Astrophys. J., 642, 1117-1130 (2006/May-2)
First attempt at spectroscopic detection of gravity modes in the long-period pulsating subdwarf B star PG 1627+017.
FOR B.-Q., GREEN E.M., O'DONOGHUE D., KISS L.L., RANDALL S.K., FONTAINE G., JACOB A.P., O'TOOLE S.J., HYDE E.A. and BEDDING T.R.
Abstract (from CDS):
The first spectroscopic campaign on a PG 1716 variable (long-period pulsating subdwarf B star) has successfully detected low-level velocity variations due to g-mode pulsations in PG 1627+017, superimposed on the known orbital motion. The strongest velocity mode is barely detectable at 1.0-1.5 km/s, although the target is one of the brightest (V=12.9) and largest amplitude (∼0.03 mag) stars in its class. Forty nights of observations on 2 m class telescopes in Arizona, South Africa, and Australia provided 84 hr of time-series spectroscopy over a time baseline of 53 days, with typical velocity errors of 5-6 km/s. The derived radial velocity amplitude spectrum, after subtracting the orbital component, shows three potential pulsational modes 3-4 σ above the mean noise level of 0.365 km/s, at 7201.0 s (138.87 µHz), 7014.6 s (142.56 µHz), and 7037.3 s (142.10 µHz). Only one feature is statistically likely to be real, but all three are tantalizingly close to, or a 1 day alias of, the three strongest periodicities found in the concurrent photometric campaign. The velocity spectrum also shows an unexpected component at twice the orbital frequency of PG 1627+017, possibly evidence of a slightly elliptical orbit, supporting Edelmann et al.'s recent results for other short-period sdB binaries. We further attempted to detect pulsational variations in the Balmer line amplitudes. The single detected periodicity of 7209 s, although weak, is consistent with theoretical expectations as a function of wavelength, and it rules out a degree index of l=3 or 5 for that mode. Given the extreme weakness of g-mode pulsations in PG1716 stars, we conclude that future efforts will require larger telescopes, higher efficiency spectral monitoring over longer time baselines, improved longitude coverage, and increased radial velocity precision.
Stars: Horizontal-Branch - stars: individual (PG 1627+017) - Stars: Interiors - Stars: Oscillations - Stars: Subdwarfs
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