Carbon, Helium and Proton Kinetic Temperatures in a Cygnus Loop Shock
  Wave
  Observations of SN1006 have shown that ions and electrons in the plasma
behind fast supernova remnant shock waves are far from equilibrium, with the
electron temperature much lower than the proton temperature and ion
temperatures approximately proportional to ion mass. In the ~360 km/s shock
waves of the Cygnus Loop, on the other hand, electron and ion temperatures are
roughly equal, and there is evidence that the oxygen kinetic temperature is not
far from the proton temperature. In this paper we report observations of the He
II lambda 1640 line and the C IV lambda 1550 doublet in a 360 km/s shock in the
Cygnus Loop. While the best fit kinetic temperatures are somewhat higher than
the proton temperature, the temperatures of He and C are consistent with the
proton temperature and the upper limits are 0.5 and 0.3 times the
mass-proportional temperatures, implying efficient thermal equilibration in
this collisionless shock. The equilibration of helium and hydrogen affects the
conversion between proton temperatures determined from H alpha line profiles
and shock speeds, and that the efficient equilibration found here reduces the
shock speed estimates and the distance estimate to the Cygnus Loop of Medina et
al. (2014) to about 800 pc.
