Physical Properties of II Zw 40's Super Star Cluster and Nebula: New
  Insights and Puzzles from UV Spectroscopy
  We analyze far-ultraviolet spectra and ancillary data of the super star
cluster SSC-N and its surrounding H II region in the nearby dwarf galaxy II Zw
40. From the ultraviolet spectrum, we derive a low internal reddening of E(B-V)
= 0.07 +/- 0.03, a mass of (9.1 +/- 1.0) x 10^5 Lsol, a bolometric luminosity
of (1.1 +/- 0.1) x 10^9 Lsol, a number of ionizing photons of (6 +/- 2) x 10^52
s^-1, and an age of (2.8 +/- 0.1) Myr. These parameters agree with the values
derived from optical and radio data, indicating no significant obscured star
formation, absorption of photons by dust, or photon leakage. SSC-N and its
nebulosity are an order of magnitude more massive and luminous than 30 Doradus
and its ionizing cluster. Photoionization modeling suggests a high ionization
parameter and a C/O ratio where C is between primary and secondary. We
calculate diagnostic emission-line ratios and compare SSC-N to local
star-forming galaxies. The SSC-N nebula does not coincide with the locus
defined by local galaxies. Rather, it coincides with the location of "Green
Pea" galaxies, objects which are often considered nearby analogs of the
galaxies reionizing the universe. Most stellar features are well-reproduced by
synthetic spectra. However, the SSC-N cluster has strong, broad, stellar He II
1640 emission that cannot be reproduced, suggesting a deficit of He-enhanced
stars with massive winds in the models. We discuss possible sources for the
broad He II emission, including very massive stars and/or enhanced mixing
processes.
