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Icy exoplanet

The Earth likely underwent several periods of planet-wide ice coverage in the past, in what’s known as snowball Earth events. A new study explores whether snowball events are also a risk for tidally locked, habitable exoplanets.

An Icy Fate

snowball Earth

Current theory suggests that the Earth underwent several snowball events in its past history. [NASA]

Snowball events can arise suddenly on a planet like Earth, driven by a rapid feedback loop. A planet that experiences a sudden drop in stellar light reaching its surface — say, due to a volcanic eruption or asteroid impact — can quickly ice over through a runaway effect: as ice coverage grows, more light is reflected from the planet’s surface. This drops the temperature of the planet, which causes ice coverage to expand even further.

Under some conditions, this runaway snowball effect can lead to a fully icy world that’s no longer able to defrost itself, even if incoming stellar light returns to original levels.

eyeball planet

Artist’s impression of a cold, tidally-locked planet. Ice covers much of the planet’s surface, but the point directly facing the planet’s host star remains ice-free. [NASA/JPL-Caltech]

Looking Beyond Our Solar System

The paradigm described above depends on specifics of how heat is transferred in the atmosphere of a rapidly rotating planet like Earth. But in searching for habitable planets beyond our solar system, we might wonder whether other types of worlds also experience snowball events.

In particular, the majority of the potentially habitable planets we’ve discovered lie around dim M-dwarf stars, and many of these planets are tidally locked, meaning the same side of the planet faces its host star at all times. Can worlds like this snowball, too?

To investigate this question, a team of scientists led by Jade Checlair (University of Chicago) used an atmospheric global climate model to conduct simulations of a tidally locked, Earth-sized planet that circles its M-dwarf host on a 50-day orbit. In particular, the team was curious whether heat transfer within a global ocean would affect the outcome — so they covered their simulated planet in a multi-layer ocean that reached a depth of 189 meters.

No Snowballs

sea ice coverage

The authors’ results show that sea-ice coverage follows a smooth relationship with stellar irradiation on tidally locked planets: for each level of stellar irradiation, the planet equilibrates to the same final state regardless of where it started. This is not the case on planets with runaway snowball events. [Checlair et al. 2019]

Checlair and collaborators found that, unlike a rapidly rotating planet, tidally locked planets are stable against runaway snowball events. In their model, as the planet experienced decreasing irradiation, its sea ice extent grew gradually — and it defrosted again as the stellar irradiation was brought back to original levels.

This means that for a tidally locked planet in its star’s habitable zone, snowball states should not be possible for extended periods of time. If a planet were to experience a catastrophic event like a volcanic eruption or asteroid impact, it may ice over briefly. But the stellar radiation concentrated on the side of the planet facing its host would quickly cause the planet to warm back up again and return to its original state.

Good or Bad?

Is the lack of tendency for tidally locked planets to snowball a good thing or a bad thing? Though a global ice age could wipe out preexisting complex life, it’s also possible that snowball events could help drive life to evolve more rapidly, by providing evolutionary pressure to adapt. The jury’s still out on the impact of snowball events, but now we know a bit more about where to expect them!

Citation

“No Snowball on Habitable Tidally Locked Planets with a Dynamic Ocean,” Jade H. Checlair et al 2019 ApJL 884 L46. doi:10.3847/2041-8213/ab487d

solar flare

Bright eruptions from the Sun’s surface can influence everything from the Sun’s own atmosphere to the Earth and beyond. The good news: we’ve got decades of detailed observations of solar flares available for study. The bad news: we may be interpreting these data incorrectly.

Distributing Flares

The distributions of properties of solar flares is a topic of great interest to solar physicists. Much of our understanding of how the Sun ejects energy into its surroundings depends on the number of flares emitted by the Sun at different energies and durations — but we’re only able to measure the larger, more energetic end of this distribution.

Solar flare

NASA’s Solar Dynamics Observatory captures a solar flare in the act. [NASA/SDO]

For this reason, scientists build databases of observed solar flares and fit power laws to the distributions of their properties. By extrapolating the power laws from the large end of the flare energy scale (which we can observe) down to the smaller end (which we can’t), scientists attempt to estimate the number of unresolved mini-flares the Sun emits. This could shed light on a number of solar mysteries, like why the Sun’s atmosphere is so much hotter than its surface.

But a team of scientists led by Cis Verbeeck (Royal Observatory of Belgium) cautions against this approach. Instead of just measuring the shape of these power-law fits for flares, they say, we first need to ask: is a power law actually the right fit to the data?

To Power Law or Not to Power Law

Power law distributions correctly describe a wide variety of astrophysical data, but Verbeeck and collaborators think we should test this assumption for solar flares. To this end, the team performed a comprehensive study of nearly 7,000 flares detected in Solar Dynamics Observatory AIA 9.4-nm images between May 2010 and March 2018, conducting statistical analyses to determine the best fit to the peak flare flux distribution.

Sure enough, the authors find that once the flare data has been background-subtracted — meaning that only the flares are included, not the non-flaring solar background — the distribution is not well fit by a power law.

peak flare intensity distribution

The peak flare intensity distribution is better described by a lognormal fit (green) than by a power law fit (red). Click to enlarge. [Verbeeck et al. 2019]

Instead, a good fit is provided by a lognormal distribution, the distribution that describes a variable that is normally distributed in log space. Lots of things are naturally described by lognormal distributions — for instance, the length of time that users will probably dwell on this post (congratulations: if you’ve made it this far, you’re likely doing better than average!).

A Lognormal Outlook

So why have we been using the wrong fit? The authors show that raw flare data that hasn’t been background-subtracted does follow a power-law distribution, so it’s possible that past studies just haven’t correctly isolated the flares from everything in the background that isn’t a flare.

Regardless of the reason, it seems clear from the work in this study that power laws are not the right approach going forward. As we continue to work to understand the flares from our nearest star, a careful treatment of the big picture is needed!

Citation

“Solar Flare Distributions: Lognormal Instead of Power Law?,” Cis Verbeeck et al 2019 ApJ 884 50. doi:10.3847/1538-4357/ab3425

NGC 1559

How has galaxy evolution changed over our universe’s history? To understand this, we need to track galaxies’ stars and gas over time. Stars are relatively easy: they’re bright and can be observed with deep optical and infrared observations. But gas? That’s a little trickier.

Atomic Challenges

M81

This image of local galaxy M81 reveals the extent of its atomic hydrogen gas — measured using the 21-cm emission line — in blue. [NASA Spitzer Space Telescope / NRAO VLA]

Reservoirs of molecular gas — like carbon monoxide — have become progressively better studied in recent years, even in galaxies that lie at huge distances. But atomic gas is a real challenge to observe beyond our local universe.

Atomic hydrogen (H I) is of particular interest: this neutral gas provides the primary fuel reservoir for star formation. But H I doesn’t have any bright emission lines, making it hard to spot. In fact, the main way to detect H I is via what’s known as the 21-centimeter line, a spectral line produced by a rare change in the energy state of the hydrogen atom. This line has such a low transition probability that you need vast amounts of hydrogen to detect it.

Since the 21-cm line is so weak, observing the neutral hydrogen from most individual galaxies beyond our local universe is out of reach until telescope technology improves. But a team of scientists led by Apurba Bera (National Centre for Radio Astrophysics, India) has used an alternative approach: stacking the observations of many distant galaxies.

uGMRT image of the EGS

The uGMRT 1.2 GHz image of the Extended Groth Strip. Red circles indicate the locations of the 445 galaxies in the authors’ sample. [Bera et al. 2019]

Amplifying a Weak Signal

Bera and collaborators used the upgraded Giant Metrewave Radio Telescope (uGMRT) in India to conduct deep radio observations of a region of the sky known as the Extended Groth Strip.

With 117 hours of observations, the team gathered detailed data for a sample of 445 blue, star-forming galaxies that lie at redshifts of 0.2 < z < 0.4. These redshifts represent galaxies from when the universe was roughly 2.5 to 4.3 billion years younger than it is now.

By stacking the spectra for these 445 galaxies, the authors were able to make a statistically significant detection of the total H I 21-cm emission — from which they could measure the average mass of H I gas in these intermediate-redshift galaxies.

How Star Factories Evolve

stacked H I 21-cm emission spectrum

The stacked H I 21-cm emission spectrum of the 445 galaxies of the authors’ sample, which allows the authors to measure the average H I mass for their galaxies. [Adapted from Bera et al. 2019]

Bera and collaborators found that the galaxies in their sample had an average H I mass of roughly 4.9 billion solar masses; for comparison, that’s around 1.2 times their average stellar mass. Based on their average star formation rate, these distant star factories should use up the fuel of their H I reservoirs in roughly 9 billion years.

How do these numbers compare to values in both the current, local universe and the distant, much older universe? The average H I mass and depletion time are consistent with the values measured in the local universe. In contrast, higher-redshift galaxies (z ~ 1.3) have been measured to have an average H I depletion time of less than a billion years.

These results therefore suggest that the efficiency of star formation evolved drastically from our universe’s early stages up to a few billion years ago, but it has held roughly steady since then. More deep radio observations like these should help us to further explore this evolution!

Citation

“Atomic Hydrogen in Star-forming Galaxies at Intermediate Redshifts,” Apurba Bera et al 2019 ApJL 882 L7. doi:10.3847/2041-8213/ab3656

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

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