arXiv subjects

Hyun-chul Lee

Publications and source records attributed to Hyun-chul Lee.

23 records · Page 2Linked to original sources

arXiv2001

Spectrophotometric Evolution of Old Stellar Systems

Theoretical integrated spectrophotometric quantities, such as spectral line indices in the Lick system (Hbeta, Mg2, Mg b, Fe5270, Fe5335), broadband color indices (in the UBVRIHK and Washington CMT1 bandpasses), and surface brightness fluctuation magnitudes and colors, ranging from far-UV to near-IR have been consistently computed for simple stellar populations from our evolutionary population synthesis code. The age range of -4 Gyr < Delta_t < +4 Gyr is explored, where Delta_t = 0 Gyr is the currently favored mean age of inner halo Galacitc globular clusters (Galactocentric radius < 8 kpc), ~ 12Gyr. The main aim of this dissertation is to investigate the effects of post-red giant branch stars on the integrated spectrophotometric quantities after employing detailed systematic variation of horizontal-branch morphology with age and metallicity.

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arXiv2001

Age Estimation of Extragalactic Globular Cluster Systems Using H beta Index

After taking into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity, our population synthesis models show that the integrated H beta index is significantly affected by the presence of blue HB stars. Our models indicate that the strength of the H beta index increases as much as 0.75 A due to blue HB stars. According to our models, a systematic difference between the globular cluster system in the Milky Way Galaxy and that in NGC 1399 in the H beta vs. Mg2 diagram is understood if globular cluster systems in giant elliptical galaxies are couple of billion years older, in the mean, than the Galactic counterpart.

astro-ph↗

arXiv2000

The H Beta Index as an Age Indicator of Old Stellar Systems: The Effects of Horizontal-Branch Stars

The strength of the H$β$ index has been computed for the integrated spectra of model globular clusters from the evolutionary population synthesis. These models take into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that the H$β$ index is significantly affected by the presence of blue HB stars. Because of the contribution from blue HB stars, the H$β$ does not monotonically decrease as metallicity increases at a given age. Instead, it reaches a maximum strength when the distribution of HB stars is centered around 9500 K, the temperature where the H$β$ index becomes strongest. Our models indicate that the strength of the H$β$ index increases as much as 0.75 Å due to the presence of blue HB stars. The comparison of the recent Keck observations of the globular cluster system in the Milky Way Galaxy with those in giant elliptical galaxies, NGC 1399 and M87, shows a systematic shift in the H$β$ against metallicity plane. Our models suggest that this systematic difference is understood if the globular cluster systems in giant elliptical galaxies are several billion years older, in the mean, than the Galactic counterpart. Further observations of globular cluster systems in the external galaxies from the large ground-based telescopes and space UV facilities will enable to clarify whether this difference is indeed due to the age difference or other explanations are also possible.

astro-ph↗

arXiv1999

Multiple Stellar Populations in the Globular Cluster omega Centauri as Tracers of a Merger Event

The discovery of the Sagittarius dwarf galaxy, which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars, and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is omega Centauri, and here we report that omega Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that omega Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that omega Centauri 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 now. Mergers probably were much more frequent in the early history of the Galaxy and omega Centauri appears to be a relict of this era.

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arXiv1999

Horizontal-Branch Stars as an Age Indicator

Surface temperature distribution of horizontal-branch (HB) stars is very sensitive to age in old stellar systems, which makes it an attractive age indicator. In this paper, we present the recent revision of our model calculations for the HB morphology of globular clusters. The result is more updated version of our earlier models (Lee, Demarque, & Zinn 1994), which suggests that the HB morphology is more sensitive (\sim 40%) to age. We also present our new model calculations on the effect of HB stars in dating old stellar systems using the H$β$ index. Our results indicate that the effect of HB stars is rather strong, and suggest a possibility for the systematic difference in age between the globular clusters in the Milky Way Galaxy and those in giant elliptical galaxies. Finally, we compare these results with the relative ages estimated from our far-UV dating technique for giant elliptical galaxies.

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