Metal pollution in white dwarf atmospheres is commonly assumed to be a signature of remnant planetary systems. Most explanations for this pollution predict a sharp decrease in the number of polluted ...systems with white dwarf cooling age. Observations do not confirm this trend, and metal pollution in old (1–5 Gyr) white dwarfs is difficult to explain. We propose an alternative, time-independent mechanism to produce the white dwarf pollution. The orbit of a wide binary companion can be perturbed by Galactic tides, approaching close to the primary star for the first time after billions of years of evolution on the white dwarf branch. We show that such a close approach perturbs a planetary system orbiting the white dwarf, scattering planetesimals on to star-grazing orbits, in a manner that could pollute the white dwarf's atmosphere. Our estimates find that this mechanism is likely to contribute to metal pollution, alongside other mechanisms, in up to a few per cent of an observed sample of white dwarfs with wide binary companions, independent of white dwarf age. This age independence is the key difference between this wide binary mechanism and others mechanisms suggested in the literature to explain white dwarf pollution. Current observational samples are not large enough to assess whether this mechanism makes a significant contribution to the population of polluted white dwarfs, for which better constraints on the wide binary population are required, such as those that will be obtained in the near future with Gaia.
Validated planet candidates need not lie on long-term stable orbits, and instability triggered by post-main-sequence stellar evolution can generate architectures which transport rocky material to ...white dwarfs, hence polluting them. The giant planet HD 131399Ab orbits its parent A star at a projected separation of about 50-100 au. The host star, HD 131399A, is part of a hierarchical triple with HD 131399BC being a close binary separated by a few hundred au from the A star. Here, we determine the fate of this system, and find the following: (i) Stability along the main sequence is achieved only for a favourable choice of parameters within the errors. (ii) Even for this choice, in almost every instance, the planet is ejected during the transition between the giant branch and white dwarf phases of HD 131399A. This result provides an example of both how the free-floating planet population may be enhanced by similar systems and how instability can manifest in the polluted white dwarf progenitor population.
Abstract
The Transiting Exoplanet Survey Satellite (TESS) mission searches for new exoplanets. The observing strategy of TESS results in high-precision photometry of millions of stars across the sky, ...allowing for detailed asteroseismic studies of individual systems. In this work, we present a detailed asteroseismic analysis of the giant star HD 76920 hosting a highly eccentric giant planet (
e
= 0.878) with an orbital period of 415 days, using five sectors of TESS light curve that cover around 140 days of data. Solar-like oscillations in HD 76920 are detected around 52
μ
Hz by TESS for the first time. By utilizing asteroseismic modeling that takes classical observational parameters and stellar oscillation frequencies as constraints, we determine improved measurements of the stellar mass (1.22 ± 0.11
M
⊙
), radius (8.68 ± 0.34
R
☉
), and age (5.2 ± 1.4 Gyr). With the updated parameters of the host star, we update the semimajor axis and mass of the planet as
a
= 1.165 ± 0.035 au and
M
p
sin
i
=
3.57
±
0.22
M
Jup
. With an orbital pericenter of 0.142 ± 0.005 au, we confirm that the planet is currently far away enough from the star to experience negligible tidal decay until being engulfed in the stellar envelope. We also confirm that this event will occur within about 100 Myr, depending on the stellar model used.
Aims. The K2 mission has recently begun to discover new and diverse planetary systems. In December 2014, Campaign 1 data from the mission was released, providing high-precision photometry for ~22 000 ...objects over an 80-day timespan. We searched these data with the aim of detecting more important new objects. Methods. Our search through two separate pipelines led to the independent discovery of K2-19b and c, a two-planet system of Neptune-sized objects (4.2 and 7.2 R⊕), orbiting a K dwarf extremely close to the 3:2 mean motion resonance. The two planets each show transits, sometimes simultaneously owing to their proximity to resonance and the alignment of conjunctions. Results. We obtained further ground-based photometry of the larger planet with the NITES telescope, demonstrating the presence of large transit timing variations (TTVs), and used the observed TTVs to place mass constraints on the transiting objects under the hypothesis that the objects are near but not in resonance. We then statistically validated the planets through the PASTIS tool, independently of the TTV analysis.
ABSTRACT
The noteworthy four-planet HR 8799 system teeters on the brink of gravitational instability and contains an A-type host star that is characteristic of the progenitors of the majority of ...known white dwarf planetary system hosts. Goździewski & Migaszewski have demonstrated that the system can retain all four planets for at least 1 Gyr along the main sequence if the planets evolve within an externally unperturbed 8:4:2:1 mean motion resonance configuration. Here, we propagate forward their most stable fit beyond the main sequence, and incorporate external effects from Galactic tides and stellar fly-bys. We find that (i) giant branch mass-loss always breaks the resonance, and usually triggers the ejection of two of the planets, (ii) stellar fly-bys and Galactic tides rarely break the resonance during the main-sequence and giant branch phases, but play a crucial role in determining the final planetary configurations around the eventual white dwarf host star, and (iii) the meanderings of the surviving planets vary significantly, occupying regions from under 1 au to thousands of au. The ubiquitous survival of at least one planet and the presence of the debris discs in the system should allow for dynamical pathways for the white dwarf to be metal-polluted.
Abstract
At least two multi-planetary systems in a 4:3 mean motion resonance have been found by radial velocity surveys.
1
These planets are gas giants and the systems are only stable when protected ...by a resonance. Additionally the Kepler mission has detected at least four strong candidate planetary systems with a period ratio close to 4:3.
This paper investigates traditional dynamical scenarios for the formation of these systems. We systematically study migration scenarios with both N-body and hydrodynamic simulations. We investigate scenarios involving the in situ formation of two planets in resonance. We look at the results from finely tuned planet-planet scattering simulations with gas disc damping. Finally, we investigate a formation scenario involving isolation-mass embryos.
Although the combined planet-planet scattering and damping scenario seems promising, none of the above scenarios is successful in forming enough systems in 4:3 resonance with planetary masses similar to the observed ones. This is a negative result but it has important implications for planet formation. Previous studies were successful in forming 2:1 and 3:2 resonances. This is generally believed to be evidence of planet migration. We highlight the main differences between those studies and our failure in forming a 4:3 resonance. We also speculate on more exotic and complicated ideas. These results will guide future investigators towards exploring the above scenarios and alternative mechanisms in a more general framework.
As a comet, asteroid or planet approaches its parent star, the orbit changes shape due to the curvature of space–time. For comets in particular, the deviation at the pericentre may noticeably change ...their ephemerides and affect the dynamics of outgassing, tidal disruption or other processes which act on orbital time-scales and are assumed to follow Newtonian gravity. By obtaining and analysing the unaveraged equations of motion in orbital elements due to the dominant post-Newtonian contribution (1PN), I derive a simple analytic expression for the maximum deviation in terms of only the stellar mass and eccentricity of the orbit. This relation can be used to assess the potential importance of including short-period relativistic terms in models containing comets, asteroids or planets, and help determine the level of precision needed in numerical integrations. The magnitude of the deviation in systems with solar-like stars is typically comparable to the size of comet nuclei, and the direction of the deviation is determined by the eccentricity. I show that for eccentricities above a critical value of
$\sqrt{19} - 4 \approx 0.359$
, the direction is away from the star.
ABSTRACT
Metal pollution in white dwarf photospheres originates from the accretion of some combination of planets, moons, asteroids, comets, boulders, pebbles, and dust. When large bodies reside in ...dynamically stagnant locations – unable themselves to pollute or even closely approach the white dwarf – then smaller reservoirs of impact debris may become a complementary or the primary source of metal pollutants. Here, we take a first step towards exploring this possibility by computing limits on the recoil mass that escapes the gravitational pull of the target object following a single impact onto an atmosphere-less surface. By considering vertical impacts only with the full-chain analytical prescription from Kurosawa & Takada, we provide lower bounds for the ejected mass for basalt, granite, iron, and water-rich target objects across the radii range 100–103 km. Our use of the full-chain prescription as opposed to physical experiments or hydrocode simulations allows us to quickly sample a wide range of parameter space appropriate to white dwarf planetary systems. Our numerical results could be used in future studies to constrain freshly generated small debris reservoirs around white dwarfs given a particular planetary system architecture, bombardment history, and impact geometries.
Binary-star exoplanetary systems are now known to be common, for both wide and close binaries. However, their orbital evolution is generally unsolvable. Special cases of the N-body problem which are ...in fact completely solvable include dynamical architectures known as central configurations. Here, I utilize recent advances in our knowledge of central configurations to assess the plausibility of linking them to coplanar exoplanetary binary systems. By simply restricting constituent masses to be within stellar or substellar ranges characteristic of planetary systems, I find that (i) this constraint reduces by over 90 per cent the phase space in which central configurations may occur, (ii) both equal-mass and unequal-mass binary stars admit central configurations, (iii) these configurations effectively represent different geometrical extensions of the Sun-Jupiter-Trojan-like architecture, (iv) deviations from these geometries are no greater than 10 degree , and (v) the deviation increases as the substellar masses increase. This study may help restrict future stability analyses to architectures which resemble exoplanetary systems, and might hint at where observers may discover dust, asteroids and/or planets in binary-star systems.
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