The XUV emission of the star HIP67522 and the evolution of its exoplanet. The study: “XUV irradiation of young planetary atmospheres. Results from a joint XMM-Newton and HST observation of HIP67522” of A. Maggio (INAF-OAPA) appeared on A&A
Despite the wide array of telescopes and satellites available to the astronomical community, there is a type of radiation to which we are virtually blind: the extreme UV band. This is high-energy radiation with a wavelength that ranges from the Lyman-α emission, caused by hydrogen atoms whose electron transitions from the innermost orbital (the ground state) to the second (n=2), to the low-energy X-ray regime (so-called soft X-rays). Extreme UV radiation does not penetrate the Earth’s atmosphere, so space missions are required to observe it.
In stars, XUV radiation (which includes both extreme UV radiation and soft X-rays) is emitted by plasma at millions of degrees present in the stellar atmosphere (corona, chromosphere, and transition region) and increases when magnetic activity-related phenomena, such as stellar flares, become more intense. The lack of telescopes capable of directly observing extreme UV radiation therefore limits our ability to study these phenomena.
This has a negative impact on several fields of astrophysics. For example, XUV radiation emitted by the most magnetically active stars, generally the youngest, influences the surrounding environment, and particularly the atmospheres of any planets orbiting these stars. XUV photons are absorbed by the outer layers of planetary atmospheres, ionizing atoms and producing a cascade of high-energy electrons that heats the atmosphere and affects its chemistry and evolution. In extreme cases, part of the heated atmosphere can escape into interplanetary space through the process of “photoevaporation.”
HIP67522 is a young star (5-20 million years old), a member of the Scorpius-Centaurus association. The star hosts two recently identified exoplanets: a gas giant with an orbital period of 6.96 days, whose mass has not yet been precisely determined (0.18-4.6 Jupiter masses), and a smaller planet with an orbital period of 14.96 days. Due to its young age, HIP67522 is magnetically active, characterized by intense XUV emission. To characterize this emission and determine its impact on the two exoplanets, a team of researchers led by astrophysicist A. Maggio (INAF – Palermo Astronomical Observatory) analyzed simultaneous observations of HIP67522 in X-rays, obtained with the European Space Agency’s XMM/Newton satellite, and in the near ultraviolet, obtained with the Hubble Space Telescope.
The analysis allowed the team to characterize the physical properties of the plasma in the stellar atmosphere, particularly the thermal stratification and the density of the emitting material (the emission measure). This enabled the modeling of the plasma responsible for producing the star extreme UV radiation and the synthesis of its emission. During the observations, the star also exhibited two coronal flares, during which X-ray emission increased thousands of times compared to its quiescent state.
Once the star XUV radiation was synthesized, the team analyzed the effects of this radiation on HIP67522 gas giant planet. The results depend on the planet mass, which, as mentioned, has not yet been precisely measured. In the case of low masses, the model developed by the authors predicts a high rate of mass loss due to the effects of incident XUV radiation. In the extreme case of a 0.18 Jupiter mass, the mass loss is about 0.00004 Jupiter masses per million years. The study is described in the paper “XUV irradiation of young planetary atmospheres. Results from a joint XMM-Newton and HST observation of HIP67522,” recently published in the journal Astronomy & Astrophysics.
The cover image (click here to view in full) shows the synthesized spectrum of HIP67522 in the X-ray regime. The XUV contribution from the stellar atmosphere is on the left, at shorter wavelengths. The photospheric contribution is shown in green.
Mario Giuseppe Guarcello
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