Cis Lagae

I am a post-doctoral research fellow in the research group of Dr. Heather Cegla, in the Astronomy and Astrophysics group. I completed my PhD at the University of Stockholm working on determining the chemical composition of the most-metal poor stars in the known universe as accurately as possible (PhD thesis). This involved the calculation of 3D models of stellar atmospheres and using these models to synthesize theoretical stellar spectra in non-local thermodynamic equilibrium. My PhD work culminated in the publication of the first public grid of 3D model atmospheres (Rodríguez Díaz et al. 2024), and a grid of stellar spectra and abundance corrections for the Ca II near-infrared triplet lines (Lagae et al. 2024).

Currently, I am applying my experience working with modelling stellar atmospheres to enable the characterization of earth-mass exoplanets using the radial-velocity technique. ESA's upcoming space mission PLATO (launch planned for 2026) is expected to detect the first earth-analogues. However, confirmation and characterization of these earth-analogue candidates requires the determination of the planet's mass. This can be done by detecting the gravitational wobble a planet induces on its host star, which is measured as a Doppler shift in the stellar spectrum. For an earth-mass exoplanet, this signal is of the order of ~10 cm/s. The last decade has seen tremendous improvements in spectrograph instruments and data reduction techniques, making it technically feasible to observe signals to a precision of sub m/s.

Figure 1: Observation of the solar surface from the Daniel K. Inouye Solar Telescope. Credit: NSO/NSF/AURA

Besides the technical challenges, the detection of a ~cm/s signal is hampered by the presence of unwanted velocity signals coming from the planet its host star. Stellar surfaces are highly dynamic, comprising of hot upflows that cool down at the surface, whereafter they sink down in cool downdrafts. This behaviour is called granulation and can be readily observed on the Sun. This granulation pattern induces spurious Doppler shifts onto the stellar spectrum of the order 30-80 cm/s, obscuring any underlying planetary signal. In order to successfully detect and characterize an earth-analogue, it is essential to find a solution to remove or reduce the granulation-induced velocity signal from observations.

To this end, our group at the University of Warwick has developed a method to simulate the Solar surface to scale for individual spectral lines. It enables us to study how granulation affects the formation and shape of spectral lines, and subsequently, to find diagnostics to remove its effect. First studies looking at a single Fe I line, found that in certain circumstances we can reduce the granulation-induces velocity noise by up to 60%. To open up the application of this method to high-resolution observations, I will expand the method to the full optical spectrum and non-solar spectral types.