Atmosphere of low-mass star radius-gap planets

Vigneshwaran Krishnamurthy ( Université McGill )

Understanding the formation and evolution of rocky planets becomes a key in our quest to finding a habitable planet. And planets around low-mass stars (<0.8 M⊙) are in general rocky and are easier to detect and characterize using the transit technique. The atmospheric characterization of such planets provides a window in understanding the evolution of the planets. In general, small planets (<3 R⊕) are expected to be accreted with 1-2% of H/He primordial atmosphere by the end of their protoplanetary disc dissipation (Owen & Wu 2017). This primordial atmosphere can be evaporated by the high-energy X-ray and EUV flux from their host star in the initial few million years. This photoevaporation process is dominant when the star is young and active (Owen & Wu 2013, 2017). Or the planet could dissipate the primordial atmosphere by its own internal heating of the cooling core. This core-powered mass-loss does not depend on the stellar environment, but just on the heat dissipating from the planet (Ginzburg et al. 2016, 2018; Gupta & Schlichting 2019, 2020). Finally, another class of models predict that the low-mass star planets essentially do not accrete enough gas during its formation (Lee et al. 2014; Lee & Chiang 2016; Lopez & Rice 2018). It is essential to distinguish between these models to have a robust prediction on the evolution of small planets. The distinction between these models can be achieved by observing planets in and around the transition region between rocky and non-rocky planets (a.k.a radius-gap; 1.5-2 R⊕). Unfortunately, the origin of radius-gap is not clearly understood in low-mass star planets due to poor sample size in this region (Cloutier & Menou 2020). Here we present near infrared (NIR) high-resolution transit observations of GJ 9827b, GJ 9827d and TOI-1235b, small planets around cool stars. We target the He I triplet lines at 1083 nm as a marker for detecting any evaporating atmospheres (Oklopčić & Hirata 2018). The observations were carried out on several open use programs on the Subaru telescope Infrared Doppler (IRD) instrument (R ~ 70,000; Tamura et al. 2012; Kotani et al. 2018). Our study on probing the primary atmospheres can provide useful insights on the atmospheric evolution of low-mass star planets.