Abstract
ABSTRACT We present an analysis of XMM–Newton observations of four stars in the young (670 Myr) open cluster Praesepe. The planets hosted by these stars all lie close in radius–period space to the radius–period valley and/or the Neptunian desert, two features that photoevaporation by X-ray and extreme ultraviolet (EUV) photons could be driving. Although the stars are no longer in the saturated regime, strong X-ray and extreme ultraviolet irradiation is still ongoing. Based on EUV time evolution slopes, we derived in a previous paper, in all four cases, two-thirds of their EUV irradiation is still to come. We compare the XMM–Newton light curves to those simultaneously measured with K2 at optical wavelengths, allowing us to search for correlated variability between the X-ray and optical light curves. We find that the X-ray flux decreases and flattens off while the optical flux rises throughout for K2-100, something that could result from active regions disappearing from view as the star spins. Finally, we also investigate possible futures for the four planets in our sample with simulations of their atmosphere evolution still to come, finding that complete photoevaporative stripping of the envelope of three of the four planets is possible, depending on the current planet masses.
Highlights
The Van Eylen et al (2018) study inferred the valley to be a clean break with few or no planets existing in the gap, suggestive of a homogeneous core composition across their sample
We have investigated four of the young planets discovered to date in the open cluster, Praesepe
X-ray analyses were performed for each of the four host stars using data taken by XMM-Newton, allowing us to estimate the XUV irradiation experienced by planets at the current epoch
Summary
The observed distribution of exoplanets in radius-orbital period space includes two regions with a deficit of planets that cannot be explained by selection biases. The other region in radius-period space lacking in observed exoplanets is the “Neptunian desert,” which is an observed dearth of intermediate-sized planets at short orbital periods (Szabó & Kiss 2011; Beaugé & Nesvorný 2013; Helled et al 2016; Lundkvist et al 2016; Mazeh et al 2016; Owen & Lai 2018) This effect, observed in the mass-period plane, has been attributed to photoevaporation (Kurokawa & Nakamoto 2014). With comparatively few such planets still discovered, the known population remains far too small for statistically meaningful comparisons of the exoplanet properties (e.g. in radius-period space) for systems of different ages This means the epoch at which features such as the radius-period valley or the Neptunian desert are forged cannot be determined in this way for the foreseeable future, even with the exoplanet discoveries coming out of the TESS mission including some young planets (e.g. Newton et al 2019; Tofflemire et al 2021). The estimated radii for most of these planets is consistent with being in or close to the radius-period valley and/or Neptunian desert, making them an ideal testing ground for these theories of atmosphere evolution
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