Variable stars in Local Group galaxies

People involved at OAB: Baldacci, Clementini, Maio.

Increasing samples of short period pulsating variable stars populating the classical instability strip from the horizontal branch of the oldest population to a few magnitudes brighter are being found in several LG galaxies, irrespective of the galaxy morphological type (Baldacci et al. 2004). The detection and study of the short period pulsating variables in a number of Local Group Galaxies (Leo I, NGC 6822, Fornax, Ursa Minor) is being carried out in collaboration with Held, Saviane, Momany and Rizzi (INAF-Padova Obs.), Poretti (INAF-Brera Obs.), Marconi, Musella, Ripepi (INAF-Napoli Obs.), Di Fabrizio (INAF-TNG), Smith (MSU), Catelan (PUC, Chile), Pritzl (Macalester Univ.).

Fundamental assets of our study are: (i) the collection of photometric data with wide field imagers and large aperture telescopes (e.g. the Wide Field Imagers of the ESO/MPI 2.2m and of the CTIO/BLANCO 4m telescopes; the ESO Very Large Telescopes and the Magellan 6.5m telescope); (ii) the use of most reliable PSF-fitting packages for the photometric reductions (DAOPHOT and ALLFRAME, Stetson 1994, 1996); (iii) the adoption of state-of-the-art techniques for the detection of variable stars in crowded fields (the Optimal Image Subtraction Method and the package ISIS2.1; Alard 2000); and the development of tools specifically designed for the study of single and double-mode pulsators (Graphycal Analyzer of TIme Series, Clementini et al. 2000) and for variable star data archive. See Clementini (2003) and Clementini et al. (2004a) for a general description of the project.

Based on multicolour WFI@2.2m time series photometry about 250 variable stars have been detected in the Leo I dwarf spheroidal galaxy. RR Lyrae stars represent about 4/5 of the sample. They provide unambiguous evidence for the presence of old metal-poor stars all the way to the innermost regions of this galaxy. The remaining 1/5 variables are Anomalous Cepheids. They trace the intermediate age ($t \sim 5$ Gyr) stellar component and appear to be more centrally concentrated (Baldacci et al. 2003).

The same type of study conducted on the dwarf irregular galaxy NGC6822 using FORS1@VLT led to the discovery of about 450 candidate variable stars (Baldacci et al. 2003, 2004). We obtained the first identification of RR Lyrae stars tracing the old stellar component in NGC 6822, and leading to the distance modulus: $(m-M)_0=23.36 \pm$0.17. We also found a significant population of small-amplitude, short-period variable stars filling the instability strip starting at luminosities only a few tenths of a magnitude brighter than the RR Lyrae stars. Given the extended star-formation history of NGC 6822, these variables are likely to originate from a population of intermediate-age, metal-poor He-burning stars younger and more massive than RR Lyrae stars (Clementini et al. 2003b).

Figure 5: Instrumental b light curves for variable stars in the field of the Fornax dwarf spheroidal galaxy (left panel) and in the galaxy globular cluster For 3 (right panel) from WFI observations. From top to bottom: ab-, c-, and d-type RR Lyrae ($<V> \sim 21.3$ mag), Anomalous Cepheids ($<V> \sim 20$ mag), and Dwarf Cepheids ($<V> \sim 23$ mag).

We obtained $B$,$V$ time series photometry of a 33' $\times$ 34' area North to the Fornax dSph center using the WFI@2.2 m ESO-MPI telescope. The selected area contains the globular clusters #3 and #6. We have detected and derived periods for 364 variables in the 2 chips of the 8-CCD WFI mosaic analyzed so far. The vast majority of these variables are RR Lyrae stars, however we also identified Anomalous and Population II Cepheids, and a large number of Dwarf Cepheids (55 objects; Clementini et al. 2004a). We estimate that the total number of variable stars in the area of Fornax we have observed is of about 1000 (lower limit; Maio et al. 2003). This represents the most extensive and deep survey of the variable stars in this galaxy. Figure 5 shows an example of the light curves of the variables in the field of Fornax dSph (left panel) and in its globular cluster # 3 (right panel; see section 1.5.3).

This study is in collaboration with Held (INAF-Padova Obs.), Poretti (INAF-Brera Obs.), Catelan (PUC), Smith (MSU), Pritzl (Macalester Univ.).

The star formation history of Ursa Minor is investigated through the study of the galaxy variable star populations. We are obtaining $V,I$ time series photometry of 3 fields of Ursa Minor with the 1.5m telescope of the Bologna Observatory in Loiano. The selected fields contain 5 of the 7 Anomalous Cepheids known in UMi, and the peak of stellar density that Kleyna et al. (2003) identify with an unbound stellar cluster sloshing back and forth within the UMi halo. This study is in collaboration with Marconi, Ripepi, Musella (INAF-Napoli Obs.).