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Simulation of a black hole warping spacetime around it, with a background of the Milky Way.

Wandering supermassive black holes — those that don’t lie at their galaxies’ centers — may be tricky to find, but not all black holes that wander are lost! A new study demonstrates how we can hope to discover these missing nomads in the future.

Photograph of two spiral galaxies colliding.

When two galaxies collide, the resulting galaxy will contain multiple supermassive black holes. [NASA/Hubble Heritage Team (STScI)]

When Galaxies Collide

We know that the center of every massive galaxy hosts a supermassive black hole weighing millions to billions of solar masses. But galactic centers aren’t the only place that supermassive black holes can lurk! In fact, we expect that the majority of galaxies host many more of these monsters beyond just the central supermassive black holes. Why? Because galaxies merge.

Structure in our universe is largely built hierarchically: over time, galaxies have frequently collided with each other, growing progressively larger with each merger. But with each of these mergers, at least two supermassive black holes — one from each of the merging galaxies — are introduced into the resulting turmoil.

While gas and stars reorder themselves neatly into a new galaxy, eventually erasing all evidence of the merger, the black holes aren’t as well-behaved. Indeed, simulations show that it can take billions of years for those supermassive black holes to make their way to the center of the newly formed galaxy and merge — if they even make it at all!

Three simulated images show the stars, gas, and X-ray emission of a simulated galaxy that contains dozens of circled wandering supermassive black holes

This extreme example shows a simulated galaxy whose halo contains dozens of black holes (circled) above a million solar masses, including five (orange circles) shining with a bolometric luminosity above 1042 erg/s. The galaxy’s stars (top) and gas (center) show no evidence of the past mergers that led to this accumulation of black holes. The simulated X-ray image of the galaxy (bottom) reveals the five brightest black holes. [Adapted from Ricarte et al. 2021]

As more galaxy collisions occur, more off-center “wandering” supermassive black holes are produced — and by present day, galaxies can potentially host dozens of black holes above a million solar masses. So how do we find this vast population of wanderers? A new study led by Angelo Ricarte (Center for Astrophysics | Harvard & Smithsonian; Black Hole Initiative) explores the possibilities.

Revealing a Hidden Population

Ricarte and collaborators use a suite of cosmological simulations called ROMULUS to produce a realistic expectation of the black holes lurking in our universe. These simulations carefully track the positions and dynamics of supermassive black holes as galaxies merge and evolve over time, allowing us to explore the population of wandering supermassive black holes predicted to arise in galaxies at different times in the universe.

From this simulated population, the authors then predict the ways in which these wanderers may betray their locations:

  1. Hyperluminous X-ray sources
    Some nearby, accreting, wandering black holes should be detectable as exceedingly bright X-ray sources.
  2. Dual active galactic nuclei
    The simulations predict that galaxies will often host more than one dramatically accreting supermassive black hole — particularly at higher redshifts.
  3. X-ray halo
    If the black holes are too distant or dim to resolve individually, we can identify wanderers by stacking images of galaxies of similar mass. The “halo” of excess X-ray radiation can then be used to describe the wandering black hole population.
  4. Tidal disruption events
    Wandering supermassive black holes can tear apart stars that come too close! These disruptions should produce transient signals offset from galactic centers.

The ROMULUS simulations show that, for black holes smaller than 10 billion solar masses, wanderers greatly outnumber the central supermassive black holes in our universe. Ricarte and collaborators’ work demonstrates that we’ll need to consider this nomadic population carefully as we analyze our observations of the universe.

Citation

“Unveiling the Population of Wandering Black Holes via Electromagnetic Signatures,” Angelo Ricarte et al 2021 ApJL 916 L18. doi:10.3847/2041-8213/ac1170

Neutron Star Merger

The binary neutron star merger that produced GW170817 was also the source of a gamma-ray burst, which was unexpectedly faint. However, it turned out that this burst was not less energetic than average; rather, the jet that produced it had an unusual structure. So what caused the jet associated with GW170817 to look like it did?

A diagram showing the electromagnetic emissions arising from the binary neutron star merger that produced GW170817. The central jet that produced the GRB is offset from the line of sight by 30 degrees. [C. Bickel/Science]

Jet Setting Out of a Merger

The environment of a binary neutron star merger is a turbulent, energetic place. The merging objects can produce powerful winds and shed large amounts of mass, and the actual merger results in strong emission across the entire electromagnetic spectrum. The electromagnetic signal from the gravitational wave event GW170817 included the relatively short gamma-ray burst (GRB) 170817A, which turned out to be much fainter — and thus less energetic — than expected.

Some astronomers suggested that GRB 170817A belonged to a class of GRBs that were simply intrinsically less energetic, but others proposed that GRB 170817A was a typical short GRB — complete with polar jets launched after the collision — that was oriented off of our line of sight. Further study has borne out the latter prediction, but it has also shown that the jets associated with GRB 170817A have a somewhat unusual structure, especially toward their outer edges. What could cause this deviation from the norm?

To probe this question, a group of researchers led by Ariadna Murguia-Berthier (University of California, Santa Cruz/University of Copenhagen, Denmark) ran simulations of the winds and central jets involved in neutron star mergers to see how the structure of jets is influenced by their environments. 

Simulations of jets that were (top to bottom) choked, marginally successful, and successful. The parameter tw is the time the winds are active and tj is the time the central engine is active. [Adapted from Murguia-Berthier et al. 2021]

Breaking Free of Winds

There are several parameters to consider while modeling how a jet would move through a merger environment. Notably, it takes a finite amount of time for the merger remnant to collapse into (most probably) a black hole with an accretion disk. The collapse triggers the jet that would produce a short GRB. The surrounding winds are dependent on this collapse time, and their density could potentially snuff out the jet, preventing it from producing a short GRB. Murguia-Berthier and collaborators focused on how the jet would be affected by these winds and disk outflows.

A jet needs time to gather enough strength to break through the surrounding winds, so the central engine that powers the jet needs to stay active till the jet is strong enough. This balance between jet strength and wind strength plays a large role in whether a jet is successful in punching out of the merger environment and forming a short GRB. Additionally, while the jet is gathering strength and making headway into the winds, extra energy is deposited into a “cocoon” around the jet. This energy cocoon evolves based on its environment and can also impact the structure of the jet.

Merging with Observations

Murguia-Berthier and collaborators explored mergers with a variety of different parameters, but the parameters associated with GW170817 were of particular interest. The kilonova that accompanied the merger and the delay between the gravitational-wave signal and the GRB helped narrow down viable scenarios for the formation of this observed explosion. With their simulations, Murguia-Berthier and collaborators found that the time it took for the merger remnant to collapse into a black hole was between 1 and 1.7 seconds. Excitingly, this range agrees with many values from previous studies that used completely different approaches to estimate the collapse time!

Murguia-Berthier and collaborators cautioned readers that the simulations could not cover every physical scenario. However, this work is a demonstration of how simulations can be used with observational constraints to better explain the outcome of neutron star mergers.

Citation

“The Fate of the Merger Remnant in GW170817 and Its Imprint on the Jet Structure,” Ariadna Murguia-Berthier et al 2021 ApJ 908 152. doi:10.3847/1538-4357/abd08e

Map of the sky shows a bright band of gamma-ray emission across the center.

The disk of the Milky Way glows with continuous emission of high-energy gamma-ray photons. Where does this diffuse emission come from? A new study suggests that we may be missing the complete picture.

Smashing Cosmic Rays

Roughly 80% of the gamma-ray photons detected by the Fermi LAT gamma-ray detector come from diffuse emission — emission produced in the plane of our galaxy that isn’t associated with specific sources. Scientists have identified speeding cosmic rays as the primary culprit: high-energy protons and atomic nuclei whiz through space at nearly the speed of light, slamming into the interstellar medium and producing byproducts of gamma rays, neutrinos, and more.

Photograph of a detector array spread over a large surface of a plateau surrounded by mountains.

Observations from this cosmic-ray observatory in Tibet reveal the diffuse gamma-ray emission in our galaxy’s disk. [Institute of High Energy Physics of the Chinese Academy of Sciences]

But does this picture tell the whole story? In a recent study, Nanjing University scientists Ruo-Yu Liu and Xiang-Yu Wang point out a possible concern with this model: if cosmic-ray collisions produce the galaxy’s diffuse gamma-ray emission … where are all the neutrinos?

A Conflict from Missing Neutrinos

By modeling the interaction of galactic cosmic rays with the interstellar medium in the Milky Way, Liu and Wang illustrate the problem: in order to reproduce the spectrum of diffuse gamma-ray emission recently observed by detectors on the Tibetan Plateau, the cosmic-ray collisions would also produce a large number of neutrinos — so many, in fact, that they should be observable by detectors on Earth, like the IceCube neutrino observatory. The problem? Based on IceCube’s most recently released results, these predicted neutrinos aren’t there!

Since the neutrino and diffuse gamma-ray observations conflict, Liu and Wang argue, then the model must be missing something. The source of the seemingly diffuse gamma-ray emission from the galactic disk cannot only be cosmic-ray collisions with the interstellar medium. Instead, there must be a contribution from some additional source that produces high-energy gamma rays without also creating lots of neutrinos.

What is that source? The authors have found a potential culprit.

Another Player

Image of a complicated star-forming nebula with bright regions of gamma-ray emission

A grayscale infrared map of the Cygnus cocoon is overlaid here with colored gamma-ray data showing the excesses of high-energy photons. [IFJ PAN / HAWC]

Of the highest-energy diffuse gamma-rays detected by the Tibet observatory, 40% come from a single region: the center of the Cygnus cocoon, a superbubble surrounding a site of massive star formation. Could this area be producing the extra gamma rays observed in the diffuse emission?

Liu and Wang describe several potential sources of gamma rays in the cocoon — like the massive star cluster Cygnus OB2, the supernova remnant γ Cygni, and a pulsar wind nebula — and demonstrate that, by adding contributions from these sources, they can successfully reproduce the diffuse gamma-ray emission we observe while not exceeding the upper limits on neutrino production set by the IceCube observations.

More exploration of this picture is still needed, but the authors’ work shows that we still plenty more to learn about the sources that produce high-energy particles in our galaxy!

Citation

“Origin of Galactic Sub-PeV Diffuse Gamma-Ray Emission: Constraints from High-energy Neutrino Observations,” Ruo-Yu Liu and Xiang-Yu Wang 2021 ApJL 914 L7. doi:10.3847/2041-8213/ac02c5

Illustration of a disk of gas spiraling onto a bright protostar.

What forces are at work in the hidden centers of clouds that are forming baby massive stars? New images reveal the roles played by gravity and whirling magnetic fields.

To Make a Massive Star

High-mass stars of up to 120 solar masses are a critical driver of galaxy evolution: they pump energy into their surroundings and enrich galaxies with heavy elements that can’t be produced elsewhere. Yet massive stars remain shrouded in mystery — in fact, we still don’t fully understand how these behemoths are born.

young stellar objects

This false-color infrared image, captured by NASA’s WISE telescope, reveals young, massive stars (pink objects near center) forming in the Rho Ophiuchi cloud complex. [NASA/JPL-Caltech/WISE Team]

What do we know? The birthplaces of massive stars are molecular clouds that, as they collapse under their own gravity, fragment into clumps. Hot cores form at the centers of these clumps as collapse continues. Accretion disks then form around these molecular cores, feeding material from the collapsing cloud onto the soon-to-be star and helping it to grow to a point where nuclear fusion can ignite.

On scales of ~2,000–20,000 au, observations suggest that magnetic fields play an important role in funneling material inward to grow the baby star. But observations become more challenging to make on smaller scales — so we still don’t know what’s happening in the innermost ~1,000 au, at the interface between the molecular core and its accretion disk. Do magnetic fields provide useful support on these small scales? Or does gravity dominate, ultimately crushing everything inward?

Peering Into the Dust

In a new study led by Patricio Sanhueza (National Astronomical Observatory of Japan; SOKENDAI, Japan), a team of scientists has now addressed these questions. Using the incredible resolving power of the Atacama Large Millimeter/submillimeter Array (ALMA), Sanhueza and collaborators have probed the gas and dust at scales of ~1,000 au around a hot molecular core embedded in the high-mass star-forming region IRAS 18089–1732.

Image showing dust distributed into spiral filaments with correspondingly spiraling magnetic field lines.

ALMA observations show the dust (color scale and contours) and the magnetic field vectors at the center of the hot molecular core IRAS 18089–1732. [Sanhueza et al. 2021]

The high-resolution ALMA observations reveal spiral-like features in both the dust and the gas distribution in this innermost region — forming a seeming whirlpool of material falling inward onto the baby star. Using polarization measurements of the dust, the authors model the magnetic field to confirm that the field lines have been dragged around with the gas, producing a configuration that includes a toroidal component wrapping equatorially around the protostar.

Gravity Is King

What does all this tell us about the physical processes happening in the inner 1,000 au around a newly forming baby star? By analyzing the energy balance of the system, Sanhueza and collaborators show that gravity overwhelms the other processes at work in this region — including turbulence, rotation, and the magnetic field, which all play roughly equal roles in trying to support IRAS 18089–1732 against collapse.

Though magnetic fields exert an important influence on larger scales, they take a back seat in this innermost region where gravity is king. Thus, in the hot, shrouded centers of collapsing molecular clouds, even magnetic whirlpools eventually succumb to the crush of gravity to help form baby stars.

Citation

“Gravity-driven Magnetic Field at ~1000 au Scales in High-mass Star Formation,” Patricio Sanhueza et al 2021 ApJL 915 L10. doi:10.3847/2041-8213/ac081c

Image of two bright blue stars side by side and one dimmer circled star below, against a background of more distant stars.

Don’t plan to knock on a door at Alpha Centauri asking to borrow a cup of sugar just yet — but with a new look at this star system using a powerful telescope, we now know a bit more about our neighbors.

Secrets Among Nearby Stars

Proxima Centauri b

This artist’s interpretation shows the planet Proxima Centauri b around its host star. You can see the binary α Cen AB in between the planet and star, as two faint white dots in the background. [ESO]

When it comes to exploring Sun-like stars that might host planets, the Alpha Centauri (α Cen) star system is an ideal target. At just over 4 light-years away, this triple system — the binary pair α Cen AB and an additional companion, Proxima Centauri — contains the closest stars to the Sun.

Recent news has hyped the discovery of two exoplanets around the red dwarf Proxima Centauri — but what other surprises might the larger stellar system harbor? Given that α Cen A and B are both very similar to the Sun, it would be particularly valuable if we could find Earth-like, potentially habitable worlds around these near neighbors.

Choosing a Method

But how to detect them? Searching for transits works only for very specific orbit orientations. Direct imaging might be an option, since α Cen is so close. But even these nearby stars are challenging: α Cen A’s habitable zone lies at about 1.2 au, or just 0.9” in angular separation, from our point of view. It’s hard to confidently detect a small, dim object at that separation!

Four images of a single source in the center of a frame.

Calibrated images of α Cen A and B taken with ALMA in October 2018 (left two panels) and in August 2019 (right two panels). [Adapted from Akeson et al. 2021]

Another method may prove useful in this case, however: astrometry. In a new study, a team of scientists led by Rachel Akeson (NASA Exoplanet Science Institute, Caltech-IPAC) has used the high resolving power of the Atacama Large Millimeter/submillimeter Array (ALMA) to make some of the most precise astrometric measurements of α Cen AB yet.

Hints of Influence

Astrometry relies on the idea that the slight gravitational tug of an orbiting planet causes a star to “wobble” in place. If this effect is large enough, we can detect it via meticulous imaging that very precisely tracks the location and motion of the star on the sky over time.

Taking advantage of the high-resolution observations provided by ALMA’s long baseline, Akeson and collaborators captured measurements of α Cen A and B during 2018 and 2019. Their results provide the first high-accuracy absolute measurements of the stars’ positions on the sky since 1991, as well as the highest-accuracy differential astrometry yet, comparing their relative separation and searching for the tiny influence of planets around the two stars. 

Four-panel plot showing the best-fit orbit for the binary stars, a closeup of the data on the model, and the residuals.

Top left: astrometric measurements (red: Hipparcos and ALMA data, blue: archival data) and best-fit orbit of α Cen B relative to α Cen A. Top right: enlargement of the 2019 ALMA measurements (the total orbit takes ~80 years). Bottom: residuals of the fit as a function of time. [Akeson et al. 2021]

The authors then combine these results with archival data to better constrain α Cen AB’s orbit and properties.

A Promising Future

Akeson and collaborators show that ALMA can produce remarkably precise astrometric measurements for the α Cen system, demonstrating the exciting potential of using ALMA for this technique. Though the observations don’t reveal signs of a planet yet, continued monitoring should allow us to ultimately be able to detect planets of a few tens of Earth masses in stable orbits between 1 and 3 au around α Cen A.

But these results go beyond our search for planets — they also refine our measurements of α Cen’s motions. This allows us to make more accurate estimates of the physical properties of α Cen A and B, filling in our understanding of our nearest neighbors.

Citation

“Precision Millimeter Astrometry of the α Centauri AB System,” Rachel Akeson et al 2021 AJ 162 14. doi:10.3847/1538-3881/abfaff

Gaia spacecraft

The Gaia satellite is making the highest precision measurements yet of stellar movement. Specifically, it is measuring the positions, velocities, and parallaxes of stars across the sky. This is incredibly useful information to have, but it doesn’t provide a complete picture of a star’s motion. For instance, how do stars move along our line of sight?

A map showing the predicted paths of 40,000 stars over the next 400,000 years. The predicted paths are based on Gaia’s early third data release. The plotted stars are within 100 parsecs of the solar system. [ESA/Gaia/DPAC]

Not On All Dimensions

Currently, Gaia returns “5D astrometry” for most of the stars it observes, which consists of two position coordinates, two velocities, and a parallax (apparent change in position as the Earth orbits the Sun). A lot of science has been done with these data already, such as probing the origins of the stellar streams surrounding the Milky Way.

However, a majority of the astrometry provided by Gaia is restricted to the plane of the sky, which means we’re missing how stars move along our line of sight — that is, whether they’re moving towards or away from us. This puts some fascinating science out of reach, like mapping out subtle stellar and dark matter structures in the Milky Way. Spectroscopic surveys can give us line-of-sight velocities, but they are time-consuming and limited in the volume of space they can cover. Additionally, not many spectroscopic surveys overlap with the region being observed by Gaia.

All’s not lost though! A recent study led by Adriana Dropulic (Princeton University) shows how machine learning could be used to predict line-of-sight velocities for stars with 5D astrometry from Gaia.

The true, predicted, and error-sampled distributions of different stellar velocity components. The error-sampled distributions trace the true distributions more closely than the predicted distributions. [Adapted from Dropulic et al. 2019]

Training a Neural Network

To develop their machine learning technique, Dropulic and collaborators started with a publicly available catalog of mock Gaia data, which included line-of-sight velocities. They also added stars similar to those in Gaia Enceladus, a prominent stellar structure that emerges when velocities are plotted. The mock catalog ended up covering the space within roughly five kiloparsecs (or 16,000 light-years) of the Sun and contained roughly 75 million stars. The catalog was then separated into training, validation, and test sets, the former resembling the real Gaia sample that has line-of-sight velocities, or “6D” information. 

Dropulic and collaborators then trained a neural network to predict the line-of-sight velocity of a given star and the associated uncertainty with that velocity. An important caveat of this method is that it does not aim to make a near-perfect guess at the line-of-sight velocity for a single star. Rather, the goal is to get a reasonable estimate of the velocity distribution of a whole group of stars.

Moving On from Mock Data

An advantage of having the network output an uncertainty was that Dropulic and collaborators were able to construct “error-sampled” velocity distributions, which are produced by averaging multiple velocity and uncertainty predictions for a single star and repeating for the entire distribution. These error-sampled velocity distributions ended up being closer to the true distributions than the predicted distributions.

The next step in this work is to train the neural network on a real catalog of Gaia data and also explore more distant regions of the Milky Way. The upcoming third Gaia data release will contain roughly 30 million stars with full 6D astrometry, so it won’t be long till this machine learning method can be put into practice!

Citation

“Machine Learning the Sixth Dimension: Stellar Radial Velocities from 5D Phase-space Correlations,”  Adriana Dropulic et al 2021 ApJL 915 L14. doi:10.3847/2041-8213/ac09ef

black hole binary

Do black holes have a preferred size? New research has explored the populations of black holes involved in catastrophic, gravitational-wave-emitting collisions — and an interesting pattern has emerged.

A Question of Mass

The population of so-called stellar-mass black holes in the universe pose an interesting puzzle: What size are they, typically, and why?

Plot showing masses of observed black hole and neutron star binary mergers. Plot includes black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange).

The “stellar graveyard”, a plot that shows the masses of the different components of observed black hole binary mergers included in GWTC–2 (blue). Also shown are the black hole masses we’d previously measured using electromagnetic observations (purple). Click to enlarge. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

Before 2015, we had measured the masses of a small number of stellar-mass black holes using electromagnetic observations. These black holes reliably weighed in at somewhere between ~5 and ~20 solar masses, providing us — or so we thought — with a fairly consistent picture of these mysterious bodies.

That picture, however, was shattered by LIGO/Virgo’s first detection of gravitational waves from a merging pair of black holes. The signal originated from a pair of black holes of ~30 and ~35 solar masses — both substantially heavier than stellar-mass black holes we’d previously observed. Since then, additional merging black holes spotted by LIGO/Virgo have continued to weigh in above 20 solar masses. Some even weigh more than 80 or 90 times the Sun!

Now that we’ve gathered a number of observations, we can start to ask what the mass distribution looks like for the underlying population of merging stellar-mass black holes. A new study by scientists Vaibhav Tiwari and Stephen Fairhurst (Cardiff University, UK) dives into the LIGO/Virgo detection catalog looking for answers.

Building a Distribution

Tiwari and Fairhurst use GWTC–2, the second LIGO/Virgo catalog of gravitational-wave detections, to analyze a population of 39 strong signals of binary black hole mergers. The authors use a statistical model to then reconstruct the underlying population of merging black holes from these data, and they explore the distributions of spins and masses for this population.

plot of the component mass distribution shows a decaying power law with 4 additional peaks marked, spaced a factor of ~2 apart

The distribution of underlying component masses for the merging black hole population shows four peaks spaced a factor of ~2 apart, rather than just a decaying power law. Click to enlarge. [Tiwari & Fairhurst 2021]

The simplest outcome would be for the black hole masses to track a decaying power law: because black holes evolve from massive stars, and smaller stars are more numerous than larger ones, we’d expect a smoothly decreasing distribution of black hole masses.

Instead, Tiwari and Fairhurst detect structure in the distribution on top of the decaying power law: a set of four peaks that fall at component masses of 9, 16, 30, and 57 solar masses. 

Clues Point to More Collisions

What’s going on? The authors show that this might be a clue as to how these black holes formed.

In a hierarchical merger scenario, where black holes are built up through successive collisions of smaller black holes, we’d expect to see a mass pile-up at the location of the first peak in the mass distribution, followed by subsequent peaks spaced roughly a factor of 2 apart.

Invisible black holes warp the space time around them in the center of a busy, dense cluster of stars.

Visualization of black holes colliding at the center of a dense stellar cluster. [Carl Rodriguez/Northwestern Visualization (Justin Muir, Matt McCrory, Michael Lannum)]

Perhaps, then, the authors’ detection of a structured distribution hints that many of the merging stellar-mass black holes in our universe didn’t evolve in isolation, but instead formed through successive collisions in dense stellar environments.

Tiwari and Fairhurst caution that their results are currently based off of a very small number of data points, and we’ll need to wait until we’ve amassed more detections to make any robust claims. But if future observations confirm these trends, this could provide valuable insight into stellar-mass black holes in the universe. 

Citation

“The Emergence of Structure in the Binary Black Hole Mass Distribution,” Vaibhav Tiwari and Stephen Fairhurst 2021 ApJL 913 L19. doi:10.3847/2041-8213/abfbe7

Illustration of a planet orbiting a star in an ellipse that traces above the poles of the star rather than in the equatorial plane.

In some planetary systems, the direction that a star spins and the direction its planets orbit don’t always line up. A new study explores what we can learn from these nonconformists.

Nature Is Trending

Much of science involves searching for patterns and trends in data. Patterns in the universe — preferences for certain shapes, locations, alignments, etc. — can often reveal hidden underlying physics that drives nature to take a non-random course. This means that patterns and trends frequently provide the key to understanding how the universe works.

illustration of dust and gas swirling around a bright, newly forming star.

A protostar lies embedded in a disk of gas and dust in this visualization. Since stars and their planets form from the same cloud, it would make sense for their rotations to be aligned. [NASA’s Goddard SFC]

Exoplanet populations are an especially intriguing place to look for trends. In recent years, our sample of observed exoplanets has grown large enough that we can now start to do useful statistical analysis — and there’s a lot we can hope to learn from this about the formation and evolution of planetary systems.

One particular curiosity among exoplanets: a planet’s orbital direction is not always aligned with its host star’s spin direction. Since a star and its planets all form out of the same rotating cloud of gas and dust, conservation of angular momentum should produce planet orbits and stellar spins that are aligned. But, while we see a large population of well-aligned systems, we also see a smaller population of misaligned systems.

diagram labeling various angles on a sphere relative to the line of sight.

Diagram illustrating the angle between the sky-projected stellar spin and planetary orbit (λ) and the actual 3D angle between the spin and orbit (Ψ). The tilt of the star relative to the observer line of sight is marked by i. [Albrecht et al. 2021]

What causes planets to become misaligned with their stars? A new study led by Simon Albrecht (Aarhus University, Denmark) examines patterns in a population of observed star–planet systems to find out.

A Polar Population

Albrecht and collaborators explored a valuable sample of 57 star–planet systems. For the majority of planetary systems with observed spin/orbital directions, we can only measure the angle between the sky-projected orbital and spin axes. But for the sample that Albrecht and collaborators used, we have independent measurements of the inclination angle of the star relative to our line of sight. Thus, for these 57 systems, the authors were able to identify the actual angle in 3D space between the planets’ orbital axes and the stars’ spin axes.

Two plots showing the two measured angles for the population of 57 planetary systems.

Left: The angle between the sky-projected orbital and spin axes (λ) for the authors’ sample. Right: The actual angle between the axes (Ψ). The actual angles show two clusterings: one near zero (aligned), and one around 90° (perpendicular). Click to enlarge. [Adapted from Albrecht et al. 2021]

The result? Albrecht and collaborators find that the majority of the systems are aligned, as expected. But the 19 misaligned systems do not have misalignments that are distributed randomly through all angles. Instead, almost all of the misalignments cluster around 90° (ranging from 80°–125°) — meaning that the planet orbits the poles of the star, perpendicular to the direction that the star spins.

What could cause this polar pileup? The authors propose several theoretical possibilities that include dynamical interactions between the planet and the star, or between the planet and an additional unseen, distant companion body. But, as we’ve seen, nature has a mind of its own — and there may be multiple mechanisms at work! We don’t yet have enough information to solve this puzzle with certainty, but a continued search for patterns is sure to point us in the right direction eventually.

Citation

“A Preponderance of Perpendicular Planets,” Simon H. Albrecht et al 2021 ApJL 916 L1. doi:10.3847/2041-8213/ac0f03

Illustration of a pulsar and disrupted star in a binary

What’s going on at the largest filled-aperture radio telescope in the world? The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has discovered a new assortment of highly magnetized, pulsating neutron stars located beyond the main disk of the Milky Way.

Globular Cluster Treasure Troves

diagram showing a pulsar with magnetic field lines and beams of emission from its poles.

Diagram of a pulsar, a rotating neutron star with a strong magnetic field. [NASA/Goddard Space Flight Center Conceptual Image Lab]

Pulsars are the compact remnants of evolved stars that spin rapidly, shining a beam of emission across the Earth like a lighthouse with an incredibly precise period. We can use these cosmic clocks to explore the galaxy around us, leveraging their precise timings to learn more about stellar evolution, their environments, the interstellar medium, and more.

But pulsars also exist beyond the relatively nearby disk of the Milky Way! A particularly interesting place to search for them is inside the globular clusters — compact, old clusters of stars that are bound together in spheres — that orbit our galaxy.

Pulsars in globular clusters are often quite exotic. The ones we’ve detected in globular clusters include some of the fastest spinners, eclipsing binaries, triple systems, and pulsars in eccentric orbits with their companions. By identifying more objects in this array of interesting systems, we can learn about the range of outcomes produced by stellar evolution in old star clusters.

FAST radio telescope

The 500-m FAST radio dish, built into a natural basin in southwest China. [Xinhua/Ou Dongqu]

The challenge? These globular cluster pulsars are far away and often faint. We need large and sensitive radio surveys to hunt for these mysterious bodies — and this is where FAST comes in.

A Giant Telescope on the Hunt

FAST, an enormous radio telescope built into the landscape in southwest China, launched its globular cluster pulsar survey in 2018. Since then, this telescope has been searching for signs of faint, pulsating neutron stars within the Milky Way globular clusters that lie in the FAST sky.

In a new study led by Zhichen Pan (NAO, Chinese Academy of Sciences), a team of scientists reports the most recent results of this survey, which include:

Plots showing pulse profiles for 12 new pulsars

Pulse profiles for some of the newly discovered pulsars. Click to enlarge. [Pan et al. 2021]

  • the discovery of 24 new pulsars in 15 globular clusters, roughly doubling the number of globular cluster pulsars known in the FAST sky
  • the first-ever detections of pulsars in globular clusters M2, M10, and M14
  • the discovery of several new black widow and redback pulsars — binary systems in which a very low-mass star orbits close to a millisecond pulsar, as the pulsar gradually consumes its companion
  • additional measurements of previously discovered pulsars

The new discoveries from these globular clusters are primarily pulsars in binaries — possibly due to a low rate of encounters in these clusters, which would allow binaries to survive for longer.

The results presented by Pan and collaborators demonstrate that FAST’s high sensitivity exceeds that of previous surveys and shows great promise for the future. We can expect our sample of globular cluster pulsars to keep growing as FAST continues its hunt alongside other telescopes. Stay tuned!

Citation

“FAST Globular Cluster Pulsar Survey: Twenty-four Pulsars Discovered in 15 Globular Clusters,” Zhichen Pan et al 2021 ApJL 915 L28. doi:10.3847/2041-8213/ac0bbd

Gravitational-wave events have allowed us to measure distances in space in a way that’s independent of previous techniques. By extension, we can also make independent measurements of the universe’s rate of expansion, expressed as the Hubble constant. Current measurements of the Hubble constant from gravitational-wave events still rely on electromagnetic information, but with new observatories on the horizon, it may be possible to make measurements of the Hubble constant with just a single gravitational-wave event.

Images of the galaxy NGC 4993 showing the electromagnetic counterpart to the gravitational wave-event GW170817. The image was taken by the Dark Energy Camera (DECam) on Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. [Adapted from M. Soares-Santos et al. 2017]

Ways to Use Gravitational-Wave Events

The gravitational-wave event GW170817 was the first observation of a binary neutron star merger. With concurrent electromagnetic observations, we were able to use information from GW170817 to make an independent measurement of the Hubble constant. This measurement coupled the distance to the event host galaxy (as determined from the gravitational-wave event) with the redshift of the host galaxy, which is derived from electromagnetic information.

While the uncertainty on this measurement of the Hubble constant is relatively large, higher precision measurements will be possible in the near future with more advanced observatories. However, our interpretation of the gravitational-wave signal in this case is very dependent on the inclination of the merging system and the distance to the merger host galaxy as measured by electromagnetic observations — and we’re currently unable to separate the influence of these two quantities.

But what if we could disentangle these influences with a single gravitational-wave event? A recent study led by Juan Calderón Bustillo (Universidade de Santiago de Compostela, Spain) explores how we could use future observations of neutron star mergers like GW170817 to make higher precision measurements of the Hubble constant.

The spectrum of gravitational-wave signals from face-on and edge-on merging systems, with certain signal components highlighted. The black line shows the sensitivity of the planned Neutron star Extreme Matter Observatory (NEMO). [Adapted from Bustillo et al. 2021]

Picking Up Subtle Signals

Like most signals, a gravitational-wave signal can be broken down into multiple simpler signals. In the case of a neutron star merger, the components of a gravitational-wave signal are dependent on properties of the merging system, such as stellar mass and the orientation of the system relative to the observer.

With current observatories, we don’t have the ability to pick up the subtler component signals in a gravitational wave, including those that appear after the merger. But if we did — and we eventually will — Bustillo and collaborators show that we could use those subtle signals to disentangle the influences of system inclination and host galaxy distance. Then, we would also be able to make more precise measurements of the Hubble constant. Most notably, being able to pick up these subtler signals would allow us to make measurements of the Hubble constant with just a single merger event!

Future Prospects

The ability to make measurements of the Hubble constant based on single events enables us to test an interesting hypothesis: what if the universe’s rate of expansion is sensitive to direction? While those particular measurements are still a few decades in the future, Bustillo and collaborators show that their method will eventually be limited by our electromagnetic observing capabilities, rather than our ability to observe gravitational-wave signals!

Citation

“Mapping the Universe Expansion: Enabling Percent-level Measurements of the Hubble Constant with a Single Binary Neutron-star Merger Detection,” Juan Calderón Bustillo et al 2021 ApJL 912 L10. doi:10.3847/2041-8213/abf502

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