Antihelium nuclei as messengers from the depths of the galaxy

How are galaxies born, and what holds them together? Astronomers assume that dark matter plays an essential role. However, as yet it has not been possible to prove directly that dark matter exists. A research team including Technical University of Munich (TUM) scientists has now measured for the first time the survival rate of antihelium nuclei from the depths of the galaxy -- a necessary prerequisite for the indirect search for Dark Matter.

Many things point to the existence of dark matter. The way in which galaxies move in galactic clusters, or how fast stars circle the center of a galaxy results in calculations which indicate that there must be far more mass present than what we can see. Approximately 85 percent of our Milky Way for example consists of a substance which is not visible and which can only be detected based on its gravitational effects. As of today it has still not been possible to directly prove the existence of this material.

Several theoretical models of dark matter predict that it could be composed of particles which interact weakly with one another. This produces antihelium-3 nuclei, which consist of two antiprotons and one antineutron. These nuclei are also generated in high-energy collisions between cosmic radiation and common matter like hydrogen and helium -- however, with energies different from those that would be expected in the interaction of dark matter particles.

In both processes, the antiparticles originate in the depths of the galaxy, several tens of thousands of lightyears away from us. After their creation, a part of them makes its way in our direction. How many of these particles survive this journey unscathed and reach the vicinity of the Earth as messengers of their formation process determines the transparency of the Milky Way for antihelium nuclei. Until now scientists have only been able to roughly estimate this value. However, an improved approximation of transparency, a unit of measure for the number and energies of antinuclei, will be important for interpreting future antihelium measurements.

LHC particle accelerator as antimatter factory

Researchers from the ALICE collaboration have now carried out measurements that have enabled them to determine the transparency more precisely for the first time. ALICE stands for A Large Ion Collider Experiment and is one of the largest experiments in the world to explore physics on the smallest length scales. ALICE is part of the Large Hadron Collider (LHC) at CERN.

The LHC can generate large amounts of light antinuclei such as antihelium. To do so, protons and lead atoms are each put on a collision course. The collisions produce particle showers which are then recorded by the detector of the ALICE experiment. Thanks to several subsystems of the detector, the researchers can then detect the antihelium-3 nuclei that have formed and follow their trails in the detector material. This makes it possible to quantify the probability that an antihelium-3 nucleus will interact with the detector material and disappear. Scientists from TUM and the Excellence Cluster ORIGINS have contributed significantly to the analysis of the experimental data.

Galaxy transparent for antinuclei

Using simulations, the researchers were able to transfer the findings from the ALICE experiment to the entire galaxy. The result: About half of the antihelium-3 nuclei which were expected to be generated in the interaction of dark matter particles would reach the vicinity of the Earth. Our Milky Way is thus 50 percent permeable for these antinuclei. For antinuclei generated in collisions between cosmic radiation and the interstellar medium, the resulting transparency varies from 25 to 90 percent with increasing antihelium-3 momentum. However, these antinuclei can be distinguished from those generated from dark matter based on their higher energy.

This means that antihelium nuclei can not only travel long distances in the Milky Way, but also serve as important informants in future experiments: Depending on how many antinuclei arrive at the Earth and with which energies, the origin of these well-travelled messengers can be interpreted as cosmic rays or dark matter thanks to the new calculations.

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Discovery of the oldest visible planetary Nebula hosted by a 500-million-year-old Galactic cluster -- a rare beauty with a hot blue heart

An international team of astronomers led by members of the Laboratory for Space Research (LSR) and Department of Physics at The University of Hong Kong (HKU), have discovered a rare celestial jewel-a so-called Planetary Nebula (PN) inside a 500 million-year-old Galactic Open Cluster (OC) called M37 (also known as NGC2099). This is a very rare finding of high astrophysical value. Their findings have just been published in the open-access paper Astrophysical Journal Letters.

PNe are the ejected, glowing shrouds of dying stars that shine with a rich emission line spectrum and display, as a result, their distinct colours and shapes that make them photogenic magnets for public interest. It was no coincidence that one of the first James Webb Space Telescope (the largest optical telescope in space) images released to the public was a PN.

The PN, with the rather ungainly name of "IPHASX J055226.2+323724," is only the third example of an association between a PN and an OC out of the ~4,000 PNe known in our Galaxy. It also appears to be the oldest PN ever found. The small team led by Professor Quentin PARKER, Director of the HKU LSR, have determined some interesting properties for their discovery: the authors found the PN has a "kinematic age" of 70,000 years. This estimate is based on how fast the nebula is expanding, as determined from the PN emission lines, and assuming this speed has remained effectively the same since the beginning, and is the time elapsed since the nebular shell was first ejected by the host, a dying star. This compares to typical PN ages of 5,000-25,000 years. It is truly a grand old dame in PN terms but of course a mere "blink of the eye" in terms of the life of the original star itself that runs to hundreds of millions of years.

Because this "grand old dame" lives in a stellar cluster, this environment enables the team to determine powerful additional parameters not possible for the general Galactic PN population. These include estimating the mass of the PN's progenitor star when it turned off the stellar main sequence, as derived from the observed properties of the thousands of stars in the cluster when plotted in a so-called colour-magnitude diagram. The team can also estimate the residual mass of the central star that ejected the PNe via theoretical isochrones and observed properties of the hot, blue central star. As a result, they figured how massive the star was that ejected the PN gaseous shell when it was born and how much mass is now left in its residual, contracting hot core (which is already a so-called 'White Dwarf' star). Fresh "Gaia" data for the hot blue, PN central star also provide a good distance estimate allowing the PN's actual size at this extreme age to be determined as 3.2pc (parsec, an astronomical unit of measure for interstellar space with 1pc equals to 3.26 light-years) in diameter -- unsurprisingly perhaps also at the extreme end of known PN physical sizes.

Former HKU PhD student Dr Vasiliki FRAGKOU, the first author of the study stated, "I am so excited to be able to work on these fascinating rare cases of OC-PN associations because they keep turning up important science results, like all three cases we have found are butterfly (bi-polar) PN in terms of shape, all are very faint and highly evolved, and all have Type-I chemistry according to their emission lines, and of course all have intermediate to high progenitor masses."

Corresponding author Professor Quentin Parker said, "This is only the third example of a PN found in a Galactic open star cluster, and my group has found all three confirmed examples. They are incredibly rare but also very important as these beautiful objects allow us to independently determine points on the so called initial to final mass relation (IFMR) for stars -- an important astrophysical relation -- independent of the traditional method of using white dwarfs in clusters. Intriguingly, all our points lie just below the empirical IFMR trend currently established but add to the "kink" in this relation found recently in the 2-3 Solar mass range for the original progenitor mass by Marigo et al in the Nature Astronomy journal. Our OC-PN points fortuitously are found in currently sparsely populated regions of the IFMR, making them even more valuable."

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