Generation of highly inclined protoplanetary discs through single stellar flybys

We study the three-dimensional evolution of a viscous protoplanetary disc which is perturbed by a passing star on a parabolic orbit. The aim is to test whether a single stellar flyby is capable to excite significant disc inclinations which would favour the formation of so-called misaligned planets. We use smoothed particle hydrodynamics to study inclination, disc mass and angular momentum changes of the disc for passing stars with different masses. We explore different orbital configurations for the perturber's orbit to find the parameter spaces which allow significant disc inclination generation. Prograde inclined parabolic orbits are most destructive leading to significant disc mass and angular momentum loss. In the remaining disc, the final disc inclination is only below $20^\circ$. This is due to the removal of disc particles which have experienced the strongest perturbing effects. Retrograde inclined parabolic orbits are less destructive and can generate disc inclinations up to $60^\circ$. The final disc orientation is determined by the precession of the disc angular momentum vector about the perturber's orbital angular momentum vector and by disc orbital inclination changes. We propose a sequence of stellar flybys for the generation of misalignment angles above $60^\circ$. The results taken together show that stellar flybys are promising and realistic for the explanation of misaligned Hot Jupiters with misalignment angles up to 60\degr.Comment: 15 pages, 15 figures, accepted for publication in MNRA

Evolutionary outcomes for pairs of planets undergoing orbital migration and circularization: second order resonances and observed period ratios in Kepler's planetary systems

In order to study the origin of the architectures of low mass planetary systems, we perform numerical surveys of the evolution of pairs of coplanar planets in the mass range (1-4)\ \rmn{M}_{\oplus}. These evolve for up to 2\times10^7 \rmn{yr} under a range of orbital migration torques and circularization rates assumed to arise through interaction with a protoplanetary disc. Near the inner disc boundary, significant variations of viscosity, interaction with density waves or with the stellar magnetic field could occur and halt migration, but allow ircularization to continue. This was modelled by modifying the migration and circularization rates. Runs terminated without an extended period of circularization in the absence of migration torques gave rise to either a collision, or a system close to a resonance. These were mostly first order with a few $\%$ terminating in second order resonances. Both planetary eccentricities were small $< 0.1$ and all resonant angles liberated. This type of survey produced only a limited range of period ratios and cannot reproduce Kepler observations. When circularization alone operates in the final stages, divergent migration occurs causing period ratios to increase. Depending on its strength the whole period ratio range between $1$ and $2$ can be obtained. A few systems close to second order commensurabilities also occur. In contrast to when arising through convergent migration, resonant trapping does not occur and resonant angles circulate. Thus the behaviour of the resonant angles may indicate the form of migration that led to near resonance.Comment: 15 pages, 12 figures, 2014, MNRAS, 449, 304

On the formation of a quasi-stationary twisted disc after a tidal disruption event

We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the $\alpha$ model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from $\sim 1$ to $\sim 10^{-3}$ which is reflected in changes in the degree of disc misalignment at the stream impact location. For maximal black hole rotation and sufficiently large values of viscosity parameter $\alpha > \sim 0.01-0.1$ the ratio of the disc inclination to that of the initial stellar orbit is estimated to be $0.1-0.2$ in the advection dominated state, while reaching of order unity in the low state. Misalignment descreases with decrease of $\alpha$, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small.MXG acknowledges support through Leopoldina fellowship programme (fellowship number LPDS 2009-50). Simulations were performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service, provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council. MXG also acknowledges the computing time granted (NIC project number 8163) on the supercomputer JUROPA at Jülich Supercomputing Centre (JSC). PBI was supported in part by RFBR grants 15-02-08476 and 16-02-01043 and also by Grant of the President of the Russian Federation for Support of the Leading Scientific Schools NSh-6595.2016.2.This is the final version of the article. It first appeared from Oxford University Pressvia https://doi.org/10.1093/mnras/stw213

Dark matter dominated dwarf disc galaxy Segue 1

Several observations reveal that dwarf galaxy Segue 1 has a dark matter (DM) halo at least ~ 200 times more massive than its visible baryon mass of only ~ 103 solar masses. The baryon mass is dominated by stars with perhaps an interstellar gas mass of < 13 solar masses. Regarding Segue 1 as a dwarf disc galaxy by its morphological appearance of long stretch, we invoke the dynamic model of Xiang-Gruess, Lou & Duschl (XLD) to estimate its physical parameters for possible equilibria with and without an isopedically magnetized gas disc. We estimate the range of DM mass and compare it with available observational inferences. Due to the relatively high stellar velocity dispersion compared to the stellar surface mass density, we find that a massive DM halo would be necessary to sustain disc equilibria. The required DM halo mass agrees grossly with observational inferences so far. For an isopedic magnetic field in a gas disc, the ratio f between the DM and baryon potentials depends strongly on the magnetic field strength. Therefore, a massive DM halo is needed to counteract either the strong stellar velocity dispersion and rotation of the stellar disc or the magnetic Lorentz force in the gas disc. By the radial force balances, the DM halo mass increases for faster disc rotation.Comment: 5 pages, 2 figures, accepted for publication in MNRA

Global non-axisymmetric perturbation configurations in a composite disc system with an isopedic magnetic field: relation between dark matter halo and magnetic field

We study global non-axisymmetric stationary perturbations of aligned and unaligned logarithmic spiral configurations in an axisymmetric composite differentially rotating disc system of scale-free stellar and isopedically magnetized gas discs coupled by gravity. The gas disc is threaded across by a vertical magnetic field $B_z$ with a constant dimensionless isopedic ratio $\lambda\equiv 2\pi\sqrt{G} \Sigma^{(g)}/B_z$ of surface gas mass density $\Sigma^{(g)}$ to $B_z$ with $G$ being the gravitational constant. Our exploration focuses on the relation between $\lambda$ and the dark matter amount represented by a ratio $f\equiv\bar{\Phi}/\Phi$ in order to sustain stationary perturbation configurations, where $\bar{\Phi}$ is the gravitational potential of a presumed axisymmetric halo of dark matter and $\Phi$ is the gravitational potential of the composite disc matter. High and low $\lambda$ values correspond to relatively weak and strong magnetic fields given the same gas surface mass density, respectively. The main goal of our model analysis is to reveal the relation between isopedic magnetic fields and dark matter halo in spiral galaxies with globally stationary perturbation configurations. Our results show that, fairly strong yet realistic magnetic fields require a considerably larger amount of dark matter in aligned and unaligned cases than weak or moderate magnetic field strengths. We discuss astrophysical and cosmological implications of our findings. For examples, patterns and pattern speeds of galaxies may change during the course of galactic evolution. Multiple-armed galaxies may be more numerous in the early Universe. Flocculent galaxies may represent the transitional phase of pattern variations in galaxies.Comment: 21 pages, 5 figures, accepted for publication in MNRA

Dark Matter and Fundamental Physics with the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a project for a next-generation observatory for very high energy (GeV-TeV) ground-based gamma-ray astronomy, currently in its design phase, and foreseen to be operative a few years from now. Several tens of telescopes of 2-3 different sizes, distributed over a large area, will allow for a sensitivity about a factor 10 better than current instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few tens of GeV to several tens of TeV, and a field of view of up to 10 deg. In the following study, we investigate the prospects for CTA to study several science questions that influence our current knowledge of fundamental physics. Based on conservative assumptions for the performance of the different CTA telescope configurations, we employ a Monte Carlo based approach to evaluate the prospects for detection. First, we discuss CTA prospects for cold dark matter searches, following different observational strategies: in dwarf satellite galaxies of the Milky Way, in the region close to the Galactic Centre, and in clusters of galaxies. The possible search for spatial signatures, facilitated by the larger field of view of CTA, is also discussed. Next we consider searches for axion-like particles which, besides being possible candidates for dark matter may also explain the unexpectedly low absorption by extragalactic background light of gamma rays from very distant blazars. Simulated light-curves of flaring sources are also used to determine the sensitivity to violations of Lorentz Invariance by detection of the possible delay between the arrival times of photons at different energies. Finally, we mention searches for other exotic physics with CTA.Comment: (31 pages, Accepted for publication in Astroparticle Physics

A Quasi-Stationary Twisted Disk Formed as a Result of a Tidal Disruption Event

In this note we briefly review the main results of our recent study of the formation of misaligned accretion disks after the tidal disruption of stars by rotating supermassive black holes. Since the accretion rates in such disks initially exceed the Eddington limit they are initially advection dominated. Assuming the α model for the disk viscosity implies that the disk can become thermally unstable when the accretion rate is comparable to, or smaller than, the Eddington value, while still being radiation pressure dominated. It then undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from ~1 to ~10−3, which is reflected in changes in the degree of disk misalignment at the stream impact location. For maximal black hole rotation and sufiociently large values of the viscosity parameter, α ≳ 0.01–0.1, the ratio of the disk inclination to that of the initial stellar orbit is estimated to be 0.1–0.2 in the advection dominated state, while reaching order unity in the low state. Misalignment decreases with decrease of α, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small