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Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Multi-phase Nature of a Radiation-driven Fountain with Nuclear Starburst in a Low-mass Active Galactic Nucleus
Authors: Keiichi Wada, Marc Schartmann, Rowin Meijerink
First Author’s Institution: Kagoshima University, Kagoshima 890-0065, Japan
Status: Accepted for publication in ApJL

What better way to start today’s astrobite than with a movie? Hit ‘play’ and I will explain.

On screen is the view around a computer-simulated active galactic nucleus (AGN) from today’s article, available at the first author’s personal webpage. An AGN is the center of a galaxy with a black hole actively feeding on gas while giving out luminous radiation across the electromagnetic spectrum. The color reflects the temperature of the gas/dust around it; red/orange parts are hot and the dark parts are cool. The video starts with a face-on view of a gas disk, followed by an inclined view showing the cold, dusty molecular gas (the clumpy dark lanes) obscuring the central source.

The traditional unified model of AGN consists of a bright central radiation source surrounded by a donut-shaped dusty torus, as shown schematically in Figure 1. Different types of AGNs could then be understood as models with various jet structures and radiation power levels viewed from different angles. However, recent mid-infrared observations found that in some AGNs dust emission comes from the polar regions, but not from the dusty tori. Since we don’t expect any dust in the polar regions, the traditional picture is therefore shown to be incomplete!

Unified AGN

Figure 1. Schematic representation of the unified AGN model. Various types of AGN can be understood as the result of different viewing angles, whether the central black hole is producing a jet, and the power level of the central source. [Beckmann & Shrader 2012]

The dusty-torus picture proves to be very useful in explaining the nature of AGNs. However, no one really understands how these tori come to be and how exactly they determine the AGN properties. The lead author of today’s paper has come up with a model explaining the production of the torus structure, known as the “radiation-driven fountain” model. In this picture, the intense radiation from the central source drives a vertical circulation of gas, naturally creating a thick disk resembling a dusty torus. We will see at the end of this astrobite that this model could produce the polar dust emission unexplained by the traditional model.

Today’s paper applies the radiation-driven fountain model with improved radiation physics to produce synthetic observations of the nearest AGN — the Circinus Galaxy — and compares them with actual observations. In particular, the major improvement is the chemistry of the X-ray dominated regions near the central source, which is crucial in producing reliable synthetic observations. Model parameters are chosen to match those of the Circinus. The simulation starts with a central black hole of 2 million solar masses surrounded by an initially thin gas disk. The radiation from the central regions stirs up and drives a circulation of gas under the gravity of the black hole. Energy input from supernova explosions is also included.

Density distributions

Figure 2. Density distributions of atomic (upper) and molecular (lower) gas in the radiation-driven fountain model. Left and right panels correspond to face-on and edge-on views, respectively. [Wada et al. 2016]

Figure 2 shows the distributions of atomic and molecular gas. On the top right panel we see the edge-on view of the disk. The thickness of the disk is comparable to its diameter, demonstrating that the fountain flows can indeed produce a geometrically thick disk with hollow cones above and below. This is a big deal because we now have a natural way of getting a structure resembling the traditional dusty torus! Supernova feedback is also shown to be required to maintain the thick disk structure for low-mass AGN like the Circinus. The authors also perform radiation transfer calculations to predict the spectral energy distributions (SEDs) of the Circinus Galaxy. The model-predicted SEDs at different inclination angles (black) are plotted together with the actual observations (blue) in Figure 3. Models with inclination angles greater than 70° match the actual observations quite well. From mid-infrared image of Circinus the inclination angle is inferred to be ~75°, confirming the SED analysis.

SEDs

Figure 3. Modeled spectral energy distributions (SEDs) of the Circinus Galaxy at various inclined angles (top to bottom 0°, 30°, 60°, 70°, 80°, 90°). [Wada et al. 2016]

From the video we can see there is irregular bright emission along the polar axes. Such emission originates from the hot dust circulating the polar regions — a feature of the fountain model. The model therefore also naturally explains the polar dust emission! Although the model does not provide a full explanation for everything about AGNs, this work is undoubtedly a beautiful effort combining advanced theoretical modeling and cutting-edge observations to learn about the nature of these structures.

About the author, Benny Tsang:

I am a graduate student at the University of Texas at Austin working with Prof. Milos Milosavljevic. Using Texas-sized supercomputers and computer simulations, I focus on understanding the effects of radiation from stars when massive star clusters are being assembled. When I am not staring at computer screens, you will find me running around Austin, exploring this beautiful city.

G29-38 Debris disc

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: A Subtle IR Excess Associated with a Young White Dwarf in the Edinburgh-Cape Blue Object Survey
Authors: E. Dennihy, John H. Debes, B. H. Dunlap et al.
First Author’s Institution: University of North Carolina at Chapel Hill
Status: Accepted for publication in the Astrophysical Journal

How do planets meet their ends? For many of the smallest worlds, it maybe as debris discs strewn around the tiny white dwarfs that are all that is left of their stars. The faint infrared glow from nearly forty such discs have been discovered, their rocky origins given away by the chemical composition of the material falling onto the parent white dwarf. Today’s paper adds another disc to the sample, although not without difficulty.

At temperatures of a few hundred to a thousand Kelvin, discs around white dwarfs emit infrared light. This aids in their detection: as the central white dwarf gives off mostly blue and ultraviolet light, the light from the disc is not washed out. However, the downside is that the Earth’s atmosphere absorbs infrared light at the wavelengths the disc emits, so such detections have to be made from space.

The authors use data from the Wide-field Infrared Survey Explorer spacecraft, or WISE. As the name suggests, WISE was a survey mission, sweeping the whole sky looking for sources of infrared light. Taking a list of white dwarfs from the ground-based Edinburgh-Cape Blue Object survey, the authors crossed-matched their positions with the infrared sources spotted by WISE. They found that the position of the white dwarf EC 05365 had a strong WISE signal, giving off much more infrared light than expected. Could this be a planetary debris disc?

fig4

Figure 1:  The left panel shows an image from the VISTA survey, with the white dwarf in the centre along with two other sources. The right panel shows the much lower-resolution WISE data, which, whilst roughly centred on the white dwarf, could be coming from the object to its left (Source A). The lines show the strength of the WISE signal building up towards the centre (Dennihy et al. 2016).

Unfortunately it wasn’t quite that simple. The resolution of WISE is low in comparison with many telescopes, such that it can be difficult to tell exactly where the infrared light is coming from between close-by objects. Figure 1 shows the WISE data on the right, and an image of the same spot from the VISTA survey on the left. EC 05365 is just off the centre of the WISE data, so is the most likely candidate for the infrared light. However, two other sources appear on the VISTA image. The top right object is too faint to matter, but the closer object to the left of the white dwarf, designated “Source A” could be contributing a portion of the WISE signal. Was it light from the second object, rather than a debris disc, that WISE was picking up?

To tease apart the two possible infrared sources, the authors took two approaches. The first was to precisely measure the strength of the WISE signal at each point. The red lines on Figure 1 show lines of equal strength of the WISE signal, building up towards the centre in a similar fashion to contour lines on a map. This technique shows the WISE signal to be roughly four times as strong at the position of the white dwarf than at Source A.

Secondly, the author used a technique called “forced photometry”, taking what they did know, such as the position of the objects, the distribution of the WISE signal, and the background noise, to simulate the relative signals of the two sources. They again found that the Source A contributed much less to the infrared signal than the white dwarf. With the two techniques argreeing, the authors are confident that they have indeed detected a debris disc around EC 05365.

fig3

Figure 2: The blue points show measurements of the light received from EC 05365 at different wavelengths, going from ultraviolet and blue light on the left to infrared on the right. The VISTA measurements of Source A are shown in red. The grey line shows the predicted signal from the white dwarf at each point. The green WISE points are clearly much higher than predicted, suggesting the presence of a debris disc. (Dennihy et al. 2016).

The detection is shown more clearly in Figure 2, which shows the amount of light detected from EC 05365 at different wavelengths, with the extra infrared light from the disc easily visible. Our sample of ruined planetary systems grows again. The authors go on to try to model the shape of the disc, as well as probe the chemical composition of the debris. They finish by looking forwards to the launch of the James Webb Space Telescope which, with its powerful infrared vision, could revolutionise our knowledge of these planetary graveyards.

Astrobiter’s note: In the interests of brevity I’ve focused on just one area of the paper here, which I hope provides an insight into the level of work behind even outwardly simple discoveries. Many more aspects of the EC 05365 system are discussed, so if you want to know more I invite you to read the paper, and I can answer questions in the comments [on the original article].  

About the author, David Wilson:

PhD student at the University of Warwick working with Professor Boris Gaensicke. I study the remnants of planetary systems at white dwarfs, looking at what they reveal about planet compositions and searching for variability. When not doing that I mostly spend my time reading, writing, playing board games and building various little plastic people.

OB120169

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: A Search for Stellar-Mass Black Holes via Astrometric Microlensing
Authors: J. R. Lu, E. Sinukoff, E. O. Ofek, A. Udalski, S. Kozlowski
First Author’s Institution: Institute for Astronomy, University of Hawai’i
Status: Accepted for publication in ApJ

When high-mass (≥8 solar masses) stars end their lives in blinding explosions known as core-collapse supernovae, they can rip through the fabric of space-time and create black holes with similar masses, known as stellar-mass black holes. These vermin black holes dwarf in comparison to their big brothers, supermassive black holes that typically have masses of 106–109 solar masses. However, as vermin usually do, they massively outnumber supermassive black holes. It is estimated that 108–109 stellar-mass black holes are crawling around our own Milky Way, but we’ve only caught sight of a few dozens of them.

black hole binary

Artist’s illustration of Cygnus X-1, a black hole in a binary system with a massive star. Black holes in binaries typically emit lots of radiation, making them easier to find. Finding isolated black holes, on the other hand, is tricky. [NASA/CXC/M.Weiss]

As black holes don’t emit light, we can only infer their presence from their effects on nearby objects. All stellar-mass black holes detected so far reside in binary systems, where they actively accrete from their companions. As matter from the companion falls onto the accretion disk of the black hole, radiation is emitted. Isolated black holes don’t have any companions, so they can only accrete from the surrounding diffuse interstellar medium, producing a very weak signal. That is why isolated black holes, which make up the bulk of the black hole population, have long escaped our discovery. Perhaps, until now.

The authors of today’s paper turned the intense gravity of black holes against themselves. While isolated black holes do not produce detectable emission, their gravity can bend and focus light from background objects. This bending and focusing of light through gravity is known as gravitational lensing. Astronomers categorize gravitational lensing based on the source and degree of lensing: strong lensing (lensing by a galaxy or a galaxy cluster producing giant arcs or multiple images), weak lensing (lensing by a galaxy or a galaxy cluster where signals are weaker and detected statistically), and microlensing (lensing by a star or planet). During microlensing, as the lens approaches the star, the star will brighten momentarily as more and more light is being focused, up until maximum magnification at closest approach, after which the star gradually fades as the lens leaves. This effect is known as photometric microlensing (see this astrobite). Check out this microlensing simulation, courtesy of Professor Scott Gaudi at The Ohio State University: the star (orange) is located at the origin, the lens (open red circle) is moving to the right, the gray regions trace out the lensed images (blue) as the lens passes by the star, while the green circle is the Einstein radius. The Einsten radius is the radius of the annular image when the observer, the lens, and the star are perfectly aligned.

Something more subtle can also happen during microlensing, and that is the shifting of the center of light (on a telescope’s detector) relative to the true position of the source — astrometric microlensing. While photometric microlensing has been widely used to search for exoplanets and MACHOs (massive astrophysical compact halo objects), for instance by OGLE (Optical Gravitational Lensing Experiment), astrometric microlensing has not been put to good use as it requires extremely precise measurements. Typical astrometric shifts caused by stellar-mass black holes are sub-milliarcsecond (sub-mas), whereas the best astrometric precision we can achieve from the ground is typically ~1 mas or more. Figure 1 shows the signal evolution of photometric and astrometric microlensing and the astrometric shifts caused by different masses.

 
fig1

Figure 1: Left panel shows an example of photometric magnification (dashed line) and astrometric shift (solid line) as function of time since the closest approach between the lens and the star. Note that the peak of the astrometric shift occurs after the peak of the photometric magnification. Right panel shows the astrometric shift as a function of the projected separation between the lens and the star, in units of the Einstein radius, for different lens masses. [Lu et al. 2016]

In this paper, the authors used adaptive optics on the Keck telescope to detect astrometric microlensing signals from stellar-mass black holes. Over a period of 1–2 years, they monitored three microlensing events detected by the OGLE survey. As astrometric shift reaches a maximum after the peak of photometric microlensing (see Figure 1), astrometric follow-up was started post-peak for each event. The authors fit an astrometric lensing model to their data, not all of which were taken under good observing conditions. Figure 2 shows the results of the fit: all three targets are consistent with linear motion within the uncertainties of their measurements, i.e. no astrometric microlensing. Nonetheless, as photometric microlensing is still present, the authors used their astrometric model combined with a photometric lensing model to back out various lensing parameters, the most important one being the lens masses. They found one lens to have comparable mass to a stellar-mass black hole, although verification would require future observations.

 
fig2

Figure 2: Results of fitting an astrometric model (blue dashed lines) to the proper motions of the three microlensing targets, where xE and xN are the observed positions in the East and North directions in milli-arcsecond. The results do not show any signs of astrometric microlensing. [Lu et al. 2016]

Despite not detecting astrometric microlensing signals, the authors demonstrated that they achieved the precision needed in a few epochs; had the weather goddess been on their side during some critical observing periods, some signals could have been seen. This study is also the first to combine both photometric and astrometric measurements to constrain lensing event parameters, ~20 years after this technique was first conceived. For now, we’ll give stellar-mass black holes a break, but it won’t be long until we catch up.

About the author, Suk Sien Tie:

I am a second year PhD student starting at the Department of Astronomy at The Ohio State University. I’m broadly interested in most things, i.e. I’m still figuring out where my interests lie. I’ve worked on X-ray transients and have had some stint in instrumentation as an undergrad. Currently, I am working on high-redshift (z ~ 6) quasars in the Dark Energy Survey (DES). Instrumentation is a prospect I intend to pursue, motivated by the observation that we need more builders. Outside of work, I like to read, run, bike, travel, and eat.

Bullet Cluster

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: ALMA-SZ Detection of a Galaxy Cluster Merger Shock at Half the Age of the Universe
Authors: K. Basu, M. Sommer, J. Erler, et al.
First Author’s Institution: Argelander Institut fur Astronomie, Bonn, Germany
Status: Submitted to the Astrophysical Journal Letters

Galaxy clusters are among the most massive objects in the Universe. Some contain thousands of galaxies, with well over a trillion stars between them. And that’s only 5% of a cluster! The vast majority (around 85%) of a cluster’s mass is made up of dark matter. The remaining 10% is hot, very low-density gas (plasma) called the “intracluster medium“, or the ICM.

We can weigh all the components with a variety of observations. The stars in galaxies are visible at optical wavelengths, while the hot ICM emits X-rays that can be observed with satellites like Chandra. It’s more difficult to measure the dark matter, which by definition doesn’t emit light. But the technique of “weak lensing” — measuring how the dark matter gravitationally distorts the light coming from background galaxies — gives us a rough estimate of where the dark matter is.

Fig1_right

Fig 2: An X-ray view of the “El Gordo” cluster, in orange/white. The shock front is highlighted with a white arc. The green contours show radio emission, including a large radio “relic” on top of the shock front. [Basu et al. 2016]

In a normal cluster, the three components (galaxies, ICM, and dark matter) all lie on top of one another. But when two clusters collide, the components can separate. Dark matter only feels the pull of gravity, but the ICM also experiences friction and gas pressure. The dark matter components whiz past one another while the ICM sticks together, as in the famous example of the Bullet Cluster, shown above. It’s like if two water balloons collide: the rubber (ICM) stays put in the middle, while the water (dark matter) is free to keep flying past.

The authors of today’s paper are catching this process in action. Specifically, they measure the shockwave in the ICM gas from the collision of two galaxy clusters, in “El Gordo”. The ICM, like any gas, has a natural sound speed (which depends on its temperature). When gas moves faster than its sound speed, it creates shockwaves, where the pressure builds significantly and the gas is superheated. This same process is what creates a sonic boom on Earth.

Fig3

Fig. 3: The measurements from X-ray and radio are shown in red, with the model shown in green. The most important panels are the top two, showing radio emission and temperature as a function of radius. There is a sharp increase in emission and temperature at the “shock front”. [Basu et al. 2016]

The authors combined two techniques to get the best constraints on the properties of the shock. First, they used X-ray measurements from Chandra to locate the shock as the brightest, hottest portion of the ICM (Figure 2). The hot ICM plasma can distort the light coming to us from the Cosmic Microwave Background, which is called the SZ (Sunyaev-Zeldovich) Effect. The authors added SZ measurements from the new ALMA radio array, which allowed them to precisely measure the gas pressure inside and outside the shockwave.

The authors generate a model of the shock which is designed to match the two sets of observations. This is shown in Figure 3. The observations show a sharp peak in radio emission and X-ray temperature, which confirms there is a shockwave that is heating up the ICM.

By comparing different models of the shock, the authors derive a “Mach number” M≈2.4, which says the shockwave is moving at 2.4 times the speed of sound. That’s quite a sonic boom, particularly given that the speed of sound in the hot plasma is around 2 million miles (3.2 million kilometers) per hour!

While this is not the first time astronomers have measured shockwaves in galaxy clusters, this is the oldest shockwave we’ve ever found. El Gordo is located about 7 billion light years away, which means we are seeing the shock happening when the universe was only half as old as it is now. With the new capabilities of ALMA, observations may be able to start seeing even farther back and see the shocks created by the formation of the very first galaxy clusters.

About the author, Ben Cook:

I’m a second year astronomy grad student at Harvard and currently study the merger and accretion histories of galaxies in simulations. I’m also interested in data science, and how machine learning can be used in astronomy. I received my bachelor’s degree from Princeton, where my senior thesis investigated the distribution of baryons in a large range of dark matter halos.

simulations of Pop III star groups

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: A Common Origin for Globular Clusters and Ultra-Faint Dwarfs in Simulations of the First Galaxies
Authors: Massimo Ricotti, Owen H. Parry, Nickolay Y. Gnedin
First Author’s Institution: University of Maryland
Status: Submitted to ApJ

Over the past century we have rapidly closed many gaps in our knowledge of the history of the universe. We can see all the way back to the epoch of recombination, just 380,000 years after the Big Bang, thanks to the Cosmic Microwave Background (CMB). Some of the first galaxies, existing just a billion years after the Big Bang, are beginning to be revealed by the largest space- and ground-based telescopes. The time between these two epochs, however, is still out of reach. This is the epoch when the first stars formed, and gradually began to light up the universe.

The first stars, confusingly named Population III, form out of gas containing no metals (astronomy parlance for elements heavier than Hydrogen and Helium). This makes them different from stars that form subsequently (known as Population I and II. I know, it’s backwards. Blame Walter Baade), as these later stars form from gas containing the metals expelled by the trailblazing Pop III stars. Because Pop III stars form from metal free gas, they have unique properties compared to stars in the universe today. One of the most striking features is their extreme mass, a hundred times the mass of the Sun in some theories. Such large stars subsequently have very short lifetimes, as they burn through their nuclear fuel rapidly. This in turn makes them difficult to detect; in the grand timeline of the universe, they blink in and out of existence. Until we get better observations one of the only ways to explore these objects, and test theories of their formation and evolution, is through simulations.

Gas Density

Figure 1: The projected density of gas for the biggest galaxies at z = 9 (roughly 500-600 Myr since the Big Bang). Where there is a disc, the top row shows a top down view of it and the bottom row shows a side view. Each image is 100 parsecs on a side.

Today’s paper is about one such simulation. The authors explored the kinds of environments that the first stars are born into, and what objects they evolve into. Unfortunately, simulating galaxies is difficult. They are large, and interact closely with their nearby environment, so you need to simulate a big volume. But ideally you would want to simulate each individual star too. Simulating this huge range of scales, from individual stars to clusters of galaxies, is currently impossible — no astrophysicist has access to a computer powerful enough — but the authors begin to push this limit by ramping up the resolution of their simulation so that each simulation particle represents a collection of stellar objects that are approximately 40 times the mass of the Sun. Previous simulations of small galaxy clusters could only resolve collections of tens of thousands of stars, so this is a big improvement.

Stellar Density

Figure 2: Each panel displays an identical view to Figure 1, but showing the projected density of stars. The disc is nowhere to be seen, and the stars extend to much greater distance than the gas.

The authors look at the morphology of their simulated objects and distinguish a few trends. The gas tends to form a disc inside a dark matter halo, and star formation is confined to the disc (see figure 1). The stars themselves though are often spread out in a wider, spherical arrangement (see figure 2). They attribute this to the fact that the stars, after eating up most of their surrounding gas in the disc, become unbound — in other words, they are no longer held together by their mutual gravity, and they begin to separate out. They then become bound within the larger dark matter halo, and the expansion stops. These objects look suspiciously like ultra faint dwarf galaxies in the local universe. The size of the spheroid can also be extended through mergers, which dynamically heat the object, adding some kinetic energy to all the constituent stars.

Certain objects tend to be smaller and more compact, containing only Pop II stars. They are triggered by nearby Pop III stars which, when they die, spread the metals they contain into the cosmos through powerful winds or supernovae. Gas polluted by these metals can cool efficiently, and therefore form Pop II stars easier. The authors suggest that these objects could be the first compact, bound stellar objects in the universe, but hesitate on what to call them — are they globular clusters, ultra compact dwarfs, or something in between? And how many of these objects will actually survive to the present day, perhaps visible in our local galaxy as “fossil” galaxies, relics directly from the first stellar objects? These simulations are only run up to a billion years after the Big Bang, so such questions will have to wait for bigger simulations in the future that follow these objects to redshift zero.

Another peculiar object the authors identify contains only Pop III stars. Since these stars are so short lived, they will rapidly die, leaving behind a dark, apparently empty halo, though it will in fact be full of the remnants of these monster stars. Hints of such objects were found last year (here’s a summary of the results, and here’s the original paper).

One of the most pertinent and urgent reasons why we need to understand these objects is for the upcoming James Webb Space Telescope, which will begin to probe this era of first star formation. We need to understand the transition from Pop III to Pop II star formation in order to know how many Pop III stars JWST could expect to see.

Many of the questions raised above could be solved by simulations with higher resolution. But the results are an exciting step towards a full understanding of the environment in which the first stars formed. The intriguing common origin of compact star clusters and ultra-faint dwarfs is worthy of further investigation, which the authors plan to publish shortly. It remains to be seen whether any relics of these first collections of baby stars have survived to the present day, and are kicking around our galactic backyard, waiting to be discovered.

About the author, Christopher Lovell:

I’m a first year PhD student at the University of Sussex, studying high redshift galaxies using hydrodynamical simulations. When I’m not reading about physics I like to read science fiction and history, and when I’m not reading I enjoy dodging London traffic on my bike.

solar system orbits

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Influence of Magnetic Field Geometry on the Formation of Close-In Exoplanets
Author: Jake Simon
First Author’s Institution: Southwest Research Institute
Status: Accepted for publication in ApJL

Our solar system is different from many others. While Mercury never gets closer than 0.3 AU from the Sun, many of the exoplanets we have discovered make our closest planet look distant. About 50% of all systems have multiple planets tightly packed within that distance, leaving astronomers to wonder why our solar system and the other half of planetary systems do not have any close-in planets.

It is a common quip in planet formation and protoplanetary disks that if you have any type of problem whatsoever that you do not understand, you should try to solve it with magnetic fields! The author of this paper — Jake Simon —  puts this problem-solving technique to good use. Simon specifically asks: Does a disk’s magnetic field alignment affect whether planetesimals can form at less than 0.3 AU?

Starting Small

Planets typically form from much smaller planetesimals (10 to 100 km-sized) that slowly aggregate over time. Likewise, planetesimals also need to form from much smaller solid dust particles (cm-sized or smaller). However, solid dust faces a barrier to grow into larger particles: the meter-size barrier. When meter-size particles collide with each other, they break up instead of combine, preventing them from growing in the usual way. The only way solid dust can cross the meter-size barrier is to circumvent it by becoming dense enough to gravitationally collapse into planetesimals. This can happen directly or through the streaming instability. After this occurs, the planetesimals that form are much larger than meter-sized and thus, free to grow to planet-size as usual through collisions.

Gravitational collapse requires a high concentration of solid material to gas — a ratio known in the field of planet formation as a disk’s metallicity (denoted by Z). If the inner region of a disk at less than 0.3 AU has a low metallicity, the disk cannot create planetesimals this close to the star. Without planetesimals, planets cannot form.

When Magnetic Fields Align

Instead of investigating a mechanism for increasing the concentration of solids, Simon looks into whether the orientation of a disk’s magnetic field can decrease the concentration of gas. Since Z is a ratio, lower gas densities also create higher metallicities that help planetesimal formation.

Magnetic fields in protoplanetary disks are vital to their evolution. They make the disk unstable to the magneto-rotational instability (MRI), which is one of the key sources of turbulence that gives the disk its viscous, fluid nature. (This is very different from most liquids and gases, which do not need magnetic fields or turbulence to behave like fluids!) Although the gas in the disk tries to act like a fluid, it struggles with the fact that the gas at larger radii moves at slower velocities due to Kepler’s 3rd Law. This creates the following situation:

  1. The gas is forced to balance itself out by exchanging momentum with the adjacent rings, causing it to slow down over time and ultimately feed all of the disk’s momentum to the outer part.
  2. As the gas loses angular momentum to the outer disk, it spirals inward.
  3. Eventually, the gas in the inner disk will spiral inward enough to accrete onto the star.

This process by which the disk accretes is known as shear flow. Disks that are more viscous will flow more easily, causing them to deplete faster than disks with lower viscosities.

metallicity

Figure 1. Radial metallicity profiles for the aligned case (solid black; top) and the anti-aligned case (dashed blue; bottom). For each case, the dotted lines (middle) of the same color show the required metallicity to form planetesimals over a range of radii. At less than 1.0 AU, the aligned case has a high enough metallicity to form planetesimals (the solid line is above dotted black line), but the anti-aligned case never has a high enough metallicity (the dashed line is below the dotted blue line).

Simon knew from previous work that the orientation of a magnetic field can greatly affect the level of viscosity in the inner disk. If a disk’s magnetic field aligns with its angular momentum (such that the two vectors are pointing in the same hemisphere), it will induce stronger magnetic winds and non-turbulent laminar flow due to the Hall effect. These additional flows create a much higher viscosity than in a disk where the two vectors are anti-aligned and the Hall effect does not manifest this way.

In the case where the field aligns, the gas in the disk is more viscous, causing it to deplete faster. This creates metallicities that are high enough to allow planetesimals — and subsequently, planets — to form at less than 0.3 AU. In the case where the field is anti-aligned, the gas in the disk is less viscous, keeping the gas density high and the metallicities at less than 0.3 AU too low to form planetesimals or planets (see Figure 1). If magnetic field orientations are distributed randomly, about half of them should be aligned and half should be anti-aligned. As a result, one would expect to find that roughly 50% of all planetary systems have planets at less than 0.3 AU, which is consistent with what we see in the population of known planetary systems.

Radial surface density distributions for aligned (black) and anti-aligned (blue) magnetic fields. The MMSN (red) matches up with the latter, suggesting our solar system may have had anti-aligned magnetic field, which would explain why we do not have any planets within Mercury's distance.

Figure 2. Radial surface density distributions for aligned (black) and anti-aligned (blue) magnetic fields. The MMSN (red) matches up better with the latter, suggesting our solar system may have had an anti-aligned magnetic field, which would explain why we do not have any planets within Mercury’s distance.

How was our Solar System’s magnetic field aligned?

When Simon calculates the disk’s metallicity distribution as a function of radius for both the aligned and anti-aligned magnetic fields, he notices that the anti-aligned case closely resembles the metallicity of the Minimum Mass Solar Nebula (MMSN), which is intended to model our solar system’s disk structure before any planets formed. If this resemblance has any bearing, our solar system may have had an anti-aligned magnetic field that prevented any planets from forming closer in than 0.3 AU, thereby offering a possible explanation as to why Mercury is so far from the Sun compared to the closest planets in roughly half of other planetary systems.

About the author, Michael Hammer:

I am a 1st-year graduate student at the University of Arizona, where I am working with Kaitlin Kratter on studying planetary dynamics and planet-disk interactions through numerical simulations. I am from Queens, NYC.

Abell 2744

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Local Analogs for High-Redshift Galaxies: Resembling the Physical Conditions of the Insterstellar Medium in High-Redshift Galaxies
Authors: Fuyan Bian, Lisa Kewley, Michael Dopita, Stephanie Juneau
First Author’s Institution: Research School of Astronomy & Astrophysics, Mt Stromlo Observatory, Australian National University
Status: Published in ApJ

Very distant galaxies can teach us a lot about the formation and evolution of some of the earliest systems in the Universe. However, they are also notoriously difficult to study as, due to their distance, they appear both dim and small. Due to the expansion of the cosmos, their spectra have been stretched and shifted redwards — giving those distant galaxies the name “high-redshift” galaxies. Studies of such objects indicate they have different properties compared to more nearby local galaxies: they are 10 times better at forming stars than local galaxies, while also being significantly smaller than those galaxies found near us. Additionally, there are indications that the interstellar medium (ISM), the matter (gas, dust, and cosmic rays) between stars, in high-redshift galaxies differs from that in nearby ones.

select

Fig. 1: Selecting the local analogues sample on the BPT diagram which plots oxygen, hydrogen and nitrogen ratios against each other. The blue dots are the selected local sample, while the black contour lines show where “normal” local galaxies live. As can clearly be seen, there is a distinct offset between the selection and the space on the plot which normal galaxies occupy. In order to mimic the ISM conditions found in high-redshift galaxies, the authors selected their local sample so as to recreate their high ionization ratios. It isn’t currently clear what causes the observed offset between the distant and local galaxy populations.

The BPT Diagram

To learn about the conditions of the ISM, the authors study some of its elements: the ISM contains hot, ionized hydrogen, oxygen, and nitrogen gases which, when ionized by radiation from e.g. stars or active galactic nuclei, give off monochromatic light at well-defined wavelenghts. By measuring the ratios and strengths of these so-called spectral lines it is possible to determine how much ionizing flux is present. The authors plot the ratios of hydrogen, oxygen, and nitrogen lines on the Baldwin-Phillips-Terlevich, or BPT, diagram (see Fig. 1). Where a galaxy falls on this diagram can give us clues about both how ionizing the radiation within it is, and whether the galaxy is powered by star formation or contains an active galactic nucleus. While this works quite well for nearby galaxies, it has been suggested that the position of both star-forming and AGN-containing galaxies in the distant universe is different to those of local galaxies on the BPT diagram. This raises the intriguing question: what causes this change in the conditions of the ISM?

Selecting Local Analogues for High-Redshift Galaxies

Given the difficulty in studying very distant galaxies, how can we study their ISM? The authors of today’s paper attempt to answer this question by selecting nearby galaxies that are located at the same place as high-redshift galaxies on the BPT diagram (see Fig. 1), and which are hence thought to share the same physical conditions of the ISM as star-forming galaxies at redshifts 2-3 (when the universe about 2-3 billion years old). If these local galaxies are indeed true analogues, they can then be used as local laboratories to study the very distant star-forming galaxies — making it easier to study them in greater detail, as they appear brighter and more extended than their high-redshift cousins. The authors of today’s paper select 252 low-redshift, i.e. local, galaxies from the Sloan Digital Sky Survey whose hydrogen, oxygen, and nitrogen spectral line ratios put them into a similar place on the BPT diagram as those found in very distant galaxies.

Physical properties

Fig. 2: The physical properties found in the local analogues sample (blue) and normal local galaxies (red). All panels show histograms of the derived properties scaled to the same height. The top panel compares the star formation rates (in log-space): the local analogues to the distant galaxies can be seen to have slightly higher star formation rates than normal nearby galaxies. The second panel shows the distribution of masses, with a clear offset between the two populations: the local analogues are almost a factor of 10 less massive than normal nearby galaxies. Finally, the ratio of star formation rate and mass, the specific star formation rate sSFR, is shown. It indicates how efficiently a galaxy is converting mass into stars. The blue population is forming stars significantly more effectively than normal local galaxies, but very similarly to the high-redshift galaxy population whose median value is indicated by a purple line.

Establishing the Properties of Local Analogues

Having selected their local sample, the authors measure various physical properties of these galaxies, and then compare them to those found in normal local galaxies and very distant star-forming ones. This allows the authors to determine whether the selected sample can indeed be used as a local laboratory to learn about very distant galaxies. They find that their selected galaxies form on average 3.6 solar masses per year (in comparison, the Milky Way forms 1 solar mass per year) and are about a factor of 10 less massive than normal nearby galaxies (see Fig. 2). Using these two measures, the authors calculate that the galaxies in their sample are very effectively transforming gas into stars (also see Fig. 2), building faster than even the distant galaxies! Previous studies have found that high-redshift galaxies have smaller sizes compared to nearby galaxies with similar masses. Determining the sizes of their local galaxy sample, the authors find that the local analogues are more compact than normal star-forming, but much like the high-redshift galaxies. Additionally, the authors compare the ratio of ionizing flux and gas density, a property known as the ionization parameter, as well as the relationship between the stellar mass and amount of metals. They find good agreement in both cases, making their analogues reliable local laboratories for studying distant galaxy populations with comparable ionization and interstellar medium conditions to the high-redshift sources.

What the Future Might Hold

Having established that their selected sample reproduces the ISM conditions found in very distant galaxies, the authors indicate that future studies of their local analogues could provide much information on the environment inside distant galaxies. Since the local analogues are relatively nearby, it might be possible to resolve individual star-forming regions in them, and hence study in detail the physical properties within them and make direct comparisons with those found in the high-redshift universe.

About the author, Steph Greis:

I’m a third (out of four) year PhD student at the University of Warwick, UK, where I study local analogues to redshift z~5 Lyman break galaxies (LBGs) which are some of the earliest galaxies in the Universe.
When I’m not thinking about galaxies far, far away, I enjoy reading, cooking, and geocaching.

white dwarf star

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Outbursts in two new cool pulsating DA white dwarfs
Authors: Keaton J. Bell, J. J. Hermes, M. H. Montgomery et al.
First Author’s Institution: Department of Astronomy, University of Texas at Austin
Status: Accepted for publication in the Astrophysical Journal

The Kepler spacecraft, launched in 2009, is best known for its monumental contributions to the science of exoplanets. But the unique observing technique of the space telescope, staring at thousands of stars all day, every day for months at a time, has led to surprising new discoveries in many fields beyond exoplanets.

Nearly a year ago, Astrobites reported on one example of Kepler’s unexpected findings: A pair of white dwarfs that were “outbursting”, becoming as much as 20 percent brighter every few days before quieting down again. Although variable white dwarfs have been studied for decades, only the continuous data obtained by Kepler provided the coverage to spot these short, unpredictable events. Today’s paper adds another two outbursting white dwarfs, and begins to explore the reason for this hitherto unobserved behaviour.

White dwarfs are the leftover cores of stars that have run out of hydrogen fuel. More than 90 percent of stars, including the Sun, will end their lives this way, a glowing ember roughly the size of the Earth. White dwarfs start off incredibly hot, millions of Kelvin, but with no more nuclear reactions taking place, they slowly cool down over many centuries.

As they cool past the (roughly) 12,500 K mark, something odd happens: The white dwarfs begin to pulse, changing in brightness by about one percent every few minutes. This carries on until, millions of years later, they cool to below around 10,600 K, and the pulsations stop.

The intriguing aspect is that those temperatures were not predicted by theoretical models of how the interiors of white dwarfs change as they cool, but come from observations. Although those models can reproduce the temperature at which pulsations start, they also predict that the pulsations should last much longer, enduring until the white dwarf reaches just 6,000 K. So why do the pulsations suddenly stop? Bell et al. suggest that the new class of outbursting white dwarfs may hold the answer.

ktwo211629697_LC

Figure 1: Kepler observations of the white dwarf EPIC 211629697. Each point is a measurement of how much light is reaching the telescope from the white dwarf, with the black data being taken every 30 seconds and the red every thirty minutes. The grey regions show the outbursts, where the brightness of the white dwarf increased by up to 14 percent. The right-hand-side figure shows an enlarged version of one outburst.

The two new outbursting white dwarfs were observed by Kepler in mid-2015. Over the roughly eighty days that each white dwarf was observed, the amount of light coming from them made sudden jumps (Figure 1), 15 times in the case of the first white dwarf observed and 33 times for the second. The outbursts were spaced seemingly randomly apart, and implied increases in brightness of up to 14 percent.

loggteff

Figure 2: A collection of white dwarfs, comparing their surface temperatures (x-axis) and gravities (a proxy for the mass, y-axis). The blue and red lines show the hot and cold boundaries between which white dwarfs have been found to pulsate. The grey crosses show white dwarfs observed with Kepler that do not pulsate, yellow dots are Kepler white dwarfs that do pulsate, and blue dots are other pulsating white dwarfs. The four red squares are the outbursting white dwarfs, with a fifth candidate shown by the orange triangle.

Although these white dwarfs are interesting in themselves, being only the third and fourth examples of what was, just a few years ago, an unknown phenomenon, the authors find that they become even more revealing when compared with other white dwarfs. Figure 2 compares the surface temperatures and gravities of the four outbursting white dwarfs (red squares), with those of other white dwarfs, both pulsating and not.

All four of the white dwarfs are found near the cool end of the temperature range where white dwarfs pulsate—exactly the place that theoretical models find difficult to predict. The authors suspect that the two behaviours are linked, with the emergence of outbursts at low temperatures leading to the cessation of pulsation soon afterwards. However, more of the outbursting white dwarfs are needed to confirm this link, as the similar temperatures of the four discovered so far could just be coincidence. With Kepler’s mission recently extended into 2018, new discoveries should not be too long in coming.

About the author, David Wilson:

PhD student at the University of Warwick working with Professor Boris Gaensicke. I study the remnants of planetary systems at white dwarfs, looking at what they reveal about planet compositions and searching for variability. When not doing that I mostly spend my time reading, writing, playing board games and building various little plastic people.

AGB star

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Pulsation-Triggered Mass Loss From AGB Stars: The 60-Day Critical Period
Authors: Iain McDonald and Albert Zijlstra
First Author’s Institution: Jodrell Bank Centre for Astrophysics
Status: Published in ApJ Letters

Background

Perhaps you’ve heard that four billion years from now, the Sun will grow into a red giant with a radius the size of Earth’s orbit before eventually shrinking into a white dwarf about the size of Earth itself. Besides being very small, the resulting white dwarf will probably only have half of the original mass of the Sun. Where does that lost mass go?

Figure 1: An HR Diagram showing the main sequence, red giant branch, horizontal branch, and asymptotic giant branch. The horizontal axis indicates the temperature, while the vertical axis indicates the luminosity. [http://www.astronomy.ohio-state.edu/~pogge/]

Figure 1: An HR Diagram showing the main sequence, red giant branch, horizontal branch, and asymptotic giant branch. The horizontal axis indicates the temperature, while the vertical axis indicates the luminosity. [http://www.astronomy.ohio-state.edu/~pogge/]

During a star’s post-main-sequence (MS) evolution, it will lose much of its starting mass through stellar winds. Currently, the Sun is constantly losing mass through solar winds—material that is being ejected from its surface—but when the Sun leaves MS and reaches the red giant branch (RGB), these solar winds will become even stronger. After the end of the RGB phase, the Sun will continue to evolve until it reaches the asymptotic giant branch (AGB)—so named because it will then asymptotically approach the same location on the Hertzsprung-Russell diagram that it does as an RGB star (see Figure 1 for an example). AGB stars have even stronger stellar winds, meaning they are losing mass at an even more rapid rate than RGB stars. It is thought that much of a star’s mass loss happens when it is on the RGB and AGB. In addition, all of this excess material being blown off of the star means that AGB stars are often surrounded by a lot of dust. 

Exactly what really drives this process, however, is not something that we understand very well. Today’s astrobite discusses some of the possible mechanisms for stellar mass loss in AGB stars, particularly the role that pulsation plays in mass loss.

Stars can pulsate in a variety of different pulsational modes. The fundamental mode is probably what you imagine when you think of stellar pulsation—all of the star is moving radially in the same direction. However, if the star has radial nodes, different parts of the star move in different directions at the same time (sort of like the nodes of an pipe). We call these pulsational modes overtone modes, and the type of overtone mode (first, second, third, etc.) tells you the number of nodes that exist in the star.

Mass Loss Above the 60-Day Critical Pulsational Period

Figure 2: The dust excess (given by K-[22] color) on the vertical axis plotted against period in days on the horizontal axis. The dotted horizontal line marks the authors' criterion for 'substantial dust excess'. The red circles show period data taken from Tabur (2009), the green squares from the International Variable Star Index, and the blue triangles from the General Catalogue of Variable Stars. Smaller light blue triangles indicate the stars for which they had GCVS data, but could not detect with Hipparcos. Starting at a period of 60 days, there is an increased number of stars with greater dust excess than their criterion. There is another increase at about 300 days. [McDonald & Zijlstra 2016]

Figure 2: The dust excess (given by K-[22] color) on the vertical axis plotted against period in days on the horizontal axis. The dotted horizontal line marks the authors’ criterion for ‘substantial dust excess’. The red circles show period data taken from Tabur (2009), the green squares from the International Variable Star Index, and the blue triangles from the General Catalogue of Variable Stars. Smaller light blue triangles indicate the stars for which they had GCVS data, but could not detect with Hipparcos. Starting at a period of 60 days, there is an increased number of stars with greater dust excess than their criterion. There is another increase at about 300 days. [McDonald & Zijlstra 2016]

Most previous studies of the effects of pulsation on mass loss have focused on stars with pulsational periods greater than 300 days, because both observation and theory have shown that to be when stars have the greatest dust production and highest mass-loss rate. However, a less-studied 60-day ‘critical period’ in the increase of dust production has also been noted as well.

Mass-loss in RGB and AGB stars seems to increase at a period of 60 days. Both RGB stars and AGB stars can pulsate (in fact, there is evidence that all stars pulsate…if only we could study them well enough to see it), but the authors find that despite inhabiting roughly the same area on the HR diagram, the 60-day period stars with strong mass loss appear to only be AGB stars and not RGB stars. This 60-day period also happens to correspond with roughly the point when AGB stars transition from second and third overtone pulsation to the first overtone pulsation mode.  Additional nodes will also result in lower pulsational amplitude (smaller change in brightness and radius over one period) for the star, leading AGB stars to have bigger amplitudes at this point. RGB stars seem to pulse only in the second and third overtone modes. This is most likely responsible for why they produce so much less dust and experience less mass-loss at the same period as their AGB star counterparts.

The relationship between dust production and infrared excess, which the authors use as a proxy for the amount of dust the star is producing, is shown in Figure 2. From this figure, we can see that at periods longer than 60 days, there appear to be more stars that are producing dust above their criterion for substantial dust excess. Figure 3 shows period-amplitude diagrams, where the pulsational amplitude is plotted against the pulsational period (where the amplitude suggests of the mode of pulsation). From this diagram, we can see that the stars with less dust production appear to also have lower amplitudes of pulsation. Together, these support the hypothesis that the pulsational mode plays a critical role in producing dust and driving mass loss. These results also confirm the increase in mass-loss at 300 days, which roughly corresponds with stars transitioning from the first overtone pulsation to the fundamental mode.

Figure 3: Amplitude in the V-band plotted against period. In both subplots, the darker colored circles are stars with substantial dust excess, and the lighter colored circles are stars without substantial dust excess. This seems to suggest that greater dust excess corresponds with greater amplitude. Greater amplitude also usually indicates fewer radial nodes. The 60 and 300-day increase in dust productions are also visible in both plots. [Adapted from McDonald & Zijlstra 2016]

Figure 3: Amplitude in the V-band plotted against period. In both subplots, the darker colored circles are stars with substantial dust excess, and the lighter colored circles are stars without substantial dust excess. This seems to suggest that greater dust excess corresponds with greater amplitude. Greater amplitude also usually indicates fewer radial nodes. The 60 and 300-day increase in dust productions are also visible in both plots. [Adapted from McDonald & Zijlstra 2016]

Conclusion

So what’s next? Well, as you might expect, the follow up to science is usually…more science! The authors point out that further study will be necessary in order to get conclusive evidence for exactly what role this critical period serves and how pulsational mode can affect it. Is it really a change in the stellar-mass loss rate, or is the stellar wind pre-existing, and the 60-day period just coincides with an increase in dust condensation? Similar studies focusing on stars with different metallicities will also be a good check to see whether these critical periods are universal.

About the author, Caroline Huang:

I’m a 2nd year graduate student in astronomy at Johns Hopkins. My main research interests lie in extragalactic astronomy, large dataset analysis, and cosmology. At the moment, I’m using galactic Cepheids to calibrate the Cepheid period-luminosity relationship.

I did my undergrad at Harvard, where I was a joint physics and astrophysics concentrator. I love traveling, cool weather, food, books, and Oxford commas.

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