Further Properties of the Candidate Progenitor of SN 2011dh in M51

ATel #3431; D. Szczygiel, R. Khan, C. S. Kochanek (The Ohio State University)
on 14 Jun 2011; 17:23 UT
Credential Certification: C.S. Kochanek (ckochanek@astronomy.ohio-state.edu)

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We report further analyses of archival Hubble Space Telescope images of the SN 2011dh progenitor candidate, following ATELs #3399 and 3401 (Li et al. 2011), as well as Spitzer mid-IR upper limits.

Analyzing the archival HST F658N image using difference imaging and either the F555W or the F814W image as the template image, there is always a positive excess at the position of the candidate. This indicates the presence of Halpha emission from the candidate, consistent with the suggestion in ATEL #3428 (Silverman et al. 2011) that the progenitor lay in an HII region.

In the difference images, we also clearly see that the bright candidate is blended with a fainter, redder star approximately 0.2 arcsec to the South which does not appear to have any associated Halpha emission. The blend is also resolved by DAOPHOT, and found to have magnitudes of F555W = 26.08 ± 0.10 mag and F814W = 23.88 ± 0.05 mag. However, astrometry between the HST images and Large Binocular Telescope images of the region both before and after the SN confirm the original identification in ATEL #3399 and rule out this fainter source as a candidate for the progenitor.

We have also analyzed the archival HST WFPC2/F336W HST images, crudely estimating F336W = 24 ± 1 mag due to difficulties in estimating the large CTE corrections given the environment. Independent of the CTE problems, the candidate is dramatically fainter in the UV than the blue star roughly 1 arcsec to the North. A sudden drop in the SED between F435W and F336W is suggestive of a Balmer break. Fits to the candidate's SED based on the Padova (Marigo et al. 2008) isochrones suggest log(L)=5.3+/-0.3, log(T_e)=3.85+/-0.07, E(B-V)=0.26+/-0.19 and a ZAMS mass of 26+/-8 msun whether the F336W value is treated as a bound or a detection. This assumes a distance modulus of 29.6+/-1 and Galactic extinction of E(B-V)=0.035. ``Binary'' fits, modeling the SED as the sum of two stars, did not lead to significant improvements. We note, however, that the SN progenitor is identified as a star cluster in Hwang \& Lee (2008, object #30182).

Finally, based on archival Spitzer IRAC data, we note that there is no mid-IR emission associated with the progenitor to 3.6, 4.5, 5.8 and 8.0 micron 3-sigma limits of 16.95, 16.90, 14.96 and 15.20 (Vega) mag. These limits are too weak to constrain whether the extinction indicated by the SED models is circumstellar.

HST images and analysis