Seventeen new very low-mass members in Taurus

The brown dwarf deficit revisited

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 , and encompassing the densest parts of the cloud as well as their surroundings, down to a mass detection limit of 15 M_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 () candidates. Our spectroscopic survey is 87% complete down to for spectral types later than M4V, which corresponds to a mass completeness limit of 30 M_J for ages ≤10 Myr and . 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  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 . This ratio now appears consistent with the value previously derived in the Trapezium cluster under similar assumptions of . 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.