In this show, we talk to Dr Sonia Anton about quasars and the Gaia mission and we find out about a mysterious radio signal from an old Jodcaster, Dr Evan Keane. As always, Megan rounds up the latest news and we hear what we can see in the December night sky from Ian Morison and John Field.
In the news this month:
There are thought to be two main formation mechanisms for the ultra-dense compact stars known as neutron stars: core collapse supernovae, where massive stars greater than eight times the mass of the Sun run out of fuel and form a massive iron-core, and electron-capture supernovae, where a lower-mass core, mainly composed of oxygen, neon and magnesium, loses outward pressure supporting it against further collapse due to the sudden capture of electrons by neon or magnesium nuclei. While the explosions caused by these two mechanisms have different characteristics, it has so far been impossible to distinguish between neutron stars formed by the two processes. Now, in a paper published in Nature on November 17th, Christian Knigge and colleagues have found evidence of two distinct sub-populations of a particular type of binary pulsar.
The researchers studied a class of binary system known as BeX binaries, a neutron star orbiting a Be-type star. These Be-stars are hot, massive B-type stars on the main sequence, with significant hydrogen emission in their spectra. They rotate very rapidly and are losing material in a surrounding disk of gas. In a binary system with a Be-star orbiting a neutron star, this disk material is accreted by the neutron star, resulting in strong X-ray emission. Such systems usually have elliptical orbits with periods between 10 and 1000 days, and the neutron stars spin rapidly with periods between 1 and 1000 seconds. These systems are useful in exploring the formation mechanisms of neutron stars because they provide a well-defined sample where the physical conditions are similar and many of the properties are easily measured.
The researchers took a sample of BeX systems from the Milky Way and the Magellanic Clouds and studied their properties. What they discovered is that both the spin period of the neutron star, and the orbital period of the binary system, fall into two groups. The split between long and short period systems is significant in the full sample, and is still significant when the sample is divided by galaxy showing that the bi-modal nature of the distribution is unlikely to be due to a selection effect.
The researchers discuss three possibilities for the split in the population. One reason could be that some neutron star spin periods are more stable than others, but this idea looks unlikely since there is a correlation between the spin period and the orbital period. Another possibility is that the orbital period of the binary system may vary over time, in this case, the two sub-populations would represent two distinct long-lived stable phases. However, the timescales over which changes would occur in such systems are much longer than the lifetimes of the Be-stars involved. The third possibility is that the two classes of BeX system are caused by the two different classes of supernova.
While core collapse supernovae occur in any star greater than a specific mass, the conditions for an electron capture supernova are far more difficult to achieve. These conditions are more easily produced in binary systems. There are significant differences in the products of the two different kinds of supernova. Electron capture supernovae result in lower mass neutron stars, moving at slower velocities than those produced in core collapse supernovae. Since the kick velocity will have a significant effect on the ellipticity of the binary orbit, the two different supernova mechanisms could naturally produce two sub-populations of BeX binaries with different orbital parameters. They conclude that core collapse supernovae should produce high-mass neutron stars in eccentric orbits, while electron capture supernovae would result in lower-mass neutron stars in more circular orbits. While only a small number of systems currently have well-measured eccentricities, the results so far appear to support this conclusion. Even so, the authors point out that their suggested explanation for the two observed populations is speculative, and there are many further observational and theoretical tests which can be carried out in order to test the hypothesis.
Very few places in the solar system are geologically active today. The Earth is one, with the tectonic plates continuously shifting. Jupiter's moon Io is another, the tidal stresses of its parent planet causing large amount of volcanic activity. Another of Jupiter's moons also shows signs of significant surface movement. With its surface covered by ice instead of rock, Europa is an unusual place. The icy shell covers a liquid ocean, but the thickness of this ice crust is debated. The Galileo spacecraft showed that Europa's surface includes features known as chaos terrains, quasi-circular regions where the surface is cracked and broken in random patterns, including some features which are unique to Europa. Some of these chaos terrains, such as the archetypal Conamara Chaos, rise above the surrounding surface and no models so far have been able to explain these features. With a thick shell potentially some 10 km deep, the buoyancy of material rising in plumes below the surface is not strong enough to create the observed features. Now, a group of researchers have found a mechanism which can explain these terrains and which suggests that, at least in some places, the crust is comparatively thin.
In a paper published in Nature on November 24th, a team led by Britney Schmidt at the University of Texas at Austin describe a model which explains the observed nature of these chaos terrains. While such features have not been found on the Earth, the researchers looked at analogous processes known to occur in terrestrial sub-glacial volcanoes and ice shelves, and applied these ideas to images of Europa's chaos terrains taken by the Galileo spacecraft a decade ago. Their analysis suggests that these chaos terrains likely form above liquid water 'lenses', lens-shaped regions of liquid water which are confined under the surface by stresses in the surrounding ice. Similar features are seen in sub-glacial eruptions in Iceland although, since the surface ice is finite in this case, the water eventually escapes. The model proposed for Europa is that a rising plume of pure ice starts to melt the salty ice layer above it. As it melts, the volume decreases and the surface collapses, forming a depression and causing fractures, creating rectangular blocks of ice. The fractures then fill with salty water, which eventually refreezes, expanding as it does and raising the resulting chaotic terrain above the surrounding surface.
This model explains the raised nature of Conamara Chaos, and suggests that the depression seen in another region known as Thera Macula, a chaos region some 100-km in diameter, is still actively forming over a liquid layer just a few kilometres below the surface. If this is the case, then the next mission to take photographs of Europa should see significant changes in the appearance of Thera Macula. The thickness of the surface ice on Europa is of key importance for future missions to the moon to search for life in the subsurface ocean, since any such mission will need to drill through this layer. This research suggests that, at least in places, the ice may be relatively thin.
And finally: November saw the launch of two separate missions to Mars. The first, Phobos-Grunt, was launched from Baikonur in Kazakhstan at 16 minutes past midnight Moscow Time on November 9th. The Russian spacecraft, designed to collect samples from Mars' moon Phobos, has been 20-years in the making, and was also carrying China's first Mars satellite, Yinghuo-1. Phobos-Grunt itself was an ambitious mission designed to return samples of the surface of the 27-km diameter satellite and return them to the Earth, giving planetary scientists around the world a means of investigating its origin. Early telemetry received from the Russian craft showed a flawless launch and entry into the correct parking orbit. However, two orbits later, when the craft was supposed to have fired its main engine to place it on a trajectory towards Mars, tracking stations failed to find the probe in the expected transfer orbit. When the craft was located several hours later, it was still in its initial parking orbit indicating a failure of the planned engine burn. While official sources of information on the status of the craft were as quiet as the spacecraft itself, contact with Phobos-Grunt was eventually made by a 15-m European Space Agency tracking station located 20-km north of Perth, Western Australia. At 2025 GMT on November 22nd, a small secondary antenna bolted to the side of the 15-m dish was used to send a weak radio signal towards the craft, commanding it to switch on its transmitter. Over the following week, several further communication sessions took place (with partial success) and the received telemetry was passed to the Russian space agency for analysis. While efforts continue to communicate with the craft and diagnose the nature of the fault, the window for sending the spacecraft on its way to Phobos closed on November 21st. If ground controllers are able to gain full control of the craft, it may be possible to re-task it to another target, or at least boost it to a higher orbit to avoid it re-entering the atmosphere. With 7.5 tonnes of propellant on board, if it does burn up in the atmosphere very little debris is expected to reach the ground.
The second Mars-bound craft was the Mars Science Laboratory, a NASA mission designed to explore the Gale crater near the equator of the red planet. Launched on November 26th at 10am Eastern Standard Time, the spacecraft carries on board the Curiosity rover, the latest of a series of ground-based missions of the planet and the largest to date. At 3-metres in length, Curiosity is twice as long and five times at heavy as the twin Mars Exploration Rovers, Spirit and Opportunity, launched in 2003. While it inherits many design features for previous missions, Curiosity contains many new experiments, including equipment for on-board analysis of rock samples including a gas chromatograph, a mass spectrometer, and a tunable laser spectrometer which will be able to identify a wide range of organic (carbon-containing) compounds and determine the ratios of different isotopes of key elements. Isotope ratios are clues to understanding the history of Mars°« atmosphere and water. On arrival at Mars in August 2012, the rover will begin its journey at the bottom of Gale crater, one of the deepest depressions on Mars and a geologically fascinating place. Over the course of the mission, the rover will investigate the old rock of the crater floor, including what may be an alluvial fan, a spread of sediment left by fluid flowing down the crater wall, before moving upwards towards the crater's central peak. As it goes it will cross many layers of rock, tracing the history of the planet's surface across more than a billion years as it goes. The mission is initially funded for two years of operation on the Martian surface, although with its on-board nuclear-powered battery it could function for much longer.
Interview with Dr Sonia Anton
Stuart talks to Dr Sonia Anton (Porto University) about how a new mission to study the Milky Way can help us to understand distant quasars. Gaia is an ESA mission scheduled for launch in 2013 that is designed to map the Milky Way with microarcsecond precision. In order to know which direction it is pointing, Gaia will use quasars as reference points because the positions of quasars are already known to very high accuracy through VLBI radio observations. In this interview, Sonia tells us about quasars (and blazars) and the Gaia mission before explaining how she hopes to use data from Gaia to investigate "jitters" in the optical positions of quasars.
Interview with Dr Evan Keane
In this interview, former Jodcaster Dr Evan Keane (now of the Max Planck Institute for Radio Astronomy) talks to Liz and Mark about a radio burst that was observed using the Parkes Telescope in Australia. This burst initially looked like an single pulse from a pulsar, but more evidence showed that this was not the case. The emission did not occur again, and so far its source has not been identified. The distance to the burst was estimated using the dispersion measure, which indicated that it originated from outside of our Milky Way, making it similar to the Lorimer burst found a few years ago. One of the possibilities is that this radio burst was produced by the evaporation of a mini black hole, so Evan explains this theory to us.
The Night Sky
Ian Morison tells us what we can see in the northern hemisphere night sky during December 2011.
The summer constellations of Cygnus and Lyra are still visible high in the north-west in the evening, with their bright stars Deneb and Vega. The Milky Way arcs overhead from Cygnus. The Square of Pegasus is in the south, while the winter constellations of Taurus and Orion follow from the east. The Pleiades star cluster lies in Taurus, and in the centre a telescope reveals a triplet of stars and a double star system nestled among the brighter members. Below Orion's Belt, the nebula M42 makes up part of Orion's Sword. The stars of the Trapezium can be seen within the nebulosity using a telescope. Sirius, the brightest star in the sky, rises to the lower left of Orion later in the evening. Above and left of Orion and Taurus is Gemini with its twin bright stars Castor and Pollux. Leo rises late in the night.
- Jupiter dominates the night sky, reaching an elevation of 50° for observers in the UK. Its height means observations of detail on its surface is less hampered by atmospheric scintillation.
- Saturn rises before dawn, reaching an elevation of about 20° when the Sun comes up at the beginning of the month. It is a few degrees from the star Spica at this time, in the constellation of Virgo. By the end of the month it is at a height of 27° in the south at dawn. Its maximum elevation will get lower over the next few months, but its rings continue to open out from our point of view and are now at 13.5° to the line of sight. The planet's disc spans 16.5".
- Mercury passes in front of the Sun (inferior conjunction) on the 4th, but reappears before dawn in the south-eastern sky near the end of the month, shining at magnitude -0.4. It is near to a thin crescent Moon and the star Antares on the 23rd.
- Mars reaches an angular diameter of 9" by month's end, allowing surface detail to be seen with a telescope. It also brightens from +0.7 to +0.2 during December, but only rises towards midnight.
- Venus is quite low in the south-west at sunset, but reaches 18° elevation by the time the Sun goes down at the end of the month. Its angular size is 13° and it is now waning in phase, but remains at a magnitude of about -3.9 all month.
- A partial lunar eclipse is visible in the UK from around 3pm GMT (Greenwich Mean Time) on the 10th, although the Moon will only be up for an hour or so before it begins to leave the Earth's shadow.
- The Geminid meteor shower peaks in the early mornings of the 14th and 15th. The Moon will obscure some of the shower, however. The meteors come from the asteroid 3200 Phaeton, rather than from a comet as is more common.
- Saturn, the star Spica and a thin, waning crescent Moon are close together an hour before dawn on the 21st. Mercury may also be visible, very low in the east.
- The Ursid meteor shower is visible on the morning of the 23rd, unhampered by the Moon's glare. 10-15 meteors per hour may be observed, but occasionally more are seen. The radiant, from where the meteors appear to originate, is near the star Kochab in Ursa Minor
- Mercury and a thin crescent Moon appear together before dawn on the 23rd.
- Venus and a thin cresent Moon are close together after sunset on the 27th.
- Our neighbouring galaxy, Andromeda, also known as M31, is visible this month. With a large angular size of around 3°, it is best viewed through binoculars or a small telescope.
John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during December 2011.
The constellations of Taurus, Orion, Canis Major and Canis Minor are in the northern sky in the evening. The fainter summer constellations of Hydrus, Mensa and Volans cluster around the south celestial pole. The ninth-brightest night-time star, a blue giant named Achernar, is high in the winding constellation of Eridanus, the River. It is not spherical, but is flattened by its high rotational speed. Hydrus, the Water Snake, contains three bright stars, each of around magnitude +3, in a triangle which crosses the constellation of Tucana and the Small Magellanic Cloud. Pi Hydri is a double star of magnitude +5.5, consisting of unconnected red and yellow stars. Mensa, the Table, contains no stars brighter then magnitude +5, but it is home to part of the Large Magellanic Cloud, the most massive dwarf satellite galaxy of the Milky Way, in which star clusters and nebulae can be seen with binoculars or a small telescope. The star Pi Mensae is orbited by a planet of around 10 times the mass of Jupiter. Volans, the Flying Fish, contains a double star called Gamma Volantis in a field of scattered stars.
The rich star fields of Carina, along the Milky Way, contain the asterism of the Diamond Cross and the open cluster IC 2602, known as the Southern Pleiades. The cluster contains over 30 stars, the brightest of which is Theta Carinae at magnitude +2.8. On the other side of the Diamond is another open cluster, NGC 3532, or the Wishing Well Cluster, which spans twice the area of the full Moon. The Carina Nebula, at the heart of which is the immense star system of Eta Carinae, is in the same constellation. Crux, the Southern Cross, is further south along the Milky Way, small but very visible. The dark and dusty cloud of the Coalsack Nebula runs alongside, and is a place where stars of the future will be born.
- Venus is low in the west after sunset, in a gibbous phase similar in shape to an almost-full Moon.
- Jupiter is high in the northern sky in the evening, allowing telescopes to make out its disc and larger moons.
- A total lunar eclipse is visible throughout Australia and New Zealand, with the Moon completely within the Earth's shadow on the 11th from 03:06 to 03:58 (New Zealand Daylight Savings Time, 13 hours ahead of GMT).
Odds and Ends
The White House has responded to a petition asking the Obama Administration to "formally acknowledge an extraterrestrial presence engaging the human race". Surprisingly enough, the answer from the White House is that there is no evidence for contact with aliens.
An article in the Astrobiology journal has ranked the habitability of other planets and moons. The top candidates include Saturn's moon Titan and the extrasolar planet Gliese 581g.
An article to appear in the December edition of the Communicating Astronomy with the Public journal has appeared on the arXiv analysing the misuse of lunar images in Christmas cards and wrapping paper. The author points out that many Christmas cards depict a waning crescent Moon which would mean that children would be out decorating Christmas trees in the early hours of the morning.
|Interview:||Dr Sonia Anton and Stuart Harper|
|Interview:||Dr Evan Keane, Liz Guzman and Mark Purver|
|Night sky:||Ian Morison and John Field|
|Presenters:||Melanie Gendre, Jen Gupta, Liz Guzman and Mel Irfan|
|Editors:||Mark Purver, Megan Argo, David Ault, Claire Bretherton, Liz Guzman and Stuart Harper|
|Intro/outro script:||David Ault|
|Baron Cardiff:||Stuart Lowe|
|Ugly Sisters:||Adam Avison and Mark Purver|
|Fairy Jodmother:||Megan Argo|
|Prince Professional Respect:||David Ault|
|Extras:||Leo Huckvale and Christina Smith|
|Segment Voice:||Mike Peel|
|Website:||Jen Gupta and Stuart Lowe|
|Cover art:||The first image from e-Merlin, showing the "Double quasar". CREDIT: Jodrell Bank Centre for Astrophysics|