9 research outputs found
Mixing along the Red Giant Branch in Metal-poor Field Stars
We have determined Li, C, N, O, Na, and Fe abundances, and 12C/13C isotopic
ratios for a sample of 62 field metal-poor stars (plus 43 taken from the
literature). This large sample was used to show that small mass lower-RGB stars
(i.e., fainter than the RGB bump) have abundances of light elements in
agreement with theoretical predictions from classical evolutionary models. A
second, distinct mixing episode occurs just after the RGB bump, reaching
regions of incomplete CNO burning. No O-Na anticorrelation, as observed in
globular cluster stars, is found in field stars. This means that the mixing
episode is not deep enough to reach regions where ON-burning occurs.Comment: 6 pages, 3 encapsulated figures, LateX, uses crckapb.sty; invited
talk, in "The Chemical Evolution of the Milky Way: Stars vs Clusters, Vulcano
(Italy), 20-24 September 1999, F. Matteucci and F. Giovannelli eds, Kluwer,
in pres
The discontinuous nature of chromospheric activity evolution
Chromospheric activity has been thought to decay smoothly with time and,
hence, to be a viable age indicator. Measurements in solar type stars in open
clusters seem to point to a different conclusion: chromospheric activity
undergoes a fast transition from Hyades level to that of the Sun after about 1
Gyr of main--sequence lifetime and any decaying trend before or after this
transition must be much less significant than the short term variations.Comment: 6 pages, 1 figure, to be published in Astrophysics and Space Scienc
Impact of Distance Determinations on Galactic Structure. I. Young and Intermediate-Age Tracers
Here we discuss impacts of distance determinations on the Galactic disk traced by relatively young objects. The Galactic disk, ∼40kpc in diameter, is a cross-road of studies on the methods of measuring distances, interstellar extinction, evolution of galaxies, and other subjects of interest in astronomy. A proper treatment of interstellar extinction is, for example, crucial for estimating distances to stars in the disk outside the small range of the solar neighborhood. We’ll review the current status of relevant studies and discuss some new approaches to the extinction law. When the extinction law is reasonably constrained, distance indicators found in today and future surveys are telling us stellar distribution and more throughout the Galactic disk. Among several useful distance indicators, the focus of this review is Cepheids and open clusters (especially contact binaries in clusters). These tracers are particularly useful for addressing the metallicity gradient of the Galactic disk, an important feature for which comparison between observations and theoretical models can reveal the evolution of the disk