Asteroseismology of eta Boötis with the AFOE

As part of a continuing search for pulsations in stars of roughly solar type, we used the AFOE to observe the radial velocity variations of the G5 subgiant eta Boötis, which has been reported to display oscillations in the equivalent width of its hydrogen Balmer lines (Kjeldsen et al. 1995). Our search revealed no clear evidence for pulsations, giving an upper limit on the mode amplitudes that is a few times smaller than the values estimated by Kjeldsen et al.. Results of this work have been published by Brown et al. (1997); The abstract of that paper appears below, along with a power spectrum illustrating the poor agreement between the frequencies reported by Kjeldsen et al. and the power in radial velocity.
Abstract The subgiant eta Boo (G5 IV) has been reported to show p-mode pulsations, as evidenced by variations in the equivalent width of its hydrogen Balmer lines (Kjeldsen et al. 1995}. In an attempt to confirm this report, we observed eta Boo's radial velocity with the AFOE spectrograph for a total of 22 hours spread over 7 successive nights in March 1995. We find no evidence for the presence of excess power at the frequencies reported by Kjeldsen et al.; our upper limit corresponds to typical mode amplitudes of 0.5 m/s, about 3 times smaller than the velocity amplitudes they inferred. Signals with amplitudes larger than 0.5 m/s may be present at other frequencies within the 0 to 1000 uHz range, but evidence for such signals is scanty, and typical mode amplitudes greater than 1.5 m/s are clearly inconsistent with our observations.

(Top panel) A portion of the power spectrum of the radial velocity of eta Boo, plotted to show only the range 700 to 1000 uHz. The frequencies identified as p-modes by Kjeldsen et al. are indicated with vertical dashed lines.
(Bottom panel) The same spectrum, with Kjeldsen et al.'s frequencies, after application of a CLEAN algorithm in an attempt to remove the effects of the window function of the observations.


  1. A Radial Velocity Search for p-Mode Pulsations in eta Boötis, Brown et al., 1997, Astrophys.J.,475, 322.
  2. Solarlike oscillations in eta Boötis, H., Bedding, T.R., Viskum, M. & Frandsen, S., 1995, Astron.J, 109, 1313.
Sylvain G. Korzennik (
Timothy M. Brown (
Last revised Apr 23 1997