[Paper Review] New and updated stellar parameters for 71 evolved planet hosts. On the metallicity - giant planet connection

Annelies Mortier, N. C. Santos+6 more|arXiv (Cornell University)|Jul 30, 2013

Stellar, planetary, and galactic studies59 references65 citations

TL;DR

This study reanalyzes stellar parameters for 71 evolved planet-host stars using two iron line list sets to assess metallicity trends in giant stars. It finds evolved planet hosts are significantly more metal-poor (by 0.24 dex) than dwarf planet hosts, with no clear metallicity enhancement in red giants with planets, suggesting a fundamental difference in planet formation or observational bias.

ABSTRACT

It is still being debated whether the well-known metallicity - giant planet correlation for dwarf stars is also valid for giant stars. For this reason, having precise metallicities is very important. Different methods can provide different results that lead to discrepancies in the analysis of planet hosts. To study the impact of different analyses on the metallicity scale for evolved stars, we compare different iron line lists to use in the atmospheric parameter derivation of evolved stars. Therefore, we use a sample of 71 evolved stars with planets. With these new homogeneous parameters, we revisit the metallicity - giant planet connection for evolved stars. A spectroscopic analysis based on Kurucz models in local thermodynamic equilibrium (LTE) was performed through the MOOG code to derive the atmospheric parameters. Two different iron line list sets were used, one built for cool FGK stars in general, and the other for giant FGK stars. Masses were calculated through isochrone fitting, using the Padova models. Kolmogorov-Smirnov tests (K-S tests) were then performed on the metallicity distributions of various different samples of evolved stars and red giants. All parameters compare well using a line list set, designed specifically for cool and solar-like stars to provide more accurate temperatures. All parameters derived with this line list set are preferred and are thus adopted for future analysis. We find that evolved planet hosts are more metal-poor than dwarf stars with giant planets. However, a bias in giant stellar samples that are searched for planets is present. Because of a colour cut-off, metal-rich low-gravity stars are left out of the samples, making it hard to compare dwarf stars with giant stars. Furthermore, no metallicity enhancement is found for red giants with planets ($\log g < 3.0$\,dex) with respect to red giants without planets.

Motivation & Objective

• To resolve inconsistencies in metallicity measurements for evolved stars by testing different iron line list sets.
• To derive homogeneous, precise stellar parameters (Teff, log g, [Fe/H], ξ) for 71 evolved planet hosts using spectroscopic analysis.
• To reassess the metallicity-giant planet connection in evolved stars, particularly comparing metallicity distributions between planet-hosting and non-hosting giants.
• To identify and quantify observational biases—especially color cuts—that may distort comparisons between dwarf and evolved star samples.
• To provide a consistent, high-precision stellar parameter catalogue (SWEET-Cat) for future exoplanet research.

Proposed method

• Performed spectroscopic analysis using Kurucz model atmospheres in LTE and the MOOG code to derive stellar parameters from high-resolution spectra.
• Compared two iron line list sets: one for cool FGK stars (TS13/SO08) and one specifically designed for giant stars (HM07), using Arcturus as a reference.
• Calculated stellar masses via isochrone fitting using the Padova evolutionary models.
• Applied Kolmogorov-Smirnov (K-S) tests to compare metallicity distributions across different samples of evolved stars and red giants.
• Assessed the impact of line list choice on derived parameters and metallicity scales, focusing on systematic offsets.
• Compiled final parameters into the SWEET-Cat stellar parameter catalogue for public use.

Experimental results

Research questions

• RQ1 Does the choice of iron line list significantly affect the derived atmospheric parameters of evolved stars?
• RQ2 Are evolved planet-host stars more metal-rich than their dwarf counterparts, as seen in the metallicity-giant planet correlation?
• RQ3 Is there a measurable metallicity enhancement in red giants with planets compared to those without planets?
• RQ4 To what extent do observational selection effects—particularly color cuts—affect the observed metallicity distribution in giant star surveys?
• RQ5 Can a homogeneous, bias-corrected sample of evolved stars reveal a consistent metallicity trend in giant planet formation?

Key findings

• The line list set from TS13 and SO08 (designed for cool FGK stars) produced more consistent and reliable results than the HM07 giant-specific line list, and was therefore adopted for final analysis.
• Evolved planet hosts are, on average, 0.24 dex more metal-poor than planet-hosting dwarf stars, indicating a significant difference in metallicity trends.
• No significant metallicity enhancement is found for red giants with planets (log g < 3.0 dex) compared to red giants without planets, suggesting a flat metallicity distribution in this group.
• A strong observational bias exists in giant star surveys: metal-rich, low-gravity stars are systematically excluded due to color cuts, distorting comparisons with dwarf stars.
• The lack of metallicity enhancement in evolved stars with planets cannot be explained by planet engulfment or stellar mass effects, pointing to possible formation mechanism differences or selection biases.
• Despite small systematic offsets in metallicity between line list sets, these differences are significant enough to affect planet frequency statistics and must be corrected in large-scale studies.