Megan's Blog

In the News this month: and finally, seasons on Pluto

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.

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This blog post is a news story from the Jodcast, aired in the March 2010 edition.

In the News this month: emission from methane in the atmosphere of an extrasolar planet

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.

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This blog post is a news story from the Jodcast, aired in the March 2010 edition.

Swain, M., Deroo, P., Griffith, C., Tinetti, G., Thatte, A., Vasisht, G., Chen, P., Bouwman, J., Crossfield, I., Angerhausen, D., Afonso, C., & Henning, T. (2010). A ground-based near-infrared emission spectrum of the exoplanet HD 189733b Nature, 463 (7281), 637-639 DOI: 10.1038/nature08775

In the News this month: the molecular content of early galaxies

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 focused 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.

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This blog post is a news story from the Jodcast, aired in the March 2010 edition.

Tacconi, L., Genzel, R., Neri, R., Cox, P., Cooper, M., Shapiro, K., Bolatto, A., Bouché, N., Bournaud, F., Burkert, A., Combes, F., Comerford, J., Davis, M., Schreiber, N., Garcia-Burillo, S., Gracia-Carpio, J., Lutz, D., Naab, T., Omont, A., Shapley, A., Sternberg, A., & Weiner, B. (2010). High molecular gas fractions in normal massive star-forming galaxies in the young Universe Nature, 463 (7282), 781-784 DOI: 10.1038/nature08773

In the News this month: the explosion mechanism behind type Ia supernovae

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.

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This blog post is a news story from the Jodcast, aired in the March 2010 edition.

Gilfanov, M., & Bogdan, A. (2010). An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate Nature, 463 (7283), 924-925 DOI: 10.1038/nature08685

Seven billion elephants

In February last year I posted about sustainability and the growing global population as part of the month-long Global Population Speak Out initiative. In the year since I last posted on the topic, the Earth has gained another 80 million people. Can you imagine that many human beings? I certainly can't.

My last post on the subject was titled "The elephant in the room" following John Feeney's article in the BBC's Green Room - it's a topic of huge importance to the sustainability of our world, but one that is frequently ignored or glossed over. The human population currently grows by about 80 million annually, right now there are more than 6.7 billion of us on this one tiny planet. Can you imagine if there were 6.7 billion elephants? Mankind would probably call it an infestation and start a cull.

One common objection to the assertion that we're heading for overpopulation is that we need a growing population to sustain the economy. This is probably a more common argument since the financial crisis, but think about it for a moment. An economy such as we currently have does require a growing population because it's designed that way, but that doesn't mean it can go on forever. Infinite growth in a closed system just cannot happen. We live on a single planet with a finite amount of land and a limited set of resources. Those resources must sustain not only us, but the rest of the ecosystem on which we fundamentally depend.

Sitting in an air-conditioned office or a heated home, the modern western world can make us feel quite disconnected from the rest of the biosphere, but we are still a part of it. What we do affects the environment around us, the decisions we make affect more than just ourselves. Whether we like it or not, we are part of that ecosystem and it simply can not sustain infinite growth. Ecology tells us this. Physics tells us this. Continued economic growth is simply unsustainable. It cannot continue indefinitely on a finite planet.

February once again sees a global effort to bring the topic into open debate. Population has been a very political topic here in Australia for various reasons - there is a big push to increase the skills base by encouraging the immigration of skilled workers, but there is also an ongoing debate as to how much population growth is physically sustainable both in terms of the economy and general infrastructure. This year, Australian MP Kelvin Thomson has added his voice to the GPSO effort, giving a speech at a public meeting of Sustainable Population Australia in Canberra on February 10th. Last October, PM Kevin Rudd sounded his approval of population growth and his allegiance to a “big Australia.” The political backlash was significant. In the meantime, Thomson released a plan for stabilizing Australia’s population at 26 million by 2050, and a new political party is reportedly being formed by an entrepreneur from Sydney - specifically to address population concerns in Australia. It will be interesting to see how the debate progresses over the next twelve months.

It's a big subject. Check out the resources and contributions at the GPSO website and join the debate.