Extrasolar Planets and Brown Dwarfs

The exoplanet brown dwarf connection - ultracool companions to Gaia 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 (we describe our overall programme in the recent publication: https://arxiv.org/abs/1706.06038). Gaia's second data release is set to come out mid 2018, 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.

Searching for ultracool objects ejected from their host stars

David Pinfield, Hugh Jones

The stability of ultracool orbital populations including brown dwarfs and giant exoplanets can sometimes be disrupted by dynamical interactions, which may lead to their ejection into the free-floating field population. Wide binary stars can be a particularly good source of such dynamical interaction because these systems often have highly elliptical orbits (as a result of impulsive interactions with passing stars). When the orbit of two binary components bring them to closest approach (perihelion) their gravitational interaction can cause havoc amongst close-in orbital populations, with surviving exoplanet population around binary stars showing clear evidence of this effect. In addition, wide binaries themselves may also be broken up if they undergo close-encounters with passing stars, offering another route to low-mass ejected companions. Wide binary stars and disintegrating multiples are thus an intriguing source for ejected brown dwarfs and exoplanets. In this project you will study samples of wide binary systems and potential disintegrating multiples identified in the Gaia 1st and 2nd data releases. By analysing astrometry, kinematics, and binary properties, you will constrain the extent of potential "emission locations" for low-mass ejecta. You will then search deep large-scale survey data (VISTA, PanSTARRS, WISE, UKIDSS, SDSS) to uncover possible ejected ultracool objects, and study their characteristics and motion.

Seeking the coolest oldest brown dwarfs in the Galaxy with NEOWISE/CatWISE

David Pinfield, Hugh Jones

Very cool brown dwarfs and free-floating planetary mass objects have temperatures of 250-900 K (with late T and Y spectral type). Their temperature depends on both mass and age, but a small fraction have cooled to this state over the full age of the Galaxy. The Galaxy's ancient thick disk and halo populations formed ~10-13 billion years ago, and constituents retain highly characteristic kinematic signatures (with space velocities of 100-300 km/s). The identification of such ancient brown dwarfs is starting to reveal the nature of substellar formation within the primordial environment, and is crucial to our big-picture understanding of galactic star-formation. Our team has an established programme identifying late T/Y dwarfs in imaging data from the WISE observatory, and has identified several of the coolest members of the thick-disk/halo. After its main mission was completed however, WISE was re-activated by popular demand to continuously/repeatedly scan the sky in the mid-infrared (NEOWISE phase). This has led to a full WISE baseline spanning 7 years (and counting), and offers huge potential for mid-infrared time-domain astronomy. Fresh NEOWISE data is released annually, and the complete WISE data-set is about to be fully combined into a new data release (called Cat-WISE). In this PhD project you will join our team, and use NEOWISE/Cat-WISE data to achieve a major increase in sensitivity/depth, and greatly expand our search-volume. By revealing much larger populations of ancient brown dwarfs, your research will seek evidence for differences in substellar formation efficiency across the full age of the Galaxy.

Revealing atmospheric structures, clouds and chemistry of brown dwarfs and free-floating planetary mass objects

Ben Burningham

Spectral inversion techniques, also known as atmospheric retrievals, were first developed for remote sensing the Earth’s atmospheric conditions, and have now been applied to Solar system planets, exoplanets and brown dwarfs. In this project you will use Ben Burningham’s atmospheric retrieval framework (“Brewster”: http://adsabs.harvard.edu/abs/2017MNRAS.470.1177B) to estimate atmospheric structure, chemistry and cloud properties of brown dwarfs, isolated planetary mass objects and planetary mass companions to stars. The results of this will then be used to test predictions from fully self-consistent radiative-convective equilibrium models and identify shortcomings in our understanding of the atmospheric physics of giant exoplanets and brown dwarfs. There is scope for the student to identify new lines of enquiry; however, promising examples include: testing for departures from chemical equilibrium and evaluating disequilibrium effects as a function of gravity; revealing cloud properties such as condensate species, cloud structure, particle sizes; testing predictions for thermal structure across a range of temperatures. These projects will initially exploit archival data, but new data will extend to new data sets as they become available.  Atmospheric retrievals using Brewster will also play a key role in analysing data from the James Webb Space Telescope (JWST) following its launch in spring 2021.