Astrophysics Formal Seminars

Formal seminars take place every Monday during full term. For a complete list of talks please visited the website.

Meteorite paleomagnetism: Constraints on the rearrangement of the planets and the formation of the first solids

It has been suggested that there were at least two major planetary rearrangements within the first 1 Gyr of our solar system. Such events are believed to have played a crucial role in shaping the present-day architecture our solar system as well as possibly those of exoplanetary systems. Within our own solar system, planetary migrations have been proposed to have brought material that formed beyond the orbit of the gas giants into the inner solar system, possibly explaining the compositional trends across the asteroid belt as well as the makeup of the Trojan asteroids. However, very few robust, accurate or quantitative estimates of the heliocentric distances of the formation of meteorite parent bodies exist. These distance estimates would also constrain the range over which the first solids (chondrules and calcium-aluminium-rich inclusions [CAIs]) may have formed or have been recycled throughout the solar system by stellar outflows. Here, we present paleomagnetic evidence that the Tagish Lake meteorite does not contain a stable magnetic remanence. Given the ancient aqueous alteration age of this meteorite ( 10 – 20 AU where the magnetic field generated by the collapse of the dust and gas within the nebula was < 0.15 µT. This distance corresponds to radii greater than the orbits of the gas giants prior to the migrations involved in Grand Tack, suggesting the Tagish Lake parent body represents outer disk bodies that now constitute the Kuiper belt and could therefore feasibly be a comet. Tagish Lake contains sparse chondrules and even rare CAIs, indicating that stellar outflows were capable of transporting solid material that formed within 1 AU of the Sun and within 1 Myr of CAI formation to distances as far as that of present-day Saturn or Uranus. Finally, our results provide a quantitative observation from the meteorite record that a body formed in the outer solar system and now resides in the inner solar system, supporting the presence of major ancient planetary migrations that altered the architecture and structure of our solar system.

Wave-Vortex Interactions, Remote Recoil, the Aharanov-Bohm Effect and the Craik-Leibovich Equation

Four of the simplest examples of interaction between a single wavetrain and a single vortex are analysed, with a focus on effective recoil forces, local and remote. All four examples comply with the pseudomomentum rule. The first three examples are two-dimensional and non-rotating (shallow-water or gas dynamical), and the fourth is rotating, with deep-water waves inducing an Ursell-Hasselmann-Pollard `anti-Stokes flow’. The Froude or Mach number, and the Rossby number in the fourth example, are assumed small. Contrary to a recent suggestion, the anti-Stokes flow does not suppress remote recoil. Remote recoil is all or part of the interaction in all four examples, except in one special limiting case. That is the only case in which, exceptionally, the effective recoil force can be regarded as entirely local, and therefore identifiable with the Craik-Leibovich vortex force. It is the case often focused on in the quantum fluids literature, in connection with phonon-vortex interactions. Another peculiarity of that case is that the only significant wave refraction effect is the Aharonov-Bohm topological phase jump.

An introduction to Asteroseismology of cool stars

Asteroseismology is one of the major success stories of space-based photometry revolution led by the Kepler and CoRoT missions. The wide-scale detection of oscillations in solar-type stars has led to breakthroughs in in many areas, e.g., the internal rotation of stars, the characterisation of exoplanet host stars, and the measurement of stellar masses and ages for field stars. In fact, Asteroseismology is has now become a reference technique to calibrate many other methods of studying stars. I will explain why this has come to be. I will review why cool stars pulsate, how they pulsate, and what information can be extracted from the pulsations. I will present some of the highlight reel science results from asteroseismology and discuss the bright future ahead with the help of TESS , PLATO, and WFIRST .

Vorticies from low-mass planet formation in "transition discs”

I will describe how so-called “transition discs” are natural sites for the trapping of pebble-sized dust particles. Consequently these pebble traps are natural sites for pebble accretion. I will argue that rapid pebble accretion can lead to the disc becoming locally baroclinic and prone to the formation of anti-cyclonic vorticies. I will then sketch out an evolutionary cycle of vortex formation and destruction and relate this mechanism to the observed asymmetries in “transition discs”.

Giant planet formation in radially structured discs

Observations have demonstrated the existence of a significant population of compact systems comprised of super-Earths and Neptune-mass planets, and a population of gas giants that appear to occur primarily in either short-period (%<%10 days) or longer period (>100 days) orbits. The broad diversity of system architectures raises the question of whether or not the same formation processes operating in standard disc models can explain these planets, or if different scenarios are required instead to explain the widely differing architectures. To explore this issue, we present recent results from a comprehensive suite of N-body simulations of planetary system formation that include the following physical processes: N-body interactions between planetary embryos and planetesimals; type I and II migration; gas accretion onto planetary cores; self-consistent viscous disc evolution and disc removal through photo-evaporation. Results indicate that the formation and survival of compact systems of super-Earths and Neptune-mass planets occur commonly in disc models where a simple prescription for the disc viscosity is assumed, but such models never lead to the formation and survival of gas giant planets due to migration into the star. Inspired in part by the ALMA observations of HL Tau, and by MHD simulations that display the formation of long-lived zonal flows, we have explored the consequences of assuming that the disc viscosity varies in both time and space. We find that the radial structuring of the disc leads to conditions in which systems of giant planets are able to form and survive. Furthermore, these giants generally occupy those regions of the mass-period diagram that are densely populated by the observed gas giants, suggesting that the planet traps generated by radial structuring of protoplanetary discs may be a necessary ingredient for forming giant planets.

Catching forming giant planets -- hydrodynamic simulations and observations of the circumplanetary disks

Recently, younger and younger planets were detected, often still embedded in gaseous circumstellar disks. In this evolutionary phase, giant planets are still accreting from their own disk, the circumplanetary disk. This disk is the key to understand the late giant planet formation, the satellite formation and to unveil how we can observe forming planets and what characteristics we can derive out from the data. Even though there is no direct observation of the circumplanetary disk yet, the trace of hot gas around gas-giants has already been detected from near-infrared emission via high contrast imaging and by H-alpha emission. The race for the first firm detection is still continuing and I will present how we can detect these disks with ALMA . In my talk, I will summarize what we learned about giant planet accretion and satellite formation from the newest radiative hydrodynamic simulations, and how the circumplanetary disk influences the entropy of the gas giant, making the “hot-start” versus “cold-start” debate even more complicated.

Title to be confirmed

Abstract not available

Title to be confirmed

Abstract not available