Browsing by Author "Vogt, N"
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- ItemAn extensive Δa-photometric survey of southern B and A type bright stars(1998) Vogt, N; Kerschbaum, F; Maitzen, HM; Faundez-Abans, MPhotoelectric photometry of 803 southern BS objects in the aa-system as detection tool for magnetic chemically peculiar (=CP2) stars has been carried out and compared to published spectral types. The statistical yield of such objects detected by both techniques is practically the same. We show that there are several factors which contaminate the search for these stars, but this contamination is only of the order of 10% in both techniques. We find a smooth transition from normal to peculiar stars.
- ItemCoupled long-term photometric and V/R variations in Be stars: evidence for prograde global one-armed disk oscillations(1997) Mennickent, RE; Sterken, C; Vogt, NWe 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.
- ItemHβ photometry of southern CP2 stars(2000) Maitzen, HM; Paunzen, E; Vogt, N; Weiss, WWWe present H beta photometry of 233 southern CP2 stars (covering the magnetic Ap stars according to the definition by Preston 1974) brighter than V < 8.5 mag from the list of Bidelman & MacConnell(1973).
- ItemLong-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, NStromgren 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.
- ItemLong-term photometry of be stars .2. Periodic variations on time scales of days to months(1996) Sterken, C; Vogt, N; Mennickent, REStromgren 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.
- ItemSimultaneous multiwavelength observations of dwarf novae .1. SU Ursae Majoris: Minihumps at a minioutburst?(1996) Echevarria, J; Tovmassian, G; Shara, M; Tapia, M; Bohigas, J; Jones, DHP; Gilmozzi, R; Costero, R; Lopez, JA; Roth, M; Alvarez, M; Rodriguez, LF; DeLara, E; Stover, RJ; MartinezRoger, C; Garzon, F; Asatrian, N; Vogt, N; Szkody, P; Zsoldos, E; Mattei, J; Bateson, FMSU Ursae Majoris-the prototype of a subgroup of dwarf novae that display superhumps in superoutbursts has been observed during an international campaign dedicated to the observation of the first day of outburst of dwarf novae during 1986 February. After the start of a brightening was reported, the star was monitored by IUE, ground-based photometry and spectrophotometry, IR-photometry, and radio observations by VLA. However, it did not undergo a normal outburst or a superoutburst.
- ItemThe cataclysmic variable WW Ceti: physical parameters and periodic phenomena(1997) Tappert, C; Wargau, WF; Hanuschik, RW; Vogt, NWe present spectroscopic and photometric data of the cataclysmic variable WW Get. By comparison of our radial velocity data with earlier published ones we are able to restrict possible orbital periods to the two choices 0.175 806 90(25)(d) and 0.175 289 02(35)(d). We derive as inclination of the system i = 48(11)degrees, which is supported by the fact that the photometric lightcurve shows periodic phenomena folded on the orbital period. These features are identified as a main and an intermediate hump which vary strongly with the brightness level of the system. Our derived periods place them at very unusual phases which are not conform with the normal explanation that the humps are caused by a hot spot which is seen from both front and back. However, our spectroscopy in quiescence indicates that these phasings are real and do not result from an error in the calculation of the ephemerides.
- ItemThe influence of a magnetic field of the secondary star on dwarf nova outbursts(1996) MeyerHofmeister, E; Vogt, N; Meyer, FMany observations indicate that fast rotating late type stars show magnetic activity. We therefore argue that some of the secondary stars in cataclysmic binaries might also have magnetic fields. Such magnetic fields would reach over to the accretion disk around the white dwarf primary. We investigate the effect on dwarf nova outbursts. The magnetic field lines will penetrate the disk and remove angular momentum. This shifts the accumulation of mass towards the inner disk, closer to the white dwarf and therefore leads to a different outburst behaviour, which can be recognized in observed light curves of dwarf novae. We detected a cycle of 18 years in the outburst behaviour of SS Aur, possibly related to magnetic activity. SS Cyg displays a similar but less pronounced cycle of 7 years while no traces of magnetic activity were found for U Gem.