Seventeen new very low-mass members in Taurus

Abstract

the date of receipt and acceptance should be inserted later Abstract. Recent studies of the substellar population in the Taurus cloud have revealed a deficit of brown dwarfs compared to the Trapezium cluster population (Briceno et al. 1998; Luhman 2000; Luhman et al. 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 ≃ 28deg 2 , and encompassing the densest parts of the cloud as well as their surroundings, down to a mass detection limits of 15 MJ. In this paper, 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 (SpT ≥ M4V ) candidates. Our spectroscopic survey is 87 % complete down to i ' =20 for spectral types later than M4V, which corresponds to a mass completeness limit of 30 MJ for ages ≤ 10 Myr and Av ≤ 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 the higher mass classical T Tauri stars. From levels of Hemission 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 Rss = 0.23 ± 0.05. This ratio now appears consistent with the value previously derived in the Trapezium cluster under similar assumptions of 0.26±0.04. We find indication 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.