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Probing the formation and evolution of brown dwarfs

Final Activity Report Summary - BERKLEY-IAC (Probing the formation and evolution of brown dwarfs)

Since their discovery 13 years ago, ultracool objects of spectral type greater than M7 were a subject of intense research. These new classes of objects with ultracool effective temperature and very low masses could be seen as a natural continuation of the classical spectral type sequence, filling the gap between low mass stars and giant planets. In spite of the progresses made on the observational and theoretical sides, our understanding of these objects is still limited. Their physical properties, i.e. temperature, mass and radius, are still very much model dependent, and the models are not reliable at young ages. The mechanisms responsible for their formation are still a matter of debate. Do they form by the collapse of very low mass pre-stellar cores? Via early ejection? By disk fragmentation, or by photo-evaporation? The latest observations and simulations showed that these mechanisms were not mutually exclusive. The next step would be to understand their respective contributions to the final population and their dependence on the initial conditions, the environment and the evolutionary stage.

In that context, the scientific goals of my fellowship were twofold:
1. confronting the theoretical and numerical predictions of formation to new observations;
2. characterising the physical properties, i.e. mass and radius, of ultracool dwarfs.

In order to achieve these goals, I used four major observational quantities, namely the multiplicity, the Initial mass function (IMF), the discs and the kinematics.

The multiplicity properties of an ultracool dwarfs’ population were directly related to the formation mechanisms. The study of multiplicity could be split in two parts, i.e. a statistical study and the measurement of dynamical masses.

Regarding the discs of ultracool dwarfs, shortly after their discovery, a number of independent observational evidence showed that brown dwarfs and Isolated planetary mass objects (IPMO) also harboured discs. Observations of these discs were critical to our understanding of their formation. Up to recently, their study was limited by the technology, their luminosities being at the limit of sensitivity of mid-infrared (mid-IR) and millimeter instruments. In that context, and as in the case of multiple systems, my work consisted in two parts, namely a statistical study of the disc occurrence and the study of the physical properties of individual discs.

In terms of the initial mass function, the mass distribution was a fundamental product of the formation. It was still unclear whether the formation of isolated objects extended down to the planetary mass domain and beyond. The theory of opacity-limited fragmentation predicted that the lowest mass objects forming in isolation should lay at a range of 3 to 8 MJup, however recent theories of dynamical star formation with turbulent fragmentation, magnetic fields etc, implied a more complicated picture. Unfortunately, the most advanced surveys were complete down to around 20 MJup only. Using the most sensitive facilities available to date, such as wide field imaging and multi-conjugated adaptive optics, we conducted a series of surveys in nearby associations to measure the planetary mass end of the IMF. This ongoing study was original as it probed the core of young massive clusters, which were so far avoided by previous studies. The shape of the IMF in the direct vicinity of massive stars would bring new constrains to the models of formation and answer the question to whether or not brown dwarfs and isolated planetary mass objects could form and survive next to massive stars.

As far as kinematics were concerned, it should be noted that our galaxy is made of various structures, like star forming regions, clusters, OB associations, etc. The study of their kinematics offered several advantages. It firstly allowed to unambiguously confirm the membership of objects to a co-moving group and to reject contaminants in the IMF study. Secondly, it allowed for comparison of the kinematics of the different classes of objects within a co-moving group, providing important information about their respective origins. Some models of formation considered that ultracool dwarfs form as aborted stellar embryos ejected from their parent proto-stellar cluster, resulting in different kinematic properties.