tokens: 0.6–2.1, corresponding to E ( B − V ) = 0.2 to 0.7. However, the dust maps of Green et al. ( 2015 ) and Lallement et al. ( 2019 ) show E ( B – V ) increasing gradually up to 0.1–0.2 at a distance of 1 kpc and remaining flat at larger distances in the direction of G70.0–21.5. The probable explanation is that Boumis et al. ( 2002 ) assumed the recombination value of about 3 for the Balmer decrement, while collisional excitation produces ratios of 5 to 3.7 for temperatures of 20,000–40,000 K (CHIANTI; Del Zanna et al. 2015 ). Thus, for the temperatures estimated in Table 2 , E ( B – V ) could be overestimated by around 0.4–0.5. Variations in the collisional contribution to the Balmer lines would produce variation in the [N ii ] to H α ratios seen in Table 2 . A complication is that the Hectochelle fibers do not cover the entire postshock cooling region. The 15 in fiber diameter corresponds to about 2.2 × 10 16 cm at a distance of 1 kpc. If the ambient density is on the order of 0.2 cm −3 , as in the NW Balmer line filaments of the halo SNR SN 1006 (Ghavamian et al. 2002 ; Bandiera et al. 2019 ), the cooling distance can be larger than the fiber diameter, though the narrow ionization and cooling zone behind the shock would still fit within a fiber. It is probable that the sharp H α filaments seen in the MDM images are just the high-emissivity region just behind the shock, and a fainter, cooler, more diffuse region trails behind. That could account for some of the variation in the strengths of the [O i ], [N ii ], and [S ii ] lines relative to H α in Figure 7 of Fesen et al. ( 2015 ) and Table 3 of Boumis et al. ( 2002 ). To further investigate the line ratios, we ran models with the code described by Raymond ( 1979 ) and Cox Raymond ( 1985 ). We assume a preshock density of 0.1 cm −3 and a magnetic field of 0.5 μ G, and we consider a range of shock speeds from 50 to 90 km s −1 and preshock neutral fractions from 0.05 to 0.6. The density choice is based on the height in the Galactic halo, and the magnetic field has little effect on the postshock region where the strong lines are produced. The preshock neutral fraction is determined not only by the overall Galactic ionizing UV background, but also by the SNR itself. Cox ( 1972a ) showed that an SNR emits a strong burst of ionizing radiation when it enters the shell formation stage, and the recombination time is longer than the SNR evolution timescale. Thus, the preshock gas will be overionized in the case of G70.0–21.5. We truncate the models at distances corresponding to 125. The density could be higher, however, and that makes the cooling region proportionally thinner, so we also consider models truncated at 25 and 50, which correspond to preshock densities of 0.2 and 0.4 cm −3 . In general, a fairly wide range of parameters can match the observed ratio of [N ii ] to H α . However, we reject shock speeds below 60 km s −1 because they predict line widths smaller than we observe. We find that the
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