Related papers: The second and third parameters of the Horizontal …
Horizontal branch (HB) stars in globular clusters offer us a probe of the mass loss mechanisms taking place in red giants. For M3 (NGC 5272), different shapes for the HB mass distribution have been suggested, including Gaussian and sharply…
Globular clusters (GCs) are fundamental for understanding the integrated light of old stellar populations and galaxy assembly processes. However, the role of hot, evolved stars, such as horizontal branch (HB), extreme HB, and blue…
Recent work, based on data from the Hubble Space Telescope (HST) UV Legacy Survey of Galactic Globular Clusters (GCs), has revealed that all the analyzed clusters host two groups of first- (1G) and second-generation (2G) stars. In most GCs,…
We use the HB morphology of 48 Galactic GCs to study the radial distributions of the different stellar populations known to exist in globular clusters. Assuming that the (extremely) blue HB stars correspond to stars enriched in Helium and…
We estimate the empirical R parameter in 26 Galactic Globular Clusters covering a wide metallicity range, imaged by WFPC2 on board the HST. The improved spatial resolution permits a large fraction of the evolved stars to be measured and…
We establish a framework for determing absolute ages of Galactic globular clusters and then use these ages to investigate the age-metallicity and age-Galactocentric distance relations for the 36 clusters with the most reliable age data. The…
Extremely hot horizontal branch (HB) stars and their progeny are widely considered to be responsible for the "ultraviolet upturn" (or UVX) phenomenon observed in elliptical galaxies and the bulges of spirals. Yet, the precise evolutionary…
We construct a speculative scenario for rotation-induced extra helium mixing to the envelope of horizontal branch (HB) stars. This scenario differs from previous ones in that the mixing occurs after the star has left the red giant branch…
Tidally enhanced stellar wind may affect horizontal branch (HB) morphology in globular clusters (GCs) by enhancing the mass loss of primary star during binary evolution. Lei et al. (2013a, 2013b) studied the effect of this kind of wind on…
We have constructed color-magnitude diagrams (CMDs) for ten globular clusters in the halo of the nearby spiral galaxy M33 based on Hubble Space Telescope Wide-Field Planetary Camera 2 observations in the F555W (~V) and F814W (~I) filters.…
In the first part of this paper we revisit the history of theoretical predictions for HB luminosities in old Population II stellar clusters, starting from the results of `old' evolutionary computations to introduce in various steps all the…
We introduce a new parameter {\Delta}{\xi} - the difference in magnitude between the red giant branch (RGB) bump and a point on the main sequence (MS) at the same color as the bump, the "benchmark" - to estimate the helium content in old…
In a pioneering effort, Preston et al. reported that the colors of blue horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance, from regions near the Galactic center to about 12 kpc away, and interpreted this as a…
Blue horizontal-branch (BHB) stars are evolved low-mass objects that have completed their core hydrogen burning main-sequence (MS) stage and have lost significant mass during the red giant phase culminating in the helium flash. We determine…
High-resolution optical spectra of thirteen blue horizontal-branch (BHB) stars in the globular cluster M13 show enormous deviations in element abundances from the expected cluster metallicity. In the hotter stars (T_eff > 12000 K), helium…
We derived chemical abundances for C, N, O, Na, Mg and Al in 20 asymptotic giant branch (AGB) stars in the globular cluster NGC 6752. All these elements (but Mg) show intrinsic star-to-star variations and statistically significant…
We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry in the V and I passbands of the recently discovered Local Group dwarf spheroidal galaxy in Cetus. Our color-magnitude diagram extends from above the first ascent red…
I list the characteristic features of Globular Cluster (GC) HR diagrams which provide a complete test of the stellar evolution of low mass stars: morphologies describing the different evolutionary phases, number ratios and luminosity…
We explore the relatiohsip between deep mixing on the red-giant branch and the second parameter that gives rise to the extended blue tail seen on the horizontal branch of the globular cluster M 13. We use the [Al/Fe] values of 96 RGB stars…
In this review, I will discuss a few problems which point to the need for improved stellar evolution models of halo stars. Current stellar evolution models do not match the observed $^7$Li abundance patterns, suggesting that the input…