The horizontal branch luminosity vs. metallicity in M 31 globular clusters

Abstract

Context. Thanks to the outstanding capabilites of the HS T , our current knowledge about the M31 globular clusters (GCs) is similar to our knowledge of the Milky Way GCs in the 1960s-1970s, which set the basis for studying the halo and galaxy formation using these objects as tracers, and established their importance in defining the cosmic distance scale. Aims. We intend to derive a new calibration of the M V (HB)-[Fe/H] relation by exploiting the large photometric database of old GCs in M31 in the HS T archive. Methods. We collected the BVI data for 48 old GCs in M31 and analysed them by applying the same methods and procedures to all objects. We obtained a set of homogeneous colour-magnitude diagrams (CMDs) that were best-fitted with the fiducial CMD ridge lines of selected Milky Way template GCs. Reddening, metallicity, Horizontal Branch (HB) luminosity and distance were determined self-consistently for each cluster. Results. There are three main results of this study: i) the relation M V (HB)=0.25(±0.02)[Fe/H]+0.89(±0.03), which is obtained from the above parameters and is calibrated on the distances of the template Galactic GCs; ii) the distance modulus to M31 of (m-M) 0 =24.42±0.06 mag, obtained by normalising this relation at the reference value of [Fe/H]=-1.5 to a similar relation using V 0 (HB). This is the first determination of the distance to M31 based on the characteristics of its GC system which is calibrated on Galactic GCs; iii) the distance to the Large Magellanic Cloud (LMC), which is estimated to be 18.54±0.07 mag as a consequence of the previous results. These values agree excellently with the most recent estimate based on HS T parallaxes of Galactic Cepheid and RR Lyrae stars, as well as with recent methods.

Toward a Better Understanding of the Distance Scale From Rr Lyrae Variable Stars: A Case Study for the Inner Halo Globular Cluster NGC 6723

“…Recently, Benedict et al (2011) The metallicity-luminosity relation by Benedict et al (2011) gives M V (RR) = 0.45 mag at [Fe/H] = −1.50 dex, which is about 0.07 mag brighter than those from others (for examples, see Federici et al 2012;Cacciari 2013;Kollmeier et al 2013). Although Benedict et al (2011) claimed that their relation gives the absolute visual distance modulus of LMC of (m − M) 0 = 18.55 ± 0.05 mag, the zero point of Benedict et al (2011), 0.45 ± 0.05 mag, and the dereddened mean visual magnitude of RRLs in LMC by Clementini et al (2003), 19.064 ± 0.064 mag at [Fe/H] = −1.50 dex, can be translated into (m − M) 0 = 18.61 ± 0.08 mag for LMC, significantly larger than those from other methods (Walker 2012;Cacciari 2013).…”

RR Lyrae stars: prime calibrators of the first rung of the distance ladder

“…This is exactly the relation referred to in Section 1, which was originally defined based on empirical evidence and provided the basis for Population II distance determinations. Past theoretical and empirical determinations of this relation showed linear or quadratic shapes (for recent summaries, see Catelan 2009;Federici et al 2012). In addition, the zero points could differ by as much as ∼0.3 mag, as shown by Gallart et al (2005, their fig.…”

Study of Integrated Spectra of Four Globular Clusters in M31

“…) with the observed CMD is shown, based on the literature values of the luminosity level of the horizontal branch VHB(Federici et al 2012), as well as on the metallicity and age determined by us as a result of the analysis of the spectra (Sections 3 and 4). The depth of the "color-magnitude" diagrams calculated according to photometry data is about 1m below the horizontal branch, which allows us to state with caution that the clusters have "red" HBs (Figs.13 and 14).…”

“…The depth of the "color-magnitude" diagrams calculated according to photometry data is about 1m below the horizontal branch, which allows us to state with caution that the clusters have "red" HBs (Figs.13 and 14). Background stars contribute to CMD of the studied clusters(Federici et al 2012). Table6shows the metal-licity and color excesses E(B-V) as well as the distance modules to Bol 6 and Bol 45 obtained as a result of our work with stellar photometry data, in comparison with the corresponding values taken from the literature.…”