We have constructed a new set of isochrones, called the Y 2 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

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 well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y 2 isochrones, that this special feature can only be reproduced by assuming a large variation (∆Y = 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 the third parameter, other than metallicity and age, also influences CMD morphology of these clusters.

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We present wide-field and high-precision BV and Ca & Strömgren by photometry of ω Centauri, which represents one of the most extensive photometric surveys to date for this cluster. The member stars of ω Cen are well discriminated from foreground Galactic field stars in the hk [=(Ca − b) − (b − y)] vs. b − y diagram. The resulting "cleaned" color-magnitude diagram (CMD) has allowed us to obtain an accurate distribution of the red horizontal branch (HB) and the asymptotic giant branch stars. We confirm the presence of several red giant branches (RGBs) with the most metal-rich sequence well separated from other bluer metal-poor ones. Our population models suggest that four populations with different metallicities can reproduce the observed nature of the RGB. The HB distribution is also found to be consistent with the multiple stellar populations of the RGB. From our population models, we propose that the most metal-rich population is about 4 Gyr younger than the dominant metal-poor population, indicating that ω Cen was enriched over this timescale. We identify, for the first time, a continuous and slanting RGB bump in the CMD of ω Cen, which is due to the metallicity spread amongst the RGB stars. Our photometry also reveals a significant population of blue straggler stars. The discovery of several populations and the internal age-metallicity relation of ω Cen provides good evidence that ω Cen was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing at the present time.Subject headings: color-magnitude diagrams -globular clusters: generalglobular clusters: individual (ω Centauri) It also appears that this anomaly of chemical inhomogeneity found in ω Cen is closely linked to the kinematics and spatial distribution of the stars. Norris et al. (1997) found that the metal-rich RGB stars are more centrally concentrated and furthermore show evidence for different velocity dispersion and rotation properties compared to the dominating population of more metal-poor stars. The metal-poor population rotates, but the metal-rich stars do not. From this apparent difference in kinematics between the metal-poor and the metal-rich stars, as well as a second peak in the metallicity distribution,