arXiv subjects

Gavin A. L. Coleman

Publications and source records attributed to Gavin A. L. Coleman.

At least 19 recordsLinked to original sources

arXiv2026

From dust to planets -- II. Effects of wide binary companions and external photoevaporation on planetesimal and embryo formation

More than half of Solar-type stars are found in binary systems. The numbers of exoplanets within binary systems in s-type orbits now numbers over 700. However, whilst the numbers have increased, there still does not exist a global model of planet formation for wide binary systems, where there does for single stars and circumbinary systems. As a precursor to such a model, that includes the necessary physical planet formation processes, it is important to understand how an outer binary companion affects the evolution of circumstellar discs, and the formation of planetesimals and planetary embryos. The main mechanism for which these processes are affected, is through truncation of the protoplanetary disc outer edges. In this paper, we determine these effects, whilst also comparing them to the effects of external photoevaporation that competes to truncate protoplanetary discs. We find that disc truncation by both a binary companion and external photoevaporation significantly reduces the efficiency to which planetary embryos are able to accrete pebbles and grow into terrestrial mass planets. This is due to the pebble supply being cut off as the pebble production front reaches the disc outer edge before planets are able to significantly increase in mass. This hindrance to planet formation occurs when the truncation radius due to the binary companion is below $\sim 30$ au, corresponding to binary separations of $\sim90$ au for equal mass, circular binary stars. For separations greater than 300 au, planet formation operates similar to that around single stars. Our results highlight the detrimental effects of a binary companion for intermediate binaries, that can provide possible explanations for the dearth of multiple planets within binary systems of separations $<100$ au

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arXiv2026

Formation of multi-planetary systems via pebble accretion in externally photoevaporating discs in stellar clusters

In this paper, we investigate how external photo-evaporation influences the formation, dynamical evolution and the resultant planetary architecture of multi-planet systems born in stellar clusters. We use a model of N-body simulations of multiple planet formation via pebble accretion coupled with a 1-D viscous disc subject to external photo-evaporation. We found that external photo-evaporation reduces the planet growth by reducing the pebble mass reservoir in discs containing multiple planetary embryos across a wide range of disc masses, and is particularly effective in suppressing planet growth in less initially massive discs (< 0.1 M$_{\odot}$). However, in more initially massive ($\geq$ 0.1 M$_{\oplus}$) discs planets lost due to planet-planet interactions dominate the outcome for final resultant total planet mass, masking the effects of external photo-evaporation in curbing the planet mass growth. In terms of the final resulting planetary architectures, the signature of external photo-evaporation is visible in less massive (< 0.1 M$_{\odot}$) discs, with fewer numbers and lower masses of planets surviving in discs irradiated with stronger external FUV radiation. External photo-evaporation also leaves a signature for the wide orbit (> 10 au) terrestrial planets (0.1 - 1 M$_{\oplus}$), with fewer planets populating this region for stronger FUV field. Finally, the 1st-order resonant pairs fraction decreases with stronger FUV radiation, although the resonant pairs occur rarely regardless of the FUV radiation environment, due to the small number of planets that survive gravitational encounters.

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arXiv2025

Using simultaneous mass accretion and external photoevaporation rates for d203-504 to constrain disc evolution processes

We cannot understand planet formation without understanding disc evolutionary processes. However, there is currently ambiguity about how protoplanetary discs transport angular momentum (e.g. via viscosity or winds) and the relative contributions and interplay of different dispersal mechanisms. A key difficulty is that for any given system only a handful of disc parameters are usually available to constrain theoretical models. Recent observations of the d203-504 disc in Orion, have yielded values of the stellar accretion rate, external photoevaporative mass loss rate, stellar mass and the disc size and mass. In particular, having the combination of accretion rate and external photoevaporative rate is new. Using this unique combination of observables, we run a suite of disc evolution simulations to constrain which scenarios can match the observed values. We explore both viscous and MHD wind-driven discs, finding that they best match observations when the angular momentum transport $α$ parameter is $3\times10^{-4}\leqα_ν\leq2\times10^{-3}$ for viscous discs, and $2\times10^{-3}\leqα_{\rm DW}\leq10^{-2}$ for MHD wind-driven discs, consistent with other estimates in the literature. As well constraining the disc properties and evolution, the d203-504 disc allows us to define a new irradiation age, since in order to match observations, it was required that the disc had only just appeared in the extreme UV environment it is currently exposed to (a known issue for proplyds referred to as the proplyd lifetime problem). This indicates that it is either very young, i.e. <0.1 Myr, or it has been shielded until recently, which would have protected the planet forming reservoir and helped facilitate planet growth despite it now residing in a harsh UV environment.

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arXiv2025

How the gradient of $M_{\rm d}$ versus UV field strength yields insights into the ages of protoplanetary disc populations

FUV radiation from massive stars launch photoevaporative winds from the outer regions of protoplanetary discs around other stars, removing gas and dust. Observations have identified a relation between the median dust disc mass and the external UV field strength. Here we use disc evolutionary models to explore how this relation evolves over time, and with respect to other stellar and disc properties. We find that the slope for the relationship $λ_{\rm UV}$ flattens over time as populations age, possibly explaining the differences seen between the L1641-N and L1641-S clusters in Orion A. We determine that $λ_{\rm UV}$ depends on the stellar mass where more massive stars exhibit steeper gradients than their lesser counterparts, in agreement with the differences seen between Herbig and T Tauri stars. Additionally, the strength of the mechanism for angular momentum transport, either viscosity or MHD disc winds, is found to significantly affect $λ_{\rm UV}$ with stronger $α$ values reducing $λ_{\rm UV}$ due to more material accreting on to the central stars in weaker UV environments. Estimates of $λ_{\rm UV}$ from observations of L1641 place preliminary constraints on $α$ to be between $10^{-3.5}$--$10^{-2.5}$, consistent with literature estimates. Further observations in different regions and better classifications of stellar masses will allow us to place stringent constraints on disc evolution properties, improving our understanding of how protoplanetary discs evolve.

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arXiv2025

BEBOP VIII. SOPHIE radial velocities reveal an eccentric, circumbinary brown dwarf

Circumbinary configurations offer a test of planet formation in an altered environment, where the inner binary has perturbed a protoplanetary disc. Comparisons of the physical and orbital parameters between the circumbinary planet population and the population of exoplanets orbiting single stars will reveal how these disc perturbations affect the assembly of planets. Circumbinary exoplanets detected thus far typically have masses $< 3 \,\rm M_{jup}$ raising the question of whether high-mass circumbinary planets are possible, and also whether population features such as the brown dwarf desert would appear in circumbinary configurations like for single star systems. Here, we report observations taken with the SOPHIE high-resolution spectrograph. These observations reveal an $m_{\rm b}\,\sin i_{\rm b} = 20.9 \,\rm M_{jup}$ outer companion, on an eccentric ($e = 0.43$), $1800\,\rm d$ orbit, which we call BEBOP-4 (AB) b. Using dynamical arguments we constrain the true mass $m_{\rm b}< 26.3 \,\rm M_{jup}$. The inner binary's two eclipsing stellar components have masses $M_{\rm A} = 1.51\,\rm M_\odot$, and $M_{\rm B} = 0.46\,\rm M_\odot$. Their orbital period is $72\,\rm d$, and their eccentricity is $0.27$. This system contains the longest period binary surveyed by the BEBOP project. BEBOP-4b is expected to be detectable using Gaia DR4 single epoch astrometric measurements. Despite a large period ratio of $\sim 25:1$, the substantial eccentricities of both orbits mean that the outer orbit is on the edge of orbital stability, and located in between two destabilising secular resonances. Should the outer companion survive, the BEBOP-4 system appears like a precursor to several post-common envelope binaries exhibiting eclipse timing variations where very massive circumbinary companions have been proposed.

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arXiv2025

Runaway origins of a disc mass gradient in $σ$ Orionis

Radiation from massive stars is known to significantly affect the evolution of protoplanetary discs around surrounding stars by driving external photoevaporative winds. Typically most studies assume that the massive stars driving these winds are comoving with their associated clusters. However, it is also known that massive stars can be runaways, after being violently ejected from their birth environment through interactions with other massive stars. In this letter, we show that the well studied system $σ~{\rm Ori~AB}$ is actually a runaway system, only now passing through $σ~{\rm Orionis}$. There are multiple observable features that indicate this is the case, including significantly larger proper motions for $σ~{\rm Orionis}$ than the surrounding stars, an infrared arc of ionising gas along the predicted velocity vector, and a disparity in protoplanetary disc masses across $σ~{\rm Orionis}$. We finally use protoplanetary disc evolution models to explain the observed disparity in disc masses, showing that those discs downstream of $σ~{\rm Ori~AB}$, i.e. those yet to encounter it, have larger masses than those upstream, consistent with observations. Overall, our work highlights the importance of understanding the dynamical history of star forming regions, since the time varying UV fields provided by runway stars results in a complex history for the evolution of the protoplanetary discs.

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arXiv2025

BEBOP VII. SOPHIE discovery of BEBOP-3b, a circumbinary giant planet on an eccentric orbit

Planetary systems orbiting close binaries are valuable testing grounds for planet formation and migration models. More detections with good mass measurements are needed. We present a new planet discovered during the BEBOP survey for circumbinary exoplanets using radial velocities. We use data taken with the SOPHIE spectrograph at the Observatoire de Haute-Provence, and perform a spectroscopic analysis to obtain high precision radial velocities. This planet is the first radial velocity detection of a previously unknown circumbinary system. The planet has a mass of $0.56$ $M_{Jup}$ and orbits its host binary in 550 days with an eccentricity of 0.25. Compared to most of the previously known circumbinary planets, BEBOP-3b has a long period (relative to the binary) and a high eccentricity. There also is a candidate outer planet with a $\sim1400$ day orbital period. We test the stability of potential further candidate signals inside the orbit of BEBOP-3b, and demonstrate that there are stable orbital solutions for planets near the instability region which is where the Kepler circumbinary planets are located. We also use our data to obtain independent dynamical masses for the two stellar components of the eclipsing binary using High Resolution Cross-Correlation Spectroscopy (HRCCS), and compare those results to a more traditional approach, finding them compatible with one another.

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arXiv2025

Solar C/O ratio in planet-forming gas at 1 au in a highly irradiated disk

The chemical composition of exoplanets is thought to be influenced by the composition of the disks in which they form. JWST observations have unveiled a variety of species in numerous nearby disks, showing significant variations in the C/O abundance ratio. However, little is known about the composition and C/O ratio of disks around young stars in clusters exposed to strong ultraviolet (UV) radiation from nearby massive stars, which are representative of the environments where most planetary systems form, including ours. We present JWST spectroscopy of d203-504, a young 0.7 $\rm M_{\odot}$ star in the Orion Nebula with a 30 au disk irradiated by nearby massive stars. These observations reveal spectroscopic signatures of CO, H$_2$O, CH$_3^+$, and PAHs. Water and CO are detected in absorption in the inner disk ($r\lesssim 1$ au), where the estimated gas-phase C/O ratio is 0.48, consistent with the Solar value and that of the Orion Nebula. In contrast, \ch{CH3+} and PAHs are found in the extended surface layers of the disk. These results suggest that gas in the inner disk is chemically shielded from UV radiation while the surface layers of the disk experience UV-induced chemistry, potentially depleting their carbon content.

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arXiv2025

The observable impact of runaway OB stars on protoplanetary discs

UV radiation from OB stars can drive ``external'' photoevaporative winds from discs in clusters, that have been shown to be important for disc evolution and planet formation. However, cluster dynamics can complicate the interpretation of this process. A significant fraction of OB stars are runaways, propagating at high velocity which might dominate over the wider cluster dynamics in setting the time variation of the UV field in part of the cluster. We explore the impact of a runaway OB star on discs and the observational impact that may have. We find that discs exposed to even short periods of strong irradiation are significantly truncated, and only rebound slightly following the ``flyby'' of the UV source. This is predicted to leave an observable imprint on a disc population, with those downstream of the OB star vector being more massive and extended than those upstream. Because external photoevaporation acts quickly, this imprint is less susceptible to being washed out by cluster dynamics for faster runaway OB stars. The Gaia proper motion vector of the B star 42 Ori in NGC 1977 is transverse to the low mass stellar population and so may make a good region to search for this signature in resolved disc observations.

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arXiv2025

How the microphysical properties of external photoevaporation influence the global evolution of protoplanetary discs

External photoevaporation is one of the dominant mechanisms for mass loss from protoplanetary discs. However this mass loss is theoretically expected to depend upon the microphysical properties of protoplanetary discs, which are currently poorly constrained in observations. In this work we explore the impact of microphysics on the bulk evolution of discs. The polycyclic aromatic hydrocarbon (PAH) abundance, and the extent to which grain growth has occurred in the disc have profound effects on the strength of mass loss rates due to external photoevaporation, which in turn can have a significant impact on the disc evolution, impacting disc radii and accretion rates over time. The strongest sensitivity is to whether grain growth has occurred in the disc, which reduces the amount of dust entrained in the wind to shield the disc, thus increasing the rate at which gas is lost. Additionally, larger PAH abundances result in stronger heating and higher mass loss rates, but to a lesser extent than grain growth. We find that plausible variations in the PAH abundance and disc dust evolution can leave observable differences in disc populations. This work highlights the importance of obtaining observational constraints of the microphysical properties of protoplanetary discs. Future observations from JWST should soon be able to provide these constraints.

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arXiv2025

Predicting the Galactic population of free-floating planets from realistic initial conditions

We present the first prediction for the mass distribution function of Galactic free-floating planets (FFPs) that aims to accurately include the relative contributions of multiple formation pathways and stellar populations. We derive our predicted distribution from dedicated simulations of planet birth, growth, migration, and ejection around circumbinary systems and extend these results to also include the contributions from single and wide binary systems. Our resulting FFP mass distribution shows several distinct features, including a strong peak at $\sim 8 M_{\oplus}$ arising from the transition between pebble and gas accretion regimes and a trough at $\sim 1 M_{\oplus}$ due to the shift in the dominant ejection process from planet-planet scattering to ejection through interactions with stars in circumbinary systems. We find that interactions with the central binary in close circumbinary systems are likely the dominant progenitor for FFPs more massive than Earth, leading to a steep power-law dependence in mass that agrees well with existing observations. In contrast, we find planet-planet scattering events in single and wide binary systems likely produce the majority of planets at Mars mass and below, resulting in a shallower power-law dependence. Our results suggest that existing extrapolations into the sub-terrestrial mass range may significantly overestimate the true FFP abundance. The features we predict in the mass distribution of FFPs will be detectable by upcoming space-based microlensing surveys and, if observed, will provide key insight into the origins of FFPs and the environments in which they form.

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arXiv2024

BEBOP VI. Enabling the detection of circumbinary planets orbiting double-lined binaries with the DOLBY method of radial-velocity extraction

Circumbinary planets - planets that orbit both stars in a binary system - offer the opportunity to study planet formation and orbital migration in a different environment compare to single stars. However, despite the fact that > 90% of binary systems in the solar neighbourhood are spectrally resolved double-lined binaries, there has been only one detection of a circumbinary planet orbitting a double-lined binary using the radial velocity method so far. Spectrally disentangling both components of a binary system is hard to do accurately. Weak spectral lines blend with one another in a time-varying way, and inaccuracy in spectral modelling can lead to an inaccurate estimation of the radial-velocity of each component. This inaccuracy adds scatter to the measurements that can hide the weak radial-velocity signature of circumbinary exoplanets. We have obtained new high signal-to-noise and high-resolution spectra with the SOPHIE spectrograph, mounted on the 193cm telescope at Observatoire de Haute-Provence (OHP) for six, bright, double-lined binaries for which a circumbinary exoplanet detection has been attempted in the past. To extract radial-velocities we use the DOLBY code, a recent method of spectral disentangling using Gaussian processes to model the time-varying components. We analyse the resulting radial-velocities with a diffusive nested sampler to seek planets, and compute sensitivity limits. We do not detect any new circumbinary planet. However, we show that the combination of new data, new radial-velocity extraction methods, and new statistical methods to determine a dataset's sensitivity to planets leads to an approximately one order of magnitude improvement compared to previous results. This improvement brings us into the range of known circumbinary exoplanets and paves the way for new campaigns of observations targeting double-lined binaries.

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arXiv2024

BEBOP V. Homogeneous Stellar Analysis of Potential Circumbinary Planet Hosts

Planets orbiting binary systems are relatively unexplored compared to those around single stars. Detections of circumbinary planets and planetary systems offer a first detailed view into our understanding of circumbinary planet formation and dynamical evolution. The BEBOP (Binaries Escorted by Orbiting Planets) radial velocity survey plays a special role in this adventure as it focuses on eclipsing single-lined binaries with an FGK dwarf primary and M dwarf secondary allowing for the highest-radial velocity precision using the HARPS and SOPHIE spectrographs. We obtained 4512 high-resolution spectra for the 179 targets in the BEBOP survey which we used to derive the stellar atmospheric parameters using both equivalent widths and spectral synthesis. We furthermore derive stellar masses, radii, and ages for all targets. With this work, we present the first homogeneous catalogue of precise stellar parameters for these eclipsing single-lined binaries.

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arXiv2024

On the properties of free floating planets originating in circumbinary planetary systems

Free-floating planets are a new class of planets recently discovered. These planets don't orbit within stellar systems, instead living a nomadic life within the galaxy. How such objects formed remains elusive. Numerous works have explored mechanisms to form such objects, but have not yet provided predictions on their distributions that could differentiate between formation mechanisms. In this work we form these objects within circumbinary systems, where these planets are readily formed and ejected through interactions with the central binary stars. We find significant differences between planets ejected through planet-planet interactions and those by the binary stars. The main differences that arise are in the distributions of excess velocity, where binary stars eject planets with faster velocities. These differences should be observable amongst known free-floating planets in nearby star-forming regions. We predict that targeted observations of directly imaged free-floating planets in these regions should be able to determine their preferred formation pathway, either by planet formation in single or multiple stellar systems, or through processes akin to star formation. Additionally the mass distributions of free-floating planets can yield important insights into the underlying planet populations. We find that for planets more massive than 20 $\rm M_{\oplus}$, their frequencies are similar to those planets remaining bound and orbiting near the central binaries. This similarity allows for effective and informative comparisons between mass distributions from microlensing surveys, to those of transit and radial velocities. Ultimately, by observing the velocity dispersion and mass distribution of free-floating planets, it will be possible to effectively compare with predictions from planet formation models, and to further understand the formation and evolution of these exotic worlds.

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arXiv2023

Photoevaporation obfuscates the distinction between wind and viscous angular momentum transport in protoplanetary discs

How protoplanetary discs evolve remains an unanswered question. Competing theories of viscosity and magnetohydrodynamic disc winds have been put forward as the drivers of angular momentum transport in protoplanetary discs. These two models predict distinct differences in the disc mass, radius and accretion rates over time, that could be used to distinguish them. However that expectation is built on models that do not include another important process - photoevaporation, both internally by the host star and externally by neighbouring stars. In this work we produce numerical models of protoplanetary discs including viscosity, magnetohydrodynamic disc winds, and internal and external photoevaporation. We find that even weak levels of external photoevaporation can significantly affect the evolution of protoplanetary discs, influencing the observable features such as disc radii, that might otherwise distinguish between viscous and wind driven discs. Including internal photoevaporation further suppresses differences in evolution between viscous and wind driven discs. This makes it much more difficult than previously anticipated, to use observations of nearby star forming regions to determine whether discs are viscous or wind driven. Interestingly we find that evolved protoplanetary discs in intermediate FUV environments may be the best cases for differentiating whether they evolve through viscosity or magnetohydrodynamic disc winds. Ultimately this work demonstrates the importance of understanding what are the key evolutionary processes and including as many of those as possible when exploring the evolution of protoplanetary discs.

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arXiv2023

Constraining the formation history of the TOI-1338/BEBOP-1 circumbinary planetary system

The recent discovery of multiple planets in the circumbinary system TOI-1338/BEBOP-1 raises questions about how such a system formed. The formation of the system was briefly explored in the discovery paper, but only to answer the question do current pebble accretion models have the potential to explain the origin of the system? We use a global model of circumbinary planet formation that utilises N-body simulations, including prescriptions for planet migration, gas and pebble accretion, and interactions with a circumbinary disc, to explore the disc parameters that could have led to the formation of the TOI-1338/BEBOP-1 system. With the disc lifetime being the main factor in determining how planets form, we limit our parameter space to those that determine the disc lifetime. These are: the strength of turbulence in the disc, the initial disc mass, and the strength of the external radiation field that launches photoevaporative winds. When comparing the simulated systems to TOI-1338/BEBOP-1, we find that only discs with low levels of turbulence are able to produce similar systems. The radiation environment has a large effect on the types of planetary systems that form, whilst the initial disc mass only has limited impact since the majority of planetary growth occurs early in the disc lifetime. With the most TOI-1338/BEBOP-1 like systems all occupying similar regions of parameter space, our study shows that observed circumbinary planetary systems can potentially constrain the properties of planet forming discs.

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arXiv2023

FRIED v2. A new grid of mass loss rates for externally irradiated protoplanetary discs

We present a new FRIED grid of mass loss rates for externally far-ultraviolet (FUV) irradiated protoplanetary discs. As a precursor to the new grid, we also explore the microphysics of external photoevaporation, determining the impact of polycyclic aromatic hydrocarbon (PAH) abundance, metallicity, coolant depletion (via freeze out and radial drift) and grain growth (depletion of small dust in the outer disc) on disc mass loss rates. We find that metallicity variations typically have a small effect on the mass loss rate, since the impact of changes in heating, cooling and optical depth to the disc approximately cancel out. The new FRIED grid therefore focuses on i) expanding the basic physical parameter space (disc mass, radius, UV field, stellar mass) ii) on enabling variation of the the PAH abundance and iii) including an option for grain growth to have occurred or not in the disc. What we suggest is the fiducial model is comparable to the original FRIED grid. When the PAH-to-dust ratio is lower, or the dust in the wind more abundant, the mass loss rate can be substantially lower. We demonstrate with a small set of illustrative disc evolutionary calculations that this in turn can have a significant impact on the disc mass/radius/ evolution and lifetime.

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arXiv2023

Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system

We report the detection of a gas-giant planet in orbit around both stars of an eclipsing binary star system that also contains the smaller, inner transiting planet TOI-1338b. The new planet, called TOI-1338/BEBOP-1c, was discovered using radial-velocity data collected with the HARPS and ESPRESSO spectrographs. Our analysis reveals it is a $65.2~\rm{M_{\oplus}}$ circumbinary planet with a period of $215.5~$days. This is the first detection of a circumbinary planet using radial-velocity observations alone, and makes TOI-1338/BEBOP-1 only the second confirmed multiplanet circumbinary system to date. We do not detect the smaller inner transiting planet with radial-velocity data, and can place an upper limit on the inner planet's mass at $21.8~\mathrm{M}_\oplus$ with $99\%$ confidence. The inner planet is the first circumbinary planet amenable for atmospheric characterisation, using the James Webb Space Telescope.

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