From AAS NOVA: “Microwaves from Solar Flares”

From AAS NOVA

15 August 2018
Susanna Kohler

A solar flare erupting on the limb of the Sun (look just below center on the right edge of the Sun in this extreme-ultraviolet image from the Solar Dynamics Observatory) in September 2017 makes for a perfect test case to see what new information we can learn from microwave observations. [NASA/Solar Dynamics Observatory]

NASA/SDO

The Sun is a rather well-studied star, so it’s always exciting when we get the opportunity to observe it in a new way. One such opportunity is upcoming, via the Parker Solar Probe that just launched last week.

NASA Parker Solar Probe Plus

154f8-sol_parkersolarprobe2_nasa

But while we wait for that new view of the Sun, we have another one to examine: the Sun in microwaves.

The spectrum over time for the first ~1 hr of the solar flare SL2017-09-10, shown at different wavelengths. [Gary et al. 2018]

Peering into Flares

In our efforts to better understand solar flares — sudden eruptions that occur when magnetic energy is abruptly released from the Sun, sending a burst of particles and radiation into space — we’ve observed these phenomena across a wide range of wavelengths. One wavelength regime known to be valuable for understanding the physics of solar flares is that of microwaves, which are emitted by high-energy electrons that are accelerated as energy is released in the flare.

But before now, the vast majority of microwave studies of solar flares have relied on data from the Nobeyama Radioheliograph in Japan, which observes the Sun at just two fixed frequencies that lie well above the peak of the microwave spectrum.

Nobeyama Millimeter Array Radioheliograph, located near Minamimaki, Nagano at an elevation of 1350m

Nobeyama Radio Telescope, located in the Nobeyama highlands in Nagano, Japan

This spectral regime explores only regions of high magnetic field strength.

What could we learn about solar flares from the lower-frequency microwaves emitted from more weakly magnetized regions? A newly upgraded array, the Expanded Owens Valley Solar Array (EOVSA) in California, is now helping us to answer this question.

Ten, now 15 antennas of NJIT’s 13-antenna Expanded Owens Valley Solar Array (EOVSA), near Big Pine, California

One of the antennas in the Owens Valley Solar Array. [Dale E. Gary]

An Upgraded Array

After its recent upgrade, which concluded in April 2017, EOVSA now consists of 15 antennas — which produce imaging and spectroscopy data that span the microwave spectrum, including lower microwave frequencies. In a recent study led by Dale Gary (New Jersey Institute of Technology), a team of scientists has presented the first example of microwave imaging spectroscopy from EOVSA, demonstrating the powerful new observations capable with this technology.

As a target to test the array’s capabilities, Gary and collaborators selected a solar flare that occurred on the limb of the Sun — i.e., the edge of its disk, as seen from Earth — in September of 2017: SOL2017-09-10.

EOVSA microwave data plotted in color over a 5’ x 5’ AIA image of the Sun during SOL2017-09-10. RHESSI hard X-ray data is shown in contours. The EOVSA data reveals the presence of high-energy electrons in multiple locations: in small reconnecting loops, well above these bright loops, and, at the north and sourth, associated with the legs of a much larger loop. [Gary et al. 2018]

High-Energy Electrons Everywhere!

High-frequency microwave observations of flares like SOL2017-09-10 had already demonstrated the presence of high-energy electrons in regions of high magnetic fields, like the small closed magnetic loops anchored in the Sun’s surface. But EOVSA’s view of the whole microwave spectrum has revealed that the spatial extent of high-energy electrons is much larger than we thought — these energetic electrons also lie well above the small reconnected loops, in the space between the loops and an erupting rope of magnetic flux associated with the flare.

This discovery indicates the necessity of some amendments to our standard model for the physics of solar flares. Though these early results from EOVSA may be preliminary, they clearly demonstrate the powerful capabilities of this new technology. We can look forward to more new observations of the Sun in the future, continuing to advance our understanding of how energy is released from our nearest star.

Citation

Dale E. Gary et al 2018 ApJ 863 83. http://iopscience.iop.org/article/10.3847/1538-4357/aad0ef/meta

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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

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From AAS NOVA: “Dark Energy Survey Reveals Stellar Streams”

From AAS NOVA

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

Over billions of years, globular clusters and dwarf galaxies orbiting the Milky Way have been torn apart and stretched out by tidal forces. The disruption of these ancient stellar populations results in narrow trails of stars called stellar streams. These stellar streams can help us understand how the Milky Way halo was constructed and what our galaxy’s dark matter distribution is like — but how do we find them?

Along with cosmological simulations, like the Millennium Simulation pictured here, stellar streams can help us understand how dark matter is distributed in galaxies like the Milky Way. [Max Planck Institute for Astrophysics]

On the Trail of Tidal Streams

Understanding how our galaxy came to look the way it does is no easy task. Trying to discern the structure and formation history of the outer reaches of the Milky Way from our vantage point on Earth is a bit like trying to see the forest for the trees — while also trying to learn how old the forest is and where the trees came from!

One way to do so is to search for the stellar streams that form when globular clusters and dwarf galaxies are disrupted and torn apart by our galaxy. Stellar streams tend to be faint, diffuse, and obscured by foreground stars, which makes them tricky to observe. Luckily, recent data releases from the Dark Energy Survey are perfectly suited to the task.

Dark Energy Survey Brings Faint Stars to Light

Nora Shipp (University of Chicago) and collaborators analyzed three years of data from the Dark Energy Survey in search of these stellar streams. The Dark Energy Survey is well-suited for stellar-stream hunts since it covers a wide area (5,000 square degrees of the southern sky) and can observe objects as faint as 26th magnitude.

Shipp and collaborators use a matched-filter technique to pinpoint the old, low-metallicity stars that belong to stellar streams. This method uses the modeled properties of stars of a certain age — synthetic isochrones — to identify stars within a background stellar stream with minimal contamination from foreground stars.

Using their matched filters, the authors found 15 stellar streams, 11 of which had never been seen before. They then estimated the age, metallicity, and distance modulus for each stream — all critical to understanding how the individual streams fit into the larger picture of galactic structure.

A closer look at the stellar streams in the first quadrant of the surveyed area. Top: Density map of stars with a distance modulus of 15.4. Bottom: Stars with a distance modulus of 17.5. [Adapted from Shipp et al. 2018]

Reconstructing the Galactic Halo

These 11 newly discovered stellar streams will greatly enhance our understanding of the history of the galactic halo. Spectroscopy can help clarify the ages of these structures, while kinematic studies can help us understand if and how these structures are associated.

Future work may also help us discern the origin of the streams; the stark dichotomy in the mass-to-light ratios of the stellar streams discovered in this work hints that it may be possible to link some streams to globular clusters and others to dwarf galaxies. Look for this and more exciting results from galactic archaeologists in the future!

Citation

N. Shipp et al 2018 ApJ 862 114. doi:10.3847/1538-4357/aacdab

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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 Imbalances in the Sun’s Magnetic Flux”

From AAS NOVA

The Sun’s activity is driven by strong magnetic field lines — but this orb also has a general magnetic field left over when you cancel out all the regions of opposite flux across its visible disk. What drives the change in this mean magnetic field? [NASA/SDO/AIA/EVE/HMI]

NASA/SDO

EVE on line. There have been plenty of opportunities for citizen scientists to join in the search for exoplanets, but this new opportunity makes a fun game out of it. Literally.
EVE Online players are helping astronomers search for real-world exoplanets.

Solar flares, coronal mass ejections, prominences, sunspots — the most exciting features of the Sun are all driven by complex magnetic activity within the Sun’s interior and at its surface. Indeed, observations of the Sun have long revealed regions of magnetic flux of various magnitudes and polarities across the Sun’s disk.

Sample full-disk image of the Sun, with various features contoured in different colors, including plages (red), enhanced networks (blue); sunspots (green), and active networks (yellow). Panel F shows all contours overplotted on a magnetogram, with the grey regions corresponding to the background field. [Bose & Nagaraju 2018]

But it wasn’t until 1970 that scientists started measuring a different aspect of the Sun’s magnetization: its mean magnetic field. By treating the Sun like a distant star and measuring its net magnetic field by integrating across its whole disk, scientists have been able to effectively measure the imbalance in the magnetic flux of opposite polarities across its visible disk. A new study explores what might create this overall imbalance.

Learning from a Mean Field

The Sun’s mean magnetic field can reveal information about global behavior of our home star, helping us to better understand how magnetic fields form and evolve in stars and providing a deeper understanding of the interplanetary magnetic field that threads the space throughout our solar system.

Scientists have now regularly monitored the Sun’s mean magnetic field variations for more than two full solar cycles, on timescales ranging from a few days to several years, and we’ve learned that the mean field doesn’t stay constant — it can vary from ± 0.2 G to ± 2 G (for reference, a typical refrigerator magnet has a strength of ~50 G).

What we haven’t yet determined is the origin of the Sun’s mean magnetic field: does it come from the Sun’s large-scale, background magnetic field structure? Or is it driven by the magnetic fields of smaller active regions, like sunspots? A new study by scientists Souvik Bose (University of Oslo, Norway) and K. Nagaraju (Indian Institute of Astrophysics) explores these relative contributions.

Active Regions and Backgrounds

By decomposing Solar Dynamics Observatory observations of the Sun’s full disk into different regions, Bose and Nagaraju track the magnetic flux resulting from three categories:

sunspots, dark spots that appear in the Sun’s photosphere as magnetic flux emerges;
plages, enhanced networks, and active networks, surface features created by emergence and dispersion of weaker magnetic fields; and
the Sun’s large-scale magnetic field — i.e., background regions that don’t fall into categories (1) and (2).

They then explore the variability of these fluxes over the span of several years.

Magnetic field variability vs. time, for A) the solar mean magnetic field; B) the background field; C) plages, enhanced networks, and active networks; and D) sunspots. The largest contributor to solar mean magnetic field variability is clearly the large-scale background field. [Bose & Nagaraju 2018]

Bose and Nagaraju’s calculations show that the variation in the solar mean magnetic field most closely tracks that of the background field, and it shows very little correlation with active regions. In particular, about 89% of the variability in the mean solar field is contributed by the background field, with only ~10% contributed by plages and the network field.

The authors point out that their work only indicates the origin of the solar mean magnetic field’s variability; its amplitude may yet be governed by the presence of sunspot activity on the surface of the Sun. Nonetheless, this study brings us a little closer to understanding the complex magnetic activity of our nearest star.

Citation

Souvik Bose and K. Nagaraju 2018 ApJ 862 35.http://iopscience.iop.org/article/10.3847/1538-4357/aaccf1/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: “Growing Black Holes Within Accretion Disks”

From AAS NOVA

1 August 2018
Susanna Kohler

Artist’s illustration of the accretion disk surrounding a supermassive black hole. A new study explores the possibility of stellar-mass black holes living and accreting mass within such disks. [Adapted from ESA/Hubble (M. Kornmesser)]

How can stellar-mass black holes attain the large sizes we’ve recently observed in merging binaries? A recent study explores an intriguing possibility: perhaps these smaller black holes grow and collide while trapped within the accretion disks that can surround enormous supermassive black holes.

The masses of the black holes observed by LIGO in the five confirmed black-hole mergers (and one trigger). Of the ten progenitor black holes, six exceed 20 solar masses. [LIGO/VIRGO]

A Windy Mass Limit

The Laser Interferometer Gravitational-wave Observatory (LIGO) has spotted five confirmed binary black-hole mergers. These mergers have provided us with a wealth of information about stellar-mass black holes, but they’ve also raised a number of new questions. One surprising property of these merging black holes is their mass: of the ten progenitor black holes that we saw colliding, six weighed more than 20 solar masses.

The common picture of how we get stellar-mass black holes is from the evolution of massive stars. But there’s a theoretical limit to the size of a black hole that can be formed this way: massive stars lose a lot of their mass over their lifetimes due to strong stellar winds, which generally constrains them to produce black holes no larger than ~10–15 solar masses. So how did the black holes spotted with LIGO form?

Possible Explanations

One proposed source of larger stellar-mass black holes is low-metallicity stars. High-mass stars with only minimal metals (i.e., elements that aren’t hydrogen or helium) are thought to have been common in the early universe, and these stars don’t lose so much mass via stellar winds. Therefore, low-metallicity, high-mass stars could develop into larger black holes — perhaps like the black holes seen by LIGO.

A recently published study led by Shu-Xu Yi (The University of Hong Kong), however, proposes an alternative explanation for the large black-hole sizes we’ve seen — an explanation that doesn’t rely on low-metallicity environments. Yi and collaborators suggest that the key to attaining large stellar-mass black holes might be the disks of gas and dust that surround supermassive black holes in active galactic nuclei (AGN).

Trapped in a Disk

Distributions of final black-hole masses at coalescence for black-hole binaries that start out at around ~7 solar masses each in the authors’ models. A significant fraction of black holes embedded in AGN disks are able to increase their mass to more than 20 solar masses. [Yi et al. 2018]

The dense centers of galaxies tend to form and accumulate large numbers of stars — and it stands to reason that many of these stars will evolve into stellar-mass black holes. But pairs of black holes that lie in the very heart of the galaxy could easily become trapped in the enormous accretion disk that surrounds the supermassive black hole at the galactic center, feeding and growing larger as they spiral into each other.

Yi and collaborators demonstrate that a pair of black holes, each starting at only ~7 solar masses, can accrete and grow to >20 solar masses before coalescing within a low-mass AGN disk. The gravitational-wave signal from this merger could therefore indicate two black holes of perhaps 20–30 solar masses, despite the fact that the black holes were both initially much smaller.

Could this be the explanation for LIGO’s large detections? Or are low-metallicity stars the more likely progenitors? Or could both models be at work? Future detections from gravitational-wave detectors, paired with electromagnetic observations, will help us to answer these questions. In the meantime, it’s exciting to watch the field unfold.

Citation

Shu-Xu Yi et al 2018 ApJL 859 L25. http://iopscience.iop.org/article/10.3847/2041-8213/aac649/meta

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See the full article for further references with links.

See the full article here .

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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: “Red Clump Stars and the History of the Galactic Bulge”

From AAS NOVA

The Sombrero Galaxy (NGC 4594) is instantly recognizable due to its prominent galactic bulge. [NASA/Hubble Heritage Team]

What’s going on in the galactic bulge? The discovery of the double red clump — two groupings of stars seen in the color-magnitude diagram of the galactic bulge — has raised questions about the structure and formation history of the stars surrounding the center of our galaxy.

A color-magnitude diagram of the galactic bulge from the Two Micron All Sky Survey. In panel b, the red, blue, and black symbols indicate the bright red clump, faint red clump, and background red giant branch stars, respectively. The double red clump is more clearly visible in the luminosity function in panel c. [Lee et al. 2018]

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.

Two Theories for the Double Red Clump

The double red clump, which is separated into bright and faint components, has inspired various theories about its origin:

1. X-shaped structure scenario: The presence of the double red clump was initially thought to indicate that the Milky Way bulge contains an X-shaped structure similar to that seen in other galaxies. The difference in apparent magnitude between the two components of the double red clump is simply due to the different distances to two of the arms of the structure as seen by an observer on Earth, with the arm reaching toward Earth forming the bright red clump and the arm stretching away from Earth forming the faint red clump.

2.Multiple-population scenario: Alternatively, the double red clump could be composed of multiple generations of stars that formed in globular clusters, where later generations of stars would be enhanced in helium, nitrogen, and sodium. Stellar evolution models predict that the first-generation stars would be fainter than subsequent generations, so the magnitude difference between the bright and faint components would be due to intrinsic luminosity differences between the two populations.

An illustration of the expected magnitude versus CN strength plots for the competing scenarios. [Lee et al. 2018]

A Closer Look at Red Clump Stars

How can we distinguish between these two scenarios? Young-Wook Lee (Yonsei University, South Korea) and collaborators posited that these theories can be separated by taking a closer look at the spectra of the double red clump stars.

Using low-resolution spectroscopy from a 2.5-meter telescope at Las Campanas Observatory, Lee and collaborators set out to measure the CN band strength of nearly 500 stars in the double red clump. Why CN? The CN band strength correlates with the nitrogen abundance, which is expected to be enhanced in later generations of stars.

Carnegie Las Campanas 2.5 meter Irénée Dupont telescope, Atacama Desert, over 2,500 m (8,200 ft) high approximately 100 kilometres (62 mi) northeast of the city of La Serena,Chile

If the two components of the double red clump represent separate generations of stars, the brighter stars will show the expected nitrogen enhancement. If, on the other hand, the double red clump arises from an X-shaped structure in the galactic bulge, there will be no difference in CN band strength between the bright and faint components.

CN indices for bright red clump (red), faint red clump (blue), and red giant branch (black) stars. [Lee et al. 2018]

From Globular Clusters to Galactic Bulges?

By comparing the spectra of the bright and faint red clump stars, Lee and collaborators found that the bright population had stronger CN absorption than the faint population at the 5.3σ level.

While precise parallax distances from Gaia and high-resolution spectroscopy are still needed to completely disentangle the stellar populations near the heart of our galaxy, this result supports the theory that the double red clump contains multiple generations of stars. The abundance patterns inferred from the stars’ spectra imply that the double red clump stars may have formed in young globular clusters, which could have profound implications for how galactic bulges are assembled — in both our home galaxy and afar.

Citation

Young-Wook Lee et al 2018 ApJL 862 L8. http://iopscience.iop.org/article/10.3847/2041-8213/aad192/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: “Shocks in the Solar Atmosphere”

From AAS NOVA

25 July 2018
Susanna Kohler

A view of the Sun’s roiling surface and its atmosphere above it, as imaged in UV light. A new study explores the shock waves that travel through the solar atmosphere. [NASA/TRACE]

NASA TRACE spacecraft

Despite the Sun’s proximity, there’s still a lot we don’t know about our nearest star! A new study, however, has brought us a step closer to understanding one of its mysteries: how the solar atmosphere is heated.

The temperatures of different layers of the Sun. The Sun’s atmosphere gets progressively warmer with increasing distance from the core, from the photosphere at 6,000 K, to the chromosphere at 10,000 K, to the corona at 1,000,000 K. [ISAS/JAXA]

Blame Shocks for Heating?

The outer layers of the Sun present an interesting conundrum: instead of continuing to drop in temperature with increasing distance from the Sun’s hydrogen-fusing core — the way the Sun’s inner layers do — the atmosphere above the Sun’s surface becomes progressively hotter with distance. How is heat getting delivered from the Sun’s center to these outer layers?

One possible culprit is shocks. Oscillations continuously travel through the Sun’s interior to its surface, launching compressible waves into its atmosphere. These waves can then evolve into shocks as they propagate outward — and the dissipation of these shocks can deposit energy into the Sun’s atmosphere, contributing to its heating.

Besides their potential role in atmospheric heating, these shocks serve a number of other purposes that should encourage us to study them. They are thought to serve as the trigger for observed phenomena, such as the tiny chromospheric jets driven in active regions. Furthermore, they can be used to probe the solar atmosphere — measuring shock properties can tell us more about the structure of the environment through which they propagate.

Observational data used by the authors to derive the properties of two successive shock waves observed by IRIS. a) IRIS image, spanning 47” x 47”. The vertical line indicates the location of the spectrograph slit. b) the Doppler velocity of Si IV 1393.76 Å in the region marked by the horizontal white lines in panel (a). c) A wavelength-time plot of Si IV 1393.76 Å. [Adapted from Ruan et al. 2018]

Theory and Observation Combined

Until now, however, we haven’t observationally measured the key parameters of shocks in the solar atmosphere — like the temperature, density, and speeds of the plasma upstream and downstream of the shock waves.

To address this challenge, a team of scientists at Peking University, the Chinese Academy of Sciences, and KU Leuven in Belgium have now developed a means of quantitatively analyzing the shocks we observe in the Sun’s atmosphere. In a recently published study led by Wenzhi Ruan, the team presents analysis that determine shock properties by using the theoretical physics of shocks in combination with quantities derived from simultaneous imaging and spectroscopic observations.

Testing the Approach

To test their analysis, the team takes two approaches. First, they apply the analysis to real shock observations — imaging and spectroscopy of two successive shock waves captured by the Interface Region Imaging Spectrograph (IRIS) observatory — and demonstrate that the shock properties they find are physically reasonable and consistent with each other.

NASA IRIS spacecraft, a spacecraft that takes spectra in three passbands, allowing us to probe different layers of the solar atmosphere

Next, the authors model shocks using numerical simulations, and they generate synthetic observations of how these shocks would appear to an observatory like IRIS from different viewing angles. They then apply their analysis to the synthetic observations and demonstrate that they can recover the original properties of the shocks in all cases, indicating that their method is reliable and independent of viewing direction.

The authors encourage other researchers to use their analysis code (you can find it here) as a standard approach for quantitatively exploring shocks in the solar atmosphere. This work is a crucial step toward better understanding how these shocks deposit heat, and how they interact with and influence the atmosphere of the Sun.

Citation

Wenzhi Ruan et al 2018 ApJ 860 99. http://iopscience.iop.org/article/10.3847/1538-4357/aac0f8/meta

Related journal articles
_________________________________________________
See the full article for further references with links.

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: “Searching for Exoplanets Around X-Ray Binaries”

From AAS NOVA

20 July 2018
Susanna Kohler

Artist’s impression of the first exoplanets discovered, which are in orbit around a magnetized neutron star, PSR B1257+12. A new study explores how we might find planets orbiting other compact objects — but compact objects that are accreting material from binary companions. [NASA/JPL-Caltech/R. Hurt (SSC)]

Finding planets around ordinary stars is great, but what if we could also hunt for planets around the black holes, neutron stars, and white dwarfs that live in binaries with companion stars? A new study shows it’s possible!

A New Place for Planets?

X-ray binaries are gravitationally interacting binary star systems in which a compact object — a white dwarf, neutron star, or black hole — accretes material from its companion star. In these systems, we can often detect the stars in optical or radio light, but the system additionally shines in X-rays as a result of radiation from the very hot, accreting gas.

Planets orbiting within binaries may be common — we’ve spotted around 70 examples so far of planets orbiting one member of a binary, and another dozen or so in which a planet orbits both members on a circumbinary path. It stands to reason, then, that some planets should survive the evolution of one of the binary stars into a compact remnant, eventually becoming a planet orbiting an X-ray binary.

Several example transit X-ray light curves for circumbinary planets, including an Earth-mass planet (solid lines) and a Jupiter-mass planet (dashed lines), for different values of μ, which relates the mass of the compact remnant to the companion star. Here the companion mass remains fixed and the different panels show different values for the remnant mass. [Imara & Dr Stefano 2018]

Looking at X-Rays

So how would we detect such a planet? Optical and radio searches for planets can be challenging, since a planet transit often means a very small dip in a light curve that might not be detectable. Two scientists from the Harvard-Smithsonian Center for Astrophysics, Nia Imara and Rosanne Di Stefano, propose an alternative: why not specifically look in the X-rays?

Because the area emitting X-rays is very compact — all smaller than the size of a white dwarf’s surface, i.e., the size of the Earth — the expected dip in the X-ray light curve due to a planet transit is quite large. This increases our chances of being able to detect it.

Exploring Challenges

A few challenges exist with this approach. To increase detection odds, the planet would ideally need to be orbiting within a similar plane to that of the binary, and preferably close to the inner cutoff for stable orbits around the binary. This is not an unreasonable assumption, however, given expected orbital dynamics as star systems evolve.

Transit probability versus binary mass ratio, μ, for planet circumbinary orbits around coplanar x-ray binaries with white dwarfs (solid lines), neutron stars (dashed lines), or black holes (dotted lines) as the primaries. The black and magenta lines represent the probability calculations for an Earth-like and Jupiter-like planet, respectively. [Imara & Di Stefano 2018]

Another potential difficulty is that X-ray photons are scarce! We’d have to observe systems for an extended time in order to gather enough light in X-rays to definitively detect light-curve dips. Imara and Di Stefano show, however, that we’ve observed a number of X-ray binaries over several hundred thousand seconds — long enough time periods that the dips would be detectable.

A Positive Outlook

With those challenges in mind, Imara and Di Stefano demonstrate through a series of calculations that circumbinary planets are reasonably likely to transit — transit probabilities range from roughly 0.1%–40%, depending on the mass ratio of the binary and the size of the X-ray-emitting region — and that our detection capabilities are such that we could actually spot these transits with present-day technology.

Future X-ray missions, like the proposed Lynx X-ray space telescope — which may have 50 times the sensitivity of the Chandra X-ray telescope! — will dramatically extend the opportunities for transit detection. Indeed, it seems like the hunt for these exotic exoplanetary systems have very good prospects.

Citation

Nia Imara and Rosanne Di Stefano 2018 ApJ 859 40. http://iopscience.iop.org/article/10.3847/1538-4357/aab903/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: “Shutting Down Star Formation”

From AAS NOVA

18 July 2018
Susanna Kohler

Image of the starburst galaxy NGC 1313, a galaxy currently undergoing an intense burst of star formation. How do galaxies transition from this state to a reddened, inactive state? [ESO]

At some point in a galaxy’s life, it transitions from a star-forming factory into an old, red, inactive relic. Can new observations of a recently transitioned galaxy help us understand what drives that change?

Spiral starburst galaxies (top) may precede red, inactive ellipticals (bottom) evolutionarily. [Adapted from Hubble/Galaxy Zoo]

Causes of Quenching

Across the nearby universe, we see two main types of galaxies: blue, active spirals, and red, quiescent ellipticals. It’s generally believed that these represent two evolutionary states: galaxies first undergo starburst periods in which many young, hot, blue stars are born. Later in their lifetimes, these galaxies then settle into red, inactive states.

But what shuts down the star formation, triggering the transition between these two states? There are a number of possible quenching explanations related to galaxy mergers:

1. Gas heating
The collision of two galaxies could heat up the gas supply via shocks, preventing it from gravitationally collapsing to form stars.
2. Compaction
Mergers may lead to compaction, in which the star-forming gas migrates inward and triggers a central starburst. The remainder of the galaxy is depleted of gas and stops forming stars.
3. Outflows
Mergers can trigger high-velocity outflows driven by radiation from new stars or from a central, feeding black hole. The outflows remove gas from the galaxy, shutting down star formation.
4.Morphological quenching
A galaxy can stabilize against star formation if the structure of the galaxy changes — say, after a merger — in specific ways, such as if the galactic disk transforms into a spheroid.

A Transitional Galaxy

These different proposed quenching mechanisms should leave distinctive signatures in the stellar populations of recently quenched galaxies. With this in mind, a team of scientists explored SDSS J0912+1523, an intermediate-redshift galaxy that shows signs of having only recently transitioned from a starburst galaxy to a quiescent one.

In a study led by Qiana Hunt, a postbaccalaureate researcher at Princeton University, the scientists combined preexisting ALMA observations of the gas within SDSS J0912+1523 with new Gemini observations of its stellar population. This combination provided a rare opportunity to gain insight into what might shut down a galaxy’s star formation.

The flux density of SDSS J0912+1523 seems to show two separate cores, which may be the sign of a past merger. [Hunt et al. 2018]

Evidence for a Merger

The authors find that SDSS J0912+1523 shows signs of containing two separate cores of stars that now rotate together — which may be evidence of a past merger.

In spite of this indication, none of the quenching scenarios above predict all of the characteristics Hunt and collaborators observed in SDSS J0912+1523. There’s lots of cold molecular gas still present in the galaxy, suggesting that neither depletion nor heating of the gas led to its quenching. Minor, gas-rich mergers or morphological quenching may be candidates still, but the kinematics of the gas and stars suggest that the gas didn’t come from an external origin. And there’s no evidence for strong outflows from SDSS J0912+1523.

For now, it looks like the mechanism that shut down star formation in this galaxy remains a mystery. But Hunt and collaborators are optimistic: a number of follow-up observations could shed more light on the problem — like radio hunts for central black-hole activity or high-resolution Hubble images of the galaxy’s morphology. Once these are conducted, we may soon understand what mechanism turns off star formation in an aging galaxy.

Citation

Qiana Hunt et al 2018 ApJL 860 L18. Stellar and Molecular Gas Rotation in a Recently Quenched Massive Galaxy at z ~ 0.7

Related journal articles
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See the full article for further references with links.

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: “When Stars Run Away”

From AAS NOVA

This false-color infrared image from the Spitzer Space Telescope shows the arched bow shock generated as blue supergiant Kappa Cassiopeiae hurtles through the interstellar medium. [NASA/JPL-Caltech]

NASA/Spitzer Infrared Telescope

The high-energy catalogs of the Fermi Large Area Telescope contain more than a thousand gamma-ray detections that have never been connected to a source. Some of these gamma rays could stem from very exotic objects: bow shocks of runaway stars.

NASA/Fermi LAT

NASA/Fermi Gamma Ray Space Telescope

Blazar. IPAC-Caltech

A Shocking Way to Generate Gamma Rays

Runaway stars get their name by careening through interstellar space after being ejected from binary or multiple systems, usually because of gravitational interactions with other stars or a kick from a nearby supernova. As runaway stars plow into the surrounding interstellar medium, a bow shock can form in front of the star. It’s these stellar bow shocks that may be the sources of galactic gamma rays.

Stellar bow shocks generate gamma rays by first accelerating electrons to relativistic speeds.

Massive Star Makes Waves
The giant star Zeta Ophiuchi is having a “shocking” effect on the surrounding dust clouds in this infrared image from NASAs Spitzer Space Telescope. Stellar winds flowing out from this fast-moving star are making ripples in the dust as it approaches, creating a bow shock seen as glowing gossamer threads, which, for this star, are only seen in infrared light.
Zeta Ophiuchi is a young, large and hot star located around 370 light-years away. It dwarfs our own sun in many ways — it is about six times hotter, eight times wider, 20 times more massive, and about 80,000 times as bright. Even at its great distance, it would be one of the brightest stars in the sky were it not largely obscured by foreground dust clouds.
This massive star is travelling at a snappy pace of about 54,000 mph (24 kilometers per second), fast enough to break the sound barrier in the surrounding interstellar material. Because of this motion, it creates a spectacular bow shock ahead of its direction of travel (to the left). The structure is analogous to the ripples that precede the bow of a ship as it moves through the water, or the sonic boom of an airplane hitting supersonic speeds. The fine filaments of dust surrounding the star glow primarily at shorter infrared wavelengths, rendered here in green. The area of the shock pops out dramatically at longer infrared wavelengths, creating the red highlights.

A bright bow shock like this would normally be seen in visible light as well, but because it is hidden behind a curtain of dust, only the longer infrared wavelengths of light seen by Spitzer can reach us.

Bow shocks are commonly seen when two different regions of gas and dust slam into one another. Zeta Ophiuchi, like other massive stars, generates a strong wind of hot gas particles flowing out from its surface. This expanding wind collides with the tenuous clouds of interstellar gas and dust about half a light-year away from the star, which is almost 800 times the distance from the sun to Pluto. The speed of the winds added to the stars supersonic motion result in the spectacular collision seen here.

Our own sun has significantly weaker solar winds and is passing much more slowly through our galactic neighborhood so it may not have a bow shock at all. NASAs twin Voyager spacecraft are headed away from the solar system and are currently about three times farther out than Pluto. They will likely pass beyond the influence of the sun into interstellar space in the next few years, though this is a much gentler transition than that seen around Zeta Ophiuchi.

For this Spitzer image, infrared light at wavelengths of 3.6 and 4.5 microns is rendered in blue, 8.0 microns in green, and 24 microns in red.
http://www.spitzer.caltech.edu/images/5517-sig12-014-Massive-Star-Makes-Waves

When a relativistic electron collides with a low-energy photon, it transfers energy to the photon, upgrading it to a gamma ray. This process, known as inverse Compton scattering, is the reverse of Compton scattering, through which electrons colliding with high-energy photons are accelerated to relativistic speeds.

Although stellar bow shocks are theoretically capable of producing gamma rays, these ultra-high-energy photons have never been definitively detected. Could the gamma rays from stellar bow shocks be hidden among the unidentified Fermi sources?

Comparison of modeled spectral energy distribution to observations for runaway star Lambda Cephei.[Sánchez-Ayaso et al. 2018]

Searching for Missing Gamma-Ray Sources

Estrella Sánchez-Ayaso (Universidad de Jaén, Spain) and collaborators embarked on a search for gamma-ray-emitting stellar bow shocks. They began by comparing the positions of unidentified Fermi sources to those of known bright stars in our galaxy and visually inspecting the matches for signs of a bow shock, which revealed two runaway star candidates.

Sánchez-Ayaso and collaborators then used the known properties of the two stars to determine whether or not it’s feasible for their bow shocks to be the source of the emission seen by Fermi. The authors modeled the gamma-ray emission generated by inverse Compton scattering of infrared photons off of electrons accelerated by the bow shocks.

By matching their model output to the Fermi observations, the authors determined that the physical conditions necessary for the two runaway stars to produce the observed gamma rays were reasonable. This suggests a promising link between the two stellar bow shocks — one of which was discovered as a result of this work — and two of the unidentified Fermi sources.

Fermi error ellipse overlaid on a composite radio, infrared, and optical image of runaway star LS 2355. [Sánchez-Ayaso et al. 2018]

Two Down, One Thousand to Go

With two of the Fermi sources potentially linked to runaway stars, can we expect more matches to be made? While runaway stars are relatively common, the conditions have to be just right for the stars to become gamma-ray sources. They must have very high velocities and be moving through a region of the interstellar medium that is dense and has a weak magnetic field, since electrons spiraling around magnetic field lines lose too much energy through synchrotron radiation to generate gamma rays through inverse Compton scattering.

While more observations are required to confirm that these two stars are the source of the gamma rays seen by Fermi, Sánchez-Ayaso and collaborators have shown that these unique stars are definitely worth exploring!

Citation

E. Sánchez-Ayaso et al 2018 ApJ 861 32. http://iopscience.iop.org/article/10.3847/1538-4357/aac7c7/meta

Related journal articles
_________________________________________________
See the full article for further references with links.

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 : “Gaia Identifies a Stellar Gap”

From AAS NOVA

11 July 2018
Susanna Kohler

New measurements of hundreds of thousands of stars have revealed a surprising gap in main-sequence stars on a Hertzsprung-Russell diagram. [NASA, ESA, and Hubble Heritage Team]

Sometimes more-precise measurements are all we need to make new discoveries in old structures! In a new study, data from the Gaia mission has revealed a surprise hidden among main-sequence stars.

An observational Hertzsprung–Russell diagram with 22,000 stars plotted from the Hipparcos Catalogue and 1,000 from the Gliese Catalogue of nearby stars. Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants. The Sun is found on the main sequence at luminosity 1 (absolute magnitude 4.8) and B−V color index 0.66 (temperature 5780 K, spectral type G2V).

Old Diagram with a New Feature

If you’ve ever taken an introductory astronomy class, you’ve probably encountered the Hertzsprung-Russell (HR) diagram: a diagram on which stellar luminosities are plotted against their colors, which serve as a proxy for their effective temperatures. The resulting positions of stars on the HR diagram reveal distinct stellar evolutionary stages — and perhaps the most striking population is the swath of main-sequence stars that cuts diagonally across the diagram.

Though we’ve constructed HR diagrams for nearby stars for more than a century, they continue to change as our data for these stars improve. In particular, today’s era of precision astrometry has significantly improved the distance measurements for the stars that surround us, allowing them to be placed more accurately on the diagram. The recent second data release from the Gaia mission presented precise astrometry measurements for billions of stars, covering virtually all types on the HR diagram — including M dwarfs, which were sparsely sampled in the past.

A team of scientists led by Wei-Chun Jao (Georgia State University) has now explored this data and discovered a surprise: there’s a gap in the HR diagram at mid-M dwarfs.

Portion of the HR diagram for stars within 100 pc in the Gaia DR2 data set. A narrow low-density gap is visible cutting through the main sequence between the two dashed lines. [Jao et al. 2018]

Mind the Gap

Jao and collaborators plotted a total of nearly 250,000 stars from the Gaia archive on an HR diagram. The new data and improved measurements revealed a previously unseen feature: a narrow, diagonal slice through the main sequence that is underpopulated. The missing stars seem to lie in the middle of the M-dwarf region.

The authors cross-match the stars against the 2MASS catalog, finding that the gap exists in other data and color bands as well — which means it’s not just a weird quirk of Gaia’s photometry. They then check whether the gap exists only in stars at a specific distance. Another no: it’s visible similarly in various populations spanning distances up to 425 light-years.

Transitioning Convection

So what’s causing this unexpected feature? The authors argue that the presence and persistence of the gap suggest that it’s due to some underlying physics that we haven’t yet thought of — which is always an exciting prospect!

In particular, Jao and collaborators suggest that the gap may be related to a known transition in mid-M dwarfs, from larger stars that are mostly convective with a thin radiative layer, to smaller stars that are fully convective. The authors propose that the missing stars in the gap may be due to subtle changes of structure that occur at this transition between partial and full convection in these M dwarfs.

In the future, the authors propose gathering more data — like dynamical masses, radii, metallicities, rotational periods, and magnetic properties — on stars in and near the gap, to better understand population trends. In the meantime, we can be excited to know that there are still some surprises left for us to discover in old structures, if we just keep improving our data.
Citation

Wei-Chun Jao et al 2018 ApJL 861 L11. http://iopscience.iop.org/article/10.3847/2041-8213/aacdf6/meta

Related journal articles
_________________________________________________
See the full article for further references with links.

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