大質量連星系内での重力崩壊型超新星爆発に関する数値シミュレーション

早大学位記番号:新7524早稲田大

The Outcome of Supernovae in Massive Binaries; Removed Mass, and its Separation Dependence

The majority of massive stars are formed in binary systems. It is hence reasonable to expect that most core-collapse supernovae (CCSNe) take place in binaries and the existence of a companion star may leave some imprints in observed features. Having this in mind, we have conducted two-dimensional hydrodynamical simulations of the collisions of CCSNe ejecta with the companion star in an almost-equal-mass ($\sim 10M_\odot$) binary to find out possible consequences of such events. In particular we pay attention to the amount of mass removed and its dependence on the binary separation. In contrast to the previous surmise, we find that the companion mass is stripped not by momentum transfer but by shock heating. Up to $25\%$ of the original mass can be removed for the closest separations and the removed mass decreases as $M_{ub} \propto a^{-4.3}$ with the binary separation $a$. By performing some experimental computations with artificially-modified densities of incident ejecta, we show that if the velocity of ejecta is fixed, the density of incident ejecta is the single important parameter that actually determines the removed mass as $M_{ub} \propto \rho_{ej} ^{1.4}$. On the other hand, another set of simulations with modified velocities of incident ejecta demonstrate that the strength of the forward shock, which heats up the stellar material and causes the mass loss of the companion star, is actually the key parameter for the removed mass.Comment: 16 pages, accepted for publication in the Astrophysical Journa

Constraining mass transfer and common-envelope physics with post-supernova companion monitoring

We present an analytical model that describes the response of companion stars after being impacted by a supernova in a close binary system. This model captures key properties of the luminosity evolution obtained from 1D stellar evolution calculations fairly well: a high-luminosity plateau phase and a decaying tail phase. It can be used to constrain the pre-supernova binary properties from the observed photometry of the companion star several years after the explosion in a relatively simple manner. The derived binary parameters are useful in constraining the evolutionary scenario for the progenitors and the physics of binary interactions. We apply our model to some known stripped-envelope supernova companions (SN1993J, SN2001ig, SN2006jc, SN2011dh, SN2013ge). Combined with other observational constraints such as the pre-supernova progenitor photometry, we find that SN1993J and SN2011dh likely had relatively massive companions on wide orbits, while SN2006jc may have had a relatively low-mass companion on a tight orbit. This trend suggests that type IIb supernova progenitors evolved from stable mass transfer channels and type Ibc progenitors may have formed from common-envelope channels. The constraints on orbital separation helps us probe the highly uncertain common-envelope physics for massive stars, especially with multiple epochs of companion observations. We also highlight possible limitations of our model due to the assumptions made in the underlying 1D models.Comment: 9 pages, 9 figures. Accepted for publication in MNRA

Comprehensive study of ejecta-companion interaction for core-collapse supernovae in massive binaries

We carry out a comprehensive study of supernova ejecta-companion interaction in massive binary systems. We aim to physically understand the kinematics of the interaction and predict observational signatures. To do this we perform simulations over a vast parameter space of binary configurations, varying the masses of the progenitor and companion, structure of the companion, explosion energy, and orbital separation. Our results were not so consistent with classical models by Wheeler et al. 1975, sometimes deviating by an order of magnitude. We construct an alternative simple model which explains the simulated results reasonably well and can be used to estimate impact velocities for arbitrary explosion profiles and companion star structures. We then investigate the long term evolution after the supernova, where the companion can be inflated by the energy injected into the star. We find that the companion can become more than an order of magnitude overluminous straight after the supernova, but quickly fades away after $\sim10$ years and returns to its original luminosity in about a thermal timescale of the star. Finally, we also discuss the possible surface contamination of heavy elements from the slower ejecta.Comment: 24 pages, 21 figures, accepted by the Astrophysical Journa

Polyhedral Clinching Auctions for Indivisible Goods

In this study, we propose polyhedral clinching auctions for indivisible goods, which has so far been studied for divisible goods. As in the divisible setting by Goel et al., our mechanism enjoys incentive compatibility, individual rationality, and Pareto optimality, and works with polymatroidal environments. A notable feature for the indivisible setting is that the whole procedure can be conducted in time polynomial of the number of buyers and goods. Moreover, we show additional efficiency guarantees, recently established by Sato for the divisible setting, that the liquid welfare (LW) of our mechanism achieves more than 1/2 of the optimal LW, and that the social welfare is more than the optimal LW

Neutron stars colliding with binary companions: formation of hypervelocity stars, pulsar planets, bumpy superluminous supernovae and Thorne-\.Zytkow objects

We perform 3D hydrodynamical simulations of new-born neutron stars (NSs) colliding with main-sequence binary companions after supernova explosions. Based on those hydrodynamical models, we construct a semi-analytical formula that describes the drag force inside stars with steep density gradients. We then compute the outcome of NS--companion collisions over a wide range of parameters using the semi-analytical formula. Depending on the direction and magnitude of the natal kick, we find that the collision may lead to various outcomes. For relatively fast kicks and high impact parameters, the NS may penetrate the companion star envelope without merging. By allowing the NS to plunge through companions, the companion can be accelerated to have runaway velocities up to $\sim10$ per cent above the theoretical upper limit considered in classical binary disruption scenarios. The NS can capture and carry away up to a few per cent of the companion envelope as it escapes, which may form pulsar planets or cause outflows through accretion to heat the ejecta from inside and power the supernova light curve. For lower impact parameters, the neutron star will directly merge with the companion and form a Thorne-\.Zytkow object. In intermediate cases, the NS penetrates the companion envelope several times before merging, possibly causing multiple bumps in the supernova light curve like in SN2015bn and SN2019stc.Comment: 13 pages, 13 figures, comments welcom