The authors present the first results on the optical identification of 89 ROSAT sources on the second Lockman hole. Because of the low Galactic H I column density towards this region the X-ray spectra of the sources are relatively unaffected by Galactic absorption, and a relation between X-ray spectral properties and the nature of the sources can

We provide a new calibration of the metallicity dependence of the Mg2 index in stellar systems. It is based on new isochrones by the Padova group and the latest stellar spectral libraries. Since the Mg2 line is one of the strongest absorption lines in the optical spectrum, it is often used to determine metallicities of composite stellar systems

Near-infrared H2 1-0 5(1) and Brγ velocity curves along the major axis of NGC 253 have revealed a central velocity gradient that is 7 times steeper than that shown by the optical velocity curve. This is interpreted as an optical depth effect due to dust. Approximately 35 mag of visual extinction in the center is required to match the southwest side

We report the first detection of Ca II H and K absorption in the high-velocity cloud complex C, the largest HVC. Absorption is detected in the spectra of the quasar PG 1351+640 and of the Seyfert galaxy Mark 290, at velocities of -163 and -137km/s (see fig.2). The implied Ca+ abundances are about 2x10^-8^, or about 0.01 times the total solar

The direct identification of structure in the cosmic microwave background (CMB) is a major objective of observational cosmology. We present a detailed discussion of observations of such structure detected on a 5 deg scale in the CMB using beam-switching radiometers at 10, 15 and 33 GHz sited in Tenerife. This long-term experiment has reached

Over the last 20 years, the surface abundance of Li has been derived for a significant number of low mass pre-main sequence (PMS) stars. Several works have pointed out the unexpected large Li spread for young stars of similar ages and masses. This spread still represents a challenge in our understanding of PMS stellar evolution. Almost in parallel