Stellar Structure and Evolution: Deductions from Hipparcos

During the last decade, the understanding of fine features of the structure and evolution of stars has become possible as a result of enormous progress made in the acquisition of high-quality observational and experimental data and of new developments and refinements in the theoretical description of stellar plasmas. The confrontation of high-quality observations with sophisticated stellar models has allowed many aspects of the theory to be validated, and several characteristics of stars relevant to Galactic evolution and cosmology to be inferred. This paper is a review of the results of recent studies undertaken in the context of the Hipparcos mission, taking benefit of the high-quality astrometric data it has provided. Successes are discussed, as well as the problems that have arisen and suggestions proposed to solve them. Future observational and theoretical developments expected and required in the field are also presented.Comment: 56 pages, including 9 figures, Ann. Rev. Astron. Astrophys. Vol. 38, September 2000 (in press

A super-Earth transiting a nearby low-mass star

A decade ago, the detection of the first transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres. As such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies and microlensing have uncovered a population of planets with minimum masses of 1.9-10 times the Earth's mass (M_Earth), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b, but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55 M_Earth and a radius 2.68 times Earth's radius (R_Earth), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen-helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. As the star is small and only 13 parsecs away, the planetary atmosphere is amenable to study with current observatories.Comment: 13 pages, 3 figures, published in Natur

Low-mass and sub-stellar eclipsing binaries in stellar clusters

We highlight the importance of eclipsing double-line binaries in our understanding on star formation and evolution. We review the recent discoveries of low-mass and sub-stellar eclipsing binaries belonging to star-forming regions, open clusters, and globular clusters identified by ground-based surveys and space missions with high-resolution spectroscopic follow-up. These discoveries provide benchmark systems with known distances, metallicities, and ages to calibrate masses and radii predicted by state-of-the-art evolutionary models to a few percent. We report their density and discuss current limitations on the accuracy of the physical parameters. We discuss future opportunities and highlight future guidelines to fill gaps in age and metallicity to improve further our knowledge of low-mass stars and brown dwarfs.Comment: 30 pages, 5 figures, no table. Review pape

Space Astrometry Missions for Exoplanet Science: Gaia and the Legacy of Hipparcos

Astrometry as a technique has so far proved of limited utility when employed as either a follow-up tool or to independently search for planetary-mass companions around stars in the solar neighborhood. However, the situation is bound to change soon. In this chapter, we provide a brief overview of past and present efforts to detect planets via milli-arcsecond (mas) astrometry, with a special focus on the legacy of the Hipparcos mission. We then focus on the Gaia mission that is poised to become a game changer in the field of exoplanets by unleashing for the first time the power of micro-arcsecond (μas) astrometry. We start by briefly describing the mission status and operation. Next, we address some of the relevant technical issues associated with the precise and accurate determination of astrometric orbits of planetary systems using Gaia data. We then present and discuss the Gaia planet-finding capabilities. We conclude by putting Gaia astrometry in context, illustrating its potential for crucial contributions to exoplanetary science in synergy with other indirect and direct methods for the detection and characterization of planetary systems