Planet-Metallicity and Planet-Stellar Mass relationships for giant stars

Precise Radial Velocities of Giant Stars VII. Occurrence Rate of Giant Extrasolar Planets as a Function of Mass and Metallicity

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(abridged) We have obtained precise radial velocities for a sample of 373 G and K type giants at Lick Observatory regularly over more than 12 years. Planets have been identified around 15 giant stars; an additional 20 giant stars host planet candidates. We investigate the occurrence rate of substellar companions around giant stars as a function of stellar mass and metallicity. We probe the stellar mass range from about 1 to beyond 3 M_Sun, which is not being explored by main-sequence samples. We fit the giant planet occurrence rate as a function of stellar mass and metallicity with a Gaussian and an exponential distribution, respectively. We find strong evidence for a planet-metallicity correlation among the secure planet hosts of our giant star sample, in agreement with the one for main-sequence stars. However, the planet-metallicity correlation is absent for our sample of planet candidates, raising the suspicion that a good fraction of them might indeed not be planets. Consistent with the results obtained by Johnson for subgiants, the giant planet occurrence rate increases in the stellar mass interval from 1 to 1.9 M_Sun. However, there is a maximum at a stellar mass of 1.9 +0.1/-0.5 M_Sun, and the occurrence rate drops rapidly for masses larger than 2.5-3.0 M_Sun. We do not find any planets around stars more massive than 2.7 M_Sun, although there are 113 stars with masses between 2.7 and 5 M_Sun in our sample (corresponding to a giant planet occurrence rate < 1.6% at 68.3% confidence in that stellar mass bin). We also show that this result is not a selection effect related to the planet detectability being a function of the stellar mass. We conclude that giant planet formation or inward migration is suppressed around higher mass stars, possibly because of faster disk depletion coupled with a longer migration timescale.

The exact nature of purported planetary companions to giant stars is a complex issue, as they show very different distributions to giant planets around MS stars. Though lack of variations in the classical activity indicators (S-index, mostly) have been used to validate or disprove planets around stars of all evolutionary states, the case of Pollux shows that a giant star with stable activity indicators can still be magnetically variable, which raises the possibility that previously confirmed planetary companions to giant stars may not be real.

This paper, however, shows evidence that at least some planets orbiting giants are real. The planet-metallicity correlation - clear evidence for planet formation in disks - has been suspiciously absent for giant stars up to this point. But this paper derives a (giant) planet - metallicity correlation similar in function to the one observed in main sequence stars. It also derives a planet - stellar mass relationship as predicted by theory (planets are rarer around more massive stars because their disks dissipate faster). Both relationships are unlikely to occur together if all these planets are not real, so even if everything is not yet explained, there do appear to be (some) giant planets around giant stars.

Thanks for posting this one!

So what's the current status of Pollux b? From a quick scan, I see that the papers about the magnetic field correlation are not cited in this one, although they do include Pollux in the list of "secure" systems at the end of section 4.

Will take a more detailed look later...

As far as I can tell, the Pollux issue hasn't been properly introduced to literature. The 2009 paper is more tentative in its result, and the 2013 thing is a poster rather than a proper analysis, so it's probably reasonable that this work doesn't reference the issue. Further processing will have to wait until the ESPaDOnS folks publish something formally, as I believe their instrument is the only comparably capable spectropolarimeter in the northern hemisphere.