Carbon exists in various forms. In addition to diamond and graphite, there are recently discovered forms with astonishing properties. For example graphene, with a thickness of just one atomic layer, is the thinnest known material, and its unusual properties make it an extremely exciting candidate for applications like future electronics and high-tech engineering. In graphene, each carbon atom is linked to three neighbours, forming hexagons arranged in a honeycomb network. Theoretical studies have shown that carbon atoms can also arrange in other flat network patterns, while still binding to three neighbours, but none of these predicted networks had been realized until now.
Researchers at the University of Marburg in Germany and Aalto University in Finland have now discovered a new carbon network, which is atomically thin like graphene, but is made up of squares, hexagons, and octagons forming an ordered lattice. They confirmed the unique structure of the network using high-resolution scanning probe microscopy and interestingly found that its electronic properties are very different from those of graphene.
In contrast to graphene and other forms of carbon, the new Biphenylene network — as the new material is named –has metallic properties. Narrow stripes of the network, only 21 atoms wide, already behave like a metal, while graphene is a semiconductor at this size. “These stripes could be used as conducting wires in future carbon-based electronic devices.” said professor Michael Gottfried, at University of Marburg, who leads the team who developed the idea. The lead author of the study, Qitang Fan from Marburg continues, “This novel carbon network may also serve as a superior anode material in lithium-ion batteries, with a larger lithium storage capacity compared to that of the current graphene-based materials.”
The team at Aalto University helped image the material and decipher its properties. The group of Professor Peter Liljeroth carried out the high-resolution microscopy that showed the structure of the material, while researchers led by Professor Adam Foster used computer simulations and analysis to understand the exciting electrical properties of the material.
The new material is made by assembling carbon-containing molecules on an extremely smooth gold surface. These molecules first form chains, which consist of linked hexagons, and a subsequent reaction connects these chains together to form the squares and octagons. An important feature of the chains is that they are chiral, which means that they exist in two mirroring types, like left and right hands. Only chains of the same type aggregate on the gold surface, forming well-ordered assemblies, before they connect. This is critical for the formation of the new carbon material, because the reaction between two different types of chains leads only to graphene. “The new idea is to use molecular precursors that are tweaked to yield biphenylene instead of graphene” explains Linghao Yan, who carried out the high-resolution microscopy experiments at Aalto University.
For now, the teams work to produce larger sheets of the material, so that its application potential can be further explored. However, “We are confident that this new synthesis method will lead to the discovery of other novel carbon networks.” said Professor Liljeroth.
Breakthrough by uOttawa researchers sees creation of light-emitting solid carbon from CO2 gas
A team of researchers at the University of Ottawa has found a way to use visible light to transform carbon dioxide gas, or CO2, into solid carbon forms that emit light. This development creates a new, low-energy CO2 reduction pathway to solid carbon that will have implications across many fields.
We talked to lead author Dr. Jaspreet Walia, Post-Doctoral Fellow in the School of Electrical Engineering and Computer Science at the University of Ottawa, and research lead Dr. Pierre Berini, uOttawa Distinguished Professor and University Research Chair in Surface Plasmon Photonics, to learn more.
Please tell us about your team’s discovery.
Pierre Berini: “We have reduced carbon dioxide, a greenhouse gas, to solid carbon on a nanostructured silver surface illuminated with green light, without the need for any other reagents. Energetic electrons excited on the silver surface by green light transfer to carbon dioxide molecules, initiating dissociation. The carbon deposits were also found to emit intense yellow light in a process known as photoluminescence.”
How did you come to these conclusions?
Jaspreet Walia: “We used a technique known as Raman Scattering to probe the reaction in real time to determine which products, if any, were forming. To our surprise, we consistently observed signatures of carbon forming on the surface, as well as bright and visible yellow light emanating from the sample.”
Why is it important?
Pierre Berini: “Recently, there has been considerable global research effort devoted to developing technologies that can transform CO2 using visible light. Our work not only demonstrates that this is possible, but also that light emitting solid carbon can be formed.”
What are the applications of this discovery in our lives?
Jaspreet Walia: “This fixed pathway for reagent-less CO2 reduction to light emitting solid carbon, driven by visible light, will be of interest to researchers involved in the development of solar driven chemical transformations, industrial scale catalytic processes, and light-emitting metasurfaces.”
“More specifically, with respect to the creation of carbon directly from CO2 gas, our findings will have an impact on research involving plasmon assisted reactions and I would expect the emergence of applications in the oil and gas industries, where catalytic transformations involving carbon-based compounds is a key focus area.”
“Next-generation reactions involving CO2 and light could also lead to other useful outcomes, such as the potential for artificial photosynthesis. Our findings could be used for light control and manipulation at the nanoscale, or to possibility realize flat light sources due to the light-emitting aspect of our discovery. The nanostructured carbon itself could also be used in catalysis.”
“Finally, the wavelength (color) of the light emitted from carbon dots on a silver surface could be very sensitive to the local environment, making it an attractive sensing platform for pollutants, for example.”
Is there anything you would like to add?
Pierre Berini: “We have learned how to form solid carbon deposits that emit light “out of thick air”, in a breakthrough enabled by light-assisted transformation of CO2 gas driven by energetic electrons. The project was entirely driven by curiosity, with no set expectations on outcomes, and benefitted from close collaboration with graduate students Sabaa Rashid and Graham Killaire, as well as Professors Fabio Variola and Arnaud Weck.”
At ultracold temperature of 4 kelvins, the carbon increased efficiency by more than 30%.
Cryocoolers are ultracold refrigeration units used in surgery and drug development, semiconductor fabrication, and spacecraft. They can be tubes, pumps, tabletop sizes, or larger refrigerator systems.
The regenerative heat exchanger, or regenerator, is a core component of cryocoolers. At temperatures below 10 kelvins (-441.67 degrees Fahrenheit), performance drops precipitously, with maximum regenerator loss of more than 50%.
In their paper, published in Applied Physics Letters, by AIP Publishing, researchers at the University of Chinese Academy of Sciences used superactivated carbon particles as an alternative regenerator material to increase cooling capability at temperatures as low as 4 kelvins.
In most cryocoolers, a compressor drives room temperature gas through the regenerator. The regenerator soaks up heat from the compression, and the cooled gas expands. The oscillating ultracold gas absorbs the heat trapped in the regenerator, and the process repeats.
Nitrogen is the most commonly used gas in cryocoolers. But for applications requiring temperatures below 10 kelvins, such as space telescope instruments and magnetic resonance imaging systems, helium is used, because it has the lowest boiling point of any gas, enabling the coldest attainable temperatures.
However, helium’s high specific heat (the amount of heat transfer needed to change the temperature of a substance) results in large temperature fluctuations during the compression and expansion cycle at low temperatures, which seriously affects cooling efficiency.
To address this problem, researchers replaced the regenerator’s conventional rare-earth metals with activated carbon, which is carbon treated with carbon dioxide or superheated steam at high temperatures. This creates a matrix of micron-size pores that increases the carbon’s surface area, enabling the regenerator to hold more helium at low temperatures and remove more heat.
The researchers used a 4 kelvins Gifford-McMahon cryocooler to test the helium adsorption capacity in superactivated carbon particles with a porosity of 0.65 within varying temperature ranges of 3-10 kelvins.
They found when they filled the regenerator with 5.6% of carbon with diameters between 50 and 100 microns, the obtained no-load temperature of 3.6 kelvins was the same as using precious metals. However, at 4 kelvins, cooling capacity increased by more than 30%.
They confirmed improved performance by placing coconut shell-activated carbon into an experimental pulse tube they built and using a thermodynamic calculation model.
“In addition to providing increased cooling capacity, the activated carbon can serve as a low-cost alternative to precious metals and could also benefit low-temperature detectors that are sensitive to magnetism,” author Liubiao Chen said.
The article “Study on the use of porous materials with adsorbed helium as the regenerator of cryocooler at temperatures below 10 K” is authored by Xiaotong Xi, Biao Yang, Yuanheng Zhao, Liubiao Chen, and Junjie Wang. The article will appear in Applied Physics Letters on April 6, 2021 (DOI: 10.1063/5.0044221). After that date, it can be accessed at https://aip.scitation.org/doi/10.1063/5.0044221.
We are made of stardust, the saying goes, and a pair of studies including University of Michigan research finds that may be more true than we previously thought.
The first study, led by U-M researcher Jie (Jackie) Li and published in Science Advances, finds that most of the carbon on Earth was likely delivered from the interstellar medium, the material that exists in space between stars in a galaxy. This likely happened well after the protoplanetary disk, the cloud of dust and gas that circled our young sun and contained the building blocks of the planets, formed and warmed up.
Carbon was also likely sequestered into solids within one million years of the sun’s birth—which means that carbon, the backbone of life on earth, survived an interstellar journey to our planet.
Previously, researchers thought carbon in the Earth came from molecules that were initially present in nebular gas, which then accreted into a rocky planet when the gases were cool enough for the molecules to precipitate. Li and her team, which includes U-M astronomer Edwin Bergin, Geoffrey Blake of Caltech, Fred Ciesla of the University of Chicago and Marc Hirschmann of the University of Minnesota, point out in this study that the gas molecules that carry carbon wouldn’t be available to build the Earth because once carbon vaporizes, it does not condense back into a solid.
“The condensation model has been widely used for decades. It assumes that during the formation of the sun, all of the planet’s elements got vaporized, and as the disk cooled, some of these gases condensed and supplied chemical ingredients to solid bodies. But that doesn’t work for carbon,” said Li, a professor in the U-M Department of Earth and Environmental Sciences.
Much of carbon was delivered to the disk in the form of organic molecules. However, when carbon is vaporized, it produces much more volatile species that require very low temperatures to form solids. More importantly, carbon does not condense back again into an organic form. Because of this, Li and her team inferred most of Earth’s carbon was likely inherited directly from the interstellar medium, avoiding vaporization entirely.
To better understand how Earth acquired its carbon, Li estimated the maximum amount of carbon Earth could contain. To do this, she compared how quickly a seismic wave travels through the core to the known sound velocities of the core. This told the researchers that carbon likely makes up less than half a percent of Earth’s mass. Understanding the upper bounds of how much carbon the Earth might contain tells the researchers information about when the carbon might have been delivered here.
“We asked a different question: We asked how much carbon could you stuff in the Earth’s core and still be consistent with all the constraints,” Bergin said, professor and chair of the U-M Department of Astronomy. “There’s uncertainty here. Let’s embrace the uncertainty to ask what are the true upper bounds for how much carbon is very deep in the Earth, and that will tell us the true landscape we’re within.”
A planet’s carbon must exist in the right proportion to support life as we know it. Too much carbon, and the Earth’s atmosphere would be like Venus, trapping heat from the sun and maintaining a temperature of about 880 degrees Fahrenheit. Too little carbon, and Earth would resemble Mars: an inhospitable place unable to support water-based life, with temperatures around minus 60.
In a second study by the same group of authors, but led by Hirschmann of the University of Minnesota, the researchers looked at how carbon is processed when the small precursors of planets, known as planetesimals, retain carbon during their early formation. By examining the metallic cores of these bodies, now preserved as iron meteorites, they found that during this key step of planetary origin, much of the carbon must be lost as the planetesimals melt, form cores and lose gas. This upends previous thinking, Hirschmann says.
“Most models have the carbon and other life-essential materials such as water and nitrogen going from the nebula into primitive rocky bodies, and these are then delivered to growing planets such as Earth or Mars,” said Hirschmann, professor of earth and environmental sciences. “But this skips a key step, in which the planetesimals lose much of their carbon before they accrete to the planets.”
Hirschmann’s study was recently published in Proceedings of the National Academy of Sciences.
“The planet needs carbon to regulate its climate and allow life to exist, but it’s a very delicate thing,” Bergin said. “You don’t want to have too little, but you don’t want to have too much.”
Bergin says the two studies both describe two different aspects of carbon loss—and suggest that carbon loss appears to be a central aspect in constructing the Earth as a habitable planet.
“Answering whether or not Earth-like planets exist elsewhere can only be achieved by working at the intersection of disciplines like astronomy and geochemistry,” said Ciesla, a U. of C. professor of geophysical sciences. “While approaches and the specific questions that researchers work to answer differ across the fields, building a coherent story requires identifying topics of mutual interest and finding ways to bridge the intellectual gaps between them. Doing so is challenging, but the effort is both stimulating and rewarding.”
Blake, a co-author on both studies and a Caltech professor of cosmochemistry and planetary science, and of chemistry, says this kind of interdisciplinary work is critical.
“Over the history of our galaxy alone, rocky planets like the Earth or a bit larger have been assembled hundreds of millions of times around stars like the Sun,” he said. “Can we extend this work to examine carbon loss in planetary systems more broadly? Such research will take a diverse community of scholars.”
Funding sources for this collaborative research include the National Science Foundation, NASA’s Exoplanets Research Program, NASA’s Emerging Worlds Program and the NASA Astrobiology Program.
J. Li, E. A. Bergin, G. A. Blake, F. J. Ciesla, M. M. Hirschmann. Earth’s carbon deficit caused by early loss through irreversible sublimation. Science Advances, 2021; 7 (14): eabd3632 DOI: 10.1126/sciadv.abd3632
Marc M. Hirschmann, Edwin A. Bergin, Geoff A. Blake, Fred J. Ciesla, Jie Li. Early volatile depletion on planetesimals inferred from C–S systematics of iron meteorite parent bodies. Proceedings of the National Academy of Sciences, 2021; 118 (13): e2026779118 DOI: 10.1073/pnas.2026779118
Carbon is one of the most ubiquitous elements in existence. As the fourth most abundant element in the universe, a building block for all known life and a material that sits in the interior of carbon-rich exoplanets, the element has been subject to intense investigation by scientists.
Decades of studies have shown that carbon’s crystal structure has a significant impact on material properties. In addition to graphite and diamond, the most common carbon structures found at ambient pressures, scientists have predicted several new structures of carbon that could be found above 1,000 gigapascals (GPa). These pressures, approximately 2.5 times the pressure in Earth’s core, are relevant for modeling exoplanet interiors but have historically been impossible to achieve in the laboratory.
That is, until now. Under the Discovery Science program, which allows academic scientists access to Lawrence Livermore National Laboratory’s (LLNL) flagship National Ignition Facility (NIF), an international team of researchers led by LLNL and the University of Oxford has successfully measured carbon at pressures reaching 2,000 GPa (five times the pressure in Earth’s core), nearly doubling the maximum pressure at which a crystal structure has ever been directly probed. The results were reported today in Nature.
“We discovered that, surprisingly, under these conditions carbon does not transform to any of the predicted phases but retains the diamond structure up to the highest pressure,” said Amy Jenei, LLNL physicist and lead author on the study. “The same ultra-strong interatomic bonds (requiring high energies to break), which are responsible for the metastable diamond structure of carbon persisting indefinitely at ambient pressure, are also likely impeding its transformation above 1,000 GPa in our experiments.”
The academic component of the collaboration was led by Professor Justin Wark from the University of Oxford, who praised the Lab’s open access policy. “The NIF Discovery Science program is immensely beneficial to the academic community — it not only allows established faculty the chance to put forward proposals for experiments that would be impossible to do elsewhere, but importantly also gives graduate students, who are the senior scientists of the future, the chance to work on a completely unique facility,” he said.
The team — which also included scientists from the University of Rochester’s Laboratory for Laser Energetics and the University of York — leveraged the unique high power and energy and accurate laser pulse-shaping of LLNL’s National Ignition Facility to compress solid carbon to 2,000 GPa using ramp-shaped laser pulses, simultaneously measuring the crystal structure using an X-ray diffraction platform to capture a nanosecond-duration snapshot of the atomic lattice. These experiments nearly double the record high pressure at which X-ray diffraction has been recorded on any material.
The researchers found that even when subjected to these intense conditions, solid carbon retains its diamond structure far beyond its regime of predicted stability, confirming predictions that the strength of the molecular bonds in diamond persists under enormous pressure, resulting in large energy barriers that hinder conversion to other carbon structures.
“Whether nature has found a way to surmount the high energy barrier to formation of the predicted phases in the interiors of exoplanets is still an open question,” Jenei said. “Further measurements using an alternate compression pathway or starting from an allotrope of carbon with an atomic structure that requires less energy to rearrange will provide further insight.”
Co-authors include David Braun, Damian Swift, Martin Gorman, Ray Smith, Dayne Fratanduono, Federica Coppari, Christopher Wehrenberg, Rick Kraus, David Erskine, Joel Bernier, James McNaney, Robert Rudd and Jon Eggert of LLNL; David McGonegle, Patrick Heighway, Matthew Suggit and Justin Wark of the University of Oxford; Ryan Rygg and Gilbert Collins of the University of Rochester’s Laboratory for Laser Energetics; and Andrew Higginbotham of the University of York.
Featured image: An artist’s rendering of 55 Cancri e, a carbon-rich exoplanet. For the first time in a laboratory setting, experiments conducted at the National Ignition Facility through the Discovery Science program reach the extreme pressures relevant to understanding the structure of carbon that sits in the interior of exoplanets. Credit: ESA/Hubble/M. Kornmesser.
NIF, the world’s highest-energy laser system, is designed to create the extreme conditions — temperatures exceeding 100 million degrees and pressures more than 100 billion times that of the Earth’s atmosphere — similar to those in stars and in detonating nuclear weapons. As the only facility that can create the conditions that are relevant to understanding the operation of modern nuclear weapons, NIF is a crucial element of the National Nuclear Security Administration’s science-based Stockpile Stewardship Program. In addition to helping ensure the reliability of the nuclear deterrent, NIF opens new frontiers in laboratory astrophysics, materials science, hydrodynamics and many other scientific disciplines.
Carbon (C), nitrogen (N), and phosphorus (P) are three bioelements with maximal accumulations in areas of abundant life. C:N:P stoichiometry in soils greatly determines nutrient availability for plants and soil microorganisms, and further reflects the functioning of terrestrial ecosystems.
Soil C:N:P ratios are very susceptible to human activities (e.g., fertilization) and climate factors (e.g., temperature and precipitation). However, how the soil C:N:P stoichiometry is affected by upland and paddy cropping over broad geographical scale remains largely unknown.
A research group led by Prof. SU Yirong from the Institute of Subtropical Agriculture (ISA) of the Chinese Academy of Sciences conducted a study to examine the soil C:N:P stoichiometry in woodland (as control), agricultural upland and paddy from four climate zones (tropics, subtropics, warm temperate, and mid-temperate) across eastern China. The study was published in Soil and Tillage Research on Dec. 30.
The researchers collected 720 surface soil samples from 240 sites with adjacent woodland, agricultural upland, and paddy at a depth of 0-15 cm. Total C, N, and P contents and their ratios were determined.
They found that among climate zones, C and N contents and C:N ratios decreased in the order of mid-temperate > tropics > subtropics > warm temperate, whereas C:P and N:P ratios followed the order of subtropics > mid-temperate and tropics > warm-temperate.
“Compared to woodland, upland agriculture decreased the C content, but increased P content, resulting in the decreases of C:N, C:P, and N:P ratios. Hence, uplands are relatively limited by C and N but enriched with P, particularly in warm temperate zone,” said Prof. SU.
By contrast, the C, N, and P contents in paddy soils were all increased compared to woodland soils, but larger N and P increase leads to the decreases in C:N and C:P ratios. The higher P content, and consequently lower C:N:P ratios in both agricultural soils are the consequences of intensive fertilization.
As a whole, the direction of soil C, N, and P contents and their stoichiometric ratios in response to agricultural use was similar in the four climate zones: P increased, but C:N:P ratios decreased. The effects of agricultural use on C:N:P stoichiometry were greater in warmer and wetter zones.
This study provides a comparable dataset on the alteration of soil C, N, and P balances in the main Chinese grain-producing areas subjected to long-term intensive cultivation, which is useful to optimize future agricultural management.
The breakthrough shows that Superman may have had a similar trick up his sleeve when he crushed coal into diamond, without using his heat ray.
An international team of scientists has defied nature to make diamonds in minutes in a laboratory at room temperature – a process that normally requires billions of years, huge amounts of pressure and super-hot temperatures.
The team, led by The Australian National University (ANU) and RMIT University, made two types of diamonds: the kind found on an engagement ring and another type of diamond called Lonsdaleite, which is found in nature at the site of meteorite impacts such as Canyon Diablo in the US.
One of the lead researchers, ANU Professor Jodie Bradby, said their breakthrough shows that Superman may have had a similar trick up his sleeve when he crushed coal into diamond, without using his heat ray.
“Natural diamonds are usually formed over billions of years, about 150 kilometres deep in the Earth where there are high pressures and temperatures above 1,000 degrees Celsius,” said Professor Bradby from the ANU Research School of Physics.
The team, including former ANU PhD scholar Tom Shiell now at Carnegie Institution for Science, previously created Lonsdaleite in the lab only at high temperatures.
This new unexpected discovery shows both Lonsdaleite and regular diamond can also form at normal room temperatures by just applying high pressures – equivalent to 640 African elephants on the tip of a ballet shoe.
“The twist in the story is how we apply the pressure. As well as very high pressures, we allow the carbon to also experience something called ‘shear’ – which is like a twisting or sliding force. We think this allows the carbon atoms to move into place and form Lonsdaleite and regular diamond,” Professor Bradby said.
Co-lead researcher Professor Dougal McCulloch and his team at RMIT used advanced electron microscopy techniques to capture solid and intact slices from the experimental samples to create snapshots of how the two types of diamonds formed.
“Our pictures showed that the regular diamonds only form in the middle of these Lonsdaleite veins under this new method developed by our cross-institutional team,” Professor McCulloch said.
“Seeing these little ‘rivers’ of Lonsdaleite and regular diamond for the first time was just amazing and really helps us understand how they might form.”
Lonsdaleite, named after the crystallographer Dame Kathleen Lonsdale, the first woman elected as a Fellow to the Royal Society, has a different crystal structure to regular diamond. It is predicted to be 58 per cent harder.
“Lonsdaleite has the potential to be used for cutting through ultra-solid materials on mining sites,” Professor Bradby said.
“Creating more of this rare but super useful diamond is the long-term aim of this work.”
Ms Xingshuo Huang is an ANU PhD scholar working in Professor Bradby’s lab.
“Being able to make two types of diamonds at room temperature was exciting to achieve for the first time in our lab,” Ms Huang said.
The team, which involved University of Sydney and Oak Ridge National Laboratory in the US, have published the research findings in the journal Small.
References : McCulloch, D. G., Wong, S., Shiell, T. B., Haberl, B., Cook, B. A., Huang, X., Boehler, R., McKenzie, D. R., Bradby, J. E., Investigation of Room Temperature Formation of the Ultra‐Hard Nanocarbons Diamond and Lonsdaleite. Small 2020, 2004695. https://doi.org/10.1002/smll.202004695
The capacity of the Amazon forest to store carbon in a changing climate will ultimately be determined by how fast trees die – and what kills them. Now, a huge new study has unravelled what factors control tree mortality rates in Amazon forests and helps to explain why tree mortality is increasing across the Amazon basin.
This large analysis found that the mean growth rate of the tree species is the main risk factor behind Amazon tree death, with faster-growing trees dying off at a younger age. These findings have important consequences for our understanding of the future of these forests. Climate change tends to select fast-growing species. If the forests selected by climate change are more likely die younger, they will also store less carbon.
The study, co-led by the Universities of Birmingham and Leeds in collaboration with more than 100 scientists, is the first large scale analysis of the causes of tree death in the Amazon and uses long-term records gathered by the international RAINFOR network.
“Understanding the main drivers of tree death allows us to better predict and plan for future trends – but this is a huge undertaking as there are more than 15,000 different tree species in the Amazon,” said lead author Dr Adriane Esquivel-Muelbert, of the Birmingham Institute of Forest Research.
Dr David Galbraith, from the University of Leeds added “We found a strong tendency for faster-growing species to die more, meaning they have shorter life spans. While climate change has provided favourable conditions for these species, because they also die more quickly the carbon sequestration service provided by Amazon trees is declining.”
Tree mortality is a rare event so to truly understand it requires huge amounts of data. The RAINFOR network has assembled more than 30 years of contributions from more than 100 scientists. It includes records from 189 one-hectare plots, each visited and monitored on average every 3 years. Each visit, researchers measure all trees above 10cm in diameter as well as the condition of every tree.
In total more than 124,000 living trees were followed, and 18,000 tree deaths recorded and analysed. When trees die, the researcher follows a fixed protocol to unravel the actual cause of death. “This involves detailed, forensic work and amounts to a massive ‘CSI Amazon’ effort conducted by skilled investigators from a dozen nations”, noted Prof. Oliver Phillips, from the University of Leeds.
Dr Beatriz Marimon, from UNEMAT, who coordinates multiple plots in central Brazil added: “Now that we can see more clearly what is going on across the whole forest, there are clear opportunities for action. We find that drought is also driving tree death, but so far only in the South of the Amazon. What is happening here should serve as an early warning system as we need to prevent the same fate overtaking trees elsewhere.”
White dwarfs are the most common fossil stars within the stellar graveyard. It is well known that more than 95% of all main-sequence stars will finish their lives as white dwarfs, earth-sized objects less massive than ~1.4 Mo—the Chandrasekhar limiting mass— supported by electron degeneracy. A remarkable property of the white-dwarf population is its mass distribution, which exhibits a main peak at ~0.6 Mo, a smaller peak at the tail of the distribution around ~0.82 Mo, and a low-mass excess near ~0.4 Mo. White dwarfs with masses lower than 1.05 Mo are expected to harbour carbon(C)-oxygen(O) cores, enveloped by a shell of helium which is surrounded by a layer of hydrogen. Traditionally, white dwarfs more massive than 1.05 Mo (ultra-massive white dwarfs) are thought to contain an oxygen-neon(Ne) core, and their formation is theoretically predicted as the end product of the isolated evolution of intermediate-mass stars with an initial mass larger than 6-9 Mo, depending on the metallicity and the treatment of convective boundaries during core hydrogen burning. Once the helium in the core has been exhausted, these stars evolve to the super asymptotic giant branch (SAGB) phase, where they reach temperatures high enough to start off-centre carbon ignition under partially degenerate conditions. A violent carbon ignition eventually leads to the formation of an ONe core, which is not hot enough to develop further nuclear burning. During the SAGB phase, the star loses most of its outer envelope by the action of stellar winds, ultimately becoming an ultra-massive ONe-core white dwarf star.
On the other hand, the existence of a fraction of ultra-massive white dwarfs harbouring CO cores is supported by different piece of evidence. This population could be formed through binary evolution channels, involving the merger of two white dwarfs. Assuming that C is not ignited in the merger event, the merger of two CO-core white dwarfs with a combined mass below the Chandrasekhar limit is expected to lead to the formation of a single CO-core white dwarf substantially more massive than any CO-core white dwarf that can form from a single evolution ( 1.05 M ∼1.05 Mo). To the date, it has not been possible to distinguish a CO-core from a ONe-core ultra-massive white dwarf from their observed properties, although a promissory avenue to accomplish this is by means of white dwarf asteroseismology. Recent studies reveal that 10% -30% of all white dwarfs are expected to be formed as a result of merger events of any kind, and that this percentage raises up to 50 % for massive white dwarfs (M>0.9Mo). In particular, double white dwarf mergers contribute to the formation of massive white dwarfs in 20-30%. These results are in line with the existence of an excess of massive white dwarfs in the mass distribution. However, the existence of ultra-massive CO white dwarfs remains to be proven, and their exact percentage is still unclear.
The formation of white dwarfs as a result of stellar mergers is particularly important given the persistent historical interest in the study of the channels that lead to the occurrence of type Ia Supernovae, which are thought to be the violent explosion of a white dwarf exceeding the Chandrasekhar limiting mass. The main pathways to type Ia Supernovae involve binary evolution, namely the single-degenerate channel in which a white dwarf gains mass from a non-degenerate companion, or the double-degenerate channel involving the merger of two white dwarfs. Moreover, ultra-massive white dwarfs resulting from merger episodes are of utmost importance in connection with the formation of rapidly spinning neutron stars/magnetars. Mergers of white dwarfs have also been invoked as the most likely mechanism for the formation of Fast Radio Bursts, which are transient intense radio pulses with duration of milliseconds. Since they have been localized at redshifts z > 0.3, it is thought that Fast Radio Bursts could replace Supernovae of type Ia to probe the expansion of the universe to higher redshifts.
Recent observations provided by Gaia space mission, indicate that a fraction of the ultra-massive white dwarfs experience a strong delay in their cooling, which cannot be attributed only to the occurrence of crystallization, thus requiring an unknown energy source able to prolong their life for long periods of time.
In this study, Maria Camisassa and colleagues showed that these strong delays in the cooling times reported for a selected population of the ultra-massive white dwarfs are caused by the energy released by the sedimentation process of ²²Ne occurring in the interior of CO-core ultra-massive white dwarfs with high ²²Ne content, providing strong sustain to the formation of CO-core ultra-massive white dwarfs through merger events.
In order to demonstrate the possible existence of these eternal youth ultra-massive CO-core white dwarfs, they have analyzed the effect of ²²Ne sedimentation on the local white dwarf population revealed by Gaia observations by means of an up-to date population synthesis code. The code, based on Monte Carlo techniques, incorporates the different theoretical white dwarf cooling sequences under study, as well as an accurate modeling of the local white dwarf population and observational biases. They have performed a population synthesis analysis of the Galactic thin disk white dwarf population within 100 pc from the Sun under different input models. In order to minimize the selection effects, they chose the 100 pc sample, given that it represents the maximum size that the sample can be considered volume-limited and thus practically complete sample.
First, they considered that all the ultra-massive white dwarfs in the simulated sample have ONe core composition. This first synthetic population is shown in the Gaia HR diagram in the upper left panel of Figure 1. The histogram of this synthetic population is shown in the upper right panel (black steps), together with the Gaia 100 pc white dwarf sample (red steps). The Q branch can easily be regarded as the main peak in the histogram of the Gaia 100 pc white dwarf sample, between 13.0 and 13.4. A first glance at these three histograms reveals that ultra-massive ONe white dwarfs fail to account for the pile-up in the Q branch, even though the ONe white dwarf sequences used include all the energy sources resulting from the crystallization process. A quantitative statistical reduced x²-test analysis of the synthetic population distribution in the Q branch reveals a value of 11.18 when compared to the observed distribution.
The middle left panel of Figure 1 illustrates the HR diagram of a typical synthetic white dwarf population realization, considering that 20% of the ultra-massive white dwarfs come from merger events. That is, 20% of the ultra-massive white dwarfs harbour a CO-core. In this model they also have assumed white dwarfs with high ²²Ne abundance, X²²Ne=0.06. The histogram of this synthetic population, shown in black steps in the middle right panel, reveals that, although a mixed white dwarf population with both CO-core and ONe-core white dwarfs is in better agreement with the observations, the pile-up is still not fully reproduced. The reduced x² value of this synthetic population is 2.60.
Finally, they have also performed a population synthesis realization in which 50% of the ultra-massive white dwarfs have a merger origin and their core-chemical composition is CO. As in the previous model, the ²²Ne content of the white dwarf sequences with merger origin was set to X²²Ne=0.06. The results of this synthetic population are shown in the lower panels of Figure 1. They found that this simulation is in perfect agreement with the observed white dwarf sample, being its reduced x²-test value 1.36.
The better agreement with the observations revealed by Gaia of the synthetic populations that include CO-core ultra-massive white dwarf sequences is in line with the longer cooling times that characterize these stars due to ²²Ne sedimentation process. They have also simulated a synthetic population that considers a fraction of merger of 50% and a ²²Ne abundance of 0.02, finding a better agreement when compared to simulations computed with only ONe-core white dwarfs, but not as good as the agreement they found for a population with high ²²Ne content (The reduced x² value of this synthetic population is 4.86). They have also generated synthetic populations considering different ²²Ne abundances in the white dwarf models and found that the best fit models are obtained for a high ²²Ne abundance (X²²Ne=0.06), as the one shown in the middle and lower panels of Figure 1. Such a high ²²Ne abundance is not consistent with the isolated standard evolutionary history channel, because it would imply that these white dwarfs come from high-metallicity progenitors. However, merger events provide a possible scenario to create such a high ²²Ne abundance. If H were burnt in C-rich layers during the merger event, it would create a high amount of ¹⁴N that could later capture He ions, creating a high ²²Ne abundance before the ultra-massive white dwarf is born.
The analysis of the ultra-massive white dwarf population revealed by Gaia shows that ONe-core white dwarfs alone are not able to account for the pile-up in the ultra-massive Q branch. Indeed, energy sources as latent heat and phase separation process due to crystallization, and ²²Ne sedimentation can not prevent the fast cooling of these stars. Their study finds that CO-core ultra-massive white dwarfs with high ²²Ne content are long-standing living objects, that should stay on the Q branch for long periods of time. Indeed, their CO core composition, combined with a high ²²Ne abundance, provides a favorable scenario for ²²Ne sedimentation to effectively operate, producing strong delays in the cooling times due to the combination of three effects: crystallization, ²²Ne sedimentation and higher thermal content, and leading to an eternal youth source.
Their study indicates that the observed evidence of these delays from Gaia provides valuable sustain on their CO chemical composition, and their past history involving merger events, whilst ONe core white dwarfs are unable to predict these delays. Moreover, the high percentage of observed carbon-rich atmosphere stars (DQ white dwarfs) on the Q branch 20 supports the hypothesis that a large fraction of the white dwarfs on the Q branch would certainly have been formed through merger events.
References: María E. Camisassa, Leandro G. Althaus, Santiago Torres, Alejandro H. Córsico, Sihao Cheng, Alberto Rebassa-Mansergas, “Forever young white dwarfs: when stellar ageing stops”, ArXiv, pp. 1-27, 2020. https://arxiv.org/abs/2008.03028
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