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SN 2009ip is now below the proposed progenitor level observed in 1999

ATel #8417; C. Thoene (IAA-CSIC), A. de Ugarte Postigo (IAA-CSIC/DARK), G. Leloudas (Weizmann/DARK), Z. Cano (Univ. of Iceland), K. Maeda (Univ. of Kyoto)
on 15 Dec 2015; 19:15 UT
Credential Certification: Christina Thoene (cthoene@iaa.es)

Subjects: Optical, Supernovae

SN 2009ip had been proposed by various authors to be either a terminal explosion of a star as a Type IIn SN or a SN impostor that ejected a large shell of mass but not leading to the core-collapse of the star. Foley et al. 2011 (ApJ, 732, 32) presented pre-explosion images from HST dating from June 29, 1999. In those images, the possible progenitor had observed magnitudes of F606W = 21.84 +/- 0.25 mag (Vega), transformation to V-band gives an absolute magnitude of M_V = -9.8 +/- 0.3 mag (assuming a distance modulus of 31.55 mag). Note that this magnitude differs by 0.5 mag from the value stated in Foley et al. 2011 who are using a different distance modulus. Unfortunately, observations were only done in one filter, so no color information is available for this epoch.

We observed SN 2009ip using OSIRIS/GTC on Nov. 29, 19:26 UT in g', r' and i' band. The object is now 1162 days after the maximum of its brightest event on Oct. 9, 2012 and has continued to fade since the last reported observations on Dec. 23, 2014 = 821d post maximum (Fraser et al. 2015, MNRAS, 453, 3886). On Nov. 29, SN 2009ip had the following magnitudes
g'= 22.44 +/- 0.07, i'=22.33 +/- 0.14 (all Vega, corrected for extinction)
corresponding to M_V = -9.2 +/- 0.07 mag which means it is now 0.6 magnitudes fainter than in the HST observations in 1999. SN 2009ip also further continued the trend towards bluer V-I colors, as it had done throughout 2014 and 2015 (see Fraser et al. 2015), and has now g'-i' = -0.11 mag (Vega).

The possible implications from our observations are that either (1) the progenitor of SN 2009ip had already been in outburst in 1999, (2) that the possible SN event in 2012 was in fact a terminal explosion or that (3) the system consists of a binary star of which only one exploded. The blue colors, however, are atypical for a late-time SN observation. As noted by e.g. Graham et al. 2014 (ApJ, 787, 163) and Fraser et al. 2015 the blue colors do not necessarily correspond to an increase in temperature if the SED is not well modeled by a black-body, though an increasing excess in V-band needs some mechanism atypical of a SN to be explained.

If the event in October 2012 was non-terminal, the remaining star is now consistent with having at most 40 solar masses (according to single-star, non-rotating Geneva stellar evolution tracks at solar metallicity), significantly lower than the 50-80 solar masses proposed by Smith et al. 2010 (ApJ, 139, 1451) and Foley et al. 2011.