SEARCH FOR PRECURSOR ERUPTIONS AMONG TYPE IIB SUPERNOVAE

The progenitor stars of several Type IIb supernovae (SNe) show indications\nfor extended hydrogen envelopes. These envelopes might be the outcome of\nluminous energetic pre-explosion events, so-called precursor eruptions. We use\nthe Palomar Transient Factory (PTF) pre-explosion observations of a sample of\n27 nearby Type IIb SNe to look for such precursors during the final years prior\nto the SN explosion. No precursors are found when combining the observations in\n15-day bins, and we calculate the absolute-magnitude-dependent upper limit on\nthe precursor rate. At the 90% confidence level, Type IIb SNe have on average\n$<0.86$ precursors as bright as absolute $R$-band magnitude $-14$ in the final\n3.5 years before the explosion and $<0.56$ events over the final year. In\ncontrast, precursors among SNe IIn have a $\\gtrsim 5$ times higher rate. The\nkinetic energy required to unbind a low-mass stellar envelope is comparable to\nthe radiated energy of a few-weeks-long precursor which would be detectable for\nthe closest SNe in our sample. Therefore, mass ejections, if they are common in\nsuch SNe, are radiatively inefficient or have durations longer than months.\nIndeed, when using 60-day bins a faint precursor candidate is detected prior to\nSN 2012cs ($\\sim2$% false-alarm probability). We also report the detection of\nthe progenitor of SN 2011dh which does not show detectable variability over the\nfinal two years before the explosion. The suggested progenitor of SN 2012P is\nstill present, and hence is likely a compact star cluster, or an unrelated\nobject.\n