The Structure of the Helium exosphere of WASP-107b
Tom Louden (University of Warwick), James Kirk (CfA Harvard & Smithsonian), Peter Wheatley (University of Warwick)
Abstract
We present an analysis of the shape and velocity structure of the metastable helium outflow of WASP-107. A single transit's ground based transmission spectrum was modelled using TERMINATOR, an algorithm designed to rapidly model arbitrary exoplanet atmosphere structures and asymmetries. We find that the average outflow velocity is 2 km/s, and the data are best explained by modelling the planet with a trailing comet-like tail of outflowing helium.
Introduction
Figure 1: An artist's impression of a Hot jupiter. Credit: Mark A Garlick
Hot jupiters are currently our best window into the atmospheres of extrasolar planets. Over the past decade, transmission spectroscopy has revealed an ever expanding list of atomic and molecular components. One of the most exciting of these is the discovery of metastable helium (Spake et al 2020), which presents an extremely strong signal, and is a key tracer of the exospheres and outflow from exoplanets. The shape and velocity structure of these outflows is crucial to understanding the physics of planet evaporation, and in some cases their long-term survival.
TERMINATOR
TERMINATOR is a fast algorithm for modelling exoplanet atmospheres with arbitrary structures, absorption heterogenity and velocity fields. The code uses a geometric algorithm and is designed to run very quickly to enable detailed and robust model comparisons.
Figure 2: A demonstration of the TERMINATOR algorithm
The code is capable of modelling telluric absorption, the Rossiter-Mclaughlin effect as well as the centre-to-limb variation on the stellar surface. These effects can contaminate transmission spectra, as shown in the pathological example in the attached video. By modelling these effects, TERMINATOR allows the true signal to be recovered. However, in the case of WASP-107b, the Helium absorption signal is so strong that it dwarfs possible stellar contamination effects by orders of magnitude. They are still included in our model, but at too low an amplitude to be easily seen.
The TERMINATOR model is fit to the data using PyMultinest, this technique fully and efficiently explores the parameter space and unlike MCMC techniques it provides the Bayesian Evidence, allowing rigerous model comparisons. Here, we use it to compare a model where the atmosphere is forced to be symetric with one where an elongated tail is allowed to form.
Data
WASP-107b was observed for a full transit with Keck II/NIRSPEC on April 6th 2019, details of the observations and the reduction can be found in (Kirk et. al. 2020). The gaps in the data are due to the A/B nodding strategy to remove the sky background.
Figure 3: The reduced data clearly showing the helium absorption signal of WASP-107b
Results
Figure 4: The recovered transmission spectrum model. Contaminant effects like the RM and CLV effects are too low amplitude to see.
The best fitting model provides an excellent match to the data, reproducing the general shape of the line as well as the slightly late ingress and gentle egress of the dataset.
Figure 5: The posterior of the model fit are plotted in green, the priors used are coloured purple.
Discussion
We performed a model comparison using the full Bayesian evidence for our two models. We found that a model with an elongated comet like tail was favoured by the data significantly over a model with a symetrical atmosphere, but both were strongly favoured over the null hypothesis of zero He absorption
Unfortunately there is insufficient data taken after egress to further constrain the shape of the tail. Future work will involve incorporating archival data from other programs to better constrain this model.
Figure 6: The best fitting structure model for the planet shows an extended tail of outflowing material with a blueshifted velocity.
Conclusion
We have applied our TERMINATOR model to recover the shape and velocity structure of the helium exosphere of WASP-107b, finding an average outflow velocity of 2 km/s and strong evidence for an assymetric atmosphere with a comet-like tail structure in the outlfow.
Acknowledgements
