Carbon, Helium and Proton Kinetic Temperatures in a Cygnus Loop Shock Wave

TL;DR

This study investigates ion and electron temperatures in a 360 km/s shock wave in the Cygnus Loop, revealing efficient thermal equilibration among ions and electrons, contrasting with observations in faster supernova remnant shocks.

Contribution

It provides new observational evidence of thermal equilibration in a supernova remnant shock at moderate velocity, impacting shock speed and distance estimates.

Findings

He and C ion temperatures are consistent with proton temperatures.

Efficient thermal equilibration reduces shock speed estimates.

Revised distance to Cygnus Loop is about 800 pc.

Abstract

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.