Cosmic-ray acceleration in a superbubble

Cosmic-rays are very energetic particles that pervade the Universe. Discovered one century ago, their origin is still unclear. In the Galaxy, the most promising sources are the powerful magnetized shock waves triggered by supernovae, which mark the cataclysmic end of a star: the thermonuclear disruption of a white dwarf (as for SN 1006) or the core-collapse of a massive star (as for RX J1713-3946). Massive stars are born in groups, called OB associations. And as they live fast and die young, they are likely to spend their whole life into clusters. Their strong stellar winds, together with their subsequent explosions, profoundly alter their environment: they carve huge, hot and tenous cavities around them, called superbubbles. It is naturally expected that these objects play a key role in the acceleration of cosmic rays in the Galaxy. And measurements of the composition of the cosmic rays indeed point to OB associations and their superbubbles as the likely source of the material of a substantial fraction of them. But the particles that we collect on the Earth don't tell where they come from: being charged particles, they wandered a lot in the Galactic magnetic field before reaching us. We can spot freshly accelerated particles next to their sources thanks to the high-energy radiation they emit. But, until recently, observations of superbubbles were very scarce.

The situation changed dramatically in the past years with the rise of a new generation of instruments. The Fermi satellite is one of them. Launched by NASA in 2008, this international mission observes the gamma-ray sky between the energies of about 100 MeV to 100 GeV with unprecedented sensitivity. It monitors all the sky, and, of particular interest for us today, a massive star forming region in the direction of Cygnus. Analyzing the data from high-energy instruments is a difficult task, especially for extended sources. After modelling and carefully removing a number of backgound diffuse emissions, scientists found a significant excess of high-energy emission from this part of the sky. They were stricken to see that the emission is coincident with the cavities formed around the numerous clusters of massive stars: it nicely follows the ionization fronts eroding the surrounding molecular clouds, as if in a "cocoon". Various objects can produce gamma-ray radiation (individual stars, pulsars and their nebulae), but after checking all possibilities the collaborators concluded that such small sources don't make a good match to the extended emission. The most likely explanation is that the cocoon is filled with cosmic-rays, that slowly diffuse inside the superbubble, and radiate some of their energy in the process (they could be either electrons or protons). The energy distribution of the observed photons is notably hard, meaning that the fraction of particles with the highest energies is higher than usual. Because it's all the more easy for particles to escape their accelerator when they are energetic, this hardness points to freshly accelerated particles. So here we have it: the Fermi observation provides the first strong direct evidence for the long-advocated hypothesis that OB associations host cosmic-ray factories.

The two main clusters in the cocoon are so young (4 to 6 million years for Cygnus OB2, 6 million years for NGC6910) that they might or not have experienced supernovae yet. But the winds of massives stars themselves can produce similar shock waves. And they generate lots of magnetic turbulence in the bubble, which traps particles in the cocoon, where they can get re-accelerated by other shocks and/or by magnetic waves. Existing models of acceleration into superbubbles predict spectra with a shape similar to that infered, which gives credance to the scenario. However it is too early for a detailed comparison of models with the cocoon spectrum: more work is needed on both fronts. Theoreticians will refine their models, and observers will have a closer look a other regions. The Arches, Quintuplet, and Sgr B2 clusters are good targets towards the Galactic centre, as are Westerlund 1 and Westerlund 2. These objects have already been detected at even higher energies, by one of the other prominent new instruments, the H.E.S.S. telescope.

Brian Schmidt's wine isn't quite as good as his science, but it is a nice tipple.  There he is pressing grapes at the top of the page - the guy with the akubra hat. And there's a bunch of astronomers picking the grapes for him at his Canberra vineyard. May his universe keep accelerating towards more and more happiness.

And the Nobel Prize in Physics goes too...

Saul Perlmutter, Brian Schmidt and Adam Riess for the discovery of the accelerating expansion of the universe.  The expansion of the universe has been known for many years now.  However Perlmutter, Schmidt and Riess, along with their supporting cast took those measurements to the next level.

Perlmutter and Schmidt investigated the light from Type Ia Supernovae explosions by setting up two rival teams to search for these energetic explosions.  Riess, working for Schmidt, took the data and converted the amount of light we received from the explosion into what the Type Ia's intrinsic luminosity.  With that in hand he could calculate the distance to these objects and compare that against their redshift.  This allowed for an extremely precise measurement of the expansion of the universe and its evolution in time.

What they found was amazing.  Instead of a fixed expansion like was expected from a simple Hubble Law, they found that the expansion changed overtime.  It changed in such a way that as one got closer to the present time the universe was expanding faster and faster.  Thus the universe's expansion was accelerating.

This discovery set off a flurry of activity in the cosmological community to explain this expansion as well as figure out what will happen if it continues.  If the expansion continues to accelerate, and given the fact that the universe appears to have a flat geometry, it seems that the eventual fate of the universe is that we will expand forever to the point where we will no longer see any other stars in the sky.  Naturally this will take hundreds of billions of years.  This also assumes that the acceleration continues and does not reverse itself in the future.  Regardless of the future fate of the now incorrectly named Hubble constant, we know for sure that the expansion is variable and currently accelerating.

ALMA First Light

The Atacama Large Millimeter/submillimeter Array (ALMA) has released it's first light images (seen above) and is now is open for business.  Located in the Atacama desert in Chile, it is in one of the driest regions in the world.  The plateau itself is 5 km above sea level and shielded by the Chilean Coastal Range.  This insures that the desert gets no water.  This is great for submillimeter astronomers, as water vapor is a significant foreground to their measurements.

ALMA itself has been about a decade in the making and is a collaboration between the National Radio Astronomy Observatory (NRAO), the European Southern Observatory (ESO), and the National Astronomical Observatory of Japan (NAOJ).  Currently made of nineteen 12 meter radio telescopes ALMA will continue to expand until it reaches completion in 2013 with 66 telescopes.  This will allow ALMA to reach resolutions 5 times smaller than the Hubble Space Telescope and 10 times better than the Very Large Array (VLA).  This will allow astronomers to get an unprecedented look at the submillimeter sky.  This view will allow us to better understand protostellar formation, planet formation, black hole physics, and starburst galaxies just to name a few.

Speaking of starburst galaxies, the first light image is of the Antennae galaxy which happens to be one.  The Antennae is the result of a major merger of two spiral galaxies of roughly the same size.  The image shows the famous Hubble image in blue with the ALMA image in yellow and orange.  The magatama shaped ALMA image shows where the protostars in this starburst galaxy are forming.  These stars are currently buried in their natal clouds and as such are invisible to Hubble due to extinction.  However, ALMA reveals these regions allowing us to probe the locations of star formation themselves, before the stars that form blow away all the surrounding gas and dust.

A Beginning and an End

China successfully launched the first piece of its upcoming space lab on Thursday.  Named Tiangong 1 (Heavenly Palace), it is intended to serve as a testing base for space technologies that China will need in the future.  This makes China the third country to successfully have launched their own space station, following Russia and the US.  The station module is currently unmanned but will see several visitors in the coming months including China's first female astronaut.

In other news, the Tevatron shuts down for the final time tomorrow.  After nearly 25 years of operation, it has seen its last ion particle beams.  Tevatron bridged the gap between the cancellation of the Superconducting Super Collider and the building of the Large Hadron Collider (LHC).  With the closure of the Tevatron the US will lack a high energy particle collider on the scale of the LHC, meaning that US particle physicists interested in high energy collisions will have to go to CERN to do their experiments.  The Tevatron itself may be recommissioned as a muon collider making it one of the first in the world to collide leptons at super high energy.

Particles faster than the speed of light???

This surprising and confusing possibility was brought up due to recent results from OPERA (Oscillation Project with Emulsion-tRacking Apparatus), a neutrino experiment at Gran Sasso Laboratory (the paper can be found here).  The experiment works by firing neutrinos from CERN at Gran Sasso through the ground.  The original intent of the experiment was to study neutrino oscillations, but they ended up with a curious result.

When the timed how long it took the neutrinos to reach the detector at Gran Sasso from CERN they clocked in at 987.8 ns (Gran Sasso is 730.085 km away from CERN).  When they checked how long light would have taken in a vacuum they got 1048.5 ns, a difference of 60.7+/-6.9(statistical)+/-7.4(systematic) ns.  In other words the neutrinos appear to be traveling 0.00248+/-0.00028(statistical)+/-0.00030(systematic) percent faster than the speed of light.  The statistical and systematics after the numbers are the calculated errors on the measurements.

This is a mind boggling result, contrary to all that we expect from physics as we know it.  Thus the physicists who work on OPERA have asked other neutrino experiments such as T2K and MINOS to confirm this result.  Naturally they are also searching for any errors in their calculations.  If it is not confirmed then the measurement by OPERA is suspect and there is likely an error in their analysis.  However if confirmed, things will get a little weird to say the least.

Like the OPERA physicists I will refrain from speculating about what this might entail for physics if true.  This result is too significant to speculate on.  We need to know for certain if this is true.  Odds are definitely on the side of it being flat out wrong. Too much of modern physics works too well, which casts significant doubt on this measurement.  Not to mention that a significant delay was not seen between the neutrinos arriving and us observing the explosion of Supernova 1987A.  This single event already ruled out this result, but only for electron neutrinos, the neutrinos in this case are muon and tau neutrinos.  Also there is history to consider, in 2007 a similar claim was made in Chicago but it had a huge margin of error that ruled out the measurement being significant.  This measurement though is way above the normal 3 sigma level of significance (basically 3 times the error bar size) usually needed for scientific data so it deserves close scrutiny.  Naturally the entire scientific community will be holding its collective breath until it is either confirmed or falsified.

Why is the Sun Yellow?

This classic childhood question actually has the same answer as, "Why the sky is blue?'', which is actually not the same answer as many teachers give when teaching introductory Astronomy courses, including myself.  This was pointed out in an article, written by Jonathan M. Marr and Francis P. Wilkin at Union College, which appeared on arXiv today.

The usual explanation for why the Sun is yellow is given by the use of Wien's Law which describes where the peak of a blackbody spectrum is given the temperature of the body.  Using Wien's Law and the surface temperature of the Sun, 5800 K, we end up with a wavelength of 500 nm, which we perceive as green.  However, the Sun isn't green by yellow.  Thus the explanation goes that the sky preferentially scatters blue light, via Rayleigh scattering, which shifts the color towards yellow.  Plus the way the human eye perceives the light and the shape of the blackbody curve naturally favor a yellow color rather than green.

This explanation turns out to be partially correct.  While Wien's Law does give the peak emission for a blackbody curve, it only does so for the intensity, B_{\lambda}, and only when done in wavelength.  If one does the emission in frequency, which is inversely related to wavelength one ends up with the blackbody peak being at 880 nm, which is red.  One can't have two peaks so what is going on here?

This ends up being a pretty subtle, but power piece of radiative transfer (I apologize in advance this gets a little technical).  It turns out what matters is the units one looks at.  If one looks at the proper units for the emission, the peaks line up.  The proper units to look at are not intensity but what is termed the spectral energy distribution (SED).  Whereas intensity, B_{\nu} in this case, is in ergs/sec/cm^2/str/Hz, the SED is measured in \nu B_{\nu} which is ergs/sec/cm^2/str.  With the per frequency dependence gone, the peak now lines up properly with the peak gotten from doing the analogous thing with \lambda B_{\lambda}.  This is because the frequency and wavelength are inversely related.  This shifts the respective intensities in different ways.  The wavelength version shifts the intensity down to smaller wavelength, higher frequency.  The frequency version does the opposite by shifting the frequency down and increasing the wavelength.  The SED formulation removes this dependency, reconciling the two.

Using the SED version of the Planck function, another name for a blackbody distribution, the peak is now in between the two at around 633 nm, which is red.  The Sun isn't red though so what's the deal?  It turns out that astronomers have been abusing Wien's Law.  The Planck function isn't sharply peaked around its peak.  Rather it slowly falls off in the immediate region surrounding the peak wavelength.  Over the visual range, which is actually quite small in terms of wavelength space at 390-750 nm, the SED of light between the red and blue end only varies by about 1% which is less than the eye can discern.  Thus the eye would see the unmodified light of the Sun as white, and not any color.

Then why does the Sun look yellow?  Well the atmosphere preferentially scatters blue light via Rayleigh scattering as noted before.  This gives the sky its blue color.  All the blue light that comes from the sky has to be removed from the light coming directly coming from the Sun.  As a result this leaves only the red and green light, which our eyes perceive as yellow.

Thus from now on whenever I, or any other astronomer for that matter, are asked why the Sun is yellow or why the sky is blue, we can give the same answer from now on.