Uncertainties in The Interstellar Extinction Curve and the Cepheid Distance to M101

TL;DR

This paper reevaluates the Cepheid-based distance to galaxy M101 considering recent debates on the interstellar extinction curve, finding that a steeper extinction law improves fit quality and slightly increases the estimated distance.

Contribution

It demonstrates that adopting a steeper interstellar extinction curve reduces scatter and metallicity dependence in Cepheid distance measurements to M101.

Findings

Steeper extinction ratio $A_{I}/E(V-I)$ is favored.

Distance modulus to M101 increases by ~0.06 mag.

Metallicity dependence of Cepheid luminosity is quantified.

Abstract

I revisit the Cepheid-distance determination to the nearby spiral galaxy M101 (Pinwheel Galaxy) of Shappee & Stanek (2011), in light of several recent investigations questioning the shape of the interstellar extinction curve at $\lambda \approx 8,000$ \AA (i.e. I-band). I find that the relatively steep extinction ratio $A_{I}/E(V-I)=1.1450$ (Fitzpatrick & Massa 2007) is slightly favoured relative to $A_{I}/E(V-I)=1.2899$ (Fitzpatrick 1999) and significantly favoured relative the historically canonical value of $A_{I}/E(V-I)=1.4695$ (Cardelli et al. 1989). The steeper extinction curves, with lower values of $A_{I}/E(V-I)$, yield fits with reduced scatter, metallicity-dependences to the dereddened Cepheid luminosities that are closer to values inferred in the local group, and that are less sensitive to the choice of reddening cut imposed in the sample selection. The increase in distance modulus to M101 when using the preferred extinction curve is ${\Delta}{\mu} \sim 0.06$ mag, resulting in an estimate of the distance modulus to M101 relative to the LMC of $ {\Delta}\mu_{\rm{LMC}} \approx 10.72 \pm 0.03$ (stat). The best-fit metallicity-dependence is $dM_{I}/d\rm{[O/H]} \approx (-0.38 \pm 0.14$ (stat)) mag dex$^{-1}$.