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11 May 2020

Signs of Collisions to Come

We know that when two neutron stars — the dense, compact cores of evolved stars — collide, they produce signals that span the electromagnetic spectrum. But could these binaries also flare before they merge, as well?

A Broad Range of Signals

Artist’s impression of the collision and merger of two neutron stars. [NSF/LIGO/Sonoma State University/A. Simonnet]

The discovery and follow-up of the gravitational-wave event GW170817, a collision of two neutron stars, provided the first direct evidence of the many forms of light that are emitted in these mergers. Between the instant of collision and the months that followed, observatories around the world recorded everything from high-energy gamma rays to late-time radio emission.

But emission might not be restricted to during and after the merger! A new study conducted by two researchers from the Flatiron Institute, Elias R. Most (also of Goethe University Frankfurt, Germany) and Alexander Philippov, explores the possibility that neutron star binaries may also produce flares of emission in the time leading up to their final impact.

This plot of the out-of-plane magnetic field density indicates the twist in flux tubes connecting the two neutron stars seen at the center of the plot. Here, an electromagnetic flare is launched from the binary after a significant twist has built up due to relative rotation of the right star. [Most & Philippov 2020]

What About Magnetic Fields?

In particular, Most and Philippov focus on how the magnetospheres of the two neutron stars — the magnetized environment surrounding each body — interact shortly before the objects collide.

The authors conduct special-relativistic force-free simulations of orbiting pairs of neutron stars in which each star is threaded with the strong dipole magnetic field expected for these bodies. The simulations then track how the stars’ magnetic fields evolve, twist, and interact as the bodies orbit each other.

A Twisted Fate

Most and Philippov find that dramatic releases of magnetic energy are a common outcome if the neutron stars orbit close enough to one another that their magnetospheres interact.

The authors show that the brightness of the flare luminosity depends only on how far apart the neutron stars are in the simulation: the smaller the separation, the brighter the flare. This dependence demonstrates that the flaring events are driven primarily by the energy stored in the twisted tube of magnetic flux that forms connecting the two neutron stars.

Here, the twisted flux tube and resultant flaring is caused by orbital motion of 45° misaligned magnetic fields, rather than by one star spinning. The bottom panel shows a 3D visualization of the field line configuration at the time of flaring. [Most & Philippov 2020]

When the two neutron stars spin at different speeds, the magnetic field loop that forms between the stars becomes progressively more twisted — until this stored rotational energy is abruptly ejected. And even if neither neutron star is spinning, the authors show that magnetic flux twist still builds up and releases as a result of the binary’s orbital motion, assuming that the magnetic fields of the two stars are not aligned.

Look for Radio Clues

So can we observe these sudden releases of energy? Most and Philippov argue that we should be able to spot the drama in radio emission: a radio afterglow will be produced behind the magnetized bubble that’s ejected from the twisted loop, and additional radio emission can be produced when the bubble collides with surrounding plasma.

Future work on this topic will explore the impacts of the neutron stars’ inspiral, and how the interactions of the magnetospheres change when the neutron stars carry unequal charge. The current study, however, indicates it’s worth keeping a radio eye out to see if we can spot signs of collisions to come!

Citation

“Electromagnetic Precursors to Gravitational-wave Events: Numerical Simulations of Flaring in Pre-merger Binary Neutron Star Magnetospheres,” Elias R. Most and Alexander A. Philippov 2020 ApJL 893 L6. doi:10.3847/2041-8213/ab8196

8 May 2020

The Case of the Missing CO

Planets start their lives in disks of gas and matter around stars, so understanding these so-called protoplanetary disks is key to decoding planet formation. One interesting feature of protoplanetary disks is that they contain less carbon monoxide gas than the typical interstellar medium. When and how does this deficit arise?

Radio observations of the disk around the star DG Tau, with separate plots for each form or isotopologue of CO. The contours designate varying intensity and the colors indicate the velocity of the gas (red is faster than blue). The arrows in the bottom-most plot indicate outflows associated with DG Tau. [Adapted from Zhang et al. 2020]

Protostellar to Protoplanetary

Carbon monoxide (CO) is one of the most common compounds found in space and can be used to trace other chemical compounds along with the structure and mass distribution of objects. However, protoplanetary disks appear to lack CO gas to a startling degree relative to the interstellar medium (ISM). CO gas can be destroyed by chemical processes or be frozen out of the gas state, but these mechanisms alone can’t explain the deficit of CO gas seen in protoplanetary disks.

Protoplanetary disks are an evolved form of protostellar disks, which are created when a cloud of gas collapses to birth a star. Could CO be dissipated at this earlier protostellar disk stage? Or does the depletion only occur when the disk is older?

This question of timing is what motivated a recent study by a group of researchers led by Ke Zhang (University of Michigan). Zhang and collaborators used radio observations of three young (less than a million years old) protostellar disks to measure their levels of CO gas and compare them to that of the typical ISM.

COuld It Be at a Higher Level?

Zhang and collaborators selected their disks based on whether the disk structure could be seen in radio observations. They searched for three different forms of CO that, taken together with models, could probe the CO content of the entire disk. Different models were used to fit the disks, with adjustments to parameters like the gas-to-dust ratio and levels of molecular hydrogen.

The CO content and ages of various protostellar and protoplanetary disks. The average ISM value is shown for comparison. The disks used in this study are TMC1A, HL Tau, and DG Tau. The circles indicate disks and the squares indicate the average value for disks in star-forming regions. Disks younger than 1 million years are considered protostellar disks and disks older than one million years are considered protoplanetary disks. Click to enlarge. [Zhang et al. 2020]

Zhang and collaborators found that the CO gas content of all three protostellar disks is similar to that of the ISM. This puts them at a higher level relative to disks that are older than a million years.

What does this mean for the missing CO problem? The dropoff in CO appears to occur around the million year mark. This means that the CO depletion process is fairly rapid — on astronomical scales — and puts tight constraints on the responsible mechanisms.It also restricts the depletion to occurring within the disk rather than in the surrounding envelope of infalling gas.

It may take exploring combinations of physical and chemical processes to solve this puzzle, as well as observing a larger sample of disks. Either way, CO continues to be a useful molecule to find (or not find) in space!

Citation

“Rapid Evolution of Volatile CO from the Protostellar Disk Stage to the Protoplanetary Disk Stage,” Ke Zhang et al 2020 ApJL 891 L17. doi:10.3847/2041-8213/ab7823

6 May 2020

Build Your Own White Dwarf Photosphere

Have you ever wanted to examine the photosphere of a white dwarf up close and personal? Now you, too, can recreate and observe the atmospheric conditions of these extreme, dense, dead stars — assuming you have access to Sandia Labs’ Z Machine.

Extreme Cores

The eye-catching planetary nebula NGC 2440 surrounds a newly formed white dwarf star. [NASA/ESA and The Hubble Heritage Team (AURA/STScI)]

When a low-mass star exhausts its nuclear fuel, it ends its life by puffing off its outer layers. The dense, scalding hot core of the star — a white dwarf — is then left exposed, emitting high-energy radiation as it gradually cools.

White dwarfs are of enormous astronomical use to scientists. By observing white dwarfs, we are able to learn about topics ranging across stellar evolution, mass-loss processes, distances to astronomical objects, and even the age of the universe. To make correct inferences, however, we need accurate measurements of these white dwarfs’ masses — which is easier said than done.

Confusing the Scale

There are multiple techniques that can be used to measure the masses of white dwarfs. One of the most widely used and broadly applicable is spectroscopy: by fitting the absorption lines observed from white dwarfs’ hydrogen atmospheres with line shape models, we can estimate the surface gravity of the white dwarf, which can then be converted into a mass.

The catch? Masses measured this way don’t agree with masses measured using other techniques. So what’s going wrong?

Schematic illustrating the sample cell of hydrogen gas. Incoming X-rays heat the gold wall and backlighter, causing them to emit radiation and turning the hydrogen gas into a dense plasma similar to that of a white dwarf photosphere. Along the red line of sight shown, scientists can measure the absorption spectrum of the plasma. [Schaeuble et al. 2019]

It’s hard to answer this question without an independent and careful look at the spectral lines created by hydrogen absorption in white dwarf atmospheres. Conveniently, there’s a way of getting this detailed and controlled view: by recreating a piece of a white dwarf in a laboratory!

To Create a Star

The Sandia Z machine, located in Albuquerque, New Mexico, is the world’s most powerful radiation source. Using this machine, a team of researchers led by Marc-Andre Schaeuble (Sandia National Laboratories) pummeled a cell of hydrogen gas with high-energy X-rays to produce an extremely hot, dense plasma, simulating the conditions in the photosphere of a white dwarf.

Schaeuble and collaborators measured the absorption spectra that formed as radiation emitted from a backlighter was absorbed by this dense hydrogen plasma. The authors then extracted the atmospheric electron density — a measurement that relates to inferred stellar mass — by fitting hydrogen line shape models to these spectra.

A Discrepancy of Lines

Top: sample line profile fits to the Hβ and Hγ absorption lines using hydrogen line calculations. Bottom: electron density inferred from fits to the Hβ line (red) is consistently higher than that inferred from the Hγ line (blue). [Schaeuble et al. 2019]

Interestingly, this carefully controlled experiment showed the same issues with mass inference that we encounter using real white dwarf observations.

Schaeuble and collaborators show that they get a different outcome for the electron density depending on which absorption line the hydrogen line shape models are fit to: from the Hβ absorption line, they infer an electron density that’s >30% higher than that inferred from the Hγ absorption line. This discrepancy would translate into significantly different mass measurements for the same white dwarf.

So which fit — if any — is correct? We don’t know yet! These experiments indicate that current hydrogen line shape models don’t capture all the intricacies at play. By continuing to study white dwarfs — both of the natural and build-your-own varieties — we may yet puzzle it out.

Citation

“Hβ and Hγ Absorption-line Profile Inconsistencies in Laboratory Experiments Performed at White Dwarf Photosphere Conditions,” M.-A. Schaeuble et al 2019 ApJ 885 86. doi:10.3847/1538-4357/ab479d

1 May 2020

Challenging a Plasma Assumption

Simplifying a problem to make it solvable is a classic trademark of scientific modeling. But what happens when cows simply aren’t spheres?

The Perpetual Struggle: Accuracy vs. Feasibility

Theoretical models can fall into the trap of becoming spherical cows — they assume simplifications that render them no longer realistic. [Ingrid Kallick]

Theoretical models are critical in astronomy: telescope observations can only take us so far without the models that allow us to interpret them.

A major challenge for theorists is to develop models that are as realistic as possible, but are still simple enough to be solvable. This often requires making simplifying assumptions, turning complex systems into spherical cows. At times, these assumptions might be good approximations. At other times, they might be oversimplifications that cause us to misinterpret observations.

In a recent study, scientist Jack Scudder (University of Iowa) challenges an especially long-standing assumption used in many models of astrophysical and space plasmas.

Equilibrium or No?

Astrophysical and space plasmas are soups of ionized gas found throughout the universe, from supernova remnants to the intergalactic medium, from the Sun’s atmosphere to Earth’s magnetosphere. We can observe the photons emitted by these plasmas, and with the help of theoretical models, we can use these observations to remotely infer the properties of the astrophysical kitchens in which they were made.

Simulation showing a 2D gas relaxing toward a Maxwellian distribution as its particles collide. [Dswartz4]

But what these photons tell us depends on key simplifying assumptions in our models — and one of the most common assumptions is that of local thermodynamic equilibrium (LTE). The LTE approximation assumes that the photons we observe originated in a region where particles bounce around frequently, colliding with one another and taking on what’s known as a Maxwellian distribution of particle speeds in the process.

Is the LTE approximation reasonable to assume for astrophysical and space plasmas? Scudder argues that it often isn’t, especially in astrophysics where gravity requires strong spatial gradients in plasma properties that aren’t permitted in thermodynamic equilibrium. This means we’re likely interpreting observations incorrectly.

A New Solution

So if LTE isn’t a good approximation, what should we be using instead? Scudder has developed a model he calls SERM — Steady Electron Runaway Model — that generalizes the Maxwellian distribution for astrophysics, but reduces to it when spatial gradients are presumed absent. This model, he says, can be used to interpret observations of plasmas that are not necessarily in LTE.

To test his model, Scudder applies it to experimental measurements of the solar wind — the plasma streaming off of the Sun’s surface and suffusing interplanetary space. He shows that SERM predictions neatly match observations, and the model additionally explains a number of odd solar wind features that have puzzled scientists for decades.

A view of the solar corona during the 2015 total solar eclipse in Svalbard, Norway. [S. Habbal, M. Druckmüller and P. Aniol]

This model also shows promise, Scudder notes, for providing answers to other longstanding mysteries relating to astrophysical and space plasmas, such as the question of how the Sun’s outer atmosphere — its corona — is heated.

There’s more work to do to flesh out this model, and it is admittedly more complicated to work with than models that rely on the simplifying assumption of LTE. But Scudder’s work shows that SERM may be the next step needed in theoretical modeling to move the field forward.

Citation

“Steady Electron Runaway Model SERM: Astrophysical Alternative for the Maxwellian Assumption,” J. D. Scudder 2019 ApJ 885 138. doi:10.3847/1538-4357/ab4882

29 April 2020

Spins Point to How Black Hole Binaries Formed

Before stellar-mass black holes merge in a spectacular burst of gravitational waves, they’re locked in a fatal dance around each other as a binary black hole. A new study uses clues from black hole spins to explore how these binaries came to be paired together in the first place.

To Build a Binary

The Hanford (top) and Livingston (bottom) LIGO facilities, which work together to detect gravitational-wave signals. [Caltech/MIT/LIGO Lab]

With ten detections of merging stellar-mass black holes made by the LIGO/Virgo gravitational-wave observatories in just their first two observing runs, these detectors have opened a new window through which we can study the evolution of massive stars.

Among the open questions we hope to answer with these and future detections is the following: How were these binary pairs of stellar-mass black holes created? There are two main formation channels proposed:

Field binary evolution
In isolation in the galactic field, the two members of a binary star system independently evolve into black holes, and they remain bound to each other through this process.
Dynamical assembly
Black holes are formed independently and then sink to the centers of high stellar density environments like globular clusters. There, dynamical interactions cause them to pair up and later get ejected from clusters as bound binaries.

Recent research suggests a combination of these two channels is likely at work to produce the black hole binaries we’ve observed. But what fraction of the LIGO/Virgo binaries are created by each channel?

A new study by scientist Mohammadtaher Safarzadeh (Center for Astrophysics | Harvard & Smithsonian; UC Santa Cruz) explores this so-called branching ratio.

Illustration of a pair of black holes with misaligned spins. [LIGO/Caltech/MIT/Sonoma State (A. Simonnet)]

Spinning an Origin Story

Safarzadeh relies on one primary clue: black hole spin. Due to conservation of angular momentum, black holes binaries that form via isolated evolution are likely to have positive black hole spins — the spins of the black holes will be in the same direction as the orbital rotation of the binary. In contrast, the chaos of dynamical assembly should result in binaries with randomly distributed spins.

Safarzadeh statistically models two populations of black hole binaries produced by these two formation channels and compares this model to LIGO/Virgo’s 10 observations of mergers from their first two observing runs. He’s careful to also take into account LIGO/Virgo’s observational biases — the detectors have an easier time observing binaries with positive effective spin.

Dynamics Weigh In

The author’s calculated posterior distribution on the parameter Q, the ratio of field binaries to the total number of observed black hole binaries, shows that contribution from the dynamical channel is more than 55% with 90% confidence. [Safarzadeh]

The result? Safarzadeh estimates that the contribution of the dynamical assembly channel to the total population of binary black holes is more than 55%, with 90% confidence — which means that random pairings of black holes in the chaotic centers of dense star clusters likely dominate the population of black hole binaries we’ve observed.

This outcome can be further refined as we increase our observed sample size — LIGO/Virgo’s third observing run, now being analyzed, is expected to contain perhaps dozens of additional systems, and future detections will hopefully add many more binaries to the list!

Citation

“The Branching Ratio of LIGO Binary Black Holes,” Mohammadtaher Safarzadeh 2020 ApJL 892 L8. doi:10.3847/2041-8213/ab7cdc

27 April 2020

Searching Pulsars for Planets

Are there more hidden exoplanets lurking around extreme pulsar hosts? A recent study explores a well-observed set of pulsars in the hunt for planetary companions.

Ushering in the Age of Exoplanets

An artist’s illustration showing a network of pulsars whose precisely timed flashes of light are observed from Earth. Could some of these pulsars host planets? [David Champion/NASA/JPL]

The first planets ever confirmed beyond our solar system were discovered in 1992 around the pulsar PSR B1257+12. By studying the pulses from this spinning, magnetized neutron star, scientists confirmed the presence of two small orbiting companions. Two years later, a third planet was found in the same system — and it seemed that pulsars showed great promise as hosts for exoplanets.

But then the discoveries slowed. Other detection methods, such as radial velocity and transits, dominated the emerging exoplanet scene. Of the more than 4,000 confirmed exoplanets we’ve discovered overall, a grand total of only six have been found orbiting pulsars.

Is this dearth because pulsar planets are extremely rare? Or have we just not performed enough systematic searches for pulsar planets? A new study led by Erica Behrens (The Ohio State University) addresses this question by using a unique dataset to explore rapidly spinning millisecond pulsars, looking for signs of hidden planets.

The Advantage of Precise Clocks

How are pulsar planets found? Pulsars have beams of hot radiation that flash across our line of sight each time they spin. The regularity of these flashes is remarkably stable, and when we observe them over long periods of time, we can predict the arrival time of the pulses with a precision of microseconds!

Sample periodograms for two pulsars. The top panel includes a simulated planet signal injected into the data, producing a strong peak at the planet’s orbital period. The bottom panel is an actual periodogram for one of the pulsars in this study, showing no evidence of a planetary companion. [Adapted from Behrens et al. 2020]

Because these pulses are so predictable, any perturbation that might change their timing can be measured and modeled. In particular, the presence of a companion body around the pulsar will cause both objects to orbit the system’s center of mass, introducing a periodic signature in the pulsar’s pulse arrival times. This fluctuation in the pulse timing allows us to measure the period and mass of potential companions.

A Multi-Use Dataset

To search for these signatures in pulse data, Behrens and collaborators turn to observations of 45 separate millisecond pulsars, which were made as part of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) project.

NANOGrav’s primary goal is to use the precise timing of these pulsars to measure the warping of spacetime caused by gravitational waves. But in the process of this work, the project has been carefully monitoring pulse arrival times for these pulsars for 11 years, producing a remarkably detailed dataset in which we can search for evidence of planets orbiting any of the 45 pulsars.

Pushing Down to Moon Masses

Lower limits of detectable masses in the 11-year NANOGrav data set, as shown with black lines. The colored data shows the masses of the least massive 10% of confirmed exoplanets we’ve detected with other methods. Pulsar timing provides the ability to detect remarkably low-mass companion bodies. Click to enlarge. [Behrens et al. 2020]

Looking for periodic signals in the data, Behrens and collaborators rule out the presence of planets that have periods between 7 and 2,000 days. By injecting simulated signals into the data, the authors show that their analysis is sensitive to companions with masses of less than the Earth — in fact, for some pulsars, they’ve eliminated the possibility of all companions with more than a fraction of the mass of our Moon!

This study shows the incredible power and sensitivity of extended pulsar monitoring in the hunt for small exoplanets. While it may well be true that pulsar planets are very rare objects, those out there can’t stay hidden for long.

Citation

“The NANOGrav 11 yr Data Set: Constraints on Planetary Masses Around 45 Millisecond Pulsars,” E. A. Behrens et al 2020 ApJL 893 L8. doi:10.3847/2041-8213/ab8121

24 April 2020

A Stellar Method of Catalog Creation

Star catalogs are critical to astronomy research. However, they’re only as reliable as the methods used to create them. As telescopes probe further and fainter regions of the sky, how can we ensure that our methods of catalog creation extract as much information as possible from the returned images?

The “first light” image from the Transiting Exoplanet Survey Satellite. This is a good example of an astronomical research image, featuring the distortion of star shapes and crowded regions. The crosses typically accompany bright stars and are caused by the instrument doing the imaging. The object on the right is the Large Magellanic Cloud and the bright star on the left is R Doradus. [NASA/MIT/TESS]

To Get to the Point

The start of any star catalog is an image taken by a telescope. These images typically look like black and white photographs of the night sky, and the wavelengths of light used to make them are set by a filter, also known as a band. To make a catalog, you just need to identify stars in an image.

This isn’t as straightforward as it sounds, though! Stars, which would ideally appear as points, look like distorted circles due to atmospheric effects and electronics. Brighter ones can drown out fainter ones. They can also overlap, making it hard to tell them apart in crowded regions like the hearts of galaxies. And as telescopes get better, the images they produce will feature fainter stars and consequently be more crowded.

To efficiently create catalogs in the future, we need improvements on our methods of star finding. In a recent study, Richard Feder (California Institute of Technology) and collaborators used a technique called probabilistic cataloging (PCAT) to identify stars in the globular cluster Messier 2 (M2) and compared their results to existing catalogs on M2.

Assuming We Can Have More Than One Catalog

A catalog is generated by applying assumptions of what a star should look like and how bright it should be. This brightness threshold is a key assumption to catalog creation — too low and false stars may be let in, too high and real stars may be left out.

Traditional methods of catalog creation are one and done; assumptions are made and a single catalog is generated per image. In PCAT, multiple catalogs are generated per image, each with a different set of assumptions. This allows PCAT to have catalogs with very low brightness thresholds and identify sources (star candidates) that, while not bright enough to be confidently identified as stars, can influence the sources around them. The multiple catalogs are eventually collapsed into a single one after source influences and other factors have been accounted for.

A Few Levels Deeper

The completeness of different catalogs obtained from the SDSS images relative to the Hubble images. The x-axis is stellar brightness in Hubble magnitudes and the y-axis is completeness, with 1.0 meaning that all the Hubble stars were recovered in the SDSS data and 0.0 meaning no Hubble stars were recovered. DAOPHOT corresponds to the traditional catalog, Portillo et al. corresponds to PCAT applied to a single band, and r+g and r+i refer to the combinations of multiband images used in this work. [Adapted from Feder et al. 2020]

Feder and collaborators wanted to examine whether PCAT could yield better results when it was simultaneously applied to images taken in multiple bands — multiband images — rather than in a single band. In line with some other PCAT studies, they chose to look at M2.

M2 has the advantage of having existing catalogs generated from images taken by the Sloan Digital Sky Survey (SDSS) and the Hubble Space Telescope (HST). Feder and collaborators worked with SDSS images taken with the g, r, and i bands, using the HST catalog as a “complete” catalog to determine how well they were finding stars. They also compared their results against a traditional catalog generated from SDSS images.

Feder and collaborators found that they recovered stars ~0.4 magnitude fainter when using the multiband images instead of the single band ones. They went 1.5 magnitudes fainter than the traditional SDSS catalog and could confidently recover stars brighter than 20th magnitude. They also developed a more reliable method of determining star positions accurately across images.

With a better understanding of what assumptions can be made for images taken by different telescopes, PCAT could be widely applied to astronomical data in the near future. Stay tuned!

Citation

“Multiband Probabilistic Cataloging: A Joint Fitting Approach to Point-source Detection and Deblending,” Richard M. Feder et al 2020 AJ 159 163. doi:10.3847/1538-3881/ab74cf

22 April 2020

Rescuing an Overlooked Planet

In every batch of detections from the Kepler spacecraft, some transit signals get relegated to “false positive” status by an automated vetting pipeline. How do we ensure that real exoplanet detections don’t accidentally get discarded by the pipeline?

The Kepler False Positive Working Group is on the case — and they just rescued quite a find from being relegated to a false-positive fate.

To Be a Planet Candidate

An illustration of some of the planetary systems discovered by the Kepler spacecraft. The stars at the centers of these systems are not pictured. [NASA Ames/UC Santa Cruz]

Since Kepler’s launch in 2009, this hard-working satellite has found signals from thousands of candidate transiting exoplanets. But all transit signals aren’t just immediately declared planet candidates!

The first hint in Kepler data of a potential transiting planet is what’s known as a “Threshold Crossing Event” (TCE). That TCE could either be a true signal from a planet transiting across the face of its host star, or it could be a false positive or false alarm — a signal mimicking a transiting planet that’s instead caused by a background eclipsing binary system, noise in the data, instrumental artifacts, etc.

Early on in the Kepler mission, every TCE was reviewed by a team of scientists and classified as a true planet candidate or a false positive. But as the mission ramped up and data volume grew, scientists turned to an automated pipeline — aptly named the Robovetter — to categorize the TCEs.

The transit signals of Kepler-1649 b (top; previously known) and c (bottom; newly discovered), in the star’s light curve. [Adapted from Vanderburg et al. 2020]

Human vs. Machine

The automated approach has many advantages: we can process larger volumes of data, and the statistical uniformity allows us to make inferences about the sample completeness. But it’s inevitable that the Robovetter will sometimes be wrong, misclassifying a true planet as a false positive.

To address this, the Kepler False Positive Working Group was established to visually inspect all signals the Robovetter classified as false positives and confirm the categorization. This process allows us to improve the Robovetter’s algorithms — and it also opens the door to new discoveries hidden in old data.

Such is the case with Kepler-1649c, a planet candidate that was incorrectly categorized by the Robovetter as a false positive. In a new study led by Andrew Vanderburg (NASA Sagan Fellow at The University of Texas at Austin), a team of scientists presents their rescue of this sneaky planet.

Earth-like Discovery

The locations of Kepler-1649 planets b and c relative to the star’s optimistic (light green) and conservative (dark green) habitable zone. [Vanderburg et al. 2020]

Kepler-1649c is a planet the same size as Earth that orbits around its M-dwarf host star once every ~20 days, placing it firmly in its host star’s habitable zone. Its star also hosts a previously known inner planet that appears to be equivalent to Venus in its size and the amount of flux it receives.

How did the Robovetter miss this important habitable-zone, Earth-like planet? Vanderburg and collaborators suspect that the pipeline was fooled into mistaking Kepler-1649’s location due to the star’s high proper motion. This introduced noise into the inferred light curve, making the Robovetter think the transit signal wasn’t real.

Vanderburg and collaborators point out that there are likely hundreds of undiscovered planets left in the extended Kepler mission data. While automated pipelines do a great job of doing the heavy lifting, the discovery of Kepler-1649c goes to show that there’s value in having a human eye checking results.

Citation

“A Habitable-zone Earth-sized Planet Rescued from False Positive Status,” Andrew Vanderburg et al 2020 ApJL 893 L27. doi:10.3847/2041-8213/ab84e5

17 April 2020

Pulsar Discovery from an Enormous Telescope

Magnetized neutron stars in distant globular clusters are a challenge to detect — but it’s a job made easier by the world’s largest filled-aperture radio telescope. Recent high-sensitivity observations have uncovered an erratic new star system.

Artist’s illustration of a pulsar (left) and its small stellar companion (right), viewed within their orbital plane. [NASA Goddard SFC/Cruz deWilde]

Pulses from Distant Clusters

Pulsars are the compact remnants of dead stars that shine powerful beams of emission into space as they spin. The brightness of these beams and the regular timing of their pulsations makes pulsars valuable targets for observatories; not only can they tell us about stellar evolution and their environments, but they also serve as probes of the interstellar medium, space-time, and more.

Since the discovery of the first pulsar in 1967, we’ve found thousands of these stellar clocks in our galaxy. While many are located relatively nearby in the galactic disk, we’ve also observed a population of pulsars in the distant globular clusters that orbit the Milky Way. These pulsars are a useful tool for probing a very different environment: the dense stellar cores made up of an old population of stars.

Hubble image of the globular cluster M92. [ESA/Hubble]

Until recently, we’d only discovered 156 pulsars in 29 globular clusters; due to these clusters’ large distances (tens to hundreds of thousands of light-years away), it takes very powerful and sensitive radio telescopes to find them using deep surveys. Now, a new observatory has entered the game.

A Powerful Telescope

The Five-hundred-meter Aperture Spherical radio Telescope (FAST), built into the hilly landscape in southwest China, is the world’s largest filled-aperture telescope. Its size dwarfs that of the Arecibo Observatory in Puerto Rico, and its dish has the advantage of being shapable — the panels that make up its surface can be tilted by a computer to change the telescope’s focus.

Comparison of the FAST (bottom) and Arecibo Observatory (top) radio dish profiles at the same scale. [Cmglee]

FAST achieved first light in 2016, and it’s been going undergoing testing and commissioning for the last few years. As of January 2020, the FAST is officially open for business, and we’re now seeing some of the major results coming from this powerful radio observatory.

Among them: the first discovery of an eclipsing binary pulsar in globular cluster M92, as reported in a recent publication led by Zhichen Pan (NAO, Chinese Academy of Sciences).

An Exotic System

Phase-folded pulse data for PSR J1717+4308A, as observed by FAST (left panel) and by the Green Bank Telescope (right panel). Eclipses are visible as breaks in the data. The difference in sensitivity between the two telescopes is starkly evident. [Adapted from Pan et al. 2020]

Pan and collaborators announce the FAST detection of a pulsar with a pulse period of 3.16 milliseconds orbiting around a low-mass companion in a globular cluster that’s about 27,000 light-years away.

This pulsar, PSR J1717+4308A, is in a close (period of 0.20 days) eclipsing orbit with its companion, making it what’s known as a “red-back pulsar”. Radiation from the pulsar has pummeled its companion star, creating a cloud of ionized material that surrounds it and causes the pulsar’s eclipses to vary in duration and timing.

The discovery of this object demonstrates the potential of FAST as a probe of the globular cluster pulsar population. More observations of M92 are planned in the future, as well as observations of dozens of even richer clusters. Keep an eye out for more FAST results as this telescope ramps up operations!

Citation

“The FAST Discovery of an Eclipsing Binary Millisecond Pulsar in the Globular Cluster M92 (NGC 6341),” Zhichen Pan et al 2020 ApJL 892 L6. doi:10.3847/2041-8213/ab799d

15 April 2020

Observations of Betelgeuse’s Dimming from the Stratosphere

This plot of V-band brightness shows Betelgeuse’s regular ~420-day pulsations, as well as the unprecedented dip in recent months. Red filled circles show the times of the three SOFIA/EXES observations compared in this study. [Harper et al. 2020]

The unprecedented dimming of the red supergiant star Betelgeuse has been making headlines since late last year. To find out what’s causing it, an airplane-borne telescope took to the skies.

A Dramatic Decline

In October 2019, Betelgeuse — identifiable as the bright, massive red supergiant lying at the left shoulder of the constellation Orion — began declining in brightness. By February 2020, it had dimmed to less than 40% of its average luminosity, leading some to speculate that this star might be preparing to end its life as a dramatic supernova.

But Betelgeuse doesn’t appear to be going anywhere just yet. In February 2020, the star stopped dimming and started to climb in brightness again — and yet we still don’t know what caused its remarkable drop.

Betelgeuse, shown here in an infrared image from the Herschel Space Observatory, is a luminous red supergiant star located about 700 light-years away. [ESA/Herschel/PACS/L. Decin et al.]

The Role of Red Supergiants

Why do we want to understand what’s happening with Betelgeuse? Red supergiants like this one represent a late evolutionary stage of massive stars. In this phase, strong winds flow off of the star, carrying away mass and populating the surrounding area with enriched stellar material.

But despite the important role these stars play in shaping galaxies and populating them with elements, the red supergiant stage is poorly understood, and there’s a lot we don’t know about the atmosphere, outflows, and timing of a star’s behavior during this phase. By tracking the evolution of Betelgeuse, a conveniently bright and nearby laboratory, we can further explore these processes.

A Telescope in Flight

Scientists have proposed two main explanations for Betelgeuse’s recent dimming: either it’s an intrinsic cooling of the star’s photosphere, or Betelgeuse has thrown off dust that’s now lying between it and us, blocking some of its light.

SOFIA, a modified Boeing 747SP carrying a 2.7-m telescope. [NASA]

Because infrared observations will be critical to exploring these options, NASA-DLR’s Stratospheric Observatory for Infrared Astronomy (SOFIA) planned an extensive campaign to look at Betelgeuse and its environment.

SOFIA consists a telescope mounted on an airplane that flies above 99% of the Earth’s infrared-blocking atmosphere. Observations of Betelgeuse were planned throughout winter/spring 2020 with all the instruments scheduled to fly on SOFIA. Now the first of these results, from the Echelon Cross-Echelle Spectrograph (EXES) instrument, have been published in a new study led by Graham Harper (University of Colorado Boulder).

Going with the Flow

Harper and collaborators explored Betelgeuse’s circumstellar envelope, the sphere of stellar material that flows off of and surrounds the star. In particular, the SOFIA/EXES observations are of two gas emission lines: singly ionized iron, and neutral sulfur. The authors compare observations of these lines from February 2020, when Betelgeuse was at its dimmest, to observations from 2015 and 2017, when Betelgeuse was at its normal brightness.

SOFIA/EXES observations of the ionized iron emission line around Betelgeuse during Cycle 2 (2015; yellow), Cycle 5 (2017; red) and Cycle 7 (February 2020; blue). [Harper et al. 2020]

The team finds that the lines from the different observing cycles are very nearly the same, suggesting that Betelgeuse’s circumstellar flow has not been affected by whatever caused the star to dim — whether that’s changes in the photosphere or the presence of new dust in the sightline to the star. The observations also indicate that the heating from the stellar wind didn’t change during the dimming.

These results are one more piece in the puzzle of Betelgeuse’s strange behavior. And with additional observations from other SOFIA instruments soon to be analyzed, we can anticipate more news to come!

Citation

“SOFIA-EXES Observations of Betelgeuse during the Great Dimming of 2019/2020,” Graham M. Harper et al 2020 ApJL 893 L23. doi:10.3847/2041-8213/ab84e6

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