Search Publications
Stellar Iron Abundances: Non-LTE Effects
Thévenin, F.; Idiart, T. P.
We report new statistical equilibrium calculations for Fe I and Fe II in the atmosphere of late-type stars. We used atomic models for Fe I and Fe II having, respectively, 256 and 190 levels, as well as 2117 and 3443 radiative transitions. Photoionization cross sections are from the Iron Project. These atomic models were used to investigate non-LTE…
The Age of β Pictoris
Barrado y Navascués, David; Stauffer, John R.; Song, Inseok +1 more
We have reanalyzed data for the proposed moving group associated with β Pictoris in order to determine if the group (or part of it) is real and, if so, to derive an improved age estimate for β Pic. By using new, more accurate proper motions from PPM and Hipparcos and a few new radial velocities, we conclude that on kinematic grounds, most of the p…
Effects of Tidal Shocks on the Evolution of Globular Clusters
Ostriker, Jeremiah P.; Lee, Hyung Mok; Gnedin, Oleg Y.
We present new Fokker-Planck models of the evolution of globular clusters, including gravitational tidal shocks. We extend our calculations beyond the core collapse by adopting three-body binary heating. Effects of the shocks are included by adding the tidal shock diffusion coefficients to the ordinary Fokker-Planck equation: the first-order heati…
The η Chamaeleontis Cluster: A Remarkable New Nearby Young Open Cluster
Feigelson, Eric D.; Mamajek, Eric E.; Lawson, Warrick A.
A cluster of young, X-ray-emitting stars is found in the vicinity of η Chamaeleontis from a deep ROSAT high-resolution imager observation. The 12 X-ray sources have prominent (R=5-14) stellar counterparts, including two early-type stars (η Cha and RS Cha) and 10 Li-rich, Hα emission-line, late-type stars (K3-M5). Hipparcos astrometry reveals that …
A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Lambert, David L.; Allende Prieto, Carlos; García López, Ramón J. +1 more
Chemical analyses of late-type stars are usually carried out following the classical recipe: LTE line formation and homogeneous, plane-parallel, flux-constant, and LTE model atmospheres. We review different results in the literature that have suggested significant inconsistencies in the spectroscopic analyses, pointing out the difficulties in deri…
Parallaxes versus p-Modes: Comparing HIPPARCOS and Asteroseismic Results for Pulsating AP Stars
Kurtz, D. W.; Matthews, Jaymie M.; Martinez, Peter
The p-mode eigenfrequency spacings of 12 rapidly oscillating Ap (roAp) stars provide estimates of their radii. By combining these with independent estimates of their effective temperatures via Hβ photometry, Martinez predicted luminosities for all but one of this sample. Hipparcos parallaxes, and hence luminosities, are now available for the entir…
Correlated Errors in HIPPARCOS Parallaxes toward the Pleiades and the Hyades
Gould, Andrew; Narayanan, Vijay K.
We show that the errors in the Hipparcos parallaxes toward the Pleiades and the Hyades open clusters are spatially correlated over angular scales of 2°-3°, with an amplitude of up to 2 mas. This correlation is stronger than expected based on the analysis of the Hipparcos catalog. We predict the parallaxes of individual cluster members, πpm
Lithium Isotope Ratios in Halo Stars. III.
Rebull, L. M.; Hobbs, L. M.; Thorburn, J. A.
New, high-quality echelle spectra of four halo stars with metallicities in the range -2.0<=[Fe/H]<=-0.9 have been obtained in the region of the Li I λ6707 lines. Upper limits on the Li isotopic ratio ranging from 6Li/7Li<=0.02 to 6Li/7Li<=0.08 are found for HD 76932, HD 218502, and HD 284248.…
Analysis of the HIPPARCOS Measurements of upsilon Andromedae: A Mass Estimate of Its Outermost Known Planetary Companion
Mazeh, Tsevi; van Leeuwen, Floor; Zucker, Shay +1 more
We present an analysis of Hipparcos astrometric measurements of υ Andromedae, a nearby main-sequence star around which three planet candidates have recently been discovered by means of radial velocity measurements. The stellar orbit associated with the outermost candidate has a period of 1269+/-9 days and a minimum semimajor axis of 0.6 mas. Using…
Revision and Calibration of MK Luminosity Classes for Cool Giants by HIPPARCOS Parallaxes
Keenan, Philip C.; Barnbaum, Cecilia
The Hipparcos parallaxes of cool giants are utilized in two ways in this paper. First, a plot of reduced parallaxes of stars brighter than 6.5, as a function of spectral type, for the first time separates members of the clump from stars in the main giant ridge. A slight modification of the MK luminosity standards has been made so that luminosity c…