Hydrodynamic atmospheric escape in HD 189733 b: Signatures of carbon and hydrogen measured with the Hubble Space Telescope

TL;DR

This study uses Hubble Space Telescope ultraviolet observations to detect and model atmospheric escape signatures, including carbon and hydrogen, from the hot Jupiter HD 189733 b, revealing details about its outflow composition and interaction with stellar wind.

Contribution

It provides the first repeatable detection of ionized carbon escape and models the hydrodynamic outflow, advancing understanding of atmospheric escape processes in hot Jupiters.

Findings

Detected in-transit C II absorption at 5.2% with repeatability.

Estimated atmospheric escape rate of ~1.1 x 10^11 g/s.

Indications of interaction with stellar wind significantly stronger than solar.

Abstract

One of the most well-studied exoplanets to date, HD 189733 b, stands out as an archetypal hot Jupiter with many observations and theoretical models aimed at characterizing its atmosphere, interior, host star, and environment. We report here on the results of an extensive campaign to observe atmospheric escape signatures in HD 189733 b using the Hubble Space Telescope and its unique ultraviolet capabilities. We have found a tentative, but repeatable in-transit absorption of singly-ionized carbon (C II, $5.2\% \pm 1.4\%$) in the epoch of June-July/2017, as well as a neutral hydrogen (H I) absorption consistent with previous observations. We model the hydrodynamic outflow of HD 189733 b using an isothermal Parker wind formulation to interpret the observations of escaping C and O nuclei at the altitudes probed by our observations. Our forward models indicate that the outflow of HD 189733 b is mostly neutral within an altitude of $\sim 2$ R$_\mathrm{p}$ and singly ionized beyond that point. The measured in-transit absorption of C II at 133.57 nm is consistent with an escape rate of $\sim 1.1 \times 10^{11}$ g$\,$s$^{-1}$, assuming solar C abundance and outflow temperature of $12\,100$ K. Although we find a marginal neutral oxygen (O I) in-transit absorption, our models predict an in-transit depth that is only comparable to the size of measurement uncertainties. A comparison between the observed Lyman-$\alpha$ transit depths and hydrodynamics models suggests that the exosphere of this planet interacts with a stellar wind at least one order of magnitude stronger than solar.