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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.

Taurus molecular cloud

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 Greater Taurus–Auriga Ecosystem. I. There is a Distributed Older Population
Authors: A.L. Kraus, G.J. Herzig, A.C. Rizzuto, et al.
First Author’s Institution: University of Texas at Austin
Status: Published in ApJ

Introduction

If only our eyes were sensitive enough, the night sky would be ablaze with nearby astronomical structures. The Andromeda Galaxy would stretch across the sky, spanning about six times the angular width of the full Moon. The Magellanic Clouds would be fireballs tens of Moons across. And the reddened molecular filaments of the nearby star-forming region Taurus-Auriga, glittering with young stars, would stretch over an area about 30 Moons across.

Taurus-Auriga is almost too close to us for its own good. It’s the nearest substantial star-forming region, lying only 145 pc (about 470 light-years) away. Much of it is extremely young, perhaps less than 5 Myr old. Its age is invaluable for studying star formation, but its size across the sky makes it a tricky business to make a full accounting of its members. But this accounting is unavoidable if we want to establish, among other things, the timescales of star formation or molecular-cloud dissipation, and the system’s initial mass function (IMF).

Young circumstellar disks re-emit stellar light in the infrared, because the disks behave roughly like a blackbody at a temperature cooler than the host star. This “infrared excess” makes young stars with disks relatively easy to identify with infrared photometry. Indeed, the Wide-field Infrared Survey Explorer (WISE) mission has gone a long way to painting in the population of Taurus-Auriga stars that possess disks. But disk-less stellar members of Taurus are much more poorly quantified. Which stars separated by entire degrees are associated?

In fact, previous attempts suggest that the Taurus IMF is a bit weird, because it’s heavy on the low-mass end. Is this indicative of deeper physics of the star formation process? Or are we just being hoodwinked and bamboozled by a masquerade of stellar interlopers? How are we to tell apart Taurus-Auriga’s serious card-carrying members from the masked ball of harlequins and mountebanks? At 30 Moons across, it’s quite a carnival.

diskless stars in Taurus

Fig. 1: Left: The author’s sample of candidate disk-less stars in Taurus. Likely members are red, likely nonmembers blue. Right: All likely members that have disks (blue) and don’t (green). The greyscale indicates dust extinction. Note the stars that have disks tend to concentrate around the dusty filaments. Note also that the full Moon is about the size of one of the zeros on the axes. [Kraus et al. 2017]

Ferreting Out the Interlopers

The authors of today’s paper began by dredging the literature for a long list of all possible diskless Taurus members, as suggested by previous studies (Fig. 1, left). They then applied multiple criteria for deciding whether they are Taurus members or not. Here’s a table of them:

Criterion Reasoning Possible complications
Kinematics Young stellar siblings will tend to move in a co-moving crowd Stellar activity or binary systems give misleading radial velocities
HR diagram position Co-eval stars will reside on isochrones There may be a prolonged period of star formation, or other field star contaminants may be passing through
H-alpha emission Young stars will emit H-alpha due to accretion and chromosphere activity Older stars that rotate rapidly may emit H-alpha
Lithium abundances Lithium is quickly burned away in late-K and M stars, so its presence is a marker of youth Older members that have already burned it away are indistinguishable from many non-members, and F-G stars keep their lithium longer
Surface gravity Surface gravity via sodium doublet lines indicate stars are still contracting

 

Interlopers may slip through any one of these criteria, but by applying all of them, this possibility is minimized. In addition to conducting literature searches, the authors acquired values for some of these criteria by taking spectra of some of candidates with a 2.2-m telescope on Mauna Kea.

The criterion of kinematics was the last to be applied. The authors’ reasoning was that, if members’ projected radial velocities and proper motions — Taurus-Auriga is so close to us and is so extended that the motions of widely separated members will exhibit perspective effects — translated into absolute velocities that are close to the average velocity of Taurus around the center of the Milky Way, then they can be considered members.

Taurus stars as disk fraction

Fig. 2: A representation of the Taurus stars in terms of disk fraction (color; see color key on the right) and density of stars (color brightness). From this you can see that there are dense (i.e., bright) agglomerations of stars with and without disks, surrounded by an extended low-density (i.e., dark) region of mostly disk-less stars. Compare this plot to Fig. 1. [Kraus et al. 2017]

Disky Stars Tend to Clump

In the end, they found their sample to contain 218 “confirmed or likely” members, and 160 stellar charlatans (Fig. 1, right). Interestingly, between angular scales of 0.03 and 3 degrees, the level of spatial clustering is the same among disk-hosting and disk-less stars. But disky stars tend to cluster more at separation scales of <0.03 degrees, and disk-less stars are more common at scales of >10 degrees. This means that, in addition to the disk-hosting stars that tend to clump in the molecular clouds, there is a widely-distributed pall of other member stars (Fig. 2). Based on their disk-hosting fractions, it appears that the more widely-distributed ones could be ~10 Myr old — still very young for a star, but older than the disky stars. In fact, three-quarters of Taurus stars appear to be disk-less.

Have disk-less stars blown off their surrounding gas, or have they been dynamically sprayed out of denser regions? The authors cannot answer this, but they point out that the precise parallaxes and proper motions from Gaia may help. In any case, it appears that star formation in Taurus-Auriga took place over an extended timescale. How much does the low-mass-heavy IMF of Taurus-Auriga change? The answer is … not a whole lot. But the authors also note that there may yet be more members out there, including early-type stars. In fact, more member candidates were found while this paper was going through peer review.

The authors finish with some arguments based on the spatial distribution of the kinematics to suggest that the star formation of Tauris-Auriga may just be a component part of a still grander star formation history over an even larger swathe of the sky. Be on the lookout for Gaia papers that will help us piece more of this history together.

About the author, Eckhart Spalding:

I am a graduate student at the University of Arizona, where I am part of the LBT Interferometer group. I went to college in Illinois, was a secondary-school physics and math teacher in Kenya’s Maasailand for two years, and got an M.S. in Physics from the University of Kentucky. My out-of-office interests include the outdoors, reading, and unicycling.

protoplanetary 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: Outer Solar System Possibly Shaped by a Stellar Fly-By
Authors: Susanne Pfalzner, Asmita Bhandare, Kirsten Vincke, & Pedro Lacerda
First Author’s Institution: Max Planck Institute for Radioastronomy, Germany
Status: Accepted to ApJ

A star is born from the gravitational collapse of a cloud of gas and dust. Yet not all of the material ends up in the star, and instead forms a flat protoplanetary disk that surrounds the new star. Over time, the materials in this disk coalesce to form planets, moons, asteroids, and most other objects you might expect to find near a typical star.

Since protoplanetary disks are flat, the expectation is that all of the planets and objects orbiting a star that formed out of a protoplanetary disk should orbit on a single plane. So when we find stars with planets that orbit at multiple different inclinations, this raises questions. A recent astrobite discussed such a case, where an exoplanet was observed orbiting on a completely different plane than the other exoplanets in that same system. But we needn’t look that far to find deviations like this — our very own solar system exhibits several features that don’t line up, so to speak.

Inclinations and Eccentricities and Truncations, Oh My!

For the first 30 AU around the Sun (until right around where Neptune orbits) things are relatively “well-behaved”: most planets’ orbital inclinations only differ from each other by 1–2 degrees, and no planet has an inclination of more than 8 degrees. But beyond Neptune, in the outer solar system, orbital inclinations are considerably higher. Pluto, recently demoted from planet to dwarf planet, is one example; its orbital inclination is more than 17 degrees. The same trend exists for orbital eccentricities, which tend to be significantly larger for objects beyond 30 AU compared to those inside the 30 AU cutoff.

A similar pattern also exists in our solar system’s surface density profile, which can be obtained by smoothing out the cumulative mass of solar system objects (planets, moons, asteroids, etc.) to approximate what the Sun’s protoplanetary disk might have looked like. The surface density profile gradually declines until ~30–35 AU, where it drops abruptly by a factor of nearly 1,000 (a phenomenon often referred to as disk truncation). Coincidence? Perhaps not.

Figure 2. The orbits of select objects in our solar system. Neptune’s orbit and the orbits of the planets interior to Neptune (not labeled in the figure) are constrained to a thin plane, but the orbital inclinations of the dwarf planets beyond Neptune (Pluto, Makemake, and Eris are shown here) are significantly larger. [journalofcosmology.com].

Astronomers seek to develop theories that can help explain these peculiar features in our solar system. One proposed explanation claims that the existence of a yet undiscovered faraway planet (sometimes called Planet 9) could cause these effects on the outer solar system. A second possible explanation, involving a supernova going off near the solar system in its early days, was covered in another recent astrobite.

Today’s paper offers yet another explanation — what if a star flew by our Sun early on, stealing a bunch of the outer material from the Sun’s protoplanetary disk with it, and throwing what was left into inclined and eccentric orbits?

A Stellar Thief

To test this so-called stellar-encounter theory, the authors simulated the Sun surrounded by a smooth, flat disk composed of test particles, and then checked what happened after this system encountered another star (a.k.a. “the perturber”). They tested a broad set of initial conditions by varying the perturber’s mass, how close it came to the Sun, and the inclination of its trajectory with respect to the disk of test particles. In all cases they started with a disk that extended much further than our modern-day solar system does, and selected only simulations in which the fly-by truncated the disk at 30–35 AU, similar to where the solar system’s density drops off. They next checked which of the remaining simulations reproduced other features of our solar system, and in particular, which were left with a sparse population of objects at inclined and eccentric orbits.

Figure 3. Three simulations from today’s paper, showing the trajectory of the perturbing star in black. Going from left to right, the perturber masses are 0.5, 1, and 5 solar masses. The top row of panels depicts the average positions of the particles that remain after the fly-by, colored by how eccentric their orbits have become (note the black circle around 30 AU, the radius within which almost all particles are blue, meaning they all have very low eccentricities). The bottom row of panels indicates the initial positions (before the fly-by) of the different eccentricity populations shown in the top row, with the grey regions signifying particles that became unbound due to the perturber [Pfalzner et a. 2018].

The authors report a good fit to the observed properties of our solar system for a broad range of initial conditions, with the best fits coming from perturbers with masses ranging from 0.5–1 solar masses. The simulation that most closely resembled our solar system was obtained from an encounter with a star half as massive as the Sun passing 100 AU away from the Sun.

In addition to this success, there are two further noteworthy triumphs of the fly-by theory. First, it only takes a single fly-by to reproduce the features the authors originally set out to explain. And second, beyond solving the questions that were originally posed, the simulations also provided natural explanations to several additional unexplained features of our solar system, including the mass ratio between Neptune and Uranus, and the existence of two distinct populations of Kuiper Belt objects.

Convinced? Well, you shouldn’t be, at least not yet. As the authors point out, showing that even only one event is enough to reproduce the observed effects is meaningless until we can quantify how likely it is for an event like that to happen in the first place.

Sunny With a Chance of Fly-Bys?

For our solar system, fly-bys have been extremely rare in the last couple of billions of years. But the good news is that stars like the Sun are typically born in large groups of stars called open clusters. Due to the higher density of stars in an open-cluster environment, the chance for fly-bys goes up significantly as we go further back in time. But just how likely are they?

To check this, the authors produced simulations of open clusters of the type in which the Sun could have formed, and checked for the likelihood of fly-bys like the ones they studied above. In Figure 4 below, they show that about 5–10 million years after the Sun’s birth, there is a probability of around 0.075% for a stellar encounter per million years. Aggregated over the first billion years of the Sun’s life, the chance of experiencing an encounter reaches 20–30% — not that unlikely at all!

Figure 4. The frequency of fly-bys that lead to a truncation of the disk around 30 AU, as a function of the age of the Sun. The dotted line at 2 million years signifies the gas expulsion phase of an open cluster, after which stellar encounters become considerably less frequent. Note that fly-bys before 5 million years should be disregarded because they do not allow enough time for the outer solar-system objects to form [Pfalzner et a. 2018].

Though today’s authors are still far from proving that a stellar fly-by caused the peculiar features of the outer solar system, they demonstrated that it is at the very least a reasonable theory — one that can reproduce many observational facts, and one that is relatively realistic.

So what comes next? The fly-by simulations produced a few additional predictions, mostly involving the detailed properties of orbits in the outer solar system, which the authors are currently attempting to confirm against observational data. Ultimately, precise observations of more objects beyond Neptune hold the key to either strengthening or disproving the theory.

About the author, Tomer Yavetz:

I am a first year graduate student at Columbia University, where I currently focus on galactic dynamics and collisions between large galaxies and smaller objects like dwarf galaxies or globular clusters. I grew up in Israel, received my bachelor’s degree from Princeton University, and spent three years solving earthly problems as a business management consultant, before deciding to return to stargazing. When I’m not busy thinking about colliding galaxies and writing horoscopes, you can usually find me either cooking, eating, or exploring new running routes through the concrete jungle that is NYC.

WISE stars

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: Wide-field Infrared Survey Explorer (WISE) Catalog of Periodic Variable
Authors: Xiaodian Chen, Shu Wang, Licai Deng, Richard de Grijs, and Ming Yang
First Author’s Institution: Key Laboratory for Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences
Status: Accepted to ApJS

Figure 1. An artist’s conception of WISE. The spacecraft “is about the height and weight of a big polar bear, only wider“. [NASA/JPL-Caltech]

Despite being such temperamental objects, variable stars can be reliable tools for probing the universe. By measuring the period of a variable star, we can learn about its composition, age, and brightness, broadly speaking. Applying that knowledge to large populations of variable stars can tell us about the structure of the Milky Way and our nearby universe (shoutout to Henrietta Swan Leavitt and the Cepheid period-luminosity relation) as well as stellar evolution.

Given how useful they are, it stands to reason that creating a large, reliable sample of variable stars would be a great help for astronomers. The Wide-field Infrared Survey Explorer (WISE) is an infrared space telescope whose purpose is to image “the entire sky in the infrared” (see this Astrobite for an early overview of WISE’s goals and accomplishments). Between its initial mission (2009–2011) and its revival mission NEOWISE (2013–present), WISE has examined all sorts of objects, from asteroids to distant galaxies. In this paper, the authors searched for variable stars in the trove of WISE data to create a catalog of variable stars spanning the Milky Way.

CLASSIFIED (Just Kidding)

Between WISE and NEOWISE, about five years of data were available to the authors. They worked primarily with data taken in the W1 and W2 bands, since data could be taken in those bands even after WISE’s cryostat had failed. The authors used the Lomb-Scargle periodogram (see this blog post by Jake VanderPlas for more on that) to identify any periodicity, and excluded long-period or unstable objects to get a total of 50,296 variables.

Figure 2. Example light curves of an eclipsing binary (left) and an RR Lyrae variable (right). The y-axis is the object magnitude in the W1 band and x-axis is the light curve phase. [Chen et al. 2018]

To sort the variables into different classes, the authors used the objects’ periods, colors (difference in an object’s magnitude when observed in different bands), and light-curve shape. The different variable classes considered were Type ab and c RR Lyrae, classical Cepheids, Type II Cepheids, (all types of intrinsic variables, which change in brightness as they expand and contract) and EW- and EA-eclipsing binaries (extrinsic variables, which change in brightness because the starlight is blocked by something in the system, like the other star in a binary pair). 

Light curves were particularly useful when it came to classifying the variables. In particular, extrinsic variables could be separated from intrinsic variables using their fairly symmetric shapes (see Figure 2). Light-curve amplitudes were also used to distinguish between the different types of variables.

Cataloging Differences

Figure 3. Map of Milky Way showing the location of the variables in the catalog. Most of the variables are concentrated near the galactic plane. [Chen et al. 2018]

After the classification process, the authors compared their objects to those in existing catalogs. They first checked if any of their objects were SIMBAD objects, then referenced the Catalina, OGLE, and ASAS catalogs to see if they had recovered any variables. They found that while they had rediscovered 15,527 objects, they had also turned up 34,769 new variables! Most of them were eclipsing binaries, which composed roughly 78% of the new objects, followed by RR Lyrae.

A particularly promising result from the WISE data was that the periods of recovered Catalina variables were within 0.001 days of their measured periods in the optical. What this implies is that periods measured from infrared data are as usable as those measured in the optical. This is good to know — especially since most of the new variables lie within 20 degrees of the galactic plane, an area that cannot be probed with optical telescopes. The rest of the objects are located close to the equatorial poles (see Figure 3).

The result of the authors’ analysis is a catalog of 50,282 periodic variables with 17,000 variable candidates (follow-up needed!). Given that each variable has upwards of 100 detections, this catalog is very robust, and the variables listed within span a region of the Milky Way that hasn’t before been studied with these sorts of objects. When all is said and done, this paper details a resource that will be extremely helpful for studying galactic structure, stellar evolution, and more.

About the author, Tarini Konchady:

I’m a first year graduate student at Texas A&M University. Currently I’m looking for variable stars to better calibrate the distance ladder. I’m also looking for somewhere to hide my excess yarn (I’m told I may have a problem).

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