Mode identification from monochromatic amplitude and phase variations for the rapidly pulsating subdwarf B star EC 20338−1925^*

¹ ESO, Karl-Schwarzschild-Str. 2, 85748, Garching bei München Germany
e-mail: srandall@eso.org
² Département de Physique, Université de Montréal, CP 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada
³ Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France

Received: 13 April 2010
Accepted: 15 July 2010

Abstract

We obtain time-series spectrophotometry observations at the VLT with the aim of partially identifying the dominant oscillation modes in the rapidly pulsating subdwarf B star EC 20338−1925 on the basis of monochromatic amplitude and phase variations.

From the data gathered, we detect four previously known pulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2 and 2.3 % of the star’s mean brightness. We also determine the atmospheric parameters of EC 20338−1925 by fitting our non-LTE model atmospheres to an averaged combined spectrum. The inferred parameters are T_eff = 34153 ± 94 K, log g = 5.966 ± 0.017 and log N(He)/N(H) = −1.642 ± 0.022, where the uncertainty estimates quoted refer to the formal fitting errors. Finally, we calculate the observed monochromatic amplitudes and phases for the periodicities extracted using least-squares fitting to the light curves obtained for each wavelength bin.

These observed quantities are then compared to the corresponding theoretical values computed on the basis of dedicated model atmosphere codes and also taking into account non-adiabatic effects. We find that the quality of the data is sufficient to identify the dominant pulsation at 146.9 s as a radial mode, while two of the lower amplitude periodicities must be low-degree modes with ℓ = 0−2. This is the first time that monochromatic amplitudes and phases have been used for mode identification in a subdwarf B star, and the results are highly encouraging.