Merger Conditions of Population III Protostar Binaries. (arXiv:2305.06843v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kirihara_T/0/1/0/all/0/1">Takanobu Kirihara</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Susa_H/0/1/0/all/0/1">Hajime Susa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hosokawa_T/0/1/0/all/0/1">Takashi Hosokawa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kinugawa_T/0/1/0/all/0/1">Tomoya Kinugawa</a>
Massive close binary stars with extremely small separations have been
observed, and they are possible progenitors of gravitational-wave sources. The
evolution of massive binaries in the protostellar accretion stage is key to
understanding their formation process. We, therefore, investigate how close the
protostars, consisting of a high-density core and a vast low-density envelope,
can approach each other but not coalesce. To investigate the coalescence
conditions, we conduct smoothed particle hydrodynamics simulations following
the evolution of equal-mass binaries with different initial separations. Since
Population (Pop) I and III protostars have similar interior structures, we
adopt a specific Pop~III model with the mass and radius of $7.75;M_{odot}$
and $61.1;R_{odot}$ obtained by the stellar evolution calculations. Our
results show that the binary separation decreases due to the transport of the
orbital angular momentum to spin angular momentum. If the initial separation is
less than about 80 per~cent of the sum of the protostellar radius, the binary
coalesces in a time shorter than the tidal lock timescale. The mass loss up to
the merging is $lesssim 3$ per~cent. After coalescence, the star rotates
rapidly, and its interior structure is independent of the initial separation.
We conclude that there must be some orbital shrinking mechanism after the
protostars contract to enter the zero-age main-sequence stage.
Massive close binary stars with extremely small separations have been
observed, and they are possible progenitors of gravitational-wave sources. The
evolution of massive binaries in the protostellar accretion stage is key to
understanding their formation process. We, therefore, investigate how close the
protostars, consisting of a high-density core and a vast low-density envelope,
can approach each other but not coalesce. To investigate the coalescence
conditions, we conduct smoothed particle hydrodynamics simulations following
the evolution of equal-mass binaries with different initial separations. Since
Population (Pop) I and III protostars have similar interior structures, we
adopt a specific Pop~III model with the mass and radius of $7.75;M_{odot}$
and $61.1;R_{odot}$ obtained by the stellar evolution calculations. Our
results show that the binary separation decreases due to the transport of the
orbital angular momentum to spin angular momentum. If the initial separation is
less than about 80 per~cent of the sum of the protostellar radius, the binary
coalesces in a time shorter than the tidal lock timescale. The mass loss up to
the merging is $lesssim 3$ per~cent. After coalescence, the star rotates
rapidly, and its interior structure is independent of the initial separation.
We conclude that there must be some orbital shrinking mechanism after the
protostars contract to enter the zero-age main-sequence stage.
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