Astronomy & Astrophysics

A&A 446, 485-500 (2006)
DOI: 10.1051/0004-6361:20053493

Seventeen new very low-mass members in Taurus

The brown dwarf deficit revisited

S. Guieu¹, C. Dougados¹, J.-L. Monin^{1, 2}, E. Magnier^{3, 4} and E. L. Martín^{5, 6}

¹ Laboratoire d'Astrophysique de Grenoble, BP 53, 38041 Grenoble, France
e-mail: sylvain.guieu@obs.ujf-grenoble.fr
² Institut Universitaire de France
³ Canada-France-Hawaii Telescope Corporation, PO Box 1597, Kamuela, USA
⁴ University of Hawaii, Institute of Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96821, USA
⁵ Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Spain
e-mail: ege@iac.es
⁶ University of Central Florida, Department of Physics, PO Box 162385, Orlando, FL 32816-2385, USA

(Received 23 May 2005 / Accepted 9 September 2005)

Abstract
Recent studies of the substellar population in the Taurus cloud have revealed a deficit of brown dwarfs compared to the Trapezium cluster population. 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 $\simeq$ $28\,{\rm deg}^2$ , and encompassing the densest parts of the cloud as well as their surroundings, down to a mass detection limit of 15 $M_{\rm J}$ . We present the optical spectroscopic follow-up observations of 97 photometrically selected potential new low-mass Taurus members, of which 27 are strong late-M spectral type ( ${\rm SpT} \ge {\rm M4V}$ ) candidates. Our spectroscopic survey is 87% complete down to $i^\prime=20$ for spectral types later than M4V, which corresponds to a mass completeness limit of 30 $M_{\rm J}$ for ages $\leq$ 10 Myr and ${\rm A}v \le 4$ . We derive spectral types, visual absorption and luminosity class estimates and discuss our criteria to assess Taurus membership. These observations reveal 5 new VLM Taurus members and 12 new BDs. Two of the new VLM sources and four of the new substellar members exhibit accretion/outflow signatures similar to higher mass classical T Tauri stars. From levels of $\rm H\alpha$ emission we derive a fraction of accreting sources of 42% in the substellar Taurus population. Combining our observations with previously published results, we derive an updated substellar to stellar ratio in Taurus of $\mathcal{R}_{\rm ss}=0.23 \pm 0.05$ . This ratio now appears consistent with the value previously derived in the Trapezium cluster under similar assumptions of $0.26 \pm 0.04$ . 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. Our findings are best explained in the context of the embryo-ejection model where brown dwarfs originate from dynamical interactions in small N unstable multiple systems.

Key words: stars: low-mass, brown dwarfs -- stars: late-type -- stars: luminosity function, mass function -- stars: pre-main sequence

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