3 Inter binary colliding loops

Figure 4: Details of Fig. 1 around 11730 JD. At that epoch close to the passage at periastron we monitored an activity lasting about a week. The separation between consecutive peaks is 3.6 and 3.4 days.

Figure 5: Sketch of the binary system V773 Tau. The orbit parameters are those of Table 2 by Welty (1995). One star is plotted in the focus of a slightly eccentric orbit, the other star is shown at its four different positions along the orbit. The coronae, here simplified as spheres, may interact and produce flares.

A possible scenario which can explain our result of the 52 $\pm$ 5 days periodicity is that the two stars, which have an orbital period of 51.075 days, possess quite large loops which collide at periastron. While for simplicity in the sketch of Fig. 5 we show spherical coronae, in reality the coronae are rather asymmetric. In fact, the observed 3.4 days modulation indicates quite confined loops coming in and out of the line of sight while the stars rotate. The extension of the loops should be large enough to allow at least for two or three collisions to explain the observed consecutive activity of about a week. In Fig. 5 we see that the separation at the periastron is about 56 $R_{\odot}$, while at the apoastron it is about 95 $R_{\odot}$. The difference between these two distances is appreciable. The question is: can one star have loops of about 25-30 $R_{\odot}$?

The pressure scale-height H is normally derived assuming constant gravity, that is $g= G{M\over r^2}$, where rvaries from the stellar radius R_* to R_*+H, is set equal to $g\simeq G{M\over R_*^2}$. However the resulting value for the pressure scale-height is underestimated for the T Tauri stars: for V773 Tau a value lower than $15~R_{\odot}$ results, even for the highest temperatures (Skinner et al. 1997), whereas VLBI measurements indicate $H> 24~R_{\odot}$(Phillips et al. 1996). That is because of the assumption of constant gravity ( $r\simeq R_*$) that actually implies $H \ll R_*$ producing as a result low values for H. Assuming the correct value of the gravitational force the pressure scale height becomes:

$$% H= {0.072 {R_*^2 T \over M} \over 1- 0.072 {R_* T\over M}}$$ (1)

with H and R_* in $R_{\odot}$, M in $M_{\odot}$ and T in 10⁶ K. This general equation for typical values of $M= 1~M_{\odot}$, $T=6.5 \times 10^6$ K and a stellar radius $R_*=2~R_{\odot}$, gives the result $H=30~R_{\odot}$. In conclusion, such a scenario of stars, with giant loops colliding at the periastron, does not contradict the theory.