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

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 $dM_{I}/drm{[O/H]} approx (-0.38 pm 0.14$ (stat)) mag dex$^{-1}$.

The X-Shaped Milky Way Bulge in OGLE-III Photometry and in N-Body Models

We model the split red clump of the Galactic bulge in OGLE-III photometry, and compare the results to predictions from two N-body models. Our analysis yields precise maps of the brightness of the two red clumps, the fraction of stars in the more distant peak, and their combined surface density. We compare the observations to predictions from two N-body models previously used in the literature. Both models correctly predict several features as long as one assumes an angle $alpha_{rm{Bar}} approx 30^{circ}$ between the Galactic bars major axis and the line of sight to the Galactic centre. In particular that the fraction of stars in the faint red clump should decrease with increasing longitude. The biggest discrepancies between models and data are in the rate of decline of the combined surface density of red clump stars toward negative longitudes and of the brightness difference between the two red clumps toward positive longitudes, with neither discrepancy exceeding $sim$25% in amplitude. Our analysis of the red giant luminosity function also yields an estimate of the red giant branch bump parameters toward these high-latitude fields, and evidence for a high rate ($sim$25%) of disk contamination in the bulge at the colour and magnitude of the red clump, with the disk contamination rate increasing toward sightlines further distant from the plane.

Red Giant Branch Bump Star Counts in Data and Stellar Models

We compare model predictions to observations of star counts in the red giant branch bump (RGBB) relative to the number density of first-ascent red giant branch at the magnitude of the RGBB, $EW_{RGBB}$. The predictions are shown to exceed the data by $(5.2 pm 4.3)$% for the BaSTI models and by $(17.1 pm 4.3)$% for the Dartmouth models, where the listed errors are purely statistical. These two offsets are brought to zero if the Galactic globular cluster metallicity scale is assumed to be overestimated by a linear shift of $sim 0.11$ dex and $sim 0.36$ dex respectively. This inference based on RGBB star counts goes in the opposite direction to the increase in metallicities of ${Delta}$[M/H]$approx$0.20 dex that would be required to fix the offset between predicted and observed RGBB luminosities. This comparison is a constraint on deep mixing models of stellar interiors, which predict decreased rather than increased RGBB star counts. We tabulate the predictions for RGBB star counts as a function of [Fe/H], [$alpha$/Fe], CNONa, initial helium abundance, and age. Though our study suggests a small zero-point calibration issue, RGBB star counts should nonetheless be an actionable parameter with which to constrain stellar populations in the differential sense. The most significant outliers are toward the clusters NGC 5025 (M53), NGC 6723, and NGC 7089 (M2), each of which shows a $sim 2 sigma$ deficit in their RGBB star counts.

Reddening and Extinction Toward the Galactic Bulge from OGLE-III: The Inner Milky Ways Rv ~ 2.5 Extinction Curve

We combine VI photometry from OGLE-III with VVV and 2MASS measurements of E(J-K_{s}) to resolve the longstanding problem of the non-standard optical extinction toward the Galactic bulge. We show that the extinction is well-fit by the relation A_{I} = 0.7465*E(V-I) + 1.3700*E(J-K_{s}), or, equivalently, A_{I} = 1.217*E(V-I)(1+1.126*(E(J-K_{s})/E(V-I)-0.3433)). The optical and near-IR reddening law toward the inner Galaxy approximately follows an R_{V} approx 2.5 extinction curve with a dispersion {sigma}_{R_{V}} approx 0.2, consistent with extragalactic investigations of the hosts of type Ia SNe. Differential reddening is shown to be significant on scales as small as as our mean field size of 6, with the 1{sigma} dispersion in reddening averaging 9% of total reddening for our fields. The intrinsic luminosity parameters of the Galactic bulge red clump (RC) are derived to be (M_{I,RC}, sigma_{I,RC,0}, (V-I)_{RC,0}, sigma_{(V-I)_{RC}}, (J-K_{s})_{RC,0}) = (-0.12, 0.09, 1.06, 0.121, 0.66). Our measurements of the RC brightness, brightness dispersion and number counts allow us to estimate several Galactic bulge structural parameters. We estimate a distance to the Galactic center of 8.20 kpc, resolving previous discrepancies in distance determinations to the bulge based on I-band observations. We measure an upper bound on the tilt {alpha} approx 40{deg}. between the bars major axis and the Sun-Galactic center line of sight, though our brightness peaks are consistent with predictions of an N-body model oriented at {alpha} approx 25{deg}. The number of RC stars suggests a total stellar mass for the Galactic bulge of 2.0*10^{10} M_{odot}, if one assumes a Salpeter IMF.