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We present X-ray, UV/optical, and radio observations of thestripped-envelope, core-collapse supernova (SN) 2011ei, one of the leastluminous SNe IIb or Ib observed to date. Our observations begin with adiscovery within 1 day of explosion and span several months afterward. Earlyoptical spectra exhibit broad, Type II-like hydrogen Balmer profiles thatsubside rapidly and are replaced by Type Ib-like He-rich features on thetimescale of one week. High-cadence monitoring of this transition suggests thatabsorption attributable to a high velocity (> 12,000 km/s) H-rich shell is notrare in Type Ib events. Radio observations imply a shock velocity of v = 0.13cand a progenitor star mass-loss rate of 1.4 x 10^{-5} Msun yr^{-1} (assumingwind velocity v_w=10^3 km/s). This is consistent with independent constraintsfrom deep X-ray observations with Swift-XRT and Chandra. Overall, themulti-wavelength properties of SN 2011ei are consistent with the explosion of alower-mass (3-4 Msun), compact (R* <= 1x10^{11} cm), He core star. The starretained a thin hydrogen envelope at the time of explosion, and was embedded inan inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. Weconclude that SN 2011ei's rapid spectral metamorphosis is indicative oftime-dependent classifications that bias estimates of explosion rates for TypeIIb and Ib objects, and that important information about a progenitor star'sevolutionary state and mass-loss immediately prior to SN explosion can beinferred from timely multi-wavelength observations.

astrophysical journal/the astrophysical journal

Multi-Wavelength Observations of Supernova 2011ei: Time-Dependent  Classification of Type IIb and Ib Supernovae and Implications for their  Progenitors