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Kepler systems

Knowing which planets can form will help us understand how planets form. So as we discover more and more exoplanets, one must wonder: what if there are certain types of planets that just don’t exist?

Good Form

We know of over 4,000 confirmed exoplanets and nearly as many exoplanet candidates. With a population of this size, we can look for trends in exoplanet characteristics much more easily than we could a decade ago.

The most helpful quantities in this regard are planet radius, planet mass, and orbital period. Plotting these quantities against each other has revealed an abundance of certain planets — like Jupiter-sized planets with short orbits — and a dearth of others. Direct analogs to Neptune appear to be missing, though Neptune-sized planets with shorter periods are common.

Kepler Earth-like planets

Earth-like planets discovered by Kepler, pictured next to the Earth. [NASA/JPL-Caltech]

Some sorts of planets may genuinely be unable to form, but we also have to consider observational bias. For instance, the transit method (utilized by the Kepler spacecraft and now by the Transiting Exoplanet Survey Satellite (TESS)) is more sensitive to larger planets with shorter orbital periods. So, any gaps and trends that emerge while plotting exoplanet characteristics have to be tested for authenticity. In a recent study, David Armstrong (University of Warwick, UK) and collaborators focus on a gap that emerges while considering short-period planets less massive than 20 Earths.

Mind the Gap!

Armstrong and collaborators looked at a sample of planets with mass under 25 Earth-masses and orbital periods of less than 20 days. Plotting mass versus orbital period yielded a gap that split their sample into two. The gap was more distinct for planets whose inclinations — and, by extension, planet radii — were known.

Given how many observational biases were involved, the authors chose to investigate the presence of the gap rather than its properties. The authors were especially concerned about whether different methods of mass determination all still produced the gap. A statistical test showed that it persisted regardless of method.

planet mass vs. radius

Planet mass in Earth-masses versus orbital period in days for the sample for which inclination was
known. The gap is illustrated with the dashed gray line. The color of the points relates to planetary radius.
The abundance of small, less massive planets below the gap suggests that the gap could be attributed to
photoevaporation. [Armstrong et al. 2019]

To better define the existence of the gap, the authors fit models to their mass–period data that allowed for multiple underlying distributions. The best fit turned out to be a model that assumed two distinct distributions, supporting the gap’s presence.

In light of this, what could be causing the gap?

Why the Gap?

The authors injected other properties of planetary systems into their mass–period plots. The gap appears impartial to nearly everything, showing trends only with planetary radius. This mild relationship could be due to photoevaporation, the process by which radiation from the host star strips gas off nearby objects. In the case of exoplanets, photoevaporation could shrink a gas giant planet — possibly to its rocky core.

In multi-planet systems, the most stable orbits may be those that don’t put planets in the gap. Another possibility is that stable regions in a system shift as it evolves.

The only way to better understand the gap is to find more exoplanets, which is just what TESS and other efforts are doing. Stay tuned!

Citation ​

“A Gap in the Mass Distribution for Warm Neptune and Terrestrial Planets,” David J. Armstrong ​et al ​ 2019 ​ApJL 880​ L1. doi:10.3847/2041-8213/ab2ba2

 

Geminga illustration

The Earth is constantly being bombarded by cosmic rays — high energy protons and atomic nuclei that speed through space at nearly the speed of light. Where do these energetic particles come from? A new study examines whether pulsars are the source of one particular cosmic-ray conundrum.

An Excess of Positrons

cosmic rays

Artist’s impression of the shower of particles caused when a cosmic ray hits Earth’s upper atmosphere. [J. Yang/NSF]

In 2008, our efforts to understand the origin of cosmic rays hit a snag: data from a detector called PAMELA showed that more high-energy positrons were reaching Earth in cosmic rays than theory predicted.

Positrons — the antimatter counterpart to electrons — are thought to be primarily produced by high-energy protons scattering off of particles within our galaxy. These interactions should produce decreasing numbers of positrons at higher energies — yet the data from PAMELA and other experiments show that positron numbers instead go up with increasing energy.

Something must be producing these extra high-energy positrons — but what?

Clues from Gamma-rays

One of the leading theories is that the excess positrons are produced by nearby pulsars — rapidly rotating, magnetized neutron stars. We know that pulsars gradually spin slower and slower over time, losing power as they spew a stream of high-energy electrons and positrons into the surrounding interstellar medium. If the pulsar is close enough to us, positrons produced in and around pulsars might make it to Earth before losing energy to interactions as they travel.

Geminga and PSR B0656+14

Observations from the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory show TeV nebulae around pulsars Geminga and PSR B0656+14. But do these sources also have extended GeV nebulae that would provide more direct constraints on positron density? [John Pretz]

Could nearby pulsars produce enough positrons — and could they diffuse out from the pulsars efficiently enough — to account for the high-energy excess we observe here at Earth? A team of scientists now addresses these questions in a new publication led by Shao-Qiang Xi (Nanjing University and Chinese Academy of Sciences).

To test whether pulsars are responsible for the positrons we see, Xi and collaborators argue that we should look for GeV emission around candidate sources. As the pulsar-produced positrons diffuse outward, they should scatter off of infrared and optical background photons in the surrounding region. This would create a nebula of high-energy emission around the pulsars that glows at 10–500 GeV — detectable by observatories like the Fermi Gamma-ray Space Telescope.

Two Pulsars Get an Alibi

gamma-ray counts

Fermi LAT gamma-ray count map (top) and residuals after the background is subtracted (bottom) for the region containing Geminga and PSR B0656+14. [Adapted from Xi et al. 2019]

Xi and collaborators carefully analyze 10 years of Fermi LAT observations for two nearby pulsars that have been identified as likely candidates for the positron excess: Geminga and PSR B0656+14, located roughly 800 and 900 light-years away from us.

The result? They find no evidence of extended GeV emission around these sources. The authors’ upper limits on emission from Geminga and PSR B0656+14 give these objects an alibi, suggesting that pulsars can likely account for only a small fraction of the positron excess we observe.

So where does this leave us? If pulsars are cleared, we will need to look to other candidate sources of high-energy positrons: either other nearby cosmic accelerators like supernova remnants, or more exotic explanations, like the annihilation or decay of high-energy dark matter.

Citation

“GeV Observations of the Extended Pulsar Wind Nebulae Constrain the Pulsar Interpretations of the Cosmic-Ray Positron Excess,” Shao-Qiang Xi et al 2019 ApJ 878 104. doi:10.3847/1538-4357/ab20c9

evolving star

Sometimes slow and steady wins the race — like in the long-term, continuous monitoring of a variable star. A collection of more than 100 years of data has now given us a rare opportunity to watch, in real time, as a star evolves.

Flashes Late in Life

AGB on HR diagram

This H-R diagram for the globular cluster M5 shows where AGB stars lie: they are represented by blue markers here. The AGB is one of the final stages in a low- to intermediate-mass star’s lifetime. [Lithopsian]

Low- to intermediate-mass stars (~0.5-8.0 solar masses) live for billions of years, so stellar evolution ordinarily occurs on timescales that are far too long to observe over our brief human lifespans.

But one particular stage of stellar evolution happens on timescales where we could see something happen, if we’re patient and watch for long enough: the end of the asymptotic giant branch (AGB). This is a stage at the end of a star’s life after it’s exhausted all of the hydrogen and helium in its core.

Low- to intermediate-mass stars aren’t able to ignite fusion of heavier elements in their cores, so at this stage, fusion proceeds only in a shell of hydrogen outside of the core. As the hydrogen shell burns, it piles up a thin layer of helium below it. But that quiet helium is deceptive: when enough of it piles up, it will ignite in a sudden flash called a thermal pulse, rapidly burning until the helium is depleted and the pile-up begins anew.

Rapidly, that is, on stellar evolution timescales — a typical thermal pulse lasts maybe a few hundred years, and these pulses occur only every 10,000 or 100,000 years. Still very difficult to observe on human timescales!

thermal pulses

Example plot showing the radius change over time in a modeled star undergoing thermal pulses. Based on observational constraints of T UMi’s radius, the region of interest in this model for T UMi’s current evolutionary stage is marked in red. [Molnár, Joyce, & Kiss 2019]

But there are some clues that we might be able to spot when a thermal pulse is just getting started — and a team of scientists led by László Molnár (Konkoly Observatory, MTA CSFK, Hungary) and Meridith Joyce (Australian National University) think that the star T Ursae Minoris (T UMi) is exhibiting those signs now.

Taking the Pulse of a Star

At the start of a pulse, the flash of igniting helium causes the inner regions of the star to expand. This, in turn, causes the outer parts of the star to cool and contract — so the star rapidly shrinks in radius and decreases in luminosity. If it’s a variable star — a star with periodic oscillations in brightness — the sudden drop in radius causes the period of its variability to plummet as the oscillating stellar envelope shrinks.

T UMi light curve

More than a century of data from variable star T UMi are shown in this AAVSO visual light curve. Changes in its period and amplitude are most evident in the bottom panel. Click to enlarge. [Molnár, Joyce, & Kiss 2019]

The variable star T UMi presents a golden opportunity to spot this process. Molnár and collaborators have compiled a dataset for this star reaching all the way back to 1904. From this, we can see that T UMi’s period started plummeting roughly 40 years ago, shortening by more than 3 days per year — and at the same time, the star’s brightness started dropping.

Using evolutionary and pulsation models, Molnár and collaborators confirmed that we’re indeed watching the onset of a thermal pulse in an AGB star. From their models, the authors predict that T UMi’s period will continue to drop for another several decades before the star begins to expand again — so be sure to check back in 50 years or so, to see if predictions pan out from this unique opportunity to watch a star evolve!

Citation

“Stellar Evolution in Real Time: Models Consistent with the Direct Observation of a Thermal Pulse in T Ursae Minoris,” László Molnár et al 2019 ApJ 879 62. doi:10.3847/1538-4357/ab22a5

TDE simulation

When a passing star gets a little too close to a massive black hole, destruction ensues. But how does the star’s age affect the outcome?

Close Encounters

At this point, we’ve witnessed a few dozen tidal disruption events (TDEs), sudden flares from massive black holes caused when a wandering star passes too close and becomes the black hole’s next meal. In TDEs, tidal forces from the black hole either partially or completely pull the star apart as it tries to edge by. Some of the star’s material then falls back onto the black hole and accretes, causing the black hole to temporarily light up.

TDE

An artist’s illustration of a tidal disruption event, in which a star is torn apart by a black hole’s tidal forces. [NASA/CXC/M.Weiss]

In order to better understand these violent, destructive events, we need accurate models of TDEs to compare our observations to. But there’s an interesting factor we need to consider in these models: young stars will behave differently in such a disruption than old stars.

To better understand these differences, a team of astronomers led by Jamie Law-Smith (UC Santa Cruz; University of Copenhagen, Denmark) has created a detailed set of simulations that capture what happens when Sun-like stars of different ages — with realistic structures and compositions — have a close encounter with a massive black hole.

A Matter of Age

Law-Smith and collaborators used the stellar evolution code MESA to build a set of stars with one and three times the mass of the Sun and ages varying from the start to the end of the main sequence — that’s 8.4 billion years apart for 1-solar-mass stars, but just 0.3 billion years apart for 3-solar-mass stars, since more massive stars evolve much more quickly. The authors then simulated the encounters of these stars with a million-solar-mass black hole, using the FLASH hydrodynamics code.

The authors found that many major observable features of the encounter depend on the star’s age, including:

  1. The likelihood of disruption
    A star’s density changes with time; older stars are more centrally concentrated and thus more difficult to fully disrupt.

  2. Mass fallback rates over time for the disruption of a 1-solar-mass star at three different ages: a star at the beginning of the main sequence (blue), a middle-aged star 4.8 Gyr later (orange), and a star at the end of the main sequence 8.4 Gyr later (green). [Adapted from Law-Smith et al. 2019]

    The timing and amount of peak accretion onto the black hole after disruption
    For close approaches, younger stars provide faster flares with larger peak accretion rates after they are disrupted.
  3. The composition of disrupted, accreting material
    Debris mixes and rotates during disruption, leading to “abundance anomalies” — enhancements and depletions of some elements — that can appear in the tidal tails accreting onto the black hole. Older and more massive stars show more of these anomalies at earlier times.

Observations of Destruction

With these results, Law-Smith and collaborators give us a host of things to look for in future TDE observations. But what about past events? The authors argue that a number of the TDEs we’ve observed show features in their ultraviolet spectra that can be naturally explained by these simulations as arising from the disruption of older, more evolved stars.

As a next step, the authors plan to construct a library of tidal disruption simulations like these for a variety of different stellar masses and ages. We’ll be ready for comparison as the next TDEs are spotted!

Bonus

Check out the video below, which shows the authors’ simulation of a 1-solar-mass young star disrupted by a million-solar-mass black hole.

Citation

“The Tidal Disruption of Sun-like Stars by Massive Black Holes,” Jamie Law-Smith et al 2019 ApJL 882 L25. doi:10.3847/2041-8213/ab379a

rogue planet

How can we measure the masses of free-floating planets wandering around our galaxy? A new study identifies one approach that combines the power of two upcoming missions.

Finding Invisible Planets

Most exoplanets we’ve found so far have relied on measurements of their host stars, either via dips in the host star’s light as the planet passes in front (transit detections), or via wiggling of lines in the host star’s spectra caused by the planet’s gravitational tug (radial velocity detections). But free-floating planets have no hosts and are therefore effectively invisible, since they don’t give off much light of their own. To find these rogues, we rely on another method: gravitational microlensing.

microlensing diagram

A diagram of how planets are detected via gravitational microlensing. In this case, the planet is in orbit around a foreground lens star, but this same diagram can also apply to a free-floating planet acting alone as the lens. Click to enlarge. [NASA]

In microlensing, the mass of a passing foreground planet — either free-floating or bound to a host star — can act as a lens, briefly gravitationally focusing the light of a background star behind it. As a result, the background star temporarily brightens (on timescales of perhaps seconds to years) in our observations. Though we never directly see the foreground planet, we can infer its presence from the spike in the background star’s brightness.

Masses from Parallax

By itself, a microlensing observation usually can’t tell us about the mass of a free-floating planet; this is because the timescale of a brightening event depends on both the mass of the lens and on the relative proper motion between the background source and the foreground lensing planet.

But if we could simultaneously observe a microlensing event from two different locations, separated by a large enough distance? Then the parallax would allow us to break that degeneracy: the differences in peak brightness and its timing at the two locations would allow us to calculate both the speed of lens relative to the source and the planet mass.

Vantage Points in Space

Where do we find two sensitive eyes located far enough apart to make this work? In space, of course!

WFIRST

Illustration of NASA’s WFIRST telescope. [NASA]

NASA’s Wide Field Infrared Survey Telescope (WFIRST) is set for launch in the mid-2020s, and one of its primary mission objectives is to perform wide-field imaging that may allow for the detection of hundreds of free-floating planets — and many additional bound planets — via microlensing.

As for the second eye, scientists Etienne Bachelet (Las Cumbres Observatory) and Matthew Penny (The Ohio State University) propose that ESA’s upcoming Euclid mission is exactly what we need. Euclid, launching in 2022, will have similar wide-field imaging capabilities to WFIRST, and it will be able to make complementary microlensing parallax measurements as long as the two satellites are 100,000 km or more apart.

Making Use of Gaps

Euclid

Artist’s illustration of ESA’s Euclid mission. [ESA/C. Carreau]

Though Euclid’s primary science goal is to study dark energy and dark matter, Bachelet and Penny demonstrate that a modest investment of Euclid observing time — approximately 60 days during its primary mission, and another 60 days during its extended mission — during scheduling gaps would be enough to obtain the masses for 20 free-floating planets and many more bound planets.

So what are we waiting for? Let’s go learn more about the rogue planets sneaking through our galaxy!

Citation

“WFIRST and EUCLID: Enabling the Microlensing Parallax Measurement from Space,” Etienne Bachelet and Matthew Penny 2019 ApJL 880 L32. doi:10.3847/2041-8213/ab2da5

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

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