We present the results of an XMM-Newton observation of the young (∼ 2 − 4 Myr) cluster around the hot star σ Orionis. In a previous paper we presented the analysis of the RGS spectrum of the central hot star; here we discuss the results of the analysis of the full EPIC field. We have detected 175 X-ray sources, 88 of which have been identified with cluster members, including very low-mass stars...

Ostreococcus spp. are extremely small unicellular eukaryotic green algae found worldwide in marine environments, and they are susceptible to attacks by a diverse group of large DNA viruses. Several biologically distinct species of Ostreococcus are known and differ in the ecological niches that they occupy: while O. tauri (representing clade C strains) is found in marine lagoons and coastal seas...

The bright pre-main sequence star HBC 515 (HD 288313) located in the L1622 cometary cloud in Orion has been studied extensively with optical/infrared imaging and ultraviolet/optical/infrared spectroscopy. The spectra indicate that HBC 515 is a weakline T Tauri star of spectral type K2V. Adaptive optics imaging in the K-band reveals that HBC 515 is a binary with two equally bright components sep...

We surveyed a sample of 75 T Tauri stars in the Taurus star forming region for companions. These stars were discovered with the help of ROSAT. The separation range covered is 0.13′′ to 13′′, where the lower limit is given by the diffraction limit of the telescope and the upper limit by confusion with background stars. Combined with the results of the preceding survey by Leinert et al. (1993), w...

We present an infrared imaging study of the low-mass pre-main-sequence binary system Haro 6-10. This system is one of a handful in which the optically visible primary has the characteristics of a normal T Tauri star, while the secondary is a so-called "infrared companion" (IRC), a strongly extincted object that emits most of its luminosity in the infrared. A speckle holographic technique was us...

We compute spectral energy distributions (SEDs) for passive T Tauri disks viewed at arbitrary inclinations. Semi-analytic models of disks in radiative and hydrostatic equilibrium are employed. Over viewing angles for which the flared disk does not occult the central star, the SED varies negligibly with inclination. For such aspects, the SED shortward of ∼80μm is particularly insensitive to orie...

We improve upon the radiative, hydrostatic equilibrium models of passive circumstellar disks constructed by Chiang & Goldreich. New features include (1) an account for a range of particle sizes, (2) employment of laboratory-based optical constants of representative grain materials, and (3) numerical solution of the equations of radiative and hydrostatic equilibrium within the original two-layer...

The aim of this paper is to quantitatively testify the “small-scale sequential star formation” hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al. (2007, Paper I), we have carried out BV Ic photometry of four more BRC aggregates along with deeper re-observations of 2 previously observed BRCs. Again quantitative age gradients are found in...

Ostreococcus tauri, a unicellular marine green alga, is the smallest known free-living eukaryote and is ubiquitous in the surface oceans. The ecological success of this organism has been attributed to distinct lowand high-light adapted ecotypes existing in different niches at a range of depths in the ocean. Viruses have already been characterised that infect the high-light adapted strains. Ostr...