Hot Jupiters Driven by High-eccentricity Migration in Globular Clusters

Hot Jupiters Driven by High-eccentricity Migration in Globular Clusters

Authors:

Hammers et al

Abstract:

Hot Jupiters (HJs) are short-period giant planets that are observed around $\sim 1 \% $ of solar-type field stars. One possible formation scenario for HJs is high-eccentricity (high-e) migration, in which the planet forms at much larger radii, is excited to high eccentricity by some mechanism, and migrates to its current orbit due to tidal dissipation occurring near periapsis. We consider high-e migration in dense stellar systems such as the cores of globular clusters (GCs), in which encounters with passing stars can excite planets to the high eccentricities needed to initiate migration. We study this process via Monte Carlo simulations of encounters with a star+planet system including the effects of tidal dissipation, using an efficient regularized restricted three-body code. HJs are produced in our simulations over a significant range of the stellar number density ${n}_{\star }$. Assuming the planet is initially on a low-eccentricity orbit with semimajor axis 1 au, for ${n}_{\star }\lesssim {10}^{3}\,{\mathrm{pc}}^{-3}$ the encounter rate is too low to induce orbital migration, whereas for ${n}_{\star }\gtrsim {10}^{6}\,{\mathrm{pc}}^{-3}$ HJ formation is suppressed because the planet is more likely ejected from its host star, tidally disrupted, or transferred to a perturbing star. The fraction of planets that are converted to HJs peaks at $\approx 2 \% $ for intermediate number densities of $\approx 4\times {10}^{4}\,{\mathrm{pc}}^{-3}$. Warm Jupiters, giant planets with periods between 10 and 100 days, are produced in our simulations with an efficiency of up to $\approx 0.5 \% $. Our results suggest that HJs can form through high-e migration induced by stellar encounters in the centers of of dense GCs, but not in their outskirts where the densities are lower.

Eating Planets Can Make Stars Appear Older Than They are

Effect of planet ingestion on low-mass stars evolution: the case of 2MASS J08095427–4721419 star in the Gamma Velorum cluster

Authors:

Tognelli et al

Abstract:

We analysed the effects of planet ingestion on the characteristics of a pre-MS star similar to the Gamma Velorum cluster member 2MASS J08095427–4721419 (#52). We discussed the effects of changing the age t0 at which the accretion episode occurs, the mass of the ingested planet and its chemical composition. We showed that the mass of the ingested planet required to explain the current [Fe/H]#52 increases by decreasing the age t0 and/or by decreasing the Iron content of the accreted matter. We compared the predictions of a simplified accretion method – where only the variation of the surface chemical composition is considered – with that of a full accretion model that properly accounts for the modification of the stellar structure. We showed that the two approaches result in different convective envelope extension which can vary up to 10 percent. We discussed the impact of the planet ingestion on a stellar model in the colour-magnitude diagram, showing that a maximum shift of about 0.06 dex in the colour and 0.07 dex in magnitude are expected and that such variations persist even much later the accretion episode. We also analysed the systematic bias in the stellar mass and age inferred by using a grid of standard non accreting models to recover the characteristics of an accreting star. We found that standard non accreting models can safely be adopted for mass estimate, as the bias is \la6 percent, while much more caution should be used for age estimate where the differences can reach about 60 percent.

Globular Clusters as Cradles of Life and Advanced Civilizations

Globular Clusters as Cradles of Life and Advanced Civilizations

Authors:

Di Stefano et al

Abstract:

Globular clusters are ancient stellar populations with no star formation or core-collapse supernovae. Several lines of evidence suggest that globular clusters are rich in planets. If so, and if advanced civilizations can develop there, then the distances between these civilizations and other stars would be far smaller than typical distances between stars in the Galactic disk. The relative proximity would facilitate interstellar communication and travel. However, the very proximity that promotes interstellar travel also brings danger, since stellar interactions can destroy planetary systems. However, by modeling globular clusters and their stellar populations, we find that large regions of many globular clusters can be thought of as "sweet spots" where habitable-zone planetary orbits can be stable for long times. We also compute the ambient densities and fluxes in the regions within which habitable-zone planets can survive. Globular clusters are among the best targets for searches for extraterrestrial intelligence (SETI). We use the Drake equation to compare globular clusters to the Galactic disk, in terms of the likelihood of housing advanced communicating civilizations. We also consider free-floating planets, since wide-orbit planets can be ejected and travel freely through the cluster. A civilization spawned in a globular cluster may have opportunities to establish self-sustaining outposts, thereby reducing the probability that a single catastrophic event will destroy the civilization or its descendants. Although individual civilizations within a cluster may follow different evolutionary paths, or even be destroyed, the cluster may always host some advanced civilization, once a small number of them have managed to jump across interstellar space.