Improved distance determination to M51 from supernovae 2011dh and 2005cs
  The appearance of two recent supernovae, SN 2011dh and 2005cs, both in M51,
provides an opportunity to derive an improved distance to their host galaxy by
combining the observations of both SNe. We apply the Expanding Photosphere
Method to get the distance to M51 by fitting the data of these two SNe
simultaneously. In order to correct for the effect of flux dilution, we use
correction factors (zeta) appropriate for standard type II-P SNe atmospheres
for 2005cs, but find zeta ~ 1 for the type IIb SN 2011dh, which may be due to
the reduced H-content of its ejecta. The EPM analysis resulted in D_M51 = 8.4
+/- 0.7 Mpc. Based on this improved distance, we also re-analyze the HST
observations of the proposed progenitor of SN 2011dh. We confirm that the
object detected on the pre-explosion HST-images is unlikely to be a compact
stellar cluster. In addition, its derived radius (~ 277$ R_sun) is too large
for being the real (exploded) progenitor of SN 2011dh. The supernova-based
distance, D = 8.4 Mpc, is in good agreement with other recent distance
estimates to M51.
