The Case of the Missing Star


When stars are born and grow in close proximity to each other, the presence of such massive neighbours can have significant consequences, especially while the stars are young.  The gravity of a neighbour can trigger flows of gas and energy off a star’s surface, spraying it far into space.

The XZ Tau star system has these impressive outflows (see below).  These messy, bubbly flows fizz out far beyond the orbits of XZ Tau A and XZ Tau B, two stars long known to exist in the system (you can see them as the two bright spots in the lower left corner of the images).  XZ Tau B orbits XZ Tau A at about the same distance that Pluto orbits the Sun.  The structure inside the bubble suggests that there was a strong pulse at some point in the 1980s that launched a vast chunk of stuff.  At one time, it was thought that the pulse was caused by XZ Tau B coming close to XZ Tau A at a specific stage in its orbit, but it was later shown that the timings didn’t match.

XZ Tau's outflows of hot gas, as imaged by the Hubble Space Telescope (image Credit: John Krist (STScI) et al.)
XZ Tau’s outflows of hot gas, as imaged by the Hubble Space Telescope (image Credit: John Krist (STScI) et al.)

So what is causing the outflows? Recently, a third star was detected in the XZ Tau system, XZ Tau C. This star was detected using the Very Large Array of radio telescopes.  It was thought to have been hidden from optical telescopes like Hubble because it was shrouded in a very thick cloud of dust and gas.  It was found to be much closer to XZ Tau A, so perhaps it collected camouflaging material direct from the source of the outflow.

The first detection of XZ Tau C at radio wavelengths.  XZ Tau A and C are in the bottom left corner of the right hand image, and B is in the top right corner.  (Image Credit: Gonzalez et al, VLA)
The first detection of XZ Tau C at radio wavelengths. Each image shows the system at different wavelengths – XZ Tau C is only detected at the longest wavelength (7 mm).  XZ Tau A and C are in the bottom left corner of the right hand image, and B is in the top right corner. (Image Credit: Gonzalez et al, VLA)

Here’s where I come in.  The original observation of XZ Tau C was made in 2004, and we wanted to see if we could detect it again.  Our observations (using the upgraded Very Large Array) were made in 2012, so we expected to see XZ Tau C move from its previous position.  Comparing 2004 and 2012 data would allow us to estimate the orbit of C around A.  When XZ Tau C was detected, the astronomers who made the detection speculated that its orbit would have the right period to be the cause of the pulse in XZ Tau A’s outflows.

So, we made our observations, and found… no XZ Tau C.

Our radio observations of XZ Tau.  Despite observing with even higher sensitivity, we saw no sign of XZ Tau C (Image Credit: Forgan et al., VLA)
Our radio observations of XZ Tau. Despite observing with even higher sensitivity, we saw no sign of XZ Tau C (Image Credit: Forgan et al., VLA)

Despite using a much improved version of the VLA, we only saw XZ Tau A and B.  So what is going on?

One thing we know for a fact is that XZ Tau C hasn’t just left the star system.  If XZ Tau C was orbiting XZ Tau A and was somehow kicked out, we would see XZ Tau A and B moved in their orbits, and the whole system would recoil, like a gun does when it fires a bullet.  We found that the positions of A and B are where we would expect them to see, given previous studies of their orbits.

Could XZ Tau C have moved in front or behind of XZ Tau A? Yes, possibly.  In fact, we found that XZ Tau A was a little dimmer than we thought it would be.  Hardly conclusive proof, though.  It’s still possible that XZ Tau C was some kind of transient phenomenon – a distant object flaring up during the first observation in 2004.

We don’t have enough data to decide which of these reasons is the correct reason we didn’t see XZ Tau C.  We’ll just have to try and look again in a few years’ time.  If we see XZ Tau C peeking out from its hiding place as it moves away from XZ Tau A, we’ll know its orbit incredibly well.  If we don’t see it, then there’ll be one less star in the universe…

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