| 其他摘要 | The most significant feature on the surface of the Sun is sunspot. Numerous studies reveal that late-type stars also have starspot activities. Starspots are the tracers of stellar magnetic activity. Studies on them are helpful to consummate and constrain the dynamo theory. Doppler imaging is a technique that can derive the distribution of starspots on the surface of a star from its time-series line profiles. It is an useful technique for the studies on starspot activity. In order to investigate how starspot activities of close binaries are related to stellar spectral types, evolution stages, and interactions between their components, we carried out high-dispersion observations on three close binaries with different orbital period, which are II Peg, SZ Psc and ER Vul, using the coud\'{e} echelle spectrograph on Xinglong 2.16m telescope. The LSD technique was also applied to improve S/N. Then the spot map of them were obtain by using the Doppler imaging code DoTS.II Peg is a single-lined binary system consisting of a K2 subgiant primary and an unseen M dwarf, in an orbital period of 6.72 d. From 2004 to 2008, we obtained 5 spot maps of II Peg and found that the latitude distribution of its starspots is very wide. The starspot activities were located from the equator to the pole of the star. The low-latitude starspots were evolving very rapidly, while the high-latitude spots were somewhat stable. We do not find a stable active longitude as well as flip-flop phenomenon from the surface images of II Peg. The longitude distribution of its starspots is freedom, which is different from that before 2004, indicating that II Peg is in a different activity epoch. Besides, we also analyzed the 7055A and 8860A TiO bands to estimate the starspot coverage in these observing seasons. The results show that the coverage of starspots is 36%-46%. These values are obviously higher than that derived from the Doppler images, which may indicates the presence of the symmetric structure of starspots that can not be detected by the Doppler imaging technique. The phase-dependent variation of starspot coverage of II Peg derived from TiO bands analysis is consistent with the results of the Doppler imaging. Besides, we find that the conjunction time of the binary system is different from the one derived by Berdyugina et al. (1998)，indicating the period change of II Peg.SZ Psc is a double-lined binary system composed of an F8 main-sequence star and a K1 subgiant star, in an orbital period of 3.97 d. the rotation of the F hotter component is non-synchronous. We obtained the LSD profiles with high S/N for 2004 November, 2006 September--December and found clear absorption features, which were contributed by the third component of the system. It confirmed that SZ Psc is a triple system. After removing the contribution of the third component of SZ Psc, we obtained the surface images of the K cooler component. The results reveal significant starspot activities on the K star, which is consistent with that derived from previous photometric studies. However, our Doppler images offer us a more complicated distribution of starspots. There are numerous spots with relatively small area on the K star. During our observations, we do not find clear relationship between the position of starspots and the orbital phases. We revealed that the starspots of K star show migrations in longitude in 2006 September--December. Besides, our combined data set of 2006 November and December show starspot activities on the F hotter component. However, its spot features are hardly resolved by the Doppler imaging, due to the slow rotational velocity of the star.ER Vul is a double-lined binary system consisting of a G0 and a G5 main-sequence stars, in an orbital period of 0.698 d. Its two components are very similar to the Sun. We derived the Doppler images of both components for 2004 November, 2006 September, 2008 November. The results reveal significant starspot activities on both components of ER Vul. As rapidly rotating stars, two components show similar latitude distribution of starspots to II Peg and SZ Psc. The high-latitude and low-latitude active regions on the surfaces of each star coexisted during our observations. Both of the 2006 and 2008 images, which have good phase sampling, show pronounced polar starspot activities. The low- and intermediate-latitude starspots of both components do not show migrations in longitude. They are concentrated in the hemisphere that facing the other star. The non-uniform longitude spot distribution show that the starspot activity of ER Vul is dominated by the interactions between its two components. |
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