Further Analysis of the archival HST images of PTF11kly in M101

ATel #3585; Weidong Li (UCB), Peter Nugent (LBL/UCB), Josh Bloom (UCB), Alexei V. Filippenko (UCB), and Saurabh Jha (Rutgers), report on behalf of a large collaboration:
on 25 Aug 2011; 15:17 UT
Credential Certification: Weidong Li (weidong@astron.berkeley.edu)

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We performed preliminary photometry on the HST/ACS data of M101 reported by Li et al. (ATEL #3582). Two different approaches, one following the recipe by Sirianni et al. (2005, PASP 117, 1049) and the other utilizing the HSTphot/dolphot pipeline developed by Andrew Dolphin (2000, PASP 112, 1383), yielded similar results. The 3-sigma detection limits (in the VEGAMAG system) for the F435W, F555W, and F814W images are B = 27.4, V = 27.0, and I = 26.3, respectively. Star "1" has I = 24.9 +/- 0.1, and is not detected in the F555W or F435W images. Star "2" has I = 25.0 +/- 0.1, V = 26.8 +/- 0.2, and is not detected in F435W.

Adopting a distance modulus of DM = 29.05 mag for M101 (Stetson et al. 1998, ApJ 508, 491; Shappee & Stanek 2011, ApJ, 733, 124) and assuming no host galaxy reddening toward the objects in the neighborhood of PTF11kly (the Milky Way reddening in the direction of M101 is negligible), the above photometry suggest the following:

(a) If the progenitor system of PTF11kly is not detected in the HST/ACS images, the limiting absolute magnitudes [M(B) > -1.7, M(V) > -2.0, and M(I) > -2.8] cannot rule out a red giant star as the binary companion to the white dwarf that exploded as PTF11kly.

(b) Both Star "1" and "2" have M(I) = -4, and (V - I) colors of 1.8 mag or redder, and are thus red giant stars. If one of the two stars is associated with PTF11kly, it would suggest at least some SNe Ia come from white dwarfs in binary systems with red giant companions.