5 Discussion and conclusions

From Figs. 2-7, it is found that the synthesized spectra can match the observational ones, except for the Na I D₁, D₂ lines, quite well. The reason for mismatched Na I D₁, D₂ lines has already been discussed by Montes et al. (1997), and they had concluded that the filled-in core of the Na I D₁, D₂ lines could be used as a chromospheric activity indicator. In the subtracted spectra, H$\alpha$ line emission profiles exhibit broad wings that can arise from microflaring (Montes et al. 1998). They result from the large-scale mass motions in the chromosphere, as in another RS CVn-type binary, DM UMa (Hatzes 1995).

In the course of spectral analysis, it was found that the primary's H$_\alpha$ profiles are deeper during orbital phase 0.60-0.93 than those during orbital phase 0.13-0.49. These features mean that the excess absorption of the primary's hydrogen lines mentioned by Huenemoerder et al. (1989) also exists in our observations, although these features are not as clear as those in the spectra of Huenemoerder et al. (1989). In addition, Montes et al.'s (1995) spectrum at phase 0.93 also supports this point. Huenemoerder et al. (1989) suggested that the secondary transfers mass to the primary, and these H$_\alpha$ excess absorptions are either caused by a cloud of material ejected by the impact or by the occultation of the mass stream. Although the absolute parameters of UX Ari derived by Duemmler & Aarum (2001) have demonstrated that the radius of the cooler component is less than its Roche lobe, Huenemoerder et al.'s (1989) interpretation to this feature is still convincing. For the component star with a very active chromosphere and corona, like the secondary of UX Ari, it is very possible that some extended circumstellar material exists that can easily extend beyond the Roche radius and cause the Roche lobe overflow.

In Table 2 and Fig. 8, it can be seen that during the two periods covered by our observations, the level of chromospheric activity is higher around the second quadrature of the binary system (phase 0.75) than at the other orbital phases. This indicates that the chromospheric activity is modulated with orbital phase and the inferred activity longitude area of UX Ari corresponds to the phase interval near the second quadrature of the system. This also is the position where there is excess absorption in the primary's hydrogen lines. The high activity level of UX Ari at this phase interval may be the consequence of the coupling between chromospheric activity and mass transfer activity. Because the Roche lobe overflow is a long-lived behavior, it can be the basis of corotating flux tubes contecting the two components. This makes it easier to create higher-level chromospheric activity. The flare-like event detected here is some weaker than the one detected by Montes et al. (1996).

Among all chromospheric activity indicators, the Ca II IRT (8542) line exhibits different behaviour to others in the second observational campaign, namely during the flare. From the EW₈₅₄₂/EW₈₄₉₈ ratio of Ca II IRT (8542 Å) and Ca II IRT (8498 Å) excess emissions, also listed in Table 2, it can be found that the values of the EW₈₅₄₂/EW₈₄₉₈ ratio are generally small, as for other active stars (Arévalo & Lázaro 1999; Montes et al. 2000), and much smaller during the phase interval of high activity and the flare (see Fig. 9). This indicates that Ca II IRT emissions arise from the plage-like regions, and the EW₈₅₄₂/EW₈₄₉₈ ratio decreases with the increase of chromospheric activity.

Figure 8: The EW of the excess emissions vs. orbital phase for H$\alpha$ and Ca II IRT lines. The solid symbols and open symbols represent the observations in Feb. and Sep.-Nov. 2000, respectively.

Figure 9: The EW8542/EW8498 ratio vs. orbital phase. The solid symbols and open symbols represent the observations in Feb. and Sep.-Nov. 2000, respectively.

Finally, we give a summary in the present study of UX Ari. The activity information is obtained from several optical spectroscopic features. The chromospheric contribution to these chromospheric activity indicators is determined by the spectral subtraction technique. A flare is detected in our observations, and it is the second time the He I D₃ line exhibits emission in the observation of the optical band (the first one was reported by Montes et al. 1996). There is a significant trend for orbital phase modulation of chromospheric emission during our observing campaigns, and an activity longitude area was found around the second quadrature, where the mass from the secondary is transferring to the primary. Perhaps the chromospheric activity and mass transfer activity are affected by each other, and the coupling between the two kinds of activities makes the total system exhibit high activity around the quadratures.

Acknowledgements

The authors would like to thank the anonymous referee for his (her) suggestions and comments, which led to large improvement in the manuscript. Gu is very grateful to Dr. Montes for providing a copy of the STARMOD program. This work is supported by the Applied and Basic Research Foundation of Yunnan Province, China.