MULTI-WAVELENGTH OBSERVATIONS OF SUPERNOVA 2011ei: TIME-DEPENDENT CLASSIFICATION OF TYPE IIb AND Ib SUPERNOVAE AND IMPLICATIONS FOR THEIR PROGENITORS

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 a timescale of one week. High-cadence monitoring of this transition suggests absorption attributable to a high-velocity ([> ~]12,000 km s super(-1)) H-rich shell, which is likely present in many Type Ib events. Radio observations imply a shock velocity of [upsilon] [asymptotically =] 0.13c and a progenitor star average mass-loss rate of M [asymptotically =] 1.4 x 10 super(-5) M sub([middot in circle]) yr super(-1) (assuming wind velocity [upsilon]w = 10 super(3) km s super(-1)). This is consistent with independent constraints from deep X-ray observations with Swift-XRT and Chandra. Overall, the multi-wavelength properties of SN 2011ei are consistent with the explosion of a lower-mass (3-4 M sub([middot in circle])), compact (R[low *] [