This thesis is an observational and theoretical study of the optical emission from nonradiative shocks in three supernova remnants: the Cygnus Loop, RCW 86 and Tycho, which together span a wide range of shock velocities (300 ≲ vS ≲ 2000 km s-1). The spectra are dominated by Balmer lines of H which have both a broad component caused by proton-neutral charge exchange and a narrow component produced by collisional excitation close to the shock front. The broad to narrow flux ratios observed in all three remnants are systematically smaller in Halpha than in Hbeta, and the narrow Balmer decrement is larger than the broad Balmer decrement. The broad component Halpha profiles of RCW 86 and Tycho are Gaussian, indicating that the postshock protons follow a Maxwellian velocity distribution.
To model the data, a new numerical shock code was developed which computes the broad and narrow Balmer line emission from a nonradiative shock in partially neutral gas. The models calculate the density, temperature and size of the postshock ionization layer for arbitrary electron-proton temperature equilibrations, and use a Monte Carlo simulation to compute narrow Balmer line enhancement from Lyman line trapping. The models constrain the shock velocity and equilibration of nonradiative shocks in each remnant using the observed Halpha and Hbeta broad to narrow ratios.
The models show that differences between the observed broad and narrow Balmer decrements can be explained by Lyman line trapping. The models also show that variations in electron-proton equilibration can reproduce the observed range of broad to narrow ratios. The results give 50--100% equilibration in nonradiative portions of the NE Cygnus Loop (vS ∼ 300 km s-1) and 40--50% equilibration in nonradiative portions of RCW 86 (vS ∼ 600 km s-1 ). In Tycho there are major discrepancies between the predicted and observed broad to narrow ratios, with only the Hbeta ratio matching the observations. The discrepancies may be due to additional narrow component emission that arises from a cosmic ray/fast neutral precursor. The observed Hbeta broad to narrow ratio implies ≲ 20% equilibration in Tycho. Hence, there is an inverse correlation between Mach number and equilibration for the three observed remnants. This correlation suggests there may be significant differences between collisionless shocks at low and high Mach numbers.
The spectroscopic observations also led to the serendipitous discovery of a photoionization precursor in Tycho. The precursor appears as a faint, diffuse region that extends several arcminutes ahead of the Balmer-dominated shocks in Tycho. A new photoionization model shows that the diffuse emission is mostly warm, neutral gas heated to ∼12,000 K by He II lambda304 photons from the nonradiative shock. High resolution spectra indicate that the upstream gas is further heated to ∼40,000 K just before entering the shock. This additional heating may arise from a second precursor produced either by cosmic rays or fast (broad component) neutrals from behind the hot postshock gas.