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Wednesday, July 18th, 2018
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| 10:59a |
X-ray data may be first evidence of a star devouring a planet For nearly a century, astronomers have puzzled over the curious variability of young stars residing in the Taurus-Auriga constellation some 450 light years from Earth. One star in particular has drawn astronomers’ attention. Every few decades, the star’s light has faded briefly before brightening again.
In recent years, astronomers have observed the star dimming more frequently, and for longer periods, raising the question: What is repeatedly obscuring the star? The answer, astronomers believe, could shed light on some of the chaotic processes that take place early in a star’s development.
Now physicists from MIT and elsewhere have observed the star, named RW Aur A, using NASA’s Chandra X-Ray Observatory. They’ve found evidence for what may have caused its most recent dimming event: a collision of two infant planetary bodies, which produced in its aftermath a dense cloud of gas and dust. As this planetary debris fell into the star, it generated a thick veil, temporarily obscuring the star’s light.
“Computer simulations have long predicted that planets can fall into a young star, but we have never before observed that,” says Hans Moritz Guenther, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research, who led the study. “If our interpretation of the data is correct, this would be the first time that we directly observe a young star devouring a planet or planets.”
The star’s previous dimming events may have been caused by similar smash-ups, of either two planetary bodies or large remnants of past collisions that met head-on and broke apart again.
“It’s speculation, but if you have one collision of two pieces, it’s likely that afterward they may be on some rogue orbits, which increases the probability that they will hit something else again,” Guenther says.
Guenther is the lead author of a paper detailing the group’s results, which appears today in the Astronomical Journal. His co-authors from MIT include David Huenemoerder and David Principe, along with researchers from the Harvard-Smithsonian Center for Astrophysics and collaborators in Germany and Belgium.
A star cover-up
Scientists who study the early development of stars often look to the Taurus-Auriga Dark Clouds, a gathering of molecular clouds in the constellations of Taurus and Auriga, which host stellar nurseries containing thousands of infant stars. Young stars form from the gravitational collapse of gas and dust within these clouds. Very young stars, unlike our comparatively mature sun, are still surrounded by a rotating disk of debris, including gas, dust, and clumps of material ranging in size from small dust grains to pebbles, and possibly to fledgling planets.
“If you look at our solar system, we have planets and not a massive disk around the sun,” Guenther says. “These disks last for maybe 5 million to 10 million years, and in Taurus, there are many stars that have already lost their disk, but a few still have them. If you want to know what happens in the end stages of this disk dispersal, Taurus is one of the places to look.”
Guenther and his colleagues focus on stars that are young enough to still host disks. He was particularly interested in RW Aur A, which is at the older end of the age range for young stars, as it is estimated to be several million years old. RW Aur A is part of a binary system, meaning that it circles another young star, RW Aur B. Both these stars are about the same mass as the sun.
Since 1937, astronomers have recorded noticeable dips in the brightness of RW Aur A every few decades. Each dimming event appeared to last for about a month. In 2011, the star dimmed again, this time for about half a year. The star eventually brightened, only to fade again in mid-2014. In November 2016, the star returned to its full luminosity.
Astronomers have proposed that this dimming is caused by a passing stream of gas at the outer edge of the star’s disk. Still others have theorized that the dimming is due to processes occurring closer to the star’s center.
“We wanted to study the material that covers the star up, which is presumably related to the disk in some way,” Guenther says. “It’s a rare opportunity.”
An iron-clad signature
In January 2017, RW Aur A dimmed again, and the team used NASA’s Chandra X-Ray Observatory to record X-ray emission from the star.
“The X-rays come from the star, and the spectrum of the X-rays changes as the rays move through the gas in the disk,” Guenther says. “We’re looking for certain signatures in the X-rays that the gas leaves in the X-ray spectrum.”
In total, Chandra recorded 50 kiloseconds, or almost 14 hours of X-ray data from the star. After analyzing these data, the researchers came away with several surprising revelations: the star’s disk hosts a large amount of material; the star is much hotter than expected; and the disk contains much more iron than expected — not as much iron as is found in the Earth, but more than, say, a typical moon in our solar system. (Our own moon, however, has far more iron than the scientists estimated in the star’s disk.)
This last point was the most intriguing for the team. Typically, an X-ray spectrum of a star can show various elements, such as oxygen, iron, silicon, and magnesium, and the amount of each element present depends on the temperature within a star’s disk.
“Here, we see a lot more iron, at least a factor of 10 times more than before, which is very unusual, because typically stars that are active and hot have less iron than others, whereas this one has more,” Guenther says. “Where does all this iron come from?”
The researchers speculate that this excess iron may have come from one of two possible sources. The first is a phenomenon known as a dust pressure trap, in which small grains or particles such as iron can become trapped in “dead zones” of a disk. If the disk’s structure changes suddenly, such as when the star’s partner star passes close by, the resulting tidal forces can release the trapped particles, creating an excess of iron that can fall into the star.
The second theory is for Guenther the more compelling one. In this scenario, excess iron is created when two planetesimals, or infant planetary bodies, collide, releasing a thick cloud of particles. If one or both planets are made partly of iron, their smash-up could release a large amount of iron into the star’s disk and temporarily obscure its light as the material falls into the star.
“There are many processes that happen in young stars, but these two scenarios could possibly make something that looks like what we observed,” Guenther says.
He hopes to make more observations of the star in the future, to see whether the amount of iron surrounding the star has changed — a measure that could help researchers determine the size of the iron’s source. For instance, if the same amount of iron appears in, say, a year, that may signal that the iron comes from a relatively massive source, such as a large planetary collision, versus if there is very little iron left in the disk.
“Much effort currently goes into learning about exoplanets and how they form, so it is obviously very important to see how young planets could be destroyed in interactions with their host stars and other young planets, and what factors determine if they survive,” Guenther says. | | 12:59p |
Light-controlled polymers can switch between sturdy and soft MIT researchers have designed a polymer material that can change its structure in response to light, converting from a rigid substance to a softer one that can heal itself when damaged.
“You can switch the material states back and forth, and in each of those states, the material acts as though it were a completely different material, even though it’s made of all the same components,” says Jeremiah Johnson, an associate professor of chemistry at MIT, a member of MIT’s Koch Institute for Integrative Cancer Research and the Program in Polymers and Soft Matter, and the leader of the research team.
The material consists of polymers attached to a light-sensitive molecule that can be used to alter the bonds formed within the material. Such materials could be used to coat objects such as cars or satellites, giving them the ability to heal after being damaged, though such applications are still far in the future, Johnson says.
The lead author of the paper, which appears in the July 18 issue of Nature, is MIT graduate student Yuwei Gu. Other authors are MIT graduate student Eric Alt, MIT assistant professor of chemistry Adam Willard, and Heng Wang and Xiaopeng Li of the University of South Florida.
Controlled structure
Many of the properties of polymers, such as their stiffness and their ability to expand, are controlled by their topology — how the components of the material are arranged. Usually, once a material is formed, its topology cannot be changed reversibly. For example, a rubber ball remains elastic and cannot be made brittle without changing its chemical composition.
In this paper, the researchers wanted to create a material that could reversibly switch between two different topological states, which has not been done before.
Johnson and his colleagues realized that a type of material they designed a few years ago, known as polymer metal-organic cages, or polyMOCs, was a promising candidate for this approach. PolyMOCs consist of metal-containing, cage-like structures joined together by flexible polymer linkers. The researchers created these materials by mixing polymers attached to groups called ligands, which can bind to a metal atom.
Each metal atom — in this case, palladium — can form bonds with four ligand molecules, creating rigid cage-like clusters with varying ratios of palladium to ligand molecules. Those ratios determine the size of the cages.
In the new study, the researchers set out to design a material that could reversibly switch between two different-sized cages: one with 24 atoms of palladium and 48 ligands, and one with three palladium atoms and six ligand molecules.
To achieve that, they incorporated a light-sensitive molecule called DTE into the ligand. The size of the cages is determined by the angle of bonds that a nitrogen molecule on the ligand forms with palladium. When DTE is exposed to ultraviolet light, it forms a ring in the ligand, which increases the size of the angle at which nitrogen can bond to palladium. This makes the clusters break apart and form larger clusters.
When the researchers shine green light on the material, the ring is broken, the bond angle becomes smaller, and the smaller clusters re-form. The process takes about five hours to complete, and the researchers found they could perform the reversal up to seven times; with each reversal, a small percentage of the polymers fails to switch back, which eventually causes the material to fall apart.
When the material is in the small-cluster state, it becomes up to 10 times softer and more dynamic. “They can flow when heated up, which means you could cut them and upon mild heating that damage will heal,” Johnson says.
This approach overcomes the tradeoff that usually occurs with self-healing materials, which is that structurally they tend to be relatively weak. In this case, the material can switch between the softer, self-healing state and a more rigid state.
“Reversibly switching topology of polymer networks has never been reported before and represents a significant advancement in the field,” says Sergei Sheiko, a professor of chemistry at the University of North Carolina, who was not involved in the research. “Without changing network composition, photoswitchable ligands enable remotely activated transition between two topological states possessing distinct static and dynamic properties.”
Self-healing materials
In this paper, the researchers used the polymer polyethylene glycol (PEG) to make their material, but they say this approach could be used with any kind of polymer. Potential applications include self-healing materials, although for this approach to be widely used, palladium, a rare and expensive metal, would likely have to be replaced by a cheaper alternative.
“Anything made from plastic or rubber, if it could be healed when it was damaged, then it wouldn’t have to be thrown away. Maybe this approach would provide materials with longer life cycles,” Johnson says.
Another possible application for these materials is drug delivery. Johnson believes it could be possible to encapsulate drugs inside the larger cages, then expose them to green light to make them open up and release their contents. Applying green light could enable recapture of the drugs, providing a novel approach to reversible drug delivery.
The researchers are also working on creating materials that can reversibly switch from a solid state to a liquid state, and on using light to create patterns of soft and rigid sections within the same material.
The research was funded by the National Science Foundation. | | 5:05p |
As brain extracts meaning from vision, study tracks progression of processing
Here’s the neuroscience of a neglected banana (and a lot of other things in daily life): Whenever you look at its color — green in the store, then yellow, and eventually brown on your countertop — your mind categorizes it as unripe, ripe, and then spoiled. A new study that tracked how the brain turns simple sensory inputs, such as “green,” into meaningful categories, such as “unripe,” shows that the information follows a progression through many regions of the cortex, and not exactly in the way many neuroscientists would predict.
The study, led by researchers at MIT’s Picower Institute for Learning and Memory, undermines the classic belief that separate cortical regions play distinct roles. Instead, as animals in the lab refined what they saw down to a specific understanding relevant to behavior, brain cells in each of six cortical regions operated along a continuum between sensory processing and categorization. To be sure, general patterns were evident for each region, but activity associated with categorization was shared surprisingly widely, say the authors of the study published in the Proceedings of the National Academy of Science.
“The cortex is not modular,” says Earl Miller, Picower Professor of Neuroscience in the Department of Brain and Cognitive Sciences at MIT. “Different parts of the cortex emphasize different things and do different types of processing, but it is more of a matter of emphasis. It’s a blend and a transition from one to the other. This extends up to higher cognition.”
The study not only refines neuroscientists’ understanding of a core capability of cognition, it also could inform psychiatrist’s understanding of disorders in which categorization judgements are atypical, such as schizophrenia and autism spectrum disorders, the authors said.
Scott Brincat, a research scientist in Miller’s Picower lab, and Markus Siegel, principal investigator at the University of Tübingen in Germany, are the study’s co-lead authors. Tübingen postdoc Constantin von Nicolai is a co-author.
From seeing to judging
In the research, animals played a simple game. They were presented with shapes that cued them to judge what came next — either a red or green color, or dots moving in an upward or downward direction. Based on the initial shape cue, the animals learned to glance left to indicate green or upward motion, or right to indicate red or downward.
Meanwhile the researchers were eavesdropping on the activity of hundreds of neurons in six regions across the cortex: prefrontal (PFC), posterior inferotemporal (PIT), lateral intraparietal (LIP), frontal eye fields (FEF), and visual areas MT and V4. The team analyzed the data, tracking each neuron’s activity over the course of the game to determine how much it participated in sensory vs. categorical work, accounting for the possibility that many neurons might well do at least a little of both. First they refined their analysis in a computer simulation, and then applied it to the actual neural data.
They found that while sensory processing was largely occurring where classic neuroscience would predict, most heavily in the MT and V4, categorization was surprisingly distributed. As expected the PFC led the way, but FEF, LIP and PIT often showed substantial categorization activity, too.
“Our findings suggest that, although brain regions are certainly specialized, they share a lot of information and functional similarities,” Siegel says. “Thus, our results suggest the brain should be thought of as a highly connected network of talkative related nodes, rather than as a set of highly specialized modules that only sparsely hand-off information to each other.”
The patterns of relative sensory and categorization activity varied by task, too. Few neuroscientists would be surprised that V4 cells were particularly active for color sensation while MT cells were active for sensing motion, but perhaps more interestingly, category signals were more widespread. For example, most of the areas were involved in in categorizing color, including those traditional thought to be specialized for motion.
The scientists also note another key pattern. In their analysis they could discern the dimensionality of the information the neurons were processing, and found that sensory information processing was highly multi-dimensional (i.e. as if considering many different details of the visual input), while categorization activity involved much greater focus (i.e. as if just judging “upward” or “downward”).
Cognition in the cortex
The broad distribution of activity related to categorization, Miller speculates, might be a sign that when the brain has a goal (in this case to categorize), that needs to be represented broadly, even if the PFC might be where the judgement is made. It’s a bit like in a business where everyone from the CEO down to workers on the manufacturing floor benefit from understanding the point of the enterprise in doing their work.
Miller also says the study extends some prior results from his lab. In a previous study he showed that PFC neurons were able to conduct highly-multidimensional information processing, while in this study they were largely focused on just one dimension. The synthesis of the two lines of evidence may be that PFC neurons are able to accommodate whatever degree of dimensionality pursuing a goal requires. They are versatile in how versatile they should be.
Let all this sink in, the next time you consider the ripeness of a banana or any other time you have to extract meaning from something you perceive.
The work was supported by National Institute of Mental Health, European Research Council, and the Center for Integrative Neuroscience.
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