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10 August 2020

Identifying a Slow Planet from a Single TESS Transit

Over the past 25 years, we’ve found thousands of worlds beyond our solar system. Nonetheless, some categories of exoplanets remain elusive — for instance, planets that orbit their hosts on long, slow paths. A new study shows how we might hunt these worlds down.

Observational Limits

Artist’s impression of a hot Jupiter transiting across the face of its host star. [ESA/C. Carreau]

Since the first exoplanet discovery a quarter century ago, we’ve found more than 4,000 confirmed planets orbiting other stars. A large number of these discoveries are planets that transit across the face of their host star — most identified by the Kepler Space Telescope or, more recently, by the Transiting Exoplanet Survey Satellite (TESS). These exoplanets are valuable targets because we can use the transits to measure properties like their radii, densities, bulk compositions, and even their atmospheres.

Unfortunately, due to the nature of transit detections, our observations are inherently biased: it’s easier to detect and confirm short-period, large planets, which means we know a lot about hot Jupiters, but relatively little about wide-orbit, cooler planets.

Because the TESS spacecraft observes a typical region for less than a month, planets on wide orbits longer than 30 days will register — at most — a single transit in TESS data before the telescope moves on to the next section of sky. But can we somehow leverage these single transits to learn more about slow, wide-orbit planets?

Avoiding Overbooking

A planet candidate with only one transit detection could be confirmed with radial-velocity measurements of its host star. But high-precision radial-velocity instruments are in high demand! Without precise knowledge of a planet’s period, confirming that planet’s existence and measuring its properties would require a huge observational time investment from already overbooked radial-velocity instruments.

Fortunately, a team of scientists led by Samuel Gill (University of Warwick, UK) has now demonstrated a more efficient way of pinning down a planet after just one TESS transit.

Light curve showing the best fit to the transit photometry of NGTS-11 from TESS and NGTS. [Gill et al. 2020]

Two Transits, Fewer Options

Gill and collaborators followed up on a planet candidate that had registered a single TESS transit in September 2018. The team conducted an intense photometric monitoring campaign of the candidate’s host star using the economical telescopes of the Next-Generation Transit Survey (NGTS) facility in Chile — and with 79 nights of observations, they detected a second transit of the candidate 390 days after the initial TESS transit.

From the combined photometric observations, the authors were able to narrow down the possible periods for the planet to just 13 options. Using these constraints, they could then wrap up with brief and efficient radial-velocity measurements to identify the properties of the planet.

A Cool Discovery

The array of twelve 0.2-meter robotic telescopes of the NGTS facility in Chile. Only one of these economical telescopes was needed to identify a second transit of NGTS-11 b. [ESO/R. West]

The result? NGTS-11 b (or TOI-1847 b), as the confirmed planet is now named, is a Saturn-mass planet on a wide, ~35-day orbit. With an equilibrium temperature of just 435 K, NGTS-11 b is one of the coolest known transiting gas giants and a valuable target for future transmission spectroscopy to explore its atmosphere.

Gill and collaborators’ identification of NGTS-11 b from just a single TESS transit shows the power of using ground-based photometry to pin down wide-orbit planets. This approach could help us to dramatically expand our understanding of the slower, long-period worlds beyond our solar system.

Citation

“NGTS-11 b (TOI-1847 b): A Transiting Warm Saturn Recovered from a TESS Single-transit Event,” Samuel Gill et al 2020 ApJL 898 L11. doi:10.3847/2041-8213/ab9eb9

7 August 2020

Tracing Gas Flows Out of Star-Forming Galaxies

The flow of gas into and out of a galaxy regulates many of its features, such as its rate of star formation and chemical content. Computer simulations have allowed us to probe these gas flows in greater detail, but there still remains much to uncover. For example, on what scales does gas flow out of actively star-forming galaxies?

Outlining Galactic Outflows

Currently, computer simulations model the flows of gas leaving a galaxy, called galactic outflows, very generally. They are typically informed either by observations or large-scale physics. However, galactic outflows need to be modeled with much finer detail to grasp their full impact on galaxies, which means that observations of outflows need to probe much smaller scales than they currently do.

A near-ultraviolet image of the galaxy J1157 taken by the Hubble Space Telescope’s Cosmic Origins Spectrograph in acquisition mode. The white circle indicates the extent of the spectrograph’s imaging capability. [Wang et al. 2020]

Typically, outflows are explored via absorption spectroscopy, which utilizes the fact that the spectra of light shining through a cloud of material will be imprinted with features unique to the cloud’s material. But this technique is only useful when we’re looking “down the barrel” of an outflow such that the outflow absorbs the light behind it — and this configuration prevents us from usefully probing the outflow size.

To study outflow sizes, we need the flip side of absorption: emission. The material spewed out in galactic outflows fluoresces — it absorbs light at specific wavelengths and reemits it at longer wavelengths. A group of researchers led by Bingjie Wang (Johns Hopkins University) traced this emitted radiation from outflows in nearby starburst galaxies using data taken by the Hubble Space Telescope (HST). They then used this emission to explore the sizes of the outflows.

How to Trace an Outflow

Starburst galaxies have much higher rates of star formation than the average galaxy (for example, the Milky Way). This means that their outflows will also be much more prominent, along with the emission used to trace the outflows.

The ratio between the column density of the material within the radius studied by the Cosmic Origins Spectrograph (COS) and the total column density, plotted versus 𝛂, which quantifies how the density of outflow material depends on radius. The color bar shows the ratio between the radius of COS’s observations and the galaxy radius. The galaxies studied are plotted as well, showing that for this sample, 𝛂 is between 1 and 1.5. This means that the outflow contains more material further away from the galaxy. [Wang et al. 2020]

By studying fluorescence emission lines, Wang and collaborators could focus on the material being swept out of the galaxy in an outflow. Comparing the strength of these fluorescence emission lines to the tracers typically used — resonance absorption lines — allowed them to then make inferences about the structure of the outflows and determine whether we’ve been interpreting them correctly.

Line, Please!

Wang and collaborators found that the fluorescence lines are systematically weaker than the resonance absorption lines. This suggests that the fluorescence lines produced within the HST’s view are more often associated with the galaxy’s stationary interstellar medium or the slow-moving central parts of outflows, rather than tracing the fast-moving material in the outer regions of the outflows.

So where are the strong fluorescence lines? Wang and collaborators suggest that since outflows contain more material the further they are from the galaxy, the fluorescence would also appear further away from a galaxy’s center. However, the HST instrument used for this study is not equipped to observe very large areas, meaning that the regions with strong fluorescence likely lie outside of the instrument’s field of view and are being excluded from the data.

This result suggests that outflow rates for distant galaxies are significantly underestimated with the technique we’ve been using to measure them, and they do not account for changing strength with distance from the galaxy. One thing’s for certain: we’re going to need a bigger telescope.

Citation:

“A Systematic Study of Galactic Outflows via Fluorescence Emission: Implications for Their Size and Structure,” Bingjie Wang et al 2020 ApJ 894 149. doi:10.3847/1538-4357/ab88b4

31 July 2020

Hints of Young Solar Systems

How did our solar system’s planets first form within the swirling disk of gas and dust that surrounded the newborn Sun? One of the best ways to answer this question is watch other solar systems as they form — and the Atacama Large Millimeter/submillimeter Array (ALMA) continues to help us do so.

A History of Large Targets

Gallery of 240 GHz (1.25 mm) continuum emission images for the disks in the DSHARP sample. The scale bars in the lower right of each image indicate 10 au. Click to enlarge. [Andrews et al. 2018]

When ALMA revealed early observations of the disks of gas and dust around young stars, we were stunned by the exquisitely detailed look this interferometer provided into newborn solar systems. Since then, again and again, ALMA has produced remarkable images of gaps, rings, and spiral arms in disks, all of which hint at how planets might be forming.

Thus far, however, we’ve mostly focused on imaging the especially large disks that give us the best look at disk substructure. As an example, the Disk Substructures at High Angular Resolution Project (DSHARP) survey used ALMA to image twenty large, bright disks with effective radii — the radius that encompasses 68% of the light from the dusty disk — of ~50 au on average.

But the vast majority of disks are faint, and their dusty disks are much more compact, with effective radii of less than 20 au. Do these more typical disks show the same wealth of substructures that we’ve spotted in larger disks? And what can this tell us about planet formation?

ALMA continuum image of the GQ Lup disk. The scale bar at the lower right indicates 5 au. [Adapted from Long et al. 2020]

Stopping the Migration

In a new study led by Deryl Long (University of Michigan), a team of scientists has used ALMA to explore one of these more typical systems: a compact disk with effective radius of 19 au around the star GQ Lup A. The high angular resolution of ALMA’s observations allow the team to resolve the dust emission even in this small disk, revealing a wealth of substructures very similar to those spotted in larger disks.

What do these substructures tell us? One challenge to planet formation theories is that larger dust grains should migrate inward through the disk in a process called radial drift, accreting onto the star before they can clump together to form planetesimals.

Long and collaborators’ observations of the GQ Lup system suggest that pebble-sized dust grains can be trapped by variations in pressure in the disk, halting the grains’ drift and giving them a chance to clump. This means that small disks may have the same opportunity as large disks to form young planets.

The Birth of a Solar System

Size-luminosity relationship for millimeter continuum sources with disk properties. DSHARP disks are marked with green circles. The new observations of GQ Lup (red diamond) move us toward a discovery space of smaller disks. [Long et al. 2020]

Long and collaborators caution that there could be other explanations for the disk substructures we see in GQ Lup. But if they are indicators of planet formation, this disk provides us with a unique opportunity to learn more about how our own solar system formed.

One of the disk gaps identified by the authors lies at ~10 au, which is roughly the distance of Saturn from our Sun. Studying GQ Lup could therefore reveal how planets like Saturn develop from a dusty disk. What’s more, these observations of GQ Lup indicate that there may be a rich population of young solar-system analogs out there, just awaiting discovery and exploration.

Citation

“Hints of a Population of Solar System Analog Planets from ALMA,” Deryl E. Long et al 2020 ApJL 895 L46. doi:10.3847/2041-8213/ab94a8

29 July 2020

Searching for Patterns from Feeding Supermassive Black Holes

Our universe is filled with distant supermassive black holes that feed on surrounding gas and dust, emitting bright gamma-ray radiation. A new study explores how many of these show periodic patterns in the variations of their high-energy light.

Putting on a Show

At the center of every galaxy lies a supermassive black hole of millions to billions of solar masses. Many of these — like the Milky Way’s own Sagittarius A* — are quiet, largely invisible lurkers. But some are considerably more attention-seeking, actively accreting material, spewing out winds and jets, and emitting radiation that spans the electromagnetic spectrum.

These active galactic nuclei (AGNs) show variability in their light curves on many different timescales — and a few have been caught flickering in a regular pattern.

Why the Patterns?

Artist’s impression of supermassive black holes that have formed a binary as they’ve sunk to the center of their merged galaxies. [NAOJ]

There are lots of potential explanations for periodic variability in an AGN’s light curve. The black hole may host powerful jets that precess about an axis, driving a lighthouse effect in their emission. The flow of accreting material onto the black hole might wax and wane periodically. And in some cases, the emission we observe might even be coming from a pair of black holes: a supermassive black hole binary whose orbital motion causes periodic patterns.

To better understand the nature of these distant, dramatic sources, we need to study the AGN that show regular variability. But, though a number of studies have explored individual variable AGN, we haven’t yet conducted a large, systematic search for periodic patterns in high-energy AGN light curves. That is, until now.

A Sample of Regular Feeders

A team of scientists led by Pablo Peñil (Complutense University of Madrid, Spain) has leveraged a powerful tool: the Fermi Large Area Telescope (LAT), a space-based gamma-ray detector with a wide field of view that has been scanning the sky consistently for more than a decade. In that time, Fermi LAT has tracked the gamma-ray emission from thousands of AGN, providing Peñil and collaborators with a wealth of data they use to systematically hunt for patterns.

Sky map showing the locations of the 11 sources with periodic emission (filled symbols) and 13 sources with lower significance (open symbols). Click to enlarge. [Peñil et al. 2020]

By applying ten different algorithms that identify periodicity, the authors discover 11 AGN that robustly show patterns in their variability that repeat on roughly 1- to 5-year timescales, as well as another 13 AGN that show strong hints of periodicity but can’t be confirmed yet with the data we have. The authors’ 11 robust detections include two AGN with previously detected variability and nine new identifications.

Peñil and collaborators’ non-detections are also interesting. There were a number of AGN that, in previous individual studies, were reported to show periodic variability — yet they did not show statistically significant periodicity in the authors’ systematic analysis.

What’s Ahead

So now that we’ve identified periodically variable AGN, what can we do with them? The authors’ consistently identified sample can now be studied to better understand what causes the variability of these sources. We can also use the predictable patterns of these AGNs’ high-energy emission to efficiently schedule follow-up observations with telescopes that have more limited fields of view and observing duty cycles.

And this is just the start! Another few years of observation with Fermi LAT should greatly expand the sample of AGN identified with periodic variability, providing even more insight into the feeding behavior of these distant monsters.

Citation

“Systematic Search for γ-Ray Periodicity in Active Galactic Nuclei Detected by the Fermi Large Area Telescope,” P. Peñil et al 2020 ApJ 896 134. doi:10.3847/1538-4357/ab910d

27 July 2020

An Infant Pulsar Defies Categorization

Pulsars have historically been classified into different categories — but the distinction between them may be blurrier than we thought. The discovery of the youngest pulsar yet observed is now raising questions about how we classify these extreme objects.

The Source of a Pulsar’s Power

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

When a massive star explodes as a supernova at the end of its lifetime, an incredibly dense remnant with the mass of one or two Suns — but spanning only 20 km or so in diameter — is left behind. If this resulting neutron star is powerfully magnetized, it can emit a beam of radiation that sweeps across the Earth as the star spins, appearing to us as a pulsar.

The pulsars that we’ve observed are classified into three categories based on what we think powers their emission:

Rotation-powered pulsars
Usually detected from their pulsed radio emission, this is the most commonly observed type of pulsar. These rapidly rotating stars gradually spin down over time. Their lost rotational energy powers the particle acceleration that produces the emission we observe.
Accretion-powered pulsars
These pulsars occur in binaries and accrete matter from their companion stars. Pulsed X-ray radiation is produced by rotating hot spots caused when the accretion flow strikes the surface of the pulsar.
Magnetically-powered pulsars
These bodies, known as magnetars, are the most magnetized objects in the universe, sporting magnetic fields of around 10¹⁴–10¹⁵ Gauss (compare this to Earth’s magnetic field, which is less than one Gauss!). The decay of their unstable magnetic field powers the emission of high-energy radiation, particularly at X-ray and gamma-ray wavelengths.

But what if these pulsar categories aren’t as distinct as we think they are? Observations of a very recently born pulsar, described in a publication led by Paolo Esposito (Scuola Superiore IUSS and INAF, Italy), are now challenging our classifications.

The source Swift J1818, as observed by the XMM-Newton spacecraft. [Adapted from Esposito et al. 2020]

Neither Here Nor There

The source Swift J1818.0–1607 was first discovered in March 2020 as a flaring outburst of X-ray radiation. Esposito and collaborators present X-ray observations of the source using the Swift Observatory, XMM-Newton, and NuSTAR, all of which paint the picture of an incredibly young — just 240 years, a relative baby on cosmic scales! — magnetar undergoing an outburst.

Profile of a bright radio pulse from the source Swift J1818, as observed by the Sardinia Radio Telescope. [Adapted from Esposito et al. 2020]

But Swift J1818 has its quirks. Of the roughly 30 magnetars we’ve discovered, Swift J1818 spins faster than any of them, with a period of just 1.36 seconds. Its quiescent luminosity is lower than we’d expect given its young age. And follow-up radio observations with the Sardinia Radio Telescope in Italy reveal that Swift J1818 also exhibits the strong and short radio pulses expected for a rotation-powered pulsar.

Esposito and collaborators’ observations lead them to conclude that Swift J1818 is a peculiar magnetar with properties that straddle those of rotationally and magnetically powered pulsars. This makes this newborn the latest in a small collection of oddball young neutron stars with diverse properties, suggesting that there may still be much we don’t know about the driving forces behind pulsar emission, and how this changes over a pulsar’s lifetime.

Citation

“A Very Young Radio-loud Magnetar,” P. Esposito et al 2020 ApJL 896 L30. doi:10.3847/2041-8213/ab9742

24 July 2020

There’s Metal in the Air

With atmospheric temperatures ranging from roughly 3,000 to 6,500 degrees Fahrenheit, ultra-hot Jupiters are ready-made laboratories for extreme planetary science. For instance, any molecules in the atmosphere of an ultra-hot Jupiter will be broken down into their component atoms and ions. So what can be found in the atmosphere of the ultra-hot Jupiter WASP-121 b?

Laboratories for Extreme Science

Still from an animation of the ultra-hot Jupiter KELT-9 b orbiting its host. [NASA/JPL-Caltech]

Ultra-hot Jupiters (UHJs) are unlike any planet in our solar system. They are massive, yet they live very close to their host stars. This proximity causes many unusual phenomena, such as chemical variations between the planet’s daysides and nightsides.

The intense heat UHJs experience also leads to their atmospheric components breaking down. Various metal atoms and ions have been identified in the atmospheres of UHJs, including neutral sodium, iron, and magnesium, and ionized titanium and calcium. However, more neutral metals ought to be detected, especially in the lower parts of these planets’ atmospheres.

Knowing which metals to expect in a UHJ would greatly aid observations and the classification of these planets. To this end, a group of researchers led by Maya Ben-Yami (University of Cambridge, UK) attempted to predict what metals could be found in the atmosphere of the UHJ WASP-121 b and then compared their results to observations of the planet.

The model transmission spectra of WASP-121 b based on the contributions from neutral iron, chromium, vanadium, and titanium. Click to enlarge. [Ben-Yami et al. 2020]

Creating a Metal Metric

WASP-121 b has been the subject of many studies over the past few years. It orbits its host star every 1.3 days and is roughly the mass of Jupiter. It’s a good candidate for transmission spectroscopy — looking at light from the host star filtered through a planet’s atmosphere to learn about the atmospheric composition — since it’s large and its host is very bright.

Ben-Yami and collaborators began their analysis by modeling atomic abundances for WASP-121 b. They then used these abundances to understand how strongly the signature of a metal would appear in a spectrum. After accounting for effects like stellar rotation, the end result is a model transmission spectrum for WASP-121 b.

With a model spectrum in hand, Ben-Yami and collaborators were then able to quantify how likely it was for a metal to show up in an observed spectrum of WASP-121 b. Assuming a reasonable spectrum quality and signal noise, they found that the most likely neutral metals to be observed in WASP-121 b would be iron, titanium, vanadium, and chromium.

Functional Metal Detectors

From left to right and top to bottom, the detections of neutral iron, neutral chromium, neutral vanadium, and ionized iron. The dashed white lines indicate where an applied template picked up the strongest signal, which, if significant enough, would be a detection. V_sys is related to the velocity of the host star. K_p quantifies the effect a planet has on its host star’s motion. [Adapted from Ben-Yami et al. 2020]

The team used observations taken by the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph in Chile to test their predictions. Based on their metric, they searched for the four aforementioned metals along with scandium, yttrium, and zirconium. They were unable to detect neutral titanium, scandium, yttrium, or zirconium, but they did recover previous detections of neutral and ionized iron. Most excitingly, they detected neutral vanadium and chromium for the first time.

These detections and non-detections provide insight into the role of vanadium oxide and titanium oxide in UHJs. Both molecules are believed to cause deviations from the expected relation between altitude and temperature. The detection of neutral vanadium suggests that vanadium oxide gets broken down while the non-detection of neutral titanium suggests the opposite is true for titanium oxide.

Aside from the new detections of neutral vanadium and chromium, this study suggests that it is viable to probe the lower atmospheres of UHJs using model-based metrics and high quality spectra. With WASP-121 b being on the cooler side, there remains a large variety of UHJs left to characterize.

Citation

“Neutral Cr and V in the Atmosphere of Ultra-hot Jupiter WASP-121 b,” Maya Ben-Yami et al 2020 ApJL 897 L5. doi:10.3847/2041-8213/ab94aa

22 July 2020

The Contradiction of a Low-Mass Massive Black Hole

The black holes we’ve observed in the universe typically fall into two categories: small star-sized black holes, and gargantuan black holes lurking at the centers of galaxies. Now, a new black-hole discovery sheds some light on the gray area between these extremes.

Growing Together

Illustrations of two types of accreting black holes: a stellar-mass black hole accreting from a binary companion (top) and a supermassive black hole accreting gas in a galaxy’s center (bottom). [Top: ESA/NASA/Felix Mirabel; Bottom: ESO/M. Kornmesser]

Stellar-mass black holes of up to 100 solar masses are scattered by the millions throughout galaxies. At the opposite end of the spectrum, most galaxies are thought to contain just one massive black hole: a black hole of millions to tens of billions of solar masses that lies in the galaxy’s core.

Intriguingly, the mass of these central black holes seems to be inherently tied to that of their host. An empirical relationship known as the M-σ relation shows a correlation between a central black hole’s mass and the spread of star velocities in its host galaxy’s bulge, which acts as a proxy for the bulge mass. The M-σ relation and other, similar relationships show that black holes seem to grow in tandem with their host galaxies throughout the universe.

If the M-σ relation holds across a broad range of masses, then we would expect to find smaller massive black holes at the hearts of especially low-mass galaxies. So far, evidence for these low-mass central black holes has been scarce. But a new study led by Ingyin Zaw (New York University Abu Dhabi, UAE) has now delivered a low-mass massive black hole for us to contemplate.

A 4’ x 4’ view of IC 750, a low-mass galaxy that hosts a massive (though less so than expected!) black hole at its center. [Sloan Digital Sky Survey]

Mass Measurement from Masers

Zaw and collaborators used the Very Long Baseline Array to obtain radio observations of the low-mass galaxy IC 750.

At the galaxy’s heart, the authors found emission from water masers, clumps of water molecules that emit light naturally in a process similar to laser emission. Light from the masers shows that they are orbiting in a disk around a compact central mass — a massive black hole — and Zaw and collaborators used their motion to measure the mass enclosed in their orbit, providing an upper limit on the black hole’s mass.

The authors then reduced and analyzed publicly available multiwavelength data to understand the location of the black hole and measure the properties of its host galaxy.

A Decidedly Low-Mass Monster

This plot of black hole mass vs. bulge stellar velocity dispersion (the M-σ relation) shows IC 750 marked with a red star, with upper mass limits indicated by downward-pointing arrows. It falls two orders of magnitude below where we’d expect it to lie on the relation. Click to enlarge. [Zaw et al. 2020]

The result? IC 750’s central massive black hole is a definite lightweight, with an upper limit of 140,000 solar masses — and it may actually be less than a third of that weight. Not only is this remarkably small for a central massive black hole, it’s also unusually light even relative to the mass of its host galaxy: IC 750’s black hole lies two orders of magnitude below where it should sit on the M-σ relation!

What’s going on with this unusual object? There are two possible explanations: either there’s more scatter at the low-mass end of the M-σ relation, or the scaling relationship is simply different for low-mass galaxies. The latter option is supported by some simulations that suggest that black holes don’t grow efficiently in low-mass galaxies.

Though we don’t yet know which explanation is more likely, more observations like those presented here will eventually fill in our picture of these low-mass massive monsters.

Citation

“An Accreting, Anomalously Low-mass Black Hole at the Center of Low-mass Galaxy IC 750,” Ingyin Zaw et al 2020 ApJ 897 111. doi:10.3847/1538-4357/ab9944

17 July 2020

A Stellar Flyby Makes Some Waves

The gaseous, dusty disks surrounding newly born stars can reveal a wealth of information about how distant stellar systems form and evolve. In a new study, scientists have now watched the interaction of two such disks in a stellar flyby.

Spotting Spirals

In the past decade, new instrumentation has led to a dramatic improvement in our views of circumstellar environments. We’ve spotted remarkable structure in the dusty disks that surround newborn stars — including, in many cases, pronounced spiral arms.

An example of spiral arms detected in a protoplanetary disk, MWC 758. [NASA/ESA/ESO/M. Benisty et al]

The presence of these spiral arms has provoked much discussion and debate. Are they caused by gravitational instabilities in the gas and dust? Or are they produced by perturbations from unseen, newborn planets orbiting within the disks? While both of these explanations could be at play in different systems, there’s an additional possibility to consider: the arms could be excited by tidal interactions with another star.

In a new study led by Luis Zapata (UNAM Radio Astronomy and Astrophysics Institute, Mexico), a team of scientists has used the sensitive and high-angular-resolution observations of the Atacama Large Millimeter/submillimeter Array (ALMA), located in Chile, to understand how tidal interactions with an orbiting star might be responsible for spiral arms observed in UX Tauri.

A New Disk Found

Located ~450 light-years away, the UX Tauri system consists of four stars: UX Tau A (the main star), UX Tau B (a binary star), and UX Tau C (a close companion that lies just to UX Tau A’s south). Past observations have revealed a disk of gas and dust around UX Tau A exhibiting distinct spiral arms.

The intensity (top) and radial velocities (bottom) of the molecular gas observed in UX Tauri reveals a disk around both UX Tau A (top star in both images) and UX Tau C (bottom star), as well as a stream of gas connecting the two. Curves tracing the spiral arms in the disk surrounding UX Tau A are overlaid in the top image. [Adapted from Zapata et al. 2020]

Zapata and collaborators have now followed up with detailed ALMA observations to explore the structure of the molecular gas and dust in UX Tau. In addition to further resolving the disk around UX Tau A, the team was also able to detect — for the first time — molecular gas swirling in a disk around UX Tau C. What’s more, the observations reveal tidal interactions between the two disks that surround these stars.

Drama in UX Tau

What do these findings mean? Zapata and collaborators suggest that we’re witnessing a close flyby of UX Tau C as it progresses on a wide, evolving, and eccentric orbit around the disk of UX Tau A. As UX Tau C plowed through UX Tau A’s circumstellar disk, it captured some of the gas, forming its own disk. Through its motion and this tidal interaction, UX Tau C also excited the observed spiral arms in UX Tau A’s disk.

The drama spotted in UX Tauri represents one of the few cases of binary disk interactions that have been mapped out in molecular gas — but this is likely a common occurrence, since stars often occur in multiple-star systems. Sensitive observations like the ALMA detections presented here will likely reveal more such interactions in the future, shining additional light on the process of star and planet formation.

Citation

“Tidal Interaction between the UX Tauri A/C Disk System Revealed by ALMA,” Luis A. Zapata et al 2020 ApJ 896 132. doi:10.3847/1538-4357/ab8fac

15 July 2020

Bent Crystals and Solar Flares

In 1984, a rescue mission took place in space — ultimately saving a spacecraft that went on to make some of the most detailed observations we have of solar flares during a highly active solar cycle. Now, more than three decades later and thanks to some clever recalibration, we’re still reaping the rewards.

A Dramatic Rescue

STS-41-C astronauts George Nelson (right) and James van Hoften (left) repair the SolarMax satellite in the Challenger shuttle’s open payload bay. [NASA]

In the first-ever attempted repair of an orbiting satellite — almost a decade before the first Hubble servicing mission — the crew of space shuttle mission STS-41-C were tasked with capturing the Solar Maximum Mission (SolarMax) spacecraft in orbit so that they could repair its fine-pointing system and several instruments.

The attempted retrieval process was anything but smooth. The astronauts’ jetpack-powered space-walk rendezvous with SolarMax (pictured above) — during which they intended to dock with the satellite and slow its gentle rotation — failed, with the spacecraft entering an uncontrolled tumble after an astronaut tried to slow it by grasping a solar panel. Unable to point its panels at the Sun, SolarMax’s batteries started to drain, and the astronauts were forced to retreat to the shuttle to regroup.

Ultimately, the team managed a last-minute rescue, retrieving the satellite using the shuttle’s robotic grappling arm and maneuvering it into the payload bay. After performing a multi-hour repair, the victorious crew returned SolarMax to orbit — and their rescue extended the mission’s lifetime by another five years.

Curved Crystals to Gather X-rays

What was SolarMax, and why was it worth repairing? The satellite carried an array of instruments all designed to observe the flaring Sun at ultraviolet, X-ray, and gamma-ray wavelengths.

Schematic of the Bent Crystal Spectrometer shows how a curved crystal was used to produce X-ray spectra. [Sylwester et al. 2020]

Launched at the peak of solar cycle 21, SolarMax gathered observations from February to November 1980, and then again from 1984 to 1989 after its repair. The combination of Solar Max’s long operation time and the especially high activity levels of the solar cycles it observed renders much of its data unparalleled for flare studies.

One of the valuable instruments on SolarMax was the Bent Crystal Spectrometer (BCS), an instrument that pioneered the concept of using curved crystals in space instrumentation to produce X-ray spectra. The spectral resolution of the BCS remains the highest of any long-operating solar X-ray spectrometer, and the instrument’s observations provide valuable insight into the properties of the hot plasma in solar flares.

Now, in a new study led by Janusz Sylwester (Space Research Center, Polish Academy of Sciences), a team of scientists has conducted a valuable recalibration of BCS spectra.

Looking Beyond the Sun

By examining data produced during the scan of a solar flare in November 1980, Sylwester and collaborators are able to identify and quantify the effect of small deformations in the crystal curvature of one of the channels. Accounting for these deformations resolves a long-standing mystery of certain anomalies in the ratios of emission lines in BCS data.

Artist’s impression of a record-setting stellar flare from the nearby star EV Lacertae. [Casey Reed/NASA]

The authors additionally improve other calibration aspects, ultimately producing high-resolution line spectra that they suggest could now be used as templates for the analysis and interpretation of future observed X-ray spectra — in particular, spectra gathered from other active, flaring stars in our galaxy.

With revitalization efforts like this, observations from BCS thus continue to be valuable many years after the mission end — well justifying the daring in-orbit rescue of SolarMax.

Citation

“A Unique Resource for Solar Flare Diagnostic Studies: the SMM Bent Crystal Spectrometer,” J. Sylwester et al 2020 ApJ 894 137. doi:10.3847/1538-4357/ab86ba

13 July 2020

Evidence for an Ancient Martian Ocean

Was Mars once partially covered in ancient seas? A recent study has found new evidence to support the Mars ocean hypothesis.

Signs Point to Seas

A map of Mars’s topography created by the Mars Orbiter Laser Altimeter shows two distinct southern and northern hemispheres. Click to enlarge. [NASA/JPL/GSFC/MOLA. Map by Emily Lakdawalla]

The debate about large bodies of water on Mars has raged for decades.

The discovery of the Martian dichotomy — a ~30 km difference in height between the lower, smooth northern hemisphere and higher, heavily cratered southern hemisphere — was an early indication that the northern hemisphere may once have been underwater.

Long, unbroken features identified as potential ancient shorelines were spotted on Mars as early as the 1970s by the Viking orbiters, further supporting a picture of vast past oceans.

And dozens of further clues have piled up: signs of stream channels and river deltas, indications of past rainfall, hints of historical tsunamis from asteroid impacts, atmospheric signatures that point to a past abundant water supply — the list goes on.

In this proposed map of an ancient ocean on Mars, underwater regions are indicated in white, and the above- and below-water regions are separated by a proposed sea level (solid line). [Saberi 2020]

A Liquid Controversy

Yet the Mars ocean hypothesis remains highly controversial.

The Martian dichotomy could have been caused by a megaimpact, or by thinning of the northern-hemisphere crust. Observed “stream channels” might instead have been created by wind erosion. And those possible ancient shorelines? They’re not actually level — they vary in elevation by a couple of kilometers along their length, which is not something a sea level should do.

On this other hand, the shorelines might have started level and been deformed more recently. Clearly, more evidence is needed in the debate over Martian oceans — and now we’ve got it, in a new study by Abbas Ali Saberi (University of Tehran, Iran; University of Cologne, Germany).

Moving the Level

Saberi argues that a giant ancient ocean covering part of Mars should have left distinct signatures in the elevation profile of the planet — signatures that we can identify in comparison with Earth’s topography. Saberi combines detailed maps of Mars’s surface altitudes from the Mars Orbiter Laser Altimeter with statistical modeling called percolation theory to search for these signatures.

As sea level height is changed in the authors’ models, the relative surface area of the largest island to the total area of Mars (red) and Earth (blue) shows a jump for both planets that corresponds to Earth’s current sea level and a sea level height of 1,400 m on Mars. [Saberi 2020]

By modeling an ocean covering Earth’s surface and varying the height of the sea level, Saberi shows that the planet undergoes a “phase transition” at a characteristic sea level when its land masses connect. For Earth, this transition occurs right around our present mean sea level, corresponding to oceans covering 71% of Earth’s surface (as we observe today).

When Saberi applies this model to Mars’s topography, a phase transition occurs in the exact same way, showing a distinct change at a sea level of 1,400 meters (measured relative to Mars’s equatorial radius of 3,396.2 km) and indicating that Mars may indeed have had an ancient sea of this height. This elevation also displays the longest contour lines in Mars’s topography, further strengthening the argument that it corresponds to an ancient sea level.

So is the question of past oceans on Mars settled? Far from it! But we can expect clues to continue to roll in with upcoming missions to the Red Planet, like the Mars 2020 mission launching later this month.

Citation

“Evidence for an Ancient Sea Level on Mars,” Abbas Ali Saberi 2020 ApJL 896 L25. doi:10.3847/2041-8213/ab982d

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