Y.-C. Kim

Y^2 ISOCHRONES WITH AN IMPROVED CORE OVERSHOOT TREATMENT

Convective core overshoot affects stellar evolution rates and the dating of stellar populations. In this paper, we provide a patch to the Y2 isochrones with an improved treatment of convective core overshoot. The new tracks cover the transition mass range from no convective core to a fully developed convective core. We compare the improved isochrones to CMDs of a few well-observed open star clusters in the Galaxy and the Large Magellanic Cloud. Finally, we discuss future prospects for improving the treatment of core overshoot with the help of asteroseismology.

Toward Better Age Estimates for Stellar Populations: The Y2 Isochrones for Solar Mixture

We have constructed a new set of isochrones, called the Y2 Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core overshoot have also been taken into consideration. This first set of isochrones is for the scaled solar mixture. A subsequent paper will consider the effects of α-element enhancement, believed to be relevant in many stellar systems. Two additionally significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main-sequence birthline instead of from the zero-age main sequence and (2) the color transformation has been performed using both the latest table of Lejeune et al., and the older, but now modified, Green et al. table. The isochrones have performed well under the tests conducted thus far. The reduction in the age of the Galactic globular clusters caused by this update in stellar models alone is approximately 15% relative to RYI-based studies. When the suggested modification for the α-element enhancement is made as well, the total age reduction becomes approximately 20%. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integrated colors are consistent with the isochrone fitting ages.

The Origin of T Tauri X-Ray Emission: New Insights from the Chandra Orion Ultradeep Project

The Chandra Orion Ultradeep Project (COUP) provides the most comprehensive data set ever acquired on the X-rayemissionofpre–main-sequencestars.Inthispaper,westudythenearly600X-raysourcesthatcanbereliably identified with optically well-characterized T Tauri stars (TTSs) in the Orion Nebula Cluster. With a detection limit of LX; min � 10 27:3 ergs s � 1 for lightly absorbed sources, we detect X-ray emission from more than 97% of the optically visible late-type (spectral types F–M) cluster stars. This proves that there is no ‘‘X-ray–quiet’’ population of late-type stars with suppressed magnetic activity. We use this exceptional optical, infrared, and X-ray data set tostudythe dependenciesoftheX-rayproperties onotherstellarparameters. AllTTSs withknownrotationperiods lie in the saturated or supersaturated regime of the relation between activity and Rossby numbers seen for mainsequence (MS) stars, but the TTSs show a much larger scatter in X-ray activity than that seen for the MS stars. Strong near-linear relations between X-ray luminosities, bolometric luminosities, and mass are present. We also

Standard solar model

A set of solar models have been constructed, each based on a single modification to the physics of a reference solar model. In addition, a model combining several of the improvements has been calculated to provide a best solar model. Improvements were made to the nuclear reaction rates, the equation of state, the opacities, and the treatment of the atmosphere. The impact on both the structure and the frequencies of the low-l p-modes of the model to these improvements are discussed. It is found that the combined solar model, which is based on the best physics available (and does not contain any ad hoc assumptions), reproduces the observed oscillation spectrum (for low-l) within the errors associated with the uncertainties in the model physics (primarily opacities).

The YY Isochrones for alpha-element Enhanced Mixtures

We present a new set of isochrones in which the effect of the alpha-element enhancement is fully incorporated. These isochrones are an extension of the already published set of YY Isochrones (Yi et al. 2001: Paper 1), constructed for the scaled-solar mixture. As in Paper 1, helium diffusion and convective core overshoot have been taken into account.The range of chemical compositions covered is 0.00001 < Z < 0.08. The models were evolved from the pre-main-sequence stellar birthline to the onset of helium burning in the core. The age range of the full isochrone set is 0.1 -- 20 Gyr, while younger isochrones of age 1 -- 80 Myr are also presented up to the main-sequence turn-off. Combining this set with that of Paper 1 for scaled-solar mixture isochrones, we provide a consistent set of isochrones which can be used to investigate populations of any value of alpha-enhancement. We confirm the earlier results of Paper 1 that inclusion of alpha-enhancement effects further reduces the age estimates of globular clusters by approximately 8 percent if [alpha/Fe]=+0.3. It is important to note the metallicity dependence of the change in age estimates (larger age reductions in lower metallicities). This reduces the age gap between the oldest metal-rich and metal-poor Galactic stellar populations and between the halo and the disk populations. The isochrone tables, together with interpolation routines have been made available via internet; this http URL this http URL this http URL

The Y2 Isochrones for α-Element Enhanced Mixtures

We present a new set of isochrones in which the effect of the alpha-element enhancement is fully incorporated. These isochrones are an extension of the already published set of YY Isochrones (Yi et al. 2001: Paper 1), constructed for the scaled-solar mixture. As in Paper 1, helium diffusion and convective core overshoot have been taken into account.The range of chemical compositions covered is 0.00001 < Z < 0.08. The models were evolved from the pre-main-sequence stellar birthline to the onset of helium burning in the core. The age range of the full isochrone set is 0.1 -- 20 Gyr, while younger isochrones of age 1 -- 80 Myr are also presented up to the main-sequence turn-off. Combining this set with that of Paper 1 for scaled-solar mixture isochrones, we provide a consistent set of isochrones which can be used to investigate populations of any value of alpha-enhancement. We confirm the earlier results of Paper 1 that inclusion of alpha-enhancement effects further reduces the age estimates of globular clusters by approximately 8 percent if [alpha/Fe]=+0.3. It is important to note the metallicity dependence of the change in age estimates (larger age reductions in lower metallicities). This reduces the age gap between the oldest metal-rich and metal-poor Galactic stellar populations and between the halo and the disk populations. The isochrone tables, together with interpolation routines have been made available via internet; this http URL this http URL this http URL

The Y2 Stellar Evolutionary Tracks

We present a database of the latest stellar models of the Y2 (Yonsei-Yale) collaboration. This database contains the stellar evolutionary tracks from the pre-main-sequence birthline to the helium core flash that were used to construct the Y2 isochrones. We also provide a simple interpolation routine that generates stellar tracks for given sets of parameters (metallicity, mass, and ?-enhancement).

Super-helium-rich populations and the origin of extreme horizontal-branch stars in globular clusters

Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster ω Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS stars well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (ΔY = 0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in ω Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super-helium-rich populations in some globular clusters suggests that a third parameter, other than metallicity and age, also influences the CMD morphology of these clusters.

Angular Momentum Loss from Cool Stars: An Empirical Expression and Connection to Stellar Activity

We show here that the rotation period data in open clusters allow the empirical determination of an expression for the rate of loss of angular momentum from cool stars on the main sequence. One significant component of the expression, the dependence on rotation rate, persists from prior work; others do not. The expression has a bifurcation, as before, that corresponds to an observed bifurcation in the rotation periods of coeval open cluster stars. The dual dependencies of this loss rate on stellar mass are captured by two functions, f(B - V) and T(B - V), that can be determined from the rotation period observations. Equivalent masses and other [UBVRIJHK] colors are provided in Table 1. Dimensional considerations and a comparison with appropriate calculated quantities suggest interpretations for f and T, both of which appear to be related closely (but differently) to the calculated convective turnover timescale, {tau}{sub c}, in cool stars. This identification enables us to write down symmetrical expressions for the angular momentum loss rate and the deceleration of cool stars and also to revive the convective turnover timescale as a vital connection between stellar rotation and stellar activity physics.

The Theoretical Calculation of the Rossby Number and the ``Nonlocal'' Convective Overturn Time for Pre-Main-Sequence and Early Post-Main-Sequence Stars

This paper provides estimates of convective turnover time scales for Sun-like stars in the pre-main sequence and early post-main sequence phases of evolution, based on up-to-date physical input for the stellar models. In this first study, all models have solar abundances, which is typical of the stars in the Galactic disk where most of the available data have been collected. A new feature of these models is the inclusion of rotation in the evolutionary sequences, thus making it possible to derive theoretically the Rossby number for each star along its evolutionary track, based on its calculated rotation rate and its local convective turnover time near the base of the convection zone. Global turnover times are also calculated for the complete convection zone. This information should make possible a new class of observational tests of stellar theory which were previously impossible with semi-empirical models, particularly in the study of stellar activity and in research related to angular momentum transfer in stellar interiors during the course of stellar evolution.