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Seminars at FGG

Observational properties of brown dwarfs: the low-mass end of the mass function

Speaker: Catia Cardoso (Oss. Astronómico di Torino)

Date and time: 2012-02-02 11:00

"Brown dwarfs are sub-stellar objects that do not sustain Hydrogen burning. They have been studied for the last 15 years but observational mass-age-luminosity degeneracy makes the full characterization of this objects difficult. Studies of large populations in cluster give us the opportunity to study a statistical meaningful population of stars in a wide range of masses, in a small area, with a similar age and chemical composition providing formation and dynamical evolution constraints. While the study of visual binaries provides independent dynamically derived masses to constrain interior and atmospheric models. I will present two projects, the study of the IMF of the double cluster h & χ Persei using a deep optical and infrared survey; and the determination of the dynamical masses of the brown dwarf binary, ε Indi Ba, Bb. h & χ Persei is the largest double cluster of the galaxy with a combine mass of 2 × 104 M⊙, it is relatively young (∼14Myr) an it is at a distance of 2.3 kpc. In this work we will present results of a I, Y, J, H and Ks survey from CFHT (Megacam and Wircam). Using the photometric data was produced the deepest mass function for the system, with more than 10 thousand photometric candidate members. The radial distribution study shows evidence of mass segregation. While the mass function shows that these clusters may be suffering from accelerated dynamical evolution due to their interaction, triggering the ejection of low mass objects. ε Indi Ba, Bb (T1 and T6) are the closest known confirmed brown dwarfs to Earth. Studying their relative motion over 7 years we have determined the system mass to be (121.16 ±0.17 ± 1.08 MJup) using high precision astrometry from infrared AO data with the VLT/NACO. The absolute movement of the binary was measured against a reference frame of field stars using optical data from VLT/FORS2, we determined the best individual masses to be MBa = 68.04 ± 0.94 MJup and MBb = 53.117 ± 0.32 MJup. The brown dwarf binary itself orbits a main sequence star allowing us to constrain the distance, metallicity and age of the system making it possible to break the sub-stellar mass-age degeneracy. Due to its proximity (∼3.62pc) and short period (10years), this is the only system where this kind of mass precision is achievable. We concluded that the masses derived isochronally were underestimating the system mass by 60%, due to the likely underestimation of the age of the system. Using system and individual dynamical masses, and the measured luminosity the parame- ters are in agreement with the evolutionary models if the system has an age ∼4Gyr. An asteroseismology run was performed earlier in the year and the results are under analyses, and they constrain the system age to ±0.5Gyr."