Type-Ia SN 2012cg: no progenitor detected in pre-explosion HST images to M_V ~ -6.0, M_I ~ -5.4 mag
ATel #4226; Or Graur (Tel-Aviv Univ., AMNH), Dan Maoz (Tel-Aviv Univ.)
on 29 Jun 2012; 13:19 UT
Credential Certification: Dan Maoz (Tel-Aviv University) (maoz@astro.tau.ac.il)
Subjects: Optical, Supernovae
We have analyzed archival Hubble Space Telescope WFPC2 images of NGC 4424 in F606W (~ V band, 600 s) and F814W (~ I band, 600 s) from 2001. We detect no point sources within a 2'' radius of the location of the Type-Ia SN 2012cg (CBET 3111, R.A.=12h27m12.83s, Decl.=+09d25'13.1"). By planting fake point sources at the location of the SN, we obtain limits of 25.5 mag (F606W)and 25.8 mag (F814W). The Tully-Fisher-based distance modulus of NGC 4424 is 30.9+/-0.3 (Cortes et al. 2008, ApJ, 683, 78), and the line of sight extinctions to the location of the SN (Silverman et al. 2012, arXiv:1206.1328) are A_V ~ 0.6 and A_I ~ 0.3. The progenitor of SN 2012cg was therefore fainter than absolute magnitudes M_V ~ -6.0 and M_I ~ -5.4, ruling out most supergiants as possible binary companions. These limits are, however, not faint enough to rule out SN interaction with a red-giant companion, as suggested by Marion et al. (Atel #4215) based on early blue colors of the event (but note differing interpretation by Silverman et al. 2012). An image showing the searched region of the HST data is at http://www.astro.tau.ac.il/~orgraur/sn2012cg.jpg.
