[Paper Review] Metallicity of M dwarfs I. A photometric calibration and impact on the mass-luminosity relation at the bottom of the main sequence

X. Bonfıls, X. Delfosse+4 more|arXiv (Cornell University)|Mar 11, 2005

Stellar, planetary, and galactic studies35 references207 citations

TL;DR

This paper presents a photometric metallicity calibration for M dwarfs using metallicity estimates from coeval F-, G-, and K-type stars in wide visual binaries, demonstrating that metallicity explains most of the intrinsic scatter in the V-band mass-luminosity relation. The calibration, accurate to ±0.2 dex, enables precise metallicity estimates for low-mass stars using V- and K-band photometry and parallaxes, and shows that known M-dwarf planet hosts Gl 876 and Gl 436 are near solar metallicity.

ABSTRACT

We obtained high resolution ELODIE and CORALIE spectra for both components of 20 wide visual binaries composed of an F-, G- or K-dwarf primary and an M-dwarf secondary. We analyse the well understood spectra of the primaries to determine metallicities ([Fe/H]) for these 20 systems, and hence for their M dwarf components. We pool these metallicities with determinations from the literature to obtain a precise (+- 0.2 dex) photometric calibration of M dwarf metallicities. This calibration represents a breakthrough in a field where discussions have had to remain largely qualitative, and it helps us demonstrate that metallicity explains most of the large dispersion in the empirical V-band mass-luminosity relation. We examine the metallicity of the two known M-dwarf planet-host stars, Gl 876 (+0.02 dex) and Gl 436 (-0.03 dex), in the context of preferential planet formation around metal-rich stars. We finally determine the metallicity of the 47 brightest single M dwarfs in a volume limited sample, and compare the metallicity distributions of solar-type and M-dwarf stars in the solar neighbourhood.

Motivation & Objective

• To develop a precise, empirical photometric calibration of M-dwarf metallicities, as direct spectroscopic analysis is hindered by molecular absorption features.
• To quantify the role of metallicity in the large intrinsic dispersion observed in the empirical V-band mass-luminosity relation for very low-mass stars.
• To determine the metallicities of known M-dwarf planet hosts Gl 876 and Gl 436 in the context of the metallicity-planet formation correlation seen in solar-type stars.
• To compare the metallicity distribution of nearby M dwarfs with that of solar-type stars in the solar neighborhood, assessing potential age or evolutionary differences.

Proposed method

• Use high-resolution ELODIE and CORALIE spectroscopy of primary stars in 20 wide visual binaries with M-dwarf secondaries to derive precise [Fe/H] metallicities.
• Assume coeval, coeval composition between binary components, leveraging the well-understood spectra of F-, G-, and K-type primaries to infer M-dwarf metallicities.
• Combine new and literature metallicity measurements to derive a photometric metallicity estimator based on V- and K-band magnitudes and parallaxes.
• Apply the calibration to 47 bright, volume-limited M dwarfs to construct a metallicity distribution for the solar neighborhood.
• Use the calibrated metallicity to re-analyze the V-band mass-luminosity relation and assess its dispersion.
• Compare the metallicity distributions of M dwarfs and solar-type stars in the solar neighborhood to infer potential differences in stellar population characteristics.

Experimental results

Research questions

• RQ1 To what extent does metallicity account for the large intrinsic scatter in the empirical V-band mass-luminosity relation for M dwarfs?
• RQ2 Can a reliable photometric metallicity calibration be established for very low-mass stars (0.2–0.8 M⊙) despite the complexity of their spectra?
• RQ3 Are the two known M-dwarf planet hosts, Gl 876 and Gl 436, metal-rich, as expected from the planet-metallicity correlation observed in solar-type stars?
• RQ4 How do the metallicity distributions of solar-type stars and M dwarfs in the solar neighborhood compare, and what might this imply about their formation or evolution?

Key findings

• The V-band mass-luminosity relation exhibits large intrinsic scatter (±1 mag), which is primarily explained by metallicity variations, not measurement errors or model deficiencies.
• A photometric metallicity calibration is established with ±0.2 dex uncertainty, valid for M dwarfs between 0.2 and 0.8 M⊙, using V- and K-band photometry and parallaxes.
• The metallicity of Gl 876 is +0.02 dex and Gl 436 is −0.03 dex, indicating they are near solar metallicity, consistent with the planet-metallicity correlation observed in more massive stars.
• The metallicity distribution of 47 bright, volume-limited M dwarfs shows a small but potentially significant difference from that of solar-type stars, possibly reflecting older average ages for M dwarfs.
• The calibration is validated by consistency with independent spectroscopic analyses of M-dwarf spectra (Woolf & Wallerstein 2005), confirming the assumption of common composition in binary systems.
• A 5% parallax uncertainty introduces an additional ∼0.2 dex uncertainty in metallicity, limiting the calibration’s utility to within ∼50 pc.