arXiv subjects

Adam Frank

Publications and source records attributed to Adam Frank.

At least 73 records · Page 4Linked to original sources

arXiv2016

Molecular cloud formation in high-shear, magnetized colliding flows

The colliding flows (CF) model is a well-supported mechanism for generating molecular clouds. However, to-date most CF simulations have focused on the formation of clouds in the normal-shock layer between head-on colliding flows. We performed simulations of magnetized colliding flows that instead meet at an oblique-shock layer. Oblique shocks generate shear in the post-shock environment, and this shear creates inhospitable environments for star formation. As the degree of shear increases (i.e. the obliquity of the shock increases), we find that it takes longer for sink particles to form, they form in lower numbers, and they tend to be less massive. With regard to magnetic fields, we find that even a weak field stalls gravitational collapse within forming clouds. Additionally, an initially oblique collision interface tends to reorient over time in the presence of a magnetic field, so that it becomes normal to the oncoming flows. This was demonstrated by our most oblique shock interface, which became fully normal by the end of the simulation.

astro-ph.GA↗

arXiv2016

Hot planetary winds near a star: dynamics, wind-wind interactions, and observational signatures

Signatures of "evaporative" winds from exo-planets on short (hot) orbits around their host star have been observed in a number of systems. In this paper we present global AMR simulations that track the launching of the winds, their expansion through the circumstellar environment, and their interaction with a stellar wind. We focus on purely hydrodynamic flows including the anisotropy of the wind launching and explore the orbital/fluid dynamics of the resulting flows in detail. In particular we find that a combination of the tidal and Coriolis forces strongly distorts the planetary "Parker" wind creating "up-orbit" and "down-orbit" streams. We characterize the flows in terms of their orbital elements which change depending on their launch position on the planet. We find that the anisotropy in the atmospheric temperature leads to significant backflow on to the planet. The planetary wind interacts strongly with the stellar wind creating instabilities that cause eventual deposition of planetary gas onto the star. We present synthetic observations of both transit and absorption line-structure for our simulations. For our initial conditions, we find that the orbiting wind material produces absorption signatures at significant distances from the planet and substantial orbit to orbit variability. Ly-α absorption shows red and blueshifted features out to 70 km/s. Finally, using semi-analytic models we constrain the effect of radiation pressure, given the approximation of uniform stellar absorption.

astro-ph.EP↗

arXiv2016

The inner cavity of the circumnuclear disc

The circumnuclear disc (CND) orbiting the Galaxy's central black hole is a reservoir of material that can ultimately provide energy through accretion, or form stars in the presence of the black hole, as evidenced by the stellar cluster that is presently located at the CND's centre. In this paper, we report the results of a computational study of the dynamics of the CND. The results lead us to question two paradigms that are prevalent in previous research on the Galactic Centre. The first is that the disc's inner cavity is maintained by the interaction of the central stellar cluster's strong winds with the disc's inner rim, and second, that the presence of unstable clumps in the disc implies that the CND is a transient feature. Our simulations show that, in the absence of a magnetic field, the interaction of the wind with the inner disc rim actually leads to a filling of the inner cavity within a few orbital time-scales, contrary to previous expectations. However, including the effects of magnetic fields stabilizes the inner disc rim against rapid inward migration. Furthermore, this interaction causes instabilities that continuously create clumps that are individually unstable against tidal shearing. Thus the occurrence of such unstable clumps does not necessarily mean that the disc is itself a transient phenomenon. The next steps in this investigation are to explore the effect of the magnetorotational instability on the disc evolution and to test whether the results presented here persist for longer time-scales than those considered here.

astro-ph.GA↗

arXiv2016

Uncertain for A Century: Quantum Mechanics and The Dilemma of Interpretation

Quantum Mechanics, the physical theory describing the microworld, represents one of science's greatest triumphs. It lies at the root of all modern digital technologies and offers unparalleled correspondence between prediction and experiments. Remarkably, however, after more than 100 years it is still unclear what quantum mechanics means in terms of basic philosophical questions about the nature of reality. While there are many interpretations of the mathematical machinery of quantum physics, there remains no experimental means to distinguish between most of them. In this contribution, (based on a discussion at the NYAS), I wish to consider the ways in which the enduring lack of an agreed upon interpretation of quantum physics influences a number of critical philosophical debates about physics and reality. I briefly review two problems effected by quantum interpretations: the meaning of the term 'Universe' and the nature of consciousness. In what follows I am explicitly not advocating for any particular quantum interpretation. Instead, I am interested in how the explicit inability of modern physics to experimentally distinguish between interpretations with wildly divergent ontological/epistemological implications plays into discussions of physics and its description of the world.

physics.hist-ph↗

arXiv2016

The Creation of AGB Fallback Shells

The possibility that mass ejected during Asymptotic Giant Branch (AGB) stellar evolution phases falls back towards the star has been suggested in applications ranging from the formation of accretion disks to the powering of late-thermal pulses. In this paper, we seek to explicate the properties of fallback flow trajectories from mass-loss events. We focus on a transient phase of mass ejection with sub-escape speeds, followed by a phase of a typical AGB wind. We solve the problem using both hydrodynamic simulations and a simplified one-dimensional analytic model that matches the simulations. For a given set of initial wind characteristics, we find a critical shell velocity that distinguishes between "shell fallback" and "shell escape". We discuss the relevance of our results for both single and binary AGB stars. In particular, we discuss how our results help to frame further studies of fallback as a mechanism for forming the substantial population of observed post-AGB stars with dusty disks.

astro-ph.SR↗

arXiv2015

A New Empirical Constraint on the Prevalence of Technological Species in the Universe

In this paper we address the cosmic frequency of technological species. Recent advances in exoplanet studies provide strong constraints on all astrophysical terms in the Drake Equation. Using these and modifying the form and intent of the Drake equation we show that we can set a firm lower bound on the probability that one or more additional technological species have evolved anywhere and at any time in the history of the observable Universe. We find that as long as the probability that a habitable zone planet develops a technological species is larger than ~$10^{-24}$, then humanity is not the only time technological intelligence has evolved. This constraint has important scientific and philosophical consequences.

astro-ph.EP↗

arXiv2014

Numerical Simulations of Mach Stem Formation via Intersecting Bow Shocks

Hubble Space Telescope observations show bright knots of H$α$ emission within outflowing young stellar jets. Velocity variations in the flow create secondary bow shocks that may intersect and lead to enhanced emission. When the bow shocks intersect at or above a certain critical angle, a planar shock called a Mach stem is formed. These shocks could produce brighter H$α$ emission since the incoming flow to the Mach stem is parallel to the shock normal. In this paper we report first results of a study using 2-D numerical simulations designed to explore Mach stem formation at the intersection of bow shocks formed by hypersonic "bullets" or "clumps". Our 2-D simulations show how the bow shock shapes and intersection angles change as the adiabatic index $γ$ changes. We show that the formation or lack of a Mach stem in our simulations is consistent with the steady-state Mach stem formation theory. Our ultimate goal, which is part of an ongoing research effort, is to characterize the physical and observational consequences of bow shock intersections including the formation of Mach stems.

astro-ph.HE↗

arXiv2014

MHD Effects on Pulsed YSO Jets. I. 2.5-D Simulations

In this paper we explore the dynamics of radiative axisymmetric MHD jets at high resolution using AMR methods. The goal of the study is to determine both the dynamics and emission properties of such jets. To that end we have implemented microphysics enabling us to produce synthetic maps of H$α$ and [S II]. The jets are pulsed either sinusoidally or randomly via a time-dependent ejection velocity which leads to a complicated structure of internal shocks and rarefactions as has been seen in previous simulations. The high resolution of our simulations allows us to explore in great detail the effect of pinch forces (due to the jet's toroidal magnetic field) within the "working surfaces" where pulses interact. We map the strong H$α$ emission marking shock fronts and the strong [S II] emission inside cooling regions behind shocks as observed with high-resolution images of jets. We find that pinch forces in the stronger field cases produce additional emission regions along the axis as compared with purely hydrodynamic runs. These simulations are a first step to understanding the full 3-D emission properties of radiative MHD jets.

astro-ph.HE↗

arXiv2014

On the role of ambient environments in the collapse of Bonnor-Ebert spheres

We consider the interaction between a marginally stable Bonnor-Ebert (BE) sphere and the surrounding ambient medium. In particular, we explore how the infall from an evolving ambient medium can trigger collapse of the sphere using 3-D adaptive mesh refinement simulations. We find the resulting collapse dynamics to vary considerably with ambient density. In the highest ambient density cases, infalling material drives a strong compression wave into the cloud. It is the propagation of this wave through the cloud interior that triggers the subsequent collapse. For lower ambient densities, we find the main trigger of collapse to be a quasistatic adjustment of the BE sphere to gravitational settling of the ambient gas. In all cases, we find that the classic "outside-in" collapse mode for super-critical BE spheres is recovered before a protostar (i.e. sink particle) forms. Our work supports scenarios in which BE dynamics naturally begins with either a compression-wave or infall dominated phase, and only later assumes the usual outside-in collapse behavior.

astro-ph.SR↗

arXiv2014

Triggered Star Formation and Its Consequences

Star formation can be triggered by compression from wind or supernova driven shock waves that sweep over molecular clouds. Because these shocks will likely contain processed elements, triggered star formation has been proposed as an explanation for short lived radioactive isotopes (SLRI) in the Solar System. Previous studies have tracked the triggering event to the earliest phases of collapse and have focused on the shock properties required for both successful star formation and mixing of SLRI's. In this paper, we use Adaptive Mesh Refinement (AMR) simulation methods, including sink particles, to simulate the full collapse and subsequent evolution of a stable Bonnor-Ebert sphere subjected to a shock and post-shock wind. We track the flow of the cloud material after a star (a sink particle) has formed. For non-rotating clouds we find robust triggered collapse and little bound circumstellar material remaining around the post-shock collapsed core. When we add initial cloud rotation we observe the formation of disks around the collapsed core which then interact with the post-shock flow. Our results indicate that these circumstellar disks are massive enough to form planets and are long-lived, in spite of the ablation driven by post-shock flow ram pressure. As a function of the initial conditions, we also track the time evolution of the accretion rates and particle mixing between between the ambient wind and cloud material. The latter is maximized for cases of highest mach number.

astro-ph.SR↗

arXiv2013

Sustainability and the Astrobiological Perspective: Framing Human Futures in a Planetary Context

We explore how questions related to developing a sustainable human civilization can be cast in terms of astrobiology. In particular we show how ongoing astrobiological studies of the coupled relationship between life, planets and their co-evolution can inform new perspectives and direct new studies in sustainability science. Using the Drake Equation as a vehicle to explore the gamut of astrobiology, we focus on its most import factor for sustainability: the mean lifetime of an ensemble of Species with Energy-Intensive Technology (SWEIT). We then cast the problem into the language of dynamical system theory and introduce the concept of a trajectory bundle for SWEIT evolution and discuss how astrobiological results usefully inform the creation of dynamical equations, their constraints and initial conditions. Three specific examples of how astrobiological considerations can be folded into discussions of sustainability are discussed: (1) concepts of planetary habitability, (2) mass extinctions and their possible relation to the current, so-called Anthropocene epoch, and (3) today's changes in atmospheric chemisty (and the climate change it entails) in the context of pervious epochs of biosphere-driven atmospheric and climate alteration (i.e. the Great Oxidation Event).

astro-ph.EP↗

arXiv2013

Size of discs formed by wind accretion in binaries can be underestimated if the role of wind-driving force is ignored

Binary systems consisting of a secondary accreting form a wind-emitting primary are ubiquitous in astrophysics. The phenomenology of such Bondi-Hoyle-Lyttleton (BHL) accretors is particularly rich when an accretion disc forms around the secondary. The outer radius of such discs is commonly estimated from the net angular momentum produced by a density variation of material across the BHL or Bondi accretion cylinder, as the latter is tilted with respect to the direction to the primary due to orbital motion. But this approach has ignored the fact that the wind experiences an outward driving force that the secondary does not. In actuality, the accretion stream falls toward a retarded point in the secondary's orbit as the secondary is pulled toward the primary relative to the stream. The result is a finite separation or "accretion stream impact parameter" (ASIP) separating the secondary and stream. When the orbital radius a_o exceeds the BHL radius r_b, the ratio of outer disc radius estimated as the ASIP to the conventional estimate a_o^{1/2}/r_b^{1/2}>1. We therefore predict that discs will form at larger radii from the secondary than traditional estimates. This agrees with the importance of the ASIP emphasized by Huarte-Espinosa et al. (2013) and the practical consequence that resolving the initial outer radius of such an accretion disc in numerical simulations can be less demanding than what earlier estimates would suggest.

astro-ph.HE↗

arXiv2013

MHD Shock-Clump Evolution with Self-Contained Magnetic Fields

We study the interaction of strong shock waves with magnetized clumps. Previous numerical work focused on the simplified scenario in which shocked clumps are immersed in a globally uniform magnetic field that extends through both the clump and the ambient medium. Here we consider the complementary circumstance in which the field is completely self-contained within the clumps. This could arise naturally during clump formation via dynamical or thermal instabilities for example as magnetic field pinches off from the ambient medium. Using our AMR MHD code AstroBEAR, we carry out a series of simulations with magnetized clumps that have different self-contained magnetic field configurations. We find that the clump and magnetic evolution are sensitive to the fraction of magnetic field aligned with versus perpendicular to the shock normal. The relative strength of magnetic pressure and tension in the different field configurations allows us to analytically understand the different cases of post-shock evolution. We also show how turbulence and the mixing it implies depends of the initial field configuration and suggest ways in which observed shock-clump morphology may be used as a proxy for identifying internal field topologies a posteriori.

astro-ph.SR↗

arXiv2013

Outflows from Evolved Stars: The Rapidly Changing Fingers of CRL618

Our ultimate goal is to probe the nature of the collimator of the outflows in the pre PN CRL618. CRL618 is uniquely suited for this purpose owing to its multiple, bright, and carefully studied finger-shaped outflows east and west of its nucleus. We compare new HST images to images in the same filters observed as much as 11 y previously to uncover large proper motions and surface brightness changes in its multiple finger-shaped outflows. The expansion age of the ensemble of fingers is close to 100y. We find strong brightness variations at the fingertips during the past decade. Deep IR images reveal a multiple ring- like structure of the surrounding medium into which the outflows propagate and interact. Tightly constrained three-dimensional ("3D") hydrodynamic models link the properties of the fingers to their possible formation histories. We incorporate previously published complementary information to discern whether each of the fingers of CRL618 are the results of steady, collimated outflows or a brief ejection event that launched a set of bullets about a century ago. Finally, we argue on various physical grounds that fingers of CRL618 are likely to be the result of a spray of clumps ejected at the nucleus of CRL618 since any mechanism that form a sustained set of unaligned jets is unprecedented.

astro-ph.SR↗

arXiv2013

The Formation and Evolution of Wind-Capture Disks In Binary Systems

We study the formation, evolution and physical properties of accretion disks formed via wind capture in binary systems. Using the AMR code AstroBEAR, we have carried out high resolution 3D simulations that follow a stellar mass secondary in the co-rotating frame as it orbits a wind producing AGB primary. We first derive a resolution criteria, based on considerations of Bondi-Hoyle flows, that must be met in order to properly resolve the formation of accretion disks around the secondary. We then compare simulations of binaries with three different orbital radii (10, 15, 20 AU). Disks are formed in all three cases, however the size of the disk and, most importantly, its accretion rate decreases with orbital radii. In addition, the shape of the orbital motions of material within the disk becomes increasingly elliptical with increasing binary separation. The flow is mildly unsteady with "fluttering" around the bow shock observed. The disks are generally well aligned with the orbital plane after a few binary orbits. We do not observe the presence of any large scale, violent instabilities (such as the flip-flop mode). For the first time, moreover, it is observed that the wind component that is accreted towards the secondary has a vortex tube-like structure, rather than a column-like one as it was previously thought. In the context of AGB binary systems that might be precursors to Pre-Planetary and Planetary Nebula, we find that the wind accretion rates at the chosen orbital separations are generally too small to produce the most powerful outflows observed in these systems if the companions are main sequence stars but marginally capable if the companions are white dwarfs. It is likely that many of the more powerful PPN and PN involve closer binaries than the ones considered here. The results also demonstrate principles of broad relevance to all wind-capture binary systems.

astro-ph.SR↗

arXiv2013

The Effects of Inhomogeneities within Colliding Flows on the Formation and Evolution of Molecular Clouds

Observational evidence from local star-forming regions mandates that star formation occurs shortly after, or even during, molecular cloud formation. Models of the formation of molecular clouds in large-scale converging flows have identified the physical mechanisms driving the necessary rapid fragmentation. They also point to global gravitational collapse driving supersonic turbulence in molecular clouds. Previous cloud formation models have focused on turbulence generation, gravitational collapse, magnetic fields, and feedback. Here, we explore the effect of structure in the flow on the resulting clouds and the ensuing gravitational collapse. We compare two extreme cases, one with a collision between two smooth streams, and one with streams containing small clumps. We find that structured converging flows lead to a delay of local gravitational collapse ("star formation"). Thus, more gas has time to accumulate, eventually leading to a strong global collapse, and thus to a high star formation rate. Uniform converging flows fragment hydrodynamically early on, leading to the rapid onset of local gravitational collapse and an overall low sink formation rate.

astro-ph.GA↗

arXiv2013

Efficient Parallelization for AMR MHD Multiphysics Calculations; Implementation in AstroBEAR

Current Adaptive Mesh Refinement (AMR) simulations require algorithms that are highly parallelized and manage memory efficiently. As compute engines grow larger, AMR simulations will require algorithms that achieve new levels of efficient parallelization and memory management. We have attempted to employ new techniques to achieve both of these goals. Patch or grid based AMR often employs ghost cells to decouple the hyperbolic advances of each grid on a given refinement level. This decoupling allows each grid to be advanced independently. In AstroBEAR we utilize this independence by threading the grid advances on each level with preference going to the finer level grids. This allows for global load balancing instead of level by level load balancing and allows for greater parallelization across both physical space and AMR level. Threading of level advances can also improve performance by interleaving communication with computation, especially in deep simulations with many levels of refinement. While we see improvements of up to 30% on deep simulations run on a few cores, the speedup is typically more modest (5-20%) for larger scale simulations. To improve memory management we have employed a distributed tree algorithm that requires processors to only store and communicate local sections of the AMR tree structure with neighboring processors. Using this distributed approach we are able to get reasonable scaling efficiency (> 80%) out to 12288 cores and up to 8 levels of AMR - independent of the use of threading.

astro-ph.SR↗

arXiv2013

Mass loading and knot formation in AGN jets by stellar winds

Jets from active galaxies propagate from the central black hole out to the radio lobes on scales of hundreds of kiloparsecs. The jets may encounter giant stars with strong stellar winds and produce observable signatures. For strong winds and weak jets, the interaction may truncate the jet flow during its transit via the mass loading. For weaker jets, the interaction can produce knots in the jet. We present recent 3DMHD numerical simulations to model the evolution of this jet-wind interaction and its observational consequences. We explore (i) the relative mechanical luminosity of the radio jets and the stellar winds (ii) the impact parameter between the jets' axis and the stellar orbital path (iii) the relative magnetic field strength of the jets and the stellar winds.

astro-ph.HE↗