Shallow jet streams

The outer ice giant planets, Uranus and Neptune, have the strongest winds in our Solar System.  Equatorial east-west zonal winds can blow at a fierce 1,000 km/h on Neptune,  while at higher latitudes the winds swap direction and become easterly jet steams. The global wind patterns on Uranus are similar but the wind speeds not as strong.  (The strongest jet streams on Earth are in the polar regions and average around 200 km/s.)

Wind speeds on Uranus (left) and Neptune (right) as a function of latitude. Data comes from Voyager 2 (circles) and HST (squares) observations. (Credit: Kaspi et al. 2013, Nature)

While these measurements were first made in the 1980′s when the Voyager 2 spacecraft fly past Uranus and Neptune, what has remained a mystery is how deep these flows penetrate into the atmospheres of the giant planets.  The zonal wind speeds are determined by following methane and ammonia clouds near the upper cloud deck of the atmosphere. Knowing the depth of these winds is vital in understanding the global dynamics as well as internal structure of the giant planets, so astronomers want to know whether the high-speed jet streams result from shallow atmospheric processes or if they extend deep into the planetary interior.

Cloud features in the upper atmosphere of Neptune are used to determine  jet stream speeds. (Credit: NASA, ESA, Hubble Heritage Team, STScI/AURA)

In this week’s edition of Nature, Kaspi et al. use gravity field data from Voyager 2, combined with atmospheric modelling, to answer this outstanding problem.  Perturbations to the gravity field of the ice giants, which were measured by small changes in the speed of Voyager 2 as it flew close to the planets, result from the oblateness of the rapidly rotating planets, as well as small changes in the density of the planet caused by the fast zonal winds. By analysing the gravity field data, Kaspi et al. show that the winds must be confined to the outer 1,000 km of both planets (which have radii of about 25,000 km).  They conclude that the dynamics controlling the zonal jet streams come from shallow processes.

What remains unknown is the energy source that drives the winds.  Being so far from the sun, solar heating is much weaker than on Earth.  Neptune has its own internal heat source, and it seems likely that the rapid rotation and internal convective motions play a part in driving the jet streams.

For more information, see:

• Neptune’s windy weather skims surface, Stuart Gary, ABC Science
• Atmospheric confinement of jet streams on Uranus and Neptune, Kaspi. et al. (2013), Nature 497, 344 [Swinburne login]
• Plumbing the depths of Uranus and Neptune, Peter Read, Nature News & View [Swinburne login]

Brightest gamma-ray burst ever

The most energetic gamma-ray burst ever detected was announced this week by the team at the Fermi Gamma-Ray Space Telescope.  The gamma-ray burst (or GRB) event was triggered on Saturday 27 April 2013, and so the GRB has the name GRB 130427A.  The Fermi Large Area Telescope (LAT) recorded energies up to 94 GeV, almost 3 times higher the energy of the previous LAT record of GRBs.  The GeV emission lasted for several hours and remained detectable by LAT for most of the day. As well as the most energetic,  GRB 130427A also set the record for the longest gamma-ray emission from a GRB measured to date.

The gamma-ray sky as see by the Fermi Gamma-ray Space Telescope before (left) and after (right) the record-setting gamma-ray burst GRB 130427A. Each frame covers 3 hours of time, with the left frame including the first 30 minutes of the GRB event. The map includes all gamma-ray sources with energies above 1 MeV. (Credit: NASA/DOE/Fermi LAT Collaboration)

When Fermi triggered the GRB event, the Swift satellite quickly determined its position towards the constellation Leo. With the position tied down, follow-up observations were immediately made by ground-based optical, infrared and radio telescopes, including finding an optical counter-part in the Catalina Real-time Transient Survey. The GRB is now know to have originated in a distant galaxy about 3.6 billion light years away.

Swift’s X-Ray Telescope image of of GRB 130427A, taken at 3:50am EDT on 27 April 2013. The image, made with a 0.1-second exposure, is 6.5 arcminutes across. (Credit: NASA/Swift/Stefan Immler)

GRBs are brief but very intense bursts of gamma radiation. There are two types of GRBs defined by the length of their burst. Long bursts (longer than two seconds) are thought to be associated with supernova explosions that result when massive stars run out of nuclear fuel and undergo sudden collapse, and short bursts (shorter than two second) are thought to originate from the merger of two compact objects such as neutron stars. Both types are GRBs are likely signal the birth of a black hole. In the case of  GRB 130427A, the long-period burst likely resulted from core collapse of a massive star.

For more information, see

• NASA’s Fermi, Swift See ‘Shockingly Bright’ Burst, NASA Features
  The “Wow” blast, HEASARC pic of the week
• Gamma-ray Coordinates Network (GNC) circulars: GCN Circular #14448,  GCN Circular #14450, GCN Circular #14471 and GCN Circular #14508
• An untriggered optical detection of GRB 130427A, The Astronomers Telegram
• GCN (Gamma-ray Coordinates Network) Report Archive (keep an eye on this for details!)
• Fermi Gamma-ray Space Telescope, NASA site
• Fermi Gamma-ray Space Telescope, Goddard site
• Swift Gamma-ray Burst Mission

[Sarah Maddison]

End of Herschel operations

On 29 April 2013, the ESA Herschel Space Observatory ran out of liquid helium. Launched on 14 May 2009, the 3.5-m infrared telescope was the largest and most sensitive infrared space telescope. Operating from 55 to 670 microns, Herschel observed in the far-infrared and sub-millimetre parts of the electromagnetic spectrum, specifically designed to study the cool Universe – cold and dusty regions of our and other galaxies where stars form and astrochemistry in the interstellar medium.  Herschel had three science  instruments: two medium resolution cameras plus spectrographs: PACS (the Photodetector Array Camera and Spectrometer) and SPIRE (the Spectral and Photometric Imaging REceiver), and HIFI (the Heterodyne Instrument for the Far Infrared).

Herschel Space Observatory, which orbited at the L2 point 1.5 million km from the Earth. (Credit: ESA/Herschel)

The 2300 litres of liquid helium was used to cool the instruments to nearly zero Kelvin to ensure that the instruments operated with great sensitivity.  The mission was planned for three years and managed to last almost four. During that time, Herschel acquired 25,000 hours of data, covering 35 000 scientific observations from almost 600 programmes since science operations started in December 2009. The rich dataset, which is publicly available after a 12 month propriety period, will keep astronomers busy for many years to come!

For more details, see

• Herschel closes it eyes to the Universe, ESA Herschel press release
• Observation gives way to examination as Herschel coolant runs out, ESA Sci&Tech press release
• Herschel Science Centre, ESA website
• Herschel Space Observatory, UK outreach site

GR still looking good

Einstein’s theory of gravity – general relativity – has been confirmed (again!) by a newly discovered relativistic binary. The theory of general relativity, which describes how matter warps space-time and thus causes gravity, predicts that objects in orbit will produce ripples in space-time called gravitational waves.  The effect is amplified and potentially detectable when the objects are massive and in a tight orbit (meaning they are very close to one another). While gravitational waves have still not been detected, the energy carried away by gravitational waves will cause the orbit of the binary to shrink by a specific amount each year.  Such a change in a binary orbit, specifically the decline in the orbital period with time, was measured for the double pulsar system PSR B1913+16 (also known as the ‘Hulse-Taylor binary pulsar’), for which Russell Hulse and Joe Taylor  were awarded the 1993 Novel Prize in Physics.

Artist’s impression of the pulsar PSR J0348+0432 (which radio jets) and its white dwarf companion. The binary emits gravitational waves, represented by ripples in (blue) spacetime. As a result, the stars spiral inwards towards each other and the binary period reduces. (Credit: ESO/L. Calçada)

The newly discovered binary contains a pulsar, called J0348+0432, which was discovered with the Green Bank Telescope with has a rotational period of 39 ms (spinning ~25.6 times every second) and a binary period of 2.46 hours.  The optical counterpart (i.e. the binary companion) was discovered in the Sloan Digital Sky Survey archive.  Both the colour and flux of the companion suggested it was a white dwarf, and follow-up spectroscopic observations with the Apache Point Optical Telescope confirmed that the companion was indeed a low-mass white dwarf. Radial velocity observations further confirmed that the pulsar and white dwarf were gravitationally bound. The short period and the fact that one stars is a pulsar suggested that the binary could be used to test general relativity if the orbital decay (resulting from the emission of gravitational waves) could be measured accurately.

Further optical observations with the ESO Very Large Telescope and precise timing observations of the pulsar with three radio telescopes (the 305-m Arecibo telescope, the 10o-m Effelsberg telescope and the 100-m Green Bank Telescope) were used to determine the masses of both components. The team, lead by PhD student John Antoniadis from the Max Planck Institute for Radioastronomy, found that the pulsar  J0348+0432 is 2.01 ± 0.04 solar masses  and the white dwarf of 0.172 ± 0.003 solar masses. With an orbital period of just 2.46 days, such a close binary should definitely be radiating gravitational waves and losing energy. Using their radio timing observations, the team determined that the orbital period of the binary is decreasing by 8.6 microseconds per year (!), which is very close to the predictions of general relativity. The team argue that these results support general relativity, and are inconsistent with some (but not all) other alternative theories of gravity.

Artist’s impression of the PSR J0348+0432 system. The 2 solar mass compact pulsar (with beams of radio emission) produces a strong distortion of spacetime (the green mesh), while spacetime around the 0.17 solar mass white-dwarf companion is less curved. (Credit: Science, Antoniadis et al.)

The new binary system also provides important information on the spin evolution of pulsars after mass accretion. The predicted merger time of the two stars is about 400 Myrs, after which time the system might evolve to become a single black hole, or a pulsar and an planet-mass remnant (like the “diamond planet” discovered in 2011 by Swinburne astronomers).

For more information, see:

• A Massive Pulsar in a Compact Relativistic Binary, Antoniadis et al. (2013), Science, 340, 1233232  [Swinburne login required]
  
• Einstein was right – so far, ESO press release
• Massive double star is latest test for Einstein’s gravity theory, Ron Cowen, Nature News
• The Green Bank Pulsar Survey data results:

1. Boyles et al. (2013),”The Green Bank Telescope 350 MHz Drift-scan survey. I. Survey Observations and the Discovery of 13 Pulsars“, ApJ, 763, 80 and
2. Lynch et al. (2013), “The Green Bank Telescope 350 MHz Drift-scan Survey II: Data Analysis and the Timing of 10 New Pulsars, Including a Relativistic Binary”, ApJ, 763, 81

Fomalhaut b remains a fascinating mystery

The large dusty ring surrounding the nearby young main-sequence star Fomalhaut was first discovered by the Infrared Astronomical Satellite (IRAS) in 1985, which found an excess of 60 µm emission above the stellar photosphere around a number of nearby stars. The excess infrared emission arises from a ring of dust that surrounds the star. The dust absorbs the stellar light and re-emits in the infrared.  When we look at the spectrum of the star (with is a measure of its brightness as a function of wavelength), stars hosting dusty disks will have more emission in the infrared than can be explained by the stellar photosphere.

Infrared excess from a dusty disk and a dusty ring surrounding a star. The top panel shows the spectrum of a star. The second panel shows the spectrum of a star surrounded by a dusty disk, while the third panel shows the spectrum of a star surrounded by a ring of dust. In both cases, the surrounding dust is seen as an excess of infrared emission above that of the stellar photosphere. A “bump” of excess infrared emission indicates the presence of a dust ring. (Credit: NASA/JPL-Caltech/T. Pyle, SSC)

In 2005, Paul Kalas and collaborators presented the first HST/ACS image of the debris disk around Fomalhaut, finally revealing the structure of the thin ring of dust.  Previous observations with the sub-millimetre telescope Jame Clerk Maxwell Telescope by Wayne Holland and collaborators showed a cavity in the dust emission, clearly demonstrating that the dust was a ring rather than a disk, but the resolution was not high enough to reveal the dusty ring. The HST observations, with a resolution of just 0.5 AU, clearly show that the dust belt in 25 AU wide, with a very sharp inner edge at a distance 133 AU from the star. The Kalas et al. results also found that the centre of the disk was off-set from star by about 15 AU. They suggest that the sharp inner edge of the dust belt and the centre of symmetry offset from the star can both be explained by an unseen planet on an eccentric orbit between the star and the inner edge of the belt.

Dust belt around Fomalhaut as seen by HST. The centre of the dust belt is offset from the star, and the inner edge of the belt is very sharp. These two facts point to sculpting of the dust belt by an unseen planet. (Credit: NASA, ESA, Kalas & Graham (UC Berkeley) & Clampin (GSFC))

Thus the hunt for the unseen planet began. Using the Keck and Gemini telescopes, along with HST’s ACS, observations were made of Fomalhaut over several years looking for common proper motion sources (basically to hunt for a blob just interior to the dust belt moving with the same proper motion across the sky as Fomalhaut).  In 2008, Paul Kalas and collaborators announced the discovery of Fomalhaut b from HST/ACS observations made in 2004 and 2006.  A small blob, situated about 18 AU from the inner edge of the dust belt, is seen to move over the course of two years and the authors suggested that they had discovered the unseen planet.  However, the HST observations detected the source at 600 and 800 nm, but no detections of the source were made in the near-infrared with either Keck or Gemini and nor with the Spitzer Space Telescope.

HST discovery image of Fomalhaut b. Is this the planet sculpting the Fomalhaut dust belt? (Credit: NASA, ESA & Kalas, UC Berkeley)

However, the fact that Fomalhaut b was seen in scattered optical light and not in the near-infrared is a mystery, as planets are expected to be detected by their thermal emission in the near-infrared.  The lack of near-infrared emission rules out a warm, massive planet and constrains the mass to be less than 3 M_jup. So what is Fomalhaut b?  Whatever it is, it must contain dust. Perhaps the HST observations are scattered light from a large circumplanetary disk around a massive planet, or a dusty disk around the planet caused by collisions between irregular satellites, or perhaps it is not a planet at all and what we are seeing in dust created by a recent collision between two Kuiper Belt-like objects.

In January this year, Paul Kalas and his team presented new HST/STIS observations which at the very least confirm the existence of Fomalhaut b.  They present four epochs of HST data over 8 years. With this new data, the team have been able to contain the orbit of Fomalhaut b, which is highly eccentric (e = 0.8 ± 0.1).

New HST/STIS observations in 2012 confirm the presence of Fomalhaut b, but its nature remains elusive. (Credit: NASA, ESA, & Kalas, UC Berkeley)

Last week’s Transformational Science with ALMA conference in Hawaii, Paul Kalas told us that while he still doesn’t know what Fomalhaut b is, he does now know that its orbit will appear to cross the dust belt around Fomalhaut in the few decades, which could undoubtedly lead to some very interesting dynamics.  With such an eccentric orbit, Fomalhaut b’s periastron may be as small as 10 AU. This information allows Kalas to determine the lower limit on the mass and size of Fomalhaut b to about 1.5e21 kg (about 1/10th that of Pluto) and 1000 km in diameter (about half the size of Pluto) to ensure that it withstands tidal shearing at periastron passage.

The new data also confirms that Fomalhaut b is not the planet that is sculpting the inner edge of the dust belt and causing the ring to be offset from the star – which means there must still be an unseen planet hiding somewhere in the Fomalhaut system.  Paul suggest that (the as yet unseen) Fomalhaut c is sculpting the dust belt, and that perhaps (the as yet undiscovered) Fomalhaut d has scattered Fomalhaut b onto its highly eccentric orbit, a bit like Centaurs in our own Solar System.  Stay tuned for more details….!

For more information, see

• Hubble Reveals Rogue Planetary Orbit for Fomalhaut b, Hubble press release (Jan 2013)
• Hubble Directly Observes Planet Orbiting Fomalhaut, Hubble press release (Nov 2008)
• Exoplanet’s misstep raises doubts, Nature News by Eric Hand (Sept 2011)
• Fomalhaut b: Independent Analysis of the Hubble Space Telescope Public Archive Data, Galicher et al. (2012), arXiv:1210.6745

Hint of dark matter

Intriguing results from the Alpha Magnetic Spectrometer aboard the International Space Station hints at evidence of annihilation from dark matter particles. Following from last week’s SAO astro news update from the Planck mission, the revised mass-energy budget of the Universe is 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Dark matter has been known about for over 80 years and is indirectly detected by its gravitational influence on ordinary matter.  But can we directly detect dark matter particles?

Composition of the Universe as measure by WMAP (“before Planck”) and as measured by Planck (“after Planck”). Normal matter like stars and galaxies comprise just 4.9% of the Universe, while dark matter, which is detected by its gravitational influence on ordinary matter, makes up 26.8% of the Universe. The remaining 68.3% is dark energy, an unknown force which accelerates the expanding Universe. (Credit: ESA & the Planck Collaboration)

Particle physicists and astronomers are trying three different approaches to directly detecting dark matter: by creating dark matter particles in particle accelerators like the Large Hadron Collider, by catching dark matter particles that whizz through the Earth in deep underground detectors, and by looking in space to find evidence of rare dark matter collision events.

When two dark matter particles collide, they will annihilate each other and transform their energy into high-energy photons and high-energy particles. These particles can be detected by the Alpha Magnetic Spectrometer (AMS), which was installed on the exterior of the International Space Station in May 2011 (in the last Space Shuttle Endeavour flight). The main aims of the AMS-02 experiment is to search for antimatter and dark matter, and it is constantly bombarded with high-energy particles, or cosmic rays.  AMS uses a large, 3-foot magnetic ring to produce a strong magnetic field which deflects the path of the incoming charged particles as they pass through various detectors which measure the speed, energy and direction of the particles. To date AMS has measured over 30 billion cosmic rays!

AMS (round instrument on the left) on the International Space Station, 31 May 2011. The AMS-02 experiment is a state-of-the-art particle physics detector. (Credit: AMS/NASA)

This week the first results from the AMS team were released, which analysed 25 billion cosmic ray events (!) over the 18 months from May 2011 to December 2012.  Of these cosmic ray events, 6.8 million were identified as electrons and their antimatter pair positrons with energies in the range 0.5 to 350 GeV. The team have measured the positron fraction, which is the ratio of the positron flux to the electron+positron flux across the 0.5-350 GeV energy range. They found that the positron fraction decreases with increasing energy, and then increases again from 10 GeV to ~250 GeV and finally appears to flatten beyond 250 GeV. This AMS data, which only represents about 10% of the expected data over the lifetime of the experiment, is of excellent quality when compared with pervious measurements of the positron fraction.

Positron fraction as a function of energy from 0.5-350 GeV. The AMS results are shown in red (with error bars) and compared with previous published measurements. (Credit: Aguilar et al. 2013, Phys. Rev. Lett.)

The team demonstrate that the positron fraction spectrum (the plot above) has no fine structure to it, nor does the spectrum vary with time. The positron-to-electron ratio is not anisotropic, which indicates that the positrons do not come from any preferred direction in space.  The researchers conclude that they are seeing some new physical phenomena, either from particle physics or astrophysics. What is not yet known is whether this position fraction spectrum originates from dark matter particle annihilation or from pulsars in our Galaxy, which also produce electrons and positrons. Extending the spectrum to higher energies will resolve this issue.

For more information, see:

• First Result from the Alpha Magnetic Spectrometer Experiment, AMS-02 press release
• Scientists report hint of dark matter in first results from $2 billion cosmic ray detector, Phys.Org
• Shining light on elusive dark matter, ESA/DAMA mission press release
• First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV, Aguilar et al. (2013), Phys. Rev. Lett., 110, 141102 [Swinburne login required]

[Sarah Maddison]

Planck supports inflation but confirms some strangeness

The European Space Agency (ESA) has just released the initial 15.5 months of data from the Planck mission that detects the relic radiation from the Big Bang when the universe was just 380 000 years old. The map of the Cosmic Microwave Background shows minute temperature fluctuations that correspond to regions of slightly different densities at a spatial resolution of 5 arcminutes. These regions evolved into the galaxies, clusters and
superclusters of today.

Anisotropies in the Cosmic Microwave Background (CMB) as observed by Planck. This CMB map, imprinted on the sky when the Universe was just 380,000 years old, shows tiny temperature fluctuations corresponding to regions of slightly different densities. These tiny overdensities represent the seeds of all structure we see in the present epoch. (Credit: ESA and the Planck Collaboration)

Planck’s new map provides an excellent confirmation of the standard model of cosmology, which is a 6 parameter, flat (Ω_total = 1 exactly) Λ-CDM model, where Λ represents dark energy and CDM = cold dark matter. The universe appears flat to a level of 0.1%. The inferred amounts of normal matter, dark matter and dark energy are in
good agreement with previous results from WMAP (the Wilkinson Microwave Anisotropy Probe). The derived Hubble constant has decreased a little, meaning the universe is slightly older (by about 100 million years or so).

What is important is that Planck has confirmed some strange features initially suggested in the WMAP data. Planck’s map is at higher resolution and it reveals some peculiar unexplained features that may well require new physics to be invoked, or even new ways of thinking about the origin of the universe .

Here are some details from Planck:

• The Universe is 13.82 billion years old (up from 13.77 billion years from WMAP)
• The Universe is expanding a bit slower than we expected (H₀ = 67 km/s/Mpc down from 69 km/s/Mpc from WMAP)
• The Universe is 4.9% normal matter, 26.8% dark matter, and 68.3% dark energy (WMAP found 4.6%, 24.0% and 71% respectively)
• The Universe is lopsided (hints of this were seen in the WMAP results)

It is the last item above that is of great interest, and has profound implications. Note that the Planck map shows temperature fluctuations just a bit stronger on the right hand side. Remember you are looking at the whole sky. It is a significant discrepancy of the CMB signal as observed in the two opposite hemispheres of the sky. Why would one part of the sky be showing this? Shouldn’t the whole map look the same? The possibilities are still being debated. The dark energy may not be constant with time. It could also be an “imprint” left over from the Big Bang. If so, this has huge ramifications for the
multiverse theory.

Sean Carroll has an excellent blog in which he discusses such a situation or as he calls it a “lopsided universe”. As you can see from his article from 5 years ago (June 2008) there were a few interesting yet speculative ideas that might be able to explain such a signature. Such proposed mechanisms may involve processes prior to Inflation.

If you navigate through the arXiv pre-print server and search the title field for “Planck 2013 results” you will find  twenty-nine — yes 29! — pre-prints uploaded by the Planck science team. Figures 11 and 37 of the Planck collaboration Paper XV shows the power spectra of the fluctuations. We reproduce the power spectra here:

Planck power spectrum. The largest angular scales are on the left side, whereas smaller and smaller scales are shown towards the right. The red dots are the Planck data. The green curve represents the best fit of the ‘standard model of cosmology’ to the Planck data. The pale green area around the curve shows the predictions of all the variations of the standard model that best agree with the data. (Credit: ESA and the Planck Collaboration)

The overwhelming response to the Planck results is one of wonder. Yes, the Λ-CDM model still looks the goods, inflation still looks valid, but the mission has thrown up new questions about the early universe, and perhaps even hinted at a multiverse. Future results from Planck will include polarisation data which will include the polarisation signal at large scales which relate to reionisation.

“The Universe seems to be simpler, but at the same time, also weirder than we ever thought. The anomalies in the CMB are telling us something fundamental: we do not know yet what this is, but we are eager to find out,” says  Jan Tauber, Planck Project Scientist at ESA.

For more details, see

• Planck reveals an almost perfect Universe, ESA press release
• Planck telescope peers into primordial Universe, Mark Peplow, Nature News

[Glen Mackie]