Extrasolar Planets and Brown Dwarfs

Ben Burningham

The last decade has seen the advent of direct characterisation of the atmospheres of giant exoplanets via high contrast imaging and spectroscopy. However, interpreting these data can be problematic due to their sparse nature and poor signal to noise resulting from the challenging nature of the observations. Unhindered by bright host stars but sharing the same temperatures, free floating brown dwarfs and planetary mass objects have become crucial laboratories for understanding the complex physics and chemistry of these atmospheres. Dr Burningham has developed software (named "Brewster") for studying these atmospheres using spectral inversion (also known as retrieval analysis) with a particular emphasis on understanding their clouds, chemistry and thermal structure. Dr Burningham is leading the retrieval analysis for two approved Cycle 1 JWST programmes using Brewster - both of these provide opportunities for PhD projects.

Spectrophotometry of brown dwarf pulsars: This project will use JWST time series data to disentangle the impacts of weather (clouds) and aurorae on the spectra of brown dwarfs that are known to have powerful radio aurorae and near-infrared variability. This will require developing a strategy for analysing the incredible quality time series data that will be supplied by our JWST programme during Cycle 1 within the Brewster framework. It will also involve extending the Brewster retrieval model to include the effects of aurorae and derive information from the model parameters that can be used to constrain how the aurorae interact with the brown dwarf atmosphere.

The coldest observed extrasolar atmospheres: This project will focus on using Brewster to understand the conditions in the atmospheres of extremely cool Y dwarfs that will be observed by JWST in Cycle 1. Y dwarfs span the temperature range from 250K up to around 500K, and represent the coldest directly observed atmospheres beyond the solar system. Their masses overlap with the planetary regime, and the population may include examples of ejected planets. The coldest examples are expected to show evidence of water clouds in their spectra, whist warmer examples may show exotic clouds formed from halides or alkali sulphides. Due to their cool temperatures, Y dwarfs are extremely faint at near-infrared wavelengths, and are essentially invisible at optical wavelengths. Since their discovery in 2011 it has only been possible to obtain relatively poor quality near-infrared spectra using Hubble. JWST will provide the first high quality spectroscopy across the wavelengths where they emit most of their light, which will revolutionise our understanding of these objects. By using Brewster to analyse JWST spectroscopy of these objects this project will provide the insights to their clouds, chemistry and thermal structure.

Ben Burningham

It is well established that to understand an exoplanet, one must understand its host star. However, the growing number of exoplanets discovered in orbit around M dwarfs via missions such as TESS highlights a significant hurdle in this context. Determining the compositions of cool M dwarfs is extremely challenging due to the complex nature of their spectra, which are dominated by overlapping molecular bands that inhibit traditional techniques that rely on the existence of a continuum against which absorption lines may be measured and modelled. Whilst a number of methods have been established for estimating aggregated metallicity as a parameter, techniques for inferring more detailed compositional information are less well developed. This project will adapt the Brewster retrieval framework written by Dr Burningham to derive compositions of M dwarfs. Starting with high-resolution spectroscopy obtained using iShell on the NASA Infrared Telescope Facility (IRTF) in Hawaii, we will first focus on M dwarfs in binary systems with FGK stars to benchmark the technique, before targeting planet host stars.

David Pinfield, Hugh Jones

Ultra-cool companions may be brown dwarfs or planetary mass objects, with known discoveries ranging in effective temperature from ~2700 K down to just a few hundred K. The question of how brown dwarfs and giant planets should be classified/understood is not yet answered, with a full understanding of the "exoplanet brown dwarf connection" being an important goal in the field. Ultracool companions are a crucial ingredient, providing excellent test-beds to helping astronomers understand the complex ultracool atmosphere physics at play. With the recent advent of the Gaia observatory we now have access to an unprecedented set of primary star measurements, which are providing constraints (through association) on companion properties including distance, temperature, surface gravity, mass, age and composition. In this project you will join a team working to fully exploit the powerful combination of Gaia with world-leading infrared surveys, and identify and study ultracool companion populations out to several hundred parsecs. Gaia's third data release is set to come out mid 2022, and will have a huge impact that the student will help exploit. We are specifically targeting companions with the most extreme properties, such as the youngest lowest mass objects, as well as those with unusual composition. Collectively these will provide the most revealing tests for the atmosphere models, and may also encompass a range of formation scenarios and provenance, offering important insight into the exoplanet brown dwarf connection.

Hugh Jones

The advent of powerful sub-mm observatories means that dust disks are now being routinely discovered around many nearby stars. The combination of alignment information with large numbers of radial velocity measurements for planets with dusta disks as well as the combination with those that transit offers the opportunity to construct a sample of nearby stars for which the planetary mass can be precisely determined. Previous determinations of planetary mass by radial velocities suffer from an orbital inclination uncertainty and those solely from transits are based only on planets at small orbital radii. This project thus represents a direct method to determine the exoplanet mass function based on nearby stars. Much of the project will be concerned with quantifying the activity and geometric biases within the sample of nearby stars considered.

Hugh Jones, Bill Martin

This project is to realise the so-called EXOhSPEC prototype spectrograph (Exoplanet high resolution spectrograph). It will be entirely built from catalogue components but will utilise technologies not deployed in astronomical spectrographs. EXOhSPEC will be tested on the Sun and local stars both in the laboratory, on local automated telescope as well as with telescopes in the Canaries and Thailand. A final version is destined for delivery to the Thai National Telescope. It is intended to have the fewest possible optical surfaces for a high resolution spectrograph and its efficiency and small size will make it a highly attractive for further development. Among several novel developments that we introduce is active metrology which enables us to construct a small athermal spectrograph from off-the-shelf parts. The long-term aim of the project is to be able to build a prototype to significantly extend the reach of precision radial velocities to higher precisions and efficiencies enabling for example a space-based radial velocity instrument. Papers describing early results and laying out future steps for the project is available (Jones, Martin et al., 2021) along with software to enable iteration through a number of designs. A wide range of experimental, practical and software skills will be necessary and further developed through this project.