Seismology and activity of the F type star HD 49933
1er février 2005
A 10-night asteroseismic observation programme has been conducted in January 2004 with the spectrometer HARPS at the ES0 3.6-m telescope. The selected target, the 6th magnitude F5V star HD 49933, was chosen among the prime candidates of COROT, the European space mission dedicated to characterize stellar oscillations mode with high precision photometry measurements. This star shows important line profiles variations, indicating a surprisingly high activity with respect to its low rotation rate. However, with the help of tools developed for disentangling the signatures of activity and oscillations, we are able to observe its oscillation spectrum in the frequency range [1.2, 2.2 mHz]. We measure the large separation (88.7
The signature of stellar activity occuring at low frequency should not affect the seismic signal at higher frequency. However, the very large distortions of the lines related to activity may mimic rapidly varying tiny Doppler shifts. A dedicated treatment was developped and applied to extract the information of a pure Doppler signal. It confirmed that the observed Fourier spectrum is not affected by distortions.
[hd-f2.png]
Figure 2 : Fourier spectrum of the velocity time series calculated with the optimum method. The window function is shown in the inset. No high pass filtering was applied, except for the removal of the mean velocity slope of each night.
The Fourier spectrum shows an excess power in the frequency range [1.2,2.2 mHz], that represents the seismic signature (Fig. 2). Its analysis exhibits a clear signature around 88.7±0.4 microHz, representative of the seismic large separation. The maximum oscillation amplitude is about 0.4±0.1 m/s rms. The agreement with the predicted scaling (0.6 m/s rms) appears to be marginal.We finally have been able to measure with HARPS the p-modes oscillation amplitude and the large separation of HD 49933. This confirms that ground-based asteroseismologic observations with HARPS provide excellent solar-like oscillations studies, even for a faint (mv=5.7) F type star exhibiting activity. For a star such as HD 49933, ground-based spectrometric measurements are less efficient than for cooler types stars, with more and narrower lines. However, we have demonstrated that spectrometric measurements are the appropriate tools for disentangling the signatures of oscillations and activity.
1) LESIA Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (CNRS/Université Paris VI/Université Paris VII/Observatoire de Paris). 2) Laboratoire d’Astrophysique de Marseille 3) Laboratoire Cassiopée de cosmologie, astrophysique stellaire et solaire, de planétologie et de mécanique des fluides 4) Observatoire de Genève Contact Benoît Mosser (Observatoire de Paris, LESIA)
Dernière modification le 4 mars 2013