Oxygen left behind: Atmospheric Enrichment due to Fractionation in Sub-Neptunes using BOREAS

Abstract

The evolution of exoplanetary atmospheres is strongly influenced by atmospheric escape, particularly for close-in planets. Fractionation during atmospheric loss can preferentially remove lighter elements such as hydrogen, while retaining heavier species like oxygen. In this study, we investigate how and under what conditions hydrodynamic escape and chemical fractionation jointly shape the mass and composition of exoplanet atmospheres, especially for mixed H2 + H2O atmospheres. We develop BOREAS, a self-consistent mass loss model coupling a 1D Parker wind formulation with a mass-dependent fractionation scheme, which we apply across a range of planet masses, radii, equilibrium temperatures, and incident XUV fluxes, allowing us to track hydrogen and oxygen escape rates at different snapshots in time. We find that oxygen is efficiently retained over most of the parameter space. Significant oxygen loss occurs under high incident XUV fluxes, while at intermediate fluxes oxygen loss is largely confined to low-gravity planets. Where oxygen is retained, irradiation is too weak to drive significant escape of hydrogen and thus limiting atmospheric enrichment. By contrast, our model predicts that sub-Neptunes undergo substantial atmospheric enrichment over approx. 200 Myr when hydrogen escape is efficient and accompanied by partial oxygen entrainment. Notably, our results imply that sub-Neptunes near the radius valley can evolve into water-rich planets, in agreement with GJ 9827 d. Present-day water-rich atmospheres may have originated from water-poor envelopes under some conditions, highlighting the need to include chemical fractionation in evolution models. BOREAS is publicly available.

Figures (9)

• Figure 1: Time evolution of stellar XUV luminosity for stars of different masses, ranging from 0.1 to 1.4 M$_\odot$.
• Figure 2: Schematic depiction of the model's atmosphere, adapted from owen_mapping_2023. For a planet undergoing significant photoevaporation, $R_B$ lies outside $R_\mathrm{XUV}$, and $R_{s} < R_B$.
• Figure 3: Mass loss rate $\dot{M}$ as a function of $F_\mathrm{XUV}$ for a range of planetary compositions and envelope structures. Each panel shows results for a different atmospheric configuration: (top left) super-Earths with 3% water mass fraction (WMF) and a purely steam H$_2$O atmosphere; (top right) sub-Neptunes with 3% atmospheric mass fraction (AMF) in H/He; (bottom left) sub-Neptunes with 3% AMF in H$_2$ and 10% envelope WMF; (bottom right) sub-Neptunes with 3% AMF and 50% envelope WMF. Each point corresponds to a specific model with varying $T_\mathrm{eq}$ and planet mass. Marker size reflects planetary mass, where the minimum, maximum, and few cases in between are plotted. Color denotes equilibrium temperature (from yellow to maroon with increasing $T_\mathrm{eq}$), and red point edges flag the recombination-limited regime. The purple indicators represent the XUV fluxes of certain scenarios and are there for context; indicators 1) and 2) correspond to the flux an Earth-like planet receives around a Sun-like star at an orbital period of 10 days, at 50 Myr and 5 Gyr, respectively; 3) and 4) are similar to 1) and 2) but for an M-star of mass 0.4$M_\odot$. Across the parameter space, higher XUV flux and lower mass lead to increased $\dot{M}$.
• Figure 4: Oxygen fractionation factor map over planetary mass (left) and radius (right), and incident $F_\mathrm{XUV}$, pooled across envelope compositions and $T_\mathrm{eq}$. Gray shaded regions and scatter points show a fractionation factor for oxygen of 0 (no oxygen loss), while the colorbar shows increasing fractionation factors towards yellow colors. A fractionation factor of 1 means that oxygen is lost according to the mixing ratio (e.g., 1 oxygen atom per 2 hydrogen atoms in a pure steam atmosphere). Red point edges flag the recombination-limited regime.
• Figure 5: Atomic escape-flux ratio $\dot{N}_O/\dot{N}_H$ for all simulated planets spanning masses $1 - 15 M_\oplus$, equilibrium temperatures $300-2{,}000$ K, and a wide range of incident $F_\mathrm{XUV}$. A denser $F_\mathrm{XUV}$ sampling was done for the data shown in this plot compared to the previous figures, for better visualization. Cases where oxygen is lost ($x_O \neq 0$) are colored by planetary type and composition as indicated. Dashed lines show the mixing-limited maximum O/H ratios corresponding to each bulk water mass fraction, representing the upper limit for oxygen escape if it occurred at the bulk atmospheric abundance with a $x_O=1$. Black points at the 0 escape flux ratio line correspond to cases where oxygen is not lost, thus the ratio drops to 0. These cases make up $\sim$ 75 % of the data, overlap with each other, and span approximately 5 orders of magnitude of hydrogen loss (atoms s$^-{1}$).