arXiv subjects

M. Kunitomo

Publications and source records attributed to M. Kunitomo.

4 recordsLinked to original sources

arXiv2026

A data-driven estimate of the protosolar helium mass fraction

The protosolar helium mass-fraction is a key ingredient of solar, planetary models and enrichment laws. However, the assumed values often rely on simplified descriptions of the transport of chemicals in solar models. They are also based on the inferred helium mass fraction in the solar convective envelope, which is itself sensitive to uncertainties in the solar equation of state. We update the reference protosolar helium abundance by including the effects of macroscopic mixing at the base of the convective zone and more recent determinations of the helium mass fraction in the convective envelope. We combine results from our inversions to spectroscopic abundances, as well as literature values to provide a robust interval of the current helium mass fraction in the convective zone. We combine this measurement to models including light element depletion to provide an udpated protosolar helium abundance. We show that macroscopic mixing at the base of the envelope is key to infer protosolar helium. We find a revised interval of primordial helium mass fraction of 0.27575 +- 0.00315 slightly lower than previous estimates when combining our latest estimate of surface helium mass fraction and spectroscopic abundances. We find that the effects of macroscopic mixing are partially compensated by an increase in the inferred solar helium mass fraction in recent studies. We also derive more precise estimates based on various reference works in the litterature. Using the usual surface helium mass fraction, the primordial helium mass fraction drops to 0.2669 +- 0.00415 due to the inclusion of macroscopic mixing. The dominant source of uncertainty is found the surface helium abundance inferred from helioseismic constraints and more specifically, the impact on the equation of state of the solar material on this inference result.

astro-ph.SR↗

arXiv2016

Searching for Li-rich giants in a sample of 12 open clusters: Li enhancement in two stars with substellar companions

The aim of this work is to search for Li-rich giants in a sample of clusters where planets have been searched, thus we can study the planet engulfment scenario to explain Li replenishment using a proper comparison sample of stars without detected giant planets. We derived Li abundances for a sample of 67 red giant stars in 12 different open clusters using standard spectral synthesis techniques and high resolution spectra (from HARPS and UVES). We also determined masses, ages and radius from PARSEC stellar isochrones to constrain the evolutionary stage of these stars. We find three stars in different clusters with clearly enhanced Li abundances compared to other stars within the cluster. Interestingly, the only two stars with a detected substellar companion in our sample belong to that group. One of the planet hosts, NGC2423No3, might lie close to the luminosity bump on the HR diagram, a phase where Li production by the Cameron-Fowler process is supported by extra-mixing to bring fresh Li up to the surface. On the other hand, NGC4349No127 is a more massive and more evolved giant that does not seem to be in the evolutionary phase where other Li-rich stars are found. We discuss the possibility that the Li enhancement of this star is triggered by the engulfment of a planet, considering that close-in planets hardly survive the RGB-tip and the early-AGB phases.

astro-ph.SR↗

arXiv2011

Planet Engulfment by ~1.5-3 Solar-Mass Red Giants

Recent radial-velocity surveys for GK clump giants have revealed that planets also exist around ~1.5-3 Msun stars. However, no planets have been found inside 0.6 AU around clump giants, in contrast to solar-type main-sequence stars, many of which harbor short-period planets such as hot Jupiters. In this study we examine the possibility that planets were engulfed by host stars evolving on the red-giant branch (RGB). We integrate the orbital evolution of planets in the RGB and helium burning (HeB) phases of host stars, including the effects of stellar tide and stellar mass loss. Then we derive the critical semimajor axis (or the survival limit) inside which planets are eventually engulfed by their host stars after tidal decay of their orbits. Especially, we investigate the impact of stellar mass and other stellar parameters on the survival limit in more detail than previous studies. In addition, we make detailed comparison with measured semimajor axes of planets detected so far, which no previous study did. We find that the critical semimajor axis is quite sensitive to stellar mass in the range between 1.7 and 2.1 Msun, which suggests a need for careful comparison between theoretical and observational limits of existence of planets. Our comparison demonstrates that all those planets are beyond the survival limit, which is consistent with the planet-engulfment hypothesis. However, on the high-mass side (> 2.1 Msun), the detected planets are orbiting significantly far from the survival limit, which suggests that engulfment by host stars may not be the main reason for the observed lack of short-period giant planets. To confirm our conclusion, the detection of more planets around clump giants, especially with masses > 2.5 Msun, is required.

astro-ph.EP↗

arXiv2011

Orbital Evolution of Planets around Intermediate-Mass Giants

Around low- and intermediate-mass (1.5-3 M_sun) red giants, no planets have been found inside 0.6 AU. Such a paucity is not seen in the case of 1 M_sun main sequence stars. In this study, we examine the possibility that short-period planets were engulfed by their host star evolving off the main sequence. To do so, we have simulated the orbital evolution of planets, including the effects of stellar tide and mass loss, to determine the critical semimajor axis, a_crit, beyond which planets survive the RGB expansion of their host star. We have found that a_crit changes drastically around 2 M_sun: In the lower-mass range, a_crit is more than 1 AU, while a_crit is as small as about 0.2 AU in the higher-mass range. Comparison with measured semimajor axes of known planets suggests that there is a lack of planets that only planet engulfment never accounts for in the higher-mass range. Whether the lack is real affects our understanding of planet formation. Therefore, increasing the number of planet samples around evolved intermediate-mass stars is quite meaningful to confirm robustness of the lack of planets.

astro-ph.EP↗