Astrobites Archives - Page 26 of 44

Astrobites RSS

31 March 2020

Interstellar Travel with Sailing (Space) Ships

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: Propulsion of Spacecrafts to Relativistic Speeds Using Natural Astrophysical Sources
Authors: Manasvi Lingam and Abraham Loeb
First Author’s Institution: Florida Institute of Technology and Harvard University
Status: Accepted to ApJ

Travelling to distant stars is one of humanity’s long-term aspirations. However, interstellar travel is a challenging endeavour due to the vast distances between objects in the universe. For example, our closest stellar neighbour, Proxima Centauri, is over four light-years away. The journey there would take over 73,000 years with the Voyager 1 spacecraft — too long for a human crew. In general, travelling to other stars is only possible at relativistic speeds.

A critical factor limiting the velocity of traditional rockets is their need to carry fuel. Faster speeds require more fuel, but the weight of the fuel also slows the rocket down. Accordingly, more fuel is needed to accelerate! This dilemma, also called the tyranny of the rocket equation, implies that interstellar travel is difficult with conventional fuel-powered rockets.

The authors of today’s paper explore a different approach to space travel — sailing spaceships. Similar to sailing boats, these spaceships do not carry any fuel. Instead, they employ sails, which are accelerated by external forces, such as photon pressure and electrostatics. One project making use of such sails is Breakthrough Starshot, which intends to use a laser array to propel a spacecraft. In addition to laser arrays, astrophysical sources can also provide “wind” for the sails. Today’s paper investigates the dynamics of these sources and how they might thrust a spacecraft to relativistic speeds!

Sailing Ships in Space: How Do They Work?

A spacecraft can use one of two types of sails: light or electric sails. Light sails are large, thin sheets of reflective material. Photons bounce off this reflective material, thereby transferring momentum to the sail and speeding up the spaceship. You can view an artist’s impression of what a light sail might look like in Figure 1, above.

Electric sails work (and appear) quite differently from light sails. Instead of a large sheet, they consist of invisible electric fields. Figure 2 shows an artist’s impression of ESTCube-1, a 2013 experimental spacecraft to test electric sails. The space probe includes multiple electrically charged tethers, between which an electric field builds up. When charged particles arrive at this electric field, the field deflects them and captures a bit of their momentum.

Figure 2: Artist’s rendering of ESTCube-1, which was the first satellite launched with an electric sail in 2013. [Taavi Torim]

These effects might seem tiny, as every single photon or charged particle carries little momentum. Yet, if enough photons or particles bombard a sail, they can accelerate it to relativistic velocities! Accordingly, the sails require a luminous photon or particle source to accelerate to high speeds.

Which Astrophysical Sources Can We Use?

The authors studied the Sun, massive stars, supernovae (SNe), and active galactic nuclei (AGNs) as potential photon sources for light sails. The luminosity of these sources determines the maximal velocity a light sail can reach, the so-called ‘terminal velocity’ (see Figure 3). While the Sun is too faint to allow for relativistic velocities, light from massive stars can accelerate a spacecraft to several percent of the speed of light. With SNe and AGNs, even higher velocities are possible. These relativistic terminal velocities could enable interstellar space travel!

Figure 3: Luminosity (in units of solar luminosity, L_☉) of sources needed for spacecraft with light sails to reach a certain terminal velocity (in units of the speed of light c). Marked in red are the peak luminosities of possible sources: active galactic nuclei (AGNs), supernovae (SNe), massive stars and the Sun. With SNe and AGNs, relativistic velocities (10–70% of the speed of light) can be reached. [Adapted from Lingam & Loeb 2020]

However, accelerating a spacecraft to high terminal velocities takes some time. Therefore, the authors studied the acceleration time to get to 5%, 10%, and 20% of the speed of light, depending on the source luminosity (Figure 4). For sources dimmer than 10⁷ L_☉, it takes several thousand years to reach relativistic terminal velocities. Comparatively, brighter sources only require a few months to reach 10% of the speed of light. This acceleration time is problematic for SNe because their peak brightness usually lasts shorter. AGNs, though, operate on timescales of 10–100 million years, so an acceleration over a few months is no problem.

Figure 4: The time needed to reach a certain final velocity v_F for a light-sail-powered spacecraft, depending on the source luminosity. Marked in red are typical luminosities of massive stars, supernovae (SNe) and active galactic nuclei (AGNs). Terminal velocities of 10% of the speed of light and more can be reached with SNe and AGNs after a few months of acceleration. [Adapted from Lingam & Loeb 2020]

What About Electric Sails?

Electric sails might be even more promising than light sails. They need charged particles, so possible astrophysical sources are stellar winds, AGN outflows, blazar jets, or pulsar wind nebulae. Since these phenomena are quite complex, it is harder to give general predictions for electric sails than for light sails. Nonetheless, the authors found that a spaceship can reach highly relativistic velocities (up to 90% of the speed of light) with blazar jets and pulsar wind nebulae. So, electric sails could provide more efficient propulsion for interstellar sailing ships.

So When Will We Have Sailing Space Ships?

Sadly, it will be a while until interstellar space travel with sailing ships is a reality. Although the authors showed that light and electric sails are compelling propulsion techniques for interstellar journeys, two caveats remain. First, the paper neglects engineering limitations, such as the space probe’s stability during the trip. Interstellar spaceships have to endure more extreme conditions than spacecraft inside the solar system, so we need to overcome various technical hurdles before building them. For example, dust in the interstellar medium can severely damage a spacecraft traveling at relativistic speeds, and a potential crew also needs to be shielded from ionizing radiation. Second, constructing a completely new kind of spacecraft will be expensive! Currently, space agencies are mainly concerned with exploring our own solar system and satellites within Earth’s orbit. Missions to stars outside of our solar system will require additional funding.

However, the paper points out another exciting possibility — advanced extraterrestrial civilizations may have already implemented interstellar travel with sailing ships! We could find signs of them by detecting the reflections from their spaceship’s sails or by measuring their radio signals near sources. Light and electric sails might accelerate not only space travel but also our search for intelligent extraterrestrial life!

About the author, Laila Linke:

I am a second year PhD Student at the University of Bonn, where I am exploring the relationship between galaxies and dark matter using gravitational lensing. Previously, I also worked at Heidelberg University on detecting galaxy clusters and theoretically predicting their abundance. In my spare time I enjoy hiking, reading fantasy novels and spreading my love of physics and astronomy through scientific outreach!

24 March 2020

Good Luck Sorting This Hat

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 Strikingly Metal-rich Halo of the Sombrero Galaxy
Authors: Roger E. Cohen, et al.
First Author’s Institution: Space Telescope Science Institute
Status: Published in ApJ

Is This Galaxy an Elliptical (Gryffindor) or Ordinary Spiral (Slytherin)?

The Sombrero galaxy, famous for its hat-like shape, has been observed many times. However, it maintains a certain level of mystery: much like the sorting hat struggled to sort Harry Potter into a Hogwarts house, the Sombrero galaxy is difficult to sort into a galaxy classification. According to Hubble’s galaxy classification system, galaxies fit into four main categories: ellipticals, ordinary spirals, barred spirals, and irregulars. We have a fantastic edge-on view of the Sombrero galaxy, which allows us to image both its disk and hazy bulge, as seen in the cover image above. Because of its disk structure and lack of developed spiral arms, many astronomers classify the Sombrero galaxy as an early-type spiral. However, there is evidence that the size of the Sombrero’s halo (its extended sphere of stars) and its number of globular clusters are more similar to values found in elliptical galaxies. This leads us to believe that the Sombrero galaxy may have two parent components that merged: a spiral disk galaxy and an elliptical galaxy, and therefore simultaneously belongs to two different Hogwarts houses.

First-Years, This Way…

In order to determine how the Sombrero galaxy formed, the authors of today’s paper used Hubble Space Telescope (HST) images to analyze the halo of the galaxy. Their fields of view for the two images were 16 and 33 kiloparsecs (kpc) above the center of the galaxy, which is pretty far from the brightest component that we usually recognize as the Sombrero.

The goal of these images was to analyze the metallicity distribution function of the galaxy, or how much the metal content of stars changes as you move further away from the Sombrero’s center. Metallicity is measured by the quantity [Z/H], which takes the log of the ratio of metals (Z) to hydrogen (H) and compares it to what we see in the Sun. A metallicity value of 0 means that a star has the same metal content as the Sun. Values above zero are very metal-rich, and metallicity drops as you move towards negative values.

In general, galaxies with massive halos and a steeper fall-off in their metal content as you move away from the center tend to have fewer parent galaxies (galaxies that merged together to form a new baby galaxy). Therefore, a metallicity distribution function can tell us about the number of parent galaxies and the formation history of the Sombrero galaxy. We can also use a metallicity distribution function to help classify the Sombrero galaxy by comparing its peak metallicity to values found in other known elliptical and spiral disk galaxies.

Scarlet and Gold or Green and Silver?

Calculating the metallicity of stars requires photometry of the stars. Basically, we take an image of a star and count how many photons we receive in each wavelength band. In general, metal-poor stars appear bluer, and metal-rich stars appear redder. However, other effects can change their colors as well, such as the age of stars (bluer = newer!) and the dust along our line of sight. To remove these effects, today’s authors use a basic dust map to correct for the foreground dust, as well as assume a uniformly old age of 12 billion years. Once they do this, they can use models to fit the metallicity of the stars in their sample.

Figure 1: The metallicity distribution function for both fields of view (16 kpc in black, 33 kpc in red). Shaded gray areas and red error bars represent the respective uncertainties. A majority of the stars are metal-rich (have [Z/H] values near zero). [Cohen et al. 2020]

Not Slytherin, Not Slytherin, Not Slytherin

Today’s authors find that the metallicity within the Sombrero’s halo decreases as you move away from the center of the galaxy, but is dominated overall by metal-rich stars, as seen in Figure 1. Using their calculated peak metallicities, they compare the Sombrero galaxy to other galaxy metallicity measurements in Figure 2. They find that the halos of disk galaxies are more metal-poor on average (they have lower [Z/H] values), and that because of its high peak metallicity, the Sombrero galaxy fits better with the population of elliptical galaxies.

Figure 2: Comparison of the Sombrero galaxy (red pentagons) to other galaxies in peak metallicity and visible magnitude. Elliptical galaxies are on the left and disk galaxies are on the right. The dashed line on the left represents the best fit to elliptical galaxies, and the line in the right panel is the same but extrapolated to a different magnitude scale. [Cohen et al. 2020]

Using the HST images, today’s authors were also able to model the stellar mass of the Sombrero’s halo and compare it to the amount of mass the galaxy has accreted, or stolen from other galaxies. The number density of stars within the images allowed the authors to calculate a total halo mass with a few basic assumptions about the age of the galaxy. Separately, the authors used their metallicity values and a known correlation with accretion mass to calculate the Sombrero galaxy’s accretion mass. They found the accretion mass to be very similar to the total halo mass, which tells us that the Sombrero likely accreted its entire halo in a single major merger event several billion years ago!

In addition to this massive merger event, the Sombrero has other properties that defy the norm. Looking back to Figure 1, the Sombrero has a complete lack of low-metallicity stars, and it also has a higher average metallicity than any galaxy halo known to date! It is possible that a population of low-metallicity stars exists further out in the halo, but we would need images even more distant from the center of the galaxy in order to find them.

Just like Hogwarts houses, the galaxy classification system isn’t exactly black and white. However, unlike Harry Potter, we can’t just decide that we want the Sombrero to fit into one of our established categories. Despite looking very much like a disk from our edge-on view, the Sombrero galaxy possesses a host of characteristics that we associate with elliptical galaxies, likely because it formed as the result of a galaxy merger. As such, the Sombrero provides a unique overall picture of what galaxies may look like after they interact. The Sombrero galaxy clearly demonstrates the variety that exists in our universe and the contributions that instruments like HST make to our understanding of astronomy!

About the author, Ashley Piccone:

I am a second year PhD student at the University of Wyoming, where I use polarimetry and spectroscopy to study the magnetic field and dust around bowshock nebulae. I love science communication and finding new ways to introduce people to astronomy and physics. In addition to stargazing at the clear Wyoming skies, I also enjoy backpacking, hiking, running and skiing.

17 March 2020

An Iced Cosmic-Ray Macchiato

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: Bottom-up Acceleration of Ultra-High-Energy Cosmic Rays in the Jets of Active Galactic Nuclei
Authors: Rostom Mbarek and Damiano Caprioli
First Author’s Institution: University of Chicago
Status: Published in ApJ

Our universe is littered with particles of unbelievably high energy, called cosmic rays. The most extreme of these particles carry the same amount of energy as a professional tennis serve, like the Oh-My-God Particle detected nearly 30 years ago. The catch: we don’t know exactly what processes can pack so much energy into a single particle. The authors of today’s article discuss how these particles might gain their energy in a way analogous to your morning trip to Dunkin’™.

Cosmic Rays at a Glance

Cosmic rays are atomic nuclei that have been accelerated to high energies in astrophysical environments, such as supernova remnants or active galactic nuclei. Although they might seem like a great tool in the multi-messenger astronomy toolbox, astronomy with cosmic rays is no simple task, as these particles get deflected by extragalactic magnetic fields.

Cosmic rays (red) consist of individual protons and nuclei of heavier elements. They are deflected by magnetic fields along their cosmological odysseys and can’t be used to point back to the place of their origin. [IceCube Neutrino Observatory]

Despite efforts to pinpoint the origins of cosmic rays, especially those of the highest energies, we’ve come up empty-handed (check out these bites for previous studies: Galactic cosmic rays, cosmic-ray anisotropy).

Even though we can’t measure where they come from, we do know their energies, and a variety of cosmic-ray experiments detect millions of these particles every year. Many of them are thousands to millions of times more energetic than the particles in the largest terrestrial particle accelerator, the Large Hadron Collider, but we don’t know how the highest energy cosmic rays get their energy.

Cosmic-Ray Acceleration: Old News

Many theories of cosmic-ray acceleration tend to revolve around the idea of Fermi acceleration. In this scenario, objects such as supernova remnants can create shocks, consisting of material moving together with supersonic speeds, and these shocks can accelerate particles to high energies. As a shock wave propagates, particles bounce back and forth across the shock boundary. Over time, successive bounces across the shock front lead to a net transfer of energy to the particles.

While Fermi acceleration does a good job of explaining cosmic rays with moderate energies and has been a staple of models for decades, it has a few pitfalls, and many argue that it can’t provide the whole story for cosmic-ray acceleration at the highest energies.

A Cosmic Cup o’ Joe

The authors of today’s paper propose a new way of looking at cosmic-ray acceleration: the espresso mechanism. Why espresso? Because instead of gradually gaining energy over time, particles gain their energy from a single shot.

In the “espresso mechanism”, particles gain tremendous amounts of energy from entering a jet for a short period of time. Here, a particle with initial momentum and energy p_i, E_i enters a jet with characteristic Lorentz factor Γ and leaves the jet with an energy equal to roughly Γ²E_i. [Caprioli et al. 2018]

Consider an object with a jet, such as an active galaxy. If a low-energy cosmic ray enters the jet (or steam), then it can be shot down the barrel of the jet and get kicked out at much higher energy. In many cases, particle energies can increase by a factor Γ², where Γ is the Lorentz factor (this reflects how fast the jet is moving). For some jets, this means particles can exit nearly 1,000 times as energetic as they were when they entered the jet.

In realistically modeled jets, material tends to clump in some regions, and these regions of overdensity (color scale in figure) cause the jet to locally move faster or slower. [Mbarek & Caprioli 2019]

While this espresso scheme sounds great in principle, many previous calculations have relied on spherical cow treatments of jets, when in reality they are remarkably dynamic and complex structures.

That’s where the authors of today’s paper come into play. These authors take a simple treatment of the espresso mechanism and complexify it by performing a full magnetohydrodynamic (MHD) simulation of ultrarelativistic jets. This takes factors like small-scale fluctuations of jet speed and jet density into account, to give a more accurate picture of the dynamics of jets.

By simulating the full structure of jets, the authors find that complex environments don’t weaken the promises of espresso acceleration. In fact, the very imperfections that manifest in realistic jets can help with particle acceleration. What’s more, jet perturbations allow particles to receive double or even triple shots of energy.

Throughout the paper, the authors describe the emergent spectra of espresso-accelerated cosmic rays. In doing this, they find that espresso acceleration is consistent with current measurements of ultra-high-energy cosmic rays in terms of energy, chemical composition, and spatial distributions, an accomplishment which no other model of cosmic-ray acceleration can boast.

Sample particle trajectories (black curves) are overlaid on top of slices of the jet, with jet velocity represented by the color in the top panels. Bottom panels show the amount of energy gained along the particle paths, showing that particles can leave jets with much more energy than they entered with. [Mbarek & Caprioli 2019]

In light of all of this, it’s probably safe to say that the future of cosmic-ray science will be very caffeinated.

About the author, Alex Pizzuto:

Alex is a PhD candidate at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. His work focuses on developing methods to locate the Universe’s most extreme cosmic accelerators by searching for the neutrinos that come from them. Alex is also passionate about local science outreach events in Madison, and enjoys hiking, cooking, and playing music when he is not debugging his code.

10 March 2020

A New Approach to Tilting Uranus

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: Tilting Ice Giants with a Spin-Orbit Resonance
Authors: Zeeve Rogoszinski and Douglas Hamilton
First Author’s Institution: University of Maryland
Status: Published in ApJ

Songs about the solar system were a significant part of my childhood. These songs taught one fact about each of the nine planets (these were the pre-Pluto’s-demotion days) and yes, I find myself singing them in the shower every now and then. The fact about Uranus is always the same: “Uranus spins on its side.” Not only is this bizarre and memorable to a child, it happens to be true. Uranus has an obliquity (tilt) of 98º, making its axis of rotation closer to the ecliptic plane than any other planet. And yet, nobody knows how it got that way.

Problems with the Current Theory

Uranus has an obliquity of 98°. [NASA]

The conventional wisdom for many years has been that one or more giant impacts must have turned Uranus onto its side when it was very young and giant impacts were common. The authors of today’s paper outline four potential problems with this theory.

If Uranus were impacted many times but Neptune were not, one would expect their rotation rates to differ significantly. The reason is that some of the impacts might have sped up or slowed down Uranus’ rotation. Instead, a day on Uranus and Neptune only differs by 6% (17.2 hours vs. 16.2 hours, respectively).
Giant impacts would disrupt the satellites orbiting Uranus. If this were true, the authors argue, the total mass of Uranus’ moons should be lower than we observe; instead, Uranus has the “appropriate” amount of moon-mass.
It’s extremely hard to design one impactor large enough to tilt Uranus. That doesn’t mean multiple impacts are out of the question, but it is a challenging scenario to model.
Giant impacts would have heated Uranus so much that a lot of the interior ice would have sublimated to gas and been ejected into space. If this occurred, Uranus’ satellites would be mostly ice, but they are actually mostly rock and only a little ice.

The authors propose a different way to tilt Uranus: spin–orbit resonance caused by a massive circumplanetary disk. To determine if this is possible, they simulated a young Uranus and Neptune evolving, each with a large orbiting disk of dust and gas.

What Is Spin–Orbit Resonance?

Figure 1. Animation showing precession of the Earth’s spin axis. [Robert Simmon/NASA]

Resonance is when two periods are integer multiples of each other. Think of pushing a kid on a swing. If you push with the right frequency, the swing will go high; if you push at other times, it won’t be as much fun. An example of orbital resonance is the system of Neptune and Pluto. Pluto orbits the Sun twice for every three times Neptune orbits, so Pluto’s orbit is reasonably stable. Spin–orbit resonance is a type of secular resonance, meaning the precession of Uranus’ spin axis resonates with Uranus’ orbit. Orbital precession means that the orbit changes orientation over time, causing the pole to point in a different direction, as shown in the animation in Figure 1.

Normally, precession happens way too slowly to resonate with orbits. For instance, Earth takes 26,000 years to precess once, but its orbital period is only one year. In this paper, the authors impart Uranus with spin–orbit resonance by increasing its precession rate — a consequence of having a circumplanetary disk.

Figure 2. The first model with a constant disk (bottom) showing changing obliquity (top) over one million years. Obliquity changes are sensitive to the exact lifetime of the disk. The middle plot shows how close the system is to perfect resonance (dashed line). [Rogoszinski & Hamilton 2020]

Growing a Tilted Ice Giant

As the ice giants formed, they each had a circumplanetary disk. Disks are relatively short-lived and only hang around for about 1 million years before the material either falls into the planet or forms a moon. That means Uranus only has one million years to turn on its side before the disk dissipates and the tilt is permanent.

In order to determine if different types of disks are capable of tilting Uranus or Neptune, the authors built computer models that capture the physical interactions of the disk and the planet. Three of the most relevant models are summarized here.

Model 1 is the simplest. It has a disk with constant mass for 1 million years before dissipating instantly. The result is shown in Figure 2, where the obliquity (top panel) can be seen changing from 0º up to about 65º and then back to 0º. The authors find that a constant disk could cause Uranus to tilt, but it could just as easily undo the tilt, making it too unpredictable.

Figure 3. The results of the second model, in which the disk slowly dissipates over one million years (bottom). Uranus’ obliquity increases and then settles around 45º (top). [Rogoszinski & Hamilton 2020]

Model 2 contains a more realistic disk that slowly dissipates over one million years, as shown in Figure 3 (bottom panel). The obliquity (top panel) reaches a maximum of about 55º and then settles to about 45º. A dissipating disk does a better job of keeping Uranus tilted, but 45º is not even half of the tilt seen today.

Model 3 is the same as model 2, but Uranus grows in mass as material from the disk falls onto it, shown in Figure 4. Here the obliquity (top panel) reaches a maximum of about 70º and settles to 60º once the disk is gone. This model gets closest to the current value of 98º, but is still not tilty enough.

One Last Tilt

Figure 4. The result of the final model, which contains a slowly dissipating disk (second from bottom) and material falling onto Uranus. After one million years, the obliquity of Uranus settles at around 60º (top). [Rogoszinski & Hamilton 2020]

The authors find that model 3 is the most realistic because it accurately captures orbital physics while plausibly tilting Uranus up to a maximum of 70º. They conclude that this supports the hypothesis that spin-orbit resonance could be responsible for Uranus’ high obliquity. They add that it would still take a massive impact, most likely, to push Uranus from 70º up to 98º, but a single impact is significantly more likely than a series of giant impacts.

The authors created many more spin-orbit models than shown here in their attempt to explain the evolution of Uranus and Neptune. As encouraging as it is that some of these models reproduce observed effects, understanding a process that occurred nearly five billion years ago, with only extremely limited observational data, will always be challenging.

About the author, Will Saunders:

I am a second year Ph.D. student at Boston University, where I study planetary atmospheres. I work with Prof. Paul Withers at BU and Dr. Mike Person at MIT using stellar occultations to measure waves and climatic changes in the atmosphere of Mars. I received my Bachelors in Physics from the University of Pennsylvania. I am also excited about co-hosting the new podcast astro[sound]bites. Check us out on Apple Podcasts, Google Play, Soundcloud, and Spotify. In my free time I enjoy traveling, visiting museums, and tasting new wines.

3 March 2020

You Were Cool, Betelgeuse

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: Betelgeuse Just Isn’t That Cool: Effective Temperature Alone Cannot Explain the Recent Dimming of Betelgeuse
Authors: Emily M. Levesque and Philip Massey
First Author’s Institution: University of Washington, Seattle
Status: Accepted to ApJL

Betelgeuse, shown here in an infrared image from the Herschel Space Observatory, is a luminous red supergiant star located only 650 light-years away from the Sun. [ESA/Herschel/PACS/L. Decin et al.]

Veteran star Betelgeuse has been getting a lot of attention in recent months. This is because this nearby red supergiant (RSG) has been looking fainter than usual. Given that Betelgeuse is (typically) the brightest RSG in the night sky, professional and amateur astronomers alike have been able to track changes in its brightness over time, plotting its light curve.

The brightness of a star is often measured in specific wavelength bands because starlight is not monochromatic. The V band is defined at a wavelength of ~ 550 nm, which closely corresponds to wavelengths seen by the human eye.

On December 7th 2019, a team of astronomers observed that Betelgeuse had a brightness of V = 1.12 magnitude, compared to its typical value of V ~ 0.2–0.3 magnitude. This registered its faintest value in 50+ years (for historical reasons, magnitude increases as brightness decreases). Over time, the decrease in brightness continued, reaching its lowest value so far on January 30th (V = 1.61 magnitude). Around this time, it was also suggested that Betelgeuse’s dimming was slowing down.

Starry Eyed or Starry Died?

Many in the scientific community and general public have interpreted Betelgeuse’s erratic behaviour as an indication of an imminent supernova, which would be visible with the naked eye and likely to last for many days. While this is an exciting interpretation, it is also the least likely explanation. It is instead argued by many that the dimming could be due to the composition of Betelgeuse itself. Specifically, variations on the surface of Betelgeuse could lower its apparent temperature temporarily, which, according to laws of blackbody emission, would push some of its emitted light into longer wavelengths that wouldn’t be observed in the V band.

Keep Your Cool

The authors of today’s paper investigate how cool Betelgeuse actually is, and whether it is enough to explain its dimming in the night sky. They start by performing optical spectrophotometry on the RSG on February 15th 2020 using the spectrograph on the 4.3-meter Lowell Discovery Telescope in Arizona. The term “spectrophotometry” simply means measuring the spectrum of the star, where its flux is scaled according to its wavelength.

Artist’s illustration of one of the most massive star clusters within the Milky Way. The center of the cluster contains 14 red supergiant stars. [NASA, ESA and A. Schaller (for STScI)]

The authors estimate the apparent temperature of a sample of 74 galactic RSGs including Betelgeuse. They use a well-known method of measuring the strength of absorption lines due to titanium-oxide (TiO) in the star and comparing them to predicted values from stellar atmosphere models.

They compare the February 2020 Betelgeuse spectrum to a previous observation taken in March 2004. During this time, Betelgeuse had V ~ 0.5 magnitude, roughly 1.1 magnitude brighter than its brightness on 2020 Feb 15. Its corresponding apparent temperature was 3,650 K in 2004, compared with 3,600 K in 2020. The authors note that while the apparent magnitude (or flux) in 2020 is considerably lower than 2004, the overall shape of the spectrum is similar (Figure 1). The appearance of stronger TiO bands correlates with a lower apparent temperature. They go on to compare the temperature of Betelgeuse with other known RSGs in their sample, concluding that Betelgeuse is slightly cooler in 2020 compared to 2004. It is important to note that these measured temperatures each have a margin of error of ~25 K, which can exaggerate or diminish the actual temperature difference considerably.

The authors find that the temperature of Betelgeuse has not decreased proportionally to its dimming. If surface convection effects were responsible, a difference of larger than 50 K would be expected. This means a temporary “cold” period on the surface of Betelgeuse is likely not the primary cause of Betelgeuse’s loss of brightness, and some other effect must be at play.

Dust in Time

The authors also measure the amount of dust content along the line of sight to Betelgeuse, as this poses another possible explanation for its dimming. Circumstellar dust created from mass loss in Betelgeuse could potentially cause it to obscure its own light and therefore appear fainter. They find that there is no apparent change in the amount of dust between 2004 and 2020, but they nevertheless acknowledge that this conclusion assumes a particular model for dust absorption.

Observations have instead demonstrated that dust produced by RSG mass loss has a much larger grain size than expected by the typical dust model. Circumstellar dust composed of larger grains would produce an extinction effect that is more “grey”, absorbing light across the optical spectrum rather than preferentially absorbing bluer wavelengths. This large-grain dust could therefore cause dimming consistent with the change in V-band magnitude of Betelgeuse.

As of February 22nd, it was noted in this Astronomer’s Telegram that Betelgeuse has officially stopped dimming and has, in fact, begun to gradually brighten again. Various multi-wavelength observations, particularly in the ultraviolet and infrared range, are needed to shed some light on the stellar processes taking place in Betelgeuse which might be responsible for this exciting period of activity, while astronomers will keep their eyes peeled for any future episodes like this. Even though Betelgeuse might not be in the spotlight anymore, I will go on the record to say I always thought it was cool.

About the author, Sunayana Bhargava:

I’m a 3rd year PhD student at the University of Sussex, looking at X-ray observations of galaxy clusters to learn more about dark matter and large-scale structure.

25 February 2020

Compton-Thick or Thin? Classifying NGC 5347

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 Hard Look at NGC 5347: Revealing a Nearby Compton-Thick AGN
Authors: E. S. Kammoun et al.
First Author’s Institution: University of Michigan
Status: Published in ApJ

Black holes are some of the most interesting and extreme objects in the universe. Fortunately, we think that almost every galaxy in the universe has a supermassive black hole (SMBH) at its center, giving us many opportunities to study their environments. As matter falls towards a black hole, it forms an accretion disk — a flattened disk of gas and other debris — outside of its event horizon. This accretion disk is hot and emits radiation, even though we can’t see any light from the black hole itself. When SMBHs at the centers (or nuclei) of galaxies are actively accreting mass and emitting a huge amount of energy, we call them active galactic nuclei (AGN). The different structural components of AGN, shown in Figure 1, emit radiation across a wide range of wavelengths, from low-energy radio to high-energy X-rays and gamma-rays.

Figure 1: The structural components of an AGN. Matter orbiting the black hole forms an accretion disk. There is also a torus, a donut-shaped cloud of neutral gas and dust, that could obscure the light emitted by the disk. [Aurore Simonnet, Sonoma State University]

Compton-Thick AGN

The X-ray emission in AGN comes from a hot atmosphere of gas called the corona, which surrounds the accretion disk. In the corona, ultraviolet (UV) photons get scattered by really fast electrons, gaining enough energy to become X-ray photons (this is the inverse of Compton scattering). As these X-ray photons leave the corona, they run the risk of being absorbed by the surrounding torus of neutral hydrogen and dust. If there is enough neutral hydrogen (at least 1.5 x 10²⁴ hydrogen atoms per cm² to be exact), most of the X-rays are absorbed and we call the gas “Compton-thick”.

In order to reproduce the observed cosmic X-ray background, we expect that 10–25% of AGN should be Compton-thick (CT AGN). However, actual observations of AGN so far have estimated the fraction of CT AGN to be less than 10%. Are there actually missing CT AGN, or are some AGN misclassified?

NGC 5347 is an AGN that has been classified as both Compton-thick and Compton-thin by different methods, with an estimated hydrogen content differing by a factor of 10 between measurements. Today’s paper re-opens this question by analyzing new high-sensitivity observations of NGC 5347, as well as incorporating more physical models. This investigation could solidify the classification of NGC 5347, as well as help explain the “missing” fraction of CT AGN.

You Have to Look Harder

X-rays can be divided into two classes depending on their energy; lower energy X-rays are called soft, while higher energy X-rays are called hard. Hard X-rays are generally considered to have energies greater than 10 keV. In CT AGN, while most of the soft X-rays get absorbed, the hard X-rays can escape and reach telescopes!

The authors use observations from three different X-ray telescopes: previously available data from the Chandra X-ray Observatory and Suzaku, as well as new observations from NuSTAR. The new observations are particularly helpful, since NuSTAR is highly sensitive to the hard X-rays that actually make it out of the CT AGN.

Figure 2: X-ray spectra for NGC 5347. The y-axis shows the number of observed photons per time, per area, per energy. The x-axis corresponds to the energy of the X-ray photons. Data from Chandra and Suzaku are shown as red and black crosses, respectively. Data from NuSTAR, shown in pink and blue, make up the hard part of the spectrum. The best-fit model is shown in gray, and can be separated into the different components labeled in the legend. These different components represent different emission and absorption processes. [Kammoun et al. 2019]

To determine the properties of the X-ray emission, the authors fit various models to the observed spectrum, shown in Figure 2 above. Fitting a simple power-law to the soft part of the spectrum finds evidence of strong absorption, but more complicated models are necessary to fully understand what’s going on. Their best-fit model, using the software MYTorus (MYTD), indicates that most of the observed X-ray emission is absorbed and reprocessed by the torus. They infer a neutral hydrogen content of 2.23 x 10²⁴ atoms per cm², leading them to classify NGC 5347 as a CT AGN.

Future Missions

One way for astronomers to classify sources as Compton-thick would be to measure the strength of iron emission lines such as Fe Kα, but current X-ray observatories are not able to resolve this emission line, at least not for NGC 5347. Simulating the spectrum of NGC 5347 as if it were observed by Athena — a future X-ray observatory — suggests that a clear Fe Kα could be resolved (Figure 3), making the classification of NGC 5347 easier.

Figure 3: Simulated X-ray spectrum for NGC 5347. Counts in the y-axis represent the number of photons, and the x-axis corresponds to the energy of the X-ray photons. The smaller squares are zoomed in to the ∼ 6 keV region of the spectrum, to show the Fe Kα emission lines. [Adapted from Kammoun et al. 2019]

The previous classification of NGC 5347 as Compton-thin could mean that other CT AGN have not been correctly classified due to a lack of high quality observations. NuStar and future X-ray missions like XRISM and Athena could provide the higher quality X-ray spectra necessary to identify CT AGN.

About the author, Gloria Fonseca Alvarez:

I’m a third year graduate student at the University of Connecticut. My current research focuses on the inner environments of supermassive black holes.

18 February 2020

The TESS Mission’s First Earth-Like Planet Found in an Interesting Trio

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 Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System
Authors: Emily A. Gilbert, Thomas Barclay, Joshua E. Schlieder, et al.
First Author’s Institution: University of Chicago
Status: Submitted to AJ

Artist’s impression of TESS observing planets orbiting a dwarf star. [NASA Goddard SFC]

Since the discovery of the first planet outside of our solar system in 1992, the field of exoplanets has been booming with interesting finds. From the diamond planet orbiting a neutron star to the giant pink planet orbiting a star in the constellation of Virgo, telescopes all over the world have been racing to find the latest gem. Of particular interest are Earth-like planets. A team led by a graduate student at the University of Chicago report the first Earth-sized planet in the habitable zone found by the TESS mission, and its surroundings were quite a surprise to astronomers.

Searching for Planets in All the Right Places

Some of the biggest questions we humans like to ask are, “Is there life out there in the universe?” and “Are there other solar systems out there with planets just like ours?” To answer these questions, astronomers have built larger and more advanced telescopes to try to find planets outside of our own neighborhood, specifically those similar to our own world. The Kepler mission was launched in 2009 specifically to search for these kinds of planets: Earth-sized planets in Earth-like orbits around Sun-like stars in order to study how common they are in the universe. The mission has made many amazing discoveries, such as an exoplanet with the density of Earth, a planet in a binary star system, and the first Earth-sized planet in the habitable zone of its star that orbits around an M-dwarf star that is about half the size of the Sun. Kepler’s extended mission, K2, focuses on low-mass stars and has led to the discovery of hundreds of small planets, some even in the habitable zone of their stars. Together, Kepler and the K2 mission have found over 3,000 new exoplanets.

Exoplanets are very small and very far away, so it is very difficult to find them. Astronomers use four methods: the transit method (which looks at how much a star dims as a planet goes in front of it, or eclipses it), the wobble method (which looks at how a planet and a star move around a common center of mass), direct imaging (which means taking a picture of the planet, straightforward but very difficult and limiting), and microlensing (which happens when light from a distant star bends around a star/planet system). TESS, or the Transiting Exoplanet Survey Satellite, was launched in 2018 and was designed to search for small planets around the Sun’s nearest neighbors using the transit method. In today’s paper, we discuss the first results of Earth-sized planets found in the habitable zone of an M-dwarf star, planets contained in an odd planetary trio.

The Host Star

The light curves observed by TESS. The pink line represents how much the star was expected to be dimmed by the eclipsing planet and the blue is the actual data. [Gilbert et al. 2020]

Understanding the properties of the host star is key in determining the habitability of the planets around it. M-dwarf stars are smaller and dimmer than the Sun but are much more common in the universe. The star that the team found the planets around is called TOI 700. To determine its fundamental properties such as mass and temperature, the team used three different methods in order to validate their results. After using known relations, checking their spectral energy distributions against known spectra, and using spectroscopy, they concluded that the star has an effective temperature of 3,480 K (which is about ⅔ of the Sun’s temperature) and a mass and radius that is about half that of our Sun. They found no flares from the star in the observations over five years, which points to a low amount of magnetic activity, making the system more likely to be habitable.

Goldilocks (Zone) and the Three Planets

By analyzing how much the star dimmed as the planets went around it, the team determined that it hosts three planets. From the inner to the outer planet (respectively called TOI 700 b, TOI 700 c, and TOI 700 d), they have radii of 1.01 ± 0.09, 2.63 ± 0.4, and 1.19 ± 0.11 Earth radii. Figure 1 shows how much the planets dim the light of the star. TOI 700 b and d are likely Earth-sized while TOI 700 c is a sub-Neptunian-type planet. TOI 700 d receives about 90% of the energy that the Earth does from the Sun, which places it in the habitable zone of the star. After finding these planets, the team performed tests with many different software packages in order to verify their discoveries. Each of the three planets passed these tests with a false-alarm probability (the probability that the signal is due to something else like instrument noise) of less than 1%. The masses of the planets were determined to be ~1.07 Earth masses, ~7.48 Earth masses, and ~1.72 Earth masses for planets b, c, and d respectively.

So How Does One Make a Neptune Sandwich…?

The fact that the largest planet is in the middle of this system is a bit puzzling. Usually planets in a given system have similar sizes — and in the case of our solar system, the inner planets are small and rocky, while the outer planets are larger and gaseous. In this system, the low-density gas planet is sandwiched between the higher-density rocky planets with similar masses. Figure 2 shows the planet in comparison to other systems. The team postulates that this could have come from the two inner planets forming faster and accreting significant gaseous envelopes and the outer one forming more slowly and accreting less gas, then the innermost planet loses its envelope somehow. It is also possible that the middle planet formed outside the outermost planet but migrated inward somehow, but how this could happen isn’t clear. This strange system may be difficult to explain, but it provides a rich laboratory for exploring the formation mechanisms of complex multi-planet systems.

Figure 2: The TOI 700 planets compared to other known systems. The bottom axis shows flux (or energy received by the planets) compared to Earth’s and the top axis shows distance. [Gilbert et al. 2020]

Thirty years ago, we did not even know planets could exist around other stars. Now, we know of thousands — and some of those planets are possibly habitable. New exoplanet discoveries like this one are shaking up the field of planetary formation and causing us to rethink our ideas of what stars could host planets and how planets form. As time goes on, new telescopes like James Webb Space Telescope will come online and further expand our understanding of exoplanets. Who knows what kind of weird extrasolar planets we will find next!

About the author, Haley Wahl:

I’m a third year grad student at West Virginia University and my main research area is pulsars. I’m currently working with the NANOGrav collaboration (a collaboration which is part of a worldwide effort to detect gravitational waves with pulsars) on polarization calibration. In my set of 45 millisecond pulsars, I’m looking at how the rotation measure (how much the light from the star is rotated by the interstellar medium on its way to us) changes over time, which can tell us about the variation of the galactic magnetic field. I’m mainly interested in pulsar emission and the weird things we see pulsars do! In addition to doing research, I’m also a huge fan of running, baking, reading, watching movies, and I LOVE dogs!

11 February 2020

TESS Reveals HD 118203 b Transits After 13 Years

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: TESS Reveals HD 118203 b to be a Transiting Planet
Authors: Joshua Pepper, Stephen R. Kane, Joseph E. Rodriguez et al
First Author’s Institution: Lehigh University
Status: Submitted to AJ

There are multiple ways to discover an exoplanet. The first exoplanet around a solar-type star was discovered by radial velocity measurements and earned the discoverers this year’s Nobel Prize. After the advent of wide-field exoplanet surveys, from SuperWASP starting in 2006 to NGTS and TESS, most exoplanets have been discovered using the transit method and confirmed soon after by radial-velocity studies. However, the exoplanet in today’s article, HD118203 b, was detected by radial velocity back in 2006 and has only now been found to transit, 13 years after its discovery.

Radial-Velocity Discovery

HD118203 b was found in 2006 by using the radial-velocity technique: measuring the amount the star’s spectrum ‘wobbles’ as the star is tugged by its orbiting planet. Over one orbit of the planet, spectra are redshifted as the planet tugs its star away from us and blueshifted as the star is tugged towards us. Radial-velocity measurements give us the planet’s orbital period as well as its eccentricity and minimum mass; the true planet mass depends on the relative inclination between star and planet. 43 radial velocity measurements from ELODIE revealed HD118203 b as an eccentric planet with an orbital period of ~6.13 days and a minimum mass of about 2 Jupiters (see Figure 1).

radial-velocity measurements of HD 118203

While most orbit orientations will produce radial-velocity signatures, only a small percentage happen to line up so that we can see the planet pass in front of its star, or transit. Transiting exoplanets block out a small fraction of light, giving us the relative radii of planet and star. If a planet transits, this constrains the planet’s inclination and means that the minimum mass from radial velocity is very close to the true mass.

A number of exoplanets discovered using radial velocities have since been found to transit — but this confirmation process is time consuming for two reasons. First, only a small fraction of exoplanets will actually transit. Second, transits only last a very small fraction of the orbit (usually a couple of hours for an orbit of less than 10 days), so telescopes must stare at a star for long periods of time to discover when the transit actually occurs.

TESS Spies a Transit (or Five)

Transits of HD 118203 b were discovered thanks to the ongoing TESS mission. TESS is a space mission that will observe most of the sky, staring at each sector for 28 days looking for transiting exoplanets. Five transits of HD 118203 b were automatically identified using the Science Processing Operations Center (SPOC; see Figure 2), and following vetting to check for false positives, it was identified as a promising candidate.

Figure 2: TESS photometry of HD 118203 b. The top plot shows the light curve as processed by SPOC, and the bottom plot shows the flattened light curve as used in EXOFASTv2. [Pepper et al. 2020]

The authors use the exoplanet fitting suite, EXOFASTv2, to fit the planet parameters — but first they need to constrain the stellar parameters. They run a preliminary fit to estimate the star’s surface gravity (log(g)), finding that the star is likely a subgiant. A spectral energy distribution (SED) model uses broadband photometry (i.e. the stellar magnitudes measured in different filters) to find the stellar temperature; the authors show that it’s very similar to the Sun’s, but the star’s radius is twice as large. The authors run a full analysis using these stellar parameters as constraints to simultaneously model the ELODIE radial velocities, the TESS photometry, and the stellar parameters using stellar evolution models.

EXOFASTv2 produces two sets of solutions that are consistent with the data: an older (5 Gyr), less massive (1.3 M_☉) star or a younger (3 Gyr) more massive (1.5 M_☉) star. The authors adopt the older, less massive star solution, as the model gives it a much higher probability (89.6% vs 10.4%). Their results are also consistent with two other codes tested.

HD 118203 b is an interesting target because it is one of few transiting exoplanets in an eccentric orbit with a bright host star (the 13^th brightest of all transiting exoplanets). Figure 3 shows all transiting exoplanets with eccentricities greater than 0.05 and places HD 118203 among the brightest host stars. The combination of a relatively short orbital period, a bright host star and an eccentric orbit makes it a good candidate for phase curve observations. Infrared phase curve observations by future space mission JWST could provide insight into the thermal properties of the planet’s atmosphere.

transiting exoplanets in eccentric orbits

Figure 3: All known transiting exoplanets with eccentricity greater than 0.05. HD 118203 b is near the top left of data. [Pepper et al. 2020]

How Many More Might TESS Find?

HD 118203 b is one exoplanet discovered by radial velocity that TESS has observed to transit, but as TESS is observing most of the sky, how many more can we expect? A paper earlier this year, led by one of the coauthors on today’s paper, investigated this question. They considered the transit probability of each radial-velocity detected system and how long TESS planned to observe each system in its primary mission. They predict that TESS would observe transits for 11 out of 677 radial velocity planets, but only three would be previously not known to transit. Only 12 radial velocity planets are known to transit, as of March 2019, so this is still a substantial increase. Today’s authors found that HD 118203 b was among the planets most likely to be observed transiting (top 2%).

It seems surprising that a relatively large, short-period planet took 13 years before it was observed to transit, but a key factor is that the transit itself is relatively shallow compared to transit discoveries from ground-based wide-field surveys of the time. Most transiting exoplanets have also been found around main-sequence stars rather than subgiant or giant stars also. Looking forward, it is clear we can expect many more interesting results from TESS, and from photometry and radial-velocity measurements working together.

About the author, Emma Foxell:

I am a PhD student at the University of Warwick. My project involves searching for transiting exoplanets around bright stars using telescopes on the ground. Outside of astronomy, I enjoy rock climbing and hiking.

4 February 2020

Why Are There so Many Sub-Neptune Exoplanets?

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: Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis
Authors: Edwin S. Kite, et al.
First Author’s Institution: University of Chicago
Status: Published in ApJL

In the few decades since the discovery of the first exoplanet in 1992, we’ve realized that our own solar system is just plain weird. We have no hot-Jupiter gas-giant planets whizzing around our star in a matter of days, nor do we have any sub-Neptune planets, the most common type of planet in the galaxy. Critically, our lack of sub-Neptunes severely hinders our understanding of the transition between Earth-like and Neptune-like planets.

The Kepler Space Telescope operated 2009—2018 and discovered over 2,600 exoplanets, nearly 1,000 of which were classified as sub-Neptunes. But Neptune-like planets are considerably rarer, despite being only slightly bigger. This “radius cliff” (Fig. 1) separates sub-Neptunes (radii < 3 R_⨁, where R_⨁ is Earth’s radius) from Neptunes (radii > 3 R_⨁). What could cause such a steep dropoff? Today’s authors explore this question.

Figure 1. The exoplanet radius distribution. The two gray bands represent two different studies. The radius cliff is denoted near 3 R_⨁ with the dashed line. [Kite et al. 2019]

Previous Model Shortcomings

Gas-giant planets are made primarily of … well … gas. Specifically, most of this gas is molecular hydrogen, H₂. Smaller gas giants like Neptune and Uranus have a larger fraction of helium and methane than planets like Jupiter or Saturn, but their atmospheres are still primarily hydrogen (Fig. 2). Previous attempts to explain the sub-Neptune radius cliff have therefore focused on atmospheric hydrogen loss and accretion.

Figure 2: Atmospheric composition of gas-giant planets in our solar system. Jupiter and Saturn primarily boast hydrogen atmospheres, which turns into metallic hydrogen under the high pressures deep in the atmosphere. Neptune and Uranus are both smaller and colder, leading to a mantle of ices, and their atmospheres contain more helium and methane than the larger two planets. [NASA/Lunar and Planetary Institute]

In one model, researchers proposed that larger atmospheres (when considered with the same core mass) are easier to strip away. However, this model can’t explain both the wide range of sub-Neptune masses and the radius cliff. Other researchers have looked instead at the flip side — accreting more atmosphere just as the protoplanetary disk dissipates. As the disk disappears, the planet’s source of atmosphere disappears, cutting off the planet’s growth. But this model depends closely on the properties of the disk and how long it lives, so the model is likely not the universal solution to the radius cliff problem. What, then, can explain it?

An Active Core

One critical assumption that both of the previous explanations incorporated is that they left the planetary core chemically and thermally inert. That is, it does not interact at all with the atmosphere. Our own experiences on Earth, from the wavy lines radiating from the road on a hot day to the very existence of the water and carbon cycles, suggest that an inert core may not be a valid assumption (even though Earth is structured differently than a gas giant).

Today’s authors throw out the assumption that the core does not interact with the atmosphere. Additionally, the deep atmospheres of gas giants insulate and slow down the cooling of their cores, which results in a magma ocean that directly touches the atmosphere. The authors then look at how the solubility of H₂ in magma depends on various atmospheric properties such as pressure and temperature.

The immense pressures at the magma–atmosphere interface mean that the properties of the gasses no longer follow the Ideal Gas Law. Instead, they behave non-linearly. In such high-pressure situations, the H₂ molecules are so squished together that they begin to repel each other and can no longer be compressed. In this case, the only place the H₂ can go is down, into the magma. Furthermore, the H₂ no longer dissolves linearly (as in Henry’s Law) due to the high pressures. Therefore, as more and more gas accretes onto the atmosphere, more and more H₂ dissolves into the magma, and the planet’s overall radius growth stalls. The authors call this non-linear solubility property of highly-pressurized H₂ the “fugacity crisis,” where fugacity refers to a gas’s tendency to dissolve into an adjacent liquid.

The authors find that sub-Neptunes with radii of 2–3 R_⨁ are so numerous because the atmospheres of planets that size reach the pressures required to force the H₂ into the magma ocean. Then, once the magma ocean saturates, planetary radius growth can resume. However, planets that have enough gas to reach beyond the saturation point are much rarer, simply because they require much more gas. Hence, the radius cliff!

Toward a More Complete Description of Sub-Neptunes

Today’s paper shows the importance of revisiting assumptions and considering additional factors to explain interesting phenomena. Even though the authors reproduced the radius cliff separating sub-Neptunes from Neptunes, they note that further research remains. Essentially no laboratory data exists about the true solubility of H₂ in magma at the temperatures and pressures that exist in the depths of sub-Neptunes because no Earth-bound container can hold that magma. The authors instead extrapolated from lower temperature and pressure measurements. Additionally, the magma–atmosphere interface likely isn’t a hard boundary but rather more fuzzy, which would likely change how the H₂ dissolves into the magma. Finally, the authors note that different core compositions would also likely change the interactions of the magma with the atmosphere.

On the plus side, the authors’ non-linear H₂ dissolving model makes a number of predictions, including how steep the radius cliff is, non-dependence on planetary disk conditions, and the ratios of molecules present in sub-Neptune atmospheres. Future data from TESS will allow astronomers to test this latest hypothesis and bring us a step closer to understanding how planets form.

About the author, Stephanie Hamilton:

Stephanie is a physics PhD graduate and former NSF graduate fellow at the University of Michigan. For her research, she studies the orbits of the small bodies beyond Neptune in order learn more about the Solar System’s formation and evolution. As an additional perk, she gets to discover many more of these small bodies using a fancy new camera developed by the Dark Energy Survey Collaboration. When she gets a spare minute in the midst of hectic grad school life, she likes to read sci-fi books, binge TV shows, write about her travels or new science results, or force her cat to cuddle with her.

28 January 2020

Combining Ground and Space-based Observations to Find EvryFlare

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: EvryFlare. I. Long-term Evryscope Monitoring of Flares from the Cool Stars across Half the Southern Sky
Authors: Ward S. Howard, Hank Corbett, Nicholas M. Law, et al.
First Author’s Institution: University of North Carolina at Chapel Hill
Status: Published in ApJ

For Sun-sized and smaller stars, energy is transported to the surface by roiling columns of convection. These convective bubbles twist up the magnetic fields at the surface and drive sudden, violent releases of energy through flares. Although flare events from the Sun are relatively inconsequential, stars smaller than the Sun have been observed to produce superflares strong enough to remove a planet’s protective ozone layer and kill all but the hardiest lifeforms.

Finding Evry Flare

Many potentially habitable planets are being discovered around low-mass stars. To better understand the viability of these worlds for harboring life, it its crucial to understand how and where these flare and superflare events can happen. Since flares happen very quickly, catching them as they happen requires a large number of stars to be continuously monitored. This is exactly the goal of the Evryscope, the details of which are described in a previous Astrobite. So far, Evryscope has collected several years of simultaneous brightness measurements for over 15 million stars. This long observing baseline makes relatively rare superflare events easier to catch. The tradeoff with such a wide, ground-based view is that the less energetic, but more frequent flare events go undetected due to the reduced photometric precision.

The TESS telescope, which is currently monitoring brightness variations for some of these same stars from above the Earth’s atmosphere, makes up for this limitation nicely. Although TESS only observes a given target for ~28 days, these less energetic flares are much more frequent. By combining the multi-year data from Evryscope with the much shorter but more precise observations from TESS, the authors of today’s paper attempt to get a handle on the frequency of a wide range of flare types from a large sample of stars. An example light curve, combining the Evryscope and TESS data, is shown in Figure 1. Even though the TESS observations only add a small chunk to the light curve, the fantastic sensitivity of TESS reveals a handful of low-energy flares that are lost in the noise with Evryscope.

Figure 1: The brightness variation of a single flare star over a period of a few years. The Evryscope data is shown by the black points and the TESS data is shown by the red points. The colored vertical lines mark the location of flare events. [Howard et al. 2019]

The authors combine ground and space-based measurements for 4,068 K and M stars that were observed by both telescopes. Using their custom-made Auto-EFLS flare detection pipeline, the authors marked 284 of these objects as flare stars, with 575 flare events detected in total. The most energetic flare detection, produced by a small M dwarf, temporarily increased the brightness of the star by a factor of nearly 100. This superflare event released hundreds of years’ worth of the Sun’s energy output in a tiny fraction of that time!

The duration of a flare as a function of total energy released. The sample is split into low mass M stars and the slightly larger K stars. The solid lines indicate a broken power law fit to each subsample. [Howard et al. 2019]

Testing Magnetic Reconnection Models

With a sample of flare detections in hand, the authors then go on to constrain the physical process driving these events. Magnetic reconnection, which is caused by the sudden snapping of twisted-up magnetic fields on the surface of a star, is the most obvious candidate; this process should produce a very specific correlation between the duration and total energy released by a flare. To test the viability of this mechanism, the authors split the sample up by spectral type and fit a power law to the energy–duration distribution of each subsample. This is shown in Figure 2. A typical flare involves a quick rise in brightness followed by a slow exponential decay. Because most of the flare duration is made up of the decay phase, the brightness of a low energy flare can quickly fall below the detection threshold of the telescope. For this reason, the authors actually model the magnetic reconnection with a broken power law, with the break location corresponding to the lowest energy for which it is possible to see the flare. This break lies at lower energy for the less luminous M stars because it is easier to detect low energy events from these stars.

Most interestingly, the power law slope for both the M and K stars is larger than that predicted by magnetic reconnection models for Solar-type stars. Additionally, the slope is larger for the slightly more massive K stars compared to the M stars. This hints that there may be a mechanism beyond magnetic reconnection operating that is responsible for some of the energy output. Unfortunately, the scatter in the data is too large to definitively say whether these differences are statistically significant.

Figure 3: Fraction of flare stars as a function of galactic latitude. The shaded gray region indicates where crowding prevents flares from being reliably detected. [Howard et al. 2019]

Flare Rates Throughout the Galaxy

Finally, the authors examine whether there is any correlation between the prominence of flare stars and their height above the disk of our galaxy. It is well known that younger stars tend to lie closer to the midplane. In addition, astronomers think that a star’s rotation, which can drive flares, slows as the star ages. Following this logic, one would expect to find most flare stars near the midplane of the galaxy. To test this, all 4,068 stars were sorted by galactic height and then authors then counted what fraction of the stars exhibited flares at each height. This is shown in Figure 3. Near the midplane, there are so many stars that brightness measurements become difficult due to crowding, and so this region is ignored. Moving out from the midplane, there does appear to be a decrease in the fraction of flare stars. The authors warn, however, that this may be due to the M stars, which are responsible for most of the flares, becoming too faint to detect at larger distances.

This study demonstrates of the combined power of ground-based and space-based observatories. The high-precision observations from TESS reveal the frequent, low-energy flare events, while the less precise but much longer Evryscope observations reveal the rare and powerful superflare events. The authors note that their most powerful superflare detection would be sufficient to completely remove the ozone layer from an Earth-like planet. This highlights the need for a better understanding of these superflare events as astronomers are finding more and more potentially habitable planets around these very active low-mass stars.

About the author, Spencer Wallace:

I’m a member of the UW Astronomy N-body shop working with Thomas Quinn to study simulations of planet formation. In particular, I’m interested in how this process plays out around M stars, which put out huge amounts of radiation during the pre main-sequence phase and are known to host extremely short period planets. When I’m not thinking about planet formation, I’m an avid hiker/backpacker and play bass for the band Night Lunch.

Previous 1 … 24 25 26 27 28 … 44 Next