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circumstellar 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 repost 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: Probing the Cold Dust Emission in the AB Aur Disk: A Dust Trap in a Decaying Vortex?
Authors: Asunción Fuente, Clément Baruteau, Roberto Neri, et al.
First Author’s Institution: Spanish National Observatory (OAN, IGN)
Status: Published in ApJL, open access

Planet formation still holds many mysteries for today’s astronomers. One of the biggest unknown is how pebbles and dust clump together to form planetesimals rather than drifting in towards the star due to its strong gravitational pull. A possible solution is dust traps — in a circumstellar disk, the inward drift of the pebbles and particles can be stopped by high pressure in the gaseous disk. A dust trap like this would be an ideal place to form planetesimals. The authors of today’s paper look at a circumstellar disks around AB Aur. They create hydrodynamical simulations of the dust and gas to see if there is a dust trap with the possible formation of small planets.

Circumstellar disks are the link between bunches of dust around a star and fully formed planetesimals. There are a few types of circumstellar disks — protoplanetary, transition, and debris. A transition disk is exactly what it sounds like — a transition from protoplanetary to debris, which means that planets would likely be forming in a transition disk. About 450 light-years away from us, AB Aur hosts an asymmetric transition disk. The unevenness of the disk could indicate a dust trap or gas vortex. At about 120 AU from the central star, the cold dust emission disk creates an asymmetric ring in the disk.

AB Aur

Figure 1: Observations of the continuum emission of AB Aur’s disk from NOEMA. [Fuente et al. 2017]

The authors use NOEMA (NOrthern Extended Millimeter Array) to observe the dust’s continuum emission at 1.12 and 2.22 mm in the AB Aur system. Their observations, shown in Figure 1, reveal that the intensity varies throughout the emission ring, and the intensity decreases at higher wavlengths. The intensity variations that the authors observed are smaller at 2.22 mm than at 1.12 mm, which is the opposite of what purely theoretical dust-trapping/gas-vortex models predict.

AB Aur simulations

Figure 2: The simulations of gas created by the authors. The four images on the left show the distribution and density of gas, and the four images on the right show the continuum emission reproduced from the simulated gas distributions. [Fuente et al. 2017]

The authors then create their own model and use their hydrodynamical simulations for both gas and dust particles in the ring. From this, they are able to they reproduce the emission observed in the disk. Their results from the simulations can be seen in Figure 2. Their simulations show that the lopsided nature of the disk can be explained if the gas vortex has started to decay due to turbulent diffusion, and dust particles are thus losing the azimuthal trapping on different timescales that depend on the particles’ sizes. The authors constrain the size distribution from the comparison between the observations and simulations. They find that there is a mass of about 30 Earth masses in the dust trap, which would be more than enough to form rocky planets.

The authors’ gas and dust simulations show a dust-trapping feature in the disk at about 96 AU. The vortex would need to be decaying as the planet formed in order to fully reproduce their observations, including the intensity varations along the ring. More observations and work are needed to fully understand the disk, but so far the authors’ simulations show that planets and planetesimals are likely currently forming in a dust trap around AB Aur.

About the author, Mara Zimmerman:

Mara is working on her PhD in Astronomy at the University of Wyoming. She has done research with binary stars, including Heartbeat stars, and currently works on modeling debris disks.

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 repost 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: Where Is the Flux Going? The Long-Term Photometric Variability of Boyajian’s Star
Authors: Joshua D. Simon, Benjamin J. Shappee, G. Pojmański, et al.
First Author’s Institution: Observatories of the Carnegie Institution for Science
Status: Submitted to ApJ, open access

Boyajian’s star loves the limelight. It splashed onto the astronomical scene in 2015 by dimming dramatically and messily, much more messily than can be explained by a transiting planet. In 2016, astronomers realized that this sensational debut was only the latest act in a century-long fade, confirmed by exquisite observations from the Kepler mission. And the drama keeps building — just this summer, Boyajian’s star has undergone (and is still undergoing!) three more dimmings, similar in shape to the dimmings reported in 2015.

Figure 1. The situation, basically.

This week, Boyajian’s star is back in the news again, the subject of two new studies. The first confirms that the slow fade observed by Kepler is happening across a broad range of colors. The second, which we’ll explore in today’s bite, points out that this consistent fade might not be so consistent after all.

When an interesting astronomical object like Boyajian’s star is discovered, the first thing astronomers do is go comb through older observations taken of the same piece of sky. They hope that the object will serendipitously appear in this older data, and that they can compare what it looked like then (before it started behaving strangely) and now. When a supernova goes off, for example, astronomers rush to the archives to try to see what it looked like before it exploded — what type of star was it, and were there any clues to hint at its spectacular demise?

Today’s authors analyze over ten years of observations taken by the two robotic telescopes of the All Sky Automated Survey, which have been scanning the northern and southern hemisphere skies in tandem since 2006. They’ve been looking for changes in stellar brightness, and Boyajian’s star certainly fits that bill. Below is a graph of what they’ve found: the brightness of Boyajian’s star over the past ten years.

Figure 2. The brightness of Boyajian’s star over time. Black points come from the All Sky Automated Survey. Red points come from the Kepler mission. Blue points come from the All-Sky Automated Survey for Supernovae (no relation). According to the black points, Boyajian’s star hasn’t just been getting straightforwardly fainter — it’s had brightening spells, too!

If you focus on the middle of the data, which overlapped with observations taken by the Kepler space telescope, you’ll see why we thought Boyajian’s star was dimming gradually over time. That’s a pretty straight downward-pointing line. But if you zoom out to the full ten years, you see that Boyajian’s Star has had at least two brightening episodes in the past decade. The first happened from late 2006 through 2008, and the second from late 2013 through early 2015.

So what, you ask? Boyajian’s star is still spending most of its time getting fainter, not brighter, and the end result is that it’s fainter now than it was in 2007. The answer is that we still don’t know why any of this is happening — and any explanation must now account not only for the crazy 20% dips in flux and the century-long net dimming, but also for these intermittent glows. Previous hypotheses — which include the idea that Boyajian’s star is dimming after gulping down a planet some time ago, and that the present-day flux dips are due to leftover orbiting debris — may struggle to explain it all. 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.

NGC 3199

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 repost 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: Hot Gas in the Wolf-Rayet Nebula NGC 3199
Authors: J.A. Toalá, A.P. Marston, M.A. Guerrero, Y.-H. Chu, R.A. Gruendl
First Author’s Institution: Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA)
Status: Accepted to ApJ, open access

As very massive stars evolve off the main sequence, they sometimes lose more than half their initial mass through dense, slow winds. The stars’ hot cores are laid bare, emitting copious ultraviolet photons that ionize the expelled material, which forms an optically bright, bubble-shaped nebula. The exposed hot core is referred to as a Wolf-Rayet (WR) star, while the discarded outer layers that surround it make up what is known as a Wolf-Rayet nebula. These objects are named after the pair of French astronomers who first discovered them via their unusual spectra, which feature broad emission lines jutting above the continuum. While WR stars are readily detectable (even out to nearby galaxies) because of their unusual emission-line spectra, the origin of the surrounding nebulosity is more difficult to pinpoint because of its close resemblance to other types of emission nebulae — especially planetary nebulae — and the interaction of the ejected material with the interstellar medium.

Theory suggests that WR nebulae should emit X-rays. After a star loses its outer layers and becomes a WR star, its stellar winds accelerate to ~1500 km/s — more than ten times the previous pace. These faster winds collide with the previously ejected material propagating into the interstellar medium. This process should generate a diffuse bubble of shocked material that should emit in the X-ray, but this predicted X-ray emission has proved elusive; only a few WR nebulae have been detected in X-rays. In this paper, the authors use narrow-band optical images and X-ray images and spectra to investigate the properties of the area surrounding WR 18, a Wolf-Rayet star closely associated with emission nebula NGC 3199. The narrow-band optical images in Figure 1 show the bright arc of the emission nebula curving around WR 18.

Figure 1. Left: False-color optical image of NGC 3199. WR 18 is circled in red to the east of the bright arc. Right: A map of the [O III] to H-alpha ratio. Two measurements of the proper motion of the star are indicated by the colored arrows. [Toalá et al. 2017]

Figure 2. False-color X-ray image of NGC 3199. The western (right) side emits significantly more in the 1.1 – 2.5 keV range than the eastern (left) side. [Toalá et al. 2017]

The authors present X-ray observations of the nebula by XMM-Newton and reveal for the first time diffuse X-ray emission suffusing NGC 3199, seemingly bounded by [O III] emission on the southwest edge. (Here, the brackets denote emission via a “forbidden” transition — one that isn’t induced by the most common mechanism and therefore rarely occurs in a laboratory setting.) This discovery cements NGC 3199’s status as a WR nebula — only the fourth such nebula to be observed in X-rays. From the false-color X-ray images in Figure 2 to the right, we can see spatial variations in the X-ray emission. The authors find that the western (right) side emits more in the 1.1–2.5 keV range than the eastern (left) side. They then use spectral fitting to extract the global and local properties of the nebula. Most importantly, they find that while the material nearest the optically bright arc is enriched in metals from the strong stellar winds, the material opposite the arc appears to be a mixture of the stellar outflow and the interstellar medium. This implies that the nebular material of NGC 3199 is being actively enriched by the stellar winds from WR 18.

WR 18: A Homebody Mistaken for a Runaway?

The authors also consider the origins of the optically bright arc shown in Figure 1. It was first thought to be a bow shock formed as WR 18 barrels into the surrounding interstellar medium, which piles up like snow before a plow. This sometimes happens in the case of runaway stars — stars ejected from their natal clusters at high velocities, careening through space with large proper motions; bow shock formation requires that the star be traveling at a very high speed (greater than the speed of sound in the material surrounding the star). However, although the bright optical arc is certainly suggestive of a bow shock, the authors extend an alternative explanation. Because the proper motion of the star is nearly perpendicular to the direction of motion that the bow shock suggests, and because the authors find no evidence that the star’s velocity is especially large, they posit that the morphology of the bright arc results simply from the interaction of the star’s fast stellar wind with a dense region of the interstellar medium.

The morphology of NGC 3199 revealed by X-ray observations (namely, the bubble of diffuse X-ray emission bounded by bright optical [O III] emission) is common to the four WR nebulae observed in X-rays. This result shows that the configuration of the local interstellar medium strongly affects the structure of stellar outflows. Future X-ray observations of WR nebulae can help us understand how massive stars enrich the interstellar medium, and how the interstellar medium can shape the outflows of these stars.

About the author, Kerrin Hensley:

I am a third year graduate student at Boston University, where I study the upper atmospheres and ionospheres of Venus and Mars. I’m especially interested in how the ionospheres of these planets change as the Sun proceeds through its solar activity cycle and what this can tell us about the ionospheres of planets around other stars. Outside of grad school, you can find me rock climbing, drawing, or exploring Boston.

HAT-P-11

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 repost 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 Starspots of HAT-P-11: Evidence for a Solar-like Dynamo
Author: B. M. Morris, L. Hebb, J. R. A. Davenport, G. Rohn, S. L. Hawley
First Author’s Institution: University of Washington, Seattle
Status: Accepted to ApJ, open access

Figure 1: A group of sunspots seen on January 7, 2013. [NASA/SDO]

Sunspots & Starspots

Although generally known for being a dependable source of light and warmth to life on Earth, the Sun shows many signs of being variable. We have good reasons to keep track of this activity, not least because violent solar phenomena can pose risks to some widespread technology. For an overview of ongoing solar monitoring, the NOAA maintains a real-time Space Weather Dashboard, which among other indicators includes a count of sunspots.

In visible light, sunspots appear as dark patches on the surface of the Sun (Figure 1) because they have lower temperatures than their surroundings, and they are directly linked to intense magnetic activity. The motivation behind today’s paper is that we could learn much more about the magnetic activity of other stars if we could study their starspots in a similar way to sunspots.

The HAT-P-11 System

For a direct comparison with sunspots, it would be interesting to know the sizes and locations of spots on a distant Sun-like star. But these properties are especially hard to measure because, for example, the spots would be expected to cover only a small fraction of the total stellar surface. However, there exists a peculiar exoplanetary system that allows such measurements to be made (and it is a favorite of astrobites authors).

Figure 2:The geometry of the HAT-P-11 system (left) and example transit light curves of HAT-P-11 b (right), observed with Kepler. The bumps visible in the light curves are occultations of starspots by the planet.

The exoplanet HAT-P-11 b is a Hot Neptune in orbit around a K4 dwarf star, which is about 80% as massive as the Sun and about 120 light-years distant. HAT-P-11 b was discovered using the transit method, i.e. by detecting a small dip in the flux of the star, caused by the planet passing in front of it and blocking a small fraction of the stellar light (0.004%). However, the observed Kepler light curve shows several bumps occurring during some of these transits (Figure 2, right panel). These anomalies are detections of starspots! When the planet passes over one, it blocks not a bright region, but a dark patch of the star’s surface, so the star briefly appears a little brighter again.

As HAT-P-11 b continues to revolve around its host star and the star itself rotates, the star’s flux measured at a certain time during a transit can be mapped unambiguously to a location on the star’s surface. This is because the planet’s orbit nearly runs over the poles of the star (Figure 2, left panel). It is this misalignment that makes it possible to estimate the positions and sizes of the starspots, by analyzing the exact timing and shape of the bumps in the light curve. In total, the authors detect 294 spots on HAT-P-11, during 138 of the 205 transits observed (over roughly 4 years). Detecting the same starspot multiple times is unlikely, due to the spots’ limited lifetimes.

The Starspots of HAT-P-11

In a series of steps, the authors first model the re-occurring transit of the planet and systematically search for any significant flux anomalies. They then estimate additional parameters such as the stellar inclination and the spot contrast, and finally they fit a detailed spot model to the light curves. Due to the geometry of the system and limited measurement precision, there can exist a few plausible spot configurations that describe a particular light curve equally well (Figure 3), but the full set of light curves holds exceptionally detailed information.

Figure 3: Left panel: The three hypothetical starspots that are shown would be indistinguishable, since the bumps they produce in the measured light curve are too similar. Middle and right panels: Examples of Kepler light curves and the inferred starspot configurations, plotted on top of each other.

The starspots of HAT-P-11 turn out to be similar to the Sun’s in several ways. They are distributed into two active strips spanning the star on either side of the equator (Figure 4), with slightly more spots appearing on the northern hemisphere. This distribution resembles that of sunspots at or around the maximum of the Sun’s 11-year activity cycle. The observed range of typical spot sizes is also consistent with that of sunspots during a solar maximum. However, in other ways, the starspot activity of HAT-P-11 is more extreme. The spot coverage of HAT-P-11 is about 3%, which is 100 times greater than the Sun’s, because HAT-P-11’s spots are more numerous. A few of these spots also have remarkable sizes, larger than the biggest sunspot ever seen.

Figure 4: Map of starspots detected in the Kepler light curves. The blue and red circles correspond to the spots in the middle and right panels of Figure 3, respectively. The green circle indicates one particularly large spot. The background shading shows the observational coverage of different parts of the stellar surface. Around 30 spots like the ones detected were spread over the entire stellar surface at any point in time between 2009 and 2013.

Future Work

The highly misaligned orbit of the transiting exoplanet HAT-P-11 b offers a unique opportunity to study starspots on its host star. It seems likely that the processes causing magnetic activity in the Sun, a G2 dwarf, operate similarly in this K4 dwarf, since both stars show such similar starspot activity. In the future, the authors are planning to observe additional transits of HAT-P-11 b from the ground, which would enable the further study of the progression of a stellar activity cycle, if HAT-P-11 does indeed follow a cycle like the Sun.

About the author, Philipp Plewa:

I am currently a graduate student at the Max-Planck-Institute for Extraterrestrial Physics in Germany. My main interest is in developing new tools for high-precision infrared astrometry, with the aim of learning more about the supermassive black hole at the center of the Milky Way and the stars in its vicinity.

Proxima Centauri b

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 repost 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: Detecting Proxima b’s Atmosphere with JWST Targeting CO2 at 15 Micron Using a High-Pass Spectral Filtering Technique
Author: I.A.G. Snellen, J.M. D’esert, L.B.F.M. Waters, T. Robinson, et al.
First Author’s Institution: Leiden Observatory, Leiden University, The Netherlands
Status: Accepted to AJ, open access

Proxima Centauri b and the Significance of CO2

This summer marks the one-year anniversary of the detection of an exoplanet orbiting our solar system’s nearest stellar neighbor, Proxima Centauri. This Earth sized exoplanet, with the oh-so-imaginative name Proxima Centauri b, lies in the habitable zone of its M-dwarf star. As we’ve previously discussed, having such an Earth-like planet in our stellar backyard is really exciting, and astronomers are keen to explore the system as thoroughly as possible for potential signs of life. The cover image above shows an artist’s rendition of this intriguing exoplanet.

Since the discovery of this close planet, researchers have been studying methods that might detect the presence of life on Proxima Centauri b. Today’s paper approaches this by devising a method to detect carbon dioxide (CO2) in the planet’s atmosphere. The authors focus on this particular molecule because it is one of the four main biomarkers used in evaluating habitability of exoplanets; water, methane, carbon dioxide, and oxygen are primarily produced during biological processes, so their presence in an atmosphere can imply life. In addition to being a biomarker molecule, CO2 has many distinguishable features that are visible in the 15 micron band, which JWST is equipped to look at.

Snellen and collaborators present a technique that can be performed with the soon-to-be-launched James Webb Space Telescope (JWST), which would reveal the presence of CO2 in the atmosphere of this nearby exoplanet. The emission in the 15 micron band will be ideal for detecting CO2, since this molecule has over 100 features within this band.

JWST and High-Pass Spectral Filtering Techniques

JWST is equipped with several extremely sensitive instruments. One of the goals of this mission is to detect and characterize atmospheres of exoplanets. With this in mind, these authors suggest using the medium resolution spectrograph (MRS) mode of the Mid-Infrared Instrument (MIRI) to detect CO2 markers in the atmosphere of Proxima Centauri b.

Figure 1: This shows an example planet spectrum and high-pass filtered spectrum. The high-pass image has more distinguishable features, which allows for greater sensitivity in molecule detection. [Snellen et al. 2017]

This new technique combines several methods to attain greater sensitivity. The cross-correlation requires a high spectral resolution to find the radial component of the planet orbital velocity, used to filter out the planet’s signal. The authors’ method cross-correlates the observed spectrum with template spectra; however, the spectral resolution is not high enough to achieve this with Proxima Centauri b, so the authors suggest a slight modification: use this method while targeting a specific feature in the spectrum. Proxima Centauri b is believed to be tidally locked, meaning that the same hemisphere always faces the star. This means that at certain alignments, Proxima b will show a contrast of up to 100 ppm with respect to the star. This contrast will be helpful in separating planet signatures from the flux of the star. The method does not require absolute flux calibration, but only depends on the relative flux of the star and the planets. However, one limitation is that the stellar spectrum must be precisely known beforehand, since the flux of the planet, even at its highest, is much smaller than the stellar flux.

The authors used atmospheric models of Proxima Centauri b to show the limits of their detection method in extreme cases and expected results which are shown in Figure 2.

Figure 2: Planet model spectra, assuming a standard Earth atmospheric model for the temperatures and gas mixing ratios. The temperature and pressure relation is shown on the right column. The left column has the observed spectra given the temperature profiles. The upper panel has stratospheric temperatures that are equal to the tropopause temperature; the middle panel shows a model for clear sky conditions with an Earth-like temperature profile; the lower panel shows the spectrum for high, thick cloud coverage, again with an Earth-like temperature profile. [Snellen et al. 2017]

A Step Forward in the Search for Extraterrestrial Life

If we can detect exoplanet life or a habitable exoplanet so close to Earth, this could tell us more about how common life is in our universe and the origins of life in different environments. Until we find a way to accurately detect the presence of life, such as detecting CO2 in atmospheres, we won’t know the extent of life in all parts of the universe. Though we don’t know for certain yet, I for one hope that we can soon discover and say hello to our new alien neighbors, hopefully on Proxima b!

About the author, Mara Zimmerman:

Mara is working on her PhD in Astronomy at the University of Wyoming. She has done research with binary stars, including Heartbeat stars, and currently works on modeling debris disks.

IceCube

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 repost 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: Constraints on Galactic Neutrino Emission with Seven Years of IceCube Data
Author: The IceCube Collaboration
Status: Submitted to ApJ, open access

Back in 2013, the IceCube Collaboration published a paper announcing their discovery of astrophysical neutrinos, i.e. ones that have an origin outside our solar system (Astrobites coverage of the paper can be read here). Since this discovery, scientists have been busily working to develop theories as to the origin of these neutrinos. The original paper noted some clustering in the area of the center of our galaxy, but it was not statistically significant. Since then, both galactic and extragalactic origins have been proposed. Star-forming galaxies have been suggested as one possible origin, which Astrobites has covered papers arguing both for and against (here and here). Other theories involve radio galaxies, transients, and dark matter.

In today’s paper, the IceCube Collaboration has analyzed more of their data and set limits on the percentage of the diffuse neutrino flux that can come from galactic sources. Theoretically, some neutrinos should be created in the galactic plane: we know that this area emits gamma rays from pion decay, and neutrinos are created in the same types of interactions that create the gamma rays.

The collaboration used an unbinned maximum likelihood method as the main analysis technique in this paper. This is a standard technique used in astrophysics; it takes a model and finds the values of all the parameters of that model that give the best likelihood of getting the data that has been observed. (A second, separate technique was used as a cross-check). They used five different catalogs of galactic sources expected to emit neutrinos to determine where to search. Sources included pulsar wind nebulae and supernovae interacting with molecular clouds. The upper limits on the flux from our galaxy can be seen below.

Figure 1: Upper limits on the neutrino flux from our galaxy, assuming a three-flavor neutrino flux and a certain emission model  known as the KRA-gamma model. The red limits are from this paper (with the grey showing how the limits change if other emission models are used); the blue are from ANTARES, which is another neutrino experiment. For comparison, the measured overall neutrino flux is also shown (black data points and the yellow band). The green band is from the data, but only data from the northern sky is used. IceCube is more sensitive in the Northern hemisphere. [Figure 2 of the paper]

It turns out that, under these assumptions, galactic contributions can’t be more than 14% of the diffuse neutrino flux. However, the authors note that there are still scenarios where the flux could originate in/near the galaxy. This paper focused on emission in the galactic plane, but cosmic ray interactions in a gas halo far from the plane, and/or dark matter annihilation or decay would change the emission templates that were used here. They also mention that the limits could be made stronger by doing a joint analysis with ANTARES. Since IceCube and ANATARES are located in different hemispheres, they are most sensitive in different areas of the sky.  The mystery continues…

About the author, Kelly Malone:

I am a fourth year physics graduate student at Penn State University studying gamma-ray astrophysics. I previously received bachelor’s degrees in physics and astronomy from UMass Amherst in 2013.

Phobos

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 repost 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: On the Impact Origin of Phobos and Deimos I: Thermodynamic and Physical Aspects
Author: Ryuki Hyodo, Hidenori Genda, Sébastien Charnoz, and Pascal Rosenblatt
First Author’s Institution: Tokyo Institute of Technology, Japan
Status: Accepted to ApJ, open access

Where Did Phobos and Deimos Come From?

Phobos and Deimos, Mars’ two small moons, were initially believed to be the result of interplanetary kidnapping. Many moons in our solar system appear to be captured objects, and the featureless reflectance spectra of Phobos and Deimos hint that they might be D-type asteroids. However, captured objects tend to have highly eccentric orbits, and both Phobos and Deimos orbit Mars in a nearly circular fashion. More recently, it has been proposed that both moons are the result of a massive impact 4.3 billion years ago — instead of having been captured from interplanetary space, they could have coalesced from the debris disk generated by the impact. Past research has shown that the masses and orbits of Phobos and Deimos can be explained by this method. This theory could also explain the presence of Borealis basin, an extended low-altitude region spanning Mars’ north pole, which can be seen in Figure 1.

Figure 1. Topographical map of Mars. Borealis basin is the low-lying (blue) region in the northern hemisphere. It encompasses many officially-named regions, such as Vastitas Borealis and Utopia Planitia. [Adapted from this image, which is made from data from the Mars Orbiter Laser Altimeter aboard Mars Global Surveyor]

In this paper, the authors use smoothed particle hydrodynamics (SPH) simulations to learn more about the thermodynamical and structural properties of the debris disk generated in the proposed impact. SPH is a common method used when simulating astrophysical fluids, or systems with a large number of particles that can be treated as a fluid, like stars in colliding galaxies. In SPH (which has been detailed in previous Astrobites like this one and this one), the properties at a given location within a fluid are extracted by weighting the properties of the particles near that point using a smoothing function (sometimes called a smoothing kernel), which is often simply a Gaussian.

Building Baby Moons

To generate a debris disk, the authors modeled a collision between young Mars and an impactor with 3% the mass of Mars. The first 20 hours of their simulation are shown in Figure 2.

Figure 2. Snapshots from the first 20 hours after the simulated impact. Top row: Positions of particles over time. Red points are Mars particles, yellow are particles that fall on to Mars, white are disk particles, and cyan are particles that escape the system. Bottom row: Temperature of the particles. Shock heating in the moments after impact liquefies much of the material. [Hyodo et al. 2017]

The material ejected in the collision is so hot that it becomes molten, but quickly cools into roughly 1.5-meter solid droplets. The initially eccentric orbits of the disk particles precess over the next 30–40 years, resulting in a collisional torus of material around the planet. The collisions within this torus heat the material once more, again rendering it molten. Most of the material cools into 100-micron-sized droplets, but a small fraction of the silicates in the disk vaporize and condense into 0.1-micron-sized particles that can coat the larger particles. Although the solid-to-gas transition is inefficient, the process of gas-to-solid condensation generates the fine silicate particles that could be responsible for the observed, asteroid-like spectral properties.

Figure 3. The cumulative fraction of Mars-originating disk particles as a function of the depth below Mars’ surface from which they originated. Beyond 4 Mars radii (solid line), there is a higher percentage of particles originating from > 50 km below the surface than in the disk as a whole (dashed line). [Hyodo et al. 2017]

Another important finding from this work is that regardless of the angle of the impact, the disk contains material from both the impactor and young Mars. Figure 3 shows the distribution of disk particles as a function of how far below Mars’ surface they originated. Regardless of the impact angle, the disk as a whole contains at least 35% Martian material by mass. In the outer disk, beyond 4 Mars radii, this fraction rises to ~70%. What’s more, the Martian material largely comes from the mantle, about 50–150 km below the surface. This means that although the angle of impact and the radial distance at which the moons form will determine how much Mars material they contain, all formation scenarios lead to the moons being composed of a mixture of the impactor and Martian mantle. Although we currently have rovers scratching the surface, our best hope of learning about the material beneath Mars’ crust could be by studying its moons.

Know Before You Go: How Can This Help Future Mars Missions?

Now that we have some idea what to expect if Phobos and Deimos were formed from a debris disk, how can we use this knowledge? These results will be valuable for the planning of future Mars-system sample-return missions, like JAXA’s planned Martian Moon eXploration (MMX) mission, which is set to launch in the early 2020s. MMX is slated to make close observations of both Phobos and Deimos before collecting a sample from one of the moons and returning it to Earth. Performing simulations like these in advance of future sample return missions will help scientists interpret their findings to learn about the origin of these two moons as well as the interior of Mars itself.

About the author, Kerrin Hensley:

I am a third year graduate student at Boston University, where I study the upper atmospheres and ionospheres of Venus and Mars. I’m especially interested in how the ionospheres of these planets change as the Sun proceeds through its solar activity cycle and what this can tell us about the ionospheres of planets around other stars. Outside of grad school, you can find me rock climbing, drawing, or exploring Boston.

cloudy 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 repost 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 Cloudiness Index for Transiting Exoplanets Based on the Sodium and Potassium Lines: Tentative Evidence for Hotter Atmospheres Being Less Cloudy at Visible Wavelengths
Author: K. Heng
Author’s Institution: Center for Space and Habitability, University of Bern
Status: Published in ApJL, open access

Background

Most astronomers hate clouds, except for those who study them. Not only do clouds on the sky ruin observing nights, clouds on other planets obscure everything underneath, too. When observing extra-solar planets during transit, different atoms, molecules, and particles absorb light at different specific wavelengths, making the apparent radii of the planets change when viewed in different colors; this is known as transmission spectroscopy. Although it is a powerful tool to infer the atmospheric composition of exoplanets, these clouds often hinder our efforts to understand such alien worlds. With clouds present at high altitude, only the atmosphere above the clouds can be seen, and it is too thin to provide any spectral features (see this popular example). It turns out that a useful strategy for dealing with a problem is to avoid it. Valuable space telescope time could be saved if we could filter cloud-free objects from ground-based measurements. Today’s paper presents an index to quantify the degree of cloudiness for transiting planets.

Aim

The author first revisits how previous studies used the slope of transmission spectra caused by scattering (blue light is scattered more than red) to distinguish cloudy and dense atmospheres. Such dense atmospheres are more packed (with smaller scale height) and can produce flat spectra that are compatible with cloud-free hydrogen-dominated atmospheres. The author demonstrates that the scattering cross sections of gaseous molecules (e.g. hydrogen, nitrogen) and aerosols or condensates (cloud particles) have the same wavelength dependence. Measuring the spectral slope alone does not resolve the ambiguity between clouds and atmospheric composition. With this motivation, the author proceeds to find a cloudiness index that does not depend on the spectral slope.

Methods

Sodium has large cross sections so it can produce a prominent spectral line without being abundant. It is, in fact, the first extrasolar atmospheric detection on HD 209458b. In a clear, cloud free atmosphere, the difference in transit radii between the line center and wing of sodium can be theoretically calculated. By measuring the actual difference in transit radii between the line center and wing (Δ Robs), the author constructs a dimensionless index (C) for the degree of cloudiness as the ratio of Δ R and Δ Robs. For an entirely cloud-free atmosphere, Δ R equals to Δ Robs and C = 1. Very cloudy atmospheres have C Gt 1. This cloudiness index is independent of the spectral slope, with the caveat that it is limited to planets with sodium or potassium line detections.

 

Figure 1: Cloudiness index plotted against equilibrium temperature (top), surface gravity (middle), planetary mass (bottom). The labels “W6,” “W17,” “W31,” “W39,” “H1,” “H12,” and “HD189” refer to WASP-6b, WASP-17b, WASP-31b, WASP-39b, HAT-P-1b, HAT-P-12b, and HD 189733b, respectively [Heng 2016].

Application to Data

Figure 1 plots the cloudiness index (C) versus equilibrium temperature, gravity, and mass of the planets. The uncertainty of the cloudiness basically stems from estimating the scale height with the equilibrium temperature in the calculation, which is larger for colder planets. An interesting trend of decreasing C with increasing equilibrium temperature (a proxy for stellar flux) is seen in the top panel. If the trend is real, it implies that more irradiated planets tend to be less cloudy. Future measurements of sodium lines at higher resolutions and for a larger sample will confirm or debunk the trend. If the trend holds, the author suggests that we can weed out cloudy objects for the detailed survey of JWST, to avoid spending lots of time (and money) for uninformative, featureless spectra.

About the author, Shang-Min Tsai:

I am a 3rd year PhD student at the University of Bern and part of the Exoplanets & Exoclimes Group led by Prof. Kevin Heng. We are developing various open-source tools to study exoplanets. I work on modelling of atmospheric chemistry and dynamics. When I am not coding or debugging, I enjoy basketball and playing board games.

LUVOIR

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 repost 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 statistical comparative planetology approach to the hunt for habitable exoplanets and life beyond the Solar System
Authors: Jacob L. Bean, Dorian S. Abbot and Eliza M.-R. Kempton [see disclaimer below] First Author’s Institution: University of Chicago
Status: Published in ApJL, open access

The discovery of the first exoplanet around a main sequence star in 1995 brought a great deal of attention to the search for life beyond our solar system (i.e., The Big Question). Since then, we have found thousands of extrasolar planets. As humans, our first instinct is to comb over this huge sample to find the best candidates for habitability (such as TRAPPIST-1 and Proxima b), and then painstakingly characterize them to check if they really are habitable. But this process is very resource-expensive and it hasn’t really paid off thus far. What if there are other, more effective ways to answer The Big Question?

The Problem with the Systems Science Approach

People are already planning and designing space telescopes whose main scientific aim is the search for habitable environments on exoplanets, such as LUVOIR and HabEx. As such, these instruments carry the weight of a world upon them. According to the authors of today’s paper, we cannot simply bring multiple detailed characterizations to bear on a handful of promising candidates; in order to be able to effectively answer The Big Question, we need to adopt a statistical approach.

Determining if an exoplanet is habitable or not goes way beyond simply pinpointing its position relative to a star: it is based on a combination of empirical inference and theoretical modeling. The objective of what the authors call the systems science approach is to identify such planets and make claims about the possibility that they harbor life.

The problem is that this framework requires considerable resources (observational and computational) to yield robust answers. In the end, it may not even reveal a definitive signature of life in your small sample of habitable planet candidates!

Lessons from Kepler and Friends

Inspired by research performed with Kepler data, Jacob Bean and collaborators propose the statistical comparative planetology approach. Its premise lies in utilising the diversity of planets in large samples to obtain information about their nature based on comparative studies. This approach has successfully been applied to identify false positive rates in Kepler data, as well as to study the water content of transiting planets. These results are robust in that they only depend on simple physical models, and they allow the identification of outliers, which reveal potential model weaknesses.

One of the examples the authors describe in today’s paper aims to test the habitable zone concept using carbon dioxide abundances. Planets that receive more irradiation from their star need less CO2 to maintain accommodating temperatures via the greenhouse effect, while those that receive less irradiation need more carbon dioxide. The idea of the test is to measure CO2 content on planets inside habitability zones and see if they are compatible with the amount expected for planets with accommodating temperatures (see the figure below). These measurements don’t need to be extremely precise given that the uncertainties in the physical properties of the planets is offset by the large sample size.

 

The habitable zone concept (blue curve) assumes a decreasing amount of CO2 as stellar irradiation increases. The black points represent hypothetical planets scrambled away from the blue curve based on the expected uncertainties of a statistical inference. The habitable zone concept can be tested if we are able to perform this kind of bulk analysis of a large sample of planets.

A similar test can be performed for water content on potentially habitable planets: if they are closer to the inner edge of the habitable zone, they should have lost all their water due to a runaway greenhouse effect, while those near the outer edge of the zone should have little water because it has frozen out. Any deviation from this means we are getting habitability wrong. Again, this test does not require a lot of precision, but does need a large number of planets.

The authors make it clear that the statistical approach is not the single best way to study habitability. For instance, this method still requires detailed analysis of a few planets to identify key diagnostics for habitability, and it is limited to planets around M-dwarf stars, whose habitable zones are the easiest to observe. However, comparative planetology still looks like a very pragmatic way to circumvent the unknowns and progress towards answering The Big Question.


Disclaimer: the leading author of today’s paper, Jacob L. Bean, was my research supervisor in 2016, but I am not in any form involved with this particular publication.

About the author, Leonardo dos Santos:

Leo is a graduate student of Astronomy at University of São Paulo, Brazil. His current research consists of studying physical and chemical properties of stars similar to the Sun, hunting for exoplanets and developing astronomical software. Not to be confused with the constellation.

Illustration of a dark body in the distant outer reaches of the solar system.

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 repost 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: OSSOS VI. Striking Biases in the detection of large semimajor axis Trans-Neptunian Objects
Authors: Shankman C., Kavelaars J. J. et al.
First Author’s Institution: University of Victoria, Canada
Status: Accepted to AJ, open access

Dubbed one of the “Wildest Alien Planet Discoveries of 2016,” the prospect of Planet Nine has been truly exciting for all planet lovers alike. The line of evidence for this elusive planet lurking in the outer reaches of our solar system has been called into question, however, by the study discussed in today’s bite.


The Case for Planet Nine

The hypothesized Planet Nine is an icy and bigger analogue of Earth (about ten times as massive, and almost the size of Neptune or Uranus) on an orbit with a period of 15,000 years, and an aphelion (farthest approach from the Sun) ~1,200x as far away as the Earth from the Sun. This far-off region in which Planet Nine spends most of its time belongs to the Kuiper belt, and it is observed to be populated by mini planet-like icy bodies known as trans-Neptunian objects (TNOs) that have been left over as relics of the early formation of our solar system.

Figure 1. The closely spaced orbits of six of the most distant previously detected TNOs, as originally identified in 2016, points towards the existence of a ninth planet whose gravity affects these movements. (Source: via Caltech)

Since the early 2000s, researchers have noted that the long-period TNOs are not uniformly distributed in space. Instead, a number of these TNOs are found on orbits that are quite eccentric, with large semi-major axes, and they are clustered together in two distinct bundles relative to other TNOs. To explain this, scientists recently performed simulations and suggested the presence of an additional planet, a.k.a. Planet Nine — which, if it exists, could have perturbed the original orbits of these TNOs and clumped them together in space (see Fig. 1). With the discovery of more such TNOs and the understanding of their exact paths around the Sun, the idea of clustering — and hence of Planet Nine — can be tested.

So, Are There Any Clusters?

Over the last four years (2013-17), a large mapping survey known as the Outer Solar System Origins Survey (OSSOS) has used the Canada-France-Hawaii Telescope to look at eight different patches of the sky, taking a tremendous leap in the direction of finding more TNOs (see Fig. 2). During its tenure, the survey discovered more than 800 new TNOs, eight of which possess the long-period properties that could show evidence for or against the existence and effects of Planet Nine.

With the help of these eight “special” TNO discoveries, Cory Shankman (an OSSOS researcher) and his team try to shed some light on the matter by performing an independent test of the idea that a long-period massive planet is shaping the outer solar system. Cory and his team report, in today’s paper, that the eight OSSOS detections have orbits oriented across a wide range of angles, and are statistically consistent with those drawn from a random distribution. Additionally, one of the observations sits at right angles to the previously proposed clusters, thereby significantly reducing the odds that a tight spatial clustering in the orbits of long-period Kuiper-belt objects exists.

Figure 2. A top-down view of the Solar System depicting the schematic of OSSOS pointings (solid gray), orbits of four of the detected long-period TNOs (red, green, cyan, yellow), and Neptune’s orbit (blue); Figure 5 in the original paper.

As part of their analysis, the team also simulated the orbits of a uniform spread of tens of thousands of distant TNOs to test which bodies would be visible to their survey and found areas of the sky, like the dense star fields of our galaxy, where certain orientations of TNO orbits become very hard to detect. Based on their findings, the OSSOS team argued that detection biases resulting from a combination of atmospheric variations (affecting the depth of observation) and ease of discovery in sparse parts of the sky could have led to false indications of clustering in previously published studies, which in turn weakens the case for the existence of Planet Nine.

The Verdict

According to the team, the OSSOS survey was not designed to exclusively look for long-period TNOs, and hence their results cannot definitively prove, or rule out, the existence of Planet Nine, but they “substantially weaken the evidence that has been used to justify the need for an additional very massive planet in our solar system”. If the previous data was indeed unbiased, incorporating these new objects in the original simulations might lend more confidence to the hypothesis. However, the final word will perhaps come from a direct detection of more long-period TNOs through a dedicated survey, or of Planet Nine itself, if it exists, as scientists continue to work hard on pushing the boundaries of how far out we can look.

About the author, Bhawna Motwani:

Third year grad student doing computational research in stellar evolution and planet formation at Caltech. In my leisure time, I like to go Latin dancing, enjoy live theatre/music, and take on cultural and culinary pursuits.

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