Could a Wolf-Rayet Star Generate a Gamma Ray Burst?

Gamma ray bursts (GRBs) are the most energetic events to be seen in the observable universe. On March 19th, a record breaking GRB was observed in the constellation of Boötes by NASA’s Swift Observatory and ground based telescope arrays (i.e. the Polish “Pie of the Sky” GRB detector). This was an explosion unparalleled with anything we have ever seen. Not only was it the brightest GRB, it was the most distant GRB – this explosion occurred 7.5 billion years ago (it was therefore located 7.5 billion light years away). Taking measurements of the spectrum of light from these events not only helps us understand what causes such a massive detonation, but also reveals the nature of the Universe when it was half the age it is now.

In a new publication headed by the University of Utrecht, in The Netherlands, the highly dynamic and self-destructive Wolf-Rayet star has been singled out as a possible GRB progenitor after some complex tidal interactions with a binary partner, spinning-up the star until it collapses and unleashes vast amounts of energy into space…

Wolf-Rayet (WR) stars are massive stars (over 20 solar masses) undergoing vast mass loss through powerful stellar winds. It can be considered to be a WR “phase” of a massive star’s lifetime, where the star becomes highly unstable as it uses up its remaining fuel. The result is a very hot (25,000-50,000K) surface temperature (compared to our Sun’s 6,000K photospheric temperature), fast stellar winds blasting into space at 7.2 million km/hr and a huge mass loss, WR stars typically shed 0.001% of their mass per year.

Previous research has pointed at WR stars as possible GRB progenitors due to their turbulent characteristics and violent deaths. These massive stars are expected to collapse as black holes soon after the WR phase, so the possibility that they may act as GRB generators seems like an obvious path to follow.

Now, GRBs are mysterious events in themselves. They are the brightest explosions observable in space, but there is no way to predict them, they are simply too far away. In fact the only way we can see their location (unless we get lucky, or unlucky, and one blows in our local universe) is when they’ve already exploded. This is cool if you’re just interested in catching the flash and afterglow, but there’s no way of understanding what they blow up from.

According to Detmers et al. (2008) currently working on the GRB/WR star relationship, GRBs often occur in star-forming regions of galaxies. Several GRBs have also been associated with Type Ic supernovae. Type Ic supernovae are believed to be triggered by WR stars as they die and form a black hole. But there’s a problem, the signature of a WR star has been found in the afterglow of only one GRB. More work obviously needs to be done to understand how GRBs are generated and to understand WR star dynamics.

The Dutch group model a variety of situations, but focus on WR stars with a compact, massive companion, like a neutron star. Most star systems come in pairs, called binary systems, and this may be a critical factor when understanding how a GRB could form. The binary system is critical as to the generation of GRBs, the star must rotate very quickly. As WR stars are constantly losing mass through the large mass loss via their strong stellar winds, a companion star is needed to maintain its rotation. The compact companion gravitationally “spins up” the WR star.

They arrive at a variety of solutions, but the main outcomes include:

  • The tidal spin-up works, reducing the orbital distance, maintaining the angular momentum. Eventually the WR star and neutron star collide. WR star collapses, producing a supernova, possibly a GRB.
  • The tidal spin up does not work. Mass loss from the WR star is too high and the binary system orbital radius increases. Stars do not collide.

Apparently after several runs, although it is possible to generate a GRB from the binary spin-up model, it isn’t easy. The conditions need to be absolutely correct for the two bodies to spin rapidly and merge. Only then can the WR star create the “collapsar” model currently assumed to drive GRBs.

Whatever the outcome of these models, without “seeing” a WR star before it explodes as a supernova (whether it creates a powerful GRB or not), we cannot be sure of the correct conditions for a GRB. However, observing supernovae in local galaxies and then looking through Hubble Space Telescope archived images can help astronomers see what local stars look like before they blow up, but the rarer GRBs will remain a mystery as their precursor stars will be too far away to observe.

Reference: Gamma-Ray Bursts from tidally spun-up Wolf-Rayet stars? – arXiv:0804.0014v1

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