Browsing by Author "Sterken, C"

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    Coupled long-term photometric and V/R variations in Be stars: evidence for prograde global one-armed disk oscillations
    (1997) Mennickent, RE; Sterken, C; Vogt, N
    We review theoretical work on global oscillations of Be star disks and summarize possible observational tests. In addition, we analyze existing records on photometric data and V/R ratio for a sample of 6 V/R variable Be stars. Five stars showed coupled long-term photometric-V/R variations. In all cases the photometric extrema coincided with times of V = R transits, the fainter state being followed by a V < R phase. V 1294 Aql, 48 Lib and MXPup showed anticorrelated c(1) and b - y colors during light cycles, the stars being redder in b - y when brightest. 48 Lib shows a complex color behaviour, exhibiting two minima during a V/R cycle and a pronounced b - y drop just at maximum light. In addition, this star shows enigmatic, non-periodic, eclipse-like features in its light curve. In general, the V/R-photometric observations can bi grouped in three main groups, depending on the degree of correlation exhibited by the V/R ratio and the photometric fluxes and the rate of change of the u flux. These groups are compatible with prograde global disk oscillations observed under different system inclinations.
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    Long-term photometry of Be stars -: III.: Evidence for periodic outbursts of λ Eri and photometric activity in HR 2142
    (1998) Mennickent, RE; Sterken, C; Vogt, N
    Stromgren differential uvby photometry of lambda Eri and HR 2142 is analyzed. The data were taken during the time interval 1983-1994 in the framework of the Long-Term Photometry of Variables project at La Silla. lambda Eri exhibited four photometric events (probably associated with line emission episodes) that last several months and which were characterized by a common pattern of change in all passbands (similar to +/-10(-3) mag/day) along with an apparent decrease of the amplitude of the short-term variability and an increase of the stellar temperature. A. search for periodicities reveals that these events fit a 486 d recurrence time. On the other hand, HR 2142 showed a relatively "quiet" long-term photometric behaviour, the orbital period being detected only marginally in the photometric data set. In contrast, a 344 d period optimally fits the data. We discuss the possibility of an internal "clock" regulating the outburst activity of lambda Eri and show that the photometric variability of HR 2142 can hardly be explained by its interacting binary nature. We also discuss an empirical relationship found between partial derivative c(1)/partial derivative u and v sini in a sample of 11 Be stars.
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    Long-term photometry of be stars .2. Periodic variations on time scales of days to months
    (1996) Sterken, C; Vogt, N; Mennickent, RE
    Stromgren differential uvby photometry of 4 Be stars is analysed. The data were taken during the time interval 1983-1994 in the framework of the Long-Term Photometry of Variables project at La Silla. We detected in these four stars periodic photometric variations with periods between 4 and 93 days. The two cases with the longest periods (FT CMa with P = 87.(d)9 and FY CMa with P = 92.(d)7) did not have any previously known periodic variability. In three of the four cases the periodic oscillation was coherent and always present during the 10 years of monitoring.