Research News & Highlights

The Local Organising Committee (LOC)

Earlier this year, the Discs group hosted the inaugural UK & Ireland Discs Meeting. We had over 100 disc experts join us on what was a fantastic few days of talks, discussion, and networking with the UK & Ireland community. We hope this conference turns into a regular occurrence and we look forward to the next iteration of the meeting at the University of Hertfordshire in September 2025!

Noah Sims, Tim Pearce, Minjae Kim

URSS student Noah Sims recently completed a10-week project focusing on the intermediate dust ring around Formalhaut:

Last year, the James Webb Space Telescope (JWST) sparked discussion on the origin of a newly observed phenomenon around the star Fomalhaut. More specifically, an undiscovered dust ring dubbed the ‘intermediate’ disc. Fomalhaut is known to contain a famous debris disc - a collection of dust and larger bodies like the Asteroid belt - in the outer region of the system. Unlike the material in the outer disc, which has a wide range of sizes from small micrometre dust (seen by JWST) to large millimetre grains (seen by the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile), the intermediate disc only contains small dust. A potential explanation is that the dust migrates inwards through well-known radiation forces. However, this model only predicts the location of the intermediate disc and not the fine structure seen. Including planets into the model could help bridge this gap. We created simulations with dust of different sizes, which have different susceptibilities to the radiation forces. We then ran the simulations to let the dust spiral inwards and interact with the planets. We found that using two planets produced similar observations to JWST and ALMA: (i) a Uranus mass near the inner edge of the intermediate disc and (ii) a Mars mass between the intermediate and main discs. This indicates the possible layout of the Fomalhaut system and gives favourable conclusions that planets could explain the intermediate disc. This may motivate future observations of Fomalhaut and potentially the discovery of new planets.

A poster will be displayed at the URSS Showcase on 13 November, and is available to view here.

Dimitri Veras, Shigeru Ida

Estimates of the frequency of planetary systems in the Milky Way are observationally limited by the low-mass planet regime. Nevertheless, substantial evidence for systems with undetectably low planetary masses now exist in the form of main-sequence stars which host debris discs, as well as metal-polluted white dwarfs. Further, low-mass sections of star formation regions impose upper bounds on protoplanetary disc masses, limiting the capacity for terrestrial or larger planets to form. Here, we use planetary population synthesis calculations to investigate the conditions that allow planetary systems to form only minor planets and smaller detritus. We simulate the accretional, collisional and migratory growth of 1017 kg embryonic seeds and then quantify which configurations with entirely sub-Earth-mass bodies (≲ 1024 kg) survive. We find that substantial regions of the initial parameter space allow for sub-terrestrial configurations to form, with the success rate most closely tied to the initial dust mass. Total dust mass budgets of up to 102M⊕ within 10 au can be insufficiently high to form terrestrial or giant planets, resulting in systems with only minor planets. Consequently, the prevalence of planetary systems throughout the Milky Way might be higher than what is typically assumed, and minor planet-only systems may help inform the currently uncertain correspondence between planet-hosting white dwarfs and metal-polluted white dwarfs.

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For an Astrobites article, see the link here: https://astrobites.org/2024/10/01/pluto-gets-revenge-a-solar-system-of-minor-planets/

Our discs group recently started the UK & Ireland (UKI) Discs community, where we aim to bring together the disc communities across the UK & Ireland for those working on astrophysical discs on various scales, from circumplanetary to galactic discs. We are excited to announce the first UKI Discs Meeting will be hosted at the University of Warwick from the 9th - 11th of September 2024 and will focus on protoplanetary discs and debris discs (including white dwarf discs). To register and find out more, follow the link hereLink opens in a new window.

The abstract submission deadline for talks/posters is the 12th of May 2024.

Grant M. Kennedy, Joshua B. Lovell, Paul Kalas, Michael P. Fitzgerald

At 7.7 pc, the A-type star Fomalhaut hosts a bright debris disk with multiple radial components. The disk is eccentric and misaligned, strongly suggesting that it is sculpted by interaction with one or more planets. Compact sources are now being detected with JWST, suggesting that new planet detections may be imminent. However, to confirm such sources as companions, common proper motion with the star must be established, as with unprecedented sensitivity comes a high probability that planet candidates are actually background objects. Here, ALMA and Keck observations of Fomalhaut are found to show significant emission at the same sky location as multiple compact sources in JWST MIRI coronagraphic observations, one of which has been dubbed the "Great Dust Cloud" because it lies within the outer belt. Since the ground-based data were obtained between 6 to 18 years prior to the JWST observations, these compact sources are unlikely to be common proper motion companions to Fomalhaut. More generally, this work illustrates that images collected at a range of wavelengths can be valuable for rejecting planet candidates uncovered via direct imaging with JWST.

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Dimitri Veras, Shigeru Ida, Evgeni Grishin, Scott J. Kenyon, Benjamin C. Bromley

The suite of over 60 known planetary debris discs which orbit white dwarfs, along with detections of multiple minor planets in these systems, motivate investigations about the migration properties of planetesimals embedded within the discs. Here, we determine whether any of the migration regimes which are common in (pre-)main-sequence protoplanetary discs, debris discs and ring systems could be active and important in white dwarf discs. We investigate both dust-dominated and gas-dominated regions, and quantitatively demonstrate that Type I and Type II migration, as well as their particulate disc analogues, are too slow to be relevant in white dwarf discs. However, we find that the analogue of Type III migration for particulate discs may be rapid in the dusty regions of asteroid- or moon-generated (>1018 kg) white dwarf discs, where a planetesimal exterior to its Roche radius may migrate across the entire disc within its lifetime. This result holds over a wide range of disc boundaries, both within and exterior to 1R⊙, and such that the probability of migration occurring increases with higher disc masses.

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Dimitri Veras, Nikolaos Georgakarakos, Ian Dobbs-Dixon

The evolution of planetary debris discs, such as extrasolar analogues of the solar system's Main Belt and Kuiper Belt, crucially determine how host stars are polluted with planetary metals. This new paper presents high-resolution resonant portraits of these debris discs as they evolve beyond the main-sequence and interact with a surviving planet in the system. We find that evolving resonant structure can be accurately bound with only main-sequence values by computing a maximum libration width as a function of asteroid longitude of pericentre. We also quantify the relative efficiency of mean motion resonances of different orders to stabilize versus destabilize asteroid orbits during both the giant branch and white dwarf phases. The 4:1, 3:1 and 2:1 resonances represent efficient polluters of white dwarfs, and even when in the orbit-crossing regime, both the 4:3 and 3:2 resonances can retain small reservoirs of asteroids in stable orbits throughout giant branch and white dwarf evolution.

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Raphael Bendahan-West -- Automated Search for Spectroscopic Exocomet Transits

Exocomets are analogues of solar system comets, with the main difference being that they orbit stars other than the Sun. These extrasolar comets can be thought of as small icy planetesimals that were left unused during the planet formation phase of a system. Although being “leftover” dust agglomerates, exocomets tend to be part of the most pristine objects in a system, enclosing primordial information about the early phases of planetary formation.

The field of exocomets has been built around the unmatched number of detections made in the circumstellar disc of the archetypal star Beta Pictoris. An exocomet detection is identified by variable absorption features around atomic lines in a stellar spectrum (typically around the ionised Calcium doublet), which can be explained by the interaction between the light emitted by a star and the transiting gas that is in and trailing an exocomet coma. Variability in the CaII doublet has been observed around approximately 30 different stars – certain detections being clearer than others, emphasising the rarity and difficulty to observe such events.

In this talk, I will show results from an automated search through the HARPS archive for signs of exocomet transits. Such a spectroscopic survey is unprecedented mainly due to the large quantity and wide variety of stars being observed. This talk will introduce our newly developed automated search algorithm that quantifies the rate of CaII exocomet absorption events in archival spectra, with the goal to estimate the occurrence rate of transiting exocomets.

Azib Norazman -- An Automated Search for Transiting Exocomets with TESS

Patrick F. Cronin-Coltsmann, Grant M. Kennedy, Paul Kalas, Julien Milli, Cathie J. Clarke, Gaspard Duchêne, Jane Greaves, Samantha M. Lawler, Jean-François Lestrade, Brenda C. Matthews, Andrew Shannon, Mark C. Wyatt

Fomalhaut C (LP 876-10) is a low mass M4V star in the intriguing Fomalhaut triple system and, like Fomalhaut A, possesses a debris disc. It is one of very few nearby M-dwarfs known to host a debris disc and of these has by far the lowest stellar mass. We present new resolved observations of the debris disc around Fomalhaut C with the Atacama Large Millimetre Array which allow us to model its properties and investigate the system's unique history. The ring has a radius of 26 au and a narrow full width at half maximum of at most 4.2 au. We find a 3σ upper limit on the eccentricity of 0.14, neither confirming nor ruling out previous dynamic interactions with Fomalhaut A that could have affected Fomalhaut C's disc. We detect no ¹²CO J=3-2 emission in the system and do not detect the disc in scattered light with HST/STIS or VLT/SPHERE. We find the original Herschel detection to be consistent with our ALMA model's radial size. We place the disc in the context of the wider debris disc population and find that its radius is as expected from previous disc radius-host luminosity trends. Higher signal-to-noise observations of the system would be required to further constrain the disc properties and provide further insight to the history of the Fomalhaut triple system as a whole.

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