[Paper Review] Chemical abundances of 451 stars from the HARPS GTO planet search program: Thin disc, thick disc, and planets

V. Neves, N. C. Santos+3 more|ArXiv.org|Feb 19, 2009

Stellar, planetary, and galactic studies44 references109 citations

TL;DR

This study presents uniform chemical abundances of 12 elements in 451 stars from the HARPS GTO planet search, finding that planet-host stars systematically exhibit higher metallicity across all elements, consistent with core-accretion models. It confirms a clear distinction between thin and thick disc populations at [Fe/H] < 0 via [X/Fe] ratios, though kinematic and abundance trends show overlapping populations, challenging simple disc separation.

ABSTRACT

We present a uniform study of the chemical abundances of 12 elements (Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg, and Al) derived from the spectra of 451 stars observed as part of one of the HARPS GTO planet search programs. Sixty eight of these are planet-bearing stars. The main goals of our work are: i) the investigation of possible differences between the abundances of stars with and without planets; ii) the study of the possible differences in the abundances of stars in the thin and the thick disc. We confirm that there is a systematically higher metallicity in planet host stars, when compared to non planet-hosts, common to all studied species. We also found that there is no difference in the galactic chemical evolution trends of the stars with and without planets. Stars that harbour planetary companions simply appear to be in the high metallicity tail of the distribution. We also confirm that Neptunian and super-Earth class planets may be easier to find at lower metallicities. A statistically significative abundance difference between stars of the thin and the thick disc was found for [Fe/H] $

Motivation & Objective

• To investigate differences in chemical abundances between stars with and without planets using a uniform analysis of 451 stars.
• To examine whether stars in the thin and thick disc populations exhibit distinct abundance patterns, particularly in [X/Fe] ratios.
• To assess whether the observed metallicity enhancement in planet-host stars arises from primordial conditions or post-formation processes.
• To evaluate the role of metallicity in the formation of different planetary types, including Neptunian and super-Earth-class planets.
• To test the validity of traditional disc population separation using kinematic and abundance data in the solar neighborhood.

Proposed method

• Spectra from the HARPS spectrograph on the 3.6m telescope at La Silla Observatory were used for abundance analysis of 12 elements: Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg, and Al.
• Stellar parameters (Teff, log g, [Fe/H]) were adopted from Sousa et al. (2008), ensuring consistency across the sample of 451 field dwarfs.
• Galactic space velocity components (U, V, W) were computed to classify stars into thin disc, thick disc, transition, or halo populations using kinematic criteria.
• Abundance ratios [X/Fe] were derived using spectral synthesis and model atmospheres, with uncertainties estimated via bootstrap resampling.
• [X/Fe] vs. [Fe/H] trends were plotted and compared between planet-host and non-planet-host stars, and between thin and thick disc populations.
• Statistical comparisons were performed to assess significance of abundance differences, particularly at [Fe/H] < 0.

Experimental results

Research questions

• RQ1 Is there a systematic difference in elemental abundances between stars hosting planets and those without planets?
• RQ2 Do stars in the thin disc and thick disc exhibit distinct [X/Fe] abundance patterns, especially at low metallicities?
• RQ3 Does the observed metallicity enhancement in planet-host stars reflect a primordial origin or a selection effect tied to formation efficiency?
• RQ4 Are there differences in the chemical trends of stars hosting Neptunian/super-Earth-class planets compared to Jovian planets?
• RQ5 Can the thin and thick disc populations be clearly separated using [X/Fe] ratios and kinematic data in the solar neighborhood?

Key findings

• Planet-host stars exhibit systematically higher [X/H] for all 12 elements studied, confirming a strong correlation between metallicity and giant planet formation.
• The abundance difference between planet-host and non-planet-host stars is consistent across all elements, supporting a primordial origin for the metallicity enhancement.
• Stars hosting only Neptunian or super-Earth-class planets are more likely found at metallicities similar to or lower than non-planet hosts, suggesting different formation pathways.
• A statistically significant distinction between thin and thick disc populations is observed at [Fe/H] < 0, with distinct [X/Fe] distributions for most elements except Cr and Ni.
• Despite the abundance difference, [X/Fe] vs. [Fe/H] plots show overlapping trends between thin and thick disc stars at the same [Fe/H], with a bifurcation in alpha elements and Mg at low metallicity.
• The bifurcation suggests two distinct evolutionary trends: a shallow slope for the low-abundance group (thin disc-dominated) and a steeper slope for the high-abundance group (thick disc-dominated), indicating different chemical evolution dynamics.