Evidence for a Universal Slope of the Period-Luminosity Relation from Direct Distances to Cepheids in the LMC

We have applied the infrared surface brightness (ISB) technique to derive distances to 13 Cepheid variables in the LMC which have periods from 3-42 days. The corresponding absolute magnitudes define PL relations in VIWJK bands which agree exceedingly well with the corresponding Milky Way relations obtained from the same technique, and are in significant disagreement with the observed LMC Cepheid PL relations, by OGLE-II and Persson et al., in these bands. Our data uncover a systematic error in the p-factor law which transforms Cepheid radial velocities into pulsational velocities. We correct the p-factor law by requiring that all LMC Cepheids share the same distance. Re-calculating all Milky Way and LMC Cepheid distances with the revised p-factor law, we find that the PL relations from the ISB technique both in LMC and in the Milky Way agree with the OGLE-II and Persson et al. LMC PL relations, supporting the conclusion of no metallicity effect on the slope of the Cepheid PL relation in optical/near infrared bands.Comment: 4 pages, to appear in the proceedings of the "Stellar Pulsation and Evolution" conference, Monte Porzio Catone, June 200

Variability of Young Massive Stars in the Arches Cluster

We present preliminary results of the first near-infrared variability study of the Arches cluster, using adaptive optics data from NIRI/Gemini and NACO/VLT. The goal is to discover eclipsing binaries in this young (2.5 +- 0.5 Myr), dense, massive cluster for which we will determine accurate fundamental parameters with subsequent spectroscopy. Given that the Arches cluster contains more than 200 Wolf-Rayet and O-type stars, it provides a rare opportunity to determine parameters for some of the most massive stars in the Galaxy.Comment: 2 pages, to appear in the proceedings of the IAUS 272 on "Active OB stars: structure, evolution, mass loss and critical limits" (Paris, July 19-23, 2010), Cambridge University Press. Editors C. Neiner, G. Wade, G. Meynet and G. Peter

Metallicity estimates of young clusters in the Magellanic Clouds from Strömgren photometry of supergiant stars

We present results obtained from Stromgren photometry of 13 young (~ 30-220 Myr) Magellanic Cloud (MC) clusters, most of them lacking in the literature from direct metallicity measurements. We derived for them [Fe/H] values from a high-dispersion spectroscopy-based empirical calibration of the Stromgren metallicity sensitive index m1 for yellow and red supergiants (SGs).  Particular care was given while estimating their respective uncertainties. In order toobtain the mean cluster metallicities, we used [Fe/H] values of selected SGs for which we required to be located within the cluster radii, placed in the expected SG region in the clustercolour-magnitude diagrams, and with [Fe/H] values within the FWHM of the observed cluster metallicitydistributions. The resulting metallicities for nearly 75 per cent of the cluster sample agree well with the most frequently used values of the mean MCs´ present-day metallicities.The remaining clusters have mean [Fe/H] values that fall near the edge of the MC present-daymetallicity distributions.  When comparing the cluster metallicities with their present positions, we found evidence that supports the claimed recent interaction of the MCs with the Milky Way, that could have caused that some clusters were scattered from their birthplaces. Indeed, we show examples of clusters with metal contents typical of the galaxy innerregions placed outward them. Like wise, we found young clusters, at present located in the inner regions of both MCs, formed out of gas that has remained unmixed since several Gyr ago.Fil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Pietrzynski, Grzegorz. Nicolaus Copernicus Astronomical Center; PoloniaFil: Narloch, Weronika. Universidad de Concepción; ChileFil: Górski, Marek. Universidad de Concepción; ChileFil: Graczyk, Dariusz. Millennium Institute of Astrophysics; Chil