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DES

What’s the eventual fate of our universe? Is spacetime destined to continue to expand forever? Will it fly apart, tearing even atoms into bits? Or will it crunch back in on itself? New results from Dark Energy Survey supernovae address these and other questions.

Uncertain Expansion

fate of our universe

The evolution of the scale of our universe; click to enlarge. Measurements suggest that the universe is currently expanding, but does dark energy behaves like a cosmological constant, resulting in continued accelerating expansion like now? Or might we instead be headed for a Big Rip or Big Crunch? [NASA/CXC/M. Weiss]

At present, the fabric of our universe is expanding — and not only that, but the its expansion is accelerating. To explain this phenomenon, we invoke what’s known as dark energy — an unknown form of energy that exists everywhere and exerts a negative pressure, driving the expansion.

Since this idea was first proposed, we’ve conducted decades of research to better understand what dark energy is, how much of it there is, and how it influences our universe.

In particular, dark energy’s still-uncertain equation of state determines the universe’s ultimate fate. If the density of dark energy is constant in time, our universe will continue its current accelerating expansion indefinitely. If the density increases in time, the universe will end in the Big Rip — space will expand at an ever-increasing acceleration rate until even atoms fly apart. And if the density decreases in time, the universe will recollapse in the Big Crunch, ending effectively in a reverse Big Bang.

Which of these scenarios is correct? We’re not sure yet. But there’s a project dedicated to finding out: the Dark Energy Survey (DES).

The Hunt for Supernovae

DES was conducted with the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile. After six years taking data, the survey officially wrapped up observations this past January.

One of DES’s several missions was to make detailed measurements of thousands of supernovae. Type Ia supernovae explode with a prescribed absolute brightness, allowing us to determine their distance from observations. DES’s precise measurements of Type Ia supernovae allow us to calculate the expansion of the space between us and the supernovae, probing the properties of dark energy.

Though DES scientists are still in the process of analyzing the tens of terabytes of data generated by the project, they recently released results from the first three years of data — including the first DES cosmology results based on supernovae.

Refined Measurements

DES dark energy constraints

Constraints on the dark energy equation of state w from the DES supernova survey. Combining this data with constraints from the cosmic microwave background radiation suggest an equation of state consistent with a constant density of dark energy (w = –1). [Abbott et al. 2019]

Using a sample of 207 spectroscopically confirmed DES supernovae and 122 low-redshift supernovae from the literature, the authors estimate the matter density of a flat universe to be Ωm = 0.321 ± 0.018. This means that only ~32% of the universe’s energy density is matter (the majority of which is dark matter); the remaining ~68% is primarily dark energy.

From their observations, the DES team is also able to provide an estimate for the dark-energy equation of state w, finding that w = –0.978 ± 0.059. This result is consistent with a constant density of dark energy (w = –1), which would mean that our universe will continue to expand with its current acceleration indefinitely.

These results are exciting, but they use only ~10% of the supernovae DES discovered over the span of its 5-year survey. This means that we can expect even further refinements to these measurements in the future, as the DES collaboration analyzes the remaining data!

Citation

“First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: Constraints on Cosmological Parameters,” T. M. C. Abbott et al 2019 ApJL 872 L30. doi:10.3847/2041-8213/ab04fa

short gamma-ray burst

What drives rapid flickering in the jets that are produced in some powerful, high-energy explosions? Recent research explores the role of magnetic fields.

Mapping an Explosion

neutron-star merger

Artist’s illustration of the gamma-ray-burst jet launched during the merger of two neutron stars in 2017. [NSF/LIGO/Sonoma State University/A. Simonnet]

Gamma-ray bursts — brief flashes of high-energy emission from beyond our galaxy — have been detected since the 1960s. Though we’ve collected many observations of this explosions through the decades, it’s only recently that new evidence has clarified what causes some gamma-ray bursts.

In 2017, the merger of two neutron stars was observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO), just before the detection of a short (less than ~2 seconds) gamma-ray burst from the same location. These observations support the following picture for short gamma-ray bursts:

  1. A neutron-star–neutron-star binary or a neutron-star–black-hole binary merges, generating a potentially observable gravitational-wave signal.
  2. The merger either immediately produces a black hole, or it produces a hypermassive neutron star that collapses into a black hole shortly thereafter.
  3. The remnant material surrounding the newly formed black hole then rapidly accretes, leading to the production of powerful jets along the black-hole rotation axis.
gamma-ray-burst jet

Snapshot from one of the authors’ simulations, in which an axisymmetric jet extends in the +z and -z directions. Only half of the jet is shown, since the simulation is axisymmetric. From left to right, panels represent density, energy distribution, magnetization, and speed of the jet. [Sapountzis & Janiuk 2019]

Jetted Mysteries

This picture, while seemingly straightforward, is loaded with uncertainties. In particular, the jets launched in the third step are not well understood. We’re not sure what drives the production of these jets in the first place, and we also don’t know what collimates the jets, causing them to become tightly beamed as they travel, rather than spraying out in all directions.

What’s more, we observe rapid variability within the gamma-ray-burst jets; for short gamma-ray bursts, the timescales for variability are just ten-thousandths to hundredths of a second! What drives this rapid flickering within the jets?

In a recent study, two researchers at the Center for Theoretical Physics of the Polish Academy of Sciences, Konstantinos Sapountzis and Agnieszka Janiuk, explore the role that magnetic fields might have in the launching and properties of short-gamma-ray-burst jets.

jet variability

Left column: Variability of jet energy at a point inside the jet as a function of time for five of the authors’ models (each shown in a different row). Right column: same as left, but for a zoomed-in time. Vertical dashed lines show the characteristic timescale of the magnetorotational instability, which matches the jet variability well in all but the bottom model. [Sapountzis & Janiuk 2019]

Magnetic Fields at Work

Sapountzis and Janiuk perform a series of general-relativistic, magnetohydrodynamic simulations of a black hole surrounded by a torus of accreting material.

The authors use these simulations to explore how the magnetic field piles up as hot, ionized gas spirals inward and falls onto the black hole. The building field eventually forms a magnetic barrier that halts the inward flow of gas, leading to the formation of jets along twisting field lines that extend down the black-hole rotation axis.

But the role of the magnetic fields isn’t over with the launch of the jet. In the authors’ simulations, they observe a magnetic instability in the accreting plasma — known as the magnetorotational instability — operating on similar timescales to the variability in gamma-ray-burst jets. This suggests a link between the activity of magnetic fields at the base of the jet and the flickering we observe in the brief gamma-ray-burst jets.

We still have a lot to learn about gamma-ray bursts — and we can hope that future observations, especially now that LIGO is back online, will shed more light on these explosions! It certainly seems clear, however, that magnetic fields have an important role to play.

Citation

“The MRI Imprint on the Short-GRB Jets,” Konstantinos Sapountzis and Agnieszka Janiuk 2019 ApJ 873 12. doi:10.3847/1538-4357/ab0107

Photograph of the rocky surface of an asteroid.

Recent space missions have served as solar-system paparazzi, stalking a number of near-Earth objects — and the images they’ve sent home give us plenty to ponder. Can we use these observations to determine how one of these photogenic asteroids obtained its shape?

Visits to Asteroids

Ryugu and Bennu

Asteroids Ryugu (top) and Bennu (bottom) have similar spinning-top shapes. Click to enlarge. [JAXA/U. of Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Ins. Tech/Meiji U./U. Aizu/AIST; GSFC/NASA/U. of Arizona]

Near-Earth objects naturally fall under public scrutiny — it’s in our best interests to learn as much as we can about these neighboring (and potentially hazardous!) bodies. Two spacecraft were launched within the past few years to scope out these objects: NASA’s OSIRIS-REx is currently examining the asteroid 101955 Bennu, and JAXA’s Hayabusa2 is exploring the asteroid 162173 Ryugu.

Both missions plan to eventually return samples of their targets to Earth to help us better understand asteroid composition. In the meantime, the spacecraft have provided stunning images of the asteroids’ surfaces and overall structures, allowing us to learn more about these kilometer-scale inhabitants of our solar system.

Details of a Spinning Top

One striking feature of these asteroids is their shape: both Bennu and Ryugu have so-called “spinning-top” shapes, characterized by a raised equatorial ridge that makes the asteroids look more diamond-like than circular when viewed from the side. Such a shape could conceivably be caused by the flow of materials to the equator, but this requires rapid rotation — quicker than Ryugu’s 7.6-hour or Bennu’s 4.3-hour periods.

Ryugu as seen by Hayabusa2

Ryugu seen from the Hayabusa2 spacecraft from different angles. The western region exhibits a smoother surface and a sharper ridge angle than the rest of the asteroid. [Hirabayashi et al. 2019 via JAXA/U. of Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Ins. Tech/Meiji U./U. Aizu/AIST]

Hayabusa2’s detailed images of Ryugu reveal other asymmetries of the asteroid: the western region — termed the western bulge — is smoother than the eastern side, and the ridge angle here is sharper: 95° rather than the 105° seen on the rest of the asteroid. Recent research led by Masatoshi Hirabayashi (Auburn University) uses Hayabusa2’s observations to explore the possibility that Ryugu’s deformation was caused by a faster spin rate in its past.

Failed Structure?

Hirabayashi and collaborators use numerical models derived from Hayabusa2’s observations to analyze Ryugu’s current structure, judging how well different regions of the asteroid would hold up at different spin rates. The authors show that the subsurface region of the western bulge is currently structurally intact, whereas other regions are sensitive to structural failure. This suggests that the western bulge’s subsurface is relaxed — likely because it already experienced structural deformation in the past.

Ryugu FMD

“Failure mode diagram” computed for Ryugu. The shaded region indicates combinations for which Ryugu cannot structurally exist because the cohesive strength of its material would be below that necessary to remain structurally intact at a given spin period. [Hirabayashi et al. 2019]

What would it take to cause this deformation and create Ryugu’s current structure? Hirabayashi and collaborators show that a past spin period of around 3 hours — more than twice the asteroid’s current spin rate — could have caused the asteroid’s structure to fail in places. As material shifted, it could have generated the smooth surface of the western region and eventually settled to form its current configuration at a spin period of ~3.5 hours. Ryugu’s spin likely slowed gradually over time after this, eventually reaching its current leisurely 7.6-hour period.

The authors acknowledge that this scenario is not the only possible explanation for Ryugu’s shape, but it’s a model that produces results consistent with Hayabusa2’s detailed images. As we gain more data from Hayabusa2 — and from OSIRIS-REx at Bennu — we can hope to refine our theories further!

Citation

“The Western Bulge of 162173 Ryugu Formed as a Result of a Rotationally Driven Deformation Process,” Masatoshi Hirabayashi et al 2019 ApJL 874 L10. doi:10.3847/2041-8213/ab0e8b

Supernova remnant G299

There’s more than just one way for a star to explode. Supernovae — perhaps the most dramatic form of star death — come in many flavors, and astronomers are still learning about the vast diversity of these stellar explosions.

When Stars Steal Mass

Type Ia supernova

This artist’s rendering depicts one kind of Type Ia supernova mechanism: the singly degenerate model, in which a white dwarf siphons mass from its companion, exceeds the Chandrasekhar mass, and explodes. [NASA/CXC/M. Weiss]

When a white dwarf accretes gas from a binary companion and gains enough mass to exceed the Chandrasekhar limit, it can ignite in a cataclysmic explosion. This is the typical scenario for a Type Ia supernova, a common curtain call for low- to intermediate-mass stars in binary systems.

However, this isn’t the only way a Type Ia supernova can happen. In the double-detonation model, the explosion of the white dwarf is triggered by the ignition of an accreted helium shell. In this case, the white dwarf can be far less massive than the Chandrasekhar limit, leading to unexpectedly dim explosions.

Past studies have explored the minimum helium shell mass necessary (~0.01 solar mass) for this process and found that helium-shell detonations can efficiently cause core detonations, but there’s still plenty we don’t know about these events. The best way to learn about supernovae — double-detonation or otherwise — is to spot them soon after they happen.

De et al. 2019 Fig. 3

A comparison of ZTF 18aaqeasu’s optical light curve (red circles) to normal (orange hexagons) and sub-luminous Type Ia supernovae. [Adapted from De et al. 2019]

A Survey Spies a Supernova

In May 2018, an unusual supernova was detected by the Zwicky Transient Facility, an optical survey that hunts for fleeting events like stellar flares, fast-rotating asteroids, and the visible-light counterparts of gravitational-wave events. Within days of its detection, a team led by Kishalay De (Caltech) began to collect photometric observations and spectra of the object.

The photometry revealed that the object, ZTF 18aaqeasu, was unusually red and less luminous than a typical Type Ia supernova, making it a good candidate for the double-detonation scenario.

Its spectra were unusual even for a sub-luminous supernova, taking much longer to develop the silicon absorption feature typically seen in this type of event. Even stranger, the spectra exhibited a never-before-seen cutoff in the flux at short wavelengths, likely due to the presence of metals like iron and titanium.

De et al. 2019 Fig. 6

Comparison of observed spectra (black) to helium-shell double-detonation models (green and orange). [Adapted from De et al. 2019]

An Unusual Event

In order to derive the properties of ZTF 18aaqeasu, De and collaborators compared their photometric and spectroscopic data to models, finding that the event was likely caused by the ignition of a 0.15 solar mass helium shell, which led to the explosion of a 0.76 solar mass white dwarf.

The combination of a massive helium shell with a low-mass white dwarf makes ZTF 18aaqeasu unique among Type Ia supernovae; SN 2016jhr (one of the only supernovae previously linked to a helium-shell detonation event) featured a much more massive white dwarf with a less massive helium shell.

Can we expect to find more supernovae like ZTF 18aaqeasu? Similarly luminous supernovae should be detectable out to about 1.3 billion light-years, but so far there have been none reported with similar spectral features and unusually red color. This may indicate that double-detonation events featuring massive helium shells might be rare — adding an elusive new member to the Type Ia supernova family.

Citation

“ZTF 18aaqeasu (SN2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar-mass White Dwarf,” Kishalay De et al 2019 ApJL 873 L18. doi:10.3847/2041-8213/ab0aec

globular cluster mosh pit

The dense, chaotic centers of star clusters may be a birthplace for binary pairs of black holes like those observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO). A new study now explores how eccentric binaries might arise and merge in these extreme environments.

A Question of Origin

LIGO detections O1/O2

The ten black-hole mergers detected thus far by LIGO/Virgo. Click to enlarge. [Teresita Ramirez/Geoffrey Lovelace/SXS Collaboration/LIGO-Virgo Collaboration]

Since the discovery of the first gravitational-wave signal in September 2015, LIGO and its European counterpart Virgo have detected nine more merging black-hole binaries. After a brief pause for upgrades, the detectors are slated to come back online in April with significantly improved sensitivities — promising many more detections to come.

Though the gravitational-wave signals provide a wealth of information about the pre-merger binaries, we haven’t yet been able to determine how these black-hole binaries formed in the first place. Did these pairs evolve in isolation? Or were they born from interactions in the dense centers of star clusters?

One overlooked piece of data might shed light on these questions in the future: eccentricity. Since black-hole binaries in isolation take a long time to merge, any initial eccentricity in the orbit will be damped by gravitational-wave emission by the time the merger happens. But what if the binary doesn’t evolve in isolation? Could we see an imprint of eccentricity on the gravitational-wave signal then?

A new study led by scientist Michael Zevin (Northwestern University and CIERA) explores one possible channel for eccentric mergers: chaotic interactions between multiple black-hole binaries in the centers of star clusters.

complex interactions

Two examples of the complex evolution of binary–binary encounters, both eventually leading to a gravitational-wave capture. An animation of the second example is shown in the video at the end of the post. [Adapted from Zevin et al. 2019]

Complex Interactions

Zevin and collaborators use models to explore what happens during strong interactions between pairs of black-hole binaries and between black-hole binaries and single black holes.

These interactions are incredibly complex (don’t believe me? Check out the video below!). Systems with more than two bodies evolve chaotically, with small changes in initial conditions leading to vastly different outcomes. To make matters worse, simple Newtonian physics won’t accurately describe these systems; to capture the effects of gravitational-wave dissipation, we must model these interactions taking general relativity into account.

Zevin and collaborators find that these complexities lead to surprising results. Though binary–binary interactions occur 10–100 times less frequently than binary–single interactions in the centers of globular clusters, the long life and complexity of binary–binary interactions means that they are significantly more likely to result in a gravitational-wave capture — the rapid inspiral and merger of a binary pair, which occurs quickly enough that the pair may still have measurable eccentricity at merger time.

An Eccentric Result

eccentricity distributions

Predicted eccentricity distributions and delay times for three populations of binary–binary produced gravitational-wave mergers. The horizontal black lines show minimum measurable eccentricities predicted for LIGO/Virgo and LISA. Solid colored lines show the eccentricities for the three populations at 10 Hz (LIGO/Virgo’s lower limit) and 0.1 Hz (the most sensitive frequency predicted for LISA). [Zevin et al. 2019]

The authors demonstrate that binary–binary interactions contribute a significant fraction (~25–45%) of the eccentric mergers that result when black holes strongly interact in cluster centers. But what are our prospects for being able to detect these eccentric collisions?

The outlook is promising! Gravitational-wave captures generally have eccentricities at merger that should be measurable by LIGO/Virgo, and binary–binary-produced mergers that occur later, either in-cluster or after being ejected from the cluster, could have eccentricities detectable by the future Laser Interferometer Space Antenna (LISA). With enough observations, eccentric binaries may soon help us better understand the origin of black-hole pairs.

Bonus

This video (click here for a larger version) of one of the authors’ binary–binary encounters follows complicated and chaotic interactions over the span of ~25 years, leading to an eventual gravitational-wave capture.

Citation

“Eccentric Black Hole Mergers in Dense Star Clusters: The Role of Binary–Binary Encounters,” Michael Zevin et al 2019 ApJ 871 91. doi:10.3847/1538-4357/aaf6ec

SDO AIA blowout jet

Our Sun often exhibits a roiling surface full of activity. But how do the different types of eruptions and disturbances we see relate to one another? Observations of one explosive jet are helping us to piece together the puzzle.

Looking for Connections

coronal blowout jet

A coronal blowout jet captured by the Solar Dynamics Observatory on 9 Mar 2011. [Miao et al. 2018]

Energy travels through and from the Sun via dozens of different phenomena. We see ultraviolet waves that propagate across the disk, loops and flares of plasma stretching into space, enormous coronal mass ejections that expel material through the solar system, and jets of all different sizes extending from the Sun’s surface and atmospheric layers. A longstanding mission for solar physicists has been to relate these phenomena into a broader picture explaining how energy is released from our closest star.

Positions of the two STEREO satellites relative to the Sun and the Earth. SDO orbits the Earth. The green arrow shows the eruption direction of the blowout jet. [Miao et al. 2018]

One particular type of jet has offered a recent window into these relations. So-called “coronal blowout jets” make up perhaps a third to a half of all jets in the corona. Observations of one of these jets, taken simultaneously with multiple space telescopes, capture not only the jet eruption, but also other phenomena as well. A new publication led by Yuhu Miao and Yu Liu (Yunnan Observatories, China) details what these observations revealed.

An Enlightening Explosion

On 9 March 2011, a coronal blowout jet erupted from the Sun’s surface. Three spacecraft were on hand to watch: the Solar Dynamics Observatory, STEREO Ahead, and STEREO Behind. These observatories were each located roughly 90° from each other, providing a view of the Sun’s surface from multiple angles at the moment of the explosion.

What did they these observatories see?

  1. The flare
    The eruption of the blowout jet — which lasted ~21 minutes — was accompanied by a class 9.4 solar flare.
  2. The wave
    Shortly after the jet launch, an arc-shaped extreme ultraviolet (EUV) wave appeared on the southeastern side of the jet. This wave lasted ~4 minutes and propagated away from the site of the jet.
  3. The jet
    The jet itself contains both bright and dark material. The dark material appears to be due to a mini-filament — a thread of cool, dense gas suspended above the Sun’s surface by magnetic fields — that erupted in the jet base. 
  4. The coronal mass ejection
    The two STEREO spacecraft captured what happened on large scales in the outer corona of the Sun, revealing an explosive coronal mass ejection spewing matter into space. The ejection consisted of two structures: a jet-like component and a bubble-like component.

Causal Ties?

STEREO CME

STEREO Ahead (left) and Behind (right) images of the coronal mass ejection in the outer corona. Both a jet-like and a bubble-like component can be seen. [Miao et al. 2018]

These observations provide an unprecedented look at multiple types of solar activity all occurring simultaneously — and they suggest causal ties between the different phenomena.

In particular, the authors propose a relation in which the EUV wave was a fast-mode magnetohydrodynamic wave driven by the blowout jet eruption. They also suggest that the jet-like component of the coronal mass ejection is the outer-corona extension of the hot part of the blowout jet body, while the bubble-like component might be associated with the eruption of the mini-filament at the jet base.

More observations like those of this event are needed to draw definitive conclusions, but this explosion has provided some definite clues about the relationship between different phenomena as the Sun lashes out into its surroundings.

Bonus

Watch the propagation of the EUV wave (top video), the eruption of the blowout jet (middle video), and the coronal mass ejections (bottom video) in the clips below.

Citation

“A Blowout Jet Associated with One Obvious Extreme-ultraviolet Wave and One Complicated Coronal Mass Ejection Event,” Y. Miao et al 2018 ApJ 869 39. doi:10.3847/1538-4357/aaeac1

planet orbiting K dwarf

Signs of life in planetary atmospheres are hard to spot! A new study suggests that the best strategy for discovering them may be to look at planets orbiting K-dwarf stars.

The Hunt for Fingerprints

Is there life beyond Earth? This remains one of the most profound scientific questions astronomers are currently poised to address, and development of ever more powerful telescopes continues to bring us closer to an answer.

One way we can hope to use these telescopes to identify the presence of life on distant exoplanets is by detecting atmospheric biosignatures. Left alone, a planet’s atmosphere ought to be in chemical equilibrium. But when life is present, the atmosphere accumulates excess gases produced by the life — telltale fingerprints that we can hope to spot.

Signatures in Methane

stellar spectra

The spectra of various types of stars used by the author in models, including the Sun (a G2V dwarf) and three types of K dwarfs. K dwarfs produce less radiation in the 200–350 nm range. [Adapted from Arney 2019]

A good example of these fingerprints is the simultaneous presence of oxygen and methane in a planet’s atmosphere — something that shouldn’t occur if life isn’t there. The hunt for this biosignature is complicated by the fact that methane in the presence of oxygen is destroyed via chemical reactions driven by stellar light; if too much of the methane is removed by these photochemical reactions, we won’t be able to detect it.

There’s hope, though: some planets may be more likely to maintain life-produced methane in their atmospheres than others. Stellar light in the 200–350 nm range triggers this reaction — so the less light a planet’s host star produces in this range, the longer methane can survive in the planet’s atmosphere. This means that the type of host star matters: G dwarfs like the Sun will destroy the methane in their planets’ atmospheres faster than smaller and cooler M dwarfs.

Model planet spectra

Spectra from two of the author’s modeled quasi-modern planets: one around the Sun, and one around a K6V star. Orange lines show the spectra with methane removed, making the methane absorption features easier to see. The absorption features are much more evident in the planet around the K dwarf. [Adapted from Arney 2019]

Unfortunately, M dwarfs have other complications hindering the potential for life — including high levels of stellar activity that drive atmospheric loss from their planets. For this reason, scientist Giada Arney (NASA Goddard SFC and NASA NExSS Virtual Planetary Laboratory) has explored the advantages of a different type of star: K dwarfs.

The K-Dwarf Advantage

K dwarfs fall between G and M dwarfs in size and temperature, and they are more abundant than G dwarfs. Dimmer than G dwarfs, K dwarfs provide better planet-to-star contrast ratios that make it easier to observe potential habitable worlds. And they are are less active than M dwarfs, providing a more hospitable environment for their planets.

In addition to these benefits, K dwarfs also produce less radiation in the 200–350 nm range than G dwarfs do. By using a one-dimensional photochemical climate model to simulate a variety of planetary atmospheres, Arney demonstrates that a planet orbiting a K6V star can support roughly an order of magnitude more methane in the presence of oxygen relative to an equivalent planet around a G2V dwarf like the Sun.

starshade

Artist’s impression for one possible configuration of the proposed HabEx mission, in which a starshade could be used to help better image distant exoplanets. [NASA/JPL/Caltech]

But is this enough to produce signatures we can soon detect? Arney uses synthesized spectra to show that, with the technologies proposed for potential future missions like LUVOIR or HabEx, we have a decent chance of spotting the simultaneous methane and oxygen signatures in planets orbiting nearby mid-late K-dwarf stars. Thus the “K-dwarf advantage” gives us a great list of promising targets for the next major space missions!

Citation

“The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets,” Giada N. Arney 2019 ApJL 873 L7. doi:10.3847/2041-8213/ab0651

Centaurus A

What’s going on in our high-energy sky? Powerful phenomena abound in our universe, and they can produce photons with tremendous energies. A new study explores a high-energy mystery from one of these sources: active galactic nuclei, or AGN.

gamma-ray spectrum

Gamma rays span a broad range of energies in the most energetic part of the electromagnetic spectrum. Very high-energy gamma rays initially emitted from AGN have energies above 100 GeV, but these are reprocessed by interactions with background photons to energies of 1–100 GeV. [Ulflund]

Where Are the Gamma Rays?

Active galactic nuclei — the accreting supermassive black holes lurking at the centers of some galaxies — dot our universal landscape, spewing out very high-energy gamma-ray photons within jets moving at nearly the speed of light. These energetic photons speed across the sky — but they don’t travel unencumbered.

Theory predicts that this energetic emission should be effectively reprocessed as it slams into the cosmic microwave background, generating a compact sheath of gamma-ray emission in the 1–100 GeV range, beamed forward in the direction of the jets emitted from each AGN. But there’s a problem: we don’t see this expected flux.

AGN sources

Galactic coordinates of the sources used to generate the authors’ stacked analysis. Two types of AGN-containing galaxies are included: FR I and FR II galaxies. [Broderick et al. 2019]

One possible explanation for the missing light is that these traveling photons could be deflected from their path by a strong, large-scale magnetic field threading through intergalactic space. This would convert the compact, forward-beamed sheath into a more diffuse, harder-to-spot gamma-ray halo around each AGN. In a new study, a team of scientists led by Avery Broderick (University of Waterloo and the Perimeter Institute for Theoretical Physics, Canada) has gone on the hunt for these missing gamma-ray halos.

Stacks of Galaxies

Though the proposed gamma-ray halos may be too faint to spot individually, Broderick and collaborators suggest that by stacking a bunch of gamma-ray observations of off-axis AGN on top of one another, we should easily be able to detect their combined halo — if it exists.

radio jet alignment

The process of aligning the jets in two different radio images: an FR I galaxy (top) and an FR II galaxy (bottom). [Broderick et al. 2019]

To do this, the AGN must first be oriented in the same direction. Broderick and collaborators use radio observations of AGN jets pointed off our line of sight to identify each jet’s orientation. They determine the transformations needed to align each of the radio jets, and then apply this transformation to corresponding Fermi-telescope gamma-ray observations of the active galaxies. The result is a sample of nearly 9,000 gamma-ray observations of AGN, all oriented in the same direction.

Broderick and collaborators then stack these observations and compare their results to a model of what we would expect to see if an intergalactic magnetic field were deflecting the gamma-ray photons, generating a faint halo around the AGN.

Still No Halos

gamma-ray halos

Top: the authors’ stacked gamma-ray observations for FR I (left) and FR II (right) galaxies. Bottom: the expected signals if gamma-ray halos were present. The observations clearly rule out the presence of faint halos. [Broderick et al. 2019]

Intriguingly, the authors find no hint of a combined gamma-ray halo. Their non-detection places strict limits on the strength of the intergalactic magnetic field allowed in this picture, and it rules out magnetic fields as an explanation for why we don’t see the gamma rays we expect from AGN.

What does this mean? Broderick and collaborators argue that this requires us to consider brand new physics in high-energy processes. There must be some unexpected mechanism that prevents the creation of the expected gamma-ray halos, either because the highest-energy emission is suppressed in gamma-ray bright AGN, or because some process affects this emission before it can lead to the generation of halos. The mystery deepens!

Citation

“Missing Gamma-Ray Halos and the Need for New Physics in the Gamma-Ray Sky,” Avery E. Broderick et al 2018 ApJ 868 87. doi:10.3847/1538-4357/aae5f2

Galactic center in infrared

How does a supermassive black hole affect its stellar neighbors? One way to explore this question is by searching for old, giant stars in the extreme environs of the galactic center.

Crowded Quarters

Galactic center in visible light

Dark dust lanes block the visible light from the galactic center, hiding the dense star cluster located there. [Dave Young]

The supermassive black hole at the center of our galaxy likely plays a huge role in the evolution and dynamics of stars in its neighborhood, as well as in how they are spatially distributed.

Theory predicts that old, giant stars near the galactic center should be arrayed in a “cusp”-like distribution, with the number of stars per square arcsecond increasing sharply toward the central black hole. Faint red giants seem to follow the expected distribution, but brighter red giants — which can be probed closer to the center of the galaxy — do not. Instead, these stars appear to follow a “core”-like distribution, with fewer stars than expected within the central arcsecond of the galaxy.

Many theories have been proposed to explain the apparent lack of bright red giants near the galactic center, from stellar collisions to tidal disruption by the supermassive black hole. While these factors may play a role, it’s also possible that observational challenges have prevented astronomers from fully cataloging the stellar population at the galactic center.

Habibi et al. 2019 Fig. 2

Giant stars from this study (black stars) on an H-R diagram with the theoretical isochrones used to determine the stellar ages. [Habibi et al. 2019]

Tracking Down Missing Stars

Observing stars so close to the galactic center is tricky — it’s crowded there, and starlight is highly extincted by dust clouds in the galactic plane at many wavelengths. In order to probe the stellar population near the galactic center, a team led by Maryam Habibi (Max Planck Institute for Extraterrestrial Physics, Germany) analyzed more than a decade’s worth of near-infrared stellar spectra from the SINFONI spectrograph on ESO’s Very Large Telescope.

The spectra used in this study were collected with the help of adaptive optics, in which the telescope’s mirror is deformed slightly to correct for the effects of turbulence in Earth’s atmosphere in close to real time — critical for observations of individual stars in a field as crowded as the galactic center!

By co-adding multiple epochs of spectra to tease out faint spectral features, the authors derived the effective temperature, spectral type, age, mass, and radius for each target star. Their deeper spectra allowed them to identify old giants that had previously been misclassified as younger stars, bringing the number of known giants to 21.

Habibi et al. 2019 Fig. 4

Observed stellar density profiles from this and other studies of the galactic center. Previously, the observed distribution was consistent with a core-like profile (blue dashed line). The inclusion of the newly identified giants shows that the distribution is instead consistent with a cusp-like distribution. [Habibi et al. 2019]

Cusp Versus Core

Combining their new observations of bright giants within the central arcsecond with previously observed giants farther from the galactic center, the authors find that the distribution of bright giants can be described by a power law with an exponent of 0.34 ± 0.04 — definitively ruling out a core-like distribution.

Does this mean the galactic center’s core–cusp problem has been solved? While many of the missing giants have been found, the authors estimate that there are still stars awaiting discovery in the crowded interior of our galaxy, including some of the brightest red giants. Future observations should help us understand the complex distribution of stellar populations in the galactic center.

Citation

“Spectroscopic Detection of a Cusp of Late-type Stars Around the Central Black Hole in the Milky Way,” M. Habibi et al 2019 ApJL 872 L15. doi:10.3847/2041-8213/ab03cf

CoRoT-2b

Exoplanets HAT-P-7b and CoRoT-2b have an unusual quirk: instead of having eastward equatorial winds, like the majority of hot Jupiters, these two hot Jupiters have westward winds. A new study explores whether magnetic fields cause this odd reversal.

Blowing the Wrong Way

HAT-P-7b

Artist’s impression of HAT-P-7b, an inflated hot Jupiter. [NASA, ESA, and G. Bacon (STScI)]

You might think that the hottest — and therefore brightest — part of a tidally locked hot Jupiter should be the part that directly faces its nearby host star. Surprisingly, our observations of hot Jupiters have generally revealed an offset for the peak brightness that’s slightly east of the point directly facing the host. These observations suggest that hot Jupiters host strong eastward-blowing winds near their equators that can displace their hottest point.

Two planets break this rule, however: HAT-P-7b and CoRoT-2b. Observations of both of these hot Jupiters instead reveal hotspots that lie west of the point facing the host. Astronomers have generally interpreted this to imply that these two planets have westward-blowing equatorial winds — but why?

There are a number of proposed explanations for this odd apparent reversal:

  1. The planet may not be tidally locked as expected; if it rotates on its axis slightly slower than it orbits its host, this could drive westward winds.
  2. The apparent offset hotspot location could be an illusion caused by asymmetric cloud distribution.
  3. Interactions of the planet’s magnetic field with its atmosphere could modify its wind pattern.

In a new study led by Alexander Hindle (Newcastle University, UK), a team of scientists explores the feasibility of this third option.

Magnetic Waves

hostspot displacement

Plot of the geopotential, which traces temperature, in the authors’ simulations, with (bottom) and without (top) the presence of magnetic fields. The hotspot (marked with a white cross) displaces to the east for the hydrodynamic case and to the west for the magnetohydrodynamic case. [Hindle et al. 2019]

Hindle and collaborators use both analytic models and simulations to show what happens in the atmosphere of a planet with a strong magnetic field. They explore a layer of atmosphere that can behave like shallow water, developing planetary-scale waves. Without a magnetic field, these waves will naturally travel eastward. But in the presence of a strong toroidal magnetic field, the wave shears as it travels, resulting in westward-tilting eddies. This drives the winds to switch direction to the west.

The authors next calculate the minimum magnetic field strength needed to create this equatorial wind reversal for planets with the properties of HAT-P-7b and CoRoT-2b. They find that an inflated hot Jupiter like HAT-P-7b would need a field strength above just 6 Gauss (for comparison, the Earth’s magnetic field is ~1 G). Estimated field strengths for inflated hot Jupiters lie in the 50–100 G range, so attributing HAT-P-7b’s wind reversal to magnetic fields is well within reason.

For an ordinary hot Jupiter like CoRoT-2b, however, a field strength of 3,000 G is needed. The maximum expected field strength for a hot Jupiter like CoRoT-2b is 250 G, which isn’t sufficient to drive the reversal. Hindle and collaborators conclude that a different mechanism is likely at work on this planet.

More observations of hot Jupiters in the future — as well as three-dimensional simulations — will help us to further understand the wind behavior in the atmospheres of these toasty planets.

Citation

“Shallow-water Magnetohydrodynamics for Westward Hotspots on Hot Jupiters,” A. W. Hindle et al 2019 ApJL 872 L27. doi:10.3847/2041-8213/ab05dd

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