[Paper Review] Chemical abundances of 1111 FGK stars from the HARPS GTO planet search program.Galactic stellar populations and planets

V. Adibekyan, S. G. Sousa+6 more|arXiv (Cornell University)|Jul 10, 2012

Stellar, planetary, and galactic studies69 references263 citations

TL;DR

This study presents a uniform, high-precision abundance analysis of 12 refractory elements in 1,111 FGK stars from the HARPS GTO planet search, using LTE spectral synthesis and MOOG. It confirms strong metallicity enhancements in giant-planet hosts across all elements, while Neptunian/super-Earth hosts show a distinct, abrupt lower limit in [X/H] for α-elements, suggesting metallicity plays a key role in their formation, and reveals chemically distinct thin- and thick-disk populations for Al, Sc, Co, and Ca.

ABSTRACT

We performed a uniform and detailed abundance analysis of 12 refractory elements (Na, Mg, Al, Si, Ca, Ti, Cr, Ni, Co, Sc, Mn and V) for a sample of 1111 FGK dwarf stars from the HARPS GTO planet search program. 109 of these stars are known to harbour giant planetary companions and 26 stars are hosting exclusively Neptunians and super-Earths. The main goals of this paper are i) to investigate whether there are any differences between the elemental abundance trends for stars of different stellar populations; ii) to characterise the planet host and non-host samples in term of their [X/H]. The extensive study of this sample, focused on the abundance differences between stars with and without planets will be presented in a parallel paper. The equivalent widths of spectral lines are automatically measured from HARPS spectra with the ARES code. The abundances of the chemical elements are determined using a LTE abundance analysis relative to the Sun, with the 2010 revised version of the spectral synthesis code MOOG and a grid of Kurucz ATLAS9 atmospheres. To separate the Galactic stellar populations we applied both a purely kinematical approach and a chemical method. We found that the chemically separated (based on the Mg, Si, and Ti abundances) thin and thick discs are also chemically disjunct for Al, Sc, Co and Ca. Some bifurcation might also exist for Na, V, Ni, and Mn, but there is no clear boundary of their [X/Fe] ratios. We confirm that an overabundance in giant-planet host stars is clear for all the studied elements.We also confirm that stars hosting only Neptunian-like planets may be easier to detect around stars with similar metallicities as non-planet hosts, although for some elements (particulary alpha-elements) the lower limit of [X/H] are very abrupt.

Motivation & Objective

• To investigate differences in elemental abundance trends between stars of different Galactic stellar populations (thin vs. thick disk).
• To characterize the chemical composition of planet-hosting stars (giant planets, Neptunians, super-Earths) versus non-hosts in terms of [X/H].
• To assess the reliability and consistency of abundance measurements across a large, homogeneous sample using consistent spectroscopic techniques.
• To determine whether kinematical and chemical population separation methods yield consistent results for stellar populations in the solar neighborhood.
• To examine the role of metallicity and α-elements in planet formation, particularly for low-mass planets.

Proposed method

• Measured equivalent widths (EWs) of spectral lines using the ARES code on high-resolution HARPS spectra.
• Determined elemental abundances via LTE analysis with the MOOG spectral synthesis code and Kurucz ATLAS9 model atmospheres.
• Applied both kinematical and chemical methods (based on Mg, Si, Ti) to separate thin- and thick-disk stars.
• Used stellar parameters (Teff, log g, [Fe/H], microturbulence) derived from consistent spectroscopic analysis in Sousa et al. (2008, 2011).
• Performed error analysis by varying atmospheric parameters to assess sensitivity of [X/H] to uncertainties in Teff, log g, and microturbulence.
• Applied cuts in [Fe/H] and Teff to minimize systematic biases in abundance trends.

Experimental results

Research questions

• RQ1 Are there significant chemical differences between thin- and thick-disk stars in refractory elements beyond α-elements?
• RQ2 Do stars hosting giant planets show consistent overabundances across all studied elements compared to non-hosts?
• RQ3 Do stars hosting only Neptunian-like or super-Earth planets exhibit distinct abundance patterns, particularly in α-elements?
• RQ4 To what extent do kinematical and chemical population separation methods agree in identifying stellar populations?
• RQ5 Is there a clear boundary or abrupt lower limit in [X/H] for certain elements in Neptunian-host stars, suggesting formation constraints?

Key findings

• Chemically separated thin- and thick-disk populations show clear disjunctions in [X/Fe] for Al, Sc, Co, and Ca, beyond the known Mg, Si, and Ti distinctions.
• The abundance trends for metal-poor α-enhanced stars differ systematically from metal-rich ones, indicating distinct formation histories.
• Giant-planet host stars show significant overabundance in all 12 studied elements, supporting the core-accretion model of planet formation.
• Neptunian/super-Earth hosts exhibit an abrupt lower limit in [X/H] for α-elements (especially Mg), with the maximum abundance difference observed for Mg ([Mg/H] ≈ 0.09 dex).
• Stars hosting only Neptunian-like planets are more easily detectable around stars of similar metallicity compared to non-hosts, suggesting a metallicity threshold for such planets.
• No clear boundary exists for Na, V, Ni, and Mn in [X/Fe] ratios between thin- and thick-disk populations, though some bifurcation is suggested.