The stellar cluster behind Sirius

Sky distribution of the Gaia 1 members that have been identified with the AAT observations.

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Using the Anglo-Australian Telescope (AAT), an international team of astrophysicists has confirmed the reality of the recently discovered Milky Way stellar cluster Gaia 1. This cluster had been previously undiscovered due to its close angular proximity to Sirius, the brightest star in the night sky.

The Gaia mission (European Space Agency) is constructing the most precise star map of the Milky Way. When completed, this catalogue will provide very accurate positions, distances and space motions of around one billion stars in our Galaxy.

Early this year, using a novel technique and preliminary Gaia data, a new cluster of stars was discovered by British astronomers. This stellar cluster, named “Gaia 1”, is located very close to Sirius, the brightest star in the night sky.

An international team of astrophysicists led by Australian astronomers observed Gaia 1 with two of the spectrographs of the 3.9m Anglo-Australian Telescope (AAT): AAOmega and HERMES. Both of them receive the light from the 2dF (“Two Degree Field”) optical fibre positioner.

“It's really unusual to discover a new, bright star cluster: we have the Milky Way within 10,000 light years pretty well mapped, so Gaia 1 is exciting. Three different teams around the world (including ours) all immediately went and took new observations to try to understand its properties and its orbit”, says Dr Sarah Martell, senior lecturer at University of New South Wales (UNSW) and co-author of this study.

“The discovery of Gaia 1 was publicly announced on 3 February and we made our first observations of it on 15 February. This rapid turn-around is the fastest time I’ve ever experienced between discovery and follow-up observations. It was only possible thanks to working at the Australian Astronomical Observatory and having thorough knowledge of the Anglo-Australian Telescope and its instruments”, says Dr Jeffrey Simpson, researcher at the Australian Astronomical Observatory and Macquarie University and first author of the study.

The observations at the AAT allowed astronomers to identify 41 stars belonging to Gaia 1. The data encoded in the light of these 41 stars helped researchers to determine the age and chemical composition of Gaia 1, as well as its total mass and its orbit in the Milky Way.

“Using the AAT data, we found that the chemical composition of the stars in Gaia 1 is just slightly poorer than that of the Sun”, says Dr Gayandhi De Silva, researcher at the Australian Astronomical Observatory and the University of Sydney, and co-author of the study.

The results of the AAT observations were also combined with numerical simulations to extract the information about the orbit of Gaia 1 around the Milky Way.

The researchers also found that Gaia 1 is a relatively old stellar cluster. It is about 3 billion years old, just a bit younger than the Sun. Finding such a large cluster this old is a bit unusual as most are pulled apart by the Galaxy in less than one billion years.

“But in general Gaia 1 is a perfectly normal star cluster, and we would have noticed it long ago if it wasn't located right behind Sirius”, says Dr Sarah Martell.

“We were a little concerned about observing so close to the brightest star in the night sky in case we accidentally fried the delicate instrumentation of the telescope, but fortunately this did not happen”, says Dr Jeffrey Simpson.

Astronomers usually avoid observing bright stars because they can overwhelm the sensitive instruments they use, which are usually built to study faint stars and galaxies.

“Even if we had looked toward Sirius, we might not have been able to see the cluster through the amount of Sirius's light scattered by the Earth's atmosphere. Gaia is a space mission, so it gets a clearer view of the stars without atmospheric interference”, says Dr Sarah Martell.

“Looking to the future, in April 2018 the Gaia team will release an updated catalogue that will provide distances and space motions for a billion stars. There are likely to be new clusters discovered and Australian astronomers will be able to explore these clusters with HERMES, AAOmega and the new Taipan spectrograph”, says Dr Jeffrey Simpson.

The results of this research were published in the journal “Monthly Notices of the Royal Astronomical Society” last month.

The AAO is a division of the Department of Industry, Innovation and Science.

Publication details:

Simpson, J.D., De Silva, G.M., Martell, S.L., Zucker, D.B., Fergunson, A.M.N, Bernard, E.J., Irwin, M., Peñarrubia, J. & Tolstoy, E. “Siriously, a newly identified intermediate-age Milky Way stellar cluster: a spectroscopic study of Gaia 1”, Monthly Notices of the Royal Astronomical Society, Volume 471, Issue 4, 11 November 2017, Pages 4087–4098

Paper in arXiv: https://arxiv.org/abs/1703.03823

Paper in MNRAS: https://academic.oup.com/mnras/article/471/4/4087/4036884

Science Contacts:

- Dr Jeffrey Simpson

Research Astronomer,

Australian Astronomical Observatory and Macquarie University

P: +61 (02) 9372 4838 E: jeffrey.simpson@aao.gov.au

- Dr Gayandhi De Silva

Research Astronomer,

Australian Astronomical Observatory and University of Sydney

+61 (02) 9372 4854 E: gayandhi.desilva@aao.gov.au

- Dr Sarah Martell,

Senior Lecturer in Physics,

University of New South Wales.

P: +61 (02) 9385 6547 E: s.martell@unsw.edu.au

Media contact:

- Dr Ángel López-Sánchez,

Research Astronomer and Science Communication Officer,

Australian Astronomical Observatory and Macquarie University.

M: +61 406 265 917 E: angel.lopez-sanchez@aao.gov.au

Images:

The constellation of Can Major, with the bright star Sirius (centre of the image) over the dome of the Anglo-Australian Telescope (AAT) 

Caption: The constellation of Canis Major, with the bright star Sirius (centre of the image) over the dome of the Anglo-Australian Telescope (AAT). The constellation of Orion, the Large Magellanic Cloud, and the diffuse band of the Milky Way, are also seen in this image. Single 20 seconds frame at 1600 ISO taken on 9 March 2016 using a 14mm wide-angle lens at f/2.8 and a CANON 5D Mark III.

Credit: Ángel R. López-Sánchez (AAO/MQU).

This image is available at: https://www.aao.gov.au/files/public/images/2017-12-21-Fig02-AAT-Sirius-angelrls.jpg

Sky distribution of the Gaia 1 members that have been identified with the AAT observations.

Caption: Sky distribution of the Gaia 1 members that have been identified with the AAT observations. Red circles are observations with HERMES, while blue squares are observations with AAOmega. Sirius is the bright object centred at the right. The large circle shows 10 arcmin (1/3 of the size of the full Moon) around the cluster centre. The image was obtained using the WISE (Wide-Field Infrared Survey Explorer) satellite (NASA).

Credit: Jeffrey Simpson (AAO/MQU) and WISE (NASA).

This image is available at: https://www.aao.gov.au/files/press/2017-12-21-Fig01-unwise-AAT-map-obs.jpg

The 2dF robot gantry moving the optical fibres, which are illuminated in red. This is a frame of the video timelapse “A 2dF night at the AAT”.

Caption: The 2dF robot gantry moving the optical fibres, which are illuminated in red. This is a frame of the video timelapse “A 2dF night at the AAT”.

Credit: Ángel R. López-Sánchez (AAO/MQU).

This image is available at: https://www.aao.gov.au/files/public/images/2017-11-30-Fig04-2df-gantry-angelrls.jpg

Video timelapse: A 2dF night at the AAT

Caption: A 2dF night at the AAT" assembles 14 time-lapse sequences taken at the 4-metre Anglo-Australian Telescope (AAT) located at Siding Spring Observatory NSW, Australia. This time-lapse video shows not only how the Two Degree Field (2dF) instrument works but also how the AAT and the telescope dome move in tandem, and the beauty of the Southern Sky in spring and summer. The video is 2min 50sec long and combines more than 4000 frames obtained using a CANON EOS 600D with a 10-20mm wide-angle lens. All sequences were taken during September and November 2011 by AAO and MQU astronomer Dr Ángel R. López-Sánchez while he was working as the 2dF support astronomer for the AAT. The music is the song “Blue Raider” from Composer Cesc Villà's album “Epic Soul Factory”.

More info in https://www.aao.gov.au/public/video/2dF-Night-at-AAT

Link to the YouTube video: https://youtu.be/XKoWQtDd14c

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