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Mass-ejection events in Be stars triggered by coupled nonradial pulsation modes

  • D. Baade (a1), Th. Rivinius (a2), A. Pigulski (a3), A. Carciofi (a4) and BEST (a5)...

Abstract

Be stars (for an in-depth review see Rivinius, Carciofi & Martayan 2013) rotate at ⩾80% of the critical velocity and are multi-mode nonradial pulsators. Magnetic dipole fields are not detected, and binaries with periods less than 30 days are rare. The name-giving emission lines form in a Keplerian decretion disk, which is viscously re-accreted and also radiatively ablated unless replenished by outburts of unknown origin.

Months-long, high-cadence space photometry with the BRITE-Constellation nanosatellites (Pablo et al. 2016) of about 10 early-type Be stars reveals the following (cf. Baade et al. 2016a, Baade et al. 2016b):

  • Many Be stars exhibit 1 or 2 so-called Δ frequencies, which are differences between two nonradial-pulsation (NRP) frequencies and much lower (mostly less than 0.1 c/d) than the parent frequencies. The associated light curves are roughly sinusoidal. The amplitudes can exceed that of the sum of the parent amplitudes.
  • Conventional beat patterns also occur.
  • Amplitudes of both Δ and beat frequencies can temporarily be enhanced. Around phases of maximal amplitude the mean brightness is in- or decreased, and the scatter can be enhanced.
  • During high-activity phases (outbursts), broad and dense groups of numerous spikes arise in the power spectra. The two strongest groups often have a frequency ratio near 2. The phase coherence seems to be low.
  • Time coverage (less than half a year) is not yet sufficient to infer whether two Δ or beat frequencies can combine to cause long-lasting (years) superoutbursts (cf. Carciofi et al. 2012).

From these observations it is concluded:

  • The variable mean brightness and the increased Δ-frequency amplitude and scatter trace the amount of near-circumstellar matter.
  • Increase or decrease of mean brightness is aspect-angle dependent (pole-on vs. equator-on).
  • Increased amounts of near-circumstellar matter are due to rotation-assisted mass ejections caused by coupled NRP modes.
  • Observations do not constrain the location of the coupling (atmosphere or stellar interior).
  • Broad frequency groups do not represent stellar pulsation modes but circumstellar variability.
  • Be stars later than B5 are less active and may in some cases even behave differently.

Copyright

References

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Baade, D., Rivinius, Th., Pigulski, A., Carciofi, A., & BEST 2016a, arXiv1610.02200
Baade, D., Rivinius, Th., Pigulski, A., Carciofi, A., Handler, G., et al. 2016b, arXiv1611.01113
Carciofi, A., Bjorkman, J. E., Otero, S., Okazaki, A., Štefl, S., et al. ApJ, 744, L15
Pablo, H., Whittaker, G. N., Popowicz, A., Mochnacki, S. M., et al. 2016, PASP, 128, 125001
Rivinius, Th., Carciofi, A. & Martayan, Ch., 2013, A&ARv, 21, 69
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