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.

Introduction

Red dwarfs are among the least massive stellar objects in the Universe. Among them, M-type stars are in the mass range from around 0.1 to 0.5 solar masses. At lower values we only find brown dwarfs or non-stellar planetary bodies. Nevertheless, low-mass stars are probably the most common type in our Galaxy, with a high potential influence in the definition of its mass function. They also provide the connection between objects with radiation generated through nuclear reactions and those which are not able to do so because of insuficiently high internal temperatures. From the structural point of view, M-type stars are very important because they are dominated by convective energy transport, and the treatment of convection is still one of the least known parts of the theory of stellar structure.

We discuss ISO observations of infrared ionic and H2 lines toward molecular shocks in the supernova remnants 3C 391, W 28, and W 44. The total surface brightness of the H2 lines toward these lines of sight exceeds that of atomic fine structure lines, showing that these lines of sight are dominated by dense molecular shocks. The H2 excitation and the presence of bright ionic lines require that there are multiple shocks into gas with a range of pre-shock densities from 10–103 cm−3.

Introduction

Massive stars end their lives in supernova explosions, and they do not live long enough to travel far from their parent molecular clouds. Therefore, supernovae frequently occur inside molecular clouds, providing compression, turbulence, cosmic rays, radiation, and heat. Using the Infrared Space Observatory, we performed a set of observations designed to search for infrared emission from the gas and dust that gets excited in molecular shock fronts. When the shock front passes through a molecular cloud, the gas cools via the most ‘convenient’ transitions available to it: low-density gas cools via atomic fine structure lines from the abundant ions, while molecular gas cools via the large number of rotational and/or vibrational transitions available. The first results of our project were the detection of bright [O I] 63 µm lines (Reach & Rho 1996), proving that abundant energy was being pumped into the gas by the shock fronts.

Shock waves in outflows are generated by the impact of jets, associated with low-mass star formation, on the surrounding molecular gas. These shocks give rise to a strong H2 rovibrational emission spectrum which has been observed by the ISO satellite in several star formation regions. The dynamical time scales associated with these outflows are estimated to be a few thousand years and can be, in some regions, as short as a few hundred years. On the other hand, the time required to reach steady state for a C-shock is about 104 years. Under such circumstances, the shocks are unlikely to have attained a state of equilibrium, and a time dependent approach has to be considered. Non stationary C-shocks are found to exhibit both C-and J-type characteristics. The H2 rotational excitation diagram can provide a measure of the age of the shock; in the case of the outflow observed in Cepheus A West by the ISO satellite, the shock age is estimated to be approximately 1.5 × 103 yr.

Time scales

Steady state shocks

Shocks propagating in the interstellar medium are expected to modify profoundly the local physical and chemical conditions. Even in the simplest case of planar shocks, the structure of the shock can take a number of different forms, from ‘jump’ or J-type structure, in which changes in density, velocity and temperature occur quasi-discontinuously, to ‘continuous’ or C-type, where the variations take place smoothly over a much larger distance scale.

The Lyman and Werner band systems of deuterated molecular hydrogen (HD) occur in the far UV range below 120 nm. This spectral window is now open at moderate resolution and high sensitivity with the FUSE satellite. FUSE spectra of hot stars with high extinction through translucent clouds will give access to the deuterium abundance inside molecular clouds where D is essentially in the form of HD. Measurement of HD/H2 ratio becomes thus a new powerful method to evaluate the D/H ratio in the interstellar medium.

An example is given with the FUSE spectrum of the high extinction O9III star HD 73882 (EB–V = 0.7). Very preliminary analysis and an estimate of the HD/H2 ratio are presented.

Introduction

It has long been recognized that the primordial abundance of deuterium represents the most sensitive probe of the baryonic density Ωb of the Universe (see, e.g., Schramm & Turner 1998; Olive et al. 1999). On the other hand, abundance of deuterium at any epoch is a lower limit to its primordial abundance, since deuterium is destroyed, not created, in stars of any mass. For this reason, deuterium abundance is also an efficient tracer of the universal star formation rate. Unfortunately, the evolution of deuterium abundance from the primordial to the solar metallicity is still unclear.

Measurements of the atomic D/H ratio have been performed in different astrophysical sites, namely in moderate to high redshift quasar absorbers, in the presolar nebula and in the local interstellar medium (for reviews see, e.g., Ferlet & Lemoine 1996; Linsky 1998; Vidal-Madjar et al. 1998a; Lemoine et al. 1999).

We present spectroscopic and imaging observations of dust and gas emission from the western edge of the ρ Ophiuchus molecular cloud facing the B2 III/IV star HD 147889. The emissions from dust heated by the external UV radiation, from collisionally excited and fluorescent H2 are resolved and observed to coincide spatially. The spectroscopic data allows to estimate the gas temperature to 350 ± 30 K in the H2 emitting layer. In the framework of a steady state model of the photo-dissociation region, a high formation rate: 210−16cm3s−1 at 350 K, seems to be required to account for this temperature. For smaller formation rates the H2 emitting layer moves into the cloud where the gas is colder due to radiation attenuation.

Introduction

ISO observations of the dust emission and H2 rotational lines are bringing a new perspective on the structure and physical conditions in regions of H2 photodissociation (PDRs) at the surface of molecular clouds illuminated by hot stars. Spectroscopic observations of bright PDRs such as NGC 2023 have allowed to build detailed excitation diagrams of H2 with numerous lines to test physical models (Draine this conference). In this paper, we present observations of a fainter PDR on the western edge of the ρ Ophiuchus molecular cloud heated by the B2III/IV star HD 147889. This is a nearby PDR (d = 135±15 pc from the star parallax) with an edge-on geometry where the observations allow to spatially resolve the layer of UV light penetration and of H2 photo-dissociation.

Using the Short-Wavelength-Spectrometer on the Infrared Satellite Observatory (ISO), we obtained near- and mid-infrared spectra toward the brightest H2 emission peak of the Orion OMC-1 outflow. A wealth of emission and absorption features were detected, dominated by 60 H2 ro-vibrational and pure rotational lines reaching from H2 0–0 S(1) to 0–0 S(25).

The total H2 luminosity in the ISO-SWS aperture is (17 ± 5) L⊙, and extrapolated to the entire outflow, (120 ± 60) L⊙. The H2 level column density distribution shows no signs of fluorescent excitation or a deviation from an ortho-to-para ratio of three. It shows an excitation temperature which increases from about 600 K for the lowest rotational and vibrational levels to about 3200 K at level energies E(v, J)/k > 14 000 K.

Introduction

The Orion molecular cloud, OMC-1, located behind the Orion M42 Nebula at a distance of ∼450 pc (Genzel & Stutzki 1989), is the best-studied massive star forming region. This cloud embeds a spectacular outflow arising from some embedded young stellar object, which can possibly be identified as the radio source “I” 0.49 arcsec south of the infrared source IRc2-A (Menten & Reid 1995; Dougados et al. 1993). The outflow shocks the surrounding molecular gas, thereby giving rise to the strongest H2 infrared line emission appearing in the sky. Peak 1 (Beckwith et al. 1978) is the brighter of the two H2 emission lobes of the outflow. Although the outflow has been studied extensively for nearly two decades, the nature of the emission mechanism remains unclear.