Guest post: Dr Chris Evans – behind an unexpected discovery

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The internet has been abuzz this week with news of a recently discovered massive star being booted from its home cluster. I have the good fortune to know one of the scientists involved in this discovery and have asked him to give us an insight in what went in to this find.

So without further delay I introduce our first guest post from Dr Chris Evans, a researcher at the UK Astronomy Technology Center.

Pick a clear night in the southern hemisphere, get out of town, and let your eyes get night adjusted – not only are you assailed by the depth and scale of the Milky Way, but you also see the faint fuzzy patches of the Magellanic Clouds.  Here’s a pretty spectacular shot from ESO, when Comet McNaught was visible.

These are two irregular galaxies outside of our own, only visible from the south, with distances of 160,000 light years to the Large Magellanic Cloud (LMC), and 195,000 to the Small Magellanic Cloud.  These are great laboratories for learning about how stars live their lives, from birth in beautiful nebulae, to their deaths, sometimes as spectacular supernovae explosions.

The fruits of some of our work in the Clouds came together early this year with some very cool new observations from the Hubble Space Telescope, leading to a journal paper and a press release on a so-called “runaway” star, that we think has been ejected from the centre of the largest stellar nursery in the local universe, 30 Doradus.

Astronomers often have unpublished data from projects that were incomplete due to bad weather, people leaving the subject, the onslaught of more immediate deadlines and so on.  In our case, we had two observations of a bright blue star on the western edge of 30 Doradus; the much bigger project we had planned was thwarted by bad weather. Months later at a conference, Nolan Walborn, based at STScI in Baltimore, was looking through the observations and recognised the tell-tale signs of one of the most massive, rare types of stars called O2 stars. Even with all of our work over the past decades we still only know of a handful of these.  A journalist once asked me if they lived fast and died young?  It’s a cheesy but accurate description. After just a few million years they effectively run out of fuel in their cores, causing a fatal collapse of their atmospheres leading to a supernova explosion. Intriguingly, the speed of this O2 star (relative to the Sun) looked peculiar when compared to the rest of the LMC, but at the time we dimissed this as perhaps being a binary system.

Cut to 2009… NASA launched the fifth servicing mission to the HST which installed some new instruments, as well as giving Hubble some much needed TLC (new batteries and gyroscopes) to keep it going for a few more years.  One of these new instruments was the Cosmic Origins Spectrograph, which takes spectra at ultraviolet wavelengths.

Our O2 star was used as one of the calibration targets immediately after the servicing mission, using some of the spectral lines to help focus COS. Nolan contacted me the moment he saw the data as the COS observation tells us that this star has a stellar wind that accelerates up to a velocity of 3450km/s – that’s 1% of the speed of light just from the processes in the atmosphere of the star, some acceleration!

By chance, the star was also observed as part of a new survey that over thirty of us across Europe (and beyond) are working on, the VLT-FLAMES Tarantula Survey, in which we’ve observed 1000 stars in that part of the LMC. It’s rare you have a real “eureka” moment of discovery, but this was one of them.

The new data, taken over the course of a year, show no changes in the velocity of the star, ruling out a massive binary companion. This tells us that its velocity is very different to the local gas, suggesting it was not formed locally and that it has been ejected from the dense core of 30 Dor, thought to be home of some of the most massive stars known. Theoreticians think that runaway stars like ours can either be ejected via interactions with other stars or via the kick of a supernova explosion. The key to this story is that the cluster at the centre of 30 Dor is thought to be too young to have hosted any supernovae, suggesting this as the first compelling evidence for a star booted-out by more massive stars.
C. J. Evans, N. R. Walborn, P. A. Crowther, V. Henault-Brunet, D. Massa, W. D. Taylor, I. D. Howarth, H. Sana, D. J. Lennon, & J. Th. van Loon (2010). A massive runaway star from 30 Doradus The Astrophysical Journal Letters, 715 (2) arXiv: 1004.5402v1


One Comment on “Guest post: Dr Chris Evans – behind an unexpected discovery”

  1. Sounds lika a setting for a great science fiction story!

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