Eta Carinae: Revolving binary stars whip up winds in space

July 03, 2020

Working with particle accelerators is part of the day-to-day business of nuclear physicists at the German Electron Synchrotron (DESY).

What is less well-known is that DESY is also concerned with completely different types of particle accelerators — those that exist naturally. And their study is primarily the task of the astrophysicists at the research center.

One such particle accelerator — much larger than the man-made facility of the European Organization for Nuclear Research (CERN) in Geneva — is the double star system Eta Carinae.

It is located 7,500 light years away in the constellation Carina in the southern sky and generates gamma radiation with an energy of up to 400 gigaelectronvolts (GeV) — about 100 billion times more than the energy of visible light.

This double star is the first known example of a source in which very high-energy gamma radiation is generated by colliding stellar winds.

Computer reconstruction

The team led by Stefan Ohm, Eva Leser and Matthias Füssling from DESY observed the double star system with a special telescope at the gamma-ray observatory High Energy Stereoscopic System (HESS) in Namibia.

Together with animation specialists from the Science Communication Lab, they then reconstructed how it works in a video animation. The composer Carsten Nicolai (artist’s name: Alva Noto) composed the soundtrack.

The researchers published their findings in the journal Astronomy Astrophysics.

A star system of superlatives

Eta Carinae consists of two giant blue suns. One of them has about 100 times the mass of our sun, the other about 30 times. They revolve about each other every 5.5 years on strongly elliptical orbits. The distance between them varies greatly, roughly between that from our sun to Mars and that from our sun to Uranus.

Read more: CERN: Large Hadron Collider gets stronger, better, brighter

Both giant stars hurl dense, supersonic stellar winds of charged particles into space. In the space of only around 5,000 years, the larger of the two loses as much mass as our sun has in total.

At the point where the two stellar winds meet every 5 1/2 years, a huge and extremely hot shock front develops with temperatures of around 50 million degrees Celsius.

The strong electromagnetic fields that prevail there accelerate subatomic particles. “Such strongly accelerated particles can also emit gamma radiation,” research director Ohm explains.

In fact, the NASA satellite Fermi and the satellite Agile of the Italian space agency ASI already detected high-energy gamma radiation up to about 10 GeV from Eta Carinae in 2009.

Where does the gamma radiation come from?

“There are different models for the production of this gamma radiation,” said co-author Füssling. “It can come from strongly accelerated electrons or from high-energy atomic nuclei.”

Read more: The KATRIN Tritium Neutrino experiment: A giant scale for the tiniest particles starts

High-energy atomic nuclei are mainly responsible for the cosmic rays that constantly hit the earth from all sides. Although these rays were discovered more than 100 years ago, it is still not completely clear where such atomic nuclei come from. Because they are electrically charged, the atomic nuclei are deflected by cosmic magnetic fields on their way through the universe. This means that the direction from which they hit Earth no longer points back to their origin.

Cosmic gamma radiation, in contrast, is not deflected. If it can be proven that gamma radiation originates from high-energy atomic nuclei, this would also mean that one of the sought-after accelerators of cosmic particle radiation has been found.

Read more: European XFEL: First images of the X-ray laser inspire researchers

Waiting for a newer gamma-ray telescope

“Analysis of the gamma radiation measured by HESS and the satellites shows that it can best be interpreted as the product of highly accelerated atomic nuclei,” says DESY doctoral student Ruslan Konno. He has published an accompanying study together with scientists from the Max Planck Institute for Nuclear Physics in Heidelberg.

“This would also make the shock regions of colliding stellar winds a new type of natural particle accelerator for cosmic rays,” he concludes.

Now the astrophysicists want to further substantiate this thesis. In addition to HESS, named after the discoverer of cosmic rays, Victor Francis Hess, a more powerful gamma-ray observatory will be available to them in the future. Construction work for the Cherenkov Telescope Array (CTA) in the Chilean highlands is already underway.

Read more: Astronomers find closest black hole to Earth

Huge digital cameras record tiny particles

Pictures of particles

The ALICE detector, owned by the European Organization for Nuclear Research (CERN) – is located more than 90 meters underneath this colorful building in Geneva. ALICE is a huge digital camera capable of photographing even the smallest building blocks of the universe – the components of an atom’s nucleus.

Huge digital cameras record tiny particles

Helmets required

In addition to ALICE, three other detector cameras, named ATLAS, CMS, and LHCb, keep a record of particle collisions at the LHC. To see them you have to go deep below the rock of the French and Swiss Alps.

Huge digital cameras record tiny particles

Did puny particles follow the Big Bang?

When protons or lead ions smash together at the speed of light the smallest elementary particles are released – and this is what it looks like to the CMS detector. Scientists believe our universe was created from such particles in the first billionth of a second after the Big Bang.

Huge digital cameras record tiny particles

On track at high speed

This is where lead ions and hydrogen protons are accelerated. They fly through a vacuum tube with the energy of a speeding train and are kept on track by massive electromagnets. The pipe has a circumference of 27 kilometers and can be accessed through the four large detectors where the particle collisions take place.

Huge digital cameras record tiny particles

The world’s largest fridge

The electromagnets that keep the particle beam on track are made of superconducting inductors. The cables must be kept at a chilly minus 271.3 degrees Celsius (minus 456 Fahrenheit) so they no longer have any electrical resistance. To cool them down, the collider sends a whole lot of liquid helium through the pipes.

Huge digital cameras record tiny particles

Precise magnets

The LHC is not a perfect circle but instead consists of long straight stretches interrupted by curves, where magnets redirect the beam. The electromagnets are extremely precise. Just before a collision they focus the beam in exactly the angle so that the probability of two particles colliding is very high. The clash then happens right in the middle of the detector.

Huge digital cameras record tiny particles

Built like a ship in a bottle

The detectors are as big as multi-level houses. But they all had to be brought into the mountain in smaller parts through narrow shafts like this one. Underneath it is a gigantic cavern where ALICE was put together.

Huge digital cameras record tiny particles

8,000 photos per second

This is the ALICE detector when it is opened for maintenance. When in operation, ion beams collide in its center. New particles are created, flying off in different directions through several layers of silicon chips, similar to the sensors of a digital camera. The chips and other detectors record the particles’ routes. ALICE can capture 1.25 gigabytes of digital data each second.

Huge digital cameras record tiny particles

Electromagnets make particles identifiable

This blue chunk is another huge electromagnet, an important part of the ALICE detector. It creates a field making it possible to identify particles that are created during the high-speed collisions. Scientists study the direction the new particles travel. For instance, they can determine whether particles were neutral or positively or negatively charged.

Huge digital cameras record tiny particles

Wings to catch a muon

The ATLAS detector has a special gauge, the so-called muon spectrometer, which lies outside the detector’s heart, just like large wings. With these wings a heavy relative of the electrons – the muon – can be caught. Muons are difficult to find because they only exist for two millionths of a second.

Huge digital cameras record tiny particles

Watching from a safe distance

All detectors have a control room, just like this one for ATLAS. Once the collider is in operation, no one is allowed to stay inside the underground facilities. An out of control proton beam can melt 500 kilograms of copper and escaped helium could cause frostbite and suffocation. The particle stream could even create radioactivity.

Huge digital cameras record tiny particles

What to do with the data?

The detectors deliver data 40 million times per second. But because not all collisions are interesting for scientists, the data has to be filtered. In the end, no more than 100 interesting particle collisions per second remain. That’s still more than 700 megabytes of data per second – about what fits on a commercial CD. All data initially lands here in CERN’s data processing center.

Huge digital cameras record tiny particles

A global computer network

CERN produces an amount of data per year that if it were stored on CD, the pile would be 20 kilometers high. Even though such a tape library can hold a lot of data, it is still not enough. So the data are distributed worldwide. More than 200 universities and research institutes have created a worldwide CERN computer network with their data processing centers.

Huge digital cameras record tiny particles

Data for everyone

Particle physicists from around the world have access to CERN data. The center sees itself as a service provider for universities and institutes conducting basic research. A common project for everyone’s benefit.

Author: Fabian Schmidt / asb