The Initial mass function of the first stars inferred from extremely metal-poor stars

This is an author-created, un-copyedited version of an article published in The Astrophysical Journal. The Version of Record is available online at https://doi.org/10.3847/1538-4357/aab3de.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 (E 51 = E/1051[erg] = 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 (E 51 = 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.Peer reviewedFinal Accepted Versio

Properties of Type II Plateau Supernova SNLS-04D2dc: Multicolor Light Curves of Shock Breakout and Plateau

Shock breakout is the brightest radiative phenomenon in a Type II supernova (SN). Although it was predicted to be bright, the direct observation is difficult due to the short duration and X-ray/ultraviolet-peaked spectra. First entire observations of the shock breakouts of Type II Plateau SNe (SNe IIP) were reported in 2008 by ultraviolet and optical observations by the {\it GALEX} satellite and supernova legacy survey (SNLS), named SNLS-04D2dc and SNLS-06D1jd. We present multicolor light curves of a SN IIP, including the shock breakout and plateau, calculated with a multigroup radiation hydrodynamical code {\sc STELLA} and an evolutionary progenitor model. The synthetic multicolor light curves reproduce well the observations of SNLS-04D2dc. This is the first study to reproduce the ultraviolet light curve of the shock breakout and the optical light curve of the plateau consistently. We conclude that SNLS-04D2dc is the explosion with a canonical explosion energy $1.2\times10^{51}$ ergs and that its progenitor is a star with a zero-age main-sequence mass $20M_\odot$ and a presupernova radius $800R_\odot$. The model demonstrates that the peak apparent $B$-band magnitude of the shock breakout would be $m_{\rm B}\sim26.4$ mag if a SN being identical to SNLS-04D2dc occurs at a redshift $z=1$, which can be reached by 8m-class telescopes. The result evidences that the shock breakout has a great potential to detect SNe IIP at z\gsim1.Comment: 5 pages, 5 figures. Accepted for publication in the Astrophysical Journal Letter

Nucleosynthesis in Core-Collapse Supernovae and GRB--Metal-Poor Star Connection

We review the nucleosynthesis yields of core-collapse supernovae (SNe) for various stellar masses, explosion energies, and metallicities. Comparison with the abundance patterns of metal-poor stars provides excellent opportunities to test the explosion models and their nucleosynthesis. We show that the abundance patterns of extremely metal-poor (EMP) stars, e.g., the excess of C, Co, Zn relative to Fe, are in better agreement with the yields of hyper-energetic explosions (Hypernovae, HNe) rather than normal supernovae. We note that the variation of the abundance patterns of EMP stars are related to the diversity of the Supernova-GRB connection. We summarize the diverse properties of (1) GRB-SNe, (2) Non-GRB HNe/SNe, (3) XRF-SN, and (4) Non-SN GRB. In particular, the Non-SN GRBs (dark hypernovae) have been predicted in order to explain the origin of C-rich EMP stars. We show that these variations and the connection can be modeled in a unified manner with the explosions induced by relativistic jets. Finally, we examine whether the most luminous supernova 2006gy can be consistently explained with the pair-instability supernova model.Comment: 15 pages, 9 figures. To appear in "Supernova 1987A: 20 Years After: Supernovae and Gamma-Ray Bursters", eds. S. Immler, K. Weiler, & R. McCray (American Institute of Physics) (2007

The Connection between Gamma-Ray Bursts and Extremely Metal-Poor Stars as Nucleosynthetic Probes of the Early Universe

The connection between the long GRBs and Type Ic Supernovae (SNe) has revealed the interesting diversity: (i) GRB-SNe, (ii) Non-GRB Hypernovae (HNe), (iii) X-Ray Flash (XRF)-SNe, and (iv) Non-SN GRBs (or dark HNe). We show that nucleosynthetic properties found in the above diversity are connected to the variation of the abundance patterns of extremely-metal-poor (EMP) stars, such as the excess of C, Co, Zn relative to Fe. We explain such a connection in a unified manner as nucleosynthesis of hyper-aspherical (jet-induced) explosions Pop III core-collapse SNe. We show that (1) the explosions with large energy deposition rate, $\dot{E}_{\rm dep}$, are observed as GRB-HNe and their yields can explain the abundances of normal EMP stars, and (2) the explosions with small $\dot{E}_{\rm dep}$ are observed as GRBs without bright SNe and can be responsible for the formation of the C-rich EMP (CEMP) and the hyper metal-poor (HMP) stars. We thus propose that GRB-HNe and the Non-SN GRBs (dark HNe) belong to a continuous series of BH-forming stellar deaths with the relativistic jets of different $\dot{E}_{\rm dep}$.Comment: 8 pages, 6 figures. To appear in "Massive Stars as Cosmic Engines", Proceedings of IAU Symposium 250 (December 2007, Kauai), eds. F. Bresolin, P.A. Crowther, & J. Puls (Cambridge Univ. Press

First Detection of Ar-K Line Emission from the Cygnus Loop

We observed the Cygnus Loop with XMM-Newton (9 pointings) and Suzaku (32 pointings) between 2002 and 2008. The total effective exposure time is 670.2 ks. By using all of the available data, we intended to improve a signal-to-noise ratio of the spectrum. Accordingly, the accumulated spectra obtained by the XIS and the EPIC show some line features around 3 keV that are attributed to the S He$\beta$ and Ar He$\alpha$ lines, respectively. Since the Cygnus Loop is an evolved ($\sim$10,000 yr) supernova remnant whose temperature is relatively low ($<$1 keV) compared with other young remnants, its spectrum is generally faint above 3.0 keV, no emission lines, such as the Ar-K line have ever been detected. The detection of the Ar-K line is the first time and we found that its abundance is significantly higher than that of the solar value; 9.0$^{+4.0}_{-3.8}$ and 8.4$^{+2.5}_{-2.7}$ (in units of solar), estimated from the XIS and the EPIC spectra, respectively. We conclude that the Ar-K line originated from the ejecta of the Cygnus Loop. Follow-up X-ray observations to tightly constrain the abundances of Ar-rich ejecta will be useful to accurately estimate the progenitor's mass.Comment: 12 pages, 9 figures, accepted for publication in PAS

A strong neutron burst in jet-like supernovae of spinstars

Some metal-poor stars have abundance patterns which are midway between the slow (s) and rapid (r) neutron capture processes. We show that the helium shell of a fast rotating massive star experiencing a jet-like explosion undergoes two efficient neutron capture processes: one during stellar evolution and one during the explosion. It eventually provides a material whose chemical composition is midway between the s- and r-process. A low metallicity 40~$M_{\odot}$ model with an initial rotational velocity of $\sim 700$~km~s$^{-1}$ was computed from birth to pre-supernova with a nuclear network following the slow neutron capture process. A 2D hydrodynamic relativistic code was used to model a $E = 10^{52}$~erg relativistic jet-like explosion hitting the stellar mantle. The jet-induced nucleosynthesis was calculated in post-processing with a network of 1812 nuclei. During the star's life, heavy elements from $30 \lesssim Z \lesssim 82$ are produced thanks to an efficient s-process, which is boosted by rotation. At the end of evolution, the helium shell is largely enriched in trans-iron elements and in (unburnt) $^{22}$Ne, whose abundance is $\sim 20$ times higher than in a non-rotating model. During the explosion, the jet heats the helium shell up to $\sim 1.5$ GK. It efficiently activates ($\alpha,n$) reactions, such as $^{22}$Ne($\alpha,n$), and leads to a strong n-process with neutron densities of $\sim 10^{19} - 10^{20}$~cm$^{-3}$ during $0.1$~second. This has the effect of shifting the s-process pattern towards heavier elements (e.g. Eu). The resulting chemical pattern is consistent with the abundances of the carbon-enhanced metal-poor r/s star CS29528-028, provided the ejecta of the jet model is not homogeneously mixed. This is a new astrophysical site which can explain at least some of the metal-poor stars showing abundance patterns midway between the s- and r-process.Comment: 9 pages, 12 figures, accepted in A&

The Unique Type Ib Supernova 2005bf at Nebular Phases: A Possible Birth Event of A Strongly Magnetized Neutron Star

Late phase nebular spectra and photometry of Type Ib Supernova (SN) 2005bf taken by the Subaru telescope at ~ 270 and ~ 310 days since the explosion are presented. Emission lines ([OI]6300, 6363, [CaII]7291, 7324, [FeII]7155) show the blueshift of ~ 1,500 - 2,000 km s-1. The [OI] doublet shows a doubly-peaked profile. The line luminosities can be interpreted as coming from a blob or jet containing only ~ 0.1 - 0.4 Msun, in which ~ 0.02 - 0.06 Msun is 56Ni synthesized at the explosion. To explain the blueshift, the blob should either be of unipolar moving at the center-of-mass velocity v ~ 2,000 - 5,000 km s-1, or suffer from self-absorption within the ejecta as seen in SN 1990I. In both interpretations, the low-mass blob component dominates the optical output both at the first peak (~ 20 days) and at the late phase (~ 300 days). The low luminosity at the late phase (the absolute R magnitude M_R ~ -10.2 mag at ~ 270 days) sets the upper limit for the mass of 56Ni < ~ 0.08 Msun, which is in contradiction to the value necessary to explain the second, main peak luminosity (M_R ~ -18.3 mag at ~ 40 days). Encountered by this difficulty in the 56Ni heating model, we suggest an alternative scenario in which the heating source is a newly born, strongly magnetized neutron star (a magnetar) with the surface magnetic field Bmag ~ 10^{14-15} gauss and the initial spin period P0 ~ 10 ms. Then, SN 2005bf could be a link between normal SNe Ib/c and an X-Ray Flash associated SN 2006aj, connected in terms of Bmag and/or P0.Comment: 16 pages, 12 figures. Accepted by the Astrophysical Journa

Fallback Supernovae: A Possible Origin of Peculiar Supernovae with Extremely Low Explosion Energies

We perform hydrodynamical calculations of core-collapse supernovae (SNe) with low explosion energies. These SNe do not have enough energy to eject the whole progenitor and most of the progenitor falls back to the central remnant. We show that such fallback SNe can have a variety of light curves (LCs) but their photospheric velocities can only have some limited values with lower limits. We also perform calculations of nucleosynthesis and LCs of several fallback SN model, and find that a fallback SN from the progenitor with a main-sequence mass of 13 Msun can account for the properties of the peculiar Type Ia supernova SN 2008ha. The kinetic energy and ejecta mass of the model are 1.2*10^{48} erg and 0.074 Msun, respectively, and the ejected 56Ni mass is 0.003 Msun. Thus, SN 2008ha can be a core-collapse SN with a large amount of fallback. We also suggest that SN 2008ha could have been accompanied with long gamma-ray bursts and long gamma-ray bursts without associated SNe may be accompanied with very faint SNe with significant amount of fallback which are similar to SN 2008ha.Comment: 9 pages, 11 figures, 2 tables, accepted by The Astrophysical Journal, proofed and some references added in v

On the Light Curve and Spectrum of SN 2003dh Separated from the Optical Afterglow of GRB 030329

The net optical light curves and spectra of the supernova (SN) 2003dh are obtained from the published spectra of GRB 030329, covering about 6 days before SN maximum to about 60 days after. The bulk of the U-band flux is subtracted from the observed spectra using early-time afterglow templates, because strong line blanketing greatly depresses the UV and U-band SN flux in a metal-rich, fast-moving SN atmosphere. The blue-end spectra of the gamma-ray burst (GRB)connected hypernova SN 1998bw is used to determine the amount of subtraction. The subtraction of a host galaxy template affects the late-time results. The derived SN 2003dh light curves are narrower than those of SN 1998bw, rising as fast before maximum, reaching a possibly fainter maximum, and then declining ~ 1.2-1.4 times faster. We then build UVOIR bolometric SN light curve. Allowing for uncertainties, it can be reproduced with a spherical ejecta model of Mej ~ 7+/-3 Msun, KE ~ (3.5+/-1.5)E52 ergs, with KE/Mej ~ 5 following previous spectrum modelling, and M(Ni56) ~ (0.4 +0.15/-0.1) Msun. This suggests a progenitor main-sequence mass of about 25-40 Msun, lower than SN 1998bw but significantly higher than normal Type Ic SNe and the GRB-unrelated hypernova SN 2002ap.Comment: 18 pages, 7 figures, published by Ap

SN 2006aj Associated with XRF 060218 At Late Phases: Nucleosynthesis-Signature of A Neutron Star-Driven Explosion

Optical spectroscopy and photometry of SN 2006aj have been performed with the Subaru telescope at t > 200 days after GRB060218, the X-ray Flash with which it was associated. Strong nebular emission-lines with an expansion velocity of v ~ 7,300 km/s were detected. The peaked but relatively broad [OI]6300,6363 suggests the existence of ~ 2 Msun of materials in which ~1.3 Msun is oxygen. The core might be produced by a mildly asymmetric explosion. The spectra are unique among SNe Ic in (1) the absence of [CaII]7291,7324 emission, and (2) a strong emission feature at ~ 7400A, which requires ~ 0.05 Msun of newly-synthesized 58Ni. Such a large amount of stable neutron-rich Ni strongly indicates the formation of a neutron star. The progenitor and the explosion energy are constrained to 18 Msun < Mms < 22 Msun and E ~ (1 - 3) 10^{51} erg, respectively.Comment: Accepted for Publication in the Astrophysical Journal Letters (2007, ApJ, 658, L5). 8 pages, including 1 table and 3 figures. Typos correcte