by Paul Gilster | Apr 16, 2010 | Interstellar Medium |
I’m always sorry when a good conference like the Royal Astronomical Society’s 2010 gathering ends, even if I’m attending it ‘virtually’ from the other side of an ocean. But virtuality has its advantages, as I’m reminded by several conference attendees who have struggled with Icelandic volcano ash when trying to book flights out of the UK. If I were with them in Glasgow, I’d praise my good fortune for extra time in Scotland and immediately take the train for Inverness, then on to Skye and the Inner Hebrides, where I’ve spent many good days and intend to spend many more.
Volcanic ash or no, it was a lively conference with tantalizing results on planetary system residues in white dwarfs and retrograde exoplanet orbits, and a number of other issues that can be found in the conference program. I’ll close our RAS coverage here with two items that deal with interstellar dust rather than the Earth-based dust and ash that closes airports. Red giants, the kind of star our Sun will eventually turn into in a later phase of its life, expel dust and gas that produces raw materials for a new generation of stars and planets. The outcome is often a beautiful nebula, but it can also be a dusty disk surrounding what the red giant finally becomes, a white dwarf.
Dusty Disks Around Aging Stars
The new work pegs disk formation around stars at various stages of their evolution, an outstanding question being how long these disks survive. The images Foteini Lykou (Jodrell Bank Centre for Astrophysics) showed at the RAS session were of disks caught early in their lives. M2-9 is a striking example, a nebula with symmetrical lobes of gas extending from it, with a binary star system at its heart. A red giant and a white dwarf are hidden by the disk, the dust originating in the red giant. Lykou believes the disk inside M2-9 is less than 2000 years old, a startling figure given the usual time-scales of astronomical observation.
Image: Top: Bipolar nebula M2-9 (credit: B. Balick/HST) with a reconstructed image of its dusty disc observed by VLTI. Bottom: Round nebula around Sakurai’s Object (credit: A. Zijlstra, University of Manchester) with its corresponding disc.
But the disk around Sakurai’s Object, a round nebula some 11,400 light years from Earth, is even younger. It’s composed of amorphous carbon (think coal or soot) and is growing rapidly. Says Lykou:
“The disc in Sakurai’s Object was created within the last 10 years, so we have the opportunity to study a newborn disc. It is expanding radially — and rapidly — in space. During our observation period in 2007, we saw the disc extend from 10 thousand million kilometers to 75 thousand million kilometers.”
We’re still speculating on what happens to these disks but current thinking is that interstellar radiation probably breaks the constituent dust grains down and thereby replenishes the interstellar medium with new materials. Observing objects like these calls for interferometry, combining the light caught by multiple instruments to sharpen the field of view. The astronomers here used the Very Large Telescope Interferometer at the European Southern Observatory in Chile, which combines four 8.2-meter telescopes and works in the infrared.
Explaining Interstellar Water
I’ll close our RAS coverage with a different kind of look at interstellar dust, one that helps to explain where water from the interstellar medium comes from. The problem is that while hydrogen atoms are extremely common in deep space, little gaseous molecular oxygen (O2) seems to be available and ozone has not been detected in these regions. Atomic oxygen (O) is plentiful, but gas phase reactions between hydrogen and atomic oxygen cannot account for the amount of water observed. Moreover, even the observed amounts of atomic oxygen show a shortage in star-forming regions when compared to the rest of interstellar space.
So while it’s one thing to say that Earth’s water was delivered by comets formed from interstellar material, the question of where that water came from in the first place has remained unanswered. And just where is the missing atomic oxygen?
Enter Victoria Frankland (Heriot-Watt University), whose team of researchers think they have found the answer in the form of the dust grains that make up approximately 1 percent of the interstellar medium. The dust, Frankland believes, provides the surface that allows the needed reactions to take place. Some molecules remain stuck to the surface so that an icy coat — mainly water ice — builds up over time. Says Frankland:
“These initial experiments are having some interesting results in that they are allowing us to look at how the ice coating develops on the dust particles. It appears that oxygen atoms may become trapped inside the icy mantles. We need to do more work, but it could be that our experiments might help solve the mystery of the missing atomic oxygen as well as where the water has come from.”
The image below is too beautiful not to run:
Image: Molecular cloud and star-forming region in the interstellar medium. Credit: NASA/JPL-Caltech/L. Allen (Harvard-Smithsonian CfA).
Interstellar dust is gorgeous to the eye when lit up in star-forming regions like this one, and we’ve often speculated here on the problem of such dust for fast moving spacecraft of the far future. Now we’re learning how dusty disks can form not just around young stars but around burnt out stars nearing the end of their fusion reactions. And we’re seeing that dust may play a role in the essential production of water, so necessary for the formation of life. How appropriate, then, that volcanic ash and dust should mark the end of the illuminating sessions in Glasgow, a reminder that scientific theory paints a real and sometimes all too frustrating world.
by Paul Gilster | Apr 15, 2010 | Astrobiology and SETI |
There is enough going on at the Royal Astronomical Society’s 2010 meeting to keep us occupied for some time, but I don’t want to go any farther without circling back to UGPS 0722-05, an unusually cool brown dwarf now thought to be the seventh closest star to the Sun. The parallax measurements of its distance are still being refined, but the dwarf is currently thought to be some 9.6 light years from Earth, roughly twice the distance of Proxima Centauri. With a temperature between 130 and 230 degrees Celsius, this is the coolest brown dwarf ever observed, its mass ranging somewhere between five and thirty times that of Jupiter.
A number of readers sent links to this discovery, for which many thanks, and I note how interest seems to be growing in the idea that a brown dwarf may exist closer to us than the Alpha Centauri stars. Brown dwarfs are now thought to be relatively common in the galaxy, perhaps as common as normal stars, which suggests that missions like WISE may well discover brown dwarfs even closer in the neighborhood. In this case, we can thank the United Kingdom Infrared Telescope on Mauna Kea for the find, with follow-up spectra from the Gemini North Telescope.
For more on this dwarf, the paper is Lucas, et al., “Discovery of a very cool brown dwarf amongst the ten nearest stars to the Solar System,” submitted to Nature and available as a preprint. Science News has a good story on the dwarf, as does New Scientist. I’m bemused by the fact that I baked a loaf of bread last night in a tiled oven that was in the same temperature range as calculated for this object, which is cool enough that it may represent the first of a new class of ultra-low temperature dwarfs. The suspicion grows that objects in this temperature range would be dim enough to have lurked within a few light years and remained unnoticed.
What other surprises are out there? At the RAS meeting in Glasgow, recent talk has focused on discovering things with new instrumentation, in this case the Low Frequency Array that will, when completed, be made up of 44 independent stations spread across the Netherlands, Germany, Sweden, France and the UK. LOFAR will study radio emitting galaxies from the early universe and look into astrophysical phenomena from cosmic rays to pulsars. But it’s intriguing to note that the project also has a SETI component, the first phase of which will be to study how to filter out contamination from Earth-based transmitters and improve the system’s sensitivity.
After that, an extended SETI effort is planned, says Alan Penny, who presented the LOFAR SETI program to an audience in Glasgow:
“LOFAR will scan nearby stars searching for radio emissions which could only be produced by artificial means — a sign that there is a civilization there and that we are not alone. Previous investigations of these stars have concentrated on higher frequencies but, as we do not know at which frequencies an extraterrestrial civilization might choose to emit radio waves, LOFAR will fill an important gap in the search. It is particularly exciting that this is being done by a European team with a pan-European telescope.”
We know comparatively little about the universe at the very low energy wavelengths LOFAR will examine, but that should change quickly as the final stations of LOFAR are completed this summer. Check the sensitivity available in the imagery below:
Image: A comparison of the LOFAR image with the results from other radio telescopes at various observing frequencies. The Very Large Array image at 74 MHz and Westerbork Synthesis Radio telescope image at 325 MHz, shown to the same scales, provided the previous state-of-the-art images at low frequencies. The image quality with LOFAR at 173 MHz is well beyond what has been done before in terms of sensitivity and resolution. Credit: van Weeren/ASTRON.
LOFAR functions in a range from 10 to 240 MHz (compare this to the 0.4 to 3 GHz range of typical radio telescopes), and offers baselines across Europe on the order of 1500 kilometers, with the bulk of the aperture array stations being located in the Netherlands. The frequencies involved are so choked with terrestrial traffic that separating signal from local noise has to be the top priority, as anyone who has worked the SWL or ham bands above 10 MHz can testify. But opening up a new frequency range invariably promises surprises down the road. Whether those surprises are all astrophysical or might involve SETI should soon become apparent.
by Paul Gilster | Apr 14, 2010 | Exoplanetary Science |
Meetings like the Royal Astronomical Society’s gathering in Glasgow can be overwhelming, with all kinds of news to track via emails, news releases and Twitter. Yesterday we looked at the possible signature of rocky planets in the atmospheres of white dwarfs. But the unusual orbits of planets newly discovered by the WASP project (and follow-up studies of older hot Jupiters) get pride of place as perhaps the most notable announcement so far. WASP (Wide Angle Search for Planets) turned up nine new planets, bumping the exoplanet total to 454), but follow-up studies of these worlds, along with other ‘hot Jupiters’ from earlier surveys, showed that six out of the 27 examined orbited opposite to the rotation of their host star. Moreover, more than half of the planets studied are misaligned with their star’s rotation axis.
Image: Exoplanets, discovered by WASP together with ESO telescopes, that unexpectedly have been found to have retrograde orbits are shown here. In all cases the star is shown to scale, with its rotation axis pointing up and with realistic colours. The exoplanets are shown during the transit of their parent star, just before mid-transit. The last object at the lower right is for comparison and has a “normal” orbital direction. The size of each image is three solar diameters. Credit: ESO/A. C. Cameron.
If misaligned orbits sound like a minor point, consider this: We’d like to know whether the existence of a hot Jupiter in a tight orbit rules out the possibility of a terrestrial world in a wider orbit. And the answer seems to depend on how these planets form. They’re surely born in the outer reaches of their solar systems, with cores made of the rock and ice particles we’d expect to exist there. If the hot Jupiters then went through gravitational interactions with the dust and debris disk from which they formed, they could migrate in to their present, tight orbits and should show alignment with the parent star’s rotation axis. That’s a scenario that allows planets like the Earth to form later, a cheery thought for life but not one enhanced by the new findings.
For if we’re looking at numerous misaligned orbits including retrograde paths around the primary star, then the other migration scenario comes into play. Here, we forego interactions with the dust disk and talk about much slower gravitational processes operating due to the effect of more distant planets or close stars. As opposed to millions of years, we’re now talking about taking hundreds of millions to move a giant planet into an orbit that eventually undergoes tidal friction, causing its orbit to circularize but remain randomly tilted close to the star.
Exoplanet hunter Didier Queloz (Geneva Observatory), who was closely involved with this work, notes the result: “A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems.” To learn more, of course, we’ll need fuller descriptions of exoplanetary systems, so we can learn how many hot Jupiters occur in systems with more distant massive companions. Andrew Cameron (University of St. Andrews), who presented the current study at the RAS meeting in Glasgow, points out that the new results present quite a challenge to conventional wisdom holding that planets should orbit in the same direction as their stars spin.
Preprints of the papers from the SuperWASP Consortium can be found here. An ESO news release is also available, as is a release from Las Cumbres Observatory. Las Cumbres used robotic telescopes located in Hawaii and Australia to provide brightness measurements that helped researchers determine the size of the planets. But follow-up radial velocity observations of these transiting worlds were performed by observatories around the world, from the Nordic Optical Telescope in the Canaries to the HARPS spectrograph on the 3.6-meter ESO telescope at La Silla. This animation showing how distant objects can perturb a gas giant’s orbit may be helpful.
by Paul Gilster | Apr 13, 2010 | Exoplanetary Science |
Kepler, CoRoT and future space missions should give us an estimate of how common small, rocky planets are in the galaxy. But there is much we can do from Earth, as Jay Farihi told the Royal Astronomical Society’s 2010 meeting today. Farihi’s team used data from the Sloan Digital Sky Survey to conclude that rocky worlds emerge around at least a small percentage of A- and F-class stars. The method: Analyze the position, motion and spectra of white dwarfs found in the SDSS survey. Farihi was interested in the presence of elements heavier than hydrogen and helium in the stellar atmospheres.
Finding calcium, magnesium or iron in the atmosphere of a white dwarf is, Farihi believes, evidence of rocky debris, and the new work shows that at least 3 percent and as much as 20 percent of all white dwarfs may be contaminated in this way. Such elements should have sunk below the photosphere in the high gravity of a white dwarf, leading to the belief that any visible contamination must be the result of external causes. Farihi sees the heavier elements as debris left over from what once may have been planetary systems containing terrestrial worlds around these stars.
Assuming this is the case, then, we can take this debris as evidence for the existence of such systems around A- and F-class stars. Moreover, the composition of the debris shows that many of these stars are polluted with material containing water. Says Farihi:
“In our own Solar System with at least one watery, habitable planet, the asteroid belt – the leftover building blocks of the terrestrial planets – is several percent water by mass. From our study of white dwarfs, it appears there are basic similarities found among asteroid-like objects around other stars; hence it is likely a fraction of these white dwarfs once harbored watery planets, and possibly life.”
Why focus on planets or planetesimals as the source of the heavy metal contamination? Farihi demonstrates in the paper on this work that there are no correlations between calcium abundances and the distribution of these stars in relation to interstellar materials that could have been the source. Two thirds of the stars under study are located above the galactic gas and dust layer, and study of their motion shows long residence in areas where interstellar materials are all but absent. Assuming planetary materials as the cause of this signature, the paper adds:
…at least 3.5% of white dwarfs appear to be polluted by circumstellar matter, the remains of rocky planetary systems. This translates directly into a similar lower limit for the formation of terrestrial planets at the main-sequence progenitors of white dwarfs, primarily A- and F-type stars of intermediate mass. While this fraction is likely to be significantly higher…, it is difficult to quantify without a commensurate examination of all the cool SDSS white dwarfs, which is beyond the scope of this paper. The appearance of hydrogen in DZA stars suggests a common origin for both heavy elements and hydrogen, and indicates DZA star pollution by water-rich minor planets may be semicontinuous on Myr timescales.
White dwarf spectral classification schemes use an initial letter D followed by letters describing secondary features of the spectrum, which is what the DZA reference above is all about. More on white dwarf classifications here.
The significance of this work is that it rules out the interstellar medium as the source for the metal pollutants in white dwarfs like these. Two conclusions follow: At least a small percentage of A- and F-class stars build terrestrial planets (‘terrestrial’ meaning small, rocky worlds) whose debris supplies the heavy elements, and “…the pattern of hydrogen abundances in DZ stars is likely a reflection of the diversity of water content in extrasolar planetesimals.”
The paper is Farihi et al., “Rocky planetesimals as the origin of metals in DZ stars,” accepted by Monthly Notices of the Royal Astronomical Society (abstract).
by Paul Gilster | Apr 12, 2010 | Outer Solar System |
I love what William Bains (University of Cambridge) has to say about extraterrestrial life and how it might appear to us. “Wouldn’t it be sad if the most alien things we found in the galaxy were just like us, but blue and with tails?” He’s thinking, of course, of some science fiction evocations of aliens and their general similarities to our own species, perhaps the result of Hollywood budgetary constraints as much as lack of imagination. But Bains is interested in alien life for more than cinematic reasons. He’s looking hard at Titan, and envisioning what life there might look like.
Image: A flat, calm, liquid methane-ethane lake on Titan is depicted in this artist’s concept. Copyright 2008 Karl Kofoed.
Life on Titan would be, by our standards, a bit unusual. Says Bains:
“Life needs a liquid; even the driest desert plant on Earth needs water for its metabolism to work. So, if life were to exist on Titan, it must have blood based on liquid methane, not water. That means its whole chemistry is radically different. The molecules must be made of a wider variety of elements than we use, but put together in smaller molecules. It would also be much more chemically reactive.”
Bains will discuss these matters at the 2010 meeting of the Royal Astronomical Society in Glasgow, a gathering that meets all week, and one whose results we’ll follow with interest. As to Titan, Bains’ talk acknowledges that the distant moon presents an enormous challenge for astrobiology, but perhaps not an insurmountable one. Although a surface temperature of -180 Celsius could not support life as we know it, we do have liquid methane and ethane available in ponds and lakes on the surface. Sunlight is only a tenth of a percent as intense on Titan’s surface as on the surface of the Earth, so energy is a problem, meaning slow-growth organisms like lichen are more theoretically plausible than fast movers.
Yet forms of life might emerge. And it’s entertaining to think of a true Titanian alien as depicted by some ambitious film director of the future. Says Bains:
“Hollywood would have problems with these aliens. Beam one onto the Starship Enterprise and it would boil and then burst into flames, and the fumes would kill everyone in range. Even a tiny whiff of its breath would smell unbelievably horrible. But I think it is all the more interesting for that reason.”
It’s hard to disagree. Aliens in the cinema are notoriously anthropomorphic or, at best, suggest odd forms of other Earth species more than serious attempts to tackle extraterrestrial beings. On Titan we have to think about the solubility of chemicals in liquid methane, which is limited and dependent on molecular weight. A metabolism functioning in liquid methane would have to be built of smaller molecules than in terrestrial biochemistry, with molecules having more than 6 non-hydrogen atoms being essentially insoluble. We would expect, says Bains, sulphur and phosphorus in diverse and unstable forms, and other elements, such as silicon. All of which leads to the potent creature described above.
Maybe the excellent ‘Exoplanets Rising’ conference jacked my expectations up to unreasonable levels, but as the RAS 2010 meeting gets going, I’m going through the site looking for evidence that video streaming or archiving of these sessions will be forthcoming. Let’s hope so. UC Santa Barbara’s Kavli Institute for Theoretical Physics did at terrific job in getting the exoplanet talks posted the next day, so that we now have a complete archive of the ‘Exoplanets Rising’ sessions, including posters. In doing so, they’ve set a high standard for future conferences. RAS take note!
