Astronomy and Astrophysics Seminars

Exploring initial properties and evolution pathways of close-in exoplanets with photoevaporation modelling

by Mr. Bihan Banerjee (TIFR)

Thursday, April 28, 2022 from to (Asia/Kolkata)
at Hybrid ( AG 66 ) Meeting ID: 847 3395 1480 Passcode: StuSem2022
As of today, more than 5000 exoplanets have been discovered. Majority of them are close-in (Orbital period < 100 days), small exoplanets (radius< 4 Earth radius) around low to medium mass (0.5-1.5 solar mass) stars. Among these planets sub-Neptunes ( 2-4 Earth radius ) and super-Earths (1.2-1.5 Earth radius) are highly abundant. But in between them, there is a region that is sparsely populated. If we plot these planets in a planet radius-orbital period plane, we see this "gap" correlates with orbital period. A similar trend is observed when plotted with insolation flux as well. The natural explanation of this bimodality is atmospheric mass loss, i.e. some planets retaining their envelope and appear bigger in size, while the second group lose their envelope and become smaller, bare cores. In the literature, two leading mechanisms of atmospheric mass loss exist. First, photoevaporation of the atmosphere with high energy radiation coming from the star, and the second, atmospheric mass loss by a luminous planetary core. In this work, we have investigated the photoevaporation mechanism and have addressed three questions. (1) Can photoevaporation explain all the observed trends of these planets? (2) If photoevaporation is the main driver of evolution of these exoplanets, what can we say about their initial conditions and evolution? (3) What are the distinct features of photoevaporation that may help us to separate from the other mechanisms? To answer these questions we have taken a forward modelling approach to constrain the set of initial conditions (such as planetary core mass and atmospheric fraction distribution) that best match the observations. Our results indicate that at early times, planets have fluffy atmospheres with diverse densities. Further, we find that both the "evaporation valley” and the mass-radius relation for planets are strong functions of age.
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