* Astronomy

Title: Multi-Wavelength Observations of Supernova 2011ei: Time-Dependent Classification of Type IIb and Ib Supernovae and Implications for their Progenitors
Authors: D. Milisavljevic, R. Margutti, A. M. Soderberg, G. Pignata, L. Chomiuk, R. Fesen, F. Bufano, N. E. Sanders, J. T. Parrent, S. Parker, T. Pickering, D. Buckley, S. Crawford, A. A. M. Gulbis, C. Hettlage, E. Hooper, K. Nordsieck, D. O'Donoghue, T.-O. Husser, S. Potter, A. Kniazev, P. Kotze, E. Romero-Colmenero, P. Vaisanen, M. Wolf, N. Bartel, M. Bietenholz, C. Fransson, P. Mazzali, A. Brunthaler, S. Chakraborti, E. M. Levesque, A. MacFadyen, C. Drescher, G. Bock, P. Marples, J. P. Anderson, S. Benetti, D. Reichart, K. Ivarsen

We present X-ray, UV/optical, and radio observations of the stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least luminous SNe IIb or Ib observed to date. Our observations begin with a discovery within ~ 1 day of explosion and span several months afterward. Early optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on the timescale of one week. High-cadence monitoring of this transition identifies an absorption feature around 6250 Angstrom to be chiefly due to hydrogen, as opposed to C II, Ne I, or Si II. Similarities between this observed feature and several SNe Ib suggest that hydrogen absorption attributable to a high velocity (>12,000 km/s) H-rich shell is not rare in Type Ib events. Radio observations imply a shock velocity of v ~ 0.13c and a progenitor star mass-loss rate of ~ 1.4 x 10^{-5} solar masses yr^{-1} (assuming wind velocity v_w=1000 km/s). This is consistent with independent constraints estimated from deep X-ray observations with Swift-XRT and Chandra. We find the multi-wavelength properties of SN 2011ei to be consistent with the explosion of a lower-mass (3-4 solar masses), compact (R* ~ 1 x 10^11 cm), He core star. The star retained a thin hydrogen envelope at the time of outburst, and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. We conclude that SN 2011ei's rapid spectral metamorphosis calls attention to time-dependent classifications that bias estimates of explosion rates for a subset of Type IIb and Ib objects. Further, that important information about a progenitor star's evolutionary state and associated mass-loss in the days to years prior to SN outburst can be inferred from timely multi-wavelength observations.

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