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Gliese 1214b

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

Title: The Formation of Mini-Neptunes
Authors: Julia Venturini and Ravit Helled
First Author’s Institution: University of Zurich, Switzerland
Status: Published in ApJL

To be a master chef, one must have an incredible amount of culinary expertise and creativity to create a wide variety of dishes. To be a computational astrophysicist building planets in simulations, it feels a lot more like going to the store and buying a box of pancake mix. Instead of buying all the ingredients, you start with a pre-made mixture. From there, all you have to do is just add water!

Figure 1: Pancake mix for Mini-Neptunes. To serve, just add a gaseous atmosphere (between 10 and 25% of the planet’s total mass). [Amazon]

Simulating planets can be quite similar. In place of the pancake mix, you can start out with a ball of rock. From there, all you have to do is just add gas (see Figure 1). When making pancakes, one thing to be careful about is how much water to add. If you add hardly any water, you will just end up with the same dried-out bowl you started out with. If you add way too much water, you will end up with pancake soup.

Unlike in cooking, there’s no recipe to follow when building a planet; you do not get to decide how much gas to add! The size of the atmosphere a planet accumulates depends on its own properties and the properties of the surrounding protoplanetary disk in which it forms. Mini-Neptunes are the “perfect pancake” of planets — they ended up with just the right amount of gas. Any less and they would have stayed Earth-sized dried-out rock; any more and they would have grown to Jupiter-sized gaseous soup.

In today’s paper, Julia Venturini and Ravit Helled explore which planet and disk conditions are best for building planets that have just the right-sized atmosphere to be classified as mini-Neptunes — the most common type of exoplanet (see Figure 2), even though there are none in our solar system.

known transiting planet sizes

Figure 2: Histogram showing the frequency of known planets of different sizes. Mini-Neptunes are the most common, followed by the slightly smaller super-Earths. [NASA Ames/W. Stenzel]

Too Much Water for Your Pancakes

Mini-Neptunes are defined to be planets that are less than 10 times the Earth’s mass and have a heavy, hydrogen-dominated atmosphere that makes up between 10 and 25% of the planet’s total mass. (Most planets that are at least 1.6 times the Earth’s radius in size are likely mini-Neptunes or bigger. Any smaller planet is more likely to have a lighter atmosphere and be classified as a super-Earth or an Earth.) With mini-Neptunes being so common, we would naturally expect a wide variety of conditions to be favorable for forming them.

However, it seems easy for planets to avoid becoming mini-Neptunes. The smallest planets have so little mass that they struggle to accumulate a significant amount of gas. For comparison, the Earth’s atmosphere only makes up <0.0001% of its total mass. On the other end, bigger planets whose atmospheres reach the mass of their rocky cores become unstable and undergo what is called “runaway gas accretion” in which they can easily grow to Jupiter-sized (318 Earth masses) or larger.

This would not be a problem if mini-Neptunes could just stop growing once their atmospheres reached a fractional mass of 10–25%. Unfortunately, as long as there is a disk from which they can accrete gas, they will keep growing. This would be like if you poured the right amount of water into your bowl of pancake mix, but then were forced to keep pouring water until you ran out! Thus, the only way a planet can end up a mini-Neptune is if the gas in the disk dissipates at just the right time for the planet to have accreted just the right amount of gas for its atmosphere.

Simulating Planetary Evolution

The authors simulate the growth of a planetary core that starts out 100 times less massive than the Earth in the presence of a depleting gaseous disk enriched with small rocky material. In the first stage, only the rocky core grows. In the second stage, the planet becomes massive enough to gravitationally attract a sizable amount of gas — at which point both the core and the atmosphere continue to grow. They then repeat this process with different planet and disk conditions.

Forming Mini-Neptunes with Pebbles

Let’s start with one of the cases where the authors grow the planet with cm-sized pebbles:

Here, the planet is placed at 5 AU with a transparent atmosphere that is just hydrogen and helium (non-enriched). The planet’s core then grows very quickly and bursts past the upper limit of 10 Earth masses in a little over 1 Myr (Case A: dashed blue lines in Figure 3). Before 2 Myr, the atmosphere’s fractional mass already exceeds the upper limit of 25% and the planet is well on its way to growing to Jupiter-sized within 3 Myr — the average lifetime of a protoplanetary disk. With these conditions, mini-Neptunes should never form.

Let’s move the planet out to 20 AU. Being farther away from the star slows down the growth of the core (since there is less material to accrete at these distances, while the planet also orbits much slower). Unfortunately, it also slows down the growth of the atmosphere for the same reasons (Case B: dashed purple lines in Figure 3), again preventing mini-Neptunes from forming.

To speed up the atmosphere’s growth without speeding up the core’s growth, the authors enrich the atmosphere with water (which is more realistic). This makes the atmosphere heavier, allowing it to gravitationally contract faster — freeing up space to accumulate more gas into the planet’s atmospheric zone. With these conditions, the planet reaches the mini-Neptune phase, but too early! Before 2 Myr, the planet has once again accumulated too much of an atmosphere (Case C: solid purple lines in Figure 3).

simulations of planet formation

Figure 3. Left: Planet mass evolution over time for planets that do not become mini-Neptunes. The black components of each line refer to when each planet crosses the mini-Neptune stage. Right: Atmosphere fractional mass as the planet grows. Each line switches from thick to thin after 3 Myr (the average disk lifetime). [Venturini & Helled 2017]

To slow down the atmosphere’s growth, the authors make the atmosphere more opaque. With a high opacity, it is harder for heat to escape. Since the atmosphere cannot cool as quickly, it cannot contract — making it harder to accumulate more gas. As a result, the planet will become and stay a mini-Neptune from 2 to 4 Myr, right when the disk is expected to dissipate (Case D: solid purple lines in Figure 4)! With these optimal conditions, Venturini and Helled find that 40% of protoplanetary disks should be able to form mini-Neptunes depending on the exact lifetime of the disk.

simulations of mini-Neptune formation

Figure 4: Case D forms a mini-Neptune, as the average disk dissipates at just the right time (vertical red line at 3 Myr) for the planet to end up with just the right amount of gas (and total mass). [Venturini & Helled 2017]

Summary

Besides finding that mini-Neptunes can form from pebbles at 20 AU as described above, the authors also find that mini-Neptunes can form from km-sized planetesimals closer in at 5 AU in 83% of systems. Between these two types of accreted material, mini-Neptunes should be able to form easily in a wide range of locations for a variety of atmospheric properties. With these results, it is no surprise mini-Neptunes are so common in the “Kepler sector” of the galaxy.

About the author, Michael Hammer:

I am a 3rd-year graduate student at the University of Arizona, where I am working with Kaitlin Kratter on simulating planets, vortices, and other phenomena in protoplanetary disks. I am from Queens, NYC; but I’m not Spider-Man…

Boyajian's star

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

Title: The First Post-Kepler Brightness Dips of KIC 8462852
Authors: Tabetha S. Boyajian et al.
First Author’s Institution: West Virginia University
Status: Published in ApJL

We’ve posted before about Boyajian’s star, one of the great unsolved mysteries of the Kepler mission. Discovered by citizen scientists in 2015, this star has everything: deeply bizarre (and bizarrely deep) dips in flux, a hundred-year fade, intermittent brightening spells. Since Kepler, investigations of this star have been hampered by the lack of new data — it’s hard to tell, for example, whether the crazy flux dips repeat if you don’t stare at the star continuously, because you might have just missed them.

All of that changed in 2016, when Dr. Boyajian began monitoring her namesake star with the Las Cumbres Observatory Global Telescope Network (LCOGT). LCOGT has two completely independent telescopes in the northern hemisphere — one in Hawaii and one in the Canary Islands — so on May 18, 2017, when both telescopes reported that Boyajian’s star was dimming anew, Dr. Boyajian could immediately rule out instrumental effects as the cause.

That dimming turned out to be the first night of a very interesting summer for Boyajian’s star. Since May, the star has dimmed four separate times. In today’s paper, Dr. Boyajian presents the new data, and offers, for the first time, a hint at a solution to the mystery.

The four dimming events of May–September 2017, named the “Elsie family.” The y-axis represents the amount of light coming from Boyajian’s Star relative to its ordinary state, and the x-axis represents time in days. Each color represents a different telescope in the Las Cumbres network — their Texas observatory, in green, came online in November 2017. [Boyajian et al. 2018]

The Case of the Elsie Family

The first thing to notice about the four dips of the Elsie family are their wonderful names. “Elsie” comes from the initials “L.C.” of “light curve.” “Celeste” (inspired by the initials “C.L.”) is so named because it’s Elsie’s opposite — instead of dimming rapidly and then brightening slowly, it dimmed slowly and brightened quickly. “Skara Brae” is named after a neolithic town in Scotland, unexpectedly unearthed by a passing storm; “Angkor” after the great abandoned Cambodian city, obscured by forest for hundreds of years, but ultimately uncovered.

It’s a mark of the exceptional and inspiring level of public engagement in this research that these events were named at all, let alone so loftily — most astronomers are happy to stick with catalog numbers and Julian dates. (The dimming events observed in Boyajian’s star during the Kepler mission were given names like D1540, for comparison.) But this project owes everything to its citizen scientists. Not only did they discover the star in the first place, but they also crowd-funded the Las Cumbres observations that revealed the Elsie family.  

The second thing to notice is that all four dips are of similar depth (the star dims to ~98% of its ordinary brightness), but drastically different shapes. In other words, Boyajian’s star looks no more like a regular old exoplanet-hosting star than it did at the end of the Kepler mission, four years ago. Skara Brae bears some resemblance to one of the dips observed by Kepler, but we won’t know if it’s truly a repeat of that earlier dip until we’ve watched it for much longer and looked for further repeats.

An Answer…

The most important thing the Las Cumbres observations tell us, though, isn’t about the number or the shape of the new dips. It’s about their color — or rather, how they appear when viewed through filters of different colors. Behold, the first color information we have about the dips of Boyajian’s Star:

The “Elsie” event as observed in bandpasses of three different colors, by two of the Las Cumbres telescopes (plotted as circles and triangles, respectively). Elsie is deepest in the B band (the bluest of the three bands) and shallowest in the i’ band (the reddest of the three). The dependence of the dimming on color suggests that circumstellar dust is responsible for the dip. [Boyajian et al. 2018]

Elsie is deeper in the blue than it is in the red! From that, we can deduce that whatever is blocking the light from Boyajian’s Star is less amenable to letting blue light through than red. It’s tough to explain that behavior with an opaque object, like a planet, transiting in front of the star — rather, Dr. Boyajian and her team argue, it’s more likely that clouds of dust grains, smaller than a micrometer across, are responsible. Think of such grains as tiny glass spheres eclipsing the star, refracting starlight off its original course, scattering away short blue wavelengths and leaving longer red wavelengths less affected.

…Or Is It?

Of course, these proposed dust grains still have to come from somewhere, and that’s an entirely new puzzle. Micrometer-sized grains are so small that they get pushed around — or, more accurately, away — by starlight itself. If dust is the answer, it must be continuously resupplied or created around Boyajian’s Star. Dusty comets, planetesimals, or collisions between such objects are one possible source of dust, so the exo-comet hypothesis (Dr. Boyajian’s original explanation for the Kepler dips!) might be back in play. Luckily, analysis of the colors of the other three dips is in the works, and Las Cumbres is still looking. Stay tuned!

About the author, Emily Sandford:

I’m a PhD student in the Cool Worlds research group at Columbia University. I’m interested in exoplanet transit surveys. For my thesis project, I intend to eat the Kepler space telescope and absorb its strength.

double pulsar

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

Title: A Direct Measurement of Sense of Rotation of PSR J0737-3039A
Authors: Nihan Pol et al.
First Author’s Institution: West Virginia University
Status: Accepted in ApJ

Disclaimer: Several of the scientists on today’s featured paper are collaborators of this article’s author; however, she had nothing to do with this work.

In 2017, the field of pulsar astrophysics turned 50 years old. In those 50 years, we’ve learned a lot about these rapidly rotating neutron stars — and they’ve facilitated some of the most exciting scientific discoveries of the last century. It’s easy to assume that in the years since 1967 we’ve confirmed most of the basic assumptions we make about these compact objects (like their evolutionary history, motion, and composition). However, today’s astrobite reveals that we still have a lot to learn.

pulsar

Diagram of a pulsar, a rotating neutron star with a strong magnetic field. [NASA/Goddard Space Flight Center Conceptual Image Lab]

We use pulsars to do unfathomably cool science, from testing general relativity and detecting gravitational waves to enabling spacecraft navigation. With such an advanced understanding of these objects, you might expect us to be able to determine a property as basic as what direction a pulsar is rotating. Remarkably, we haven’t been able to — at least not with much certainty. This study from Pol et al. presents the first direct determination of a pulsar’s sense of rotation (i.e. if it’s rotating prograde or retrograde with respect to its orbit). It also provides support for the widely accepted rotating lighthouse model of pulsars, which describes pulsars as rotating neutron stars with radiation emitted from their magnetic poles. Their beams sweep across our line of sight, creating the rapid pulsing signals we detect.

The authors of this work exploited a one-of-a-kind system, called PSR J0737–3039 (the Double Pulsar). It is the only known binary that consists of two detectable radio pulsars, and its fast orbital period (~2.5 hours) makes it the most relativistic pulsar binary we know of. More subtle features (detailed below) make this system ideal for the determination of a pulsar’s sense of rotation.

The Double Pulsar system consists of a pulsar with a 22.7-millisecond period (pulsar A) in orbit with a pulsar of period 2.8 seconds (B). Magnetic dipole radiation (for a review, scroll to Energetics here) from pulsar A has been shown to introduce a new signal (the “modulation signal”) when A’s radiation hits B’s magnetosphere, which is the subtle feature that enables Pol et al.’s analysis. In general terms, they are able to deduce the direction in which pulsar A is spinning by looking at the modulation signal. If the modulation signal has a slightly longer period than if the pulsar were not rotating, they can conclude that it is rotating in the same direction (prograde) as its orbital motion. If the period is shorter, it implies that pulsar A’s rotation is retrograde relative to its orbit.

Brave astrobites readers will want to refer to the paper that describes the algorithm the authors used to resample and transform the data. After these transformations, the main analysis occurs by looking at Fourier power spectra. A Fourier transform splits a signal into its constituent frequencies, and the power spectrum shows how much signal (technically, the average signal squared, or power) is present at any given frequency. In short, the scientists perform three tests by looking at the Fourier power spectrum corresponding to prograde, retrograde, and no rotation. The three possibilities are denoted by a value s, which could be -1, 0, or 1 (retrograde, none, or prograde). The signal from A is corrected in three separate trials, once for each value of s, to see which sense of rotation pushes the most power into the frequency of A’s main signal (i.e. which correction works the best). They find that s = 1, indicating prograde motion, is the best correction (see Figure 1).

Figure 1. The Fourier power spectrum of the modulation signal for s = -1, 0, and 1. More power appears in the s = 1 trial (cyan) than the other two; therefore, the authors conclude that the pulsar is rotating in the same direction as its orbit (prograde). [Pol et al. 2018]

This unique analysis is the first time the sense of rotation of a pulsar has been directly determined, which is an exciting result in its own right. However, this information can be used in several applications. For example, knowing the direction of rotation of a pulsar with respect to its orbit can help inform theories describing the evolutionary history of these systems. Pulsar B is much younger than A, so it might be interesting to know how much the supernova that birthed pulsar B affected pulsar A. The results of today’s paper suggest that the “kick” from B’s supernova did very little to disturb A’s motion. Additionally, the direction of rotation can (along with other measured orbital parameters) help determine the moment of inertia of A. With this information and an accurate mass measurement, it is possible to deduce the radius of the pulsar. Very little is known about the structure and composition of neutron stars, and a radius measurement can help inform and rule out proposed theories.

In a field as strange and complicated as astronomy, it’s wise not to assume what we know and don’t know about the universe. Today’s astrobite reminds us that despite our solid understanding of many complex issues in astrophysics, innovation still continues in understanding basic traits of the systems we study.

About the author, Thankful Cromartie:

I am a graduate student at the University of Virginia and completed my B.S. in Physics at UNC-Chapel Hill. As a member of the NANOGrav collaboration, my research focuses on millisecond pulsars and how we can use them as precise tools for detecting nanohertz-frequency gravitational waves. Additionally, I use the world’s largest radio telescopes to search for new millisecond pulsars. Outside of research, I enjoy video games, exploring the mountains, traveling to music festivals, and yoga.

galaxy disruption

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

Title: Empirical Determination of Dark Matter Velocities using Metal-Poor Stars
Authors: Jonah Herzog-Arbeitman, Mariangela Lisanti, Piero Madau, Lina Necib
First Author’s Institution: Princeton University
Status: Submitted to ApJL

Our galaxy is embedded in a cloud of dark matter, thought to consist of tiny particles traveling along orbits through the halo. These dark matter particles permeate all regions of the galaxy, extending far beyond the edge of the bright central spiral, but also orbiting through our solar system, and even passing right through the Earth. This is why scientists build giant detectors, hoping to trap some of these dark matter particles as they pass by. So far, these experiments have not detected dark matter, but that lack of detection is actually quite interesting. Finding out what dark matter is not, and thereby narrowing down the possibilities, is an important step towards revealing the true nature of these mysterious particles.

In order to really understand what it means when a detecter does not see dark matter, it is important to have a clear prediction for how much dark matter should be detected. For example, if we expect very few dark matter particles to pass through the Earth in a given amount of time, then maybe the lack of detections over a few years doesn’t actually mean those particles don’t exist. One essential piece of information in this prediction is the velocity of dark matter particles as they orbit past our solar system.

So, how can we determine the speed of these particles that we haven’t even directly detected? Well, let’s look back at where these particles actually come from. Dark matter halos grow over time by consuming other dark matter halos. This process is called hierarchical structure formation. The Milky Way is continuously pulling in smaller galaxies and then tearing them apart, thoroughly mixing their stars and dark matter particles into the Milky Way halo (Figure 1).

This understanding of the origin of these particles reveals an important piece of information: when dark matter particles join the Milky Way, they are often accompanied by stars. This is great news, because stars, unlike dark matter particles, are not invisible, and we can directly measure their velocities. If we can confirm that dark matter particles tend to move at similar velocities to their stellar companions, then this problem of determining the local dark matter velocity is much simpler! Finding out if this is in fact the case is exactly the goal of today’s paper.

The tricky thing here is that the Milky Way is continuously forming new stars, so the authors need to find a way to distinguish between the stars formed within the Milky Way and stars that formed in smaller galaxies and were then consumed by the Milky Way along with the corresponding dark matter. This turns out to be fairly straightforward: stars that form in smaller galaxies tend to have a different chemical composition than stars that are currently being formed in the Milky Way. This is because Milky Way stars are forming from materials that have been enriched with heavier elements by generations of star formation, while the stars in smaller galaxies are not. The stars we are interested in are therefore what astronomers call “metal-poor.” The prediction is therefore that metal-poor stars and dark matter particles should have similar velocities.

Figure 2. A simulated Milky Way-like galaxy, from the ERIS simulation used in this paper. [Simone Callegari]

The authors use simulations of Milky Way-like galaxies (Figure 2) to compare the velocities of dark matter and stars, and find that this prediction holds up! Figure 3 shows the distributions of velocities for dark matter and different stellar populations. The black histogram is dark matter, the cyan histogram is all stars, and the orange histogram is only metal-poor stars. The black and orange histograms line up pretty well, meaning the velocity of metal-poor stars does tend to match that of the dark matter. This means that by observing the velocities of these stars near the Sun, we can improve our understanding of the dark matter velocity. This will improve our interpretation of the results of dark matter experiments. In particular, based on preliminary calculations, the authors show that the velocity is lower than previously thought. They suggest that this may weaken the significance of non-detections at smaller dark matter particle masses.

Figure 3. Velocity histograms of different components of the Milky Way, as seen in the ERIS simulation. The black histogram shows the velocity distribution of dark matter. The cyan histogram illustrates the velocity of all stars, and has a much larger central peak than the dark matter distribution. The orange histogram, however, which includes only metal-poor stars, is very similar to the dark matter velocity distribution. [Herzog-Arbeitman et al. 2018]

This is a really exciting result. Previous estimates of the dark matter velocity all came from simulations and theoretical predictions, so this new method, which uses observations of our actual galaxy, rather than a simplified model, should really improve the accuracy of these calculations. Furthermore, current experiments like Gaia are greatly improving our understanding of the local stellar velocity distribution, which will continue to increase the power of this method to determine the local dark matter velocity.

About the author, Nora Shipp:

I am a 2nd year grad student at the University of Chicago. I work on combining simulations and observations to learn about the Milky Way and dark matter.

dwarf galaxy

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

Title: Hunting Faint Dwarf Galaxies in the Field Using Integrated Light Surveys
Authors: S. Danieli, P. van Dokkum, C. Conroy
First Author’s Institution: Yale University
Status: Submitted to ApJ

One marvelous fact about our universe is that at the largest scales, it is fractal. Unlike true fractals, which exhibit exact self-similarity, the universe is only statistically self-similar. If you looked at the most massive objects in the universe, a record held by the gargantuan, invisible dark matter blobs holding clusters of galaxies together, which clock in with masses upwards of 1015 times the mass of the Sun, you’d find that they’re rife with smaller blobs, or “halos” of dark matter. Many of these smaller dark matter halos are inhabited by a galaxy, including giant bright elliptical galaxies, the smaller and fainter spiral galaxies, and hordes of yet smaller, fainter galaxies. Peering closer at, say, one of the dark matter halos of a Milky Way-like spiral galaxy, which clocks in at about 1012 times the mass of the Sun, you’d find that it in turn is surrounded by a similar but down-sized army of even smaller dark matter halos, which may contain even fainter “dwarf” galaxies. And the halos of each of these dwarf galaxies in turn can host their own army of even tinier dark matter halos. If you just looked at the dark matter of a galaxy cluster, a single spiral galaxy, or a dwarf galaxy, it would be hard to tell which was which — they would roughly look like scaled up (or down) versions of each other.

How far down does this fractal structure go? We can search part of the way down by searching for the smallest, faintest galaxies that live within them — which is an incredibly difficult task. To go further down to the smallest dark matter halos, which may be completely dark, and thus unobservable by usual means (i.e. by light), we’ll have to turn to more exotic methods. The faintest dwarf galaxies we’ve found thus far have been discovered by hunting for clusters, or “overdensities,” of stars. This technique can only uncover dwarfs in which we can observe individual stars, which we can distinguish only out to a pitiful distance — just to up to about 5 Mpc away, which is a little beyond the edge of the Local Group of neighborly galaxies.

What about the faint dwarfs that live even further away, far enough that they appear as fuzzy patches of light, and not as collections of stars? The authors of today’s paper discuss our prospects for finding these “integrated light” images of dwarfs (see Fig. 1) as far away as 10 Mpc. To do this, they set out to ask a simple question: how many galaxies could they find, given a telescope with a particular resolution and sensitivity?

Figure 1. Simulated observations of a faint dwarf galaxy at different distances from the Milky Way. If the dwarf is just outside the Milky Way, at about 500 kpc, we can see the individual stars within the galaxy. However, if such a galaxy is farther away, it becomes increasingly difficult to resolve individual stars. At 4 Mpc, it only appears as a fuzzy blob of light, and we can no longer see the individual stars in the galaxy. Detecting such faraway, faint dwarfs requires new search methods. [Danieli et al. 2017]

To carry out this calculation, the authors assume — for lack of data — that the faint galaxies as far as 10 Mpc look like the ones we’ve seen that are close by. Based on these nearby galaxies, they estimate how large and faint the distant dwarfs they’re searching for would be, then determine whether or not we could see them. They also estimate the mass in stars each of these faraway dwarfs have — a key quantity that allows them to guess the mass of the dark matter halos the dwarfs inhabit. It’s as yet unclear how much dark matter a galaxy with a given mass in stars has — a mapping we call the stellar mass-halo mass (SMHM) relation — so the authors adopt two different ones. With the dark matter mass of the dwarf galaxies, it’s simple enough to determine the number of such dwarfs that should exist out to 10 Mpc from simulations of the Milky Way and its surroundings.

Figure 2. The number of dwarfs between 3–10 Mpc that we could see with a telescope of a given resolution and sensitivity. The angular resolution is shown on the horizontal axis, and the sensitivity, here quantified as the surface brightness μ, is shown on the vertical axis. The colors and black contour lines denote the number of dwarfs you can see per square degree in the sky (an area equivalent to five times the Moon’s). The two panels show results from two different SMHM relations (see above paragraph for details). Brighter dwarf galaxies — those with smaller μ and thus at the bottom of the plots — can be seen no matter the resolution of your telescope. Fainter dwarfs — those with larger μ and higher up on the plot — are found in greater abundance, and we need good spatial resolution (fewer arcsecs) to detect them all. [Danieli et al. 2017]

The authors find (see Fig. 2) that, as you might expect, brighter dwarfs can be discovered no matter how good the resolution of your telescope is. However, when attempting to discover fainter dwarfs, the resolution really begins to matter. If, say, you had a telescope with a resolution of 9 arcsec versus one that was twice as good, you could detect up to six times as many of the faintest dwarfs. They also find that the SMHM they assume can affect the number of the faintest dwarfs they expect to find by as much as a factor of five.

The authors calculate that using this “integrated light” method to hunt for faint dwarfs using the Dragonfly Telescope Array, a telescope that was designed for the task, we could find a similar number of galaxies — if not more — as with surveys that rely on the traditional method of finding clusters of individually resolved stars. This is an exciting result. The few smallest, faintest galaxies we’ve found so far currently puzzle astronomers: how many of them are there? Why are they so faint? How did their stars form? We could begin to unravel these mysteries once we find more of these tiny galaxies.

About the author, Stacy Kim:

I am a fourth-year graduate student in The Ohio State University’s Department of Astronomy. On a day-to-day basis, you can typically find me attempting to smash clusters of galaxies together inside big supercomputers with Dr. Annika Peter to see if cluster mergers are good testbeds for dark matter collisionality. As an undergraduate at Caltech, I spent a few years chasing photons where planets are thought to form (or, as they say, performing Monte Carlo radiative transfer calculations of protoplanetary disks) with Dr. Neal Turner of the Jet Propulsion Laboratory. When I’m not sitting in front of a computer trying to translate cosmic thoughts into pithy lines of code, you can find me in the kitchen or on the walls of a climbing gym.

Icy exoplanet

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

Title: Towards a Galactic Distribution of Planets. I: Methodology & Planet Sensitivities of the 2015 High-Cadence Spitzer Microlens Sample
Authors: Wei Zhu, A. Udalski, S. Calchi Novati et al.
First Author’s Institution: Ohio State University
Status: Published in AJ

I don’t know if you’ve heard, but astronomers have found quite a few exoplanets in the last couple of decades. However, most of these are clustered in our tiny corner of the galaxy. For the 2,043 planets with stellar distance listed on exoplanets.org today (yes, I know this article will be out of date in a week…) the average distance from to the host star from Earth is 624 pc. The center of the galaxy, meanwhile, is ~8,000 pc away. That’s further than even the furthest known exoplanet, OGLE-05-390L b, which is 6,500 pc from us.

And we’d really like to have a better understanding of the exoplanets in the galactic bulge, because their presence — or lack thereof — helps us to understand planet formation. Planet formation is believed to be affected by several external factors such as the host star’s metallicity, the stellar mass, the stellar multiplicity, and the stellar environment. That final category is what we’re going to consider today: does the presence of a large number of nearby stars interrupt the formation of planets? The galactic bulge, as the part of the galaxy with the highest number density of stars, is an ideal place to test this — if only we could detect enough planets out there…

OGLE telescope at the Las Campanas Observatory in Chile. [Krzysztof Ulaczyk]

Any readers particularly clued-up on exoplanet surveys might have recognised the phrase ‘OGLE’ in the name of planet ‘OGLE-05-390L b’. OGLE is the Optical Gravitational Lensing Experiment, a microlensing project run by Warsaw University. Although the mission was initially designed for dark-matter surveys, it has also made several serendipitous exoplanet discoveries. This astrobite describes microlensing for exoplanet detection in more detail, but for today all we really need to know is that sometimes nearby stars and distant stars happen to be really well aligned on the sky for a short time. In these cases, the nearby star’s gravity bends the light from the faraway star, causing it to be brighter for a short time; this is the process we call microlensing. If the nearby star also has a planet, which is also well aligned with the distant star, then the gravitational influence of the planet plus star system causes the brightness of the microlensing event to vary in a particular way. The planet/star mass ratio can be inferred from the precise shape of that brightness plot. This is, of course, the same physics that produces the stunning, strong-lensing Einstein Rings — but with a slightly weaker requirement for close alignment.

Unlike the radial-velocity, transit or direct-imaging methods for exoplanet hunting, the microlensing technique is able to detect exoplanets at huge distances. Meanwhile, a field with physically more stars is a great place for microlensing experiments, since a large number of stars need to be monitored for a long time so as to catch some of these chance alignments of foreground and background stars. As such, OGLE has been staring at the center of the galaxy for over a decade.

More recently, the microlensing community has become particularly interested in microlensing detections that have been measured by multiple different telescopes simultaneously. In a typical microlensing event, the mass and the distance of the foreground lensing star are degenerate. However, this degeneracy can be broken by comparing several simultaneous observations of the microlensing event with physically separated telescopes. The wider the separation between the different telescopes measuring the microlensing detection, the better — so why not use a telescope in space? The Spitzer telescope is almost 200,000,000 km away, giving an impressive distance baseline for this kind of work.

Figure 1: Sensitivities to planets for a subset of the survey. Red, green, blue, purple and black curves show the depth to which 15, 30, 45, 60 and 75% of planets could be detected; q and s represent the mass ratio and projected separation of the planets. The bottom left corner lists the OGLE catalog number (bold) and the impact parameter, which represents the closest on-sky separation of the two stars during the entire event. [Adapted from Zhu et al. 2017]

Today’s authors carry out a pilot study laying out the methodology for a microlensing survey exploring how the galactic bulge affects planet formation. After data validation, removal of instrumental systematics, and a check that the distance to each star is well defined, the authors use a sample of 41 microlensing events — all of which have been observed by OGLE, Spitzer and a third telescope, KMTNet. Each of these microlensing events consists of a distant and a nearby star — and in this case no planets are detected around the nearby stars.

The authors model each of the lensing events with a variety of planets orbiting the nearby star, so as to determine how sensitive the survey is to exoplanets as a function of mass and separation. Some of these sensitivity curves are shown in Figure 2 above. The authors then carry out a statistical analysis; for this they use a simple parametrised model of the galaxy as a bulge and a disk, a couple of different assumptions about the stellar mass function, the footprint of the survey on the sky, some beastly Bayes calculations and information about their survey’s sensitivity to planets.

On the assumption that planetary frequency is the same in the galactic bulge, the authors find that roughly a third of all planetary detections in a survey like this one should come from bulge events. Since they have no planet detections in this sample, they aren’t yet able to calculate what fraction of detections actually come from the galactic bulge — this is left as future work. If the number turns out to be significantly different from the value of one third calculated here, it will reveal crucial information about planet formation in crowded regions of the galaxy. This work is currently ongoing. And, if they get lucky, maybe OGLE-05-390L b’s record as most-distant-planet will soon be broken!

About the author, Elisabeth Matthews:

I’m a third year PhD student at the University of Exeter, in the south of England, where I’m aiming to detect and characterise extrasolar planets and debris disks via direct imaging. So far this has meant lots of detecting background stars that happen to be well aligned with bright, nearby stars and no detecting of actual planets — but hopefully my luck will change soon!

Proxima Centauri

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

Title: ALMA Discovery of Dust Belts Around Proxima Centauri
Authors: Guillem Anglada, Pedro J. Amado, Jose L. Ortiz, and collaborators
First Author’s Institution: Institute of Astrophysics of Andalusia, Spain
Status: Published in ApJL

Hiding behind the gleaming light of a pale red dot lies one of the world’s favorite exoplanets, Proxima b, whose orbital motion imprints a barely detectable wobble in its host star, Proxima Centauri. Following the seminal discovery of a planet in its habitable zone, this undistinguished low-mass red dwarf star became a target for tireless examination, despite the challenges involved in observing it. In today’s paper, we will see that one of these searches, however, seems to have paid off … in dust?

Figure 1. Observations of Proxima Centauri with the ALMA observatory suggest that it may not only host a rocky planet but also at least one dust ring around it. [Anglada et al. 2017]

The One Ring

Okay, I will be honest and admit that, indeed, few people in the world would be enthusiastic about dust — in space, of all things! But, let me tell you, this is exciting dust. Guillem Anglada and his collaborators (including his spacefaring self from a parallel universe, Guillem Anglada-Escudé, who is the first author of Proxima b’s discovery paper) used the Atacama Large Millimeter Array (ALMA) to observe a dust ring around Proxima Centauri. Such an object is a signpost of terrestrial planet formation and could clue us in on what its planetary system looks like and how it came to be.

You may recall that the lore of Middle Earth tells the story of The One Ring to Rule Them All™, whose (un)fortunate wearer is granted the power to become invisible. In a funny exchange of roles, the dust ring around Proxima Centauri may actually help us uncover more information about the well-hidden planet Proxima b, such as its orbital inclination and mass.

But leaving western epic novels aside, let’s talk about the instrument. The ALMA observatory has two types of antennas: the smaller 7-meter diameter detectors, and the bigger 12-meter ones. The data that comes from these antennas differ in that the smaller ones produce images with fewer details but with a wider field of view; the bigger antennas produce more detailed images but see a limited field of view.

Following analysis of the data depicted in Figure 2 below, the authors found that Proxima Centauri appears to emit more infrared light than it should, which they conclude must be produced by a “belt,” or ring, of cold (50 K) dust. The dust belt has a radius four times the distance of the Earth from the Sun (i.e. 4 AU). In reality, this dust ring seems to be an analog to the Solar System’s Kuiper Belt. They estimate that the mass contained in this belt is 1% that of Earth’s, which is similar to the mass of the Kuiper Belt around the Sun.

Figure 2. Image of Proxima Centauri (represented by the + mark) using ALMA’s 12-meter array. Although not clearly separated from the central source, the observed infrared excess strongly suggests the presence of a dust belt around the star with a radius smaller than 4 AU. Its shape also hints at the presence of a warmer belt closer to the central star, but this hypothesis needs confirmation. The identity of the detached turquoise blob to the left of the star is unknown: it could be either a real object or plain noise fluctuation (see main text). [Anglada et al. 2017]

Rings for Days

In addition to the “One Ring” described above, the elongated shape of the source in Figure 2 suggests the presence of a warmer (T ~ 90 K) dust belt with size 0.8 AU, but the authors don’t seem to be completely sure about its nature yet.

Now, if you’re asking yourself what that blob of emission to the lower left of Proxima Cen in Figure 2 could possibly be, the short answer is: we don’t know yet. The long answer is that it could either be a real source or it could just be random noise fluctuation. If it is a real source then the authors suggest many possible explanations, such as a background galaxy or a collision between large bodies. But the most intriguing explanation for this potential source is that we could be looking at the rings of a Saturn-like planet orbiting Proxima Cen. More observations will be needed to confirm this exciting possibility, though.

But wait, there’s more! Figure 3 below depicts the image as observed by the compact array configuration of ALMA, using the 7-meter antennas: what seems to be a second dust belt is seen as a series of green smudges at a distance of approximately 30 AU, represented by the white ellipse. Although the detection is very marginal, the authors propose that this could be an outer dust ring with 1/10,000 the mass of Earth.

Figure 3. Image of Proxima Centauri using ALMA’s compact array configuration (7-meter antennas). The dotted white ellipse marks the position of a dust belt of radius 30 AU. The green regions are the positions where the signal produced by the dust belt is stronger.

I know, I know, that was a lot to take in! Proxima Centauri suddenly seems a lot busier than we previously thought, likely sporting a rocky planet in its habitability zone and a Kuiper Belt-analog. The other possibilities, such as the Saturn-like ringed planet and the other dust belts are still a bit speculative, so further observations of the system are a no-brainer at this point.

About the author, Leonardo dos Santos:

Leo is an exoplanet scientist and Ph.D. candidate at the Geneva Observatory. His current research involves characterization of exoplanets, physical and chemical properties of stars similar to the Sun and developing astronomical software. Not to be confused with the constellation.

turbulent disk

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

Title: The Effects of Protostellar Disk Turbulence on CO Emission Lines: A Comparison Study of Disks With Constant CO Abundance vs. Chemically Evolving Disks
Authors: Mo (Emma) Yu, Neal Evans, Sarah Dodson-Robinson, Karen Willacy, Neal Turner
First Author’s Institution: University of Texas at Austin
Status: Accepted to ApJ

We know it happens. We see that protoplanetary disks — the birthplace of planets — spill the gaseous material at their inner edge onto the young stars around which they orbit. This process of accretion persists throughout the disk’s lifetime and within 1 to 5 million years (or 10 Myr in rare cases), a disk will feed all of its gas to its star and completely fade away — leaving behind only the planets that formed along the way and any leftover rocky material that remains.

The rate at which accretion occurs — as well as why accretion occurs — are both driving forces behind determining what types of planets can form quickly enough in a protoplanetary disk’s relatively short lifetime. While we can measure accretion rates onto stars directly, we still do not know why disks accrete! This is one of the most important unsolved problems in planet formation. Until we can solve it, our models for planet formation are incomplete.

There are two leading potential explanations for why accretion occurs: (1) turbulence, and (2) magnetic winds. In the last two years, several attempts have been made to measure turbulence in a nearby disk for the first time using CO (carbon monoxide) spectral lines. These measurements showed that the turbulence in that system is not strong enough to be responsible for accretion, winning favor for the other idea of magnetic winds.

However, today’s paper led by Emma Yu argues that those measurements were not interpreted properly since they did not take into account the relatively rapid rate at which CO depletes over time (see Figure 1). This paper asks: What can we really learn about turbulence from CO spectral lines if we include CO depletion in our model?

CO depletion

Figure 1: Density of CO and H2 at a range of distances from the star over time (taken from chemical models). Previous measurements of turbulence assumed the ratio of CO-to-H2 to be constant over time, but this is not true beyond 15 AU after 1 Myr. [Adapted from Yu et al. 2017]

Background: Why Do Protoplanetary Disks Accrete onto Their Stars?

It is understood that disks must accrete by transporting angular momentum outwards. Naively, we might expect this to happen through “viscous shear flow.” In this process, the gaseous disk struggles to behave like a fluid because it rotates at a slower rate further away from the star (due to Kepler’s 3rd law). As a result, material in the outer disk will try to speed up to catch up to the inner disk — thereby increasing its angular momentum. Meanwhile to compensate for that increase (and conserve angular momentum), the material in the inner disk will slow down. Since this gas is rotating too slowly to obey Kepler’s 3rd law, it will spiral inwards and eventually accrete.

There is just one problem: disks are too sparse to interact viscously like a fluid! For shear flow to occur, there must be some source of turbulence to make the disk viscous. It is widely accepted that magnetic fields can create turbulence by triggering an instability, but it is not clear if this instability actually occurs in the entire disk. If it does not occur in most of the disk, turbulence most likely would not be responsible for making the disk accrete.

Instead, magnetic winds — which transport angular momentum outward in a completely different way by flinging material out of the disk on magnetic field lines pointed away from the star while the disk rotates (see also here and here for more) — would be the favored reason for the disk accreting.

Introduction to Spectral Lines with Turbulence

We would know why protoplanetary disks accrete if we could just measure their levels of turbulence, but we did not have the telescope power to do this until the ALMA telescope array was turned on several years ago. In this astrobite, Tim discussed a new method for measuring turbulence with ALMA by looking at the shape of a specific CO spectral line.

Spectral lines are one of the most important tools in astronomy. They are used for everything from inferring compositions of atmospheres to measuring distances to distant galaxies, among many other things. Molecules (and atoms) emit spectral lines when electrons that were excited to a high energy state transition back to a lower energy state. While these lines are emitted at the precise wavelength corresponding to a transition, they never appear perfectly thin. They always exhibit some level of “broadening” due to the gas molecules moving towards us or away from us as they emit.

The velocities of individual molecules are mostly random. Thermal effects create most of the random motion for gas molecules. Besides that, turbulence also creates a little bit of random motion, causing spectral lines to broaden more than normal. As a result, we can use the broadened shapes of spectral lines (that are very well-resolved) to measure turbulence!

Spectral Lines with CO Depletion

The paper covered in Tim’s astrobite found that turbulence can be probed by measuring the CO spectral line’s peak-to-trough ratio (see Figure 2). Specifically, disks that are less turbulent have higher peak-to-trough ratios. However, this analysis assumed that the amount of CO in the disk relative to hydrogen is fixed over time, whereas chemical models predict it should actually drop significantly in older disks (see Figure 1).

CO spectral line profiles

Figure 2: CO spectral line profiles with different levels of turbulence, with lower levels producing a higher peak-to-trough ratio. (Note: The double-peak structure arises from part of the disk rotating towards us and part of the disk rotating away from us.) [Adapted from Yu et al. 2017]

Coincidentally, Emma Yu et al. find that the depletion of CO also creates spectral lines with higher peak-to-trough ratios (see Figure 3). As a result, the authors of the other study may have thought that they detected low levels of turbulence by measuring a high peak-to-trough ratio when they may really only be seeing evidence that CO has been depleted!

two disk stages

Figure 3: CO spectral line profiles at different stages in the disk lifetime. Left: Includes CO depletion. Right: Assumes constant CO-to-H2 ratio. With a constant CO ratio, the peak-to-trough ratio does not change after 1 Myr. With CO depletion, the peak-to-trough ratio increases over time just like with decreasing levels of turbulence. [Adapted from Yu et al. 2017]

Figures 2 and 3 (and also Figure 7 of the paper) show how easy it is to confuse a disk with no turbulence at all with a disk that has a moderate level of turbulence. If that extra turbulence is present, it might be strong enough to explain the observed levels of accretion in the previous study, giving favor to the long-assumed idea that turbulence drives accretion — not magnetic winds. However, the paper shows that our current measurements may not be able to distinguish these levels of turbulence, leaving the question of why disks accrete still unsolved.

Future Work

All hope is not lost! The authors point out that a different isotopologue of carbon monoxide (C18O) may be more useful for measuring turbulence. However, its spectral lines are weaker and thus, more difficult to resolve. They also point out that CO would be more helpful if we could also measure the precise level of CO depletion in a disk (rather than infer it from chemical models).

This is an exciting time for studying protoplanetary disks because we are finally beginning to scratch the surface of measuring turbulence. Getting proper measurements though, will require more digging.

About the author, Michael Hammer:

I am a 3rd-year graduate student at the University of Arizona, where I am working with Kaitlin Kratter on simulating planets, vortices, and other phenomena in protoplanetary disks. I am from Queens, NYC; but I’m not Spider-Man…

Aurorae on Jupiter

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

Title: The Detectability of Radio Auroral Emission from Proxima b
Authors: Blakesley Burkhart & Abraham Loeb
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Published in ApJL, open access

Dazzling auroral displays are not uncommon in our solar system. In fact, when the Sun sends highly energetic particles out in to the solar system, any planets with a substantial magnetic field will interact with the particles, resulting in the emission of radio waves. Detecting this emission allows us to determine many interesting planetary properties, such as orbital parameters, habitability, plate tectonics and atmospheric compositions. Yet we have not observed any auroral activity from planets that lie outside our solar system (we have, however, detected radio auroral emission on a brown dwarf star!).

Proxima b

Figure 1: Artist’s impression showing Proxima b orbiting its red dwarf host Proxima Centauri. [ESO/M.Kornmesser]

Today’s bite investigates our nearest stellar neighbour, Proxima Centauri, to answer one very important question: can we detect auroral emissions from its exoplanet, Proxima b?

What Are We Looking For?

To answer this question, the authors first employ radiometric Bode’s law to estimate the magnitude of radio waves released during stellar wind and magnetosphere interactions. Radiometric Bode’s law, derived from observations of magnetic planets within our own solar system, indicates that the brightness of radio waves increases with size of the planetary magnetosphere. Blackett’s law then suggests the size of the magnetosphere scales with mass and rotation speed, which translates to a predicted radio power of 1013 for Proxima b. When combined with Proxima b’s estimated magnetic field strengths of 0.007 – 1 G, comparable to the 0.5 G observed at the Earth’s equator, scientists expect the frequency of these radio waves to be between 0.02 – 2.8 MHz.

radio wave brightness

Figure 2: Predicted brightness of radio waves versus emission frequency, in accordance with Bode’s law, for 106 exoplanets. The expected values for Proxima b are highlighted in the blue square, with the yellow star representing a magnetic field value of 0.3 G. The black dashed line indicates the cut-off frequency for Jupiter. [Burkhart & Loeb 2017]

Figure 2 highlights the first hurdle for observations of radio auroral emissions from Proxima b. All radiometric modelling points to radio waves being emitted at frequencies between 0.02 – 2.8 MHz for this exoplanet. However observations below 10 MHz are not possible from Earth, as these wavelengths are blocked by our atmosphere — so even at the extremes of likely magnetic field strengths, we cannot use ground-based instrumentation. A secondary issue with observing aurorae on Proxima b is that we’re now observing in a low-frequency regime that tends to be absorbed by the interstellar medium (ISM).

Modelling a Magnetosphere

So far, the authors have only estimated the brightness of auroral activity on Proxima b. With its close-in orbit creating a highly variable magnetosphere radius, we expect the variation in the observed radio waves to be rather large. To characterise this variability, authors implemented models of the wind and magnetic field around Proxima b as a function of various orbital parameters. An example of the results obtained from the simulations is shown below in Figure 3.

Figure 3: Variation in the observed radio brightness as the radius of the magnetosphere caries over Proxima b’s short 11.2 day orbit. Eccentricity = 0 and inclination = 10 degrees. [Burkhart & Loeb 2017]

Here we see how the simulated brightness of radio waves changes over one orbit. The two dips correspond to periods where the magnetospheric radius suddenly changes as the planet passes through streamer regions, located near Proxima Centauri’s equator, where stellar winds become denser. The authors also considered four different magnetic field strengths and found that a weaker magnetic field, and therefore lower emission frequency, results in brighter radio emission.

From modelling the variability in radio flux for Proxima b, the authors concluded two things:

  1. Proxima b’s radio auroral emissions vary by almost an order of magnitude over one full orbit.
  2. The amplitude of the variation depends on orbital parameters — the eccentricity and inclination of the orbit — as well as on parameters of the stellar wind and planetary magnetic field.

Can We Detect Aurorae Around Proxima b?

There are a number of issues associated with undertaking observations of Proxima b’s radio emission. As previously mentioned, the ISM is highly problematic, as electrons within the ISM are more likely to absorb photons at low frequencies due to free-free absorption. Thankfully, this shouldn’t be an issue above 0.3 MHz for nearby planetary systems, like Proxima b, as there is less ISM to contend with. We also have to overcome the 10 MHz atmospheric cut-off introduced by absorption of photons in the ionosphere. Clearly the only solution here is to make observations from space. The authors mention several interesting proposals including a radio observatory on the Moon (one example given is ROLSS) and clusters of low-cost CubeSats to form a very large telescope (think of experiments like ALMA which combined lots of smaller telescopes to form one big one, but in space).

Figure 4: Example CubeSat with hands for scale. [NASA]

The take-home message of this paper is that the brightness of radio emissions around Proxima b are substantial enough to be detected here on Earth. This is fantastic because if aurorae, caused by interactions between stellar winds and the planet’s magnetosphere, are detected on Proxima b, we will be able to further constrain the planet’s orbital inclination, eccentricity and generally gain insight into its magnetosphere. Now we just have to wait for the right instrumentation to put into space, allowing scientists to overcome the pesky 10 MHz limit imposed by our own atmosphere.

About the author, Amber Hornsby:

First year postgraduate researcher based in the Astronomy Instrumentation Group at Cardiff University. Currently I am working on detectors for future observations of the Cosmic Microwave Background. Other interests include coffee, Star Trek and pizza.

NGC 1300

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

Title: Galaxy-Scale Bars in Late-Type Sloan Digital Sky Survey Galaxies Do Not Influence the Average Accretion Rates of Supermassive Black Holes
Authors: A.D. Goulding, E. Matthaey, J.E. Greene, et al.
First Author’s Institution: Princeton University
Status: Accepted to ApJ, open access

When it comes to picking their host galaxies, active galactic nuclei (or AGN) are rather promiscuous. They reside in all types of galaxies: ellipticals, irregulars, and spirals. AGN of the same feather tend to flock together — the more luminous and radio-loud ones are found in elliptical galaxies while the lower luminosity ones are more often found in spiral galaxies. This is a manifestation of the black hole mass-host galaxy luminosity correlation, where spiral galaxies like our Milky Way tend to have less massive black holes than elliptical galaxies. Besides spiral arms, spiral galaxies sometimes also boast of having bars, if the right mood strikes. How are bars related to their AGN? Could they trigger the central black holes to light up as AGN?

Galactic bars are thought to contribute to the dynamical evolution of their host galaxies. Numerical studies show that they can funnel in gas from the outskirts to the central regions of the galaxies, triggering star formation and possibly AGN activity. It is still unclear whether bars actually help trigger AGN, as previous studies have produced conflicting results and tend to suffer from small number statistics and biased AGN diagnostics. In today’s paper, the authors bring better tools to bear on the problem, by utilizing the large wealth of information from the SDSS Galaxy Zoo citizen science project and X-ray stacking analyses.

The authors first selected a sample of ~100,000 local spiral galaxies from SDSS that have been visually classified as such in Galaxy Zoo by at least 20 people. This sample is later divided into three redshift bins that are each complete in stellar mass, i.e. there are galaxies that span the whole range of stellar masses in each redshift bin. This is to ensure the final results are representative and unbiased. They distinguished sources with and without bars in these three redshift bins using information from Galaxy Zoo, based on the fraction of votes to the question “Is there a sign of a bar feature through the center of the galaxy?” If the fraction of votes is equal or greater than 0.25, the galaxy is defined to have a bar; if the fraction of votes is less than 0.1, the galaxy is defined to be bar-less. Galaxies with fraction of votes between these two numbers are defined to be ambiguous. Figure 1 shows some examples of spiral galaxies with bars, no bars, and ambiguous bars in their sample.

Fig. 1: Sample unbarred (blue borders), ambiguously barred (yellow borders), and barred (red borders) spiral galaxies from the Galaxy Zoo project, as determined by fbar, which is the fraction of votes by citizen scientists for the presence of bars. [Goulding et al. 2017]

In contrast to optical light which is absorbed by dust, X-rays from an AGN can more easily pierce through dust obscuration. Stacking lots of X-ray observations help to reveal heavily-obscured or low-luminosity AGN. Using data from the Chandra X-ray observatory, the authors performed X-ray stacking analyses to investigate the presence of AGN in their barred, unbarred, and ambiguously barred samples. Figure 2 shows the X-ray luminosity of their samples after subtracting the contributions from star formation processes. The three types of spiral galaxies do not show obvious differences in their X-ray luminosities, suggesting that AGN are no more common in one type of galaxy than the others. For galaxies with X-ray detections (i.e. hosting AGN), the authors further investigated the distributions of their specific black hole accretion rates, which are the X-ray luminosities divided by the host galaxy stellar mass shown in Figure 3. This ratio removes the dependence on stellar mass and instead probes the dependence on the host galaxy properties. There is again no difference in the accretion rates between the barred and unbarred samples.

Fig. 2: Star-formation subtracted X-ray luminosities vs. redshift in three X-ray energy bands. As with Figure 1, blue markers refer to unbarred galaxies, yellow to ambiguously barred, and red to barred galaxies. Open markers are sources with X-ray detections while filled markers are the luminosities produced by stacking sources without X-ray detections. As a comparison, the predicted mean X-ray luminosities due to stellar processes are shown by the dotted lines. [Adapted from Goulding et al. 2017]

Fig. 3: Distributions of specific black hole accretion rates for galaxies with AGN. The different line colors again refer to the presence or absence of bars. Dashed, solid, and dotted lines refer to different cuts in the X-ray luminosity. [Goulding et al. 2017]

Well, all that is a bummer — the presence of AGN in spirals seems to be independent of the presence of bars. For those with AGN, there is also no difference in the specific accretion rates of their host galaxies on the basis of a bar existence. As stacking analyses tend to wash away short timescale events, any bar contributions to AGN activities would need to be very short-lived. This study shows that over the lifetime of the galactic bars, they do not play significant role in triggering AGN — astronomers need to turn their eyes to other means of growing black holes in spiral galaxies.

About the author, Suk Sien Tie:

I am a third year PhD student at the Department of Astronomy at The Ohio State University. I am currently working on quantitative analyses of various quasar selection methods using the Dark Energy Survey (DES) and quasar variability via microlensing.

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