Virginie Faramaz

HD 8799 is one of the rare systems in which planets have been directly imaged. Even rarer, it is a system where both planets and debris components are visible. The structure of the HR 8799 system mimics that of the Solar System: an Asteroid-belt analogue is surrounded by four giant planets, themselves enclosed by a Kuiper-belt analogue. HR 8799 is somewhat a slightly younger, amore massive, and more extended version of the Solar System. It has therefore been a great playground for many astronomer in the past decade. Why is this system twice as extended as the Solar System? Why are the giant planets all at least five times more massive than Jupiter? Are there other planets in this system? While the access to terrestrial-like planets close to the star and below the asteroid belt is still difficult, there seemed to be clues for an extra planet to be beyond the outermost giant HR 8799 b. Indeed, measurements carried out on the Kuiper-belt analogue with ALMA at 1.3mm indicated that the inner edge was too far out to be sculpted by planet b, and that there was room for an additional less massive planet that remained to be discovered, but this remained relatively uncertain. Thanks to the ALMA facility, I was able to image the disk at 870 microns and get a clearer picture of it. I found that the inner edge of the disk is not abrupt as was expected. This leaves room either for a planet that is little massive and has not completely cleared its surroundings, or material trapped in resonance. The mystery is not entirely solved, but future modelling work will now have a reliable map of the disk at SNR high enough for accurate comparison between models to be made. You will find a link to the publication below.

• Improving mid-infrared thermal background subtraction with principal component analysis -- Rousseau et al. 2024
• The first scattered light images of HD 112810, a faint debris disk in the Sco-Cen association -- Matthews et al. 2023
• An inner warp discovered in the disk around HD 110058 using VLT/SPHERE and HST/STIS -- Stasevic et al. 2023
• Long-term Evolution of Warps in Debris Disks-Application to the Gyr-old System HD 202628 -- Brady et al. 2023
• The clumpy structure of ϵ Eridani's debris disc revisited by ALMA -- Booth et al. 2023
• Imaging nearby, habitable-zone planets with the Large Binocular Telescope Interferometer -- Ertel et al. 2022
• PoZoLE: A Tiny Space Telescope to Snap our Solar System's Ultraviolet Selfie -- Turner et al. 2022
• Fomalhaut b could be massive and sculpting the narrow, eccentric debris disc, if in mean-motion resonance with it -- Pearce et al. 2021
• The debris disk of HR 8799: do we need an extra planet? -- Faramaz et al. 2021
• From scattered-light to millimeter emission: A global view of the Gyr-old system of HD 202628 and its eccentric debris ring -- Faramaz et al. 2019
• A Planetary Mass Companion in an Edge-on Circumstellar Disk System -- Stapelfedlt et al. 2019
• A gap in HD 92945's broad planetesimal disc revealed by ALMA -- Marino et al. 2019
• A gap in the planetesimal disc around HD 107146 and asymmetric warm dust emission revealed by ALMA -- Marino et al. 2018
• Inner mean-motion resonances with eccentric planets: a possible origin for exozodiacal dust clouds -- Faramaz et al. 2017

HD 202628 is a Gyr-old star, and thus a mature star comparable to our Sun. Similar to the Solar System, it hosts an analogue to the Kuiper Belt, which was first revealed by the Hubble Space Telescope. Actually, this is the faintest belt ever resolved in scattered light. However, the Kuiper Belt analogue around HD 202628 is intrinsically eccentric (its geometrical center is offset from the star), which indicates it is sculpted by a planet on an eccentric orbit, which contrasts with the planets on circular orbits contained in the Solar System. This system is one of the steps needed to unravel the whole diversity of mature planetary systems, understand different outcomes and evolutions, and put our own Solar System in perspective. Here I have obtained ALMA observations of this debris disk as Principal Investigator: the wavelengths investigated by ALMA reveal dust grains that are little affected by stellar radiation effects, and thus the gravitational imprint of the planet is seen more clearly. This allows us in turn to set constraints on the yet invisible planet. At this stage, we suspect that the source S1 might be material orbiting this planet, potentially under the form of a giant ring system. Combining our ALMA data with Hubble and Herschel data allowed us to carry out a thorough analysis. You will find a link to the publication below.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), the team led by Sebastian Marino has identified gaps in two debris disks, those of HD 107146 (top) and HD 92945 (bottom), questioning how far planets can form from their host stars.

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.

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

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 ζ² Ret 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).

On the other hand, ALMA observations have been carried out as well for the debris disk of ζ² Ret (PI: V. Faramaz). It appears that a double lobe structure is recovered with ALMA, but that it does not surround the star as seen in Herschel images. Taking into account the star's proper motion, it appears that the star would have been surrounded by the double lobe structure seen with ALMA at the time of the Herschel observations, leading us to believe that the lobes seen with Herschel were in fact background confusion and not a debris disk (Faramaz et al., 2018).

However, everything is not lost in the quest to unravel the diversity of mature Gyr-old systems, as an eccentric debris disk has been revealed by the Hubble Space Telecope around the star HD 202628. Observations have been carried out with ALMA (PI: V. Faramaz), have been presented at the AAS Meeting early 2019 (Faramaz et al., 2019), and will be soon published.

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 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) .

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) .