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5 April 2017

ALMA Examines a Distant Quasar Host

The dust continuum (top) and the [CII] emission (bottom) maps for the region around J1120+0641. [Adapted from Venemans et al. 2017]

A team of scientists has used the Atacama Large Millimeter/submillimeter Array (ALMA) to explore the host galaxy of the most distant quasar known. Their observations may help us to build a picture of how the first supermassive black holes in the universe formed and evolved.

Faraway Monsters and Their Galaxies

We know that quasars — the incredibly luminous and active centers of some distant galaxies — are powered by accreting, supermassive black holes. These monstrous powerhouses have been detected out to redshifts of z ~ 7, when the universe was younger than a billion years old.

Though we’ve observed over a hundred quasars at high redshift, we still don’t understand how these early supermassive black holes formed, or whether the black holes and the galaxies that host them co-evolved. In order to answer questions like these, however, we first need to gather information about the properties and behavior of various supermassive black holes and their host galaxies.

A team of scientists led by Bram Venemans (Max-Planck Institute for Astronomy, Germany) recently used the unprecedented sensitivity and angular resolution of ALMA — as well as the Very Large Array and the IRAM Plateau de Bure Interferometer — to examine the most distant quasar currently known, J1120+0641, located at a redshift of z = 7.1.

A High-Resolution Look

The team’s observations of the dust and gas emission from the quasar’s host galaxy revealed a number of intriguing things:

The red and blue sides of the [CII] emission line are shown here as contours, demonstrating that there’s no ordered rotational motion of the gas on kpc scales. [Adapted from Venemans et al. 2017]
The majority of the galaxy’s emission is very compact. Around 80% of the observed flux came from a region of only 1–1.5 kpc in diameter.
Despite the fact that the 2.4-billion-solar-mass black hole at the galaxy’s center is accreting at a high rate, the heating in the galaxy is dominated not by the black hole’s accretion, but by star formation.
There’s no sign of the expected structure of a rotating disk on kpc scales.
The authors estimate a dynamical mass of the host galaxy of 43 billion solar masses — and the black hole at the galaxy’s center makes up ~6% of that. This ratio is roughly 10x higher than the black-hole-to-bulge mass ratio in local early-type galaxies.
In the very central region, the black hole accounts for around 20% of the galaxy’s dynamical mass, and gas and dust likely accounts for most of the remainder. This doesn’t leave much room for massive stars in the center of the galaxy.

ALMA’s capabilities have enabled these first efforts to spatially resolve the host galaxy of the most distant quasar known, resulting new and unexpected information. The authors now look hopefully to the future, when even longer baselines of ALMA may allow us a still-higher-resolution look at this distant quasar, possibly providing answers to some of the questions it has raised.

Citation

Bram P. Venemans et al 2017 ApJ 837 146. doi:10.3847/1538-4357/aa62ac

31 March 2017

An Exo-Venus in the Solar Neighborhood

A size comparison of Venus and Earth. Though they are nearly the same size and density, these two planets evolved very differently. [NASA]

Earth is great place for life — but Venus definitely isn’t. Both planets have similar masses and densities. So why did one evolve to support life, while the other turned into a barren and inhospitable hothouse? This is a question we might be able to answer if we can gather observations of other planets similar to Earth and Venus. The recent discovery of an exo-Venus in our solar neighborhood brings us one step closer to this goal!

A New Neighbor

A team of scientists led by Isabel Angelo (SETI Institute, NASA Ames Research Center, and UC Berkeley) has announced the discovery of Kepler-1649b, an exoplanet transiting a star located just 219 light-years away from us. Kepler-1649b is unique in being roughly the same size as Earth and Venus and also receiving a similar amount of starlight as Venus does from our Sun.

Phase-folded light curve showing the transit of Kepler-1649b. [Angelo et al. 2017]

Angelo and collaborators conducted follow-up observations after Kepler’s detection of 1649b to verify its planetary nature and pin down its properties. They found that Kepler-1649b has a radius of 1.08 times that of Earth, and it receives an incident flux of 2.3 times Earth’s — which is very similar to the incident flux received by Venus. Kepler-1649b orbits a star that’s only a quarter of our Sun’s radius, however, and it therefore orbits significantly closer to its star in order to receive the same flux, circling its host once every 8.7 days.

Differences Due to a Small Host

It’s worth identifying how this planet might differ from Venus. The authors suggest a few key factors:

Kepler-1649b may be more prone to effects of host-star variability. M-dwarf stars like this one are typically more magnetically active than our Sun, and Kepler-1649b is orbiting very close to its star.
Kepler-1649b receives comparatively low-energy radiation, compared to Venus. This is because its cooler host emits more light at lower frequencies than the Sun.
Kepler-1649b may be subject to larger tidal effects from its host star. Because it orbits so close in, it might experience tidal heating, synchronous rotation, and tidal locking — all of which can influence its seasons and geologic activity.

Target for the Future

The colored contours show the most likely radius and incident flux measured for Kepler-1649b. Earth, Venus, Mars, and several other exoplanets are plotted for comparison. [Angelo et al. 2017]

In spite of these differences, Kepler-1649b still qualifies as the most similar exoplanet we’ve found to Venus in terms of its size and incident radiation. This marks our first opportunity to study such a target to understand how it differs from Earth-like planets and what conditions might lead to habitability on a planet.

We will be able to gain more information on Kepler-1649b with upcoming missions. The Transiting Exoplanet Survey Satellite (TESS) will observe more transits, and Gaia’s improved-accuracy distance measurements should also improve our measurements of the star’s and planet’s properties. What’s more, Kepler-1649b will make an excellent target for the James Webb Space Telescope (launching in 2018) to examine in the hopes of learning about its atmosphere.

Citation

Isabel Angelo et al 2017 AJ 153 162. doi:10.3847/1538-3881/aa615f

29 March 2017

Counting Rogue Asteroids and Comets

How many comets, asteroids, and planetary bodies are floating through interstellar space, not gravitationally bound to any solar system? A new study explores this question and what its answer means for our understanding of how solar systems form.

Rough Beginnings

Do all solar systems form with gas-giant migration causing a large fraction of the system’s material to be ejected? [ESO/L. Calçada]

Our solar system was not a great place to be when it was first born. Simulations of its formation suggest that early orbital migration of the gas and ice giants caused up to 99% of the original planetesimals in our solar system to be ejected out into space, gravitationally unbound from our star.

If this model is correct, and if other solar systems formed in the same way, then it stands to reason that interstellar space should be littered with free-floating, rogue planets, asteroids, and comets. So why have we never yet observed a sizable interstellar object? Do these non-detections mean that our models are wrong? Or just that our ability to spot interstellar objects isn’t good enough yet?

Trajectories of eight synthetic interstellar objects modeled by the authors as they pass within 50 AU of the Sun. [Engelhardt et al. 2017]

Predicting Detections

To answer these questions, a team of scientists led by Toni Engelhardt (University of Hawaii’s Institute for Astronomy and Technical University of Munich, Germany) and Robert Jedicke (University of Hawaii’s Institute for Astronomy) set out to find an observational upper limit on the spatial number density of interstellar objects using data from three different contemporary wide-field solar system surveys.

The team first simulated a synthetic population of interstellar objects out to a distance of 750 AU from the Sun. The authors then propagated the positions of this population forward in time to account for the effects of our Sun’s gravitational pull on passing objects, known as “gravitational focusing”.

Using the resulting distribution of interstellar objects within 50 AU of the Sun, the authors then simulated the detection of these objects with three surveys: Pan-STARRS1, the Mt. Lemmon Survey, and the Catalina Sky Survey.

The authors calculate the number of interstellar objects in their synthetic population that the surveys should have been able to detect in their 19 cumulative survey years. From the surveys’ non-detections, they then estimate an upper limit to the number density of interstellar objects in the solar neighborhood.

Not Many Interstellar Objects

Orbital elements for the synthetic interstellar objects that the authors find Pan-STARRS1 should be able to detect. Black lines denote active comets and grey lines denote inactive asteroids. [Engelhardt et al. 2017]

The authors find that the number density of interstellar objects larger than 1 km in radius must be lower than ~1.4 per every 10,000 AU³.

What does this mean? This is a stricter upper limit than the number densities predicted by many currently accepted solar system formation models. There are two explanations for this that the authors deem most likely:

The properties we expect for interstellar objects based on what we observe in our own solar system — in particular, the distribution of their sizes — are wrong.
Other solar systems didn’t form like ours did, ejecting the vast majority of its protoplanetary material into interstellar space.

While this remains an unsatisfying conclusion, observations from powerful future sky surveys like the Large Synoptic Survey Telescope (LSST) may help us to further home in on the answer to this puzzle.

Citation

Toni Engelhardt et al 2017 AJ 153 133. doi:10.3847/1538-3881/aa5c8a

27 March 2017

Observations from Sunrise

A crane hoists the Sunrise II payload in preparation for its 2013 flight. [Adapted from Solanki et al. 2017]

On a cloudy but windless day in early summer 2013, a team of scientists gathered in Sweden to launch a balloon carrying the largest solar telescope to leave Earth. Now the team has published 13 papers describing what they learned from the spectacularly high-resolution ultraviolet observations of the Sun during the Sunrise II mission.

Airborne Again

This was not Sunrise’s first flight; the 1-meter Gregorian telescope with its two instruments — an imager that took photos at various ultraviolet wavelengths, and a magnetograph that imaged the Sun’s magnetic features — were hoisted into the air once before in a similar flight in 2009. Sunrise I, however, flew during an unexpectedly long activity minimum for the Sun, preventing it from making observations of anything besides the quiet Sun. In contrast, Sunrise II flew at the ideal time to observe the emerging active region NOAA AR 11768 as it developed (see the video below).

The flight paths of the two Sunrise missions. Both launched in northern Sweden, and came down in slightly different locations in northern Canada. [Adapted from Solanki et al. 2017]

Both Sunrise flights were launched from northern Sweden. Sunrise II floated for a total of 122.3 hours — a little over 5 days — at an altitude of roughly 36 km before landing in northern Canada. During this time, its imager and magnetograph respectively recorded a whopping 60,806 and 48,129 high-resolution images from above 99% of Earth’s atmosphere.

Outcomes from Flight

What did scientists discover from the Sunrise II flight? The mission’s primary purpose was to learn about the Sun’s magnetic field and its influence on the solar atmosphere. A few highlights from the mission include:

Observations of an Ellermann bomb, an explosive event like a tiny solar flare that generally occurs in developing solar active regions. Simulations accompanying Sunrise’s data may help identify where the changes in magnetic field originate that cause these explosions.
Observations of the dynamics of moving magnetic features close to a pore, a small sunspot that doesn’t have a penumbra.
Observations that help pinpoint the footpoints of coronal loops (the points where the loops are anchored to the Sun’s photosphere). These regions are discovered to be places of strong magnetic contrasts, and the data indicate the presence of jets at the base of the loop base that might supply plasma to the overlying loop.
Sample data from the imaging instrument on board Sunrise II, recorded in three different wavelengths. The two Ca II H channels, shown in the middle and right panels, reveal the presence of long, slender fibrils, many of which seem to emanate from the large pore just to the left of the image. [Solanki et al. 2017]
Observations of a network of small, slender fibrils that together form a large-scale magnetic canopy. These fibrils were found to be much longer-lasting than previously realized, and they carry copious amounts of energy via different kinds of waves — possibly playing an important role in heating the chromosphere in active regions.

More analysis is still planned of the data from the first and second flight, so there remain many more discoveries to be made from the first two flights of Sunrise. Meanwhile, since the mission was able to again land with the payload operationally undamaged, the team has already begun plans for a third flight!

For more information on Sunrise II’s results, you can check out the 13 papers the team published in a special issue of the Astrophysical Journal Supplements this week, or you can read the mission overview provided by Sunrise’s PI, Sami Solanki (Max Planck Institute for Solar System Research and Kyung Hee University in Korea).

Bonus

Check out this video from the Solar Dynamics Observatory’s HMI instrument, which shows the evolution of the active region NOAA AR 11768 over the period of time around when Sunrise II observed it.

Citation

Special ApJS Issue on Sunrise

S. K. Solanki et al 2017 ApJS 229 2. doi:10.3847/1538-4365/229/1/2

24 March 2017

Challenging the Model for Galactic Bulges

Galaxies of similar stellar mass to our own don’t all have the same bulge and black hole masses. So what determines how much mass will end up in the bulge and the black hole at the center of a Milky-Way-like galaxy?

The Role of Mergers

One theory is that major and minor mergers build up the bulge and black-hole masses for some galaxies. It’s often argued that massive, centrally concentrated “classical” bulges are caused by merger activity, whereas less massive, more disk-like “pseudobulges” might be caused by other means, such as violent disk instabilities in early gas-rich disks, or misaligned infall of gas throughout cosmic time.

Bulge mass (top) and BH mass (bottom) as a function of stellar halo mass. Red denotes galaxies with low-mass pseudobulges, black shows galaxies with higher-mass classical bulges. The grey shaded area in the bottom plot shows what would be expected if there were a 1:1 correlation between bulge mass and stellar halo mass. [Bell et al. 2017]

A team of scientists led by Eric Bell (University of Michigan) set out to test the role of major and minor mergers in bulge formation by examining the stellar halos of a sample of 18 Milky-Way-mass galaxies — six with classical bulges expected to have grown through mergers and 12 with pseudobulges expected to have grown through a variety of other mechanisms.

Halos as Historical Record

Stellar halos offer a useful way of tracking the merger history of a galaxy. It’s believed that as major mergers with larger satellites occur, a galaxy’s stellar halo will increase in both mass and metallicity as it retains the stars of the satellite.

Bell and collaborators first verify this picture in their sample by plotting the stellar halo metallicities against the stellar halo masses. This check reveals a strong correlation between the two properties that’s consistent with the outcomes from simulations — so the stellar halos indeed encode the merger history of the galaxies. This means that from their halos, we can infer the masses of the largest satellites accreted by these galaxies.

Laboratories for Quiet Accretion

The authors then search for any indication of correlation between the stellar halo mass and the galaxy’s bulge mass or black hole mass. They find that their galaxy sample has a wide range in stellar halo masses that don’t correlate significantly with the bulge-to-total ratio, bulge mass, or black hole mass of the galaxy. This is true not only for the pseudobulges, but also for the classical bulges.

The galaxy M81 has a massive classical bulge but an anemic stellar halo containing only 2% of its total stellar mass. This galaxy may be a useful laboratory for studying quiet accretion events. [Subaru Telescope (NAOJ)/HST/R. Colombari/R. Gendler]

This outcome suggests that not even the classical bulges form primarily via minor and major merging activity. Instead, the bulges all form from a variety of mechanisms: a few are likely created by mergers, but the remainder are probably caused by quieter means like secular evolution, disk instabilities or misaligned gas accretion.

These findings challenge the classical models of massive bulge formation and suggest that more detailed simulations and observations are necessary to unravel how the bulges and black holes at the centers of Milky-Way-like galaxies are grown. In particular, the galaxies with massive classical bulges but without massive stellar halos (the galaxy M81 is suggested as an example) may be ideal laboratories for studying quiet growth mechanisms.

Citation

Eric F. Bell et al 2017 ApJL 837 L8. doi:10.3847/2041-8213/aa6158

17 March 2017

More Unusual Light Curves from Kepler

Twenty-three new objects have been added to the growing collection of stars observed to have unusual dips in their light curves. A recent study examines these stars and the potential causes of their strange behavior.

An Influx of Data

The primary Kepler mission provided light curves for over 100,000 stars, and its continuation K2 is observing another 20,000 stars every three months. As we enter an era where these enormous photometric data sets become commonplace — Gaia will obtain photometry for millions of stars, and LSST billions — it’s crucial that we understand the different categories of variability observed in these stars.

The authors find three different types of light curves among their 23 unusual stars. Scallop-shell curves (top) show many undulations; persistent flux-dip class curves (middle) have discrete triangularly shaped flux dips; transient, narrow dip class curves (bottom) have only one dip that is variable in depth. The authors speculate a common cause for the scallop-shell and persistent flux-dip stars, and a different cause for the transient flux-dip stars. [Stauffer et al. 2017]

After filtering out the stars with planets, those in binary systems, those with circumstellar disks, and those with starspots, a number of oddities remain: a menagerie of stars with periodic variability that can’t be accounted for in these categories. Some of these stars are now famous (for instance, Boyajian’s star); some are lesser known. But by continuing to build up this sample of stars with unusual light curves, we have a better chance of understanding the sources of variability.

Building the Menagerie

To this end, a team of scientists led by John Stauffer (Spitzer Science Center at Caltech) has recently hunted for more additions to this sample in the K2 data set. In particular, they searched through the light curves from stars in the ρ Oph and Upper Scorpius star-forming region — a data set that makes up the largest collection of high-quality light curves for low-mass, pre-main-sequence stars ever obtained.

In these light curves, Stauffer and collaborators found a set of 23 very low-mass, mid-to-late-type M dwarfs with unusual variability in their light curves. The variability is consistent with the stars’ rotation period where measured — which suggests that whatever causes the dips in the light curve, it’s orbiting at the same rate as the star spins.

Causes of Variability?

These plots show how the properties of these 23 stars compare to those of the rest of the stars in their cluster (click for a closer look!). For all but the rotation rate, they are typical. But the stars with scallop-shaped light curves have among the shortest periods in Upper Sco, with some near the theoretical break-up for stars of their age. [Stauffer et al. 2017]

The authors categorize the 23 stars into two main groups.

The first group consists of 19 stars with short periods; more than half of them rotate within a factor of two of their predicted breakup period! Many of these show sudden changes in their light-curve morphology, often after a stellar flare. The authors propose that the variability in these light curves might be caused by warm coronal gas clouds that are organized into a structured toroidal shape around the star.
The second group consists of the remaining four stars, which have slightly longer periods. The light curves show a single short-duration flux dip — with highly variable depth and shape — superposed on normal, spotted-star light curves. The authors’ best guess for these four stars is that there are clouds of dusty debris circling the star, possibly orbiting a close-in planet or resulting from a recent collisional event.

Stauffer and collaborators are currently developing more detailed models for these stars based on the possible variability scenarios. The next step, they state, is to determine if the gas in these structures have properties necessary to generate the light-curve features we see.

Citation

John Stauffer et al 2017 AJ 153 152. doi:10.3847/1538-3881/aa5eb9

15 March 2017

A Connection Between Corona and Jet

The structure immediately around a supermassive black hole at the heart of an active galaxy can tell us about how material flows in and out of these monsters — but this region is hard to observe! A new study provides us with clues of what might be going on in these active and energetic cores of galaxies.

In- and Outflows

In active galactic nuclei (AGN), matter flows both in and out. As material flows toward the black hole via its surrounding accretion disk, much of this gas and dust can then be expelled from the vicinity via highly collimated jets.

Top: The fraction of X-rays that is reflected decreases as jet power increases. Bottom: the distance between the corona and the reflecting part of the disk increases as jet power increases. [Adapted from King et al. 2017]

To better understand this symbiosis between accretion and outflows, we examine what’s known as the “corona” — the hot, X-ray-emitting gas that’s located in the closest regions around the black hole. But because the active centers of galaxies are generally obscured by surrounding gas and dust, it’s difficult for us to learn about the structure of these inner regions near the black hole.

Where are the X-rays of the corona produced: in the inner accretion flow, or at the base of the jet? How far away is this corona from the disk? And how does the corona’s behavior relate to that of the jet?

Reflected Observations

To address some of these questions, a group of scientists led by Ashley King (Einstein Fellow at Stanford University) has analyzed X-ray observations from NuSTAR and XMM-Newton of over 40 AGN. The team examined the reflections of the X-rays off of the accretion disk and used two measurements to learn about the structure around the black hole:

the fraction of the corona’s X-rays that are reflected by the disk, and
the time lag between the original and reflected X-rays, which reveals the distance from the corona to the reflecting part of the disk.

A visualization of the authors’ model for an AGN. The accretion disk is red, corona is green, and jet is blue. The corona shines on the disk, causing the inner regions (colored brighter) to fluoresce, “reflecting” the radiation. As the accretion rate increases from the top to the bottom panel, the jet power increases and the dominant reflective part of the disk moves outward due to the ionization of the inner region (which puffs up into a torus). [Adapted from King et al. 2017]

King and collaborators find two interesting relationships between the corona and the jet: there is an inverse correlation between jet power and reflection fraction, and there is a correlation between jet power and the distance of the corona from the reflecting part of the disk the disk. These observations indicate that there is a relationship between changes in the corona and jet production in AGN.

Modeling the Corona

The authors use these observations to build a self-consistent model of an AGN’s corona. In their picture, the corona is located at the base of the jet and moves mildly relativistically away from the disk, propagating into the large-scale jets.

As the velocity of the corona increases, more of its radiation is relativistically beamed away from the accretion disk, which decreases the fraction of X-rays that are reflected — explaining the inverse correlation between jet power and reflection fraction.

At the same time, the increased mass accretion further ionizes the inner disk region, pushing the dominant reflection region to further out in the disk — which explains the correlation between jet power and the distance from corona to reflection region.

King and collaborators show that this model is fully consistent with the X-ray observations of the 40 AGN they examined. Future X-ray observations of the strongest radio jet sources will help us to further pin down what’s happening at the heart of active galaxies.

Citation

Ashley L. King et al 2017 ApJ 835 226. doi:10.3847/1538-4357/835/2/226

13 March 2017

How Stable is a Light Sail Riding on a Laser Beam?

The Breakthrough Starshot Initiative made headlines last year when the plan was first announced to send tiny spacecraft to our nearest stellar neighbors. But just how feasible is this initiative? A new study looks at just one aspect of this plan: whether we can propel the spacecraft successfully.

Propelling a Fleet

The Alpha Centauri star system, which consists of Alpha (left) and Beta (right) Centauri as well as Proxima Centauri (circled). [Skatebiker]

The goal behind the Breakthrough Starshot Initiative is to build a fleet of tiny, gram-scale spacecraft to travel to the Alpha Centauri star system — a system in which a planet was recently discovered around Proxima Centauri, the star nearest to us.

To propel the spacecraft, the team plans to attach a reflective sail to each one. When a high-power laser beam is pointed at that sail from Earth, the impulse of the photons bouncing off the sail can accelerate the lightweight spacecraft to a decent fraction of the speed of light, allowing it to reach the Alpha Centauri system within decades.

Among the many potential engineering challenges for such a mission, one interesting one is examined in a recent study by Zachary Manchester and Avi Loeb of Harvard University: how do we keep the spacecraft’s light sail centered on the laser beam long enough to accelerate it?

Beam profile (left) and corresponding potential function (right) for a laser beam made up of four Gaussians. With this configuration, the potential well pushes the spacecraft back to the center if it drifts toward the edges of the well. [Manchester & Loeb 2017]

The Search for Stability

Manchester and Loeb argue that any slight perturbations to the light sail’s position relative to the laser beam — in the form of random disturbances, misalignments, or manufacturing imperfections — could cause it to slide off the beam, preventing it from continuing to accelerate. Ideally, the project would use a sail that could be passively stable: the sail wants to stay centered on the beam, rather than requiring active interference to keep it there.

The scenario that’s been proposed and studied in the past is that of a conical sail propelled by a Gaussian beam. But Manchester and Loeb perform analytic stability calculations to show that such a system will not, in fact, be stable — if the beam gets knocked off the center of the sail, it will not be able to recover its centered position.

Spheres on the Go

Sail position during beam-riding simulations for a spherical sail on the 4-Gaussian beam. Left: When the sail begins with a 5-cm offset from the center of the beam, it oscillates around the center but successfully remains bounded in the x-y plane (rather than drifting off the beam). Right: When noise is added to the beam, the sail oscillates more, but it still remains stable and bounded over several minutes of acceleration. [Manchester & Loeb 2017]

So if a conical sail won’t work, what will instead? Manchester and Loeb propose an intriguing alternative: a light sail in the shape of a spherical shell around the spacecraft, propelled by a beam that is constructed from the sum of four Gaussians. This more complex configuration has the benefit that if the spacecraft is knocked off the center of the beam, it will experience a restoring force that pushes it back to the center. The spherical shape of the sail means that it won’t destabilize if it’s tilted.

The authors perform a series of numerical simulations to test this configuration, demonstrating that it remains stable even when they introduce deliberate noise into the beam. The simulations show that the beam can stay successfully centered on the spherical sail for at least several minutes — sufficient for the spacecraft to be accelerated to a sizable fraction of the speed of light.

So does this approach make Starshot feasible? It may be a step in the right direction, but challenges still remain. We can undoubtedly look forward to seeing further clever innovations as planning for this project continues!

Citation

Zachary Manchester and Abraham Loeb 2017 ApJL 837 L20. doi:10.3847/2041-8213/aa619b

10 March 2017

Building Magnetic Fields in White Dwarfs

White dwarfs, the compact remnants left over at the end of low- and medium-mass stars’ lifetimes, are often found to have magnetic fields with strengths ranging from thousands to billions of times that of Earth. But how do these fields form?

Multiple Possibilities

Around 10–20% of white dwarfs have been observed to have measurable magnetic fields with a wide range of strengths. There are several theories as to how these fields might be generated:

The fields are “fossil”.
The original weak magnetic fields of the progenitor stars were amplified as the stars’ cores evolved into white dwarfs.
The fields are caused by binary interactions.
White dwarfs that formed in the merger of a binary pair might have had a magnetic field amplified as a result of a dynamo that was generated during the merger.
The fields were produced by some other internal physical mechanism during the cooling of the white dwarf itself.

In a recent publication, a team of authors led by Jordi Isern (Institute of Space Sciences, CSIC, and Institute for Space Studies of Catalonia, Spain) explored this third possibility.

Dynamos from Crystallization

The inner and outer boundaries of the convective mantle of carbon/oxygen white dwarfs of two different masses (top vs. bottom panel) as a function of luminosity. As the white dwarf cools (toward the right), the mantle grows thinner due to the crystallization and settling of material. [Isern et al. 2017]

As white dwarfs have no nuclear fusion at their centers, they simply radiate heat and gradually cool over time. The structure of the white dwarf undergoes an interesting change as it cools, however: though the object begins as a fluid composed primarily of an ionized mixture of carbon and oxygen (and a few minor species like nickel and iron), it gradually crystallizes as its temperature drops.

The crystallized phase of the white dwarf is oxygen-rich — which is denser than the liquid, so the crystallized material sinks to the center of the dwarf as it solidifies. As a result, the white dwarf forms a solid, oxygen-rich core with a liquid, carbon-rich mantle that’s Rayleigh-Taylor unstable: as crystallization continues, the solids continue to sink out of the mantle.

By analytically modeling this process, Isern and collaborators demonstrate that the Rayleigh-Taylor instabilities in the convective mantle can drive a dynamo large enough to generate the magnetic field strengths we’ve observed in white dwarfs.

Magnetic field density as a function of the dynamo energy density. The plots show Earth and Jupiter (black dots), T Tauri stars (cyan), M dwarf stars (magenta), and two types of white dwarfs (blue and red). Do these lie on the same scaling relation? [Isern et al. 2017]

A Universal Process?

This setup — the solid core with an unstable liquid mantle on top — is exactly the structure expected to occur in planets such as Earth and Jupiter. These planets’ magnetic fields are similarly thought to be generated by convective dynamos powered by the cooling and chemical separation of their interiors — and the process can also be scaled up to account for the magnetic fields of fully convective objects like T Tauri stars, as well.

If white-dwarf magnetic fields are generated by the same type of dynamo, this may be a universal process for creating magnetic fields in astrophysical objects — though other processes may well be at work too.

Citation

Jordi Isern et al 2017 ApJL 836 L28. doi:10.3847/2041-8213/aa5eae

3 March 2017

Where Do Messy Planetary Nebulae Come From?

If you examined images of planetary nebulae, you would find that many of them have an appearance that is too “messy” to be accounted for in the standard model of how planetary nebulae form. So what causes these structures?

Examples of planetary nebulae that have a low probability of having been shaped by a triple stellar system. They are mostly symmetric, with only slight departures (labeled) that can be explained by instabilities, interactions with the interstellar medium, etc. [Bear and Soker 2017]

A Range of Looks

At the end of a star’s lifetime, in the red-giant phase, strong stellar winds can expel the outer layers of the star. The hot, luminous core then radiates in ultraviolet, ionizing the gas of the ejected stellar layers and causing them to shine as a brightly colored “planetary nebula” for a few tens of thousands of years.

Planetary nebulae come in a wide variety of morphologies. Some are approximately spherical, but others can be elliptical, bipolar, quadrupolar, or even more complex.

It’s been suggested that non-spherical planetary nebulae might be shaped by the presence of a second star in a binary system with the source of the nebula — but even this scenario should still produce a structure with axial or mirror symmetry.

A pair of scientists from Technion — Israel Institute of Technology, Ealeal Bear and Noam Soker, argue that planetary nebulae with especially messy morphologies — those without clear axial or point symmetries — may have been shaped by an interacting triple stellar system instead.

Examples of planetary nebulae that might have been shaped by a triple stellar system. They have some deviations from symmetry but also show signs of interacting with the interstellar medium. [Bear and Soker 2017]

Departures from Symmetry

To examine this possibility more closely, Bear and Soker look at a sample of thousands planetary nebulae and qualitatively classify each of them into one of four categories, based on the degree to which they show signs of having been shaped by a triple stellar progenitor. The primary signs the authors look for are:

Symmetries
If a planetary nebula has a strong axisymmetric or point-symmetric structure (i.e., it’s bipolar, elliptical, spherical, etc.), it was likely not shaped by a triple progenitor. If clear symmetries are missing, however, or if there is a departure from symmetry in specific regions, the morphology of the planetary nebula may have been shaped by the presence of stars in a close triple system.
Interaction with the interstellar medium
Some asymmetries, especially local ones, can be explained by interaction of the planetary nebula with the interstellar medium. The authors look for signs of such an interaction, which decreases the likelihood that a triple stellar system need be involved to produce the morphology we observe.

Examples of planetary nebulae that are extremely likely to have been shaped by a triple stellar system. They have strong departures from asymmetry and don’t show signs of interacting with the interstellar medium. [Bear and Soker 2017]

Examples of planetary nebulae that are extremely likely to have been shaped by a triple stellar system. They have strong departures from symmetry and don’t show signs of interacting with the interstellar medium. [Bear and Soker 2017]

Influential Trios

From the images in two planetary nebulae catalogs — the Planetary Nebula Image Catelog and the HASH catalog — Bear and Soker find that 275 and 372 planetary nebulae are categorizable, respectively. By assigning crude probabilities to their categories, the authors estimate that the total fraction of planetary nebulae shaped by three stars in a close system is around 13–21%.

The authors argue that in some cases, all three stars might survive. This means that we may be able to find direct evidence of these triple stellar systems lying in the hearts of especially messy planetary nebulae.

Citation

Ealeal Bear and Noam Soker 2017 ApJL 837 L10. doi:10.3847/2041-8213/aa611c

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