79 research outputs found
Probing self-gravitating protostellar discs using smoothed particle hydrodynamics and radiative transfer
No full textStars are likely to form with non-zero initial angular momentum, and will consequently
possess a substantial gaseous protostellar disc in the early phases of their evolution.
At this early stage, the disc mass is expected to be comparable to the mass of the
protostar. The disc’s self-gravity therefore plays an important role in the
subsequent evolution of the system, regulating the accretion of matter onto the
protostar, as well as being potentially capable of forming low mass stars and massive
planets by disc fragmentation. The protostellar disc may later evolve into a protoplanetary
disc, providing the feedstock for planet formation. Therefore, if the current
stellar populations and exoplanetary systems are to be understood, an understanding
of the evolution of protostellar discs is crucial, especially their earliest self-gravitating
phases. I have used various methods of numerical simulation to probe the physics of
self-gravitating protostellar discs and their constituents.
When constructing a model for self-gravitating protostellar discs, including detailed
thermodynamics and radiative transfer is essential. I have developed two distinct numerical
techniques for incorporating radiative transfer into Smoothed Particle Hydrodynamics
(SPH) simulations. The first allows the modelling of frequency-averaged
radiative transfer during the SPH simulation, in effect approximating radiative SPH
(RSPH) with only a marginal increase in runtime (around 6%). The second takes the
output from SPH simulations, and creates synthetic, wavelength-dependent telescope
images and spectra of SPH systems. This allows the direct construction of observables
from SPH simulations, providing, for the first time, a direct connection between the
output of SPH simulations and observations.
I have used these numerical methods to analyse, in detail, the local angular momentum
transport induced by self-gravity in protostellar discs, testing the robustness
of the “pseudo-viscous” analytical approximation for local disc stresses. I confirm that
semi-analytical disc modellers are justified in using the pseudo-viscous approximation
in some cases, but I also outline the limits in which non-local transport effects causes
the approximation to fail.
Also, I have investigated the evolution of protostellar discs when perturbed by a
secondary companion, in particular identifying whether such events will in general
trigger a) a disc fragmentation event, or b) a stellar outburst event. For case a), I found
no significant evidence that perturbation by a companion improves the possibility of
disc fragmentation in compact discs - in case b), I found that stellar outburst events do
indeed occur, but they are unlikely to be seen by observers due to their rare occurrence,
as well as due to self-obscuration effects
The nature of angular momentum transport in radiative self-gravitating protostellar discs
No full textSemi-analytic models of self-gravitating discs often approximate the angular momentum transport generated by the gravitational instability using the phenomenology of viscosity. This allows the employment of the standard viscous evolution equations, and gives promising results. It is, however, still not clear when such an approximation is appropriate.
This paper tests this approximation using high-resolution 3D smoothed particle hydrodynamics (SPH) simulations of self-gravitating protostellar discs with radiative transfer. The nature of angular momentum transport associated with the gravitational instability is characterized as a function of both the stellar mass and the disc-to-star mass ratio. The effective viscosity is calculated from the Reynolds and gravitational stresses in the disc. This is then compared to what would be expected if the effective viscosity were determined by assuming local thermodynamic equilibrium or, equivalently, that the local dissipation rate matches the local cooling rate.
In general, all the discs considered here settle into a self-regulated state where the heating generated by the gravitational instability is modulated by the local radiative cooling. It is found that low-mass discs can indeed be represented by a local α-parametrization, provided that the disc aspect ratio is small (H/r≤ 0.1) which is generally the case when the disc-to-star mass ratio q≲ 0.5. However, this result does not extend to discs with masses approaching that of the central object. These are subject to transient burst events and global wave transport, and the effective viscosity is not well modelled by assuming local thermodynamic equilibrium. In spite of these effects, it is shown that massive (compact) discs can remain stable and not fragment, evolving rapidly to reduce their disc-to-star mass ratios through stellar accretion and radial spreading
The dynamical fate of self-gravitating disc fragments after tidal downsizing
No full textDF and KR acknowledge support from STFC grant ST/J001422/1. DF also acknowledges support from the ECOGAL ERC Advanced Grant Programme. RJP acknowledges support from the Royal Astronomical Society in the form of a research fellowship.The gravitational instability model of planet/brown dwarf formation proposes that protostellar discs can fragment into objectswith masses above a few Jupiter masses at large semimajor axis. Tidal downsizing may reduce both the object mass and semimajor axis. However, most studies of tidal downsizing end when the protostellar disc disperses, while the system is embedded in its parent star-forming region. To compare disc fragment descendants with exoplanet and brown dwarf observations, the subsequent dynamical evolution must be explored. We carry out N-body integrations of fragment-fragment scattering in multi-object star systems, and star systems embedded in substructured clusters. In both cases, we use initial conditions generated by population synthesis models of tidal downsizing. The scattering simulations produce a wide range of eccentricities. The ejection rate is around 25 per cent. The ejecta mass distribution is similar to that for all objects, with a velocity dispersion consistent with those produced by full hydrodynamic simulations. The semimajor axis distribution after scattering extends to parsec scales. In the cluster simulations, 13 per cent of the objects are ejected from their planetary system, and around 10 per cent experience significant orbit modification. A small number of objects are recaptured on high-eccentricity, high-inclination orbits. The velocity distribution of ejecta is similar to that produced by fragment-fragment scattering. If fragment-fragment scattering and cluster stripping act together, then disc fragmentation should be efficient at producing free-floating substellar objects, and hence characterizing the free-floating planet population will provide strong constraints on the frequency of disc fragmentation.Peer reviewe
Exoplanet transits as the foundation of an interstellar communications network
No full textThe author gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG, and the STFC grant ST/J001422/1.Two fundamental problems for extraterrestrial intelligences (ETIs) attempting to establish interstellar communication are timing and energy consumption. Humanity's study of exoplanets via their transit across the host star highlights a means of solving both problems. An ETI ‘A’ can communicate with ETI ‘B’ if B is observing transiting planets in A's star system, either by building structures to produce artificial transits observable by B, or by emitting signals at B during transit, at significantly lower energy consumption than typical electromagnetic transmission schemes. This can produce a network of interconnected civilizations, establishing contact via observing each other's transits. Assuming that civilizations reside in a Galactic Habitable Zone (GHZ), I conduct Monte Carlo Realization simulations of the establishment and growth of this network, and analyse its properties in the context of graph theory. I find that at any instant, only a few civilizations are correctly aligned to communicate via transits. However, we should expect the true network to be cumulative, where a ‘handshake’ connection at any time guarantees connection in the future via e.g. electromagnetic signals. In all our simulations, the cumulative network connects all civilizations together in a complete network. If civilizations share knowledge of their network connections, the network can be fully complete on timescales of order a hundred thousand years. Once established, this network can connect any two civilizations either directly, or via intermediate civilizations, with a path much less than the dimensions of the GHZ.Peer reviewe
The Galactic Club or Galactic Cliques? Exploring the limits of interstellar hegemony and the Zoo hypothesis
No full textThe author gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG and the STFC grant ST/J001422/1.The Zoo solution to Fermi's Paradox proposes that extraterrestrial intelligences (ETIs) have agreed to not contact the Earth. The strength of this solution depends on the ability for ETIs to come to agreement, and establish/police treaties as part of a so-called ‘Galactic Club’. These activities are principally limited by the causal connectivity of a civilization to its neighbours at its inception, i.e. whether it comes to prominence being aware of other ETIs and any treaties or agreements in place. If even one civilization is not causally connected to the other members of a treaty, then they are free to operate beyond it and contact the Earth if wished, which makes the Zoo solution ‘soft’. We should therefore consider how likely this scenario is, as this will give us a sense of the Zoo solution's softness, or general validity. We implement a simple toy model of ETIs arising in a Galactic Habitable Zone, and calculate the properties of the groups of culturally connected civilizations established therein. We show that for most choices of civilization parameters, the number of culturally connected groups is >1, meaning that the Galaxy is composed of multiple Galactic Cliques rather than a single Galactic Club. We find in our models for a single Galactic Club to establish interstellar hegemony, the number of civilizations must be relatively large, the mean civilization lifetime must be several millions of years, and the inter-arrival time between civilizations must be a few million years or less.Peer reviewe
splash-tools-wrapper: Initial Release
No full text<p>Coded in a brief interlude at Comparing Apples with Apples, hosted at the Lorentz Center in Leiden. Basic functionality in place - happy to include more at user request.</p>
dh4gan/oberon: Version 1.1
No full text<p>This release now contains the Carbonate-Silicate Cycle modelling formalism from Haqq-Misra et al (2016) (<a href="https://arxiv.org/abs/1605.07130v1">https://arxiv.org/abs/1605.07130v1</a>).</p>
<p><code>parFile</code> has been completely rewritten for greater flexibility in adding new input parameters, and the output to the terminal screen has been vastly improved.</p>
Milankovitch cycles of terrestrial planets in binary star systems
No full textDHF gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG.The habitability of planets in binary star systems depends not only on the radiation environment created by the two stars, but also on the perturbations to planetary orbits and rotation produced by the gravitational field of the binary and neighbouring planets. Habitable planets in binaries may therefore experience significant perturbations in orbit and spin. The direct effects of orbital resonances and secular evolution on the climate of binary planets remain largely unconsidered. We present latitudinal energy balance modelling of exoplanet climates with direct coupling to an N-Body integrator and an obliquity evolution model. This allows us to simultaneously investigate the thermal and dynamical evolution of planets orbiting binary stars, and discover gravito-climatic oscillations on dynamical and secular time-scales. We investigate the Kepler-47 and Alpha Centauri systems as archetypes of P- and S-type binary systems, respectively. In the first case, Earth-like planets would experience rapid Milankovitch cycles (of order 1000 yr) in eccentricity, obliquity and precession, inducing temperature oscillations of similar periods (modulated by other planets in the system). These secular temperature variations have amplitudes similar to those induced on the much shorter time-scale of the binary period. In the Alpha Centauri system, the influence of the secondary produces eccentricity variations on 15 000 yr time-scales. This produces climate oscillations of similar strength to the variation on the orbital time-scale of the binary. Phase drifts between eccentricity and obliquity oscillations creates further cycles that are of order 100 000 yr in duration, which are further modulated by neighbouring planets.Peer reviewe
dh4gan/taktent: Full implementation of Galactic Habitable Zone
No full text<p>This version fully implements the Galactic Habitable Zone as a specified distribution. It also allows Observer objects to be placed inside the same spatial distribution as the Transmitters.</p>
<p>The Transmitter object is also more flexible in the properties of its emitted radiation, allowing decay according to an arbitrary power law. This is essential for modelling e.g. gravitational wave signals, which decay as 1/r rather than 1/r^2.</p>
The habitable zone for Earthlike exomoons orbiting Kepler-1625b
No full textThe recent announcement of a Neptune-sized exomoon candidate orbiting the
Jupiter-sized object Kepler-1625b has forced us to rethink our assumptions
regarding both exomoons and their host exoplanets. In this paper I describe
calculations of the habitable zone for Earthlike exomoons in orbit of
Kepler-1625b under a variety of assumptions. I find that the candidate exomoon,
Kepler-1625b-i, does not currently reside within the exomoon habitable zone,
but may have done so when Kepler-1625 occupied the main sequence. If it were to
possess its own moon (a "moon-moon") that was Earthlike, this could potentially
have been a habitable world. If other exomoons orbit Kepler-1625b, then there
are a range of possible semimajor axes/eccentricities that would permit a
habitable surface during the main sequence phase, while remaining dynamically
stable under the perturbations of Kepler-1625b-i. This is however contingent on
effective atmospheric CO regulation.Comment: 17 pages, 5 figures, accepted to IJA. arXiv admin note: text overlap
with arXiv:1601.00789, arXiv:1404.449
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