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

Privitera, Giovanni; Meynet, Georges; Eggenberger, P.; Georgy, Cyril; Ekström, Sylvia; Vidotto, A. A.; Bianda, M.; Villaver, E.; ud‐Doula, Asif

doi:10.1051/0004-6361/201629142

Cited by 36 publications

(24 citation statements)

References 33 publications

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“…A deficit of giant planets close to evolved stars (Kunitomo et al 2011) as well as the peculiar characteristics of some RGB stars (rapid rotation, magnetic fields, and lithium abundance) have been explained as the result of orbital decay and ingestion of giant planets (Carlberg et al 2009;AguileraGómez et al 2016aAguileraGómez et al , 2016bPrivitera et al 2016aPrivitera et al , 2016b). …”

Section: Selection Effects and The Similarity Of Planet Parametersmentioning

confidence: 99%

Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars

Grunblatt¹,

Huber²,

Gaidos³

et al. 2017

102

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Despite more than 20 years since the discovery of the first gas giant planet with an anomalously large radius, the mechanism for planet inflation remains unknown. Here, we report the discovery of K2-132b, an inflated gas giant planet found with the NASA K2 Mission, and a revised mass for another inflated planet, K2-97b. These planets orbit on ≈9day orbits around host stars that recently evolved into red giants. We constrain the irradiation history of these planets using models constrained by asteroseismology and Keck/High Resolution Echelle Spectrometer spectroscopy and radial velocity measurements. We measure planet radii of 1.31±0.11 R J and 1.30±0.07 R J , respectively. These radii are typical for planets receiving the current irradiation, but not the former, zero age main-sequence irradiation of these planets. This suggests that the current sizes of these planets are directly correlated to their current irradiation. Our precise constraints of the masses and radii of the stars and planets in these systems allow us to constrain the planetary heating efficiency of both systems as 0.03% 0.02% 0.03% -+. These results are consistent with a planet re-inflation scenario, but suggest that the efficiency of planet re-inflation may be lower than previously theorized. Finally, we discuss the agreement within 10% of the stellar masses and radii, and the planet masses, radii, and orbital periods of both systems, and speculate that this may be due to selection bias in searching for planets around evolved stars.

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Section: Selection Effects and The Similarity Of Planet Parametersmentioning

confidence: 99%

Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars

Grunblatt¹,

Huber²,

Gaidos³

et al. 2017

102

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“…García et al 2014;Ceillier et al 2017) with the orbital periods, G14 associated short-period systems with mode depletion. When the surface rotation and orbital motion become synchronised in close binaries, the red-giant component could be spun up and the magnetic field generated by the dynamo (Privitera et al 2016) can be responsible for the mode damping. Figure 3 shows that these four non-oscillating primaries are located in short period and quasi-circularised systems (15 ≤ P orbit ≤ 45 d and e ≤ 0.02).…”

Section: Synergies With Space Photometrymentioning

confidence: 99%

Testing tidal theory for evolved stars by using red giant binaries observed by Kepler

Beck

Mathis

Gallet

et al. 2018

Monthly Notices of the Royal Astronomical Society: Letters

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Tidal interaction governs the redistribution of angular momentum in close binary stars and planetary systems and determines the systems evolution towards the possible equilibrium state. Turbulent friction acting on the equilibrium tide in the convective envelope of low-mass stars is known to have a strong impact on this exchange of angular momentum in binaries. Moreover, theoretical modelling in recent literature as well as presented in this paper suggests that the dissipation of the dynamical tide, constituted of tidal inertial waves propagating in the convective envelope, is weak compared to the dissipation of the equilibrium tide during the red-giant phase. This prediction is confirmed when we apply the equilibrium-tide formalism developed by Zahn (1977), Verbunt & Phinney (1995), and Remus, Mathis & Zahn (2012) onto the sample of all known red-giant binaries observed by the NASA Kepler mission. Moreover, the observations are adequately explained by only invoking the equilibrium tide dissipation. Such ensemble analysis also benefits from the seismic characterisation of the oscillating components and surface rotation rates. Through asteroseismology, previous claims of the eccentricity as an evolutionary state diagnostic are discarded. This result is important for our understanding of the evolution of multiple star and planetary systems during advanced stages of stellar evolution.

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“…We determined the initial conditions required for an engulfment to occur during the red giant phase. We also computed the evolution of the star after the engulfment (Privitera et al 2016c) and explored the possibility that the fast rotation of the red giant after an engulfment might produce an observable surface magnetic field (Privitera et al 2016a). In the present work, we focus on the short phase during which the orbit is shrinking before the planet engulfment.…”

Section: Introductionmentioning

confidence: 99%

Star-planet interactions

Meynet

Eggenberger

Privitera

et al. 2017

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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 M J 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 yr 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 min per year in the last 3000 yr before engulfment. During this period, the spin period of the star is shorter than 0.7 yr. During this same period, the variation in orbital period, which is shorter than 0.18 yr, 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 changes of the beginning or end of the transit. For the relatively long orbital periods expected around red giants, long observing runs of typically a few years are needed. Interesting star-planet systems that currently are in this stage of orbit-shrinking would be red giants with fast rotation (above typically 4−5 km s −1 ), a low surface gravity (log g lower than 2), and having a planet at a distance typically smaller than about 0.4−1 au, depending on log g. A space mission like PLATO might be of great interest for detecting planets that are on the verge of being engulfed by red giants. The discovery of a few systems, even only one, would provide very interesting clues about the physics of tidal interaction between a red giant and a planet.

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

Cited by 36 publications

References 33 publications

Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars

Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars

Testing tidal theory for evolved stars by using red giant binaries observed by Kepler

Star-planet interactions

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