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neutron star merger

Short-duration gamma-ray bursts (GRBs) are brief flashes of high-energy emission lasting less than two seconds. These events have long been hypothesized to occur when two neutron stars spiral in and collide, launching enormously powerful jets of light and matter. Can we now definitively make this connection?

A Singular Event?

gamma-ray burst

Artist’s illustration of a gamma-ray burst. Could short gamma-ray bursts all be the product of neutron-star mergers? [NASA/Swift/Mary Pat Hrybyk-Keith/John Jones]

In 2017, the LIGO/Virgo gravitational-wave detectors made the first observations of two neutron stars merging, in a chirp of ringing spacetime known as GW170817. Just 1.7 seconds thereafter, scientists spotted a weak, short-duration gamma-ray burst coming from the same part of the sky, now labeled as GRB170817A.

In the ensuing months and years, we’ve carefully analyzed the data supplied by this event, attempting to understand what makes this GRB light curve look the way it does. While GW170817’s gamma-ray emission shares some traits with the known population of observed short GRBs, it has a number of weird quirks.

GW170817’s host galaxy lies much closer than that of any other short GRB known, and its gamma-ray energies were perhaps 10,000 times lower than is typical among the short GRB population. GRB170817A’s afterglow — a long-lived glow that normally follows right after the initial gamma-ray flash of a GRB — only showed up 9 days after the flash.

So is the outflow from GW170817 actually the same as that in other short GRBs? And if so, does this mean that all short GRBs arise from neutron-star mergers?

Angles Matter

To address these questions, New York University scientists Yiyang Wu and Andrew MacFadyan explored the structure of GW170817’s outflow in comparison with a sample of 27 observed short GRBs. The authors modeled the GRB light curves allowing for a range of outflow structures — from spherical flows that splay out in all directions, to jetted structures that are tightly beamed in a specific direction.

From their fits, Wu and MacFadyan showed that the differences between GRB170817A’s light curve and those of the other GRBs can all be explained by one thing: viewing angle.

jet angles

Plot of the viewing angle vs. opening angle for the jets of 17 known short GRBs and GW170817. While all have similar opening angles, GW170817 is the only one viewed significantly off-axis. [Wu & MacFadyan 2019]

GRB170817A and all the other GRBs were found to be best fit by similar models with fast-moving jet-like structures, where the jet itself is extremely narrow: less than 10° wide. But according to Wu and MacFadyan’s fits, the “normal” GRBs are viewed from an angle of ~0–6° off the axis of the jet — which means we’re looking nearly straight down the jet. For GW170817, however, we’re seeing the jet off-axis, from an angle of about 30°.

Complications from Relativity

Why does viewing angle matter? Since these outflows are moving at nearly the speed of light, relativistic effects come into play. This means that energies and timescales can all look different depending on whether we’re viewing the jet on-axis or off-axis.

If GW170817 and the other GRBs can indeed all be explained by the exact same jet model, this is a powerful indicator that they all represent the same event. If true, that means that all short GRBs we’ve observed were also caused by the violent collision of two neutron stars somewhere in the distant universe.

Citation

“GW170817 Afterglow Reveals that Short Gamma-Ray Bursts are Neutron Star Mergers,” Yiyang Wu and Andrew MacFadyen 2019 ApJL 880 L23. doi:10.3847/2041-8213/ab2fd4

Boyajian's star

Remember KIC 8462852, better known as Boyajian’s star (or you may have seen it referred to as the “alien megastructure” star)? We still don’t have a definitive explanation for this source’s odd behavior — in part because we thought that Boyajian’s star was one-of-a-kind.

But what if it isn’t?

Unusual Dimming

comet swarm

In this artist’s illustration, a star’s light is blocked by orbiting comets. A swarm of comets is just one of the many explanations that have been proposed for Boyajian’s unusual light-curve dips. [NASA/JPL-Caltech]

Boyajian’s star was first discovered in 2016 by a group of citizen scientists examining light curves in the Planet Hunters Project. This seemingly ordinary source was doing something rather extraordinary: its light curve showed a series of unusual and irregular dips scattered across 1,460 days of observation by Kepler.

More dips were found later in ensuing data. The dips last anywhere from a few days to a week, didn’t appear to be periodic, and block out 0.5–22% of the star’s light — presenting a combination of traits that seem to rule out most ordinary explanations, such as transiting planets or intrinsic stellar variability. So what causes this bizarre star’s odd behavior?

The Hunt for Analogs

Thus far, our search for conclusive answers remains hindered by a scarcity of data. Future observations of Boyajian’s star will continue to help us better understand its behavior — but that still leaves us working with a sample size of one star, which makes it hard to determine anything with certainty.

But what if there are more stars like Boyajian’s star?

A new study published by scientist Edward Schmidt (University of Nebraska, Lincoln) details the hunt for analogs of this unusual dipper — with a number of successes!

slow dipper light curves

Light curves for the 15 slow-dipper candidates identified in the study. Click to enlarge. [Schmidt 2019]

More Dippers Found!

Schmidt looked for Boyajian’s star analogs by applying an automated search algorithm to a subset of the Northern Sky Variable Survey dataset, a catalog of light curves for more than 14 million objects observed in a vast photometric survey of variable stars.

The algorithm identified light curves that showed dipping events similar to Boyajian’s star. Schmidt then followed up on promising candidates manually, exploring their light curves from the All Sky Automated Survey for Supernovae (ASAS-SN) and rejecting sources whose dips could be readily explained by things like a binary companion or intrinsic variability.

In the end, Schmidt was left with a total of 21 stars showing possible unusual, unexplained dipping.

Further Observations, Please

The new dimming stars fall into two categories:

  1. HR diagram dippers

    An H-R diagram showing the locations of the 21 candidate dipping stars (circles represent slow dippers; triangles represent rapid dippers) found in this study. They seem to cluster in two distinct regions of the diagram. [Schmidt et al. 2019]

    15 slow dippers, whose light curves show dimming with rates similar to Boyajian’s star. These sources may well be analogs of Boyajian’s star.
  2. 6 rapid dippers, whose light curves show even more frequent variability. These sources may be more extreme versions of Boyajian’s star.

Data from the Gaia Data Release 2 show that the dipper candidates cluster in two locations on an HR diagram, suggesting that dippers tend to be main-sequence stars of ~1 solar mass or evolved red giants of ~2 solar masses.

So what’s next? Schmidt encourages other astronomers to follow up on these candidates and observe their behavior over longer timescales! Perhaps one of these new dimming stars can reveal what’s going on with Boyajian’s star — and its analogs.

Citation

“A Search for Analogs of KIC 8462852 (Boyajian’s Star): A Proof of Concept and the First Candidates,” Edward G. Schmidt 2019 ApJL 880 L7. doi:10.3847/2041-8213/ab2e77

M31 and satellites

Though dark matter appears to be common in the universe, there’s still a lot we don’t know about it. A new study has now shed some light on this mysterious topic using faint satellite galaxies around the Milky Way.

Prolific Yet Unseen

dark matter

The relative amounts of the different constituents of the universe. Dark matter makes up roughly 27%. [ESA/Planck]

Our universe is composed almost entirely of dark energy, dark matter, and ordinary matter. While ordinary matter makes up a scant 5% of the universe, dark matter appears to be more common, accounting for 27%. But while dark matter reveals itself through its gravitational effects — adding bulk to galaxy halos that helps hold galaxies together and changes how they move, for instance — we’ve yet to detect it directly.

This challenge means that we’re still working to understand the nature of this unseen substance. Is dark matter made up of primordial black holes? An as-yet undiscovered subatomic particle? Or something else entirely? 

Abell 1689

Strong gravitational lensing like that observed in this image of Abell 1689 provides evidence for dark matter, but we still don’t understand its nature. [NASA/N. Benitez/T. Broadhurst/H. Ford/M. Clampin/G. Hartig/G. Illingworth/the ACS Science Team/ESA]

The Hunt for the Right Model

Based on our observations and models of our universe, the standard picture of dark matter is the ΛCDM model, in which dark matter is described as cold (it moves slowly, forming structures only gradually) and collisionless (it doesn’t scatter off of ordinary matter, instead effectively passing through it).

The cold, collisionless dark-matter model has held up to a number of tests, and it neatly explains the large-scale structure of our universe. But some challenges to the model exist, and astronomers are still considering a number of alternative pictures.

In a new study led by Ethan Nadler (Kavli Institute for Particle Astrophysics and Cosmology, Stanford University), a team of scientists has now tested alternative theories by asking whether dark matter might not be collisionless, but instead interacts with ordinary matter.

Suppressing Structure

Nadler and collaborators point out that alternative models that treat dark matter as a collisional fluid come with a catch: in this picture, as dark matter scatters off of particles in the early universe, heat and momentum are transferred. This transfer smooths out perturbations in the distribution of matter, suppressing the very glitches that would later grow to become small-scale structure in the universe today.

In effect, the more that dark matter collides with baryons, the less small-scale structure there should be today — limiting the number of low-mass dark-matter halos in our galactic neighborhood and constraining how many small, faint galaxies reside within them.

dark-matter–proton scattering cross sections

Upper limits on the velocity-independent dark-matter–proton scattering cross section as a function of dark-matter particle mass. The different shaded regions show values excluded by various observations. The blue shaded exclusion region is the new constraint placed by observations of Milky Way satellites in the current study. Click to enlarge. [Nadler et al. 2019]

Collisions Limited

So what do observations tell us? By combining the observed population of classical and Sloan Digital Sky Survey (SDSS)-discovered Milky-Way satellite galaxies with some clever probabilistic modeling of the population, Nadler and collaborators were able to place strict limits on the scattering cross sections for different-sized dark-matter particles, thereby constraining just how “collisional” dark matter can be.

The authors’ work continues to support the standard, collisionless picture of dark matter — but there’s plenty of room for deeper constraints. As data arrives from upcoming imaging programs like the Large Synoptic Survey Telescope (LSST), we’re sure to learn more about the small-scale structure of our surroundings and what it means for the nature of mysterious dark matter.

Citation

“Constraints on Dark Matter Microphysics from the Milky Way Satellite Population,” Ethan O. Nadler et al 2019 ApJL 878 L32. doi:10.3847/2041-8213/ab1eb2

Saturn's rings from Cassini

Saturn may appear calm and motionless from afar, but the immense planet is subtly pulsing and oscillating — and those oscillations impose a pattern on the planet’s rings that could tell us about Saturn’s history.

A Planet in Motion

Close-up of Saturn's rings

This extreme close-up of Saturn’s rings from Cassini shows the alternating dark and light bands of spiral density waves. [NASA/JPL-Caltech/Space Science Institute]

As the Cassini spacecraft orbited Saturn, it watched light trickle through the planet’s icy rings as they passed in front of distant stars. The flickering starlight revealed density waves — alternating stripes of compacted and loose material. Those density waves tell us much more than just what’s going on in the rings — they also tell us about the motions of Saturn’s surface.

Yanqin Wu (University of Toronto, Canada) and Yoram Lithwick (Northwestern University) combined observations and theory to study Saturn’s surface oscillations. They found that impacts from small objects were the most likely cause of the oscillations, with convection and atmospheric storms playing a minor role. Each of those impacts caused Saturn to “ring” like a bell, and the volume of the “sound” that we hear now depends on how hard it was struck, how many times, how long ago, and how quickly it fades.

Wu & Lithwick 2019 Fig. 4

Energies associated with different oscillation modes as derived from Cassini observations (black squares) and theory (colored circles and grey dashed line). While the impact theory matches the observations well for high l-values, it’s several orders of magnitude too low at low l-values. Alternative explanations, shown in the right-hand plot, match the data more closely at those low l-values. Click to enlarge. [Wu & Lithwick 2019]

Ringing Like a Bell

Saturn’s oscillations diminish as energy is carried away by the density waves in its rings, a process that can take up to 20 million years. By considering the expected frequency and size of impacts over that time period, the authors find that collisions in the distant past could have imparted enough energy to set Saturn ringing in the way we see today — with the exception of a few oscillation modes.

The authors explored several possibilities to explain the mismatch. Saturn could have experienced a once-in-a-million-year impact within the past 40,000 years — a so-called “lucky” strike. It’s also possible that some oscillation modes fade away more quickly than others or that energy is transferred between modes.

Another intriguing possibility is that those missing modes are excited not by impacts but by something more exotic: rock storms. These massive storms might begin deep within Saturn, where the atmospheric pressure is roughly ten thousand times higher than the pressure at Earth’s surface. Since it’s still not clear whether these massive storms actually exist, the authors acknowledge that the theory can’t yet be proved or disproved.

Wu & Lithwick 2019 Fig. 6

Simulations of two potentially observable signatures of the impact of a 150-km object: gravitational moments (left) and radial velocity (right). [Wu & Lithwick 2019]

From One Gas Giant to Another

Could oscillations be used to learn about the impact history of other planets? Since Jupiter lacks an extensive ring system to act as a dampener, any impact-induced oscillations would last far longer — potentially as long as billions of years — and we may be able to spot them.

To show this, Wu and Lithwick estimated how Jupiter would respond to a collision with a 150-km body a billion years ago. They found that the resulting changes in Jupiter’s gravitational field and surface velocity should be detectable by Juno and ground-based spectroscopy, respectively. With further study, we may be able to read the oscillations of Saturn and Jupiter to look back in time.

Citation

“Memoirs of a Giant Planet,” Yanqin Wu and Yoram Lithwick 2019 ApJ 881 142. doi:10.3847/1538-4357/ab2892

Coronal mass ejection

With energies a thousand times greater than an average solar flare, stellar superflares can strip away atmospheres and endanger life on planetary surfaces. These violent outbursts often go hand in hand with coronal mass ejections, shaping the evolution of planets near and far.

A Bit of Solar System History

Aurora borealis

Astronauts on the International Space Station captured this view of the aurora over Canada. [NASA]

In 1859, a solar storm launched a coronal mass ejection toward Earth. Named after one of the British astronomers who dutifully recorded the associated solar flare, the Carrington event is one of the most powerful solar storms to impact the Earth in recorded history.

Luckily for us, our middle-aged Sun rarely throws tantrums that explosive; if it happened today, the Carrington event would fry spacecraft electronics and power grids, causing trillions of dollars of damage.

In the Sun’s wilder youth, however, superflares were likely common. Studying younger Sun-like stars, like 700-million-year-old κ1Cet, can help us understand what the Sun was like billions of years ago — and what the planets orbiting young stars may be subjected to today.

Lynch et al. 2019 Fig. 5

Simulation of the coronal mass ejection magnetic flux rope propagating away from the star, as seen from the star’s north pole. [Lynch et al. 2019]

Simulating a Global Superflare

A team of astronomers led by Benjamin Lynch (University of California-Berkeley) used three-dimensional magnetohydrodynamic models to study the eruption of a superflare and a coronal mass ejection from the young Sun-like star κ1Cet.

They started with observations of κ1Cet’s surface magnetic field, then modeled how the stellar wind drags the magnetic field outward. By setting the star’s surface in motion, they caused the field lines to become twisted and tangled, building up the magnetic energy.

The magnetic energy gradually increased until reconnection kicked in and launched a coiled rope of magnetic flux into space. The eruption of the modeled coronal mass ejection lasted 10 hours and released a whopping 3 × 1026 Joules — about the same amount of energy estimated to have been released by the Sun in the 1859 Carrington event.

Lynch et al. 2019 Fig. 11

Radial cuts showing the speed (left) and density (right) of the coronal mass ejection. [Adapted from Lynch et al. 2019]

An Effect to Consider

Lynch and coauthors note that their model captures the most extreme superflare that κ1Cet can release based on past observations of the star’s surface magnetic flux. However, because starspots aren’t resolved in those observations, it’s possible that they could contain even more magnetic flux than expected, leading to even more energetic outbursts.

The authors hope that models of stellar superflares and coronal mass ejections can be used to understand how stellar activity affects planetary systems. They pointed to the importance of taking into consideration the effects of flares and coronal mass ejections on the habitability of exoplanets, not all of which are bad: when energetic particles from these events enter a planet’s atmosphere, they can generate compounds like hydrogen cyanide, which may play a role in the formation of the building blocks of life.

Citation

“Modeling a Carrington-scale Stellar Superflare and Coronal Mass Ejection from k1Cet,” Benjamin J. Lynch et al 2019 ApJ 880 97. doi:10.3847/1538-4357/ab287e

Pisces–Eridanus stellar stream

stellar stream

Artist’s impression of a stellar stream arcing high in the Milky Way’s halo. The Pisces-Eridanus stream has been discovered much closer to Earth than those illustrated here. [NASA]

Pisces–Eridanus may try to pass itself off as a billion years old, but scientists are calling its bluff. The Transiting Exoplanet Survey Satellite (TESS) has now carded this nearby stream of stars, revealing that it’s actually a relative baby!

Looking for Age

Stellar streams are faint associations of stars that were born together and move together, but they’ve been stretched into long tails across the sky. Some stellar streams likely originated as dense, compact clusters of stars that were pulled into streams by tidal interactions; others may have formed in a decentralized fashion and been spread further apart with time.

To understand the evolution of stars in streams and clusters, we use benchmarks: sample star clusters of different ages that we’ve explored in high detail. Unfortunately, most star clusters and associations that we can observe closely are young. Known older clusters all lie at larger distances — the 1-Gyr-old benchmark cluster NGC 6811, for example, is 3,600 light-years away — which limits what we can learn from them.

distance vs. age

Plot of distance vs. age for a selection of benchmark open star clusters. Pisces–Eridanus was originally identified as being 1 Gyr old (Meingast et al. 2019 red marker on the plot), which would make it the oldest cluster within 300 pc (~1,000 light-years). [Curtis et al. 2019]

Can I See Your ID?

It’s for this reason that the recent discovery of the Pisces–Eridanus stream — a faint stellar stream that spans 120° in the sky, is located just 260–740 light-years away, and was originally aged at 1 billion years — was met with a warm welcome. This unexpectedly close stream could prove to be a critical new 1-Gyr-old benchmark that would help us better understand stellar evolution.

Acting as bouncers for the 1 Gyr+ club, however, is a team of astronomers led by Jason Curtis (NSF Astronomy and Astrophysics Postdoctoral Fellow at Columbia University). They’ve set out to check that Pisces–Eridanus is as old as it initially led us to believe — and it turns out we’ve been deceived.

Revealing Rotation

Curtis and collaborators used TESS light curves of more than 100 members of the Pisces-Eridanus stream to identify how rapidly the stars are spinning. In a process called gyrochronology, the authors used the stars’ measured rotation rates to determine the age of the stream by comparing the distribution of rotation periods to the distributions for benchmark clusters with known ages.

rotation period distributions

Rotation period distributions for Pisces–Eridanus (red) and three benchmark clusters: 120 Myr Pleiades (blue), 670 Myr Praesepe (cyan), and 1 Gyr NGC 6811 (orange). Pisces–Eridanus stars clearly overlap with the Pleiades stars, indicating the two clusters have the same age. [Adapted from Curtis et al. 2019]

They found that Pisces-Eridanus’s distribution precisely overlapped the distribution for the stars of the Pleiades, indicating that these two groups are the same age: a mere 120 million years old!

Curtis and collaborators then used Gaia data combined with past radial-velocity measurements to hunt for new members of the Pisces-Eridanus stream. They identified 34 new high-mass candidate members — and the colors and brightnesses of these stars also support a young age of around 120 million years.

A Target for Planet-Hunting

Does the Pisces-Eridanus stream’s newly revealed youth mean that it’s no good to us after all? Not at all, according to Curtis and collaborators. One particular value of this stream is as an exploration ground in the hunt for exoplanets; planet discoveries here will allow us to learn about planet formation in a unique, diffuse environment.

What else have we learned? This study marks the first gyrochronology study conducted using TESS data — demonstrating the valuable role TESS has to play in the future as we continue to work to understand stellar and planetary birth and evolution.

Citation

“TESS Reveals that the Nearby Pisces–Eridanus Stellar Stream is only 120 Myr Old,” Jason L. Curtis et al 2019 AJ 158 77. doi:10.3847/1538-3881/ab2899

Barnard 68

Let’s be honest: the universe has an awful lot of gas. But the gas discovered in a new study isn’t your run-of-the-mill atomic gas! We’ve now found dense, star-formation-enabling molecular gas farther out than ever before.

A Crucial Ingredient

Carina Nebula

A Hubble view of a molecular cloud, roughly two light-years long, that has broken off of the Carina Nebula. [NASA/ESA, N. Smith (University of California, Berkeley)/The Hubble Heritage Team (STScI/AURA)]

Interstellar gas fills galaxies, lingering in the space between stars. Most of this material is in atomic form — primarily low-density ionized hydrogen and helium. But in some regions, conditions are right for atoms to join together into molecules, forming reservoirs of molecular gas. Less than 1% of the Milky Way’s interstellar medium is molecular gas by volume — yet this gas is critical to the galaxy’s development.

You can’t get star formation without molecular gas; this cold, dense material forms the fuel that can eventually collapse into hydrogen-fusing cores. This means that hunting molecular gas can give us insight into how galaxies build up and form their stellar populations: molecular gas reservoirs actively feed violently starbursting galaxies throughout the universe.

Molecular gas is also often associated with the host galaxies of distant quasars, supermassive black holes accreting vast amounts of matter and shining brightly. By studying the properties of this molecular gas, we can learn more about how supermassive black holes evolve with their host galaxies.

CO emission around PSO145+19

Intensity maps of CO line emission show two locations of molecular gas: PSO145+19 and PSO145+19N. The blue cross marks the location of the known quasar. Click to enlarge. [Koptelova & Hwang 2019]

Looking Back in Time

Because galaxy formation and evolution is very much a big-picture question, we might wonder how molecular gas was different in the early universe. Did early star-forming galaxies contain more molecular gas than today’s galaxies? What were the properties of the gas? How did early galaxies form and evolve, creating young stars and feeding their central black holes?

To answer these questions, we need to hunt for large reservoirs of molecular gas at high redshifts. But this is challenging! The most common component of molecular gas, molecular hydrogen, isn’t easily detectable. For this reason, we turn to carbon monoxide (CO) as a tracer of molecular gas reservoirs. 

So far, the most distant detections we’ve made of molecular gas using CO emission are at redshifts of z = 6–6.9. But now a pair of scientists from National Central University in Taiwan have looked even farther.

Drama of a Distant Interaction

Using observations of CO emission lines, Ekaterina Koptelova and Chorng-Yuan Hwang have discovered two sources containing molecular gas at a redshift of z = 7.09. That’s 13 billion light-years away, or from a time when the universe was just ~700,000 years old!

ALMA spectra of PSO145+19

ALMA spectra of PSO145+19 (top panels) and PSO145+19N (bottom panels) reveal spectral lines corresponding to CO emission and water emission. [Koptelova & Hwang 2019]

Koptelova and Hwang estimate the two molecular-gas sources to be roughly 27,000 and 41,000 light-years across. One of the two sources is coincident with a previously discovered quasar, and the other is located about 68,000 light-years to the side of the quasar — a very close neighbor, on cosmic scales!

The properties of the sources lead the authors to suggest that the gas may be tracing two or more star-forming galaxies that are interacting in the early universe. These colliding monsters contain reservoirs of molecular gas to fuel their star formation, as well as at least one quasar.

Future observations will hopefully confirm this picture and help us to better understand the role that molecular gas plays in the dramatic formation and evolution of galaxies in the early universe. 

Citation

“A Luminous Molecular Gas Pair beyond Redshift 7,” Ekaterina Koptelova and Chorng-Yuan Hwang 2019 ApJL 880 L19. doi:10.3847/2041-8213/ab2ed9

self-lensing

Though Kepler’s primary mission ended years ago, the resulting dataset remains a vast playground in which astronomers continue to discover new surprises in stellar light curves. The latest? Evidence of a white dwarf that defies all expectations.

Forming a Lightweight

ELM white dwarf

Artist’s impression of an extremely low-mass white dwarf (foreground) orbiting a more typical white dwarf (background). [CfA/David A. Aguilar]

White dwarfs come in a range of different sizes. A typical white dwarf might be around 0.6 solar mass and arise when an isolated star of perhaps a few times the mass of the Sun expands into a red giant, exhausts its fuel supply, and puffs off its outer layers, leaving behind its dead, dense core.

But some observed white dwarfs have much lower masses — say, between 0.15 and 0.3 solar mass. To produce such a small remnant mass, the mass of the initial progenitor star would also have to be very low. But this poses a problem: smaller stars take longer to evolve, so a star of such low mass would need longer than the age of the universe to exhaust its fuel supply!

Since isolated stellar evolution can’t explain extremely low-mass white dwarfs, astronomers have settled on another explanation: binary interactions. In this scenario, the close orbit of two stars in a binary results in material being stripped away from the progenitor star, accelerating its mass loss and allowing it to evolve into a very low-mass white dwarf.

So far, this explanation has fit our observations. But now, the discovery of a new low-mass white dwarf is challenging our understanding.

self lensing diagram

Example diagram of a different self-lensing binary system, KOI-3278. When the white dwarf passes between us and the primary star, gravitational magnification causes a brightening in the light curve that we detect. For KIC 8145411, we do not observe an occultation, because the light from the white dwarf is too faint to detect directly. [Eric Agol]

Self-Lensing Surprise

In a new publication, a team of scientists led by Kento Masuda (NASA Sagan Fellow at Princeton University) present the discovery of the binary system KIC 8145411 from Kepler data. This unique binary is one of only five known self-lensing systems: one object in the binary gravitationally lenses the light of the other as it passes in front once per orbit.

Masuda and collaborators use follow-up observations from the Fred Lawrence Whipple Observatory in Arizona and the Subaru telescope in Hawaii to pin down the properties of the system, confirming that we’re looking at a 0.2 solar-mass white dwarf orbiting a Sun-like star in an edge-on, eclipsing orbit.

But here’s the catch: KIC 8145411’s orbit is quite wide, at 1.28 AU (a period of ~450 days) — ten times too wide for the primary and the white-dwarf progenitor to have interacted in the way we’d expect. How, then, did this “impossible” white dwarf come to exist?

Tip of the Iceberg

white dwarfs in binaries

Masses of known white dwarfs in binaries and their orbital periods. The KIC 8145411 system is a clear outlier, having both a low mass and a very wide orbit. Click to enlarge. [Masuda et al. 2019]

Masuda and collaborators discuss a few proposed formation mechanisms — like interactions with a since-ejected or swallowed tertiary object — but none of them are especially satisfying.

So what’s next? The authors point out that we had only a 1 in 200 chance of detecting this particular system, due to its edge-on orientation — which likely means that KIC 8145411 is just the tip of the iceberg. Now that we know what we’re looking for, dedicated searches may turn up many more of these systems in the future — hopefully helping us to explain why this white dwarf is possible after all!

Citation

“Self-lensing Discovery of a 0.2 M⊙ White Dwarf in an Unusually Wide Orbit around a Sun-like Star,” Kento Masuda et al 2019 ApJL 881 L3. doi:10.3847/2041-8213/ab321b

black hole neutron star binary

There’s still a lot we don’t know about the internal structure and behavior of neutron stars, the compact remnants of giant, collapsed stars. Can the mergers of neutron stars with another type of exotic object, black holes, reveal important information?

Uncharted Remnants

gamma-ray bursts

Illustration of a short gamma-ray burst, which could be produced during the collision of a neutron star with a black hole. [ESO/A. Roquette]

Since neutron stars were first theorized, we’ve observed about 2,000 of them in the Milky Way and the Magellanic Clouds. Due to the exotic high-density conditions inside neutron stars, however, we still don’t well understand their interior structure and behavior.

A star’s internal properties are characterized by its “equation of state”. To better constrain the equation of state for neutron stars, we need accurate measurements of their masses and radii. But though we know that neutron stars are typically have the mass of a couple of Suns packed into a sphere of order 10 km in radius, it’s challenging to get precise enough radius measurements to constrain the equation of state for these dense, distant objects.

Can we do a better job of measuring neutron-star radii using future observations of merging neutron-star–black-hole binaries? A team of scientists led by Stefano Ascenzi (Tor Vergata University of Rome, INAF Rome Observatory, and Sapienza University of Rome, Italy) thinks the answer is yes — and in a new study, the team outlines how.

A Well-Marked Collision

It’s expected that the merger of a neutron star with a black hole would result in the destruction of the neutron star, the brief formation of a torus around the black hole as the neutron-star matter rains back down, and the rapid accretion of the torus.

This process would produce a pair of observable signals: 

  1. a gravitational-wave chirp from the merger, visible to detectors like the Laser Interferometer Gravitational-wave Observatory (LIGO) and its European counterpart Virgo, and
  2. a short gamma-ray burst (GRB) caused by the accretion of the torus.
equations of state

Example of how the various curves of mass vs. radius described by different neutron-star equations of state (colored curves) could be eliminated based on mass and radius measurements of a neutron star during a merger. In this test run, the authors’ injected neutron star properties are shown by the blue dot and the recovered properties are shown by the red dot. The red dashed lines show the 68% and 90% credible regions for the recovered properties — which, in this example, eliminate several of the possible equations of state. [Adapted from Ascenzi et al. 2019]

According to Ascenzi and collaborators, we can use these two signals in tandem to obtain information about the neutron star. Fitting the gravitational-wave signal of such a merger will reveal the neutron-star and black-hole masses, as well as the spin of the black hole. Measurements of the short GRB energy will tell us how much mass was in the torus that accreted onto the black hole.

The combination of this information allows us to infer the radius of the neutron star in the initial binary. Ascenzi and collaborators show that this method would let us estimate a neutron star radius to within 20% accuracy for a gravitational-wave detection with a signal-to-noise ratio of 10. For comparison, the signal-to-noise ratio for GW 170817, the first observed binary neutron star merger, was more than 30.

Looking for a New Type of Merger

How lucky do we have to get to observe a neutron-star–black-hole merger simultaneously in gravitational waves and electromagnetic radiation? Theory predicts these joint detections will perhaps occur at rates of 0.1 to 10 per year with current technology, and future telescopes and gravitational-wave detectors should increase our odds.

Here’s hoping these spectacular explosions will reveal more about neutron-star interiors soon!

Citation

“Constraining the Neutron Star Radius with Joint Gravitational-wave and Short Gamma- Ray Burst Observations of Neutron Star–Black Hole Coalescing Binaries,” Stefano Ascenzi et al 2019 ApJ 877 94. doi:10.3847/1538-4357/ab1b15

Beehive Cluster

Many people start to slow down as they age, and stars seem to be the same way. Astronomers measure how fast stars spin to determine their ages, but new measurements suggest that some stars may wind down more quickly than others.

Color-magnitude diagram

Color-magnitude diagram for globular cluster M55. Stars with masses higher than the turn-off point mass have lived long enough to evolve off the main sequence. [B.J. Mochejska, J. Kaluzny (CAMK), 1-m Swope Telescope]

Assigning Ages

It’s relatively straightforward to determine the age of a star that belongs to a cluster. The age of the cluster — and the stars that comprise it — is equal to the hydrogen-burning lifetime of the stars that are just about to swerve off the main sequence and balloon into red giants.

It’s much more challenging to measure the age of an isolated star. Astronomers rely on a method called gyrochronology — measuring a star’s age from the seemingly predictable slowdown of its rotation over time.

Stellar rotation periods appear to increase with the square root of the star’s age, but recent observations have hinted that that may not be true for all stars.

Evidence from Star Clusters

A team led by Jason Lee Curtis (Columbia University) studied F, G, and K stars to determine if the slowdown rate depends on mass. They used light curves from the Kepler spacecraft to measure the rotation periods of stars in NGC 6811, an open cluster in the constellation Cygnus. Curtis and collaborators used the observed rotation periods of the F and G stars in NGC 6811 to pinpoint the cluster’s age at 1.04 billion years.

Curtis et al. 2019 Fig. 5

Comparison of rotation periods for stars in NGC 6811 (blue, red, and cyan symbols) to models of Praesepe at 670 million and 1 billion years old (gray and black curves). The observed stellar rotation rates begin to diverge from the expected behavior around 5,400 K. [Curtis et al. 2019]

Next, they turned to a younger open cluster, Praesepe, which clocks in at around 670 million years old. Using a gyrochronology model derived from Praesepe’s observed stellar rotation periods, the authors artificially aged the cluster to 1 billion years old — the same age as NGC 6811. If all stars spin down the same way, the rotation rates of the artificially aged cluster should match those from NGC 6811.

Instead, although the rotation rates of the F and G stars from the two clusters matched neatly, NGC 6811’s lower-mass K stars were spinning faster than expected. In fact, they lined up almost perfectly with the K stars in the much younger cluster. In other words, while the F and G stars had slowed down with age, the K stars seemed to have not slowed down a bit in at least 330 million years.

Curtis et al. 2019 Fig. 7

Stellar ages derived from gyrochronology for NGC 6811 and the Hyades Cluster, compared to the 670-million-year-old Praesepe model. [Adapted from Curtis et al. 2019]

Why the Slow Pace?

One interpretation of this finding is that a star’s spin-down rate depends on its mass, with lower-mass stars winding down more slowly. However, the authors cautioned that it may not be that simple. Instead, they posited that stars experience a period during which their rotation rates don’t slow down — and lower-mass stars spend more time in this stalled spin-down state than higher-mass stars.

The authors aren’t sure what causes the stars to take a break from spinning down. They suggested that the stars either temporarily experience less magnetic braking or their outer layers gain angular momentum from their interiors.

Curtis and collaborators hope that studies of even older clusters, like 2.5-billion-year-old Ruprecht 147, will reveal at what point K dwarfs resume their usual spin-down and lead to a better understanding of how rotation rates can be used to determine the ages of stars.

Citation

“A Temporary Epoch of Stalled Spin-down for Low-mass Stars: Insights from NGC 6811 with Gaia and Kepler,” J. L. Curtis, M. A. Agüeros, S. T. Douglas, and S. Meibom 2019 ApJ 879 49. doi:10.3847/1538-4357/ab2393

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