DRUM Community: Astronomy

Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback

2012-10-12T02:33:19Z

Title: Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback Authors: Park, KwangHo Abstract: This thesis focuses on radiation-regulated gas accretion onto black holes (BHs) from galactic scales emphasizing the role of thermal and radiation pressure in limiting gas supply to the BH. Assuming quasi-spherical symmetry, we explore how the gas accretion depends on free parameters such as radiative efficiency, BH mass, ambient gas density/temperature, and the spectral index of the radiation. Our numerical simulations show an oscillatory behavior of the accretion rate, and thus the luminosity from the BH. We present a model for the feedback loop and provide analytical relationships for the average/maximum accretion rate and the period of the accretion bursts. The thermal structure inside the str sphere is a key factor for the regulation process, while with increasing ambient gas density and mass of BHs eventually the accretion rate becomes limited by radiation pressure. The period of the luminosity bursts is proportional to the average size of the ionized hot bubble, but we discover that there are two distinct modes of oscillations with very different duty cycles that are governed by different depletion processes of the gas inside the ionized bubble. We also study how angular momentum of the gas affects the accretion process. In the second part of the thesis, we study the growth rate and luminosity of BHs in motion with respect to their surrounding medium. Contrary to the case without radiation feedback, we find that the accretion rate increases with increasing BH velocity, v, reaching a maximum value at v ~ 20-30 km/s, before decreasing as v^{-3}. The increase of the accretion rate with v is produced by the formation of a D-type (density) ionization front (I-front) preceded by a standing bow-shock that reduces the downstream gas velocity to nearly sub-sonic values. Interestingly, there is a range of densities and velocities in which the dense shell downstream of the bow-shock is unstable; its central part is destroyed and reformed intermittingly, producing a periodic accretion rate with peak values about 10 times the mean. This effect can significantly increase the detectability of accreting intermediate mass BHs from the interstellar medium (ISM) in nearby galaxies. We find that the maximum accretion rate for a moving BH is larger than that of a stationary BH of the same mass, accreting from the same medium, if the medium temperature is T<10^4 K. This result could have an important impact on our understanding of the growth of seed BHs in the multi-phase medium of the first galaxies and for building and early X-ray background that may affect the formation of the first galaxies and the reionization process.

Magnetic Field Effects on the Motion of Circumplanetary Dust

2012-10-12T02:32:23Z

Title: Magnetic Field Effects on the Motion of Circumplanetary Dust Authors: Jontof-Hutter, Daniel Abstract: Hypervelocity impacts on satellites or ring particles replenish circumplanetary dusty rings with grains of all sizes. Due to interactions with the plasma environment and sunlight, these grains become electrically charged. We study the motion of charged dust grains launched at the Kepler orbital speed, under the combined effects of gravity and the electromagnetic force. We conduct numerical simulations of dust grain trajectories, covering a broad range of launch distances from the planetary surface to beyond synchronous orbit, and the full range of charge-to-mass ratios from ions to rocks, with both positive and negative electric potentials. Initially, we assume that dust grains have a constant electric potential, and, treating the spinning planetary magnetic field as an aligned and centered dipole, we map regions of radial instability (positive grains only), where dust grains are driven to escape or collide with the planet at high speed, and vertical instability (both positive and negative charges) whereby grains launched near the equatorial plane and are forced up magnetic field lines to high latitudes, where they may collide with the planet. We derive analytical criteria for local stability in the equatorial plane, and solve for the boundaries between all unstable and stable outcomes. Comparing our analytical solutions to our numerical simulations, we develop an extensive model for the radial, vertical and azimuthal motions of dust grains of arbitrary size and launch location. We test these solutions at Jupiter and Saturn, both of whose magnetic fields are reasonably well represented by aligned dipoles, as well as at the Earth, whose magnetic field is close to an anti-aligned dipole. We then evaluate the robustness of our stability boundaries to more general conditions. Firstly, we examine the effects of non-zero launch speeds, of up to 0.5 km s$^{-1}$, in the frame of the parent body. Although these only weakly affect stability boundaries, we find that the influence of a launch impulse on stability boundaries strongly depends on its direction. Secondly, we focus on the effects of higher-order magnetic field components on orbital stability. We find that vertical stability boundaries are particularly sensitive to a moderate vertical offset in an aligned dipolar magnetic field. This configuration suffices as a model for Saturn's full magnetic field. The vertical instability also expands to cover a wider range of launch distances in slightly tilted magnetic dipoles, like the magnetic field configurations for Earth and Jupiter. By contrast, our radial stability criteria remain largely unaffected by both dipolar tilts and vertical offsets. Nevertheless, a tilted dipole magnetic field model introduces non-axisymmetric forces on orbiting dust grains, which are exacerbated by the inclusion of other higher-order magnetic field components, including the quadrupolar and octupolar terms. Dust grains whose orbital periods are commensurate with the spatial periodicities of a rotating non-axisymmetric magnetic field experience destabilizing Lorentz resonances. These have been studied by other authors for the largest dust grains moving on perturbed Keplerian ellipses. With Jupiter's full magnetic field as our model, we extend the concept of Lorentz resonances to smaller dust grains and find that these can destabilize trajectories on surprisingly short timescales, and even cause negatively-charged dust grains to escape within weeks. We provide detailed numerically-derived stability maps highlighting the destabilizing effects of specific higher-order terms in Jupiter's magnetic field, and we develop analytical solutions for the radial locations of these resonances for all charge-to-mass ratios. We include stability maps for the full magnetic field configurations of Jupiter, Saturn, and Earth, to compare with our analytics. We further provide numerically-derived stability maps for the tortured magnetic fields of Uranus and Neptune. Relaxing the assumption of constant electric charges on dust, we test the effects of time-variable grain charging on dust grain motion in two distinct environments. Firstly, we examine orbital stability in the tenuous plasma of Jupiter's main ring and gossamer ring where sunlight, the dominant source of grain charging, is periodically interrupted by transit through the planetary shadow. This dramatically expands dynamical instabilities to cover a large range of grain sizes. Secondly, we study the motion of dust grain orbits in the dense plasma environment of the Io torus. Here dust grain charges deviate little from equilibrium, and our stability map conforms closely to that of constant, negatively-charged dust grains. Finally, we focus on the poorly understood spokes in Saturn's B ring, highlighting the observational constraints on spokes, and present our hypothesis for spoke formation.

N-body Simulations with Cohesion in Dense Planetary Rings

2012-02-18T03:33:11Z

Title: N-body Simulations with Cohesion in Dense Planetary Rings Authors: Perrine, Randall Abstract: This dissertation is primarily focused on exploring whether weak cohesion among icy particles in Saturn's dense rings is consistent with observations--and if so, what limits can be placed on the strength of such cohesive bonds, and what dynamical or observable consequences might arise out of cohesive bonding. Here I present my numerical method that allows for N-body particle sticking within a local rotating frame ("patch")--an approach capable of modeling hundreds of thousands or more colliding bodies. Impacting particles can stick to form non-deformable but breakable aggregates that obey equations of rigid body motion. I then apply the method to Saturn's icy rings, for which laboratory experiments suggest that interpenetration of thin, frost-coated surface layers may lead to weak bonding if the bodies impact at low speeds--speeds that happen to be characteristic of the rings. This investigation is further motivated by observations of structure in the rings that could be formed through bottom-up aggregations of particles (i.e., "propellers" in the A ring, and large-scale radial structure in the B ring). This work presents the implementation of the model, as well as results from a suite of 100 simulations that investigate the effects of five parameters on the equilibrium characteristics of the rings: speed-based merge and fragmentation limits, bond strength, ring surface density, and patch orbital distance (specifically the center of either the A or B ring), some with both monodisperse and polydisperse particle comparison cases. I conclude that the presence of weak cohesion is consistent with observations of the A and B rings, and present a range of parameters that reproduce the observed size distribution and maximum particle size. The parameters that match observations differ between the A and B rings, and I discuss the potential implications of this result. I also comment on other observable consequences of cohesion for the rings, such as optical depth and scale height effects, and discuss the unlikelihood that very large objects are grown bottom-up from cohesion of smaller ring particles. Lastly, I include a brief summary of other projects in ring dynamics I have undertaken before and during my thesis work.

The Dynamics of Dense Stellar Systems with a Massive Black Hole

2011-10-09T02:33:22Z

Title: The Dynamics of Dense Stellar Systems with a Massive Black Hole Authors: Gill, Michael Allen Abstract: In this work, we explore the dynamics of two similar types of dense stellar systems with a central black hole of mass much greater than a typical stellar object. In particular, we use numerical N-body simulations to examine the effects that the massive black hole (MBH) has on the surrounding stars and compact objects as they pertain to indirectly observable signals. The first systems we consider are the highly uncertain cusps likely comprised of primarily massive compact objects that surround the MBHs at the center of typical galaxies. The gradual inspiral of a compact object by emission of gravitational radiation, called an extreme mass-ratio inspiral (EMRI), will produce a signal that falls in the peak detection range of the space-bound laser interferometer space antenna (LISA). Despite a veritable gold mine of astrophysical data that could be gleaned from such a detection, previous investigations in the literature have left the predicted rate of these events uncertain by several orders of magnitude. We present direct N-body simulations of the innermost ≤ 100 objects with the inclusion of the first-order Post-Newtonian correction with the aim of reducing one of the key uncertainties in the dynamics of these systems - the efficiency of resonant relaxation. We find that relativistic pericenter precession prevents a significant enhancement of the EMRI rate; the rate we derive during this work is consistent with those derived in the literature from less direct methods. We do find, however, that our EMRI progenitors originate from much closer to the MBH than previous investigations have suggested was likely. Our second investigation delves into the possibility of finding intermediate-mass black holes (IMBHs), with masses ∼ 102−4 M_sun, at the center of dense star clusters. Because of the substantial investment of telescope time needed to perform the multiyear proper motion studies that are likely needed to achieve a definitive detection, careful selection of candidate clusters is prudent. We provide a new observational signature of the presence of an IMBH in a dense star cluster - a quenching of mass segregation. Our ensemble of direct N-body simulations with N ≤ 32768 objects and highly varied initial conditions show that the existence of an IMBH with mass ∼ 1% of the total cluster mass limits the mass segregation in visible stars, as measured by the radial gradient in average stellar mass. This effect is consistently visible in systems that have had enough time to reach their equilibrium value of mass segregation, usually about 5 initial half-mass relaxation times. In practical terms, our method will apply to Galactic globular clusters that are fairly small, and that are unlikely to have lost a significant portion of their mass to Galactic tidal stripping. We apply this method to two of the ∼ 30 Galactic globular clusters that fit our conservative criteria for application of this method, NGC 2298 and NGC 6254(M10). Thanks to deep observations by the Hubble Space Telescope Advanced Camera for Surveys, data exist that are sufficient to allow a good comparison to our simulation data. We find that the degree of mass segregation we observe in NGC 2298 is clearly inconsistent with simulations harboring an IMBH at about the 3−σ level. In contrast, application of the method to NGC 6254 reveals a mass segregation profile that can only be explained by the presence of either an IMBH or a significant population of primordial binaries (≥ 5%). Unfortunately, a reliable measure of the binary fraction of NGC 6254 does not exist; however, NGC 6254 is a good candidate for follow-up proper motion studies.