The impact of stellar mass on the formation of planets

Context Having recently reached the impressive milestone of 5000 confirmed extrasolar planets, the exoplanetary field has attained in less than 30 years a remarkable degree of maturity. While a purely detection-oriented phase is giving way to a subtler and more intense characterization phase, the quest for statistical trends connecting the observed properties of the exoplanet population is rapidly emerging as the next big step forward for the field. Unveiling the physical processes lurking behind the multifaceted hues of observed planetary architectures — and the limits outside which these processes no longer work — is indeed the ultimate purpose of exoplanet demographics. Still, the role played by the central star in carving these processes, and hence in shaping its own planetary system, is not completely understood. Disks around more massive stars are known to be more massive, shorter-lived and more prone to fragmentation; several formation mechanisms, differing in the accretion timescale and in the resulting mass distribution, have been proposed. In this tension between matter, time and gravity lies the kismet of planet formation for stars with increasing mass, at the borderline between becoming more efficient and being abruptly halted. Aims of this work This thesis revolves around the B-Star Exoplanet Abundance STudy (BEAST), an ongoing direct-imaging survey that is searching for a wideorbit giant planet population around B-type (2.4 M⊙ ≲ M ≲ 16 M⊙) members of the young (5 − 30 Myr) Scorpius-Centaurus association. Previous radial velocity (RV) and direct imaging campaigns have mostly focused on less massive targets; the few existing RV-based indications suggest a turnoff in the occurrence frequency of giant planets at about M ∼ 2M⊙, consistent with the early disk dissipation predicted by core accretion; the argument does not apply to gravitational instability, whose wide-orbit massive products may have escaped detection in RV surveys that are virtually insensitive to orbital periods ≳ 20 yr. By targeting the outer regions around young B stars, BEAST is ideally positioned to investigate the frontiers of planet formation. Results As a first step in preparation for forthcoming detections, we focused our efforts in constraining a stellar property that is crucial to companion characterization: namely, age. In order to circumvent the well-known issues of direct age determination for B stars, we explored an indirect technique which hinges upon the membership of BEAST targets to small groups of stars within the association. By computing group ages through isochronal fitting, we were able to refine the age estimates for the majority of the targets. This kinematic analysis, enabled by the extreme precision of data delivered by the Gaia satellite, was later extended to encompass the whole Upper Scorpius (US), one of the three subregions in which Scorpius-Centaurus is classically divided. Impressed with the prominent degree of substructuring discernible in the subregion, we developed a trace-back model to understand whether US still retained traces of its initial velocity structure. We discovered that about one half of US appears composed of many smaller entities, which were in a more compact configuration in the past. The presence of a kinematic duality is reflected into an age spread between this younger clustered population and an older diffuse population, in turn confirmed by a different fraction of disk-bearing stars. Star formation in US appears to have lasted more than 10 Myr and proceeded in small groups that, after a few Myr, dissolve in the field of the older population but retain for some time memory of their initial structure. Prompted by the necessity to evaluate isochronal ages for large lists of stars in a fast and robust way, we began developing a tool, madys, to automatize the entire process. The tool gathers from the literature a large set of stellar evolutionary models and puts them in a unified framework, allowing extensive customization of input parameters. With an eye on the study of star-forming regions and the other on directly imaged substellar objects such as those expected from BEAST, we assembled the list of models to encompass substellar evolutionary models as well. In this way, the versatility of madys turned it into the ideal tool to determine the physical parameters of direct-imaged substellar companions based on measured contrasts to their parent stars. While BEAST is still in progress, its provisional results are already intriguing. A 10.9±1.6 MJ object was found around the 6−10 M⊙ binary b Centauri, setting the record for the most massive planet-bearing system to date. Shortly thereafter, when analyzing high-contrast images of µ2 Scorpii, we found evidence of a comoving substellar companion at a projected separation of 290±10 au. In order to precisely determine the properties of this companion, we first undertook a complete reassessment of the properties of the star. Based on kinematic information, we established its membership to a small group of stars and hence constrained its distance, which in turn allowed us to refine the precision on the remaining stellar parameters. We determined the mass of the companion to be 14.4±0.8 MJ, slightly above the deuterium-burning limit that classically marks the transition between planets and brown dwarfs. Lurking beneath the glaring light of the star, a second companion candidate was tentatively spotted at an extremely small separation (0.12′′≈20 au). If confirmed, its luminosity and age would be consistent with a mass M=18.5±1.5 MJ. Conclusions The nature of these objects is uncertain, and challenges our current view of planet formation. While their masses are near the deuterium burning limit, their properties better resemble those of giant planets around less massive stars and are better reproduced by assuming that they formed under a planet-like, rather than a star-like scenario. When putting this finding in the context of core accretion and gravitational instability formation scenarios, we conclude that the current modeling of both mechanisms is not able to produce this kind of companion and needs being extended to higher stellar masses. The BEAST survey has already shown that B stars can possess planetary — or at least planet-like — systems, challenging many of our prior expectations. In the next few years, we will know how frequent these systems are, and the combination of thorough follow-up efforts and dedicated models will hopefully shed light on their elusive origin