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Illustration of a giant planet with a fuzzy surrounding layer passing in front of the face of an active yellow star.

Low-density planets struggle to hold on to their atmospheres when they’re blasted with high-energy radiation from a close-by host star. New observations have caught a view of one such escaping atmosphere using a powerful tracer: helium.

Atmosphere on the Run

When a planet orbits close to its star, incoming ultraviolet radiation can heat and puff up the planet’s atmosphere, extending it so far that the gravitational pull of the planet can no longer hold it in. The mass loss that results from this process dramatically shapes the population of short-period exoplanets — so understanding atmospheric escape is critical to our understanding of planetary evolution.

transmission spectroscopy

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

But measuring a planet’s escaping atmosphere is challenging! At high altitudes, the atmosphere is thin and low-pressure, which means that most of the spectral signatures of this escaping mass — produced during transits when the planetary atmosphere absorbs background stellar light — are faint.

In 2018, however, a new discovery provided some hope: the first detection of helium in an exoplanet atmosphere.

Letting Helium Lead

Why is helium helpful? When a low-density planet is pelted with extreme ultraviolet radiation, this can produce a population of helium atoms in the planet’s upper atmosphere that exist in a long-lived excited state. This metastable helium absorbs photons even at the low pressures that accompany high altitudes, creating a prominent absorption feature at the near-infrared wavelength of 1,083 nm.

By hunting for this absorption line — which, since it falls in the infrared, can be observed even through the Earth’s atmosphere using ground-based telescopes — we can probe the extended atmosphere of close-in transiting planets, measuring how much mass the planets are losing through atmospheric escape.

Two light curves for an exoplanet transit.

Folded data and best-fit models showing the transit light curve and residuals for HAT-P-18b at 1,083 nm (top) and the corresponding broad-band light curve from TESS (bottom). The transit depth in the helium bandpass exceeds that in the TESS bandpass by roughly half a percent. [Adapted from Paragas et al. 2021]

This is precisely the detection made in 2018 for the gas giant orbiting WASP-107, and it’s now what a team of scientists led by Wesleyan University undergraduate Kimberly Paragas has succeeded in doing for the similar — but fainter — system HAT-P-18.

Loss from a Giant

HAT-P-18 is a K-type star located about 540 light-years away. The star hosts a gas-giant planet, HAT-P-18b, on a close-in, transiting orbit of just 5.5 days. Though the planet is roughly the size of Jupiter, it contains only 20% of Jupiter’s mass — making it very low-density and an excellent target to search for an escaping atmosphere.

Paragas and collaborators observed two transits of HAT-P-18b with the 200” Hale Telescope at Palomar Observatory in California, using an ultra-narrow band filter centered on the 1,083-nm line. In these observations, the team successfully detected excess helium absorption that allowed them to measure the planet’s escaping upper atmosphere.

By applying wind models to these observations, the authors show that HAT-P-18b is losing less than 2% of its mass per billion years.

HAT-P-18b is one of only a handful of planets whose extended atmosphere has been measured using helium, and it’s the faintest yet. This study therefore demonstrates the effectiveness of using mid-sized, ground-based telescopes to survey planets that lie close in around faint stars, providing a valuable opportunity to learn more about the evolution of this population.

Citation

“Metastable Helium Reveals an Extended Atmosphere for the Gas Giant HAT-P-18b,” Kimberly Paragas et al 2021 ApJL 909 L10. doi:10.3847/2041-8213/abe706

Computer simulation shows two elongated spheres starting to combine as their internal matter jumbles.

What’s the largest mass that a neutron star — the dense, collapsed core of a massive star — can grow to before further collapsing into a black hole? Recent gravitational-wave events are providing new insight.

Finding the Maximum

magnetar

Artist’s impression of a strongly magnetized neutron star. [NASA/Penn State University/Casey Reed]

Neutron stars consist almost entirely of neutrons packed together at the density of atomic nuclei. This extreme mass in such a small space results in an extraordinary inward gravitational pull that increases as more neutrons are packed in. When the crushing gravitational force exceeds the combined quantum and nuclear forces pushing outward, the star collapses to form a black hole.

What is the maximum mass limit above which a neutron star collapses? Theory suggests that, for a non-rotating neutron star, it’s somewhere around 2 or 3 times the mass of the Sun — but the precise value relies on the unknown state of matter inside the neutron star. To get around this missing information, we need observational constraints to help us pin down how heavy a neutron star can be.

Collisional Clues

In recent years, gravitational waves have provided valuable new insight. Two particular mergers of compact objects have tempted us with clues:

  1. GW170817
    In this event, two neutron stars in the range of 1.1–1.6 solar masses merged to form a larger object, which we think collapsed into a black hole shortly after merger. The gravitational-wave and electromagnetic observations of this process point to a maximum neutron star mass that’s less than 2.3 solar masses.
  2. GW190814
    In this event, a black hole of more than 20 solar masses merged with an object of just 2.5–2.7 solar masses — but we don’t know whether that smaller object was a black hole or a neutron star. If it was a non-rotating neutron star, then this would imply that the upper limit for neutron star mass is above 2.5 solar masses.

Can we reconcile these two potentially conflicting pieces of information? A study led by Antonios Nathanail (Institute for Theoretical Physics, Germany) presents new analysis that further explores what these mergers tell us about neutron star limits.

A Lower Upper Limit

Nathanail and collaborators analyzed these two mergers by employing a genetic algorithm — an algorithm that explores a large parameter space and looks for optimized solutions by mimicking the process of natural selection. Using this algorithm, the authors identified which maximum mass solutions are consistent with gravitational-wave and electromagnetic observations of GW170817 and GW190814 and numerical simulations of mergers.

Plot showing a probability distribution function centered at 2.2 solar masses.

Probability distribution function for the maximum mass of a non-rotating neutron star, as estimated by the authors’ genetic algorithm (blue curve) and in a previous study of GW170817 (purple curve). [Nathanail et al. 2021]

From their systematic investigation, the authors show that a large maximum neutron star mass — like the 2.5 solar masses required if GW190814’s secondary was a non-rotating neutron star — doesn’t mesh with our observations of GW170817 or with expectations from numerical simulations of gravitational wave production.

Instead, the authors find that a maximum neutron star mass of about 2.2 solar masses neatly reproduces the observations of GW170817 and is consistent with numerical simulations. This upper limit implies that GW190814’s secondary was too large to have been a non-rotating neutron star. Instead, GW190814 was likely the merger of two unequal-mass black holes.

Citation

“GW170817 and GW190814: Tension on the Maximum Mass,” Antonios Nathanail et al 2021 ApJL 908 L28. doi:10.3847/2041-8213/abdfc6

The European Space Agency–NASA Solar Orbiter was launched at the start of 2020 to study the Sun and its influence on the solar system. One of its instruments, Metis, has the ability to study the immediate surroundings of the Sun. Could Metis soon provide a new window into coronal mass ejections? 

A coronal mass ejection as seen by two coronagraphs (orange: LASCO C2, blue: LASCO C3) on the Solar and Heliospheric Observatory on February 27, 2000. [ESA & NASA/SOHO]

Mysterious Magnetic Fields Meet Their Match?

Coronal mass ejections (CMEs) are massive releases of magnetized plasma from the Sun’s surface. They are highly energetic and move extremely fast, sometime at millions of miles per hour. CMEs likely originate from interactions within the Sun’s complex magnetic environment, but there still remains a lot to be learned about them.

To study magnetic fields in the Sun, we can use a tool called spectropolarimetry to examine the polarization of certain wavelengths of light. Polarization is key here: it is the measurement of these oscillations in electromagnetic waves that allow us actually probe magnetic fields. Until now, CMEs have never been studied in this way — but that might change very soon!

The Metis instrument on the Solar Orbiter is a coronagraph; it is built to study the Sun’s corona at visible light and ultraviolet wavelengths simultaneously. Notably, Metis’s capability in visible light could allow it to measure the polarization of the D3 emission line, which is associated with helium. In a study, a group of researchers led by Petr Heinzel (The Czech Academy of Sciences, Czech Republic) examined how well Metis could observe this D3 line in the context of CMEs as well as what we could learn from those observations.

Fractional polarization signals EQ/EI (top) and EU/EI (bottom) versus magnetic field strength intensity at a temperature of 30,000 K. The contributions of the D3 line and visible light are both included. The dashed and solid lines correspond to combinations of different heights from the solar surface and plasma pressures. [Adapted from Heinzel et al. 2020]

Reading Between the Lines

Heinzel and collaborators first modeled CMEs and determined how the D3 line could be produced. Then, they considered how the D3 line would appear to Metis. At temperatures between 30,000 K and 50,000 K — very attainable for CMEs — the intensity of the D3 line is much greater than the intensity of the rest of the visible light wavelengths available to Metis. However, the polarizations associated with these two components of the visible light observations are all tangled up with each other. So what now?

The polarization of the D3 line will be affected by the CME magnetic field, and so the total observed polarization will change with magnetic field strength (and other relevant factors). These changes are very evident when we compare the intensities of different polarization parameters. Interestingly, the contribution of the D3 line can result in “depolarization”, or the lowering of certain polarization parameters.

So will Metis be able to tell us about the magnetic fields of CMEs? It’s likely! The authors of this study plan to do more detailed analysis on how CME features will affect what Metis can see, and if we can bring Metis’s ultraviolet capabilities into this science. They also note the exciting prospect of combining Metis’s data with those from the upcoming Proba-3 mission. Perhaps the mystery of CME magnetic fields won’t stay a mystery for long!

Citation

“On the Possibility of Detecting Helium D3 Line Polarization with Metis,” Petr Heinzel et al 2020 ApJ 900 8. doi:10.3847/1538-4357/aba437

Illustration of a reddened disk of matter surrounding a large black hole. A bright flash of white light lies in one region of the disk.

The swirling disks of material that surround supermassive black holes are likely home to massive stars, neutron stars, and black holes. A new study explores whether we can detect the signatures of fiery explosions produced by these uniquely situated stars and stellar remnants.

An Unusual Home

merger in AGN disk

Artist’s illustration of two merging black holes embedded in the gas disk surrounding a supermassive black hole. [Caltech/R. Hurt (IPAC)]

Recently, scientists detected gravitational waves from the merger of unexpectedly large black holes. One proposed explanation — that these monsters grew to their large sizes while embedded within the accretion disk surrounding an even larger supermassive black hole — has piqued interest in studying the evolution of stars hosted within the violent disks of these active galactic nuclei (AGN).

AGN accretion disks are dense, turbulent environments that produce bright, high-energy radiation as disk material spirals inwards toward the black hole. Yet these seemingly hostile surroundings may still host stars that arise either in situ — the gas within accretion disks can become unstable and fragment into self-gravitating clumps that become stars — or are captured from the nuclear star cluster that surrounds an AGN.

Explosive Ends

Once stars form or are trapped in an AGN disk, the dense environment increases the likelihood that the stars pair off into binaries. As disk-hosted stars evolve, some fraction of them should end their lives in spectacular explosions — either as long gamma-ray bursts (GRBs) caused by the deaths of massive stars, or as short GRBs produced when two evolved stellar remnants collide.

Two panel schematic diagram of an AGN disk and an expanding relativistic GRB outflow.

Schematic illustrating the location of an exploding star in an AGN disk, shown in cross section. Bottom: Illustration of relevant radii in observed GRBs. RIS is the location of internal shocks that usually powers prompt emission, and RES marks the location where the expanding outflow runs into the surrounding medium, powering the afterglow. The relative locations of these radii can change in a dense surrounding environment, leading to different emission signatures. [Perna et al. 2021]

The possibility of these relativistic explosions occurring within AGN disks is intriguing. Does the unique environment of the disk influence the explosion? If so, could we expect to see specific, identifiable features from GRBs produced within the disks around supermassive black holes?

A team of scientists led by Rosalba Perna (Stony Brook University and Flatiron Institute) has explored these questions by modeling how the properties of GRB explosions are changed when they occur within disks.

Searching for Signatures

Perna and collaborators explore a standard model of a GRB in which prompt emission is produced first as a series of internal shocks are driven by colliding shells of speeding material. The prompt emission is then followed by a long, decaying afterglow as this relativistic outflow is slowed when it plows into the surrounding matter.

The authors show that the properties of the AGN disk environment can change the behavior of both of those emission components. The high density of the disk material can cause a powerful reverse shock to be driven backwards early in the explosion, powering the prompt emission in place of internal shocks. And the later afterglow of the GRB can end up brighter and peaking earlier than is the case for typical GRBs observed in a low-density environment like the interstellar medium.

These features and other signatures identified by Perna and collaborators may help us to determine whether future observed GRBs exploded in typical environments, or instead in the extreme surroundings of an AGN disk. This will help us to better understand how some stars may be evolving in their unusual homes around supermassive black holes.

Citation

“Electromagnetic Signatures of Relativistic Explosions in the Disks of Active Galactic Nuclei,” Rosalba Perna et al 2021 ApJL 906 L7. doi:10.3847/2041-8213/abd319

Photograph of a radio telescope dish set into the landscape, surrounded by towers that support suspended cables across its face.

Some observatories — like the recently collapsed Arecibo Telescope in Puerto Rico — examine nearby objects by bouncing radio light off of them. A new study has now improved how we analyze these observations to learn about near-Earth asteroids.

Clues from Reflections

There’s plenty we can learn about the universe from passive radio astronomy, in which we observe the radio signals emitted by distant sources. But when it comes to objects that lie near the Earth, we have another option: active radio astronomy.

Grayscale image of a rocky surface.

Asteroid surfaces are complex, as evidenced by this up-close image from OSIRIS-REx of the surface regolith of asteroid Bennu. [NASA]

With radar astronomy, we’re in the driver’s seat: we send a beam of radio light in the direction of our target — perhaps a close planet like Venus, or a nearby asteroid — and then observe the reflected light that returns to us. By measuring timing differences in the reflected signal, we can map out the shape of the object and its motion.

What’s more, measurements of the polarization of the reflected light — the direction the light waves are vibrating — tell us about how the light was scattered from the surface and near-surface of the body. This, in turn, provides information about the outer material properties of the object. Does this material consist of fine-grained dust, or large boulders? How porous is it? How reflective?

The answers to these questions help us to comprehend the nature of bodies close to the Earth. This is especially useful in the context of near-Earth asteroids, where understanding the structure and composition of these potential hazards could be critical for mitigation tactics or spacecraft visitation.

Set of 15 images of an asteroid based on different polarization components.

Radar data for near-Earth asteroid 1999 JM8, broken down into different polarization components for three different observation dates. [Hickson et al. 2021]

Separating the Pieces

The catch? Interpreting radar polarimetry isn’t easy. To disentangle the combined information about a body’s surface roughness, particle shape, ice content, boulder abundance, composition, and viewing geometry, we often make inferences based on well-characterized surfaces like the Moon’s. But when the surfaces we’re studying are more complex — like those of near-Earth asteroids — the lunar analog may not apply.

To address this, a team of scientists led by Dylan Hickson (Arecibo Observatory) recently developed an improved methodology to analyze the ground-based radar polarimetry of near-Earth asteroids. Hickson and collaborators show how we can decompose the reflected radio images of asteroids to derive specific polarimetric products, and they then use numerical simulations to improve their interpretations of these signals.

The authors apply their methodology to archived radar observations of three near-Earth asteroids obtained by Arecibo, demonstrating that they can retrieve a wealth of information about the physical properties of the asteroids’ surfaces using this approach.

An Uncertain Future

Histogram showing the increasing detections of near-Earth asteroids each year, with Arecibo contributing a large fraction of recent detections.

This plot shows the number of near-Earth asteroids detected by radar each year between 1980 and 2021 (last updated 4 February 2021). Despite the collapse of Arecibo (blue), we can still expect future detections from Goldstone (red). [NASA JPL]

So what’s next for radar astronomy now that Arecibo has collapsed? This giant’s demise has left the Goldstone Solar System Radar in California as the only remaining radar astronomy facility in regular use at the moment.

Fortunately, we have archives that contain past data for more than 1,100 radar-detected asteroids and comets. Reanalysis of this content using the authors’ new methodology is certain to provide valuable information while the field of radar astronomy reshapes itself going forward.

Citation

“Polarimetric Decomposition of Near-Earth Asteroids Using Arecibo Radar Observations,” Dylan C. Hickson et al 2021 Planet. Sci. J. 2 30. doi:10.3847/PSJ/abd846

illustration of two merging black holes in the night sky above an L-shaped detector on earth.

Gravitational waves have revealed a wealth of information about distant black holes and neutron stars — but they can also provide large-scale insights into how our universe works. A new study explores how gravitational-wave detections may soon resolve the long-lived tension in measurements of our universe’s expansion.

An Expanding Problem

plot showing the different measurements made of H0 through the years. Local and global measurements are clearly not in agreement.

Some past measurements of H0 (click to enlarge). Black data points are local-universe distance-ladder measurements, which cluster around 73 km/s/Mpc; red data points are early-universe CMB measurements, which cluster around 67 km/s/Mpc. [Renerpho]

We know the universe is expanding, but we’re still not sure how quickly. The empirically derived value H0 — referred to as the Hubble constant or the Hubble-Lemaître constant — parametrizes the universe’s expansion rate. This controversial parameter, which describes how quickly galaxies are receding from us as a function of their distance from us, is traditionally measured in one of two ways:

  1. In the local universe, by determining the distances to and recession speeds of visible astronomical objects. This method relies on the distance ladder: the distances measured to far-off objects are built upon measured distances to nearer objects.
  2. On global scales, estimated by modeling measurements of the cosmic microwave background (CMB), relic radiation from the Big Bang.

The trouble? The values we come up with for H0 from these two methods are not consistent with one another! To resolve this tension, we need another way of measuring H0 that’s independent of these approaches. Enter: dark sirens.

The Call of Hidden Collisions

Dark sirens are the collisions of two black holes that, though they produce no light, can provide us with valuable distance information. When black holes merge, their gravitational-wave signal encodes a distance luminosity. By combining this piece of information with the physical distance to the merging black holes — identifiable if we can precisely pinpoint the host galaxy of the collision — we can get an independent measure of H0.

map showing the localization of GW170817 on the sky

This map shows the localization of the gravitational-wave, gamma-ray, and optical signals of the neutron-star merger detected on 17 August 2017. Upgrades to LIGO and Virgo should improve gravitational-wave localization, making it possible to identify the host galaxy of a merger even without electromagnetic counterparts. [Abbott et al. 2017]

The catch? Right now, our gravitational-wave technology isn’t quite good enough to identify a precise value of H0. This problem is rooted in two issues: the large uncertainty on the measured distance luminosity in the gravitational-wave signal, and the difficulty in accurately identifying the host galaxy of a black hole merger, which produces no electromagnetic counterpart.

But there’s hope! Future advancements in technology at gravitational-wave detectors may soon bring these precise measurements into reach, according to the calculations of a team of scientists led by Ssohrab Borhanian (Pennsylvania State University).

A Precise Future

Borhanian and collaborators explore a series of models for future detections of gravitational-wave events using current and upcoming detectors with increasingly advanced technology.

plot of delta(H0)/H0 for four different networks.

Estimated distribution of fractional errors in the measurement of H0 after 2 years of observing time for different networks of detectors. The left cluster (HLV+) represents 2G+ technology on a network consisting of only LIGO-Livingston, LIGO-Hanford, and Virgo detectors; the right cluster (ECC) represents the 3G technology of a future detector network. The different colors represent the authors’ different population models. [Adapted from Borhanian et al. 2020]

Upgrades to the Laser Interferometer Gravitational-wave Observatory (LIGO) and the European detector Virgo are currently underway. The authors show that this advancement to next-generation (known as 2G+) technology could allow these detectors to uniquely identify the host galaxies of binary black hole mergers, without the need for an electromagnetic counterpart to the merger.

Borhanian and collaborators estimate that with 2G+ technology, we’ll be able to measure H0 to a precision of 2% within 5 years — which is sufficient to resolve the Hubble tension. And if you want something to dream about, consider this: third-generation detector technology (which will include the proposed Einstein Telescope and Cosmic Explorer) will be able to measure H0 to within less than a percent. The call of dark sirens is leading us to a beautifully precise future!

Citation

“Dark Sirens to Resolve the Hubble–Lemaître Tension,” Ssohrab Borhanian et al 2020 ApJL 905 L28. doi:10.3847/2041-8213/abcaf5

A quasar sits on a sparse background of distant stars. The black hole is represented by a white circle at the center of a flat, pancake-like cloud of pink and blue dust and clouds. The black hole shoots out white jets and debris towards the upper-right and lower-left.

Some quasar host galaxies live in the early universe. This makes them especially interesting, since they had to have accumulated a lot of mass very quickly. Luckily for us, radio telescopes like ALMA can peer back in time and tell us more about these galaxies and their environments.

Finding Far-Off Quasars

ALMA

Antennas of the ALMA observatory under the Magellanic Clouds. [ESO/C. Malin]

Quasars are absurdly energetic objects. They are a version of the supermassive black holes at the centers of galaxies, and what makes them unique is the large amounts of energy they emit while actively accreting material. A significant portion of this energy is emitted as short-wavelength ultraviolet (UV) light, which is the key to studying quasars that live in the early universe.

The farther away an object is, the more its light becomes redshifted as it travels to us — that is, the wavelength at which light from the object was first emitted is shorter than the wavelength we observe when that light reaches us. So in the case of far-off quasars, their UV emission will be redshifted into radio wavelengths, where we can still observe it!

FIR (left) and [C II] (right) emission maps (distances shown in arcseconds) for two galaxies from this study. The redder regions indicate higher emission levels; bluer regions point to the absence of emission. [Adapted from Venemans et al. 2020]

In a recent study, a group of researchers led by Bram P. Venemans (Max-Planck Institute for Astronomy, Germany) used radio observations of distant quasar host galaxies to learn more about them, as well as conditions in the early universe.

Evidence from Emissions

All 27 galaxies in this study live at a redshift of roughly z = 6, or when the universe was just under a billion years old. Venemans and collaborators were especially interested in two types of emission from these galaxies: singly ionized carbon ([C II]) emission, which tracks the gas of the interstellar medium; and the general continuum brightness in the far-infrared (FIR), which is associated with dust. The spatial extent of the [C II] emission in particular is also sensitive to the motions of a galaxy and its surroundings.

The galaxies were observed by Atacama Large Millimeter/submillimeter Array (ALMA) in September 2019. The observations had a resolution of roughly a kiloparsec (or 19 trillion miles), which is pretty high definition for the early universe! This allowed Venemans and collaborators to examine the central regions of their galaxies. They were also able to probe the surrounding space for any companion galaxies.

Seeing Into the Center

Star formation rates versus distance from galaxy center. Each track represents a quasar host galaxy, with the color of the track corresponding to the FIR brightness of the galaxy. [Venemans et al. 2020]

It turned out that for the galaxies in this study, the central dust regions mapped closely onto the positions of central supermassive black holes. This may not sound like a profound observation, but it is observational evidence to support that these central black holes live at the hearts of dark matter halos, which are cosmological building blocks.

The [C II] emission revealed that about half of the quasar-hosting galaxies in this sample had companions. The FIR emission also allowed Venemans and collaborators to determine that in the central regions of their galaxies, star formation peaks at the center and then declines moving outward. The outer regions of these distant galaxies currently remain elusive, but as Venemans and collaborators noted, ALMA is quite capable of probing these galaxies further!

Citation

“Kiloparsec-scale ALMA Imaging of [C II] and Dust Continuum Emission of 27 Quasar Host Galaxies at z ~ 6,” Bram P. Venemans et al 2020 ApJ 904 130. doi:10.3847/1538-4357/abc563

Composite image showing an explosive outflow that looks like a firework set against a backdrop of stars.

There’s still much we don’t know about the birth of massive stars — stars with more than 8 times the mass of the Sun. A recent study reveals details of a thousand-year-old explosion that might provide clues about the formation of these giants.

An Unexpected Explosion

Orion Nebula

The clouds of molecular gas in regions like the Orion nebula provide nurseries in which massive stars form and evolve. [ESO/G. Beccari]

Several decades ago, astronomers discovered something odd. In a region inside the Orion nebula where massive star formation is underway, scientists detected signs of an explosive outflow: dense molecular gas streaming outward from a central point at rapid speeds. Surprisingly, there was nothing at the center of this explosion.

This one-off discovery was intriguing. One could imagine a number of sudden, energy-liberating events that could occur in a massive star-forming environment — like the formation of a close massive stellar binary, or the merger of two young, massive protostars. And the discovery of several candidate runaway stars at the fringes of the explosion provided another hint to a dynamical origin.

Could this explosion help us understand the process of how massive stars form in their birth environments? Or was it just a fluke event? As years passed without astronomers finding evidence of another, similar outflow, these questions remained unanswered.

Map of SiO molecular gas shows streams of material moving outward from a central point.

This ALMA SiO map of the star-forming region G5.89 shows outflowing molecular gas surrounding an expanding, shell-like HII region (white contours). Two stars moving away from the origin are marked in magenta and cyan. [Adapted from Zapata et al. 2020]

Two of a Kind

Forty years later, we now have proof of another such explosive outflow in a massive star-forming environment. In a recent publication led by Luis Zapata (UNAM Radio Astronomy and Astrophysics Institute, Mexico), a team of scientists has used the Atacama Large Millimeter/submillimeter Array (ALMA) to confirm the presence of streamers of molecular gas flowing isotropically outward from a central point in the massive stellar birthplace G5.89, which lies roughly 10,000 light-years away from us.

Zapata and collaborators measured 34 molecular filaments in this explosive outflow, finding that the streamers are accelerating as they expand outward. This is consistent with behavior of the Orion explosion and shows that the density of the ejecta is substantially larger than the surrounding medium.

As with the Orion explosive outflow, the point of origin of the filaments contains no source. Previous studies, however, have identified several young, massive stars in the periphery of the G5.89 explosion that are speeding away from the point of origin at roughly the right speed to have been at the center 1,000 years previously at the time of explosion.

Learning about Stellar Birth

illustration of dust and gas swirling around a bright, newly forming star.

A protostar lies embedded in a disk of gas and dust in this visualization. The collision of two protostars could release enough energy to power an explosive molecular outflow — and produce a massive star. [NASA’s Goddard SFC]

What does all this tell us about the origins of massive stars? Explosive outflows like this — caused by dynamical interactions during the birth of massive stars — may be more common than we previously thought!

The authors estimate a rate for such outflows based on our limited observations, finding that there should be one every ~100 years. The fact that this is very close to the rate of supernovae further solidifies the connection of explosive molecular outflows to massive star formation.

Dedicated, high-sensitivity searches for more such outflows in nearby massive star-forming regions will certainly go a long way toward confirming this theory. In the meantime, the authors argue, we should consider revising high-mass star formation models to include dynamical interactions, as these stellar explosions may prove to be regular occurrences!

Bonus

The animation below shows a different view of the authors’ ALMA-observed streamers, traced by CO gas. Two axes give the position of observations, while the third axis and the colors show the radial velocity at each point in the streamers, showing how the ejecta are accelerating as they expand outward. The star marks the origin of the explosive outflow.

Citation

“Confirming the Explosive Outflow in G5.89 with ALMA,” Luis A. Zapata et al 2020 ApJL 902 L47. doi:10.3847/2041-8213/abbd3f

Left: Drawing of a disk representing the sun's surface, with several dark clusters of spots colored in. Right: image of the sun taken at 193 Å.

Astronomers have drawn detailed maps of dark spots on the Sun’s surface since Galileo’s time. Today, we have a host of modern spacecraft that make these observations for us, continuously charting the shifts in sunspot patterns and solar magnetic fields. Can computers help us to bridge between these historical and modern datasets?

A Long-Lived Record

photograph of a tall tower with a telescope dome at the top, surrounded by pine trees

Photograph of the 150-ft solar tower at Mt. Wilson Observatory, where daily sunspot drawings have been produced since 1912. [Susanna Kohler]

Every clear day since 1912, an observer at the Mt. Wilson Observatory near Los Angeles has hand-drawn a map of the dark spots on the face of the Sun — tracers of magnetic activity at and beneath the solar surface. This meticulous practice dates back to long ago: the first known sunspot drawings are from the year 1128 AD! Perhaps most famous among the astronomers who have undertaken this task is Galileo, whose early telescope allowed him to record detailed changes in sunspot geometry over a span of several months in 1612.

Historical sunspot records have provided valuable insight into the behavior of our nearest star. But today, we can also gather more sophisticated solar data. Space-based telescopes like the Solar Dynamics Observatory (SDO) monitor the Sun’s emission at a variety of wavelengths and produce magnetograms that reveal the magnetic field arrangements across the Sun’s surface. These modern observations allow us to explore the Sun’s magnetic flux and the light emitted from solar active regions — information that can tell us more about how the Sun’s activity evolves and how it impacts the Earth.

What if we could gain this same level of insight from historical daily sunspot drawings? Led by Harim Lee, a team of scientists from Kyung Hee University in the Republic of Korea has undertaken the challenge of translating sunspot drawings into something that more closely resembles modern satellite data.

photograph of a sunspot drawing covered in labels of regions

Photograph of a sunspot drawing produced at the Mt. Wilson Observatory. [Mt. Wilson Observatory]

Teaching a Computer to Translate Drawings

Lee and collaborators recognized that the 100+ years of daily sunspot drawings from Mt. Wilson Observatory have a significant benefit: there is overlap between these drawings and modern satellite data. The authors compiled a set of more than a thousand Mt. Wilson sunspot drawings from 2011 to 2015 that they then paired with the corresponding daily ultraviolet/extreme ultraviolet (UV/EUV) images and magnetograms captured with SDO.

The next step: train a computer to map between the datasets. Using a training set of 1,046 pairs of sunspot drawings and SDO images, Lee and collaborators developed a deep learning model that takes a sunspot drawing as input, and generates a magnetogram and a set of seven mock SDO images at different UV/EUV wavelengths as output.

eight-frame set of pairs of observations showing real vs. mock images of the sun.

Comparison between real SDO images (left image of each pair) and the model-generated ones (right image of each pair) for sunspot observations on 8 June, 2014. Click to enlarge. [Lee et al. 2021]

The authors then used the remaining 204 pairs of sunspot drawings and SDO observations to evaluate the success of their model, demonstrating that it accurately reproduces the bipolar structures of the magnetograms and the approximate geometry of active regions on the Sun.

Finally, Lee and collaborators apply their model to Galileo’s original sunspot drawings from 1612, generating mock SDO images and magnetograms for a time more than four centuries ago.

The authors note that this unique method of modernizing historical data is, of course, limited in what it can reproduce — but it does provide us with unusual insight into the long-term evolution of our Sun’s magnetic fields and radiation.

Citation

“Generation of Modern Satellite Data from Galileo Sunspot Drawings in 1612 by Deep Learning,” Harim Lee et al 2021 ApJ 907 118. doi:10.3847/1538-4357/abce5f

Illustration of magnetic field lines extending in a tail beyond the earth. the moon lies within the region shielded by the magnetic field.

Given plans for future manned missions to the Moon — and interest in the potential for longer-term lunar habitation — the presence of water on the Moon is of critical importance. Studies over the last few decades have revealed water lurking on our satellite in numerous forms. But how does it get there?

Water In, Water Out

Two maps, each showing a lunar pole, that indicate the locations of water measured by M3.

Overview of the lunar OH/H2 abundance in the polar regions of the Moon, as derived from M3 observations in January/February 2009. [Adapted from Wang et al. 2021]

Lunar water has been found locked in ice form in the cold, permanently shadowed craters at the Moon’s poles, and drifting in gas form in the very thin lunar atmosphere. In addition, we’ve discovered that water exists in trace amounts across the Moon’s surface, bound to lunar minerals.

But lunar water is more complicated than its mere presence or absence. The Moon is also thought to have a water cycle — water is continuously created on or delivered to the Moon’s surface, and then destroyed on or removed from it.

Understanding the driving processes in this cycle will enable us to best leverage the Moon’s resources and deepen our insight into the physics that influences airless rocky bodies throughout our solar system and beyond.

Identifying Processes

Based on laboratory experiments and lunar observations, here’s our understanding so far:

  1. Production
    We think the continuous production of lunar surface water may largely be driven by incoming protons — hydrogen nuclei — from the solar wind, which then bind with the oxygen in lunar minerals to form water. Other processes may also contribute, like production from additional sources of incoming protons, or episodic delivery of water via comets and asteroids.
  2. Removal
    Water on the Moon’s surface is primarily removed through continuous processes like photodissociation — the decomposition of water molecules by sunlight.

With the rich observations recently produced by missions like NASA’s Moon Mineralogy Mapper (M3) spectrometer on India’s Chandrayaan-1 orbiting probe, we’re currently in an excellent position to test this understanding.

In a new publication led by Huizi Wang (Shandong University and Chinese Academy of Sciences), a joint team of space physicists and planetary scientists presents an exploration of water production on the surface of the Moon.

Windy Production

Schematic showing the orbit of the moon around the earth and the portion of the lunar orbit where the Moon is shielded from the solar wind by the earth's magnetosphere

Schematic showing the Moon’s orbit around the Earth. The Moon spends 3–5 days each orbit passing through the Earth’s magnetosphere, where it is shielded from the solar wind. [Adapted from Wang et al. 2021]

As the Moon circles the Earth, it spends 3–5 days each month shielded from the solar wind by the Earth’s magnetosphere. If incoming protons from the solar wind are the primary driver of lunar water production, Wang and collaborators argue, then measurements of lunar water abundance should show a decrease during those 3–5 days, assuming water continues to be destroyed at the same rate via photodissociation.

Instead, the authors find that spectroscopy from M3 reveals no change in water abundances over the complete lunar orbit, despite observations showing the expected drop in incoming solar wind energy when the Moon passes through Earth’s magnetosphere.

Could another source contribute to production of water on the Moon, keeping abundances constant? Wang and collaborators demonstrate that when the Moon is shielded from the solar wind, incoming protons from the Earth’s wind — a weaker stream of charged particles from the Earth’s magnetosphere — could provide the protons needed to maintain observed water abundances on the Moon’s surface.

There are still many open questions, but the future holds more opportunities to refine our understanding. The Chinese Chang’e 5 lunar mission successfully measured lunar material and brought samples back to Earth late last year, and the planned Artemis missions to the Moon will soon provide further insight.

Citation

“Earth Wind as a Possible Exogenous Source of Lunar Surface Hydration,” H. Z. Wang et al 2021 ApJL 907 L32. doi:10.3847/2041-8213/abd559

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