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VLT/X-shooter spectroscopy of SN 2009ip: Narrow + intermediate width emission lines and NIR excess emission relative to a black body

ATel #4454; Christa Gall (GSFC, DARK/NBI), Jens Hjorth (DARK/NBI), Giorgos Leloudas (OKC Stockholm, DARK/NBI)
on 5 Oct 2012; 09:21 UT
Credential Certification: Jens Hjorth (jens@dark-cosmology.dk)

Subjects: Infra-Red, Optical, Supernovae

Referred to by ATel #: 4479, 4491

The recent outburst of the LBV SN 2009ip is likely a genuine Type IIn supernova (Smith & Mauerhan ATel #4412, ATel #4427, Mauerhan et al. arXiv:1209.6320). We observed the transient with the X-shooter spectrograph (Vernet et al. 2011, A&A, 536, A105) at the VLT on 2012 September 30.03 UT, covering the wavelength range 320 nm to 2400 nm, at a resolving power ranging from 5100 to 8800. The spectrum shows a richness of prominent hydrogen emission lines from the Balmer and Paschen series as well as Bracket gamma. We also detect a large number of He I lines (see also Burgasser et al. ATel #4431 and Vinko et al. #4435). The lines are well characterized by combinations of intermediate-width Lorentzians (FWHM ~ 800-1200 km/s) and narrow-width Gaussians or Lorentzians (FWHM ~ 100 km/s). The narrow lines likely arise from unshocked circumstellar material. However, they do not exhibit P Cygni profiles, such as those seen in, e.g., the Type IIn SN 2010jl (Smith et al. 2011, ApJ, 732, 63). From a black-body fit to the continuum emission we infer a high temperature of T ~ 14000 K for the emitting region. The spectrum shows a conspicuous NIR excess above 1000 nm and a slight ultraviolet excess below ~ 4000 nm relative to the black-body fit. Such excess emission at an early phase of a SN has been attributed to an extended atmosphere (Dwek et al. 1983, ApJ, 274, 168). Alternatively, the NIR excess is consistent with hot dust emitting at ~ 1700-2000 K.