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Semester 1, 2004

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26th May, 2004: The Protoplanetary Disk around AB Aurigae

The star AB Aurigae is about 470 light years distant. Astronomers using the 8m Subaru telescope on Mauna Kea have successfully imaged the protoplanetary disk around the star using the Coronagraphic Imager with Adaptive Optics (CIAO) instrument which blocks out the light from the host star.

What is of great interest is that there is a large amount of structure in the disk. Circumstellar emission extending out to a radius of r=580 AU is seen, with a double spiral structure detected at r=200-450 AU. There are four major spiral arms, which are trailing if the brighter southeastern part of the disk is the near side.

Near-infrared direct imaging of the protoplanetary disk surrounding the star AB Aurigae. FOV 8 arcsec x 8 arcsec. Copyright Subaru Telescope, National Astronomical Observatory of Japan (NAOJ)

What could cause such structure? There are two current theories. In the first a nearby companion star may gravitationally induce such patterns. (This effect has been detected in the circumstellar disk around HD 141569A by Clampin et al. 2003).

The second theory requires the protoplanetary disk to be inhomogeneous so that small density enhancements grow into a spiral pattern as the disk rotates. There does not appear to be a nearby star to AB Aurigae, hence the structure appears to have an intrinsic origin.

These results were published by Misato Fukagawa and collaborators in the Astrophysical Journal, Letters on April 10, 2004. (Ap. J. 605, L53).

For more information see: NAOJ Press Release

17th May, 2004: Permian-Triassic Extinction Impact Site? Subtitled: Where were you Bruce?

OK, Bruce W. may be able to save us these days but where was he 250 million years ago?

About 250 million years ago the Permian era ended and the Triassic era began. This coincided with the largest mass extinction of flora and fauna in Earth's history. More than 95% of terrestrial and marine species disappeared during what is now referred to as the "Great Dying".

Bedout crater site Credit: CNN

Scientists believe they have found an impact crater at "Bedout High" [pronounced Bedoo], a 200 kilometre-wide dent under the seabed, 250 km due north of Port Hedland in Western Australia that could provide the explanation for the extreme decrease in plant and animal life immediately after the Permian era.

The scientific team was inspired by previous work. In 1998, a geologist named John Gorter wrote a short paper about Bedout crater saying that based on seismic research the geological formation could very well be an impact crater. The team led by geologist Luann Becker from the University of California, Santa Barbara, has published an article in the journal Science stating that the Australian crater contains evidence of the impact of a meteor 4 to 7 miles wide about the time of the Great Dying.

False colour image of the Bedout site Credit: Science

By it's nature this work is not definitive. Seismic studies, carried out by team member Robert Iasky of the Geological Survey of Western Australia in Perth, shows the Bedout structure has a central uplift of rocks surrounded by a basin. However a type of disfigured quartz, (which is unmistakably formed from an extraterrestrial impact and has been found at the famous Chicxulub crater site) has not been found.

"The structure looks like it could quite well be an impact crater - although it could be a volcanic uplift," said Iasky. "Although I'm a co-author, I'm sitting on the fence."

This work is described in detail in a research paper "Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia" by L. Becker, R. J. Poreda, A. R. Basu, K. O. Pope, T. M. Harrison, C. Nicholson, and R. Iasky Published online (Science Express) May 13 2004; 10.1126/science.1093925 Science www.sciencemag.org

For more information see: ABC Space News * CNN Science & Space

30th April, 2004: Titan Unveiled

Recent observations by the ESO VLT have improved our knowledge of Titan, Saturns largest moon. Discovered in 1665 by Christiaan Huygens, it was not until 1944 when Gerald Kuiper detected methane that astronomers knew that it was the only satellite in the Solar System to have an appreciable atmosphere. Subsequent observations showed that as well as methane, its primary constituent was nitrogen. However the atmosphere is very thick, effectively hiding any surface features from us until now.

A narrow "window" is available in the near-infrared near 1.575 microns which allows a reasonably, though not perfectly clear, view of surface features. In February 2004, an international research team working at the ESO VLT obtained images of the surface with excellent spatial resolution.

ESO PR Photo 11c/04 shows simultaneous images of Titan, obtained on February 7, 2004, with NACO in SDI mode. Left: at 1.575 microns with a clear view towards the surface. Right: at 1.625 microns, where the atmosphere appears entirely opaque. Credit: ESO Education & Public Relations Department

The image resolution 0.06 arcsec corresponds to 360 km on the surface. The brightness is proportional to the surface reflectivity. The nature of the various regions is still unknown although it is speculated that the darkest areas may indicate the extent of reservoirs of liquid hydrocarbons.

PR Photo 11f/04 identifies the low-reflection areas seen on the surface of Titan and which were given provisional names by the research team. Credit: ESO Education & Public Relations Department

This work is described in detail in a research paper "First surface map of Titan at 1.575 microns" by M. Hartung et al., submitted to the European research journal Astronomy & Astrophysics.

Original Press Release: ESO Press Release

For more information see: Bill Arnetts Nine Planets Titan page * ESA Huygens probe

14th April, 2004: Once more around the Galaxy

A team of astronomers have just released results of a program of observations of 14,000 F and G type stars. The stars are currently close to the Sun but the observations have allowed their orbital history to be modelled.

Most of the stars are located within about 500 light-years and were already observed by Hipparcos thus giving us their distances and motions. However their radial velocities were not measured, so only their 2D motions were known. The observations have now been done and for the first time, we know the 3D space motion of a complete sample of typical stars in the solar neighbourhood. From their space motions, the team was able to compute the positions of these stars at different points in the Milky Way's history.

For an animated gif image see: A&A Press Release

The 14,000 observed stars projected on the plane of the sky Credit: Birgitta Nordstrom (Niels Bohr Institute for Astronomy, Physics and Geophysics, Denmark and Lund Observatory, Sweden), Michel Mayor, F. Pont, Stephane Udry and Nami Mowlavi (Geneva Observatory, Switzerland), Johannes Andersen and J. Holmberg (Astronomical Observatory, Copenhagen, Denmark and Nordic Optical Telescope Scientific Association, Spain), B. Rosenkilde Jorgensen (Lund Observatory, Sweden), and Erik Olsen (Astronomical Observatory, Copenhagen, Denmark).

The research has been published in the Astronomy \& Astrophysics journal.

The team has now determined the characteristics of these stars, including ages, heavy-element content, and their orbital motion around the Galaxy. A first analysis reveals that objects in the galactic disk, such as molecular clouds, spiral arms, or possibly a central bar, have stirred up the stellar velocities. Hence the evolution of the Milky Way was probably far more complex and chaotic than suggested by traditional, more simplified models.

For more information see: A&A Press Release * ABC Space and Astronomy News * "The Geneva-Copenhagen survey of the Solar neighbourhood: Ages, metallicities and kinematic properties of ~14,000 F and G dwarfs" by B. Nordstrom et al.

26th March, 2004: Sedna

The Solar System's most distant object has been found. The object, about 13 billion kilometers from Earth, has been given the provisional name of Sedna, after the Inuit goddess who created sea creatures of the Arctic.

Sedna has a diameter of 1,700 kilometres (Pluto has a diameter of just under 2,300 kilometres) and is probably made of ice and rock, with surface temperatures never rising above -240 degrees Celsius.

Associate Professor Mike Brown, a U.S. astronomer at the California Institute of Technology, led the research team that detected Sedna in November 2003 during a survey of the outer Solar System.

The astronomers think Sedna is the first detection of the hypothetical Oort cloud, a group of small icy objects that supplies some of the comets that streak by Earth. But Sedna is much closer to the Sun than predicted, so could form part of an inner Oort cloud, the astronomers said.

Sedna (arrowed) Credit: NASA/Caltech/M Brown

Sedna is part of the Solar System, but that doesn't mean it's a planet, according to Dr Brian Marsden, director of the Minor Planet Center of the International Astronomical Union.

"I think it would be misleading to call it the 10th planet," Marsden said. "Just as I think it's misleading to call Pluto the ninth planet."

Sedna and other Solar System bodies Credit: M Brown

Planets must "participate" in the events of the Solar System, and there again, he feels Pluto does not qualify; its orbit is neither circular nor in the same plane as the other planets. As Sedna is smaller and far more eccentric in its path than Pluto, Marsden questioned its potential planetary status. Marsden questioned the planetoid's suggested name, given by its discoverers and not yet approved by the astronomical union. The formal designation for Sedna is 2003 VB12.

For more information see: ABC Space and Astronomy News * CNN Science and Space * Mike Brown Sedna page

18th March, 2004: Traffic Jam at Mars

There is unprecedented activity on and above Mars at present! NASA's two rovers, Spirit and Opportunity continue to explore the surface at Gusev crater and Meridiani Plateau respectively.

Scientists have concluded the part of Mars that Opportunity rover is exploring was soaking wet in the past. A rock outcrop (below) near the rovers landing spot provides important evidence. Clues from the rocks' composition, such as the presence of sulfates, and the rocks' physical appearance, such as niches where crystals grew, help make a strong case for a watery history.

"Liquid water once flowed through these rocks. It changed their texture, and it changed their chemistry" said Dr. Steve Squyres of Cornell University, Ithaca, N.Y.

Rock outcrop near Opportunity rover Credit: NASA/JPL/Cornell

From orbit ESA's Mars Express captured some impressive high resolution images. One, taken by the High Resolution Stereo Camera (HRSC) from a height of 273 km shows the eastern part of the Hellas basin. The area is 100 km across, and the image has a resolution of 12 m per pixel, and shows a channel (Reull Vallis). The channel was probably formed by flowing water.

Reull Vallis - HRSC image 15 January 2004 Credit: ESA/DLR/FU Berlin (G. Neukum)

Unfortunately Japan's Nozomi spacecraft failed to obtain orbit in January 2004. As well there is still no contact with the ESA Beagle 2 lander (part of the ESA Mars Express mission), that landed on the 25th December 2003. Beagle 2 was to have landed in the Isidis Planitia basin (about 90 degrees East, 10 degrees North).

For more information see: Mars Exploration Rover Mission * ESA Mars Express * ISAS Nozomi Mission * Beagle 2 homepage

11th March, 2004: HST goes deep, ... way deep ... Ultra Deep!

Hubble Space Telescope has one-upped itself! After the Hubble Deep Fields it was only natural to go deeper, ... way deeper. The Hubble Ultra Deep Field (HUDF) required 800 exposures of the Advanced Camera for Surveys (ACS) taken over the course of 400 Hubble orbits. The total amount of exposure time was 11.3 days, taken between Sept. 24, 2003 and Jan. 16, 2004.

The HUDF will be used to detect galaxies that existed between 400 and 800 million years (corresponding to a redshift range of 7 to 12) after the big bang. This data will greatly enhance the quest to understand galaxy evolution over the majority of time since the universe began. The HUDF field, located in the constellation Fornax, contains an estimated 10,000 galaxies. In ground-based images, the patch of sky (just one-tenth the diameter of the full Moon) is largely empty.

"Hubble takes us to within a stone's throw of the big bang itself" says Massimo Stiavelli of the Space Telescope Science Institute in Baltimore, Md., and the HUDF project lead.

Subimages of the Hubble Ultra Deep Field. Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team

Hubble's ACS allows astronomers to see galaxies two to four times fainter than in the previous Hubble Deep Fields (taken with WFPC2), and is also very sensitive to the near-infrared radiation that allows astronomers to detect some of the farthest observable galaxies in the universe. The HUDF will hold the record as the deepest-ever view of the universe until ESA, together with NASA, launches the James Webb Space Telescope in 2011.

For more information see: Hubble Site Newscenter

27th February, 2004: Galaxy Entree or Main Course?

In June 1990 Stephen Hawking conceeded a bet (made in 1974) with Kip Thorne. He agreed that the Cygnus X-1 system contained a black hole. For over 200 years since John Michell described "dark stars" (stars massive enough to stop even light escaping from their surfaces), and Einstein's General Theory of Relativity predicted them, black holes had been an astronomical enigma. However, even since Hawking conceeded the bet the inability to directly observe black holes left some astronomers still unconvinced. Slowly the case for the existence of black holes has been built by indirect observational evidence.

An enormous X-ray burst in the centre of the galaxy RX J1242-11 has been observed with the Chandra and XMM-Newton X-ray telescopes by an international team of astronomers. This outburst, one of the most extreme ever detected in a galaxy, was most likely caused by gas from a destroyed star that was heated to millions of degrees Celsius before being swallowed by a centrally located supermassive black hole. The energy liberated in the process was equivalent to a supernova explosion.

"Now, with all the data in hand, we have the smoking gun proof that this spectacular event has occurred," said coauthor Gunther Hasinger, of Max Planck Institute for Extraterrestrial Physics (MPE).

(Top) Artists impression of the centre of the galaxy RX J1242-11 showing the evolution of the star as it nears the central supermassive black hole (Bottom left) Chandra X-ray image, (Bottom right) ESO optical image. Credit: Illustration: NASA/CXC/M.Weiss; X-ray: NASA/CXC/MPE/S.Komossa et al.; Optical: ESO/MPE/S.Komossa

The black hole in the centre of the galaxy RX J1242-11 is estimated to have a mass of about 100 million solar masses. The astronomers believe about one percent of the star's mass was accreted, by the black hole. This small amount is consistent with predictions that the momentum and energy of the accretion process will cause most of the destroyed star's gas to be flung away from the black hole.

For more information see: Chandra Press Release * Animations (QuickTime and MPEG) - Chandra site * Science@NASA Press Release