Spectroscopy of Very Low Mass Stars and Brown Dwarfs in Young Clusters

Summary

Spectroscopy provides key information about the membership of brown dwarf candidates in clusters, and allows the study of important evolutionary and structural properties like atmospheric composition, chromospheric activity, lithium depletion and rotation. Indeed, spectroscopy is the technique that has allowed the unambiguous confirmation (mainly via the lithum test) of all the known brown dwarfs. In this review, the spectroscopic observations that have been taken to date on very low-mass stars and brown dwarf candidates in clusters are summarized. Particular attention has been paid to the information that we are obtaining on the early evolution of very low-mass objects.

Introduction

Substellar terminology

What do we understand by “Brown Dwarf”? What is a transition object? How do we distinguish between brown dwarfs and planets? The answers to all these questions rely on conventions. With the discoveries of the first unambiguous substellar objects, language ambiguities that may lead to widespread confusion and misunderstanding should be avoided. Here, some definitions are favored for sake of simplicity, even though there is not yet a general consensus among researches in the field. Throughout this paper, I will use the following terminology which relies on clear-cut mass ranges:

Brown Dwarf (BD): A gaseous object with enough mass to kindle nuclear reactions in its core (H, Li and/or D burning), but these reactions are never sufficiently energetic to halt gravitational contraction. A BD never reaches a main sequence equilibrium state, and cools forever.