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. This thesis revolves around the B-Star Exoplanet Abundance Study (BEAST), an ongoing direct-imaging survey that is searching for the first time for a wide-orbit giant planet population around young B-type (2.4 M⊙ ≲ M ≲ 16 M⊙) stars, belonging to the Scorpius-Centaurus association. As a first step in preparation for forthcoming detections, we focused our efforts in constraining stellar ages, a crucial parameter for companion characterization. In order to circumvent the well-known issues of direct age determination for B stars, we devised an indirect technique which hinges upon the membership of BEAST targets to small groups of stars within the association. 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. 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. The versatility of MADYS turned it into the ideal tool to determine the physical parameters of direct-imaged substellar companions. While BEAST is still in progress, its provisional results are already intriguing. A 10.9 ± 1.6 MJ object was found around the 6 - 10M⊙ binary b Centauri, setting the record for the most massive planet-bearing system to date. Shortly thereafter, we found evidence of a comoving substellar companion to µ2 Scorpii at a projected separation of 290 ± 10 au. After undertaking a complete reassessment of the properties of the star, 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). 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.
Superato di recente il considerevole traguardo del 5000° pianeta extrasolare confermato, la semplice rivelazione sta cedendo il passo a una fase di più intensa e minuziosa caratterizzazione e di ricerca di relazioni statistiche in grado di connettere le proprietà osservate della popolazione di esopianeti. Portare alla luce i processi fisici nascosti dietro la multiforme varietà delle architetture planetarie – e i limiti al di fuori dei quali tali processi smettono di operare – è, difatti, il fine ultimo della demografia esoplanetaria. Eppure, il ruolo della stella centrale nel plasmare questi processi non è ancora completamente compreso. Questa tesi è incentrata sullo studio dell’abbondanza di pianeti attorno a stelle B (BEAST): basato sulla tecnica del direct imaging, esso sta cercando le prime prove dell’esistenza di una popolazione di pianeti giganti nelle regioni esterne attorno a stelle B (2.4 M⊙ ≲ M ≲ 16 M⊙) appartenenti alla giovane (5 – 30 milioni di anni) associazione Scorpius-Centaurus. Passaggio necessario per la caratterizzazione delle potenziali scoperte, ci siamo concentrati anzitutto sulla determinazione delle età stellari. Onde evitare di incorrere nei noti problemi delle stime dirette di età per le stelle B, abbiamo sviluppato una tecnica indiretta, basata sull’appartenenza delle stelle di BEAST a piccoli gruppi di stelle all’interno dell’associazione. Tale analisi cinematica, resa possibile dalla grande precisione dei dati forniti dal satellite Gaia, è stata in seguito estesa all’intero Scorpione Superiore (US), una delle tre sottoregioni di Scorpius-Centaurus. Spinti dalla necessità di determinare le età stellari per un gran numero di stelle, abbiamo poi iniziato a sviluppare un programma, MADYS, capace di automatizzare l’intero processo. Il programma mette insieme una larga collezione di modelli di letteratura in una cornice omogenea, e ciò lo rende particolarmente adatto a determinare i parametri fisici dei compagni substellari scoperti tramite imaging. I risultati preliminari di BEAST, attualmente in corso, sono già estremamente interessanti. Un oggetto di 10.9 ± 1.6 MJ è stato scoperto nel sistema binario (M = 6 - 10M⊙) b Centauri, stabilendo il record del sistema planetario più massiccio noto ad oggi. Poco dopo, abbiamo trovato evidenze dell’esistenza di un compagno stellare comovente con µ2 Scorpii a una separazione proiettata di 290 ± 10 au. Dopo aver ricalcolato le proprietà della stella, abbiamo determinato la massa del compagno: M = 14.4±0.8 MJ, poco sopra il limite di bruciamento del deuterio usato per distinguere tra pianeti e nane brune. Nascosto dall’intensa luce stellare, abbiamo probabilmente osservato un secondo candidato compagno a una separazione molto ridotta (0.12′′ ≈ 20 au). La natura di questi oggetti è incerta e mette parzialmente in discussione l’attuale concezione della formazione planetaria. Se da una parte questi oggetti sono vicini al limite di bruciamento del deuterio, dall’altra le loro proprietà ricordano quelle dei pianeti giganti attorno a stelle meno massicce e sono riprodotte più facilmente assumendo che si siano formati in uno scenario simil-planetario piuttosto che in uno scenario simil-stellare. Mettendo queste considerazioni nel contesto dei modelli di formazione di core accretion e gravitational instability, concludiamo che l’attuale modellizzazione di entrambi i meccanismi non è ancora in grado di produrre questo tipo di compagni e necessita pertanto di un’estensione a masse stellari maggiori. La survey BEAST ha già dimostrato che le stelle B possono possedere sistemi planetari — o almeno simil-planetari –, mettendo in discussione molte delle nostre aspettative iniziali. Nei prossimi anni sapremo quanto sono frequenti questi sistemi, e la combinazione di minuziosi studi di follow-up e di nuovi modelli potrà fare chiarezza sulla loro sfuggevole origine.
L'impatto della massa stellare sulla formazione dei pianeti / Squicciarini, Vito. - (2022 Dec 19).
L'impatto della massa stellare sulla formazione dei pianeti
SQUICCIARINI, VITO
2022
Abstract
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. This thesis revolves around the B-Star Exoplanet Abundance Study (BEAST), an ongoing direct-imaging survey that is searching for the first time for a wide-orbit giant planet population around young B-type (2.4 M⊙ ≲ M ≲ 16 M⊙) stars, belonging to the Scorpius-Centaurus association. As a first step in preparation for forthcoming detections, we focused our efforts in constraining stellar ages, a crucial parameter for companion characterization. In order to circumvent the well-known issues of direct age determination for B stars, we devised an indirect technique which hinges upon the membership of BEAST targets to small groups of stars within the association. 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. 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. The versatility of MADYS turned it into the ideal tool to determine the physical parameters of direct-imaged substellar companions. While BEAST is still in progress, its provisional results are already intriguing. A 10.9 ± 1.6 MJ object was found around the 6 - 10M⊙ binary b Centauri, setting the record for the most massive planet-bearing system to date. Shortly thereafter, we found evidence of a comoving substellar companion to µ2 Scorpii at a projected separation of 290 ± 10 au. After undertaking a complete reassessment of the properties of the star, 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). 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.File | Dimensione | Formato | |
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PhD_Thesis.pdf
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Descrizione: PhD Thesis
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