HD 203608, a quiet asteroseismic target in the old galactic disk

TL;DR

This study presents a detailed asteroseismic analysis of the old, low-metallicity star HD 203608, revealing its oscillation properties and refining its age estimate through new observational and analytical methods.

Contribution

It introduces a new statistical approach for extracting significant oscillation peaks and provides detailed mode identification for HD 203608, improving stellar parameter constraints.

Findings

Detected significant oscillation excess power between 1.5 and 3.0 mHz.

Measured a large frequency spacing of about 120.4 μHz.

Determined the star's age as approximately 7.25 Gyr.

Abstract

A short observing run with the spectrometer Harps at the ESO 3.6-m telescope was conducted in order to continue exploring the asteroseismic properties of F type stars. In fact, Doppler observations of F type on the main sequence are demanding and remain currently limited to a single case (HD 49933). Comparison with photometric results obtained with the CoRoT mission on similar stars will be possible with an enhanced set of observations. We selected the 4th magnitude F8V star HD 203608, in order to investigate the oscillating properties of a low-metallicity star of the old galactic disk. A 5-night asteroseismic observation program has been conducted in August 2006 with Harps. Spectra were reduced with the on-line data reduction software provided by the instrument. A new statistical approach has been developed for extracting the significant peaks in the Fourier domain. The oscillation spectrum shows a significant excess power in the frequency range [1.5, 3.0 mHz]. It exhibits a large spacing about 120.4 $\mu$Hz at 2.5 mHz. Variations of the large spacing with frequency are clearly identified, which require an adapted asymptotic development. The modes identification is based on the unambiguous signature of 15 modes with $\ell = 0$ and 1. This observation shows the potential diagnostic of asteroseismic constraints. Including them in the stellar modeling enhances significantly the precision on the physical parameters of \cible, resulting in a much more precise position in the HR diagram. The age of the star is now determined in the range $7.25\pm0.07$ Gyr.