Wednesday, January 23, 2013

1301.4996 (A. C. Day-Jones et al.)

The sub-stellar birth rate from UKIDSS    [PDF]

A. C. Day-Jones, F. Marocco, D. J. Pinfield, Z. H. Zhang, B. Burningham, N. Deacon, M. T. Ruiz, J. Gallardo, H. R. A. Jones, P. W. L. Lucas, J. S. Jenkins, J. Gomes, S. L. Folkes, J. R. A. Clarke
We present a new sample of mid L to mid T dwarfs with effective temperatures of 1100 to 1700K selected from the UKIDSS Large Area Survey and confirmed with infrared spectra from X-Shooter/VLT. This effective temperature range is especially sensitive to the formation history of Galactic brown dwarfs and allows us to constrain the form of the sub-stellar birth rate, with sensitivity to differentiate between a flat (stellar like) birth rate, and an exponentially declining form. We present the discovery of 63 new L and T dwarfs from the UKIDSS LAS DR7, including the identification of 12 likely unresolved binaries, which form the first complete sub-set from our program, covering 495 sq degrees of sky, complete to J=18.1. We compare our results for this sub-sample with simulations of differing birth rates for objects of mass 0.10-0.03Msol and ages 1-10Gyrs. We find that the more extreme birth rates (e.g. a halo type form) can likely be excluded as the true form of the birth rate. In addition we find that although there is substantial scatter we find a preference for a mass function, with a power-law index, alpha in the range -1 < alpha < 0 that is consistent (within the errors) with the studies of late T dwarfs.
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