12 new substellar members in Taurus. Clues for substellar formation models

Guieu, S.; Monin, J.-L.; Dougados, C.; Magnier, E.; Martín, E. L.
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Astronomische Nachrichten, Vol.326, Issue 10, p.1068-1071

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Recent studies of the substellar population in the Taurus cloud have revealed a deficit of brown dwarfs compared to the Trapezium cluster population (citealt{Briceno-1998,Luhman-2000,Luhman-2003a,Luhman-2004}) However, these works have concentrated on the highest stellar density regions of the Taurus cloud. We have performed a large scale optical survey of this region, covering a total area of ≃ 28 deg2, and encompassing the densest parts of the cloud as well as their surroundings. In this paper, we present optical spectroscopic follow-up observations of 97 photometrically selected potential new low-mass Taurus members. We derive spectral types, visual absorption and luminosity class estimates and discuss our criteria to assess Taurus membership. These observations reveal 5 new very low mass stars (VLMs) and 12 new Brown Dwarfs (BDs) on the Taurus cloud. We derive a new substellar to stellar ratio in Taurus of mathcal{R}ss =0.23 ± 0.05. This revisited ratio appears consistent with the value of 0.26 ± 0.04 previously derived in the Trapezium cluster under similar assumptions. We find indications that the relative numbers of BDs with respect to stars is decreased by a factor 2 in the central regions of the aggregates with respect to the more distributed population.