(Pinfield, Barnes, Jones, Lucas)

We are carrying out a new large scale survey that will probe for habitable rocky planets around cool stars, as well as extrasolar giant planets. This survey employs the so called transit method - by monitoring the brightness of large numbers of stars and searching for the characteristic periodic decrease in brightness as a planet passes in front of (or transits) its host star.

Cool star planet-scape, by J. Pinfield, 2009, property of the RoPACS network at the University of Hertfordshire: A terrestrial planet orbits a cool red star. A small cratered moon is also seen in orbit around the planet. In its close orbit the planet is tidally locked to the star with the same hot side always in day light. The other side of the planet is cold, dark and icy. In between these two extremes lies the terminator region in which an ambient climate and liquid water have taken hold.

Previous transit searches have been made in the optical, and have identified about 10 close in gas giant planets transiting solar type stars. These planets are known as "hot Jupiters", since they are strongly irradiated by the star. By using the new Wide Field Infrared camera on the UK Infrared Telescope, our survey is far more sensitive to planets transiting cool stars (M dwarfs) which are brightest in the near infrared. M dwarfs are 5-10 times smaller than solar type stars, and therefore smaller rocky planets can produce detectable transits. Also, the habitable zone (where liquid water may flow on an orbiting planet) around cool stars is much closer in than around solar type stars, and cool dwarf transits could be habitable rocky worlds.

(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.

Relative probability chi^2 map of planet-star flux ratio log10 eta versus planet velocity Kp. The corresponding NICMOS/HST contrast ratio for this region is marked by a + symbol with Fp/F* 1/1340. A candidate signature is detected with 97.2 per cent confidence close to the expected Kp velocity, although the contrast ratio of log10(0) = -3.449 (Fp/F*= 1/2810) is lower than expected.

(Jones, Tuomi, Barnes, Pinfield)

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.

The mass-period distribution of known exoplanets (open circles) and a recently discovered system around a naked-eye star discovered by our searches (filled circles). The dashed line corresponds to 1m/s RV amplitude of a Solar-mass star. J, V and E show the respective position of Jupiter, Venus and Earth. .(Tuomi et al., 2011, submitted)

(Lucas, Hough)

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 on 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:

1. Measure sin(i) and hence the planetary mass, being sensitive at any orbital inclination (i).
2. Determine planetary albedo and radius. Detection at BVRI will provide good precision for these observables.
3. Determine the optical properties, size and perhaps composition of the reflecting particles in the planet's atmosphere.

(Pinfield, Lucas, Burningham, Jones)

Our group is heavily involved with the UKIDSS and VISTA surveys. These are major near infrared surveys leading to coverage of a large fraction of the sky. They probe several magnitudes deeper than the 2-Micron All Sky Survey, searching an unprecedented volume for brown dwarfs. We have discovered large numbers of L dwarfs (2300-1300 K) and T dwarfs (1300-500 K). However, we expect to also discover fainter and cooler than the known L and T classes. For temperatures below 500 K atmospheric ammonia and water clouds are expected, and any resulting changes in spectral character would warrant a new spectral class, pre-emptively called the Y dwarfs.

These new record breaking Y dwarfs could include brown dwarfs that formed when the galaxy was young, as well as brown dwarfs with masses in the planetary regime - so called free-floating planets. We are carrying out follow-up programs (8m spectroscopy, photometry and astrometry) to confirm and study UKIDSS Y dwarfs, and reveal the nature of these objects.

The J-H, Y-J 2-colour diagram for sources we expect in the LAS. The location of A-M (III-V) stars, late M, L and T dwarfs and z=2-7 QSOs have been determined using colours synthesized from available spectroscopy. And expected Y dwarf colours have been synthesized from the latest theoretical model spectra by the Lyon group (priv. comm.), the Marley group (priv. comm.; see Marley et al 2002, ApJ, 568, 335), the Burrows group (Burrows et al., 2003, ApJ, 596, 587) and Tsuji (private comm.). We note that we have adjusted the Lyon J-H colours by -0.3 so that they pass through the known T dwarfs (Pinfield et al. 2007 in prep).

(Pinfield, Burningham, Jones, Lucas, Napiwotzki)

Brown dwarf atmospheres are very complex. With their relatively low temperatures, numerous molecular species appear and atmospheric dust grains condense out. It is very difficult to accurately model such atmospheres, and it is thus an ongoing challenge to observationally constrain the physical properties of brown dwarfs - a task that is vital if we are to measure the disk IMF and formation history.

The group is carrying out a number of programs to identify brown dwarfs whose physical properties may be inferred independently of spectral analysis - so called "benchmark brown dwarfs". By finding brown dwarfs with known age, composition and distance, it becomes possible to determine their temperature, surface gravity and metallicity without reliance on atmospheric models. These benchmarks thus become observational reference points in temperature-gravity-metallicity space.

A UKIDSS JHK false colour image of Cancri AB with a separation of 164 arcsec. Cancri A is the brightest object in this field, the L dwarf Cancri B is in the white circle (Zhang et al. 2010)

We are specifically searching for benchmarks brown dwarfs as

• Members of open clusters, where the age, composition and distance of the cluster is known.
• Nearby members of kinematic moving groups, where the age and composition may be inferred from other group members, and distances can be measured astrometrically.
• Wide companions to subgiant stars, where the age, composition and distance can be inferred from the subgiant.
• Wide companions to high mass white dwarfs, where the age and distance can be inferred from the white dwarf (in general, high mass white dwarf progenitor lifetimes will be much less than the white dwarf cooling age).

(Lucas, Pinfield, Burningham, 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.

(Jones, Pinfield, Lucas, Barnes)

The group has several programs to search for and study very nearby brown dwarfs. Such objects are amongst the brightest (apparent) in their spectral class, and thus provide ideal targets for detailed study including

• High resolution spectroscopy to constrain atmospheric physics and search for companions using the Doppler wobble technique.
• High spatial resolution imaging (e.g., adaptive optics) to study binarity and attempt to image any planetary companions.
• Photometric and spectroscopic monitoring to reveal cool dwarf atmospheric weather phenomena.
• Optical and infrared parallax programmes to measure their distances.

We have also begun a program to search for distant brown dwarfs within the galaxy, to investigate if the sub-stellar IMF varies spatially within the galactic disk. The challenge here is to identify L dwarfs in the galactic plane, which is difficult because there are many sources of photometric contamination that share L dwarf colour. But by combining optical and near infrared photometry with astrometry and morphological analysis, we are able to select optimal L dwarf samples in the plane. We plan to use these techniques to identify L dwarfs out to distances of about 500pc in the UKIDSS Galactic Plane Survey.

Computed space densities for different spectral types from Monte Carlo simulations of the field population of T dwarfs for a uniform birthrate (i.e. Β = 0.0) and various underlying mass functions that are indicated on the plot. The observed range of space densities is indicated by the solid black lines, with the lower and upper values indicating the range implied by the different likely binary fractions. The T9 space density is that calculated using the relation of Liu et al. (2006), to keep it consistent with the method used to derive observed properties from the simulations. Uncertainties on the maximum and minimum densities are indicated with bars at the midpoint of each spectral type bin, and reflect volume uncertainties and Poisson counting uncertainties. (Burningham et al. 2010)