We present surface photometry and stellar kinematics of NGC 4431, a barred dwarf galaxy in the Virgo Cluster undergoing a tidal interaction with one of its neighbors, NGC 4436. We measured its bar pattern speed using the Tremaine-Weinberg method and derived the ratio of the corotation radius, DL, to the bar semimajor axis, aB. We found DL/aB=0.6+1

We present the first results of a large Advanced Camera for Surveys (ACS) survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N (signal-to-noise ratio) gtrsim10 for main-sequence stars with masses gtrsim0.2 Modot in a sample of globulars using the ACS Wide Field Channel

The nature of the ultra-luminous X-ray sources (ULXs) discovered in significant numbers in nearby galaxies by XMM and CXO continues to be a controversial topic. Interpreted simply as Eddington-limited accreting binaries implies compact object masses substantially in excess of those derived for typical galactic black-hole X-ray binaries. Whilst this

We propose a calibration plan for the GTC based on techniques similar to the ones used for space calibration. After studying the telescope and instrument requirements, and observing during intensive campaigns we have begun to prepare the GTC catalogue of standard stars. In this article we show the calibration strategy for the telescope and the

This paper presents semiempirical models of various solar magnetic structures, extending from the photosphere to the chromosphere. The models have been derived from non-LTE inversions of high-resolution spectropolarimetric observations of four Ca II and Fe I lines. The observed targets are dark and bright components of a sunspot umbra; dark and

Aims:In this paper we investigate the dependence of umbral core brightness, as well as the mean umbral and penumbral brightness on the phase of the solar cycle and on the size of the sunspot. Methods: Albregtsen & Maltby (1978, Nature, 274, 41) reported an increase in umbral core brightness from the early to the late phase of solar cycle from the