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Crab nebula

The same physical phenomenon that causes bumpy airplane rides also pervades our universe, jumbling stellar atmospheres, interstellar clouds, and even the magnetized sheath surrounding the Earth. Now, a new study brings us a little closer to understanding turbulence.

turbulence

This image captures the transition between laminar and turbulent flow in the convection plume above a candle flame. [Gary Settles]

A Complex Phenomenon

Have you ever watched the entrancing wisps of smoke rising above a candle flame? What you’re looking at is turbulence — and despite this phenomenon’s prevalence throughout the universe, a complete description of turbulence remains one of the unsolved problems in physics.

The difficulty is that turbulent motion — characterized by rapid and chaotic fluctuations of fluid properties — is incredibly complex. Turbulence begins when energy is injected on large scales, causing field-level fluctuations. This energy then cascades down to smaller and smaller scales, creating chaotic motions all the way down to microscales. When the energy reaches small enough scales, it can dissipate, accelerating individual particles and converting into heat.

But scientists don’t fully understand the physical mechanisms at work in turbulence that inject the energy, transfer it to smaller scales, and eventually dissipate it. Worse yet, these processes take a different form when we’re no longer talking about fluids, but instead about astrophysical plasmas.

Plasmas, Plasmas Everywhere

Astrophysical plasmas are soups of ionized gas found everywhere from supernova remnants to the compressed solar wind surrounding the Earth in its magnetosheath — and in these plasmas, energy could be dissipated through a variety of mechanisms related to interactions between particles and waves.

magnetosphere

This diagram of the Earth’s magnetosphere shows the location of the magnetosheath, the region behind the bow shock where the compressed solar wind detours around the Earth. [NASA/Goddard/Aaron Kaase]

How can we tell which mechanisms are at work? The key is to explore the rate at which turbulence in a plasma is dissipated across different length scales. In a recent study, a team of scientists led by Jiansen He (Peking University, China) has now developed a new approach to examine this spectrum and applied it within the Earth’s magnetosheath.

Measuring a Fluctuating Environment

The authors’ approach takes advantage of unprecedented, high-quality measurements made by the Magnetospheric Multiscale mission, a constellation of four spacecraft exploring the plasma environment around the Earth. As these spacecraft — separated by a distance of about 10 km — pass through magnetosheath plasma, they make measurements of the three-dimensional electric and magnetic fields, tracking the field fluctuations caused by turbulence.

MMS

Artist depiction of the Magnetospheric Multiscale Mission spacecraft. [NASA/GSFC]

He and collaborators present a method that uses these measured fluctuations to investigate how the dissipation rate is distributed across various length scales within the plasma. This spectrum of dissipation rates can then tell us which physical processes are most likely at play, driving the dissipation.

While we still have a lot to learn, He and collaborators’ work indicates that ion cyclotron waves — waves generated when ions oscillate in a magnetized plasma — play an important role in dissipating turbulent energy in the Earth’s magnetosheath.

More importantly, the authors’ approach for measuring the dissipation rates at different scales can be widely applied to different space plasma environments — so we can hope for more insight into turbulence in space in the future!

Citation

“Direct Measurement of the Dissipation Rate Spectrum around Ion Kinetic Scales in Space Plasma Turbulence,” Jiansen He et al 2019 ApJ 880 121. doi:10.3847/1538-4357/ab2a79

Being able to make precise measurements of distances and redshifts will help us understand how the universe is evolving. With the advent of gravitational wave observatories, we can make these measurements by using black holes in a very different way than before.

Standard Sirens

To measure how the universe is expanding, we need to simultaneously obtain the distances and redshifts to sources. When it comes to measuring large distances in space, astronomers have typically leaned on “standard candles” — objects whose intrinsic brightness is known. The dimmer a standard candle appears, the farther away it is.

Merging binary black holes (BBHs) can serve as standard candles, in a way. When compact objects like black holes merge, they produce gravitational waves, which can be picked up by observatories like the Laser Interferometer Gravitational-Wave Observatory (LIGO). The emitted gravitational waves have a characteristic energy, meaning that these mergers could be used to measure distances as “standard sirens”.

The trouble comes when trying to simultaneously measure the redshift of these sources. The gravitational wave detection on Earth gives us a mass measurement for the black holes that’s a combination of their redshift and their true masses in the source frame. If we know the true masses, we can disentangle these variables and determine the redshift. To achieve this, Will Farr (Stony Brook University and Flatiron Institute) and collaborators propose using a particular constraint on the masses of BBHs.

True black hole mass (not measured mass) versus redshift as obtained from one year of simulated BBH merger observations. The blue line indicates the maximum mass of black holes as set by PISNs, with the dark and light bars showing the confidence intervals. [Farr et al. 2019]

Capping Masses

When we model the population of merging black holes we’ve detected via gravitational wave observations, we see a drop-off in black hole mass above 45 solar masses. Farr and collaborators suggest this upper limit could be tied to one specific route of black hole creation: pair instability supernovae (PISNs).

Only massive stars can die as PISNs. In these events, the core of a star gets hot enough to allow electron–positron pairs to pop into existence, which lowers the star’s internal pressure enough for gravity to trigger the trademark explosion of a supernova. The remnants left behind by PISNs peak in mass around 45 solar masses. 

By taking advantage of the mass scale imprinted on the population of BBH mergers by the PISN process, Farr and collaborators argue, we can extract redshifts from our detector measurements. Simulating 5 years of detections, the authors show that we could potentially constrain the Hubble parameter — our measurement of the expansion of the universe — at a specific redshift to within an impressive 2.9%.

Distributions of the Hubble parameter at a redshift of 0.8 as estimated by one year of observations (blue) and five years of observations (orange). The true value of the Hubble parameter at that redshift is indicated by the black vertical line. [Adapted from Farr et al. 2019]

Paring Down Parameters

The authors find that BBHs are most useful for constraining the Hubble parameter at a redshift of z = 0.8 (redshifts that can be explored with the current capabilities of gravitational wave observatories are between z = 0 and 1.5). This is because at that redshift the models peg the uncertainty on the Hubble parameter at a minimum. Additionally, the uncertainty is halved when going from one year of observations to five years.

The authors note that a change of 1–2 solar masses in their maximum black hole mass does not change their results drastically. Their method would also work with a different maximum mass — so long as there is some mass scale, BBH mergers can be used to measure distances.

New gravitational-wave detectors will extend our sample of BBH mergers enormously. With a larger sample and a better understanding of the utility of black holes, we will be closer to pinning down the fate of the universe.

Citation

“A Future Percent-level Measurement of the Hubble Expansion at Redshift 0.8 with Advanced LIGO,” Will M. Farr et al 2019 ApJL 883 L42. https://doi.org/10.3847/2041-8213/ab4284

exoplanet system

Though we’ve discovered thousands of planets beyond our solar system, we still have a lot to learn about how these bodies form and evolve. Now, a newly discovered baby planetary system may provide some insight.

Multiplanet Wealth

Kepler systems

An illustration of some of the planetary systems discovered by the Kepler spacecraft. The stars at the centers of these systems are not pictured. [NASA Ames/UC Santa Cruz]

The Kepler mission has been instrumental in our exploration of worlds beyond our solar system, helping us to discover nearly 5,000 confirmed and candidate exoplanets. In particular, Kepler’s gaze has revealed a wealth of compact, multiplanet systems that share both intriguing similarities and striking differences with our own solar system.

Kepler multiplanet systems tend to be coplanar, with nearly circular orbits and low obliquities. There is often a high degree of intrasystem uniformity — planets of similar sizes, masses, and orbital spacing are more likely to be found together in the same system. And, intriguingly, most Kepler compact multiplanet systems tend to consist of small planets that have radii of less than 3 Earth radii.

Could these systems’ traits point to how they form and evolve? Might these planets once have had larger sizes, before they cooled and contracted or lost some of their atmospheres to photoevaporation? In order to answer these questions, we need to explore multiplanet systems much earlier in their lifetimes.

V1298 Tau transit light curves

Phase-folded transits for each of the four V1298 Tau planet candidates. [Adapted from David et al. 2019]

New Baby Planets Found

Now, a team of scientists led by Trevor David (Jet Propulsion Laboratory, California Institute of Technology) has identified a system of planets that might be exactly what we’re looking for.

David and collaborators re-analyzed Kepler data from 2015 to identify three new planets transiting the young solar analog V1298 Tau, which was already known to host one Jupiter-sized planet on a 24 day orbit. The newly discovered planets have periods of 8.25 days, 12.46 days, and somewhere between 36 and 223 days (we only have one transit for this last one, so its orbit isn’t yet well-constrained).

Critically, V1298 Tau is a very young star, at just 23 million years old — so we’re examining this planetary system early in its formation.

A Valuable Laboratory

Young transiting exoplanets

Young transiting exoplanets in the period–radius plane. The new planets discovered around V1298 Tau are indicated by yellow stars, and they occupy sparsely populated regions of the plane. Click to enlarge. [David et al. 2019]

What makes the planets of the V1298 Tau system interesting is their unusually large sizes: though their masses are low, these planets are Neptune-to-Saturn-sized, clocking in at 5.6, 6.4, and 8.7 Earth radii. V1298 Tau’s planets are therefore significantly larger than the planets found in the vast majority of Kepler multiplanet systems.

The authors speculate that these planets may still be radiatively cooling and contracting, and perhaps losing atmosphere. The V1298 Tau system could, in fact, be the precursor to the compact multiplanet systems Kepler has found throughout the galaxy.

V1298 Tau provides a valuable laboratory to explore a stellar system in the early stages of its evolution. By following up with additional observations — such as planet mass measurements and atmospheric characterization — we stand to learn much more about how this baby planetary system and others like it formed and evolved.

Citation

“Four Newborn Planets Transiting the Young Solar Analog V1298 Tau,” Trevor J. David et al 2019 ApJL 885 L12. doi:10.3847/2041-8213/ab4c99

Kepler-186f

How can we identify life on other planets? The Earth might be able to help with that — specifically with something called the red edge.

The Green Light for Life

One of the most exciting prospects of exoplanet science is discovering another planet that can harbor life. However, this necessitates us knowing how to identify life at a distance, which is quite a challenge!

The light reflected by an exoplanet is one of the most useful observations to have on this quest. The reflected light is dependent on the surface and atmospheric conditions of the planet, and it may hold key features that point to the existence of life. What those features are, however, is another question.

Blue Marble

Photograph of the Earth taken from space. [NASA/Goddard Space Flight Center/Reto Stöckli]

This is where the Earth comes in handy. With our intimate understanding of the Earth, we can simulate it as an exoplanet fairly accurately. Those simulations can help us better pick out Earth-like planets from reflected-light observations.

The Earth’s reflected-light spectrum contains a unique feature: something called the red edge, a region of rapid change in the near-infrared part of the spectrum. The red edge is caused by chlorophyll in the Earth’s organisms, which has the quirk of strongly reflecting red light. Could this red edge be used to identify chlorophyll-containing life on other planets? In a recent study, Jack O’Malley-James and Lisa Kaltenegger (Carl Sagan Institute, Cornell University) considered the effects of various organisms on the red edge and what that means for the red edge’s detectability.

Exploring Scenarios

A surprising number of organisms contain chlorophyll, but at present, land-based vegetation is most responsible for the red edge. Aside from trees (a catchall term for land-based vegetation), O’Malley-James and Kaltenegger considered other organisms like cyanobacteria, algae, and lichens.

Initially, the authors used a simplified model of the Earth to understand the impact of each organism. They assumed that the entirety of the planet was covered by just one organism and determined how the red edge would appear for an atmosphere that was still like the Earth’s. They then tried the same scenario with a more realistic Earth, which had a surface that was 30% land and 70% ocean.

The authors also considered the effect of clouds. They tried two cases for each planet scenario, one with clear skies and the other with 60% cloud cover. The difference in cloud cover was more significant for the realistic planet model.

reflected light spectrum

The fraction of light reflected at different wavelengths by different chlorophyll-containing organisms (corals; trees; elysia viridis, a photosynthetic sea slug; lichens; algae; cyanobacteria). The gray area shows the wavelength range in which the red peak appears. [O’Malley-James and Kaltenegger 2019]

Earths at Different Times

The red edge likely evolved with life on the Earth. Trees are relative newcomers, having only established themselves ~500–725 million years ago. Algae and lichens are much older — ~1 billion years old — and cyanobacteria likely appeared at least 2 billion years ago. These staggered arrival times imply that planets that are similar to the early Earth could still produce red edges.

There may not be a lot of planets exactly like the present-day Earth, but O’Malley-James and Kaltenegger suggest that this isn’t a setback. We could potentially identify planets that have just begun to harbor life — and that’s a big step in the right direction.

Citation

“Expanding the Timeline for Earth’s Photosynthetic Red Edge Biosignature,” Jack T. O’Malley-James and Lisa Kaltenegger 2019 ​ApJL​ ​879​ L20. doi:10.3847/2041-8213/ab2769

Neutron Star Merger

To predict how often binary neutron star mergers occur, we need to know when binary neutron stars are born and how long it takes them to merge. An avenue for understanding this is to carefully study their host galaxies. 

Where To Look?

Neutron stars are the seemingly anticlimactic remnants of supernovae. However, aside from containing fascinating states of matter, they may also be responsible for creating some of the elements that can’t be created in the cores of normal stars. This would happen when a pair of neutron stars — binary neutron stars (BNS) — merge, emanating characteristic gravitational waves.

Proving this hypothesis of element formation requires an understanding of where and when BNS form and collide. This is where Delay Time Distributions come in. The Delay Time Distribution for binary neutron stars predicts how long after a binary birth two neutron stars will spend spiraling around each other before they finally merge. If we obtained a well-constrained Delay Time Distribution for BNS, we would have a more complete idea of how often BNS form and merge.

BNS merger likelihood

The likelihood of different rates of BNS mergers, given Delay Time Distributions with different parameters. The top plot assumes a slow, continuous star formation history and the bottom plot assumes a single burst of star formation. Click to enlarge. [Adapted from Safarzadeh ​et al. ​2019]

Mohammadtaher Safarzadeh (Harvard-Smithsonian Center for Astrophysics and Arizona State University) and collaborators have studied the Delay Time Distribution for BNS quite extensively over a series of recent publications. Most recently, they examined the star formation history of galaxies that have hosted BNS mergers through simulations, exploring how this could be used to constrain the BNS Delay Time Distribution.

Modeling Star Formation Histories

Past efforts by Safarzadeh and collaborators have previously studied the BNS Delay Time Distribution using the properties of BNS merger hosts — specifically galaxy mass and redshift. Both quantities can be broadly tied to a galaxy’s star formation history, which is key to constraining the Delay Time Distribution. In this work, the authors attempt to more directly examine the star formation histories of the merger hosts.

They start by modeling star formation histories for about 6,000 galaxies that were observed in the Galaxy and Mass Assembly survey. From this modeling, two sorts of histories emerge: one where stars formed quickly and nearly all at once and the other where star formation happened slowly and continuously.

A given star formation history can be used to estimate the number of BNS that are born in a galaxy over time. The authors then use a subset of their galaxy sample with the different star formation histories to simulate several sets of BNS mergers. By comparing these simulations to current and future observations of BNS merger rates, the authors succeed in placing new constraints on the BNS Delay Time Distribution parameters.

Constraints on Delay Time Distribution parameters obtained using a sample of 300 BNS merger host galaxies. The input function is marked by the yellow circle, the red region comes from assuming a burst of star formation, and the blue region comes from assuming slow, continuous star formation. [Safarzadeh ​et al. ​2019]

In Search of More

Using star formation histories to constrain Delay Time Distributions proves to be an improvement over using galaxy masses. Additionally, the simulations provide a larger sample of BNS host galaxies to work with. However, the best results will be obtained when we eventually build a larger sample of observed BNS mergers that spans a much larger volume of space.

Given that gravitational wave astronomy is in its infancy, our sample of BNS mergers is likely to explode as new observatories come online. Will this tell us more about how binary neutron stars form, collide, and brew the chemical elements that pervade our universe? Likely so!

Citation ​

“Measuring the Delay Time Distribution of Binary Neutron Stars. III. Using the Individual Star Formation Histories of Gravitational-wave Event Host Galaxies in the Local Universe,” Mohammadtaher Safarzadeh ​et al ​2019 ​ApJL 878​ L14. https://doi.org/10.3847/2041-8213/ab24e3

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

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