Abstract
Extremely metal-poor (EMP) stars are the living fossils with records of chemical enrichment history at the early epoch of galaxy formation. By the recent large observation campaigns, ...statistical samples of EMP stars have been obtained. This motivates us to reconsider their classification and formation conditions. From the observed lower limits of carbon and iron abundances of Acr(C) ∼ 6 and Fe/Hcr ∼ −5 for C-enhanced EMP (CE-EMP) and C-normal EMP (CN-EMP) stars, we confirm that gas cooling by dust thermal emission is indispensable for the fragmentation of their parent clouds to form such low mass, i.e. long-lived stars, and that the dominant grain species are carbon and silicate, respectively. We constrain the grain radius $r_i^{\rm cool}$ of a species i and condensation efficiency fij of a key element j as $r_{\rm C}^{\rm cool} / f_{\rm C,C} = 10 \ {\rm \mu m}$ and $r_{\rm Sil}^{\rm cool} / f_{\rm Sil,Mg} = 0.1 \ {\rm \mu m}$ to reproduce Acr(C) and Fe/Hcr, which give a universal condition 10C/H − 2.30 + 10Fe/H > 10−5.07 for the formation of every EMP star. Instead of the conventional boundary C/Fe = 0.7 between CE-EMP and CN-EMP stars, this condition suggests a physically meaningful boundary C/Feb = 2.30 above and below which carbon and silicate grains are dominant coolants, respectively.
We compare the elemental abundance patterns of ∼200 extremely metal-poor (EMP; Fe/H < −3) stars to the supernova yields of metal-free stars, in order to obtain insights into the characteristic masses ...of the first (Population III or Pop III) stars in the universe. The supernova yields are prepared with nucleosynthesis calculations of metal-free stars with various initial masses (M = 13, 15, 25, 40 and 100 M ) and explosion energies (E51 = E/1051erg = 0.5-60), to include low-energy, normal-energy, and high-energy explosions. We adopt the mixing-fallback model, to take into account possible asymmetry in the supernova explosions, and the yields that best fit the observed abundance patterns of the EMP stars are searched by varying the model parameters. We find that the abundance patterns of the EMP stars are predominantly best-fitted by the supernova yields with initial masses M < 40 M , and that more than than half of the stars are best-fitted by the M = 25 M hypernova (E51 = 10) models. The results also indicate that the majority of the primordial supernovae have ejected 10−2-10−1 M of 56Ni, leaving behind a compact remnant (either a neutron star or a black hole), with a mass in the range of ∼1.5-5 M . These results suggest that the masses of the first stars responsible for the first metal enrichment are predominantly <40 M . This implies that the higher-mass first stars were either less abundant, directly collapsed into a black hole without ejecting heavy elements, or a supernova explosion of a higher-mass first star inhibits the formation of the next generation of low-mass stars at Fe/H < −3.
ABSTRACT
The first generation of metal-free (Population III) stars are crucial for the production of heavy elements in the earliest phase of structure formation. Their mass scale can be derived from ...the elemental abundance pattern of extremely metal-poor (EMP) stars, which are assumed to inherit the abundances of uniformly mixed supernova (SN) ejecta. If the expanding ejecta maintains its initial stratified structure, the elemental abundance pattern of EMP stars might be different from that from uniform ejecta. In this work, we perform numerical simulations of the metal enrichment from stratified ejecta for normal core-collapse SNe (CCSNe) with a progenitor mass $25 \ {\rm M_{\bigodot }}$ and explosion energies 0.7–10 B ($1 \ {\rm B} = 10^{51} \ \rm erg$). We find that SN shells fall back into the central minihalo in all models. In the recollapsing clouds, the abundance ratio M/Fe for stratified ejecta is different from the one for uniform ejecta only within ±0.4 dex for any element M. We also find that, for the largest explosion energy (10 B), a neighbouring halo is also enriched. Only the outer layers containing Ca or lighter elements reach the halo, where C/Fe = 1.49. This means that C-enhanced metal-poor stars can form from the CCSN even with an average abundance ratio C/Fe = −0.65.
ABSTRACT We study the number and the distribution of low-mass Population III (Pop III) stars in the Milky Way. In our numerical model, hierarchical formation of dark matter minihalos and ...Milky-Way-sized halos are followed by a high-resolution cosmological simulation. We model the Pop III formation in H2 cooling minihalos without metal under UV radiation of the Lyman-Werner bands. Assuming a Kroupa initial mass function (IMF) from 0.15 to 1.0 M for low-mass Pop III stars, as a working hypothesis, we try to constrain the theoretical models in reverse by current and future observations. We find that the survivors tend to concentrate on the center of halo and subhalos. We also evaluate the observability of Pop III survivors in the Milky Way and dwarf galaxies, and constraints on the number of Pop III survivors per minihalo. The higher latitude fields require lower sample sizes because of the high number density of stars in the galactic disk, the required sample sizes are comparable in the high- and middle-latitude fields by photometrically selecting low-metallicity stars with optimized narrow-band filters, and the required number of dwarf galaxies to find one Pop III survivor is less than 10 at <100 kpc for the tip of red giant stars. Provided that available observations have not detected any survivors, the formation models of low-mass Pop III stars with more than 10 stars per minihalo are already excluded. Furthermore, we discuss the way to constrain the IMF of Pop III stars at a high mass range of 10 M .
We present new nucleosynthesis yields as functions of the stellar mass, metallicity, and explosion energy (corresponding to normal supernovae and hypernovae). We apply the results to the chemical ...evolution of the solar neighborhood. Our new yields are based on the new developments in the observational/theoretical studies of supernovae (SNe) and extremely metal-poor (EMP) stars in the halo, which have provided excellent opportunities to test the explosion models and their nucleosynthesis. We use the light curve and spectra fitting of individual SN to estimate the mass of the progenitor, explosion energy, and produced
56Ni mass. Comparison with the abundance patterns of EMP stars has made it possible to determine the model parameters of core-collapse SNe, such as mixing-fallback parameters. More specifically, we take into account the two distinct new classes of massive SNe: (1) very energetic hypernovae, whose kinetic energy (KE) is more than 10 times the KE of normal core-collapse SNe, and (2) very faint and low energy SNe (faint SNe). These two new classes of SNe are likely to be “black-hole-forming” SNe with rotating or non-rotating black holes. Nucleosynthesis in hypernovae is characterized by larger abundance ratios
(
Zn
,
Co
,
V
,
Ti
)
/
Fe
and smaller
(
Mn
,
Cr
)
/
Fe
than normal SNe, which can explain the observed trends of these ratios in EMP stars. Nucleosynthesis in faint SNe is characterized by a large amount of fall-back, which explains the abundance pattern of the most Fe-poor stars. These comparisons suggest that black-hole-forming SNe made important contributions to the early galactic (and cosmic) chemical evolution.
ABSTRACT
Recent observations of active galactic nuclei (AGNs) have shown a high Fe ii/Mg ii line-flux ratio in their broad-line regions, nearly independent of redshift up to z ≳ 6. The high flux ...ratio requires rapid production of iron in galactic nuclei to reach an abundance ratio of Fe/Mg ≳ 0.2 as high as those observed in matured galaxies in the local universe. We propose a possible explanation of rapid iron enrichment in AGNs by massive star formation that follows a top-heavy initial mass function (IMF) with a power-law index of Γ larger than the canonical value of Γ = −2.35 for a Salpeter IMF. Taking into account metal production channels from different types of SNe, we find that the high value of Fe/Mg ≳ 0.2 requires the IMF to be characterized with Γ ≳ −1 (Γ ≳ 0) and a high-mass cutoff at Mmax ≃ 100–150 M⊙ (Mmax ≳ 250 M⊙). Given the conditions, core-collapse SNe with M* ≳ 70 M⊙ and pair-instability SNe give a major contribution for iron enrichment. Such top-heavy stellar IMFs would be a natural consequence from mass growth of stars formed in dense AGN discs under Bondi-like gas accretion that is regulated by feedback at M* ≳ 10 M⊙. The massive stellar population formed in AGN discs also leave stellar-mass black hole remnants, whose mergers associated with gravitational-wave emission account for at most 10 per cent of the merger rate inferred from LIGO/Virgo observations to simultaneously explain the high Fe/Mg ratio with metal ejection.
Abstract
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, HE 1327−2326, with
. We determine Zn/Fe = 0.80 ± 0.25 from a UV Zn
i
line at 2138 Å, detected at 3.4
σ
. Yields of a 25
M
⊙
aspherical supernova model with artificially modified densities exploding with
E
= 5 × 10
51
erg best match the entire abundance pattern of HE 1327−2326. Such high-entropy hypernova explosions are expected to produce bipolar outflows, 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 HE 1327−2326 might have formed in such externally enriched environments.
Context. The nature of the early generation of massive stars can be inferred by investigating the origin of the extremely metal-poor (EMP) stars, likely formed from the ejecta of one or a few ...previous massive stars. Aims. We investigate the rotational properties of early massive stars by comparing the abundance patterns of EMP stars with massive stellar models including rotation. Methods. Low metallicity 20 M⊙ massive stellar models with eight initial rotation rates between 0 and 70% of the critical velocity are computed. Explosions with strong fallback are assumed. The ejected material is considered to fit individually the abundance patterns of 272 EMP stars with −4 < Fe/H < −3. Results. With increasing initial rotation, the C/H, N/H, O/H, Na/H, Mg/H, and Al/H ratios in the massive star ejecta are gradually increased (up to ∼4 dex) while the 12C/13C ratio is decreased. Among the 272 EMP stars considered, ∼40 − 50% are consistent with our models. About 60 − 70% of the carbon-enhanced EMP star sample can be reproduced against ∼20 − 30% for the carbon-normal EMP star sample. The abundance patterns of carbon-enhanced EMP stars are preferentially reproduced with a material coming from mid to fast rotating massive stars. The overall velocity distribution derived from the best massive star models increases from no rotation to fast rotation. The maximum is reached for massive stars having initial equatorial velocities of ∼550 − 640 km s−1. Conclusions. Although subject to significant uncertainties, these results suggest that the rotational mixing operating in between the H-burning shell and the He-burning core of early massive stars played an important role in the early chemical enrichment of the Universe. The comparison of the velocity distribution derived from the best massive star models with velocity distributions of nearby OB stars suggests that a greater number of massive fast rotators were present in the early Universe. This may have important consequences for reionization, the first supernovae, or integrated light from high redshift galaxies.
Abstract
Superluminous supernovae can be explained by the interaction of their ejecta with a dense circumstellar medium. The resulting shock boosts the radiative luminosity of the supernova by ...converting mechanical energy into radiative energy. Accurate modeling of the shock, which suffers high radiative losses, requires the use of radiation hydrodynamics. High-precision methods have a large computational cost, so approximations are generally used. In this paper, we describe the implementation of the M1 approximation of radiation transfer using the hydrodynamics code,
front
. Basic tests show good agreement with reference solutions and with results from other codes. Additional tests were undertaken to show some cases where the M1 method produces unphysical results, such as in the regions where the light beams intersect each other. Tests with outgoing rays are also presented to validate the use of the M1 approach in supernova simulations. Further, a simple initial model for a superluminous supernova was created to study the shock-interacting mechanism. It is shown that the M1 approach correctly reproduces both the bolometric light curve of the supernova in one-dimensional, spherically symmetric simulations, as well as the dynamics of the thin dense layer that arises in this scenario due to extreme radiative cooling. The thin layer is unstable in multidimensional simulations, but the perturbations do not drastically change the photosphere’s parameters at the beginning of the simulation and do not strongly affect the light curve during the first 50 days.
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