Brown Dwarfs and Extrasolar Planets
(Jones, Lucas, Napiwotzki, Pinfield)
We are leaders in a wide range of programmes to detect and characterise brown dwarfs and extrasolar planets. We have discovered a variety of record breaking low-temperature brown dwarfs and examples with unusual properties, and are one of only a few groups worldwide to discover nearby extrasolar planets. We are leading a number of major brown dwarf and extrasolar planet projects including; the detection and follow-up of brown dwarfs from the UKIDSS, VISTA and WISE surveys, an infrared planet transit survey with over 100 nights worth of UKIRT observations, new techniques to find extrasolar planets using our own polarimeter to observe their reflected light, and red-optical spectra to push radial velocity surveys to later spectral types. We have a variety of scientific objectives, with long-term ones being; a proper understanding of ultracool atmospheres (spanning the brown dwarf and giant planet regime), a full characterisation of Galactic brown dwarf populations (mass function, birth-rate, multiplicity and kinematics), and the discovery and characterisation of the full range of multiple planet systems and Earth analogues around different types of star.
(Pinfield, Burningham, Lucas, Jones)
We are leaders in the exploitation of new generation infrared sky surveys for brown dwarf discovery and study. Together UKIDSS, VISTA and WISE are providing an unprecedented view of the infrared sky which is probing a large volume of new parametre-space and revealing free-floating objects with masses and temperatures all the way into the planetary regime, and ages as old as the Galaxy itself.
- In recent years we doubled the number of known T dwarfs (Teff<1500K) through our follow-up and spectroscopic confirmation programmes.
- We identified the coolest T dwarfs in the Teff range down to 500K, and have ongoing parallax programmes to determine their luminosities.
- We were the first to identify an unexpected dearth in mid to late T dwarf numbers (compared to expectations from open cluster populations), that is either high-lighting problems with the latest evolutionary models or resulting from a varying initial mass function. Our ongoing studies are target this open question.
- We are carrying out a spectroscopic study of a major magnitude limited sample of UKIDSS L to mid T dwarfs to constrain the sub-stellar formation history of the Galaxy. This programme has to-date been awarded 20 nights of XShooter time on ESO's VLT.
- Our infrared proper motion catalogue combined with radial velocity measurements is revealing the kinematics of brown dwarf populations, giving important clues to their origin. We have recently identified the first T dwarfs with halo kinematics.
- The multiplicity of brown dwarfs is also crucial for revealing sub-stellar formation mechanisms. We study such multiplicity over a range of separations, using spectroscopy, high resolution imaging, and proper motion measurements.
- And we study brown dwarf atmospheric weather phenomena through photometric and spectroscopic monitoring.
(Pinfield, Burningham, Lucas, Jones, Napiwotzki)
The most complete understanding of brown dwarfs and indeed giant planets with ultra-cool temperatures will come from a good understanding of the physics of their atmospheres, and the ability this brings to fit their physical properties (Teff, logg, [M/H]) from observation (e.g. spectroscopy). However, ultracool atmospheres are very complex, with molecular opacities, dust grain condensation, and non-equilibrium chemistry remaining major challenges (as our high resolution spectroscopic studies have shown). To test and guide the models, we populate the parameter-space of physical properties directly, using benchmark bown dwarfs - whose atmospheric and/or physical properties can be inferred in a relatively model free way. We search for benchmark brown dwarfs of several varieties.
- Brown dwarfs in moving group associations and clusters consitute young benchmarks with well constrained ages and uniform (within any group/cluster) composition.
- Widely separated brown dwarf compnions in multiple systems can have their properties (distance, age, composition) constrained from studies of the other component stars.
- In particular, wide companions to subgiant stars and high-mass white dwarfs can have ages and/or compositions known to a high degree of accuracy. Wide companions to young stars (e.g. active M dwarfs) can have Teff and logg in the giant planet domain.
- Closer binary systems may be resolved with high resolution imaging and yield dynamical mass measurements.
(Lucas, Burningham, Pinfield, Riaz)
This group was responsible for the first detection of free floating planetary mass objects (IPMOs) in the Trapezium Cluster in Orion, in collaboration with Pat Roche of Oxford University.
Our research in this area is continuing, using the twin Gemini telescopes to search for less massive IPMOs in Orion and other star formation regions. The main aim of this research is to further our understanding of star formation and establish whether there is a minimum mass for objects forming via the star formation process. We are also engaged in further follow-up spectroscopy in order to better determine the temperatures, ages, masses and surface gravities of the IPMOs.
Studies in regions where star formation is still ongoing have the disadvantage that source masses cannot be determined with high precision. However this is offset by the benefits of observing the star and brown dwarf formation process in action. Features such as the Initial Mass Function (IMF), the velocity dispersion of brown dwarfs, brown dwarf accretion disks and the sub-clustering of these sources all help us to piece together the star formation process.
We are conducting WISE based searches for young brown dwarf disks in nearby star-forming regions and associations. Our focus is on the age interval of ~5-10Myr, which is a critical intermediate age when most disks begin to show a transition from a primordial to a debris disk system. We have constructed a WISE SED classification scheme, based on the Ks and WISE bands of 3.4-12mu. We have determined certain thresholds in the WISE spectral slope versus spectral type diagrams to distinguish between the red population of Class I/II systems and the Class III sequence. We have found the WISE [3.4]- color to provide the best distinction between the photospheric and the disk population. Based on the results obtained so far, the disk frequencies for brown dwarfs are not found to show any clear dependence on the age, stellar density or the BD/star number ratio in a cluster. This is in contrast to the higher mass stars where the inner disk frequency shows an evolutionary decline with age, and the disk fractions are lower in the denser clusters. Also, primordial BD disks are still visible at ages of ~10 Myr, whereas the higher mass stars have all transitioned to the debris stage by this age. We have looked into the various formation mechanisms for BDs, which could lead to different disk properties, and have found that the observed BD disk fraction may only partially be due to an age evolution. The large differences in the fractions between regions may well be due to different BD formation mechanisms, and therefore different initial disk fractions/properties.
Our surveys with the Herschel Space Telescope are focused on brown dwarf disks at relatively older ages of ~10-50 Myr. With these far-infrared observations, we aim to obtain robust disk masses for these older disk sources, and to determine if brown dwarf disk masses show any decline with the age of the system. Also, obtaining a better constraint on the outer disk radius or the spatial extent of the disk will help understand the possible formation mechanism for these brown dwarfs. This work will also highlight new transition disk systems with inner opacity holes, which may have been missed by previous Spitzer surveys. These Herschel surveys will be important to map the evolution of brown dwarf disks over a wider age range, and to determine if the brown dwarf disk decay time scale is longer than that observed for the earlier-type stars.
(Pinfield, Barnes, Jones, Napiwotzki)
Pinfield leads a large scale infrared transit survey searching for planets around cool stars. The survey uses the Wide-field Camera on the UK Infrared Telescope to monitor the brightness of many thousand M stars and searches for the characteristic periodic decrease in brightness as a planet passes in front of its host star.
Other transit surveys use optical ligfht, and are sensitive to planets transiting stars similar to the Sun. Cooler M dwarf stars are much fainter and 5-10 times smaller, and the transit technique is thus more sensitive to smaller planets around M stars, that could have lower temperatures and may be in the habitable zone. With 100+ nights of observations the survey has built up to a critical sensitivity at which new planets can be found.
(Jones, Barnes, Pinfield, Tuomi)
We are engaged in a number of radial velocity projects, in particular, the Anglo-Austrailian Planet Search is targeting 250 nearby stars brighter than V=7.5 in the Southern Hemisphere. A Jupiter-like planet exerts a small gravitational pull on its parent star, causing the star to wobble with a velocity of 1 to 100 meters per second depending on the orbital distance and mass of the planet. This motion can be detected via the Doppler Effect. The light emitted by a star moving toward the Earth will be Doppler shifted to shorter (bluer) wavelengths, while a star receding from the Earth will emit light shifted to longer (redder) wavelengths. The effect is extremely subtle and has no effect on the apparent colour of the star. A star with a Jupiter-mass planet will be revealed by the periodic Doppler shift of its light. After one or two orbital periods the information from the Doppler measurements allows us to calculate the orbit and the mass of the unseen planet. Our current measurement precision is 3 meters per second (a brisk walk). For comparison, Jupiter causes the Sun to wobble with a velocity of 12.5 meters per second over a 12 year period. Saturn induces a 2.7 meter per second wobble on the Sun with a 30 year period. The other planets are too small to produce a measurable effect on the Sun.
(Jones, Barnes, Pinfield)
Direct imaging techniques will have great difficulty detecting such planets, because the planet is so much fainter than the star and because their two images are never separated on the sky by more than 0.003 seconds of arc. Fortunately, however, the starlight scattered from the planet can be distinguished from the direct starlight because the scattered light is Doppler shifted by virtue of the close-in planet's relatively fast orbital velocity (~ 150 km/sec). Superimposed on the pattern given by the planet's albedo changing slowly with wavelength, the spectrum of the planet's light will retain the same pattern of photospheric absorption lines as in the direct starlight.
Following the detection of extrasolar planets by the radial velocity method in the mid-1990s a global effort began to try and detect these planets directly. The radial velocity method pioneered by Mayor & Queloz and Marcy & Butler provides a great deal of information about planetary orbits, including the fact that many planets orbit very close to the parent star (which are known as "hot Jupiters"). However this indirect technique leaves an uncertainty in the mass of the planets and their orbital inclination, measuring only M.sin(i). It also provides no information about the composition of the planets themselves.
A number of groups are attempting to remedy this by directly observing extrasolar planets using high resolution interferometric imaging to pick out faint companions, or high resolution spectroscopy to distinguish reflected light from the planet using the wavelength shift caused by the Doppler effect.
When PLANETPOL is mounted at the Cassegrain focus of a 4-8m telescope it receives sufficient photons from a V=5 star to measure such a signal in less than 1 hour. A number of larger polarisation signals are also detected: eg. telescope polarisation, instrumental polarisation, interstellar polarisation and sky polarisation. However, only the signal from the planet will rotate at the planet's orbital period, which is known from radial velocity measurements. PLANETPOL has a sky channel to measure sky polarisation so that this can be subtracted if necessary (it is insignificant in dark time). The object and sky channels are both dual beam systems, using 3-wedge Wollaston prisms to send the light to four separate thermoelectrically cooled Avalanche Photodiode detectors. The modulating element in each channel is a Photoelastic Modulator or PEM, which provides a retardance oscillating sinusoidally from zero to Pi at 20 kHz. The polarised signal is recovered from the first harmonic at 40 kHz using lock-in amplifiers. These PEMs provide the cleanest polarisation signal of any type of polarimeter, providing an instantaneous measurement of the I and Q or I and U components of the Stokes vector with each detector. The dual beam nature of the system merely serves to double the throughput.
Owing to the differential nature of PEM-based detection and the use of large apertures the instrument can operate in non-photometric conditions and in poor seeing.
PLANETPOL has the potential to:
- Measure sin(i) and hence the planetary mass, being sensitive at any orbital inclination (i).
- Determine planetary albedo and radius. Detection at BVRI will provide good precision for these observables.
- Determine the optical properties, size and perhaps composition of the reflecting particles in the planet's atmosphere.