tokens: ). CSM densities and radii of well-known Type Ia SNe and confirmed Type Ia SNRs studied in radio or X-rays, along with SN 1885A and G1.9+0.3 from this work (shown in red). The blue dashed and dotted lines indicate different CSM profiles of the form ( is mass-loss rate, u w is wind velocity, μ = 1.4) created by isotropic mass loss at constant rates and constant velocities given in the legend box. The horizontal gray bars show examples of cavity sizes created by accretion-driven outflows from the progenitor due to fast and slow winds (Badenes et al. 2007 ). The ruler on top shows the timescales on which these CSM profiles formed in the pre-supernova stage. The gray region shows the typical extent of the warm ISM phase (Ferrière 2001 ). The red dashed line indicates the average ISM density of the M31* circumnuclear region where SN 1885A is located (Li et al. 2009 ). Object references: SN 2011fe (Chomiuk et al. 2012 ), SN 2014J (Pérez-Torres et al. 2014 ), Tycho (Slane et al. 2014 ), SN 1006 (Raymond et al. 2007 ). Our upper limit on density of 0.04 ± 0.03 (sys) cm −3 derived in Section 5.1 suggests that, on average, SN 1885A has interacted with very little CSM material. A significantly denser CSM, if present, would have been captured in our measurements of n 0 in Figure 3 (a), and could have produced detectable radio emission. A low-density CSM (≲0.1 cm −1 ) around SN 1885A was also inferred from the non-detection of X-rays from SN 1885A (Perets et al. 2011 ). Our inferred density is lower than that of well-known Type Ia SNRs, and well-studied, recent nearby Type Ia SNe such as SN 2011fe and SN 2014J, although the density values are in the range of the warm ISM phase (Figure 5 ). The strong density limit rules out some portion of the SD parameter space for SN 1885A. For example, if the progenitor binary emitted a wind, the mass-loss rate, , would have been 10 −4 M ⊙ yr −1 (assuming wind velocity u w = 1000 km s −1 ) or 10 −6 M ⊙ yr −1 (assuming u w = 20 km s −1 , see Figure 5 ). Modeling the shock dynamics in such an r −2 profile can lead to even deeper constraints on , because the SNR would have evolved in much denser CSM in the past, and this would have left an observable effect on the radius and velocity of the SNR. However, as noted in Badenes et al. ( 2007 ), it is likely that the CSM structure on parsec scales resulting from an SD configuration is more complex than either a uniform or r −2 profile. Accounting for realistic CSM would require detailed modeling to predict the radius, velocity, and radio luminosity of the SNR, and is beyond the scope of this paper. In the future, it would be productive to revisit the observational constraints from this paper and Fesen et al. ( 2015 ) when a grid of models become available, predicting the CSM structure out to ∼10 pc for a variety of SN Ia progenitors (C. Badenes et al. 2019, in preparation). Such an analysis would allow us to make definitive statements about the progenitor of SN 1885A. As an
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