Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Magnetic white dwarfs appear younger than they are


Scientists from Göttingen University link magnetic fields to atmospheric convection

An international group of astronomers including a scientist from the University of Göttingen has found an explanation of the long-standing mystery of why magnetic fields are more common among cool white dwarf stars than among young and hotter ones.

Reconstructed distribution of the magnetic field (red) and temperature (grey) on the surface of white dwarf star WD 1953-011 at different rotation phases.

Foto: Universität Göttingen

Dr. Denis Shulyak

Foto: Universität Göttingen

The researchers showed that strong magnetic fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic ones, making them appear younger than they truly are. The results were published in Nature.

White dwarf (WD) stars are the remnants of intermediate mass stars at the final stage of their evolution. Since the white dwarf does not burn any fusion in its interior, it cools down from the time it is born – pretty much like a pot of hot water left out the fire. Therefore, the surface temperature of any white dwarf star can be uniquely linked to its age.

If a star-progenitor has a magnetic field, then the contraction process during the formation of the WD will amplify this field by many orders of magnitude. This is how magnetic white dwarf stars (MWD) appear. Because magnetic fields are expected to decay with time, and because surface temperature also drops when WD cools down, one might expect to detect more non-magnetic or weakly magnetic objects at cool temperatures, but the opposite is observed.

The researchers found that the magnetic field may have a global control of surface convection in cool MWD stars which explains their puzzling characteristics. „By analyzing the light variability of the cool dwarf WD 1953-011 we found a direct link between the strength of local magnetic field and the local surface temperature,“ explains Dr. Denis Shulyak from Göttingen University’s Institute for Astrophysics. This suggests that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas similar to that occurring in sunspots.

However, in contrast to sunspots that have short life times from weeks to months, the magnetic details and associated temperature distribution patterns in WD 1953-011 are stable and do not change over at least ten years. This implies that the majority of convective MWD stars should demonstrate photometric variability. „And this is indeed what astronomers observed,“ says Shuylak.

But if the global magnetic field is very strong (hundreds of kilogauss and above), it can then inhibit convective motions everywhere over the stellar surface and deep into the interior of the star. „Because convection transfers a significant fraction of the total energy flux from subphotospheric layers to the surface in WD stars with surface temperatures below approximately 12,000 K, its suppression by strong magnetic fields will result in decrease of the stellar luminosity.

If we now remember that cooling times of WD stars are inversely proportional to luminosities, then objects with globally suppressed convection should have longer cooling timescales than their non-magnetic or weakly magnetic twins. Therefore, magnetic suppression of cooling provides a natural explanation for the increase in number of MWD stars at cool temperatures where convection is the dominant energy transport mechanism. This result fully agrees with our theoretical predictions,“ says Shulyak.

The analysis of photometric variability of cool MWD stars and their unexpectedly high frequencies compared to non-magnetic stars, as well as the high dispersion of their space velocities (which carries the information about the stellar age) – all these observational facts ultimately point towards the existence of a magnetic suppression of cooling in strongly magnetic, isolated WD stars.

„If we imagine the WD star as an open pot with hot water left on the table to cool, then covering it with a lid will slow its cooling time. Strong magnetic field is this kind of lid in WD stars which suppresses convection and therefore heat loses. Our findings imply that the ages of most magnetic and cool MWD stars can be underestimated. This prompts a revision of our interpretation of the MWD cooling sequence that, in turn, may require tuning of our understanding of the evolution of the Galaxy and the Universe,“ concludes Shulyak.

Original publication: Gennady Valyavin et al. Suppression of cooling by strong magnetic fields in white dwarf stars. Nature 2014. Doi: 10.1038/nature13863.

Dr. Denis Shulyak
University of Göttingen
Faculty of Physics – Institute for Astrophysics
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany, Phone +49 551 39-5055

Weitere Informationen:

Thomas Richter | idw - Informationsdienst Wissenschaft

More articles from Physics and Astronomy:

nachricht Hubble discovers mysterious black hole disc
12.07.2019 | ESA/Hubble Information Centre

nachricht What happens when you explode a chemical bond?
12.07.2019 | University of California - Berkeley

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Artificial neural network resolves puzzles from condensed matter physics: Which is the perfect quantum theory?

For some phenomena in quantum many-body physics several competing theories exist. But which of them describes a quantum phenomenon best? A team of researchers from the Technical University of Munich (TUM) and Harvard University in the United States has now successfully deployed artificial neural networks for image analysis of quantum systems.

Is that a dog or a cat? Such a classification is a prime example of machine learning: artificial neural networks can be trained to analyze images by looking...

Im Focus: Extremely hard yet metallically conductive: Bayreuth researchers develop novel material with high-tech prospects

An international research group led by scientists from the University of Bayreuth has produced a previously unknown material: Rhenium nitride pernitride. Thanks to combining properties that were previously considered incompatible, it looks set to become highly attractive for technological applications. Indeed, it is a super-hard metallic conductor that can withstand extremely high pressures like a diamond. A process now developed in Bayreuth opens up the possibility of producing rhenium nitride pernitride and other technologically interesting materials in sufficiently large quantity for their properties characterisation. The new findings are presented in "Nature Communications".

The possibility of finding a compound that was metallically conductive, super-hard, and ultra-incompressible was long considered unlikely in science. It was...

Im Focus: Modelling leads to the optimum size for platinum fuel cell catalysts: Activity of fuel cell catalysts doubled

An interdisciplinary research team at the Technical University of Munich (TUM) has built platinum nanoparticles for catalysis in fuel cells: The new size-optimized catalysts are twice as good as the best process commercially available today.

Fuel cells may well replace batteries as the power source for electric cars. They consume hydrogen, a gas which could be produced for example using surplus...

Im Focus: The secret of mushroom colors

Mushrooms: Darker fruiting bodies in cold climates

The fly agaric with its red hat is perhaps the most evocative of the diverse and variously colored mushroom species. Hitherto, the purpose of these colors was...

Im Focus: First results of the new Alphatrap experiment

Physicists at the Max Planck Institute for Nuclear Physics in Heidelberg report the first result of the new Alphatrap experiment. They measured the bound-electron g-factor of highly charged (boron-like) argon ions with unprecedented precision of 9 digits. In comparison with a new highly accurate quantum electrodynamic calculation they found an excellent agreement on a level of 7 digits. This paves the way for sensitive tests of QED in strong fields like precision measurements of the fine structure constant α as well as the detection of possible signatures of new physics. [Physical Review Letters, 27 June 2019]

Quantum electrodynamics (QED) describes the interaction of charged particles with electromagnetic fields and is the most precisely tested physical theory. It...

All Focus news of the innovation-report >>>



Industry & Economy
Event News

2nd International Conference on UV LED Technologies & Applications – ICULTA 2020 | Call for Abstracts

24.06.2019 | Event News

SEMANTiCS 2019 brings together industry leaders and data scientists in Karlsruhe

29.04.2019 | Event News

Revered mathematicians and computer scientists converge with 200 young researchers in Heidelberg!

17.04.2019 | Event News

Latest News

Hubble discovers mysterious black hole disc

12.07.2019 | Physics and Astronomy

Super salty, subzero Arctic water provides peek at possible life on other planets

12.07.2019 | Life Sciences

UC San Diego cancer scientists identify new drug target for multiple tumor types

12.07.2019 | Health and Medicine

Science & Research
Overview of more VideoLinks >>>