IoA Institute of Astronomy Talk Lists

A major hurdle in planet formation theory is that we do not understand how small pebbles congregate into big planetesimals. A promising way to overcome this metre-scale barrier involves a fluid dynamics phenomenon called the streaming instability (SI). It concentrates the pebbles into clumps that are dense enough to collapse gravitationally, thereby forming planetesimals.

Unfortunately, the mechanism responsible for the onset of the instability remains mysterious. This makes it hard to evaluate the robustness of the instability, or to understand how it saturates. It has recently been shown that the SI is a Resonant Drag Instability (RDI) involving inertial waves. In the first part of this talk, I build on this insight to produce a clear physical picture of how the SI develops.

Another problem is that the SI can only devellop in regions containing a high density of similar-sized pebbles. Those conditions are met in large-scale vortices, but no one knows if the SI can feed on vorticial flows. Indeed, any instability can only devellop in specific flows, and a priori the SI is tailored to Keplerian disc flows, not vortex flows. I answer this question in the second part of the talk. To do so, I develop a simple pen-and-paper model of a dust-laden vortex in a protoplanetary disc. I find that if the vortex is weak and anticyclonic, dust drifts towards its centre. I then build a vortex analog of the shearing box to analyse the local linear stability of my dusty vortex. I find that the dust’s drift powers an instability which closely resembles the SI. This result strengthens the case for vortex-induced planetesimal formation.

Speaker: Nathan Magnan (DAMTP)
Tuesday 23 April 2024, 13:00-14:00
Venue: Hoyle Committee Room + ONLINE - Details to be sent by email.
Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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Thu 02 May 16:00: Experimental Studies of Black Holes: Status & Prospects

Fri, 12/04/2024 - 11:45

Experimental Studies of Black Holes: Status & Prospects

More than a century ago, Albert Einstein presented his general theory of gravitation. One of the predictions of this theory is that not only particles and objects with mass, but also the quanta of light, photons, are tied to the curvature of space-time, and thus to gravity. There must be a critical mass density, above which photons cannot escape. These are black holes. It took fifty years before possible candidate objects were identified by observational astronomy. Another fifty years have passed, until we finally can present detailed and credible experimental evidence that black holes of 10 to 10^10 times the mass of the Sun exist in the Universe. Three very different experimental techniques have enabled these critical experimental breakthroughs. It has become possible to investigate the space-time structure in the vicinity of the event horizons of black holes. I will summarize these interferometric techniques, and discuss the spectacular recent improvements achieved with all three techniques. In conclusion, I will sketch where the path of exploration and inquiry may lead to in the next decades.

Speaker: Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics)
Thursday 02 May 2024, 16:00-17:00
Venue: Hoyle Lecture Theatre, Institute of Astronomy (and online - details to be sent by e-mail).
Series: The Kavli Lectures; organiser: Alison Wilson.

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Fri 19 Apr 11:30: Probing the epoch of galaxy assembly with MUSE

Thu, 11/04/2024 - 13:14

Probing the epoch of galaxy assembly with MUSE

Abstract not available

Speaker: Trevor Mendel (ANU)
Friday 19 April 2024, 11:30-12:30
Venue: Ryle seminar room + online.
Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Fri 31 May 11:30: Title to be confirmed

Tue, 09/04/2024 - 17:11

Title to be confirmed

Abstract not available

Speaker: Sinan Deger (KICC)
Friday 31 May 2024, 11:30-12:30
Venue: Ryle seminar room + online.
Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Wed 22 May 11:30: Hierarchical star cluster assembly boosts intermediate-mass black hole formation

Mon, 08/04/2024 - 09:08

Hierarchical star cluster assembly boosts intermediate-mass black hole formation

Observations and high-resolution hydrodynamical simulations indicate that massive star clusters assemble hierarchically from sub-clusters with a universal power-law cluster mass function. We study the consequences of such assembly for the formation of intermediate-mass black holes (IMBHs) and massive black hole (MBH) seeds at low metallicities (1% of the solar value) with our updated direct N-body code BIFROST in simulations up to N = 2.35 million stars. The GPU -accelerated code BIFROST is based on the hierarchical fourth-order forward integrator. Few-body systems are treated using secular and regularized techniques including post-Newtonian equations of motion up to order PN3 .5 and gravitational-wave recoil kicks for merging BHs. Stellar evolution is provided by the fast population synthesis code SEVN . IMBHs with masses up to 2200 solar masses form rapidly mainly via the collapse of very massive stars (VMSs) assembled through repeated collisions of massive stars followed by growth through tidal disruption events (TDEs) and BH mergers. Later the IMB Hs form subsystems resulting in gravitational-wave BH-BH, IMBH -BH and IMBH -IMBH mergers with a 1000 solar mass gravitational-wave detection being the observable prediction. Our simulations indicate that the hierarchical formation of massive star clusters in metal poor environments naturally results in formation of potential seeds for supermassive black holes.

Speaker: Antti Rantala (MPA)
Wednesday 22 May 2024, 11:30-12:30
Venue: Martin Ryle Meeting Room, KICC + Online.
Series: Kavli Institute for Cosmology Seminars; organiser: Alison Wilson.

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Thu 16 May 16:00: Inside Astronomically Realistic Black Holes

Fri, 05/04/2024 - 23:35

Inside Astronomically Realistic Black Holes

I will use a real-time general relativistic Black Hole Flight Similator to show what really happens inside astronomically realistic black holes. The inner horizon of a rotating black hole is the most violent place in the Universe, easily reaching and surpassing energy densities attained in the Big Bang. What does Nature do at this extraordinary place?