Discovery of non-radial pulsations in the spectroscopic binary Herbig Ae star RS Chamaeleontis

Abstract

Context: To understand the origin of stellar activity in pre-main-sequence Herbig Ae/Be stars and to get a deeper insight into the interior of these enigmatic stars, the pulsational instability strip of Palla and Marconi is investigated. In this article we present a first discovery of non radial pulsations in the Herbig Ae spectroscopic binary star RS Cha. Aims: The goal of the present work is to detect non-radial pulsations in a Herbig Ae star for the first time directly by spectrographic means and to identify the largest amplitude pulsation modes. Methods: The spectroscopic binary Herbig Ae star RS Cha was monitored in quasi-continuous observations during 14 observing nights (Jan. 2006) at the 1 m Mt. John (New Zealand) telescope with the Hercules high-resolution echelle spectrograph. The cumulative exposure time on the star was 44 h, corresponding to 255 individual high-resolution echelle spectra with R = 45 000. Least-square deconvolved spectra (LSD) were obtained for each spectrum, representing the effective photospheric absorption profile modified by pulsations. Difference spectra were calculated by subtracting rotationally broadened artificial profiles, these residual spectra were analysed and non-radial pulsations detected. A subsequent analysis with two complementary methods, namely Fourier Parameter Fit (FPF) and Fourier 2D (F2D) has been performed and first constraints on the pulsation modes derived. Results: For the very first time, we discovered by direct observational means using high-resolution echelle spectroscopy, non-radial oscillations in a Herbig Ae star. In fact, both components of the spectroscopic binary are Herbig Ae stars and both show NRPs. The FPF method identified 2 modes for the primary component with (degree l, azimuthal order m) couples ordered by decreasing probability: f1 = 21.11 d-1 with (l, m) = (11, 11), (11, 9) or (10, 6) and f2 = 30.38 d-1 with (l, m) = ( 10, 6) or (9, 5). The F2D analysis indicates for f1 a degree l = 8-10. For the secondary component, the FPF method identified 3 modes with (l,m) ordered by decreasing probability: f1 = 12.81 d-1 with (l, m) = (2, 1) or (2, 2), f_2b = 19.11 d-1 with (l, m) = (13, 5) or (10, 5) and f3 = 24.56 d-1 with (l, m) = (6, 3) or (6,5). The F2D analysis indicates for f1 a degree l = 2 or 3, but proposes a contradictory identification of f_2b as a radial pulsation (l = 0). Based on observations collected at the 1 m McLellan telescope at Mt John, NZ.