Echoes of ancient exploding star

Apr 26, 2016

by Dr Miszalski & Prof Woudt

SALT helps to pinpoint echoes of an ancient exploding star on our stellar doorstep

Image: Location of Te 11 in the constellation Orion (background image by Rogelio Bernal Andreo).

When amateur astronomers found a peculiar looking nebula (called Te 11) in the Orion constellation in 2010, astronomers weren’t quite sure what to make of it. It could have been the envelope of an old star like the Sun or the result of a violent stellar explosion. As explained in a joint SALT/SAAO/UCT press release, combined observations taken primarily by telescopes of the South African Astronomical Observatory (SAAO) in Sutherland, South Africa, have resolved the conundrum. According to a recently published paper by a team of astronomers based in South Africa, England, Chile, Spain and Mexico, the nebula seems to have been formed in a stellar explosion just over 1500 years ago.

Putting together the pieces of this cosmic puzzle has been a detective story bringing together the very latest astronomical equipment and millenia-old Chinese records of the variable night sky. First observations of this star at the centre of the nebula showed it to be a double star system in a close orbit of almost 3 hours. The team responsible for this discovery, a group of astronomers with expertise in the envelopes of old stars led by Dr Brent Miszalski at the SAAO, were soon joined by astronomers Prof. Patrick Woudt, Prof. Brian Warner and Ms Mokhine Motsoaledi at the University of Cape Town (UCT), who happened to have also directed their attention to this unusual star. By noticing variations in the brightness of this star over several years, Woudt and colleagues had amassed high-speed observations recording the changes in the binary’s light.

The jumps in the light curve imply that the star is a dwarf nova (a star hiccuping material nibbled off the companion). Such a combination of a dwarf nova with a nebula is however extremely rare. So….what’s going on!?


Image: Colour-composite image of Te 11 made from images taken with Halpha+[NII] (red, VLT FORS2), [OIII] (green, VLT FORS2) and SDSS g (blue, SDSS) filters.

Key to working out the nature of the Te 11 nebula was calculating its distance. Here the huge light collecting power of the 11-m Southern African Large Telescope (SALT) in Sutherland was used to peer into the heart of the nebula, determining the temperature of one of the stars present. This was worked into the modelling of the light curve to yield a distance of around 1000 light years – putting Te 11 on the edge of the Orion-Eridanus superbubble. This distance confirmed the nebula could not be the envelope of an old star like the Sun.

Looking back in time at Chinese historical records, Prof. Brian Warner found that there was a bright guest star in Orion during 483 CE near the position of Te 11. This remarkable connection suggests that Te 11 is the leftovers of this explosion of more than 1500 yrs ago – at which time the star would have outshone all the stars of the Orion constellation, reaching a similar brightness to Jupiter in the night sky. Such pairings in astronomy as found in Te 11 are exceedingly rare, but it is anticipated that planned studies of the night sky will find a whole lot more. South African astronomers are looking forward to using the upcoming MeerLICHT facility in Sutherland that may help find similar objects to Te 11. MeerLICHT will be a dedicated wide-field optical telescope that will robotically follow the gaze of the Meerkat radio telescope array, a precursor array that will form part of the Square Kilometre Array (to start early science this year). Prof Woudt says: “Planned surveys with MeerKAT and MeerLICHT will scan the southern skies for more of these unusual objects, which can tell us more about the formation and the evolution of these compact binaries in the Milky Way”. With a binary period of only 2.9 hours, the two stars in Te11 are separated only by about twice the Earth-Moon distance.

There is a long history of scientific collaborations between the University of Cape Town and the South African Astronomical Observatory (SAAO). UCT and SAAO have a number of joint staff positions, and both are partners in the MeerLICHT project. MeerLICHT is a robotic telescope that will soon be installed in Sutherland and which will be permanently tied to the observing schedule of MeerKAT. As demonstrated in the case of Te 11, the Southern African Large Telescope proved fundamental in understanding the faint source of the transient signal; the unique combination of SALT, MeerKAT and MeerLICHT bodes very well for peering deeper into the transient southern skies in the coming years.

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Dr Brent Miszalski
SALT Astronomer, SAAO
Telephone: +27 (0) 21 460 6292

Prof. Patrick Woudt
Head of Astronomy Department, UCT
Telephone: +27 (0) 21 650 2392