Exoplanets and Stellar Physics

Stellar activity in Radial Velocity surveys
Suzanne Aigrain
Description: Stellar activity is a broad term that refers to time-variable phenomena due to the interplay of magnetism and convection in the surface layers of stars. Activity gives rise to variability on wide range of times scales and wavelengths, and these are a major nuisance for exoplanet searches, particularly using the radial velocity method. At the same time, the time-series of spectra obtained during RV planet searches contain very valuable information on activity-related phenomena - rotation rates, activity cycle, effect of different types of surface structures on different spectral lines. In this project we will use the vast database of over 250000 spectra collected to date by the HARPS and HARPS-N RV spectrographs to explore the dependency of different activity indicators, and their variability, on stellar parameters. One initial goal of the project will be to measure activity cycle periods and stellar rotation periods from activity indicators such as the log R'_HK index, in order to calibrate the relationship between the these two periodicities and the stellar mass and overall activity level. Another goal will be to look for correlations between the activity indicators and the RV variability on different timescales, and identify which indicators are likely to be most useful to correct activity-induced RV variations for different types of stars.

Discovery of new microquasars
Katherine Blundell
The proposed project will make use of the Many Commensal Cameras (MC2) data streams from the worldwide Global Jet Watch observatories that are making sensitive, successive observations of the signatures of precessing jets exhibited by microquasars in our Galaxy. Promising candidates will be followed-up and investigated via spectroscopy to discern dynamical confirmation of precessing jets moving at relativistic speeds. Unique data streams for this student project are assured, from the five Global Jet Watch observatories around the planet (PI K Blundell; www.GlobalJetWatch.net) although in addition there will be scope to obtain complementary data at radio wavelengths with milli-arcsec-scale angular resolution.

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Circumbinary discs and planets
Katherine Blundell

In recent years the existence and significance of circumbinary discs, orbiting outside of pairs of binary stars in orbit around one another, has emerged. Not only are these purported to have significant dynamical back-reactions on their inner binary stars (and hence their evolution) and in the development of nova explosions but they are in some cases likely to be the breeding ground of Tatooine-like circumbinary planets. The goal is to explore and understand the nature of binary star systems that host such circumbinary structures, using data from the Global Jet Watch telescopes (PI K Blundell; www.GlobalJetWatch.net).

Exoplanet Atmospheres in High Resolution
Jayne Birkby

When we observe exoplanets in high resolution, be it via direct imaging or by spectroscopy, a wealth of information about the exoplanetary atmosphere awaits us. These powerful high resolution techniques can reveal the composition, structure, and dynamics of exoplanets atmospheres, including their global winds patterns and rotation, alongside their variability over time that may indicate their appearance or even presence of exomoons. This project seeks a student eager to explore exoplanet atmospheres in detail through observational and simulated data, with input from theoretical modelling. The goal is to explore the properties of a range of different planets, from gas giants, to mini-Neptunes and super-Earths, in a goal to understand the incredible diversity of exoplanet atmospheres. The DPhil timeline aligns with the final stages of preparation for the first light of the Extremely Large Telescope (ELT), which will host instruments highly suited to survey small rocky planets in our local neighbourhood using the techniques learnt during this DPhil. With this in mind, the student will not only join Prof Birkby’s team of DPhils and postdocs working on exoplanet atmospheres, but will be encouraged to explore connections with researchers in atmospheric physics and Earth Sciences as part of the Oxford Network for Exoplanets too.

Caroline Terquem
Dissipation of tides in the convective envelope of stars

A large proportion of stars are found in binary systems. When the distance between the two stars in such systems is small enough, oscillations are excited in each of the stars by the tidal potential of its companion. These tidal waves are dissipated in the convective regions of the stars. Such dissipation of energy leads to circularisation of the orbits. Observations do show that close orbits are circular whereas wider orbits have eccentricities. The period at which the transition occurs for a type of stars is called the 'circularisation period'. Until now, theoretical studies, which have relied on mixing length theory to model convection, have predicted circularisation periods significantly smaller than the observed ones. However, we have just developed a new description of the interaction between tides and convection that leads to the observed values of the circularisation period. There is a large number of problems that should be revisited using this new description, and this is the aim of the project. These studies can be applied to a variety of systems, including binary systems with two stars, or with one star and a giant planet, or with a giant planet and a satellite. The project will use analytical and numerical tools.

Interstellar Objects in a Galactic Context
Chris Lintott (Oxford) & Michele Bannister (U. Canterbury, New Zealand)

The discovery of the first interstellar objects observed travelling through the Solar System - 1I/‘Oumuamua and 2I/Borisov has created great interest. These small worlds are samples of the building blocks of planet formation that took place at other stars, and come close enough for the kind of detailed physical characterisation hitherto reserved for our own Solar System's comets and asteroids. The upcoming Vera Rubin Observatory’s LSST survey will offer the first chance to characterise this population. The aim of this PhD project is to build on a novel insight: we can use the EAGLE simulations of the star-formation history of analogues of the Milky Way, together with models of how planetesimals form, to understand the population of interstellar objects that Rubin will detect. This offers an opportunity for a truly novel test of our understanding of two very different areas of astrophysics — the structure and history of the Milky Way, and planet formation. The project would suit a student with broad interests in observational astronomy, and offers the chance to do something unique. The Rubin survey is currently scheduled to start in 2023, so there is also the chance to be directly involved in testing the predictions created in this PhD.

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Artist's concept of interstellar object1I/2017 U1 ('Oumuamua) as it passed through the solar system after its discovery in October 2017. The aspect ratio of up to 10:1 is unlike that of any object seen in our own solar system. Image Credit: European Southern Observatory / M. Kornmesser

Exoplanet projects are also available within the sub-department of Atmospheric, Oceanic and Planetary Physics. Students interested in these projects should apply to AOPP; applications which ask to be considered by both astrophysics and AOPP are encouraged. Note that these projects will be computationally intensive using Fortran, IDL and other software packages, so a physics/computing/mathematics degree is preferred.