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 planetary mass objects, and the coolest brown dwarfs with temperatures down to room temperature.

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.

A Search for Ejected Planets and Brown Dwarfs

David Pinfield, Hugh Jones

Giant planets are generally found in stable orbits around host stars. However, it is an interesting prospect that such orbits can sometimes destabilise, ejecting planets into the free-floating population. Ultracool brown dwarfs can also be found as companions, and may undergo similar ejection leading to a warmer (more detectable) population of ejected objects. Catching such dynamical ejections in the act would offer important evidence about the full origins of the ultracool field population, and also provide a unique opportunity to study orbital objects that have become much more accessible. Wide binary stars should be good targets for an ejected object search, because they naturally evolve into elliptical orbits with perihelia interactions that could lead to ejected companions. Cool ejected objects would appear as a third members of triple systems (along with 2 widely separated stars) but in highly unstable orbital configurations. In this project you will target wide binary stars from ESA's Gaia survey, and search for additional cool companions in infrared surveys. You will further constrain interesting systems using proper motion and spectroscopy where available, and assess stability by considering separation and projection effects.