Exoplanets and Stellar Physics

Abstract:

Whether a planet's volcanic gas is oxidising or reducing is inherited from redox conditions in the planet's mantle. It is often presumed that reactions between iron species control mantle oxygen fugacity. However, iron alone need not be the sole dictator of how oxidising the interior of a planet is. Carbon is a powerful redox element, with great potential to feed back upon the mantle redox state as it melts. Despite Earth being carbon-poor, it has been proposed that the oxygen fugacity of Earth's upper mantle is in part controlled by carbon (Holloway et al., 1992; Stagno et al., 2013); a slightly-higher volatile endowment could make carbon-powered geochemistry inescapable. Indeed, a number of known rocky exoplanets are predicted to have formed with carbon contents greater than Earth (Bergin et al., 2023). We offer a framework for how carbon is transported from solid planetary interior to atmosphere, tracking redox couplings between carbon and iron. We also incorporate a coupled 1D energy- and mass-balance model to provide first-order predictions of the rate of volcanism. We show that carbon-iron redox coupling would maintain interior oxygen fugacity in a narrow range: more reducing than Earth magma, but not reducing enough to prevent CO2 outgassing entirely.Bergin, E. A., Kempton, E. M.-R., Hirschmann, M., Bastelberger, S. T., Teal, D. J., Blake, G. A., Ciesla, F. J., & Li, J. (2023). Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation. The Astrophysical Journal, 949, L17. Holloway, J. R., Pan, V., & Gudmundsson, G. (1992). High-pressure fluid-absent melting experiments in the presence of graphite: Oxygen fugacity, ferric/ferrous ratio and dissolved CO2. European Journal of Mineralogy, 4(1), 105–114. Stagno, V., Ojwang, D. O., McCammon, C. A., & Frost, D. J. (2013). The oxidation state of the mantle and the extraction of carbon from Earth’s interior. Nature, 493(7430).

Abstract:

Abstract Ultra-short period lava worlds offer a unique window into the coupled evolution of planetary interior and atmospheres under extreme irradiation. In this study, we investigate the mantle dynamics, nightside volcanism, and volatile outgassing on lava world K2-141 b (1.54 R⊕, 5.31 M⊕) using two-dimensional convection models with tracer-based volatile tracking. Our simulations explore a range of interior configurations, including models with and without plastic yielding, basal versus mixed heating, core cooling, and melt intrusion. In models without plastic yielding (i.e. with a strong lithosphere), we find that mantle upwellings form at the substellar and antistellar points, while downwellings form near the day-night terminators at the boundary between the magma ocean and cold, solid nightside. These downwellings facilitate the recycling of crustal material, representing a form of asymmetric, single-lid tectonics. The resulting magma ocean thickness varies from 200 to 300 km depending on the model parameters, corresponding to about 2-3 % of the planet’s radius. Continuous nightside volcanism produces a basaltic crust and gradually depletes the mantle of volatiles. We find that over a billion years, volcanic eruptions can outgas tens of bars of CO2 and H2O. We show that even relatively large volcanic eruptions on the nightside produce thermal emission signals of no more than 1 ppm, remaining below the current detectability threshold in thermal phase curves. However, for most models, outgassing rates are increased near the day-night terminators and future studies should assess whether such localised outgassing could lead to atmospheric signatures in transmission spectroscopy.

Abstract:

We present a new stellar-dynamical measurement of the supermassive black hole (SMBH) mass in the nearby spiral galaxy NGC 4258 (M106), a critical benchmark for extragalactic mass measurements. We use archival James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec) integral field unit data (G235H/F170LP grating) to extract high-resolution two-dimensional stellar kinematics from the CO bandhead absorption features within the central 3″ × 3″. We extract the stellar kinematics after correcting for instrumental artifacts and separating the stellar light from the nonthermal active galactic nucleus (AGN) continuum. We employ Jeans anisotropic models to fit the observed kinematics, exploring a grid of 12 models to systematically test the impact of different assumptions for the point-spread function, stellar mass-to-light ratio profile, and orbital anisotropy. All 12 models provide broadly acceptable fits, albeit with minor differences. The ensemble median and 68% (1σ) bootstrap confidence interval of our 12 models yield a black hole mass of MBH=(4.08−0.33+0.19)×107 M⊙. This paper showcases the utility of using the full model ensemble to robustly account for systematic uncertainties, rather than relying on formal errors from a single preferred model, as has been common practice. Our result is just 5% larger than, and consistent with, the benchmark SMBH mass derived from water-maser dynamics, validating the use of NIRSpec stellar kinematics for robust SMBH mass determination. Our analysis demonstrates JWST’s ability to resolve the SMBH’s sphere of influence and deliver precise dynamical masses, even in the presence of significant AGN continuum emission.

Abstract:

Plain Language Summary: In hot and wet “hothouse” climate conditions, rainfall transitions from a pattern that fluctuates from about a mean of 3 mm day − 1 ${\text{day}}^{-1}$ to more intense outbursts that are separated by multi‐day dry spells. Previous studies on hothouse climates did not consider the role of the diurnal cycle even though it strongly controls precipitation in Earth's current climate. This study uses radiative‐convective equilibrium simulations to investigate the impact of rising temperatures on the transition to hothouse conditions, incorporating the diurnal cycle with both swamp‐like and open ocean surface conditions. We find that episodic precipitation occurs at surface temperatures above 322 K even when accounting for the diurnal cycle. However, the diurnal cycle significantly influences the timing of convection and rainfall at high temperatures with precipitation primarily starting late at night or in the early morning.