On the nature of the upper atmospheric variability in the rapidly oscillating Ap star HD 134214★

Abstract

Recently, we discovered an entirely new type of variability in the upper atmospheres of rapidly oscillating Ap (roAp) stars. This manifests itself in amplitude modulation of the radial velocities that has not been previously detected in photometric studies of the same stars. To study this new variability further we obtained a full night, 8.85 h, of high time resolution (70 s), high spectral resolution (R = 105 000), high signal-to-noise ratio (on average S/N ∼ 130) data with Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) for the roAp star HD 134214. We also obtained 4.2 h of new photometric data in Johnson B with the South African Astronomical Observatory (SAAO) 0.5-m telescope 2 d later. HD 134214 has been known for years to be singly periodic with a relatively stable amplitude in photometry; it has the highest pulsation frequency of any roAp star of 2.949 mHz (P = 5.65 min). Our new UVES data show this principal frequency, plus five other frequencies in amplitude spectra of rare earth elements lines and the Hα line. The new frequencies are stable over the 8.85 h of observation, and the highest of them ν 2 = 2.782 mHz is the same as found in data taken 2 yr earlier with UVES presented in the discovery paper. The amplitudes of the new frequencies drop faster with atmospheric depth than does the amplitude of the principal frequency, hence explaining why they are generally not seen in broad-band photometric measurements that sample on average more deeply in the atmosphere. Our new photometric measurements also detect ν 2 for the first time in photometric data. Our analysis suggests that the new frequencies are associated with pulsation modes, but the nature of those modes and why they increase in amplitude with atmospheric height more strongly than the principal frequencies is not yet known.

MOST observations of the roAp stars HD 9289, HD 99563, and HD 134214

“…8, our results of a very conservative analysis for HD 134214 show that we can confirm most frequencies reported by Kurtz et al (2007). There is a more or less A135, page 9 of 11 regular frequency spacing that can be found throughout the region between 2600 and 3000 µHz.…”

“…Most of the frequencies line up at ν mod 67.63 µHz ≈ 10 µHz, but the primary frequency ν 1 does not! Considering only the frequencies that are consistent with Kurtz et al (2007), ν 1 is the only mode that clearly seems to relate to a different spherical degree from the remaining frequencies. Moreover, two of the new candidate frequencies seem to continue the ridge of ν 1 towards lower frequencies.…”

“…Schematic pulsation spectrum of HD 134214. Squares indicate the frequencies reported in Kurtz et al (2007). The error bars show our estimated upper limits for their uncertainties derived from their data set length.…”

“…This strongly suggests that HD 134214 pulsates in at least two different dominant spherical degrees. Kurtz et al (2007) could not yet decide between stable and unstable frequencies, and why most of them could not be observed in ground-based photometry. However, from the point of view of the long-timebase MOST observations, the frequencies seem to be stable, and HD 134214 reveals an impressive p-mode spectrum.…”

“…A 10-h run with the MOST satellite could not confirm any of the newly reported signals, but it set an upper 2-σ detection limit of 0.36 mmag for the corresponding amplitudes in the MOST bandpass (Cameron et al 2006). A later spectroscopic run dedicated to HD 134214 (Kurtz et al 2007), was intended to clarify the values of the newly seen frequencies. Due to the relatively short duration of the observations, and the apparent dependence of the frequency solution on the used spectral lines, no general list of new modes could be given.…”