Two-site simultaneous observations of solar-like stellar oscillations in radial velocities with a Fabry-Pérot calibration system ^*

¹ Service d'Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France
² Observatoire de Haute Provence, 04870 Saint Michel L'Observatoire, France
³ Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland

Corresponding author: J.-L. Bertaux, jean-loup.bertaux@aerov.jussieu.fr

Received: 15 July 2002
Accepted: 1 April 2003

Abstract

We report simultaneous measurements of the radial velocity V_r of a star, ζ Herculis, with two different telescopes and spectrometer systems, both located at Observatoire de Haute Provence, France. We believe that we detected for the first time in a solar type star a correlated signal in the time series of V_r observed simultaneously with the two different instruments, due to the stellar oscillations. The objective of our observations was to compare the performance of the two systems, for the detection of stellar oscillations (astero-seismology). Both systems use a large portion of the visible spectrum of the star to increase the number of spectral lines on which to compute variations of V_r, and a white-light channelled spectrum produced by a Fabry-Pérot as a calibration of spectrometer drifts. The two calibration systems that we designed and implemented are however different. With the 193 cm telescope, we calibrated the ELODIE spectrometer with a fixed Fabry-Pérot, while for the EMILIE spectrometer, we use the full AAA (Astronomical Absolute Accelerometer) system of Pierre Connes, with a tunable Fabry-Pérot and various servo-controls. We first show, with three stars observed with ELODIE (Procyon, η Cas and ζ Her), that the auto-correlation function on a single-night time series is clearly significant when the periodogram shows significant peaks (Procyon and ζ Her), while for η Cas, nothing significant is seen in the periodogram nor in the auto-correlation function. Then we show evidence of the correlation of the two simultaneous, but independent, time-series obtained on a single night on ζ Her, that we believe is the first reported clear case of simultaneous detection of stellar oscillations (solar type), which paves the way to future multi-site observations with a network of longitude spread telescopes. Finally, the observed fluctuations are compared, yielding an estimate of the instrumental (plus atmospheric effects) short term random error: <2.1 m s^-1 for EMILIE + AAA system, and <2.9 m s^-1 for ELODIE + fixed Fabry-Pérot system.