Influence Of The Sun-like Magnetic Cycle On Exoplanetary Atmospheric Escape

Solar XUV spectra over a time span of a few years [2002 -2019]. (a) Six months averaged of full XUV spectra (5-1940 ˚A) is plotted with time, with colour denoting the log of the spectral energy density (F(λ)), given in units of erg/cm2/s/˚A. The black solid line shows the sunspot cycle for the same time interval with sunspot number ranges from 3 to 180. The vertical black dashed lines mark the solar minima and blue vertical dashed line shows solar maximum. (b) The variation of the XUV flux (Fxuv) integrated over the wavelength range of 5-915 ˚A as a function of time (grey dashed line) and its six-month smoothed curve (blue solid line). (c) Solar SED over the XUV wavelength range at two different times: blue dashed line shows the spectra during a solar maximum and red solid line represents the spectra during a solar minimum.

Stellar high-energy radiation (X-ray and extreme ultraviolet, XUV) drives atmospheric escape in close-in exoplanets.

Given that stellar irradiation depends on the stellar magnetism and that stars have magnetic cycles, we investigate how cycles affect the evolution of exoplanetary atmospheric escape.

Firstly, we consider a hypothetical HD209458b-like planet orbiting the Sun. For that, we implement the observed solar XUV radiation available over one and a half solar cycles in a 1D hydrodynamic escape model of HD209458b. We find that atmospheric escape rates show a cyclic variation (from 7.6 to 18.5 × 1010 g s−1), almost proportional to the incident stellar radiation. To compare this with observations, we compute spectroscopic transits in two hydrogen lines. We find non-detectable cyclic variations in Lyα transits. Given the temperature sensitiveness of the Hα line, its equivalent width has an amplitude of 1.9 mA variation over the cycle, which could be detectable in exoplanets such as HD209458b.

We demonstrate that the XUV flux is linearly proportional to the magnetic flux during the solar cycle. Secondly, we apply this relation to derive the cyclic evolution of the XUV flux of HD189733 using the available magnetic flux observations of the star from Zeeman Doppler Imaging over nearly a decade. The XUV fluxes are then used to model escape in HD189733b, which shows escape rate varying from 2.8 to 6.5 × 1010 g s−1. Like in the HD209458b case, this introduces variations in Lyα and Hα transits, with Hα variations more likely to be observable. Finally, we show that a strong stellar flare would enhance significantly Lyα and Hα transit depths.

Gopal Hazra, Aline A. Vidotto, Carolina Villarreal D'Angelo

Comments: 17 pages, 11 figures, accepted for publication in MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2006.10634 [astro-ph.SR] (or arXiv:2006.10634v1 [astro-ph.SR] for this version)
Submission history
From: Gopal Hazra
[v1] Thu, 18 Jun 2020 15:57:42 UTC (444 KB)

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