2014MNRAS.439.4057F

We present new observations of the XZ Tau system made at high angular resolution (55 mas) with the Karl G. Jansky Very Large Array (VLA) at a wavelength of 7 mm. Observations of XZ Tau made with the VLA in 2004 appeared to show a triple-star system, with XZ Tau A resolved into two sources, XZ Tau A and XZ Tau C. The angular separation of XZ Tau A and C (0.09 arcsec) suggested a projected orbital separation of around 13 au with a possible orbital period of around 40 yr. Our follow-up observations were obtained approximately 8 yr later, a fifth of this putative orbital period, and should therefore allow us to constrain the orbital parameters of XZ Tau C, and evaluate the possibility that a recent periastron passage of C coincided with the launch of extended optical outflows from XZ Tau A. Despite improved sensitivity and resolution, as compared with the 2004 observations, we find no evidence of XZ Tau C in our data. Components A and B are detected with a signal-to-noise ratio greater than 10; their orbital motions are consistent with previous studies of the system, although the emission from XZ Tau A appears to be weaker. Three possible interpretations are offered: either XZ Tau C is transiting XZ Tau A, which is broadly consistent with the periastron passage hypothesis, or the emission seen in 2004 was that of a transient, or XZ Tau C does not exist. A fourth interpretation, that XZ Tau C was ejected from the system, is dismissed due to the lack of angular momentum redistribution in the orbits of XZ Tau A and XZ Tau B that would result from such an event. Transients are rare but cannot be ruled out in a T Tauri system known to exhibit variable behaviour. Our observations are insufficient to distinguish between the remaining possibilities, at least not until we obtain further VLA observations at a sufficiently later time. A further non-detection would allow us to reject the transit hypothesis, and the periastron passage of XZ Tau C as agent of XZ Tau A's outflows.