M. Maio

Distance to the Large Magellanic Cloud: The RR Lyrae Stars

New photometry and spectroscopy for more than a hundred RR Lyrae stars in two fields located close to the bar of the Large Magellanic Cloud (LMC) are used to derive new accurate estimates of the average magnitude, the local reddening, the luminosity-metallicity relation, and the distance to the LMC. The average apparent luminosity of the RR Lyraes with complete V and B light curves is V(RR) = 19.412 ± 0.019 (σ = 0.153), B(RR) = 19.807 ± 0.022 (σ = 0.172) in our field A (62 stars) and V(RR) = 19.320 ± 0.023 (σ = 0.159), B(RR) = 19.680 ± 0.024 (σ = 0.163) in our field B (46 stars). The average V apparent luminosity of the clump stars in the same areas is 0.108 and 0.029 mag brighter than the RR Lyrae level (Vclump = 19.304 ± 0.002 and 19.291 ± 0.003, in field A: 6728 stars, and B: 3851 stars, respectively). Metallicities from low-resolution spectra obtained with the Very Large Telescope have been derived for 101 RR Lyrae stars, finding an average value of [Fe/H] = -1.48 ± 0.03 (σ = 0.29, on the Harris metallicity scale). An estimate of the reddening within the two fields was obtained (1) from the Sturch method applied to the fundamental-mode pulsators (RRabs) with known metal abundance and (2) from the colors of the edges of the instability strip defined by the full sample of RR Lyrae variable stars. We obtained E(B-V) = 0.116 ± 0.017 and 0.086 ± 0.017 mag in fields A and B, respectively, with a clear-cut indication of a 0.03 mag differential reddening between the two fields. We find that reddening in field A is 0.028 mag smaller than derived by OGLE-II in the same area. On average, the new reddenings are also 0.035 mag larger than derived from Cepheids with projected distances within 2° from the centers of our fields. The new metallicities were combined with the apparent average V0 luminosities to determine the slope of the luminosity-metallicity relation for the RR Lyrae stars. We derived ΔMV(RR)/Δ [Fe/H] = 0.214 ± 0.047, with no clear evidence for the change in slope at [Fe/H] = -1.5, as recently suggested by evolutionary/pulsation and horizontal-branch models. The dereddened apparent average luminosity of the RR Lyraes defined by the present photometry is V(RR)0 = 19.064 ± 0.064 at [Fe/H] = -1.5. When coupled with the absolute magnitude derived from the Baade-Wesselink and the statistical parallaxes methods (MV(RR) = 0.68 ± 0.15 and 0.76 ± 0.13 mag at [Fe/H] = -1.5), both methods known to favor the short distance scale, this value leads to distance moduli for the LMC of μLMC = 18.38 ± 0.16 and μLMC = 18.30 ± 0.14, respectively. If we use instead the absolute magnitude from the new main-sequence fitting of Galactic globular clusters from Gratton et al. [MV(RR) = 0.61 ± 0.07 mag at [Fe/H] = -1.5], we derive μLMC = 18.45 ± 0.09. The average I apparent luminosity of the clump stars derived by the present photometry is Iclump = 18.319 ± 0.002 and 18.307 ± 0.003, in field A (σ = 0.190, 6728 stars) and B (σ = 0.184, 3851 stars), respectively. These values, once corrected for our new reddening estimates, lead to I0 = 18.12 ± 0.06 mag and move the clump distance modulus to the LMC to 18.42 ± 0.07 and 18.45 ± 0.07 when Udalski or Popowski metallicity-I luminosity relations for the clump stars are adopted. All these values are only 1 σ shorter than provided by the Population I distance indicators and make it possible to reconcile the short- and long-distance scale on a common value for the distance modulus of the LMC of μLMC = 18.515 ± 0.085 mag.

Metal abundances of RR Lyrae stars in the bar of the Large Magellanic Cloud

Metallicities ((Fe/H)) from low resolution spectroscopy obtained with the Very Large Telescope (VLT) are presented for 98 RR Lyrae and 3 short period Cepheids in the bar of the Large Magellanic Cloud. Our metal abundances have typical errors of ±0.17 dex. The average metallicity of the RR Lyrae stars is (Fe/H) = −1.48 ± 0.03 ± 0.06 on the scale of Harris (1996). The star-to-star scatter (0.29 dex) is larger than the observational errors, indicating a real spread in metal abundances. The derived metallicities cover the range −2.12 −1. For the ab-type variables we compared our spectroscopic abundances with those obtained from the Fourier decomposition of the light curves. We find good agreement between the two techniques, once the systematic offset of 0.2 dex between the metallicity scales used in the two methods is taken into account. The spectroscopic metallicities were combined with the dereddened apparent magnitudes of the variables to derive the slope of the luminosity-metallicity relation for the LMC RR Lyrae stars: the resulting value is 0.214 ± 0.047 mag/dex. Finally, the 3 short period Cepheids have (Fe/H) values in the range −2.0 < (Fe/H) < −1.5. They are more metal-poor than typical LMC RR Lyrae stars, thus they are more likely to be Anomalous Cepheids rather than the short period Classical Cepheids that are often found in a number of dwarf Irregular galaxies.

Variable Stars in the Fornax dSph Galaxy. II. Pulsating Stars below the Horizontal Branch

We have carried out an intensive survey of the northern region of the Fornax dwarf spheroidal galaxy with the aim of detecting the galaxys short-period pulsating stars (P < 0.25 days). Observations collected over three consecutive nights with the Wide Field Imager of the 2.2 m MPI telescope at ESO allowed us to detect 85 high-amplitude (0.20-1.00 mag in B light) variable stars with periods in the range from 0.046 to 0.126 days, similar to SX Phoenicis stars in Galactic metal-poor stellar populations. The plots of the observed periods vs. the B and V magnitudes show a dispersion largely exceeding the observational errors. To disentangle the matter, we separated the first-overtone from the fundamental-mode pulsators and tentatively identified a group of subluminous variables, about 0.35 mag fainter than the others. Their nature as either metal-poor intermediate-age stars or stars formed by the merging of close binary systems is discussed. The rich sample of the Fornax variables also led us to reconstruct the period-luminosity relation for short-period pulsating stars. An excellent linear fit, MV = − 1.83(± 0.08) − 3.65(± 0.07) log PF, was obtained using 153 δ Scuti and SX Phoenicis stars in a number of different stellar systems.

Variable Stars in the Fornax dSph Galaxy. III. The Globular Cluster Fornax 5

We present a new study of the variable star population in globular cluster 5 of the Fornax dSph, based on B and V time series photometry obtained with the MagIC camera of the 6.5 m Magellan Clay telescope and complementary Hubble Space Telescope archive data. Light curves and accurate periodicities were obtained for 30 RR Lyrae stars and one SX Phoenicis variable. The RR Lyrae sample includes 15 fundamental-mode pulsators, 13 first-overtone pulsators, one candidate double-mode pulsator and one RR Lyrae star with uncertain type classification. The average and minimum periods of the ab-type RR Lyrae stars, Pab = 0.590 days, P ab,min = 0.53297 days and the position in the horizontal branch type-metallicity plane, indicate that the cluster has Oosterhoff-intermediate properties, basically confirming previous indications by Mackey & Gilmore, although with some differences both in the period and type classification of individual variables. The average apparent magnitude of the Fornax 5 RR Lyrae stars is V(RR) = 21.35 ± 0.02 mag (σ = 0.07 mag, average on 14 stars more likely belonging to the cluster, and having well sampled light curves). This value leads to a true distance modulus of μ0 = 20.76 ± 0.07 (d = 141.9+4.6 –4.5 kpc) if we adopt for the cluster the metal abundance by Buonanno et al. ([Fe/H] = –2.20 ± 0.20), or μ0 = 20.66 ± 0.07 (d = 135.5+4.4 –4.3 kpc), if we adopt Strader et al.s metal abundance ([Fe/H] = –1.73 ± 0.13).

Looking for the Building Blocks of the Galactic Halo: Variable stars in the Fornax, Bootes I, Canes Venatici II Dwarfs and in NGC2419

Looking for the building blocks of the Galactic Halo we have investigated and compared the properties of the RR Lyrae stars in a number of different stellar systems inside and outside the Milky Way.

Variable Stars in the Globular Clusters and in the Field of the Fornax dSph Galaxy

claudia.greco@bo.astro.it 2 INAFO.A.Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy 3 INAFO.A.Brera, Via E. Bianchi 46, 23807 Merate, Italy 4 Departamento de Astronomia y Astrofisica, Pontificia Universidad Catolica de Chile, Avenida Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile 5 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, USA 6 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-2320, USA 7 Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA 8 Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile

The Oosterhoff types of the Fornax dSph Globular Clusters

We have applied the Image Subtraction to B,V time-series photometry of four globular clusters of the Fornax dwarf spheoridal galaxy, which were observed with the 2.2m ESO-MPI, the 6.5m Magellan/Clay, the 4m CTIO/Blanco telescopes and the WFPC2 on board of HST. We have identified RR Lyrae stars and obtained well sampled light curves for the clusters’ variables. From the periods and pulsation properties of their RR Lyrae stars Fornax globular clusters seem to be of Oosterhoff intermediate type.

Distance scale, variable stars and stellar populations in Local Group galaxies

We present an overview of our study of the short period variable stars in the Large Magellanic Cloud, and in the dwarf galaxies Fornax, Leo I, and NGC 6822. Light curves are presented for RR Lyrae stars, Anomalous Cepheids and, for the first time, for Dwarf Cepheids in the field and in the globular cluster #3 of the Fornax galaxy.

RR Lyrae Variables in Local Group Galaxies: LMC, Leo I. New Results and Distances to these Galaxies 1

We present new results on a sample of RR Lyrae variables in the bar of the Large Magellanic Cloud (LMC), and report on the first detection of RR Lyrae in the dwarf spheroidal galaxy Leo I. Emphasis is given to the discussion of the distances to these galaxies as derived from their RR Lyrae populations, and of the impact on the “short” and “long” distance scale dichotomy.