Extrasolar Planets and Brown Dwarfs

Supervisory team: Hugh Jones, Richard Smart, David Pinfield

Very low temperature brown dwarfs allow us to discern the shape of the low-mass end of the field mass function and define the low-mass cutoff. We are doing this with a volume-limited sample of the coldest brown dwarfs. Our international multi-telescope campaign to obtain high-quality astrometry is active on GTC+EMIR, VLT+HAWK, NTT+SOFI and UKIRT+WFCAM combinations. The distances and temperatures for all the coldest objects in the Solar neighbourhood will address a range of important outstanding questions: What is the shape of the mass function? How can star formation create objects of extremely low mass? With what efficiency? Is there a low-mass cutoff to star formation? This position is co-funded by the University of Hertfordshire and the Istituto Nazionale di Astrofisica (INAF), will be registered for a PhD at the University of Hertfordshire and will require spending at least 1/3 of their time at each of the supporting institutions.

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 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.

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: Ben Burningham, Fei Wang, Hugh Jones

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, Vos+ 2023) 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 Cycle 1 and Cycle 2 programmes.

In this project, we will seek to identify the specific cloud species in a set of benchmark brown dwarfs that orbit FGK stars that will be observed during 2024 on JWST Cycle 2 approved programme 3670.  We will place the chemistry of these clouds within the context of the expected bulk composition of these atmospheres based on that of their host stars. Non-equilibrium chemistry and vertical transport are also expected to play a key role, and will also be constrained by our analysis.  This work will aim to place firm constraints on our ability to infer bulk compositions for extrasolar worlds based on atmospheric observations.

Supervisory team: Ben Burningham, Fei Wang, Hugh Jones

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.

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 extremely 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).