Homogeneous comparison of planet candidates imaged directly until 2008

We present a compilation of the planet candidates currently known from direct imaging. We have gathered available data from the literature and derive the luminosity of all candidates in a homogeneous way using a bolometric correction, the distances and the K band magnitudes of the objects. In a final step we find the masses of the candidates from a comparison of the luminosity or, if not available, an absolute brightness and several well known hot-start evolutionary models.Comment: 4 pages, 1 figure, Proceedings of the 2nd Subaru International Conference on Exoplanets and Disks: Their Formation and Diversity, Keauhou - Hawaii - USA, 9-12 March 2009; 2nd version: Several typos correcte

Low-Mass Pre-Main Sequence Stars and Their X-ray Emission

To investigate the formation and early evolution of stars, astronomers study the x-ray emission of T Tauri stars, which are young, solar-mass stars called pre–main sequence stars. Two Earth-orbiting x-ray satellites, the Röntgen X-ray Satellite (ROSAT) and the Advanced Satellite for Cosmology and Astrophysics (ASCA), have discovered x-ray emission from young protostars, called Class I objects. Many T Tauri stars were detected as x-ray sources by ROSAT. X-ray luminosity functions and correlations with other stellar parameters can be studied and used to investigate the x-ray emission mechanism. From the ROSAT data hundreds of T Tauri stars have been discovered, some of which are located outside regions of ongoing star formation. Stellar x-rays also irradiate circumstellar disks, regions where planets may form, so x-ray emission data from T Tauri stars may also be used to investigate the formation of planets.</jats:p

ROSAT X-ray Detection of a Young Brown Dwarf in the Chamaeleon I Dark Cloud

Photometry and spectroscopy of the object Cha Hα 1, located in the Chamaeleon I star-forming cloud, show that it is a ∼10 6 -year-old brown dwarf with spectral type M7.5 to M8 and 0.04 ± 0.01 solar masses. Quiescent x-ray emission was detected in a 36-kilosecond observation with 31.4 ± 7.7 x-ray photons, obtained with the Röntgen Satellite (ROSAT), with 9σ detection significance. This corresponds to an x-ray luminosity of 2.57 × 10 28 ergs per second and an x-ray to bolometric luminosity ratio of 10 −3.44 . These are typical values for late M-type stars. Because the interior of brown dwarfs may be similar to that of convective late-type stars, which are well-known x-ray sources, x-ray emission from brown dwarfs may indicate magnetic activity. </jats:p