[Paper Review] Chemical abundances of planet-host stars: Results for alpha and Fe-group elements

A. Bodaghee, N. C. Santos+2 more|ArXiv.org|Apr 21, 2003

Stellar, planetary, and galactic studiesPhysics and Astronomy25 references71 citations

TL;DR

This study presents a uniform analysis of alpha and iron-peak elements (Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni) in 77 planet-host stars and 42 field stars without known planets, using high-resolution spectroscopy from multiple observatories. The key finding is that, except for minor differences in V, Mn, and to a lesser extent Ti and Co, planet-host stars show no significant chemical distinctions from field stars at the same [Fe/H], supporting the idea that planetary systems form preferentially in metal-rich environments without anomalous elemental trends beyond iron-peak elements.

ABSTRACT

In this paper, we present a study of the abundances of Si, Ca, Sc, Ti, V, Cr, Mn, Co, and Ni in a large set of stars known to harbor giant planets, as well as in a comparison sample of stars not known to have any planet ary-mass companions. We have checked for possible chemical differences between planet hosts and field stars without known planets. Our results show that overall, and for a given value of [Fe/H], the abundance trends for the planet hosts are nearly indistinguishable from those of the field stars. In general, the trends show no discontinuities,and the abundance distributions of stars with giant planets are high [Fe/H] extensions to the curves traced by the field dwarfs without planets. The only elements that might present slight differences between the two groups of stars are V, Mn, and to a lesser extent Ti and Co. We also use the available data to describe galactic chemical evolution trends for the elements studied. When comparing the results with former studies, a few differences emerge for the high [Fe/H] tail of the distribution, a region that is sampled with unprecedented detail in our analysis.

Motivation & Objective

• To investigate whether planet-host stars exhibit chemical anomalies in alpha and iron-peak elements beyond iron-peak abundance trends.
• To reduce systematic errors by using a uniform analysis framework for both planet-host and field stars, avoiding comparisons across heterogeneous literature studies.
• To explore galactic chemical evolution trends using high-[Fe/H] stars, which are sampled in unprecedented detail.
• To assess the reliability of abundance determinations by testing for dependencies on stellar parameters such as effective temperature.
• To identify elements most affected by non-local thermodynamic equilibrium (NLTE) effects that could bias comparisons between planet-host and non-host stars.

Proposed method

• High-resolution spectroscopy was obtained using the CORALIE, FEROS, UVES, TNG, William Herschel, and ELODIE instruments at multiple observatories.
• Stellar atmospheric parameters (Teff, log g, ξt) were derived uniformly using the same methods applied in prior works (Santos et al. 2000, 2001, 2003).
• Abundances of Si, Ca, Sc, Ti, V, Cr, Mn, Co, and Ni were determined using spectral line analysis with consistent model atmospheres and line lists.
• The analysis ensured excitation equilibrium for Fe I and Fe II lines to minimize NLTE-related systematic errors.
• Linear least-squares fits were applied to [X/Fe] vs. Teff plots to quantify temperature-dependent abundance trends for each element.
• The results were compared between planet-host stars and a volume-limited comparison sample of field stars with no known planetary companions.

Experimental results

Research questions

• RQ1 Do planet-host stars show significant chemical differences in alpha and iron-peak elements compared to field stars with the same [Fe/H]?
• RQ2 Are the observed abundance trends in planet-host stars consistent with those of field stars, or do they exhibit discontinuities or anomalies?
• RQ3 Which elements are most sensitive to systematic effects such as NLTE or temperature dependence in abundance determinations?
• RQ4 How do the abundance trends in high-[Fe/H] stars inform galactic chemical evolution models?
• RQ5 To what extent do differences in stellar parameter derivation methods introduce biases in previous comparative studies of planet-host stars?

Key findings

• For most elements—Si, Ca, Sc, Ti, Cr, Co, and Ni—no significant differences were found between planet-host stars and field stars at the same [Fe/H], indicating that abundance trends are indistinguishable.
• The abundance distributions of planet-host stars extend the trends seen in field dwarfs without planets, suggesting that planet hosts are simply metal-rich extensions of the field star population.
• The only elements showing potential differences are V, Mn, and to a lesser extent Ti and Co, with [V/Fe] and [Mn/Fe] showing slight but systematic offsets in planet hosts.
• Significant temperature dependence in derived abundances was observed for V, Ti, Co, and Mn, with differences of up to 0.3 dex between K and F dwarfs, indicating sensitivity to Teff and possible NLTE effects.
• The slope of [Ca/Fe] vs. Teff is significantly higher for metal-rich stars ([Fe/H] ≥ 0) than for metal-poor stars, suggesting a complex behavior possibly linked to NLTE or line formation effects.
• The analysis confirms that NLTE effects are non-negligible for certain elements (e.g., V, Ti, Co), and that future comparisons between planet-host and non-host stars may require NLTE corrections for reliable results.