Balmer‐dominated Shocks Revisited

We present a new formalism to describe the ratios and profiles of emissionlines from hydrogen in Balmer-dominated shocks. We use this model to interpretthe measured widths and ratios of broad and narrow H-alpha, H-beta and Ly-alphaemission lines in supernova remnants (SNRs). Our model results agree fairlywell with those obtained previously by Chevalier, Kirshner & Raymond (1980) andare consistent with observations of several SNRs. The same model fails toaccount for the ratio of broad to narrow line emission from the reverse shockin SNR 1987A as observed by Heng et al. (2006). We suggest that thisdiscrepancy between theory and observation results from a faulty assumptionthat Balmer-dominated shocks can be treated as sharp discontinuities. If thespatial structure of the shock transition zone is taken into account, thepredicted ratios of broad to narrow line emission in most SNRs will change bymodest factors, but the ratio in SNR 1987A will increase substantially.Significantly greater shock velocities will be required to account for theobserved full widths at half-maximum of the broad emission lines in most SNRs.Comment: 34 pages, 12 figures. Accepted by Ap