[Paper Review] Detailed chemical composition of Galactic Cepheids. A determination of the Galactic abundance gradient in the 8-12 kpc region

Bertrand Lemasle, P. François+4 more|ArXiv.org|Jan 17, 2007

Stellar, planetary, and galactic studies42 references47 citations

TL;DR

This study presents high-resolution spectroscopic abundance measurements of iron and six light elements (O, Na, Mg, Al, Si, Ca) in 30 Galactic Cepheids using FEROS data from the ESO 1.52m telescope. It determines a Galactic radial abundance gradient of -0.061 dex kpc⁻¹ in the 8–12 kpc region, confirming a steep iron gradient consistent with other Cepheid and open cluster studies, while highlighting discrepancies in oxygen and sodium gradients compared to prior B-type star tracers.

ABSTRACT

The recent introduction of high resolution/large spectral range spectrographs provided the opportunity to investigate in detail the chemical composition of classical Cepheids. This paper is focussed on new abundance determinations for iron and 6 light metals (O, Na, Mg, Al, Si, Ca) in 30 Galactic Cepheids. We also give a new estimate of the Galactic radial abundance gradient. The stellar effective temperatures have been determined using the method of line depth ratios, while the surface gravity and the microturbulent velocity v$_{t}$ by imposing the ionization balance between Fe I and Fe II and the help of the curves of growth. Abundances were calculated with classical LTE atmosphere models. Abundances are obtained with RMS accuracies of the order of 0.05-0.10 dex for Fe, and 0.05-0.20 dex for the other elements. Cepheids in our sample have solar-like abundances and current measurements agree quite-well with previous determinations. We computed ``single zone'' Galactic radial abundance gradients for the 8-12 kpc region and found a slope for iron of -0.061 dex kpc$^{-1}$.

Motivation & Objective

• To determine precise chemical abundances of iron and six light elements (O, Na, Mg, Al, Si, Ca) in a sample of 30 Galactic Cepheids using high-resolution spectroscopy.
• To measure the Galactic radial abundance gradient in the 8–12 kpc region, focusing on iron and key light elements.
• To compare the derived abundance gradients with previous studies based on different tracers (e.g., B-type stars, open clusters, planetary nebulae) to assess consistency and potential evolution of the gradient.
• To evaluate the reliability of Cepheids as tracers of the present-day Galactic disk abundance distribution, given their young age and luminosity.
• To investigate whether discrepancies in reported abundance gradients stem from measurement uncertainties or intrinsic variations linked to stellar age or radial distribution.

Proposed method

• Stellar parameters (effective temperature, surface gravity, microturbulent velocity) were derived using line depth ratios for temperature and ionization balance between Fe I and Fe II for gravity and microturbulence.
• Abundances were computed using classical LTE atmospheric models and curve-of-growth techniques applied to high-resolution (R ≈ 48,000) spectra from the FEROS spectrograph.
• Spectra were obtained with the 1.52m ESO telescope at La Silla, covering 370–920 nm with signal-to-noise ratios between 40 and 136.
• The radial abundance gradient was computed using a 'single zone' model for the 8–12 kpc region, with uncertainties estimated via bootstrap resampling.
• Results were compared with previous studies using Cepheids (Andrievsky et al. 2002a), B-type stars, open clusters, and HII regions to assess consistency and discrepancies.
• The analysis included both the current sample and an extended sample incorporating Cepheids from Andrievsky et al. (2002a) to improve statistical robustness.

Experimental results

Research questions

• RQ1 What is the precise radial abundance gradient for iron in the 8–12 kpc region of the Galactic disk, as traced by Cepheids?
• RQ2 How do the abundance gradients for light elements (O, Na, Mg, Al, Si, Ca) compare with that of iron in the same radial range?
• RQ3 Are the abundance gradients derived from Cepheids consistent with those derived from other tracers such as B-type stars or open clusters?
• RQ4 Do discrepancies between previous studies on the abundance gradient stem from measurement uncertainties or intrinsic differences in tracer populations?
• RQ5 To what extent do Cepheids provide a reliable and homogeneous probe of the current Galactic disk abundance distribution?

Key findings

• The iron abundance gradient in the 8–12 kpc region is measured at -0.061 dex kpc⁻¹, with a slightly flatter value of -0.056 dex kpc⁻¹ when including 69 Cepheids with homogeneous distance determinations.
• The derived iron gradient is steeper than the value reported by Andrievsky et al. (2002a) for the central region of the multi-zonal gradient model.
• For oxygen, the gradient is -0.041 ± 0.034 dex kpc⁻¹, which is steeper than the -0.022 ± 0.009 dex kpc⁻¹ found by Andrievsky et al. (2002a), and inconsistent with flatter values reported by Fitzsimmons et al. (1992) and Kilian-Montenbruck et al. (1994).
• Sodium shows a gradient of -0.042 ± 0.029 dex kpc⁻¹, steeper than the -0.023 ± 0.006 dex kpc⁻¹ reported by Andrievsky et al. (2002a), indicating potential discrepancies in tracer populations or measurement systems.
• Silicon and calcium gradients are in good agreement with previous studies: Si at -0.031 ± 0.029 dex kpc⁻¹ and Ca at -0.014 ± 0.029 dex kpc⁻¹, matching values from Andrievsky et al. (2002a) within uncertainties.
• The overall radial gradient computed over a broader baseline is -0.070 ± 0.008 dex kpc⁻¹, consistent with other studies using open clusters and Cepheids, supporting a steep gradient in the solar neighborhood.