1:45 PM - 3:15 PM
[PEM11-P19] Atmospheric characterization of a sub-Neptune using Helium transmission spectroscopy
Keywords: Sub-Neptune, Transmission spectroscopy, Planetary atmosphere
The population of small planets, sub-Neptunes (2-4 Earth radii) and super-Earths (1-1.5 Earth radii) become a majority among exoplanets after the launch of Kepler and TESS space telescopes. The Radius Valley (1.5-2 Earth radii) is thought to be the boundary of them if they could maintain their primary H/He atmosphere or not. However, the actual boundary has not been confirmed directly by atmospheric detection, and the mechanisms that form the diverse composition of the small planets are unknown.
Helium triplet is an effective absorption line to detect the primary atmosphere on the small planets since the X-rays and EUV stellar radiation makes the planets’ H/He atmosphere expand up to several planetary radii, and it is not affected by interstellar absorption so that it is accessible from the ground. K dwarfs and early M dwarfs are considered as ideal spectral types for He triplet’s formation.
Here we conducted high-resolution (~70,000) transmission spectroscopy using Subaru/IRD. We observed a transit of a short-period sub-Neptune candidate TOI 654.01 around an early-M dwarf, which is on the upper edge of the Radius Valley. We corrected the effects of heliocentric, stellar proper motional and planetary radial velocity, the telluric contamination and removed the stellar spectrum from the planetary spectra.
We set an upper limit of 3.3% of the excess absorption of He triplet, at 90% confidence. We could not rule out the absence of a He atmosphere (which means neither the planet is formed by a primordial atmosphere, nor its He atmosphere survives from atmospheric escape). However, we suggest that even if He triplet exists, weak stellar EUV might cause the absence of a He triplet, resulting in a non-detection. The presentation will also include future implications from this research.
Helium triplet is an effective absorption line to detect the primary atmosphere on the small planets since the X-rays and EUV stellar radiation makes the planets’ H/He atmosphere expand up to several planetary radii, and it is not affected by interstellar absorption so that it is accessible from the ground. K dwarfs and early M dwarfs are considered as ideal spectral types for He triplet’s formation.
Here we conducted high-resolution (~70,000) transmission spectroscopy using Subaru/IRD. We observed a transit of a short-period sub-Neptune candidate TOI 654.01 around an early-M dwarf, which is on the upper edge of the Radius Valley. We corrected the effects of heliocentric, stellar proper motional and planetary radial velocity, the telluric contamination and removed the stellar spectrum from the planetary spectra.
We set an upper limit of 3.3% of the excess absorption of He triplet, at 90% confidence. We could not rule out the absence of a He atmosphere (which means neither the planet is formed by a primordial atmosphere, nor its He atmosphere survives from atmospheric escape). However, we suggest that even if He triplet exists, weak stellar EUV might cause the absence of a He triplet, resulting in a non-detection. The presentation will also include future implications from this research.