First evidences of IR excess and disk mass accretion (strong Hα emission) around brown dwarfs seem to indicate the existence of circumstellar disks around these sub-stellar objects. Nothing is known at the present time about outflows which potentially might be launched from brown dwarfs, although jets are typically associated with the accretion in standard T Tauri star disks. In this paper we c...

Using a multi-object spectrograph on the Schmidt telescope in Tautenburg, we study the flare-activity of T Tauri stars and of zero-age main sequence stars. The sample comprises both classical, and weak-line T Tauri stars (cTTSs, wTTSs) in the Taurus-Auriga star forming region, as well as zero-age mainsequence stars (ZAMSSs) in the open cluster α Persei. The properties of the flares detected on ...

We present the ROSAT PSPC pointed and ROSAT All-Sky Survey (RASS) observations and the results of our low and high spectral resolution optical followup observations of the T Tauri stars (TTS) and X-ray selected T Tauri star candidates in the region of the high galactic latitude dark cloud MBM12 (L1453-L1454, L1457, L1458). Previous observations have revealed 3 “classical” T Tauri stars and 1 “w...

We have analysed newly calibrated IUE and ROSAT data for three T Tauri stars from diierent sub-classes: TW Hya, V410 Tau and CS Cha, a Classical T Tauri star (CTTS), a Weak T Tauri star (WTTS) and a T Tauri star intermediate between CTTS and WTTS, respectively. In the ultraviolet the continuum seems to be well explained by the sum of the stellar black body emission plus a hydrogenic component w...

We report here on a series of adaptive optics observations of T Tauri made from Dec. 1993 to Nov. 1997. Observations show an arc-like structure extending toward north and north-west. On the south side a bright tail-like structure extends radially in the south-west direction. Both structures seem to be produced by dust scattering. Close to T Tauri, the south structure forms bright clumps near a ...

We present Doppler imaging and Balmer line analysis of the weak-line T Tauri star TWA 6. Using this data we have made one of the first attempts to measure differential rotation in a T Tauri star, and the first detection of a slingshot prominence in such a star. We also show the most direct evidence to date of the existence of solar-type plages in a star other than the Sun. Observations were mad...

The green picoalga Ostreococcus is emerging as a simple plant model organism, and two species, O. lucimarinus and O. tauri, have now been sequenced and annotated manually. To evaluate the completeness of the metabolic annotation of both species, metabolic networks of O. lucimarinus and O. tauri were reconstructed from the KEGG database, thermodynamically constrained, elementally balanced, and f...

We present results of X-ray spectroscopy for TWA 5, a member of the young TW Hydrae association, observed with XMM-Newton. TWA 5 is a multiple system which shows Hα emission, a signature typical of classical T Tauri stars, but no infrared excess. From this analysis of the RGS and EPIC spectra, we have derived the emission measure distribution vs. temperature of the X-ray emitting plasma, its ab...

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 low- and high-light-adapted ecotypes existing in different niches at a range of depths in the ocean. Viruses have already been characterized that infect the high-light-adapted strains. Os...

Context. Mass accretion rate determinations are fundamental for an understanding of the evolution of pre-main sequence star circumstellar disks. Aims. Magnetospheric accretion models are used to derive values of the mass accretion rates in objects of very different properties, from brown dwarfs to intermediate-mass stars; we test the validity of these models in the brown dwarf regime, where the...