We present ground-based spectra of early-type star candidates in the Local Group galaxies M 33 and NGC 6822 taken at the 4.2 m William Herschel Telescope (La Palma, Spain). Photospheric parameters Teff and log g were derived by LTE modeling of the Balmer lines. When possible, mass-loss rates were calculated from the Halpha profiles computing

We describe the technique that will be used to develop a set of on-orbit calibrators for the Infrared Array Camera (IRAC) and demonstrate the validity of the method for stars with spectral types either K0-M0 III or A0-A5 V. For application to SIRTF, the approach is intended to operate with all available optical, near-infrared (NIR), and mid

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En la tesis se presentan los resultados de un estudio comparativo de la formación estelar que tiene lugar en dos galaxias espirales distintas: una floculenta (NGC 4395) y una de gran diseño (NGC 5457). Para ello se obtuvo una serie de imágenes en banda estrecha en diferentes líneas de emisión de las regiones HII. De esta forma no sólo se obtuvo

Investigating the reliability of the assumption of instantaneous chemical equilibrium (ICE) for calculating the CO number density in the solar atmosphere is of crucial importance for the resolution of the long-standing controversy over the existence of ``cool clouds'' in the chromosphere and for determining whether the cool gas owes its existence

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