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A new interpretation of the giant planet-metallicity correlation

1er juin 2009 A new interpretation of the giant planet-metallicity correlation

Since the discovery, at the Observatoire de Haute-Provence, of a planet orbiting 51 Peg in 1995, almost 300 planetary systems have been discovered. The only characteristic that distinguishes the host stars is their metallicity (the abundance of elements heavier than helium in the star atmosphere). These stars, which are mainly dwarfs (stars in the main sequence phase), are, in the mean, more metal-rich than most field objects. Up to now, this peculiarity has been interpreted as due to the enhanced rate of giant planets, or "Jupiters", formation in metal-rich circumstellar disks. A new interpretation suggests that the rate of detection of Jupiters around stars could depends on the density of H2 in the galactic disk, decreasing from the inner disk outwards. The observed correlation would then result from the presence in the solar neighbourhood of stars originating from the inner galactic disk.

Two recent observational facts have mitigated the evidence of the correlation between metallicity and the probability of detection of exo-planets : (a) the first is that, contrary to dwarfs, giant and ’massive’ stars which were found to have an orbiting exoplanet do not show an overabundance of heavy elements. (b) the second is that at [Fe/H]<-0.2 dex, for moderately deficient stars, more planets are being detected on stars of the thick disk population than on thin disk (Haywood 2008 A&A, 482, 673) objects, here again breaking down the correlation between metallicity and planets. We show in a recent article (Haywood 2009) that these two pecularities can be explained if the planet-metallicity correlation is given a radically different interpretation. In the last years, radial mixing has proved to be a phenomenon of importance in the galactic disk. Radial mixing is a consequence of badly identified dynamical processes, which are responsible for the ’migration’ of stars in the disk. Stars born in the inner parts of the disk may drift to the outer parts within a few billion years, and vice-versa. The inner disk evolving at a more rapid pace, stars born in these regions are, in the mean, more metal-rich than those born in the outer disk. Radial mixing then provides a natural explanation to point (a) and (b). Indeed, giants and massive stars are in the mean younger than dwarfs. Radial mixing being a secular process, its effects are proportional to time. Hence, samples containing older objects are more polluted by wanderers from the inner disk. It is also possible to explain point (b) above, because if thick disk stars are deficient in metals, their origin is also thought to be local or in the inner disk, i.e very different from the thin disk stars of the same metallicity. All this suggest that, rather than metallicity, it is the distance to the galactic center that governs the rate of Jupiters. The new question is then : what is the cause of this dependence ? A interesting candidate is the density of H2 gas. Molecular hydrogen is indeed the main ingredient of circumstellar disks and Jupiters. 70% of galactic H2 resides within the solar galactocentric radius. Its density rises towards the inner disk to reach a maximum at 3-5 kpc from the Sun (the molecular ring), at 4 to 5 times the estimated local density, in proportion to the rate of exoplanets on local stars (4%) and metal-rich ones (25%). 

Figure 1 : La correlation planète-metallicité observée (en rouge) peut être facilement produite par un modèle (en noir) où le taux d’exoplanètes dépend de la distance au centre galactique, les étoiles riches en métaux provenant du disque interne ayant un taux élevé (environ 25%) de planètes géantes. Aucune dépendence particulière entre planètes géantes et métallicité n’est alors requise. La densité de gaz H2 dans le disque galactique pourrait être la raison de la dépendance du taux planétaire avec la distance au centre galactique.

On the correlation between metallicity and the presence of giant planets Haywood M. ApJ Letter, 698, L1

Référence

  • On the correlation between metallicity and the presence of giant planets Haywood M. ApJ Letter, 698, L1

Contact

  • Misha Haywood
    Observatoire de Paris, GEPI, et CNRS
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