A general study of the fermionic structure of the 331 models with eta arbitrary shows the possibility of obtaining 331 vector-like models with mirror fermions. On one hand, the existence of mirror fermions gives a possible way to fit the discrepancy in the bottom quark asymmetries from the prediction of the standard model. On the other hand, the

We report on the first pointed X-ray observations with XMM-Newton and RXTE satellites of the X-ray source RX J0757.0+6306 = HT Cam. We detect a strong 515 s X-ray modulation confirming the optical photometric period found in 1998, which definitively assigns this source to the intermediate polar class of magnetic cataclysmic variables. The lack of

We present accurate and deep multiband B,R,Hα data for the globular cluster ω Cen collected with the Advanced Camera for Surveys on HST. The photometric catalogue includes more than one million stars. By adopting severe selection criteria we identified more than 600 bona fide White Dwarfs (WDs). Empirical evidence suggests that a small sample of

Los problemas relacionados con la inhomogeneidad en densidad de las regiones HII gigantes, y sus consecuencias sobre la pérdida de los fotones Lyman, sientan el punto de partida de esta tesis. En ella, en primer lugar, realizamos un modelo estadístico para las regiones HII inhomogéneas. Posteriormente, comparamos las predicciones de los modelos con

We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi-Element Spectrograph (FLAMES) instrument at the Very Large Telescope (VLT). Here we present observations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph (R ≃ 25 000), in fields centered on the open clusters NGC 3293, NGC

The details of the early star formation histories of tiny dwarf galaxies can shed light on the role in galaxy formation of the reionization which occured at high redshift. Isolated dwarfs are ideal probes since their evolution is not complicated by environmental effects owing to the vicinity of the Milky Way and M31. In addition, dwarf galaxies are