The full spectral radiative properties of Proxima Centauri ?

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Context. The discovery of Proxima b, a terrestrial temperate planet, presents the opportunity of studying a potentially habitable world in optimal conditions. A key aspect in the modeling of its habitability is to understand the radiation environment of the planet in the full spectral domain. Aims. We aim to characterize the X-rays to mid-IR radiative properties of Proxima with the goal of providing the top-of-atmosphere fluxes on the planet. We also aim at constraining the fundamental properties of the star, namely its mass, radius, effective temperature and luminosity. Methods. We have employed observations from a large number of facilities and made use of different methodologies to piece together the full spectral energy distribution of Proxima. In the high-energy domain, we payed particular attention to the contributions of rotational modulation, activity cycle, and flares so that the data provided are representative of the overall radiation dose received by the atmosphere of the planet. Results. We present the full spectrum of Proxima covering 0.7 to 30 000 nm. The integration of the data shows that the top-of-atmosphere average XUV irradiance on Proxima b is 0.293 W m-2, that is, nearly 60 times higher than Earth, and that the total irradiance is 877 ± 44 W m-2, or 64 ± 3% of the solar constant but with a significantly redder spectrum. We also provide laws for the XUV evolution of Proxima corresponding to two scenarios, one with a constant XUV-to-bolometric luminosity value throughout its history and another one in which Proxima left the saturation phase at an age of about 1.6 Gyr and is now in a power-law regime. Regarding the fundamental properties of Proxima, we find M = 0.120 ± 0.003 M, R = 0.146 ± 0.007 R, Teff = 2980 ± 80 K, and L = 0.00151 ± 0.00008 L. In addition, our analysis reveals a ~20% excess in the 3-30 μm flux of the star that is best interpreted as arising from warm dust in the system. Conclusions. The data provided here should be useful to further investigate the current atmospheric properties of Proxima b as well as its past history, with the overall aim of firmly establishing the habitability of the planet.

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Astronomy and Astrophysics

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