Research Projects

During my PhD I have specialized in using high-dispersion spectroscopic methods to find signatures of the atmospheres of exoplanets. Such characterization studies are currently limited to planets that are large and hot: Either hot-Jupiters because they have a large transit-depth, large atmospheric scale heights and glow in the infra-red due to the intense stellar irradiation. Or heavy gas giants in the early stages of their formation, but further away from the star so that they can be spatially resolved.

Reflected light of Tau Boo b

Over the years, various groups have tried to use high-dispersion spectroscopy to detect optical reflected light of the non-transiting planet Tau Boo b. Depending on its albedo and the orbital phase, the planet reflects a scaled version of the stellar spectrum towards the observer, that is Doppler-shifted to the instantaneous radial velocity of the planet due to its orbital motion. At high spectral resolution, the star's absorption lines are individually resolved, and cross-correlation can be used to lift the weak reflected spectrum above the photon noise of the dominating stellar spectrum. Depending on what is assumed for the radius and the albedo, the contrast between the planet's reflection spectrum and the star can be as low as several parts per million!

Past searches for the reflected light of Tau Boo b have been unsuccessful, but have left a trail of high-quality high-dispersion optical spectra in the public data archives. I am currently combining data from UVES, HARPS south, HARPS north, ESPaDOnS, NARVAL and the now decomissioned Utrecht Echelle Spectrograph to make one final attempt to detect this planet. These data amount to many hours of exposure time, taken across a wide range of epochs between 1998 and 2011. Although the analysis is not yet complete, it looks like this planet will evade detection once again. Excitingly though, by combining all this data we can reach a 1-sigma sensitivity of about 5 parts per million, equivalent to good space-telescope data! This means that the planet we are trying to observe just happens to be darker than charcoal...

We are preparing a paper that will describe this work in detail, but I already presented a poster at the Exoplanets 1 workshop in July 2016.

Molecular mapping with AO-assisted IFU's

The cross-correlation technique alluded to above makes use of the fact that the flux in individual absorption lines of the target planet's spectrum can be co-added into a single cross-correlation signal. This co-addition greatly enhances the planet's signal relative to the photon noise of the star (which is not co-added constructively). However, it requires that the individual spectral lines of the planet be spectally resolved. At lower spectral resolution, this technique works less well. However, we have shown that even at modest spectral resolutions of a few thousand, the cross-correlation technique still yields a significant contrast enhancement.

This makes the technique applicable to integral-field spectrographs such as SINFONI (VLT) and OSIRIS (Keck). These instruments are AO-assisted, and thus do high-contrast imaging and spectroscopy at the same time. We can use these instruments to filter out the stellar photons by combining spatial resolution and the absorption spectrum of the planet to discriminate between the planet and the star. We have applied this method to archival SINFONI data of the well studied young gas giant Beta Pictoris b, and are able to independently retrieve absorption signals due to water and carbon monoxide absorption in the planet's emission spectrum at K-band. In the spring of 2017, we hope to observe a number of new targets to test this method on systems with other parameters. This method is very exciting because the E-ELT will employ one or several similar IFU spectrographs! We are preparing a paper to publish these results.

Cross-correlation of Beta pic SINFONI data

Cross-correlation of beta-pictoris SINFONI data-cubes with absorption models of carbon monoxide, water and methane. The planet is not visible in the wavelength-averaged image (upper-left), but when cross-correlating, the star is suppressed and the planet is revealed in CO and H2O.

TiO / VO in hot-Jupiter atmospheres?

There has been an ongoing discussion in the literature about the presence of temperature inversion layers in the atmospheres of hot-Jupiters. A temperature inversion is caused by chemical constituents that efficiently absorb short-wavelength stellar radiation and heat the atmosphere at altitudes above a particular pressure level. Molecules of TiO and VO are commonly invoked as likely suspects, but have not yet been ubiquitously detected.

I have searched for TiO / VO absorption in a transit transmission-spectroscopy dataset of HD 209458 b obtained by the HDS spectrograph. In our paper we show that through the cross-correlation technique (above), high resolution spectra are sensitive to extremely low TiO / VO concentrations in the upper atmosphere of hot-Jupiters (down to the part-per-billion level). However, we are unable to retrieve the TiO / VO absorption spectra due to inaccuracies in the modelling of the energy levels of the TiO and VO molecules. These inaccuracies lead to tiny errors in the wavelengths of the absorption lines in the modelled absorption spectra of these molecules and these models are thus not valid at high spectral resolution. Recently though, new cross-section data of TiO and VO has been made available through the Exomol project. I am therefore re-analyzing the archival data of HD 209458 b with these new models, as well as data of WASP-34 b and WASP-79 b, which we observed with UVES at the VLT.

Cross-correlation with TiO model

The cross-correlation function of the HD 209458 b HDS data with a TiO model, with TiO absorption injected into the data to test the sensitivity of the cross-correlation method. The injected TiO model is retrieved down to concentrations less than 1 part-per-billion.

Observing the Earth's polarization spectrum from the Moon

Apart from all the observations and data analysis described above, I have also spent time in the optical lab of Leiden Observatory. Here I developed a prototype of the Lunar Observatory for Unresolved Polarimetry of Earth. The goal of this project was to develop a space-based integral-field spectrograph, that not only combines spatial imaging and spectroscopy, but also measures wavelength-dependent linear polarization! The linear polarization of an incoming ray of light can be encoded into the spectrum using a method called channeled spectropolarimetry or spectral modulation. Obtaining spectro-polarimetric measurements of the Earth is important because spectro-polarimetric observations of exoplanets will be obtained in the future. Spectro-polarimetric data is hard to interpret, and without knowing the polarimetric appearance of the Earth, it will be difficult to identify Earth-like biomarkers in future spectropolarimetric data. The design and construction of this instrument was recently published in Optics Express.

Impression of the LOUPE setup in the lab.