17 new very low-mass members in Taurus. The brown dwarf deficit revisited

TL;DR

This study conducted a large optical survey in Taurus, identifying new very low-mass stars and brown dwarfs, and found the substellar to stellar ratio aligns with the Trapezium cluster, supporting the embryo-ejection formation model.

Contribution

It provides new spectroscopic data on low-mass objects in Taurus and updates the substellar to stellar ratio, revealing environmental effects on brown dwarf populations.

Findings

Identified 5 new very low-mass Taurus members.

Discovered 12 new brown dwarfs in Taurus.

Substellar to stellar ratio in Taurus is similar to Trapezium.

Abstract

Recent studies of the substellar population in the Taurus cloud have revealed a deficit of brown dwarfs (BD) 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 30 deg^2, and encompassing the densest part of the cloud as well as their surroundings, down to a mass detection limits of 15 Jupiter Masses (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 (SpT < M4V) candidates. These observations reveal 5 new very low mass (VLM) Taurus members and 12 new BDs. 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 strong 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.