Study of a sample of faint Be stars in the exofield of CoRoT

Abstract

Context. The class of Be stars are the epitome of rapid rotators in the main sequence. These stars are privileged candidates for studying the incidence of rotation on the stellar internal structure and on non-radial pulsations. Pulsations are considered possible mechanisms to trigger mass-ejection phenomena required to build up the circumstellar disks of Be stars. Aims. Time series analyses of the light curves of 15 faint Be stars observed with the CoRoT satellite were performed to obtain the distribution of non-radial pulsation (NRP) frequencies in their power spectra at epochs with and without light outbursts and to discriminate pulsations from rotation-related photometric variations. Methods. Standard Fourier techniques were employed to analyze the CoRoT light curves. Fundamental parameters corrected for rapid-rotation effects were used to study the power spectrum as a function of the stellar location in the instability domains of the Hertzsprung–Russell (H-R) diagram. Results. Frequencies are concentrated in separate groups as predicted for g-modes in rapid B-type rotators, except for the two stars that are outside the H-R instability domain. In five objects the variations in the power spectrum are correlated with the time-dependent outbursts characteristics. Time-frequency analysis showed that during the outbursts the amplitudes of stable main frequencies within 0.03 c d−1 intervals strongly change, while transients and/or frequencies of low amplitude appear separated or not separated from the stellar frequencies. The frequency patterns and activities depend on evolution phases: (i) the average separations between groups of frequencies are larger in the zero-age main sequence (ZAMS) than in the terminal age main sequence (TAMS) and are the largest in the middle of the MS phase; (ii) a poor frequency spectrum with f ≲ 1 cd−1 of low amplitude characterizes the stars beyond the TAMS; and (iii) outbursts are seen in stars hotter than B4 spectral type and in the second half of the MS. Conclusions. The two main frequency groups are separated by δf = (1.24 ± 0.28) × frot in agreement with models of prograde sectoral g-modes (m = −1, −2) of intermediate-mass rapid rotators. The changes of amplitudes of individual frequencies and the presence of transients correlated with the outburst events deserve further studies of physical conditions in the subatmospheric layers to establish the relationship between pulsations and sporadic mass-ejection events.

γCas stars: Normal Be stars with discs impacted by the wind of a helium-star companion?

“…There is no such report for γ Cas (apart from the optical broad-band flux). Radial pulsations are not known in Be stars but both short periods are well within the range of NRPs found in other Be stars (Rivinius et al 2016;Baade et al 2017;Semaan et al 2018). Since the 1.216 d variability faded while the 0.403 d variability rose, it is plausible to believe that both are of the same nature, which can, then, only be NRPs.…”

“…Since the 1.216 d variability faded while the 0.403 d variability rose, it is plausible to believe that both are of the same nature, which can, then, only be NRPs. In fact, space photometry (Rivinius et al 2016;Baade et al 2017;Semaan et al 2018) has detected multiple low-order NRP modes in many Be stars over the full range of B-type stars.…”

Short-term variability and mass loss in Be stars

“…Still cooler is β CMi at B8, for which MOST recorded many NRP modes with similarly low amplitudes (Saio et al 2007). In observations of the B7 V star CoRoT 101486436, Semaan et al (2018) find this star's variability to resemble that of β CMi; however, the peak-to-valley amplitude of a 0.03 c/d beat (perhaps: difference) frequency has an amplitude of 12 000 ppm with individual frequencies near 5 c/d reaching up to 3000 ppm.…”

“…Broad consensus exists that the near-critical rotation (Frémat et al 2005;Meilland et al 2012;Rivinius et al 2013) is a necessary condition for the mass loss. Numerous observational studies Huat et al 2009;Baade et al 2017Baade et al , 2018a suggest that much of the star-to-disk mass-transfer process takes place in discrete events, which cover a wide range in cadence and amplitude (Semaan et al 2018;Bernhard et al 2018). Although having the appearance of mass-loss events, they may actually be more accurately described as angular-momentum-loss events.…”

“…Frequency groups are characteristic of Be stars (Balona et al 2011;Baade et al 2017;Semaan et al 2018) but do not seem limited to them , and not all pulsating Be stars display frequency groups. The latter authors proposed that frequency groups may be understood as clusters of combination frequencies of NRP modes.…”