Star-planet interactions. IV. Possibility of detecting the orbit-shrinking of a planet around a red giant

Star-planet interactions. IV. Possibility of detecting the orbit-shrinking of a planet around a red giant

Authors:

Meynet et al

Abstract:
The surface rotations of some red giants are so fast that they must have been spun up by tidal interaction with a close companion, either another star, a brown dwarf, or a planet. We focus here on the case of red giants that are spun up by tidal interaction with a planet. When the distance between the planet and the star decreases, the spin period of the star decreases, the orbital period of the planet decreases, and the reflex motion of the star increases. We study the change rate of these three quantities when the circular orbit of a planet of 15 MJ that initially orbits a 2 M⊙ star at 1 au shrinks under the action of tidal forces during the red giant phase. We use stellar evolution models coupled with computations of the orbital evolution of the planet, which allows us to follow the exchanges of angular momentum between the star and the orbit in a consistent way. We obtain that the reflex motion of the red giant star increases by more than 1 m s−1 per year in the last ∼40 years before the planet engulfment. During this phase, the reflex motion of the star is between 660 and 710 m s−1. The spin period of the star increases by more than about 10 minutes per year in the last 3000 y before engulfment. During this period, the spin period of the star is shorter than 0.7 year. During this same period, the variation in orbital period, which is shorter than 0.18 year, is on the same order of magnitude. Changes in reflex-motion and spin velocities are very small and thus most likely out of reach of being observed. The most promising way of detecting this effect is through observations of transiting planets, that is, through{\it } changes of the beginning or end of the transit. A space mission like PLATO might be of great interest for detecting planets that are on the verge of being engulfed by red giants.

Hot-Jupiter Core Mass from Roche-lobe Overflow

Hot-Jupiter Core Mass from Roche-lobe Overflow

Authors:

Ginzburg et al

Abstract:

The orbits of many observed hot Jupiters are decaying rapidly due to tidal interaction, eventually reaching the Roche limit. We analytically study the ensuing coupled mass loss and orbital evolution during the Roche-lobe overflow and find two possible scenarios. Planets with light cores Mc≲6M⊕ (assuming a nominal tidal dissipation factor Q∼106 for the host star) are transformed into Neptune-mass gas planets, orbiting at a separation (relative to the stellar radius) a/R⋆≈3.5. Planets with heavier cores Mc≳6M⊕ plunge rapidly until they are destroyed at the stellar surface. Remnant gas-Neptunes, which are stable to photo-evaporation, are absent from the observations, despite their unique transit radius (5−10R⊕). This result suggests that Mc≳6M⊕, providing a useful constraint on the poorly-known core mass that may distinguish between different formation theories of gas giants. Alternatively, given a prior estimate of Mc≈6M⊕ from the core-accretion theory, our results exclude the range 106≲Q≲107.

The Fate of Exomoons in White Dwarf Planetary Systems

The Fate of Exomoons in White Dwarf Planetary Systems

Authors:

Payne et al

Abstract:

Roughly 1000 white dwarfs are known to be polluted with planetary material, and the progenitors of this material are typically assumed to be asteroids. The dynamical architectures which perturb asteroids into white dwarfs are still unknown, but may be crucially dependent on moons liberated from parent planets during post-main-sequence gravitational scattering. Here, we trace the fate of these exomoons, and show that they more easily achieve deep radial incursions towards the white dwarf than do scattered planets. Consequently, moons are likely to play a significant role in white dwarf pollution, and in some cases may be the progenitors of the pollution itself.

K2-98 b: A 32 Earth Mass Neptune-sized Exoplanet in a 10-day orbit around an F8 star

K2-98 b: A 32-M⊕ Neptune-sized planet in a 10-day orbit transiting an F8 star

Authors:

Barragán et al

Abstract:

We report the discovery of K2-98 b (EPIC 211391664 b), a transiting Neptune-sized planet monitored by the K2 mission during its campaign 5. We combine the K2 time-series data with ground-based photometric and spectroscopic follow-up observations to confirm the planetary nature of the object and derive its mass, radius, and orbital parameters. K2-98 b is a warm Neptune-like planet in a 10-day orbit around a V=12.2~mag F-type star with M⋆=1.074±0.042, R⋆=1.311+0.083−0.048, and age of 5.2+1.2−1.0~Gyr. We derive a planetary mass and radius of Mp=32.2±8.1 and Rp=4.3+0.3−0.2. K2-98 b joins the relatively small group of Neptune-sized planets whose both mass and radius have been derived with a precision better than 25 %. We estimate that the planet will be engulfed by its host star in ∼3~Gyr, due to the evolution of the latter towards the red giant branch.

High surface magnetic field in red giants as a new signature of exoplanet engulfment?

High surface magnetic field in red giants as a new signature of planet engulfment?

Authors:

Privitera et al

Abstract:

Context.

Red-giant stars may engulf planets. This may increase the rotation rate of their convective envelope, which could lead to strong dynamo-triggered magnetic fields. Aims. We explore the possibility of generating magnetic fields in red giants that have gone through the process of a planet engulfment. We compare them with similar models that evolve without any planets. We discuss the impact of stellar wind magnetic braking on the evolution of the surface velocity of the parent star.

Methods.

With rotating stellar models with and without planets and an empirical relation between the Rossby number and the surface magnetic field, we deduce the evolution of the surface magnetic field along the red-giant branch. The effects of wind magnetic braking is explored using a relation deduced from MHD simulations.

Results.

The stellar evolution model of a 1.7 M⊙ without planet engulfment and that has a time-averaged rotation velocity during the Main-Sequence equal to 100 km s−1, shows a surface magnetic field triggered by convection larger than 10 G only at the base of the red giant branch, that means for gravities log g>3. When a planet engulfment occurs, such magnetic field can also appear at much lower gravities, i.e. at much higher luminosities along the red giant branch. Typically the engulfment of a 15 MJ planet produces a dynamo triggered magnetic field larger than 10 G for gravities between 2.5 and 1.9. We show that for reasonable wind magnetic braking laws, the high surface velocity reached after a planet engulfment may be maintained sufficiently long for being observable.

Conclusions.

High surface magnetic fields for red giants in the upper part of the red giant branch is a strong indication of a planet engulfment or of an interaction with a companion. Our theory can be tested by observing fast rotating red giants and check whether they show magnetic fields.

HIP 68468 is a Death Star, ate Exoplanets

An international team of scientists, including researchers from the University of Chicago, has made the rare discovery of a planetary system with a host star similar to Earth's sun. Especially intriguing is the star's unusual composition, which indicates it ingested some of its planets.

"It doesn't mean that the sun will 'eat' the Earth any time soon," said Jacob Bean, assistant professor of astronomy and astrophysics at UChicago and co-author of an Astronomy & Astrophysics article on the research. "But our discovery provides an indication that violent histories may be common for planetary systems, including our own."

Unlike the artificial planet-destroying Death Star in the movie "Star Wars," this natural version could provide clues about how planetary systems evolve over time.

Astronomers discovered the first planet orbiting a star other than the sun in 1995. Since then, more than two thousand exoplanets have been identified. Rare among them are planets that orbit a star similar to Earth's sun. Due to their extreme similarity to the sun, these so-called solar twins are ideal targets for investigating the connections between stars and their planets.

Bean and his colleagues studied star HIP68468, which is 300 light years away, as part of a multi-year project to discover planets that orbit solar twins. It's tricky to draw conclusions from a single system, cautioned Megan Bedell, a UChicago doctoral student who is co-author of the research and the lead planet finder for the collaboration. She said the team plans "to study more stars like this to see whether this is a common outcome of the planet formation process."

Computer simulations show that billions of years from now, the accumulated gravitational tugs and pulls between planets will eventually cause Mercury to fall into the sun, said Debra Fischer, a professor of astronomy at Yale University who was not involved in the research. "This study of HIP68468 is a post-mortem of this process happening around another star similar to our sun. The discovery deepens our understanding of the evolution of planetary systems."

link.

Could Fast Rotating Red Giants be a Sign of Having Eaten an Exoplanet?

Star-planet interactions: II. Is planet engulfment the origin of fast rotating red giants?

Authors:

Privitera et al

Abstract:

Context.

Fast rotating red giants in the upper part of the red giant branch have surface velocities that cannot be explained by single star evolution.

Aims.

We check whether tides between a star and a planet followed by planet engulfment can indeed accelerate the surface rotation of red giants for a sufficient long time in order to produce these fast rotating red giants.

Methods.

Using rotating stellar models, accounting for the redistribution of the angular momentum inside the star by different transport mechanisms, for the exchanges of angular momentum between the planet orbit and the star before the engulfment and for the deposition of angular momentum inside the star at the engulfment, we study how the surface rotation velocity at the stellar surface evolves.

Results.

We show that the surface velocities reached at the end of the orbital decay due to tidal forces and planet engulfment can be similar to values observed for fast rotating red giants. This surface velocity then decreases when the star evolves along the red giant branch but at a sufficiently slow pace for allowing stars to be detected with such a high velocity. More quantitatively, star-planet interaction can produce a rapid acceleration of the surface of the star, above values equal to 8 km s−1, for periods lasting up to more than 30% the red giant branch phase. The changes of the surface carbon isotopic ratios produced by the dilution of the planetary material into the convective envelope is quite modest. Much more important might be the increase of the lithium abundance due to this effect. However lithium may be affected by many different, still uncertain, processes.

Conclusions.

The acceleration of the stellar surface to rotation velocities above limits that depend on the surface gravity does appear at the moment as the clearest signature of a star-planet interaction.

On Lithium-Rich Red Giants. I. Engulfment of Sub-Stellar Companions

On Lithium-Rich Red Giants. I. Engulfment of Sub-Stellar Companions

Authors:

Aguilera-Gómez et al

Abstract:

A small fraction of red giants are known to be lithium (Li) rich, in contradiction with expectations from stellar evolutionary theory. A possible explanation for these atypical giants is the engulfment of a Li-rich planet or brown dwarf by the star. In this work, we model the evolution of Li abundance in canonical red giants including the accretion of a sub-stellar mass companion. We consider a wide range of stellar and companion masses, Li abundances, stellar metallicities, and planetary orbital periods. Based on our calculations, companions with masses lower than 15 M_J dissolve in the convective envelope and can induce Li enrichment in regimes where extra mixing does not operate. Our models indicate that the accretion of a substellar companion can explain abundances up to A(Li)~2.2, setting an upper limit for Li-rich giants formed by this mechanism. Giants with higher abundances need another mechanism to be explained. For reasonable planetary distributions, we predict the Li abundance distribution of low-mass giants undergoing planet engulfment, finding that between 1% to 3% of them should have A(Li)>1.5. We show that depending on the stellar mass range, this traditional definition of Li-rich giants is misleading, as isolated massive stars would be considered anomalous while giants engulfing a companion would be set aside, flagged as normal. We explore the detectability of companion engulfment, finding that planets with masses higher than ~7 M_J produce a distinct signature, and that descendants of stars originating in the Li-dip and low luminosity red giants are ideal tests of this channel.