Earth and Planetary Astrophysics (astro-ph.EP)

Abstract: Transmission spectroscopy is one of the most successful methods of learning about exoplanet atmospheres. The process of retrievals using transmission spectroscopy consists of creating numerous forward models and comparing them to observations to solve the inverse problem of constraining the atmospheric properties of exoplanets. We explore the impact of one simplifying assumption commonly employed by forward models of transiting exoplanets: namely that the planet can be treated as an isolated, non-rotating spherical body. The centrifugal acceleration due to a planet's rotation opposes the gravitational pull on a planet's atmosphere and increases its scale height. Conventional forward models used for retrievals generally do not include this effect. We find that atmospheric retrievals produce significantly different results for close-in planets with low gravity when this assumption is removed, e.g., differences between true and retrieved values of gas abundances greater than 1$\sigma$ for a simulated planet analogous to WASP-19 b. We recommend that the correction to the atmospheric scale height due to this effect be taken into account for the analysis of high precision transmission spectra of exoplanets in the future, most immediately JWST Cycle 1 targets WASP-19 b and WASP-121 b.

Abstract: In this paper we report on the follow-up of five potential exoplanets detected with Gaia astrometry and provide an overview of what is currently known about the nature of the entire Gaia astrometric exoplanet candidate sample, 72 systems in total. We discuss the primary false-positive scenario for astrometric planet detections: binary systems with alike components that produce small photocenter motions, mimicking exoplanets. These false positives can be identified as double-lined SB2 binaries through analysis of high resolution spectra. Doing so we find that three systems, Gaia DR3 1916454200349735680, Gaia DR3 2052469973468984192, and Gaia DR3 5122670101678217728 are indeed near equal mass double star systems rather than exoplanetary systems. The spectra of the other two analyzed systems, HD 40503 and HIP 66074, are consistent with the exoplanet scenario in that no second set of lines can be found in the time series of publicly available high resolution spectra. However, their Gaia astrometric solutions imply radial-velocity semi-amplitudes $\sim$\,3 (HD 40503) and $\sim$\,15 (HIP 66074) larger than what was observed with ground based spectrographs. The Gaia astrometry orbital solutions and ground-based radial-velocity measurements exhibit inconsistencies in six out of a total of 12 exoplanet candidate systems where such data are available, primarily due to substantial differences between observed ground-based radial-velocity semi-amplitudes and those implied by the Gaia orbits. We investigated various hypotheses as to why this might be the case, and though we found no clear perpetrator, we note that a mismatch in orbital inclination offers the most straightforward explanation.

Abstract: We aim at searching for exoplanets on the whole ESO/VLT-SPHERE archive with improved and unsupervised data analysis algorithm that could allow to detect massive giant planets at 5 au. To prepare, test and optimize our approach, we gathered a sample of twenty four solar-type stars observed with SPHERE using angular and spectral differential imaging modes. We use PACO, a new generation algorithm recently developed, that has been shown to outperform classical methods. We also improve the SPHERE pre-reduction pipeline, and optimize the outputs of PACO to enhance the detection performance. We develop custom built spectral prior libraries to optimize the detection capability of the ASDI mode for both IRDIS and IFS. Compared to previous works conducted with more classical algorithms than PACO, the contrast limits we derived are more reliable and significantly better, especially at short angular separations where a gain by a factor ten is obtained between 0.2 and 0.5 arcsec. Under good observing conditions, planets down to 5 MJup, orbiting at 5 au could be detected around stars within 60 parsec. We identified two exoplanet candidates that require follow-up to test for common proper motion. In this work, we demonstrated on a small sample the benefits of PACO in terms of achievable contrast and of control of the confidence levels. Besides, we have developed custom tools to take full benefits of this algorithm and to quantity the total error budget on the estimated astrometry and photometry. This work paves the way towards an end-to-end, homogeneous, and unsupervised massive re-reduction of archival direct imaging surveys in the quest of new exoJupiters.

Abstract: In the context of a recently proposed contact-binary model of the Kreutz system, all its members are products of the process of cascading fragmentation of the two lobes of the parent, Aristotle's comet of 372 BC. This process presumably began with the lobes' separation from each other near aphelion. However, not every object in a Kreutz-like orbit is a Kreutz sungrazer. Any surviving sungrazer that had split off from the progenitor before the lobes separated, as well as its surviving fragments born in any subsequent tidal or nontidal event, are by definition not members of the Kreutz system. Yet, as parts of the same progenitor, they belong -- as do all Kreutz sungrazers -- to a broader assemblage of related objects, which I refer to as a super-Kreutz system. After estimating the ratio of the number of super-Kreutz members to nonmembers among potential historical sungrazers, I generate representative extended pedigree charts for both the Kreutz system and super-Kreutz system. While the fragmentation paths and relationships among the individual sungrazers or potential sungrazers in the two charts are (with at most a few exceptions) arbitrary, the purpose of the exercise is to suggest that the Kreutz system proper could in effect represent an ultimate deagglomeration stage of the super-Kreutz system.

Abstract: The orbits of the main satellites of Uranus are expected to slowly drift away owing to tides raised in the planet. As a result, the 5/3 mean motion resonance between Ariel and Umbriel was likely encountered in the past. Previous studies have shown that, in order to prevent entrapment in this resonance, the eccentricities of the satellites must be larger than $\sim 0.01$ at the epoch, which is hard to explain. On the other hand, if the satellites experience some temporary capture and then escape, the inclinations rise to high values that are not observed today. We have revisited this problem both analytically and numerically focussing on the inclination, using a secular two-satellite model with circular orbits. We show that if the inclination of Umbriel was around $0.15^{\circ}$ at the time of the 5/3 resonance encounter, capture can be avoided in about $60\%$ of the cases. Moreover, after the resonance crossing, the inclination of Umbriel drops to a mean value around $0.08^{\circ}$, which is close to the presently observed one. The final inclination of Ariel is distributed between $0.01^{\circ}$ and $0.25^{\circ}$ with a nearly equal probability, which includes the present mean value of $0.02^{\circ}$.