Showing papers by "Hideyuki Umeda published in 2016"

THE FINAL FATES of ACCRETING SUPERMASSIVE STARS

Abstract: The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have a less concentrated structure than a fully convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit 105 M ⊙ derived for the fully convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with . With , the star becomes GR unstable during the helium-burning stage at M 2–3.5 × 105 M ⊙. In an extreme case with 10 , the star does not collapse until the mass reaches 8.0 × 105 M ⊙, where it is still in the hydrogen-burning stage. We expect that BHs with roughly the same mass will be left behind after the collapse in all the cases.

Mass ejection by pulsational pair instability in very massive stars and implications for luminous supernovae

Abstract: Massive stars having a CO core of $\sim$40-60 M$_\odot$ experience pulsational pair-instability (PPI) after carbon-burning. This instability induces strong pulsations of the whole star and a part of outer envelope is ejected. We investigate the evolution and mass ejection of metal-poor very massive stars which experience PPI. We use stellar models with initial masses of 140, 200, and 250 M$_\odot$ and the metallicity Z=0.004. Their masses decrease to 54.09, 58.65, and 61.03 M$_\odot$ before the neon-burning owing to mass-loss and He mass fraction at the surface becomes about 20%. During the PPI period of $\sim$1-2000 yr, they experience six, four, and three pulsations, respectively. The larger CO-core model has the longer PPI period and ejects the larger amount of mass. Since almost all surface He has been lost by the pulsations, these stars become Type Ic supernovae if they explode. Light curves during the PPI stage and supernovae are investigated and are implicated in luminous supernovae. The luminosity created by the interaction of different PPI ejecta becomes $M_{\rm bol} \sim -16$ to $-20$. The interaction between the circumstellar shell ejected by PPI and the supernova ejecta can be more luminous. These luminous transients could be an origin of Type I superluminous supernovae and supernovae with precursor.

Exact and approximate expressions of energy generation rates and their impact on the explosion properties of pair instability supernovae

Abstract: Energetics of nuclear reaction is fundamentally important to understand the mechanism of pair instability supernovae (PISNe). Based on the hydrodynamic equations and thermodynamic relations, we derive exact expressions for energy conservation suitable to be solved in simulation. We also show that some formulae commonly used in the literature are obtained as approximations of the exact expressions. We simulate the evolution of very massive stars of ~100-320 Msun with zero- and 1/10 Zsun, and calculate further explosions as PISNe, applying each of the exact and approximate formulae. The calculations demonstrate that the explosion properties of PISN, such as the mass range, the 56Ni yield, and the explosion energy, are significantly affected by applying the different energy generation rates. We discuss how these results affect the estimate of the PISN detection rate, which depends on the theoretical predictions of such explosion properties.

The Final Fates of Accreting Supermassive Stars

Abstract: The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have less concentrated structure than fully-convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit $\gtrsim 10^5~M_\odot$ derived for the fully-convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with $\dot M \lesssim 0.1~M_\odot~{\rm yr}^{-1}$. With $\dot{M} \simeq 0.3 - 1~M_\odot~{\rm yr}^{-1}$, the star becomes GR-unstable during the helium-burning stage at $M \simeq 2 - 3.5~\times 10^5~M_\odot$. In an extreme case with $10~M_\odot~{\rm yr}^{-1}$, the star does not collapse until the mass reaches $\simeq 8.0\times 10^5~M_\odot$, where it is still in the hydrogen-burning stage. We expect that BHs with roughly the same mass will be left behind after the collapse in all the cases.

Presupernova neutrino events relating to the final evolution of massive stars

Abstract: When a supernova explosion occurs in neighbors around hundreds pc, current and future neutrino detectors are expected to observe neutrinos from the presupernova star before the explosion. We show a possibility for obtaining the evidence for burning processes in the central region of presupernova stars though the observations of neutrino signals by current and future neutrino detectors such as KamLAND, JUNO, and Hyper-Kamiokande. We also investigate supernova alarms using neutrinos from presupernova stars in neighbors. If a supernova explodes at $\ensuremath{\sim}200\text{ }\text{ }\mathrm{pc}$, future 20 kton size liquid scintillation detectors are expected to observe hundreds neutrino events. We also propose a possibility of the detection of neutrino events by Gd-loaded Hyper-Kamiokande using delayed $\ensuremath{\gamma}$-ray signals. These detectors could observe detailed time variation of neutrino events. The neutrino emission rate increases by the core contraction in the final evolution stage. However, the O and Si shell burnings suppress the neutrino emission for a moment. The observed decrease in the neutrino event rate before hours to the explosion is possibly evidence for the shell burnings. The observations of detailed time evolution of presupernova neutrino events could reveal properties of burning processes in the central region of presupernova stars.

Gravitational waves from supermassive stars collapsing to a supermassive black hole

Abstract: We derive the gravitational waveform from the collapse of a rapidly rotating supermassive star (SMS) core leading directly to a seed of a supermassive black hole (SMBH) in axisymmetric numerical-relativity simulations. We find that the peak strain amplitude of gravitational waves emitted during the black hole formation is $\ensuremath{\approx}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ at the frequency $f\ensuremath{\approx}5\text{ }\text{ }\mathrm{mHz}$ for an event at the cosmological redshift $z=3$, if the collapsing SMS core is in the hydrogen-burning phase. Such gravitational waves will be detectable by space laser interferometric detectors like eLISA with signal-to-noise ratio $\ensuremath{\approx}10$, if the sensitivity is as high as LISA for $f=1--10\text{ }\text{ }\mathrm{mHz}$. The detection of the gravitational wave signal will provide a potential opportunity for testing the direct-collapse scenario for the formation of a seed of SMBHs.

Neutrino emission from nearby supernova progenitors

Abstract: Neutrinos have an important role for energy loss process during advanced evolution of massive stars. Although the luminosity and average energy of neutrinos during the Si burning are much smaller than those of supernova neutrinos, these neutrinos are expected to be detected by the liquid scintillation neutrino detector KamLAND if a supernova explosion occurs at the distance of ~100 parsec. We investigate the neutrino emission from massive stars during advanced evolution. We calculate the evolution of the energy spectra of neutrinos produced through electron-positron pair-annihilation in the supernova progenitors with the initial mass of 12, 15, and 20 M ⊙ during the Si burning and core-collapse stages. The neutrino emission rate increases from ~ 1050 s-1 to ~ 1052 s-1. The average energy of electron-antineutrinos is about 1.25 MeV during the Si burning and gradually increases until the core-collapse. For one week before the supernova explosion, the KamLAND detector is expected to observe 12-24 and 6-13 e events in the normal and inverted mass hierarchies, respectively, if a supernova explosion of a 12-20 M ⊙ star occurs at the distance of 200 parsec, corresponding to the distance to Betelgeuse. Observations of neutrinos from SN progenitors have a possibility to constrain the core structure and the evolution just before the core collapse of massive stars.

Pre-SN neutrino emissions from ONe cores in the progenitors of CCSNe

Abstract: In order to investigate the distinguishability about the progenitors of FeCCSNe and ECSNe, we calculate the luminosities and spectra of their pre-SN neutrinos and estimate the number of events at neutrino detectors.