Variable stars in the Andromeda galaxy

People involved at OAB: Bellazzini, Cacciari, Clementini, Contreras, Federici, Fusi Pecci, Tosi

As the nearest giant spiral galaxy, Andromeda (M31) provides a unique opportunity to study the structure and evolution of a massive galaxy and, by comparison with the Milky Way (MW), to address the question of variety in the evolutionary histories of massive spirals. Our team is carrying out an observational programme aimed at monitoring the pulsation characteristics of the short and intermediate period pulsating variable stars in the M31 halo, in its giant tidal stream and in the M31 GCs. Nine hours of observing time with the Large Binocular Telescope (LBC@LBT) were awarded to the project during the Science Demonstration Time of the blue camera (PI G. Clementini) and a further 20 hours were awarded in 2008. Time-series observations of the selected fields of Andromeda were obtained in October 2007 and September 2008.

Figure 16: $V,B-V$ color magnitude diagram of a stream region of M31 observed during the October 2007 SDT run with LBC@LBT, showing the comparison with isochrones from Girardi et al. (2000, and following updates) for ages in the range from 63 to 700 million years which well fit the young and intermediate-age stellar components, and with the mean ridge lines of the Galactic globular clusters (GCs) M15, M3, 47 Tuc and NGC6553 for the old stellar component. Examples of the different types of variable stars detected in this portion of M31 are shown by larger filled dots; they include RR Lyrae stars (at $V \sim 25$ mag), Anomalous Cepheids (at $V \sim 24$ mag), Classical Cepheids ($V \ge 22$ mag) and a couple of binary systems. Stars mark variables for which we have a good sampling of the light curves (see Fig. 15). The long-dashed lines show the boundaries of the theoretical instability strips for RR Lyrae stars, and for Anomalous Cepheids with $\rm Z=0.0004$ and $\rm 1.3 < M< 2.2 M_\odot$ (from Marconi et al. 2004).

Fig.16 shows the color-magnitude diagram of one of our M31 stream fields. On the other hand, 78 orbits with WFPC2 on board of the HST were awarded in HST Cycle 15 (PI G. Clementini) to study the variable star population of six properly selected globular clusters of M31. Observations were obtained from June to September 2007. Data reduction has been completed for all 6 clusters, analysis of the light curves is in progress. In B514 we have discovered more than a hundred RR Lyrae stars with light curves of excellent quality (see Contreras et al. 2008).

The study is in collaboration with: Marconi, Ripepi (INAF-NApoli Obs.), Smith (MSU), Catelan (PUC, Chile), Pritzl (Macalester Univ.), Kinemuchi (Univ. Wyoming).