arXiv subjects

Richard J. Parker

Publications and source records attributed to Richard J. Parker.

At least 19 recordsLinked to original sources

arXiv2026

White dwarf planets in star clusters: gravitational scattering versus mass-loss effects

White dwarfs are unique laboratories for understanding the formation, evolution and survivability of planetary systems. Post-main sequence mass-loss will change planetary orbital properties and stir up debris discs, leading to the observed pollution of white dwarf atmospheres. However, to date, very few studies have investigated the impact of the stellar birth environment on white dwarf planetary systems. In this paper we simulate the evolution of giant planets around white dwarf progenitors from their formation in a star-forming region until 1Gyr, when the most massive stars ($>$2M$_\odot$) have left the main sequence. Our simulations self-consistently model $N$-body interactions between stars and planets while stars evolve into white dwarfs within the cluster lifetime. We find that although scattering interactions in dense star-forming regions create free-floating planets, and alter the orbital properties of up to 20 per cent of the surviving planets, the effects of mass-loss from the star dominate the dynamics. This behaviour is independent of the stellar density of the birth star-forming region, and largely independent of the initial planet orbital properties. Our simulations produce both captured planets around white dwarfs (potentially similar to WD 0806-661b), and triple systems with white dwarfs and planets (potentially similar to PSR B1620-26(AB)b), and our results yield a population synthesis of giant planets from 1 - 100au that may be relevant to Roman, Gaia and JWST observations.

astro-ph.EP↗

arXiv2026

Planet or brown dwarf? Constraints on the formation of H-type objects in IC348

The formation mechanism(s) of substellar objects, such as brown dwarfs and free-floating planets, remains an ongoing puzzle in stellar and planetary physics. Recent observational and theoretical work points towards a star-like origin for brown dwarfs, though several authors posit that they could form like planets in a circumstellar disc, and then subsequently be ejected into a star-forming region or the Galactic field. Recently, JWST observations have discovered nine substellar objects in the IC348 star-forming region with a spectral absorption feature at 3.4$μ$m from an unidentified aliphatic hydrocarbon, detected for the first time in planetary atmospheres outside of the Solar System. It is unclear whether these hydrocarbon absorption features in these 'H-type' objects indicate a different formation mechanism compared to more massive brown dwarfs. We quantify the spatial distribution of these objects and find they are indistinguishable from the spatial distribution of stars and other brown dwarfs in IC348. We use N-body simulations to test whether the H-type objects could have formed as planets in circumstellar discs and then been dynamically ejected by stellar fly-bys. We show that a similar number of free-floating planets could be produced if those planets initially resided at ~5au from their host stars. However, these free-floating planets have a much more dispersed spatial distribution than the stars and brown dwarfs, inconsistent with the spatial distribution of the H-type objects in IC348. We therefore conclude that the H-type objects are unlikely to have a planetary-like origin.

astro-ph.EP↗

arXiv2026

The Effect of External Photoevaporation on the Disk Fraction in M17

A major obstacle to improving models of planet formation is understanding how the local environment influences the lifetime of the disks in which they form. The spread in observed disk lifetimes is caused by effects both observational (e.g., target selection, survey sensitivity) and physical (e.g., disk destruction by internal and external photoevaporation); however, the degree to which each plays a role remains poorly constrained. Isolating the impact of external photoevaporation on the disk lifetime benefits from the inclusion of low-mass ($\lesssim0.5$ M$_{\odot}$) YSOs, for which this effect is most predominant. In this work, we measure the inner disk fraction from JHK excess in the ~6000 M$_{\odot}$, ~1 Myr-old star-forming region M17. Using VLT/HAWK-I, we perform a deep photometric survey of an ~8$^{\prime}\times$8$^{\prime}$ field towards the region. The ~4 times greater sensitivity and ~2-3 times higher resolution than previous surveys of M17 reveal 10,339 sources. We select cluster members using the Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) catalog and find a disk fraction of 28$\pm$2%: the first X-ray-selected disk fraction measurement in M17 to include low-mass YSOs, and only the second such measurement in any high-mass star-forming region. After correcting for observational biases, we find no correlation between disk fraction and incident UV flux within M17, likely due to dynamical mixing within the region. However, when compared to other regions of similar age, we find lower disk fractions in regions with higher UV fields, suggesting that external photoevaporation decreases the average disk lifetime.

astro-ph.SR↗

arXiv2026

Drifters on the edge of town: $λ$ Boötis stars in clusters

$λ$ Boötis stars are a subset of chemically peculiar A-stars that display Solar abundances in lighter elements (C, N, O, S, etc.) but a deficiency in Iron-peak elements. This difference has been attributed to the A-stars accreting pristine (metal deficient) gas from the Interstellar Medium. However, the recent discovery of $λ$ Boötis stars in clusters challenges this theory, due to the presence of ionising radiation from intermediate/massive ($>$5 M$_\odot$) stars, which could prevent accretion of pristine ISM gas. We use $N$-body simulations to track the dynamical histories of A-stars during the evolution of a star cluster. We find that some stars leave the confines of the cluster and travel beyond the tidal radius, where they may be able to accrete pristine ISM gas. These A-stars then sometimes move back into the inner regions of the cluster, but the photoionising radiation flux they receive is not high enough to prevent $λ$ Boötis abundances from occurring in these A-stars. We find that A-stars can develop $λ$ Boötis abundances and subsequently form a wide ($>100$ au) binary system, meaning that observations of binary systems that have different abundances between the component stars would not rule out the ISM accretion scenario. Whilst we have shown that $λ$ Boötis stars can reside in and around star clusters, further research is required to assess the validity of the accretion rates required to explain their abundance patterns.

astro-ph.SR↗

arXiv2025

The long-term effects of gas removal from hydrodynamic simulations of star formation

The removal of gas left over from star formation has long been thought to dominate the dynamical evolution, and dissolution of star-forming regions. Feedback from massive stars from their stellar winds, photoionising radiation and supernovae is postulated to expel significant amounts of gas, altering the gravitational potential energy of the star-forming region and causing a supervirial expansion, which disperses the stars into the Galaxy on rapid timescales (<10Myr). The majority of previous work has utilised N-body simulations with a background potential to model the effects of gas removal. Here, we adopt a different approach where we take the end point of hydrodynamic simulations of star formation in which stars form with and without feedback from massive stars and then evolve the stars as N-body simulations. We also scale the velocities of the stars to various virial ratios, to mimic slower or faster removal of gas, and evolve these as additional N-body simulations. We find that the simulations where the stars inherit the velocities of the sink particles from the hydrodynamic simulations predominantly evolve more like a simulation in virial equilibrium, rather than the supervirial behaviour we would expect after gas removal. We see no significant differences in the dynamical evolution between the simulations where the stars inherit velocities directly from the hydrodynamical simulations and the simulations with (sub)virial velocities. This strongly suggests that gas removal by feedback processes does not lead to rapid expansion of star-forming regions, beyond the expansion caused by dynamical relaxation in star-forming regions.

astro-ph.GA↗

arXiv2025

Short-Lived Radioisotopic enrichment from AGB interlopers in low-mass star-forming regions

The decay of Short-Lived Radioisotopes (SLRs) can be a significant source of heating early in protoplanetary systems, though how a protoplanetary disk becomes enriched with these SLRs far above the galactic background level remains an open question. Observational evidence suggests that this enrichment occurs during the period from when the disk forms to when it progresses into a protoplanetary system, and is homogenous throughout the resultant planetary system. Whilst SLRs such as 26Al and 60Fe can be injected into disks through interaction with Wolf-Rayet winds and supernovae, these outflows can disrupt disks, and in the case of low-mass star-forming regions high-mass stars may not form at all. Recent research has determined the existence of AGB ``interlopers'', Asymptotic Giant Branch stars that pass close to or through star-forming regions that could be an alternative source of SLR enrichment to WR winds and supernovae. In this paper we study the effect of AGB interlopers on star-forming regions from a dynamical perspective, determining the enrichment amount of 26Al and 60Fe in disks within small clusters via numerous N-body simulations via a parameter space exploration. We find that enrichment is widespread from AGB stars, with efficient enrichment dependent on the time at which the interloper intersects the star-forming region. Velocity is a factor, though interlopers travelling at 30 km/s are capable of enriching many disks in a star-forming region assuming they encounter a disk when the interloper is more evolved.

astro-ph.EP↗

arXiv2025

Hidden binaries in star-forming regions

A significant fraction of, and possibly all, stars form in binary or multiple systems. For Solar-mass stars in the Galactic field, the distribution of orbital separations is log-normal over seven orders of magnitude, from $10^{-2} - 10^5$ au. In contrast, the separation distributions of systems in nearby star-forming regions paints a much more confusing picture. There appears to be an excess of systems in the separation range 10 - 1000 au, and recent high-resolution spectroscopic observations of close (<10 au) systems suggest a field-like distribution in some star-forming regions, but a possible excess with respect to the field in other regions. Furthermore, the resolution limit of numerical simulations of binary star formation is $\sim$1\,au, and consequently comparisons with the binary distributions in star-forming regions and in the field are restricted. In this paper, we demonstrate that these observational uncertainties, and limitations in the simulations, are potentially a much bigger problem than previously realised. We show that the log-normal separation distribution in the field can be reproduced by combining constituent binary populations whose initial separation distributions have a very different form to a log-normal. We also argue that the observed excess of binaries in the range 10 - 62 au in the ONC compared to the Galactic field is not necessarily inconsistent with the field population, because the ONC is only one of many star-forming regions that populate the field. We propose that further observations of spectroscopic binaries in star-forming regions to probe and complete the <10 au parameter space are urgently needed.

astro-ph.SR↗

arXiv2025

Can planet-planet binaries survive in star-forming regions?

Significant numbers of free-floating planetary-mass objects have been discovered in nearby star-forming regions by the James Webb Space Telescope, including a substantial number (42) of Jupiter Mass Binary Objects ('JuMBOs') in the Orion Nebula Cluster. The JuMBOs have much wider separations than other populations of substellar binaries, and their existence challenges conventional theories of substellar and planetary-mass object formation. Whilst several theories have been proposed to explain their formation, there has yet to be a study that determines whether they could survive the dynamical encounters prevalent within a dense star-forming region. We place a population of planet-planet binaries in N-body simulations of dense star-forming regions and calculate their binary fraction over time. We find that between 50-90 per cent of planet-planet binaries are destroyed on timescales of a few Myr, which implies that many more must form if we are to observe them in their current numbers. Furthermore, if the ONC was much more dense at formation, the initial separation distribution of the JuMBOs must have been even wider (and less similar to other substellar binaries) than the observed distribution.

astro-ph.SR↗

arXiv2025

The past, present and future of observations of externally irradiated disks

Recent years have seen a surge of interest in the community studying the effect of ultraviolet radiation environment, predominantly set by OB stars, on protoplanetary disc evolution and planet formation. This is important because a significant fraction of planetary systems, potentially including our own, formed in close proximity to OB stars. This is a rapidly developing field, with a broad range of observations across many regions recently obtained or recently scheduled. In this paper, stimulated by a series of workshops on the topic, we take stock of the current and upcoming observations. We discuss how the community can build on this recent success with future observations to make progress in answering the big questions of the field, with the broad goal of disentangling how external photoevaporation contributes to shaping the observed (exo)planet population. Both existing and future instruments offer numerous opportunities to make progress towards this goal.

astro-ph.SR↗

arXiv2024

Deviations from the universal Initial Mass Function in binary star clusters

The stellar mass distribution in star-forming regions, stellar clusters and associations, the Initial Mass Function (IMF), appears to be invariant across different star-forming environments, and is consistent with the IMF observed in the Galactic field. Deviations from the field, or standard, IMF, if genuine, would be considered strong evidence for a different set of physics at play during the formation of stars in the birth region in question. We analyse N-body simulations of the evolution of spatially and kinematically substructured star-forming regions to identify the formation of binary star clusters, where two (sub)clusters which form from the same Giant Molecular Cloud orbit a common centre of mass. We then compare the mass distributions of stars in each of the subclusters and compare them to the standard IMF, which we use to draw the stellar masses in the star-forming region from which the binary cluster(s) form. In each binary cluster that forms, the mass distributions of stars in one subcluster deviates from the standard IMF, and drastically so when we apply similar mass resolution limits as for the observed binary clusters. Therefore, if a binary subcluster is observed to have an unusual IMF, this may simply be the result of dynamical evolution, rather than different physical conditions for star formation in these systems.

astro-ph.GA↗

arXiv2024

Formation of Jupiter-Mass Binary Objects through photoerosion of fragmenting cores

The recent discovery of tens of Jupiter-mass binary objects (JuMBOs) in the Orion Nebula Cluster with the James Webb Space Telescope has intensified the debate on the origin of free-floating planetary mass objects within star-forming regions. The JuMBOs have masses below the opacity limit for fragmentation, but have very wide separations (10s - 100s au), suggesting that they did not form in a similar manner to other substellar mass binaries. Here, we propose that the theory of photoerosion of prestellar cores by Lyman continuum radiation from massive stars could explain the JuMBOs in the ONC. We find that for a range of gas densities the final substellar mass is comfortably within the JuMBO mass range, and that the separations of the JuMBOs are consistent with those of more massive (G- and A-type) binaries, that would have formed from the fragmentation of the cores had they not been photoeroded. The photoerosion mechanism is most effective within the HII region(s) driven by the massive star(s). The majority of the observed JuMBOs lie outside of these regions in the ONC, but may have formed within them and then subsequently migrated due to dynamical evolution.

astro-ph.EP↗

arXiv2024

Signatures of mass segregation from competitive accretion and monolithic collapse

The two main competing theories proposed to explain the formation of massive ($>10$M$_\odot$) stars -- competitive accretion and monolithic core collapse -- make different observable predictions for the environment of the massive stars during, and immediately after, their formation. Proponents of competitive accretion have long predicted that the most massive stars should have a different spatial distribution to lower-mass stars, either through the stars being mass segregated, or being in areas of higher relative densities, or sitting deeper in gravitational potential wells. We test these predictions by analysing a suite of SPH simulations where star clusters form massive stars via competitive accretion with and without feedback. We find that the most massive stars have higher relative densities, and sit in deeper potential wells, only in simulations in which feedback is not present. When feedback is included, only half of the simulations have the massive stars residing in deeper potential wells, and there are no other distinguishing signals in their spatial distributions. Intriguingly, in our simple models for monolithic core collapse, the massive stars may also end up in deeper potential wells, because if massive cores fragment the stars are still massive, and dominate their local environs. We find no robust diagnostic test in the spatial distributions of massive stars that can distinguish their formation mechanisms, and so other predictions for distinguishing between competitive accretion and monolithic collapse are required.

astro-ph.SR↗

arXiv2024

No signature of the birth environment of exoplanets from their host stars' Mahalanobis phase space

The architectures of extrasolar planetary systems often deviate considerably from the ``standard" model for planet formation, which is largely based on our own Solar System. In particular, gas giants on close orbits are not predicted by planet formation theory and so some process(es) are thought to move the planets closer to their host stars. Recent research has suggested that Hot Jupiter host stars display a different phase space compared to stars that do not host Hot Jupiters. This has been attributed to these stars forming in star-forming regions of high stellar density, where dynamical interactions with passing stars have perturbed the planets. We test this hypothesis by quantifying the phase space of planet-hosting stars in dynamical N-body simulations of star-forming regions. We find that stars that retain their planets have a higher phase space than non-hosts, regardless of their initial physical density. This is because an imprint of the kinematic substructure from the regions birth is retained, as these stars have experienced fewer and less disruptive encounters than stars whose planets have been liberated and become free-floating. However, host stars whose planets remain bound but have had their orbits significantly altered by dynamical encounters are also primarily found in high phase space regimes. We therefore corroborate other research in this area which has suggested the high phase space of Hot Jupiter host stars is not caused by dynamical encounters or stellar clustering, but rather reflects an age bias in that these stars are (kinematically) younger than other exoplanet host stars.

astro-ph.SR↗

arXiv2024

The dynamical evolution of star-forming regions measured with INDICATE

Observations of star-forming regions provide snapshots in time of the star formation process, and can be compared with simulation data to constrain the initial conditions of star formation. In order to make robust inferences, different metrics must be used to quantify the spatial and kinematic distributions of stars. In this paper, we assess the suitability of the INDICATE (INdex to Define Inherent Clustering And TEndencies) method as a diagnostic to infer the initial conditions of star-forming regions that subsequently undergo dynamical evolution. We use INDICATE to measure the degree of clustering in N-body simulations of the evolution of star-forming regions with different initial conditions. We find that the clustering of individual stars, as measured by INDICATE, becomes significantly higher in simulations with higher initial stellar densities, and is higher in subvirial star-forming regions where significant amounts of dynamical mixing has occurred. We then combine INDICATE with other methods that measure the mass segregation, relative stellar surface density ratio and the morphology (Q-parameter) of star-forming regions, and show that the diagnostic capability of INDICATE increases when combined with these other metrics.

astro-ph.GA↗

arXiv2024

Devolatilization of extrasolar planetesimals by 60Fe and 26Al heating

Whilst the formation of Solar system planets is constrained by meteoritic evidence, the geophysical history of low-mass exoplanets is much less clear. The bulk composition and climate states of rocky exoplanets may vary significantly based on the composition and properties of the planetesimals they form from. An important factor influenced by planetesimal composition is water content, where the desiccation of accreting planetesimals impacts the final water content of the resultant planets. While the inner planets of the Solar system are comparatively water-poor, recent observational evidence from exoplanet bulk densities and planetary formation models suggest that rocky exoplanets engulfed by substantial layers of high-pressure ices or massive steam atmospheres could be widespread. Here we quantify variations in planetesimal desiccation due to potential fractionation of the two short-lived radioisotopes 26Al and 60Fe relevant for internal heating on planetary formation timescales. We focus on how order of magnitude variations in 60Fe can affect the water content of planetesimals, and how this may alter the formation of extrasolar ocean worlds. We find that heating by 26Al is the dominant cause of planetesimal heating in any Solar system analogue scenario, thus validating previous works focussing only on this radioisotope. However, 60Fe can become the primary heating source in the case of high levels of supernova enrichment in massive star-forming regions. These diverging scenarios can affect the formation pathways, bulk volatile budget, and climate diversity of low-mass exoplanets.

astro-ph.EP↗

arXiv2023

A dependence of binary and planetary system destruction on subtle variations in the substructure in young star-forming regions

Simulations of the effects of stellar fly-bys on planetary systems in star-forming regions show a strong dependence on subtle variations in the initial spatial and kinematic substructure of the regions. For similar stellar densities, the more substructured star-forming regions disrupt up to a factor of two more planetary systems. We extend this work to look at the effects of substructure on stellar binary populations. We present $N$-body simulations of substructured, and non-substructured (smooth) star-forming regions in which we place different populations of stellar binaries. We find that for binary populations that are dominated by close ($<$100au) systems, a higher proportion are destroyed in substructured regions. However, for wider systems ($>$100au), a higher proportion are destroyed in smooth regions. The difference is likely due to the hard-soft, or fast-slow boundary for binary destruction. Hard (fast/close) binaries are more likely to be destroyed in environments with a small velocity dispersion (kinematically substructured regions), whereas soft (slow/wide) binaries are more likely to be destroyed in environments with higher velocity dispersions (non-kinematically substructured regions). Due to the vast range of stellar binary semimajor axes in star-forming regions ($10^{-2} - 10^4$au) these differences are small and hence unlikely to be observable. However, planetary systems have a much smaller initial semimajor axis range (likely $\sim$1 -- 100au for gas giants) and here the difference in the fraction of companions due to substructure could be observed if the star-forming regions that disrupt planetary systems formed with similar stellar densities.

astro-ph.SR↗

arXiv2023

On the origin of planetary-mass objects in NGC1333

The dominant formation mechanism of brown dwarfs and planetary mass objects in star-forming regions is presently uncertain. Do they form like stars, via the collapse and fragmentation of cores in Giant Molecular clouds, or do they form like planets in the discs around stars and are ejected via dynamical interactions? In this paper, we quantify the spatial distribution of substellar objects in NGC1333, in particular focusing on planetary-mass objects that have been the target of recent deep imaging observations. We find that these objects have a spatial distribution that is indistinguishable from the stars, and more massive brown dwarfs. We also analyse N-body simulations and find that a population of ejected planets would have a significantly different spatial and kinematic distribution to stars, and brown dwarfs that also formed through gravitational collapse and fragmentation. We therefore conclude that the low-mass substellar objects in NGC1333 formed more like stars than planets, although we predict that a population of hitherto undetected ejected planetary mass objects may be lurking in this, and other star-forming regions.

astro-ph.SR↗

arXiv2023

Isotopic enrichment of planetary systems from Asymptotic Giant Branch stars

Short-lived radioisotopes, in particular 26-Al and 60-Fe, are thought to contribute to the internal heating of the Earth, but are significantly more abundant in the Solar System compared to the Interstellar Medium. The presence of their decay products in the oldest Solar System objects argues for their inclusion in the Sun's protoplanetary disc almost immediately after the star formation event that formed the Sun. Various scenarios have been proposed for their delivery to the Solar System, usually involving one or more core-collapse supernovae of massive stars. An alternative scenario involves the young Sun encountering an evolved Asymptotic Giant Branch (AGB) star. AGBs were previously discounted as a viable enrichment scenario for the Solar System due to the presumed low probability of an encounter between an old, evolved star and a young pre-main sequence star. We report the discovery in Gaia data of an interloping AGB star in the star-forming region NGC2264, demonstrating that old, evolved stars can encounter young forming planetary systems. We use simulations to calculate the yields of 26-Al and 60-Fe from AGBs and their contribution to the long-term geophysical heating of a planet, and find that these are comfortably within the range previously calculated for the Solar System.

astro-ph.EP↗