Research

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The Gaia-GRAVITY Synergy

Check out the reporting on our results about the synergy between Gaia and GRAVITY enabling the detection and characterisation of substellar companions.

Combining Gaia and GRAVITY

Predicting astrometry of Gaia-inferred substellar companions

High-precision astrometric time series data, that is the repeated and precise measurements of stellar positions, have the potential to revolutionise the field of direct imaging. The recently published Gaia DR3 Non-Single-Star (NSS) two-body orbit catalogue contains orbital solutions to several hundred thousand stars around their system's centre-of-mass. In combination with an estimate of the two bodies' mass ratio, the respective orbital solutions enable the inferrence of the companion's momentary position. Depending on the uncertainties associated to and the correlations between the individual orbital elements the resulting position predictions are more or less constrained in the sky plane.

The GRAVITY follow-up

Using dedicated proprietary tools we can filter for such companion candidates that exhibit a sufficiently confined position prediction, a suitable companion-to-host separation, sky position, distance, host magnitude etc. Mining the Gaia data set allows for identifying suitable targets for a follow-up with VLTI/GRAVITY. We could show that the synergy between Gaia and GRAVITY is particularly strong. Indeed, when following-up on eight different target systems we were able to directly detect three stellar and five substellar companions (all of which previously unknown), establishing the method to be highly efficient. Furthermore, these objects are the closest-in directly imaged companions to date. Thus, the Gaia-GRAVITY ensemble is capable of "filling in" the parameter space of close-in substellar companions which is unaccessible to classical imaging instruments.

Orbital constraints and dynamical masses

After detecting and thereby confirming the companion, we can use the high-precision astrometry epoch obtained by GRAVITY to constrain the orbital solution and set tight constraints on the companion mass. We use an MCMC approach to sample a posterior distribution that reflects both the Gaia and GRAVITY likelihood information. This method has shown that a single GRAVITY detection suffices to shatter the Gaia-inherent degeneracy on the host and companion mass. This degeneracy is caused by the fact that the host movement around the centre-of-mass can conceivable be caused by an inifinite set of companion mass and host-to-companion separations. Measuring the separation between the bodies we are able to constrain the companion mass to a relative error of a few percent. Considering that dynamical masses are still a scarce resource, achieving such precision on a routine basis is a remarkable case in point of how Gaia nad GRAVITY complement each other.

Fig. 1: Position prediction of an arbitrary companion candidate. The contour line shows the one-sigma confidence region.

Fig. 2: Companion orbit as per Gaia (black) as well as per Gaia-GRAVITY combination (green).

Fig. 3: Constrained orbital elements resulting from the combination of the Gaia and GRAVITY data (green) as well as the unconstrained Gaia orbital elements (black). Note the extreme accuracy improvement in the total, host and companion mass estimates.

Hunting for Gaia planets

Stay tuned! We are currently applying the methodology laid out above to potentially planet-hosting target systems. Such observations are limited to target systems of young age to ensure the contrast required for detection. Currently, the lack of reliable age estimates is the main bottleneck preventing a massive follow-up and confirmation campaign. But we are working on it.

Fig. 4: Excerpt of a mosaic plot showing an injected reflection signature's signal-to-noise retrieved using CCF methods as a function of the rotational broadening velocity and the assumed planetary geometrical albedo.

Reflection Spectroscopy

Detecting planetary reflection signatures in high-resolution spectra of exoplanet-hosting systems is notoriously difficult. The extremely low expected contrasts are the main issue here. In the course of my Master's thesis I investigated pathways to facilitating bona-fide reflection detections in an effort to establishing reflection spectroscopy as a means of exoplanet characterisation. The work resulted in the definition of a new figure of merit, the Reflection Spectroscopy Metric (RSM), that aims to quantify a system's favourability to reflection studies, the building of a reflection template, an attempt at detection and an investigation of rotational broadening effects on reflection studies. I am currently distilling these findings into a publication to be submitted soon.

Modelling Europa Plume Detections with JUICE

ESA's JUpiter ICy moons Explorer (JUICE) is set to perform two flybys of Europa, one of the most intruiging bodies in the Solar System. Its icy surface is believed to cover a global ocean of liquid water. Recent observations of active plumes (outgassing events ejecting clouds of water vapour into space) raise the question of whether the instruments flying aboard JUICE will be capable of detecting these plumes if the mission happens to fly through one of them. I explored this question during an internship at ESA's Space Research and Technology Centre ESTEC. This work resulted in a paper which you can find here.

To summarise: it depends on the flyby geometry as well as the mass flux at the plume source but in principle, JUICE can be expected to separate the plumes from Europa's background atmosphere, thereby sampling material directly originating from the moon's subsurface ocean. Who knows what is swimming around down there...

Fig. 5: Region of plume detectability as a function of the plume source mass flux.

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