Sun and Bass

A new member of the stellar orchestra has been identified by astronomers and the star, xi Hya in the constellation of hydra 130 light-years away, plays a very different tune to our own star, the Sun.

The Sun resonates like a giant, spherical organ pipe. It produces well-defined notes although they are far too deep to be seriously described as music. The energy that produces these enormous waves of sound comes from the turbulent region just below the Sun’s visible surface. Solar scientists have been studying these sound waves with the art of helioseismology for about thirty years. As with the Earth’s own vibrations before and after an earthquake, the researchers can use the frequency and movement of the waves to indirectly explore the Sun’s interior.

Oscillation frequencies

Oscillation frequencies

Astronomers are now tuning into the pitch of stars elsewhere in the heavens and have applied the general variation on the theme – asteroseismology – to stars that resemble the sun. The first observations of a star very different from the sun have been made on xi Hya also known rather enigmatically as CD-31 9083 among some audiences.

An international team of astronomers has observed xi Hya with the Swiss 1.2 metre Euler telescope at the European Southern Observatory’s La Silla Observatory, in Chile, and discovered that the star behaves like a giant sub-ultra-bass instrument. Xi Hya is twenty times the diameter of our Sun and is about 60 times more luminous; it is also, sadly close to retirement from the firmament, so a very different instrument indeed. The team has measured the timbre of xi Hya and found that it oscillates with several periods of around 3 hours.

Views of the ESO La Silla Observatory

Views of the ESO La Silla Observatory

It is estimated that the star’s outer envelope will soon, within hundreds of thousands of years, anyway, expand and the star will take on the stature of a red giant. Asteroseismology hopes to become the scientific method of choice for reading the underlying score of such changes and providing a more detailed understanding of stellar interiors and the overall evolution of stars.

View of the ESO La Silla Observatory (2)

View of the ESO La Silla Observatory (2)

The team led by Conny Aerts of the Catholic University of Leuven, in Belgium, Soeren Frandsen of the University of Aarhus in Denmark, and Fabien Carrier of the Geneva Observatory in Sauverny, Switzerland and their colleagues used the telescope’s CORALIE spectrograph to measure the oscillation velocities of the stellar surface.

Soeren Frandsen

Soeren Frandsen

xi Hya is a giant star so the waves need more time to propagate through from deep within the star but the surface reveals a broad distribution of about a dozen different frequency sound waves. The team point out that it is more difficult to model the interior of a giant star than our familiar sun because the giant’s core has changed a lot during its evolution. The sound waves in the sun are mostly concentrated in the outer parts of the Sun but for xi Hya there are also gravity modes to take into account deep in the interior of the star.

Non-radial oscillations

Non-radial oscillations

The team is planning to use the CORALIE and the, soon to be installed, HARPS instruments to listen to the sound show of other stars at different stages in their evolution from those still tuning up to the middle-aged virtuosi.

Acoustic waves in a Solar-like star

Acoustic waves in a Solar-like star

The researchers have taken the bold step of announcing their results ahead of publication in Astronomy & Astrophysics Letters. We oppose that researchers keep their results for a long time before informing the press, hence we have given our first results free for the ESO publication relations office to give an example that this [technique] is feasible, Aerts told Spotlight, It is a little risky for us, but I do not like the chase for high impact factors and citations.

The MP3 file linked below raises the pitch of xi Hya’s voice 1 million-fold revealing what some have described as the thundering approach of the Four Horsemen of the Apocalypse rather than the music of the spheres.

Further reading

European Southern Observatory’s La Silla Observatory
http://www.eso.org/sci/facilities/lasilla/

Conny Aerts
http://www.ster.kuleuven.ac.be/~conny/mons/mons.html

Soeren Frandsen
http://www.phys.au.dk/~srf/

MP3 file
http://www.psigate.ac.uk/spotlight/images/PSI4-stellarsound.mp3

Suggested searches

Asteroseismology
Helioseismology
Stellar evolution

The Gem of Siberia

Lake Baikal is the deepest lake on Earth and is one of Asia’s largest bodies of fresh water. But, it seems to be getting bigger, faster than sediment supply can fill it. Russian scientists have taken a closer look at why.

For millions of years the chilling and crystal clear waters of Lake Baikal, the Gem of Siberia, have harboured a deep-water repository of unique and indigenous fauna, such as the Baikal seal, local forms of Arctic cisco and gobies, and a staggering one-fifth of the Earth’s fresh water. Alluvion constantly pours into the lake through landslides, mudflows and river flooding.

Lake Baikal

Lake Baikal

A Russian team has now shown that the volume of the incoming alluvion is four times less than the increase in size of the basin, which can only be explained by the changing crust of the Earth at the bottom of the lake and thankfully means that Baikal will still be on maps for years to come.

Boris Agafonov of the Institute of Earth’s Crust of the Siberian Branch of the Russian Academy of Sciences located in Irkutsk says that Lake Baikal is in a seismically active but comparatively young area. Geologically speaking, the elongated basin of the lake is a rift trough like those found at the bottom of the oceans.

Agafonov and his colleagues believe that the lake might be considered a prototype ocean. Satellite data have revealed that the Baikal basin is extending at a rate of 5 mm a year, which is equivalent to an increase in volume of about 20 million cubic metres but more than that Agafonov reckons the volume of the lake is also increasing as the original basin bed subsides through earthquake activity.

Agafonov’s calculations reveal that movements of the earth’s crust have resulted in the hollow of the lake having increased immensely since the massive earthquake of 1862. During the 139-year period, from 1862 to 2001, the volume of the lake increased by 3.95 billion cubic metres while the volume of water has increased by 2.9 billion cubic metres, Agafonov explained to Spotlight. The lake is, one would assume, likely to remain the Gem of Siberia for the foreseeable future.

Further reading

Doklady RAN (Reports of Russian Academy of science), V. 382, 4, pp. 540-542; (in Russian)

Suggested searches

Lake Baikal
Lakes

Pinpoint acidity

An infrared laser can make acid at a single point when it is tightly focused in a polymer resin developed by US researchers. The research demonstrates for the first time the possibility of engineering molecules so that they efficiently absorb two particles of light, or photons, and efficiently trigger a chemical change.

Seth Marder and Joseph Perry of the University of Arizona in Tucson have developed new molecules that generate acid at the focus of a near-infrared laser beam. The intensity of a laser is highest at the focus and falls off quadratically with distance from the focus, explains Marder. Two photon absorption, however, scales quadratically with intensity and therefore falls off at the fourth power of distance from the focus.

Seth Marder

Seth Marder

This means the researchers can pinpoint very precise locations. The technique could be used to sculpt three-dimensional structures on the microscale for use in making microelectromechanical devices, tiny medical devices, and in optical information technology for a future optical molecular computer.

Chemists Marder and Perry have for several years been working to design and synthesise molecules that are able to absorb two photons at once. More importantly though, the team has now managed to incorporate a reactivity into these dye molecules so that when they absorb two, and only two photons of appropriate energy, the molecules end up in a higher energy state. In this excited state they can activate chemical reactions by transferring their electrons to another part of the molecule and causing the break-up of that part to produce reactive fragments including acid.

Molecular structures

Molecular structures

The two-photon absorbing molecules designed by the team are thus known as photoacid generators. Until now, photoacid generators have been ineffectual under two-photon excitation with near infrared lasers and have found little technological application. Acid is one of the most ubiquitous reagents in chemistry, Marder explains. Protons (hydrogen ions, the simplest acid) can be used to start reactions that string monomers together to make polymers, or rip polymers into smaller fragments, or change solubility. We now have the ability to put protons anywhere in materials with three-dimensional pinpoint control afforded by the two-photon-absorption process.

Microstructures of interest in photonics and sensing, such as the stack-of-logs photonic crystal can be created using the two-photon absorbing molecules (Credit: JW Perry, U Arizona)

Microstructures of interest in photonics and sensing, such as the stack-of-logs photonic crystal can be created using the two-photon absorbing molecules (Credit: JW Perry, U Arizona)

The Arizona molecules are up to a few hundred times more sensitive for two-photon absorption than their one-photon predecessors, which relied on destructive ultraviolet light for their excitation. Working with Christopher Ober and Tianyue Yu at Cornell University, the team has developed specially designed resins that can be etched away after exposure to acid.

Joseph Perry

Joseph Perry

The possibility of etching a resin solid containing embedded two-photon molecules will open the way to three-dimensional microfabrication by allowing specific points in a resin block to be activated, release protons and so etch a hole or channel inside the block. To demonstrate their prowess the team created networks of microchannels, as well as free-standing microstructures, by exposing solid resins of the new materials to the laser beam.

The two-photon molecules are added in low concentration, about one percent, to a polymer starting material to make a resin block. The acid generation process can then be used to manipulate the resin, changing its properties, etching holes or channels or even changing its transparency or making it mechanical stronger. The two-photon process can also be used to change the solubility of the resin so that it becomes soluble in water or organic solvents.

Imagine that we start with a plastic, totally impervious to water, says Perry. We can wash it all day and nothing happens. We scan with the laser, protons nip off parts hanging from the side of the polymer, and now there are polymer chains that can be dissolved in water. Instead of ending up with a little framework of stick-like structures, we have a bunch of little channels buried in the object. He adds, From an architectural point of view, it’s the difference between erecting a building or digging a mine. If all you want to do is dig a mine, you don’t want to have to build the mountain first just so you can tunnel.

The researchers also point out that the process could be used in photodynamic therapy in medicine to change the acidity, the pH, of specific points in the body, bursting open an injected drug capsule using a near infra-red laser focused at the target site, a tumour, for instance. The University of Arizona scientist have formed a company, Focal Point Microsystems, to commercialise this and related two-photon technology.

Further reading

Seth Marder
http://www.chemistry.gatech.edu/faculty/Marder/

Joseph Perry
http://www.chemistry.gatech.edu/faculty/Perry/

Christopher Ober
http://people.ccmr.cornell.edu/~cober/

Suggested searches

Photonics
Two photon excitation