We derive inequalities which restrict the precision of the Stokes parameters imposed by photon noise. The signal-to-noise ratio of the measured Stokes parameters is never better than the square root of the number of detected photons. In case of weakly polarized sources (i.e., most astronomical sources), such an inequality imposes no practical limit

Narowband H alpha imaging and long-slit optical and near-infrared spectroscopy are analyzed in a study of the starburst galaxy NGC 7714. The data indicate that the galaxy as a whole is experiencing intense star formation activity; this is more conspicuous in the nucleus, where the star formation rate and the star formation rate per unit area are

IAUC 6135 available at Central Bureau for Astronomical Telegrams.

We follow the interaction of massive stars with their circumstellar gas over their entire life-times by combining hydrodynamic stellar evolution calculations for 35 and 60M &sun; stars and one- and two-dimensional gas dynamical calculations for the circumstellar medium.

Supernova 1991F appeared in the lenticular galaxy NGC 3458 (UGC 6037). On discovery, it was classified as type I, although no subtype (i.e., Ia or Ib/c) was assigned to it. In this paper, we present several low resolution spectra of this supernova. One of them was recorded ten days after the discovery. Two others show the supernova spectral

HD 36705 (AB Doradus) and HD 155555 (V824 Ara) are two nearby very active cool stars. Both have strong Li I (lambda)6708 resonance lines, implying high surface Li abundances, comparable to those of very young T Tauri stars. They also have late M-type visual companions which share their proper motion, as confirmed by new astrometric measurements