Improved distance determination to M 51 from supernovae 2011dh and 2005cs

Aims. The appearance of two recent supernovae, SN 2011dh and 2005cs, both in M 51, provides an opportunity to derive an improved distance to their host galaxy by combining the observations of both SNe. Methods. We apply the Expanding Photosphere Method to get the distance to M 51 by fitting the data of these two SNe simultaneously. In order to correct for the effect of flux dilution, we use correction factors (ζ) appropriate for standard type II-P SNe atmospheres for 2005cs, but find ζ ∼ 1 for the type IIb SN 2011dh, which may be due to the reduced H-content of its ejecta. Results. The EPM analysis resulted in D M5 1 = 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� ) is too large for being the real (exploded) progenitor of SN 2011dh. Conclusions. The supernova-based distance, D = 8.4 Mpc, is in good agreement with other recent distance estimates to M 51.

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