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'Oumuamua

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

High-drag Interstellar Objects and Galactic Dynamical Streams

Published March 2019

Main takeaway:

Remember the first known interstellar asteroid, 1I/’Oumuamua? A study by scientist Marshall Eubanks (Space Initiatives, Inc.) explores the possibility that the asteroid is a very lightweight, high-drag object that long ago orbited the galaxy, became caught up within dense interstellar gas, and was then released in our direction as part of the Pleiades dynamical stream.

Why it’s interesting:

'Oumuamua velocity

‘Oumuamua’s incoming velocity is consistent with the dynamics of the Pleiades stream. [Eubanks 2019]

This interstellar object visited our solar system for only a brief time, and it displayed a number of perplexing behaviors — like its unexpected acceleration boost as it left the solar system again. Eubanks’ hypothesis provides a plausible natural explanation: if ‘Oumuamua is a body with a large area-to-mass ratio, solar radiation pressure could have provided the acceleration boost (note that this is the same explanation used by Bialy & Loeb to argue that ‘Oumuamua could be a light sail). And this same lightweight, high-drag structure makes it easy for the asteroid to have become entrained in interstellar gas before being sent into our solar system with the Pleiades stream.

Why This Could Provide Cool Future Opportunities:

In Eubanks’ model, ‘Oumuamua may not be unique. Instead, this object could be just one of an entire population of light asteroids with large area-to-mass ratios — all of which are likely to be entrained with dense gas and could be ejected toward us in streams. By searching stellar streams for bodies like this one, we could identify future interstellar visitors like ‘Oumuamua before they arrive, providing us with more time to study them from afar — or even prepare a fly-by mission.

Citation

T. M. Eubanks 2019 ApJL 874 L11. doi:10.3847/2041-8213/ab0f29

KIC 10544976

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Orbital Period Variation of KIC 10544976: Applegate Mechanism versus Light Travel Time Effect

Published March 2019

Main takeaway:

In a study led by Leonardo Almeida (Federal University of Rio Grande do Norte and University of São Paulo, Brazil), scientists announce evidence for a 13-Jupiter-mass planet around an evolved binary system, KIC 10544976, that consists of a white dwarf and a red dwarf star orbiting each other once every 0.35 days.

Why it’s interesting:

This is the first planet found orbiting an evolved binary like this one, and it raises questions as to how it formed. Was the planet born at the same time as the stars, and somehow survived the end of life of the binary member that evolved into a white dwarf? Or was the planet instead born later, out of the gas ejected by this star as it died? By studying the KIC 10544976 planet with next generation telescopes, we should be able to answer this question.

How the planet was discovered:

Observations of the eclipsing binary stars show timing variations in the eclipses. This change in orbit could be caused by one of two things: either the gravitational tug of an additional unseen, massive body, or period fluctuations in the magnetic field of the red dwarf. By studying the magnetic activity cycle for the red dwarf using years of flare and starspot data, Almeida and collaborators were able to rule out the hypothesis that magnetic activity caused the eclipse timing variations. This made the presence of a giant planet the most likely explanation.

Citation

L. A. Almeida et al 2019 AJ 157 150. doi:10.3847/1538-3881/ab0963

ANTARES

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Search for Multimessenger Sources of Gravitational Waves and High-energy Neutrinos with Advanced LIGO during Its First Observing Run, ANTARES, and IceCube

Published January 2019

Main takeaway:

No significant detections of high-energy neutrinos and gravitational waves coming from the same astrophysical source were found during the Laser Interferometer Gravitational-Wave Observatory’s (LIGO’s) first observing run. This was established from detailed analysis and comparison of IceCube and Antares neutrino candidates and LIGO gravitational-wave candidates over the ~130-day observing period.

Why it’s interesting:

neutron-star merger

Artist’s impression of two merging neutron stars producing a gamma-ray burst. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

Null results matter too! Core-collapse supernovae, binary neutron star mergers, and neutron star–black hole mergers are all among the astrophysical sources that we expect to produce both gravitational waves (which come from the source shedding angular momentum) and neutrinos (which come from astrophysical outflows like jets). By searching for joint neutrino/gravitational-wave signals from these sources, non-detections help us set limits on the rates and energies for these cosmic catastrophes.

Why this is only the start:

IceCube and Antares are already remarkable neutrino detectors located beneath kilometers of ice at the South Pole and water in the Mediterranean Sea, respectively. But the future holds hope for a significant upgrade to IceCube (IceCube-Gen2), work on an expanded neutrino observatory in the Mediterranean Sea is already underway (KM3NeT), and we’re even constructing a neutrino observatory in the deepest lake in the world, Lake Baikal in Russia (Baikal-GVD). Observations from these new detectors, combined with data from the upgraded LIGO/Virgo observatories, should place even further constraints on our understanding of astrophysical neutrino and gravitational-wave sources in the future.

Citation

A. Albert et al 2019 ApJ 870 134. doi:10.3847/1538-4357/aaf21d

black widow pulsar

One of the fastest spinning radio pulsars known has now been detected to pulse in gamma rays, too. What can we learn about this extreme pulsar from new observations?

Pushing the Record for Spin

pulsar

Artist’s illustration of a pulsar, a fast-spinning, magnetized neutron star. [NASA]

Pulsars are rapidly spinning, magnetized neutron stars left behind at the end of a star’s lifetime. Pulsar J0952-0607, a pulsar in a binary orbit with a very low-mass companion star, has the second-fastest known pulsar spin, rotating 707 times each second. For comparison, that’s about 70 times faster spin than the fastest helicopter rotors — and it’s an object that’s 10 km across and weighs more than the Sun!

As it spins, PSR J0952-0607 flashes a beam of radio waves across the path of the Earth, radiating from a hot spot on its surface. In a recent study, a team of scientists led by Lars Nieder (Albert Einstein Institute and Leibniz University Hannover, Germany) have now hunted through years of data from the Fermi Gamma-ray Space Telescope to see if we can spot pulsations from a gamma-ray beam as well.

Fermi

An artist’s illustration of the Fermi Gamma-ray Space Telescope. [NASA/General Dynamics]

Finding High-Energy Pulses

The radio observations of PSR J0952-0607’s pulsations span only 100 days, which isn’t long enough to precisely constrain its properties. The Fermi Gamma-ray Space Telescope launched in 2008, and its Large Area Telescope (LAT) has been providing all-sky images on a regular basis since then. Nieder and collaborators reasoned that if they could spot PSR J0952-0607 in gamma rays in the Fermi LAT data, then they’d be able to observe the pulsar over a much longer baseline than its radio observations provide.

The catch? PSR J0952-0607 is very faint in gamma-rays — which is why its pulsations weren’t previously detected. Nieder and collaborators had to develop novel search and timing methods with greater sensitivity, ultimately using the computational equivalent of 24 years on a single-core computer to search for a signal. Their efforts paid off, however — they managed to detect faint gamma-ray pulsations from PSR J0952-0607 spanning from July 2011 to the end of the dataset in January 2017.

Some Answers and Some New Puzzles

P–Pdot diagram

Plot of the spin-down rate vs. the spin for the known pulsar population outside of globular clusters. PSR J0952-0607 is marked by an orange star. [Nieder et al. 2019]

From the gamma-ray observations, Nieder and collaborators were able to measure a precise spin-down rate for the pulsar (it slows by less than 4.6 x 10-21 seconds each second), as well as other properties. PSR J0952-0607’s inferred magnetic field is among the 10 lowest magnetic fields measured for pulsars — an extreme that is predicted by theory based on this pulsar’s remarkably fast spin.

Though we’ve gained a lot of information about PSR J0952-0607 from its gamma-ray pulsations, new mysteries have also been introduced. The fact that its pulsations are undetectable before July 2011 is one of these — could the pulsar’s flux have changed? Or its orbit around its companion star? We’ll need more data to be able to solve this puzzle.

We still have more to learn about PSR J0952-0607, but the newly discovered gamma-ray pulsations have provided us with unique insight into the extremes that arise when compact astrophysical bodies spin at such high speeds. With luck, future observations of this pulsar — and others like it — will help us to further probe the physics of these unusual sources.

Citation

“Detection and Timing of Gamma-Ray Pulsations from the 707 Hz Pulsar J0952−0607,” L. Nieder et al 2019 ApJ 883 42. doi:10.3847/1538-4357/ab357e

exoplanet system

One of the goals of the Transiting Exoplanet Survey Satellite (TESS) is to identify exoplanets whose atmospheres can be characterized by other telescopes. Part of this process entails measuring planetary masses to some degree of precision. So just how well do we need to know an exoplanet’s mass to understand its atmosphere?

The Use of Transmission Spectra

One way to study the atmosphere of an exoplanet is to observe the light from its host star that passes through the planet’s atmosphere. Comparing the resulting spectrum — called a transmission spectrum — with the spectrum of the host star alone can tell us about what’s in the planet’s atmosphere.

A planet’s mass plays an important role in how far its atmosphere extends. This has prompted studies on whether a planet’s mass could be inferred from its transmission spectrum alone. In some cases, this approach works. But in other cases, the transmission spectra of very different planets can appear to be alike.

exoplanet mass uncertainty vs mass

The precision of exoplanet mass measurements versus their most likely mass. The seven planets used in this study are highlighted. [Adapted from Batalha et al. 2019]

So should we know the mass of a planet before trying to characterize its atmosphere? If so, how well? And how do these answers change for different types of planets? Natasha Batalha (University of California, Santa Cruz) and collaborators attempt to tackle these questions with simulated James Webb Space Telescope (JWST) transmission spectra.

Seven Special Planets

For their study, Batalha and collaborators chose seven known planets that span the gamut of exoplanets we’ve observed. Their sample included three hot Jupiters (WASP-17b, HAT-P-1b, WASP-12b), three Neptune-like planets (HAT-P-26 b, GJ 436b, GJ 1214b), and one Earth-like planet (TRAPPIST-1e). 

To simulate transmission spectra, the authors started with models that are consistent with Hubble spectroscopy of their chosen planets. They then used these models to simulate the analogous JWST spectra.

Aside from mass, the sample planets also varied in composition. Their host stars are also different, meaning that in real life the JWST would have to adopt different observing strategies to get quality transmission spectra.

The accuracy with which different atmospheric properties are recovered from the simulated transmission spectra. From top left, clockwise: temperature, metallicity (the abundance of elements that are not hydrogen or helium), radius, and mass. The shaded regions correspond to mass being known and the unfilled regions correspond to mass not being known. The colors of the curves indicate different planets. Click to enlarge. [Batalha et al. 2019]

A Matter of Caution

To test what role mass played in the usefulness of transmission spectra, the authors attempted to measure atmospheric properties from their modeled spectra. They tried different precisions on mass (how far off the assumed mass could be from the true mass) as well as not knowing a planet’s mass at all.

The authors found that transmission spectra alone could not reliably characterize a planet’s atmosphere. Hot Jupiters required the loosest mass constraints to infer atmospheric properties, though cloud cover  — such as in the case of WASP-12b — could make that untrue. For the other Neptunes and the Earth-like planet, mass had to be known with at least a 50% precision to get accurate atmospheric properties.

A recurring theme was that a mass measurement is necessary to distinguish one planet from others with similar transmission spectra. To this end, the authors recommend that any planets selected for atmospheric characterization have their mass known to at least 50% precision.

One of TESS’s goals is to measure the mass of fifty Earth-sized planets, and Batalha and collaborators have set a benchmark for those measurements. This sort of groundwork is critical to exoplanet science and should contribute to great results not too long from now!

Citation

“The Precision of Mass Measurements Required for Robust Atmospheric Characterization of Transiting Exoplanets,” Natasha E. Batalha et al 2019 ApJL 885 L25. https://doi.org/10.3847/2041-8213/ab4909

tidal disruption event

Close encounters between stars and supermassive black holes generally don’t end well for the stars. Under the influence of a black hole’s strong gravitational forces, an unsuspecting passing star can be completely shredded, resulting in a spectacular tidal disruption flare. But what happens when the star is only partially destroyed?

Unfortunate Encounters

Tidal disruption event

This simulated TDE shows the looping tidal stream caused when a star is pulled apart by the gravitational forces of a black hole. When this stream intersects with itself, material collides and rains onto the black hole, causing it to light up. [NASA/S. Gezari (JHU)/J. Guillochon (UCSC)]

Stars don’t often wander close to supermassive black holes — which means that tidal disruption events (TDEs) are relatively infrequent. Nonetheless, we’ve spotted around 40 of these destructive encounters so far, and they’re being discovered at an ever-increasing rate.

During a TDE, a passing star is torn apart and stretched into a stream of debris by the tidal forces of the supermassive black hole. Part of this stellar material escapes the black hole’s pull and scatters; the rest collides with itself during looping orbits, eventually raining down on the black hole and accreting. This accretion emits radiation, causing the black hole to briefly flare, producing a characteristic light curve that we might observe.

A Universal Decay?

The shape of the light curve is what makes TDEs distinctive. When we search for tidal flares, we hunt for transient signals that feature a sharp rise in the light curve followed by a long, decaying tail with a shape that’s governed by the physics of the fallback and accretion of the stellar material. For TDEs, that decay asymptotes to a characteristic power-law slope — a slope that was thought to be universal for all such stellar destruction.

But is it truly? A new study by scientists Eric Coughlin (Princeton University; Columbia Astrophysics Laboratory) and Chris Nixon (University of Leicester) explores whether we can expect to see differences when a star is only partially destroyed in its encounter with a supermassive black hole.

Survival Under Stress

fallback rates

Fallback rate onto a million-solar-mass supermassive black hole as a function of time for a disrupted Sun-like star. Different color curves represent different masses of surviving stellar cores. The curve representing a fully destroyed star (µ=0, red) asymptotes to a shallower slope (~t-5/3). The other curves, which all represent partial disruptions of varying degrees, all asymptote to a more steeply decaying slope (~t-9/4). Click to enlarge. [Coughlin & Nixon 2019]

In a partial stellar destruction, material is stripped from the star, but some fraction remains bound together as a stellar core. This core then orbits around the black hole along with the stream of colliding, accreting debris. Coughlin and Nixon show that the gravitational pull of this surviving stellar core affects the rate at which material falls back onto the black hole, causing different behavior than if the star had been fully destroyed.

What does this mean for observations? The authors argue that we should expect to see two types of TDEs: those representing complete stellar disruptions, whose light curves asymptote to a shallower slope, and those representing partial stellar disruptions, whose light curves asymptote to a steeper slope.

Coughlin and Nixon estimate that, for stars that undergo tidal disruptions, just under half of low-mass stars and around 70% of high-mass stars will be only partially disrupted. They therefore expect that a substantial fraction of TDEs detected by future facilities — like the Large Synoptic Survey Telescope (LSST) coming online in 2020 — will represent stars that partially survived their close encounter with a supermassive black hole … though perhaps a little the worse for wear.

Citation

“Partial Stellar Disruption by a Supermassive Black Hole: Is the Light Curve Really Proportional to t −9/4?,” Eric R. Coughlin and C. J. Nixon 2019 ApJL 883 L17. doi:10.3847/2041-8213/ab412d

M-dwarf planets

What determines the climate of an Earth-like planet orbiting its host star? And how is that climate affected by the type of star the planet orbits? A new study explores how distant terrestrial worlds are shaped by their hosts.

Radiation In, Radiation Out

The climate for a planet like Earth is largely set by the delicate balance between incoming radiation from the planet’s star, and outgoing radiation in the form of heat emitted into space. The amount of energy absorbed, reflected, and emitted by a planet’s surface and atmosphere dictate how this balance plays out.

mean energy budget

Diagram describing the annual mean energy budget for a planet orbiting a G dwarf star. Click to enlarge. [Adapted from Shields et al. 2019]

The pathways that govern this global energy budget for our own planet have been worked out through many decades of modeling and analysis of observations — to the point where we can identify sources of imbalance in the Earth’s system, like those currently caused by anthropogenic CO2 emissions.

But these climate models don’t apply directly to other planets, because the factors that determine a planet’s global energy budget all depend on the wavelength distribution of incoming light. Since stars of different temperatures emit varying amounts of radiation at different wavelengths, models that describe the energy budget for a planet around a Sun-like G dwarf won’t accurately describe a planet around a cooler M dwarf or hotter F dwarf.

So how do the climates of distant, Earth-like worlds change when orbiting a different type of host star? A team of scientists led by Aomawa Shields (University of California, Irvine) has now used detailed 3D global climate models to find out.

A Difference of Hosts

Shields and collaborators’ models of terrestrial planets take into account details like the interaction between the incoming host star’s radiation and gases like CO2 and H2O in the planet’s atmosphere, as well as with icy and snowy surfaces on the ground.

global mean surface temp

Plot of the global mean surface temperature as a function of the amount of incoming stellar radiation at the top of the planet’s atmosphere, shown for a planet orbiting an F dwarf (blue triangles), a G dwarf (black plus symobls), and an M dwarf (red x symbols). [Adapted from Shields et al. 2019]

The authors show that M-dwarf planets absorb more of their hosts’ radiation, both in their atmospheres and their surfaces, whereas F-dwarf planets absorb less. As a result, a planet can have a climate similar to that of modern-day Earth if it’s receiving current solar amounts of incoming radiation from a G-dwarf star — but to achieve the same climate around an M-dwarf star, it would need to receive 12% less incoming radiation. Around an F-dwarf star, it would need to receive 8% more.

What about rotation? The above models assumed that the planets all had 24-hour rotation rates, but Shields and collaborators also test how this compares to a tidally locked planet that always shows the same face to its host. For an M-dwarf host, a tidally locked planet has lower minimum and maximum dayside temperatures when compared with a planet with a 24-hour rotation period; the average dayside temperature is around 37 K colder on the tidally locked planet.

As we continue to discover more planets around a variety of stars, a constant question is whether these distant worlds have the potential to support life. Understanding how these planets’ global climates are shaped by their host stars is an important part of this exploration!

Citation

“Energy Budgets for Terrestrial Extrasolar Planets,” Aomawa L. Shields et al 2019 ApJL 884 L2. doi:10.3847/2041-8213/ab44ce

black hole accretion

In April of this year, the Event Horizon Telescope captured the first detailed images of the shadow of a black hole. In a new study, a team of scientists has now explored what determines the size and shape of black hole shadows like this one.

M87 EHT image

The first detailed image of a black hole, M87, taken with the Event Horizon Telescope. [Adapted from EHT collaboration et al 2019]

Imaging a Shadow

The stunning new radio images of the supermassive black hole in nearby galaxy Messier 87, released this spring by the Event Horizon Telescope team, revealed a bright ring of emission surrounding a dark, circular region.

This distinct structure is a result of the warped spacetime around massive objects like black holes. The ring of light is comprised of photons from the hot, radiating gas that surrounds the black hole, whose paths have been bent around the black hole before arriving at our telescopes. The dark region in the center is termed the black hole’s “shadow”; this is the collection of paths of photons that did not escape, but were instead captured by the black hole.

black hole disk comparison

Comparison of conceptions of a black hole surrounded by a thin accretion disk vs. a thick accretion disk. [Top: NASA, bottom: Nicolle R. Fuller/NSF]

The Shape of Accretion

While some previous studies have explored what a black hole shadow looks like when the black hole is surrounded by a very thin disk of accreting gas (think the black hole + disk from the movie Interstellar), most supermassive black holes — like M87, or our own supermassive black hole, Sagittarius A* — are more likely to be surrounded by hot, accreting gas that is more broadly distributed, forming a thick or quasi-spherical disk.

Does the geometry and motion of the accreting gas affect the size and shape of a black hole’s shadow?

Models of Monsters

In a new study, three scientists — Ramesh Narayan and Michael Johnson (Harvard-Smithsonian Center for Astrophysics) and Charles Gammie (University of Illinois at Urbana–Champaign) — have teamed up to explore how a black hole’s shadow changes based on the behavior of the hot gas around it.

black hole shadows

The image of the black hole shadow for three of the authors’ models: non-relativistic spacetime (top), relativistic spacetime with static surrounding gas (center), and relativistic spacetime with accreting gas flowing radially inwards (bottom). [Adapted from Narayan et al. 2019]

Narayan, Johnson, and Gammie built analytical models of a black hole surrounded by hot, optically thin gas (which means that the radiation escapes the gas and is observable). They then analyzed how the shadow would appear using different spacetimes, with different gas motions, and with different behaviors of the gas close to the black hole.

Reducing Complications

Intriguingly, the authors found that the appearance of the black hole’s shadow doesn’t depend on the details of the gas accretion close to the black hole. The size of the shadow was primarily determined by the spacetime itself (which is impacted by the mass of the black hole). But how the gas is distributed around the black hole, and whether that gas is stationary or accreting, doesn’t hugely affect the appearance of the shadow.

Real life is a little messier than this simple, spherically symmetric model; black hole spin and the presence of jets or outflows will cause asymmetries in the shadow. But the authors’ results generally tell us that the close-in details of accretion flows aren’t complicating what we’re seeing. And that’s valuable information we can use as we interpret future observations of black hole shadows! 

Citation

“The Shadow of a Spherically Accreting Black Hole,” Ramesh Narayan et al 2019 ApJL 885 L33. doi:10.3847/2041-8213/ab518c

fast transient

A supernova-like transient was observed to decline stupendously fast. What could have caused it?

Fast Transients

“Fast transients” are objects whose brightness rises and then falls drastically, usually on the order of weeks. They are not regularly varying objects; they have more in common with supernovae, which brighten once and then fade. However, fast transients change more rapidly than supernovae do, suggesting they have different explosive progenitors. 

With the advent of large astronomical surveys, fast transients are spotted more often now than ever before. The “fastest” optical fast transient is kilonova AT2017gfo — the result of the first observed binary neutron star merger. A recent study by Owen McBrien (Queen’s University Belfast) and collaborators discusses a transient that’s right on the heels of AT2017gfo in terms of the speed of its variation: a supernova-like object named SN2018kzr.

Magnetars and Nickel

The host galaxy of SN2018kzr, as seen more than two months after the transient appeared. The image was constructed using data taken by the ESO (European Southern Observatory) Faint Object Spectrograph and Camera on the New Technology Telescope. [Adapted from McBrien et al. 2019]

SN2018kzr was discovered independently by the Zwicky Transient Factory and the Asteroid Terrestrial-impact Last Alert System. Observers were tipped off by its rapid brightening, which took place over hours. SN2018kzr was then observed extensively over the next two weeks by multiple observatories, yielding a wealth of photometric and spectroscopic data. It began declining in brightness the same night it was first detected. 

To explain SN2018kzr’s rise and fall, McBrien and collaborators consider mechanisms that have previously been used for fast transients. They start with the reasonable assumption that nickel-56 is involved. The isotope nickel-56 can form in the explosions associated with fast transients and supernovae, and its radioactive decay can contribute greatly to a transient’s brightness. However, SN2018kzr dims too rapidly for it to be explained by nickel-56 alone, and the decay of other radioactive isotopes don’t explain observations either.

One solution is to tweak the progenitor scenario to include a massive remnant: a rotating neutron star with a strong magnetic field, known as a magnetar. A magnetar can contribute to the energy put out by SN2018kzr by slowing its own rotation. When coupled with nickel-56 decay, a magnetar’s spin-down could explain the shape of SN2018kzr’s light curve.

Choose Your Scenario

Fits to the light curve of SN2018kzr assuming different progenitor scenarios. The red points are associated with SN2018kzr and the white diamonds are associated with another fast transient, SN2005ek. SN2005ek is better fit by the He star model, while SN2018kzr is better fit by the nickel-56–magnetar scenario. [Adapted from McBrien et al. 2019]

Assuming that nickel-56 and a magnetar are involved in the progenitor of SN2018kzr, the authors present three possible scenarios: the core-collapse of a helium-rich star, the collapse of a white dwarf that’s accreted too much matter (accretion induced collapse, or AIC), and the merger of a white dwarf and a neutron star.

The first scenario isn’t favored since any remnant it produces wouldn’t rotate fast enough to explain SN2018kzr’s rapid decline. The authors favor the other scenarios, though the AIC model is on shaky ground based on previous studies.

The more fast transients we discover, the better our understanding becomes of how they form. Stay tuned!

Citation

“SN2018kzr: a rapidly declining transient from the destruction of a white dwarf,” Owen R. McBrien et al 2019 ApJL 885 L23. https://doi.org/10.3847/2041-8213/ab4dae

solar magnetic field

General models of the Sun’s atmosphere have a fundamental problem: the magnetic flux they predict here at Earth is much lower than what we actually observe. Has a new study found the missing flux?

When Models and Observations Don’t Match

The Sun offers us a unique opportunity to study stellar magnetic fields up close, testing the match-up between our models and observations — but, unfortunately, the outcome isn’t always ideal.

Sun's magnetic field lines

Magnetic field lines drawn from the solar surface to two times the Sun’s radius. The Sun’s magnetic field permeates our solar system as an interplanetary magnetic field. [Adapted from Riley et al. 2019]

Global models of the Sun’s atmosphere match the observed magnetic fields at its photosphere (the solar surface), and they do a great job of reproducing many features of the corona (the Sun’s extended upper atmosphere) and the solar wind (the stream of energetic particles that flows from the Sun into interplanetary space).

The trouble comes when we take a closer look at the predicted strength of the modeled interplanetary magnetic field — the natural extension of the solar magnetic field into the solar system. It’s there that our global solar models have a problem: they often underpredict the strength of the interplanetary magnetic field at a distance of 1 AU (i.e., at Earth, where we can easily measure it) by a factor of two or more.

So where’s the missing magnetic flux?

solar magnetic field

Line-of-sight component of the solar magnetic field, mapped out for models without (top row) and with (bottom row) extra flux added at the poles, as seen from two different vantage points: the view from Earth in the Sun’s equatorial plane (left column), and the view from a point 30° above the equatorial plane (right column). From Earth the difference is not visible; from 30° above, it will be. [Adapted from Riley et al. 2019]

Adding to the Poles

Led by Pete Riley, a team of scientists at Predictive Science Inc. in San Diego have proposed a solution: what if this “missing” flux takes the form of concentrated bundles of magnetic flux at the solar poles that we just can’t see from our angle?

Earth’s position near the plane of the Sun’s equator makes it difficult for us to observe much of what’s happening at its poles. It’s entirely possible that there’s extra magnetic flux at the poles of the Sun — where we can’t resolve it with ground-based observatories or Earth-based spacecraft — that our models are missing. By adding this flux into our models, maybe we’ll be able to reproduce the interplanetary magnetic field measured at Earth.

A Better View Ahead

To test this theory, Riley and collaborators explore a series of global models of the Sun, reconstructing the coronal magnetic field and measuring the resulting magnetic flux at 1 AU. In their models, the authors add extra bundles of open magnetic field lines at the poles of the Sun, and they then test whether this addition creates any other changes that would conflict with current solar observations.

Solar Orbiter

Artist’s impression of the ESA/NASA Solar Orbiter mission. [ESA/C. Carreau]

The authors find that their added flux significantly reduces the problem of the underpredicted interplanetary magnetic field — their values are much closer to the measured values. What’s more, the added flux also doesn’t produce any new observational discrepancies; the authors’ models still produce features in the corona consistent with what we see (and in some cases, even provide a better reproduction of observations).

So how can we tell if this is a realistic solution? The upcoming joint ESA/NASA Solar Orbiter mission, launching in February 2020, is just what we need: this spacecraft will eventually orbit the Sun at an inclination of 25° and above, allowing us a more definitive look at the Sun’s poles.

Citation

“Can an Unobserved Concentration of Magnetic Flux Above the Poles of the Sun Resolve the Open Flux Problem?,” Pete Riley et al 2019 ApJ 884 18. doi:10.3847/1538-4357/ab3a98

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