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exoplanetary 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 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: AN EXCESS OF JUPITER ANALOGS IN SUPER-EARTH SYSTEMS
Authors: M. L. Bryan, H. A. Knutson, B. Fulton, E.J. Lee, K. Batygin, H. Ngo, T. Meshkat
First Author’s Institution: California Institute of Technology
Status: Submitted to ApJ

Various studies throughout the years have established the fact that the gas giants significantly influenced the formation and evolution of our solar system. Jupiter, in particular, is thought to have played a considerable role in shaping the formation of the inner solar system. Astronomers believe that during the formation of the solar system, Jupiter blocked material from flowing into the inner disk, altered the velocity distribution of this material, and disrupted planet formation within several AU of the sun, leaving our solar system with no planets out to 0.39 AU. Because the gas giants played such an integral role in the formation of our own solar system, scientists are now interested in whether similar processes occur in exoplanetary systems.

Figure 1. An artist’s representation of Gliese 832c, a super-Earth, as compared with Earth. [PHL/UPR Arecibo]

The authors of today’s paper are interested in how long-period gas giants, or Jupiter analogs, disrupt planet formation close to the host star. In particular, the authors are interested in exoplanetary systems containing both Jupiter analogs and super-Earths, or planets with masses larger than Earth’s, but significantly less than that of Uranus or Neptune (see Figure 1). In today’s paper, the authors use radial velocity (RV) observations to search for such systems.

It is difficult to constrain the effect that Jupiter analogs have on inner planetary system evolution. Transit and radial-velocity surveys require the observation of one or more complete orbits of these planets, and current surveys have not been around long enough to observe the whole orbit of Jupiter analogs in exoplanetary systems.

In order to combat this lack of observational constraints, the authors collect published RV data for 65 systems that each have at least one confirmed super-Earth. Those systems are where the researchers believe they will find the Jupiter analogs they are hunting for. Here the authors define a super-Earth as a planet with a radius of 1–4 Earth radii, and a mass of 1–10 Earth masses. A Jupiter analog has a mass of 0.5–20 Jupiter masses, and a semi-major axis of 1–20 AU.

Within the sample of 65 systems, the authors were able to recognize Jupiter analogs by analyzing long term trends in the RV data. The researchers then obtain adaptive optics (AO) imaging data to search for companions to the system’s host star, as companion stars could cause RV trends that the researchers might misattribute to Jupiter analogs. The authors determine whether the measured RV trends exhibit a correlation with the star’s emission lines to see whether any of the observed trends are due to stellar activity. Finally, the researchers account for uncertainty introduced by the inability to pinpoint the precise locations of the Jupiter analogs.

Figure 2. The Jupiter analog occurrence rate found in this paper as compared with the rate estimates published in Wittenmyer et al. (2016) and Rowan et al. (2016). This study finds a much higher occurrence rate of long-period gas giants in systems hosting inner super-Earths than would be expected by chance alone. [Bryan et al. 2018]

After performing this analysis, the authors find nine systems with statistically significant trends indicating the presence of a Jupiter analog. They find that in systems hosting inner super-Earths, the occurrence rate of Jupiter analogs is 39±7%, which is a significantly higher rate than one would expect to see by chance alone (see Figure 2). The authors also find an occurrence rate of 44±17% for systems with an M-type host star, which are the smallest type of stars. This indicates that a system having a host star of a different spectral type does not alter the Jupiter analog occurrence rate in systems hosting an inner super-Earth. The apparent correlation between the occurrence of inner super-Earths and outer Jupiter analogs suggests that long-period gas giants do not hinder super-Earth formation by any of the processes previously mentioned. Conversely, it seems as though these two population of planets seem to be correlated with one another, and as though outer gas giants may even facilitate super-Earth formation.

This is an intriguing result, because it implies that systems with long-period gas giants identified in RV surveys likely contain an inner super-Earth as well, providing a compelling place to continue the search for these mid-sized planets. We have a lot to learn about how exoplanetary systems form, but observing and understanding correlations like the one found in this study help researchers understand what to expect in the wealth of exoplanetary systems we now know exist.

About the author, Catherine Clark:

Today’s post was written by an Astrobites guest author: Catherine Clark, a second-year graduate student at Northern Arizona University. She uses speckle imaging and long-baseline interferometry to characterize exoplanet host stars. When she’s not Fourier transforming, she enjoys yoga, climbing, and photography.

ALMA observations

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: Multiple Disk Gaps and Rings Generated by a Single Super-Earth: II. Spacings, Depths, and Number of Gaps, with Application to Real Systems
Authors: Ruobing Dong, Shengtai Li, Eugene Chiang, & Hui Li
First Author’s Institution: Steward Observatory, University of Arizona; University of Victoria, Canada
Status: Accepted in ApJ

Taking a Closer Look at Protoplanetary Disks

It’s a little too late for anyone to witness how Earth, and its planetary neighbors, came to be — at least, not without some sort of time machine. That’s one reason scientists study protoplanetary disks out in space: to learn more about how planets form.

A protoplanetary disk is a fluffy disk of dust and gas orbiting around a young star. These disks are believed to be sites of planet formation. The solar system we live in was once a protoplanetary disk, waaay back in the day before its planets formed.

In the past, it’s been extremely difficult to directly catch disks in the act of forming planets. But powerful new instruments of this (and the next) decade are helping to change that. With the awesome might of one such instrument, known as the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, scientists have very recently been able to observe protoplanetary disks at higher resolution than ever before. Their observations show all sorts of substructures in disks, like holes, spirals, and gaps. Figure 1 above gives a taste of these intricate substructures in a handful of disks, which were all observed with ALMA a few years ago.

It’s still not clear what exactly is causing these substructures in protoplanetary disks.  But one really, really exciting prospect is that they’re caused by planet formation.

Reading this, you might take a second look at the gaps observed in Figure 1 … and then quickly get overwhelmed, trying to imagine a planet within every single gap. But a group of researchers recently found that it’s possible for a single planet to clear out multiple gaps. Through protoplanetary disk simulations, they found that a single super-Earth-like planet, with a mass between those of Earth and Neptune, could lead to up to five dust gaps in a disk — all of which could be seen by ALMA. So orbiting planets might have caused groups of the gaps that we see in Figure 1.

Today’s authors set out to more fully explore the relationship between a growing planet and the gaps that it can form. Using simulations of disks with evolving planets, they studied how gap characteristics vary with disk and planet parameters. They also determined how scientists can use these gap characteristics to infer properties of planets from observed disk substructures — assuming that those observed substructures were caused by planets to begin with!

Putting Theory into Practice

Today’s authors performed two-dimensional simulations of both the gas and the dust in a protoplanetary disk.  In each simulation, the authors placed a single planet at a fixed orbital radius. They then ran the simulation, letting the planet orbit and grow for about 0.1 to 1 million years. They kept track of the gaps that formed due to the planet over time, through the gaps’ widths, depths, and radial locations.

They varied the planet masses (Mp) and disk height vs. disk radius ratios (h/r) across their simulations. Figure 2 shows an example simulation run, where Mp = 1.8 Earth masses and h/r = 0.03 (meaning that the total height of the disk at a given disk radius is always 3% of that disk radius). We can see that the planet produced many gaps in both the disk’s gas and dust over time, but that the dust gaps are much larger in magnitude than those in the gas.

Figure 2: A snapshot of one disk simulation, giving a bird’s-eye-view of the disk after 0.26 million years. The left and right plots show the gas surface density and the dust surface density of the disk, respectively. The green plus marks the star’s location, which is masked out with a large black circle. The green dot marks the planet’s location. We can see gaps carved out by the planet in the dust, and much fainter gaps carved out in the gas. [Dong et al. 2018]

The authors analyzed how variations in Mp and h/r affect characteristics of the gaps. They found a number of interesting trends from their results, including:

  • As Mp decreases or h/r increases, gaps become more widely spaced and shift away from the planet.
  • As Mp increases or h/r increases, gaps are opened more quickly.
  • As h/r decreases, gaps become narrower.

The authors also found that, among the three gap characteristics that they explored — width, depth, and location — gap location is the easiest and most robust characteristic for comparing simulations to observations. Of the three, gap location changes the least with time in their simulation runs. And for particularly narrow gaps that aren’t resolved well in observations or models, gap locations can still be more accurately determined than gap width or depth.

Figure 3: Comparisons between real disk observations on the left and the authors’ best-matching models on the right. In each of the models, the green plus marks the star’s location, which is masked out with a black circle, while the green dot marks the planet’s location. Each row corresponds to a different disk. Read from top to bottom, the disks are named HL Tau, TW Hya, and HD 163296, and the modeled planets have masses of 57, 29, and 65 Earth masses. We can see that some (though not all) of the gaps in the models match the gaps in the observations. [Dong et al. 2018]

With gap location as their major tool of analysis, the authors went on to compare their simulation results to three real protoplanetary disks, known as HL Tau, TW Hya, and HD 163296.

The authors emphasized that their models were not tailored or fitted to the unique structures of these disks. Instead, the authors used the best-matching simulations from the grid of Mp and h/r values that they’d already explored. More work would need to be done to specifically simulate each disk.

Figure 3 compares the three real protoplanetary disks to the authors’ best-matching models. They found that in each case, a simulated planet of sub-Saturn mass could produce gaps that roughly match the gaps we observe in these disks.

For the future, the authors point to observational microlensing surveys, as a way to learn about the planets possibly forming within protoplanetary disk gaps. With the help of their simulations, those surveys may have a better idea of where to look first.

About the author, Jamila Pegues:

Hi there! I’m a 2nd-year grad student at Harvard. I focus on the evolution of protoplanetary disks and extra-solar systems. I like using chemical/structural modeling and theory to explain what we see in observations. I’m also interested in artificial intelligence; I like trying to model processes of decision-making and utility with equations and algorithms. Outside of research, I enjoy running, cooking, reading stuff, and playing board/video games with friends. Fun Fact: I write trashy sci-fi novels! Stay tuned — maybe I’ll actually publish one someday!

Cas A

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 distribution of radioactive 44Ti in Cassiopeia A
Authors: Brian Grefenstette et al.
First Author’s Institution: California Institute of Technology
Status: Published in ApJ

NuSTAR

Artist’s concept of the NuSTAR X-ray satellite.

Massive stars die as core-collapse supernovae: the star can no longer produce the nuclear reactions that balance its strong gravity, and the star collapses onto its core. When this happens, large amounts of energy and neutrons are available to form elements heavier than iron. The distribution of elements produced in the deepest layers of the star as it goes supernova is key to understanding the mechanism by which the collapse of the star leads to an explosion. Radioactive decay powers the optical light emitted by the supernova around 50−100 days after the explosion. In fact, we can still see radioactive signatures in remnants that are hundreds of years old. In today’s paper, the authors use observations from the high energy X-ray satellite NuSTAR to study the distribution of 44Ti in the young supernova remnant Cassiopeia A (Cas A). The current distribution of radioactive elements and their decay products is linked to the local conditions in which they were synthesised when the explosion took place. Therefore, knowing where the 44Ti is now can shed light on the details of the supernova event that ended the life of Cas A’s progenitor star.

The Cas A Supernova Remnant

Cas A is known by radio astronomers as one of the brightest radio sources in the sky, although the bulk of its energy is emitted as thermal X-rays. These are produced as the ejecta from the star encounter the supernova remnant shock, and it heats them to X-ray emitting temperatures. Shocks are transition layers in which the thermodynamic properties of a plasma change rapidly; they arise whenever some material moves faster than the local speed of sound. Supernova remnants can show two kinds of shocks: the forward shock, which is the blast wave from the supernova explosion, and a reverse shock that forms as the forward shock bounces back when it encounters dense circumstellar material. We think Cas A’s explosion date is 1672, although there are no definite records of it being observed. It is one of the few historical supernova remnants found to be of type IIb from its light echoes (read this Astrobite for the details of supernova classification and this Astrobite to find out what makes light echoes such a mind-blowing astronomical technique).

Radioactive Elements and the Explosion

The production of radioactive elements is very sensitive to local conditions of density and temperature during explosive nucleosynthesis and can give us clues on the nature of the explosion mechanism. 44Ti has a half life of ~58 years, which means there is still a small amount of 44Ti in Cas A decaying. We can see these decays as high-energy lines within the frequency range of NuSTAR. In fact, the mass of 44Ti we measure today is directly proportional to its original mass, independent of what has happened to the ejecta in the 340 years since explosion. Another important radioactive product of explosive nucleosynthesis is 56Ni. Unlike 44Ti, 56Ni has a short half-life, ~6 days, which means we can only know its original abundance from the abundance of its stable decay product 56Fe.

Figure 1: X-ray features of Cas A. The thin rim in the outermost layer is continuum synchrotron emission from the forward shock. Note that much of the X-ray emission forms a bright circle. This is because the ejecta are being heated by the reverse shock, which was generated when the supernova blast wave ran into the surrounding interstellar medium. [Robert Hurt, NASA/JPL-Caltech]

The authors use the NuSTAR satellite to map 44Ti with 18″ resolution (see Figure 1; the size of Cas A is 5′). Their aim is to get a velocity for each 44Ti clump. The velocity vector has two components in the plane of the sky, which they can get from interpolating the current position of a clump to the center of expansion (which for Cas A has been measured quite reliably), assuming that the ejecta expand freely. The third component of the velocity vector, the line-of-sight component, can be measured from the red and blue Doppler shifting of the lines coming from each spatially resolved clump — recall that an X-ray telescope labels the energy of each incoming photon.

The authors find that almost all of the 44Ti is moving in the opposite direction of Cas A’s central compact object (CCO). The CCO is a neutron star that we think is the compact remnant of the supernova explosion. It is off-centre, so we believe that it received a ‘kick’ at the time of the explosion. Since the 44Ti is moving in the opposite direction, it can be part of the material that gave the neutron star its kick (from momentum conservation). They also find that there are regions where they see 44Ti and Fe, regions with 44Ti and no Fe, and regions with Fe and no 44Ti (recall that Fe is the decay product of 56Ni). Since the ratio of 56Ni to 44Ti is sensitive to the local conditions during the explosion, these observations suggest that the local conditions of the supernova shock during explosive nucleosynthesis were varied, and so there were large-scale asymmetries in densities of the innermost ejecta when the star went supernova.

Perhaps it is not so surprising that core-collapse supernovae are complicated events. In the case of Cas A, 4–6 solar masses of material collapsed on itself, with part of it forming a neutron star and part of it being expelled outward to interact with the medium around it. The asymmetries evident from element distributions result in a shell that is surprisingly spherical. There are clearly many details left to iron out as to how all of this happens. Works such as this one can offer us a (radioactive) glimpse into how it all fits together.

About the author, Maria Arias:

Today’s post was written by an Astrobites guest author: Maria Arias, a third year PhD student at the University of Amsterdam. She studies supernova remnants at low radio frequencies with the LOFAR telescope. She’s always happy to take a break from data reduction, though, and go for a yoga class, a run, or a beer.

debris disk

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

Title: On the dynamics of pebbles in protoplanetary disks with magnetically-driven winds
Authors: Mohsen Shadmehri, Fazeleh Khajenabi, Martin Pessah
First Author’s Institution: Golestan University, Iran
Status: Published in ApJ

Do you take Jupiter for granted? When you were taking your dog for a walk last night, did you stop and think about how your dog could have been hit by a meteorite had Jupiter not been ejecting asteroids from the solar system for the last few billion years — conveniently protecting life on Earth from frequent giant-asteroid impacts? Or when you were eating breakfast this morning, did you stop and appreciate how you would not have been able to have breakfast if it weren’t for the fact that Jupiter migrated into the inner solar system shortly after it formed, dumping excess planetesimals into the Sun — and conveniently preventing the Earth from growing past its current size and becoming uninhabitable?

We may have Jupiter to thank for shaping the conditions that allow life to thrive on our planet. However, surveys of exoplanets in other star systems have found giant planets to be rare (though a new study is more optimistic). In smaller protoplanetary disks (such as those around smaller stars), the lack of gas giants is easy to understand, as there simply may not have been enough planet-forming material in the disk to form such a large planet. However even in larger disks, there is a longstanding unanswered theoretical question of how planets of this size can form so quickly before the disk fades away in a few million years. If real protoplanetary disks cannot solve this problem, Jupiter-sized planets may indeed be few and far between.

The Pebble Solution

It was long thought that the rocky cores of gas-giant planets (5 to 10 times the mass of the Earth) formed from city-sized planetesimals (>1 km) merging together, but this takes too long. The reason this process is so slow is that there is a limited supply of planetesimals in any given area of the disk. In the last decade, it has been suggested that the largest dust particles (1 cm to 1 m), called “pebbles”, can act as a catalyst to speed up the gas-giant growth process, because they do not stay where they formed. Pebbles drift towards their stars faster than objects of any other size. This change of location makes it possible for pebbles all the way near the outer edge of the disk to reach a rocky core much further inwards and help it grow. Even though pebbles are small, they also make up a large fraction of the mass in a disk — allowing them to readily speed up a planet’s growth process.

However, today’s paper by Mohsen Shadmehri et al. suggests pebbles may not be a good solution to this problem in all cases.

Turbulence vs. Magnetic Winds

Shadmehri et al. challenge the robustness of using pebbles as a catalyst by more accurately modeling how disks accrete onto their stars. Protoplanetary disks are also called accretion disks because the gas and dust at the inner edge can fall into and be consumed by the star at the center, while disk material throughout the disk also tends to fall further inwards (see Figure 1). There are two main explanations as to why a disk’s material accretes onto its star — turbulence and magnetic winds — but it is unclear which effect is dominant (see this bite for a detailed overview).

While most studies in the past have assumed that turbulence is primarily responsible for the disk’s accretion, the explanation of magnetic winds has gained traction in part because theoretical studies have had trouble finding a source of turbulence strong enough to account for the observed high levels of accretion onto young stars. Previous studies supporting the idea of pebbles as a catalyst for the growth of rocky cores completely neglected the role of magnetic winds in their models.

Figure 1. Diagram of a disk around a young star. Magnetic fields fling a little bit of gas out of the disk (green) as it rotates. To conserve angular momentum, the rest of the disk flows inwards. Meanwhile, dust drifts inwards for a different reason (drag forces). With more vertical diffusion (such as with strong winds), the dust will spread more out of the midplane in the up and down directions. [Scott et al. 2018]

A Pebble Problem

In order to test the effects of magnetic winds on how much pebbles can contribute to the growth of planet cores, Shadmehri et al. develop an analytic model of the evolution of a disk with magnetic winds of different strengths. They keep the total rate of accretion fixed to match observations, implying that disks with stronger magnetic winds also have less turbulence (while disks with weaker winds are more turbulent). Interestingly, in disks with stronger winds, they find that pebbles do not speed up the growth of rocky cores enough for them to ultimately grow into gas-giant planets later on.

Pebbles are a less effective catalyst with strong winds because it takes too long for the smallest dust particles (1 µm) to grow to pebble-size (1 cm to 1 m). This slower rate of growth is not because of the winds themselves, but because there is less turbulence in these disks. With less turbulence, dust particles stay closer to their usual near-circular orbits. As a result, they also have slower relative velocities, which in turn makes them less likely to collide with each other and merge into bigger particles large enough to be considered pebbles.

Additionally, the authors account for how stronger magnetic winds spread the dust towards the surface of the disk, instead of the dust staying flat in the midplane (see Figures 1 and 2). With the dust more spread out, the dust density throughout the disk drops, which in turn leads to a lower amount of pebbles growing in the disk.

Figure 2. Dust diffusion coefficient (αD) as a function of distance from the central star. With the strongest winds (β0 = 103), there is more diffusion, which spreads out the dust particles in the disk — thereby making it harder for them to grow to pebbles. [Shadmehri et al. 2018]

The net effect of stronger magnetic winds causing pebbles to grow both slower and in lower numbers is ultimately to make them less effective in contributing to the growth of cores in the disk (see Figure 3).

Figure 3. Pebble accretion rates over time. The case with the strongest wind (β0 = 103) has the lowest accretion rate. Over 1 Myr, the authors calculate that this case contributes a total of only 0.1 Earth masses of pebbles to a rocky core in the disk — not nearly enough to help it grow to the 5+ Earth masses needed for it to become a gas giant. In contrast, the medium wind case contributes 56 Earth masses, which is more than enough. The red dashed line shows the accretion rate from a previous study with no winds. [Shadmehri et al. 2018]

Reconciling with Jupiter

Fortunately, pebbles can still be excellent catalysts in other cases, including some with strong magnetic winds. For example, if the total accretion rate onto the star is higher (which would imply strong turbulence in addition to strong winds), pebbles can still effectively aid cores in growing large enough to eventually become gas giants. These higher accretion rates are preferentially found around younger stars, which might suggest that pebbles are better at solving the mystery of gas-giant growth for cores that grow earlier on in a young star’s lifetime.

All in all, disks with strong magnetic winds and low accretion rates may prevent pebbles from helping Jupiter-sized planets grow, supporting observational evidence that these planets may be rare. That is all the more reason to be thankful that the Jupiter-sized planet in our own solar system is there to make our lives better.

About the author, Michael Hammer:

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

Omega Centauri

Editor’s note: This article, written by AAS Media Fellow Kerry Hensley, was originally published on Astrobites.

Tadpole Galaxy

Figure 1. Tidal forces don’t only have an effect on globular clusters! The dramatic tail of the Tadpole Galaxy (Arp 188) is also the result of a gravitational tug. [NASA]

The most ancient stellar populations in our galaxy are being ripped apart. Globular clusters — massive gravitationally bound collections of hundreds of thousands of stars — have occupied the Milky Way halo for billions of years. Studying globular clusters can help us understand not only how our galaxy formed, but also how it has evolved over the history of the universe. As the Milky Way has evolved, its gravitational potential has changed as well — and the changes in our galaxy’s gravitational pull are recorded in the behavior of globular clusters.

As the stars in globular clusters interact gravitationally, some gain enough kinetic energy to be ejected from the cluster entirely. The shrinking of globular clusters through this process is called evaporation. When the ejected stars escape the gravitational confines of the cluster, the gravitational pull of the Milky Way starts to take over. As the cluster orbits the galactic center, it experiences tidal forces. Much like an unwitting spacefarer approaching a black hole, globular clusters get stretched out by these tidal forces, stringing those escaped stars into a tidal tail or stream (see Figure 1).

We see these tidal tails in many globular clusters, but the question remains: How can we figure out when the tidal disruption began?

Bose, Ginsburg & Loeb 2018 Figure 1

Figure 2. Simulated globular-cluster tidal streams at a redshift of 0. All else being equal, a more compact cluster (bottom row) experiences less tidal disruption than a more extended cluster (top row). [Bose et al. 2018]

Tracing Tidal Evolution

Today’s paper introduces a new technique to estimate the age of tidally disrupted globular cluster streams. While the ages of the globular clusters themselves are usually determined using stellar evolution models, it can be challenging to figure out when the gravitational pull of the Milky Way began to tear them apart.

The authors of today’s paper found a way to pinpoint the time of tidal disruption by considering the evolution of simulated globular clusters orbiting a Milky-Way-like galaxy. They varied the initial position and velocity (with respect to the center of the Milky Way’s gravitational potential) of six otherwise identical globular clusters — they have the same mass (100,000 solar masses) and initial mass function.

The simulated globular clusters were then allowed to evolve forward in time from roughly 13 billion years ago to today. Figure 2 shows the results of the simulations for three of the six clusters, demonstrating the importance of the initial conditions.

Tidal Disruption in 3… 2… 1…

While the simulated globular clusters give us a good sense of what happens during tidal disruption, they aren’t a good representation of what we would actually observe; telescopes aren’t infinitely sensitive, and their magnitude cutoffs can impose some interesting restrictions on observations. To explore this, the authors simulated what the Gaia survey would see when observing these clusters, assuming a sensitivity limit of 20 magnitudes. Figure 3 compares observable and unobservable stars from three of the simulated clusters.

Bose, Ginsburg & Loeb Figure 2

Figure 3. Maps of the sky in galactic coordinates showing three of the simulated clusters. Stars unobservable by Gaia are shown in grey, while observable stars are shown in color, with the color corresponding to each star’s radial velocity. The number of observable stars decreases as the distance to the cluster increases (counterclockwise from top right). [Bose et al. 2018]

Now considering only the stars that Gaia would observe, the authors found that it’s possible to relate the time at which the tidal destruction of the cluster happened to the proper motions and parallaxes of stars in the tidal tails. Specifically, the more scattered the positions of the stars and the less scattered their velocities along the tidal stream, the longer the time elapsed since the cluster was gravitationally disrupted.

Bose, Ginsburg & Loeb Figure 4

Figure 4. Comparison of the disruption time estimated using the positions and proper motions of stars (horizontal axis) and searching backward through the simulations. The proper motion/position method tends to slightly underestimate the time since tidal disruption. [Bose et al. 2018]

To test that their method recovers the correct disruption time, the authors also searched backward in time in their simulations. They were looking for the time at which the motions of the stars in the tidal stream switched from being controlled by the gravitational potential of the cluster to the gravitational potential of the Milky Way — in other words, the point in time at which the cluster was disrupted. Figure 4 shows that the two methods agree reasonably well, but that the magnitude cutoff of their simulated observations skewed the estimate to be smaller than the true value.

This happens because larger stars are brighter and more easily observed, and tend to be found closer to the center of the cluster. As a result, smaller stars are drawn into the tidal stream before larger stars, and the inferred time since disruption tends to be a bit too short. Still, this is a huge step forward along the path to accurately dating the tidal disruption of globular clusters — an exciting prospect for learning more about the Milky Way’s history!

Citation

“Dating the Tidal Disruption of Globular Clusters with GAIA Data on Their Stellar Streams,” Sownak Bose et al 2018 ApJL 859 L13. doi:10.3847/2041-8213/aac48c

P352-15

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 Powerful Radio-Loud Quasar at the end of Cosmic Reionization
Authors: Eduardo Banados, Chris Carilli, Fabian Walter, et al.
First Author’s Institution: The Observatories of the Carnegie Institution for Science
Status: Published in ApJL

Quasars are some of the most interesting astronomical objects, able to provide us with information across both astrophysics and cosmology. When a quasar with unique properties — such as being exceptionally luminous — is discovered, it’s particularly eye-catching. This is because it can provide an opportunity to make measurements that wouldn’t be possible otherwise, as we will explore in today’s astrobite.

What’s All the Fuss About Quasars?

Astronomers look to quasars because of the extreme conditions surrounding their existence. They are exceptionally bright and emit across the entire span of the electromagnetic spectrum — luminous emissions that are thought to be due to the accretion of gas onto a supermassive black hole at the center. You might be asking yourself: if these are so bright, why do we even need telescopes to see them? Most quasars are at incredible distances, located at redshifts of z > 0.1 — meaning relative to anything local to us, they’ll appear very dim. In fact, the closest quasar to us, Markarian 231, is still at a measurable redshift of z = 0.04.

Figure 1: Flux density of a quasar’s emission across observed wavelengths. The rapidly varying peaks from ~4300 to 4900 Angstroms is known as the Lyman-alpha forest, where dips correspond to neutral hydrogen that has absorbed Lyman-alpha photons. [http://www.aip.de/groups/cosmology/im.html]

What we want to highlight today are quasars on the opposite end of that spectrum: those that are very far away, at high redshift. These types of quasars can give us direct probes to intermediate periods in the universe’s history, like a certain epoch that may have left the universe reionized. One way a quasar can tell us about the timeline of the universe is through the Lyman-alpha forest, which provides us with hints about neutral hydrogen content in the intervening intergalactic medium (IGM). This happens when higher-energy ultraviolet (UV) emissions (shorter wavelength than the Lyman-alpha line) from high redshift galaxies get redshifted into the Lyman-alpha absorption range. This leaves dips in the flux density when observed here on Earth, like in the example in Figure 1. We can directly relate these dips in the flux density to the redshift location of neutral hydrogen.

Cosmic Reionization

The Epoch of Reionization (EoR) is a period in cosmic history when neutral hydrogen from the early universe became reionized. This was due to the earliest stars and galaxies generating an abundance of ionizing UV radiation which systematically reionzied the surrounding hydrogen beginning somewhere around a redshift of z ~ 12 (300 Myr after the Big Bang). This process, based on our best “guess”, has reionization completing somewhere in the region of redshift z ~ 6. Having a rough idea when the EoR ended gives us a great starting point to begin searching for additional evidence of the reionization process. Thus why we are very excited about discovering quasars at very high redshift, because they can directly probe the amount of neutral hydrogen present in the IGM.

Radio-Brightest Quasar To Date

Figure 2: The relative log luminosities of quasars at 4,000 Angstroms to radio emissions at 5GHz. This shows that the quasar P352–15 is radio-brightest of all competing high-redshift quasars. [Banados et al. 2018]

Now moving onto the new discovery: the brightest high-redshift quasar at radio wavelengths, PSO J352.4034–15.3373. Only 15–20% of all quasars are considered radio loud (i.e., radio emissions are the brightest components of the quasar spectrum). This newly discovered quasar, which was determined to be at redshift z = 5.82, has a radio flux density an order of magnitude greater than the next best radio-loud high-redshift quasar (see Figure 2)! The peak flux density of PSO J352–15 was measured to be approximately 100 mJy in the observing frequencies of 3 GHz to 230 MHz. In comparison, Cygnus A, one of the brightest radio galaxies nearby, has a flux density on the order of 10–100 Jy, which demonstrates why these high-redshift quasars are difficult to see.

Figure 3: Flux density measurement over observed wavelengths of the quasar PSO J352.4034-15.3373. A sharp drop-off at a wavelength of ~8300 Angstroms indicates that between us and the quasar is a very dense lyman alpha absorbing environment. [Banados et al. 2018]

Of particular interest to reionization is the spectroscopic follow-up of PSO J352–15. In Figure 3 we can see that, compared to the quasar template (blue dashed line), there is a sharp drop-off in flux density at 8,300 Angstroms. Similar to the Lyman-alpha forest mentioned before, this sharp absorption feature hints to a dense nearby environment, which means lots of neutral hydrogen. This interesting feature of PSO J352–15 puts it in a good position to probe for 21-cm EoR measurements using radio telescopes such as the Murchison Widefield Array or the Giant Meterwave Radio Telescope.

Going beyond cosmological implications, the authors additionally point to future potential studies for PSO J352–15 that could include radio-jet measurements to understand how supermassive black holes form, and how important radio-mode feedback is to the earliest galaxies. To see additional information on PSO J352-15 you can check out the companion paper, which goes into the components and morphology of the quasar.

About the author, Joshua Kerrigan:

I’m a 5th year PhD student at Brown University studying the early universe through the 21cm neutral hydrogen emission. I do this by using radio interferometer arrays such as the Precision Array for Probing the Epoch of Reionization (PAPER) and the Hydrogen Epoch of Reionization Array (HERA).

Gaia view

Editor’s note: This article, written by AAS Media Fellow Kerry Hensley, was originally published on Astrobites.

Dyson spheres

Figure 1. A primer on Dyson spheres. Click to enlarge. [Karl Tate]

Signs of extraterrestrial intelligence don’t appear in the astrophysical literature very often. One of the most well-known signposts of advanced spacefaring civilizations, a Dyson sphere (see Figure 1), named after physicist Freeman Dyson, is a theorized structure surrounding a star, through which a highly technologically advanced civilization could harness the full energy output of its star.

Most Dyson sphere searches to date have looked for excess infrared radiation. Since a large portion of the star is covered, the amount of visible light emitted drops sharply. However, the emission from the Dyson sphere itself, which has an estimated temperature between 50 and 1,000 K, peaks in the infrared. So far, searches for infrared excesses have come up empty.

Science with a Side of Sci-fi

In today’s paper, Zackrisson and coauthors looked for Dyson spheres with little or no infrared excess, just the sort that would have been overlooked by past searches. Specifically, they considered the case of a Dyson sphere made of a gray absorber — a material that dims the star’s light equally at all wavelengths. An observer will see the same overall shape of the star’s spectrum, but the flux will be lower everywhere.

This means that if you try to measure the distance to the star spectrophotometrically — by comparing the star’s observed flux and spectrum to stellar emission models — your measurements will tell you that the star is farther away than it actually is. However, the dimming of the star by the Dyson sphere won’t fool the parallax method, which uses the apparent movement of the target star against the background of more distant stars seen as Earth orbits the Sun. The greater the difference in distances from these two methods, the larger the fraction of the star’s surface is covered by the Dyson sphere.

Zackrisson and collaborators compared parallax distances from the first data release of Gaia, the European Space Agency’s spacecraft tasked with charting the positions and motions of a billion stars, to the spectrophotometric distances from the Radial Velocity Experiment (RAVE), which takes spectra of stars in the Milky Way. By comparing the stars’ spectrophotometric distances from RAVE to their parallax distances from Gaia, the authors estimated the fraction of each star covered by Dyson sphere material. As Figure 2 shows, this resulted in a wide range of covering fractions for the stars in their sample.

covering fractions

Figure 2. Distribution of covering fractions for all stars in the Gaia-RAVE database overlap (left) and just those stars with less than 10% error in their Gaia parallax distance and less than 20% error in their RAVE spectrophotometric distance (right). If, due to large errors or other reasons, the parallax distance is smaller than the spectrophotometric distance, the analysis interprets this as a negative covering fraction. [Zackrisson et al. 2018]

Will the Real Dyson Sphere Please Stand Up?

To narrow down their list of candidates, the authors limited their search to main-sequence stars of spectral types F, G, and K, with a covering fraction greater than 0.7; the spectrophotometric distances for giant stars tend to be overestimated compared to main-sequence stars, so they got the boot. This left just six stars. A further four stars fell due to issues with the data, leaving only two Dyson sphere candidates. Of these two stars, the authors selected TYC 6111-1162-1, an F dwarf with a temperature of 6,200 K, as the most promising candidate.

TYC 6111-1162-1 seemed to be a garden-variety late-F dwarf, and follow-up high-resolution spectroscopy overwhelmingly confirmed the star’s known parameters. With no apparent fishiness in the star’s parameters, the distance discrepancy between RAVE and Gaia still stood. Have we found the first sign of extraterrestrial intelligence?! Given the lack of news stories, you probably already know the answer! TYC 6111-1162-1’s distance discrepancy may be due to the fact that it’s not one star but two — a binary system with a hidden white-dwarf companion — which the authors of today’s paper discovered using radial velocity measurements. It’s still not totally clear what’s going on with this star, but future Gaia data releases may hold the answer — and if we’re lucky, subtle signs of a distant civilization…

Citation

“SETI with Gaia: The Observational Signatures of Nearly Complete Dyson Spheres,” Erik Zackrisson et al 2018 ApJ 862 21. doi:10.3847/1538-4357/aac386

Comet NEAT

Editor’s note: This article, written by AAS Media Fellow Kerry Hensley, was originally published on Astrobites.

Nearly two hundred and fifty years ago, Charles Messier, renowned comet hunter and chronicler of deep-sky objects, cataloged the passage of the first known Near-Earth Object: comet D/Lexell. Calculations of its orbit revealed that the massive chunk of ice and dust had hurtled past Earth at a distance of only 1.4 million miles — just under six times the Earth-Moon distance. It was due to return within the decade but was never seen again. How did we lose a comet?!

In the following several decades, a trio of scientists independently showed that a rendezvous with Jupiter likely threw D/Lexell into a new orbit — and possibly out of the solar system entirely. While the mystery of D/Lexell’s disappearance has remained unsolved since, modern astronomical and computational methods might be able to crack this cold case.

Lost in Space?

In today’s paper, authors Ye, Wiegert, and Hui describe their efforts to retrace the steps of the first-discovered Near-Earth Object. They begin with Messier’s record of the comet’s passage, which encompasses about three months in the year 1770. Messier tracked the comet’s progress across the sky, usually aided by a refracting telescope but occasionally making the measurements by eye (once excusing himself from a gathering at the Minister of State’s house to do so).

The authors used Messier’s observations to reconstruct the comet’s orbit as it was two and a half centuries ago. They assigned reasonable errors to Messier’s measurements and propagated the motion of 10,000 “clones” of D/Lexell forward to the year 2000. This process took into account the gravitational nudges from the Sun, Earth, Moon, and seven of the planets — the exact sort of interactions that were thought to have thrown the comet off course.

The authors found that it was very likely that the comet remained in the solar system; as shown in their simulation results in Figure 1, only 2% of the clones escaped the solar system or were lost in collisions with the planets or the Sun. Even more exciting, of the remaining 98% of clones that remained bound to the solar system, roughly 40% had orbits that brought them close to Earth. This result persisted even when they considered non-gravitational effects such as the release of jets of gas and dust as the Sun warmed the comet. If the comet hasn’t left the solar system… where is it?

Figure 1. Simulation results for 10,000 clones of D/Lexell during a gravitational encounter with Jupiter. Orange dots represent clones that were ejected from the solar system, while grey dots show clones that remain in the solar system but move into orbits that do not cross Earth’s. Green dots representing clones remaining in bound, near-Earth orbits make up about 40% of the clones. Click on the image to enlarge. A video of the simulation can be downloaded from this link. [Ye et al. 2018]

Is D/Lexell Masquerading as Another Object?

Figure 2. The relationship between a comet’s absolute magnitude and its active area. The authors derived an absolute magnitude of 7 for D/Lexell, which corresponds to an active area of 50–1600 square kilometers. [Ye et al. 2018]

In his observations, Messier kept track of D/Lexell’s brightness over time. Combining the apparent magnitude with known values for its distance from both Earth and the Sun gives its absolute magnitude. Choosing reasonable values for its total active area — the amount of its surface that is releasing gas and dust — based on the relation shown in Figure 2 and combining this estimate with a typical value for the active surface fraction, they find that the comet has a diameter on the order of 10 kilometers.

A comet of this size should be detectable by modern surveys, even if it is no longer actively shedding gas and dust. The authors propagate the orbits of known Near-Earth Objects back to the year 1770 and compare them to Messier’s observations of D/Lexell’s orbit. While the paths of four of the objects known today line up with the orbit of D/Lexell, a statistical analysis revealed that only one case of orbital alignment (that of 2010 JL33) is unlikely to be a coincidence.

However, as Figure 3 shows, even applying both constant and time-varying non-gravitational effects can’t bring the orbits of D/Lexell and 2010 JL33 into perfect agreement. While it may be possible to find an orbital solution that links the two objects, the authors acknowledge that proving the solution to be unique might not be possible.

Figure 3. Comparison of Messier’s 1770 observations of D/Lexell with the extrapolated position of JL33. Even with non-gravitational effects, the positions of the two objects fail to line up perfectly. [Ye et al. 2018]

Dust Footprints in the Sky

Although no known Near-Earth Objects have orbits satisfactorily similar to that of D/Lexell, we don’t have to declare the search over. Comets leave traces wherever they go by shedding dust as they travel; when cometary dust passes through Earth’s atmosphere, it produces meteors (see Figure 4). Could meteor showers betray the location of D/Lexell?

Figure 4. Cartoon showing how three well-known meteor showers form, including the identities of their parent comets. [Professor Kenneth R. Lang, Tufts University]

The authors scoured modern meteor surveys and historical records such as the Draft History of the Qing but came up short. This could mean that D/Lexell’s orbit evolved to the point where it no longer crosses Earth’s path. However, the authors show that the dust-trail orbits vary more widely for comets in close-in orbits, so even if the comet still crosses Earth’s orbit, its trail of dust could have been dispersed through gravitational interactions.

While D/Lexell still eludes modern astronomers, it’s possible that future observations of meteor showers could place further constraints on the comet’s orbit. Having given astronomers the slip once more, D/Lexell is free to wander the solar system unidentified. For now.

Citation

“Finding Long Lost Lexell’s Comet: The Fate of the First Discovered Near-Earth Object,” Quan-Zhi Ye et al 2018 AJ 155 163. doi:10.3847/1538-3881/aab1f6

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: Tycho’s supernova: the view from Gaia
Authors: Pilar Ruiz-Lapuente, Jonay I. Gónzalez Hernández, Mercè Romero-Gómez, et al.
First Author’s Institution: Institute of Fundamental Physics, Spanish National Research Council
Status: Submitted to ApJ

In the last 2000 years, only 8 supernovae have occurred within our galaxy that were bright enough to be recorded by humans. Among these is SN 1572, which was first spotted in the year 1572. It was observed around the world, but is perhaps most famously associated with the Danish astronomer Tycho Brahe, who wrote a small book about it titled De Nova Stella. As an aside, the title of that book is where the modern-day terms ‘nova’ and hence ‘supernova’ come from. The appearance of a new star in the sky helped to challenge the old Aristotelian understanding that the heavens were unchanging. Even today, there is a lot that we can learn from this supernova.

At the site where SN 1572 occurred, we see today a supernova remnant — a cloud of gas that was thrown off by the supernova (see Figure 1). In fact, the gas shell is still visibly expanding, as you can see in this video. By studying light emitted by the supernova and reflected from surrounding material, researchers in 2008 were able to tell that SN 1572 was a type Ia supernova. Supernovae of this type are used to measure the distances to far-away galaxies, because of their unique feature that each explosion has almost the same luminosity. We know that Type Ia supernovae are caused by exploding white dwarfs, but we don’t fully understand what triggers the explosion. There are two important models: either the white dwarf collides with another white dwarf, or it grows in mass by pulling in material from a companion star.

In the second of those two models, the companion star should survive the explosion and be flung away at a relatively high speed. Apart from having a high velocity, it would look just like a normal star near the supernova remnant. Therefore, by looking for a star near the supernova remnant that might be a surviving companion, researchers can hope to tell which of the two models triggered the explosion in SN 1572.

Just Add Gaia

Figure 2: All of the stars studied for today’s paper, lettered according to how far away they are from the SN 1572 remnant. The remnant is at the centre of the image. [Ruiz-Lapuente et al. 2018]

The team behind today’s paper set out to do just this using data from Gaia. Regular readers will no doubt have heard of Gaia by now, but a brief refresher: Gaia is a satellite that measures the positions of stars, the movements of stars due to their own innate velocity (the “proper motion” of a star), and the apparent movements of the stars created by the Earth’s movement around the Sun (the “parallax” of a star, which effectively tells you the distance of the star from the Earth). Combining these gives you information on each star’s position in 3D, and its velocity in 2D — the third component, its radial velocity (towards and away from the Earth), can be found from spectroscopy.

Today’s authors studied all the stars close to the SN 1572 remnant using the Gaia data, plus spectroscopy for stars where it was available. Figure 2 shows which stars were studied. Their aim was to determine whether any star appears to be an ejected companion to the supernova. They considered various criteria, most importantly the distance of each star from the supernova remnant, the velocity of each star, and the direction in which each star is moving. Figure 3 shows the distance of each star from the Earth compared to the distance of the supernova remnant from the Earth.

Figure 3: The distance of each of the stars studied from Earth (x-axis) and the proper motion of each star (y-axis). Vertical lines show the range of distances estimated for the SN 1572 remnant. Stars where the distance is consistent with the SN 1572 remnant are shown in blue, while those that disagree are shown in black dashes or dots. [Ruiz-Lapuente et al. 2018]

Considering these various criteria, the authors selected star G (see the labels in Figure 2) as the best candidate for being an ejected companion. This agrees with most of the previous studies, and with the Gaia data is now more certain. Star G is quite close to the supernova remnant, is moving away, and is moving at a higher speed than most of the stars in the area. It is moving at a speed which is quite unusual given its young age, as young stars tend to belong to the slower-moving ‘thin disk’ of the galaxy. In most ways star G looks much like any star, which is generally what we’d expect, so we can’t say for sure whether it is an ejected companion or just an innocent passer-by of a star.

The authors also consider another possibility for an ejected companion: what if the companion that was ejected was another white dwarf? A theory called D6 (dynamically driven double-degenerate double-detonation) predicts that this should be possible, and white dwarfs that look like they’ve been ejected have already been found. Ejected white dwarfs can end up moving a lot faster, because their lower masses mean they get a bigger kick from the supernova. Today’s authors performed a search for high-speed white dwarfs in the area around the supernova remnant. They found none. The result is not necessarily conclusive, given that such a white dwarf would be faint and there’s still a chance that Gaia would have missed it.

Overall, it seems that star G is more likely than any other star to have been the companion to SN 1572. Whether it truly was the companion remains to be seen, however — if it was not, then it becomes likely that SN 1572 formed by the collision of two white dwarfs which were completely obliterated, leaving no star to be ejected. Further study of star G will hopefully be able to tell us for sure, giving us an exciting new piece of information on how type Ia supernovae work. Even a 450-year-old supernova still has something to tell us!

About the author, Matthew Green:

I am a PhD student at the University of Warwick. I work with white dwarf binary systems, and in particular with AM CVn-type binaries. In my spare time I enjoy writing of all kinds, as well as playing music, board games and rock climbing. For more things written by me, take a look at my website.

SN 2011fe

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: Red vs Blue: Early observations of thermonuclear supernovae reveal two distinct populations?
Authors: Maximilian Stritzinger, Benjamin Shappee, Anthony Piro, et al.
First Author’s Institution: Aarhus University, Denmark
Status: Submitted to ApJ

Type Ia (pronounced “one-A”) supernovae are powerful explosions caused by a stellar remnant known as a white dwarf undergoing runaway nuclear fusion so violent that it blows the star apart. They are an important part of astronomy, as they can help astronomers estimate distances to far-away galaxies. (In fact, observations of Type Ia supernovae led to the 2011 Nobel Prize in physics for the discovery of the acceleration of the expansion of the universe.)

Type Ia supernovae are useful because we can use them as standardizable candles — objects whose inherent luminosity we can figure out based on various properties such as how long it takes for them to fade after brightening. Once we know their luminosity (how much light they actually emit) we can calculate their distance by measuring their brightness (how much light we measure from them here on Earth) and applying the inverse-square law. Actually figuring out their luminosity requires some care, however. We think that Type Ia supernovae can happen in at least two different ways (see this bite from 2012 for an explanation), and astronomers using them for distance measurements need to make various empirical corrections in order to do so.

Figure 1: This plot shows the light curves of the thirteen supernovae examined in the paper. The red and blue colors show the “Red” and “Blue” populations and how their light curves diverge within the first four days since exploding. All light curves have been normalized to correct for reddening and time-dilation. [Stritzinger et al. 2018]

Today’s paper offers another potential factor to consider, by looking at thirteen supernovae that were discovered very early after the initial explosion (within a few days, sooner after the explosion than most supernovae are discovered). The authors compared the light curves of the supernovae after normalizing for reddening caused by intervening dust and time-dilation caused by the expansion of the universe and found what appear to be two distinct populations. Based on their colors (in the astronomical sense of measuring the difference in brightness of their light between two standard filters) the two populations were named “Red” and “Blue.” Interestingly, after about five days the light curves of both populations were mostly indistinguishable; the difference was only seen prior to that time.

The authors present four possible explanations for why these two distinct populations may exist: interaction with a stellar companion, the presence of radioactive nickel-56 in the outer layers of the star*, interactions with the circumstellar medium (gas and dust around the star), and simple differences between the composition or opacity of the progenitor white dwarfs. They note, however, that none of these explanations completely explain all the evidence, so we still have more to learn about these intriguing explosions.

Part of the problem is the small number of objects to work with. Although there are hundreds of Type Ia supernovae known, most of them aren’t caught soon enough after the initial explosion to see the dichotomy found in today’s paper, as it only shows up very early on. This situation should be remedied in the future, however, as more — and more sensitive — automated surveys come online that will enable us to find more supernovae sooner after their initial explosions.

*It bears noting that much of the light in the later stages of a Type Ia supernova’s lightcurve comes from the decay of large amounts of radioactive nickel-56 (and iron-56 and cobalt-56) created during the explosion. However, this nickel-56 would have been created from material deep in the interior of the white dwarf. The proposed explanation involves nickel-56 existing in the outer layers of the white dwarf very soon after the start of the explosion, perhaps from being dredged up from the center due to strong mixing or from being created by a double-degenerate merger where two white dwarfs collide and explode.

supernovae

Examples of supernovae in the authors’ early red vs. blue sample, with the supernovae positions indicated by stars. [Stritzinger et al. 2018]

About the author, Daniel Berke:

I’m a first-year grad student at Swinburne University of Technology in Melbourne, where I search for variation in the fine-structure constant on the galactic scale. When I’m not at uni I enjoy a variety of creative enterprises including photography, blogging, and video editing, or just relaxing with a good video game or some classical music.

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