From AAS NOVA: “Featured Image: Hunting for Past Fireworks” NGC 6946

From AAS NOVA

The face-on spiral galaxy NGC 6946 is revealed in the stunning false-color image above (click for the full view), constructed from observations of three emission lines with the WIYN telescope at Kitt Peak Observatory.

NOAO WIYN 3.5 meter telescope at Kitt Peak, AZ, USA, Altitude 2,096 m (6,877 ft)

NGC 6946 is undergoing a major starburst, earning it the moniker “the Fireworks Galaxy”; evidence of its star-forming activity can be seen in the 10 supernovae (labeled in yellow) that have been observed within it since 1917. In a recent study led by Knox Long (Space Telescope Science Institute and Eureka Scientific, Inc.), scientists have undertaken a new optical search of NGC 6946 for supernova remnants — the ghostly remains of past stellar explosions. The resulting data to the right shows that NGC 6946 is positively rich with supernova remnants; green circles indicate remnant candidates previously found in a radio study, and blue circles indicate new remnant candidates discovered in optical in the current study. To learn more, check out the article below.

Supernova remnants in the spiral galaxy NGC 6946. Field is the same as that shown in the full image above. [Long et al. 2019]

Citation

“A New, Larger Sample of Supernova Remnants in NGC 6946,” Knox S. Long et al 2019 ApJ 875 85.
https://iopscience.iop.org/article/10.3847/1538-4357/ab0d94/meta

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: “Compact Objects Charging Toward Merger”

From AAS NOVA

19 April 2019
Susanna Kohler

Artist’s illustration showing two inspiralling neutron stars shortly before they merge. Could electric charge play a role in the radiation we see from compact-binary mergers? [Goddard Media Studios/NASA]

When two compact objects — neutron stars or black holes — merge, will they emit light? A recent study looks at a neglected factor that could affect the answer: electric charge.

Dark or Light?

Gamma-ray burst credit NASA SWIFT Cruz Dewilde

Most theories agree that a compact binary containing a neutron star can emit light when it merges. This is because these systems contain lots of neutron-rich matter that can then radiate in the final stages of merger, in the form of gamma-ray bursts, kilonovae, and afterglows.

But what about compact binaries containing two black holes? Or so-called “plunging” black-hole–neutron-star mergers in which the neutron star plunges directly into the black hole before it can be disrupted? Are these mergers all doomed to darkness?

Possible Charge

Not according to Bing Zhang, a scientist at University of Nevada Las Vegas. Recently, Zhang proposed [The Astrophysical Journal Letters] that black holes might carry electric charge in a surrounding magnetosphere. As charged black holes spiral around and around each other during a merger, they could generate electromagnetic radiation: a characteristic signal that rises sharply just before merger.

Now Zhang is back with a generalized model for the merger of charged compact objects, which also explores possible signatures from electrically charged neutron stars. In a new study, he works out the details and reports on where we might be able to detect these signals.

Searching for a Signal

All compact binaries containing a neutron star should emit radiation from electric charge, since neutron stars are definitely charged — they’re essentially spinning magnets. But for most systems containing a neutron star, Zhang demonstrates, the radiation associated with the object’s charge will be non-detectable, since it’s so much dimmer than other electromagnetic signatures from merger (like a gamma-ray burst).

The Crab pulsar is a highly magnetized, spinning neutron star that powers the Crab nebula seen in this composite image. [X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz]

There’s hope, though, in the scenario of a plunging neutron-star–black-hole merger. If the neutron star is less than 20% the size of the black hole, it can be consumed whole, preventing any of the typical electromagnetic signatures from occurring. In this case, the radiation from the charged, inspiralling neutron star is the only electromagnetic signal present.

If the neutron star in such a system has a magnetic field similar to that of the Crab pulsar — possible in young star clusters — the charge signal can reach detectable levels, according to Zhang’s calculations. In fact, it’s possible that we could observe such a signal as a fast radio burst, the mysterious millisecond radio bursts that we’ve seen originating from beyond our galaxy.

Looking Ahead

Many unknowns are still present in this picture. How is the electric radiation converted into observable emission? How commonly do we expect plunging neutron-star–black-hole mergers to occur as described? Will we be able to link radiation from charged mergers to a gravitational-wave chirp?

One thing is for certain: if we can, indeed, observe the light from charge in a compact-binary merger, this would provide an exciting new opportunity to further probe these distant, exotic systems.

Citation

“Charged Compact Binary Coalescence Signal and Electromagnetic Counterpart of Plunging Black Hole–Neutron Star Mergers,” Bing Zhang 2019 ApJL 873 L9.
https://iopscience.iop.org/article/10.3847/2041-8213/ab0ae8/meta

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: “Exploring Filaments on the Sun”

From AAS NOVA

17 April 2019
Susanna Kohler

This image of the Sun’s chromosphere reveals dark cuts across its surface: solar filaments. A new study explores how these filaments are built. [NOAA/SEL/USAF]

Images of the Sun’s chromosphere often reveal dark threads cutting across the Sun’s face. New research has now explored how these solar filaments are built from magnetic fields and plasma.

Two-Faced Structures

A solar eruptive prominence as seen in extreme UV light on March 30, 2010, with Earth superimposed for a sense of scale. [NASA/SDO]

NASA/SDO

Solar filaments may look like deep cracks in the Sun’s façade, but in reality, they are enormous arcs of hot plasma that extend above the Sun’s surface. Because this plasma is slightly cooler than the solar surface below, they appear dark against the hotter background.

Unfamiliar with filaments? You’ve likely seen plenty of them in images — but from a different angle! Filaments are the same structures as solar prominences, the loops of plasma we can see suspended above the Sun’s limbs. When prominences appear on the side of the Sun facing us, they take the form of filaments from our point of view.

Shaped by Fields

Filaments are often associated with various forms of solar activity. They last for days, frequently hanging above active regions of the Sun; filament channels are often the origin of eruptions from the Sun’s surface. To better understand our active and energetic Sun, understanding the structures of filaments is an important step.

Unfortunately, this is challenging! We know that filament structure is largely due to the magnetic fields — whose forces suspend the filaments against the downward pull of gravity — but we don’t have the ability to directly measure the magnetic field in the Sun’s atmosphere. A team of scientists at the University of Science and Technology of China has instead taken an indirect approach: they explored filaments by looking at the motion of plasma along them.

Top: time-distance map characterizing the oscillations at one position on the filament spine. Bottom: a Doppler map, averaged over time, that shows the rotation around the spine of the filament. Blue indicates motion toward the observer, red away. [Adapted from Awasthi et al. 2019]

A Double Decker?

Scientists Arun Awasthi, Rui Liu, and Yuming Wang examined observations of a filament that appeared near active region AR 12685 in October 2017, captured with the 1-m New Vacuum Solar Telescope in China. The team used these high-resolution images to explore bulk motions of plasma in the filament.

Awasthi and collaborators found that the filament displayed two different types of motion: rotation around its central spine, and longitudinal oscillations along its spine. The longitudinal oscillations in the eastern segment of the filament were distinct from those in the west, suggesting that the magnetic field lines underneath these two segments have different lengths and curvatures.

On the whole, the motions observed in the filament indicate that magnetic structure for filaments is complicated. The authors argue that more than one model is likely at work; they propose a “double-decker” picture for the filament in which a flux rope (a twisted bundle of magnetic field lines) sits on top of a sheared arcade (a series of distorted loops).

Awasthi and collaborators conclude with specific predictions of indicators we can look for in future filament observations to test this model. If correct, this view of filament structure brings us a little closer to understanding the complex magnetic fields that control solar activity.

Citation

“Double-decker Filament Configuration Revealed by Mass Motions,” Arun Kumar Awasthi et al 2019 ApJ 872 109.
https://iopscience.iop.org/article/10.3847/1538-4357/aafdad/meta

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: “NuSTAR Explores the Aftermath of a Supernova”

From AAS NOVA

15 April 2019
Susanna Kohler

Composite view of the supernova remnant W49B, combining X-rays from NASA’s Chandra X-ray Observatory in blue and green, radio data from the NSF’s Very Large Array in pink, and infrared data from Caltech’s Palomar Observatory in yellow. [X-ray: NASA/CXC/MIT/L.Lopez et al.; Infrared: Palomar; Radio: NSF/NRAO/VLA]

NASA/Chandra X-ray Telescope

NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

Caltech Palomar Observatory, located in San Diego County, California, US, at 1,712 m (5,617 ft)

There’s plenty to learn from the skeletons left behind after supernova explosions tear through their surroundings. An X-ray view from space has revealed new details about a particularly extreme supernova remnant.

Unexpected Plasmas

NuSTAR observations showing the spatial distribution of flux that indicates where overionized plasma resides in supernova remnant W49B. The overionized plasma is more highly concentrated on the western side of the remnant. [Yamaguchi et al. 2018]

NASA/DTU/ASI NuSTAR X-ray telescope

When some stars explode as powerful supernovae at the end of their lifetimes, they expel material into their surroundings, enriching the galaxy with heavy elements. As this matter is flung outwards at high speeds, it slams into the interstellar medium, generating shocks that heat the gas and ionize it.

We can study the young remnants of supernovae — the structures of gas and dust left behind shortly after these explosions — to learn more about how supernovae interact with the interstellar medium. One type of source is particularly intriguing: very young, hot supernova remnants that are “overionized”.

Overionized plasmas send us mixed signals: their level of ionization is higher than what we expect from the temperature we measure from their electrons. This is most likely an indication that the plasma has recently started cooling very rapidly.

But what might cause this sudden cooling of the remnant? To learn more, we need detailed observations of a young, hot remnant. Luckily, we’ve got an ideal target — and an ideal instrument.

Setting Sights

Supernova remnant W49B is one of the first sources in which we discovered signs of an overionized plasma. It’s the youngest (just ~1,000 years old!), hottest, and most highly ionized among all such objects exhibiting this trait. But W49B’s hot plasma is challenging to observe, and we haven’t yet managed to constrain its detailed high-energy properties.

A powerful telescope is up to the task, however: the Nuclear Spectroscopic Telescope Array (NuSTAR). This versatile space observatory is ideally suited for exploring the spectroscopic details of the X-rays emitted from the hot plasma in W49B.

Sudden Expansion

Top panel: Diagram of the 1’ x 1’ box regions NuSTAR resolved for spectral analysis (overlaid on a color image of W49B from the Wide Field Infrared Camera at Palomar Observatory). Bottom panel: Plot of the electron temperature (bottom) vs. the density (right) measured for each of the labeled regions. [Adapted from Yamaguchi et al. 2018]

Led by Hiroya Yamaguchi (Institute of Space and Astronautical Science, JAXA, Japan), a team of scientists used NuSTAR to capture detailed images and spectroscopy of the W49B remnant.

Yamaguchi and collaborators first confirmed that the overionized plasma is most highly concentrated on the western side of the remnant. They then show that lower electron temperatures — i.e., signs of rapid cooling — are found in the same regions that also have lower density. They measure a gradient from lowest electron temperature and density in the west, to highest in the east.

Taken together with previous observations that reveal that W49B’s surroundings also have lower density on the western side, these results provide strong evidence that the remnant is cooling via adiabatic expansion. In this picture, the supernova blast wave punched through dense circumstellar matter in early stages of the explosion, expanding slowly. Now it’s suddenly breaking out into the lower-density interstellar medium — on the west side first, because it’s not exactly symmetric — leading to sudden expansion and cooling.

Does this explanation apply to overionized plasmas in the skeletons of other, similar supernovae? We’ll need more observations to be sure — but NuSTAR has proven itself up to the task!

Citation

“Evidence for Rapid Adiabatic Cooling as an Origin of the Recombining Plasma in the Supernova Remnant W49B Revealed by NuSTAR Observations,” Hiroya Yamaguchi et al 2018 ApJL 868 L35.
https://iopscience.iop.org/article/10.3847/2041-8213/aaf055/meta

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: “Supernovae, Dark Energy, and the Fate of Our Universe”

From AAS NOVA

5 April 2019
Susanna Kohler

Dark Energy Survey

Dark Energy Camera [DECam], built at FNAL

NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

What’s the eventual fate of our universe? Is spacetime destined to continue to expand forever? Will it fly apart, tearing even atoms into bits? Or will it crunch back in on itself? New results from Dark Energy Survey supernovae address these and other questions.

Uncertain Expansion

The evolution of the scale of our universe. Measurements suggest that the universe is currently expanding, but does dark energy behaves like a cosmological constant, resulting in continued accelerating expansion like now? Or might we instead be headed for a Big Rip or Big Crunch? [NASA/CXC/M. Weiss]

At present, the fabric of our universe is expanding — and not only that, but the its expansion is accelerating. To explain this phenomenon, we invoke what’s known as dark energy — an unknown form of energy that exists everywhere and exerts a negative pressure, driving the expansion.

Since this idea was first proposed, we’ve conducted decades of research to better understand what dark energy is, how much of it there is, and how it influences our universe.

In particular, dark energy’s still-uncertain equation of state determines the universe’s ultimate fate. If the density of dark energy is constant in time, our universe will continue its current accelerating expansion indefinitely. If the density increases in time, the universe will end in the Big Rip — space will expand at an ever-increasing acceleration rate until even atoms fly apart. And if the density decreases in time, the universe will recollapse in the Big Crunch, ending effectively in a reverse Big Bang.

Which of these scenarios is correct? We’re not sure yet. But there’s a project dedicated to finding out: the Dark Energy Survey (DES).

The Hunt for Supernovae

DES was conducted with the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile. After six years taking data, the survey officially wrapped up observations this past January.

One of DES’s several missions was to make detailed measurements of thousands of supernovae. Type Ia supernovae explode with a prescribed absolute brightness, allowing us to determine their distance from observations. DES’s precise measurements of Type Ia supernovae allow us to calculate the expansion of the space between us and the supernovae, probing the properties of dark energy.

Though DES scientists are still in the process of analyzing the tens of terabytes of data generated by the project, they recently released results from the first three years of data — including the first DES cosmology results based on supernovae.

Refined Measurements

Constraints on the dark energy equation of state w from the DES supernova survey. Combining this data with constraints from the cosmic microwave background radiation suggest an equation of state consistent with a constant density of dark energy (w = –1). [Abbott et al. 2019]

Using a sample of 207 spectroscopically confirmed DES supernovae and 122 low-redshift supernovae from the literature, the authors estimate the matter density of a flat universe to be Ωm = 0.321 ± 0.018. This means that only ~32% of the universe’s energy density is matter (the majority of which is dark matter); the remaining ~68% is primarily dark energy.

From their observations, the DES team is also able to provide an estimate for the dark-energy equation of state w, finding that w = –0.978 ± 0.059. This result is consistent with a constant density of dark energy (w = –1), which would mean that our universe will continue to expand with its current acceleration indefinitely.

These results are exciting, but they use only ~10% of the supernovae DES discovered over the span of its 5-year survey. This means that we can expect even further refinements to these measurements in the future, as the DES collaboration analyzes the remaining data!

Citation

“First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: Constraints on Cosmological Parameters,” T. M. C. Abbott et al 2019 ApJL 872 L30.
https://iopscience.iop.org/article/10.3847/2041-8213/ab04fa/meta

Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex Mittelmann Cold creation

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: “The Variable Jets of Gamma-Ray Bursts”

From AAS NOVA

3 April 2019
Susanna Kohler

This artist’s illustration shows the powerful jets emitted during a short gamma-ray burst. A new study explores the role of a magnetic instability in driving variability of these jets. [NASA/GSFC]

What drives rapid flickering in the jets that are produced in some powerful, high-energy explosions? Recent research explores the role of magnetic fields.

Gamma-ray burst credit NASA SWIFT Cruz Dewilde

Mapping an Explosion

Artist’s illustration of the gamma-ray-burst jet launched during the merger of two neutron stars in 2017. [NSF/LIGO/Sonoma State University/A. Simonnet]

Artist’s iconic conception of two merging black holes similar to those detected by LIGO Credit LIGO-Caltech/MIT/Sonoma State /Aurore Simonnet

Gamma-ray bursts — brief flashes of high-energy emission from beyond our galaxy — have been detected since the 1960s. Though we’ve collected many observations of this explosions through the decades, it’s only recently that new evidence has clarified what causes some gamma-ray bursts.

In 2017, the merger of two neutron stars was observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO), just before the detection of a short (less than ~2 seconds) gamma-ray burst from the same location. These observations support the following picture for short gamma-ray bursts:

A neutron-star–neutron-star binary or a neutron-star–black-hole binary merges, generating a potentially observable gravitational-wave signal.
The merger either immediately produces a black hole, or it produces a hypermassive neutron star that collapses into a black hole shortly thereafter.
The remnant material surrounding the newly formed black hole then rapidly accretes, leading to the production of powerful jets along the black-hole rotation axis.

Snapshot from one of the authors’ simulations, in which an axisymmetric jet extends in the +z and -z directions. Only half of the jet is shown, since the simulation is axisymmetric. From left to right, panels represent density, energy distribution, magnetization, and speed of the jet. [Sapountzis & Janiuk 2019]

Jetted Mysteries

This picture, while seemingly straightforward, is loaded with uncertainties. In particular, the jets launched in the third step are not well understood. We’re not sure what drives the production of these jets in the first place, and we also don’t know what collimates the jets, causing them to become tightly beamed as they travel, rather than spraying out in all directions.

What’s more, we observe rapid variability within the gamma-ray-burst jets; for short gamma-ray bursts, the timescales for variability are just ten-thousandths to hundredths of a second! What drives this rapid flickering within the jets?

In a recent study, two researchers at the Center for Theoretical Physics of the Polish Academy of Sciences, Konstantinos Sapountzis and Agnieszka Janiuk, explore the role that magnetic fields might have in the launching and properties of short-gamma-ray-burst jets.

Left column: Variability of jet energy at a point inside the jet as a function of time for five of the authors’ models (each shown in a different row). Right column: same as left, but for a zoomed-in time. Vertical dashed lines show the characteristic timescale of the magnetorotational instability, which matches the jet variability well in all but the bottom model. [Sapountzis & Janiuk 2019]

Magnetic Fields at Work

Sapountzis and Janiuk perform a series of general-relativistic, magnetohydrodynamic simulations of a black hole surrounded by a torus of accreting material.

The authors use these simulations to explore how the magnetic field piles up as hot, ionized gas spirals inward and falls onto the black hole. The building field eventually forms a magnetic barrier that halts the inward flow of gas, leading to the formation of jets along twisting field lines that extend down the black-hole rotation axis.

But the role of the magnetic fields isn’t over with the launch of the jet. In the authors’ simulations, they observe a magnetic instability in the accreting plasma — known as the magnetorotational instability — operating on similar timescales to the variability in gamma-ray-burst jets. This suggests a link between the activity of magnetic fields at the base of the jet and the flickering we observe in the brief gamma-ray-burst jets.

We still have a lot to learn about gamma-ray bursts — and we can hope that future observations, especially now that LIGO is back online, will shed more light on these explosions! It certainly seems clear, however, that magnetic fields have an important role to play.
Citation

“The MRI Imprint on the Short-GRB Jets,” Konstantinos Sapountzis and Agnieszka Janiuk 2019 ApJ 873 12.
https://iopscience.iop.org/article/10.3847/1538-4357/ab0107/meta

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: “A Rare Double-Detonation Supernova Caught in the Act”

From AAS NOVA

29 March 2019
Kerry Hensley

This representative-color X-ray and infrared image shows supernova remnant G299, which is all that’s left after a massive explosion roughly 4,500 years ago. Like the supernova studied in today’s paper, G299 met its end when a white dwarf underwent a thermonuclear detonation. [X-ray: NASA/CXC/U.Texas/S.Post et al, Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF]

NASA/Chandra X-ray Telescope

Caltech 2MASS Telescopes, a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center (IPAC) at Caltech, at the Whipple Observatory on Mt. Hopkins south of Tucson, AZ, Altitude 2,606 m (8,550 ft) and at the Cerro Tololo Inter-American Observatory at an altitude of 2200 meters near La Serena, Chile.

There’s more than just one way for a star to explode. Supernovae — perhaps the most dramatic form of star death — come in many flavors, and astronomers are still learning about the vast diversity of these stellar explosions.

When Stars Steal Mass

This artist’s rendering depicts one kind of Type Ia supernova mechanism: the singly degenerate model, in which a white dwarf siphons mass from its companion, exceeds the Chandrasekhar mass, and explodes. [NASA/CXC/M. Weiss]

When a white dwarf accretes gas from a binary companion and gains enough mass to exceed the Chandrasekhar limit, it can ignite in a cataclysmic explosion. This is the typical scenario for a Type Ia supernova, a common curtain call for low- to intermediate-mass stars in binary systems.

However, this isn’t the only way a Type Ia supernova can happen. In the double-detonation model, the explosion of the white dwarf is triggered by the ignition of an accreted helium shell. In this case, the white dwarf can be far less massive than the Chandrasekhar limit, leading to unexpectedly dim explosions.

Past studies have explored the minimum helium shell mass necessary (~0.01 solar mass) for this process and found that helium-shell detonations can efficiently cause core detonations, but there’s still plenty we don’t know about these events. The best way to learn about supernovae — double-detonation or otherwise — is to spot them soon after they happen.

A comparison of ZTF 18aaqeasu’s optical light curve (red circles) to normal (orange hexagons) and sub-luminous Type Ia supernovae. [Adapted from De et al. 2019]

A Survey Spies a Supernova

In May 2018, an unusual supernova was detected by the Zwicky Transient Facility, an optical survey that hunts for fleeting events like stellar flares, fast-rotating asteroids, and the visible-light counterparts of gravitational-wave events.

Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Courtesy Caltech Optical Observatories

Edwin Hubble at Caltech Palomar Samuel Oschin 48 inch Telescope, (credit: Emilio Segre Visual Archives/AIP/SPL)

Caltech Palomar Samuel Oschin 48 inch Telescope, located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

Within days of its detection, a team led by Kishalay De (Caltech) began to collect photometric observations and spectra of the object.

The photometry revealed that the object, ZTF 18aaqeasu, was unusually red and less luminous than a typical Type Ia supernova, making it a good candidate for the double-detonation scenario.

Its spectra were unusual even for a sub-luminous supernova, taking much longer to develop the silicon absorption feature typically seen in this type of event. Even stranger, the spectra exhibited a never-before-seen cutoff in the flux at short wavelengths, likely due to the presence of metals like iron and titanium.

Comparison of observed spectra (black) to helium-shell double-detonation models (green and orange). [Adapted from De et al. 2019]

An Unusual Event

In order to derive the properties of ZTF 18aaqeasu, De and collaborators compared their photometric and spectroscopic data to models, finding that the event was likely caused by the ignition of a 0.15 solar mass helium shell, which led to the explosion of a 0.76 solar mass white dwarf.

The combination of a massive helium shell with a low-mass white dwarf makes ZTF 18aaqeasu unique among Type Ia supernovae; SN 2016jhr (one of the only supernovae previously linked to a helium-shell detonation event) featured a much more massive white dwarf with a less massive helium shell.

Can we expect to find more supernovae like ZTF 18aaqeasu? Similarly luminous supernovae should be detectable out to about 1.3 billion light-years, but so far there have been none reported with similar spectral features and unusually red color. This may indicate that double-detonation events featuring massive helium shells might be rare — adding an elusive new member to the Type Ia supernova family.

Citation

“ZTF 18aaqeasu (SN2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar-mass White Dwarf,” Kishalay De et al 2019 ApJL 873 L18.
https://iopscience.iop.org/article/10.3847/2041-8213/ab0aec

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: “Merging Eccentric Pairs of Black Holes”

From AAS NOVA

27 March 2019
Susanna Kohler

This scene from a computer simulation shows the dense, chaotic center of a stellar cluster. What happens when black-hole binaries encounter each other in this extreme environment? [Carl Rodriguez/Northwestern Visualization (Justin Muir, Matt McCrory, Michael Lannum)]

The dense, chaotic centers of star clusters may be a birthplace for binary pairs of black holes like those observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

A Question of Origin

Since the discovery of the first gravitational-wave signal in September 2015, LIGO and its European counterpart Virgo have detected nine more merging black-hole binaries. After a brief pause for upgrades, the detectors are slated to come back online in April with significantly improved sensitivities — promising many more detections to come.

A new study now explores how eccentric binaries might arise and merge in these extreme environments.

The ten black-hole mergers detected thus far by LIGO/Virgo.[Teresita Ramirez/Geoffrey Lovelace/SXS Collaboration/LIGO-Virgo Collaboration]

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

Gravity is talking. Lisa will listen. Dialogos of Eide

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

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)

Though the gravitational-wave signals provide a wealth of information about the pre-merger binaries, we haven’t yet been able to determine how these black-hole binaries formed in the first place. Did these pairs evolve in isolation? Or were they born from interactions in the dense centers of star clusters?

One overlooked piece of data might shed light on these questions in the future: eccentricity. Since black-hole binaries in isolation take a long time to merge, any initial eccentricity in the orbit will be damped by gravitational-wave emission by the time the merger happens. But what if the binary doesn’t evolve in isolation? Could we see an imprint of eccentricity on the gravitational-wave signal then?

A new study led by scientist Michael Zevin (Northwestern University and CIERA) explores one possible channel for eccentric mergers: chaotic interactions between multiple black-hole binaries in the centers of star clusters.

Two examples of the complex evolution of binary–binary encounters, both eventually leading to a gravitational-wave capture. An animation of the second example is shown in the video at the end of the post [only available at the full article]. [Adapted from Zevin et al. 2019]

Complex Interactions

Zevin and collaborators use models to explore what happens during strong interactions between pairs of black-hole binaries and between black-hole binaries and single black holes.

These interactions are incredibly complex (don’t believe me? Check out the video below!). Systems with more than two bodies evolve chaotically, with small changes in initial conditions leading to vastly different outcomes. To make matters worse, simple Newtonian physics won’t accurately describe these systems; to capture the effects of gravitational-wave dissipation, we must model these interactions taking general relativity into account.

Zevin and collaborators find that these complexities lead to surprising results. Though binary–binary interactions occur 10–100 times less frequently than binary–single interactions in the centers of globular clusters, the long life and complexity of binary–binary interactions means that they are significantly more likely to result in a gravitational-wave capture — the rapid inspiral and merger of a binary pair, which occurs quickly enough that the pair may still have measurable eccentricity at merger time.

Predicted eccentricity distributions and delay times for three populations of binary–binary produced gravitational-wave mergers. The horizontal black lines show minimum measurable eccentricities predicted for LIGO/Virgo and LISA. Solid colored lines show the eccentricities for the three populations at 10 Hz (LIGO/Virgo’s lower limit) and 0.1 Hz (the most sensitive frequency predicted for LISA). [Zevin et al. 2019]

An Eccentric Result

Predicted eccentricity distributions and delay times for three populations of binary–binary produced gravitational-wave mergers. The horizontal black lines show minimum measurable eccentricities predicted for LIGO/Virgo and LISA. Solid colored lines show the eccentricities for the three populations at 10 Hz (LIGO/Virgo’s lower limit) and 0.1 Hz (the most sensitive frequency predicted for LISA). [Zevin et al. 2019]
The authors demonstrate that binary–binary interactions contribute a significant fraction (~25–45%) of the eccentric mergers that result when black holes strongly interact in cluster centers. But what are our prospects for being able to detect these eccentric collisions?

The outlook is promising! Gravitational-wave captures generally have eccentricities at merger that should be measurable by LIGO/Virgo, and binary–binary-produced mergers that occur later, either in-cluster or after being ejected from the cluster, could have eccentricities detectable by the future Laser Interferometer Space Antenna (LISA). With enough observations, eccentric binaries may soon help us better understand the origin of black-hole pairs.

Citation

“Eccentric Black Hole Mergers in Dense Star Clusters: The Role of Binary–Binary Encounters,” Michael Zevin et al 2019 ApJ 871 91.
https://iopscience.iop.org/article/10.3847/1538-4357/aaf6ec/meta

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: ” Featured Image: A Choice of Projections”

From AAS NOVA

25 March 2019
Susanna Kohler

This unique way of looking at the sky (click for the full view) shows an octahedron-based projection of the 408 MHz all-sky map. Map projections are how we represent a spherical surface like the sky on flat surfaces like sheets of paper and computer monitors. Different projections are designed to meet different goals — and because astronomers use many different projections, large data sets should ideally be stored in a way that can easily be converted to the projection of choice. The Tessellated Octahedral Adaptive Spherical Transformation (TOAST) Projection, demonstrated above, is used by projects like the WorldWide Telescope to store data. In a recent study, a team of scientists led by Thomas McGlynn (NASA Goddard SFC) proposes that octahedron-based projections like this one make for an ideal intermediate representation; from this, it’s easy to convert the data to whatever projection is requested by the user. For a closer look at the fascinating math acrobatics behind projections — and for more awesome images of what those projections look like — check out the article below.

Citation

“Octahedron-based Projections as Intermediate Representations for Computer Imaging: TOAST, TEA, and More,” Thomas McGlynn et al 2019 ApJS 240 22.
https://iopscience.iop.org/article/10.3847/1538-4365/aaf79e/meta

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009

From AAS NOVA: “Flares, Waves, Jets, and Ejections”

From AAS NOVA

22 March 2019
Susanna Kohler

Solar Dynamics Observatory images at 171 Å of a blowout jet erupting in the solar corona on 9 Mar 2011. The dashed white line shows the direction of jet eruption. [SDO/Miao et al. 2018]

NASA/SDO

Our Sun often exhibits a roiling surface full of activity. But how do the different types of eruptions and disturbances we see relate to one another? Observations of one explosive jet are helping us to piece together the puzzle.

Looking for Connections

A coronal blowout jet captured by the Solar Dynamics Observatory on 9 Mar 2011. [Miao et al. 2018]

Energy travels through and from the Sun via dozens of different phenomena. We see ultraviolet waves that propagate across the disk, loops and flares of plasma stretching into space, enormous coronal mass ejections that expel material through the solar system, and jets of all different sizes extending from the Sun’s surface and atmospheric layers. A longstanding mission for solar physicists has been to relate these phenomena into a broader picture explaining how energy is released from our closest star.

Positions of the two STEREO satellites relative to the Sun and the Earth. SDO orbits the Earth. The green arrow shows the eruption direction of the blowout jet. [Miao et al. 2018]

An Enlightening Explosion

On 9 March 2011, a coronal blowout jet erupted from the Sun’s surface. Three spacecraft were on hand to watch: the Solar Dynamics Observatory, STEREO Ahead, and STEREO Behind.

NASA/STEREO spacecraft

These observatories were each located roughly 90° from each other, providing a view of the Sun’s surface from multiple angles at the moment of the explosion.

What did they these observatories see?

The flare
The eruption of the blowout jet — which lasted ~21 minutes — was accompanied by a class 9.4 solar flare.
The wave
Shortly after the jet launch, an arc-shaped extreme ultraviolet (EUV) wave appeared on the southeastern side of the jet. This wave lasted ~4 minutes and propagated away from the site of the jet.
The jet
The jet itself contains both bright and dark material. The dark material appears to be due to a mini-filament — a thread of cool, dense gas suspended above the Sun’s surface by magnetic fields — that erupted in the jet base.
The coronal mass ejection
The two STEREO spacecraft captured what happened on large scales in the outer corona of the Sun, revealing an explosive coronal mass ejection spewing matter into space. The ejection consisted of two structures: a jet-like component and a bubble-like component.

Causal Ties?

STEREO Ahead (left) and Behind (right) images of the coronal mass ejection in the outer corona. Both a jet-like and a bubble-like component can be seen. [Miao et al. 2018]

These observations provide an unprecedented look at multiple types of solar activity all occurring simultaneously — and they suggest causal ties between the different phenomena.

In particular, the authors propose a relation in which the EUV wave was a fast-mode magnetohydrodynamic wave driven by the blowout jet eruption. They also suggest that the jet-like component of the coronal mass ejection is the outer-corona extension of the hot part of the blowout jet body, while the bubble-like component might be associated with the eruption of the mini-filament at the jet base.

More observations like those of this event are needed to draw definitive conclusions, but this explosion has provided some definite clues about the relationship between different phenomena as the Sun lashes out into its surroundings.

Bonus

Watch the propagation of the EUV wave (top video), the eruption of the blowout jet (middle video), and the coronal mass ejections (bottom video) in the clips below. Videos can not be
Copied and presented here. You can view them at the full article.

Citation

“A Blowout Jet Associated with One Obvious Extreme-ultraviolet Wave and One Complicated Coronal Mass Ejection Event,” Y. Miao et al 2018 ApJ 869 39.
https://iopscience.iop.org/article/10.3847/1538-4357/aaeac1/meta

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

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.

Adopted June 7, 2009