EMCep的快周期光变

１９９０年和１９９２年本文作者用北京天文台兴隆站６０厘米反光望远镜对Ｂｅ星ＥＭＣｅｐ作了ＵＢＶ光电测光，取得数据２７７组。结合历年来文献给出的光变极小时刻数据作了Ｏ－Ｃ分析，得到了改进的周期为０．８０６１８４天或单波周期为０．４０３０９２天。分析还表明可能存在周期变快的趋势，且周期变率ｄＰ／ｄｔ＝－８．８５１４×１０－１０；光变曲线的形状呈现出极小阶段尖锐而极大阶段较宽。此外Ｕ－Ｂ色指数随光变周期位相有可察觉的变化。

RAPID PERIODIC LIGHT VARIATIONS OF EM CEP

UBV photometric observations of EM Cep were made with 60 cm telescope at Beijing Astronomical Observatory in 1990 and 1992.277 data sets have been obtained.HD208218 was used as the comparison star.Journal of the observations is presented in Table 1.The time of minimum light is derived at HJD 2448912.0452.Combining this value with all published ones previously,an improved linear ephemeris is derived with a RMS of 0.01382:T min.light=HJD 2440134.7338+0.8061840*E From this ephemeris the values of O-C are given in Table 2,the O-C diagram for this improved period 0.806184 days is shown in Figure 1.For a comparision,the residuals of O-C from period 0.806187 days sungested in 1980 by Breinhorst and Karimie 10] are also presented.Based on our observational data,the phase diagrams of EM Cep plotted with periods 0.403092 days and 0.806184 days (two minima and two manima in each cycle) are displayed in Figure 2.The phase diagrame of (B-V) and (U-B) are plotted in Figure 3.From our observations,the features of light variations of EM Cep are summarised as follows:(1)The period of light variations seems to be quite stable covering an interval of more than 34 years,nevertheless,it is of interest to note that there is also a possibility of decrease tendency of the period.We have derived an ephemeris by using a binomial fitting with a RMS of 0.01155:T min.light=HJD 2440134.7368+0.8061861*E-3.5679*10-10*E2 it means the dp/dt=-8.8514*10-10.Figure 4 demonstrats the O-C diagram.Comparing this figure with figure 1 and also their fitting errors,it seems that the binomial fitting is better than the linear more or less.We expect to confirm this results in further observations.(2)The light-curve of EM Cep has such a shape:the minima of it are quite sharp and maxima of it are flater.This indicates that it may be difficult to be interpreted by using the pulsation model.(3)It can be seen from Figure 3,EM Cep exhibits phase-dependent color variations in U passband,which is reminiscent of the light variations of SIGMA Ori E and favours the oblique rotator model for interpretation.