Horizontal branch (HB) models were evolved using the Yale stellar evolution code, YREC, to test the possibility that mass loss during the RR Lyrae phase is able to produce the observed color (mass) dispersion on the HB (Willson and Bowen 1984) and the anomalous period changes in RR Lyrae stars (Laskarides 1974). Models of total mass 0.64, 0.66, 0.68, 0.70, and 0.72 M⊙ (YMS = 0.23, Z = 0.001) were evolved with constant mass loss rates of 0, 10-10, and 10-9 M⊙ yr-1. Mass loss was assumed to occur only in the RR Lyrae phase, and the instability strip was defined by 3.800 < log Teff < 3.875.

HB stars which lose mass evolve further to the blue. Low mass loss rates do not affect the shape of the tracks significantly. Stars, which without mass loss could not become blue HB stars, were able to emerge from the instability strip on the blue side.

As tracers of old stellar populations and as primary Population II standard candles, RR Lyrae stars have played an important role in the development of modern astronomy. Our knowledge of stellar evolution has identified these variable stars in a core helium burning phase of low-mass star evolution, the hoiizontal-branch (HB) phase. Consequently, not only to understand fully the nature of RR Lyrae stars, but also to apply them correctly as population probes and distance indicators, we must understand the underlying evolutionary effect of HB stars.

In this paper, I briefly review the most important properties of RR Lyrae stars predicted from the HB evolutionary models, and present many pieces of supporting evidence for these models. For the implications of these models on the chronology of the Galactic formation and on the cosmological distance scale, the reader is referred to several recent publications by Lee (1992a,b,c,d).

In their recent, high-quality spectroscopic observations of 59 RR Lyrae stars in Baade's window field of the Galactic nuclear bulge, Walker and Terndrup (1991) found that the metallicity distribution for their whole sample is sharply peaked at [Fe/H] = −1.0. Comparison of their data with the sample of halo RR Lyraes (Suntzeff et al. 1991) suggests that the metallicity at which the stellar population is most likely to form RR Lyraes increases with decreasing galactocentric distance.

The horizontal-branch (HB) model calculations indicate that this is what one would expect if the radial variation in HB morphology observed in the halo continues to the very center of the Galaxy. In particular, the observed peak of the bulge RR Lyrae abundance distribution at [Fe/H] = −1.0 can only be reproduced by the unique relationship between the HB type and [Fe/H] that is shifted to the right of the mean relationship for inner halo globular clusters (see Fig. 7a of Lee 1991).

Recent UV observations of elliptical galaxies are interpreted as evidence for the global second parameter phenomenon of horizontal-branch (HB) morphology within, as well as between, these galaxies. In this picture, the origin of the UV radiation is mostly due to hot HB stars and their post-HB progeny produced by the metal-poor tail of the wide metallicity distribution expected to be present in these systems. The attractive feature of this model is that the bimodal temperature distributions of HB stars (and their progeny), required to generate the 2000 Å dip of the spectral energy distribution (SED), can naturally be reproduced from the standard HB population models with large range of metal abundance (see Lee 1994, ApJ, 430, L113). Detailed population synthesis models are presented, which reproduce the systematic variation of UV upturn among elliptical galaxies (Fig 1). If age is the major second parameter, as suggested by the fossil record in our Galaxy, the observed UV color gradient and the UV upturn-total mass (mean metallicity) correlation, within and between the early-type systems, would imply, respectively, (1) that most galaxies formed from the inside out, and (2) that there is age spread among galaxies, in the sense that more massive galaxies are older (and more metal-rich in the mean) than less massive galaxies as a result of more efficient star formation (and metal enrichment) in denser environments.

For the first time, we have taken into account the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity in our population synthesis models and they result that the integrated Hβ index is significantly affected by the presence of blue HB stars. As a matter of fact, due to the systematic HB morphology variation, it is found that Hβ does not monotonically decrease as metallicity increases at given ages, but shows a kind of wavy feature. According to our models, a systematic difference between the globular cluster system in the Milky Way Galaxy and that in NGC 1399 in the Hβ vs. Mg2 plane is explained if globular cluster systems in giant elliptical galaxies are a couple of billion years older, in the mean, than the Galactic counterpart.

We present our recent revision of model constructions for the horizontal-branch (HB) morphology of globular clusters, which suggests the HB morphology is more sensitive to age compared to our earlier models. We also present our high precision CCD photometry for the classic second parameter pair M3 and M13. The relative age dating based on this photometry indicates that M13 is indeed older than M3 by 1.7 Gyr. This is consistent with the age difference predicted from our new models, which provides a further support that the HB morphology is a reliable age indicator in most population II stellar systems.

In order to investigate the origin of multiple stellar populations in the halo and bulge of the Milky Way, we have constructed chemical evolution models for the low-mass proto-Galactic subsystems such as globular clusters (GCs). Unlike previous studies, we assume that supernova blast waves undergo blowout without expelling the pre-enriched gas, while relatively slow winds of massive stars (WMS), together with the winds and ejecta from low and intermediate mass asymptotic-giant-branch stars (AGBs), are all locally retained in these less massive systems. We find that the observed Na-O anti-correlations in metal-poor GCs can be reproduced, when multiple episodes of starbursts are allowed to continue in these subsystems. A specific form of star formation history (SFH) with decreasing time intervals between the stellar generations, however, is required to obtain this result, which is in good agreement with the parameters obtained from our stellar evolution models for the horizontal-branch. The “mass budget problem” is also much alleviated by our models without ad-hoc assumptions on star formation efficiency (SFE) and initial mass function (IMF). We also applied these models to investigate the origin of super-helium-rich red clump stars in the metal-rich bulge as recently suggested by Lee et al. (2015). We find that chemical enrichments by the WMS can naturally reproduce the required helium enhancement (ΔY/ΔZ = 6) for the second generation stars. Disruption of proto-GCs in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field.

A number of recent studies have claimed that the double red clump observed in the Milky Way bulge is a consequence of an X-shaped structure. In particular, Ness & Lang (2016) report a direct detection of a faint X-shaped structure in the bulge from the residual map of the Wide-Field Infrared Survey Explorer (WISE) image. Here we show, however, that their result is seriously affected by a bulge model subtracted from the original image. When a boxy bulge model is subtracted, instead of a simple exponential bulge model as has been done by Ness & Lang, we find that most of the X-shaped structure in the residuals disappears. Furthermore, even if real, the stellar density in the claimed X-shaped structure is way too low to be observed as a strong double red clump (dRC) at l = 0°.

To trace past changes in flooding frequency, we investigated fluvial sediments in the middle reach of the Nakdong River, South Korea. Sediments with larger grain size, lower total organic carbon percentage, and depleted δ13C values in the recovered sediment cores were interpreted as periods of more frequent flooding. Patterns of decreased long-term flooding frequency and vegetation changes during the early to late Holocene were similar to the decreasing regional summer monsoon intensity. Multicentennial frequent flooding periods in the study area (2900–3400 cal yr BP, 3600–3900 cal yr BP, 4600–5300 cal yr BP, and 5800–6400 cal yr BP) corresponded to stronger El Niño–Southern Oscillation (ENSO) activity periods. Based on previous studies showing that high-frequency tropical typhoon-driven coastal inundation along the western coast of Japan during the middle to late Holocene was coupled with stronger ENSO activity, it is likely that the observed centennial-to-millennial-scale flood frequency change in South Korea was influenced mainly by changes in the genesis and tracks of tropical typhoons at centennial-to-millennial time scales. This suggests that the centennial-to-millennial-scale hydrologic changes in East Asia were linked to the remote atmospheric-oceanic circulation changes represented by an ENSO-like pattern.

New synthetic horizontal branch (HB) models are presented for some globular clusters known to have bimodal HB distributions. These models are based on new Yale HB evolutionary tracks for Y=0.25 and the core masses appropriate for the compositions. The distribution of stellar masses along the HB is given by a slightly modified version of Rood's (1973) function. Figure 1 compares the synthetic and the observed color-magnitude diagrams and the generalized histograms of the distribution of HB stars over (B-V)o (observational data from Alcaino & Liller 1984, Buonanno et al. 1981, Stetson 1981, and Menzies 1974 for clusters M4, M5, N1851, and N6723 respectively). Following Norris (1981), we have used the period-luminosity-color relations to estimate the colors of some RR Lyrae variables. Since none of the parameters in the models has a bimodal distribution, the excellent agreement between the color distributions of the models and the observations suggests that the observed bimodal distributions are a consequence of the evolution from the zero-age HB. Contrary to the suggestion of Norris (1981) and Smith & Norris (1983), there is no need to connect the bimodality of the HB with the observed bimodal CN distributions of the red giants in some of these clusters. The clusters in Figure 1 span a narrow range in metallicity ([Fe/H]=-1.40 to −1.09; Zinn 1985), and we are investigating other, less well observed, clusters in this range (e.g., N2808) to see if their bimodal HB distributions also have simple explanations.

There is a compelling evidence for old ages of giant elliptical galaxies, about 3 Gyrs older than our Galaxy, from space ultraviolet observations and new population synthesis models. Alternative interpretation requires ad-hoc assumptions yet it produces poor fitting to observational data. If confirmed by future space UV mission, this would provide evidence for non-zero cosmological constant from time scale test.

The suggestion of Lee (1994, ApJ, 430, L113) that the age spread among galaxies is responsible for the systematic variation of ultraviolet (UV) upturn among the early-type systems is confirmed here with the detailed population synthesis models. Our models suggest that the far-UV spectra of these systems are composite of hot metal-poor horizontal-branch (HB) stars (and their post-HB progeny) and metal-rich post-asymptotic giant-branch (PAGB) stars. The systematic variation of UV upturn, however, depends on the contribution from hot metal-poor HB stars and their progeny, which in turn depends on the ages of oldest stellar populations in galaxies. Consequently, our models predict that the strength of absorption features, such as C IV and Si IV, is anticorrelated with the strength of UV upturn. This is consistent with the far-UV spectra for NGC1399 and M31 obtained by the Hopkins Ultraviolet Telescope aboard the Astro-1 space shuttle mission. We note that the opposite trend is expected in other's models that favor metal-rich solution for the UV upturn phenomenon without age spread among galaxies. Our result implies a prolonged epoch of galaxy formation, in the sense that more massive galaxies (in denser environments) formed first. With the assumption that the UV upturn phenomenon is solely due to the age variations among galaxies, we tentatively estimate the difference in age between the giant ellipticals and our Galaxy to be 4 billion years or more. This suggests that the best estimate for the lower limit of the age of the Universe is close to 20 Gyrs, which of course would be in conflict with the current estimate of the Ho together with the standard cosmological models with zero cosmological constant. The reader is referred to Park & Lee (1995, ApJL, in press) for details of this work.

The genus Vicia L., one of the earliest domesticated plant genera, is a member of the legume tribe Fabeae of the subfamily Papilionoideae (Fabaceae). The taxonomic history of this genus is extensive and controversial, which has hindered the development of taxonomic procedures and made it difficult to identify and share these economically important crop resources. Species identification through DNA barcoding is a valuable taxonomic classification tool. In this study, four DNA barcodes (ITS2, matK, rbcL and psbA-trnH) were evaluated on 110 samples that represented 34 taxonomically best-known species in the Vicia genus. Topologies of the phylogenetic trees based on an individual locus were similar. Individual locus-based analyses could not discriminate closely related Vicia species. We proposed a concatenated data approach to increase the resolving power of ITS2. The DNA barcodes matK, psbA-trnH and rbcL were used as an additional tool for phylogenetic analysis. Among the four barcodes, three-barcode combinations that included psbA-trnH with any two of the other barcodes (ITS2, matK or rbcL) provided the best discrimination among Vicia species. Species discrimination was assessed with bootstrap values and considered successful only when all the conspecific individuals formed a single clade. Through sequencing of these barcodes from additional Vicia accessions, 17 of the 34 known Vicia species could be identified with varying levels of confidence. From our analyses, the combined barcoding markers are useful in the early diagnosis of targeted Vicia species and can provide essential baseline data for conservation strategies, as well as guidance in assembling germplasm collections.

A total of 27 accessions from ten Vicia species were investigated for flavonoid contents, total polyphenol contents, and DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS [2,2′-azinobis(3-ethylbenzothiazoline 6-sulfonic acid)] free radical-scavenging activities. The results revealed that NAC17 (V. monantha) and NAC14 (V. hyrcanica) had the highest total flavonoid content (1.42 ± 0.09 mg/g) and total polyphenol content [124.2 ± 0.5 μg/gallic acid equivalents (GAE) mg], respectively. Among four flavonoids, naringenin was detected at high concentrations in Vicia species. The DPPH and ABTS assays showed values in the range of 57.2 (IC50) (NAC13, V. faba) to 6530.0 (IC50) (NAC24, V. sativa subsp. nigra) and 19.1 μg/Trolox mg (NAC7, V. cracca) to 253.4 μg/Trolox mg (NAC13, V. faba), respectively. Among ten Vicia species, V. monantha and V. hyrcanica had the highest flavonoid content (1.31 ± 0.09 mg/g) and total polyphenol content (116.5 ± 2.0 μg/GAE mg), respectively. The highest antioxidant activity was detected in V. faba. These results will expand the flavonoid database and provide valuable information on Vicia species for the development of functional foods or feed-additive resources.

Scale-bridging analysis on deformation behavior of high-nitrogen austenitic Fe–18Cr–10Mn–(0.39 and 0.69)N steels was performed by neutron diffraction, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). Two important modes of deformation were identified depending on the nitrogen content: deformation twinning in the 0.69 N alloy and strain-induced martensitic transformation in the 0.39 N alloy. The phase fraction and deformation faulting probabilities were evaluated based on analyses of peak shift and asymmetry of neutron diffraction profiles. Semi in situ EBSD measurement was performed to investigate the orientation dependence of deformation microstructure and it showed that the variants of ε martensite as well as twin showed strong orientation dependence with respect to tensile axis. TEM observation showed that deformation twin with a {111}⟨112⟩ crystallographic component was predominant in the 0.69 N alloy whereas two types of strain-induced martensites (ε and α′ martensites) were observed in the 0.39 N alloy. It can be concluded that scale-bridging analysis using neutron diffraction, EBSD, and TEM can yield a comprehensive understanding of the deformation mechanism of nitrogen-alloyed austenitic steels.

Examining both optical and optical-infrared color distributions of the globular cluster (GC) systems in large elliptical galaxies is the key to study how non-linearities in the color-metallicity relations of their GC systems are linked to bimodal optical color distributions. In order to do this for the core of the Coma cluster of galaxies (Abell 1656), centered on the giant elliptical galaxy NGC 4874, we have combined F160W (H160) near-infrared (NIR) imaging data acquired with the Wide Field Camera 3 IR Channel (WFC3/IR), installed on Hubble Space Telescope (HST) in 2009, with F475W (g475) and F814W (I814) optical imaging data from the HST Advanced Camera for Surveys (ACS). Since optical-NIR color distributions of extragalactic GC systems reflect the underlying features of the metallicity distributions, we have probed not only optical g475–I814 and optical-NIR I814–H160 color distributions but also the color-color relation for this GC system. The features of these color distributions have been quantitatively analyzed using the Gaussian Mixture Modeling code. We find that brighter GCs have a much redder mean color than fainter ones. The optical color distribution of the GC system in the Coma cluster core shows the typical bimodality, while the evidence for bimodality is significantly weaker in the optical-NIR color distribution.

Increasing number of massive globular clusters (GCs) in the Milky Way are now turned out to host multiple stellar populations having different heavy element abundances enriched by supernovae. Recent observations have further shown that [CNO/Fe] is also enhanced in metal-rich subpopulations in most of these GCs, including ω Cen and M22 (Marino et al. 2011, 2012). In order to reflect this in our population modeling, we have expanded the parameter space of Y2 isochrones and horizontal-branch (HB) evolutionary tracks to include the cases of normal and enhanced nitrogen abundances ([N/Fe] = 0.0, 0.8, and 1.6). The observed variations in the total CNO content were reproduced by interpolating these nitrogen enhanced stellar models. Our test simulations with varying N and O abundances show that, once the total CNO sum ([CNO/Fe]) is held constant, both N and O have almost identical effects on the HR diagram (see Fig. 1).

The surface brightness fluctuation (SBF) method at near-infrared (NIR) wavelengths is a powerful tool for estimating distances to unresolved stellar systems with high precision. The IR channel of the Wide Field Camera 3 (WFC3), installed on board the Hubble Space Telescope (HST) in 2009, has a greater sensitivity and a wider field of view than the previous generation of HST IR instruments, making it much more efficient for measuring distances to early-type galaxies in the Local Volume. To take full advantage of its capabilities, we need to empirically calibrate the SBF distance method for WFC3's NIR passbands. We present the SBF measurements for the WFC3/IR F160W bandpass filter using observations of 16 early-type galaxies in the Fornax and Virgo Clusters. These have been combined with existing (g475–z850) color measurements from the Advanced Camera for Surveys Virgo and Fornax Cluster Surveys to derive a space-based H160-band SBF relation as a function of color. We have also compared the absolute SBF magnitudes to those predicted by evolutionary population synthesis models in order to study stellar population properties in the target galaxies.