CALCIUM-RICH GAP TRANSIENTS IN THE REMOTE OUTSKIRTS OF GALAXIES

Abstract

From the first two seasons of the Palomar Transient Factory, we identify\nthree peculiar transients (PTF09dav, PTF10iuv, PTF11bij) with five\ndistinguishing characteristics: peak luminosity in the gap between novae and\nsupernovae (M_R = 15.5 to -16.5), rapid photometric evolution (rise-time\n~12--15 days), large photospheric velocities (~6000 to 11000 km/s), early\nspectroscopic evolution into nebular phase (~1 to 3 months) and peculiar\nnebular spectra dominated by Calcium. We also culled the extensive decade-long\nLick Observatory Supernova Search database and identified an additional member\nof this group, SN 2007ke. Our choice of photometric and spectroscopic\nproperties was motivated by SN 2005E (Perets et al. 2010). To our surprise, as\nin the case of SN 2005E, all four members of this group are also clearly offset\nfrom the bulk of their host galaxy. Given the well-sampled early and late-time\nlight curves, we derive ejecta masses in the range of 0.4--0.7 Msun.\nSpectroscopically, we find that there may be a diversity in the photospheric\nphase, but the commonality is in the unusual nebular spectra. Our extensive\nfollow-up observations rule out standard thermonuclear and standard\ncore-collapse explosions for this class of "Calcium-rich gap" transients. If\nthe progenitor is a white dwarf, we are likely seeing a detonation of the white\ndwarf core and perhaps, even shockfront interaction with a previously ejected\nnova shell. In the less likely scenario of a massive star progenitor, a very\nnon-standard channel specific to a low-metallicity environment needs to be\ninvoked (e.g., ejecta fallback leading to black hole formation). Detection (or\nlack thereof) of a faint underlying host (dwarf galaxy, cluster) will provide a\ncrucial and decisive diagnostic to choose between these alternatives.\n