[Paper Review] Metallicity of M dwarfs III. Planet-metallicity and planet-stellar mass correlations of the HARPS GTO M dwarf sample

V. Neves, X. Bonfıls+5 more|Archive ouverte UNIGE (University of Geneva)|Dec 14, 2012

Stellar, planetary, and galactic studies63 references73 citations

TL;DR

This study investigates planet-metallicity and planet-stellar mass correlations in a volume-limited sample of 102 M dwarfs using high-resolution HARPS spectroscopy. It develops a new method achieving 0.08 dex precision in [Fe/H] measurements, confirming a strong planet-metallicity correlation for Jovian planets (α ≈ 1.26–2.94), while finding no significant correlation for Neptunian or smaller planets—though an anti-correlation is suggested with low statistical confidence. Stellar mass trends are shown to be driven by detection bias, not intrinsic correlation.

ABSTRACT

Aims. The aim of this work is the study of the planet-metallicity and the planet-stellar mass correlations for M dwarfs from the HARPS GTO M dwarf subsample Methods. We use a new method that takes advantage of the HARPS high-resolution spectra to increase the precision of metallicity, using previous photometric calibrations of [Fe/H] and effective temperature as starting values. Results. In this work we use our new calibration (rms = 0.08 dex) to study the planet-metallicity relation of our sample. The well-known correlation for Giant planet FGKM hosts with metallicity is present. Regarding Neptunians and smaller hosts no correlation is found but there is a hint that an anti-correlation with [Fe/H] may exist. We combined our sample with the California Planet Survey late-K and M-type dwarf sample to increase our statistics but found no new trends. We fitted a power law to the frequency histogram of the Jovian hosts for our sample and for the combined sample, f_p = C10^α[Fe/H], using two different approaches: a direct bin fitting and a bayesian fitting procedure. We obtained a value for C between 0.02 and 0.04 and for αbetween 1.26 and 2.94. Regarding stellar mass, an hypothetical correlation with planets was discovered, but was found to be the result of a detection bias.

Motivation & Objective

• To investigate the planet-metallicity correlation in M dwarfs, particularly for low-mass planets, using a volume-limited sample.
• To assess whether stellar mass correlates with planet occurrence in M dwarfs, accounting for observational biases.
• To improve metallicity precision for M dwarfs using high-resolution HARPS spectra and photometric calibrations.
• To test whether the planet-metallicity relation observed in FGK stars extends to M dwarfs, especially for Neptunian and smaller planets.
• To determine whether observed correlations are physical or driven by detection biases in the sample.

Proposed method

• A new metallicity calibration method is developed using HARPS high-resolution spectra, combining photometric [Fe/H] and Teff estimates with spectral fitting to improve precision.
• The method achieves a root-mean-square (rms) precision of 0.08 dex in [Fe/H], significantly improving over previous estimates for M dwarfs.
• Planet occurrence frequencies are binned by [Fe/H] and fitted with a power-law model: fp = C × 10^(α[Fe/H]), using both direct bin fitting and Bayesian inference.
• The HARPS sample is combined with the California Planet Survey (CPS) late-K and M dwarf sample to improve statistical power for low-significance trends.
• Detection bias in planet frequency with stellar mass is quantified by comparing observed trends with simulated detection limits.
• Statistical significance of correlations is assessed using both frequentist (bin fitting) and Bayesian approaches, with uncertainty propagation.

Experimental results

Research questions

• RQ1 Is there a significant planet-metallicity correlation for Jovian planets around M dwarfs, and how does it compare to FGK stars?
• RQ2 Does a correlation exist between metallicity and the occurrence of Neptunian or smaller planets around M dwarfs?
• RQ3 Is there an intrinsic correlation between stellar mass and planet occurrence in M dwarfs, or is it an artifact of detection bias?
• RQ4 Can the planet-metallicity relation be described by a power-law functional form fp = C × 10^(α[Fe/H]) in M dwarfs, and what are the values of C and α?
• RQ5 What is the role of detection bias in shaping apparent correlations between planet frequency and stellar mass in M dwarf samples?

Key findings

• A significant planet-metallicity correlation is confirmed for Jovian planets around M dwarfs, with α values ranging from 1.26 to 2.94 depending on fitting method and sample, consistent with FGK stars.
• For Neptunian and smaller planets, no significant correlation is found; however, an anti-correlation with [Fe/H] is suggested (α ≈ -0.79 to -0.57), though with low statistical confidence.
• The frequency of Jovian planets increases with metallicity, with observed frequencies rising from 1.9% in [-0.5, -0.1] dex to 5.6% in [-0.1, 0.4] dex in the HARPS sample.
• Neptunian-host frequencies are highest in the lowest metallicity bin (12.5% in HARPS sample), decreasing to 2.8% in the highest bin, suggesting a possible anti-correlation.
• A positive trend in planet detection frequency with increasing stellar mass is observed, but this is shown to be driven by detection bias, not a physical correlation.
• The combined HARPS+CPs sample does not reveal new trends, and the statistical power remains insufficient to confirm an anti-correlation for Neptunian hosts with high confidence.