From The University of Canterbury [Te Whare Wānanga o Waitaha] (NZ)
13 May 2022
A quadruple star system discovered in 2017 and recently observed at the University of Canterbury Mt John Observatory could represent a new channel by which thermonuclear supernova explosions can occur in the Universe, according to results published in Nature Astronomy today (13 May NZ time) by an international team of astronomers.
The rare double-binary star system HD74438 was discovered in the Vela constellation in 2017 using the Gaia-ESO Survey which characterized over 100,000 stars in our Milky Way Galaxy.
Follow-up observations of HD 74438 were obtained over several years to precisely track the orbits of the stars in the quadruple star system. Observations were taken with high-resolution spectrographs at the University of Canterbury Mt John Observatory in New Zealand, and the Southern African Large Telescope in South Africa.
Mt John University Observatory (NZ)
The astronomers were able to determine that this stellar quadruple is made up of four gravitationally bound stars: a short-period binary orbiting another short-period binary on a longer orbital period (2+2 configuration).
The quadruple system is a member of the young open star cluster IC 2391, making it the youngest (only 43-million years old) spectroscopic quadruple discovered in the Milky Way Galaxy to date, and among the quadruple systems with the shortest outer orbital period (six years).
In the Nature Astronomy paper published today, the authors have shown that the gravitational effects of the outer binary system is changing the orbits of the inner binary, causing it to become more eccentric. State-of-the art simulations of this system’s future evolution show that such gravitational dynamics can lead to one or multiple collisions and merger events producing evolved dead stars (white dwarfs) with masses just below the Chandrasekhar limit. As a result of mass transfer or mergers, these white dwarf stars can produce a thermonuclear supernova explosion.
Astronomers involved in this study include the Director of the University of Canterbury Mt John Observatory, Associate Professor Karen Pollard of the School of Physical and Chemical Sciences, University of Canterbury; UC alumni Dr C. Clare Worley and Professor Gerry Gilmore (the first UC student to receive a doctorate in astronomy), both of the Institute of Astronomy, The University of Cambridge (UK).
Associate Professor Pollard says high-precision and high-resolution spectroscopic observations were taken with the Hercules spectrograph on the 1.0m McLellan Telescope at the University of Canterbury Mt John Observatory in Tekapo.
“A star like our Sun will end its life as a small dense dead star known as a white dwarf, and the mass of white dwarfs cannot go above the so-called Chandrasekhar limit (about 1.4 times the mass of the Sun),” she says. “If it does, because of mass transfer or merger events, it can collapse and produce a thermonuclear supernova. Interestingly, 70% to 85% of all thermonuclear supernovae are now suspected to result from the explosion of white dwarfs with sub-Chandrasekhar masses. As a result of mass transfer or mergers, these white dwarf stars can explode as a thermonuclear supernova explosion.”
The evolution of stellar quadruples such as HD 74438 thus represents a new promising channel to form thermonuclear supernova explosions in the Universe, Associate Professor Pollard says.
Binary stars are now recognized to play a major role in a large range of astrophysical events, and mergers of binaries are the cause of the recent gravitational wave emission detection. Binary stars also allow us to derive fundamental stellar parameters like masses, radii and luminosities with a better accuracy compared to single stars. They represent the gems on which various astrophysics topics rely.
Stellar quadruples only represent a marginal fraction (a few percent) of all multiple systems. The complex evolution of such high-order multiples involves mass transfer and collisions, leading to mergers that are also possible progenitors of thermonuclear supernovae. These supernovae represent standard candles for fixing the Universe distance scale, even though the evolutionary channel(s) leading to the progenitors of such supernova explosions are still highly debated.
See the full article here.
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The University of Canterbury [Māori: Te Whare Wānanga o Waitaha] is a public research university based in Christchurch, New Zealand. It was founded in 1873 as Canterbury College, the first constituent college of the University of New Zealand. It is New Zealand’s second-oldest university, after the University of Otago, itself founded four years earlier in 1869.
Its original campus was in the Christchurch Central City, but in 1961 it became an independent university and began moving out of its original neo-gothic buildings, which were re-purposed as the Christchurch Arts Centre. The move was completed on 1 May 1975 and the university now operates its main campus in the Christchurch suburb of Ilam.
The university is well known for its Engineering and Science programmes, with its Civil Engineering programme ranked 9th in the world (Academic Ranking of World Universities, 2021). The university also offers a wide range of other courses including degrees in Arts, Commerce, Education (physical education), Fine Arts, Forestry, Health Sciences, Law, Criminal Justice, Antarctic Studies, Music, Social Work, Speech and Language.
Canterbury College, University of New Zealand, 1873–1960
On 16 June 1873, the university was founded in the centre of Christchurch as Canterbury College, the first constituent college of the University of New Zealand and was funded by the then Canterbury Provincial Council. It became the second institution in New Zealand providing tertiary-level education (following the University of Otago, established in 1869), and the fourth in Australasia. It was founded on the basis of the Oxbridge college system, but it differed from Oxbridge in that it admitted female students from its foundation. Its foundation professors arrived in 1874, namely, Charles Cook (Mathematics, University of Melbourne, St John’s College, Cambridge), Alexander Bickerton (Chemistry and Physics, School of Mining, London), and John Macmillan Brown (University of Glasgow, Balliol College, Oxford). A year later the first lectures began and in 1875 the first graduations took place. In 1880, Helen Connon was the first woman to graduate from the college, and in 1894, Apirana Ngata became the first Māori-born student to graduate with a degree. The School of Art was founded in 1882, followed by the faculties of Arts, Science, Commerce, and Law in 1921, and Mental, Moral, and Social Sciences in 1924. The Students’ Union, now known as the University of Canterbury Students Association, was founded in 1929 operating out of the Arts Centre of Christchurch Old Student Union Building, and the first edition of the student magazine Canta was published in 1930. In 1933, the name changed from Canterbury College to Canterbury University College.
Independence of the University of Canterbury, 1961–2010
Until 1961, the university formed part of the University of New Zealand (UNZ), and issued degrees in its name. That year saw the dissolution of the federal system of tertiary education in New Zealand, and the University of Canterbury became an independent University awarding its own degrees. Upon the UNZ’s demise, Canterbury Agricultural College became a constituent college of the University of Canterbury, as Lincoln College. Lincoln College became independent in 1990 as a full university in its own right and is now known as Lincoln University.
Rankings
In the 2017 Academic Ranking of World Universities (ARWU), UC was dropped completely from the world’s top 500 universities. In the 2017 QS World University Rankings, UC was rated 214th overall in the world, and third highest among New Zealand universities. Its individual global faculty rankings for 2015/2016 were: 146th in Arts & Humanities, 161st in Engineering & IT, 211th in Natural Sciences, and 94th in Social Sciences and Management. By 2018, these faculty rankings had all fallen considerably, and as of the release of the 2019 world university rankings, the three major university ranking organizations, ARWU, QS and THE, had all placed UC squarely in the middle of the pack of NZ universities at fourth place overall out of eight institutions, and in one case just two numerical positions above NZ’s fifth-place university in the nation’s lower division. In the 2016–2017 Times Higher Education World University Rankings, UC was ranked in the world’s top 400 universities, up from being in just the world’s top 500 universities in 2015. By 2021, however, UC had fallen back into just the top 600. Similarly, ARWU dropped UC from the top 400 universities in 2018 to just the top 500 in 2019, where it has remained ever since.
The university was the first in New Zealand to be granted five stars by QS Stars. Unlike the QS World University Rankings, QS Stars ratings are only given to universities that pay a fee; the programme is designed to give “those institutions that are not highly ranked or do not appear in the rankings an opportunity to reach out to their prospect students, to stand out and to be recognised for their excellence.”
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