6,593 research outputs found
New insights on the origin of multiple stellar populations in globular clusters
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-correlation in metal-poor
GCs can be reproduced, when multiple episodes of starbursts are allowed to
continue in these subsystems. Specific 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 (dY/dZ = 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.Comment: To appear in the proceedings of the IAU Symposium 334 "Rediscovering
our Galaxy", Potsdam, 10-14 July 201
Enrichment by supernovae in globular clusters with multiple populations
The most massive globular cluster in the Milky Way, omega Centauri, is
thought to be the remaining core of a disrupted dwarf galaxy, as expected
within the model of hierarchical merging. It contains several stellar
populations having different heavy elemental abundances supplied by supernovae
-- a process known as metal enrichment. Although M22 appears to be similar to
omega Cen, other peculiar globular clusters do not. Therefore omega Cen and M22
are viewed as exceptional, and the presence of chemical inhomogeneities in
other clusters is seen as `pollution' from the intermediate-mass
asymptotic-giant-branch stars expected in normal globular clusters. Here we
report Ca abundances for seven globular clusters and compare them to omega Cen.
Calcium and other heavy elements can only be supplied through numerous
supernovae explosions of massive stars in these stellar systems, but the
gravitational potentials of the present-day clusters cannot preserve most of
the ejecta from such explosions. We conclude that these globular clusters, like
omega Cen, are most probably the relics of more massive primeval dwarf galaxies
that merged and disrupted to form the proto-Galaxy.Comment: Published in Natur
Explaining the multiple populations in globular clusters by multiple episodes of star formation and enrichment without gas expulsion from massive star feedback
In order to investigate the origin of multiple stellar populations found in
globular clusters (GCs) in the halo and bulge of the Milky Way, we have
constructed chemical evolution models for their putative low-mass progenitors.
In light of recent theoretical developments, we assume that supernova blast
waves undergo blowout without expelling the pre-enriched ambient gas, while
relatively slow winds of massive stars, together with the winds and ejecta from
low to high mass asymptotic-giant-branch stars, are all locally retained in
these less massive systems. Interestingly, we find that the observed Na-O
anti-correlations in metal-poor GCs can be reproduced when multiple episodes of
starburst and enrichment are allowed to continue in these subsystems. A
specific form of star formation history with decreasing time intervals between
the successive stellar generations, however, is required to obtain this result,
which is in good agreement with the parameters obtained from synthetic
horizontal-branch models. The "mass budget problem" is also much alleviated by
our models without ad-hoc assumptions on star formation efficiency, initial
mass function, and the preferential loss of first-generation stars. We also
applied these models to investigate the origin of super-He-rich red clump stars
in the metal-rich bulge suggested by Lee et al. (2015). We find that chemical
enrichment by the winds of massive stars can naturally reproduce the required
strong He enhancement in metal-rich subsystems. Our results further underscore
that gas expulsion or retention is a key factor in understanding the multiple
populations in GCs.Comment: Accepted for publication in Ap
Star Formation Histories of Globular Clusters with Multiple Populations. I. \omega\ Cen, M22, and NGC 1851
There is increasing evidence that some massive globular clusters (GCs) host
multiple stellar populations having different heavy element abundances enriched
by supernovae. They usually accompany multiple red giant branches (RGBs) in the
color-magnitude diagrams (CMDs), and are distinguished from most of the other
GCs which display variations only in light element abundances. In order to
investigate the star formation histories of these peculiar GCs, we have
constructed synthetic CMDs for \omega\ Cen, M22, and NGC 1851. Our models are
based on the updated versions of Yonsei-Yale (Y^2) isochrones and horizontal
branch (HB) evolutionary tracks which include the cases of enhancements in both
helium and the total CNO abundances. To estimate ages and helium abundances of
subpopulations in each GC, we have compared our models with the observations on
the Hess diagram by employing a \chi^2 minimization technique. We find that
metal-rich subpopulations in each of these GCs are also enhanced in helium
abundance, and the age differences between the metal-rich and metal-poor
subpopulations are fairly small (~0.3-1.7 Gyr), even in the models with the
observed variations in the total CNO content. These are required to
simultaneously reproduce the observed extended HB and the splits on the main
sequence, subgiant branch, and RGB. Our results are consistent with the
hypothesis that these GCs are the relics of more massive primeval dwarf
galaxies that merged and disrupted to form the proto-Galaxy.Comment: 40 pages, 16 figures, 4 tables, accepted for publication in Ap
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.Comment: 3 pages, 2 figures. Contribution to `Astrophysical Ages and
Timescales' ASP Conf. Ser., Eds. T. von Hippel, N. Manset, C. Simpson, EPS
files are available at http://csaweb.yonsei.ac.kr/~hclee/Hawai
The CN-CH positive correlation in the globular cluster NGC 5286
We performed low-resolution spectroscopy for the red giant stars in the
Galactic globular cluster (GC) NGC 5286, which is known to show intrinsic heavy
element abundance variations. We found that the observed stars in this GC are
clearly divided into three subpopulations by CN index (CN-weak,
CN-intermediate, and CN-strong). The CN-strong stars are also enhanced in the
calcium HK' (7.4{\sigma}) and CH (5.1{\sigma}) indices, while the
CN-intermediate stars show no significant difference in the strength of HK'
index with CN-weak stars. From the comparison with high-resolution
spectroscopic data, we found that the CN- and HK'-strong stars are also
enhanced in the abundances of Fe and s-process elements. It appears, therefore,
that these stars are later generation stars affected by some supernovae
enrichment in addition to the asymptotic giant branch ejecta. In addition,
unlike normal GCs, sample stars in NGC 5286 show the CN-CH positive
correlation, strengthening our previous suggestion that this positive
correlation is only discovered in GCs with heavy element abundance variations
such as M22 and NGC 6273.Comment: 8 pages, 6 figures, 1 table, accepted for publication in Ap
The Milky Way without X: An alternative interpretation of the double red clump in the Galactic bulge
The presence of two red clumps (RCs) in high latitude fields of the Milky Way
bulge is interpreted as evidence for an X-shaped structure originated from the
bar instability. Here we show, however, that this double RC phenomenon is more
likely to be another manifestation of multiple populations observed in globular
clusters (GCs) in the metal-rich regime. As in the bulge GC Terzan 5, the
helium enhanced second generation stars (G2) in the classical bulge component
of the Milky Way are placed on the bright RC, which is about 0.5 mag brighter
than the normal RC originated from the first generation stars (G1), producing
the observed double RC. In a composite bulge, where a classical bulge can
coexist with a boxy pseudo bulge, our models can also reproduce key
observations, such as the dependence of the double RC feature on metallicity
and Galactic latitude and longitude. If confirmed by Gaia trigonometric
parallax distances, this would indicate that the Milky Way bar is not
sufficiently buckled to form the X-shaped structure in the bulge, and suggest
that the early-type galaxies would be similarly prevailed by super-helium-rich
subpopulation.Comment: Accepted for publication in MNRA
Globular clusters with the extended horizontal-branch as remaining cores of galaxy building blocks
The relics of building blocks that made stellar halo and bulge are yet to be
discovered unless they were completely disrupted throughout the history of the
Galaxy. Here we suggest that about 25% of the Milky Way globular clusters have
characteristics of the remaining cores of these early building blocks rather
than genuine star clusters. They are clearly distinct from other normal
globular clusters in the presence of extended horizontal-branch and multiple
stellar populations, in mass (brightness), and most importantly in orbital
kinematics. Based on this result, a three-stage formation picture of the Milky
Way is suggested, which includes early mergers, collapse, and later accretion.Comment: Invited paper presented at the conference "New Quests in Stellar
Astrophysics II: Ultraviolet Properties of Evolved Stellar Populations", M.
Chavez, E. Bertone, D. Rosa-Gonzalez, & L. H. Rodriguez-Merino ed
Yonsei evolutionary population synthesis (YEPS). II. Spectro-photometric evolution of helium-enhanced stellar populations
The discovery of multiple stellar populations in Milky Way globular clusters
(GCs) has stimulated various follow-up studies on helium-enhanced stellar
populations. Here we present the evolutionary population synthesis models for
the spectro-photometric evolution of simple stellar populations (SSPs) with
varying initial helium abundance (). We show that
brings about {dramatic} changes in spectro-photometric properties of SSPs. Like
the normal-helium SSPs, the integrated spectro-photometric evolution of
helium-enhanced SSPs is also dependent on metallicity and age for a given
. {We discuss the implications and prospects for the
helium-enhanced populations in relation to the second-generation populations
found in the Milky Way GCs.} All of the models are available at
\url{http://web.yonsei.ac.kr/cosmic/data/YEPS.htm}.Comment: Accepted for publication in ApJ, 21 pages, 12 figures, 3 table
Nonlinear Color--Metallicity Relations of Globular Clusters. VI. On Calcium II Triplet Based Metallicities of Globular Clusters in Early-type Galaxies
The metallicity distribution function of globular clusters (GCs) in galaxies
is a key to understanding galactic formation and evolution. The calcium II
triplet (CaT) index has recently become a popular metal abundance indicator
thanks to its sensitivity to GC metallicity. Here we revisit and assess the
reliability of CaT as a metallicity indicator using our new stellar population
synthesis simulations based on empirical, high-resolution fluxes. The model
shows that the CaT strength of old ( 10 Gyr) GCs is proportional to below . In the modest metal-rich regime, however, CaT does not
increase anymore with due to the little contribution from
coolest red giant stars to the CaT absorption. The nonlinear nature of the
color-- relation is confirmed by the observations of GCs in nearby
early-type galaxies. This indicates that the CaT should be used carefully when
deriving metallicities of metal-rich stellar populations. Our results offer an
explanation for the observed sharp difference between the color and
distributions of GCs in the same galaxies. We take this as an analogy to the
view that metallicity--color and metallicity--Lick index nonlinearity of GCs is
primarily responsible for their observed "bimodal" distributions of colors and
absorption indices.Comment: 27 pages, 10 figures, 2 tables, accepted for publication in Ap
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