arXiv subjects

S. Wanajo

Publications and source records attributed to S. Wanajo.

14 recordsLinked to original sources

arXiv2020

Detailed abundances in a sample of very metal poor stars

Unevolved metal poor stars are the witness of the early evolution of the Galaxy. The determination of their detailed chemical composition is an important tool to understand the chemical history of our Galaxy. The study of their chemical composition can also be used to constrain the nucleosynthesis of the first generation of supernovae that enriched the interstellar medium. The aim is to observe a sample of extremely metal poor stars (EMP stars) candidates selected from SDSS DR12 release and determine their chemical composition. We obtained high resolution spectra of a sample of five stars using HDS on Subaru telescope and used standard 1D models to compute the abundances. The stars we analysed have a metallicity [Fe/H] between -3.50 dex and -4.25 dex . We confirm that the five metal poor candidates selected from low resolution spectra are very metal poor. We present, the discovery of a new ultra metal-poor star (UMP star) with a metallicity of [Fe/H]= -4.25 dex (SDSS~J1050032.34$-$241009.7). We measured in this star an upper limit of lithium ( log(Li/H) <= 2.0. We found that the 4 most metal poor stars of our sample have a lower lithium abundance than the Spite plateau lithium value. We obtain upper limits for carbon in the sample of stars. None of them belong to the high carbon band. We measured abundances of Mg and Ca in most of the stars and found three new alpha-poor stars.

astro-ph.SR↗

arXiv2019

A New Model for Electron-Capture Supernovae in Galactic Chemical Evolution

We examine the contribution of electron-capture supernovae (ECSNe), low-mass SNe from collapsing Fe cores (FeCCSNe), and rotating massive stars to the chemical composition of the Galaxy. Our model includes contributions to chemical evolution from both thermonuclear ECSNe (tECSNe) and gravitational collapse ECSNe (cECSNe). We show that if ECSNe are predominantly gravitational collapse SNe but about 15% are partial thermonuclear explosions, the model is able to reproduce the solar abundances of several important and problematic isotopes including $^{48}$Ca, $^{50}$Ti and $^{54}$Cr together with $^{58}$Fe, $^{64}$Ni, $^{82}$Se and $^{86}$Kr and several of the Zn--Zr isotopes. A model in which no cECSNe occur, only tECSNe with low-mass FeCCSNe or rotating massive stars, proves also very successful at reproducing the solar abundances for these isotopes. Despite the small mass range for the progenitors of ECSNe and low-mass FeCCSNe, the large production factors suffice for the solar inventory of the above isotopes. Our model is compelling because it introduces no new tensions with the solar abundance distribution for a Milky Way model -- only tending to improve the model predictions for several isotopes. The proposed astrophysical production model thus provides a natural and elegant way to explain one of the last uncharted territories on the periodic table of astrophysical element production.

astro-ph.SR↗

arXiv2018

Chemical analysis of very metal-poor turn-off stars from SDSS-DR12

The most metal-poor stars are the relics of the early chemical evolution of the Galaxy. Their chemical composition is an important tool to constrain the nucleosynthesis in the first generation of stars. The aim is to observe a sample of extremely metal-poor star (EMP stars) candidates selected from the Sloan Digital Sky Survey Data Release 12 (SDSS DR12) and determine their chemical composition. We obtain medium resolution spectra of a sample of six stars using the X-Shooter spectrograph at the Very Large Telescope (VLT) and we used ATLAS models to compute the abundances. Five stars of the sample have a metallicity [Fe/H] between -2.5 dex and -3.2 dex. We confirm the recent discovery of SDSS J002314.00+030758.0. As a star with an extremely low [Fe/H] ratio. Assuming the alpha-enhancement [Ca/Fe]=+0.4 dex, we obtain [Fe/H]=-6.1 dex. We could also determine its magnesium abundance and found that this star exhibits a very high ratio [Mg/Fe] >= +3.60~dex assuming [Fe/H] = -6.13 dex. We determined the carbon abundance and found A(C)=6.4 dex. From this carbon abundance, this stars belongs to the lower band of the A(C)-[Fe/H] diagram.

astro-ph.GA↗

arXiv2018

Mass Measurements of Neutron-Deficient Y, Zr, and Nb Isotopes and Their Impact on $rp$ and $νp$ Nucleosynthesis Processes

Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of $^{82}$Zr and $^{84}$Nb were measured for the first time with an uncertainty of $\sim 10$ keV, and the masses of $^{79}$Y, $^{81}$Zr, and $^{83}$Nb were re-determined with a higher precision. %The latter differ significantly from their literature values. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low $α$ separation energies for neutron-deficient Mo and Tc isotopes, making the formation of Zr-Nb cycle in the $rp$-process unlikely. The new proton separation energy of $^{83}$Nb was determined to be 490(400)~keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the $p$-nucleus $^{84}$Sr relative to the neutron-deficient molybdenum isotopes in the previous $νp$-process simulations.

nucl-ex↗

arXiv2017

Simulations of Electron Capture and Low-Mass Iron Core Supernovae

The evolutionary pathways of core-collapse supernova progenitors at the low-mass end of the spectrum are beset with major uncertainties. In recent years, a variety of evolutionary channels has been discovered in addition to the classical electron capture supernova channel of super-AGB stars. The few available progenitor models at the low-mass end have been studied with great success in supernova simulations as the peculiar density structure makes for robust neutrino-driven explosions in this mass range. Detailed nucleosynthesis calculations have been conducted both for models of electron capture supernovae and low-mass iron core supernovae and revealed an interesting production of the lighter trans-iron elements (such as Zn, Sr, Y, Zr) as well as rare isotopes like Ca-48 and Fe-60. We stress the need to explore the low-mass end of the supernova spectrum further and link various observables to understand the diversity of explosions in this regime.

astro-ph.SR↗

arXiv2017

Production and Distribution of $^{44}$Ti and $^{56}$Ni in a Three-dimensional Supernova Model Resembling Cassiopeia A

The spatial and velocity distributions of nuclear species synthesized in the innermost regions of core-collapse supernovae can yield important clues about explosion asymmetries and the operation of the still disputed explosion mechanism. Recent observations of radioactive $^{44}$Ti with high-energy satellite telescopes (Nuclear Spectroscopic Telescope Array [NuSTAR], INTEGRAL) have measured gamma-ray line details, which provide direct evidence of large-scale explosion asymmetries in SN 1987A and in Cassiopeia A (Cas A) even by mapping of the spatial brightness distribution (NuSTAR). Here we discuss a 3D simulation of a neutrino-driven explosion, using a parameterized neutrino engine, whose $^{44}$Ti distribution is mostly concentrated in one hemisphere pointing opposite to the neutron star (NS) kick velocity. Both exhibit intriguing resemblance to the observed morphology of the Cas A remnant, although neither the progenitor nor the explosion was fine-tuned for a perfect match. Our results demonstrate that the asymmetries observed in this remnant can, in principle, be accounted for by a neutrino-driven explosion, and that the high $^{44}$Ti abundance in Cas A may be explained without invoking rapid rotation or a jet-driven explosion, because neutrino-driven explosions generically eject large amounts of high-entropy matter. The recoil acceleration of the NS is connected to mass ejection asymmetries and is opposite to the direction of the stronger explosion, fully compatible with the gravitational tugboat mechanism. Our results also imply that Cas A and SN 1987A could possess similarly "one-sided" Ti and Fe asymmetries, with the difference that Cas A is viewed from a direction with large inclination angle to the NS motion, whereas the NS in SN 1987A should have a dominant velocity component pointing toward us.

astro-ph.HE↗

arXiv2014

High-resolution abundance analysis of very metal-poor r-I stars

Moderately r-process-enriched stars (r-I) are at least four times as common as those that are greatly enriched in r-process elements (r-II), and the abundances in their atmospheres are important tools for obtaining a better understanding of the nucleosynthesis processes responsible for the origin of the elements beyond the iron peak. The main aim of this work is to derive abundances for a sample of seven metal-poor stars with classified as r-I stars, to understand the role of these stars for constraining the astrophysical nucleosynthesis event(s) that is(are) responsible for the production of the r-process, and to investigate whether they differ, in any significant way, from the r-II stars. We carried out a detailed abundance analysis based on high-resolution spectra obtained with the VLT/UVES spectrograph. The OSMARCS LTE 1D model atmosphere grid was employed, along with the spectrum synthesis code Turbospectrum. We have derived abundances of light elements Li, C, and N, alpha-elements, odd-Z elements, iron-peak elements, and the trans-iron elements from the first peak, the second peak, the third peak, and the actinides regions. The results are compared with values for these elements for r-II and normal very and extremely metal-poor stars reported in the literature, ages based on radioactive chronometry are explored using different models, and a number of conclusions about the r-process and the r-I stars are presented. Hydrodynamical models were used for some elements, and general behaviors for the 3D corrections were presented.

astro-ph.SR↗

arXiv2012

First stars XVI. STIS/HST abundances of heavy-elements in the uranium-rich star CS 31082-001

Detailed abundances of the elements produced by r-process nucleosynthesis in various circumstances are our best observational clues to their origin, since the site(s) of r-element production is(are) still not known with certainty. A small fraction of extremely metal-poor (EMP) stars exhibit excesses of heavy neutron-capture elements produced in the r-process, and CS 31082-001 is among the 4 well-known r-process-enhanced EMP stars. Observations with HST/STIS provide abundances for elements observable only from the UV region. Here we aim to supplement the optical data with abundances from near-UV spectroscopy of the first and second peak of the r-elements, which are crucial to giving insight into the nucleosynthesis of the elements beyond iron. The UVES spectrum provided additional measurements, thereby improving the previous results. The spectra were analyzed with the OSMARCS LTE model atmosphere and with a consistent approach based on the spectrum synthesis code Turbospectrum to derive abundances of heavy elements in CS 31082-001, using updated oscillator strengths from the recent literature. We computed synthetic spectra for all lines of the elements of interest, checking for proper intensities and possible blends. We combined the abundances of heavy elements derived in previous works with the derivation of abundances from all reliable new list of lines, for the first and second peaks of r-elements. We were able to derive new abundances for 23 n-elements, 6 of them - Ge, Mo, Lu, Ta, W, and Re - were not available in previous works, making this star the most complete r-II object studied, with a total of 37 detections of n-capture elements. We also present the first NLTE+3D lead abundance in this star. The results provide improved constraints on the nature of the r-process.

astro-ph.SR↗

arXiv2009

Nucleosynthesis in Electron Capture Supernovae of AGB Stars

We examine nucleosynthesis in the electron capture supernovae of progenitor AGB stars with an O-Ne-Mg core (with the initial stellar mass of 8.8 M_\odot). Thermodynamic trajectories for the first 810 ms after core bounce are taken from a recent state-of-the-art hydrodynamic simulation. The presented nucleosynthesis results are characterized by a number of distinct features that are not shared with those of other supernovae from the collapse of stars with iron core (with initial stellar masses of more than 10 M_\odot). First is the small amount of 56Ni (= 0.002-0.004 M_\odot) in the ejecta, which can be an explanation for observed properties of faint supernovae such as SNe 2008S and 1997D. In addition, the large Ni/Fe ratio is in reasonable agreement with the spectroscopic result of the Crab nebula (the relic of SN 1054). Second is the large production of 64Zn, 70Ge, light p-nuclei (74Se, 78Kr, 84Sr, and 92Mo), and in particular, 90Zr, which originates from the low Y_e (= 0.46-0.49, the number of electrons per nucleon) ejecta. We find, however, that only a 1-2% increase of the minimum Y_e moderates the overproduction of 90Zr. In contrast, the production of 64Zn is fairly robust against a small variation of Y_e. This provides the upper limit of the occurrence of this type of events to be about 30% of all core-collapse supernovae.

astro-ph↗

arXiv2006

Neutron-capture elements in the very metal-poor star HD122563

We obtained high resolution, high S/N spectroscopy for the very metal-poor star HD122563 with the Subaru Telescope High Dispersion Spectrograph. Previous studies have shown that this object has excesses of light neutron-capture elements, while its abundances of heavy ones are very low. In our spectrum covering 3070 - 4780 A of this object, 19 neutron-capture elements have been detected, including seven for the first time in this star (Nb, Mo, Ru, Pd, Ag, Pr, and Sm). Upper limits are given for five other elements including Th. The abundance pattern shows a gradually decreasing trend, as a function of atomic number, from Sr to Yb, which is quite different from those in stars with excesses of r-process elements. This abundance pattern of neutron-capture elements provides new strong constraints on the models of nucleosynthesis responsible for the very metal-poor stars with excesses of light neutron-capture elements but without enhancement of heavy ones.

astro-ph↗

arXiv2005

Enrichment of Very Metal-Poor Stars with Both r-Process and s-Process Elements from 8-10 M_\odot Stars

Recent spectroscopic studies have revealed the presence of numerous carbon-enhanced, metal-poor stars with [Fe/H] < -2.0 that exhibit strong enhancements of s-process elements. These stars are believed to be the result of a binary mass-transfer episode from a former asymptotic giant-branch (AGB) companion that underwent s-process nucleosynthesis. However, several such stars exhibit significantly lower Ba/Eu ratios than solar s-process abundances. This might be explained if there were an additional contribution from the r-process, thereby diluting the Ba/Eu ratio by extra production of Eu. We propose a model in which the double enhancements of r-process and s-process elements originate from a former 8-10 M_\odot companion in a wide binary system, which may undergo s-processing during an AGB phase, followed by r-processing during its subsequent supernova explosion. The mass of Eu (as representative of r-process elements) captured by the secondary through the wind from the supernova is estimated, which is assumed to be proportional to the geometric fraction of the secondary (low-mass, main-sequence) star with respect to the primary (exploding) star. We find that the estimated mass is in good agreement with a constraint on the Eu yield per supernova event obtained from a Galactic chemical evolution study, when the initial orbital separation is taken to be \sim 1 year. If one assumes an orbital period on the order of five years, the efficiency of wind pollution from the supernova must be enhanced by a factor of \sim 10. This may, in fact, be realized if the expansion velocity of the supernova's innermost ejecta, in which the r-process has taken place, is significantly slow, resulting in an enhancement of accretion efficiency by gravitational focusing.

astro-ph↗

arXiv2003

The r-process in supernova explosions from the collapse of O-Ne-Mg cores

We examine r-process nucleosynthesis in a "prompt supernova explosion" from an 8-10 Msun progenitor star, as an alternative scenario to the "neutrino wind" mechanism. In the present model, the progenitor star has formed an oxygen-neon-magnesium core at its center. The core-collapse simulations are performed with a one-dimension, Newtonian hydrodynamic code. We obtain a very weak prompt explosion, in which no r-processing occurs. We further simulate energetic prompt explosions by enhancement of the shock-heating energy, in order to investigate conditions necessary for the production of r-process nuclei in such events. The highly neutronized ejecta (Ye = 0.14-0.20) leads to robust production of r-process nuclei; their relative abundances are in excellent agreement with the solar r-process pattern. Our results suggest that prompt explosions of 8-10 Msun stars with oxygen-neon-magnesium cores can be a promising site of r-process nuclei.

astro-ph↗

arXiv2001

R-Process Nucleosynthesis in Core-Collapse Supernova Explosion

We study the r-process nucleosynthesis in neutrino-driven winds of gravitational core collapse SNeII. Appropriate physical conditions are found for successful r-process nucleosynthesis, which meet with several features of heavy elements discovered recently in metal-deficient halo stars. We find also several difficulties which are not explained in the present wind models. We discuss quests for new insights in nuclear physics, astrophysics, and astronomy.

astro-ph↗

arXiv1999

Nucleosynthesis in ONeMg Novae: Models versus Observations to Constrain the Masses of ONeMg White Dwarfs and Their Envelopes

Nucleosynthesis in ONeMg novae has been investigated with the wide ranges of three parameters, i.e., the white dwarf mass, the envelope mass at ignition, and the initial composition. A quasi-analytic one-zone approach is used with an up-to-date nuclear reaction network. The nucleosynthesis results show correlation with the peak temperatures or the cooling timescales during outbursts. Among the combinations of white dwarf and envelope masses which give the same peak temperature, the explosion is more violent for a lower white dwarf mass owing to its smaller gravitational potential. Comparison of the nucleosynthesis results with observations implies that at least two-third of the white dwarf masses for the observed ONeMg novae are $\simeq 1.1 M_\odot$, which are significantly lower than estimated by previous hydrodynamic studies but consistent with the observations of V1974 Cyg. Moreover, the envelope masses derived from the comparison are $\gtrsim 10^{-4} M_\odot$, which are in good agreement with the ejecta masses estimated from observations but significantly higher than in previous hydrodynamic studies. With such a low mass white dwarf and a high mass envelope, the nova can produce interesting amounts of $γ$-ray emitters $^7$Be, $^{22}$Na, and $^{26}$Al. We suggest that V1974 Cyg has produced $^{22}$Na as high as the upper limit derived from the COMPTEL survey. In addition, a non-negligible part of the Galactic $^{26}$Al may originate from ONeMg novae, if not the major contributors. Both the future INTEGRAL survey for these $γ$-ray emitters and abundance estimates derived from ultraviolet, optical, and near infrared spectroscopies will impose a severe constraint on the current nova models.

astro-ph↗