arXiv subjects

A. Schneeberger

Publications and source records attributed to A. Schneeberger.

5 recordsLinked to original sources

arXiv2026

Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways?

The formation of Uranus and Neptune remains poorly constrained largely due to uncertain deep elemental abundances. Carbon monoxide (CO), a disequilibrium species in the upper troposphere, provides an indirect constraint on the deep oxygen abundance. We investigate the deep O/H and C/O ratios of the ice giants and their formation history by accounting for meridional variations in atmospheric structure and uncertainties in chemical kinetics. We extended a 1D thermochemical and diffusion model into a pseudo-2D model by including latitudinal variations in key model parameters. The O/H ratio was inferred by matching the modeled upper-tropospheric CO mole fractions to observations, and combined with the deep carbon abundance to derive the C/O ratio. For Uranus, varying tropospheric methane alone yielded O/H $\sim [47-57] \times$ protosolar, whereas allowing $K_{zz}$ to vary expanded the range to O/H $\sim [62-177] \times$ protosolar. For Neptune, the corresponding ranges are O/H $\sim [182-215] \times$ protosolar and O/H $\sim [222-342] \times$ protosolar, respectively, supporting lower oxygen enrichment in Uranus than Neptune. Chemical-network uncertainties have a more modest effect, amounting to at most $\sim 10\%$ of the retrieved oxygen ranges, comparable to the uncertainty associated with the measured CO abundance on Neptune, but larger on Uranus. We computed C/O latitudinal ranges and found C/O $\sim [0.06-0.52]$ on Uranus and $\sim [0.02-0.12]$ on Neptune. Comparison with a protoplanetary disk model suggests different formation or evolutionary pathways for the two planets. Our results highlight the dominant role of vertical mixing in constraining deep oxygen abundance and the importance of accounting for meridional variability and chemical uncertainties. This work also provides a framework for selecting the entry latitude of a future Uranus Orbiter and Probe mission.

astro-ph.EP↗

arXiv2024

Journey of complex organic molecules: Formation and transport in protoplanetary disks

Complex organic molecules serve as indicators of molecular diversity. Their detection on comets, planets, and moons has prompted inquiries into their origins, particularly the conditions conducive to their formation. One hypothesis suggests that the UV irradiation of icy grains in the protosolar nebula generates significant molecular complexity, a hypothesis supported by experiments on methanol ice irradiation. We investigated the irradiation of methanol ice particles as they migrate through the protosolar nebula. Our objective is to ascertain whether the encountered conditions facilitate the formation of complex organics molecules, and we leverage experimental data in our analysis. We developed a two-dimensional model that describes the transport of pebbles during the evolution of the protosolar nebula, employing a Lagrangian scheme. This model computes the interstellar UV flux received by the particles along their paths, which we compared with experimental values. On average, particles ranging from 1 to 100 micrometers in size, released at a local temperature of 20 K, undergo adequate irradiation to attain the same molecular diversity as methanol ice during the experiments within timescales of 25 kyr of protosolar nebula evolution. In contrast, 1 cm sized particles require 911 kyr of irradiation to reach similar molecular diversity, making comparable molecular complexity unlikely. Similarly, particles ranging from 1 to 100 micrometers in size, released at a local temperature of 80 K, receive sufficient irradiation after 141 and 359 kyr. The particles readily receive the irradiation dose necessary to generate the molecular diversity observed in the experiments within the outer regions of the disk. Our model, combined with future irradiation experiments, can provide additional insights into the specific regions where the building blocks of planets form.

astro-ph.EP↗

arXiv2024

Fresh view of the hot brown dwarf HD 984 B through high-resolution spectroscopy

Context. High-resolution spectroscopy has the potential to drive a better understanding of the atmospheric composition, physics, and dynamics of young exoplanets and brown dwarfs, bringing clear insights into the formation channel of individual objects. Aims. Using the Keck Planet Imager and Characterizer (KPIC; R = 35,000), we aim to characterize a young brown dwarf HD 984 B. By measuring its C/O and 12CO/13CO ratios, we expect to gain new knowledge about its origin by confirming the difference in the formation pathways between brown dwarfs and super-Jupiters. Methods. We analysed the KPIC high-resolution spectrum (2.29-2.49 μm) of HD 984 B using an atmospheric retrieval framework based on nested sampling and petitRADTRANS, using both clear and cloudy models. Results. Using our best-fit model, we find C/O = 0.50+0.01-0.01 (0.01 is the statistical error) for HD 984 B which agrees with that of its host star within 1σ (0.40+0.20-0.20). We also retrieve an isotopolog 12CO/13CO ratio of 98+20-25 in its atmosphere, which is similar to that of the Sun. In addition, HD 984 B has a substellar metallicity with [Fe/H] = -0.62+0.02-0.02. Finally, we find that most of the retrieved parameters are independent of our choice of retrieval model. Conclusions. From our measured C/O and 12CO/13CO, the favored formation mechanism of HD 984 B seems to be via gravitational collapse or disk instability and not core accretion, which is a favored formation mechanism for giant exoplanets with m < 13 MJup and semimajor axis between 10 and 100 au. However, with only a few brown dwarfs with a measured 12CO/13CO ratio, similar analyses using high-resolution spectroscopy will become essential in order to determine planet formation processes more precisely.

astro-ph.SR↗

arXiv2022

Moonraker -- Enceladus Multiple Flyby Mission

Enceladus, an icy moon of Saturn, possesses an internal water ocean and jets expelling ocean material into space. Cassini investigations indicated that the subsurface ocean could be a habitable environment having a complex interaction with the rocky core. Further investigation of the composition of the plume formed by the jets is necessary to fully understand the ocean, its potential habitability, and what it tells us about Enceladus' origin. Moonraker has been proposed as an ESA M-class mission designed to orbit Saturn and perform multiple flybys of Enceladus, focusing on traversals of the plume. The proposed Moonraker mission consists of an ESA-provided platform, with strong heritage from JUICE and Mars Sample Return, and carrying a suite of instruments dedicated to plume and surface analysis. The nominal Moonraker mission has a duration of 13.5 years. It includes a 23-flyby segment with 189 days allocated for the science phase, and can be expanded with additional segments if resources allow. The mission concept consists in investigating: i) the habitability conditions of present-day Enceladus and its internal ocean, ii) the mechanisms at play for the communication between the internal ocean and the surface of the South Polar Terrain, and iii) the formation conditions of the moon. Moonraker, thanks to state-of-the-art instruments representing a significant improvement over Cassini's payload, would quantify the abundance of key species in the plume, isotopic ratios, and physical parameters of the plume and the surface. Such a mission would pave the way for a possible future landed mission.

astro-ph.EP↗

arXiv2020

K-Stacker, an algorithm to hack the orbital parameters of planets hidden in high-contrast imaging. First applications to VLT SPHERE multi-epoch observations

Recent high-contrast imaging surveys, looking for planets in young, nearby systems showed evidence of a small number of giant planets at relatively large separation beyond typically 20 au where those surveys are the most sensitive. Access to smaller physical separations between 5 and 20 au is the next step for future planet imagers on 10 m telescopes and ELTs in order to bridge the gap with indirect techniques (radial velocity, transit, astrometry with Gaia). In that context, we recently proposed a new algorithm, Keplerian-Stacker, combining multiple observations acquired at different epochs and taking into account the orbital motion of a potential planet present in the images to boost the ultimate detection limit. We showed that this algorithm is able to find planets in time series of simulated images of SPHERE even when a planet remains undetected at one epoch. Here, we validate the K-Stacker algorithm performances on real SPHERE datasets, to demonstrate its resilience to instrumental speckles and the gain offered in terms of true detection. This will motivate future dedicated multi-epoch observation campaigns in high-contrast imaging to search for planets in emitted and reflected light. Results. We show that K-Stacker achieves high success rate when the SNR of the planet in the stacked image reaches 7. The improvement of the SNR ratio goes as the square root of the total exposure time. During the blind test and the redetection of HD 95086 b, and betaPic b, we highlight the ability of K-Stacker to find orbital solutions consistent with the ones derived by the state of the art MCMC orbital fitting techniques, confirming that in addition to the detection gain, K-Stacker offers the opportunity to characterize the most probable orbital solutions of the exoplanets recovered at low signal to noise.

astro-ph.EP↗