[Paper Review] 17 new very low-mass members in Taurus. The brown dwarf deficit revisited

Sylvain Guieu, C. Dougados+3 more|CERN Bulletin|Sep 13, 2005

Astrophysics and Star Formation Studies58 citations

TL;DR

This study presents 17 new very low-mass (VLM) and brown dwarf (BD) members in the Taurus star-forming region through a large-scale optical survey and spectroscopic follow-up, finding a substellar-to-stellar ratio of 0.23 ± 0.05—now consistent with the Trapezium cluster—suggesting BDs are not intrinsically underabundant but may be spatially segregated due to dynamical ejection from dense cores.

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.

Motivation & Objective

• To resolve the long-standing 'brown dwarf deficit' in Taurus by extending surveys beyond high-density regions.
• To assess the true substellar initial mass function (IMF) in Taurus by identifying VLM and BD members across a large area.
• To investigate the spatial distribution and accretion properties of substellar objects in relation to dynamical ejection models.
• To compare the substellar-to-stellar ratio in Taurus with that in the Trapezium cluster under consistent assumptions.
• To evaluate whether environmental dependence in BD abundance is due to observational bias or physical processes like dynamical ejection.

Proposed method

• Conducted a wide-field optical survey of ~28 deg² in Taurus using CFHT and Megacam, reaching i′ ≈ 20–24 and z′ ≈ 21–23, corresponding to a mass detection limit of 15 M_J for ages ≤5 Myr.
• Selected 97 photometric candidates based on color-magnitude and color-color diagrams, focusing on late-M and substellar types (SpT ≥ M4V).
• Performed medium-resolution optical spectroscopy to derive spectral types, visual extinction, and luminosity class via spectral fitting and Na I equivalent width analysis.
• Assessed membership using color, spectral type, extinction, and radial velocity consistency, with 87–90% completeness for M4V and later types.
• Identified accretion signatures via Hα emission excess and forbidden line emission (e.g., [O I] 6300 Å), indicating ongoing accretion in 42% of substellar members.
• Combined results with prior surveys to compute the updated substellar-to-stellar ratio (ℛ_ss) and analyzed spatial gradients in BD abundance across the cloud.

Experimental results

Research questions

• RQ1 Is the apparent deficit of brown dwarfs in Taurus due to observational bias in high-density regions, or does it reflect a true physical deficiency?
• RQ2 What is the true substellar-to-stellar ratio in Taurus when surveyed over a large, representative area including low-density regions?
• RQ3 Do newly identified brown dwarfs and very low-mass stars show accretion signatures similar to classical T Tauri stars?
• RQ4 Is there a spatial gradient in the BD-to-star ratio, with lower BD abundance in dense cluster cores compared to the more diffuse regions?
• RQ5 Can the observed distribution and abundance of BDs in Taurus be explained by dynamical ejection from unstable multiple systems (embryo-ejection model)?

Key findings

• The study identifies 17 new Taurus members, including 12 brown dwarfs and 5 very low-mass stars, with 42% of the substellar members showing Hα emission indicative of accretion.
• The updated substellar-to-stellar ratio in Taurus is ℛ_ss = 0.23 ± 0.05, now statistically consistent with the value of 0.26 ± 0.04 found in the Trapezium cluster under similar assumptions.
• There is a significant 2× reduction in the BD-to-star ratio in the central 0.5 pc aggregates compared to the more spatially distributed population, indicating spatial segregation.
• Two VLM members (CFHT-Tau 19 and 21) and four BDs (CFHT-Tau 6, 8, 11, 12) show near-infrared excess and optical forbidden line emission, confirming ongoing accretion processes.
• The similarity in ℛ_ss between Taurus and the Trapezium cluster, combined with spatial gradients in BD abundance, supports the embryo-ejection model as a viable explanation for BD formation.
• The results suggest that dynamical ejection from unstable small-N systems can reproduce the observed spatial distribution and relative abundance of BDs in Taurus, with ejection velocities of ≥1 km/s capable of dispersing BDs over 1° in 2–3 Myr.