The formation history of brown dwarfs and implications thereof on the low-mass initial mass function
Final Report Abstract
The results obtained strongly indicate that most BDs form according to a different channel to that of star formation. Observed mass distributions show evidence for a discontinuity near 0.1 M , and the small fraction of BD–BD binaries amongst BDs as well as the lack of star–BD pairs also shows that BDs follow different pairing rules than stars, which are born with a high star–star binary fraction. The hydrodynamical simulations performed here for the first time show that the theoretical BDs do have the observed properties (low binary fraction and a narrow separation distribution). Thus, BDs form following their own IMF which needs to be added to the stellar IMF, but it is not a continuous extension thereof. An analogy is the mass function of planets which also form with stars but not as an extension of the stellar IMF. The definition of the residual mass function allowed us to compare the IMFs predicted by gravo-turbulent models which assume BDs from as an extension of the stellar population. Such models predict a lack of BDs compared to observations. This is another statement that most BDs form from a different mode. Because some very low mass stars form as BDs do (they are in a sense “massive BDs”, sufficiently massive to become stars) which follow the same pairing rules as BDs, high-resolution star–star binary surveys ought to show a narrow, BD-like peak in the separation distribution amongst very low mass star binaries. The results obtained here show that the fragmentation of accretion disks around proto-stars appears to be the most realistic origin for most BDs. The research continues by calculating how BDs spread with time relative to stars once formed, as a possible diagnostic for formation models.
Publications
- 2010, ApJ 717, 577–585, “Tidally Induced Brown Dwarf and Planet Formation in Circumstellar Disks”
Thies, I., Kroupa, P., Goodwin, S. P., Stamatellos, D., & Whitworth, A. P.
- 2013, Planets, Stars and Stellar Systems. Volume 5: Galactic Structure and Stellar Populations, Vol.5, ISBN 978-94-007-5611-3. Springer Science+Business Media Dordrecht, “The Stellar and Sub-Stellar Initial Mass Function of Simple and Composite Populations”
Kroupa, P., Weidner, C., Pflamm-Altenburg, J., et al.
- 2015, ApJ 800, 72–880, “Characterizing the Brown Dwarf Formation Channels from the Initial Mass Function and Binary-star Dynamics”
Thies, I., Pflamm-Altenburg, J., Kroupa, P., & Marks, M.
(See online at https://doi.org/10.1088/0004-637X/800/1/72) - 2015, MNRAS 452, 1014–1025, “M-dwarf binaries as tracers of star and brown dwarf formation”
Marks, M., Janson, M., Kroupa, P., Leigh, N., & Thies, I.
(See online at https://doi.org/10.1093/mnras/stv1361)
