arXiv subjects

Duncan H. Forgan

Publications and source records attributed to Duncan H. Forgan.

17 recordsLinked to original sources

arXiv2019

Predator-Prey Behaviour in Self-Replicating Interstellar Probes

The concept of a rapid spread of self-replicating interstellar probes (SRPs) throughout the Milky Way adds considerable strength to Fermi's Paradox. A single civilisation creating a single SRP is sufficient for a fleet of SRPs to grow and explore the entire Galaxy on timescales much shorter than the age of the Earth - so why do we see no signs of such probes? One solution to this Paradox suggests that self-replicating probes eventually undergo replication errors and evolve into predator-prey populations, reducing the total number of probes and removing them from our view. I apply Lotka-Volterra models of predator-prey competition to interstellar probes navigating a network of stars in the Galactic Habitable Zone to investigate this scenario. I find that depending on the local growth mode of both populations and the flow of predators/prey between stars, there are many stable solutions with relatively large numbers of prey probes inhabiting the Milky Way. The solutions can exhibit the classic oscillatory pattern of Lotka-Volterra systems, but this depends sensitively on the input parameters. Typically, local and global equilibria are established, with prey sometimes outnumbering the predators. Accordingly, we find this solution to Fermi's Paradox does not reduce the probe population sufficiently to be viable.

physics.pop-ph↗

arXiv2018

Observing substructure in circumstellar discs around massive young stellar objects

Simulations of massive star formation predict the formation of discs with significant substructure, such as spiral arms and clumps due to fragmentation. Here we present a semi-analytic framework for producing synthetic observations of discs with substructure, in order to determine their observability in interferometric observations. Unlike post-processing of hydrodynamical models, the speed inherent to our approach permits us to explore a large parameter space of star and disc parameters, and thus constrain properties for real observations. We compute synthetic dust continuum and molecular line observations probing different disc masses, distances, inclinations, thermal structures, dust distributions, and number and orientation of spirals and fragments. With appropriate spatial and kinematic filtering applied, our models predict that ALMA observations of massive YSOs at <5 kpc distances should detect spirals in both gas and dust in strongly self-gravitating discs (i.e. discs with up to two spiral arms and strong kinematic perturbations). Detecting spirals will be possible in discs of arbitrary inclination, either by directly spatially resolving them for more face-on discs (inclinations up to ~50 degrees), or through a kinematic signature otherwise. Clumps resulting from disc fragmentation should be detectable in the continuum, if the clump is sufficiently hotter than the surrounding disc material.

astro-ph.SR↗

arXiv2018

The fate of formamide in a fragmenting protoplanetary disc

Recent high-sensitivity observations carried out with ALMA have revealed the presence of complex organic molecules (COMs) such as methyl cyanide (CH$_{\rm 3}$CN) and methanol (CH$_{\rm 3}$OH) in relatively evolved protoplanetary discs. The behaviour and abundance of COMs in earlier phases of disc evolution remains unclear. Here we combine a smoothed particle hydrodynamics simulation of a fragmenting, gravitationally unstable disc with a gas-grain chemical code. We use this to investigate the evolution of formamide (NH$_{\rm 2}$CHO), a pre-biotic species, in both the disc and in the fragments that form within it. Our results show that formamide remains frozen onto grains in the majority of the disc where the temperatures are $<$100 K, with a predicted solid-phase abundance that matches those observed in comets. Formamide is present in the gas-phase in three fragments as a result of the high temperatures ($\geq$200\,K), but remains in the solid-phase in one colder ($\leq$150 K) fragment. The timescale over which this occurs is comparable to the dust sedimentation timescales, suggesting that any rocky core which is formed would inherit their formamide content directly from the protosolar nebula.

astro-ph.GA↗

arXiv2018

Clumpy shocks as the driver of velocity dispersion in molecular clouds: the effects of self-gravity and magnetic fields

We revisit an alternate explanation for the turbulent nature of molecular clouds - namely, that velocity dispersions matching classical predictions of driven turbulence can be generated by the passage of clumpy material through a shock. While previous work suggested this mechanism can reproduce the observed Larson relation between velocity dispersion and size scale ($σ\propto L^Γ$ with $Γ\approx 0.5$), the effects of self-gravity and magnetic fields were not considered. We run a series of smoothed particle magnetohydrodynamics experiments, passing clumpy gas through a shock in the presence of a combination of self-gravity and magnetic fields. We find powerlaw relations between $σ$ and $L$ throughout, with indices ranging from $Γ=0.3-1.2$. These results are relatively insensitive to the strength and geometry of magnetic fields, provided that the shock is relatively strong. $Γ$ is strongly sensitive to the angle between the gas' bulk velocity and the shock front, and the shock strength (compared to the gravitational boundness of the pre-shock gas). If the origin of the $σ-L$ relation is in clumpy shocks, deviations from the standard Larson relation constrain the strength and behaviour of shocks in spiral galaxies.

astro-ph.GA↗

arXiv2018

Are Elias 2-27's spiral arms driven by self-gravity, or by a companion? A comparative spiral morphology study

The spiral waves detected in the protostellar disc surrounding Elias 2-27 have been suggested as evidence of the disc being gravitationally unstable. However, previous work has shown that a massive, stable disc undergoing an encounter with a massive companion are also consistent with the observations. We compare the spiral morphology of smoothed particle hydrodynamic simulations modelling both cases. The gravitationally unstable disc produces symmetric, tightly wound spiral arms with constant pitch angle, as predicted by the literature. The companion disc's arms are asymmetric, with pitch angles that increase with radius. However, these arms are not well-fitted by standard analytic expressions, due to the high disc mass and relatively low companion mass. We note that differences (or indeed similarities) in morphology between pairs of spirals is a crucial discriminant between scenarios for Elias 2-27, and hence future studies must fit spiral arms individually. If Elias 2-27 continues to show symmetric tightly wound spiral arms in future observations, then we posit that it is the first observed example of a gravitationally unstable protostellar disc.

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arXiv2017

Interstellar communication. VI. Searching X-ray spectra for narrowband communication

We have previously argued that targeted interstellar communication has a physical optimum at narrowband X-ray wavelengths $λ\approx1\,$nm, limited by the surface roughness of focusing devices at the atomic level (arXiv:1711.05761). We search 24,247 archival X-ray spectra (of 6,454 unique objects) for such features and present 19 sources with monochromatic signals. Close examination reveals that these are most likely of natural origin. The ratio of artificial to natural sources must be <0.01%. This first limit can be improved in future X-ray surveys.

astro-ph.IM↗

arXiv2017

Interstellar communication. III. Optimal frequency to maximize data rate

The optimal frequency for interstellar communication, using "Earth 2017" technology, was derived in papers I and II of this series (arXiv:1706.03795, arXiv:1706.05570). The framework included models for the loss of photons from diffraction (free space), interstellar extinction, and atmospheric transmission. A major limit of current technology is the focusing of wavelengths $λ<300\,$nm (UV). When this technological constraint is dropped, a physical bound is found at $λ\approx1\,$nm ($E\approx\,$keV) for distances out to kpc. While shorter wavelengths may produce tighter beams and thus higher data rates, the physical limit comes from surface roughness of focusing devices at the atomic level. This limit can be surpassed by beam-forming with electromagnetic fields, e.g. using a free electron laser, but such methods are not energetically competitive. Current lasers are not yet cost efficient at nm wavelengths, with a gap of two orders of magnitude, but future technological progress may converge on the physical optimum. We recommend expanding SETI efforts towards targeted (at us) monochromatic (or narrow band) X-ray emission at 0.5-2 keV energies.

astro-ph.IM↗

arXiv2017

Photogravimagnetic assists of light sails: a mixed blessing for Breakthrough Starshot?

Upon entering a star system, light sails are subject to both gravitational forces and radiation pressure, and can use both in concert to modify their trajectory. Moreover, stars possess significant magnetic fields, and if the sail is in any way charged, it will feel the Lorentz force also. We investigate the dynamics of so-called "photogravimagnetic assists" of sailcraft around $α$ Centauri A, a potential first destination en route to Proxima Centauri (the goal of the Breakthrough Starshot program). We find that a 10m$^2$ sail with a charge-to-mass-ratio of around 10 $μ$C/g or higher will need to take account of magnetic field effects during orbital maneouvres. The magnetic field can provide an extra source of deceleration and deflection, and allow capture onto closer orbits around a target star. However, flipping the sign of the sailcraft's charge can radically change resulting trajectories, resulting in complex loop-de-loops around magnetic field lines and essentially random ejection from the star system. Even on well-behaved trajectories, the field can generate off-axis deflections at $α$ Centauri that, while minor, can result in very poor targetting of the final destination (Proxima) post-assist. Fortunately for Breakthrough Starshot, nanosails are less prone to charging en route than their heavier counterparts, but can still accrue relatively high charge at both the origin and destination, when travelling at low speeds. Photogravimagnetic assists are highly non-trivial, and require careful course correction to mitigate against unwanted changes in trajectory.

astro-ph.IM↗

arXiv2017

On The Feasibility of Exomoon Detection Via Exoplanet Phase Curve Spectral Contrast

An exoplanet-exomoon system presents a superposition of phase curves to observers - the dominant component varies according to the planetary period, and the lesser varies according to both the planetary and the lunar period. If the spectra of the two bodies differs significantly, then it is likely there are wavelength regimes where the contrast between the moon and planet is significantly larger. In principle, this effect could be used to isolate periodic oscillations in the combined phase curve. Being able to detect the exomoon component would allow a characterisation of the exomoon radius, and potentially some crude atmospheric data. We run a parameter survey of combined exoplanet-exomoon phase curves, which show that for most sets of planet-moon parameters, the lunar component of the phase curve is undetectable to current state-of-the-art transit observations. Even with future transit survey missions, measuring the exomoon signal will most likely require photometric precision of 10 parts per million or better. The only exception to this is if the moon is strongly tidally heated or in some way self-luminous. In this case, measurements of the phase curve at wavelengths greater than a few microns can be dominated by the lunar contribution. Instruments like the James Webb Space Telescope and its successors are needed to make this method feasible.

astro-ph.EP↗

arXiv2016

Grand challenges in protoplanetary disc modelling

The Protoplanetary Discussions conference --- held in Edinburgh, UK, from 7th --11th March 2016 --- included several open sessions led by participants. This paper reports on the discussions collectively concerned with the multiphysics modelling of protoplanetary discs, including the self-consistent calculation of gas and dust dynamics, radiative transfer and chemistry. After a short introduction to each of these disciplines in isolation, we identify a series of burning questions and grand challenges associated with their continuing development and integration. We then discuss potential pathways towards solving these challenges, grouped by strategical, technical and collaborative developments. This paper is not intended to be a review, but rather to motivate and direct future research and collaboration across typically distinct fields based on \textit{community driven input}, to encourage further progress in our understanding of circumstellar and protoplanetary discs.

astro-ph.EP↗

arXiv2014

Can Collimated Extraterrestrial Signals be Intercepted?

The Optical Search for Extraterrestrial Intelligence (OSETI) attempts to detect collimated, narrowband pulses of electromagnetic radiation. These pulses may either consist of signals intentionally directed at the Earth, or signals between two star systems with a vector that unintentionally intersects the Solar System, allowing Earth to intercept the communication. But should we expect to be able to intercept these unintentional signals? And what constraints can we place upon the frequency of intelligent civilisations if we do? We carry out Monte Carlo Realisation simulations of interstellar communications between civilisations in the Galactic Habitable Zone (GHZ) using collimated beams. We measure the frequency with which beams between two stars are intercepted by a third. The interception rate increases linearly with the fraction of communicating civilisations, and as the cube of the beam opening angle, which is somewhat stronger than theoretical expectations, which we argue is due to the geometry of the GHZ. We find that for an annular GHZ containing 10,000 civilisations, intersections are unlikely unless the beams are relatively uncollimated. These results indicate that optical SETI is more likely to find signals deliberately directed at the Earth than accidentally intercepting collimated communications. Equally, civilisations wishing to establish a network of communicating species may use weakly collimated beams to build up the network through interception, if they are willing to pay a cost penalty that is lower than that meted by fully isotropic beacons. Future SETI searches should consider the possibility that communicating civilisations will attempt to strike a balance between optimising costs and encouraging contact between civilisations, and look for weakly collimated pulses as well as narrow-beam pulses directed deliberately at the Earth.

physics.pop-ph↗

arXiv2014

Surface Flux Patterns on Planets in Circumbinary Systems, and Potential for Photosynthesis

Recently, the Kepler Space Telescope has detected several planets in orbit around a close binary star system. These so-called circumbinary planets will experience non-trivial spatial and temporal distributions of radiative flux on their surfaces, with features not seen in their single-star orbiting counterparts. Earthlike circumbinary planets inhabited by photosynthetic organisms will be forced to adapt to these unusual flux patterns. We map the flux received by putative Earthlike planets (as a function of surface latitude/longitude and time) orbiting the binary star systems Kepler-16 and Kepler-47, two star systems which already boast circumbinary exoplanet detections. The longitudinal and latitudinal distribution of flux is sensitive to the centre of mass motion of the binary, and the relative orbital phases of the binary and planet. Total eclipses of the secondary by the primary, as well as partial eclipses of the primary by the secondary add an extra forcing term to the system. We also find that the patterns of darkness on the surface are equally unique. Beyond the planet's polar circles, the surface spends a significantly longer time in darkness than latitudes around the equator, due to the stars' motions delaying the first sunrise of spring (or hastening the last sunset of autumn). In the case of Kepler-47, we also find a weak longitudinal dependence for darkness, but this effect tends to average out if considered over many orbits. In the light of these flux and darkness patterns, we consider and discuss the prospects and challenges for photosynthetic organisms, using terrestrial analogues as a guide.

astro-ph.EP↗

arXiv2013

On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves

The Class A stellar engine (also known as a Shkadov thruster) is a spherical arc mirror, designed to use the impulse from a star's radiation pressure to generate a thrust force, perturbing the star's motion. If this mirror obstructs part of the stellar disc during the transit of an exoplanet, then this may be detected by studying the shape of the transit light curve, presenting another potential means by which the action of extraterrestrial intelligence (ETI) can be discerned. We model the light curves produced by exoplanets transiting a star which possesses a Shkadov thruster, and show how the parameters of the planet and the properties of the thruster can be disentangled. provided that radial velocity follow-up measurements are possible, and that other obscuring phenomena typical to exoplanet transit curves (such as the presence of starspots or intrinsic stellar noise) do not dominate. These difficulties aside, we estimate the a priori probability of detecting a Shkadov thruster during an exoplanet transit, which even given optimistic assumptions remains stubbornly low. Despite this, many exoplanet transit surveys designed for radial velocity follow-up are on the horizon, so we argue that this remains a useful serendipitous SETI technique. At worst, this technique will place an upper limit on the number of Class A stellar engines in the Solar neighbourhood; at best, this could help identify unusual transiting exoplanet systems as candidates for further investigation with other SETI methods.

astro-ph.EP↗

arXiv2012

The Effect of Probe Dynamics on Galactic Exploration Timescales

The travel time required for one civilisation to explore the Milky Way using probes is a crucial component of Fermi's Paradox. Previous attempts to estimate this travel time have assumed that the probe's motion is simple, moving at a constant maximum velocity, with powered flight producing the necessary change in velocity required at each star to complete its chosen trajectory. This approach ignores lessons learned from interplanetary exploration, where orbital slingshot maneouvres can provide significant velocity boosts at little to no fuel cost. It is plausible that any attempt to explore the Galaxy would utilise such economising techniques, despite there being an upper limit to these velocity boosts, related to the escape velocity of the object being used to provide the slingshot. In order to investigate the effects of these techniques, we present multiple realisations of single probes exploring a small patch of the Milky Way. We investigate 3 separate scenarios, studying the slingshot effect on trajectories defined by simple heuristics. These scenarios are: i) standard powered flight to the nearest unvisited star without using slingshot techniques; ii) flight to the nearest unvisited star using slingshot techniques, and iii) flight to the next unvisited star which provides the maximal velocity boost under a slingshot trajectory. We find that adding slingshot velocity boosts can decrease the travel time by up to two orders of magnitude over simple powered flight. In the third case, selecting a route which maximises velocity boosts also reduces the travel time relative to powered flight, but by a much reduced factor. From these simulations, we suggest that adding realistic probe trajectories tends to strengthen Fermi's Paradox.

physics.pop-ph↗

arXiv2012

Turbulent Linewidths as a Diagnostic of Self-Gravity in Protostellar Discs

We use smoothed particle hydrodynamics simulations of massive protostellar discs to investigate the predicted broadening of molecular lines from discs in which self-gravity is the dominant source of angular momentum transport. The simulations include radiative transfer, and span a range of disc-to-star mass ratios between 0.25 and 1.5. Subtracting off the mean azimuthal flow velocity, we compute the distribution of the in-plane and perpendicular peculiar velocity due to large scale structure and turbulence induced by self-gravity. For the lower mass discs, we show that the characteristic peculiar velocities scale with the square root of the effective turbulent viscosity parameter, as expected from local turbulent-disc theory. The derived velocities are anisotropic, with substantially larger in-plane than perpendicular values. As the disc mass is increased, the validity of the locally determined turbulence approximation breaks down, and this is accompanied by anomalously large in-plane broadening. There is also a high variance due to the importance of low-m spiral modes. For low-mass discs, the magnitude of in-plane broadening is, to leading order, equal to the predictions from local disc theory and cannot constrain the source of turbulence. However, combining our results with prior evaluations of turbulent broadening expected in discs where the magnetorotational instability (MRI) is active, we argue that self-gravity may be distinguishable from the MRI in these systems if it is possible to measure the anisotropy of the peculiar velocity field with disc inclination. Furthermore, for large mass discs, the dominant contribution of large-scale modes is a distinguishing characteristic of self-gravitating turbulence versus MRI driven turbulence.

astro-ph.SR↗

arXiv2010

The Entropy Principle and the Influence of Sociological Pressures on SETI

We begin with the premise that the law of entropy could prove to be fundamental for the evolution of intelligent life and the advent of technological civilization. Building on recent theoretical results, we combine a modern approach to evolutionary theory with Monte Carlo Realization Techniques. A numerical test for a proposed significance of the law of entropy within the evolution of intelligent species is performed and results are compared with a neutral test hypothesis. Some clarifying aspects on the emergence of intelligent species arise and are discussed in the framework of contemporary astrobiology.

physics.pop-ph↗

arXiv2010

Numerical Testing of The Rare Earth Hypothesis using Monte Carlo Realisation Techniques

The Search for Extraterrestrial Intelligence (SETI) has thus far failed to provide a convincing detection of intelligent life. In the wake of this null signal, many "contact pessimistic" hypotheses have been formulated, the most famous of which is the Rare Earth Hypothesis. It postulates that although terrestrial planets may be common, the exact environmental conditions that Earth enjoys are rare, perhaps unique. As a result, simple microbial life may be common, but complex metazoans (and hence intelligence) will be rare. This paper uses Monte Carlo Realisation Techniques to investigate the Rare Earth Hypothesis, in particular the environmental criteria considered imperative to the existence of intelligence on Earth. By comparing with a less restrictive, more optimistic hypothesis, the data indicates that if the Rare Earth hypothesis is correct, intelligent civilisation will indeed be relatively rare. Studying the separations of pairs of civilisations shows that most intelligent civilisation pairs (ICPs) are unconnected: that is, they will not be able to exchange signals at lightspeed in the limited time that both are extant. However, the few ICPs that are connected are strongly connected, being able to participate in numerous exchanges of signals. This may provide encouragement for SETI researchers: although the Rare Earth Hypothesis is in general a contact-pessimistic hypothesis, it may be a "soft" or "exclusive" hypothesis, i.e. it may contain facets that are latently contact-optimistic.

astro-ph.EP↗