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A planet-metallicity correlation for low-mass planets

February 20th, 2011

The planet — host star metallicity connection has been one of the most secure and enduring results from the radial velocity planet surveys. In 1997, soon after the detection of the first planets, Guillermo Gonalez pointed out that the host stars were significantly enriched in elements heavier than hydrogen and helium, and suggested that a planet-metallicity connection exists.

Over the years, the correlations have been refined by many different workers, and a clear set of facts has emerged:

(1) Giant planet hosts, all the way from low-mass red dwarf stars through stars that are somewhat hotter and more massive than the Sun, tend to be metal rich.

(2) The occurrence rate for giant planets increases with stellar mass.

(3) Among stars with mass similar to the Sun, there’s no evidence that the presence of sub-Neptune/super-Earth is correlated with host star metallicity.

Taken together, these facts provide basic support for the core-accretion mechanism of giant planet formation. A planet like Jupiter forms by first assembling a core of icy/rocky/metallic material. When the core mass grows to of order 10-20 Earth masses, the core gains the ability to very rapidly accrete hydrogen and helium, and increases its mass by a significant, multiplicative factor to become a full-blown giant planet. Core accretion is a threshold phenomenon in the sense that the eventual presence or absence of a giant planet depends sensitively on whether the core is assembled while nebular gas is still present. Sufficiently rapid core growth is strongly aided by larger disk masses (which is the source of the planet-stellar mass connection) and by larger surface densities of solids in the disk (which is the source of the planet-stellar metallicity connection).

“Better late than never.” You hear that a lot when the chronic under-performance of super-Earths and sub-Neptunes is being discussed. The planet census makes it clear, however, that when not pressured to succeed while the gas is still there, sub-Neptunes and super-Earths regularly grow to 5-15 Earth masses and migrate to various locations in protoplanetary disks. The observations, furthermore, show that for host stars lying close to a solar mass, there’s no evidence for any metallicity dependence in the occurrence rate of these lower-mass planets.

At some point, however, metallicity has to play a role. An early-bird Population II star with [Me/H]=-3 started out with only ~0.1% as much iron, molybdenum, oxygen and carbon as did the Sun. Super-Earths won’t be found orbiting such stars because the raw planet-building materials flat-out weren’t there. Likewise, for low-mass disks orbiting low-mass stars, the overall metal budgets are tight enough that it’s quite reasonable to expect that a planet-metallicity connection for non-giant planets should be detectable.

Last year, Kevin Schlaufman and I looked into this issue and we found a tantalizing hint that among the red dwarf stars, a planet-metallicity connection does exist for planets with ~Neptune mass and below. The statistics were too sparse, however, to have anything more than ~1-sigma confidence.

Enter the Kepler results. In the course of an afternoon, 1,235 planet candidates flooded the market, completely upending the old business-as-usual model for the planet hunters. Correlations no longer emerge, they pop out.

In addition to being numerous, the Kepler stars are quite well characterized, and Sloan photometry has been published for the ~150,000 stars with Q1+Q2 public-domain light curves. At a given J-H color (obtained from the 2-Mass catalog) a star’s Sloan g-r color is significantly dependent on metallicity (see, e.g. here). It’s thus informative to make JHg-r color-color plots with (1) a control sample of 10,000 stars drawn from the Kepler 156K star target list, (2) the Kepler giant planet (Rp>5R_earth) hosts, and (3) the Kepler low-mass planet (Rp<5R_earth) hosts:

These plots, which are from a paper that Kevin and I just submitted to the Astrophysical Journal, demonstrate quite convincingly that a metallicity correlation does exist for low-mass planets orbiting lower-mass stars. (The correlation starts to kick in below ~0.8 solar masses.) Assuming that the probability of forming a planet is proportional to the total amount of solids in its protoplanetary disk, the correlation indicates that a late K-dwarf with 70% of the Sun’s mass needs to have a metallicity [Fe/H]=0.15 to have the same chance of forming a planetary system as a solar metallicity star of similar mass.

I’m pretty excitied about these results. A quantitative statistical link between relative disk conditions and planet outcomes for the huge super-Earth population gives us direct information about how the really interesting systems — the ones harboring large terrestrial planets — are assembled.

We’ll put the paper on astro-ph once it’s gone through review. It contains a lot of work to establish that the correlations are real, rather than due to reddening or the various observational biases inherent in the Kepler target list.

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  1. February 21st, 2011 at 02:17 | #1

    To what extent is the planetary gravitational field needed to trap He dependent on the surface temperature of the planet?

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