arXiv subjects

Kohei Inayoshi

Publications and source records attributed to Kohei Inayoshi.

148 records · Page 9Linked to original sources

arXiv2013

Pulsational instability of supergiant protostars: Do they grow supermassive by accretion?

Supermassive stars (SMSs; M>10^5 Msun) and their remnant black holes are promising progenitors for supermassive black holes (SMBHs) observed in the early universe at z>7. It has been postulated that SMSs forms through very rapid mass accretion onto a protostar at a high rate exceeding 0.01 Msun/yr. According to recent studies, such rapidly accreting protostars evolve into "supergiant protostars", i.e. protostars consisting of a bloated envelope and a contracting core, similar to giant star. However, like massive stars as well as giant stars, both of which are known to be pulsationally unstable, supergiant protostars may also be also unstable to launch strong pulsation-driven outflows. If this is the case, the stellar growth via accretion will be hindered by the mass loss. We here study the pulsational stability of the supergiant protostars in the mass range M<10^3 Msun through the method of the linear perturbation analysis. We find that the supergiant protostars with M>600 Msun and very high accretion rate Mdot>1.0 Msun/yr are unstable due to the kappa mechanism. The pulsation is excited in the He^+ ionization layer in the envelope. Even under a conservative assumption that all the pulsation energy is converted into the kinetic energy of the outflows, the mass-loss rate is ~10^-3 Msun/yr, which is lower than the accretion rate by more than two orders of magnitude. We thus conclude that the supergiant protostars should grow stably via rapid accretion at least in the mass range we studied. As long as the rapid accretion is maintained in the later stage, protostars will become SMSs, which eventually produce seeds for the high-z SMBHs.

astro-ph.SR↗

arXiv2012

Supermassive black hole formation by the cold accretion shocks in the first galaxies

We propose a new scenario for supermassive star (SMS;>10^5Msun) formation in shocked regions of colliding cold accretion flows near the centers of first galaxies. Recent numerical simulations indicate that assembly of a typical first galaxy with virial temperature (~10^4K) proceeds via cold and dense flows penetrating deep to the center, where the supersonic streams collide each other to develop a hot and dense (~10^4K, ~10^3/cc) shocked gas. The post-shock layer first cools by efficient Ly alpha emission and contracts isobarically until 8000K. Whether the layer continues the isobaric contraction depends on the density at this moment: if the density is high enough for collisionally exciting H2 rovibrational levels (>10^4/cc), enhanced H2 collisional dissociation suppresses the gas to cool further. In this case, the layer fragments into massive (>10^5Msun) clouds, which collapse isothermally (~8000K) by the Ly alpha cooling without subsequent fragmentation. As an outcome, SMSs are expected to form and evolve eventually to seeds of supermassive black holes (SMBH). By calculating thermal evolution of the post-shock gas, we delimit the range of post-shock conditions for the SMS formation, which can be expressed as: T>6000K/(n/10^4/cc) for n<10^4/cc and T>5000-6000K for n>10^4/cc, depending somewhat on initial ionization degree. We found that metal enrichment does not affect the above condition for metallicity below 10^-3Zsun if metals are in the gas phase, while condensation of several percent of metals into dust decreases this critical value of metallicity by an order of magnitude. Unlike the previously proposed scenario for SMS formation, which postulates extremely strong ultraviolet radiation to quench H2 cooling, our scenario naturally explains the SMBH seed formation in the assembly process of the first galaxies, even without such a strong radiation.

astro-ph.CO↗

arXiv2011

Effect of cosmic ray/X-ray ionization on supermassive black hole formation

We study effects of external ionization by cosmic rays (CRs) and X-rays on the thermal evolution of primordial clouds under strong far-ultraviolet (FUV) radiation. A strong FUV radiation dissociates H2 and quenches its cooling. Even in such an environment, a massive cloud with Tvir>10^4 K can contract isothermally at 8000 K by Lyman alpha cooling. This cloud collapses monolithically without fragmentation, and a supermassive star (>10^5 Msun) is believed to form at the center, which eventually evolves to a supermassive black hole (SMBH). However, candidates of FUV sources, including star-forming galaxies, are probably sources of strong CRs and X-rays, as well. We find that the external ionization promotes H2 production and elevates the threshold FUV intensity Jcr needed for the SMBH formation for CR energy density U_CR>10^-14 erg/cm^3 or X-ray intensity J_X>10^-24 erg/s/cm^2/sr/Hz at 1 keV. The critical FUV flux increases in proportion to U_CR^{1/2} (J_X^{1/2}) in the high CR (X-ray, respectively) limit. With the same value of FUV intensity at the Lyman limit (13.6 eV), the H^- photodissociation rate, with threshold of 0.755 eV, increases and the H2 molecules decrease with decreasing effective temperature of the FUV sources T*. The lower value of T* thus results in the lower critical FUV flux at 13.6 eV. Using an empirical relation between intensities of FUV and CR/X-ray from nearby star-forming galaxies, we find that external ionization effect remarkably enhances the critical FUV flux for sources with T* as high as 10^5 K and composed of stars with <100 Msun to a level that is not realized in any halo. This indicates that, to induce SMBH formation, the FUV sources must be either Pop II/I galaxies with low brightness temperature, Pop III galaxies with a very top-heavy IMF, or Pop III galaxies too young to harbor sources of CR/X-ray, e.g., supernova remnants or high-mass X-ray binaries.

astro-ph.CO↗

arXiv2011

Testing two-component jet models of GRBs with orphan afterglows

In the \swift era, two-component jet models were introduced to explain the complex temporal profiles and the diversity of early afterglows. In this paper, we concentrate on the two-component jet model; first component is the conventional afterglow and second is the emission due to the late internal dissipation such as the late-prompt emission. We suggest herein that the two-component jet model can be probed by the existence of two optical peaks for orphan GRB afterglows. Each peak is caused by its respective jet as its relativistic beaming cone widens to encompass the off-axis line of sight. Typically, the first peak appears at $10^4-10^5$ s and the second at $10^5-10^6$ s. Furthermore, we expect to observe a single, bright X-ray peak at the same time as the first optical peak. Because orphan afterglows do not have prompt emission, it is necessary to monitor the entire sky every $10^4$ s in the X-ray regime. We can test the model with orphan afterglows through the X-ray all-sky survey collaboration and by using ground-based optical telescopes.

astro-ph.HE↗