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Register a new userThe Final Fate of Supermassive $M sim 5 times 10^4 ; M_odot$ Pop III Stars: Explosion or Collapse?
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We investigate the possibility of a supernova in supermassive ($5 times 10^4 ;M_odot$) population III stars induced by a general relativistic instability occurring in the helium burning phase. This explosion could occur via rapid helium burning during an early contraction of the isentropic core. Such an explosion would be visible to future telescopes and could disrupt the proposed direct collapse formation channel for early universe supermassive black holes. We simulate first the stellar evolution from hydrogen burning using a 1D stellar evolution code with a post Newtonian approximation; at the point of dynamical collapse, we switch to a 1D (general relativistic) hydrodynamics code with the Misner-Sharpe metric. In opposition to a previous study, we do not find an explosion in the non rotating case, although our model is close to exploding for a similar mass to the explosion in the previous study. When we include slow rotation, we find one exploding model, and we conclude that there likely exist additional exploding models, though they may be rare.
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We calculate the neutrino signal from Population III supermassive star collapse using a neutrino transfer code originally developed for core collapse supernovae and massive star collapse. Using this code, we are able to investigate the supermassive star mass range thought to undergo neutrino trapping ($sim 10^4$ M$_odot$), a mass range which has been neglected by previous works because of the difficulty of neutrino transfer. For models in this mass range, we observe a neutrino-sphere with a large radius and low density compared to typical massive star neutrino-spheres. We calculate the neutrino light-curve emitted from this neutrino-sphere. The resulting neutrino luminosity is significantly lower than the results of a previous analytical model. We briefly discuss the possibility of detecting a neutrino burst from a supermassive star or the neutrino background from many supermassive stars and conclude that the former is unlikely with current technology, unless the SMS collapse is located as close as 1 Mpc, while the latter is also unlikely even under very generous assumptions. However, the supermassive star neutrino background is still of interest as it may serve as a source of noise in proposed dark matter direct detection experiments.
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 $simeq 8.0times 10^5~M_odot$, 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.
The formation of supermassive Population III stars with masses $gtrsim$ 10,000 Msun in primeval galaxies in strong UV backgrounds at $z sim$ 15 may be the most viable pathway to the formation of supermassive black holes by $z sim$ 7. Most of these stars are expected to live for short times and then directly collapse to black holes, with little or no mass loss over their lives. But we have now discovered that non-rotating primordial stars with masses close to 55,000 Msun can instead die as highly energetic thermonuclear supernovae powered by explosive helium burning, releasing up to 10$ ^{55}$ erg, or about 10,000 times the energy of a Type Ia supernova. The explosion is triggered by the general relativistic contribution of thermal photons to gravity in the core of the star, which causes the core to contract and explosively burn. The energy release completely unbinds the star, leaving no compact remnant, and about half of the mass of the star is ejected into the early cosmos in the form of heavy elements. The explosion would be visible in the near infrared at $z lesssim$ 20 to {it Euclid} and the Wide-Field Infrared Survey Telescope (WFIRST), perhaps signaling the birth of supermassive black hole seeds and the first quasars.
We investigated stellar winds from zero/low-metallicity low-mass stars by magnetohydrodynamical simulations for stellar winds driven by Alfven waves from stars with mass $M_{star}=(0.6-0.8)M_{odot}$ and metallicity $Z=(0-1)Z_{odot}$, where $M_{odot}$ and $Z_{odot}$ are the solar mass and metallicity, respectively. Alfvenic waves, which are excited by the surface convection, travel upward from the photosphere and heat up the corona by their dissipation. For lower $Z$, denser gas can be heated up to the coronal temperature because of the inefficient radiation cooling. The coronal density of Pop.II/III stars with $Zle 0.01Z_{odot}$ is 1-2 orders of magnitude larger than that of the solar-metallicity star with the same mass, and as a result, the mass loss rate, $dot{M}$, is $(4.5-20)$ times larger. This indicates that metal accretion on low-mass Pop.III stars is negligible. The soft X-ray flux of the Pop.II/III stars is also expected to be $approx (1-30)$ times larger than that of the solar-metallicity counterpart owing to the larger coronal density, even though the radiation cooling efficiency is smaller. A larger fraction of the input Alfvenic wave energy is transmitted to the corona in low $Z$ stars because they avoid severe reflection owing to the smaller density difference between the photosphere and the corona. Therefore, a larger fraction is converted to the thermal energy of the corona and the kinetic energy of the stellar wind. From this energetics argument, we finally derived a scaling of $dot{M}$ as $dot{M}propto L R_{star}^{11/9}M_{star}^{-10/9}T_{rm eff}^{11/2}left[max (Z/Z_{odot},0.01)right]^{-1/5}$, where $L$, $R_{star}$, and $T_{rm eff}$ are stellar luminosity, radius, and effective temperature, respectively.
Supermassive black holes observed at high redshift $zgtrsim6$ could grow from direct collapse black holes (DCBHs) with mass $sim10^5,M_{odot}$, which result from the collapse of supermassive stars (SMSs). If a relativistic jet is launched from a DCBH, it can break out of the collapsing SMS and produce a gamma-ray burst (GRB). Although most of the GRB jets are off-axis from our line of sight, we show that the energy injected from the jet into a cocoon is huge $sim10^{55-56},{rm{erg}}$, so that the cocoon fireball is observed as ultra-luminous supernovae of $sim10^{45-46}rm{,erg,s^{-1}}$ for $sim5000 [(1+z)/16] rm{,days}$. They are detectable by the future telescopes with near infrared bands, such as, $Euclid$, $WFIRST$, $WISH$, and $JWST$ up to $zsim20$ and $sim 100$ events per year, providing a direct evidence of the DCBH scenario.
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