arXiv subjects

Ethan Schreyer

Publications and source records attributed to Ethan Schreyer.

6 recordsLinked to original sources

arXiv2026

Using observations of escaping H/He to constrain the atmospheric composition of sub-Neptunes

The internal composition of sub-Neptunes remains a prominent unresolved question in exoplanetary science. We present a technique to place constraints on envelope mean molecular weight that utilises observations of escaping hydrogen or helium exospheres. This method is based on a simple timescale argument, which states that sub-Neptunes require a sufficiently large hydrogen or helium reservoir to explain on-going escape at their observed rates. This then naturally leads to an upper limit on atmospheric mean molecular weight. We formalise this argument within a Bayesian inference model and apply it to the archetypal sub-Neptunes GJ-436 b, TOI-776 b and TOI-776 c, which have all been observed to be losing significant hydrogen content as well as relatively featureless transit spectra when observed with JWST. Combining constraints from atmospheric escape and transit spectroscopy in the case of TOI-776 c allows us to tentatively rule out the high mean molecular weight scenario, pointing towards a low mean molecular weight atmosphere with high-altitude aerosols muting spectral features in the infra-red. Finally, we reframe our analysis to the hycean candidate K2-18 b, which has also been shown to host a tentative escaping hydrogen exosphere. If such a detection is robust, we infer a hydrogen-rich envelope mass fraction of $\log_{10} f_\text{env} = -1.67\pm0.78$, which is inconsistent with the hycean scenario at the $\sim 4σ$ level. This latter result requires further observational follow-up to confirm.

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arXiv2026

The First Dedicated Survey of Atmospheric Escape from Planets Orbiting F Stars

Hydrodynamic escape can strip the envelopes of close-in exoplanets, but most observations of atmospheric mass loss to date have been confined to planets orbiting K and M dwarfs. A growing body of detections of atmospheric escape from planets orbiting early-type stars indicates that they may have significantly stronger and more extended outflows than planets orbiting cooler stars. However, it is unclear whether this limited sample of planets is representative of all gas giants orbiting early-type stars. Motivated by this question, we initiated the first dedicated survey of atmospheric escape from gas giants orbiting F stars in order to understand how their distinct radiation environments shape planetary outflows. We observed ten transits of six planets in an ultra-narrowband filter centered on the metastable helium line using Palomar/WIRC. We report strong ($>3σ$) detections of atmospheric escape for WASP-12~b and WASP-180~A~b, tentative ($>2σ$) detections for WASP-93~b and HAT-P-8~b, and non-detections for WASP-103~b and KELT-7~b. We fit these measurements with a 1D Parker wind model to derive corresponding mass-loss rates, and combine our results with literature measurements to obtain an updated picture of mass loss from planets orbiting early-type stars. Our results indicate that the observed variation in mass-loss rates can be explained by a combination of Roche filling factor and XUV luminosity, and disfavors NUV-driven escape models.

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arXiv2026

Observational Signatures of Circumstellar Gas Tori Formed by Planetary Mass-Loss from Close-In Exoplanets

Close-in exoplanets with H/He atmospheres often undergo hydrodynamic escape. In extreme cases, it is hypothesized that the mass loss can be high enough for the escaping planetary material to wrap around the star, forming a long-lasting circumstellar torus. In this work, we develop a physical model of such circumstellar tori and use a ray tracing scheme to calculate the attenuation of stellar light passing through them. We show that the presence of a circumstellar torus significantly increases the equivalent width of the observed stellar He I 10830~Å~line. When combined with observations of the star's Ca II H & K lines, these systems can typically be distinguished from field stars. Based on these results, we propose a survey of stars hosting close-in planets, combining observations of the He I 10830~Å~and Ca II H & K lines to search for circumstellar tori generated from planetary mass-loss in these systems.

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arXiv2024

Using Lyman-$α$ transits to constrain models of atmospheric escape

Lyman-$α$ transits provide an opportunity to test models of atmospheric escape directly. However, translating observations into constraints on the properties of the escaping atmosphere is challenging. The major reason for this is that the observable parts of the outflow often comes from material outside the planet's Hill sphere, where the interaction between the planetary outflow and circumstellar environment is important. As a result, 3D models are required to match observations. Whilst 3D hydrodynamic simulations are able to match observational features qualitatively, they are too computationally expensive to perform a statistical retrieval of properties of the outflow. Here, we develop a model that determines the trajectory, ionization state and 3D geometry of the outflow as a function of its properties and system parameters. We then couple this model to a ray tracing routine in order to produce synthetic transits. We demonstrate the validity of this approach, reproducing the trajectory of the outflows seen in 3D simulations. We illustrate the use of this model by performing a retrieval on the transit spectrum of GJ 436 b. Our model constrains the sound speed of the outflow $\gtrsim 10 \text{ km s}^{-1}$, indicating that we can rule out core-powered mass loss as the mechanism driving the outflow for this planet. The bound on planetary outflow velocity and mass loss rates are consistent with a photoevaporative wind.

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arXiv2024

Using helium 10830 Å transits to constrain planetary magnetic fields

Planetary magnetic fields can affect the predicted mass loss rate for close-in planets that experience large amounts of UV irradiation. In this work, we present a method to detect the magnetic fields of close-in exoplanets undergoing atmospheric escape using transit spectroscopy at the 10830 Angstrom line of helium. Motivated by previous work on hydrodynamic and magneto-hydrodynamic photoevaporation, we suggest that planets with magnetic fields that are too weak to control the outflow's topology lead to blue-shifted transits due to day-to-night-side flows. In contrast, strong magnetic fields prevent this day-to-night flow, as the gas is forced to follow the magnetic field's roughly dipolar topology. We post-process existing 2D photoevaporation simulations to test this concept, computing synthetic transit profiles in helium. As expected, we find that hydrodynamically dominated outflows lead to blue-shifted transits on the order of the sound speed of the gas. Strong surface magnetic fields lead to unshifted or slightly red-shifted transit profiles. High-resolution observations can distinguish between these profiles; however, eccentricity uncertainties generally mean that we cannot conclusively say velocity shifts are due to the outflow for individual planets. The majority of helium observations are blue-shifted, which could be a tentative indication that close-in planets generally have surface dipole magnetic field strengths $\lesssim 0.1$ gauss. More 3D hydrodynamic and magneto-hydrodynamic are needed to confirm this conclusion robustly.

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arXiv2022

The fundamentals of Lyman-alpha exoplanet transits

Lyman-$α$ transits have been detected from several nearby exoplanets and are one of our best insights into the atmospheric escape process. However, due to ISM absorption, we typically only observe the transit signature in the blue-wing, making them challenging to interpret. This challenge has been recently highlighted by non-detections from planets thought to be undergoing vigorous escape. Pioneering 3D simulations have shown that escaping hydrogen is shaped into a cometary tail receding from the planet. Motivated by this work, we develop a simple model to interpret Lyman-$α$ transits. Using this framework, we show that the Lyman-$α$ transit depth is primarily controlled by the properties of the stellar tidal field rather than details of the escape process. Instead, the transit duration provides a direct measurement of the velocity of the planetary outflow. This result arises because the underlying physics is the distance a neutral hydrogen atom can travel before it is photoionized in the outflow. Thus, higher irradiation levels, expected to drive more powerful outflows, produce weaker, shorter Lyman-$α$ transits because the outflowing gas is ionized more quickly. Our framework suggests that the generation of energetic neutral atoms may dominate the transit signature early, but the acceleration of planetary material produces long tails. Thus, Lyman-$α$ transits do not primarily probe the mass-loss rates. Instead, they inform us about the velocity at which the escape mechanism is ejecting material from the planet, providing a clean test of predictions from atmospheric escape models.

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