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Stars born into Milky Way’s violent centre

Star formation is a steady process, requiring vast clouds of cold gas to gradually accumulate and compact. So in the region around the supermassive black hole at the centre of our galaxy, which is awash with violent gravitational “tidal” forces, one would expect stars to be few and far between.

Over the years, however, many stars have been spotted near the galactic centre — which raises the question: were the stars able to survive a perilous birth, or did they somehow migrate inwards from elsewhere?

Now, a group of astronomers from the Harvard–Smithsonian Center for Astrophysics, US, and the Max Planck Institute for Radio Astronomy, Germany, has evidence that stars can indeed be born near the galactic centre. Using the Very Large Array of radio telescopes in New Mexico, they have discovered two baby stars, or “protostars”, just a few light-years from the Milky Way’s black hole.

It solves what has been a hotly debated mystery for astrophysicists Elizabeth Humphreys, Harvard–Smithsonian Center for Astrophysics

“It solves what has been a hotly debated mystery for astrophysicists,” Elizabeth Humphreys of the Harvard–Smithsonian centre told physicsworld.com. “Observing stars where they shouldn’t have been able to form has meant that exotic theories have been proposed, in which massive clusters of stars form much farther away from the black hole and then migrate inwards to the galactic centre before breaking apart into individual stars.”

Seeing through the dust

A protostar marks the embryonic stage in a star’s formation, before it has begun nuclear fusion and while it is still hidden within a collapsing cloud of dust and gas. This cloud absorbs visible light, so to see protostars astronomers have to search within infrared and radio wavelengths, such as those monitored by the Very Large Array. They normally appear as “water masers”, which result from the cloud — like a laboratory laser or maser — amplifying radio waves to the extent that they become beacons for detection on Earth.

The findings, which Humphreys presented at a meeting of the American Astronomical Society in Long Beach, California, earlier this week, were of two masers seven and ten light-years away from the Milky Way’s central black hole. Although they are not the first protostars close to a galactic centre to be discovered — one was detected by Karl Menten, also of the Max Planck Institute for Radio Astronomy, in 1997 — they do prove that star formation in such regions is no fluke.

Perhaps more interesting of one of the newly discovered protostars, however, is that it lies is a ring of material orbiting the central black hole known as a circumnuclear disk. This protostar would therefore seem to support some recent simulations of cloud collisions, which implies that star formation in these disks is theoretically possible. “[As] the theoretical models are getting closer to reality, that is to say including gas cloud collisions that might well happen in the chaotic region near the black hole, the stars can form where they are observed,” says Humphreys.

Too few stars

One theorist who has successfully modelled star formation in the circumnuclear disk is Sergei Nayakshin of the University of Leicester, UK. He agrees that the observations of Humphreys and colleagues support the simulations, but thinks that they also raise new questions. “The result from simulations is that you get many tens or hundreds of protostars at once, so I would have expected us to have seen a lot more of them,” he explains. “If they are there, I think they would have been discovered by now.”

What’s interesting is that the stars must be formed in a slightly different way to what we were thinking about, and this might shed new light on star formation theories Sergei Nayakshin, University of Leicester

Nayakshin suggests that there may be unknown physics at work that reduces the clumping of gas and dust in the Milky Way’s inner regions. If this were accounted for in the models it might become apparent that the stars are formed individually and not in groups. “What’s interesting is that the stars must be formed in a slightly different way to what we were thinking about, and this might shed new light on star formation theories,” he says.

Nevertheless, the fact that stars close to the galactic centre can form in situ does not diminish the possibility that many of the older observed stars migrated inwards from elsewhere in the galaxy. “In principle, this process could have operated in earlier days of the Galaxy, and might work in other galactic centres as well,” adds Nayakshin.

Casimir effect goes negative

Since the 1950s physicists have been able to show that quantum fluctuations in a vacuum will cause two surfaces to attract one another, a phenomenon known as the Casimir effect. Now researchers in the US have demonstrated the opposite — that under certain conditions surfaces can also repel each other. Given the importance of the Casimir effect over separations of tens to hundreds of nanometres, the result could lead to new types of nanotechnology device with extremely low levels of friction.

The vacuum force was first predicted by Dutch physicist Hendrik Casimir in 1948. Casimir considered what would happen when two uncharged, perfectly conducting metal plates were placed opposite one another in a vacuum. According to quantum mechanics, the energy of an electromagnetic field in a vacuum is not zero but continuously fluctuates around a certain mean value (equal to half a photon at a temperature of absolute zero). Resonance means that only certain wavelengths will exist between two plates separated by a particular distance. What Casimir worked out was that the radiation pressure of the field outside the plates will tend to be slightly greater than that between the plates and therefore the plates will be attracted to one another.

Casimir’s prediction was generalized for real materials by Evgeny Lifshitz in 1956, whose work was further generalized to show that the vacuum can in fact be replaced by a material. Moreover, it was shown that if the plates and the material between them, generally a liquid, have particular dielectric permitivities the force between the plates will be negative.

It’s all in the permittivity

Dielectric permittivity reflects how easily the atoms and molecules in a material can be polarized. A fluctuating electromagnetic field will induce fluctuating electric dipoles whose strength is proportional to this polarizability, so the force between two materials will be proportional to the product of their permitivities. By making the permittivity of the liquid lower than that of one of the plates (the first plate) but higher than that of the other, it will be attracted to the first plate more than the two plates will be attracted to each other. This will allow it to come between the two plates and therefore in effect make them repel one another.

Now, Harvard University’s Federico Capasso and Jeremy Munday (now at Caltech), and Adrian Parsegian of the National Institutes of Health in Bethesda, Maryland, have demonstrated this effect using gold and silica separated by the liquid bromobenzene. The liquid was placed in a cell between a plate of silica and a 40 µm diameter polystyrene sphere that was coated with a 200 nm thick gold film and suspended from an atomic force microscope cantilever (in principle a sphere produces less accurate results than a second plate, but in practice it is more useful because two plates are so hard to align accurately). By bouncing a laser beam off the cantilever, any bending of the cantilever caused by interactions between the sphere and the plate led to a change in the reflected laser signal (Nature 457 170).

In building their experiment, Capasso and colleagues had to minimize potentially harmful electrostatic effects, such as charge build up on the silica plate. They also had to find a way of calibrating their experiment; in other words find a known force that they could use to convert their reflected laser signals into force measurements. For this they used a hydrodynamic force generated in the liquid, which is proportional to the speed with which sphere and plate are moved apart. “The calibration can be performed with large speeds and at large distances where the Casimir force is relatively small,” says Capasso. “Then the speed can be reduced and the sphere brought closer to the plate to measure only the Casimir force.”

Gold and silica repel

The researchers carried out measurements of the Casimir force using separations from 20 nm up to several hundred nanometres. They found that, as the gold sphere and silica were brought together, they clearly repelled one another. By contrast, they found a clear attraction between the gold sphere and a gold plate that they put in the place of the silica.

Steve Lamoreaux of Yale University, writing in a News and Views article to accompany the research paper, says that by mixing together two or more liquids it might be possible to tune the Casimir force so that it is attractive over large separations but repulsive over shorter distances. “This would provide the means for quantum levitation of an object in a fluid at a fixed distance above another object, and so could lead to the design of ultra-low friction devices,” he says. Lamoreaux also believes that the work could have implications for fundamental physics, pointing out that a Casimir-like force is predicted to be caused by density fluctuations in binary-liquid phase transitions.

Magnetic fields could reveal exoplanets

In the 400th year since Galileo pointed his first telescope heavenwards, a new way of looking at the skies — based on magnetism — could bring into focus significantly more small stars as well as planets orbiting stars other than the Sun (exoplanets).

The majority of astronomical objects are believed to possess magnetic fields. We know this because of synchrotron radiation emitted by particles trapped in them. Magnetic fields of Earth-like planets are relatively strong and are believed to originate from convection patterns in the planetary interiors. Solar magnetic fields, on the other hand — like the Sun’s — are relatively weak and thought to emerge from a layer of intense shear lying between the inner and outer sections of stars.

Now, a group of German theorists claims that this divide is too simplistic in the case of small stars and very large planets. They suggest that some planets and stars, less than a third as massive as the Sun, generate strong dipole fields like the Earth’s. If true, this would lead to very large emissions of synchrotron radiation which could be detected on earth.

This is a significant advance for it connects planetary dynamo theory to stellar dynamo theory Chris Jones, University of Leeds

Ulrich Christensen from the Max Plank Institute for Solar System Research in Northeim, Germany and his colleagues have successfully verified their model with known magnetic measurements for Jupiter, a group of young contracting stars, and a group of rapidly rotating brown dwarfs (Nature 457 167).

A matter of scale

In ‘standard’ solar dynamos, a magnetic field is generated inside the star at a region known as the ‘tachocline’; a zone dividing star interiors where conduction dominates heat transfer from the exterior where convection takes over. A sudden change in rotation rate at this layer creates high shear which intensifies a residual magnetic dynamo.

Unfortunately this model doesn’t work for certain classes of stars such as fast-rotating ones that have a mass that is less than a third of that of the Sun. Christensen and his colleagues suggest that in these cases dynamo-generating shear could originate from large-scale convection cells: regions of ionized gas rise and fall ‘rubbing shoulders’ as they pass.

This effect had been predicted previously but Christensen’s model goes much further because it also predicts the strength of field. “The key difference is our proposed scaling relationship: we directly link the energy field strength at the surface with the available energy flux from the interior,” Christensen told physicsworld.com.

Illuminate the heavens?

After correlating their model with existing observations from the T Tauri Stars and old M Dwarfs, Christensen noted that their model also requires stars and planets to be rapidly-rotating. The reason for this is yet to be fully understood.

Given these caveats, Christopher Johns-Krull of Rice University warns, “Although Christenson and colleagues’ work is an impressive step forward in our understanding of magnetic dynamo behaviour in celestial objects, it is far from a complete description of the process.”

Perhaps the most promising aspect is the radio waves stemming from strong magnetic fields. This year — the International Year of Astronomy — a new central European radio telescope array known as LOFAR will begin scanning the skies for low frequency radio waves. Christensen said, “I hope projects like LOFAR will take notice of our research and this may lead astronomers to detecting new stars and possibly extrasolar planets.”

It's noisy up there

By Hamish Johnston

A few days ago I mentioned an ad campaign to make the public aware of how events in the far-off cosmos affect us here on Earth. One ad points out that the some of the snowy noise on the screen of a poorly-tuned television is actually “microwave afterglow from the origin of the universe”.

It seems, however, that the universe contains more static than expected — six times more “radio noise” according to a team of astrophysicists in the US.

NASA’s Alan Kogut and colleagues launched the balloon borne ARCADE radio telescope with the hope of detecting emissions from the first stars formed after the Big Bang. Instead they found a booming signal that they couldn’t pin down to early stars or other known radio sources — a genuine mystery.

The team announced their findings at the 213th Meeting of the American Astronomical Society, going on this week in California.

The study of other types of cosmic noise has led to major breakthroughs in our understanding of the universe…so watch this space.

Let those neutrinos through

By Hamish Johnston

It’s the International Year of Astronomy and here at physicsworld.com we are travelling the world (from the comfort of our desks mostly) in search of weird and wonderful astronomy events.

First stop is my hometown (sort of), where buses, subway trains and trams are adorned with advertisements illustrating the mysteries of the cosmos. As well as neutrinos, the Toronto transit ads explain how you can watch evidence of the Big Bang on an old TV and why we are all star dust.

The ads are placed by the CoolCosmos programme at the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics. Take a look at their website for the other ads.

Interpreting Newton: brilliant ideas wanted

Home to a famous apple tree

By Hamish Johnston

I was just speaking to Susan Haimes, who is property manager of Woolsthorpe Manor in Lincolnshire — the birthplace of Sir Isaac Newton. The Manor, which is now owned by the National Trust and open to the public, is also the place where that legendary apple fell and inspired Newton to think hard about the nature of gravity.

Susan has issued a call to physicists for help in revamping the Manor’s interactive science discovery centre, which opened in 2000 and “is in need of updating”. Work is underway to redesign it and the new centre will be opened in March 2010.

New exhibits will include interactive models that demonstrate planetary orbits, the movement of points, calculus, gravity, prisms (including lenses, refraction and the problem of chromatic aberration), forces and telescopes.

However, Haimes is also keen to hear from any physicists who may have “a brilliant idea for interpreting an aspect of Newton’s work in a way that just hadn’t occurred to us”.

Time is short, though, so if you’ve got a brilliant idea for demonstrating an aspect of Newton’s work to families, send your sketch or idea off to susan.haimes@nationaltrust.org.uk by Friday 16 January, with your contact details.

If your idea is chosen and used in the centre, you will be given a one-year visiting pass entitling you and your family to visit National Trust properties around the country.

Please contact Susan for more information.

And in case your wondering, the apple in question was of the variety “Flower of Kent” and according to Susan is a rather large fruit. Legend has it that the original tree died in 1820, but its roots produced a second tree that is still there today.

Graphene transistor speeds up

Scientists at IBM have made the fastest graphene transistor to date. The device operates at frequencies as high as 26 GHz and could find use in wireless communications equipment.

A transistor’s operation speed depends on the size of the device — smaller devices can run faster — and the speed at which electrons travel in it. This size dependence has been one of the major driving forces for making ever smaller silicon transistors. However, the challenges of making practical silicon transistors smaller than about 40 nm will ultimately put the brakes on future speed-ups.

Some researchers believe that the ‘wonder material’ graphene — which is a sheet of carbon just one atom thick — could offer a way forward. As well as being extremely thin and a semiconductor, electrons move through graphene at extremely high speeds. This is because they behave like relativistic particles that have no rest mass. Because of this, and other unusual physical properties, graphene is often touted to replace silicon as the electronic material of choice and might be used to make faster transistors than any that exist today.

Real progress since 2004

Yu-Ming Lin and colleagues at IBM’s T J Watson Research Center have now demonstrated graphene transistors exhibiting a cut-off frequency of 26 GHz, the highest frequency reported for graphene so far (arXiv 0812.1586). While this is about 10 times slower than the fastest silicon transistor, it is remarkable progress because graphene was only discovered in 2004, whereas the silicon transistor has been around for over 50 years and has benefited from decades of intense research and development.

The team also revealed for the first time that the operation frequency of these devices increases as gate length size decreases. The value of 26 GHz was found for transistors with a gate length of 150 nm. This result provides guidance for improving the performance of graphene devices, said Lin.

The team also found that the devices behave like conventional field-effect transistors — that is the measured current gain decreases with frequency, f, according to 1/f. This will be important for modelling graphene transitors and for designing circuits based on graphene devices, explained Lin.

Scaling the gate length down

Most graphene devices made previously were back-gated, using a 300 nm thick silicon dioxide layer as the gate dielectric. Lin and colleagues’ device is different because it employs a top-gate dielectric structure, made of aluminium oxide, which allows the transistor to operate at lower voltages. Scaling the gate length down to 150 nm, from tens of micrometres previously, also contributes to the high cut-off frequency of 26 GHz, Lin told physicsworld.com.

The IBM researchers now hope to optimize the gate dielectric materials to further improve the electrical properties of the final devices and make radio-frequency circuits from the structures. They even reckon that graphene transistors operating at terahertz frequencies could be made in devices that have a gate length of just 50 nm. “We also plan to demonstrate graphene transistors made by other approaches,” added Lin.

Reasons to be cheerful

It is always easy to be optimistic at the start of a year, a credit crunch and likely recession notwithstanding. In the US, President Barack Obama has nominated the Nobel-prize-winning physicist Steven Chu as secretary of the US Department of Energy — a move that augurs well in giving science, and physics in particular, a prominent role in the new administration’s thinking. In April, the European Space Agency’s Planck satellite, which will measure the cosmic microwave background in unprecedented detail, is due to take off, while 2009 will also see space missions to study early galaxies (Herschel), exoplanets (Kepler) and the Earth itself (GOCE).

We can also look forward to the Large Hadron Collider at CERN finally colliding its first particles in the summer, although — given the complexity involved in repairing its damaged magnets — that date may well stretch into the autumn or even beyond. Meanwhile, Japanese science will take a giant step forward in June with the opening of the Japan Particle Accelerator Research Complex (J-PARC) — a huge facility combining an intense neutron source, a proton synchrotron and neutrino-beam generator all rolled into one.

Another highlight of 2009 is its designation by the United Nations as the International Year of Astronomy (IYA2009) — it being exactly 400 years since Galileo carried out the first-ever studies of the heavens using a telescope. IYA2009 will probably not have quite the impact on physicists that the International Year of Physics did in 2005, but there will still be plenty to get excited about (see “Let the global astronomy celebrations begin“). Keep an eye out for Physics World’s contribution to IYA2009 — an astronomy special issue in March and extra astronomy coverage on physicsworld.com. One cloud on the horizon is that this year physics could be eclipsed in the public’s mind by biology: 2009 marks the 150th anniversary of Charles Darwin’s On the Origin of Species and 200 years since his birth. Not wishing to miss out, Physics World will be running a special issue on physics and biology in the summer, which underlines the fact that physics remains as vital as ever.

Science toys

“I have a low boredom threshold,” Tim Rowett explains, ushering in my son Alex and me. Rowett is a jovial, professorish-looking man with wire-rimmed glasses and a short, white beard. Alex and I have gone to his flat in Twickenham, on the edge of London, to see his collection of fun stuff — jokes, games, puzzles and other toys related to science. When I ask what they have in common, Rowett has a ready, if not illuminating, answer: “They’re just things that make people go ‘Wow!’.”

The how of the wow

During my visit, Rowett hands Alex a pair of inverse goggles, which turn your field of vision upside down. While my son gets to grips with those, I look round Rowett’s small, three-room flat. At first glance, it seems like the lost-and-found corner of a 1950s airport. One room has metal racks stacked to the ceiling with old-fashioned, cloth-covered valises. Another has shelves crammed with books, with a jumble of other objects strewn across the mantelpiece, piano and other flat surfaces.

Many people collect toys of one kind or another, but Rowett is different on three counts. First, he has a lot — 18,000 toys in fact. Second, he has managed to eke a living out of them, entertaining children until his retirement in 1996, then selling toys over the Web (www.grand-illusions.com). Third, while most of us focus our fascination on a particular kind of amusement — magnetic toys, optical illusions or puzzles — Rowett is equally interested in each kind. He will discuss a simple toy made of clothes pegs — its inventor, variants and how these evolved over time — with the same passion that he discusses complex perceptual-trick devices made of arrays of lenses.

While Alex bumps about in his goggles, I manage to elicit some of Rowett’s own story out of him as he shows me his toys. He was born in Farnham in 1941, and his father was in the Royal Signal Corps and then the clergy. Rowett studied engineering at King’s College, London, but kept failing, eventually graduating in six rather than three years. “I was easily distracted,” he shrugs. Rowett found short-lived employment as an engineer, followed by odd jobs ranging from selling central heating (nine months) to peddling encyclopedias (three days).

All along, he loved entertaining children as a hobby. But it took him a while to discover his talent: his magic tricks never met the demands of his audience, and he did not have the personality for puppets. One day in the early 1970s, he brought to a party a toy dog equipped with a simple electroacoustic switch. “When you called, it went waggle waggle waggle, walk walk walk,” Rowett explains. “The kids were all over it. I had discovered that they don’t need high technology, they want things they can touch. That’s how I started.”

Alex, still wearing the inverse goggles, asks us to throw him a pen. He misses a few times and goes back to practising. Meanwhile, Rowett shows me his Klein bottle used to store fennel seeds; a baton-sized, battery-powered van de Graaff generator able to make a sheet of silver Mylar float; and a tiny Stirling engine: “Look at that — chug, chug, chug — it runs on the heat of your hand!”

Rowett cross-catalogues everything, and reels off a list: “Topological toys, string-climbing toys, disentanglement toys, nothing novelties, kaleidoscopes, spectacles, magnetic toys, rattlebacks, jigsaws, literary novelties, light sticks, geophysical toys, mirror toys, water siphons, spinners, soap bubbles, electroacoustic switches, aroma games, optical scopes…” On and on he goes, with dozens of overlapping categories.

We wander into the kitchen, which is another wonderland. It has pitchers that will not pour unless you know which holes to cover with your fingers, spoons that bend when inserted in hot liquid and a can of “primordial soup” from Fermilab (“Ingredients: quarks, force-carriers, electron-like particles, neutrinos, Higgs bosons…”). Rowett then shows me his latest acquisition: a few square inches of an extraordinary super-cushiony, sticky material. He drops an egg on it from a height of about two metres and it lands on the square without cracking or bouncing. “Astonishing!” Rowett says. When I ask him what it is good for, he gives me his usual answer: “For the ‘Wow!’.”

Alex asks us to throw him something again, and this time he catches it. We applaud.

The critical point

A few months later, Rowett and I meet again in a New York City café. He rattles off interesting optical effects that he has witnessed, including a balcony on 29th Street and 3rd Avenue that appears to slope upwards when viewed from uptown and downwards from downtown, and a place from which the Empire State Building looks like it is a rocket blasting off. Rowett has been scouring the stalls of toy fairs, and begins pulling his acquisitions out of a bag: a Japanese-made penlight that projects a star chart; a device that relies on the Bernoulli principle to make a rocket pop out of a box when you blow on it; and real Mexican jumping beans — which Rowett calls “nature’s best novelty” — the bizarre and unexpected movements of which are caused by newly hatched caterpillars.

The last items in his bag are plastic straws. Rowett pinches each end of one with his fingers and winds it up like a crank until it bulges. “Flick it with your finger,” he commands. I do, and it bursts with a bang. Startled coffee-drinkers look us over warily. “Adiabatic compression,” Rowett explains to anyone caring to listen. “It heats the wall of the straw, weakening it, so all it takes is your finger to make the trapped air pop it. I had a physics professor in Munich explain it all to me!”

Suddenly I know how to describe Rowett’s things. They each have a hidden order that, however familiar, shows itself in an unexpected and apparently magical way. His toys allow us to appreciate the order, and the magic too.

Web life: The Periodic Table of Videos

Eagle-eyed readers may spot a change in this column. Previously known as Blog life, it highlighted top picks from the physics blogosphere, and was itself an outgrowth of an earlier column on physics books, Shelf life. The new Web life column will continue to feature the best of physics blogging, but it will also include other types of Web content of interest to Physics World readers. First up is a periodic table of videos from Nottingham University in the UK.

What is it?

The Periodic Table of Videos

The website’s main page contains a periodic table with links to short (about five minute) videos on all 118 chemical elements. It is the brainchild of video-journalist Brady Haran, who teamed up with a group of chemists — most notably the Einstein-haired Martyn Poliakoff — from Nottingham University to produce the short films.

Can you describe a typical video?

Almost all of the videos are structured around anecdotes from Poliakoff, a veritable chemical raconteur who even manages to spend nearly three minutes talking about unniloctium, otherwise known as element-118. Videos of the more common elements often feature trips to the laboratory or chemical stockroom, where chemists Pete Licence, Stephen Liddle and Debbie Barnes examine carefully wrapped samples, play with gases and/or blow things up. The science they present is serious and well explained but the researchers are also clearly having fun.

Who is it aimed at?

The most obvious answer is chemists, but there is plenty here for the more physics-minded as well. For example, the video on helium has a nice demonstration of gas-law physics (as well as the obligatory squeaky voices), and the lanthanides and actinides beloved of nuclear physicists and engineers are not neglected — in fact, they are Liddle’s speciality. Many of the videos also contain laboratory stunts that should definitely not be tried at home, and these should appeal to science teachers with limited budgets (or nerve!) for, say, dropping lumps of caesium into water.

Why should I visit?

In an introductory clip, Poliakoff compares the periodic table to a family: some members you know well, while others (ruthenium, anyone?) may not be as familiar. Whether your most recent peek at Dmitry Mendeleev’s table came last week or last century, you are sure to find something here you did not know. After watching a few videos, you may even find yourself thinking “Oh, just one more…”.

How often is it updated?

The team finished the periodic table in summer 2008, but the researchers have not rested on their laurels. Updated videos crop up every few weeks, and the site maintains a list of elements soon to be refreshed. In late 2008 Liddle and Haran travelled to Ytterby, Sweden, to make a special video about the mine where four elements — yttrium, erbium, terbium and ytterbium — were discovered.

Can you give me a sample quote?

“Potassium is very reactive. One of my colleagues who used to work with it describes it as ‘evil’,” says Poliakoff.

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