regions: evidence for
  an overabundance of very massive stars
  As deep spectroscopic campaigns extend to higher redshifts and lower stellar
masses, the interpretation of galaxy spectra depends increasingly upon models
for very young stellar populations. Here we present new HST/COS ultraviolet
spectroscopy of seven nearby ($<120$ Mpc) star-forming regions hosting very
young stellar populations ($\sim$ 4-20 Myr) with optical Wolf-Rayet stellar
wind signatures, ideal laboratories in which to test these stellar models. We
detect nebular C III] in all seven, but at equivalent widths uniformly $< 10$
{\AA}. This suggests that even for very young stellar populations, the highest
equivalent width C III] emission at $\geq 15$ {\AA} is reserved for
inefficiently-cooled gas at metallicities at or below that of the SMC. The
spectra also reveal strong C IV P-Cygni profiles and broad He II emission
formed in the winds of massive stars, including some of the most prominent He
II stellar wind lines ever detected in integrated spectra. We find that the
latest stellar population synthesis prescriptions with improved treatment of
massive stars nearly reproduce the entire range of stellar He II wind strengths
observed here. However, we find that these models cannot simultaneously match
the strongest wind features alongside the optical nebular line constraints.
This discrepancy can be naturally explained by an overabundance of very massive
stars produced by a high incidence of binary mass transfer and mergers
occurring on short $\lesssim 10$ Myr timescales, suggesting these processes may
be crucial for understanding the highest-sSFR galaxies in the early Universe.
Reproducing both the stellar and nebular light of young systems such as these
will be a crucial benchmark for the next generation of stellar population
synthesis models.
