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Helium Detection in IRTF Spectra of SN 2011dh

ATel #3435; G. H. Marion (CfA), R. Kirshner (CfA), J. C. Wheeler (UT Austin), J. Vinko (U. Szeged), R. Chornock (CfA), R. J. Foley (CfA), P. Challis (CfA), A. Friedman (CfA), P. Garnavich (Notre Dame), and K. Krisciunas (TAMU)
on 16 Jun 2011; 19:28 UT
Credential Certification: Ryan Chornock (rchornock@cfa.harvard.edu)

Subjects: Infra-Red, Supernovae

We observed SN 2011dh with the IRTF 3.5m and the SpeX instrument on June 8, 12, & 16. No helium was detected on June 8. Weak evidence for helium was present on June 12 and He I is unambiguously detected from the 1.0830 & 2.0581 micron lines in the spectrum obtained June 16. The He I velocities on June 16 are about 8000 km/s while H velocities remain above 11000 km/s. O I & Fe II are also clearly detected. A figure displaying these data can be viewed at http://www.cfa.harvard.edu/supernova/sn11dh.html We thank John Rayner (IFA) for making time available for the IRTF observations.

A series of optical spectra (3300-7400 A) was obtained with the FAST spectrograph on the FLWO 1.5m Tillinghast telescope between June 4 and June 9. These spectra reveal no evidence of He I. Additional optical spectra were obtained at the 9.2m Hobby-Eberly Telescope at the McDonald Observatory using the Marcario Low-Resolution Spectrograph (4000-10000 A) on June 12 & 14. The June 12 spectrum shows weak evidence for He I 5876 & 6678 while the June 14 spectrum displays clear evidence of He I 5876 and 6678, and possibly He I 7065.

These results are consistent with the spectral classification of SN 2011dh as a Type IIb supernova (Arcavi et al. 2011, ATEL 3413). The emergence of helium in spectra of SN 2011dh occurred at a later epoch than it did for SN 2008ax but earlier than in SN 1993J.