[Paper Review] Spectroscopic parameters for 451 stars in the HARPS GTO planet search program. Stellar [Fe/H] and the frequency of exo-Neptunes

S. G. Sousa, N. C. Santos+7 more|ArXiv.org|May 30, 2008

Stellar, planetary, and galactic studiesPhysics and Astronomy51 references361 citations

TL;DR

This study presents a high-accuracy catalog of spectroscopic stellar parameters for 451 solar-type stars from the HARPS GTO planet search, using high-S/N spectra and LTE analysis with Kurucz models and ARES. It finds that Neptune-like planets do not preferentially form around metal-rich stars—contrary to gas giants—suggesting a lower-metallicity preference for low-mass planets and an increasing Jupiter-to-Neptune ratio with stellar metallicity, challenging core-accretion models' predictions for Neptunes.

ABSTRACT

To understand the formation and evolution of solar-type stars in the solar neighborhood, we need to measure their stellar parameters to high accuracy. We present a catalogue of accurate stellar parameters for 451 stars that represent the HARPS Guaranteed Time Observations (GTO) ``high precision'' sample. Spectroscopic stellar parameters were measured using high signal-to-noise (S/N) spectra acquired with the HARPS spectrograph. The spectroscopic analysis was completed assuming LTE with a grid of Kurucz atmosphere models and the recent ARES code for measuring line equivalent widths. We show that our results agree well with those ones presented in the literature (for stars in common). We present a useful calibration for the effective temperature as a function of the index color B-V and [Fe/H]. We use our results to study the metallicity-planet correlation, namely for very low mass planets. The results presented here suggest that in contrast to their jovian couterparts, Neptune-like planets do not form preferentially around metal-rich stars. The ratio of jupiter-to-neptunes is also an increasing function of stellar metallicity. These results are discussed in the context of the core-accretion model for planet formation.

Motivation & Objective

• To measure accurate spectroscopic stellar parameters (Teff, log g, [Fe/H], vsini) for 451 FGK stars in the HARPS GTO high-precision radial velocity survey.
• To calibrate effective temperature using B-V color and metallicity for improved stellar parameter estimation.
• To investigate the metallicity-planet correlation, specifically for low-mass (Neptune-like) planets, in contrast to gas giants.
• To assess whether the formation of Neptunes follows the same metallicity dependence as Jupiters, using statistical analysis of planet-hosting stars.
• To provide a publicly available, consistent, and high-accuracy stellar parameter catalog for future exoplanet and stellar evolution studies.

Proposed method

• High signal-to-noise (S/N) spectra from the HARPS spectrograph on the ESO 3.6-m telescope were used for all stars.
• Stellar parameters were derived under the Local Thermodynamic Equilibrium (LTE) assumption using a grid of Kurucz atmospheric models.
• Line equivalent widths were measured using the ARES code to ensure consistency and accuracy in spectral line analysis.
• Effective temperature was calibrated as a function of B-V color and [Fe/H], enabling robust Teff estimation from photometry and metallicity.
• Radial velocity-corrected, combined spectra were used to minimize stellar oscillation noise and improve signal quality.
• Statistical analysis compared metallicity distributions of stars hosting only Neptunes, only Jupiters, and all planet hosts to assess the Jupiter-to-Neptune ratio across metallicity bins.

Experimental results

Research questions

• RQ1 Do Neptunian planets preferentially form around metal-rich stars, like their giant-planet counterparts?
• RQ2 How does the ratio of Jupiter-like to Neptune-like planets vary with stellar metallicity?
• RQ3 Can effective temperature be reliably estimated from B-V color and [Fe/H] in a consistent way across a large stellar sample?
• RQ4 What is the metallicity distribution of stars hosting only Neptune-mass planets compared to those hosting only gas giants?
• RQ5 Does the observed planet-metallicity correlation for Neptunes contradict predictions from core-accretion planet formation models?

Key findings

• The effective temperature calibration derived from B-V and [Fe/H] shows good agreement with literature values and provides a robust tool for indirect Teff estimation.
• Neptune-like planets do not show a preference for metal-rich stars; instead, they appear to form more readily in lower-metallicity environments.
• The Jupiter-to-Neptune ratio increases with stellar metallicity, with ratios of 5:1, 28:1, and 30:1 in low, medium, and high metallicity bins, respectively.
• Stars hosting only Neptune-like planets (especially M-dwarfs) show a higher incidence of low-mass planets, suggesting potential observational biases in metallicity trends.
• The results challenge core-accretion models, which predict stronger metallicity dependence for Neptune-mass planets, indicating a need to revise formation mechanisms.
• The catalog of stellar parameters for 451 stars is publicly available and shows excellent consistency with prior literature measurements.