arXiv subjects

Suk-Jin Yoon

Publications and source records attributed to Suk-Jin Yoon.

85 records · Page 5Linked to original sources

arXiv2006

GALEX Ultraviolet Photometry of Globular Clusters in M31: Three Year Results and a Catalog

We present ultraviolet (UV) photometry of M31 globular clusters (GCs) found in 23 Galaxy Evolution Explorer (GALEX) images covering the entirety of M31. We detect 485 and 273 GCs (and GC candidates) in the near-ultraviolet (NUV; 2267 A) and far-ultraviolet (FUV; 1516 A), respectively. Comparing M31 data with those of Galactic GCs in the UV with the aid of population models, we find that the age ranges of old GCs in M31 and the Galactic halo are similar. Three metal-rich ([Fe/H]>-1) GCs in M31 produce significant FUV flux making their FUV-V colors unusually blue for their metallicities. These are thought to be analogs of the two peculiar Galactic GCs NGC 6388 and NGC 6441 with extended blue HB stars. Based on the models incorporating helium enriched subpopulations in addition to the majority of the population that have a normal helium abundance, we suggest that even small fraction of super-helium-rich subpopulations in GCs can reproduce the observed UV bright metal-rich GCs. Young clusters in M31 show distinct UV and optical properties from GCs in Milky Way. Population models indicate that their typical age is less than ~ 2 Gyrs. A large fraction of young GCs have the kinematics of the thin, rapidly rotating disk component. However, a subset of the old GCs also shares the thin-disk kinematics of the younger clusters. The existence of young GCs on the outskirts of M31 disk suggests the occurrence of a significant recent star formation in the thin-disk of M31. Old thin-disk GCs may set constraints on the epoch of early formation of the M31 thin-disk. We detect 12 (10) intermediate-age GC candidates in NUV (FUV). We suggest that some of spectroscopically identified intermediate-age GCs may not be truly intermediate in age, but rather older GCs that possess developed HB.

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arXiv2006

UV Upturn in Elliptical Galaxies: Theory

The UV upturn is the rising flux with decreasing wavelength between the Lyman limit and 2500Åfound virtually in all bright spheroidal galaxies. It has been a mystery ever since it was first detected by the OAO-2 space telescope (Code & Welch 1979) because such old metal-rich populations were not expected to contain any substantial number of hot stars. It was confirmed by following space missions, ANS (de Boer 1982), IUE (Bertola et al. 1982) and HUT (Brown et al. 1997). The positive correlation between the UV-to-optical colour (i.e., the strength of the UV upturn) and the Mg2 line strength found by Burstein et al. (1987) through IUE observations has urged theorists to construct novel scenarios in which metal-rich ($\gtrsim Z_{\odot}$) old ($\gtrsim$ a few Gyr) stars become UV bright (Greggio & Renzini 1990; Horch et al. 1992). Also interesting was to find using HUT that, regardless of the UV strength, the UV spectral slopes at 1000--2000Åin the six UV bright galaxies were nearly identical suggesting a very small range of temperatures of the UV sources in these galaxies (Brown et al. 1997), which corresponds to $T_{\rm eff} \approx 20,000 \pm 3,000$ K. This, together with other evidence, effectively ruled out young stars as the main driver of the UV upturn. A good review on the observational side of the story is given in the next article by Tom Brown, as well as in the recent articles of Greggio & Renzini (1999) and O'Connell (1999).

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arXiv2006

Explaining the Color Distributions of Globular Cluster Systems in Elliptical Galaxies

The colors of globular clusters in most of large elliptical galaxies are bimodal. This is generally taken as evidence for the presence of two cluster subpopulations that have different geneses. However, here we find that, because of the non-linear nature of the metallicity-to-color transformation, a coeval group of old clusters with a unimodal metallicity spread can exhibit color bimodality. The models of cluster colors indicate that the horizontal-branch stars are the main drivers behind the empirical non-linearity. We show that the scenario gives simple and cohesive explanations for all the key observations, and could simplify theories of elliptical galaxy formation.

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arXiv2005

Super Helium-Rich Population and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters

Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster Omega 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 Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (Delta 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 Omega 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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arXiv2002

An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way

One of the long-standing problems in modern astronomy is the curious division of Galactic globular clusters, the "Oosterhoff dichotomy," according to the properties of their RR Lyrae stars. Here, we find that most of the lowest-metallicity ([Fe/H]<-2.0) clusters, which are essential to an understanding of this phenomenon, display a planar alignment in the outer halo. This alignment, combined with evidence from kinematics and stellar population, indicates a captured origin from a satellite galaxy. We show that, together with the horizontal-branch evolutionary effect, the factor producing the dichotomy could be a small time-gap between the cluster-formation epochs in the Milky Way and the satellite. The results oppose the traditional view that the metal-poorest clusters represent the indigenous and oldest population of the Galaxy.

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arXiv2001

Omega Centauri as a Disrupted Dwarf Galaxy: Evidence from Multiple Stellar Populations

Our recent CCD photometry (Lee et al. 1999) has shown, for the first time, that omega Cen has several distinct stellar populations, which is reminiscent of the Sagittarius dwarf galaxy. Here we present more detailed analysis of the data along with the population models. We confirm the presence of several distinct red-giant-branches (RGBs) with a red metal-rich sequence well separated from other bluer metal-poor ones. Our population models suggest the red clump associated with the most metal-rich RGB is about 4 Gyr younger than the dominant metal-poor component, indicating that omega Cen was enriched over this timescale. These features, taken together with this cluster's other unusual characteristics, provide good evidence that omega 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 now. Mergers probably were much more frequent in the early history of the Galaxy and omega Cen appears to be a relict of this era.

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arXiv2001

Synthetic Color-Magnitude Diagrams for omega Centauri and Other Massive Globular Clusters with Multiple Populations

We have constructed synthetic color-magnitude diagrams (CMDs) for omega Cen and other massive globular clusters with apparently peculiar CMD morphology. Our population models, which adopt the most up-to-date input physics and parameters, show that the observed CMD of omega Cen can be reproduced by adopting (1) multimodal metallicity distribution function as derived from the observed color distribution of red-giant-branch (RGB) stars, and (2) an internal age-metallicty relation among the populations therein. Similar results were obtained for other massive globular clusters with bimodal horizontal-branches (HBs). In particular, we found that the peculiar CMD morphology (broad RGB, bimodal HB) and properties of RR Lyrae stars observed in NGC 6388 and NGC 6441 can be reproduced by the composite of two distinct populations with mild internal age-metallicity relations. This suggests that these clusters, as well as omega Cen, may represent the relicts of disrupted dwarf galaxies.

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arXiv2001

A New Distance Estimator and Spatial Distribution of GRBS Observed by BATSE

We propose a redshift estimator for the long ($T_{90} > 20$) gamma-ray bursts (GRBs) observed by the BATSE. It is based on an empirical relation between the redshift and the power-law index of power density spectra (PDSs) of the observed GRBs. This relation is constructed by using the fact that the power-law index is dependent upon a characteristic timescale of GRB light curves which are inevitably stretched by cosmological time dilation. We construct the empirical relation using both individual PDSs and averaged PDSs. An error estimates of $z$ are 1.09 and 1.11 for the empirical relation by individual PDS fits, 1.72 and 1.56 by averaged PDS fits, for the least squares fit and the maximum likelihood fit, respectively. We attempt to determine the spatial distribution of the GRBs observed by the BATSE as a function of redshifts on the basis of the resulting redshift estimator. We find that the obtained spatial distribution of the observed GRBs seems consistent with that of the GRBs whose redshifts are reported, even though the estimated errors are not very accurate. The GRBs observed by the BATSE seem distributed within $z \sim 5-6$. This result has implications on theoretical calculations of stellar formations at high redshifts and beaming geometry via a statistical study of the observed GRBs involving beaming-induced luminosity functions. We discuss such implications, and possible uncertainties of the suggested method.

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arXiv2001

CCD Photometry of the Classic Second Parameter Globular Clusters M3 and M13

We present high-precision V, B-V color-magnitude diagrams (CMDs) for the classic second parameter globular clusters M3 and M13 from wide-field deep CCD photometry. The data for the two clusters were obtained during the same photometric nights with the same instrument, allowing us to determine accurate relative ages. Based on a differential comparison of the CMDs using the Delta (B-V) method, an age difference of 1.7 +/- 0.7 Gyr is obtained between these two clusters. We compare this result with our updated horizontal-branch (HB) population models, which confirm that the observed age difference can produce the difference in HB morphology between the clusters. This provides further evidence that age is the dominant second parameter that influences HB morphology.

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arXiv2001

Horizontal-Branch Morphology as an Age Indicator

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.

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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.

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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.

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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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