[Paper Review] Spectroscopic [Fe/H] for 98 extra-solar planet-host stars: Exploring the probability of planet formation

N. C. Santos, G. Israelian+1 more|CERN Bulletin|Nov 24, 2003

Stellar, planetary, and galactic studiesPhysics and Astronomy38 references602 citations

TL;DR

This study presents spectroscopic metallicities ([Fe/H]) for 98 planet-host stars and 41 non-host stars, confirming that giant planet frequency strongly increases with stellar metallicity—rising from ~3% at solar metallicity to over 25% at [Fe/H] > +0.3—supporting metallicity-dependent core accretion models of planet formation.

ABSTRACT

We present stellar parameters and metallicities, obtained from a detailed spectroscopic analysis, for a large sample of 98 stars known to be orbited by planetary mass companions (almost all known targets), as well as for a volume-limited sample of 41 stars not known to host any planet. For most of the stars the stellar parameters are revised versions of the ones presented in our previous works. However, we also present parameters for 18 stars with planets not previously published, and a compilation of stellar parameters for the remaining 4 planet-hosts for which we could not obtain a spectrum. A comparison of our stellar parameters with values of Teff, log(g), and [Fe/H] available in the literature shows a remarkable agreement. The derived [Fe/H] values are then used to confirm the previously known result that planets are more prevalent around metal-rich stars. Furthermore, we confirm that the frequency of planets is a strongly rising function of the stellar metallicity, at least for stars with [Fe/H]>0. While only about 3% of the solar metallicity stars in the CORALIE planet search sample were found to be orbited by a planet, this number increases to more than 25% for stars with [Fe/H] above +0.3. Curiously, our results also suggest that these percentages might remain relatively constant for values of [Fe/H] lower than about solar, increasing then linearly with the mass fraction of heavy elements. These results are discussed in the context of the theories of planetary formation.

Motivation & Objective

• To determine precise stellar metallicities ([Fe/H]) for a large sample of planet-host stars and control stars using high-resolution spectroscopy.
• To assess the statistical relationship between stellar metallicity and the frequency of giant planet companions.
• To test whether the observed metallicity-metallicity correlation in planet hosts is consistent with core accretion models of planet formation.
• To improve stellar parameter estimates (Teff, log g, [Fe/H]) by comparing with literature and trigonometric parallaxes.
• To evaluate whether the Sun is typical in metallicity among planet-host stars, given its low [Fe/H] value.

Proposed method

• Acquired high-resolution spectra using multiple instruments: CORALIE, FEROS, UVES, ELODIE, TNG, and William Herschel Telescopes.
• Performed detailed spectroscopic analysis to derive stellar parameters: effective temperature (Teff), surface gravity (log g), and iron abundance ([Fe/H]).
• Calibrated Teff using B-V colors and [Fe/H] via a new empirical relation: T_eff = 8423 - 4736(B-V) + 1106(B-V)^2 + 411[Fe/H], valid for log g > 4.0 and -0.70 < [Fe/H] < 0.43.
• Compared derived log g values with trigonometric estimates from Hipparcos parallaxes, finding agreement within ~0.03 dex.
• Compiled a volume-limited sample of 41 non-planet-host stars for comparison with the 98 planet-host stars.
• Analyzed the frequency of planets as a function of [Fe/H], identifying a strong, non-linear rise above solar metallicity.

Experimental results

Research questions

• RQ1 Is there a significant correlation between stellar metallicity ([Fe/H]) and the presence of giant planets?
• RQ2 How does the frequency of planet-host stars vary with increasing [Fe/H], particularly above and below solar metallicity?
• RQ3 Are the derived stellar parameters (Teff, log g, [Fe/H]) consistent with those from other independent methods?
• RQ4 Does the Sun lie within the typical metallicity range of planet-host stars, or is it an outlier?
• RQ5 What do the observed trends imply for the dominant mechanism of giant planet formation?

Key findings

• The derived [Fe/H] values confirm that planet-host stars are significantly more metal-rich than average field dwarfs, with an average excess of ~0.25 dex.
• The frequency of giant planets increases sharply with metallicity: ~3% of solar-metallicity stars host planets, rising to >25% for stars with [Fe/H] > +0.3.
• The planet occurrence rate appears relatively flat for [Fe/H] < 0.0 (Z < 0.02), but increases linearly with increasing metallicity for higher Z values.
• Spectroscopic log g values agree within ~0.03 dex of trigonometric estimates from Hipparcos parallaxes, validating the precision of the analysis.
• The new Teff calibration based on B-V and [Fe/H] has a root mean square of only 43 K, and matches previous calibrations (e.g., Alonso et al. 1996) within 21 K dispersion.
• The results support core accretion models of planet formation, as higher metallicity enhances solid core growth and gas disk instability is less likely to explain the trend.