Pre-explosion spiral mass loss of a binary star merger

Paper with Brian Metzger, Jacob Tyles and Kengo Tomida on the best-studied example of stellar mergers V1309 Sco. We explain the pre-maximum behavior as due to runaway increase of mass loss from the outer Lagrange L2 of a contact binary. The resulting spiral stream initially obscures the binary and modifies the light curve. Then, mass-loss rate is so high that the binary variability disappears and we see only shocks from collision of the spiral arms. We can track mass-loss rate by looking at the luminosity (with some caveats).  When the merger becomes dynamical, the faster more-spherical explosion collides with the previous equatorially-focused mass loss. The resulting shock interaction can power bright second peaks in many similar transients. Paper is at