Photometric surveys in nearby, young open clusters have provided a large amount of very low-mass stars and brown dwarfs over the last two decades. These clusters offer a number of advantages like known distance, metallicity and age, which make feasible the identification of such objects. Furthermore, deep searches do constitute one of the most direct means for measuring the mass function through the whole stellar (and brown dwarf) mass range. In this paper it will be reviewed the progress of recent work on several young open clusters leading to the findings of unambiguous brown dwarfs and very low mass stars approaching the substellar mass limit. These discoveries, particularly in the Pleiades, imply a rising mass function (α = 0.75 ± 0.25, dN/dM ∼ M−α) in the very low mass stellar and substellar domains down to 0.04 M⊙. The detection of reliable free-floating candidate members with estimated masses of only 0.04−0.015 M⊙ does provide substantial evidence on the formation of such low mass objects and thus, on the extension of the initial mass function down to the deuterium burning mass limit.

Introduction

Our knowledge of the low mass stellar content in open clusters has increased considerably during the last decade. For a relatively large amount of nearby young clusters, like α Per, Pleiades, Praesepe and Hyades, membership lists extending down to the hydrogen-burning limit (∼0.08 M⊙) are now available (see the reviews by Stauffer 1996, and Hambly 1998).

Introduction

This paper has three parts. First will be an estimate of how many brown dwarfs there ought to be in globular clusters, by following their observed mass functions as close as possible to the hydrogen-burning limit, and then naïvely extrapolating the mass function beyond that. Next will be a discussion of the H-burning limit and how we can try to locate it observationally, by pushing luminosity functions as faint as possible. This part will conclude with a demonstration of how the observations can guide the theoreticians toward more accurate models in that region, by telling us something about how the MLR must go. And finally we will give a brief description of a microlensing experiment that some one else has underway, that may actually tell us how many brown dwarfs one particular globular cluster contains.

Introduction

The next few years will be a marvellous time for X-ray astronomy, with the launch of AXAF (Advanced X-ray Astrophysics Facility) in spring 1999 and of XMM (X-ray Multi Mirror Mission) and ASTRO-E (the new Japanese X-ray mission) in early 2000. These new powerful missions will produce a great leap forward in all fields of X-ray astronomy, from nearby stars to the most distant objects in the Universe. They will be far more sensitive than past and ongoing X-ray missions and will be equipped with new detectors (CCD and microchannel plate cameras, transmission and reflection gratings, and X-ray microcalorimeters) that will allow detection of fainter objects as well as detailed medium to high-resolution spectroscopy of the brightest sources.

Introduction

We have been carrying out large surveys of M dwarfs in the field (Reid et al. 1995, hereafter PMSU1, Hawley, Gizis & Reid 1996, hereafter PMSU2) and in nearby open clusters (Hawley, Tourtellot & Reid 1998, hereafter HTR98). Although these stars, on the whole, might not qualify as “very low mass” (VLM) stars for this conference, they are interesting to study in order to understand the properties that might affect stars even further down the main sequence.

Introduction

After waiting three decades since Kumar (1963) proposed their existence, we are gratified to see literally dozens of candidates probably or definitely below the stellar mass limit being found in young clusters and associations. Here one has the big advantages that the age, the distance and luminosity of a cluster member are generally known. In this presentation, complementary to the topic of this meeting, we report the finding of a large number of candidates in one of the first infrared surveys of the field population.

We review the current theory of very low mass stars model atmospheres including the coolest known M dwarfs, M subdwarfs, and brown dwarfs, i.e. Teff ≤ 5,000K and −2.0 ≤ [M/H] ≤ +0.0. We discuss ongoing efforts to incorporate molecular and grain opacities in cool stellar spectra, as well as the latest progress in deriving the effective temperature scale of M dwarfs. We especially present the latest results of the models related to the search for brown dwarfs.

Very low mass star models and the Teff scale

Very Low Mass stars (VLMs) with masses from about 0.3 M⊙ to the hydrogen burning minimum mass (0.075 M⊙, Baraffe et al. 1995) and young substellar brown dwarfs share similar atmospheric properties. Most of their photospheric hydrogen is locked in H2 and most of the carbon in CO, with the excess oxygen forming important molecular absorbers such as TiO, VO, and H2O. They are subject to an efficient convective mixing often reaching the uppermost layers of their photosphere. Their energy distribution is governed by the millions of absorption lines of TiO, VO, CaH, and FeH in the optical to near-infrared, and H2O and CO in the infrared, which leave no window of true continuum. But as brown dwarfs cool with age, they begin to differentiate themselves with the formation of methane (CH4) in the infrared (Tsuji et al. 1995; Allard et al. 1996).

In this paper we review the results of the denis survey on very low mass stars and brown dwarfs. The analysis of denis catalogs for 1500 square degrees has produced a sample of ∼ 100 very-late M dwarfs and 15 L dwarfs. Spectroscopy of these objects has established a spectroscopic classification sequence for L dwarfs, and determined the underlying effective temperature scale.

We use this sample to obtain the local luminosity function of the very low mass stars and brown dwarfs, with particular attention to correcting possible error sources and Malmquist-like biases. This first denis luminosity function has good statistical accuracy down to the limit between M and L dwarfs.

Introduction

Very low mass stars and brown dwarfs can be looked for around known brighter stars, in clusters, or in the general field, with advantages and disadvantages which have been repeatedly discussed in detail (for instance, Hambly 1998). Companion searches have historically identified the coolest object known at any given time, though usually not the least massive (which are found in clusters, where they haven't yet cooled much). Companion searches in the immediate solar neighbourhood also provide the information needed to correct cluster and field samples for the contribution of unresolved companions to more distant objects, and as such they are an essential complement to both field and cluster surveys. Cluster searches benefit both from an increased source density and from the much larger luminosity of younger brown dwarfs, and as a consequence they are sensitive to much lower mass objects (e.g. Zapatero-Osorio et al. 1999).

The first of the “Three-Island” Euroconferences on Stellar Clusters and Associations was dedicated to Very Low-Mass Stars and Brown Dwarfs. It was held in the island of La Palma (May 11–15, 1998) where the Observatory of Roque de los Muchachos is located. These series of Euroconferences, an initiative led by Roberto Pallavicini (co-ordinator), Thierry Montmerle and Rafael Rebolo, are aimed to cover a very broad range of astrophysical problems where research on Stellar Clusters and Associations is crucial. In the first Euroconference, we reviewed, in a beautiful location, problems related to the formation, evolution and characterization of objects at the bottom of the Main Sequence and beyond. The first discoveries of brown dwarfs in 1995 have been followed by numerous detections in stellar clusters and in the solar vicinity. The drastic increase in the number of known examples of these fascinating objects, which suggests they are indeed very numerous in the Galaxy, has allowed a better comparison with theoretical predictions and a better and faster development of our knowledge about their physical conditions.

Some of the questions addressed in the papers compiled in this volume and delivered by active researchers in the field are: how very low-mass stars and brown dwarfs form, how many there are in the Galaxy, how they evolve, what the physical conditions of their atmospheres and interiors are, how magnetic activity develops in fully convective objects, if they generate magnetic fields, if brown dwarfs are chromospherically active and show coronae.

In stellar convection zones and fully convective stars, the rotation profiles are determined by the balance between the Reynolds stress and the meridional circulation. Due to the Coriolis force, the Reynolds stress has a non-diffusive component called ∧-effect that drives both differential rotation and meridional motions. The solar differential rotation pattern is almost perfectly reproduced by a mixing-length model of the convection zone that takes into account the influence of the Coriolis force on the convective motions. The same model also yields the turbulent electromotive force that together with rotational shear drives the solar dynamo.

The model has recently been applied to a fully convective pre-main sequence star. We find that for a strictly spherical star without any latitudinal gradients in temperature, density and pressure the rotation is very close to the rigid-body state. We conclude that the stellar magnetic field must be generated by a mechanism quite different from that in the Sun, namely an α2 rather than an αΩ-dynamo. It is thus very likely to have non-axisymmetric geometry and not to show cyclic behavior.

We study the analogous problem for M dwarfs. Like the T Tauri stars, these objects are fully convective and may hence be expected to have similar rotational profiles and magnetic field structures, respectively. As their Coriolis numbers are, however, closer to solar values than to those of pre-main sequence stars, the rotation may also be of solar-type.

Introduction

There is a tendency in the wider astronomical community to view astrometrists in general, and those who measure parallaxes in particular, as musty old fuddy-duddies doing valuable and worthy, if dull, work. This has always seemed to me a strange attitude, given that such observations set the foundations for almost all areas of astronomy. Personally, I find measuring parallaxes to be just about the most rewarding type of observation I've ever performed. There's a certain satisfaction to be had in measuring a fundamental quantity whose only model dependence is on Euclidean geometry - there aren't many other areas in astronomy where that's possible.

This general view of astrometry is particularly surprising in view of the phenomenal demand amongst the astronomical community for the rapid release of HIPPARCOS results. To my mind the constant discussion to be heard over observatory dinner tables world-wide asking “just when would the HIPPARCOS data go public?” reinforces the fact that such data is of fundamental importance to every field of astronomy – from the study of the faintest “non-quite stars”, to the study of galaxy formation and the early universe.

Introduction

The study of temporal variations is a very powerful tool to characterize and study the properties of a population of X-ray sources. Studies of typical time scales and amplitude of the observed variability can provide useful information on dimensions and physical conditions of the regions where X-ray emission originates. Comparative studies of the variability properties within a homogeneous class of X-ray sources are useful to determine or constrain the mechanisms generating their X-ray emission. To pursue such studies, a large number of homogenous observations are required.

As part of our ongoing research into low–mass star formation in Orion, we have obtained deep photometric and spectroscopic observations of PMS objects in the σ Orionis cluster and near the other O stars in the belt of Orion. The photometry indicates the existence of objects with masses as low as 0.01 M⊙ (100 Mjup). Spectroscopic follow-up has confirmed the sub-stellar nature of the candidate object tested.

Introduction

The Orion OB associations are one of the richest star forming regions in the local galaxy. Recently, we have made a concentrated effort to study the stars near belt of Orion within the Orion OB1a and OB1b associations. ROSAT observations totaling 100 Ksec of this region have been supplemented with ground based spectroscopy and photometry (Wolk 1996). These data demonstrated the clear existence of a 1-5 Myr old pre–main sequence of stars with common space motion and density of sources reaching a maximum at the location of σ Orionis. The confluence of this data lead us to conclude that the stars near σ Orionis form a young stellar cluster with a central density of at least 50 stars/pc3 (Walter et al. 1997, Walter et al. 1998). Similar clusters of older stars seem to exist near the other O stars in the belt of Orion.

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.

ROSAT has allowed the detection in X–rays of a large fraction of M dwarfs both in young open clusters such as the Pleiades and α Persei, and in the older Hyades. No decline of the average X–ray luminosity occurs between α Per and the Pleiades, while a rather steep decay is seen between the latter and the Hyades. The similarity of the Pleiades and α Per M dwarfs X–ray activity distributions simply reflects the similarity in their rotation distributions. It is more difficult to understand, instead, why the Hyades are, on average, significantly underluminous with respect to the Pleiades, since, due to the long spin-down timescales for M dwarfs, a large fraction of moderate or even rapid rotators are still present in the Hyades.

Although fully convective stars as active as stars with a radiative core have been observed, based on the Hyades, there might be an indication for a slight drop of the average X–ray emission level below the fully convective boundary mass, indicating a possible loss of efficiency of the mechanism of magnetic field generation.

Stars with masses down to 0.13 M⊙ and 0.19 M⊙ have been detected in the Pleiades and the Hyades, respectively: These detections, together with that of a 0.04 M⊙ brown dwarf in the Chamaleon I star forming region and of very-low mass dwarfs in the field, support the idea that there is not a cut-off mass below which stars do not have coronae anymore.

Introduction

Open clusters provide the astronomer with a rich source of objects for studying stellar structure over the full mass range of stable, hydrogen burning stars; furthermore, stellar evolution can be studied as the higher mass stars evolve away from, and as the low mass stars contract onto, the main sequence. Moreover, open cluster studies of objects that have too low a mass to stabilise on the hydrogen burning main sequence (i.e. brown dwarfs) have recently come of age, so now it is possible to study the physics of coeval objects having masses ranging over three orders of magnitude (and luminosities over eight orders of magnitude). Properties of very low mass (VLM) stars being studied in open clusters include lithium evolution, angular momentum evolution, spotting and variability, choronal activity, the binary fraction, and, most fundamentally, the mass function.

Introduction: why would we want to resolve stellar surfaces?

Doppler imaging for stars that have spots of cooler or greater temperature on their surface, amounts to recovering the surface temperature distribution from the integral equation that relates the distribution of surface temperature to the observed line-profile and light-curve variations.

We have conducted an extensive program of optical and IR imaging and spectroscopy targeted at the low-mass populations of nearby (≤ 300 pc) young (∼ 1-10 Myr old) clusters: L1495E, IC 348, and ρ Oph. By combining the spectroscopic data with IR luminosity function modeling, we arrive at mass functions which are roughly flat or slowly declining in logarithmic mass units below ∼0.4 M⊙ into the substellar regime. With the discovery of several likely brown dwarfs, we demonstrate the potential of young clusters in studying the formation and mass functions of substellar objects.

Introduction

Young, nearby (< 500 pc) clusters offer unique advantages in the search for brown dwarfs and the study of the low-mass initial mass function (IMF). Young (< 10 Myr) low-mass stars and brown dwarfs are quite luminous relative to evolved (> 1 Gyr) objects found in the field. Because young clusters often occupy small regions on the sky (D ∼ 10′), many low-mass candidates can be identified in only a limited amount of imaging. In addition, the mass function can be studied in the context of a compact, well-defined region of star formation where the stars have a common history and origin. Compared to open cluster studies, contamination by background stars is reduced significantly by extinction of the natal molecular cloud and the compact nature of the cluster. These factors also facilitate completeness estimates, which can be highly problematic in studies of low-mass objects in the field.