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South African Clone of Penn State Telescope Makes First Scientific Discovery

25 August 2006

The Southern African Large Telescope Hobby-Eberly Telescope

The Southern African Large Telescope (top) was patterned after the Hobby-Eberly Telescope (bottom).

 

The new Southern African Large Telescope (SALT), an international partnership that includes Penn State, has released its first public research results, which will be published in the journal Monthly Notices of the Royal Astronomical Society. The research gives new insight into an exotic pair of stars that are closely orbiting each other, known as a "polar" binary-star system. These two stars are orbiting each other so closely that the entire system would fit inside our Sun.

SALT was patterned after the successful design of the William P. Hobby-Robert E. Eberly Telescope — one of the largest and most powerful telescopes in the world — which was conceived by Lawrence Ramsey, department head and professor of astronomy and astrophysics at Penn State, and Daniel W. Weedman, formerly a professor of astronomy and astrophysics at Penn State. The Hobby-Eberly Telescope (HET) is located at the McDonald Observatory in a remote area of West Texas, where night skies are among the darkest in the continental United States. "It is gratifying to everyone involved with the Hobby Eberly Telescope that the first scientific results with the Southern African Large Telescope will be published soon," Ramsey comments.

One of the stars in the polar system studied with SALT is an ordinary star like the Sun, but cooler, redder, and about one-third of the Sun's mass and radius. The other star is a compact object called a white dwarf — a star that shrunk to about one millionth of the volume of our Sun after it used up its original store of nuclear energy. This white dwarf is hundreds of thousands of times as dense as the Earth. A chunk of white dwarf as large as a pair of dice would weigh as much as two small trucks. This density gives the white dwarf an intense gravitational field that sucks in material from the larger star. This particular white dwarf has a huge magnetic field — 30 million times as strong as the Earth's. "These are incredibly fascinating objects," remarked Penn State Professor of astronomy and astrophysics Donald Schneider, who has worked on these types of binary stars. "The magnetic fields are so powerful that they compress the infalling material into a funnel-like structure that impacts the white dwarf at a single spot: the magnetic pole of the star."

Polars are the most readily accessible objects for studying gas accretion in strong magnetic fields. They also are among the most closely orbiting pairs of stars known. The polar that astronomers studied with SALT takes only one-and-a-half hours to complete an orbit, compared to a month for the Earth and moon, and a year for the Earth and Sun.

The radiation emitted as the gravitational field of a compact star gobbles gas from its companion star is one of the indirect ways astronomers detect black holes. It also is the way that mass builds up on some compact stars until supernova explosions blow them apart, producing the 'Type Ia' supernovas that astronomers have used recently to show that the expansion of the universe is speeding up.

The innovative, precision design of the HET dramatically reduces construction costs and has opened the door to a new era for such large-sized telescopes, including SALT, which was inaugurated in November 2005. The HET received the Discover Magazine Award for Technological Innovation in 1997.

This first research result from SALT uses a strength of the telescope's design that is rare among large telescopes, the ability to take 'snapshots' of stars in very quick succession, making it possible to study the rapidly changing properties of compact stars, especially as they pull in gas from their companions or surroundings. SALT's advantages for this type of research are expected to allow SALT astronomers to be among the leaders in probing the mysteries of these 'cannibal stars'.

CONTACTS:

Lawrence Ramsey: lramsey@astro.psu.edu, (+1)814-865-0410

Barbara K. Kennedy (PIO): science@psu.edu, (+1)814-863-4682

Full scientific details are in the first scientific paper from SALT, which has been accepted for publication in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society. An electronic preprint of the article is available online at

http://xxx.lanl.gov/abs/astro-ph/06072666.

MORE ABOUT THE SALT TELESCOPE

The Southern African Large Telescope is an international collaboration comprising:

  • * National Research Foundation of South Africa

    * Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences and Jagiellonian University, Nicolaus Copernicus University, Adam Mickiewicz University

    * The Hobby-Eberly Telescope Board comprising: University of Texas at Austin, Penn State University, Stanford University, Ludwig Maximilians Universität München, Georg-August-Universität Göttingen

    * Rutgers, the State University of New Jersey

    * Georg-August-Universität Göttingen, Germany

    * The University of Wisconsin-Madison

    * University of Canterbury, New Zealand

    * University of North Carolina-Chapel Hill

    * Dartmouth College

    * Carnegie Mellon University

    * United Kingdom SALT Consortium, comprising: Armagh Observatory, University of Keele, University of Central Lancashire, University of Nottingham, Open University, University of Southampton

MORE ABOUT THE HOBBY-EBERLY TELESCOPE

The Hobby-Eberly Telescope-named in honor of its principal benefactors, William P. Hobby, the former Lieutenant Governor of Texas, and Robert E. Eberly of Pennsylvania, an industrialist and philanthropist-is a joint project of Penn State, the University of Texas at Austin, Stanford University, Ludwig-Maxmilians-Universität in München, Germany, and Georg-August-Universität in Göttingen, Germany.

More information about the first science results from SALT can be found in the following SALT press release.


First Science with SALT: Observations of Eclipsing Polar

The artist Bob Watson's painting of a 'Polar' binary.

The artist Bob Watson's painting of a 'Polar' binary.

 

16 August 2006 — The Southern African Large Telescope (SALT), inaugurated in November 2005, is today releasing its first public research results, giving new insight into an exotic pair of stars closely orbiting one another.

This research uses a strength of the SALT design which is rare among large telescopes, the ability to take 'snapshots' of stars in very quick succession, so that we can study the rapidly changing properties of compact stars, especially as they pull in gas from their companions or surroundings.

The gravitational field of a compact star commonly pulls in gas from a companion star — the radiation (especially X-ray) emitted as this happens is one of the indirect ways we use to detect black holes. It's also the way that mass builds up on some compact stars until supernova explosions blow them apart, giving us the 'Type Ia' supernovae recently used to show that the expansion of the universe is speeding up.

The new SALT results are for a 'polar' binary star system, which contains a compact star called a 'white dwarf' – a star which has used up its original store of nuclear energy, then shrunk to about one millionth of the volume of a star like our sun. In a polar this 'white dwarf' also has a very strong magnetic field, which strongly influences how the hot gases from its relatively ordinary companion reach the white dwarf surface.

Polars are the most readily accessible objects we know for studying gas accretion in strong magnetic fields, and are among the closest orbiting pairs of stars we know: both stars and their orbits would fit inside the Sun!

The polar which SALT has studied takes only one and a half hours to complete an orbit (compared to a month for the earth and moon, and a year for the earth and sun). Despite being a pair of stars, they are so close you would see them as only one star in a telescope. One of the stars is an ordinary star like the Sun, but cooler, redder and about 1/3 of the Sun's mass and radius. Its 'white dwarf' companion is hundreds of thousands of times as dense of the Earth — a chunk of white dwarf as large as a pair of dice would weigh as much as two small trucks. This gives the white dwarf an intense gravitational field that sucks in material from the larger star. But it is the white dwarf's huge magnetic field (30 million times as strong as the Earth's) that forces the gas from the cool star to impact at the white dwarf's magnetic poles. Figure 1 is an artist's impression of what such a typical such binary system might look like: the cool, red star is in the background with the stream of gas being sucked off by gravity shown in white, finding its way down to the white dwarf along a path shaped by magnetic forces.

Earth Observer's view of a polar at the start (left) and end (right) of eclipse.

Earth Observer's view of a polar at the start (left) and end (right) of eclipse.

 

Imagine now that you are looking at a binary system like this from "behind" the cool, red star with your viewing angle such that the red star, once an orbit, passes in front of the white dwarf and cuts off your view of it. If you had a telescope like SALT, and a camera on it like SALTICAM, which can make brightness measurements every 100 milliseconds, you would see the brightness of the system dim quite drastically because the light from the gas crashing on to the magnetic poles of the white dwarf completely outshines the light from everything else.

Figure 2 shows a cartoon of your view of the system at the start of eclipse (left) when the red star is just about to block our view of one magnetic pole, labeled Spot 2, and at the end of eclipse (right) when the red star has just uncovered Spot 2.

Sequence of brightness measurements of the polar. Each point is a 112 millisecond exposure.

Sequence of brightness measurements of the polar. Each point is a 112 millisecond exposure.

 

Figure 3 is a sequence of brightness measurements and the evidence for what has just been described can be seen in the sequence. If you look closely at Figure 3, you will see it has a first sudden brightness drop (Spot 2 disappearing), followed about 25 seconds later by a second sudden brightness drop (Spot 1 disappearing). Towards the end of the sequence there are sudden rises in brightness corresponding to the earlier sudden drops as the spots are uncovered. The gas stream between the stars also gives some light, and this accounts for the rounded shape of the bottom of the eclipse.

This sequence of measurements is better than anything that has been obtained before, and SALT's advantages over all other large telescopes for this type of research should allow SALT astronomers to lead in probing the mysteries of these 'cannibal stars'.

Full scientific details are in the first scientific paper (or report) from SALT, which has been accepted for publication in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society. An electronic preprint of the article is available online at xxx.lanl.gov/archive/astro-ph, entry number 0607266.

Auroral oval shown in green

Auroral oval shown in green

SALT 4 large

 

CONTACTS:

For further information, contact Dr. Darragh O'Donoghue: Tel: +27 21 447 0025

E-mail: dod@saao.ac.za