The non-monotonic, strong metallicity dependence of the wide-binary fraction

The metallicity dependence of the wide-binary fraction in stellar populations plays a critical role in resolving the open question of wide binary formation. In this paper, we investigate the metallicity ([Fe/H]) and age dependence of the wide-binary fraction (binary separations between 103 and 104 AU) for field F and G dwarfs within 500 pc by combining their metallicity and radial velocity measurements from LAMOST DR5 with the astrometric information from Gaia DR2. We show that the wide-binary fraction strongly depends on the metallicity: as metallicity increases, the wide-binary fraction first increases, peaks at [Fe/H]≃ 0, and then decreases at the high metallicity end. The wide-binary fraction at [Fe/H]=0 is about two times larger than that at [Fe/H]=−1 and [Fe/H]=+0.5. This metallicity dependence is dominated by the thin-disk stars. Using stellar kinematics as a proxy of stellar age, we show that younger stars have a higher wide-binary fraction at fixed metallicity close to solar. We propose that multiple formation channels are responsible for the metallicity and age dependence. In particular, the positive metallicity correlation at [Fe/H]<0 and the age dependence may be due to the denser formation environments and higher-mass clusters at earlier times. The negative metallicity correlation at [Fe/H]>0 can be inherited from the similar metallicity dependence of close binaries, and radial migration may play a role in enhancing the wide-binary fraction around the solar metallicity.

On the geometry and environment of repeating FRBs

ABSTRACT We propose a geometrical explanation for periodically and non-periodically repeating fast radio bursts (FRBs) under neutron star (NS)–companion systems. We suggest a constant critical binary separation, rc, within which the interaction between the NS and companion can trigger FRBs. For an elliptic orbit with the minimum and maximum binary separations, rmin and rmax, a periodically repeating FRB with an active period could be reproduced if rmin < rc < rmax. However, if rmax < rc, the modulation of orbital motion will not work due to persistent interaction, and this kind of repeating FRBs should be non-periodic. We test relevant NS–companion binary scenarios on the basis of FRB 180916.J0158+65 and FRB 121102 under this geometrical frame. It is found that the pulsar–asteroid belt impact model is more suitable to explain these two FRBs since this model is compatible with different companions (e.g. massive stars and black holes). At last, we point out that FRB 121102-like samples are potential objects that can reveal the evolution of star-forming region.

Basic considerations for the observability of kinematically offset binary AGN

ABSTRACT The gravitational waves from massive black hole (MBH) binaries are expected to be detected by pulsar timing arrays in the next few years. While they are a promising source for multimessenger observations as binary active galactic nuclei (AGNs), few convincing candidates have been identified in electromagnetic surveys. One approach to identifying candidates has been through spectroscopic surveys searching for offsets or time-dependent offsets of broad emission lines (BLs), which may be characteristic of Doppler shifts from binary orbital motion. In this study, we predict the parameter space of MBH binaries that should be kinematically detectable. There is a delicate trade-off between requiring binary separations to be large enough for BL regions to remain attached to one of the AGNs, but also small enough such that their orbital velocity is detectable. We find that kinematic signatures are only observable for the lower mass secondary AGN, for binaries with total masses above about $10^8 \, \textrm {M}_{\odot }$, and separations between 0.1 and 1 pc. We motivate our usage of a kinematic offset sensitivity of 103 km s−1, and a sensitivity to changing offsets of 102 km s−1. With these parameters, and an Eddington ratio of 0.1, we find that $0.5{{\ \rm per\ cent}}$ of binaries have detectable offsets, and only $0.03{{\ \rm per\ cent}}$ have detectable velocity changes. Overall, kinematic binary signatures should be expected in fewer than one in 104 AGNs. Better characterizing the intrinsic variability of BLs is crucial to understanding and vetting MBH binary candidates. This requires multi-epoch spectroscopy of large populations of AGNs over a variety of time-scales.

Binary–binary scattering in the secular limit

ABSTRACT Binary–binary interactions are important in a number of astrophysical contexts including dense stellar systems such as globular clusters. Although less frequent than binary–single encounters, binary–binary interactions lead to a much richer range of possibilities such as the formation of stable triple systems. Here, we focus on the regime of distant binary–binary encounters, i.e. two binaries approaching each other on an unbound orbit with a periapsis distance Q much larger than the internal binary separations. This ‘secular’ regime gives rise to changes in the orbital eccentricities and orientations, which we study using analytic considerations and numerical integrations. We show that ‘direct’ interactions between the three orbits only occur starting at a high expansion order of the Hamiltonian (hexadecupole order), and that the backreaction of the outer orbit on the inner two orbits at lower expansion orders is weak. Therefore, to good approximation, one can obtain the changes of each orbit by using previously known analytic results for binary–single interactions, and replacing the mass of the third body with the total mass of the companion binary. Nevertheless, we find some dependence of the ‘binarity’ of the companion binary, and derive explicit analytic expressions for the secular changes that are consistent with numerical integrations. In particular, the eccentricity and inclination changes of orbit 1 due to orbit 2 scale as ϵSA, 1(a2/Q)2[m3m4/(m3 + m4)2], where ϵSA, 1 is the approximate quadrupole-order change, and a2 and (m3, m4) are the companion binary orbital semimajor axis and component masses, respectively. Our results are implemented in several python scripts that are freely available.

Physics of the Applegate mechanism: Eclipsing time variations from magnetic activity

Since its proposal in 1992, the Applegate mechanism has been discussed as a potential intrinsical mechanism to explain transit-timing variations in various types of close binary systems. Most analytical arguments presented so far focused on the energetic feasibility of the mechanism while applying rather crude one- or two-zone prescriptions to describe the exchange of angular momentum within the star. In this paper, we present the most detailed approach to date to describe the physics giving rise to the modulation period from kinetic and magnetic fluctuations. Assuming moderate levels of stellar parameter fluctuations, we find that the resulting binary period variations are one or two orders of magnitude lower than the observed values in RS-CVn like systems, supporting the conclusion of existing theoretical work that the Applegate mechanism may not suffice to produce the observed variations in these systems. The most promising Applegate candidates are low-mass post-common-envelope binaries with binary separations ≲1 R⊙ and secondary masses in the range of 0.30 M⊙ and 0.36 M⊙.

On the Induced Gravitational Collapse

The induced gravitational collapse (IGC) paradigm has been applied to explain the long gamma ray burst (GRB) associated with type Ic supernova, and recently the Xray flashes (XRFs). The progenitor is a binary systems of a carbon-oxygen core (CO) and a neutron star (NS). The CO core collapses and undergoes a supernova explosion which triggers the hypercritical accretion onto the NS companion (up to 10-2 M⊙s-1). For the binary driven hypernova (BdHNe), the binary system is enough bound, the NS reach its critical mass, and collapse to a black hole (BH) with a GRB emission characterized by an isotropic energy Eiso > 1052 erg. Otherwise, for binary systems with larger binary separations, the hypercritical accretion onto the NS is not sufficient to induced its gravitational collapse, a X-ray flash is produced with Eiso < 1052 erg. We’re going to focus in identify the binary parameters that limits the BdHNe systems with the XRFs systems.