Extreme-ultraviolet Radiation from A-stars: Implications for Ultra-hot Jupiters

• Published in: Astrophysical journal. Letters Vol. 868; no. 2; p. L30
• Main Authors: Fossati, L., Koskinen, T., Lothringer, J. D., France, K., Young, M. E., Sreejith, A. G.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.12.2018; IOP Publishing
• Subjects: A stars; Absorption; Atmospheric heating; Convection; Extrasolar planets; Extreme ultraviolet radiation; Fluxes; Gas giant planets; Literature reviews; Optical measurement; Planetary atmospheres; Planetary mass; Planets; planets and satellites: atmospheres; planets and satellites: gaseous planets; planets and satellites: individual (KELT-9b, WASP-33b); stars: activity; stars: early-type; Stellar temperature; Ultraviolet absorption; Ultraviolet radiation
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/aaf0a5

Extremely irradiated, close-in planets to early-type stars might be prone to strong atmospheric escape. We review the literature showing that X-ray-to-optical measurements indicate that for intermediate-mass stars (IMSs) cooler than 8250 K, the X-ray and extreme-ultraviolet (XUV) fluxes are on average significantly higher than those of solar-like stars, while for hotter IMSs, because of the lack of surface convection, it is the opposite. We construct spectral energy distributions for prototypical IMSs, comparing them to solar. The XUV fluxes relevant for upper-planet atmospheric heating are highest for the cooler IMSs and lowest for the hotter IMSs, while the ultraviolet (UV) fluxes increase with increasing stellar temperature. We quantify the influence of this characteristic of the stellar fluxes on the mass loss of close-in planets by simulating the atmospheres of planets orbiting EUV-bright (WASP-33) and EUV-faint (KELT-9) A-type stars. For KELT-9b, we find that atmospheric expansion caused by heating due to absorption of the stellar UV and optical light drives mass-loss rates of 1011 g s−1, while heating caused by absorption of the stellar XUV radiation leads to mass-loss rates of 1010 g s−1, thus underestimating mass loss. For WASP-33b, the high XUV stellar fluxes lead to mass-loss rates of 1011 g s−1. Even higher mass-loss rates are possible for less massive planets orbiting EUV-bright IMSs. We argue that it is the weak XUV stellar emission, combined with a relatively high planetary mass, which limit planetary mass-loss rates, to allow the prolonged existence of KELT-9-like systems.