小犬座AD星的光电观测及其周期变化

PHOTOELECTRIC PHOTOMETRY OF AD CMI AND ITS PERIOD CHANGE

作者对AD CMi进行了大量的V波段光电观测,结果表明其极大和极小光度均有微小的变化.用新定出的12个光度极大时刻与前人的极大时刻综合分析表明该星脉动周期正以(1.89±0.09)×10^(-8)天/年的速率持续地增长着.根据它在"变幅-周期"图上的位置推知该星已脉动至少8..6×10~6年.

From 1981 to 1986, we have got 732 V band photoelectric observations of AD CMi in 6 nights, and have determined 12 times of maxima. Figs. 1 to 4 are the light curves on 4 good nights. Table 1 list all the photoelectric times of maxima available. Using the least squares method we find the linear solution (1) and the quadratic solution (2). In table 1 the Q(O-C) s are much smaller than L(O-C)s, and the parabolic curve in Fig. 5 fits the observations quite well. So, the pulsation period seems to be increasing continually with a rate of (1.89±0.09)×10^(-s) days per year. As we can see in Fig. 5 and table 1, there are still some fluctuation although the metal content of AD CMI is normal. These may be mainly caused by the fourth time of maximum in table 1 which is given by Breger as the new primary ephemeris in consideration of old observations with a linear equation. But actually the period is increasing all the time, so this primary ephemeris must be larger than the true time of maxima at that time. Fig. 6 is the "amplitude-period" diagram of Delta Scuti stars. In this diagram AD CMi is near the middle line, and the distance with border line in short period side is 0.102 days, so the time required for the increase from 0.021 to 0.123 days is T=5.4×10~6 years. This is rather small compared with the main sequence life time of normal A to F type stars. If it is true, the possibility for A star in the instability strip to show high amplitude Delta Scuti type pulsation will be about 0.1%. To date we have found some 50 Delta Scuti variables with amplitudes larger than or near 0.3 mag. With V≤10.5 mag. and the total number of A3 to F5 within this magnitude is about 1.2×10~5, so the incidence of Delta Scuti variables with high amplitude is about 0.04%. These two values are not very different in view of the incompleteness of our sample. It seems that the period variation is mainly caused by stellar evolution. But there are still some problem: according to Breger for the variables in clusters the period variation rate is only about 10^(-9) days per year. Because there are only very few variables in clustere having higher amplitude, so this rate may be confined principally to the low amplitude variables. Unfortunately, the measurement of period variation for low amplitude variables is very difficult, only for 28 And we can find it using the data collected by Garrido et al.: dp/dt=(1.23±0.52)×10^(-8). This is still 10 times larger. Let me suppose there sre really some differences between these two Kinds of Delta Scuti variables, the pulsatiog life time for larger amplitude variables will be much smaller than that of the low amplitude variables, and the age will be about 10~8 years. This is supported by the fact that so far we have found several high amplitude Delta Souti variables only in some relatively old clusters such as NGC 7789 and Omega Centauri.