arXiv subjects

Wako Aoki

Publications and source records attributed to Wako Aoki.

At least 55 records · Page 3Linked to original sources

arXiv2019

Evidence for the accretion origin of halo stars with an extreme r-process enhancement

Small stellar systems like dwarf galaxies are suggested to be the main building blocks of our Galaxy by numerical simulations in Lambda CDM models. The existence of star streams like Sagittarius tidal stream indicates that dwarf galaxies play a role in the formation of the Milky Way. However, it is unclear how many and what kind of stars in our Galaxy are originated from satellite dwarf galaxies, which could be constrained by chemical abundances of metal-poor stars. Here we report on the discovery of a metal-poor star with an extreme r-process enhancement and alpha-element deficiency. In this star, the abundance ratio of the r-process element Eu with respect to Fe is more than one order of magnitude higher than the Sun and the metallicity is 1/20 of the solar one. Such kind of stars have been found in present-day dwarf galaxies, providing the clearest chemical signature of past accretion events. The long timescale of chemical evolution of the host dwarf galaxy expected from the abundance of alpha element with respect to Fe suggests that the accretion occurred in a relatively late phase compared to most of the accretions that formed the bulk of the Milky Way halo.

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arXiv2019

Evidence for an aspherical Population III supernova explosion inferred from the hyper metal-poor star HE1327-2326

We present observational evidence that an aspherical supernova explosion could have occurred in the First stars in the early universe. Our results are based on the First determination of a Zn abundance in a Hubble Space Telescope/Cosmic Origins Spectrograph high-resolution UV spectrum of a hyper metal-poor (HMP) star, HE1327-2326, with [Fe/H](NLTE) = -5.2. We determine [Zn/Fe] = 0.80$\pm$0.25 from a UV Zn I line at 2138 detected at $3.4σ$. Yields of a 25M$_{\odot}$ aspherical supernova model with artificially modified densities exploding with E = 5x10$^{51}$ ergs best match the entire abundance pattern of HE1327-2326. Such high-entropy hypernova explosions are expected to produce bipolar outfows which could facilitate the external enrichment of small neighboring galaxies. This has already been predicted by theoretical studies of the earliest star forming minihalos. Such a scenario would have significant implications for the chemical enrichment across the early universe as HMP Carbon Enhanced Metal-Poor (CEMP) stars such as HE1327-2326 might have formed in such externally enriched environments.

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arXiv2019

Origin of the excess of high-energy retrograde stars in the Galactic halo

We report on the very low $α$-element abundances of a group of metal-poor stars with high orbital energy and with large retrograde motion in the Milky Way halo, whose excess has been reported recently from metallicity and kinematics. We constructed a sample of halo stars with measured abundances and precise kinematics, including $\sim 880$ stars with [{{Fe}/{H}}]$<-0.7$, by crossmatching the Stellar Abundances for Galactic Archaeology database to the second data release of Gaia. Three regions in the energy-angular momentum space have been selected: innermost halo, Gaia Enceladus/Sausage, and high-energy retrograde halo. While the innermost halo and Gaia Enceladus regions have chemical abundances consistent with high- and low-$α$ populations in the halo, respectively, chemical abundances of stars in the high-energy retrograde halo are different from the two populations; their [{X}/{Fe}], where X represents Na, Mg, and Ca, are even lower than those in Gaia Enceladus. These abundances, as well as their low mean metallicity, provide a new support for the idea that the retrograde component is dominated by an accreted dwarf galaxy which has a longer star formation timescale and is less massive than Gaia Enceladus/Sausage.

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arXiv2019

Catching Element Formation In The Act

Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions.

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arXiv2018

Tracing the origin of moving groups I. The γ Leo moving group with high resolution spectra from the Subaru Telescope

We present chemical abundances of 15 stars in the γ Leo moving group based on high-resolution spectra with the Subaru High Dispersion Spectrograph. The sample was picked up by applying wavelet transform to UVW velocity compo- nents of stars in the solar neighbourhood. Both photometric and spectroscopic method have been used to determine the stellar parameters of stars. Abun- dances of 11 elements including Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y and Ba are measured. Our results show that the member stars display a wide metallicity distribution with abundance ratios similar to Milky way disk stars. We presume that the γ Leo moving group is originated from dynamical effects probably related to the Galactic spiral arms.

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arXiv2018

LAMOST J2217+2104: a new member of carbon-enhanced extremely metal-poor stars with excesses of Mg and Si

We report on the elemental abundances of the carbon-enhanced metal-poor (CEMP) star J2217+2104 discovered by our metal-poor star survey with LAMOST and Subaru. This object is a red giant having extremely low Fe abundance ([Fe/H]=-4.0) and very large enhancement of C, N, and O with excesses of Na, Mg, Al, and Si. This star is a new example of a small group of such CEMP stars identified by previous studies. We find a very similar abundance pattern for O-Zn in this class of objects that shows enhancement of elements up to Si and normal abundance of Ca and Fe-group elements. Whereas the C/N ratio is different among these stars, the (C+N)/O ratio is similar. This suggests that C was also yielded with similar abundance ratios relative to O-Zn in progenitors, and was later affected by the CN-cycle. By contrast, the heavy neutron-capture elements Sr and Ba are deficient in J2217+2104, compared to the four objects in this class previously studied. This indicates that the neutron-capture process in the early Galaxy, presumably the r-process, has no direct connection to the phenomenon that has formed such CEMP stars. Comparisons of the abundance pattern well determined for such CEMP stars with those of supernova nucleosynthesis models constrain the progenitor mass to be about 25Msun, which is not particularly different from typical mass of progenitors expected for extremely metal-poor stars in general.

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arXiv2018

Optical high-resolution spectroscopy of 14 young $α$-rich stars

We report chemical abundances of 14 young $α$-rich stars including neutron-capture elements based on high-quality optical spectra from HIRES/Keck I and differential line-by-line analysis. From the comparison of the abundance patterns of young $α$-rich stars to those of nearby bright red giants with a similar metallicity range ($-0.7<[\mathrm{Fe/H}]<-0.2$), we confirm their high $α$-element abundances reported by previous studies based on near-infrared spectroscopy. We reveal for the first time low abundances of $s$-process elements and high abundances of $r$-process elements. All the abundances are consistent with those seen in typical $α$-rich population of the Galactic disk, and no abundance anomalies are found except for Li-enhancement in one object previously reported and mild enhancement of Na in two stars. In particular, the lack of $s$-process enhancement excludes the hypothesis that mass transfer from asymptotic giant branch stars plays an important role in the formation of young $α$-rich stars. The high frequency of radial velocity variation (more than $50\%$) is also confirmed. We argue that mass transfer from low-mass red giants is the likely dominant formation mechanism for young $α$-rich stars.

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arXiv2018

Subaru Hyper Suprime-Cam Survey for An Optical Counterpart of GW170817

We perform a $z$-band survey for an optical counterpart of a binary neutron star coalescence GW170817 with Subaru/Hyper Suprime-Cam. Our untargeted transient search covers $23.6$ deg$^2$ corresponding to the $56.6\%$ credible region of GW170817 and reaches the $50\%$ completeness magnitude of $20.6$ mag on average. As a result, we find 60 candidates of extragalactic transients, including J-GEM17btc (a.k.a. SSS17a/DLT17ck). While J-GEM17btc is associated with NGC 4993 that is firmly located inside the 3D skymap of GW170817, the other 59 candidates do not have distance information in the GLADE v2 catalog or NASA/IPAC Extragalactic Database (NED). Among 59 candidates, 58 are located at the center of extended objects in the Pan-STARRS1 catalog, while one candidate has an offset. We present location, $z$-band apparent magnitude, and time variability of the candidates and evaluate the probabilities that they are located inside of the 3D skymap of GW170817. The probability for J-GEM17btc is $64\%$ being much higher than those for the other 59 candidates ($9.3\times10^{-3}-2.1\times10^{-1}\%$). Furthermore, the possibility, that at least one of the other 59 candidates is located within the 3D skymap, is only $3.2\%$. Therefore, we conclude that J-GEM17btc is the most-likely and distinguished candidate as the optical counterpart of GW170817.

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arXiv2017

Enormous Li-enhancement preceding red giant phases in low-mass stars in the Milky Way halo

Li abundances in the bulk of low-mass metal-poor stars are well reproduced by stellar evolution models adopting a constant initial abundance. However, a small number of stars have exceptionally high Li abundances, for which no convincing models have been established. We report on the discovery of 12 very metal-poor stars that have large excesses of Li, including an object having more than 100 times higher Li abundance than the values found in usual objects, which is the the largest excess in metal-poor stars known to date. The sample is distributed over a wide range of evolutionary stages, including five unevolved stars, showing no abundance anomaly in other elements. The results indicate the existence of an efficient process to enrich Li in a small fraction of low-mass stars at the main-sequence or subgiant phase. The wide distribution of Li-rich stars along the red giant branch could be explained by dilution of surface Li by mixing that occurs when the stars evolve into red giants. Our study narrows down the problem to be solved to understand the origins of Li-excess found in low-mass stars, suggesting the presence of unknown process that affects the surface abundances preceding red giant phases.

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arXiv2017

Enrichment in r-process elements from multiple distinct events in the early Draco dwarf spheroidal galaxy

The stellar record of elemental abundances in satellite galaxies is important to identify the origin of r-process because such a small stellar system could have hosted a single r-process event, which would distinguish member stars that are formed before and after the event through the evidence of a considerable difference in the abundances of r-process elements, as found in the ultra-faint dwarf galaxy Reticulum II (Ret II). However, the limited mass of these systems prevents us from collecting information from a sufficient number of stars in individual satellites. Hence, it remains unclear whether the discovery of a remarkable r-process enrichment event in Ret II explains the nature of r-process abundances or is an exception. We perform high-resolution spectroscopic measurements of r-process abundances for twelve metal-poor stars in the Draco dwarf galaxy in the metallicity range of -2.5<[Fe/H]<-2. We found that these stars are separated into two groups with r-process abundances differing by one order of magnitude. A group of stars with high abundances of r-process elements was formed by a single r-process event that corresponds to the event evidenced in Ret II. On the other hand, the low r-process abundance group was formed by another sporadic enrichment channel producing a far fewer r-process elements, which is clearly identified for the first time. Accordingly, we identified two populations of stars with different r-process abundances, which are built by two r-process events that enriched gases at levels that differ by more than one order of magnitude.

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arXiv2017

J-GEM observations of an electromagnetic counterpart to the neutron star merger GW170817

The first detected gravitational wave from a neutron star merger was GW170817. In this study, we present J-GEM follow-up observations of SSS17a, an electromagnetic counterpart of GW170817. SSS17a shows a 2.5-mag decline in the $z$-band from 1.7 days to 7.7 days after the merger. Such a rapid decline is not comparable with supernovae light curves at any epoch. The color of SSS17a also evolves rapidly and becomes redder for later epochs; the $z-H$ color changed by approximately 2.5 mag in the period of 0.7 days to 7.7 days. The rapid evolution of both the optical brightness and the color are consistent with the expected properties of a kilonova that is powered by the radioactive decay of newly synthesized $r$-process nuclei. Kilonova models with Lanthanide elements can reproduce the aforementioned observed properties well, which suggests that $r$-process nucleosynthesis beyond the second peak takes place in SSS17a. However, the absolute magnitude of SSS17a is brighter than the expected brightness of the kilonova models with the ejecta mass of 0.01 $\Msun$, which suggests a more intense mass ejection ($\sim 0.03 \Msun$) or possibly an additional energy source.

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arXiv2017

High-resolution spectroscopy of extremely metal-poor stars from SDSS/SEGUE. III. Unevolved Stars with $[\mathrm{Fe/H}]\lesssim -3.5$

We present elemental abundances for eight unevolved extremely metal-poor stars with $T_{\rm eff}>5500\,\mathrm{K}$, among which seven have $[\mathrm{Fe/H}]<-3.5$. The sample is selected from the Sloan Digital Sky Survey / Sloan Extension for Galactic Understanding and Exploration (SDSS/SEGUE), and our previous high-resolution spectroscopic follow-up with the Subaru Telescope (Aoki et al.). Several methods to derive stellar parameters are compared, and no significant offset in the derived parameters is found in most cases. From an abundance analysis relative to the standard extremely metal-poor star G 64-12, an average Li abundance for stars with $[\mathrm{Fe/H}]<-3.5$ is $A(\mathrm{Li}) =1.90$, with a standard deviation of $σ=0.10$ dex. This result confirms that lower Li abundances are found at lower metallicity, as suggested by previous studies, and demonstrates that the star-to-star scatter is small. The small observed scatter could be a strong constraint on Li-depletion mechanisms proposed for explaining the low Li abundance at lower metallicity. Our analysis for other elements obtained the following results: i) A statistically significant scatter in $[\mathrm{X/Fe}]$ for Na, Mg, Cr, Ti, Sr, and Ba, and an apparent bimodality in $[\mathrm{Na/Fe}]$ with a separation of $\sim 0.8\, \mathrm{dex}$, ii) an absence of a sharp drop in the metallicity distribution, and iii) the existence of a CEMP-$s$ star at $[\mathrm{Fe/H}]\simeq -3.6$ and possibly at $[\mathrm{Fe/H}]\simeq-4.0$, which may provide a constraint on the mixing efficiency of unevolved stars during their main-sequence phase.

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arXiv2017

Stellar Abundances for Galactic Archaeology Database IV - Compilation of Stars in Dwarf Galaxies

We have constructed the database of stars in the local group using the extended version of the SAGA (Stellar Abundances for Galactic Archaeology) database that contains stars in 24 dwarf spheroidal galaxies and ultra faint dwarfs. The new version of the database includes more than 4500 stars in the Milky Way, by removing the previous metallicity criterion of [Fe/H] <= -2.5, and more than 6000 stars in the local group galaxies. We examined a validity of using a combined data set for elemental abundances. We also checked a consistency between the derived distances to individual stars and those to galaxies in the literature values. Using the updated database, the characteristics of stars in dwarf galaxies are discussed. Our statistical analyses of alpha-element abundances show that the change of the slope of the [alpha/Fe] relative to [Fe/H] (so-called "knee") occurs at [Fe/H] = -1.0+-0.1 for the Milky Way. The knee positions for selected galaxies are derived by applying the same method. Star formation history of individual galaxies are explored using the slope of the cumulative metallicity distribution function. Radial gradients along the four directions are inspected in six galaxies where we find no direction dependence of metallicity gradients along the major and minor axes. The compilation of all the available data shows a lack of CEMP-s population in dwarf galaxies, while there may be some CEMP-no stars at [Fe/H] <~ -3 even in the very small sample. The inspection of the relationship between Eu and Ba abundances confirms an anomalously Ba-rich population in Fornax, which indicates a pre-enrichment of interstellar gas with r-process elements. We do not find any evidence of anti-correlations in O-Na and Mg-Al abundances, which characterises the abundance trends in the Galactic globular clusters.

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arXiv2017

Diversity of Abundance Patterns of Light Neutron-capture Elements in Very-metal-poor Stars

We determine the abundances of neutron-capture elements from Sr to Eu for five very-metal-poor stars (-3<[Fe/H]<-2) in the Milky Way halo to reveal the origin of light neutron-capture elements. Previous spectroscopic studies have shown evidence of at least two components in the r-process; one referred to as the "main r-process" and the other as the "weak r-process," which is mainly responsible for producing heavy and light neutron-capture elements, respectively. Observational studies of metal-poor stars suggest that there is a universal pattern in the main r-process, similar to the abundance pattern of the r-process component of solar-system material. Still, it is uncertain whether the abundance pattern of the weak r-process shows universality or diversity, due to the sparseness of measured light neutron-capture elements. We have detected the key elements, Mo, Ru, and Pd, in five target stars to give an answer to this question. The abundance patterns of light neutron-capture elements from Sr to Pd suggest a diversity in the weak r-process. In particular, scatter in the abundance ratio between Ru and Pd is significant when the abundance patterns are normalized at Zr. Our results are compared with the elemental abundances predicted by nucleosynthesis models of supernovae with parameters such as electron fraction or proto-neutron-star mass, to investigate sources of such diversity in the abundance patterns of light neutron-capture elements. This paper presents that the variation in the abundances of observed stars can be explained with a small range of parameters, which can serve as constraints on future modeling of supernova models.

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arXiv2016

Lithium in CEMP-no stars: A new constraint on the lithium depletion mechanism in the early universe

Most of relatively warm, unevolved metal-poor stars ($T_{\rm eff}\gtrsim 5800\,\mathrm{K}$ and $[{\rm Fe/H}]\lesssim -1.5$) exhibit almost constant lithium abundances, irrespective of metallicity or effective temperature, and thus form the so-called Spite plateau. This was originally interpreted as arising from lithium created by the Big Bang nucleosynthesis. Recent observations, however, have revealed that ultra metal-poor stars (UMP stars; $[\mathrm{Fe/H}]<-4.0$) have significantly lower lithium abundances than that of the plateau. Since most of the UMP stars are CEMP-no stars, carbon-enhanced metal-poor stars with no excess of neutron-capture elements, a connection between the carbon enhancement and lithium depletion is suspected. A straightforward approach to this question is to investigate carbon-normal UMP stars. However only one object is known in this class. As an alternative, we have determined lithium abundances for two CEMP-no main-sequence turn-off stars with metallicities $[{\rm Fe/H}]\sim -3.0$, where there are numerous carbon-normal stars with available lithium abundances that can be considered. Our 1D LTE analysis indicates that the two CEMP-no stars have lithium abundances that are consistent with values near the plateau, which suggests that carbon enhancement and lithium depletion are not directly related. Instead, our results suggest that extremely low iron abundance is a fundamental cause to depleted lithium in UMP stars.

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arXiv2016

High-resolution spectroscopy of the extremely iron-poor post-AGB star CC Lyr

High-resolution optical spectroscopy was conducted for the metal-poor post-AGB star CC Lyr to determine its chemical abundances and spectral line profiles. Our standard abundance analysis confirms its extremely low metallicity ([Fe/H]<-3.5) and a clear correlation between abundance ratios and the condensation temperature for 11 elements, indicating that dust depletion is the cause of the abundance anomaly of this object. The very low abundances of Sr and Ba, which are detected for the first time for this object, suggest that heavy neutron-capture elements are not significantly enhanced in this object by the s-process during its evolution through AGB phase. Radial velocity of this object and profiles of some atomic absorption lines show variations depending on pulsation phases, which could be formed by dynamics of the atmosphere rather than by binarity or contributions of circumstellar absorption. On the other hand, the H-alpha emission with double peaks shows no evident velocity shift, suggesting that the emission is originating from the circumstellar matter, presumably the rotating disk around the object.

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arXiv2016

Line-of-sight velocity and metallicity measurements of the Palomar 5 tidal stream

We present Subaru/FOCAS and Keck/DEIMOS medium-resolution spectroscopy of a tidally disrupting Milky Way (MW) globular cluster Palomar 5 and its tidal stream. The observed fields are located to cover an angular extent of $\sim 17$\arcdeg along the stream, providing an opportunity to investigate a trend in line-of-sight velocities ($V_{\rm los}$) along the stream, which is essential to constrain its orbit and underlying gravitational potential of the Milky Way's dark matter halo. A spectral fitting technique is applied to the observed spectra to obtain stellar parameters and metallicities ([Fe/H]) of the target stars. The 19 stars most likely belonging to the central Pal 5 cluster have a mean $V_{\rm los}$ of $-58.1\pm 0.7$ km s$^{-1}$ and metallicity [Fe/H]$=-1.35\pm 0.06$ dex, both of which are in good agreement with those derived in previous high-resolution spectroscopic studies. Assuming that the stream stars have the same [Fe/H] as the progenitor cluster, the derived [Fe/H] and $V_{\rm los}$ values are used to estimate the possible $V_{\rm los}$ range of the member stars at each location along the stream. Because of the heavy contamination of the field MW stars, estimated $V_{\rm los}$ range depends on prior assumptions about the stream's $V_{\rm los}$, which highlights the importance of more definitely identifying the member stars using proper motion and chemical abundances to obtain an unbiased information of $V_{\rm los}$ in the outer part of the Pal 5 stream. The models for the gravitational potential of the MW's dark matter halo that are compatible with the estimated $V_{\rm los}$ range are discussed.

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arXiv2016

Spatially-Resolved Spectroscopy of a Balmer-Dominated Shock in the Cygnus Loop: An Extremely Thin Cosmic-Ray Precursor?

We present high-resolution long-slit spectroscopy of a Balmer-dominated shock in the northeastern limb of the Cygnus Loop with the Subaru high dispersion spectrograph. By setting the slit angle along the shock normal, we investigate variations of the flux and profile of the H-alpha line from preshock to postshock regions with a spatial resolution of about 4 times 10^{15} cm. The H-alpha line profile can be represented by a narrow (28.9+/-0.7 km/s) Gaussian in a diffuse region ahead of the shock, i.e., a photoionization precursor, and narrow (33.1+/-0.2 km/s) plus broad (130-230 km/s) Gaussians at the shock itself. We find that the width of the narrow component abruptly increases up to 33.1+/-0.2 km/s, or 38.8+/-0.4 km/s if we eliminate projected emission originating from the photoionization precursor, in an unresolved thin layer (< 4 times 10^{15} cm at a distance of 540 pc) at the shock. We show that the sudden broadening can be best explained by heating via damping of Alfven waves in a thin cosmic-ray precursor, although other possibilities are not fully ruled out. The thickness of the cosmic-ray precursor in the Cygnus Loop (a soft gamma-ray emitter) is an order of magnitude thinner than that in Tycho's Knot g (a hard gamma-ray emitter), which may be caused by different energy distribution of accelerated particles between the two sources. In this context, systematic studies might reveal a positive correlation between the thickness of the cosmic-ray precursor and the hardness of the cosmic-ray energy distribution.

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