Nearby Supernova Factory Observations of SN 2005gj: Another Type Ia Supernova in a Massive Circumstellar Envelope

Abstract

We report Nearby Supernova Factory observations of SN 2005g j, the second confirmed case of a ''hybrid'' Type Ia/IIn supernova, which we interpret as the explosion of a white dwarf interacting with a circumstellar medium. Our early-phase photometry of SN 2005gj shows that the interaction is much stronger than for the prototype, SN 2002ic. Our first spectrum shows a hot continuum with broad and narrow H emission. Later spectra, spanning over 4 months from outburst, show clear Type Ia features combined with broad and narrow H , H , H , and He i kk5876, 7065 in emission. At higher resolution, P Cygni profiles are apparent. Surprisingly, we also observe an inverted P Cygni profile for [O iii] k5007. We find that the light curve and measured velocity of the unshocked circumstellar material imply mass loss as recently as 8 years ago. The early light curve is well described by a flat radial density profile for the circumstellar material. However, our decomposition of the spectra into Type Ia and shock emission components allows for little obscuration of the supernova, suggesting an aspherical or clumpy distribution for the circumstellar material. We suggest that the emission-line velocity profiles arise from electron scattering rather than the kinematics of the shock. This is supported by the inferred high densities and the lack of evidence for evolution in the line widths. Ground-and spacebased photometry and Keck spectroscopy of the host galaxy are used to ascertain that the host galaxy has low metallicity (Z /Z < 0:3; 95% confidence) and that this galaxy is undergoing a significant star formation event that began roughly 200 AE 70 Myr ago. We discuss the implications of these observations for progenitor models and cosmology using Type Ia supernovae.

Type Ia Supernovae Strongly Interacting with Their Circumstellar Medium

“…At later epochs, the EW tends to increase with time until ∼1 yr past maximum, when the EW possibly begins to decrease. This overall behavior has been seen previously in PTF11kx (Dilday et al 2012;Silverman et al 2013), SN 2002ic (Wang et al 2004, and SN 2005gj (Aldering et al 2006;Prieto et al 2007). …”

“…As already mentioned, the two prototypical and best-studied SNe Ia-CSM are SN 2002ic (Hamuy et al 2003;Deng et al 2004;Kotak et al 2004;Wang et al 2004;Wood-Vasey et al 2004) and SN 2005gj (Aldering et al 2006;Prieto et al 2007). The photometric and spectral evolution of these two objects has been well studied in the works cited above.…”

“…The Balmer decrement in SNe Ia-CSM also appears to increase with time before eventually decreasing, peaking at ages anywhere from about a few months to a year and a half past maximum brightness. PTF11kx had Hα/Hβ > 7 in all of its late-time spectra (Silverman et al 2013), and large Balmer decrements were also observed in SN 2005gj (Aldering et al 2006).…”

“…Previously calculated Hα luminosities of SNe Ia-CSM are consistent with this range. Specifically, SN 2002ic had a luminosity of ∼5 × 10 40 erg s −1 (Kotak et al 2004), SN 2005gj had luminosities of (1-10) × 10 40 erg s −1 (Aldering et al 2006;Prieto et al 2007), and SN 2008J had a luminosity of ∼1.3 × 10 41 erg s −1 (Taddia et al 2012).…”

“…Their light curves are broad and quite long-lived, peaking at absolute magnitudes brighter than about −19 mag. The two best-studied objects in this class are SNe 2002ic and 2005gj (Hamuy et al 2003;Deng et al 2004;Kotak et al 2004;Wang et al 2004;Wood-Vasey et al 2004;Aldering et al 2006;Prieto et al 2007). Until recently there was some debate in the literature as to whether these objects are truly SNe Ia or are in fact some strange flavor of core-collapse SNe (CCSNe; e.g., Benetti et al 2006;Trundle et al 2008).…”