[Paper Review] Precise radial velocities of giant stars. III. Spectroscopic stellar parameters

S. Hekker, J. Meléndez|ArXiv.org|Sep 7, 2007

Stellar, planetary, and galactic studiesPhysics and Astronomy40 references148 citations

TL;DR

This study presents uniformly derived spectroscopic stellar parameters—effective temperature, surface gravity, metallicity, and rotational velocity—for 380 G and K giant stars using high-resolution, high-S/N spectra from the Lick Observatory radial velocity survey. The authors achieve consistent results by applying excitation and ionization equilibrium with model atmospheres and calibrating rotational velocities via FWHM measurements, finding systematic offsets in T_eff and log g compared to literature, and confirming enhanced metallicity in planet-hosting giants by 0.13 ± 0.03 dex.

ABSTRACT

Context: A radial velocity survey of about 380 G and K giant stars is ongoing at Lick observatory. For each star we have a high signal to noise ratio template spectrum, which we use to determine spectroscopic stellar parameters. Aim: The aim of this paper is to present spectroscopic stellar parameters, i.e. effective temperature, surface gravity, metallicity and rotational velocity for our sample of G and K giant stars. Methods: Effective temperatures, surface gravities and metallicities are determined from the equivalent width of iron lines. Rotational velocities are determined from the full width at half maximum (FWHM) of moderate spectral lines. A calibration between the FWHM and total broadening (rotational velocity and macro turbulence) is obtained from stars in common between our sample and the sample from Gray (1989). Results: The metallicity we derive is essentially equal to the literature values, while the effective temperature and surface gravity are slightly higher by 56 K and 0.15 dex, respectively. Our rotational velocities are comparable with the ones obtained by Gray (1989), but somewhat higher than the ones obtained by de Medeiros & Mayor (1999), consistent with the different diagnostics used. Conclusions: We are able to determine spectroscopic stellar parameters for about 380 G and K giant stars in a uniform way (112 stars are being analysed spectroscopically for the first time). For stars available in the literature, we find reasonable agreement between literature values and values determined in the present work. In addition, we show that the metallicity enhancement of companion hosting stars might also be valid for giant stars, with the planet-hosting giants being 0.13 +/- 0.03 dex (i.e. 35 +/- 10%) more metal-rich than our total sample of stars.

Motivation & Objective

• To determine accurate spectroscopic stellar parameters—effective temperature, surface gravity, metallicity, and rotational velocity—for a large sample of G and K giant stars.
• To improve consistency and reduce systematic errors in stellar parameter determination by applying excitation and ionization equilibrium with model atmospheres.
• To calibrate rotational velocities using FWHM measurements of moderate lines and macro turbulence relations, ensuring compatibility with prior studies.
• To investigate whether planet-hosting giant stars exhibit enhanced metallicity, as observed in main-sequence stars.
• To provide a homogeneous parameter set for 112 stars analyzed spectroscopically for the first time.

Proposed method

• Determine effective temperature, surface gravity, and metallicity by imposing excitation and ionization equilibrium on iron line equivalent widths using LTE model atmospheres.
• Use a calibration between FWHM of moderate spectral lines and total line broadening (vsini + macroturbulence) derived from stars in common with Gray (1989).
• Estimate macroturbulence using temperature-dependent relations from Gray (2005), with separate calibrations for luminosity classes III and IV.
• Assign luminosity class based on log g and T_eff to select appropriate macroturbulence relation.
• Apply a 1:1 comparison with literature values to validate results, particularly for T_eff, log g, and vsini.
• Use high-resolution, high-S/N template spectra from the Lick radial velocity survey to ensure measurement precision.

Experimental results

Research questions

• RQ1 Do the derived spectroscopic stellar parameters for G and K giant stars show systematic offsets compared to literature values?
• RQ2 Is the metallicity of giant stars hosting planetary companions significantly higher than that of non-hosting giants?
• RQ3 How do rotational velocities derived from FWHM measurements compare with those from other methods, such as cross-correlation or Fourier transforms?
• RQ4 To what extent do model atmosphere uncertainties affect T_eff and log g determinations in cooler giant stars?
• RQ5 Can a consistent and homogeneous parameter set be established for a large sample of giant stars, including those previously unanalyzed?

Key findings

• The effective temperature derived in this work is approximately 56 K higher than literature values, with the largest offset observed in cooler stars.
• Surface gravity values are 0.15 dex higher than literature values, consistent with the temperature shift.
• Metallicity measurements show excellent agreement with literature values, indicating no severe systematic errors in the metallicity scale.
• Rotational velocities are consistent with those from Gray (1989) but systematically higher than those from de Medeiros & Mayor (1999), due to differences in diagnostic methods.
• The mean metallicity of planet-hosting giant stars is 0.13 ± 0.03 dex higher than the overall sample, supporting the metallicity-planet connection in evolved stars.
• For 112 stars, this work provides the first spectroscopic analysis of stellar parameters, enhancing the homogeneous dataset for giant star studies.