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Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have made the first high-resolution image of the cometary belt (a region analogous to our own Kuiper belt) around HR 8799, the only star where multiple planets have been imaged directly.


Here are links to my most recent publications.


My name is Virginie and I am a French astrophysicist, originally from Savoie. I defended my thesis in 2014 in Grenoble, France, and I am now a FONDECYT postdoctoral Fellow in Santiago, Chile. My work consists of studying planetary system formation and evolution in circumstellar discs, the interactions between the planets and these discs, and the stability of planetary systems, using numerical tools.

Herschel vs Synthetic image of the debris disc of ζ2Ret (Faramaz et al., 2014)

Prints of planets in debris discs

At least ~20 % of Main-Sequence stars are known to harbor debris discs analogues to the Kuiper Belt. These discs are proof that the accretion of solids has permitted the formation of at least km-sized bodies. It is thus not surprising that several of these discs are accompanied by planets, which may reveal themselves by setting their dynamical imprints on the spatial structure of debris disks. Therefore, the detection of an eccentric debris disk surrounding ζ2Ret by the Herschel space telescope provides evidence for the presence of a massive perturber in this system. ζ2 Ret being a mature Gyr-old system, and in that sense, analogous to our own Solar System, it offers a different example of long-term dynamical evolution. During my PhD, I carried out a detailed modelling of the structure of the debris disc of ζ2 Ret, which lead to constraints on the mass and orbital characteristics of the putative perturber. This study also reveals that eccentric structures in debris discs can survive on Gyr timescales (Faramaz et al., 2014).

Detailed modelling of the structure of debris disks can allow the posterior discovery of hidden planets, as is the case for the Fomalhaut system (see next section). However, this requires first to obtain detailed informations on the spatial structure of those discs. In the case of ζ2 Ret, resolved images of the disc were obtained with Herschel, that is, in space. Today, the observatory that certainly has the better capabilities to obtain detailed resolved images of debris disc is the Radio-interferometer ALMA. Moreover, with ALMA, mm-sized grain sizes are probed. These grains are much little sensitive to stellar radiation effects, and their spatial distribution bears a "purer" gravitational prints of planets than that of micron-sized grains. Using ALMA, a team led by Mark Booth (PI) obtained the first high-resolution image of the cometary belt around HR 8799, the only star where multiple planets have been imaged directly. The disc inner edge position has been found to be too far out to be explained simply by interactions with the outermost planet, HR 8799 b. It suggest either that the system has a more complicated dynamical history, or that there is an extra planet beyond HR 8799 b(Booth et al., 2016).

Debris disc and orbit of Fom b (Kalas et al. 2013) and combined HST and ALMA observations of the debris disc (Boley et al. 2012)

The case of the Fomalhaut system

The eccentric shape of the debris disc observed around the star Fomalhaut was first attributed to Fom b, a companion detected near the belt inner-edge, which revealed to be highly eccentric (e ~ 0.6-0.9), and thus very unlikely shaping the belt (Beust et al., 2014) . This hints at the presence of another massive body in this system, Fom c, which drives the debris disk shape. The resulting planetary system is highly unstable, which involves a recent scattering of Fom b on its current orbit, potentially with the yet undetected Fom c. This scenario was investigated during my PhD, and its study revealed that by having resided in inner mean-motion resonance with a Neptune or Saturn-mass belt-shaping eccentric Fom c and therefore have suffered a gradual resonant eccentricity increase on timescales comparable to the age of the system (~440 Myr), Fom b could have been brought close enough to Fom c and suffered a recent scattering event, which, complemented by a secular evolution with Fom c, explains its current orbital configuration (Faramaz et al., 2015) .

Zodiacal light seen from Paranal (ESO/Y. Beletsky)

Exozodis and Exocomets

The three-step scenario unraveled in the context of the study of the dynamical history of the Fomalhaut system can to occur for a large range of planetary masses and semi-major axes. This implies that material may be set very generically on extremely eccentric orbits through this mechanism, which in return could feed in dust the inner parts of the system. Therefore, this mechanism may also explain the presence of inner dust belts in the Fomalhaut system (Lebreton et al., 2013) , but also the discovery a significant population of very bright hot dust belts, especially in systems older than 100 Myr. Indeed, starting from a realistic reservoir, that could have remained unseen, This mechanism can set km-sized bodies on cometary orbits such that the replenishment rate of an exozodi is compatible with observations, and can be sustained over Gyr timescales (Faramaz et al., submitted) .

Dynamical evolution of planetary systems

The planetary systems discovered so far exhibit a great variety of architectures, and our solar system is far from being a generic model. One of the main mechanism that determines a planetary system morphology is planetary migration. The presence of a stellar binary companion - which our solar system is deprived of - is expected to affect planetary migration conditions, and potentially lead to the formation of very different planetary systems. This phenomenon is obviously non-negligible since binary systems represent at least half of stellar systems. At late stages of planetary systems evolution, planetary migration may occur as the result of interactions with remaining solid planetesimals and the impact of binarity on this planetesimal-driven migration is explored in this thesis. A stellar binary companion may in fact reverse the tendency for planets in single star systems to migrate inwards, and bring them closer to regions perturbed by the binary companion, where they could not have formed in situ (Faramaz et al., 2014) . This may give an explanation for the detection of planets which present signs of outward migration in close binary systems.