arXiv subjects

Toshikazu Shigeyama

Publications and source records attributed to Toshikazu Shigeyama.

106 records · Page 6Linked to original sources

arXiv2006

A method to identify the companion stars of type Ia supernovae in young supernova remnants

We propose a method to identify the companion stars of type Ia supernovae (SNe Ia) in young supernova remnants (SNRs) by recognizing distinct features of absorption lines due to Fe I appearing in the spectrum. If a sufficient amount of Fe I remains in the ejecta, Fe I atoms moving toward us absorb photons by transitions from the ground state to imprint broad absorption lines exclusively with the blue-shifted components in the spectrum of the companion star. To investigate the time evolution of column depth of Fe I in the ejecta, we have performed hydrodynamical calculations for SNRs expanding into the uniform ambient media, taking into account collisional ionizations, excitations, and photo-ionizations of heavy elements. As a result, it is found that the companion star in Tycho's SNR will exhibit observable features in absorption lines due to Fe I at $λλ= 371.9935$ nm and 385.9911 nm if a carbon deflagration SN model is taken. However, these features may disappear by taking another model that emits a few times more intense ionizing photons from the shocked outer layers. To further explore the ionization states in the freely expanding ejecta, we need a reliable model to describe the structure of the outer layers.

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arXiv2006

Relics of metal-free low mass stars exploding as thermonuclear supernovae

Renewed interest in the first stars that were formed in the universe has led to the discovery of extremely iron-poor stars. Since several competing scenarios exist, our understanding of the mass range that determines the observed elemental abundances remains unclear. In this study, we consider three well-studied metal-poor stars in terms of the theoretical supernovae (SNe) model. Our results suggest that the observed abundance patterns in the metal-poor star BD +80 245 and the pair of stars HD 134439/40 agree strongly with the theoretical possibility that these stars inherited their heavy element abundance patterns from SNe initiated by thermonuclear runaways in the degenerate carbon-oxygen cores of primordial asymptotic giant branch stars with \~3.5-5 solar masses. Recent theoretical calculations have predicted that such SNe could be originated from metal-free stars in the intermediate mass range. On the other hand, intermediate mass stars containing some metals would end their lives as white dwarfs after expelling their envelopes in the wind due to intense momentum transport from outgoing photons to heavy elements. This new pathway for the formation of SNe requires that stars are formed from the primordial gas. Thus, we suggest that stars of a few solar masses were formed from the primordial gas and that some of them caused thermonuclear explosions when the mass of their degenerate carbon-oxygen cores increased to the Chandrasekhar limit without experiencing efficient mass loss.

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arXiv2005

Self-Similar Evolution of Relativistic Shock Waves Emerging from Plane-Parallel Atmospheres

We study the evolution of the ultra-relativistic shock wave in a plane-parallel atmosphere adjacent to a vacuum and the subsequent breakout phenomenon. When the density distribution has a power law with the distance from the surface, there is a self-similar motion of the fluid before and after the shock emergence. The time evolution of the Lorentz factor of the shock front is assumed to follow a power law when the time is measured from the moment at which the shock front reaches the surface. The power index is found to be determined by the condition for the flow to extend through a critical point. The energy spectrum of the ejected matter as a result of the shock breakout is derived and its dependence on the strength of the explosion is also deduced. The results are compared with the self-similar solution for the same problem with non-relativistic treatment.

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arXiv2004

Roles of Supernova Ejecta in Nucleosynthesis of Light Elements, Li, Be, and B

Explosions of type Ic supernovae (SNe Ic) are investigated using a relativistic hydrodynamic code to study roles of their outermost layers of the ejecta in light element nucleosynthesis through spallation reactions as a possible mechanism of the "primary" process. We have confirmed that the energy distribution of the outermost layers with a mass fraction of only 0.001 % follows the empirical formula proposed by previous work when the explosion is furious. In such explosions, a significant fraction of the ejecta ($>$0.1 % in mass) have the energy greater than the threshold energy for spallation reactions. On the other hand, it is found that the outermost layers of ejecta become more energetic than the empirical formula would predict when the explosion energy per unit ejecta mass is smaller than $\sim 1.3\times 10^{51}{ergs/}\Msun$. As a consequence, it is necessary to numerically calculate explosions to estimate light element yields from SNe Ic. The usage of the empirical formula would overestimate the yields by a factor of $\gtsim 3$ for energetic explosions such as SN 1998bw and underestimate the yields by a similar factor for less energetic explosions like SN 1994I. The yields of light elements Li, Be, and B (LiBeB) from SNe Ic are estimated by solving the transfer equation of cosmic rays originated from ejecta of SNe Ic and compared with observations.

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arXiv2003

Accretion of dust grains as a possible origin of metal-poor stars with low alpha/Fe ratios

The origin of low alpha/Fe ratios in some metal-poor stars, so called low-alpha stars, is discussed. It is found that most of low-alpha stars in the Galaxy are on the main-sequence. This strongly suggests that these stars suffered from external pollution. It is also found that the abundance ratios Zn/Fe of low-alpha stars both in the Galaxy and in dwarf spheroidal galaxies are lower than the average value of Galactic halo stars whereas damped Ly alpha absorbers have higher ratios. This implies that some low-alpha stars accreted matter depleted from gas onto dust grains. To explain the features in these low-alpha stars, we have proposed that metal-poor stars harboring planetary systems are the origin of these low-alpha stars. Stars engulfing a small fraction of planetesimals enhance the surface content of Fe to exhibit low alpha/Fe ratios on their surfaces while they are on the main-sequence, because dwarfs have shallow surface convection zones where the engulfed matter is mixed. After the stars leave the main-sequence, the surface convection zones become deeper to reduce the enhancement of Fe. Eventually, when the stars ascend to the tip of the red giant branch, they engulf giant planets to become low-alpha stars again as observed in dwarf spheroidal galaxies. We predict that low-alpha stars with low Mn/Fe ratios harbor planetary systems.

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arXiv2003

Excavation of the first stars

The external pollution of the first stars in the Galaxy is investigated. The first stars were born in clouds composed of the pristine gas without heavy elements. These stars accreted gas polluted with heavy elements while they still remained in the cloud. As a result, it is found that they exhibit a distribution with respect to the surface metallicity. We have derived the actual form of this distribution function. This metallicity distribution function strongly suggests that the recently discovered most metal-deficient star HE0107-5240 with [Fe/H]=-5.3 was born as a metal-free star and accreted gas polluted with heavy elements. Thus the heavy elements such as Fe in HE0107-5240 must have been supplied from supernovae of later generations exploding inside the cloud in which the star had been formed. The elemental abundance pattern on the surface of stars suffering from such an external pollution should not be diverse but exhibit the average pattern of numerous supernovae. Future observations for a number of metal-deficient stars with [Fe/H]<-5 will be able to prove or disprove this external pollution scenario. Other possibilities to produce a star with this metallicity are also discussed.

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arXiv2003

Star Formation History of Omega Centauri Imprinted in Elemental Abundance Patterns

The star formation history of the globular cluster Omega Centauri is investigated in the context of an inhomogeneous chemical evolution model in which supernovae induce star formation. The proposed model explains recent observations for Omega Cen stars, and divides star formation into three epochs. At the end of the first epoch, ~ 70% of the gas was expelled by supernovae. AGB stars then supplied s-process elements to the remaining gas during the first interval of ~300 Myr. This explains the observed sudden increase in Ba/Fe ratios in Omega Cen stars at [Fe/H] ~ -1.6. Supernovae at the end of the second epoch were unable to expel the gas. Eventually, Type Ia supernovae initiated supernova-induced star formation, and remaining gas was stripped when the cluster passed through the newly formed disk of the Milky Way. The formation of Omega Cen is also discussed in the framework of globular cluster formation triggered by cloud-cloud collisions. In this scenario, the relative velocity of clouds in the collision determines the later chemical evolution in the clusters. A head-on collision of proto-cluster clouds with a low relative velocity would have converted less than 1% of gas into stars and promoted the subsequent chemical evolution by supernova-driven star formation. This is consistent with present observed form of Omega Cen. In contrast, the other Galactic globular clusters are expected to have formed from more intense head-on collisions, and the resultant clouds would have been too thin for supernovae to accumulate enough gas to form the next generation of stars. This explains the absence of chemical evolution in these other globular clusters.

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arXiv2003

Relics of subluminous supernovae in metal-poor stars

The unique elemental abundance pattern of the carbon-rich stars CS29498-043 and CS22949-037 is characterized by a large excess of magnesium and silicon in comparison with iron. This excess is investigated in the context of a supernova-induced star formation scenario, and it is concluded that these stars were born from the matter swept up by supernova remnants containing little iron and that such supernovae are similar to the least-luminous SNe ever observed, SNe 1997D and 1999br. Comparison of the observed abundance pattern in iron-group elements of subluminous supernovae with those of other supernovae leads to an intriguing implication for explosion, nucleosynthesis, and mixing in supernovae. The observed invariance of these ratios can not be accounted for by a spherically symmetric supernova model.

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arXiv2002

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.

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arXiv2001

Supernova SN1987A Revisited as a Major Production Site for r-Process Elements

The origin of nucleosynthesis products of rapid neutron capture reactions (the r-process) is a longstanding astrophysical problem. Recent analyses of elemental abundances for extremely metal-poor stars shed light on the elemental abundances of individual supernovae. Comparison of the abundance distributions of some extremely metal-poor stars with those of the best-observed supernova SN 1987A clearly indicates that the overabundances of barium and strontium found in SN 1987A that have been ascribed to the slow neutron capture process must be results of r-process nucleosynthesis. The mass of freshly synthesized barium in SN 1987A is estimated to be 6x10^-6 solar mass based on the observed surface abundance and detailed hydrodynamical models for this supernova. These new findings lead to the conclusion that 20 solar mass stars, one of which is the progenitor star of SN 1987A, are the predominant production sites for r-process elements in the Galaxy and the r-process element donors for notable neutron-capture-rich giant stars, CS22892-052 and CS31082-001.

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arXiv2001

Chemical Evolution of Odd Elements in an Inhomogeneous Early Galaxy

We investigate the chemical evolution of odd-numbered elements such as sodium (Na) and aluminum (Al) during the early epochs of the Galactic halo with the use of a model that reproduces the observed box-shaped distribution of extremely metal-poor stars in the [Na, Al/Mg] versus [Mg/H] plane. Our model is constructed under the assumptions that those stars retain the elemental abundance patterns produced by individual Type II supernovae (SNe), and that the yields of the odd elements depend on the initial metallicity, z, of their SN progenitors. As a result, recent abundance determinations that clarify how the [Na, Al/Mg] ratios of field stars have evolved to the solar values enable us to deduce how the yields of these odd elements depend on z. The observed trends in these abundances, in particular the very large scatter (over 1 dex in [Al/Mg]) requires that the Al yield scales as m_Al proportional to z^0.6 for [Mg/H]<-1.8, while the observed [Na/Mg] trend requires that the Na yield scales as m_Na proportional to z^-0.4 for [Mg/H]<-1.8 and m_Na proportional to z^0.4 for [Mg/H]>-1.8. It is found that the predicted frequency distribution of stars in the [Na/Mg] versus [Mg/H] diagram is very sensitive to the assumed form of the primordial IMF, and that its slope is steeper than the Salpeter IMF. The necessity to match the observed abundance patterns of odd elements and the frequency distribution of extremely metal-poor stars should provide useful constraints on nucleosynthesis calculations of metal-free massive stars as well as on theories of their formation.

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arXiv2000

Metal Enrichment of The Primordial Interstellar Medium through 3-D Hydrodynamical Evolution of The First Supernova Remnant

The long-term evolution of supernova remnants (SNRs) in the primordial interstellar medium (ISM) with an inhomogeneous structure is calculated to investigate metal enrichment of the primordial gas. For this purpose, we have constructed a parallel 3-D hydrodynamics code incorporating the radiative cooling and self-gravity. The self-gravity and radiative cooling develop the inhomogeneous structure of the ISM from a small perturbation with a power-law spectrum. The resultant density ranges from 0.5 cm$^{-3}$ to 10$^6$ cm $^{-3}$. Calculations for a supernova (SN) with the progenitor mass of 20 solar mass are performed as the first step of a series of our study. It is found from the results that a single SN distributes some of newly synthesized heavy elements into a dense filament of the ISM with densities ranging from 100 to 10$^4$ cm$^{-3}$ depending on where the SN explodes. Thus, the metallicity [Mg/H] of the dense filaments polluted by the SN ejecta becomes $-2.7\pm0.5$. From these filaments, the first Population II stars will form. This value is in accordance with the previous analytical work (Shigeyama & Tsujimoto 1998) with accuracy of $\sim 0.3$ dex.

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arXiv2000

Probing the Site for r-Process Nucleosyntheis with Abundances of Barium and Magnesium in Extremely Metal-Poor Stars

We suggest that if the astrophysical site for r-process nucleosynthesis in the early Galaxy is confined to a narrow mass range of Type II supernova (SN II) progenitors, with a lower mass limit of Mms = 20 Msun, a unique feature in the observed distribution of [Ba/Mg] vs.[Mg/H] for extremely metal-poor stars can be adequately reproduced. We associate this feature, a bifurcation of the observed elemental ratios into two branches in the Mg abundance interval -2.7 < [Mg/H] < -2.3, with two distinct processes. The first branch, which we call the ``y''-branch, is associated with the production of Ba and Mg from individual massive supernovae. We conclude that SNe II with Mms = 20 Msun are the dominant source of r-process nucleosynthesis in the early Galaxy. An SN-induced chemical evolution model with this Mms-dependent Ba yield creates the y-branch, reflecting the different nucleosynthesis yields of [Ba/Mg] for each SN II with Mms > 20 Msun. The second branch, which we call the ``i''-branch, is associated with the elemental abundance ratios of stars which were formed in the dense shells of the interstellar medium swept up by SNe II with Mms < 20 Msun that do not synthesize r-process elements, and applies to stars with observed Mg abundances in the range [Mg/H] < -2.7. The Ba abundances in these stars reflect those of the interstellar gas at the (later) time of their formation. The existence of a [Ba/Mg] i-branch strongly suggests that SNe II which are associated with stars of progenitor mass Mms < 20 Msun are infertile sources for the production of r-process elements. We predict the existence of this i-branch for other r-process elements, such as europium (Eu), to the extent that their production site is in common with Ba.

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arXiv1999

Chemical Evolution of the Galactic Halo through Supernova-Induced Star Formation and Its Implication for Population III Stars

A model for Galactic chemical evolution, driven by supernova-induced star formation, is formulated and used to examine the nature of the Galactic halo at early epochs. In this model, new stars are formed following each supernova event, thus their abundance pattern is determined by the combination of heavy elements ejected from the supernova itself and those elements which are already present in the interstellar gas swept up by the supernova remnant. The end result is a prediction of large scatter in the abundance ratios among low-metallicity stars, reflecting a different nucleosynthesis yield for each Type II supernova with a different progenitor mass. Formation of new stars is terminated when supernova remnants sweep up too little gas to form shells. We show from calculations based on the above scenario that (i) the observed [Fe/H] distribution for the Galactic halo field stars can be reproduced without effectively decreasing the heavy-element yields from Type II supernovae by some manipulation required by previous models (e.g., via mass loss from the early Galaxy, or later mixing with ``pristine'' hydrogen clouds), (ii) the large observed scatter in the abundance ratio [Eu/Fe] for the most metal-poor stars can also be reproduced, and (iii) the frequency distribution of stars in the [Eu/Fe]-[Fe/H] plane can be predicted. Our model suggests that the probability of identifying essentially metal-free stars (Population III) in the local halo is around one in 10^{3-4}, provided that star formation in the halo is confined to individual gas clouds with mass of 10^{6-7} Msun and that the initial mass function of metal-free stars is not significantly different from the Salpeter mass function.

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arXiv1998

New Insights into the Early Stage of the Galactic Chemical Evolution

The supernova yields of several heavy elements including alpha-, iron-group, and r-process elements are obtained as a function of the mass of their progenitor main-sequence stars M_ms from the abundance patterns of extremely metal-poor stars with a procedure recently proposed by Shigeyama and Tsujimoto (1998). The ejected masses of alpha- and iron-group elements increase with M_ms, whereas more Eu is ejected from supernovae with lower M_ms. For these several heavy elements, it is shown that the average abundance ratios weighted by the Salpeter initial mass function coincide with the ratios observed in stars with -2<[Fe/H]<-1 within 0.1 dex. It follows that the correlations of stellar abundance ratios with the metallicity are twofold. One is the abundance ratios for [Fe/H] <-2.5 imprinted by the nucleosynthesis in individual supernovae on the timescale 10^7yr and the other for [Fe/H]>-2 results from the mixing of the products from a whole site of the nucleosynthesis, taking place on the timescale longer than 10^9yr.

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arXiv1998

Fossil Imprints of the First Generation Supernova Ejecta in Extremely Metal-Deficient Stars

Using results of nucleosynthesis calculations for theoretical core-collapse supernova models with various progenitor's masses, it is shown that abundance patterns of C, Mg, Si, Ca, and H seen in extremely metal-deficient stars with [Fe/H] < -2.5 follow those seen in the individual first generation supernova remnants (SNRs). This suggests that most of the stars with [Fe/H] < -2.5 were made from individual supernova (SN) events. To obtain the ratio of heavy elements to hydrogen, a formula is derived to estimate the mass of hydrogen swept up by a SNR when it occurs in the interstellar matter with the primordial abundances. We use [Mg/H] to indicate the metallicities instead of [Fe/H]. The metallicities [Mg/H] predicted from these SNRs range from ~-4 to ~-1.5 and the mass of Mg in a SN is well correlated with its progenitor's mass. Thus the observed [Mg/H] in an extremely metal deficient star has a correspondence to the progenitor's mass. A larger [Mg/H] corresponds to a larger progenitor's mass. Therefore, so called `age-metallicity relation' does not hold for stars with [Fe/H] < -2.5. In contrast, the [Mg/Fe] ratios in the theoretical SNRs have a different trend from those in extremely metal-deficient stars. It is also shown that the observed trend of [Mg/Fe] can predict the Fe yield of each SN given the correspondence of [Mg/H] to the progenitor's mass. The Fe yields thus obtained are consistent with those derived from SN light curve analyses. This indicates that there is still a problem in modelling a core-collapse supernova at its beginning of explosion or mass cut.

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