The ACS LCID Project: Variable Stars in Tucana and LGS3

We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the one chip per galaxy we studied so far, a total of 230 and 80 candidates variables were found, respectively. For Tucana, we identified 134 of them as RR Lyrae stars (RRL) pulsating in the fundamental mode (RRab), 51 in the first-overtone mode (RRc), and 37 in both modes simultaneoulsy (RRd), as well as four candidate anomalous Cepheids (AC). In the case of LGS3, we found 45 RRab and 5 RRc, plus three candidates RRd and five candidate AC. The metallicities obtained from the mean period of the RRab are [Fe/H]_{Tuc}=-1.7 and [Fe/H]_{LGS3}=-1.8.Comment: 2 pages, 1 figure. To appear in the Proceedings of IAU Symposium 241: "Stellar Populations as Building Blocks of Galaxies", 10-16 December, 2006 at La Palma, Canary Islands, Spai

Variable stars in the dwarf irregular galaxy NGC 6822: the photometric catalogue

Deep B,V time-series photometry obtained with the ESO Very Large Telescope has been used to identify variable stars in the dwarf irregular galaxy NGC 6822. We surveyed a 6.8x6.8 arcmin area of the galaxy and detected a total number of 390 candidate variables with the optimal image subtraction technique (Alard 2000). Light curves on a magnitude scale were obtained for 262 of these variables. Differential flux light curves are available for the remaining sample. In this paper we present the photometric catalogue of calibrated light curves and time-series data, along with coordinates and classification of the candidate variables. A detailed description is provided of the procedures used to identify the variable stars and calibrate their differential flux light curves on a magnitude scale.Comment: 22 pages, 19 figures only as JPEG. Revised version with corrected eq. 5. Full text with better resolution .ps figures available upon request from the authors. Uses aa.cls (included), in press on A&A. Table 2 will only be published at the CDS, Appendix A, Tables 4,5,6 will only be available in the electronic edition of the Journa

A continuous population of variable stars up to about 1.5 mag above the horizontal branch?

Increasing samples of pulsating variable stars populating the classical instability strip from the horizontal branch to a few magnitudes brighter are being found in several Local Group galaxies, irrespective of the galaxy morphological type. We will review the observational scenario focusing in particular on the Anomalous Cepheids and related objects.Comment: 5 pages, 2 figures, invited review in JENAM 2003, "Minisymposium: Asteroseismology and Stellar Evolution", Communications in Asteroseismology, in pres

Variability and star formation in Leo T, the lowest luminosity star-forming galaxy known today

We present results from the first combined study of variable stars and star formation history (SFH) of the Milky Way (MW) "ultra-faint" dwarf (UFD) galaxy Leo T, based on F606W and F814W multi-epoch archive observations obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We have detected 14 variable stars in the galaxy. They include one fundamental-mode RR Lyrae star and 10 Anomalous Cepheids with periods shorter than 1 day, thus suggesting the occurrence of multiple star formation episodes in this UFD, of which one about 10 Gyr ago produced the RR Lyrae star. A new estimate of the distance to Leo T of 409 $^{+29}_{-27}$ kpc (distance modulus of 23.06 $\pm$ 0.15 mag) was derived from the galaxy's RR Lyrae star. Our V, V-I color-magnitude diagram of Leo T reaches V~29 mag and shows features typical of a galaxy in transition between dwarf irregular and dwarf spheroidal types. A quantitative analysis of the star formation history, based on the comparison of the observed V,V-I CMD with the expected distribution of stars for different evolutionary scenarios, confirms that Leo T has a complex star formation history dominated by two enhanced periods about 1.5 and 9 Gyr ago, respectively. The distribution of stars and gas shows that the galaxy has a fairly asymmetric structure.Comment: 31 pages, 14 figures, accepted for publication in Ap