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Galactic cosmic rays consist of highly energetic particles coming from sources outside our solar system. The particles that make up these rays have some electric charge, so when the rays enter our solar system they are affected by the Sun’s magnetic field. But how much influence does the Sun actually exert on galactic cosmic rays?

The Sun’s sphere of influence as it travels through the interstellar medium, with the heliosphere, heliopause, and heliosheath marked. The Voyager spacecraft are also shown. The blue circle around the Sun shows the termination shock, where the solar wind goes from supersonic to subsonic speeds. [NASA/JPL-Caltech]

Making Space with Magnetic Fields

The Sun is a mighty presence, and that’s not just in terms of its gravitational influence! With the solar wind, the Sun carves out a roughly elliptical region in the interstellar medium that extends at least 11 billion miles in one direction (for context, the Earth is usually 93 million miles from the Sun). This region is called the heliosphere.

The Sun is able to create the heliosphere thanks to its powerful magnetic field. A quirk of this magnetic field is that its poles switch every 11 years or so, corresponding to what we call the solar cycle. This is a very short timescale when it comes to astronomical phenomena, so with the right instruments, we could study how the Sun’s changing magnetic field affects the passage of charged particles like those in galactic cosmic rays, and how that effect changes with time. 

We can detect galactic cosmic rays using Earth-based observatories, but we’re limited in that we can only see these rays as they appear at 1 astronomical unit (au) from the Sun. But what if we had another galactic ray observatory in a different part of the solar system?

The Cassini spacecraft happens to fit the bill! In a recent study, researchers led by Elias Roussos (Max Planck Institute for Solar System Research, Germany) used Cassini data to augment Earth-based observations of galactic cosmic rays and determine how these rays travel through the solar system over time.

The particle flux as measured by Cassini and the Earth-based observatories over time. The normalization point indicates when all the observatories were at roughly 1 au from the Sun. [Adapted from Roussos et al. 2020]

Cosmic Rays as Seen by Cassini and Others

The Cassini spacecraft observed Saturn from 2004 to 2017, putting it at 9.5 au from the Sun during its mission. One of Cassini’s instruments was the Low Energy Magnetospheric Measurement System (LEMMS), which could measure galactic cosmic ray fluxes. LEMMS has been used in cosmic ray studies before, but Roussos and collaborators made sure to utilize LEMMS data that was taken during Cassini’s flyby of Earth on its way to Saturn. This allowed them to compare the LEMMS data taken at 1 au to data taken at Earth-based observatories.

Aside from Cassini, Roussos and collaborators also used data from the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), the Alpha Magnetic Spectrometer-2 (AMS-02), the Balloon-Borne Experiment with a Superconducting Spectrometer (BESS), and BESS-Polar. Luckily, BESS happened to be operating around the time of Cassini’s flyby, allowing for even better calibration of the LEMMS data.

The galactic cosmic ray gradient over time. The shaded rectangles highlight regions where gradient “enhancement” is seen. [Adapted from Roussos et al. 2020]

An Evolving Gradient

The specific quantity Roussos and collaborators wanted to track over time, as the Sun’s magnetic field changed, was the radial intensity gradient of galactic cosmic rays: how the strength of rays varies with distance from the Sun. The data they used were taken over one solar cycle, with 2006–2014 corresponding to negative polarity in the Sun’s magnetic field and 2014–2017 corresponding to positive polarity. 

Roussos and collaborators found that during negative polarity, the galactic cosmic ray gradient is roughly 3.5–4% per au, while during positive polarity it drops down to 2% per au. The gradient reached 4% per au during what we call solar maximum, when the Sun shows the most surface activity during the solar cycle. Yearly to biennial variations in the gradient were also observed.

Roussos and collaborators noted that their measurements of the galactic cosmic ray gradient weren’t meant to be prescriptive of all solar cycles and particles — just the observed solar cycles and protons with specific energies. However, spacecraft like the Mars orbiters and New Horizons will offer even more insight into galactic cosmic rays. So stay tuned!

Citation

“Long- and Short-term Variability of Galactic Cosmic-Ray Radial Intensity Gradients between 1 and 9.5 au: Observations by Cassini, BESS, BESS-Polar, PAMELA, and AMS-02,” Elias Roussos et al 2020 ApJ 904 165. doi:10.3847/1538-4357/abc346

Photo of an edge-on galaxy encircled by a faint stream of stars.

The Milky Way is enwreathed in long streams of stars that hold clues to everything from our galaxy’s history to the nature of dark matter. New research has now identified the likely origins of some of these subtle ribbons.

Streams Across the Sky

Plot showing 23 stellar streams in orbital phase space.

The orbital energy vs. angular momentum of the stars in 23 of the Milky Way’s stellar streams (colored and labeled data), as compared to field stars (black data). [Bonaca et al. 2021]

Stellar streams are associations of stars that are grouped into elongated filaments arcing around a host galaxy. These filaments are thought to be produced when a stream progenitor — like a globular cluster or a satellite dwarf galaxy — is disrupted by its host galaxy’s tidal forces. Stars are drawn out from the progenitor into a tidal stream that then orbits the host galaxy; the progenitor itself may remain connected to the stream, orbit separately, or disrupt entirely.

We’ve observed stellar streams in other galaxies (like NGC 5907, shown above), but we needn’t look that far away — our own Milky Way is host to more than 60 catalogued streams. Of these thin trails, only a handful have been connected to a known progenitor, like a surviving globular cluster. The rest have unknown origins, leaving a number of open questions that only now, with current observations, have answers within reach.

In a recent study led by Ana Bonaca (Center for Astrophysics | Harvard & Smithsonian), a team of scientists has leveraged the incredible precision of the Gaia space observatory to hunt for the origins of 23 cold stellar streams in the Milky Way halo.

Orbital phase space plot showing locations of streams and possible progenitor galaxies and clusters.

Locations in orbital phase space of the 23 stellar streams, labeled by whether they have a dwarf galaxy progenitor (pentagon) or globular cluster progenitor (star). Only one stream, Svöl, falls into the region associated with possible in situ formation (rather than having been brought in via a dwarf galaxy). [Adapted from Bonaca et al. 2021]

A Disrupted Home

Bonaca and collaborators make use of improved proper motions provided in Gaia’s Early Data Release 3 for stars in these 23 streams. By analyzing the energies and 3D angular momenta of these streams, and by examining how the streams are distributed in physical space, the authors are able to identify the probable progenitors for most of the streams.

According to the authors’ results, only 1 of the streams plausibly originated from a globular cluster that was born in the Milky Way. The vast majority instead originated from dwarf galaxies that the Milky Way has accreted. Some of the streams were produced from the dwarf galaxies themselves; others were likely formed from disrupted globular clusters that orbited those dwarf galaxies.

Several of the 23 streams have similar properties, suggesting that many originated from the same progenitors. The authors identify original host dwarf galaxy candidates for 20 of the streams, and they point to 6 specific globular clusters as the origin of 8 of the streams.

Illuminating Dark Matter

Sky map showing locations of 8 stellar streams.

Sky map showing the 6 globular clusters (crosses) that the authors associate with 8 stellar streams (circles). [Adapted from Bonaca et al. 2021]

What can we do with this information? Understanding the origin of these stellar streams allows us to better trace their paths, how long they’ve been orbiting, and what other gravitational interactions they may have had over time. These details are valuable not just for understanding galaxy evolution, but also for mapping out the big-picture distribution of dark matter in our galaxy and studying the small-scale structure of dark matter in the streams’ host galaxies.

Further expansion of Bonaca and collaborators’ work to the other stellar streams orbiting the Milky Way will rely on continued high-quality proper motion measurements of these faint and distant sources. Look for more results as future Gaia data is released!

Citation

“Orbital Clustering Identifies the Origins of Galactic Stellar Streams,” Ana Bonaca et al 2021 ApJL 909 L26. doi:10.3847/2041-8213/abeaa9

Photo of the craters on mars's surface.

On 4 March 2021, the Perseverance rover began rolling through Jezero crater on the surface of Mars. How old is the terrain it’s exploring? A recent study provides an updated answer.

An Impactful History

Photo of the surface of mars centered around Jezero crater.

The landing site of the Perseverance rover on the floor of the Jezero crater. [NASA/JPL-Caltech/University of Arizona]

On Earth, we can determine the ages of surfaces by physically interacting with them. To establish how old a region is, we take a sample of the surface rocks and measure the decay of naturally occurring radiative isotopes within the sample; this technique allows us to identify the rocks’ absolute age. 

But how do we analyze the ages of more distant surfaces that we can’t touch, like planetary bodies in our solar system? This is where impact craters come in. Throughout their lifetimes, bodies in our solar system are peppered with small and large impactors that leave their mark in the form of impact craters. We can use these signatures to build a crater chronology — a timeline that allows us to interpret the ages of different surfaces on a body.

Photograph of a basalt rock in a display case

Lunar olivine basalt, collected by the crew of Apollo 15 and brought back to Earth. [Wknight94]

Building a Timeline

Relative aging of cratered surfaces is straightforward: older surfaces have a higher density of craters than newer ones, because older surfaces have had more time to be bombarded by rocky impactors. Areas of newer geology — for instance, where a lava flow resurfaced a region — have had less time to accumulate impact craters.

But how do we anchor these relative ages in an absolute timescale? That gets trickier. To start with, we need a calibration point. The Moon is ideal for this: we’ve brought back samples of lunar rocks and radiometrically dated them. These absolute dates then provide anchor points that allow us to establish a chronology for the lunar surface.

The Crux Is the Flux

To extrapolate this chronology to other bodies in our solar system — like Perseverance’s new home, Mars — we need two main things: good observations of the body’s crater counts, and an understanding of how the flux of impactors of different sizes has evolved over our solar system’s history.

Two maps showing simulated crater distribution on Mars's surface.

These two simulated crater maps represent different potential bombardment histories for Mars: the top represents an early heavy bombardment ~4.5 billion years ago, and the bottom represents a late heavy bombardment ~4.1 billion years ago. [Marchi 2021]

This latter point is especially challenging. Our best guess as to the past fluxes of different populations of impactors depends on our understanding of the dynamical evolution of the early solar system; as our models change, so do the estimated fluxes. In a new study, scientist Simone Marchi (Southwest Research Institute) has used the latest dynamical models to update Mars’s crater chronology.

Traversing Old Terrain

Marchi’s updated timelines change our predicted ages for Mars’s surface — including for the crater that Perseverance is now exploring. According to the author’s chronology, the dark regions of Jezero crater may be ~3.1 billion years old, which is up to 0.5 billion years older than was previously thought.

Mars’s crater chronology will doubtless be updated again as we continue to improve our models. But we have another prospect for anchoring this planet’s timeline: Perseverance’s goals include caching samples for future return to Earth. If successful, we’ll eventually have some Mars rocks on hand for radiometric dating, providing valuable insight into our solar system’s evolution.

Citation

“A New Martian Crater Chronology: Implications for Jezero Crater,” Simone Marchi 2021 AJ 161 187. doi:10.3847/1538-3881/abe417

Four images of the event horizon of a black hole, with lines drawn in to show the encircling magnetic fields.

Almost two years ago, the Event Horizon Telescope team grabbed the world’s attention with a stunning first picture of the inner regions around a supermassive black hole. Now the team is sharing new insight from their unprecedented observations.

A Giant Telescope’s Target

Four photos of a bright ring of emission around a black hole's shadow.

EHT observations of M87 taken over 4 days revealed a bright, asymmetric ring; north is up and east is left. [EHT Collaboration et al 2019]

The Event Horizon Telescope (EHT), a joint network of observatories spanning the globe, recently published its first images of the supermassive black at the center of the nearby active galaxy M87. This series of 4 images, captured in April of 2017, revealed an eerie, glowing ring of hot, magnetized plasma at the event horizon surrounding the “shadow” of the black hole M87*.

Why target M87*? This black hole is relatively nearby, it’s enormous (6.5 billion solar masses!), and it doesn’t vary too quickly. What’s more, M87* is the source of a spectacular, 5,000-light-year-spanning jet.

Looking for Launch

Jets are produced when accreting material is flung out from the poles of a supermassive black hole at incredible speeds — but the means by which they are launched, accelerated, and shaped, and even how they emit light, are all still open questions.

Could M87 help us to better understand these dramatic phenomena? The EHT team has now released two exciting new publications that provide additional insight into the environment close around M87*, from which its jet originates.

Photograph of a long, narrow jet emitted from a bright source.

M87’s supermassive black hole produces a collimated jet, visible in this Hubble image. Its counter-jet isn’t seen because relativistic effects make the receding jet appear less bright. [The Hubble Heritage Team (STScI/AURA) and NASA/ESA]

An Added Dimension

When the EHT observed M87* in 2017, it didn’t just capture the data that led to the total intensity images we’ve all seen; it also captured information about the polarization of the light observed.

When light is emitted from hot, magnetized plasma, it is linearly polarized — the magnetic field leaves an imprint on the direction the electromagnetic waves oscillate. As the light travels, this polarization can then rotate or become scrambled as it moves through magnetized matter.

The direction and amount of polarization that we ultimately observe from a source like M87* — if properly disentangled and analyzed — reveals information about the structure of the magnetic fields and the plasma properties close around this black hole.

What We Found and What It Means

Four polarization maps for M87*.

Polarization maps of M87* captured by the EHT on 4 different days. The ticks show the polarization direction and fraction. [EHT Collaboration et al. 2021]

The EHT team’s carefully constructed polarized intensity maps around M87*’s shadow provide a few key insights:

  1. The emission ring around the black hole exhibits polarization, but the polarization fraction is relatively low.
    This confirms the picture of a hot, magnetized plasma in the inner regions around the black hole, and it indicates that the polarization is scrambled inside the emission region, on scales smaller than the telescope can resolve.
  2. Across the 4 different observations spanning roughly a week, the polarization evolves.
    These changes over time are expected, and while the team doesn’t analyze it here, this evolution will likely help us to further constrain models in the future.
  3. In all observations, the polarization pattern is largely azimuthal, wrapping around the black hole.
    This is important: this directionality places significant limits which models can feasibly describe the structure of the magnetic fields and the accretion flow immediately around the black hole.

A MAD Flow?

The authors compare the observed polarization maps for M87* to a gallery of 72,000 snapshot images from numerical simulations probing 120 different models of the accretion flow and jet. The tight constraints from the EHT’s intensity and polarization observations dramatically reduce the set of feasible models, suggesting that the surroundings of M87* are best described by a model called a magnetically arrested disk (MAD).

schematic illustrating magnetic field lines threading through an accretion disk

Schematic illustrating the MAD model, as viewed from within the plane of the accretion disk around a black hole. Poloidal magnetic field lines pile up close to the black hole, pushing back on the infalling matter. [Narayan et al. 2003]

In the MAD model, piled-up poloidal magnetic fields close to the black hole are strong enough to affect how matter accretes onto the black hole. This influence and the arrangement of magnetic fields implied by this model constrain the possible mechanisms that lead to the launch of the jet.

With possible models in hand, the authors conclude by using them to estimate the properties of the plasma around M87* and the accretion rate onto this supermassive black hole — finding it’s likely accreting around a Jupiter mass each year.

Looking to the Future

It’s clear that there’s still a lot we can learn from the EHT’s first observations — and with follow-up observations and analysis, we can continue to use M87* as a laboratory for exploring supermassive black holes and their accretion flows and jets.

What’s more, we know the EHT is still working to create images of our own supermassive black hole at the center of the Milky Way, Sgr A*. If successful, this endeavor will provide complementary insight into a somewhat quieter supermassive black hole. EHT continues to brighten our outlook on black holes!

For more information, you can see the complete collection of EHT results in the ApJL focus issue:
Focus on the First Event Horizon Telescope Results

Citation

“First M87 Event Horizon Telescope Results VII: Polarization of the Ring,” EHT Collaboration et al 2021 ApJL 910 L12. doi:10.3847/2041-8213/abe71d
“First M87 Event Horizon Telescope Results VIII: Magnetic Field Structure Near the Event Horizon,” EHT Collaboration et al 2021 ApJL 910 L13. doi:10.3847/2041-8213/abe4de

FRB

Fast radio bursts are mysterious astronomical phenomena — for now. To understand how they form, we need to take a closer look at where they live. A new study does just that, with the help of some very sensitive astronomical instruments.

The Gran Telescopio Canarias during the day. [University of Florida]

The Fascination of Fast Radio Bursts

Fast radio bursts (FRBs) are exactly what they say they are: short, bright radio signals that last milliseconds at most. Their energy levels make them especially intriguing, since there aren’t many processes that can produce such large amounts of energy so quickly. Another constraint is that FRBs have been detected in all kinds of galaxies, meaning that whatever produces FRBs can’t be overly unique.

Radio telescopes today have the capability to precisely isolate FRBs in their host galaxies, meaning that we can probe the environments that produce FRB sources. The closest known FRB we’ve confidently isolated is called FRB 20180916B (though see this post for a new discovery that may be closer!), which is nearly 500 million light-years away. High-resolution observations have shown that FRB 20180916B is located in a distinct star-forming region, but what can we see if we look even closer?

In a recent study, a group of researchers led by Shriharsh P. Tendulkar (Tata Institute of Fundamental Research, India) studied the surroundings of FRB 20180916B in the highest detail yet, getting down to a scale of hundreds of light-years.

The velocity of gas in the host galaxy of FRB 20180916B, with the location of the FRB shown by a red cross. The contours come from Hubble images of the galaxy and depend on the flux detected in the image. [Adapted from Tendulkar et al. 2021]

Searching Through Gas and Stars

For their study, Tendulkar and collaborators used the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope and the MEGARA spectrograph on the Gran Telescopio Canarias. Taken together, the observations span mainly optical wavelengths, which are sensitive to gas and stars.

The gas serves two important functions: it can be used to determine how much star formation is happening in a region, and it can also be used to measure motion. Tendulkar and collaborators used the latter property to determine that FRB 20180916B’s home region is likely rotating with the large galaxy in its vicinity. This rules out the possibility that the FRB source is actually hosted in a smaller, less distinct satellite galaxy.

The star-forming region closest to FRB 20180916B as seen by Hubble, with its V-shape highlighted. The FRB’s location is shown by the green ellipse with a green arrow pointing towards it. [Adapted from Tendulkar et al. 2021]

Running Away from Home

Tendulkar and collaborators also found that the star formation happening around FRB 20180916B is at an interesting stage: it’s not extremely active, but it hasn’t gone placid either, suggesting that the region is still rather young.

FRB 20180916B is also a significant distance from the nearest group of stars. So, if the FRB source was born in that group, it had to have traveled between 800,000 to 7 million years to get to where it is now. This puts constraints on what the source of FRB 20180916B is, since not many astronomical objects can remain as energetic as FRB sources as they age.

So what’s behind FRB 20180916B? After considering possible scenarios, Tendulkar and collaborators zero in on X-ray or gamma-ray binaries, which consist of a neutron star and a massive companion star. However, to be certain that these sorts of objects are FRB sources, we’d need large samples of well-studied binaries — which is certainly doable with the radio telescopes we have now!

Citation

“The 60 pc Environment of FRB 20180916B,” Shriharsh P. Tendulkar et al 2021 ApJL 908 L12. doi:10.3847/2041-8213/abdb38

Illustration of a white dwarf surrounded by a disk and emitting jets, siphoning mass off of a large red companion star.

“One of the advantages to being disorganized is that one is always having surprising discoveries.”
—A.A. Milne

So begins a recent publication exploring the mystery of CN Cha, an unexpected discovery found in the “disorganization” of vast archives of data. What did we find, and how can we learn from it? The story starts with an unexpectedly luminous star in the Gaia mission’s second data release.

Morphing Appearances

Two images showing CN Cha in 1991 and 2016.

A comparison of two images of CN Cha (red crosshairs), the top taken in April 1991 and the bottom in June 2016. [Adapted from Lancaster et al. 2020]

CN Cha is a star located in the direction of the Chamaeleon constellation. This object was first recorded in 1963 as a Mira variable — a massive red-giant star that varies in luminosity regularly as the star expands and contracts.

But when a team of scientists led by Lachlan Lancaster (Princeton University) combed through a recent Gaia catalog looking for interesting bright and distant objects for spectroscopic follow-up, they found a different description of CN Cha: an unusually luminous star that’s not variable.

What followed for Lancaster and collaborators was a detailed dive into decades of archival photometric data — data produced by more than a dozen different observatories and ranging from infrared to ultraviolet wavelengths.

An Eruptive History

By cobbling together this archival data, Lancaster and collaborators were also able to piece together CN Cha’s unusual story.

CN Cha started out with all the properties of a Mira variable star — but then, in 2013, it underwent a spectacular outburst, brightening by about 8 optical magnitudes. For roughly 3 years, it maintained this brightened state, before starting to dim at a rate of 1.4 magnitudes per year.

The authors use this history and new spectroscopic observations to identify the most likely explanation for this mystery: CN Cha is probably a symbiotic binary system that recently experienced a long-duration nova eruption.

Plot showing light curves from 3 surveys for CN Cha.

Photometry from the ASAS (black), APASS (light-blue) and the ASAS–SN (pink) surveys showing the outburst and 3-year plateau in CN Cha’s optical luminosity. Click to enlarge. [Adapted from Lancaster et al. 2020]

Puzzling Behavior

Symbiotic binaries consist of an evolved star — in the case of CN Cha, presumably a Mira variable — in an orbit with a white dwarf. As mass is transferred onto the white dwarf, it can ignite thermonuclear fusion, causing the system to go into outburst.

The identification of CN Cha as a slow symbiotic nova is intriguing because there are only a few known examples of these outbursts. And though most of CN Cha’s properties are perfectly consistent with those of other slow symbiotic novae, the 3-year extent of its optical brightness plateau is unusually short for this class of transients: one to several decades is more common.

Organizing the Future

Photo of a telescope dome and associated building on a tall mountaintop at sunset.

The Vera Rubin Observatory, pictured under construction in this image from May 2019, will soon be a new source of large quantities of time-domain survey data. [LSST Project/NSF/AURA]

So what can we learn from this mysterious source? First, further study of CN Cha may provide valuable insights into symbiotic novae, the evolution of stars in the galactic thick disk, and even the possible progenitors of Type Ia supernova eruptions.

But what’s more, CN Cha’s story underscores how many discoveries are still hidden in the vast — and rapidly growing — quantities of human-collected astronomical data.

The astronomy community is making strides toward developing better systems and tools that centralize different data archives and make them accessible and searchable. Perhaps as we become more organized, stories like CN Cha’s will become routine rather than surprising.

Citation

“A Mystery in Chamaeleon: Serendipitous Discovery of a Galactic Symbiotic Nova,” Lachlan Lancaster et al 2020 AJ 160 125. doi:10.3847/1538-3881/aba435

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

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