Paper detail

The Distance to M101 Hosting Type Ia SN 2011fe Based on the Tip of the Red Giant Branch

We present a new determination of the distance to M101, host of the type Ia SN 2011fe, based on the tip of the red giant branch method (TRGB). Our determination is based on {\it Hubble Space Telescope} archival $F555W$ and $F814W$ images of nine fields within the galaxy. Color-magnitude diagrams of arm-free regions in all fields show a prominent red giant branch (RGB). We measure the $I$-band magnitudes of the TRGB, obtaining a mean value of $I_{\rm TRGB}=25.28\pm0.01$ (where the error is a standard error), using an edge-detection method. We derive a weighted mean value of distance modulus $(m-M)_0=29.30\pm0.01 ({\rm random})\pm0.12 ({\rm systematic})$, corresponding to a linear distance of $7.24\pm0.03\pm0.40 $ Mpc. While previous estimates for M101 show a large range (TRGB distances of $(m-M)_0=29.05$ to 29.42 and Cepheid distances of $(m-M)_0=29.04$ to 29.71), our measurements of the TRGB distances for nine fields show a small dispersion of only 0.02. We combine our distance estimate and photometry in the literature to derive absolute peak magnitudes in optical and near-infrared bands of SN 2011fe. Absolute maximum magnitudes of SN 2011fe are $\sim0.2$ mag brighter in the optical band and much more in the NIR than the current calibrations of SNe Ia in the literature. From the optical maximum magnitudes of SN 2011fe we obtain a value of the Hubble constant, $H_0=65.0\pm0.5({\rm random})\pm5.7({\rm systematic})$ \kmsMpc, slightly smaller than other recent determinations of $H_0$.