This still from a simulation captures binary star formation in action. Researchers have long speculated on the processes that lead to clouds of gas and dust breaking up into smaller pieces to form multiple-star systems — but these take place over a large range of scales, making them difficult to simulate. In a new study led by Leonardo Sigalotti (UAM Azcapotzalco, Mexico), researchers have used a smoothed-particle hydrodynamics code to model binary star formation on scales of thousands of AU down to scales as small as ~0.1 AU. In the scene shown above, a collapsing cloud of gas and dust has recently fragmented into two pieces, forming a pair of disks separated by around 200 AU. In addition, we can see that smaller-scale fragmentation is just starting in one of these disks, Disk B. Here, one of the disk’s spiral arms has become unstable and is beginning to condense; it will eventually form another star, producing a hierarchical system: a close binary within the larger-scale binary. Check out the broader process in the four panels below (which show the system as it evolves over time), or visit the paper linked below for more information about what the authors learned.
Evolution of a collapsed cloud after large-scale fragmentation into a binary protostar: (a) 44.14 kyr, (b) 44.39 kyr, (c) 44.43 kyr, and (d) 44.68 kyr. The insets show magnifications of the binary cores. [Adapted from Sigalotti et al. 2018]
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
Artist’s illustration of two merging neutron stars. Astronomers witnessed such a merger in August 2017, and we’re now trying to interpret these observations. [University of Warwick/Mark Garlick]
The first optical image of a gravitational wave source was taken by a team led by Ryan Foley of UC Santa Cruz using the Swope Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars. (Image credit: 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)
Carnegie Institution Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile
A neutron star forms when a massive star runs out of fuel and explodes as a supernova, throwing off its outer layers and leaving behind a collapsed core composed almost entirely of neutrons. Neutrons are the uncharged particles in the nucleus of an atom, where they are bound together with positively charged protons. In a neutron star, they are packed together just as densely as in the nucleus of an atom, resulting in an object with one to three times the mass of our sun but only about 12 miles wide.
“Basically, a neutron star is a gigantic atom with the mass of the sun and the size of a city like San Francisco or Manhattan,” said Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz.
These objects are so dense, a cup of neutron star material would weigh as much as Mount Everest, and a teaspoon would weigh a billion tons. It’s as dense as matter can get without collapsing into a black hole.
THE MERGER
Like other stars, neutron stars sometimes occur in pairs, orbiting each other and gradually spiraling inward. Eventually, they come together in a catastrophic merger that distorts space and time (creating gravitational waves) and emits a brilliant flare of electromagnetic radiation, including visible, infrared, and ultraviolet light, x-rays, gamma rays, and radio waves. Merging black holes also create gravitational waves, but there’s nothing to be seen because no light can escape from a black hole.
Foley’s team was the first to observe the light from a neutron star merger that took place on August 17, 2017, and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Caltech/MIT Advanced aLigo Hanford, WA, USA installation
Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project
Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib
ESA/eLISA the future of gravitational wave research
Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)
Now, for the first time, scientists can study both the gravitational waves (ripples in the fabric of space-time), and the radiation emitted from the violent merger of the densest objects in the universe.
The UC Santa Cruz team found SSS17a by comparing a new image of the galaxy N4993 (right) with images taken four months earlier by the Hubble Space Telescope (left). The arrows indicate where SSS17a was absent from the Hubble image and visible in the new image from the Swope Telescope. (Image credits: Left, Hubble/STScI; Right, 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)
It’s that combination of data, and all that can be learned from it, that has astronomers and physicists so excited. The observations of this one event are keeping hundreds of scientists busy exploring its implications for everything from fundamental physics and cosmology to the origins of gold and other heavy elements.
A small team of UC Santa Cruz astronomers were the first team to observe light from two neutron stars merging in August. The implications are huge.
ALL THE GOLD IN THE UNIVERSE
It turns out that the origins of the heaviest elements, such as gold, platinum, uranium—pretty much everything heavier than iron—has been an enduring conundrum. All the lighter elements have well-explained origins in the nuclear fusion reactions that make stars shine or in the explosions of stars (supernovae). Initially, astrophysicists thought supernovae could account for the heavy elements, too, but there have always been problems with that theory, says Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz.
The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram, that give rise to the different types of radiation seen by astronomers, including a gamma-ray burst and a kilonova explosion seen in visible light. (Image credit: Murguia-Berthier et al., Science)
A theoretical astrophysicist, Ramirez-Ruiz has been a leading proponent of the idea that neutron star mergers are the source of the heavy elements. Building a heavy atomic nucleus means adding a lot of neutrons to it. This process is called rapid neutron capture, or the r-process, and it requires some of the most extreme conditions in the universe: extreme temperatures, extreme densities, and a massive flow of neutrons. A neutron star merger fits the bill.
Ramirez-Ruiz and other theoretical astrophysicists use supercomputers to simulate the physics of extreme events like supernovae and neutron star mergers. This work always goes hand in hand with observational astronomy. Theoretical predictions tell observers what signatures to look for to identify these events, and observations tell theorists if they got the physics right or if they need to tweak their models. The observations by Foley and others of the neutron star merger now known as SSS17a are giving theorists, for the first time, a full set of observational data to compare with their theoretical models.
According to Ramirez-Ruiz, the observations support the theory that neutron star mergers can account for all the gold in the universe, as well as about half of all the other elements heavier than iron.
RIPPLES IN THE FABRIC OF SPACE-TIME
Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, but until recently they were impossible to observe. LIGO’s extraordinarily sensitive detectors achieved the first direct detection of gravitational waves, from the collision of two black holes, in 2015. Gravitational waves are created by any massive accelerating object, but the strongest waves (and the only ones we have any chance of detecting) are produced by the most extreme phenomena.
Two massive compact objects—such as black holes, neutron stars, or white dwarfs—orbiting around each other faster and faster as they draw closer together are just the kind of system that should radiate strong gravitational waves. Like ripples spreading in a pond, the waves get smaller as they spread outward from the source. By the time they reached Earth, the ripples detected by LIGO caused distortions of space-time thousands of times smaller than the nucleus of an atom.
The rarefied signals recorded by LIGO’s detectors not only prove the existence of gravitational waves, they also provide crucial information about the events that produced them. Combined with the telescope observations of the neutron star merger, it’s an incredibly rich set of data.
LIGO can tell scientists the masses of the merging objects and the mass of the new object created in the merger, which reveals whether the merger produced another neutron star or a more massive object that collapsed into a black hole. To calculate how much mass was ejected in the explosion, and how much mass was converted to energy, scientists also need the optical observations from telescopes. That’s especially important for quantifying the nucleosynthesis of heavy elements during the merger.
LIGO can also provide a measure of the distance to the merging neutron stars, which can now be compared with the distance measurement based on the light from the merger. That’s important to cosmologists studying the expansion of the universe, because the two measurements are based on different fundamental forces (gravity and electromagnetism), giving completely independent results.
“This is a huge step forward in astronomy,” Foley said. “Having done it once, we now know we can do it again, and it opens up a whole new world of what we call ‘multi-messenger’ astronomy, viewing the universe through different fundamental forces.”
IN THIS REPORT
Neutron stars
A team from UC Santa Cruz was the first to observe the light from a neutron star merger that took place on August 17, 2017 and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)
Graduate students and post-doctoral scholars at UC Santa Cruz played key roles in the dramatic discovery and analysis of colliding neutron stars.Astronomer Ryan Foley leads a team of young graduate students and postdoctoral scholars who have pulled off an extraordinary coup. Following up on the detection of gravitational waves from the violent merger of two neutron stars, Foley’s team was the first to find the source with a telescope and take images of the light from this cataclysmic event. In so doing, they beat much larger and more senior teams with much more powerful telescopes at their disposal.
“We’re sort of the scrappy young upstarts who worked hard and got the job done,” said Foley, an untenured assistant professor of astronomy and astrophysics at UC Santa Cruz.
David Coulter, graduate student
The discovery on August 17, 2017, has been a scientific bonanza, yielding over 100 scientific papers from numerous teams investigating the new observations. Foley’s team is publishing seven papers, each of which has a graduate student or postdoc as the first author.
“I think it speaks to Ryan’s generosity and how seriously he takes his role as a mentor that he is not putting himself front and center, but has gone out of his way to highlight the roles played by his students and postdocs,” said Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and the most senior member of Foley’s team.
“Our team is by far the youngest and most diverse of all of the teams involved in the follow-up observations of this neutron star merger,” Ramirez-Ruiz added.
Charles Kilpatrick, postdoctoral scholar
Charles Kilpatrick, a 29-year-old postdoctoral scholar, was the first person in the world to see an image of the light from colliding neutron stars. He was sitting in an office at UC Santa Cruz, working with first-year graduate student Cesar Rojas-Bravo to process image data as it came in from the Swope Telescope in Chile. To see if the Swope images showed anything new, he had also downloaded “template” images taken in the past of the same galaxies the team was searching.
Ariadna Murguia-Berthier, graduate student
“In one image I saw something there that was not in the template image,” Kilpatrick said. “It took me a while to realize the ramifications of what I was seeing. This opens up so much new science, it really marks the beginning of something that will continue to be studied for years down the road.”
At the time, Foley and most of the others in his team were at a meeting in Copenhagen. When they found out about the gravitational wave detection, they quickly got together to plan their search strategy. From Copenhagen, the team sent instructions to the telescope operators in Chile telling them where to point the telescope. Graduate student David Coulter played a key role in prioritizing the galaxies they would search to find the source, and he is the first author of the discovery paper published in Science.
Matthew Siebert, graduate student
“It’s still a little unreal when I think about what we’ve accomplished,” Coulter said. “For me, despite the euphoria of recognizing what we were seeing at the moment, we were all incredibly focused on the task at hand. Only afterward did the significance really sink in.”
Just as Coulter finished writing his paper about the discovery, his wife went into labor, giving birth to a baby girl on September 30. “I was doing revisions to the paper at the hospital,” he said.
It’s been a wild ride for the whole team, first in the rush to find the source, and then under pressure to quickly analyze the data and write up their findings for publication. “It was really an all-hands-on-deck moment when we all had to pull together and work quickly to exploit this opportunity,” said Kilpatrick, who is first author of a paper comparing the observations with theoretical models.
César Rojas Bravo, graduate student
Graduate student Matthew Siebert led a paper analyzing the unusual properties of the light emitted by the merger. Astronomers have observed thousands of supernovae (exploding stars) and other “transients” that appear suddenly in the sky and then fade away, but never before have they observed anything that looks like this neutron star merger. Siebert’s paper concluded that there is only a one in 100,000 chance that the transient they observed is not related to the gravitational waves.
Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, is first author of a paper synthesizing data from a range of sources to provide a coherent theoretical framework for understanding the observations.
Another aspect of the discovery of great interest to astronomers is the nature of the galaxy and the galactic environment in which the merger occurred. Postdoctoral scholar Yen-Chen Pan led a paper analyzing the properties of the host galaxy. Enia Xhakaj, a new graduate student who had just joined the group in August, got the opportunity to help with the analysis and be a coauthor on the paper.
Yen-Chen Pan, postdoctoral scholar
“There are so many interesting things to learn from this,” Foley said. “It’s a great experience for all of us to be part of such an important discovery.”
Abbott et al., Nature, A gravitational-wave standard siren measurement of the Hubble constant (The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration)
Writing: Tim Stephens
Video: Nick Gonzales
Photos: Carolyn Lagattuta
Header image: Illustration by Robin Dienel courtesy of the Carnegie Institution for Science
Design and development: Rob Knight
Project managers: Sherry Main, Scott Hernandez-Jason, Tim Stephens
NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet
Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet
Noted in the video but not in the article:
NASA/Chandra Telescope
NASA/SWIFT Telescope
NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA
CTIO PROMPT telescope telescope built by the University of North Carolina at Chapel Hill at Cerro Tololo Inter-American Observatory in Chilein the Chilean Andes.
Lick Automated Planet Finder telescope, Mount Hamilton, CA, USA
The University of California, Santa Cruz, opened in 1965 and grew, one college at a time, to its current (2008-09) enrollment of more than 16,000 students. Undergraduates pursue more than 60 majors supervised by divisional deans of humanities, physical & biological sciences, social sciences, and arts. Graduate students work toward graduate certificates, master’s degrees, or doctoral degrees in more than 30 academic fields under the supervision of the divisional and graduate deans. The dean of the Jack Baskin School of Engineering oversees the campus’s undergraduate and graduate engineering programs.
Lick Observatory’s Great Lick 91-centimeter (36-inch) telescope housed in the South (large) Dome of main building
Search for extraterrestrial intelligence expands at Lick Observatory
New instrument scans the sky for pulses of infrared light
March 23, 2015
By Hilary Lebow
The NIROSETI instrument saw first light on the Nickel 1-meter Telescope at Lick Observatory on March 15, 2015. (Photo by Laurie Hatch) UCSC Lick Nickel telescope
Astronomers are expanding the search for extraterrestrial intelligence into a new realm with detectors tuned to infrared light at UC’s Lick Observatory. A new instrument, called NIROSETI, will soon scour the sky for messages from other worlds.
“Infrared light would be an excellent means of interstellar communication,” said Shelley Wright, an assistant professor of physics at UC San Diego who led the development of the new instrument while at the University of Toronto’s Dunlap Institute for Astronomy & Astrophysics.
Wright worked on an earlier SETI project at Lick Observatory as a UC Santa Cruz undergraduate, when she built an optical instrument designed by UC Berkeley researchers. The infrared project takes advantage of new technology not available for that first optical search.
Infrared light would be a good way for extraterrestrials to get our attention here on Earth, since pulses from a powerful infrared laser could outshine a star, if only for a billionth of a second. Interstellar gas and dust is almost transparent to near infrared, so these signals can be seen from great distances. It also takes less energy to send information using infrared signals than with visible light.
UCSC alumna Shelley Wright, now an assistant professor of physics at UC San Diego, discusses the dichroic filter of the NIROSETI instrument. (Photo by Laurie Hatch)
Frank Drake, professor emeritus of astronomy and astrophysics at UC Santa Cruz and director emeritus of the SETI Institute, said there are several additional advantages to a search in the infrared realm.
“The signals are so strong that we only need a small telescope to receive them. Smaller telescopes can offer more observational time, and that is good because we need to search many stars for a chance of success,” said Drake.
The only downside is that extraterrestrials would need to be transmitting their signals in our direction, Drake said, though he sees this as a positive side to that limitation. “If we get a signal from someone who’s aiming for us, it could mean there’s altruism in the universe. I like that idea. If they want to be friendly, that’s who we will find.”
Scientists have searched the skies for radio signals for more than 50 years and expanded their search into the optical realm more than a decade ago. The idea of searching in the infrared is not a new one, but instruments capable of capturing pulses of infrared light only recently became available.
“We had to wait,” Wright said. “I spent eight years waiting and watching as new technology emerged.”
Now that technology has caught up, the search will extend to stars thousands of light years away, rather than just hundreds. NIROSETI, or Near-Infrared Optical Search for Extraterrestrial Intelligence, could also uncover new information about the physical universe.
“This is the first time Earthlings have looked at the universe at infrared wavelengths with nanosecond time scales,” said Dan Werthimer, UC Berkeley SETI Project Director. “The instrument could discover new astrophysical phenomena, or perhaps answer the question of whether we are alone.”
NIROSETI will also gather more information than previous optical detectors by recording levels of light over time so that patterns can be analyzed for potential signs of other civilizations.
“Searching for intelligent life in the universe is both thrilling and somewhat unorthodox,” said Claire Max, director of UC Observatories and professor of astronomy and astrophysics at UC Santa Cruz. “Lick Observatory has already been the site of several previous SETI searches, so this is a very exciting addition to the current research taking place.”
NIROSETI will be fully operational by early summer and will scan the skies several times a week on the Nickel 1-meter telescope at Lick Observatory, located on Mt. Hamilton east of San Jose.
The NIROSETI team also includes Geoffrey Marcy and Andrew Siemion from UC Berkeley; Patrick Dorval, a Dunlap undergraduate, and Elliot Meyer, a Dunlap graduate student; and Richard Treffers of Starman Systems. Funding for the project comes from the generous support of Bill and Susan Bloomfield.
Now that the hubbub of GW170817 — the first coincident detection of gravitational waves and an electromagnetic signature — has died down, scientists are left with the task of taking the spectrum-spanning observations and piecing them together into a coherent picture. Researcher Iair Arcavi examines one particular question: what caused the blue color in the early hours of the neutron-star merger?
Early Color
When the two neutron stars of GW170817 merged in August of last year, they produced not only gravitational waves, but a host of electromagnetic signatures. Chief among these was a flare of emission thought to be powered by the radioactive decay of heavy elements formed in the merger — a kilonova.
The emission during a kilonova can come from a number of different sources — from the heavy-element-rich tidal tails of the disrupting neutron stars, or from fast, light polar jets, or from a wind or a disk outflow — and each of these components could reveal different information about the original neutron stars and the merger.
It’s therefore important that we understand the sources of the emission that we observed in the GW170817 kilonova. In particular, we’d like to know where the early blue emission came from that was spotted in the first hours of the kilonova.
Comparing Models
To explore this question, Iair Arcavi (Einstein Fellow at University of California, Santa Barbara and Las Cumbres Observatory) compiled infrared through ultraviolet observations of the GW170817 kilonova from nearly 20 different telescopes. To try to distinguish between possible sources, Arcavi then compared the resulting combined light curves to a variety of models.
LCOGT Las Cumbres Observatory Global Telescope Network, Haleakala Hawaii, USA, Elevation 10,023 ft (3,055 m)
Arcavi found that the light curves for the GW170817 kilonova indicate an initial ~24-hour rise of emission. This rise is best matched by models in which the emission is produced by radioactive decay of ejecta with lots of heavier elements (likely from tidal tails). The subsequent decline of the emission, however, is fit as well or better by models that include lighter, faster outflows, or additional emission due to shock-heating from a wind or a cocoon surrounding a jet.
Missing Ultraviolet
The takeaway from Arcavi’s work is that we can’t yet eliminate any models for the GW170817 kilonova’s early blue emission — we simply don’t have enough data.
Why not? It turns out we had some bad luck with GW170817: a glitch in one of the detectors slowed down localization of the source, preventing earlier discovery of the kilonova. The net result was that the electromagnetic signal of this merger was only found 11 hours after the gravitational waves were detected — and the ultraviolet signal was detected 4 hours after that, when the kilonova light curves are already decaying.
If we had ultraviolet observations that tracked the earlier, rising emission, Arcavi argues, we would be able to differentiate between the different emission models for the kilonova. So while this may be the best we can do with GW170817, we can hope that with the next merger we’ll have a full set of early observations — allowing us to better understand where its emission comes from.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
Composite optical and infrared image of Milky Way star-forming region S106. Tracing the magnetic fields threaded through star-forming regions like this one can help us learn more about how stars form. [NASA/ESA/Hubble Heritage Team (STScI/AURA)/Subaru Telescope (NAOJ)]
NASA/ESA Hubble Telescope
NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA,4,207 m (13,802 ft) above sea level
Deep within giant molecular clouds, hidden by dense gas and dust, stars form. Unprecedented data from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal the intricate magnetic structures woven throughout one of the most massive star-forming regions in the Milky Way.
ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres
How Stars Are Born
The Horsehead Nebula’s dense column of gas and dust is opaque to visible light, but this infrared image reveals the young stars hidden in the dust. [NASA/ESA/Hubble Heritage Team]
Simple theory dictates that when a dense clump of molecular gas becomes massive enough that its self-gravity overwhelms the thermal pressure of the cloud, the gas collapses and forms a star. In reality, however, star formation is more complicated than a simple give and take between gravity and pressure. The dusty molecular gas in stellar nurseries is permeated with magnetic fields, which are thought to impede the inward pull of gravity and slow the rate of star formation.
How can we learn about the magnetic fields of distant objects? One way is by measuring dust polarization. An elongated dust grain will tend to align itself with its short axis parallel to the direction of the magnetic field. This systematic alignment of the dust grains along the magnetic field lines polarizes the dust grains’ emission perpendicular to the local magnetic field. This allows us to infer the direction of the magnetic field from the direction of polarization.
Tracing Magnetic Fields
Magnetic field orientations for protostars e2 and e8 derived from Submillimeter Array observations (panels a through c) and ALMA observations (panels d and e). [Adapted from Koch et al. 2018]
CfA Submillimeter Array Mauna Kea, Hawaii, USA,4,207 m (13,802 ft) above sea level
Patrick Koch (Academia Sinica, Taiwan) and collaborators used high-sensitivity ALMA observations of dust polarization to learn more about the magnetic field morphology of Milky Way star-forming region W51. W51 is one of the largest star-forming regions in our galaxy, home to high-mass protostars e2, e8, and North.
The ALMA observations reveal polarized emission toward all three sources. By extracting the magnetic field orientations from the polarization vectors, Koch and collaborators found that the molecular cloud contains an ordered magnetic field with never-before-seen structures. Several small clumps on the perimeter of the massive star-forming cores exhibit comet-shaped magnetic field structures, which could indicate that these smaller cores are being pulled toward the more massive cores.
These findings hint that the magnetic field structure can tell us about the flow of material within star-forming regions — key to understanding the nature of star formation itself.
Guiding Star Formation
Maps of sin ω for two of the protostars (e2 and e8) and their surroundings. [Adapted from Koch et al. 2018]
Do the magnetic fields in W51 help or hinder star formation? To explore this question, Koch and collaborators introduced the quantity sin ω, where ω is the angle between the local gravity and the local magnetic field.
When the angle between gravity and the magnetic field is small (sin ω ~ 0), the magnetic field has little effect on the collapse of the cloud. If gravity and the magnetic field are perpendicular (sin ω ~ 1), gravity can slow the infall of gas and inhibit star formation.
Based on this parameter, Koch and collaborators identified narrow channels where gravity acts unimpeded by the magnetic field. These magnetic channels may funnel gas toward the dense cores and aid the star-formation process.
The authors’ observations demonstrate just one example of the broad realm ALMA’s polarimetry capabilities have opened to discovery. These and future observations of dust polarization will continue to reveal more about the delicate magnetic structure within molecular clouds, further illuminating the role that magnetic fields play in star formation.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
Artist’s illustration of a galaxy with bent jets in a galaxy cluster. A new study finds that more such bent-jet galaxies exist closer to cluster centers. [NASA/JPL-Caltech].
Powerful jets emitted from the centers of distant galaxies make for spectacular signposts in the radio sky. Can observations of these jets reveal information about the environments that surround them?
Signposts in the Sky
VLA FIRST images of seven bent double-lobed radio galaxies from the authors’ sample. [Adapted from Silverstein et al. 2018]
NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)
An active supermassive black hole lurking in a galactic center can put on quite a show! These beasts fling out accreting material, often forming intense jets that punch their way out of their host galaxies. As the jets propagate, they expand into large lobes of radio emission that we can spot from Earth — observable signs of the connection between distant supermassive black holes and the galaxies in which they live.
These distinctive double-lobed radio galaxies (DLRGs) don’t all look the same. In particular, though the jets are emitted from the black hole’s two poles, the lobes of DLRGs don’t always extend perfectly in opposite directions; often, the jets become bent on larger scales, appearing to us to subtend angles of less than 180 degrees.
Can we use our observations of DLRG shapes and distributions to learn about their surroundings? A new study led by Ezekiel Silverstein (University of Michigan) has addressed this question by exploring DLRGs living in dense galaxy-cluster environments.
Living Near the Hub
Projected density of DLRG–central galaxy matches (black) compared to a control sample of random positions–central galaxy matches (red) for different distances from a cluster center. DLRGs have a higher likelihood of being located close to a cluster center. [Silverstein et al. 2018]
To build a sample of DLRGs in dense environments, Silverstein and collaborators started from a large catalog of DLRGs in Sloan Digital Sky Survey quasars with radio lobes visible in Very Large Array data. They then cross-matched these against three galaxy catalogs to produce a sample of 44 DLRGs that are each paired to a nearby massive galaxy, galaxy group, or galaxy cluster.
To determine if these DLRGs’ locations are unusual, the authors next constructed a control sample of random galaxies using the same selection biases as their DLRG sample.
Silverstein and collaborators found that the density of DLRGs as a function of distance from a cluster center drops off more rapidly than the density of galaxies in a typical cluster. Observed DLRGs are therefore more likely than random galaxies to be found near galaxy groups and clusters. The authors speculate that this may be a selection effect: DLRGs further from cluster centers may be less bright, preventing their detection.
Bent Under Pressure
The angle subtended by the DLRG radio lobes, plotted against the distance of the DLRG to the cluster center. Central galaxies (red circle) experience different physics and are therefore excluded from the sample. In the remaining sample, bent DLRGs appear to favor cluster centers, compared to unbent DLRGs. [Silverstein et al. 2018]
In addition, Silverstein and collaborators found that location appears to affect the shape of a DLRG. “Bent” DLRGs (those with a measured angle between their lobes of 170° or smaller) are more likely to be found near a cluster center than “unbent” DLRGs (those with angles of 170°–180°). The fraction of bent DLRGs is 78% within 3 million light-years of the cluster center, and 56% within double that distance — compared to a typical fraction of just 29% in the field.
These results support the idea that ram pressure — the pressure experienced by a galaxy as it moves through the higher density environment closer to the center of a cluster — is what bends the DLRGs.
What’s next to learn? This study relies on a fairly small sample, so Silverstein and collaborators hope that future deep optical surveys will increase the completeness of cluster catalogs, enabling further testing of these outcomes and the exploration of other physics of galaxy-cluster environments.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
Hubble image of the densely-packed stars in the nuclear star cluster at the center of the Milky Way. [NASA, ESA, and Hubble Heritage Team (STScI/AURA, Acknowledgment: T. Do, A.Ghez (UCLA), V. Bajaj (STScI)].
NASA/ESA Hubble Telescope
Andrea Ghez, UCLA Galactic Center Group
Far from the galactic suburbs where the Sun resides, a cluster of stars in the nucleus of the Milky Way orbits a supermassive black hole. Can chemical abundance measurements help us understand the formation history of the galactic center nuclear star cluster?
Studying Stellar Populations
Metallicity distributions for stars in the inner two degrees of the Milky Way (blue) and the central parsec (orange). [Do et al. 2018]
While many galaxies host nuclear star clusters, most are too distant for us to study in detail; only in the Milky Way can we resolve individual stars within one parsec of a supermassive black hole. The nucleus of our galaxy is an exotic and dangerous place, and it’s not yet clear how these stars came to be where they are — were they siphoned off from other parts of the galaxy, or did they form in place, in an environment rocked by tidal forces?
Studying the chemical abundances of stars provides a way to separate distinct stellar populations and discern when and where these stars formed. Previous studies using medium-resolution spectroscopy have revealed that many stars within the central parsec of our galaxy have very high metallicities — possibly higher than any other region of the Milky Way. Can high-resolution spectroscopy tell us more about this unusual population of stars?
Comparison of the observed spectra of the two galactic center stars (black) with synthetic spectra with low (blue) and high (orange) [Sc/Fe] values.[Do et al. 2018]
In order to constrain the metallicities of these stars, Do and collaborators compared the observed spectra to a grid of synthetic spectra and used a spectral synthesis technique to determine the abundances of individual elements. They found that while one star is only slightly above solar metallicity, the other is likely more than four times as metal-rich as the Sun.
The features in the observed and synthetic spectra generally matched well, but the absorption lines of scandium, vanadium, and yttrium were consistently stronger in the observed spectra than in the synthetic spectra. This led the authors to conclude that these galactic center stars are unusually rich in these metals — trace elements that could reveal the formation history of the galactic nucleus.
Old Stars, New Trends?
Scandium to iron ratio versus iron abundance for stars in the disk of the Milky Way (blue) and the stars in this sample (orange). The value reported for this sample is a 95% lower limit. [Do et al. 2018]
For stars in the disk of the Milky Way, the abundance of scandium relative to iron tends to decrease as the overall metallicity increases, but the stars investigated in this study are both iron-rich and anomalously high in scandium. This hints that the nuclear star cluster might represent a distinct stellar population with different metallicity trends.
However, it’s not yet clear what could cause the elevated abundances of scandium, vanadium, and yttrium relative to other metals. Each of these elements is linked to a different source; scandium and vanadium are mainly produced in Type II and Type Ia supernovae, respectively, while yttrium is likely synthesized in asymptotic giant branch stars. Future observations of stars near the center of the Milky Way may help answer this question and further constrain the origin of our galaxy’s nuclear star cluster.
Discovery of Low-metallicity Stars in the Central Parsec of the Milky Way doi: 10.1088/0004-637X/809/2/143
Detailed Abundances for the Old Population near the Galactic Center. I. Metallicity Distribution of the Nuclear Star Cluster doi: 10.3847/1538-3881/aa970a
Detailed Abundance Analysis of a Metal-poor Giant in the Galactic Center doi: 10.3847/0004-637X/831/1/40
Chemical Evolution of the Inner 2 Degrees of the Milky Way Bulge: [α/Fe] Trends and Metallicity Gradients doi: 10.3847/0004-6256/151/1/1
Abundances, Stellar Parameters, and Spectra from the SDSS-III/APOGEE Survey doi: 10.1088/0004-6256/150/5/148
Chemical Abundances of Luminous Cool Stars in the Galactic Center from High-Resolution Infrared Spectroscopy doi: 10.1086/521813
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
Artist’s impression of the black hole binary system in NGC 3201. [ESO/L. Calçada/spaceengine.org]
How many black holes lurk within the dense environments of globular clusters, and how do these powerful objects shape the properties of the cluster around them? One such cluster, NGC 3201, is now helping us to answer these questions.
Hunting Stellar-Mass Black Holes
Since the detection of merging black-hole binaries by the Laser Interferometer Gravitational-Wave Observatory (LIGO), the dense environments of globular clusters have received increasing attention as potential birthplaces of these compact binary systems.
The central region of the globular star cluster NGC 3201, as viewed by Hubble. The black hole is in orbit with the star marked by the blue circle. [NASA/ESA]
In addition, more and more stellar-mass black-hole candidates have been observed within globular clusters, lurking in binary pairs with luminous, non-compact companions. The most recent of these detections, found in the globular cluster NGC 3201, stands alone as the first stellar-mass black hole candidate discovered via radial velocity observations: the black hole’s main-sequence companion gave away its presence via a telltale wobble.
Now a team of scientists led by Kyle Kremer (CIERA and Northwestern University) is using models of this system to better understand the impact that black holes might have on their host clusters.
A Model Cluster
The relationship between black holes and their host clusters is complicated. Though the cluster environment can determine the dynamical evolution of the black holes, the retention rate of black holes in a globular cluster (i.e., how many remain in the cluster when they are born as supernovae, rather than being kicked out during the explosion) influences how the host cluster evolves.
Kremer and collaborators track this complex relationship by modeling the evolution of a cluster similar to NGC 3201 with a Monte Carlo code. The code incorporates physics relevant to the evolution of black holes and black-hole binaries in globular clusters, such as two-body relaxation, single and binary star evolution, galactic tides, and multi-body encounters. From their grid of models with varying input parameters, the authors then determine which fit best to NGC 3201’s final observational properties.
Surface brightness profiles for all globular-cluster models at late times compared to observations of NGC 3201 (yellow circles). Blue lines represent models with few retained black holes; black lines represent models with many retained black holes. [Kremer et al. 2018]
Retention Matters
Kremer and collaborators find that the models that best represent NGC 3201 all retain more than 200 black holes at the end of the simulation; models that lost too many black holes due to natal kicks did not match observations of NGC 3201 as well. The models with large numbers of retained black holes also harbored binaries just like the one recently detected in NGC 3201.
Models that retain few black holes, on the other hand, may instead be good descriptions of so-called “core-collapsed” globular clusters observed in the Milky Way. The authors demonstrate that these clusters could contain black holes in binaries with stars known as blue stragglers, which may also be detectable with radial velocity techniques.
Kremer and collaborators’ results suggest that globular clusters similar to NGC 3201 contain hundreds of invisible black holes waiting to be discovered, and they indicate some of the differences in cluster properties caused by hosting such a large population of black holes. We can hope that future observations and modeling will continue to illuminate the complicated relationship between globular clusters and the black holes that live in them.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
Artist’s illustration demonstrating the distance traveled by Voyager 1. [NASA/JPL.]
In 2012, Voyager 1 zipped across the heliopause. Five and a half years later, Voyager 2 still hasn’t followed its twin into interstellar space. Can models of the heliopause location help determine why?
NASA/Voyager 1
NASA/Voyager 2
Artist’s conception of the heliosphere with the important structures and boundaries labeled. [NASA/Goddard/Walt Feimer.]
How Far to the Heliopause?
As our solar system travels through the galaxy, the solar outflow pushes against the surrounding interstellar medium, forming a bubble called the heliosphere. The edge of this bubble, the heliopause, is the outermost boundary of our solar system, where the solar wind and the interstellar medium meet. Since the solar outflow is highly variable, the heliopause is constantly moving — with the motion driven by changes in the Sun.
NASA’s twin Voyager spacecraft were poised to cross the heliopause after completing their tour of the outer planets in the 1980s. In 2012, Voyager 1 registered a sharp increase in the density of interstellar particles, indicating that the spacecraft had passed out of the heliosphere and into the interstellar medium. The slower-moving Voyager 2 was set to pierce the heliopause along a different trajectory, but so far no measurements have shown that the spacecraft has bid farewell to our solar system.
In a recent study, a team of scientists led by Haruichi Washimi (Kyushu University, Japan and CSPAR, University of Alabama-Huntsville) argues that models of the heliosphere can help explain this behavior. Because the heliopause location is controlled by factors that vary on many spatial and temporal scales, Washimi and collaborators turn to three-dimensional, time-dependent magnetohydrodynamics simulations of the heliosphere. In particular, they investigate how the position of the heliopause along the trajectories of Voyager 1 and Voyager 2 changes over time.
Modeled location of the heliopause along the paths of Voyagers 1 (blue) and 2 (orange). Click for a closer look. The red star indicates the location at which Voyager 1 crossed the heliopause. The current location of Voyager 2 is marked with a red circle. [Washimi et al. 2017]
A Time-Varying Barrier
The authors consider the impact that solar flares, coronal mass ejections, and other disturbances in the solar outflow have on the heliopause distance. These solar disturbances intermingle as they travel outward to form what the authors call global merged interaction regions.
Using their hydrodynamical simulations, Washimi and collaborators capture the complex behavior of the global merged interaction regions as they propagate through the termination shock and collide with the heliopause. Part of the shock is transmitted into the local interstellar medium, while part of it is reflected back toward and collides with the termination shock, which is pushed toward the Sun. This complex interplay of transmitted and reflected shocks — combined with the nonuniformity of the local interstellar medium — causes the heliopause location to vary dramatically in time as well as space.
What Does this Mean for Voyager 2?
Washimi and collaborators find that the location of the heliopause along the trajectories of Voyagers 1 and 2 has changed considerably over the past decade. In particular, they find that the heliopause has been pushed outward over the past few years due to an increase in the solar wind ram pressure. According to their simulations, Voyager 2 is currently traveling outward faster than the heliopause is advancing, which means that the spacecraft should soon cross the boundary — perhaps even this year — to become Earth’s second interstellar messenger.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
Hubble image of the Cassiopeia A supernova remnant. Studies of this and other supernova remnants have revealed that complex structure and asymmetries are common in stellar remains. [NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]
NASA/ESA Hubble Telescope
Supernova explosions enrich the interstellar medium and can even briefly outshine their host galaxies. However, the mechanism behind these massive explosions still isn’t fully understood. Could probing the asymmetry of supernova remnants help us better understand what drives these explosions?
Hubble image of the remnant of SN 1987A, one of the first remnants discovered to be asymmetrical. [ESA/Hubble, NASA]
Stellar Send-Offs
High-mass stars end their lives spectacularly. Each supernova explosion churns the interstellar medium and unleashes high-energy radiation and swarms of neutrinos. Supernovae also suffuse the surrounding interstellar medium with heavy elements that are incorporated into later generations of stars and the planets that form around them.
The bubbles of expanding gas these explosions leave behind often appear roughly spherical, but mounting evidence suggests that many supernova remnants are asymmetrical. While asymmetry in supernova remnants can arise when the expanding material plows into the non-uniform interstellar medium, it can also be an intrinsic feature of the explosion itself.
Simulation results clockwise from top left: Mass density, calcium mass fraction, oxygen mass fraction, nickel-56 mass fraction. Click to enlarge. [Adapted from Wollaeger et al. 2017]
Coding Explosions
The presence — or absence — of asymmetry in a supernova remnant can hold clues as to what drove the explosion. But how can we best observe asymmetry in a supernova remnant? Modeling lets us explore different observational approaches.
A team of scientists led by Ryan T. Wollaeger (Los Alamos National Laboratory) used radiative transfer and radiative hydrodynamics simulations to model the explosion of a core-collapse supernova. Wollaeger and collaborators introduced asymmetry into the explosion by creating a single-lobed, fast-moving outflow along one axis.
Their simulations showed that while some chemical elements lingered near the origin of the explosion or were distributed evenly throughout the remnant, calcium was isolated to the asymmetrical region, hinting that spectral lines of calcium may be good tracers of asymmetry.
Bolometric (top) and gamma-ray (bottom) synthetic light curves for the authors’ model for a range of simulated viewing angles. [Adapted from Wollaeger et al. 2017]
Synthesizing Spectra
Wollaeger and collaborators then generated synthetic light curves and spectra from their models to determine which spectral features or characteristics indicated the presence of the asymmetric outflow lobe. They found that when an asymmetric outflow lobe is present, the peak luminosity of the explosion depends on the angle at which you view it; the highest luminosity occurs when the lobe is viewed from the side, while the lowest luminosity — nearly 40% dimmer — is seen when the explosion is viewed “down the barrel” of the lobe. The dense outflow shades the central radioactive source from view, lowering the luminosity.
This effect also plays out in the gamma-ray light curves; when viewed down the barrel, the shading of the central source by a high-density lobe slows the rise of the gamma-ray luminosity and changes the shape of the light curve compared to views from other vantage points.
Another promising avenue for exploring asymmetry is a near-infrared band encompassing an emission line of singly-ionized calcium near 815 nm. Since calcium is confined within the outflow lobe in the simulation, its emission lines are blueshifted when the lobe points toward the observer.
The authors point out that there is much more to be done in their models, such as including the effects of shock heating of circumstellar material, which can contribute strongly to the light curve, but these simulations bring us a step closer to understanding the nature of asymmetrical supernova remnants — and the explosions that create them.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
Artist’s illustration of the final stages of a neutron-star merger. Scientists have now caught a binary-neutron-star system about 46 million years before this stage. NASA/Goddard Space Flight Center.
Got any plans in 46 million years? If not, you should keep an eye out for PSR J1946+2052 around that time — this upcoming merger of two neutron stars promises to be an exciting show!
Survey Success
Average profile for PSR J1946+2052 at 1.43 GHz from a 2 hr observation from the Arecibo Observatory. Stovall et al. 2018
It seems like we just wrote about the dearth of known double-neutron-star systems, and about how new surveys are doing their best to find more of these compact binaries. Observing these systems improves our knowledge of how pairs of evolved stars behave before they eventually spiral in, merge, and emit gravitational waves that detectors like the Laser Interferometer Gravitational-wave Observatory might observe.
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Caltech/MIT Advanced aLigo Hanford, WA, USA installation
Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project
Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib
ESA/eLISA the future of gravitational wave research
Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)
Today’s study, led by Kevin Stovall (National Radio Astronomy Observatory), goes to show that these surveys are doing a great job so far! Yet another double-neutron-star binary, PSR J1946+2052, has now been discovered as part of the Arecibo L-Band Feed Array pulsar (PALFA) survey. This one is especially unique due to the incredible speed with which these neutron stars orbit each other and their correspondingly (relatively!) short timescale for merger.
An Extreme Example
The PALFA survey, conducted with the enormous 305-meter radio dish at Arecibo, has thus far resulted in the discovery of 180 pulsars — including two double-neutron-star systems.
NAIC/Arecibo Observatory, Puerto Rico, USA, at 497 m (1,631 ft) , built into the landscape at Arecibo, Puerto Rico. NOAO/AURA/NSF/H. Schweiker/WIYN
The most recent discovery by Stovall and collaborators brings that number up to three, for a grand total of 16 binary-neutron-star systems (confirmed and unconfirmed) known to date.
The newest binary in this collection, PSR J1946+2052, exhibits a pulsar with a 17-millisecond spin period that whips around its compact companion at a terrifying rate: the binary period is just 1.88 hours. Follow-up observations with the Jansky Very Large Array and other telescopes allowed the team to identify the binary’s location to high precision and establish additional parameters of the system.
PSR J1946+2052 is a system of extremes. The binary’s total mass is found to be ~2.5 solar masses, placing it among the lightest binary-neutron-star systems known. Its orbital period is the shortest we’ve observed, and the two neutron stars are on track to merge in less time than any other known neutron-star binaries: in just 46 million years. When the two stars reach the final stages of their merger, the effects of the pulsar’s rapid spin on the gravitational-wave signal will be the largest of any such system discovered to date.
More Tests of General Relativity
What can PSR J1946+2052 do for us? This extreme system will be especially useful as a gravitational laboratory. Continued observations of PSR J1946+2052 will pin down with unprecedented precision parameters like the Einstein delay and the rate of decay of the binary’s orbit due to the emission of gravitational waves, testing the predictions of general relativity to an order of magnitude higher precision than was possible before.
As we expect there to be thousands of systems like PSR J1946+2052 in our galaxy alone, better understanding this binary — and finding more like it — continue to be important steps toward interpreting compact-object merger observations in the future.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
The Cygnus-X star-forming region, as imaged by the Spitzer space telescope. This region has recently been discovered to host over 500 outflows from young, newly forming stars. [NASA/JPL-Caltech/Harvard-Smithsonian CfA.]
NASA/Spitzer Infrared Telescope
How do you spot very young, newly formed stars? One giveaway is the presence of jets and outflows that interact with the stars’ environments. In a new study, scientists have now discovered an unprecedented number of these outflows in a nearby star-forming region of our galaxy.
CO map of the Cygnus-X region of the galactic plane, with the grid showing the UWISH2 coverage and the black triangles showing the positions of the detected outflows. [Makin & Froebrich 2018.]
Young Stars Hard at Work
The birth and evolution of young stars is a dynamic, energetic process. As new stars form, material falls inward from the accretion disks surrounding young stellar objects, or YSOs. This material can power collimated streams of gas and dust that flow out along the stars’ rotation axes, plowing through the surrounding material. Where the outflows collide with the outside environment, shocks form that can be spotted in near-infrared hydrogen emission.
Though we’ve learned a lot about these outflows, there remain a number of open questions. What factors govern their properties, such as their lengths, luminosities, and orientations? What is the origin of the emission features we see within the jets, known as knots? What roles do the driving sources and the environments play in the behavior and appearance of the jets?
A selection of previously unknown outflows discovered as a result of this survey. [Makin & Froebrich 2018.]
Jackpot in Cygnus-X
In a recent publication, Sally Makin and Dirk Froebrich (University of Kent, UK), present results from UWISH2’s latest release: a survey segment targeting a 42-square-degree region in the galactic plane known as the Cygnus-X star-forming region.
The team’s search for shock-excited emission in Cygnus-X yielded spectacular results. They found a treasure trove of outflows — a remarkable 572 in total, representing a huge increase over the 107 known previously.
Makin and Froebrich then measured properties of the outflows themselves — such as length, orientation, and flux — as well as properties of the sources that appear to drive them.
This low-mass bright-rimmed cloud near IRAS 20294+4255 contains a number of stellar outflows. It may warrant further study as a classical example of triggered star formation. [Makin & Froebrich 2018.]
Pinning Down Properties
Of the 572 outflows, the authors found that 27% are one-sided jets and 46% are bipolar. The bipolar outflows are typically ~1.5 light-years in total length, and they are frequently asymmetric, with the shorter jet lobe averaging only 70% the length of the longer one. The flux from the two sides of bipolar jets is also often asymmetric: typically one side is brighter by about 50%.
Exploring the knots of bright emission within the outflows, the authors found that they are typically closely spaced, suggesting that the material generating them is ejected every 900–1,400 years. This rapid production — faster than what has been found in YSO outflows in other regions — rules out some models of how these knots are produced.
Based on the fraction of UWISH2 data analyzed so far, the authors estimate that the entire UWISH2 survey will uncover a total of ~2,000 jets and outflows from YSOs. This large, unbiased new sample is finally allowing astronomers to build out the statistics of YSO outflows to better understand them.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.
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