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26 June 2020

Orbits Evolving Under Gravity

The solar system extends well beyond Pluto, encompassing small objects on their own unusual orbits around the Sun. How did they get there? A new study attempts to answer this question with simulations.

Models and Moving Objects

The largest objects in the solar system wield the most influence. Models that account for the Sun and the outer planets — Jupiter, Saturn, Uranus, and Neptune — can produce realistic approximations of the solar system’s overall gravitational influence.

So if you have a model of the major gravitational forces at play, you can drop in orbiting objects and see what they do over time. This sounds simple, but it’s a powerful tool when it comes to understanding the current structure of our solar system.

The evolution of the surface density of the disk with time (starting from the upper-left) as seen face-on (top) and edge-on (bottom). Click to enlarge. Yellow regions have a higher density than blue regions. The timescale P represents 1,000 years. The authors note the “cone” of orbits present prior to t = 4,300 P, as well as the coherent ring of orbits most prominent at t = 9,900 P, which corresponds to an “m = 1 mode”. [Zderic et al. 2020]

A new study led by Alexander Zderic (University of Colorado Boulder) looks at what would happen to a large disk of small objects orbiting in the outskirts of our solar system. This study is the latest in a line of similar studies attempting to understand large scale structure in the solar system.

A Disk on the Outskirts

The disk being examined by Zderic and collaborators consists of objects orbiting at roughly 100 to 1,000 astronomical units (au) from the Sun. For context, Pluto’s farthest distance from the Sun is just 50 au, so these distances definitely qualify as the outer solar system. The orbits of the disk objects all start off in the same plane (which is also the plane in which the solar system’s planets orbit), but they have a higher than average eccentricity (as conditions in the outer solar system require).

In previous studies with higher mass disks, the disk conditions have been shown to reach a consistent state within 660 million years of simulated time. Zderic and collaborators were interested in this consistent state, which reflects the long-term behavior of the disk. To reach this state more quickly, the authors used an equivalent setup: they started with a less massive disk, and they ran their simulation for just under 10 million years.

Evolution of two orbital parameters for a particular object, specifically eccentricity (y-axis) and the longitude of perihelion (x-axis; the sum of two other orbital properties that sets the orientation of the orbit relative to a plane). Between 5,000 P and 9,000 P, the object under consideration is in the m = 1 mode. [Zderic et al. 2020]

Modes in Models

As the disk of objects evolves, the authors show that the collective gravity of the small bodies can induce an instability. As a result, the final state of the disk has a significant feature: orbits appear to cluster in a particular region. This is called being in a “mode”, which is shorthand for a group of orbital parameters having specific values. Zderic and collaborators note that later in the simulation, objects tend to settle into the m = 1 mode, though objects also fall in and out of the mode. Additionally, adding more particles to the simulation shows that objects stay in the mode longer. Extrapolating to the solar system, the mode may be stable for as long as the solar system is around.

Why is this interesting? These simulations show that the collective gravity of small bodies in a disk can naturally reproduce many of the observed behaviors of objects in our outer solar system — including extreme trans-Neptunian objects (TNOs), small bodies beyond the orbit of Neptune that are on very unusual orbits.

Schematic showing the observed alignment of the orbits of detached extreme TNOs and the proposed orbit of a hypothetical super-Earth-mass planet (in green). But is Planet Nine actually necessary to explain the extreme TNO orbits? [Sheppard et al. 2019]

In particular, extreme TNOs have been observed to have clustered orbital properties — a fact that has been used to argue for the presence of an additional, hypothesized giant planet in our outer solar system, Planet Nine. But if Zderic and collaborators are correct, there’s no need for a hidden planet to explain extreme TNO alignments.

Further work will require the simulation of high mass disks that are more similar to the early solar system. Keep an eye out for future studies exploring the cause of our solar system’s structure!

Citation

“Apsidal Clustering following the Inclination Instability,” Alexander Zderic et al 2020 ApJL 895 L27. doi:10.3847/2041-8213/ab91a0

23 June 2020

LIGO-Virgo’s New Find Shakes Things Up

Neutron star or black hole? That’s the question scientists are asking about the latest gravitational-wave detection announced from the Laser Interferometer Gravitational-wave Observatory (LIGO) and its sister observatory, Virgo. In a new publication, scientists detail this newest addition to the list of confirmed collisions — and explain why it’s rather unexpected.

Artist’s impression of two merging neutron stars. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

Still More from O3

Things have been decidedly quiet on the LIGO-Virgo front lately. The gravitational-wave detectors’ third observing run, O3, wrapped up in March (cut unfortunately short due to COVID-19). Since then, the collaboration has announced only two discoveries from this run: another binary neutron star merger, and the collision of two black holes of very unequal masses.

Yet there remain many dozens of potential candidates recorded during O3 that are still undergoing analysis to confirm whether they’re “true” detections — and, if they are, to identify the properties and astrophysical implications of the mergers.

Despite GW190814’s relatively small sky localization (shown here), no associated electromagnetic signature was found — but we also wouldn’t expect one from such an unequal-mass binary, as the secondary would likely have been swallowed whole. [Abbott et al. 2020]

Today, another one of these is officially confirmed: GW190814, an especially unusual collision of a large, ~23-solar-mass black hole with a smaller, ~2.6-solar-mass mystery object.

Thwarting Expectations

GW190814 was detected in August of last year by all three of the LIGO-Virgo detectors. The signal was localized to a region of just 18.5 square degrees, but follow-up observations didn’t detect any corresponding electromagnetic signatures from the area. Despite the lack of fireworks, however, GW190814 is anything but mundane — in fact, it’s a source unlike any other known compact binary merger.

What makes GW190814 so weird? There are three main factors:

Very unequal masses
The two objects that collided to produce GW190814 had a mass ratio of q = 0.112. This is the most unequal mass ratio yet measured in a merger; most mergers consist of two objects that are nearly the same mass.
The mass gap between the heaviest neutron stars and the lightest black holes is clearly visible in this diagram of the masses of known compact binary components, compiled at the end of 2018. [LIGO-Virgo/Frank Elavsky/Northwestern]
Low primary spin
The primary object in the binary, the 23-solar-mass black hole, has an extremely low measured spin of χ₁ ≤ 0.07. This is the tightest constraint we’ve ever placed on the spin of the primary component of a gravitational-wave source.
Mass-gap secondary
The secondary object in the binary, the 2.6-solar-mass body, falls right in the middle of what’s known as the mass gap: its mass is heavier than the heaviest confirmed neutron star (~2.5 solar masses), but lighter than the lightest confirmed black hole (~5 solar masses). So which is it: a neutron star or a black hole?

The marginalized posterior distribution describing the likely mass of the secondary object, for different waveform models. The 90% credible range is 2.50–2.67 solar masses, which lies firmly in the mass gap. [Abbott et al. 2020]

A Challenge to the Paradigm

Regardless of whether this unusual merger occurred between two black holes or between a black hole and a neutron star, GW190814 challenges our current models of compact binaries and their components.

How could a binary containing such a heavy black-hole primary and such a light neutron-star or black-hole secondary form? And what about the perceived mass gap between neutron stars and black holes — is it real? Or is it just an observing bias?

We have a lot of questions, and a lot of work to do to account for GW190814 in our theoretical models. But if GW190814 does, indeed, represent a whole new kind of compact binary merger, then we also have an exciting road ahead as LIGO-Virgo inevitably discovers more of them, allowing us to gradually piece together the puzzle of these unexpected collisions.

Citation

“GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass Black Hole with a 2.6 Solar Mass Compact Object,” R. Abbott et al 2020 ApJL 896 L44. doi:10.3847/2041-8213/ab960f

19 June 2020

Exploring Links Between Nearby Asteroids

It’s not easy being a speeding rock in our solar system.

Illustration of an asteroid breaking apart into smaller fragments. [NASA/JPL]

Over their lifetimes, the millions of minor rocky bodies of our solar system — asteroids — are subject to extreme conditions. Some experience dramatic collisions, some are spun up to such high rotation speeds that they fly apart, and some venture so close to the Sun that our star’s heat cracks them into pieces.

Over time, these violent processes create families of asteroids that dance around our solar system on similar paths. Where one rock once orbited, there might now be a group of genetically linked asteroids that follow similar trajectories — all produced by the splitting of one parent rock.

In a new study, scientists have explored two especially nearby asteroids to determine whether they might be linked.

A Visit to a “Potentially Hazardous” Neighbor

The orbital path of the near-Earth asteroid Phaethon. [Sky&Tel]

Asteroids whose orbits bring them close to the Earth are of particular interest to us: we like to keep an eye on those bodies that might threaten our planet.

Perhaps 22,000 near-Earth asteroids are currently known, with just over 2,000 that are large enough and swing close enough to Earth’s orbit to be considered “potentially hazardous” — though it should be noted that the vast majority of these have been ruled out as being an impact threat in at least the next 100 years.

Artist’s illustration of the DESTINY+ spacecraft. [JAXA]

To learn more about these nearby bodies, the Japanese Aerospace Exploration Agency is sending a spacecraft, DESTINY+, to fly by a large (~5-km) near-Earth asteroid. The target is 3200 Phaethon — an unusual blue-toned, dust-producing asteroid thought to be the source of the Geminid meteor stream — and other minor bodies that might be associated with it.

As DESTINY+ is currently scheduled to launch in 2022, scientists are currently preparing by learning all they can about the possible mission targets using ground- and space-based observatories. In a new study led by Maxime Devogèle (Lowell Observatory), a team of scientists presents detailed observations of (155140) 2005 UD, another near-Earth object and potential DESTINY+ target that might be related to Phaethon.

Signs Point to a Linked Pair

Devogèle and collaborators gathered an impressive array of observations of 2005 UD, using dozens of telescopes to obtain photometry, polarimetry, and spectroscopy, and also reanalyzing thermal imaging.

2005 UD and Phaethon exhibit very similar spectra, including rare spectroscopic (B-type) signatures. [Devogèle et al. 2020]

By combining new observations with archival data and detailed modeling, the team constrained 2005 UD’s size (just over 1 km across) and rotation rate (it spins roughly once every 5.2 hours), as well as many other properties like its albedo, spectroscopic class, and even the size of the grains on its surface — knowledge that will all help with mission planning for DESTINY+.

But what about 2005 UD’s potential link to Phaethon? Based on Devogèle and collaborators’ observations, 2005 UD and Phaethon appear to share more than just orbital characteristics. They also have very similar — and rare, among asteroids — physical properties as shown by their spectroscopy and polarimetry.

More study is needed, but the data suggest that the two are, indeed, genetically linked — perhaps 2005 UD and Phaethon both split from the same parent thousands of years ago. With any luck, DESTINY+ will soon reveal more about these close-swinging rocky bodies!

Citation

“New Evidence for a Physical Link between Asteroids (155140) 2005 UD and (3200) Phaethon,” Maxime Devogèle et al 2020 Planet. Sci. J. 1 15. doi:10.3847/PSJ/ab8e45

17 June 2020

Spotted: A Galactic PeVatron?

Speeding charged particles — far more energetic than any we can create in laboratory particle accelerators — constantly bombard the Earth’s atmosphere. But what extreme environments produce these high-energy particles? A new study may have identified one cosmic accelerator in our galaxy.

Diagram showing cosmic ray flux as a function of energy. The lower-energy cosmic rays (yellow region) are thought to be produced by the Sun. Intermediate-energy cosmic rays (blue region) are likely of galactic origin, and the highest-energy cosmic rays (pink region) likely come from beyond the galaxy. [Sven Lafebre]

Charged Arrivals

At any given moment, protons and atomic nuclei are whizzing through our galaxy, sometimes at nearly the speed of light. These charged particles — cosmic rays —span a wide range of energies, with the most energetic packing the same punch as a 90 kilometer-per-hour (56 mph) baseball!

More modest cosmic rays reach “only” peta-electron-volt (PeV) energies — that’s 10¹⁵ eV, still more than 100 times more energetic than the particles accelerated by the record-holding Large Hadron Collider. We think that these PeV particles were produced somewhere within our own galaxy.

If we could unravel their secrets, these cosmic rays could provide clues about how stars evolve and how energy is transported throughout the galaxy. First, however, we need to figure out where they came from. Are their sources supernova remnants? Microquasars? Superbubbles? What galactic PeVatrons accelerated these particles to their tremendous speeds?

Cosmic rays (red) consist of individual protons and nuclei of heavier elements. They are deflected by magnetic fields along their cosmological odysseys and can’t be used to point back to the place of their origin. Click to enlarge diagram. [IceCube Neutrino Observatory]

Road Map to a Birthplace

Unfortunately, we can’t just trace cosmic rays backwards to figure out their origins. Because these particles are charged, their trajectories are deflected by interstellar magnetic fields — which means that the direction a cosmic ray arrived from probably isn’t the direction of its source.

To address this challenge, high-energy astronomers search for more direct messengers that are produced as cosmic rays are accelerated — like extremely energetic gamma-ray radiation.

When PeV particles accelerated by a galactic PeVatron collide with gas and dust in the vicinity of their origin, they should produce very high-energy tera-electron-volt (TeV, or 10¹² eV) gamma-ray photons. These photon by-products won’t be deflected by magnetic fields, so their arrival at gamma-ray observatories on Earth provides a clearer path back to the source of the PeV cosmic rays.

Top: significance map from HAWC showing the location of gamma-ray emission from near SNR G106.3+2.7. Bottom: Molecular hydrogen column density around the HAWC-detected source (shown in gray contours). The detectors VERITAS and Milagro have also observed very high-energy gamma-ray emission from this region; their detection centers are also marked. [Adapted from Albert et al. 2020]

Hunting for Galactic Accelerators

So how’s the search for these characteristic TeV gamma-rays going? With one possible success on the books so far — scientists think there’s a galactic PeVatron at the center of our galaxy, but we haven’t yet determined the source — we’ve now identified a new potential galactic PeVatron: the remnant produced by a past supernova explosion just 2,600 light-years from Earth.

In a new publication, a team of scientists from the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) announces the detection of TeV gamma-ray emission from the same region as supernova remnant SNR G106.3+2.7.

Though the team can’t rule out other causes of the emission, this signal has a spectrum that’s consistent with what we’d expect to be produced by PeV protons colliding with gas and dust. The origin near SNR G106.3+2.7 supports a picture in which charged particles can be accelerated across the shocks of supernova remnants and flung into space with PeV energies.

So might the mystery of galactic PeVatrons be solved with supernova remnants? We don’t know for sure yet, but future high-energy gamma-ray observations are sure to help us further identify the sources of the speeding charged particles in our galaxy.

Citation

“HAWC J2227+610 and Its Association with G106.3+2.7, a New Potential Galactic PeVatron,” A. Albert et al 2020 ApJL 896 L29. doi:10.3847/2041-8213/ab96cc

15 June 2020

The Traits of Supernova Siblings

What can we learn about the large-scale properties of our universe from Type Ia supernovae — cosmic flashes thought to be caused by the explosions of white dwarfs? The answer may depend on the shared traits of pairs of supernova siblings.

Standardizable Candles

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

How do we measure vast distances — and, correspondingly, the cosmological properties of our universe? One of astronomers’ go-to methods relies on Type Ia supernovae, explosions thought to have a fixed intrinsic luminosity.

By comparing the measured brightness of these explosions to their expected intrinsic luminosity, we can obtain a quantity known as the distance modulus, which tells us how far away the supernova occurred.

But there’s a catch: though Type Ia supernovae all have roughly the same intrinsic luminosity, they still exhibit some variation — scatter — in their natural peak brightness. This intrinsic scatter reduces how accurately we can measure their distance.

Taking the Blame for Scatter

Astronomers are still working to understand what causes the intrinsic scatter in supernova distance modulus. Is it differences in the atmospheres of the white dwarfs that exploded? Or variations in the explosion process?

Another possibility is that variations in the host galaxy environment could impact the observed supernova signal. This would be important: since galaxy properties evolve with redshift, that means that if host galaxy properties do influence the supernova signals we see, we’d need to account for this when we infer supernova distances.

The locations of the eight pairs of Type Ia supernova siblings found by the authors in the DES-SN data set, marked in red and yellow on images of the eight host galaxies. Each plot is 16”x16”. [Scolnic et al. 2020]

But how can we test this possibility? In a new study, a team of authors led by Daniel Scolnic (Duke University) has looked for the influence of galaxy hosts on distance modulus scatter by exploring the traits of supernova siblings.

On The Hunt for Siblings

Supernovae are rare — a typical galaxy only hosts a few per century. But with modern supernova surveys, we can monitor millions of galaxies over multiple years, collecting many observations of these explosions.

Scolnic and collaborators leveraged one such survey, the Dark Energy Survey Supernova Program (DES-SN) conducted at the Cerro Tololo Inter-American Observatory in Chile, to find eight parent galaxies that each hosted two Type Ia supernovae during the survey.

The authors then examined the light curves of these supernovae to see if these sibling pairs had more light-curve properties in common than a random pair of two supernovae drawn from a simulated set.

Expanding the Supernova Crowd

Plots show the comparison of different light curve parameters (c, x₁, and m_B) and the distance modulus µ, for each of the eight pairs of matched sibling supernovae. The authors’ results show that the light curves of sibling pairs are not much more alike than those of random pairs. [Scolnic et al. 2020]

The result? Scolnic and collaborators found that, for the most part, pairs of supernova siblings hosted by the same galaxy were no more likely to be similar than a random pair of supernovae. From their analysis, the authors argue that no more than half of the intrinsic scatter seen in supernova distance moduli can be due to the properties of their host galaxies.

Though these results are based on just eight pairs of supernova siblings, more robust findings will be possible in the future! The Vera Rubin Observatory (formerly LSST) is expected to detect ~100 times this number of Type Ia supernova pairs, vastly increasing our sample size and further allowing us to more accurately measure the cosmological properties of the universe.

Citation

“Supernova Siblings: Assessing the Consistency of Properties of Type Ia Supernovae that Share the Same Parent Galaxies,” D. Scolnic et al 2020 ApJL 896 L13. doi:10.3847/2041-8213/ab8735

12 June 2020

Seeing Things in Threes

GW Ori is a system of three stars that are gravitationally bound. Aside from being a triple system, GW Ori also stands out for another reason — it harbors a circumtriple disk, which is a disk of gas and dust surrounding all three stars.

A Tricky Triple

The dust component of GW Ori’s disk as seen by ALMA, showing the three rings discussed in this study. The x- and y-axes of the plot are position offsets, with (0,0) being the position of GW Ori. The color of the rings indicate intensity of emission, with yellow being more intense than purple. The circle in the lower left corner shows the size of the beam used by ALMA to image the disk. [Bi et al. 2020]

GW Ori lives in a star cluster called Lambda Orionis, which appears near Betelgeuse on the sky. The inner stars of the system, GW Ori A and GW Ori B, orbit each other and are separated by about 1 astronomical unit (au). The third star, GW Ori C, revolves around its two companions at a distance of roughly 8 au.

GW Ori’s circumtriple disk is enormous relative to the orbits of its stars. The dust component of the disk is about 400 au across, with the gas component spanning roughly 1,300 au. For scale, Neptune is only about 30 au from the Sun!

Models of GW Ori have suggested a gap in the disk between 25 and 55 au from its center. A recent study led by Jiaqing Bi (University of Victoria) attempted to test these models and probe the structure of GW Ori directly using observations by the Atacama Large Millimeter/submillimeter Array (ALMA).

Finding Rings in Radio

Bi and collaborators used ALMA observations taken at multiple frequencies to probe the gas and dust of the circumtriple disk. The dust component has characteristic emission that can be observed at 1.3 millimeters, while the gas can be studied using a particular transition of carbon monoxide.

A schematic showing the likely alignment of GW Ori’s dust rings. The diagram on the left is how the disk appears on the sky and the diagram on the right is how the disk would appear if seen edge-on. The red ellipse is the plane of the orbits of GW Ori AB and GW Ori C. The ring of white dots represents the gap between the inner dust ring and middle dust ring. The orientation axes for both diagrams are located in their respective lower left corners. [Bi et al. 2020]

The gas observations showed the expected disk rotation, and additional structure in the disk was immediately apparent in the dust observations. Bi and collaborators identified three dust rings in GW Ori’s disk at roughly 46, 188, and 338 au from its center, the innermost ring being the one that was suggested by past models. Additionally, an unexpected result was brought to fore — the dust rings may be very misaligned relative to one another!

Out of Balance But It’s Fine

Bi and collaborators found that the dust rings showed significant inclinations relative to the plane of the orbit of GW Ori A and B — specifically 11, 35, and 40 degrees starting from the innermost ring. The gas observations back this up, requiring a model that assumes some distortion from an undisturbed disk.

Additional analysis and simulations by Bi and collaborators suggest that the stars of GW Ori alone could not be responsible for this misalignment. The innermost ring also adds another puzzle to this system: in addition to being misaligned, it also has a non-zero eccentricity, meaning its center is different than those of the other rings.

A possible explanation could be additional companions to GW Ori, which are also carving out paths in the disk. This phenomenon has been observed by ALMA before in protoplanetary disks. If this is the case, it would be the first time circumtriple companions were detected. Only time will tell!

Citation

“GW Ori: Interactions between a Triple-star System and Its Circumtriple Disk in Action,” Jiaqing Bi et al 2020 ApJL 895 L18. doi:10.3847/2041-8213/ab8eb4

27 May 2020

Are We Watching a Planet Disintegrate?

Among the wealth of exoplanets we’ve discovered beyond our solar system, some are temperate, some less so. New observations have now revealed what may be a particularly inhospitable environment: a planet literally disintegrating as it orbits its host.

Peering Through the Shroud

Artist’s illustration of another DMPP-discovered planetary system, DMPP-2. [Mark A. Garlick/Haswell/ Barnes/Staab/Open University]

With initial observations in 2015, the Dispersed Matter Planet Project (DMPP) promised an innovative approach to hunting for exoplanets closely orbiting their hosts. Using high-cadence, high-precision radial velocity measurements, the project targets bright nearby stars that shows signatures of being shrouded in hot circumstellar gas. By looking for tiny radial-velocity wiggles in the star’s signal, the DMPP team hopes to detect small planets that are losing mass as they orbit close to their hot hosts.

In December 2019, DMPP announced its first discoveries: six planets orbiting around three different target stars. Now, in a new publication led by scientist Mark Jones (The Open University, UK), the team has revisited the first of these systems, DMPP-1, with follow-up photometry from the Transiting Exoplanet Survey Satellite (TESS).

Intriguingly, the radial-velocity-detected planets are not the only signals from this system.

Missing the Expected, but Finding the Unexpected

DMPP-1 is a 2-billion-year-old star located just over 200 light-years away. The radial-velocity observations of this system revealed the gravitational tugs of four planets all orbiting with periods of less than 19 days. The radial-velocity data suggest that this system is probably near edge-on and contains three super-Earths and one Neptune-like planet.

Phase-folded TESS light curve for DMPP-1, identifying a weak transit signal with a period of P = 3.2854 days. [Jones et al. 2020]

Jones and collaborators began their photometric follow-up by searching TESS data for evidence of these four planets transiting across the host star’s face. Interestingly, they found no sign of transits at the predicted periods — indicating that the four radial-velocity planets are either smaller than expected, or that the system isn’t quite edge-on after all, so the planets don’t pass directly in front of the star.

The authors did, however, find a new signal: a weak transit detection with a period of just ~3.3 days. This signal doesn’t match any of the known radial-velocity planets.

A Disappearing Planet?

What might this marginal detection be? Its variable transit depths, short period, and apparent small size are all consistent with a catastrophically disintegrating exoplanet — a close-in, small, rocky planet that is so irradiated by its host that its rocky surface is being sublimated. As time goes on, such a planet will eventually disintegrate into nothing.

The depths of the seven detected transits are highly variable, with one even consistent with a depth of zero! This variability is common for disintegrating planets, as the cloud of ablated material is the primary cause of the transits. Click to enlarge. [Jones et al. 2020]

This transit signal still needs to be confirmed with additional follow-up photometric observations. Assuming it proves to be a true detection, however, such a disintegrating, rocky planet orbiting a bright nearby star would provide a veritable gold mine of information.

By exploring the transit signals from DMPP-1 with future technology like the James Webb Space Telescope, we will be able to examine the composition of the ablated material, potentially revealing clues as to how hot, rocky inner planets form and evolve.

Citation

“A Possible Transit of a Disintegrating Exoplanet in the Nearby Multiplanet System DMPP-1,” Mark H. Jones et al 2020 ApJL 895 L17. doi:10.3847/2041-8213/ab8f2b

22 May 2020

Making Galaxies of All Sorts

It’s hard to translate an image taken by a telescope to a physical object like a galaxy, but, like with most things, computers have made the job easier. Astronomers can code models of galaxies and determine the most realistic models based on observations. A recent study showcases this, presenting a code that can model all sorts of galaxies along with their internal motions.

Hubble view of NGC 1300, a barred spiral galaxy. [NASA, ESA, and The Hubble Heritage Team (STScI/AURA)]

Physical Frameworks and Altered Orbits

Any model begins with some assumed framework of physics. In a 1979 paper, Martin Schwarzschild proposed a method of modeling groups of stars called the orbit-superposition method. This method assumed that a system could be modeled as a gravitational potential that had objects orbiting in it and that properties of the system could be studied by seeing how the orbits changed with time.

While Schwarzschild used the orbit-superposition method to consider elliptical galaxies, other researchers have used it to study galaxies with more distinct structures like disks and bars. Since the 1990s, the Schwarzschild orbit-superposition method has been used extensively in galaxy modeling.

In a new study, Eugene Vasiliev (University of Cambridge) and Monica Valluri (University of Michigan) present a new code called Forstand, an implementation of the Schwarzschild method that can be applied to all sorts of galaxies without excessive computational cost.

An example of modeling results (top) and mock kinematic data (bottom). In the results plot, β and α describe the orientation of the mock galaxy. Ω is pattern speed and ɣ is mass-to-light ratio. The gray dots are the models considered and the red cross is the true value. The blue contours show the likeliest values for the pattern speed and mass-to-light ratio. The mock data plot is a velocity map, highlighting the x-, y-, and z-axes. The velocities changing from negative to positive indicates that the galaxy is rotating. [Adapted from Vasiliev & Valluri 2020]

Mockups of Mock Galaxies

The code was tested with two different data sets of mock galaxies — one that was low-resolution but spanned large regions of the component galaxies, and one that was high resolution but only covered the central regions of galaxies. The mock galaxies were all roughly the size of the Milky Way and had variations in their structure and orientation.

Vasiliev and Valluri assumed that the 3D shapes of their galaxies were known, acknowledging that this assumption can’t easily be made in the case of observational data. However, in all other ways the mock data were very similar to data taken by actual astronomical instruments.

The code was able to accurately recover the mass-to-light ratios and the pattern speeds of the mock galaxies. For galaxies, the mass-to-light ratio lets astronomers understand how much of the matter in a galaxy is dark matter. The pattern speed has to do with how the overall structure of a galaxy — spiral arms, for example — moves.

Vasiliev and Valluri wrapped up with avenues for improvement on their code. Forstand wasn’t able to recover the properties of certain galaxy components, specifically central black holes and dark matter haloes. There’s also still the issue of determining 3D galaxy shapes, which can be tricky to constrain observationally.

However, room for improvement doesn’t take away from the fact that the Schwarzschild method is a powerful tool when it comes to modeling galaxies. And Vasiliev and Valluri seem to have come up with a code that makes the Schwarzschild method far more accessible than before.

Citation

“A New Implementation of the Schwarzschild Method for Constructing Observationally Driven Dynamical Models of Galaxies of All Morphological Types,” Eugene Vasiliev and Monica Valluri 2020 ApJ 889 39. doi:10.3847/1538-4357/ab5fe0

20 May 2020

Cosmic Rays as the Source of Life’s Handedness

You might expect living cells to be composed of a random soup of materials — but look closely and you’ll find they’re built from molecules with distinct orientation preferences. How did life’s preferred “handedness” arise?

Living Single-Handedly

Billions of years ago, perhaps on the barely solidified crust of young Earth, life somehow arose from non-living matter. Today, we still haven’t solved the puzzle of how this transition from non-living to living occurred — but nature has left behind some helpful clues, including the intriguing geometry of life’s building blocks.

This generic amino acid exists in both a left-handed and a right-handed form: all the same things are attached in the same order, but the two forms are mirror images and cannot be superimposed. If both forms exist, why is life built from only left-handed amino acids? [Public Domain]

Many organic molecules come in two forms: left-handed and right-handed versions that are mirror images of each other. Just as you can’t put a left-handed glove onto your right hand, the mirror images of these chiral molecules are distinct and cannot be superimposed.

But while the pre-biological soup of material on early Earth contained both left-handed and right-handed forms of chiral molecules, living organisms are homochiral: they’re built almost exclusively from left-handed amino acids and right-handed sugars and nucleotides, which preferentially construct right-handed DNA and RNA.

A Cosmic Influence

What determined the handedness of Earth’s life? Was it a random initial disequilibrium — perhaps an overabundance of right-handed nucleotides trapped in a prebiotic nursery — that was then copied to all subsequent life? Or might the bias only have arisen later, driven by an outside source?

In a new study, scientists Noemie Globus (New York University and Flatiron Institute) and Roger Blandford (Stanford University and KIPAC) explore how cosmic rays may have shaped the evolution of early life forms.

In this schematic, the authors illustrate how the decay of a cosmic ray proton produces a shower of spin-polarized particles. When these particles reach the Earth’s surface, they affect right-handed and left-handed molecules differently, imparting a chirality bias. [Adapted from Globus & Blandford 2020]

Mutating for Survival

Cosmic rays are highly energetic, charged particles that careen in from space and slam into the Earth’s atmosphere, producing a cascade of secondary particles that can reach the planet’s surface.

Irradiation from cosmic-ray cascades is thought to drive gene mutation in DNA — an exploratory process that is critical for evolution. Many successive gene mutations in the earliest stages of life likely allowed proto-life forms to adapt to their environment, increasing their complexity and their chances of survival.

Intriguingly, the physical force that governs cosmic-ray cascades naturally introduces a preferential handedness in the secondary particles that reach the planet’s surface. Could this handedness somehow have introduced homochirality for life forms?

Pick a Hand

Globus and Blandford propose a model in which prebiotic chemistry produced both of the mirror versions of DNA and RNA. As these proto-lifeforms worked to self-replicate and evolve, cosmic rays rained down, introducing minor mutations.

Prebiotic chemistry may have produced both the left-handed and right-handed forms of RNA (which the authors cleverly label “evil” and “live” to underscore their mirror-image nature). [Adapted from Globus & Blandford 2020]

The handedness of the cosmic rays caused a slight difference in the mutation rate of the two different chiral forms, giving a tiny edge to the right-handed DNA. Over many generations of self-replication, this small chiral bias eventually resulted in early life dominated by a single handedness.

Can we test this model? We might find supporting evidence from laboratory experiments that simulate the influence of cosmic-ray irradiation on bacteria mutation rates. And, of course, we can hope that we’ll one day explore life’s building blocks in samples from asteroids and other planets, further pinpointing what led to life as we know it.

Citation

“The Chiral Puzzle of Life,” Noemie Globus and Roger D. Blandford 2020 ApJL 895 L11. doi:10.3847/2041-8213/ab8dc6

11 May 2020

Signs of Collisions to Come

We know that when two neutron stars — the dense, compact cores of evolved stars — collide, they produce signals that span the electromagnetic spectrum. But could these binaries also flare before they merge, as well?

A Broad Range of Signals

Artist’s impression of the collision and merger of two neutron stars. [NSF/LIGO/Sonoma State University/A. Simonnet]

The discovery and follow-up of the gravitational-wave event GW170817, a collision of two neutron stars, provided the first direct evidence of the many forms of light that are emitted in these mergers. Between the instant of collision and the months that followed, observatories around the world recorded everything from high-energy gamma rays to late-time radio emission.

But emission might not be restricted to during and after the merger! A new study conducted by two researchers from the Flatiron Institute, Elias R. Most (also of Goethe University Frankfurt, Germany) and Alexander Philippov, explores the possibility that neutron star binaries may also produce flares of emission in the time leading up to their final impact.

This plot of the out-of-plane magnetic field density indicates the twist in flux tubes connecting the two neutron stars seen at the center of the plot. Here, an electromagnetic flare is launched from the binary after a significant twist has built up due to relative rotation of the right star. [Most & Philippov 2020]

What About Magnetic Fields?

In particular, Most and Philippov focus on how the magnetospheres of the two neutron stars — the magnetized environment surrounding each body — interact shortly before the objects collide.

The authors conduct special-relativistic force-free simulations of orbiting pairs of neutron stars in which each star is threaded with the strong dipole magnetic field expected for these bodies. The simulations then track how the stars’ magnetic fields evolve, twist, and interact as the bodies orbit each other.

A Twisted Fate

Most and Philippov find that dramatic releases of magnetic energy are a common outcome if the neutron stars orbit close enough to one another that their magnetospheres interact.

The authors show that the brightness of the flare luminosity depends only on how far apart the neutron stars are in the simulation: the smaller the separation, the brighter the flare. This dependence demonstrates that the flaring events are driven primarily by the energy stored in the twisted tube of magnetic flux that forms connecting the two neutron stars.

Here, the twisted flux tube and resultant flaring is caused by orbital motion of 45° misaligned magnetic fields, rather than by one star spinning. The bottom panel shows a 3D visualization of the field line configuration at the time of flaring. [Most & Philippov 2020]

When the two neutron stars spin at different speeds, the magnetic field loop that forms between the stars becomes progressively more twisted — until this stored rotational energy is abruptly ejected. And even if neither neutron star is spinning, the authors show that magnetic flux twist still builds up and releases as a result of the binary’s orbital motion, assuming that the magnetic fields of the two stars are not aligned.

Look for Radio Clues

So can we observe these sudden releases of energy? Most and Philippov argue that we should be able to spot the drama in radio emission: a radio afterglow will be produced behind the magnetized bubble that’s ejected from the twisted loop, and additional radio emission can be produced when the bubble collides with surrounding plasma.

Future work on this topic will explore the impacts of the neutron stars’ inspiral, and how the interactions of the magnetospheres change when the neutron stars carry unequal charge. The current study, however, indicates it’s worth keeping a radio eye out to see if we can spot signs of collisions to come!

Citation

“Electromagnetic Precursors to Gravitational-wave Events: Numerical Simulations of Flaring in Pre-merger Binary Neutron Star Magnetospheres,” Elias R. Most and Alexander A. Philippov 2020 ApJL 893 L6. doi:10.3847/2041-8213/ab8196

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