117 research outputs found
History of Milky Way Dwarf Spheroidal Galaxies Imprinted on Abundance Patterns of Neutron-Capture Elements
Stellar abundance pattern of neutron-capture elements such as barium is used
as a powerful tool to infer how star formation proceeded in dwarf spheroidal
(dSph) galaxies. It is found that the abundance correlation of barium with iron
in stars belonging to dSph galaxies orbiting the Milky Way, i.e., Draco,
Sextans, and Ursa Minor have a feature similar to the barium-iron correlation
in Galactic metal-poor stars. The common feature of these two correlations can
be realized by our inhomogeneous chemical evolution model based on the
supernova-driven star formation scenario if dSph stars formed from gas with a
velocity dispersion of ~26 km/s. This velocity dispersion together with the
stellar luminosities strongly suggest that dark matter dominated dSph galaxies.
The tidal force of the Milky Way links this velocity dispersion with the
currently observed value <10 km/s by stripping the dark matter in dSph
galaxies. As a result, the total mass of each dSph galaxy is found to have been
originally ~25 times larger than at present. Our inhomogeneous chemical
evolution model succeeds in reproducing the stellar [Fe/H] distribution
function observed in Sextans. In this model, supernovae immediately after the
end of the star formation epoch can expel the remaining gas over the
gravitational potential of the dSph galaxy.Comment: 5 pages including 2 figures, to appear in ApJ Letters Vol.57
Chemical evolution of galaxies with radiation-driven dust wind
We discuss how the removal of interstellar dust by radiation pressure of
stars influences the chemical evolution of galaxies by using a new one-zone
chemical evolution models with dust wind. The removal efficiency of an element
(e.g., Fe, Mg, and Ca) through radiation-driven dust wind in a galaxy is
assumed to depend both on the dust depletion level of the element in
interstellar medium and the total luminosity of the galaxy in the new model. We
particularly focus on the time evolution of [alpha/Fe] and its dependence on
model parameters for dust wind in this study. The principal results are as
follows. The time evolution of [Ca/Fe] is significantly different between
models with and without dust wind in the sense that [Ca/Fe] can be
systematically lower in the models with dust wind. The time evolution of
[Mg/Fe], on the other hand, can not be so different between the models with and
without dust wind owing to the lower level of dust depletion for Mg. As a
result of this, [Mg/Ca] can be systematically higher in the models with dust
wind. We compare these results with the observed elemental features of stars in
the Large Magellanic Cloud (LMC), because a growing number of observational
studies on [alpha/Fe] for the LMC have been recently accumulated for a detailed
comparison. Based on the present new results, we also discuss the origins of
[alpha/Fe] in the Fornax dwarf galaxy and elliptical galaxies in the context of
radiation-driven dust wind.Comment: 18 pages, 10 figures, accepted for the publication of MNRA
Galactic r-process abundance feature shaped by radial migration
Growing interests in the chemical feature of r-process elements among nearby
disk stars represented by the [Eu/Fe] vs. [Fe/H] diagram have sprouted since it
can assess the origin of r-process elements through the comparison with
theoretical models, including a test as to if neutron star mergers can be the
major site of r-process nucleosynthesis. On the other hand, recent studies
reveal that local chemistry is strongly coupled with the dynamics of Galactic
disk, which predicts that stars radially move on the disk where the observed
elemental feature is different at various Galactocentric distances. Here, we
show that radial migration of stars across the Galactic disk plays a crucial
role in shaping the r-process abundance feature in the solar vicinity. In this
proposed scenario, we highlight the importance of migration from the outer disk
where [r-process/Fe] of some old stars is predicted to be enhanced to the level
beyond the expectation from the observed Galactic Fe and Eu radial gradient,
which results in a large span of [r-process/Fe] among nearby disk stars. The
variation in the [r-process/Fe] ratio seen across the Galactic disk as well as
in dwarf galaxies may be an outcome of different stellar initial mass functions
which change the occurrence frequency between supernovae leaving behind neutron
stars and ones ending with black holes. Here we propose that enhancement in
[Eu/Fe] is attributed to the initial mass function lacking high-mass stars such
as > 25 solar masses in the scheme for which neutron star mergers are a major
source of r-process elements.Comment: 11 pages including 6 figures, accepted for publication in Ap
The r-process in Magnetorotational Supernovae
One of the hottest open issues involving the evolution of r-process elements
is fast enrichment in the early Universe. Clear evidence for the chemical
enrichment of r-process elements is seen in the stellar abundances of extremely
metal poor stars in the Galactic halo. However, small-mass galaxies are the
ideal testbed for studying the evolutionary features of r-process enrichment
given the potential rarity of production events yielding heavy r-process
elements. Their occurrences become countable and thus an enrichment path due to
each event can be found in the stellar abundances. We examine the chemical
feature of Eu abundance at an early stage of in the
Draco and Sculptor dwarf spheroidal (dSph) galaxies. Accordingly, we constrain
the properties of the Eu production in the early dSphs. We find that the Draco
dSph experienced a few Eu production events, whereas Eu enrichment took place
more continuously in the Sculptor dSph due to its larger stellar mass. The
event rate of Eu production is estimated to be about one per -
core-collapse supernovae, and a Eu mass of \ms per
single event is deduced by associating this frequency with the observed plateau
value of for . The observed
plateau implies that early Eu enrichment ceases at .
Such a selective operation only in low-metallicity stars supports
magnetorotational supernovae, which require very fast rotation, as the site of
early Eu production. We show that the Eu yields deduced from chemical evolution
agree well with the nucleosynthesis results from corresponding supernovae
models.Comment: 5 pages, 3 figures, published in ApJL 811:L10 (2015); Title is
change
Chemical Signature of a Major Merger in the Early Formation of Small Magellanic Cloud
The formation history of the Small Magellanic cloud (SMC) is unraveled based
on the results of our new chemical evolution models constructed for the SMC,
highlighting the observed anomaly in the age-metallicity relation for star
clusters in the SMC. We first propose that evidence of a major merger is
imprinted in the age-metallicity relation as a dip in [Fe/H]. Our models
predict that the major merger with a mass ratio of 1:1 to 1:4 occurred at ~7.5
Gyr ago, with a good reproduction of the abundance distribution function of
field stars in the SMC. Furthermore, our models predict a relatively large
scatter in [Mg/Fe] for -1.4 < [Fe/H] <-1.1 as a reflection of a looping feature
resulting from the temporally inverse progress of chemical enrichment, which
can be tested against future observational results. Given that the observed
velocity dispersion (~30 km/s) of the SMC is much smaller than that (~160 km/s)
of the Galactic halo, our finding strongly implies that the predicted merger
event happened in a small group environment that was far from the Galaxy and
contained a number of small gas-rich dwarfs comparable to the SMC. This
theoretical view is extensively discussed in the framework that considers a
connection with the formation history of the Large Magellanic cloud.Comment: 5 pages including 4 figures, to appear in ApJ Letter
Diversity of Type Ia Supernovae Imprinted in Chemical Abundances
A time delay of Type Ia supernova (SN Ia) explosions hinders the imprint of
their nucleosynthesis on stellar abundances. However, some occasional cases
give birth to stars that avoid enrichment of their chemical compositions by
massive stars and thereby exhibit a SN Ia-like elemental feature including a
very low [Mg/Fe] (~-1). We highlight the elemental feature of Fe-group elements
for two low-Mg/Fe objects detected in nearby galaxies, and propose the presence
of a class of SNe Ia that yield the low abundance ratios of [Cr,Mn,Ni/Fe]. Our
novel models of chemical evolution reveal that our proposed class of SNe Ia
(slow SNe Ia) is associated with ones exploding on a long timescale after their
stellar birth, and gives a significant impact on the chemical enrichment in the
Large Magellanic Cloud (LMC). In the Galaxy, on the other hand, this effect is
unseen due to the overwhelming enrichment by the major class of SNe Ia that
explode promptly (prompt SNe Ia) and eject a large amount of Fe-group elements.
This nicely explains the different [Cr,Mn,Ni/Fe] features between the two
galaxies as well as the puzzling feature seen in the LMC stars exhibiting very
low Ca but normal Mg abundances. Furthermore, the corresponding channel of slow
SN Ia is exemplified by performing detailed nucleosynthesis calculations in the
scheme of SNe Ia resulting from a 0.8+0.6 solar mass white dwarf merger.Comment: 5 pages including 3 figures, accepted for publication in ApJ Letter
Chemical evolution of the Large Magellanic Cloud
We adopt a new chemical evolution model for the Large Magellanic Cloud (LMC)
and thereby investigate its past star formation and chemical enrichment
histories. The delay time distribution of type Ia supernovae recently revealed
by type Ia supernova surveys is incorporated self-consistently into the new
model. The principle results are summarized as follows. The present gas mass
fraction and stellar metallicity as well as the higher [Ba/Fe] in metal-poor
stars at [Fe/H]<-1.5 can be more self-consistently explained by models with
steeper initial mass functions. The observed higher [Mg/Fe] (> 0.3) at [Fe/H] ~
-0.6 and higher [Ba/Fe] (>0.5) at [Fe/H] ~ -0.3 can be due to significantly
enhanced star formation about 2 Gyr ago. The observed overall [Ca/Fe]-[Fe/H]
relation and remarkably low [Ca/Fe] (-0.6 are consistent with
models with short-delay supernova Ia and with the more efficient loss of Ca
possibly caused by an explosion mechanism of type II supernovae. Although the
metallicity distribution functions do not show double peaks in the models with
a starburst about 2 Gyr ago, they show characteristic double peaks in the
models with double starbursts at ~200 Myr and ~2 Gyr ago. The observed apparent
dip of [Fe/H] around ~1.5 Gyr ago in the age--metallicity relation can be
reproduced by models in which a large amount (~10^9 M_{sun}) of metal-poor
([Fe/H]<-1) gas can be accreted onto the LMC.Comment: 39 pages and 22 figures, accepted in Ap
Stripping of nitrogen-rich AGB ejecta from interacting dwarf irregular galaxies
Dwarf irregular galaxies (dIrrs) including the Magellanic Clouds in the local
Universe, in many cases, exhibit an unusually low N/O abundance ratio (log N/O
~ -1.5) in H II regions as compared with the solar value (~-0.9). This ratio is
broadly equivalent to the average level of extremely metal-poor stars in the
Galactic halo, suggesting that N released from asymptotic giant branch (AGB)
stars is missing in the present-day interstellar matter of these dIrrs. We find
evidence for past tidal interactions in the properties of individual dIrrs
exhibiting low N/O ratios, while a clear signature of interactions is unseen
for dIrrs with high N/O ratios. Accordingly, we propose that the ejecta of
massive AGB stars that correspond to a major production site of N can be
stripped from dIrrs that have undergone a strong interaction with a luminous
galaxy. The physical process of its stripping is made up of two stages: (i) the
ejecta of massive AGB stars in a dIrr are first merged with those of the
bursting prompt SNe Ia and pushed up together to the galaxy halo, and (ii)
subsequently through tidal interactions with a luminous galaxy, these ejecta
are stripped from a dwarf galaxy's potential well. Our new chemical evolution
models with stripping of AGB ejecta succeed in reproducing the observed low N/O
ratio. Furthermore, we perform N-body + hydrodynamical simulations to trace the
fate of AGB ejecta inside a dIrr orbiting the Milky Way, and confirm that a
tidal interaction is responsible for the efficient stripping of AGB ejecta from
dIrrs.Comment: 11 pages including 6 figures, accepted for publication in MNRA
Formation of globular clusters with internal abundance spreads in r-process elements: strong evidence for prolonged star formation
Several globular clusters (GCs) in the Galaxy are observed to show internal
abundance spreads in r-process elements (e.g., Eu). We here propose a new
scenario which explains the origin of these GCs (e.g., M5 and M15). In this
scenario, stars with no/little abundance variations first form from a massive
molecular cloud (MC). After all of the remaining gas of the MC is expelled by
numerous supernovae, gas ejected from asymptotic giant branch stars can be
accumulated in the central region of the GC to form a high-density
intra-cluster medium (ICM). Merging of neutron stars then occurs to eject
r-process elements, which can be efficiently trapped in and subsequently mixed
with the ICM. New stars formed from the ICM can have r-process abundances quite
different from those of earlier generations of stars within the GC. This
scenario can explain both (i) why r-process elements can be trapped within GCs
and (ii) why GCs with internal abundance spreads in r-process elements do not
show [Fe/H] spreads. Our model shows that (i) a large fraction of Eu-rich stars
can be seen in Na-enhanced stellar populations of GCs, as observed in M15, and
(ii) why most of the Galactic GCs do not exhibit such internal abundance
spreads. Our model demonstrates that the observed internal spreads of
-process elements in GCs provide strong evidence for prolonged star
formation (~10^8 yr).Comment: 19pages, 11 figures, accepted for publication in Ap
Enrichment history of r-process elements shaped by a merger of neutron star pairs
The origin of r-process elements remains unidentified and still puzzles us.
The recent discovery of evidence for the ejection of r-process elements from a
short-duration gamma-ray burst singled out neutron star mergers (NSMs) as their
origin. In contrast, core-collapse supernovae are ruled out as the main origin
of heavy r-process elements (A>110) by recent numerical simulations. However,
the properties characterizing NSM events - their rarity and high yield of
r-process elements per event - have been claimed to be incompatible with the
observed stellar records on r-process elements in the Galaxy. We add to this
picture with our results, which show that the observed constant [r-process/H]
ratio in faint dwarf galaxies and one star unusually rich in r-process in the
Sculptor galaxy agree well with this rarity of NSM events. Furthermore, we
found that a large scatter in the abundance ratios of r-process elements to
iron in the Galactic halo can be reproduced by a scheme that incorporates an
assembly of various protogalactic fragments, in each of which r-process
elements supplied by NSMs pervade the whole fragment while supernovae
distribute heavy elements only inside the regions swept up by the blast waves.
Our results demonstrate that NSMs occurring at Galactic rate of 12-23 per Myr
are the main site of r-process elements, and we predict the detection of
gravitational waves from NSMs at a high rate with upcoming advanced detectors.Comment: 4 pages including 2 figures, accepted for publication in A&A Letter
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