arXiv subjects

Toshinori Hayashi

Publications and source records attributed to Toshinori Hayashi.

9 recordsLinked to original sources

arXiv2025

Compact Binary Formation in Open Star Clusters III: Probability of Binary Black Holes Hidden in Gaia Black Hole Binary

The Gaia mission and its follow-up observations have discovered a few candidates of non-interacting single black holes (BHs) and visible stars, Gaia BH1, BH2, and BH3, collectively called ``astrometric BH binaries''. This paper investigates whether any of these candidates harbor binary BHs (BBHs), namely, whether any such candidates are previously undiscovered ``astrimetric BBH triples''. Focusing on open star clusters, which are promising formation sites of astrometric BH binaries, we estimate the formation rate of astrometric BBH triples through gravitational $N$-body simulations. We find a competitively high formation efficiency of astrometric BBH triples ($\sim 10^{-6} {\rm M}_\odot^{-1}$ or $\sim 10$\% of astrometric BH binaries) in low-metallicity environments but no astrometric BBH triples in solar-metallicity environments. Most of the astrometric BBH triples in our simulations were dynamically stable for $10$ Gyrs, indicating that $\sim10$\% of astrometric BH binary candidates may indeed harbor inner BBHs if they originate from open star clusters in low-metallicity environments. Astrometric BBH triples can be distinguished from astrometric BH binaries through radial velocity follow-up of the tertiary star. According to the statistics of our simulated samples, a small percent of astrometric BH binary candidates should exhibit detectable radial-velocity modulations generated by inner BBHs. Such candidates preferentially exhibit ``outer'' orbital periods of $\gtrsim 10^3$ days and moderately high ``outer'' orbital eccentricities ($\gtrsim 0.7$). Our current result will strongly motivate the search for astrometric BBH triples in the upcoming Gaia Data Release 4 and Gaia Final Data Release.

astro-ph.GA↗

arXiv2025

Stability of hierarchical triples comprising a central massive body and a tight binary: the effect of inner and outer eccentricities on the binary breakup condition

We explore the stability of gravitational triple systems comprising a central massive body and a tight binary of less massive pairs. In the present paper, we focus on improving the Hill-type stability criterion for the binary in those systems, with particular attention to the effects of the eccentricities of the inner and outer orbits. We perform direct Newtonian N-body simulations over much longer integration times than previous studies, which is essential to determine the stability and breakup timescale distributions of those systems in a reliable fashion. As a result, we obtain an empirical fitting formula of the binary breakup condition that incorporates effects of the inner and outer eccentricities, the mutual inclination of the inner and outer orbits, the mass ratios of the three bodies, and the breakup timescale.

astro-ph.SR↗

arXiv2023

Constraining the binarity of black hole candidates: a proof-of-concept study of Gaia BH1 and Gaia BH2

Nearly a hundred of binary black holes (BBHs) have been discovered with gravitational-wave signals emitted at their merging events. Thus, it is quite natural to expect that significantly more abundant BBHs with wider separations remain undetected in the universe, or even in our Galaxy. We consider a possibility that star-BH binary candidates may indeed host an inner BBH, instead of a single BH. We present a detailed feasibility study of constraining the binarity of the currently available two targets, Gaia BH1 and Gaia BH2. Specifically, we examine three types of radial velocity (RV) modulations of a tertiary star in star-BBH triple systems; short-term RV modulations induced by the inner BBH, long-term RV modulations induced by the nodal precession, and long-term RV modulations induced by the von Zeipel-Kozai-Lidov oscillations. Direct three-body simulations combined with approximate analytic models reveal that Gaia BH1 system may exhibit observable signatures of the hidden inner BBH if it exists at all. The methodology that we examine here is quite generic, and is expected to be readily applicable to future star-BH binary candidates in a straightforward manner.

astro-ph.HE↗

arXiv2022

Lagrange vs. Lyapunov stability of hierarchical triple systems: dependence on the mutual inclination between inner and outer orbits

While there have been many studies examining the stability of hierarchical triple systems, the meaning of ``stability'' is somewhat vague and has been interpreted differently in previous literatures. The present paper focuses on ``Lagrange stability'', which roughly refers to the stability against the escape of a body from the system, or ``disruption'' of the triple system, in contrast to ``Lyapunov-like stability'' that is related to the chaotic nature of the system dynamics. We compute the evolution of triple systems using direct $N$-body simulations up to $10^7 P_\mathrm{out}$, which is significantly longer than previous studies (with $P_\mathrm{out}$ being the initial orbital period of the outer body). We obtain the resulting disruption timescale $T_\mathrm{d}$ as a function of the triple orbital parameters with particular attention to the dependence on the mutual inclination between the inner and outer orbits, $i_\mathrm{mut}$. By doing so, we have clarified explicitly the difference between Lagrange and Lyapunov stabilities in astronomical triples. Furthermore, we find that the von Zeipel-Kozai-Lidov oscillations significantly destabilize inclined triples (roughly with $60^\circ < i_\mathrm{mut} < 150^\circ$) relative to those with $i_\mathrm{mut}=0^\circ$. On the other hand, retrograde triples with $i_\mathrm{mut}>160^\circ$ become strongly stabilized with much longer disruption timescales. We show the sensitivity of the normalized disruption timescale $T_\mathrm{d}/P_\mathrm{out}$ to the orbital parameters of triple system. The resulting $T_\mathrm{d}/P_\mathrm{out}$ distribution is practically more useful in a broad range of astronomical applications than the stability criterion based on the Lyapunov divergence.

astro-ph.SR↗

arXiv2022

Dynamical disruption timescales and chaotic behavior of hierarchical triple systems

We examine the stability of hierarchical triple systems using direct $N$-body simulations without adopting a secular perturbation approximation. We estimate their disruption timescales in addition to the mere stable/unstable criterion, with particular attention to the mutual inclination between the inner and outer orbits. First, we improve the fit to the dynamical stability criterion by \citet{Mardling1999,Mardling2001} widely adopted in the previous literature. Especially, we find that that the stability boundary is very sensitive to the mutual inclination; coplanar retrograde triples and orthogonal triples are much more stable and unstable, respectively, than coplanar prograde triples. Next, we estimate the disruption timescales of triples satisfying the stability condition up to $10^9$ times the inner orbital period. The timescales follow the scaling predicted by \citet{Mushkin2020}, especially at high $e_\mathrm{out}$ where their random walk model is most valid. We obtain an improved empirical fit to the disruption timescales, which indicates that the coplanar retrograde triples are significantly more stable than the previous prediction. We furthermore find that the dependence on the mutual inclination can be explained by the energy transfer model based on a parabolic encounter approximation. We also show that the disruption timescales of triples are highly sensitive to the tiny change of the initial parameters, reflecting the genuine chaotic nature of the dynamics of those systems.

astro-ph.SR↗

arXiv2022

Radial-velocity variation of a tertiary star orbiting a binary black hole in coplanar and non-coplanar triples: short- and long-term anomalous behavior

A number of ongoing surveys are likely to discover star-black hole binaries in our Galaxy in the near future. A fraction of them may be triple systems comprising an inner binary, instead of a single black hole, which might be progenitors of binary black holes (BBHs) routinely discovered now from the gravitational wave. We extend our previous proposal to locate inner BBHs from the short-term radial-velocity (RV) variation of a tertiary star in coplanar triples, and we consider noncoplanar triples and their long-term RV variations as well. Specifically, we assume coplanar and noncoplanar triples with an inner BBH of the total mass $20~M_\odot$, whose outer and inner orbital periods are 80 days and 10 days, respectively. We perform a series of N-body simulations and compare the results with analytic approximate solutions based on quadrupole perturbation theory. For coplanar triples, the pericenter shift of the outer star can be used to detect the hidden inner BBH. For noncoplanar triples, the total RV semi-amplitude of the outer star is modulated periodically on the order of $100$km/s due to its precession over roughly the Kozai-Lidov oscillation timescale. Such long-term modulations would be detectable within a decade, independent of the short-term RV variations on the order of of $100$ m/s at roughly twice the orbital frequency of the inner binary. Thus the RV monitoring of future star-black hole binary candidates offers a promising method for searching for their inner hidden BBHs in optical bands.

astro-ph.HE↗

arXiv2020

Unveiling the architecture of a pulsar - binary black-hole triple system with pulsar arrival time analysis

A large number of binary black holes (BBHs) with longer orbital periods are supposed to exist as progenitors of BBH mergers recently discovered with gravitational wave (GW) detectors. In our previous papers, we proposed to search for such BBHs in triple systems through the radial-velocity modulation of the tertiary orbiting star. If the tertiary is a pulsar, high precision and cadence observations of its arrival time enable an unambiguous characterization of the pulsar -- BBH triples located at several kpc, which are inaccessible with the radial velocity of stars. The present paper shows that such inner BBHs can be identified through the short-term Rømer delay modulation, on the order of $10$ msec for our fiducial case, a triple consisting of $20~M_\odot$ BBH and $1.4~M_\odot$ pulsar with $P_\mathrm{in}=10$ days and $P_\mathrm{out}=100$ days. If the relativistic time delays are measured as well, one can determine basically all the orbital parameters of the triple. For instance, this method is applicable to inner BBHs of down to $\sim 1$ hr orbital periods if the orbital period of the tertiary pulsar is around several days. Inner BBHs with $\lesssim 1$ hr orbital period emit the GW detectable by future space-based GW missions including LISA, DECIGO, and BBO, and very short inner BBHs with sub-second orbital period can be even probed by the existing ground-based GW detectors. Therefore, our proposed methodology provides a complementary technique to search for inner BBHs in triples, if exist at all, in the near future.

astro-ph.HE↗

arXiv2020

Architecture of three-planet systems predicted from the observed protoplanetary disk of HL Tau

A number of protoplanetary disks observed with ALMA potentially provide direct examples of initial conditions for planetary systems. In particular, the HL Tau disk has been intensively studied, and its rings/gaps are conventionally interpreted to be a result of unseen massive planets embedded in the gaps. Based on this interpretation, we carried out N-body simulations to investigate orbital evolution of planets within the protoplanetary disk and after the disk dispersal. Before the disk dispersal, our N-body simulations include both migration and mass-growth of the planet coupled with evolution of the disk. By varying the disk parameters, we produce a variety of widely-separated planetary systems consisting of three super-Jupiters at the end of disk dispersal. We found the outer planet is more massive than the inner one, and the migration of the innermost planet is inefficient due to the accretion of outer planet(s). We also showed how the final configuration and the final planetary mass depend on disk parameters. The migration is found to be convergent and no planet-pair has a period ratio less than 2. After the disk dispersal, we switch to pure gravitational N-body simulations and integrate the orbits up to 10 Gyr. Most simulated systems remain stable for at least 10 Gyr. We discuss implications of our result in terms of the observed widely-separated planetary systems HR 8799 and PDS 70.

astro-ph.EP↗

arXiv2020

A strategy to search for an inner binary black hole from the motion of the tertiary star

There are several on-going projects to search for stars orbiting around an invisible companion. A fraction of such candidates may be a triple, instead of a binary, consisting of an inner binary black hole (BBH) and an outer orbiting star. In this paper, we propose a methodology to search for a signature of such an inner BBH, possibly a progenitor of gravitational-wave sources discovered by {\it LIGO}, from the precise radial velocity (RV) follow-up of the outer star. We first describe a methodology using an existing approximate RV formula for coplanar circular triples. We apply this method and constrain the parameters of a possible inner binary objects in 2M05215658+4359220, which consists of a red giant and an unseen companion. Next we consider co-planar but non-circular triples. We compute numerically the RV variation of a tertiary star orbiting around an inner BBH, generate mock RV curves, and examine the feasibility of the BBH detection for our fiducial models. We conclude that the short-cadence RV monitoring of a star-BH binary provides an interesting and realistic method to constrain and/or search for possible inner BBHs. Indeed a recent discovery of a star--BH binary system LB-1 may imply that there are a large number of such unknown objects in our Galaxy, which are ideal targets for the methodology proposed here.

astro-ph.EP↗