The Local Organising Committee (LOC)

Earlier this year, the Discs group (led by Farzana) 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!

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.

Article

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.

Sahl Rowther, Rebecca Nealon, Farzana Meru

We conduct gas and dust hydrodynamical simulations of protoplanetary discs with one and two embedded planets to determine the impact that a second planet located further out in the disc has on the potential for subsequent planet formation in the region locally exterior to the inner planet. We show how the presence of a second planet has a strong influence on the collection of solid material near the inner planet, particularly when the outer planet is massive enough to generate a maximum in the disc’s pressure profile. This effect in general acts to reduce the amount of material that can collect in a pressure bump generated by the inner planet. When viewing the inner pressure bump as a location for potential subsequent planet formation of a third planet, we therefore expect that the mass of such a planet will be smaller than it would be in the case without the outer planet, resulting in a small planet being sandwiched between its neighbours – this is in contrast to the expected trend of increasing planet mass with radial distance from the host star. We show that several planetary systems have been observed that do not show this trend but instead have a smaller planet sandwiched in between two more massive planets. We present the idea that such an architecture could be the result of the subsequent formation of a middle planet after its two neighbours formed at some earlier stage.

Article

Matthew Pritchard, Farzana Meru

We conduct gas and dust hydrodynamical simulations of protoplanetary discs with one and two embedded planets to determine the impact that a second planet located further out in the disc has on the potential for subsequent planet formation in the region locally exterior to the inner planet. We show how the presence of a second planet has a strong influence on the collection of solid material near the inner planet, particularly when the outer planet is massive enough to generate a maximum in the disc’s pressure profile. This effect in general acts to reduce the amount of material that can collect in a pressure bump generated by the inner planet. When viewing the inner pressure bump as a location for potential subsequent planet formation of a third planet, we therefore expect that the mass of such a planet will be smaller than it would be in the case without the outer planet, resulting in a small planet being sandwiched between its neighbours – this is in contrast to the expected trend of increasing planet mass with radial distance from the host star. We show that several planetary systems have been observed that do not show this trend but instead have a smaller planet sandwiched in between two more massive planets. We present the idea that such an architecture could be the result of the subsequent formation of a middle planet after its two neighbours formed at some earlier stage.

Press Release

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.

Article

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.

Article

Rebecca Nealon, Enrico Ragusa, Davide Gerosa, Giovanni Rosotti, Riccardo Barbieri

The inspiral of supermassive black hole (BH) binaries in a gas-rich environment is driven by the presence of an accretion disc and viscous interactions tend to align the spin of the BHs with the orbital angular momentum of the disc. Recent work introduced a new iterative approach to describe the alignment process and the resulting non-linear evolution of the surrounding warped accretion disc. Their model predicted that BH spins reach either full alignment or a 'critical obliquity' where solutions to the warp equations cease to exist. In this paper, we show that this critical region corresponds to the disc breaking phenomenon, where the disc is disrupted into two or more discrete sections. We use 3D hydrodynamical simulations to (i) recover the predictions of the semi-analytic model and (ii) unveil a richer phenomenology where the disc exhibits either unsuccessful, single and multiple breaks. We additionally identify hydrodynamic effects such as spiral arms that are able to stabilize the disc against breaking beyond criticality. Our results show that when disc breaking occurs, the ability of BHs and disc to align is compromised and in some cases even prevented as the binary inspirals.

Article

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.

Article