6.2 Flip-flop cycles

Although active longitudes endure for a long time, the active regions they consist of evolve in size, indicating possible cyclic variations as observed in Doppler images (Berdyugina et al., 1998a1999aJump To The Next Citation Point). While one active longitude reduces its activity level, the other increases, which suggests a redistribution of the spotted area between the opposite hemispheres (see Figure 15View Image). When the active longitudes have about the same activity level a switch of the dominant activity from one longitude to the opposite one occurs. Such a phenomenon was first observed on FK Com (Jetsu et al., 1991) and was tentatively called flip-flop. Berdyugina and Tuominen (1998Jump To The Next Citation Point) have analysed long time series of photometric data for four RS CVn stars and discovered that flip-flops are regularly repeated and, thus, indicate a new type of stellar cycle which is related to active longitudes, i.e., a flip-flop cycle. Korhonen et al. (2002Jump To The Next Citation Point) inferred a flip-flop cycle on FK Com. In addition to RS CVn stars and FK Com, such cycles have been detected on young solar analogues (Berdyugina et al., 2002Jump To The Next Citation PointJärvinen et al., 2005bJump To The Next Citation PointBerdyugina and Järvinen, 2005), examples of which are shown in Figure 11View Image. A list of stars exhibiting a flip-flop phenomenon is given in Table 4
View Image

Figure 15: Doppler images of the RS CVn star II Peg in the pole-on projection illustrating a flip-flop that occurred between the end of 1997 and mid of 1998. From Berdyugina et al. (1999aJump To The Next Citation Point).

The frequency ratio between the spot cycle and flip-flop cycle appears to be different for binary components and single stars. In RS CVn-type stars exhibiting both types of cycles, flip-flops appear to occur at the frequency of the spot cycle, implying the flip-flop cycle to be twice as long as the spot cycle. In young dwarfs flip-flop cycles are 3 – 4 times shorter than the spot cycle.

The solar active longitudes also exhibit a flip-flop cycle. On the Sun major spot activity alternates the active longitudes in about 1 – 3 yr, which results in flip-flop cycles of 3.8 and 3.65 yr in the northern and southern hemispheres, respectively (Berdyugina and Usoskin, 2003Jump To The Next Citation Point). This is about 1/3 of the 11-yr sunspot cycle and agrees with the results obtained for young solar analogues. The difference between flip-flop cycle lengths in the north and south is significant and produces an oscillating effect in the north-south asymmetry on a century time scale.

Star Sp M ∕M ⊙ R∕R ⊙ Ω ∕Ω⊙ Prot, d Pspot, yr Pff, yr Reference
Binary stars
II Peg K2 IV 0.8 3.4 3.8 6.7066 9.3 [1]
IM Peg K2 III 1.5 13.3 1.05 24.65 13 [2]
EI Eri G5 IV ∼ 1.5 > 3.4 13.3 1.9510 9.0 [1]
σ Gem K2 III ∼ 2 ∼ 13 19.604 14.9 [1]
HR 7275 K2 IV ∼ 1 > 8 0.90 28.263 17.5 [1]
HR 1099 K1 IV 9.2 2.841 [3]
HD 37824 K0 III 2.0 12.6 0.5 53.12 ∼ 13.5 [9]
HD 181809 K0 III-IV 1.7 6.5 0.4 59.85 ∼ 4 [9]
Single stars
Sun G2 V 1.0 1.0 1.0 25. 11 3.7 [4]
LQ Hya K2 V ∼ 0.7 ∼ 0.8 16.3 1.601 15 5.2 [5]
AB Dor K0 V ∼ 0.8 ∼ 0.8 50.5 0.515 20 5.5 [6]
EK Dra G1 V ∼ 1 ∼ 1 10.0 2.606 10.5 4.5 [7]
FK Com G5 III ∼ 1.5 ∼ 10 10.8 2.400 13/30 6.4 [8]

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