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.

We combine the results from two CCD surveys covering a large area of the custer at I and Z wavebands. We have obtained follow-up K photometry for many of the numerous brown dwarf candidates discovered in these surveys which we employ as a test for cluster membership. From these data we derive the mass function of the whole Pleiades cluster down to 0.04 M⊙. We emphasise the importance of a careful consideration of the spatial distribution within the cluster and find the core radius for brown dwarfs to be 2±1 parsecs. The contribution of brown dwarfs to the total mass of the cluster is about 1%.

Introduction

The Pleiades has long been recognised as one of the best places to search for brown dwarfs, e.g. Jameson & Skillen (1989), Stauffer et al. (1989, 1994), Simons & Becklin (1992), Rebolo et al. (1995), Cossburn et al. (1997), Zapatero Osorio et al. (1997), Bouvier et al. (1998), Festin (1998).

The cluster is both reasonably close (but not so close as to cover too large an area of the sky) and young, so that brown dwarfs are not too faint. Controversy still rages over the precise distance to the Pleiades, which Hipparcos places significantly closer (at 118 parsecs) than ground based measurements (at typically 133 parsecs). The Hipparcos results have been published by Van Leeuwen & Hansen Ruiz (1997) and Mermilliod et al. (1997) and critically discussed by Pinnsoneault et al. (1998).

We present results of the analysis of X-ray observations of the active M dwarfs AD Leo and EV Lac. The PSPC spectra can be fitted with one- (EV Lac) or two-component (AD Leo) isothermal mekal models, and very low metallicity (∼ 0.1 solar); during an intence flare the spectrum of EV Lac can be fitted only by adding a second component with log T ∼ 7.5. The SAX light-curves of AD Leo and EV Lac also show the occurrence of several flares. The fits of the SAX spectra require at least three thermal mekal components and best-fit coronal plasma metallicity below solar for AD Leo and only marginally below solar for EV Lac.

We have also fitted the SAX spectra of AD Leo and EV Lac with model spectra from constant cross-section static coronal loops. One-loop models fail to fit the observed spectra. A second loop component, that accounts for most of the plasma emission at high energy, is required to obtain an acceptable fit. We interpret the fit results as pointing toward the existence of various (at least two) dominant classes of coronal emitting structures: the dominant one is composed of hundreds of compact loops, with lower maximum temperature and length smaller than 0.1 the stellar radius, covering no more than 1% of stellar surface; the second one, responsible for the high energy emission, is composed at least of tens of quite elongated loops, covering a very small fraction of stellar surface.

Introduction

Since the announcement of the first brown dwarfs in 1995, the field has been moving very quickly. The number of known brown dwarfs has increased rapidly, along with a growing collection of stars at the bottom of the main sequence.

Important progress has been made within the past few years regarding the theory of low mass stars (m < 1M⊙) and brown dwarfs. The main improvements concern the equation of state of dense plasmas and the modelling of cool and dense atmospheres, necessary for a correct description of such objects. These theoretical efforts now yield a better understanding of these objects and good agreement with observations regarding color-magnitude diagrams of globular clusters, mass-magnitude relationships and near-IR color-magnitude diagrams for young open clusters. However uncertainties still remain regarding synthetic optical colors and the complex problem of dust formation in the coolest atmosphere models.

Improvement of the theory

Very low mass (VLM) stars and brown dwarfs (BD) are dense and cool objects, with typical central densities of the order of 100−1000 gr.cm−3 and central temperatures lower than 107 K. Under such conditions, a correct equation of state (EOS) for the description of their inner structure must take into account strong correlations between particles, resulting in important departures from a perfect gas EOS (cf. Chabrier & Baraffe (1997)). Important progress has been made in this field, in particular by Saumon, Chabrier & Van Horn (1995) who developed an EOS specially devoted to VLM stars, BD and giant planets. Since the EOS determines essentially the mechanical structure of these objects, and thus the mass-radius relationship, it can be tested against observations of eclipsing binary systems.