Megan's Blog http://www.rigel.org.uk/blog/ A blog about astronomy, science generally, and anything else that looks interesting. en-gb Fri, 23 Jul 2010 09:54:00 GMT Fri, 23 Jul 2010 09:54:00 GMT http://blogs.law.harvard.edu/tech/rss Marzipan (v0.5.7) Data tsunami http://www.rigel.org.uk/blog/000305.shtml While radio telescope arrays make some of the highest resolution maps of the sky, one thing they are not generally very good at is making images that cover large areas. But new telescopes that are planned or currently under construction (such as the Australian SKA Pathfinder, ASKAP, being built here in Western Australia) are going to make wide field images as standard. This is exciting stuff for radio astronomy as, for one thing, it allows surveys to be conducted much faster. If you can only look at very small patches of sky at a time, it's going to take you a long time to cover the whole sky, but if you can widen your field of view then you can survey the same total area in a much shorter time (it's kinda like doing a jigsaw - the smaller the pieces, the longer it takes). This means that with new telescopes we will be able to do both more sensitive surveys (staring at patches of sky for longer to detect fainter sources), and more efficient searches for transient sources such as radio supernovae and gamma ray bursts (and other things we probably don't even know about yet).

It is possible to produce wide field images from existing radio arrays, but it's not exactly "normal operating procedure". With any kind of telescope, the resolution is determined by the wavelength you're observing at and the diameter of your telescope. For an interferometer, the diameter used is the largest separation between your antennas, which can be several thousand kilometres for large arrays. This makes them capable of very high resolution images but, due (amongst other things) to averaging effects, the standard data outputs from a correlator are only usable over a relatively small field of view around the pointing centre, the point on the sky your telescopes were aimed directly at during the observation. In principle though, the observations are sensitive to sources over a much larger area, it's just that using normal methods we can't make reliable images far from the pointing centre.

Well, earlier this month we observed two fields in the nearby Andromeda galaxy (M31), one using the US-based Very Long Baseline Array, the other using the European VLBI Network. The data are on the way to us, and when we get our hands on them we are going to have some fun. Instead of just using the correlated data around the pointing centre, we are going to use a software correlator, running on a computer cluster here, to re-correlate the baseband (raw) data wherever we like over the area the observations are sensitive to, with the ultimate goal of imaging the entire primary beam of the array. This means we will be able to make reliable images of sources across the primary beam of the array, essentially the area covered by the beam size of the individual telescopes in the array - in the case of the VLBA observation, we will be able to make images of sources with a resolution of 5 mas (1 mas = 1/1000 arcseconds = 0.0000003 degrees) over an area about half a degree in diameter.

M31 in both optical and radio
The nearby spiral galaxy M31 (Andromeda) in both optical (blue) and radio (orange). Our new radio observations will have a resolution some 7000 times greater than the radio observations used to make the above composite, but covering a similar total area. CREDIT: Optical: DSS; radio: VLA, Beck et al

Think about that for a moment. Half a degree - that's the size of the full Moon. If we were looking at the Moon, this is equivalent of being able to resolve features less than nine metres in size. We'd almost be able to spot the lunar landers! Imagine mapping the surface of the Moon at a resolution of nine metres. That's a lot of pixels.

So what are we hoping to see in our terabytes of data? Lots! Supernova remnants, star forming regions, planetary nebulae, and heaps of background quasars... may be something unexpected. This is going to be sooo cool!

 Fri, 23 Jul 2010 09:54:00 GMT http://www.rigel.org.uk/blog/000305.shtml In the news this month: a drop in eta Carinae's stellar wind http://www.rigel.org.uk/blog/000304.shtml
A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star Eta Carinae.
A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star Eta Carinae. CREDIT: NASA, ESA and Jon Morse (University of Colorado)

Supernovae are the violent explosions of massive stars, so bright that the events can be seen in distant galaxies. But not all apparent explosions are genuine supernovae. Some fall into the category of supernova impostors, the sudden increase in brightness of a star without the terminal explosion. One such impostor event was the great eruption of eta Carinae, a star which is amongst the most massive known in the Milky Way. Located 7,500 light years away in the constellation of Carina, the star is five million times more luminous than the Sun and an estimated 100 times as massive. Eta Carinae underwent a massive but non-terminal explosion 150 years ago, allowing the close-up study of a supernova impostor. During the eruption, the star lost about ten percent of its mass, throwing off the outer layers in the surrounding nebula. Since then, the star has been enshrouded in a thick cloud of dusty debris and has been losing material at the rate of one Jupiter-mass per year in a strong stellar wind.

Now, a team of researchers, led by Andrea Mehner at the University of Minnesota, have observed dramatic changes in the star's spectrum. Observations over the last decade have shown an increase in the star's magnitude, but with no major long-term changes in its spectrum, something that might be expected following an event causing a major change in brightness. New observations carried out with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope by Mehner's team show a substantial change in the emission lines, caused by specific elements in the star's atmosphere. According to the team, the sudden rapid decrease in the brightness of the emission lines (dropping to a third of their original strength in just ten years) suggests a decrease in the strength of the stellar wind, possibly signifying a much more rapid return to the pre-explosion state than was previously anticipated. With the wind density decreasing, the nebula should begin to thin and the star itself may become visible to modern telescopes for the first time, possibly within the next decade.

There are other explanations which may account for the unusual spectroscopic developments, including a change in the latitude dependence of the wind, but the complicated nature of the surrounding nebula and the difficulties constructing accurate models make an accurate assessment problematic.


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

Mehner, A., Davidson, K., Humphreys, R., Martin, J., Ishibashi, K., Ferland, G., and; Walborn, N. (2010). A SEA CHANGE IN ETA CARINAEThe Astrophysical Journal, 717 (1) DOI: 10.1088/2041-8205/717/1/L22

 Fri, 02 Jul 2010 09:45:00 GMT http://www.rigel.org.uk/blog/000304.shtml The awesomeness of Astronomy.FM http://www.rigel.org.uk/blog/000303.shtml I finally found time to test the updated version of Marzipan, but I can't get it to work yet. So, having rolled back to the old version, here's what I was going to post anyway:

Occasionally you come across something that just makes you stop and say "wow, that is a really neat idea". Not much on the internet gets that reaction out of me, but occasionally I'm surprised. One such gem is Astronomy.FM, an online radio station mainly devoted to astronomy but with plenty of other juicy bits of science thrown in for good measure.

Astronomy.FM, full of sciency goodness
Astronomy.FM, full of sciency goodness CREDIT: Astronomy.FM

Much of the content is made up of various astronomy and science podcasts such as the 365 Days of Astronomy, Naked Science, Slacker Astronomy, the Jodcast, and so on, but there are a number of original programmes as well, usually transmitted live and then replayed later the same day for listeners in other time zones. The schedule is currently arranged in four-hour programme blocks which repeat throughout the day.

I first came across AFM during 2009 when I took part in a live on-air discussion with Adrian West, Elias Jordan and Michael Foerster during #moonwatch. I'm pretty sure I'd heard about it before that, but it's the first time I had any actual involvement. Earlier this year Michael asked if the Jodcast could be included in their schedule, and it has been airing weekly on AFM for a few months now. In May I was a guest on Event Horizon, one of AFM's original shows broadcast live at 0200 GMT on Saturdays, and have popped up in the chat room regularly ever since. There's even talk of having a live show from down under, but that's a story for another day.

Anyway, there's heaps of interesting content and some entertaining hosts. It's all run by volunteers, and some of them put in a seriously amazing amount of their own time and effort to keep it running. So, next time it's a cloudy night and you've run out of Patrick Moore books to read, give it a try.

 Wed, 16 Jun 2010 11:50:00 GMT http://www.rigel.org.uk/blog/000303.shtml Spam spam spam spam http://www.rigel.org.uk/blog/000302.shtml Apologies to anyone (human) who has made a genuine comment here recently, I may well have accidentally deleted it while trying to clean up the torrent of comment spam that has been plaguing this site for quite a while now. A lot of it is coming from IP addresses in China, and contains some very child-unfriendly text and links. Up to now, I've been deleting it from the comments file every few days, but quite frankly it's getting ridiculous and I've had enough. If I had the time I'd try and shoe-horn reCAPTCHA or something similar into the code, but I haven't what with everything else that's going on. So, as it's unlikely to stop, I may have to find another solution. That may involve switching to different software, but it might just end up with me disabling comments altogether, possibly just closing the blog down completely.

If you've got any suggestions, please do let me know.
 Wed, 09 Jun 2010 03:12:00 GMT http://www.rigel.org.uk/blog/000302.shtml In the news this month... and finally: Trans-Tasman VLBI http://www.rigel.org.uk/blog/000301.shtml
Supernova 1987A in the Large Magellanic Cloud, before (right) and ten days after the event (left)
Colour composite image of Centaurus A, revealing the lobes and jets emanating from the active galaxy;s central black hole. This is a composite of images obtained with three instruments, operating at very different wavelengths. The 870-micron submillimetre data, from LABOCA on APEX, are shown in orange. X-ray data from the Chandra X-ray Observatory are shown in blue. Visible light data from the Wide Field Imager (WFI) on the MPG/ESO 2.2 m telescope located at La Silla, Chile, show the background stars and the galaxy's characteristic dust lane in close to "true colour". CREDIT: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)
Astronomers have connected up the largest ever array of radio telescopes in the Southern hemisphere and made the highest resolution image of the core of the nearby active galaxy Centaurus A. The project linked up new telescopes in New Zealand and in mid-west Western Australia, with the existing long baseline array, including the Parkes radio telescope in New South Wales, to form an array more than 5,500 kilometres across, the first time telescopes have have been connected over such large distances in the southern hemisphere.

At 14 million light years from Earth, Centaurus A is the nearest example of a galaxy containing an active black hole at its core. Observations show two enormous jets moving out from the core at close to the speed of light, but probing the physics of the core itself requires very high resolution observations, only possible by linking up radio telescopes over many thousands of kilometres.

The new telescope at Warkworth is the first research-quality radio telescope in New Zealand, while the new antenna in Western Australia is the first of many that will make up the Australia Square Kilometre Array Pathfinder. Using the same technique, the Square Kilometre Array will consist of radio telescopes spread out over many thousands of kilometres and will be located either in Australia or Southern Africa.


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

 Mon, 07 Jun 2010 08:42:00 GMT http://www.rigel.org.uk/blog/000301.shtml In the news this month... Hubble spots a planet-eating star http://www.rigel.org.uk/blog/000300.shtml
An artists concept of the exoplanet WASP-12b
An artists concept of the exoplanet WASP-12b, the hottest known planet in the Milky Way galaxy, and potentially the shortest lived. CREDIT: NASA, ESA, and G. Bacon (STScI)
Most of the extra-solar planets discovered so far are in the class known as "hot Jupiters", large gas giants orbiting close to their parent stars, since many of the search techniques used are most sensitive to this type of planet. Usually these planets are located close enough to their parent star that they orbit in just a few days, but a team have now discovered one that is orbiting so close to its parent star that it is actually being disrupted.

The planet, known as WASP-12b, is located in the constellation of Auriga and was discovered in the Wide-Area Search for Planets survey, or WASP, operated by a consortium of eight academic institutions. WASP consists of two robotic observatories, one located at La Palma in the Canary Islands, the other at the South African Astronomical Observatory at Sutherland in South Africa, both scanning the sky for the tiny dimming effects caused when a planet transits in front of a star. This particular planet orbits its parent star, a yellow dwarf known as WASP-12, in just 1.1 Earth days, and shows evidence of an atmosphere which extends far further from the planet than would be expected for a body of this size.

Previous observations have shown that at least one other exoplanet displays evidence of such an extended atmosphere, and two different mechanisms have been suggested: either heating from the parent star, or an interaction with the stellar wind. This new planet was first discovered by the WASP survey in 2008, and was predicted to be physically distorted by its proximity to the host star. These new observations, made with the Cosmic Origins Spectrograph on the Hubble Space Telescope and published in the Astrophysical Journal during May, have verified the prediction. WASP-12b is so close to the star that the tidal forces exerted on it have heated and deformed it far from the normal almost-spherical planetary shape, so far in fact that the internal heating has caused the atmosphere to expand far enough that it is being dragged off onto the surface of the star.

Absorption from elements such as sodium, magnesium, aluminium and tin was expected in the atmosphere of the star, and the increase in absorption during the transit allowed the astronomers to calculate how common these elements are in the planet's atmosphere. The research, led by Luca Fossati at the Open University in the UK, examined the ultra violet spectrum of the planet's atmosphere and found a much greater abundance of heavy elements than expected from models of planetary atmospheres. The suggested reason for this unexpected result is that the high amount of incident radiation due to the close proximity of the star, together with tidal effects, cause a large amount of mixing within the atmosphere, pulling heavy elements higher in the atmosphere than they would normally be found. The heating also causes the atmosphere to expand, overflowing what is known as the Roche limit, the point beyond which particles escape the gravitational pull of the planet and are lost to the surrounding space.

From the evidence provided by their ultra-violet observations, the researchers conclude that the planet is probably undergoing photo-evaporation by its host star, and the material lost from the atmosphere is forming a diffuse ring around the star along the planet's orbit. While few examples of such systems are currently known, further observations and detailed modelling will help to determine exactly what is going on in these peculiar atmospheres.


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

Fossati, L., Haswell, C., Froning, C., Hebb, L., Holmes, S., Kolb, U., Helling, C., Carter, A., Wheatley, P., Cameron, A., Loeillet, B., Pollacco, D., Street, R., Stempels, H., Simpson, E., Udry, S., Joshi, Y., West, R., Skillen, I., and; Wilson, D. (2010). METALS IN THE EXOSPHERE OF THE HIGHLY IRRADIATED PLANET WASP-12b The Astrophysical Journal, 714 (2) DOI: 10.1088/2041-8205/714/2/L222

 Mon, 07 Jun 2010 08:20:00 GMT http://www.rigel.org.uk/blog/000300.shtml In the news this month... runaway star in 30 Doradus http://www.rigel.org.uk/blog/000299.shtml
The Tarantula Nebula in the Large Magellanic Cloud (LMC) and its surroundings.
The Tarantula Nebula in the Large Magellanic Cloud (LMC) and its surroundings. CREDIT: ESO / J. Alves (Calar Alto, Spain), B. Vandame, and Y. Beletski (ESO) Processing by B. Fosbury (ST-ECF)
One of the most spectacular examples of a star formation region in the nearby universe is 30 Doradus, also known as the Tarantula nebula, located in the Large Magellanic Cloud. This region is a giant stellar nursery, similar to the Orion Nebula, but much larger, containing many clusters of recently formed young, hot stars. Some of the young stars in the nebula are many tens of times more massive than the Sun, making them some of the most massive stars known. New observations, reported in the Astrophysical Journal on May 5th, show that one particular star is travelling away from the nebula at high velocity.

The star, known as 30 Dor 016, was first spotted in 2006 when it was observed by the Anglo-Australian Telescope at Siding Spring Observatory in Australia. It was found to be an exceptionally hot, massive blue-white star, located relatively far from any cluster in which such stars are usually found. More recent observations made during the calibration of the Cosmic Origins Spectrograph, installed on the Hubble Space Telescope during servicing mission four in May 2009, showed that the star had an unusually fast stellar wind, almost 3500 km/s, one of the most powerful ever detected and a strong indication that the star is incredibly massive - it is estimated to be roughly 90 times the mass of the Sun. Its size means that it must be young - stars this large only live for a few million years before exploding as core collapse supernovae.

Archive images taken by Hubble's Wide Field Planetary Camera 2 in 1995, show that the star is at one end of an egg-shaped cavity in the surrounding interstellar gas which points towards 30 Doradus, in the direction of a cluster of massive stars known as R136, the likely birthplace of the star. Further observations, made with the Very Large Telescope in Chile, have shown that the star's velocity is more than 400,000 kilometres an hour, a speed that would get you from the Earth to the Moon in an hour. The measured velocity could have been due to orbital motion if the star had a companion, but the VLT observations show that it is a single massive star, and the velocity is due to motion away from the nebula.

Stars can end up with such high velocities as a result of nearby explosions. In the case of 016 however, this is unlikely since the stars in 30 Doradus are still too young to have exploded as supernovae. The more likely explanation, say the team, led by Chris Evans at the UK Astronomy Technology Centre in Edinburgh, is that it was ejected from the cluster by dynamical interactions with other massive stars, one of the clearest examples yet of such a process.


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

Evans, C., Walborn, N., Crowther, P., Hénault-Brunet, V., Massa, D., Taylor, W., Howarth, I., Sana, H., Lennon, D., and; van Loon, J. (2010). A MASSIVE RUNAWAY STAR FROM 30 DORADUS The Astrophysical Journal, 715 (2) DOI: 10.1088/2041-8205/715/2/L74

 Mon, 07 Jun 2010 08:00:00 GMT http://www.rigel.org.uk/blog/000299.shtml In the news this month... a possible new class of supernova http://www.rigel.org.uk/blog/000298.shtml
SN 2005E discovery image
SN 2005E discovery image, discovered by the Lick Observatory Supernova Search CREDIT: LOSS

Most supernovae are classified as one of two different types of explosion: single massive short-lived stars that explode when their cores run out of fuel at the end of their lives and undergo gravitational collapse, and old evolved white dwarfs in binary systems which accrete hydrogen from a companion star before exploding catastrophically.; Core collapse supernovae are generally seen only in regions of ongoing star formation since, by stellar standards, their supergiant progenitors do not live for very long. In contrast, type Ia supernovae in binary systems are produced by old, evolved stars and so are seen in all galaxy types, even those which show no signs of recent star formation. However, in the May 20th issue of the journal Nature, two groups of astronomers report stellar explosions with characteristics that do not fit into existing categories of supernovae, and come to very different conclusions about their progenitors.

The first event, SN 2005E, was observed in the edge-on spiral galaxy NGC 1032 in 2005 and was initially classified as a type Ib (core collapse) supernova based on the chemical elements detected in its optical spectrum soon after explosion. Located in the halo, rather than the disk of the galaxy, the surrounding environment is composed of an old stellar population with no recent star formation, an unlikely location for a core collapse supernova. While some of its properties show similarities to type Ia explosions, the lightcurve shows a much faster decline than is expected for the thermonuclear explosion of a white dwarf. The mass ejected in the explosion, less than a third of a solar mass, is also low for this class of supernova, and analysis of the spectra showed significant differences from what is expected from either explosion mechanism. This evidence lead Dr Perets' team to conclude that the progenitor was something unusual, likely to be a helium-rich low mass star, probably a helium-accreting white dwarf, making 2005E the first example of a new class of supernova.

However, 2005E is not the only supernova with these unusual characteristics; several other calcium-rich, subluminous supernovae, spectroscopically classified as type Ib/c events, have also been observed. One such event is SN 2005cz, reported by Professor Kawabata and colleagues in the same issue of Nature. While 2005cz shares many properties with 2005E, Kawabata's team reaches a different conclusion on the cause of the explosion. According to their study, supernovae in this class are more likely to originate via the core collapse mechanism, but from stars with masses at the lower end of the range of those that explode. Unlike 2005E, SN2005cz is located in an elliptical galaxy. These galaxies are generally made up of old stellar populations, but NGC4589 has a relatively young stellar population (for an elliptical galaxy) so the explosion of a star by core collapse is not ruled out.

Since most supernova searches are more likely to detect bright events, the number of faint 2005E-like events currently known is small. More sensitive surveys are planned, however, and these should result in many more examples and further insights into this non-standard class of supernova.


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

Perets, H., Gal-Yam, A., Mazzali, P., Arnett, D., Kagan, D., Filippenko, A., Li, W., Arcavi, I., Cenko, S., Fox, D., Leonard, D., Moon, D., Sand, D., Soderberg, A., Anderson, J., James, P., Foley, R., Ganeshalingam, M., Ofek, E., Bildsten, L., Nelemans, G., Shen, K., Weinberg, N., Metzger, B., Piro, A., Quataert, E., Kiewe, M., and; Poznanski, D. (2010). A faint type of supernova from a white dwarf with a helium-rich companion Nature, 465 (7296), 322-325 DOI: 10.1038/nature09056

Kawabata, K., Maeda, K., Nomoto, K., Taubenberger, S., Tanaka, M., Deng, J., Pian, E., Hattori, T., and; Itagaki, K. (2010). A massive star origin for an unusual helium-rich supernova in an elliptical galaxy Nature, 465 (7296), 326-328 DOI: 10.1038/nature09055

 Mon, 07 Jun 2010 07:45:00 GMT http://www.rigel.org.uk/blog/000298.shtml