13 research outputs found
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