Astronomy and Astrophysics, volume 294, 135-154 (1995/2-1)
Simultaneous photometric and spectroscopic monitoring of rapid variations of the Be star η Centauri.
STEFL S., BAADE D., HARMANEC P. and BALONA L.A.
Abstract (from CDS):
Result. of the simultaneous monitoring of line-profile and light and colour variations of the bright southern Be star ηCen in May-June 1992 are reported. The new data were analyzed along with numerous earlier b and uvby observations of the star, secured between 1987 and 1991. The brightness of ηCen varies on three distinct time scales but only rapid variations on a time scale of hours to days are analyzed here. They can be described as a superposition of a periodic component with period 0.6424241d, which is secularly stable over the whole six-year interval covered by our data, and systematic but obviously aperiodic cycle-to-cycle disturbances. The 0.6424-d light curve is slightly non-sinusoidal (light minima occur 0.64P after light maxima) and has an average peak-to-peak amplitude of 0.05mag in y, b and v and about 0.10mag in u. The u-b and 1 indices also vary with the same period, with full amplitudes of 0.04mag and 0.03mag, respectively, while the variations of b-y and .1mag - if present - are within 0.01mag only. During light maximum, the star is bluest in u-b. The radial velocity of Si III 455.26nm as measured in the outer wings of the profile exhibits a nearly sinusoidal variation with the 0.6424-d period and definitively excludes the triple-wave photometric period of 1.927d considered earlier. For variations of other line-profile parameters of the Si III 455.26 nm line, the most significant periods differ by up to ±5% from the 0.6424-d period. More extended spectroscopic material will be needed to confirm our preliminary conclusion that this disparity is of stochastic nature only and that the entire line-profile variability may be reconciled with the photometric period. The exactly simultaneous monitoring shows that radial velocity, first moment, equivalent width, and line depth of the Si III 455.26nm line increase when the star is getting brighter and bluer. Very pronounced higher-order line-profile variations of the Si III line were also detected. They have the character of apparent absorption or emission bumps, which move across the line profile from blue to red. Most of them disappear close to the line center. The analysis of their radial velocities leads to a super-period, which is close to the photometric one. Occurrence and strength of the bumps also seem to follow the same period. The light, colour, radial-velocity and equivalent-width variations can only be due to the low-order line profile variations or duplicity. The observed radial-velocity amplitude seems to be larger than expected for the rotational modulation and nonradial pulsation models. The equivalent-width variation cannot be understood unless a phase-dependent microturbulent velocity is assumed in both models. Available data cannot also exclude a model of a contact binary with 0.6-day orbital period.
stars: Be - stars, activity - stars: individual: η Cen
VizieR on-line data:
<Available at CDS (J/A+A/294/135): table1 table3 table4>
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