METAMORPHOSIS OF SN 2014C: DELAYED INTERACTION BETWEEN A HYDROGEN POOR CORE-COLLAPSE SUPERNOVA AND A NEARBY CIRCUMSTELLAR SHELL

Abstract

We present optical observations of supernova SN 2014C, which underwent an\nunprecedented slow metamorphosis from H-poor type Ib to H-rich type IIn over\nthe course of one year. The observed spectroscopic evolution is consistent with\nthe supernova having exploded in a cavity before encountering a massive shell\nof the progenitor star's stripped hydrogen envelope. Possible origins for the\ncircumstellar shell include a brief Wolf-Rayet fast wind phase that overtook a\nslower red supergiant wind, eruptive ejection, or confinement of circumstellar\nmaterial by external influences of neighboring stars. An extended high velocity\nHalpha absorption feature seen in near-maximum light spectra implies that the\nprogenitor star was not completely stripped of hydrogen at the time of core\ncollapse. Archival pre-explosion Subaru Telescope Suprime-Cam and Hubble Space\nTelescope Wide Field Planetary Camera 2 images of the region obtained in 2009\nshow a coincident source that is most likely a compact massive star cluster in\nNGC 7331 that hosted the progenitor system. By comparing the emission\nproperties of the source with stellar population models that incorporate\ninteracting binary stars we estimate the age of the host cluster to be 30 - 300\nMyr, and favor ages closer to 30 Myr in light of relatively strong Halpha\nemission. SN 2014C is the best-observed member of a class of core-collapse\nsupernovae that fill the gap between events that interact strongly with dense,\nnearby environments immediately after explosion and those that never show signs\nof interaction. Better understanding of the frequency and nature of this\nintermediate population can contribute valuable information about the poorly\nunderstood final stages of stellar evolution.\n