Extrasolar Planets and Brown Dwarfs

Supervisory team: 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 a series of 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.

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 parts of the array at different speeds allowing arbitrary dynamic range to be reached (e.g., Wocial et al. 2022), (2) developing tapered fibre technologies (e.g., Choochalerm et al. (2021, 2022), (3) optical design without curvature in the spectral orders. The immediate aim of the project is to be able to build a prototype to with precision radial velocities of a cm/s and efficiencies to enable 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.

Supervisory team: Hugh Jones, Jan Forbrich, Ben Burningham

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, we often 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 offer the opportunity to determine the correlation between radio emission and age. These objects can also provide a sample to derive parallaxes at radio wavelengths and constrain any companions. These studies can be seen as a precursor to the large-scale radio parallax determination of brown dwarfs and discovery of their companions which are becoming feasible with new large radio telescopes such as FAST and Greenbank as well as arrays such as the Square Kilometre Array.

Supervisory team: David Pinfield, Bartek Gauza, Hugh Jones

Proper motion is the angular velocity at which astronomical objects move across the sky, and can be used to select local or fast-moving objects of interest. However, such objects can also have extreme colours and be much fainter in one wave-band compared to another. So in the fainter wave-band an object may only be detectable in very deep "image stacks" (constructed by adding together many individual images observed over several years). In such “image stacks” high proper motion objects may appear as extended sources at offset positions (due to their movement between individual images in the stack), and a very high proper motion object may even manifest as an apparent "trail" of objects in an image stack.

In this project you will combine deep optical and near infrared/infrared survey data from ground/space-based surveys to search for high proper motion brown dwarfs. Brown dwarfs have extreme colours (e.g optical-NIR or NIR-IR) and can have high proper motion if they are nearby or are members of the thick-disk/halo kinematic populations. You will develop a method to search for brown dwarfs that are point-like objects in one survey but are extended (or forming a trail) in deep-stack survey/s. And you will then explore the nature of these objects using colour/magnitude and proper motions to assess spectra type, distance and age (from e.g. kinematic group membership, companionship, and moving group membership).

Supervisory team: Ben Burningham, Hugh Jones

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, Vos+ 2023, Faherty+ 2024) 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 coordinating retrieval analysis for a range of approved JWST programmes.

This project will focus on disentangling the impact of clouds, chemistry and thermal structure on the transition between the and L and T spectral types that occurs around T_eff = 1200 K. This transition occurs across a narrow temperature range, and is widely attributed to the rapid clearance of clouds from the observable photosphere of objects with decreasing temperature. However, this explanation leaves a number of puzzles unsolved, and alternative explanations have been offered that rely on changes to the thermal structure of the atmospheres due to dramatic changes in chemistry that are also taking place across this range. Atmospheric retrievals offer the possibility of distinguishing these possibilities, and high-quality JWST data will provide the ideal basis for this study.