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CCD work begins on the WiNCam instrument

In the past week, a milestone was reached on the WiNCam (Wide-field Nasmyth Camera) instrument. This is an imaging camera system to be mounted on the new 1-m Lesedi telescope in Sutherland. It allows a wide field of imaging, 43-arcmin diameter, with a standard suite of astronomical filters. Notably, this is the largest charge-coupled device […]

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Discovery of one of the most luminous eruptions from a dying star: SALT contributes to a major international multi-wavelength effort led by South Africa.

A multinational group of astronomers have discovered what it might be the brightest nova (a luminous stellar eruption) ever observed. This mighty eruption was first detected on the 14th of October 2016 by the MASTER-instrument in Argentina. Since then it has attracted the interests of astronomers around the world, who have pointed their ground-based and space telescopes, in an attempt to understand the different aspects of the eruption.

Novae are actually binary stars, involving a white dwarf (dead star) pulling material from a close companion star that is very similar to the Sun. The material transferred to the white dwarf builds up on its surface leading to an increase in the pressure and density. When the pressure and density reach a critical stage, a thermonuclear eruption is triggered on the surface of the white dwarf equivalent to a million hydrogen bombs. During the eruption, the brightness of the star increases dramatically sometimes appearing as a new naked-eye star in the night sky hence the name nova or “new star”. The system survives the eruption and after a year or so from the nova event, the white dwarf resumes pulling material from its donor.

The new study led by Elias Aydi, a PhD student jointly affiliated with the South African Astronomical Observatory (SAAO) and the University of Cape Town (UCT), has been accepted for publication in the Monthly Notices of Royal Astronomical Society journal today. The study is based on optical observations from the Southern African Large Telescope (SALT) in Sutherland (South Africa), MASTER-instruments in Sutherland and Argentina, the Las Cumbres Observatory (LCO) telescope in Australia, the SMARTS, OGLE, and SOAR telescopes in Chile, and X-ray and ultraviolet observations from the Swift satellite.

The newly discovered very bright nova, named SMCN 2016-10a, is located toward the Small Magellanic Cloud (SMC), a small galaxy near the Milky Way, at a distance of around 200,000 light years away from us. When a Nova eruption takes place, the brightness of the star increases in a matter of a few days to reach a maximum and then decreases gradually. Nova SMCN 2016-10a reached a maximum brightness no previous nova in the SMC has ever attained. It is also at least as bright as Nova CP Pup and nova V1500 Cyg, the two brightest novae ever observed. “We are very excited to discover such extreme events which occur only rarely, especially in the SMC,” said the study’s leading author Elias Aydi. He then adds, “a combination of several factors such as the mass of the white dwarf, its temperature and chemical composition might be responsible for such a luminous eruption”. Despite the extreme brightness of the eruption, at a distance of 200,000 light years in a neighbouring galaxy, it is not possible for our naked eyes to see the nova. Such a distance also means that the eruption took place around 200,000 years ago and the light has just reached us now.
According to Aydi, “we used observations from the MASTER-instruments at Sutherland and Argentina along with the SMARTS telescope to estimate the maximum brightness reached during the eruption leading us to the main conclusion.” After its unprecedented peak brightness, the brightness of Nova SMCN 2016-10a decreased quickly in a matter of few days which tells us that the white dwarf is quite massive, probably about 1.3 times the mass of the Sun. The study co-author Patricia Whitelock, from the SAAO and UCT, stated that “In such systems, if the white dwarf keeps soaking up matter and grows beyond 1.4 times the mass of the Sun, there is a good chance that it will detonate in a really big explosion, what we call a supernova, blowing up both of the stars and anything else that gets in the way.”
SAAO astronomer, and co-author, Dr. David Buckley, explained how “The SALT high-resolution spectroscopy along with other data from the Las Cumbres Observatory have helped us understand the material involved in the eruption and even to measure their speeds, which reached just under a million kilometre per hour.” Buckley also elaborated on the ability of SALT, the biggest single optical telescope in the south, to follow up rapidly on such events providing us with particularly high-quality data.

According to study co-author Kim Page, an astronomer from the Swift satellite team and the University of Leicester in the UK, “Swift’s ability to respond rapidly, together with its daily planned schedule, makes it ideal for the follow-up of transients, including novae. We were able to follow Nova SMCN 2016-10a throughout its eruption, starting to collect very useful X-ray and UV data within a day of the eruption being reported.” The X-ray observations were essential to constrain the mass of the white dwarf in the system.

Nova events are frequently observed in the Milky Way with a rate of around 35 eruptions per year, but they are much rarer in the SMC. Nova SMCN 2016-10a is the first nova to erupt in the SMC since 2012, which adds importance to the event. From the Mullard Space Science Laboratory of the University College London, study co-author Paul Kuin says that “the present observations provide the kind of coverage in time and in spectral colour coverage what is needed to make progress in gaining an understanding of a nova in the neighbouring galaxy.” Kuin emphasizes that “Observing the nova in different wavelengths using world-class telescopes such as Swift and the SALT help us reveal the condition of matter in nova ejecta as if it were nearby.”
For more information contact:
Elias Aydi
phone: +27 (0)21 460 9302
South African Astronomical Observatory and the University of Cape Town.

Dr. David Buckley
phone: +27 (0)21 460 6286

The article can also be found on

Other authors on this study include:
M. J. Darnley from Liverpool John Moores University, F. M. Walter from Stony Brook University, P. Mróz & A. Udalski from Warsaw University Observatory, S. Mohamed from the South African Astronomical Observatory and the University of Cape Town, P. Woudt from the University of Cape Town, S. C. Williams from Lancaster University and Liverpool John Moores University, M. Orio from the INAF–Osservatorio di Padova and the University of Wisconsin, R. E. Williams from the Space Telescope Science Institute (Baltimore), A. P. Beardmore and J. P. Osborne from the X-ray and Observational Astronomy Group – University of Leicester, A. Kniazev from the SALT, the South African Astronomical Observatory and the Special Astrophysical Observatory of RAS, V. A. R. M. Ribeiro from Universidade de Aveiro, University of Santiago and Botswana International University of Science & Technology, J. Strader and L. Chomiuk from Michigan State University.

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Recovery of nova spotted by ancient Koreans illuminates many stages of star system’s life cycle

On a cold March night in Seoul almost 600 years ago, Korean astronomers spotted a bright new star in the tail of the constellation Scorpius. It was seen for just 14 days before fading from visibility. From these ancient records, modern astronomers determined that what the Royal Imperial Astronomers saw was a nova explosion, but they have been unable to find the binary star system that caused it—until now.

The new study, published in the journal Nature today, is based on observations from the Southern African Large Telescope (SALT)  and the SAAO 1-meter telescope in Sutherland, Northern Cape, as well as the Las Campanas Observatories’ Swope and Dupont telescopes in Chile. The study pinpoints the location of the old nova, which now undergoes smaller-scale “dwarf nova” eruptions. The work supports the idea that novae go through a very long-term life cycle after erupting, fading to obscurity for thousands of years and then building back up to become full-fledged novae once more.

“This is the first nova that’s ever been recovered with certainty based on the Chinese, Korean, and Japanese records of almost 2,500 years,” said the study’s lead author Michael Shara, Chair of the SALT Board and a curator in the American Museum of Natural History’s Department of Astrophysics.

A nova is a colossal hydrogen bomb produced in a binary system where a star like the Sun is being cannibalized by a white dwarf—a dead star. It takes about 100,000 years for the white dwarf to build up a critical layer of hydrogen that it steals from the sun-like star, and when it does, it blows the envelope off, producing a burst of light that makes the star up to 300,000 times brighter than the sun for anywhere from a few days to a few months.

For years, Shara has tried to pinpoint the location of the binary star that produced the nova eruption in 1437, along with Durham University’s Richard Stephenson, a historian of ancient Asian astronomical records, and Liverpool John Moores University astrophysicist Mike Bode. Recently, they expanded the search field and found the ejected shell of the classical nova. They confirmed the finding with another kind of historical record: a photographic plate from 1923 taken at the Harvard Observatory station in Peru and now available online as part of the Digitizing a Sky Century at Harvard (DASCH) project.

“With this plate, we could figure out how much the star has moved in the century since the photo was taken,” Shara said. “Then we traced it back five centuries, and bingo, there it was, right at the center of our shell. That’s the clock, that’s what convinced us that it had to be right.”

Other DASCH plates from the 1940s helped reveal that the system is now a dwarf nova, indicating that so-called “cataclysmic binaries”—novae, novae-like variables, and dwarf novae—are one and the same, not separate entities as has been previously suggested. After an eruption, a Nova becomes “nova-like,” then a dwarf nova, and then, after a possible hibernation, comes back to being nova-like, and then a nova, and does it over and over again, up to 100,000 times over billions of years.

To get a better look at the present state of the binary system, Joanna Mikołajewska and Krystian Iłkiewicz, from the Copernicus Astronomical Center of the Polish Academy of Sciences, obtained several SALT spectra of the binary and the shell.  These data allowed them to identify the white dwarf companion and to determine its temperature and distance, to constrain the binary components’ masses, as well as to estimate the temperature, density and mass of the shell.

In August 2016, photometric monitoring of the system was carried out with three telescopes, and in particular, by Lisa Crause with the SAAO 1-meter telescope.  These observations revealed deep eclipses which allowed the team to very accurately derive the system’s orbital period.  In addition, the SAAO photometry (obtained over 11 days) demonstrated that the white dwarf rotates with a period of 1859 sec, the same periodicity that is present in X-ray observations.

According to Crause, “We knew from the data obtained in Chile that we were dealing with an eclipsing system, but since we didn’t know the orbital period, each night’s data from the 1-metre amounted to an additional piece of the puzzle.  After collecting light curves on several nights and putting all the data together we could work out the period – a key parameter for understanding the binary system.”

“In the same way that an egg, a caterpillar, a pupa and a butterfly are all life stages of the same organism, we now have strong support for the idea that these binaries are all the same thing seen in different phases of their lives,” Shara said. “The real challenge in understanding the evolution of these systems is that unlike watching the egg transform into the eventual butterfly, which can happen in just a month, the lifecycle of a nova is hundreds of thousands of years. We simply haven’t been around long enough to see a single complete cycle. The breakthrough was being able to reconcile the 580-year-old Korean recording of this nova event to the dwarf nova and Nova shell that we see in the sky today.”

Other authors on this study include K. Ilkiewicz, J. Mikolajewska, and K. Drozd from the Polish Academy of Sciences; Ashley Pagnotta, Jackie Faherty, and D. Zurek from the American Museum of Natural History; L.A. Crause from the South African Astronomical Observatory; I. Fuentes-Morales and C. Tappert from the Instituto de Fisica y Astronomia; J.E. Grindlay from the Harvard-Smithsonian Center for Astrophysics; A.F.J. Moffat from the Universite de Montreal; M.L. Pretorius from the South African Astronomical Observatory and the University of Cape Town; and L. Schmidtobreick from the European Southern Observatory.

Funding for this study was provided in part by the Polish NCN grant #DEC-2013/10/M/ST9/00086, the National Sciences and Engineering Research Council of Canada, FQRNT (Quebec), and the American Museum of Natural History’s Kathryn W. Davis Postdoctoral Scholar program, which is supported in part by the New York State Education Department and by the National Science Foundation under grant #s DRL-1119444 and DUE-1340006. 

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Catching a Glimpse of New Horizons

In the early hours of the morning on 03 June 2017, while most locals are sleeping, dozens of astronomers across South Africa will be looking up at the night sky in hopes of viewing a shadow from a mysterious, distant object.  They are part of a large, international effort to study the target of NASA’s New Horizons spacecraft extended mission.  New Horizons stunned the world in 2015 when it passed through the Pluto system and returned unprecedented images and information about Pluto and its five moons.  The extended mission will allow the spacecraft to collect data on an even more remote object, called 2014MU69 (Figure 1).

2014MU69 was discovered by the Hubble Space Telescope. It is dynamically classified as a classical Kuiper Belt object.  Because it is very faint, with a  26.3 visual magnitude, little is known about its physical properties.  A stellar occultation, during which the light of a distant star is blocked by 2014MU69, has been predicted to be observable from specific regions of the Southern Hemisphere on 03 June (Figure 2).  If successfully observed, this fleeting event could allow measurement of 2014MU69‘s size and shape, could detect rings, dust or debris, and could improve the positional accuracy, all of which would be vital information for planning the spacecraft encounter.  2014MU69 is located nearly 6.5 billion kilometers away with an estimated diameter of approximately 40 km, and the entire occultation will only last roughly 2 seconds. Thus, the observers need to be in exactly the right place, at the right time, with the right equipment to detect the brief dip in the starlight.

To optimize the likelihood of successful observing opportunities (and to avoid bad weather!), observers will be stationed in South Africa, Namibia, Chile, Brazil, Uruguay, and Argentina. Locally, SAAO observers will be using both the 74-inch telescope and the three Las Cumbres Observatory telescopes in Sutherland.  Multiple members of the ASSA (Astronomical Society of South Africa) are also contributing to the effort.  In Southern African alone, there are roughly three dozen overseas visitors who pre-shipped more than a dozen portable telescopes and specialized cameras (Figure 3).

“It is impressive to have so many people visiting South Africa and to see the amount of effort put in to taking the data. It’s a particularly challenging observation that can’t be done without international collaborators. If we are successful, this will be an occultation observation by the faintest object ever and will allow new, accurate measurements to help support New Horizons,” says Anja Genade, an SAAO/UCT student in the NASSP program whose research involves stellar occultations.

Currently, the spacecraft is approximately halfway between Pluto and 2014MU69, and it is scheduled to fly by the target on 01 January 2019. This will be the most distant Solar System object ever encountered by a spacecraft.  Pluto was revealed to be a dynamic, exciting object by New Horizons, and we are sure to expect further surprises and enlightenments from the extended mission as it explores the edge of our Solar System.

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Our new 1-metre telescope has a name – Lesedi

The South African Astronomical Observatory (SAAO) has concluded the naming competition for its new 1-metre telescope; a recent addition to the many national and international telescopes on our observing plateau near Sutherland, in the Northern Cape.

Since the day we announced the competition (which ran 26 January – 31 March), we received an overwhelming response from enthusiastic learners countrywide. The suggested names were accompanied by thoughtful motivations, many of which highlighted the keen interest learners have in astronomy, and in the advancement of technology through the development of astronomy in South Africa. The judges had a very tough decision to make.

Lesedi, a name put forward by Sam Mpho Mthombeni, a Grade 9 learner at Lentheng Middle School in North West Province, was chosen from the nine shortlisted names. Upon receiving the news, Sam had this to say, “I am very happy to be the winner. I am proud that my name, Lesedi, will be the name used for the telescope. I believe this will motivate other learners to also want to study science”, according to Dr. Ramotholo Sefako who made the call.

His motivation was “The new 1-metre telescope should be named Lesedi because it’s the first South African telescope that will be remotely operable and potentially robotic. The instrument will even help S.A. university students go places in their future visually and one cannot visualize in darkness.”  Lesedi means light in Sesotho.

In support of the telescope naming competition, the Department of Science and Technology has invited Sam Mpho Mthombeni and his parent to the DST budget debate in Parliament in Cape Town on 16 May. Thereafter, he and his parent will travel to the Sutherland Observing site for the dedication and unveiling ceremony. Later in the evening, Sam will have his very own 1-metre telescope for the night, with which to view the wonderful Sutherland night-sky.

Project scientist, Dr. Hannah Worters says,This is the first new telescope for over forty years that is owned and operated solely by South Africa.  The next generation of astronomers will train with this telescope — and use it to make their own great discoveries — so it was important to ask our young people for a name that they can relate to, and that means something to them”.  She went on to say, “I have loved reading every one of the suggestions, and thank all the participants for their effort and creativity. The competition has truly been a highlight of this three-year project, and we are all extremely happy to go forth with Lesedi.”

Mr. Sivuyile Manxoyi, the outreach manager at SAAO thinks, “This new 1-metre telescope is a welcome addition to an already very important and interesting theme in the curriculum where learners from grade 6 learn about the historical and modern telescopes of South Africa. Learners will likely feel a stronger connection with Lesedi, especially when learning about modern telescopes because it was developed and built during their time.”

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New highly inflated exoplanet spotted around nearby star

Researchers at the South African Astronomical Observatory and others from around the world, found a new exoplanet orbiting a star 320 light years away. The planet, called KELT-11b, is a gas giant similar to Jupiter and Saturn.

However, KELT-11b is very different from the gas giants in our solar system. The new exoplanet orbits its host star – called KELT-11 – so closely that it completes an orbit in less than five days. KELT-11b has only a fifth of Jupiter’s mass, but is 40% larger in radius. This means that this new bloated planet has about the same density as styrofoam!

This puffed up planet also has a very large atmosphere, providing researchers the opportunity to study its atmospheric properties in detail. These studies will be useful for developing tools to assess Earth-like planets for signs of life in future.

The KELT (Kilodegree Extremely Little Telescope) project consists of two small, robotic telescopes. One of the telescopes, KELT-North, is located in Arizona in the USA while the other telescope, KELT-South, is located in Sutherland, South Africa. The exoplanet was first discovered with the KELT-South telescope and thereafter monitored by many telescopes around the world operated by researchers at universities as well as telescopes operated by amateur astronomers.

The KELT telescopes scan the sky every night, measuring the brightness of about five million stars. Astronomers search for stars that seem to dim slightly at regular intervals, which can indicate a planet is orbiting that star and eclipsing it. Much larger telescopes are then used to measure the gravitational “wobble” of the star – the slight tug a planet exerts on the star as it orbits – to verify that the dimming is due to a planet, and to measure the planet’s mass.

Dr. Rudi Kuhn of SAAO, who helped in the construction of KELT-South, had this to say: ”This is a very exciting discovery. The planet KELT-11b orbits one of the brightest stars known to host an exoplanet and is one of the most inflated planets ever discovered. This enables us to make some very detailed observations of the atmospheric composition of the exoplanet using much larger telescopes, like the Southern African Large Telescope (SALT). This will help us understand how these giant planets are formed, why they have such small orbits as well as what might happen to them in the future.”


Original Paper: J, Pepper et al.: KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the V = 8 Subgiant HD 93396


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SAAO helps to reveal seven new Earth-sized planets

A new system of seven Earth-sized planets orbiting a star 40 light years away has been discovered using data from South African Astronomical Observatory (SAAO) telescope, as well as other instruments around the world. Three of these planets are located in the star’s “habitable zone”. This means that they could have liquid water on their surface, which increases the chances of them hosting life. This new planetary system now holds the record for the largest number of Earth-sized planets found.

SAAO’s 1m telescope was used to take observations of the planetary system over several nights in June and July 2016. The 1m telescope is equipped with a special camera, called the Sutherland High Speed Optical Camera (SHOC), which can take up to 70 images per second.

Dr. Amanda Sickafoose, Head of Instrumentation at SAAO, had this to say about SAAO’s involvement in this exciting work: ”This is a remarkable discovery. To find multiple, possibly habitable exoplanets orbiting the same star is exciting. This system is quite different from our Solar System, which also raises new questions. The SAAO is proud to have played a small role in this advancement in our understanding of planetary systems.”

Other telescopes used in this research include NASA’s Spitzer Space Telescope and ground-based telescopes in Chile, Morocco, Hawaii and the Canary Islands.

The planets were observed as they moved in front of their host star, called TRAPPIST-1, blocking out its light. By carefully measuring the amount of light blocked out as each planet passes in front of the star, astronomers were able to determine the sizes of the planets and the way in which they orbited TRAPPIST-1.

The researchers, lead by Michaël Gillon of the University of Liège in Belgium, also report that the three planets in the habitable zone are likely to be rocky planets, like the Earth and Mars, Venus and Mercury, making this the system with the highest number of rocky planets in the habitable zone of their parent star.

All seven planets orbit TRAPPIST-1 at a distance smaller than the orbit of Mercury, the closet planet to our Sun. The planets are able to orbit so near to TRAPPIST-1 is because it is a small, red dwarf star with temperatures much cooler than the Sun. The full details of this new discovery have been published in the journal Nature.

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A robotic all-sky monitor to observe one star for one year

For a period spanning 200 days from April 2017 extending up to January 2018 astronomers will observe beta Pictoris, the second brightest star in the constellation Pictor to detect rings from the planet beta Pictoris b. Beta Pictoris is a star located 63.4 light years from our Solar System with luminosity that is equal to that of the Sun. What is curious about beta Pictoris is, in 1981 its brightness diminished making astronomers think there must have been a huge object passing in front of the star, then the giant planet Pictoris b, was discovered in 2008.

In anticipation, a small robotic all sky monitor with two camera systems, the beta Pictoris b Ring project – bRing for short, will be dedicated to looking at beta Pictoris at the SA Astronomical Observatory in Sutherland, Northern Cape. The first light image of bRing proves that the instrument is ready for observations.

This year, the planet will move again in front of the star and pass almost directly between the star and us. If the planet has a ring system, we may be able to see the shadows of giant rings surrounding the planet, if and when they move into our line of sight.

The images taken by the cameras will be analysed on a set of computers inside bRing and will monitor any changes in the brightness of beta Pictoris. If a change in brightness is detected, this will allow the triggering of a host of observations using larger telescopes and more advanced instrumentation to study the details of the suspected ring system in-depth. Blaine Lomberg, UCT and SAAO PhD student, will trigger observations with the High Resolution Spectrograph on the Southern African Large Telescope (SALT) to see if a transit of the ring system is detected to determine the composition of the rings

Dr. Steve Crawford who is among the team who worked on the installation of bRing in Sutherland says, “In addition to monitoring beta Pictoris, bRing will also provide regular monitoring of the southern sky and the conditions of the night sky at the Sutherland observatory. These data will be available to astronomers in South Africa allowing them to search for new phenomena and also monitoring the performance of their own observations.”

The bRing project, is funded by NOVA and Leiden University, enabled by a collaboration grant from the Netherlands Organisation for Scientific Research (NWO) and National Research Foundation (NRF), the two funding institutions of South Africa and the Netherlands. Later in the year the second station will be installed in Australia led by astronomers from Rochester University.

The design, construction, installation and operation of bRing has been made possible by funding from NWO and NRF. South African astronomers will host the bRing instrument that was built by Leiden astronomers Matthew Kenworthy, Remko Stuik, John I. Bailey III and Patrick Dorval and hosted by the South African astronomer Steve Crawford and Blaine Lomberg of SAAO.

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SAAO’s New 1 Metre Telescope Needs A Name

The South African Astronomical Observatory (SAAO) is running a telescope naming competition for its new 1 metre telescope which is a recent addition to the collection of many national and international telescopes on our observing plateau near Sutherland in the Northern Cape.

The competition is open now till March 31 (extended from March 18) to learners in grade 6 to 12 countrywide; participants are encouraged to send their suggested names in any of the South African eleven official languages accompanied by a 60 word motivation stating why their name should be considered. “We are looking forward to the many exciting names that the learners will come up with, as well as meeting the winner of the competition at the telescope naming ceremony at Sutherland”, said Dr. Hannah Worters, an astronomer responsible for commissioning the new telescope.

The new telescope is the first South African optical telescope that will be remotely operable and potentially robotic, since the establishment of the SAAO observing location near Sutherland. It will be capable of taking images of areas of the sky seventy times larger than our existing 1m telescope. It uses the SHOC (Sutherland High Speed Optical Camera), a very high-speed camera which can take 70 images in one second to study rapid changes in star systems.

Our telescope operations manager, Dr. Ramotholo Sefako, says, “It is important to share our excitement about the arrival of this new telescope in some way with the rest of the South African community by having its name given by one of South Africa’s young learners.” He continued to say, “We also hope everyone will be as delighted about it as we are. Hopefully, a few school learners may end up being motivated enough to pursue careers in science in the future.”

The 1 metre will also be a teaching telescope mostly used by postgraduate astronomy students from South African universities to develop skills in observing, processing images taken with the telescope, where possible writing up results and publishing them in scientific journals, as well as acquiring technical expertise on aspects of operating a telescope.

Important information:

Competition is open to learners in grades 6 to 12 countrywide (South Africa).

Learner’s entry should be accompanied by a motivation (maximum 60 words); entries without motivation will not be considered.

Learners should include their full names, grade, name of their school and contact number in their entry.

For online click on this website: or post their entry by no later than 31 March to:

South African Astronomical Observatory
P.O. Box 9

Prize: The winner and one parent or guardian will travel to Sutherland to attend the naming ceremony. The learner’s name will be inscribed on a plaque attached to the building that houses telescope. After the ceremony, the winner will be shown around the facility; if the weather permits she/he will get a chance to observe the spectacular Sutherland night sky using our visitor’s telescopes.

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Dwarf-Star Wars: the revenge of the degenerates

Nature Astronomy, Volume 1, Article Number 0029

A bizarre binary star system has been discovered where a degenerate white dwarf pulsar is “lashing” its red dwarf companion with its strong magnetic field and beamed radiation every minute as it spins on its axis. This is the conclusion reached by a small team of three South African and two UK astronomers who have just published a paper in the new journal Nature Astronomy, announcing their discovery of strongly polarized pulsed optical emission from a white dwarf, a so-called degenerate star, in the binary system known as AR Scorpii, establishing it to be a white dwarf pulsar.

Last July, the UK co-authors of the current paper, Professors Tom Marsh and Boris Gänsicke from the University of Warwick, together with their collaborators, announced in Nature the discovery of strongly pulsed emission, across wavelengths from the radio to ultraviolet, from the fast spinning white dwarf, which rotates once every 2 minutes. Their conclusions were that the system was dominated by non-thermal emission, characteristic of pulsars. The current paper firmly establishes the pulsar link with the discovery of pulsed polarization at extremely high levels, reaching 40%, which is amongst the highest polarization levels detected in astronomical objects.

The discovery was made in March 2016 with the venerable and modest sized 1.9-m diameter telescope at the South African Astronomical Observatory (SAAO) in Sutherland, formerly known as the Radcliffe reflector when it began operating about 70 years ago in Pretoria. The observations used the HIPPO photopolarimeter, an instrument which is only one of a few in the world capable of making such observations of fast varying polarization. “This is a demonstration that forefront science can still be done with modest sized telescopes and niche instruments”, said Dr David Buckley, lead author of the paper and an astronomer at the SAAO. “HIPPO was really the ideal instrument for this study”, confirmed Dr Stephen Potter, also at the SAAO, who, together with Dr Buckley, had conceived of this instrument over 15 years ago and managed its design, construction and commissioning, which was completed in 2007. He went on to say, “Polarimetry is an often overlooked discipline in astronomy, with perceptions of it being difficult to do, but the results can sometimes be revolutionary”, as in the case of the results presented in their Nature Astronomy paper.

One of Dr Buckley’s UK collaborators, Professor Tom Marsh of the University of Warwick, had alerted him to their discovery of the fast pulsations discovered in AR Scorpii during their 2015 observations, and plans were put in place for a more detailed follow-up campaign in 2016, utilizing telescopes across the globe, including in South Africa. “It occurred to me from the outset that this object was ripe for polarization observations”, said Dr Buckley, “so we successfully applied for observing time to do this in May”. However, after having a spell of good weather during an earlier observing week in March and having observed all of his main targets for the week, Dr Buckley decided to observe AR Scorpii for an hour or so on his penultimate night, “Just to take a quick peek since it was too tempting to wait”, he said. He was astounded to see, in only a matter of a few minutes of observing, huge values of linear polarization, changing over the 2 minute rotation period of the white dwarf. “It was extremely exciting to see this in real time and was more than my wildest expectations”, he said. Further observations were done on the following night and it is these two nights of observations which feature in their Nature Astronomy paper.

The detection of the strongly pulsed optical polarization, which varies periodically at the spin period of the white dwarf and its beat period with the 3.6 orbital period, has been explained in terms two mechanisms. One of them is beamed radiation and the other due to magnetic interactions between the two stars. Professor Pieter Meintjes, from the Physics Department of the University of the Free State, who took the lead on the theoretical modelling and interpretation, commented: “AR Sco shows all of the hallmarks of a pulsar (dense stellar objects about 20 km in size consisting of a spinning neutron star), including being dominated by synchrotron emission from relativistic particles, both from the white dwarf and in the stellar wind produced by its interactions with its red dwarf companion”.

But the big difference in the case of the AR Scorpii white dwarf pulsar is that at a diameter of ~6000 km, it is about 300 times larger than any neutron star pulsar. “This is why it is able to provide the huge energy generation seen over all wavelengths”, says Dr Buckley, “because its moment of inertia is 100,000 times higher than for a neutron star”. The conclusions in the paper, based on the slowing down of the white dwarf’s spin period, have led to the suggestion that the magnetic field of the white dwarf is very high, up to 500 million Gauss (the Earth’s and Sun’s field strengths are 0.5 and 1 Gauss, respectively, and a fridge magnetic is about 50 Gauss). “This will produce radiation due to the white dwarf’s strong magnetic field”, says Professor Meintjes, “and is also strong enough to pump the weaker field of the red dwarf companion, producing periodic emission from the coronal loops, producing radio emission”. It was the radio pulsations, first discovered from observations with the Australia Telescope and presented in the original Nature discovery paper, that were particularly intriguing. “The radio data was what really started to make us believe that this object had some unique properties, not unlike pulsars”, said Professor Marsh.  Although there is one other similar binary star, also with a fast spinning white dwarf, called AE Aquarii, “That object is quite different and, importantly, not polarized, so with weaker pulsar credentials”, says Professor Gänsicke.

Professor Meintjes calculations imply that so-called magnetohydrodynamic (MHD) instabilities occur in the surface layers of the red dwarf, due to the magnetic field of the white dwarf sweeping by every minute. This results in energy loss which can account for some of the observed optical properties and explain why the white dwarf is slowing down so fast, on the relatively short timescale of 10 million years. “In a very real sense, this is a tug of war between two dwarf stars”, says Dr Buckley, “where right now the red dwarf is being “slapped in the face” once a minute by its rapidly rotating degenerate white dwarf companion.” Eventually these strong interactions will slow down the white dwarf until it is synchronously locked to the 3.6 hour orbital period of the pair. Perhaps the biggest puzzle, however, is why the white dwarf is spinning so fast in the first place. This is most likely a result of mass transfer from the red dwarf during a previous evolutionary phase, but as Professor Marsh says, “The evolutionary path that AR Scorpii took to its current configuration is still an open question.”

Picture credit: University of Warwick and Mark A. Garlick : An artist’s impression of the binary star AR Scorpii, with the spinning white dwarf pulsar in the upper right emitting a beam of energetic particles and radiation from its two magnetic poles. This beam and the strong magnetic field of the white dwarf lashes the larger red dwarf companion star as the white dwarf rotates, once every 2 minutes. The magnetic interactions between the two stars produces strongly polarized and pulsed radiation and also causes strong electrical field generation, powering the ejection of charged particles at close to the speed of light. The drag by the red dwarf on the magnetic field of the spinning white dwarf is slowing its rotation, which will eventually synchronize with the 3.6 h orbital period in about 10 million years.
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