Extrasolar Planets and Brown Dwarfs

Ben Burningham, Hugh Jones, +PDRA

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 (nicknamed "Brewster" - see Burningham+ 2021; Gonzales+ 2020 , 2021; Calamari+ 2022) 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 three approved Cycle 1 JWST programmes using Brewster.

This project will focus on using Brewster to understand the conditions in the atmospheres of extremely cool Y dwarfs that are being 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, while warmer examples may show exotic clouds formed from halides (e.g. alkali salts) 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, Hugh Jones, +PDRA

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.

Hugh Jones, William 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. One 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.

We introduce several novel developments including active metrology which enables us to construct a small athermal spectrograph from off-the-shelf parts. We are in the process of developing several accompanying technologies including (1) being able to read out different part of the array at different speeds allowing arbitrary dynamic range to be reached (e.g., Wocial et al. 2022) and (2) developing tapered fibre technologies (e.g., Choochalerm et al. (2021, 2022). 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. A paper by Jones et al. (2021) describes early developments. A wide range of experimental, practical and software skills will be necessary and further developed through this project.

Hugh Jones, Jan Forbrich

Brown dwarfs are an enigmatic class of astronomical objects with a range of observed spectral type including M, L, T and Y with properties between stars and planets. They are relatively poorly studied due to their intrinsic faintness. The last few years has seen a dramatic improvement of their understanding due to the availability of distance determinations for the hotter nearby examples using the Gaia space telescope. However, for later spectral types of T and beyond, we have to use space infrared measurements and brighter companions to obtain their distances and other properties. In principle radio detections of brown dwarfs offer a new route to detection and determination of distance. Based on existing observations of M to T dwarfs there is the potential to reliably determine distance down to the lowest mass examples using radio parallaxes. So far it is found that the fraction of dwarfs that are radio emitters is independent of their spectral type. This might arise from a characteristic beaming angle and also youth. We have discovered a sample of M dwarfs (e.g., Wang et al. 2022) which based on the detection of lithium in their atmospheres are definitively young and seek to determine whether they are radio emitters and carry out parallaxes at radio wavelengths to determine their distances. This search can be seen as a precursor to the large-scale radio investigations of brown dwarfs which are becoming feasible with new large radio telescopes such as FAST and Greenbank as well as arrays such as LOFAR and the Square Kilometre Array.