Characterisation of the upper of HD 209458 b by means of helium triplet absorption spectra M. Lampón, M. López-Puertas, L.M. Lara, A. Sánchez-López and the CARMENES Consortium IAA-CSIC, Granada, Spain ([email protected]) The upper atmosphere of HD 209458b undergoes that is the efficient atmospheric -loss process. Previous works on its characterisation were mainly based on Ly-⍺ observations, which provide limited information due to interstellar medium absorption and geocoronal contamination. However, He(23S) absorption at 10830 Å is not affected by these processes and hence it is very suitable for obtaining information of planetary upper . In this work, we significantly improve the characterisation of the upper atmosphere of HD 209458b by analysing mid- He(23S) spectral absorption measurements. Our study shows that hydrodynamic expands the thermosphere from the thermobase (at ~1.04 planetary radius, Rp) to the Roche lobe (at 4.22 Rp), with a strong and light wind (H/He of ~98/2), almost fully ionised beyond 2.9 Rp.

Results published in Lampón, M. et al. 2020, A&A, 636, A13

Virtual Meeting, 21 Sep-9 Oct 2020 Introduction

The He triplet absorption at 10830Å offers a new window for studying the atmospheric escape of exo- atmospheres (Nortmann et al., 2018). Atmospheric and helium gases of that orbit close to their host stars can escape after suffering strong irradiation at XUV. So far the evaporation has been studied using the H Ly-⍺ in the UV. The He NIR line provides more information since its directly probes the region where the escape is originating (see Figure). Brogi, M., Science, 362, 6421, 2018.

Lampón et al., He triplet absorption in HD 209458b 2 2 Observations andOklop ciˇModellingc&Hirata´

• We analyse the observations1. INTRODUCTION of the He triplet absorptionClose-in of exoplanetsHD209458b give measured us a new insight with the into high the - resolutionmechanisms spectrograph of atmospheric CARMENES escape and at mass the loss. 3.5m In Calar Alto highlyTelescope irradiated (Alonso planets,-Floriano atmosphericet al., escape 2019). can be • We developedvery ecient a and 1D acthydrodynamic collectively on model the atmosphere of the thermosphere,as a fluid (e.g. withOwen spherical & Jackson symmetry,2012), instead and of coupled on witha a particle-by-particle non-LTE model basis. that Thiscalculates hydrodynamic the escape may be important for the planetary evolution, especially profiles of H, H+, He, He+ and He(23S). in low-mass planets which are more vulnerable to pho- • Hydrodynamictoevaporation continuity compared to equations massive planets are solved with deep Oklopcic & Hirata, 2018 assuminggravitational a constant wells. speed This process of sound, has been similar proposed to the isothermalas an explanation Parker wind for themodel observed (Parker, paucity 1958). of short- • The periodinputs sub- of the model planets are and , the bimodal distribution mass-loss rate of(MLR) planet and radii the (Owen H/He & Wu composition.2013; Lundkvist et al. 2016; Fulton et al. 2017). Improving our knowledge of how • The production and loss processes of He(23S) are sketchedFigure in 1. theStructure figure. of the helium atom, indicating the hydrodynamic escape works and how it a↵ects a broad With the He(23S) density, we computed synthetic spectraradiative and and compared collisional transitions to that includedmeasured. in our analysis. • range of atmospheres is therefore necessary for better The transition shown in red depicts the 10830 A˚ absorption • In orderunderstanding to constrain the demographics the temperate, of planetary MLR and systems H/He, weline. run simulations for T=4000-11500 K, MLR=10and their8 -10 evolution.12 g/s, and H/He=90/10, 95/5 and 98/2. Observational evidence for atmospheric escape has Ly↵ line core—and the valuable information content it been obtained for a handful of exoplanets to date in the might carry—irretrievable (e.g. Ehrenreich et al. 2015). Lampón et al., He triplet absorption in HD 209458b 3 form of a strong absorption signal detected in the wings Here we investigate the possibility of probing the es- of the hydrogen Ly↵ line, but also in some UV lines of caping atmospheres of exoplanets with the absorption metals (Vidal-Madjar et al. 2003, 2004; Lecavelier Des line of helium at 10830 A.˚ This line may provide a new Etangs et al. 2010; Linsky et al. 2010; Fossati et al. 2010; wavelength window for studying the hydrodynamic es- Kulow et al. 2014). The first observations of this kind cape and atmospheric mass loss. Its main advantages were obtained for a transiting HD 209458b over the UV lines include weaker interstellar absorption1 by Vidal-Madjar et al. (2003). Strong absorption in the and the possibility of ground-based observations. wings of the Ly↵ line resulted in transit depth about 2. HELIUM METASTABLE STATE AND THE an order of magnitude greater than the optical transit, 10830 A˚ LINE suggesting that the observed cloud of hydrogen extends far away from the planet. Even greater transit depths The helium atom can exist in two configurations based in the wings of Ly↵ have been reported for a warm Nep- on the relative orientation of its electrons’ spin, singlet tune GJ 436b (Kulow et al. 2014; Ehrenreich et al. 2015; (anti-parallel) and triplet (parallel). The lowest-lying 3 Lavie et al. 2017). triplet level (2 S) is almost decoupled from the sin- 1 Several groups have developed theoretical models of glet ground state (1 S) because radiative transitions be- escaping atmospheres (e.g. Lammer et al. 2003; Yelle tween them are strongly suppressed. Due to relativistic 2004; Garc´ıa Mu˜noz 2007; Murray-Clay, Chiang & Mur- and finite-wavelength corrections to the magnetic dipole 3 ray 2009; Koskinen et al. 2010; Bourrier & Lecavelier transition formula, the 2 S triplet helium can radiatively des Etangs 2013; Tripathi et al. 2015; Salz et al. 2016; decay into the singlet ground state with an exceptionally Carroll-Nellenback et al. 2017). The methodology and long lifetime of 2.2 hours (Drake 1971). 3 2 the complexity varies greatly between these models, and The 2 S state can be populated by recombination hence their predictions, such as the expected mass loss or by collisional excitation from the ground state (see rate, can di↵er by orders of magnitude. More detailed Figure 1). Depopulation of this state progresses slowly observations are required to place more stringent con- making it metastable, and hence a promising origin of straints on theoretical models. Ly↵ observations have been immensely valuable for 1 Indriolo et al. (2009)measuredthecolumndensityof providing evidence of atmospheric escape. However, metastable helium through di↵use interstellar clouds and obtained 9 2 there are inherent limitations of using this line. Ly↵ an upper limit of N . 10 cm ,whichisroughlythreeorders su↵ers from extinction by the ISM and contamination of magnitude lower than our prediction for escaping at- mospheres (see Figure 3,bottompanel). from geocoronal emission, rendering the signal from the 2 Around 75% of helium recombinations result in the triplet configuration (Osterbrock & Ferland 2006). Results and Discussion (1) Spectral transmission of the He(23S) at mid-transit

(black line and error bars) and the synthetic 1.002 spectrum for T=6000 K, MLR=1.9x109 g/s and 1.000 H/He=90/10 (dashed orange). The magenta curve is an additional absorption needed to reproduce 0.998

the signal. The cyan curve is the total absorption. 0.996 Transmission

χ2 0.994 100 1012 0.992

1011 1.0829 1.0830 1.0831 Wavelength (air) (µm)

1010 10 Map of the constrained and mass- dM/dt (g/s) loss rates derived from the He triplet absorption 9 10 measurements for H/He=90/10. The white line represents the constrained (T, MLR) pairs.

8 10 1

4000 6000 8000 10000 Temperature (K)

Lampón et al., He triplet absorption in HD 209458b 4 Results and Discussion (2)

Degeneracy in MLR, temperature and H/He ratio Derived He(23S) concentrations for H/He=98/2

12 10 H/He=98/2 4000 K, 2.37x108 4500 K, 1.0x109 5000 K, 3.16x109 100.0 5500 K, 7.50x109

98/2 95/5 ) 6000 K, 1.33x1010 11 -3 10 6500 K, 2.37x1010 90/10 7000 K, 4.22x1010 7500 K, 5.62x1010 8000 K, 1.0x1011 8500 K, 1.33x1011 10 10.0 9000 K, 1.33x1011 10 9500 K, 1.78x1011 10000 K, 2.37x1011 11 S) density (cm dM/dt (g/s) 10500 K, 2.37x10 3 11000 K, 3.16x1011 11500 K, 4.22x1011 109 1.0 He(2

8 10 0.1 4000 6000 8000 10000 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Temperature (K) Radius (Rp) • We reduced the degeneracy shown above (left panel) by comparing our H density derived from the He absorption with previous results obtained from Ly-⍺ measurements (Salz et al., 2016; García-Muñoz, 2007; Koskinen et al. 2013). • Our results suggest an H/He composition of 98/2, higher than the canonical 90/10. • We further reduce the T/MLR degeneracy by considering heating efficiencies previously derived of 0.1-0.2 (Shematovich, 2014). Thus, we found a temperature range from 7125 to 8125 K and a MLR of (0.42-1.00)x1011 g/s. Lampón et al., He triplet absorption in HD 209458b 5 Conclusions and Future work

Conclusions: We found the following constraints for the upper atmosphere of HD209458b: • The MLR is in the range of (0.42-1.00)x1011g/s. • The maximum thermospheric temperatures are from 7125 to 8125 K. • The H/He composition is about 98/2. • The H/H+ transition altitude is in the range of 1.2-1.9 Rp. • The atmosphere is almost fully ionised beyond 2.9 Rp. • The effective radii at which XUV absorption takes place is about 1.16-1.3 Rp. • The average of the mean molecular weight is in the range of 0.61-0.73g/mole. Future work: • We foresee the extension of this analysis to other planets with different physical parameters and XUV irradiation levels with detected He 10833Å absorption, e.g., HD189733b and GJ3470b (Salz et al. 2018; Pallé et al. 2020).

Lampón et al., He triplet absorption in HD 209458b 6