A Nitrogen-rich SNR in M31: SNR Interaction with the CSM at Late Times

We present the discovery of a supernova remnant (SNR) in M31 which is unlike any other remnant known in that galaxy. An optical MMT spectrum of WB92-26 sampling most of this marginally resolved object reveals strong lines of [O II], [Ne III], H I, [O III], [O I], [N II] and [S II], though the H I lines are very weak and the [N II] lines are very strong. Multiple velocity components are visible in those lines, with broad wings extending to $-2000$ and $+1500$ or $2000$ km/s (the heliocentric velocity of M31 is $-300$ km/s). The lines show strong peaks or shoulders near $-750$ km/s, $-50$ km/s, and $+800$ km/s in the M31 frame. The density implied by the [S II] ratio combined with the X-ray luminosity, FUV flux and optical size lead us to conclude that the optical emission lines are generated by shock waves, not photoionization. Consideration of the velocity structure indicates that the emission is from a shock in the circumstellar medium (CSM). This CSM must be depleted in H and enriched in He and N through CNO processing, and it must have had a high velocity before the explosion of the parent star, to explain the broad wings in the emission lines. We estimate the CSM shell to have a mass of 2 Msun, implying a Core Collapse SN. It is likely that Eta Car will produce a remnant resembling WB92-26 a few thousand years after it explodes.