Ákos Szölgyén

We have analyzed how drag forces modify the orbits of objects moving through extended gaseous distributions. We considered how hydrodynamic (surface area) drag forces and dynamical friction (gravitational) drag forces drive the evolution of orbital eccentricity. While hydrodynamic drag forces cause eccentric orbits to become more circular, dynamical friction drag can cause orbits to become more eccentric. We developed a semi-analytic model that accurately predicted these changes by comparing the total work and torque applied to the orbit at periapse and apoapse. We useed a toy model of a radial power-law density profile, to determine that there is a critical power index (γ) that separates the eccentricity evolution in dynamical friction: orbits become more eccentric for γ < 3 and circularize for γ > 3. We applied these findings to the infall of a Jupiter-like planet into the envelope of its host star. The hydrostatic envelopes of stars are defined by steep density gradients near the limb and shallower gradients in the interior. Under the influence of gaseous dynamical friction, an infalling object's orbit will first decrease in eccentricity, then increase. The critical separation that delineates these regimes is predicted by the local density slope and is linearly dependent on the polytropic index. More broadly, our findings indicated that binary systems may routinely emerge from common envelope phases with non-zero eccentricities that were excited by the dynamical friction forces that drove their orbital tightening.

• NASA ADS

• arXiv

• Monthly Notices of the Royal Academic Society

We investigated the dynamical evolution of an intermediate-mass black hole (IMBH) in a nuclear star cluster hosting a supermassive black hole (SMBH) and both a spherical and a flattened disk-like distribution of stellar-mass objects. We used a direct N-body (phiGPU) and an orbit-averaged (N-ring) numerical integrator to simulate the orbital evolution of stars and the IMBH. We found that the IMBH's orbit gradually aligns with the stellar disk if their mutual initial inclination is less than 90 degrees. If it is larger than 90 degrees, i.e. counterrotating, the IMBH does not align. Initially, the rate of orbital reorientation increases linearly with the ratio of the mass of the IMBH over the SMBH mass and it is orders of magnitude faster than ordinary (i.e. Chandrasekhar) dynamical friction, particularly for high SMBH masses. The semimajor axes of the IMBH and the stars are approximately conserved. This suggested that the alignment is predominantly driven by orbit-averaged gravitational torques of the stars, a process that may be called resonant dynamical friction. The stellar disk is warped by the IMBH and ultimately increases its thickness. This process may offer a test for the viability of IMBH candidates in the Galactic Center. Resonant dynamical friction is not limited to IMBHs; any object much more massive than disk particles may ultimately align with the disk. This may have implications for the formation and evolution of black hole disks in dense stellar systems and gravitational wave source populations for LIGO, VIRGO, KAGRA, and LISA.

• NASA ADS

• arXiv

• The Astrophysical Journal

We investigated the internal dynamics of anisotropic, rotating globular clusters with a multi-mass stellar population by performing new direct N-body simulations. In addition to the well-known radial mass segregation effect, where heavy stars and stellar remnants sink toward the center of the cluster, we found mass segregation in the distribution of orbital inclinations as well. This newly discovered anisotropic mass segregation led to the formation of a disk-like structure of massive objects near the equatorial plane of a rotating cluster. This result has important implications for the expected spatial distribution of black holes in globular clusters.

• NASA ADS

• arXiv

• The Astrophysical Journal

Gravitational torques among objects orbiting a supermassive black hole drive the rapid reorientation of orbital planes in nuclear star clusters (NSCs), a process known as vector resonant relaxation. We determined the statistical equilibrium of systems with a distribution of masses, semimajor axes, and eccentricities. We averaged the interaction over the apsidal precession time and constructed a Monte Carlo Markov chain method to sample the microcanonical ensemble of the NSC. We examined the case of NSCs formed by 16 episodes of star formation or globular cluster infall. We found that the massive stars and stellar-mass black holes form a warped disk, while low mass stars resemble a spherical distribution with a possible net rotation. This may explain the origin of the clockwise disk in the Galactic center and predicts a population of black holes (BHs) embedded within this structure. The rate of mergers among massive stars, tidal disruption events of massive stars by BHs, and BH-BH mergers are highly increased in such disks. The first two may explain the origin of the observed G1 and G2 clouds, the latter may be important for gravitational wave detections with LIGO and VIRGO. More generally, black holes are expected to settle in disks in all dense spherical stellar systems assembled by mergers of smaller systems including globular clusters.

• NASA ADS

• arXiv

• Physical Review Letters

We introduced two novel time-dependent figures of merit for both online and offline optimizations of advanced gravitational-wave (GW) detector network operations with respect to (i) detecting continuous signals from known source locations and (ii) detecting GWs of neutron star binary coalescences from known local galaxies, which thereby have the highest potential for electromagnetic counterpart detection. For each of these scientific goals, we characterized an N-detector network, and all its (N−1)-detector subnetworks, to identify subnetworks and individual detectors (key contributors) that contribute the most to achieving the scientific goal. Our results showed that aLIGO-Hanford is expected to be the key contributor in 2017 to the goal of detecting GWs from the Crab pulsar within the network of LIGO and Virgo detectors. For the same time period and for the same network, both LIGO detectors were key contributors to the goal of detecting GWs from the Vela pulsar, as well as detecting signals from 10 high-interest pulsars. Key contributors to detecting continuous GWs from the Galactic Center can only be identified for finite time intervals within each sidereal day with either the 3-detector network of the LIGO and Virgo detectors in 2017, or the 4-detector network of the LIGO, Virgo, and KAGRA detectors in 2019-2020. Characterization of the LIGO-Virgo detectors with respect to the goal (ii) identified the two LIGO detectors as key contributors. Additionally, for all analyses, we identified time periods within a day when lock losses or scheduled service operations could result in the least amount of signal-to-noise or transient detection probability loss for a detector network.

• NASA ADS

• arXiv

• Classical and Quantum Gravity