astro-ph/0511166 [abs, ps, pdf, other] :

Title: Ultra low-mass star and substellar formation in sigma Orionis

Authors: J. A. Caballero

Comments: 4 pages, 4 figures. Proceeding at the IAC-TNG Workshop on Ultra low-mass star formation and evolution, 28 June - 1 July 2005, La Palma, Canary Islands, Spain. Accepted for publication in Astron. Nachr

The nearby young sigma Orionis cluster (~360 pc, ~3 Ma) is becoming one of the most important regions for the study of ultra low-mass star formation and its extension down to the mass regimes of the brown dwarfs and planetary-mass objects. Here, I introduce the sigma Orionis cluster and present three studies that the JOVIAN group is developing: a pilot programme of near-infrared adaptive-optics imaging, intermediate-resolution optical spectroscopy of a large sample of stars of the cluster and a study of the mass function down to the planetary-mass domain. This paper is a summary of the content of four posters that I presented in the Ultra low-mass star formation and evolution Workshop, as single author or on behalf of different collaborations.

http://arxiv.org/abs/astro-ph/0511166

astro-ph/0511168 [abs, ps, pdf, other] :

Title: X-ray emission of brown dwarfs: Towards constraining the dependence on age, luminosity, and temperature

Authors: B. Stelzer (1,2), G. Micela (2), E. Flaccomio (2), R. Neuhaeuser (3) R. Jayawardhana (4); (1) - Uni Palermo, (2) - OA Palermo, (3) - Uni Jena, (4) - Uni Toronto

Comments: accepted for publication in Astronomy & Astrophysics

Three brown dwarfs in different evolutionary stages have been observed with the Chandra X-ray Observatory. Combining the new observations with previous studies presented in the literature yields a brown dwarf sample that reaches well down into the cooling phase of brown dwarfs, allowing to investigate the possible influence of effective temperature on X-ray activity. Combining our results with published data allows us to consider a subsample of high-mass brown dwarfs (with 0.05-0.07 Msun), and another one characterized by similar effective temperature (with 2400-2800 K). Our findings support the idea that effective temperature plays a critical role for the X-ray activity in brown dwarfs. This underlines an earlier suggestion based on observations of chromospheric Ha emission in ultracool dwarfs that the low ionization fraction in the cool brown dwarf atmospheres may suppress magnetic activity.

http://arxiv.org/abs/astro-ph/0511168