March 2010: Explosion

We have an interview with Professor Mike Barlow from UCL. As ever we have the latest astronomical news, and what you can see in the March night sky.

The News

In the news this month:

• Supernova explosions are initially classified by the chemical signatures in their optical spectra. While some are caused by the catastrophic collapse of stars more than eight times as massive as the Sun, others are thought to be caused by white dwarfs, stars like the Sun which have already evolved off the main sequence and shrunk in size. Called type Ia supernovae, such explosions are thought to have a fixed brightness, allowing them to be used as standard candles to measure distances to galaxies and test cosmological models of the expansion of the universe. There are two possible models for these type Ia supernovae, both involving the explosion of white dwarf stars. Of these theories, the one thought to be the most likely involves the accumulation of material from a companion star onto the surface of a white dwarf. When the mass of the white dwarf exceeds a certain limit, known as the Chandrasekhar limit, it becomes unstable and explodes. The second theory is that the explosion is caused by the merger of two white dwarfs in orbit around each other. While the first theory was thought to be the most likely explanation, research published in Nature on the 18th February suggests that the second model may, in fact, be far more likely than was previously assumed.

  The X-ray signatures of these two different explosion mechanisms are quite different, with far more pre-explosion X-ray emission expected from an accreting white dwarf than from the merger scenario, so two researchers at the Max Planck Institute for Astrophysics in Germany used data from the Chandra X-ray Observatory and the Spitzer Space Telescope to examine several nearby galaxies. The ongoing accretion process prior to a supernova explosion would generate significant amounts of X-ray emission detectable by Chandra, while a binary white dwarf system heading towards a merger would not generate such emission. The infra-red luminosity of a particular galaxy, taken from the Spitzer data, gives an estimate of the number of white dwarfs in the galaxy, leading to an estimate of the expected X-ray luminosity if accretion is the dominant mechanism. The astronomers examined observations of five nearby elliptical galaxies, as well as the bulge of M31, the nearest spiral galaxy to the Milky Way, and found in all cases that the predicted X-ray luminosity was between 30 and 50 times lower than expected if the accretion scenario was the main cause of type Ia supernovae.

  The results imply that, at least in elliptical galaxies, the dominant mechanism behind type Ia supernovae is white dwarf merger rather than accretion. The researchers calculate that, in ellipticals, it may be that less than five per cent of type Ia supernovae explosions are caused by accretion. The story is slightly different in spiral galaxies however, where clouds of neutral gas and thick dust lanes typical of star formation in spiral galaxies could be obscuring the X-ray radiation created in the pre-explosion phase of the accretion scenario.

  These new results may have implications for cosmological studies, since the assumed standard luminosity of type Ia supernovae is used to calculate the expansion velocity of the universe. Since the two merging stars may have slightly different masses in different systems, the total explosion luminosity may not be as standard as thought.

• A long-standing question in the study of star formation is whether the process was more efficient in the early universe than it is today.

  Stars form through the collapse of clouds of cold gas. As the collapse progresses, the core of the cloud gets denser and hotter until nuclear fusion begins and a star is born. In the local universe, however, cold molecular gas is relatively rare so star formation occurs slowly; the Milky Way forms new stars at a rate of only a few per year. More distant galaxies formed stars at a much higher rate, but in order to determine whether this is due to a more efficient star formation process or a more ready supply of molecular gas, it is necessary to investigate their gas content.

  Star formation within these clouds is very difficult to observe directly since the gas absorbs much of the visible light produced by young proto-stars. Once they begin to shine, the radiation pressure of young stars begins to dispel the surrounding gas and the star becomes visible. The gas itself is hard to detect but some molecules, such as carbon monoxide, are visible through the radiation they emit at infrared wavelengths.

  A team of researchers used the Plateau de Bure interferometer to examine the gas content of two samples of galaxies which are so distant that we see them as they were when the universe was only 40 and 24 percent of its current age. Because they are so distant, the infrared radiation from the carbon monoxide molecules in these galaxies is shifted into the part of the spectrum where wavelengths are measured in millimetres. Using new receivers recently installed on the antennas of the interferometer at the Plateau de Bure in France, Linda Tacconi and colleagues imaged the molecular gas content of these galaxies. Many previous studies have focussed on highly extreme examples, galaxies forming stars at very high rates due to powerful central black holes or systems where galaxies are merging, but Tacconi's team studied more modest examples likely to be more typical of normal star forming galaxies.

  Published in the journal Nature on February 11th, their results show that distant star forming galaxies were in fact gas rich, containing three to ten times more cold gas (as a fraction of the galaxy's total mass) than equivalent galaxies in the local universe today. Their results also show that the fraction of gas does not vary greatly with redshift: the galaxies in the more distant sample, seen when the universe was just three billion years old, contained 44 percent molecular gas while those in the closer sample, seen when the universe was 5.5 billion years old, contained 34 percent gas.

  The results also suggest that there is a mechanism replenishing the molecular gas in these galaxies. The rate at which stars are forming can be used to estimate how long it would take to use up the entire supply of molecular gas, the timescale turns out to be less than the time interval between the two samples, suggesting that either the gas is replenished, or that the two galaxy populations studied have experienced different evolutionary paths.

• In just fifteen years, several hundred planets have been discovered around stars other than the Sun using a variety of techniques. Even without the ability to directly image these other worlds, some of their properties can be determined. Most extra solar planets found so far are massive gas giants orbiting close to their parent stars, since these are the types of planets that the detection methods are most sensitive to. As techniques develop and improve, astronomers are finding out more and more about these other worlds, including the composition of their atmospheres.

  The chemical make-up of planetary atmospheres can provide clues to a whole variety of processes, including both geological and biological effects, but often our own atmosphere gets in the way, hampering attempts to detect the spectral signatures of certain molecules. To get a full picture of what is going on often requires both ground-based and space-based observations. Satellite observations have previously detected the absorption signatures of water, carbon dioxide, carbon monoxide and methane in the atmospheres of two so-called hot Jupiters, planets with masses similar to or greater than that of Jupiter, but orbiting far closer to their parent star.

  In research published in Nature on the 4th of February, a team led by Mark Swain of the Jet Propulsion Laboratory in California, have detected the signature of emission from methane in the atmosphere of one particular exoplanet known as HD-189-733-b. Using the NASA Infrared Telescope Facility located on Mauna Kea, the team discovered an unexpectedly strong emission feature at a wavelength of 3.25 microns, corresponding to the presence of methane in the planet's atmosphere.

  This is not the first time that methane fluorescence has been seen, but it is the first time it has been detected in the spectrum of an exoplanet. It has previously been seen in our own solar system in the atmospheres of Jupiter, Saturn and Titan, although HD-189-733-b is much closer to its parent star and so offers a chance to study a planetary atmosphere under very different physical conditions.

• And finally: On February 4th, new images of Pluto were released showing a surprisingly dynamic surface. Actually taken in 2002 and 2003, the new images from the Hubble Space Telescope aren't sharp enough to pick out individual surface features, even Hubble lacks the resolution to image Pluto in that amount of detail, but they do reveal a varied surface with patches that have changed in brightness considerably since the previous set of images taken in 1994. The images suggest that Pluto's surface and atmosphere undergo dramatic seasonal variations. In the nine years since the previous images were taken, Pluto has become significantly redder and the northern hemisphere has increased in brightness. The changes in surface brightness are likely caused by the seasonal effects of surface ice sublimating at one pole and refreezing on the other as Pluto moves in its 248-year orbit around the Sun. Observations like this will be used to plan the images taken by the New Horizons probe as it flies past Pluto at high speed in 2015.

Interview

In January 2010, Jen got Professor Mike Barlow (UCL) into the Jodcast studio to tell us about evolved stars, supernovae and what they tell us about dust in the universe. Dust is everywhere in the universe but we still don't know how it is formed. For a long time, it was thought that the dominant source of dust was low mass stars at the end point of their lives. However young, far away galaxies have been observed to be producing large amounts of dust. As we are observing these galaxies in the early stages of the Universe, low mass stars will not have had time to evolve (and therefore produce dust) so it is now thought that the dust is created when high mass stars end their lives as Type II supernovae. Mike and his colleagues are studying supernovae in nearby galaxies to see if they are producing enough dust to explain the amounts seen in the distant galaxies. As it stands, the nearby supernovae don't produce quite enough dust to account for the quantities seen.

Mike Barlow also tells us about the different types of supernovae, why we don't see as many supernovae in our own galaxy as we would expect and his work with the Spitzer and Herschel Space Telescopes.

The Night Sky

Ian Morison tells us what we can see in the night sky during March 2010.

Northern Hemisphere

As the Sun sets there is a lovely skyscape to the south, with Orion just a little to the west of south. Below to its left is the very bright star Sirius in Canis Major and up to its left are the heavenly twins Gemini. High overhead is Auriga with the bright star Capella. As the night moves on, Leo rises at about 9pm and getting high overhead is Ursa Major.

The Planets

• Jupiter passed behind the sun at the end of February but will reappear in the predawn sky at the very end of the month.
• Saturn is coming to the best few months and can be seen at magnitude +0.6 for much of the night. It reaches opposition on March 21st. The angular size stays pretty constant at 19.5 arcseconds and the extent of the rings is 44 arcseconds. The rings are close to edge on, at about 4 degrees and will drop to 3 degrees by the end of the month.
• Mercury passes behind the Sun on March 14th but reappears again in the twilight sky along with Venus in the last week of March. Might spot it with binoculars on March 22nd about 20 mins after sunset.
• Mars is fairly high in the south east after sunset. It will be well up in the south and highest in the sky at about about 8.30 in the evening. It continues to move westward into Cancer until March 8th and then it resumes its eastern track across the sky.
• Venus becomes prominent in the evening sky as it climbs higher at magnitude -3.9.

Highlights

• Look towards Leo at about 10pm for a really nice skyscape. Leo is in the centre of the field with Saturn down to the lower left in Virgo. Up to the right we see Mars in Cancer. The asteroid Vesta continues across the sickle up to the top right hand star of the arc of the head of the lion. Vesta is the 5th brightest object within the sickle.
• After sunset on March 31st, Venus and Mercury will be quite close, just 3.3 degrees apart. They'll stay around that until April 4th when they are just 3 degrees apart. Binoculars will help to pick out Mercury.
• The Hyades and Pleiades open clusters are well up after sunset.
• With a small telescope or 8x40 binoculars you may be able to see a white dwarf in the constellation Eridanus not long after sunset. Find the star Omicron-2, to the right of Rigel and you might be able to spot a white 9th magnitude companion which is a white dwarf.
• One binocular field below and a touch to the left of Sirius is the open cluster M41. Amongst the blue stars of the cluster is one red giant star.

Southern Hemisphere

After sunset, the Milky Way is running up from just to the left of due south towards the zenith. Not far from the horizon is Alpha Centauri at magnitude 0.01. With Beta Centauri, they are the pointers to the Southern Cross just above and to the left. Higher in the sky and a little bit to the right of due south is the constellation Carina, and the bright star at top is Canopus. This is one of the brightest stars in the nearby part of our galaxy and is used as a navigation beacon for spacecraft.

Odds and Ends

As we have mentioned before, the Spirit rover is staying put on Mars but is now being used in a rather innovative way to help determine whether the core of Mars is solid or liquid.

Soichi Noguchi (@Astro_Soichi) has been making use of the new internet connection to the ISS to put images from orbit directly online via twitpic. Our favourites include photos of the Moon rise, the Patagonia glacier and the space shuttle Endeavour undocking.

Create your own Solar System! with the flash-based Solar System simulator

GLOBE at Night is happening again this month, 3rd-16th March. Do your part to monitor light pollution around the world simply by going out and looking at Orion.

The West of London Astronomical Society is holding its annual Planet Watch on the 19th-21st March 2010.

Show Credits