Influence of the Sun-like magnetic cycle on exoplanetary Atmospheric escape

Gopal Hazra (Trinity College Dublin), Aline A. Vidotto (Trinity College Dublin), Carolina Villarreal D'Angelo (Observatorio Astronomico de Cordoba (OAC-UNC))

Context

  • X-ray and EUV (hence XUV) radiation drives the atmospheric escape on exoplanets due to photoionization.
  • This XUV radiation is modulated by the magnetic activity of the host star.
  • Most of the planet-hosting stars show magnetic activity and many of them show a magnetic cycle like the Sun.
  • Therefore, we study influence of stellar cycle on the XUV radiation and hence on atmospheric escape of exoplanets.

Radiation Hydrodynamic Model of Planetary Escape

We consider a hypothetical system where a hot Jupiter is orbiting our Sun at a = 0.05 AU

A hot jupitor is orbiting the Sun at a =0.05 AU. The model equations for the planetary escape including heating and cooling are shown

Figure 1: Schematic diagram of our model with equations used

XUV radiation (an input in our model) from the Sun over solar cycles 23 and 24.

Left: full solar XUV spectra over solar cycle 23 and 24 is shown from TIMED/SEE data. Black solid line shows the solar cycle. Right: the integrated solar XUV spectrum over wavelength range 5-915 Angstrom.

Figure 2: XUV spectrum of the Sun

Solar XUV radiation is highly correlated with the solar cycle

Planetary atmospheric evaporation over the magnetic cycle

As the Fxuv varies with cycle, we simulate 34 cases covering solar cycle 23-24.

  • Cyclic Fxuv --> variation in heating --> different temperature and velocity profiles --> different mass loss rate
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    Left: Planetary mass-loss rate over solar cycle 23-24 and Right: terminal velocity of escaping planetary ourflow over the cycle. The colorbar shows the amount of XUV luminosity during the cycle.

    Figure 3: Cyclic behavior of planetary properties

    Magnetic field as a probe of XUV radiation for other stars

    The solar surface magnetic flux is highly correlated with the XUV flux

    Left: Azimuthally-averaged high resolution surface magnetic field from HMI magnetogram over solar cycle 24 with meanflux in black solid line. Middle: The direct comparison of smoothed mean magnetic flux and XUV flux over 1 year window and Right: The correlation plot between mean magnetic flux and XUV flux.

    Figure 4: Full scale magnetic flux vs XUV flux

    For other stars, we can only get large-scale magnetic components (upto lmax = 10)

    Is the large-scale component of solar magnetic field also correlated? Yes

    Left: Azimuthally-averaged large-scale surface magnetic field (l<sub>max</sub> =10) from HMI magnetogram over solar cycle 24 with meanflux in black solid line. Middle: The direct comparison of smoothed mean large-scale magnetic flux and XUV flux over 1 year window and Right: The correlation plot between mean magnetic flux and XUV flux.

    Figure 5: Large-scale magnetic flux vs XUV flux

    The XUV radiation F xuv ∝ (magnetic flux) 1.04

    XUV flux from HD189733

    The observed correlation between large scale magnetic flux of the Sun with XUV flux is extrapolated to obtain XUV flux from HD189733 based on ZDI surface magnetic flux of the star. We calculated XUV fluxes from the star HD189733 using above formula at different epochs (when ZDI observations are available).

  • Stellar XUV flux varies from [7,17] x 105 erg/cm2/s
  • Planetary escape rate varies over a factor of 2.3

    • Can we observe the escape variability induced by the magnetic cycle?

    Flare case: five times stronger Fxuv from "minimum" magnetic activity

    Left: Transit depth of the Ly α line at mid-transit as a function of Doppler velocity over magnetic cycle, Middle: Transit depth of Hα line over the same magnetic cycle and Right: the equivalent widths are plotted at the same epoch. The colors in the Equivalent width plots are associated with the magnitudes of the magnetic field that are shown in the colorbar, except for the flare case, shown in red.

    Figure 7: Spectral transit of Lyα and Hα lines over magnetic cycle

  • Ly α is not directly comparable (line center is not observable)
  • Barnes et al. 2016: temporal variation of 0.37% for observations of 1 year apart: probably caused by change in stellar magnetic activity?
  • Symmetric profile in our model is due to lack of stellar wind interaction [See posters by Stephen Carolan and Carolina Villarreal D'angelo ]

    • Conclusions

  • Stellar activity cycle plays an important role in modulating atmospheric escape rate
  • It introduces a temporal variation in the Ly α and H α transit spectra
  • Surface magnetic flux is a good proxy for probing XUV radiation from stars
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