J/MNRAS/545/F1855 iota Hor SED modelling (Allan+, 2026)
The effects of stellar activity cycles on planetary atmospheric escape and the
HeI 1083nm transit signature.
Allan A.P., Vidotto A.A., Sanz-Forcada J., Villarreal D'Angelo C.
<Mon. Not. R. Astron. Soc. 545, staf1855 (2026)>
=2026MNRAS.545F1855A 2026MNRAS.545F1855A (SIMBAD/NED BibCode)
ADC_Keywords: Stars, G-type ; Stars, F-type ; Exoplanets ;
X-ray sources ; Extreme UV sources ; Models ; Energy distributions
Keywords: hydrodynamics - exoplanets - planets and satellites: atmospheres -
planets and satellites: gaseous planets - stars: activity
Abstract:
The HeI 1083nm transit signature is commonly used in tracing escaping
planetary atmospheres. However, it can be affected by stellar
activity, complicating detections and interpretations of atmospheric
escape. We model how stellar activity cycles affect the atmospheric
escape and HeI 1083nm signatures of four types of highly irradiated
exoplanets, at 0.025 and 0.05au, during minimum and maximum cycle
phases. We consider two stars, exhibiting different cycle behaviours:
the Sun and the more active star iota Hor, for which we reconstruct
its spectral energy distributions at minimum and maximum phases using
X-ray observations and photospheric models. We show that over a
modulated activity cycle, the release of extreme ultraviolet photons,
responsible for atmospheric escape, varies substantially more than
that of mid-UV photons, capable of photoionising HeI(23S). This leads
to consistently stronger helium signatures during maximum phases. We
show that planets at the largest orbit are more affected by cycles,
showing larger variations in escape rates and absorptions between
minimum and maximum. We also confirm the counter-intuitive behaviour
that, despite the fall-off in escape rate with orbital distance, the
HeI 1083nm absorption is not significantly weaker at further orbits,
even strengthening with orbital distance for some iota Hor planets. We
partially explain this behaviour with the lower mid-UV fluxes at more
distant orbits, leading to less HeI(23S) photoionisations. Finally, we
propose that stellar cycles could explain some of the conflicting HeI
1083nm observations of the same planet, with detections more likely
during a phase of activity maximum.
Description:
The table provides the modelled Spectral Energy Distribution (SED) of
iota Hor during a minimum and maximum phase of its activity cycle, as
described in Section 2.2 of the paper. The first column represent the
wavelength, in Angstroms, and the second and third columns represent
the density flux at 1 a.u. in units of erg/s/cm2/Å, of the minimum
and maximum of the cycle respectively.
Objects:
-----------------------------------------------
RA (2000) DE Designation(s)
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02 42 33.46 -50 48 01.0 iota Hor = HD 17051
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
sed.dat 34 1300 Modelled Spectral Energy Distribution (SED) of
iota Hor during a minimum and maximum phase of
its activity cycle
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Byte-by-byte Description of file: sed.dat
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Bytes Format Units Label Explanations
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1- 8 F8.3 0.1nm lambda [1.0/3190.0] Wavelength
10- 21 E12.4 10mW/m2/nm FCycleMin iota Hor density flux at 1a.u during
minimum phase of activity cycle
23- 34 E12.4 10mW/m2/nm FCycleMax iota Hor density flux at 1a.u during
maximum phase of activity cycle
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Acknowledgements:
Andrew Allan, allan(at)strw.leidenuniv.nl
References:
Sanz-Forcada et al., 2019A&A...631A..45S 2019A&A...631A..45S
(End) Patricia Vannier [CDS] 29-Oct-2025