Detection and Characterisation of Extrasolar Planets

I am PI of a long term effort to discover and characterise benchmark extrasolar planets (aka. exoplanets) in the southern hemisphere. The main goal is to detect short period transiting planets around bright stars where follow-up ground and space-based analysis can lead to further breakthroughs into the physics of exoplanetary atmospheres. In addition, we also aim to study and detect the lowest mass planets, down into the Earth-mass regime. We employ the radial-velocity technique to detect such systems, utilising instruments such as the HARPS spectrograph. HARPS is shown to be stable down to around 80cm/s in the long term on real stars and we aim to make use of such precision to detect planets down into the low mass rocky regime.

The Calan-Hertfordshire Extrasolar Planet Search (CHEPS) has contributing collaborators in Chile, the UK and Poland, with publications already accepted, like the discovery of a companion to the star HD191760, which is either an eccentric brown dwarf in the desert or an example of the first extreme-Jovian deuterium burning planet (Jenkins et al. 2009, MNRAS, 398, 911; see figure). This result was announced as a press release in the popular UK astronomy magazine Astronomy Now. We also perform system stability tests to look for hidden planets in the data, or regions where other planets could be located.

I am also a member of the Anglo-Australian Planet Search, an international collaboration setup to detect planets around Sun-like stars in the southern hemisphere. Currently we have detected over 30 planets ranging from planets like Jupiter to short period low-mass rocky planets (see the Exoplanet Encyclopedia).

Top panels show the radial-velocity variations for HD191760, along with the bisector velocity corrections applied to the data. The bottom panels show our stability tests for the system (light parts are unstable regions for any other possible companions)

Stellar Atmospheres: Activities & Metallicities

In preparation for an exoplanet search it is desirable to have prior knowledge of each target star's chromospheric activity and photospheric metallicity. It has been well established that chromospheric activity is a useful tracer of the expected level of radial-velocity noise for a given solar-type star. Hence, I also work on the determination of southern solar-type star activities, both for the CHEPS sample and also the AAPS project (Jenkins et al. 2006, MNRAS, 372, 163; Jenkins et al. 2008, A&A, 485, 571).

The photospheric metallicities ([Fe/H]) of solar-type stars can also be used to determine the probability of any star hosting a gas giant planet, whereby the higher a star's [Fe/H], the higher the probability of hosting a planet (Fischer et al. 2005, ApJ, 622, 1102). I have measured the metallicities of all stars on the CHEPS planet search list, along with a number of other stars in the southern hemisphere (totaling around 900 stars), and use this knowledge to both select stars for future planet search and also to study the properties of stars with and without planets. The figure below shows the comparison of these values with those in the literature and highlights the robustness of our method.

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Direct Imaging of Exoplanets and Brown Dwarfs

Direct imaging of exoplanets and brown dwarfs as companions to well studied stars can provide vital information about these objects that can't be determined any other way. Only a handful of such objects have been detected due to the faintness of the companion compared to the closeby bright star. Contrasts of millions to billions are required to detect faint exoplanets and low-mass brown dwarfs and although this is extremely difficult, the knowledge that can be gained from the host star is invaluable.

As part of this project we have observed with the VLT NACO-SDI instrument, which utilises narrowband imaging across the methane band to subtract away the stellar light and reveal low-mass methane rich companions. Instead of hunting blindly for such companions we go after known companions that have been detected by radial-velocity surveys, such as the AAPS, Magellan Planet Search, etc. The benefit of this is that we know each star has a companion on a long period orbit. The drawback is that the stars in most radial-velocity planet search projects tend to be old, since the activity of a Sun-like star decreases with time, along with the noise in an associated radial-velocity dataset, and since the brightness of substellar companions decrease with time also, it makes imaging them more difficult. However, the combined knowledge that can be gained from the radial-velocity data, the imaging data and the high resolution spectral data of the host star mean that we can test in detail evolutionary and atmospheric physics through comparison of the data with current models.

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M Dwarf Evolution

Another area of research I am interested in is studying the evolutionary properties of cool M dwarf stars. I focus my study towards the rotational evolution of M dwarfs, along with their activities and magnetic properties. Also I study M dwarf metallicities through the use of high resolution spectroscopy. One of the main aims of this work is to select future planet search targets for precision radial-velocity programs working in the near-infrared part of the electromagnetic spectrum. We plan to survey the most inactive and hence slowest rotating stars, since activity and rotation is shown to be coupled for early to late type M dwarfs.

We employ a deconvolution procedure to generate a line profile for each star that is constructed from a large number of lines in each spectrum and then compare this profile to a non-rotating star. The non-rotating profile is scaled and broadened using a rotational profile to determine a Chi Squared and then we determine the minimum of the Chi Squared through polynomial fitting in rotation velocity space, and utilise this to determine the uncertainties in the procedure. The figures below show the results of this work in our publication (Jenkins et al. 2009, ApJ, in press) and highlight the changing rotational distribution from early to late M dwarf stars.

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Precision Radial Velocity Spectrometer

Finally I am also involved in collaborative work between the University of Hertfordshire in the UK and Penn State University in the USA to develop a system that can measure radial-velocities down to the 1m/s level and detect habitable Earth-mass planets around M dwarf stars. Currently we have developed a Pathfinder instrument that is housed at Penn State and using this instrument we have we have shown we can reach precisions of around 4m/s using Solar observations (Ramsey et al. 2008, PASP, 120, 887; see figure below) to measure the Earth's rotational velocity.

Although Pathfinder is far from the finished article, such a demonstration shows we are nearing the regime that optical spectrographs, such as HARPS, operate at, only in an unexplored waveband. We plan to operate a similar instrument to Pathfinder on the Hobby-Eberly Telescope in Texas, to begin exploring such velocities on cool M dwarf stars, which make up the bulk of objects within 10 pc distance of the Sun. As mentioned above, we are curently preparing our target list of M stars that should provide ideal targets for such a project.