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News Article | August 22, 2016
Site: www.sciencenews.org

A star that made headlines for its bizarre behavior has got one more mystery for astronomers to ponder. Tabby’s star, also known as KIC 8462852, has been inexplicably flickering and fading. The Kepler Space Telescope caught two dramatic drops in light — by up to 22 percent — spaced nearly two years apart. Photographs from other telescopes dating back to 1890 show that the star also faded by roughly 20 percent over much of the last century. Possible explanations for the behavior range from mundane comet swarms to fantastical alien engineering projects (SN Online: 2/2/16). A new analysis of data from Kepler, NASA’s premier planet hunter, shows that Tabby’s star steadily darkened throughout the telescope’s primary four-year mission. That’s in addition to the abrupt flickers already seen during the same time period. Over the first 1,100 days, the star dimmed by nearly 1 percent. Then the light dropped another 2.5 percent over the following six months before leveling off during the mission’s final 200 days. Astronomers Benjamin Montet of Caltech and Josh Simon of the Observatories of the Carnegie Institution of Washington in Pasadena, Calif., report the findings online August 4 at arXiv.org. The new data support a previous claim that the star faded between 1890 and 1989, a claim that some researchers questioned. “It’s just getting stranger,” says Jason Wright, an astronomer at Penn State University. “This is a third way in which the star is weird. Not only is it getting dimmer, it’s doing so at different rates.” The slow fading hadn’t been noticed before because data from Kepler are processed to remove long-term trends that might confuse planet-finding algorithms. To find the dimming, Montet and Simon analyzed images from the telescope that are typically used only to calibrate data. “Their analysis is very thorough,” says Tabetha Boyajian, an astronomer at Yale University who in 2015 reported the two precipitous drops in light (and for whom the star is nicknamed). “I see no flaws in that at all.” While the analysis is an important clue, it doesn’t yet explain the star’s erratic behavior. “It doesn’t push us in any direction because it’s nothing that we’ve ever encountered before,” says Boyajian. “I’ve said ‘I don’t know’ so many times at this point.” An object (or objects) moving in front of the star and blocking some of the light is still the favored explanation — though no one has figured out what that object is. The drop in light roughly 1,100 days into Kepler’s mission is reminiscent of a planet crossing in front of a star, Montet says. But given how slowly the light dropped, such a planet (or dim star) would have to live on an orbit more than 60 light-years across. The odds of catching a body on such a wide, slow orbit as it passed in front of the star are so low, says Montet, that you would need 10,000 Kepler missions to see just one. “We figure that's pretty unlikely.” An interstellar cloud wandering between Earth and KIC 8462852 is also unlikely, Wright says. “If the interstellar medium had these sorts of clumps and knots, it should be a ubiquitous phenomenon. We would have known about this for decades.” While some quasars and pulsars appear to flicker because of intervening material, the variations are minute and nothing like the 20 percent dips seen in Tabby’s star. A clump of gas and dust orbiting the star — possibly produced by a collision between comets — is a more likely candidate, although that doesn’t explain the century-long dimming. “Nothing explains all the effects we see,” says Montet. Given the star’s unpredictable nature, astronomers need constant vigilance to solve this mystery. The American Association of Variable Star Observers is working with amateur astronomers to gather continuous data from backyard telescopes around the globe. Boyajian and colleagues are preparing to monitor KIC 8462852 with the Las Cumbres Observatory Global Telescope Network, a worldwide web of telescopes that can keep an incessant eye on the star. “At this point, that’s the only thing that’s going to help us figure out what it is,” she says.

News Article | October 31, 2016
Site: motherboard.vice.com

Halloween is all about irreverently confronting our greatest fears, from the inevitability of death to the slow march of candy-induced tooth decay. But though witches, ghosts, zombies, chainsaw-massacres, and Trump masks make for adequately scary symbols of the season, they pale in comparison to the frightfest offered by the wider universe. Out there beyond our skies, galaxies eat each other alive and the light of long-dead stars shines on like a spectral cemetery. There are eerie pareidolic apparitions written with the guts of exploded stars, and worlds more hellish than anything dreamed up on our tranquil little planet. In that spirit, here's our guide to some of the most Halloweeny of astronomical phenomena, running the gamut from flesh-vaporizing lava planets to Orwellian nightmare nebulas. Enjoy the good old-fashioned existential cosmic dread. We have zombie movies, zombie television shows, zombie comic books, zombie games, zombie reboots of classic literary works, and "Zombie" by the Cranberries. Should we draw the line at zombie stars? No, said the universe. The term "zombie" has been applied to a few different types of star systems, but the most well-known scenario involves a low-mass white dwarf star exploding in a Type Iax supernova, which means that the star survives what is normally a fatal blast. After that, it is deemed an undead zombie star. Zombie stars can also refer to the "mysterious glow of high-energy X-rays that [...] could be the 'howls' of dead stars as they feed on stellar companions," according to NASA. So metal. Nebulas are enormous clouds of dust and gas that often act as stellar nurseries for successive generations of stars. They are particularly captivating phenomenon because they form ghostly, ethereal shapes, which people frequently anthropomorphize for kicks. Exhibit A: The Witch Head Nebula, located 900 light years from Earth and illuminated by the nearby giant star Rigel. Nebulaic flying monkeys not pictured. About 250 million light years away lies a galaxy called UGC 1382 that appears to be slapped together out of "spare parts," according to NASA. Its insides are younger than its outsides, and it's various components are so dislocated that it stretches out over 718,000 light years across, around seven times as wide as the Milky Way. "This rare, 'Frankenstein' galaxy formed and is able to survive because it lies in a quiet little suburban neighborhood of the universe, where none of the hubbub of the more crowded parts can bother it," said researcher Mark Seibert, based at the Observatories of the Carnegie Institution for Science in California, in a JPL statement. "It is so delicate that a slight nudge from a neighbor would cause it to disintegrate." This is the part where we pedantically point out that UGC 1382 would be more accurately called 'Frankenstein's monster' galaxy. We Earthlings are wise to unforgiving nightmare planets, given that our closest neighbor, Venus, is an acid-soaked pressure-cooker that would melt your face off like in Raiders of the Lost Ark. But with the ongoing discovery and characterization of thousands of worlds beyond our solar system, astronomers have learned that Venus is far from the only torture planet out there. For instance, take Kepler-78b, or the "Hell Planet," as some have taken to calling it. Only slightly larger than Earth, Kepler-78b is stuck in an orbit that brings it 40 times closer to its star than Mercury is to the Sun. Its entire surface is expected to be covered in lava, reaching temperatures of over 5,000 degrees Fahrenheit, which makes the 800 degree weather on Venus seem chilly by comparison. In keeping with biblical themes of penance and suffering, it's worth noting that the Hell Planet itself is doomed to be eaten by its star within the next three billion years. The universe is wonderful; the universe is terrifying. READ MORE: The Science of Being Scared to Death Just in case you forgot, a skull-shaped dead comet zipped by Earth last year on Halloween night. Perhaps we'll receive a similar trick-or-treater at our planetary door this year. My money's on a pumpkin-shaped asteroid that sounds like a theremin. We are all used to having only one Sun, but single parent stars are not the only type of solar hosts in the universe. Binary and multiple star systems are common, especially when massive O-type stars are involved. More often than not, these hulking giants are accompanied by smaller stars, and their lower-mass companions capitalize on this proximity by sucking fresh hydrogen gas off their O-type partners to revitalize and sustain them for longer, more active lives. This draining of lifeforce has earned them the name "vampire stars." It's not all bad for the "victim" O-type stars, because losing that outer layer of hydrogen makes them look much several million years younger. And in any case, these unstable stellar relationships are ultimately doomed to self-immolate in the fiery mutual destruction of a supernova, the way all good vampire love stories end. Galaxies can contain several hundred billion luminous stars, so they are not exactly known for being difficult to spot. But "ghost galaxies," a newly identified phenomenon, are another matter entirely. Made up primarily of dark matter, these small globs of mass are thought to be ancient, small, and relatively undisturbed galactic fossils from the early universe. They float around in our midst, but because they contain so few stars, it takes specialized instruments like the Hubble Space Telescope to root them out. Scientists think they likely date back over 13 billion years ago, when their star-forming fires were possibly snuffed out by the reionization epoch. "These galaxies are all ancient and they're all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies," said Tom Brown, an astronomer based at the Space Telescope Science Institute in Baltimore, in a NASA statement. "The most likely explanation is reionization." Who has time to worry about the NSA spying on them when this gigantic eyeball is out there watching our every move? This source of this intense cosmic staredown is the Helix Nebula, located 700 light years away. It is sometimes referred to as the Eye of God or the Eye of Sauron, depending on whether you think it looks like a benevolent cross-cultural deity or a rogue Maiar intent on destroying Middle Earth. Let's face it: The universe is pro-cannibalism. Stars eat planets like tapas, and frequently binge on other stars until they're overweight with stellar blubber. Galaxies certainly enjoy feasting on their brethren—our own Milky Way is still digesting some of the smaller galaxies it has eaten in the past. Even black holes have been spotted wolfing down other black holes. I'm not saying this justifies cannibalism on Earth (though for the record, it's common as hell here). I'm just saying that the universe would have a hard time judging you if you decided to go that route. Get six of our favorite Motherboard stories every day by signing up for our newsletter.

Mok A.,University of Waterloo | Balogh M.L.,University of Waterloo | Balogh M.L.,Leiden University | Mcgee S.L.,Leiden University | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present new analysis from the Group Environment Evolution Collaboration 2 (GEEC2) spectroscopic survey of galaxy groups at 0.8 < z < 1. Our previous work revealed an intermediate population between the star-forming and quiescent sequences and a strong environmental dependence in the fraction of quiescent galaxies. Only ∼5 per cent of star-forming galaxies in both the group and field sample show a significant enhancement in star formation, which suggests that quenching is the primary process in the transition from the star-forming to the quiescent state. To model the environmental quenching scenario, we have tested the use of different exponential quenching time-scales and delays between satellite accretion and the onset of quenching. We find that with no delay, the quenching time-scale needs to be long in order to match the observed quiescent fraction, but then this model produces too many intermediate galaxies. Fixing a delay time of 3 Gyr, as suggested from the local Universe, produces too few quiescent galaxies. The observed fractions are best matched with a model that includes a delay that is proportional to the dynamical time and a rapid quenching time-scale (∼0.25 Gyr), but this model also predicts intermediate galaxies Hδ strength higher than that observed. Using stellar synthesis models, we have tested other scenarios, such as the rejuvenation of star formation in early-type galaxies and a portion of quenched galaxies possessing residual star formation. If environment quenching plays a role in the GEEC2 sample, then our work suggests that only a fraction of intermediate galaxies may be undergoing this transition and that quenching occurs quite rapidly in satellite galaxies (≲0.25 Gyr). © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Hou A.,McMaster University | Parker L.C.,McMaster University | Wilman D.J.,Max Planck Institute for Extraterrestrial Physics | Mcgee S.L.,Durham University | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

The presence of substructure in galaxy groups and clusters is believed to be a sign of recent galaxy accretion and can be used to probe not only the assembly history of these structures, but also the evolution of their member galaxies. Using the Dressler-Shectman (DS) test, we study substructure in a sample of intermediate-redshift (z∼ 0.4) galaxy groups from the Group Environment and Evolution Collaboration (GEEC) group catalogue. We find that four of the 15 rich GEEC groups, with an average velocity dispersion of ∼525kms -1, are identified as having significant substructure. The identified regions of localized substructure lie on the group outskirts and in some cases appear to be infalling. In a comparison of galaxy properties for the members of groups with and without substructure, we find that the groups with substructure have a significantly higher fraction of blue and star-forming galaxies and a parent colour distribution that resembles that of the field population rather than the overall group population. In addition, we observe correlations between the detection of substructure and other dynamical measures, such as velocity distributions and velocity dispersion profiles. Based on this analysis, we conclude that some galaxy groups contain significant substructure and that these groups have properties and galaxy populations that differ from groups with no detected substructure. These results indicate that the substructure galaxies, which lie preferentially on the group outskirts and could be infalling, do not exhibit signs of environmental effects, since little or no star formation quenching is observed in these systems. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Balogh M.L.,University of Waterloo | Mcgee S.L.,University of Waterloo | Mcgee S.L.,Durham University | Wilman D.J.,Max Planck Institute for Extraterrestrial Physics | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We introduce our survey of galaxy groups at 0.85 < z < 1, as an extension of the Group Environment and Evolution Collaboration. Here we present the first results, based on Gemini GMOS-S nod-and-shuffle spectroscopy of seven galaxy groups selected from spectroscopically confirmed, extended XMM detections in COSMOS. We use photometric redshifts to select potential group members for spectroscopy, and target galaxies with r < 24.75. In total, we have over 100 confirmed group members, and four of the groups have >15 members. The dynamical mass estimates are in good agreement with the masses estimated from the X-ray luminosity, with most of the groups having 13 < logMdyn/M⊙ < 14. We compute stellar masses by template-fitting the spectral energy distributions; our spectroscopic sample is statistically complete for all galaxies with Mstar≳ 1010.1M⊙, and for blue galaxies we sample masses as low as Mstar∼ 108.8M⊙. The fraction of total mass in galaxy starlight spans a range of 0.25-3per cent, for the six groups with reliable mass determinations. Like lower redshift groups, these systems are dominated by red galaxies, at all stellar masses Mstar > 1010.1M⊙. A few group galaxies inhabit the 'blue cloud' that dominates the surrounding field; instead, we find a large and possibly distinct population of galaxies with intermediate colours. The 'green valley' that exists at low redshift is instead well populated in these groups, containing ∼30per cent of the galaxies. These do not appear to be exceptionally dusty galaxies, and about half show prominent Balmer absorption lines. Furthermore, their Hubble Space Telescope morphologies appear to be intermediate between those of red-sequence and blue-cloud galaxies of the same stellar mass. Unlike red-sequence galaxies, most of the green galaxies have a disc component, but one that is smaller and less structured than discs found in the blue cloud. We postulate that these are a transient population, migrating from the blue cloud to the red sequence, with a star formation rate that declines with an exponential time-scale 0.6 < τ < 2Gyr. Such galaxies may not be exclusive to the group environment, as we find examples also amongst the non-members. However, their prominence among the group galaxy population, and the marked lack of blue, star-forming galaxies, provides evidence that the group environment either directly reduces star formation in member galaxies or at least prevents its rejuvenation during the normal cycle of galaxy evolution. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Mok A.,University of Waterloo | Balogh M.L.,University of Waterloo | Mcgee S.L.,Durham University | Mcgee S.L.,Leiden University | And 10 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present deep Gemini Multi-Object Spectrograph-South spectroscopy for 11 galaxy groups at 0.8 < z < 1.0, for galaxies with rAB < 24.75. Our sample is highly complete (>66 per cent) for eight of the 11 groups. Using an optical-near-infrared colour-colour diagram, the galaxies in the sample were separated with a dust insensitive method into three categories: passive (red), star-forming (blue) and intermediate (green). The strongest environmental dependence is observed in the fraction of passive galaxies, which make up only ∼20 per cent of the field in the mass range 1010.3 < Mstar/M⊙ < 1011.0, but are the dominant component of groups. If we assume that the properties of the field are similar to those of the 'pre-accreted' population, the environment quenching efficiency (ερ) is defined as the fraction of field galaxies required to be quenched in order to match the observed red fraction inside groups. The efficiency obtained is ∼0.4, similar to its value in intermediate-density environments locally. While green (intermediate) galaxies represent ∼20 per cent of the star-forming population in both the group and field, at all stellar masses, the average specific star formation rate of the group population is lower by a factor of ∼3. The green population does not show strong Hδ absorption that is characteristic of starburst galaxies. Finally, the high fraction of passive galaxies in groups, when combined with satellite accretion models, require that most accreted galaxies have been affected by their environment. Thus, any delay between accretion and the onset of truncation of star formation (τ) must be ≲ 2 Gyr, shorter than the 3-7 Gyr required to fit data at z 0. The relatively small fraction of intermediate galaxies require that the actual quenching process occurs quickly, with an exponential decay time-scale of τq ≲ 1 Gyr. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

News Article | August 23, 2016
Site: news.yahoo.com

Artist's illustration of cometary material crossing the face of a star — one possible explanation for the strange dimming exhibited by "Tabby's star." Nearly a year after first making headlines around the world, "Tabby's star" is still guarding its secrets. In September 2015, a team led by Yale University astronomer Tabetha Boyajian announced that a star about 1,500 light-years from Earth called KIC 8462852 had dimmed oddly and dramatically several times over the past few years. These dimming events, which were detected by NASA's planet-hunting Kepler space telescope, were far too substantial to be caused by an orbiting planet, scientists said. (In one case, 22 percent of the star's light was blocked. For comparison, when huge Jupiter crosses the sun's face, the result is a dimming of just 1 percent or so.) [13 Ways to Hunt Intelligent Alien Life] Boyajian and her colleagues suggested that a cloud of fragmented comets or planetary building blocks might be responsible, but other researchers noted that the signal was also consistent with a possible "alien megastructure" — perhaps a giant swarm of energy-collecting solar panels known as a Dyson sphere. Astronomers around the world soon began studying Tabby's star with a variety of instruments, and reanalyzing old observations of the object, in an attempt to figure out what, exactly, is going on. But they have yet to solve the puzzle. "I'd say we have no good explanation right now for what's going on with Tabby's star," Jason Wright, an astronomer at Pennsylvania State University, said earlier this month during a talk at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California. "For now, it's still a mystery." In fact, that mystery may have deepened over the past 12 months. For example, in January, Bradley Schaefer, a professor of physics and astronomy at Louisiana State University, determined that, in addition to the weird short-term dimming events, the brightness of Tabby's star had dropped by about 20 percent overall between 1890 and 1989. That pattern is very difficult for known natural phenomena to explain, he said. Schaefer came to this conclusion after poring over old photographic plates of the night sky that captured Tabby's star. Other researchers suggested that the trend Schaefer saw could have been caused by changes in the instruments used to take those photos over the century-long timespan. However, a new study bolsters Schaefer's interpretation. In the new work, Benjamin Montet (of the California Institute of Technology and the Harvard-Smithsonian Center for Astrophysics) and Joshua Simon (of the Observatories of the Carnegie Institution of Washington) reanalyzed Kepler observations of Tabby's star from 2009 through 2013. They found that the object dimmed by 3 percent over that span, with a rapid 2-percent brightness dip over one 200-day period. "Of a sample of 193 nearby comparison stars and 355 stars with similar stellar parameters, 0.6 percent change brightness at a rate as fast as 0.341 percent [per year], and none exhibit either the rapid decline by > 2 percent or the cumulative fading by 3 percent of KIC 8462852," Montet and Simon wrote in the new study, which they uploaded to the online preprint site ArXiv on Aug. 5. "No known or proposed stellar phenomena can fully explain all aspects of the observed light curve." Schaefer's results, combined with those of Montet and Simon, make the comet hypothesis look less and less likely, Wright said in his SETI talk. "Why would comets, over a century, make the star dimmer?" he said. "What's going on?" [5 Bold Claims of Alien Life] The sustained dimming of Tabby's star is still consistent with at least some variants of the "alien megastructure" hypothesis, Wright said. "Some people have sort of facetiously offered that perhaps this is a Dyson sphere under construction: You're seeing lots of material getting built," he said. "In just 100 years, they've blotted out 20 percent of the starlight. That seems kind of fast to me — but, you know, aliens, right?" It's also possible that the alien megastructure — if it exists — is fully constructed, and some parts are just denser than others, Wright added. "That would naturally make the star get brighter and dimmer, as dense parts of the swarm came around," he said. "So if I had to invoke megastructures to explain it, that seems consistent. You've got lots of panels of different shapes, different sizes, and the big ones make big dips and the little ones make little dips, and the whole swarm is sort of like a translucent screen that makes the whole thing dimmer." But Wright and others have always stressed that the "E.T. did it" scenario is very unlikely, and that a more prosaic explanation will probably rise to the top eventually. And indeed, other recent observations throw some cold water on the alien-megastructure idea — and any other hypothesis that invokes some object or phenomenon near Tabby's star. Any structure surrounding the star, be it alien-made or naturally occurring, would heat up and give off infrared radiation, Wright said. But he and his colleagues saw no signatures of such "waste heat" in data gathered by NASA's Wide-field Infrared Survey Explorer spacecraft. And another research team — which analyzed observations by the Submillimeter Array telescope and the Submillimeter Common-User Bolometer Array-2 instrument, both of which are in Hawaii — also came up empty. Whatever is blocking the starlight from Tabby's star is "not surrounding the whole star — it must be along our line of sight," Wright said. "So you can do that if it's in a disk of some kind. And that hopefully will help constrain what the heck is going on." Wright has a hunch that the answer lies far away from Tabby's star, out in the dark depths of space. "I think I've all but abandoned circumstellar explanations, and I think now we're going to have to talk about [some] bizarre structure in the interstellar medium, and stuff like that," he said. Still, Wright hasn't given up on the alien-megastructure hypothesis. While the lack of waste heat is "almost a fatal blow" for the idea, he said, it's still viable if the purported aliens are doing something with the waste heat — turning it into matter, for example, or converting the heat into radio waves for communication purposes. Astronomers have already searched for such signals coming from Tabby's star using the Allen Telescope Array, a network of radio dishes in northern California operated by the SETI Institute. They found nothing. But Wright and his colleagues plan to conduct another search beginning in October; they've secured time on West Virginia's huge Green Bank Telescope for this purpose. "This is a 1-in-300,000 object," Wright said. "People have gone looking for more, and it's the only one. So that also says you're allowed to invoke one really rare thing, because it is a rare phenomenon." SETI: All About the Search for Extraterrestrial Intelligence (Infographic) Copyright 2016 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

News Article | March 22, 2016
Site: phys.org

Dark Energy Survey image of the region surrounding the faint dwarf galaxy Reticulum II. The nine brightest known stars in the galaxy are marked with red circles. Spectra showing the unique chemical content of three stars are shown. Credit: Alex Ji. Background image: Fermilab/Dark Energy Survey Reticulum II is an ancient and faint dwarf galaxy discovered in images taken as part of the Dark Energy Survey. It orbits the Milky Way galaxy about 100,000 light years away from us. Though the galaxy looks unassuming at first, the chemical content of its stars may hold the key to unlocking a 60-year-old mystery about the cosmic origin of the heaviest elements in the periodic table. Today in the journal Nature, a team of astronomers at MIT's Kavli Institute for Astrophysics and Space Research and the Observatories of the Carnegie Institution of Washington report on observations of this unique galaxy using the Magellan telescopes at the Las Campanas Observatory in Chile's Atacama Desert. Lead author and MIT physics graduate student Alex Ji explains more. Q: How are the heaviest elements in the periodic table created in the cosmos? A: Carl Sagan popularized the notion that we are all made of star stuff. He could say so with confidence because we actually know where nearly every element in the periodic table is made in the universe. But there's a hole in our understanding. The heaviest elements are made in what is called the "rapid neutron-capture process," or "r-process" for short, in which heavy elements are quickly built up from lighter seed nuclei. Gold, platinum, and uranium are r-process elements, as are more exotic elements like europium, neodymium, and gadolinium. The nuclear physics of the r-process was mostly worked out by 1957, but for almost 60 years astronomers have debated about the astrophysical site that could provide the extreme conditions for the r-process to occur. Synthesizing the r-process elements requires environments with a very large number of neutrons. The two best candidate sites are supernovae and merging neutron stars. Supernovae are the explosions that mark the end of a massive star's life. They often leave behind a remnant called a neutron star. During the formation of a neutron star, a large amount of neutrons is released. If two of these neutron stars happen to be orbiting each other, they will eventually merge to form one giant neutron star. During that explosion neutrons are released and r-process elements can form. Q: How does this dwarf galaxy help us understand the site of the r-process? A: Reticulum II is not the first ancient dwarf galaxy to have its chemical content examined; it's actually the 10th. But its chemical composition differs completely from those other galaxies. The stars in those first nine galaxies have unusually low amounts of r-process elements. Reticulum II, on the other hand, is chock-full of r-process elements. Its stars display some of the highest r-process enhancements we have ever seen. It's almost literally a gold mine. What this means is that a single rare event produced a rather large amount of this r-process material. All those elements were then incorporated into the surrounding gas and from there into the next generations of stars. It is those stars that we can still observe today. The single, prolific r-process event in this galaxy implies that a neutron star merger could have produced these elements in the early universe. A normal supernova would have produced less, and the observed enhancement could not have been as high, though it's also hypothesized that rare, magnetically-driven supernovae might be able to produce much more r-process material. Interestingly, there is indirect evidence that neutron star mergers do also synthesize r-process elements in the universe today. So it looks like neutron star mergers could be the primary r-process sites throughout cosmic time. It's amazing to think that Reticulum II preserved a signature of that extraordinary event for more than 12 billion years, just waiting for us to dig it up. Q: What was it like to be at the telescope and realize what you had found? A: Based on studying the other ultra-faint galaxies, I had expected to find stars with essentially none of these r-process elements and to further establish that these types of dwarf galaxies are devoid in these elements. So we had a plan to get some really good, low upper limits on the r-process content to push this issue. When we realized the stars in this galaxy were the complete opposite, and instead full of r-process elements, I was certain I had screwed something up. From the telescope in Chile I called my advisor Anna Frebel in Cambridge [Mass.] in the middle of the night to urgently talk about what was going on. Telescope time is precious and expensive after all and shouldn't be wasted. During the hour-long discussions that followed, I kept observing more stars while carrying out preliminary analyses of the data at hand to ensure that this was a real signal. At the observatory, astronomers work all night and sleep during the day. But after seeing the r-process elements in the first few stars, I couldn't sleep anymore; all I could do was stare out the window and hope the incoming clouds would go away again and the wind would die down. We were very lucky that it ended up being clear most of the four nights we had available. My last night there, the weather forecast, as translated from Spanish by Google, read "rain and wind." So I prepared myself to get no data that night. But it turned out Google had translated the word "despejado" incorrectly and in fact it was supposed to be "clear and wind." An important translation to get right, especially for astronomers! More information: Alexander P. Ji et al. R-process enrichment from a single event in an ancient dwarf galaxy, Nature (2016). DOI: 10.1038/nature17425

Andersson B.-G.,NASA | Piirola V.,University of Turku | De Buizer J.,NASA | Clemens D.P.,Boston University | And 6 more authors.
Astrophysical Journal | Year: 2013

In the interstellar medium (ISM), molecular hydrogen is expected to form almost exclusively on the surfaces of dust grains. Due to that molecule's large formation energy (-4.5 eV), several dynamical effects are likely associated with the process, including the alignment of asymmetric dust grains with the ambient magnetic field. Such aligned dust grains are, in turn, believed to cause the broadband optical/infrared polarization observed in the ISM. Here, we present the first observational evidence for grain alignment driven by H2 formation, by showing that the polarization of the light from stars behind the reflection nebula IC 63 appears to correlate with the intensity of H2 fluorescence. While our results strongly suggest a role for "Purcell rockets" in grain alignment, additional observations are needed to conclusively confirm their role. By showing a direct connection between H 2 formation and a probe of the dust characteristics, these results also provide one of the first direct confirmations of the grain-surface formation of H2. We compare our observations to ab initio modeling based on Radiative Torque Alignment (RAT) theory. © 2013. The American Astronomical Society. All rights reserved..

Fraser W.C.,California Institute of Technology | Fraser W.C.,Herzberg Institute for Astrophysics | Batygin K.,California Institute of Technology | Batygin K.,Harvard - Smithsonian Center for Astrophysics | And 3 more authors.
Icarus | Year: 2013

Here we present new adaptive optics observations of the Quaoar-Weywot system. With these new observations we determine an improved system orbit. Due to a 0.39day alias that exists in available observations, four possible orbital solutions are available with periods of ∼11.6, ∼12.0, ∼12.4, and ∼12.8days. From the possible orbital solutions, system masses of 1.3-1.5±0.1×1021kg are found. These observations provide an updated density for Quaoar of 2.7-5.0gcm-3. In all cases, Weywot's orbit is eccentric, with possible values ∼0.13-0.16. We present a reanalysis of the tidal orbital evolution of the Quaoar-Weywot system. We have found that Weywot has probably evolved to a state of synchronous rotation, and has likely preserved its initial inclination over the age of the Solar System. We find that for plausible values of the effective tidal dissipation factor tides produce a very slow evolution of Weywot's eccentricity and semi-major axis. Accordingly, it appears that Weywot's eccentricity likely did not tidally evolve to its current value from an initially circular orbit. Rather, it seems that some other mechanism has raised its eccentricity post-formation, or Weywot formed with a non-negligible eccentricity. © 2012.

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