An analysis of Hα and Hβ spectra in a sample of 30 cool dwarf and subgiant stars is presented using MARCS model atmospheres based on the most recent calculations of the line opacities. A detailed quantitative comparison of the solar flux spectra with model spectra shows that Balmer line profile shapes, and therefore the temperature structure in the

The European Space Agency (ESA) has undertaken the development of Optical Data Relay payloads, aimed at establishing free space optical communication links between satellites. The first of such systems put into orbit is the SILEX project, in which an experimental link between a GEO satellite (ARTEMIS) and a LEO satellite (SPOT IV) will be used to

We present high resolution spectroscopic data of the two giant extragalactic H II regions NGC 5461 and NGC 5471 in M101, which have been obtained with the 2.1-m telescope of the Observatorio Astronómico Nacional in San Pedro Martir, Baja California. We measured the intensities of several H and He recombination lines, and of forbidden lines of a

Its combination of good spatial resolution and soft X-ray sensitivity make Chandra an ideal instrument for the detection of luminous supersoft X-ray sources (SSSs)in external galaxies. M31 may provide the best opportunity to study a galactic population of SSSs. We have recently completed a survey of ~ 2560 square arcmin ( ~ 131 kpc^2) of M31

We present a summary of Chandra monitoring observations of M31, and partial results of our HST program to search for UV counterparts to M31 X-ray Transients. The first 8 epochs of monitoring spread over nearly 2 years reveals more than a dozen transient sources, with peak luminosities ranging from 1 × 10^37ergs sec-1 to 3 × 10^38ergs sec-1. While

We report very intense and variable Hα emission (pseudoequivalent widths of ~180 and 410 Å) of S Ori 55, a probable free-floating, M9-type substellar member of the young σ Orionis open star cluster. After comparison with state-of-the-art evolutionary models, we infer that S Ori 55 is near or below the cluster deuterium-burning mass borderline