124 research outputs found
Impact of photoevaporative mass loss on masses and radii of water-rich sub/super-Earths
Recent progress in transit photometry opened a new window to the interior of
super-Earths. From measured radii and masses, we can infer planetary internal
compositions. It has been recently revealed that super-Earths are diverse in
composition. Such a diversity is thought to arise from diversity in volatile
content. The stability of the volatile components is to be examined, because
hot super-Earths undergo photo-evaporative mass loss. While several studies
investigated the impact of photo-evaporative mass loss on hydrogen-helium
envelopes, there are few studies as to the impact on water-vapor envelopes. To
obtain theoretical prediction to future observations, we also investigate the
relationships among masses, radii, and semimajor axes of water-rich
sub/super-Earths that have undergone photo-evaporative mass loss. We simulate
the interior structure and evolution of sub/super-Earths that consist of a
rocky core surrounded by a water envelope, including mass loss due to the
stellar XUV-driven energy-limited hydrodynamic escape. We find that the
photo-evaporative mass loss has a significant impact on the evolution of hot
sub/super-Earths. We then derive the threshold planetary mass and radius below
which the planet loses its water envelope completely as a function of the
initial water content, and find that there are minimums of the threshold mass
and radius. We constrain the domain in the parameter space of planetary mass,
radius, and semimajor axis in which sub/super-Earths never retain water
envelopes in 1-10 Gyr. This would provide an essential piece of information for
understanding the origin of close-in low-mass planets. The current
uncertainties in stellar XUV flux and its heating efficiency, however, prevent
us from deriving robust conclusions. Nevertheless, it seems to be a robust
conclusion that Kepler planet candidates contain a significant number of rocky
sub/super-Earths.Comment: 13 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Orbital Evolution of Close-in Super-Earths Driven by Atmospheric Escape
The increasing number of super-Earths close to their host stars have revealed a scarcity of close-in small planets with 1.5–2.0 R⊕ in the radius distribution of Kepler planets. The atmospheric escape of super-Earths by photoevaporation can explain the origin of the observed “radius gap.” Many theoretical studies have considered the in situ mass loss of a close-in planet. Planets that undergo atmospheric escape, however, move outward due to the change in the orbital angular momentum of their star–planet systems. In this study, we calculate the orbital evolution of an evaporating super-Earth with a H₂/He atmosphere around FGKM-type stars under stellar X-ray and extreme-UV irradiation (XUV). The rate of increase in the orbital radius of an evaporating planet is approximately proportional to that of the atmospheric mass loss during a high stellar XUV phase. We show that super-Earths with a rocky core of ≲10 M⊕ and a H₂/He atmosphere at ≲0.03–0.1 au (≲0.01–0.03 au) around G-type stars (M-type stars) are prone to outward migration driven by photoevaporation. Although the changes in the orbits of the planets would be small, they would rearrange the orbital configurations of compact, multiplanet systems, such as the TRAPPIST-1 system. We also find that the radius gap and the so-called “Neptune desert” in the observed population of close-in planets around FGK-type stars still appear in our simulations. On the other hand, the observed planet population around M-type stars can be reproduced only by a high stellar XUV luminosity model
High-contrast Imaging around a 2 Myr-old CI Tau with a Close-in Gas Giant
Giant planets around young stars serve as a clue to unveiling their formation
history and orbital evolution. CI Tau is a 2\,Myr-old classical T-Tauri star
hosting an eccentric hot Jupiter, CI Tau\,b. The standard formation scenario of
a hot Jupiter predicts that planets formed further out and migrated inward. A
high eccentricity of CI Tau b may be suggestive of high- migration due to
secular gravitational perturbations by an outer companion. Also, ALMA
1.3\,mm-continuum observations show that CI Tau has at least three annular gaps
in which unseen planets may exist. We present high-contrast imaging around CI
Tau taken from Keck/NIRC2 -band filter and vortex coronagraph that
allows us to search for an outer companion. We did not detect any outer
companion around CI Tau from angular differential imaging (ADI) using two deep
imaging data sets. The detection limits from ADI-reduced images rule out the
existence of an outer companion beyond \,au that can cause the
Kozai-Lidov migration of CI Tau\,b. Our results suggest that CI Tau\,b may have
experienced Type II migration from \,au in Myrs. We also confirm
that no planets with are hidden in two outer gaps.Comment: 7 pages, 6 figures, accepted in A
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