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side-by-side images of Venus's surface today and an imagining of what its surface might have looked like in the past

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Was Venus Ever Habitable? Constraints from a Coupled Interior–Atmosphere–Redox Evolution Model
Authors: Joshua Krissansen-Totton, Jonathan J. Fortney, and Francis Nimmo
First Author’s Institution: University of California, Santa Cruz
Status: Published in PSJ

Where Oh Where Did the Water Go? (And Was It There To Begin With?)

Despite sometimes being called “Earth’s twin,” Venus isn’t very similar to Earth beyond its size and composition. With a thick toxic atmosphere filled with CO2 and a volcano-laden surface, it’s definitely more like Earth’s evil twin. Even spacecraft can only survive on its surface for a maximum of 2 hours before succumbing to the high pressure and temperature of a planet plagued by the runaway greenhouse effect.

But was Venus always such a hellish place? For a long time, we’ve theorized that Venus boasted an ocean of liquid water on its surface many billions of years ago, but being closer to the Sun, a runaway greenhouse effect took hold: as the Sun got brighter over time, more solar radiation hit the planet’s surface and led to an increase of surface water evaporation and more water vapor in Venus’s atmosphere. As the presence of water vapor heated the planet even more, intense radiation from the Sun split the water molecules, causing hydrogen to escape into space. This left room for carbon escaping from the planet’s surface to combine with some of the free oxygen left over and build up CO2 in the atmosphere, trapping even more heat and leading to a runaway greenhouse effect.

Different models of Venus’s climate evolution have led to conflicting stories about its past. Some models that incorporate the effects of clouds responding to the warming or cooling of the planet have found it possible for habitable conditions to have existed on the planet as late as 700 million years ago. Also, unlike the Moon or Earth, whose craters are weathered or otherwise degraded, most of Venus’s craters are in pristine condition and randomly distributed across its surface. From this, we infer that most of Venus’s geological history has been erased due to resurfacing events like volcanic outbursts and lava flows that happened very recently. This means that the surface we can see is very young (<1 billion years old), which makes it difficult to use observations of the surface to uncover information about Venus’s elusive history.

However, scientists have found evidence of felsic crust: igneous rocks on Venus’s surface that are relatively rich in feldspar and quartz, whose presence may indicate past surface water. That said, Venus’s atmosphere is almost completely devoid of molecular oxygen (O2). But if water vapor in the atmosphere was broken down by radiation and most hydrogen escaped into space, then this would mean there should be some leftover oxygen in the atmosphere. So what happened to all the oxygen?

Let’s Have PACMAN Eat All Our Doubts Away…

The authors of today’s article try to reconcile all the clues we have about Venus by using a coupled atmospheric–interior model called PACMAN (Planetary Atmosphere, Crust, and MANtle) to reproduce its climate conditions over time to see if the planet could have ever sustained liquid water on its surface. All this means is that they keep track of the conditions in both Venus’s atmosphere and its interior while accounting for any effect one system has on the other. People have used these kinds of models to study Venus before, but none of them have ever looked at the possibility of water on its surface.

The model is split into two phases (see Figure 1). Initially, Venus had a magma ocean on its surface created from impacts with other pieces of space rocks that were abundant during the planet’s formation. The magma ocean was a giant layer of molten, bubbly rock that you definitely wouldn’t want to dip your toes in. As this ocean cooled and released gases into the atmosphere, the temperature dropped to a point where this ocean “froze” and became a solid mantle, initiating phase two of the model.

schematic of the two phases that make up the authors' model

Figure 1: A simplified schematic of the PACMAN model the authors used. On the left is the magma-ocean phase that consists of (from innermost to outermost layer) the core, a solid mantle, magma ocean, and atmosphere. On the right is the solid mantle phase which occurs after the magma ocean solidifies, consisting of the core, solid mantle, and the atmosphere/hydrosphere. Different colored arrows show what components leave and enter each layer in the model. [Adapted from Krissansen-Totton et al. 2021]

The authors calculate quantities like the surface temperature, the amount of radiation emitted and absorbed by the planet, how much water vapor is in the atmosphere, and the amount of water on the surface during both phases. They also keep track of the abundance of various molecules containing carbon, hydrogen, and oxygen (carbon dioxide, water, O2, etc.) and calculate their flux between the atmosphere and the interior (i.e., how many of these molecules enter or exit over time). In addition, they also calculate the accumulation of 40Ar and 4He in the atmosphere, which tell us about the total magmatic activity and more recent magmatic activity, respectively. Together, these enable us to better determine whether a habitable or uninhabitable past is better at reproducing Venus’s current atmosphere.

There are lots of unknown parameters and initial conditions in the model such as CO2 pressure and planetary albedo (reflectiveness), so the authors run their model 10,000 thousand times to sample all 24 of these unknown parameters. Out of all of these runs, only 10% ended successfully in a state that mirrors Venus’s modern atmospheric and surface conditions and chemical abundances. What’s interesting about these successful models is that they suggest Venus’s current state is compatible with two different histories: some of the models tell us Venus was never habitable in its past, while others claim that Venus was transiently habitable, meaning it could have contained an ocean up to ~100 meters deep on its surface for anywhere between 0.04 and 3.5 billion years before succumbing to the runaway greenhouse effect. The latter scenario should have left salt or mineral deposits on the surface after all the water evaporated, leaving these materials potentially accessible to future remote sensing observations!

And the Winner Is…

So which model is correct? Unfortunately, there is no definitive answer since the authors found that both models are favored under different conditions. CO2 tends to make it difficult for hydrogen to escape if the water concentration is too low. Therefore, in the uninhabitable scenarios where no surface water is present, H2O in the atmosphere has a hard time escaping because CO2 continually dominates the atmosphere instead of being locked in the surface. That means that these scenarios can’t reproduce the modern water-less and oxygen-less Venus that we see today. But, if CO2 is allowed to radiatively cool the upper atmosphere, then water can condense on the surface and CO2 is removed from the atmosphere and stored away in the interior of the planet, giving Venus a chance to have a period of enhanced water loss that can then initiate the runaway greenhouse effect before the CO2 is outgassed back into the atmosphere.

On the other hand, most modern models assume that when the magma ocean phase ends, virtually all the carbon and water from the magma (so-called volatiles) live in the atmosphere. But it is possible that some of these volatiles are trapped in the resulting solid mantle instead. If this is allowed, then far fewer models allow for Venus to have been habitable. This is because it would take longer for water to then be released back into the atmosphere, making it hard to explain Venus’s current almost non-existent water abundance.

The bottom line here is that either of these two scenarios is possible and consistent with modern observations. Which scenario wins depends on our assumptions and model parameters. Though this might seem a bit anticlimactic, understanding and constraining Venus’s evolution is important for interpreting the atmospheres and histories of other exoplanets out there that might have gone through similar processes. JWST might be capable of constraining what the atmospheres of other so-called exo-Venuses are, like some of the TRAPPIST-1 system planets. Hopefully, our studies of both Venus and exo-Venuses can symbiotically help shine a light on planetary evolution!

Original astrobite edited by Ishan Mishra.

About the author, Katya Gozman:

Hi! I’m a second-year PhD student at the University of Michigan. I’m originally from the northwest suburbs of Chicago and did my undergrad at the University of Chicago. There, my research primarily focused on gravitational lensing and galaxies while also dabbling in machine learning and neural networks. Nowadays I’m working on galaxy mergers and stellar halos, currently studying the spiral galaxy M94. I love doing astronomy outreach and frequently volunteer with a STEAM education non-profit in Wisconsin called Geneva Lake Astrophysics and STEAM.

visualization of the milky way's magnetic field

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Evolution of Primordial Magnetic Fields During Large-Scale Structure Formation
Authors: Salome Mtchedlidze et al.
First Author’s Institution: Ilia State University, Georgia; University of Göttingen, Germany; and Abastumani Astrophysical Observatory, Georgia
Status: Published in ApJ

Magnetic fields — often denoted by a B in physics shorthand — are ubiquitous throughout the universe, playing a part in the physics of planets, stars, galaxies, and beyond. But, where did these magnetic fields come from? Were they born in the Big Bang, or did they arise sometime later in cosmic history? The short answer: we don’t know! This question of cosmic magnetogenesis remains one of the most important unsolved problems in modern astronomy and is intimately connected to the underlying cosmology and fundamental physics of our universe.

The Birth of Magnetic Fields

Very broadly speaking, there are two competing avenues for cosmic magnetogenesis: the astrophysical scenario and the primordial scenario. In the astrophysical scenario, weak, small-scale magnetic fields are produced around local astronomical systems — like stars and galaxies — and are then amplified and spread across large scales; these initially tiny seed fields could be generated via naturally circulating electric currents (so-called dynamos), the turbulent flow of intergalactic or interstellar gas, or spontaneous processes in unstable plasmas. By contrast, magnetic fields in the primordial scenario are generated at the dawn of cosmic time — before stars, galaxies, or any structure in the universe came to be — and grow with the universe itself. Hypotheses of primordial magnetogenesis involve highly theoretical quantum phenomena, like the violation of fundamental symmetries of nature or the coupling and decoupling of the fundamental forces. Despite each of the primordial models taking place shortly after the Big Bang, the precise mechanism of field generation is hotly contested.

Nevertheless, the presence of magnetic fields in the early universe could have vitally important cosmological consequences. For one, these fields would tamper with the cosmic microwave background — the oldest, most distant light we can see — fundamentally affecting our inferences of the state of the infant universe. Primordial magnetic fields would also alter the thermal properties of the material between galaxies, thus shifting the time at which the universe transitioned from neutral to reionized. Recently, it’s even been suggested that early magnetic fields could explain the Hubble tension — the notorious mismatch between local and global measurements of the expansion rate of the universe — and, if these fields are sufficiently twisty (i.e., if the fields are helical), they could also explain why the universe contains so much more matter than antimatter. In other words, figuring out magnetogenesis could solve many of the universe’s biggest puzzles for the price of one!

Baby Photos of the Cosmic Magnetic Field

Evidently, primordial magnetic fields deserve some attention. As such, the authors of today’s article seek to understand how magnetic fields would evolve from the very early universe to the present day. In particular, the authors use computer simulations to trace how a primordial seed field would interact with the largest-scale structures in the universe — the components of the cosmic web, like massive galaxy clusters; long, thin filaments; and vast, empty cosmic voids — as they develop over cosmic time. By comparing current observations of large-scale magnetic fields to the patterns predicted by these simulations, we can rule out different models of primordial magnetogenesis.

The authors consider four different models for the primordial magnetic field:

  1. A completely uniform and homogeneous field that could be produced during the rapid inflation of the universe
  2. A scale-invariant field (a field possessing equal contributions from waves with small wavelengths and waves with large wavelengths) that could result from a different inflationary scenario
  3. A random, non-helical field that could originate from a phase transition in the early universe, when some fundamental force became independent from the rest
  4. A random, helical field that could also arise from a phase transition

These scenarios set the initial conditions of the authors’ simulations, and thus each model is expected to evolve in a different way.

maps of the temperature, mass density, and magnetic field strength at a redshift of z=0.02 for the four scenarios

Figure 1: Maps of the present-day cosmic web as predicted from simulations of primordial magnetic field evolution. From left to right: uniform magnetic field case, scale-invariant case, helical phase-transitional case, and non-helical phase-transitional case; from top to bottom: magnetic field, density, and temperature. Click to enlarge. [Mtchedlidze et al. 2022]

Magnetic Fields All Grown Up

Figure 1 shows the imprint of the simulated primordial magnetic fields on the present-day cosmic web with respect to field strength, density, and temperature. The authors find that the two inflationary magnetic field models develop stronger evolved fields than the two phase-transitional models, with the overall magnetization in galaxy clusters and in the bridges between clusters differing by orders of magnitude between the two field-generation scenarios. Additionally, the inflationary magnetic fields stretch to much larger scales than do the phase-transitional cases. While the helical phase-transitional fields evolve to higher strengths than the non-helical fields, the authors note that, at least according to their models, it should be difficult to distinguish between helical and non-helical fields observationally.

four-panel plot of simulated rotation measure

Figure 2: Predicted present-day rotation measure from simulations of primordial magnetic field evolution. From top to bottom: uniform magnetic field case, scale-invariant case, helical phase-transitional case, and non-helical phase-transitional case. The color bar is in units of radians per square meters. [Mtchedlidze et al. 2022]

The authors also produce simulated maps of the present-day rotation measure based on the evolved primordial fields (Figure 2). When a radio wave passes through a magnetic field on its way to an observer, its polarization is rotated by an amount proportional to the magnetic field’s strength; therefore, by measuring the degree to which an extragalactic radio wave’s polarization has been affected (quantified by the aptly named rotation measure) one can deduce the strength of astronomical magnetic fields. By comparing their rotation measure maps to recent observations, the authors find that the two inflationary magnetic field models, which produce larger magnetization levels in cosmic filaments, are favored over the phase-transitional models.

While their modeling of magnetic field evolution over cosmic time neglects some key physical processes, such as gas cooling, chemical evolution, and high-energy outflow from stars and black holes, the authors still decisively show that different models of primordial magnetogenesis leave unique imprints on the universe’s largest scales.

Since the Low-Frequency Array has already started taking rotation measure data of distant radio waves passing through cosmic filaments, it’s only a matter of time before we can start ruling out models of early magnetic field creation. Even better, when the Square Kilometre Array comes online in the next decade, it’ll collect exquisite rotation measure data from the entirety of the cosmic web. With the power of the Square Kilometre Array at our disposal, we’ll be solving the mysteries of magnetogenesis B-fore you know it!

Original astrobite edited by Zili Shen.

About the author, Ryan Golant:

I am a second-year astronomy Ph.D. student at Columbia University. My current research involves the use of particle-in-cell (PIC) simulations to study magnetic field growth in gamma-ray burst afterglows and closely related plasma systems. I completed my undergraduate at Princeton University, and I am originally from Northern Virginia. Outside of astronomy, I enjoy learning about art history, playing violin and video games, and watching cat videos on the internet.

picture of a globular cluster

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: NGC5846-UDG1: A Galaxy Formed Mostly by Star Formation in Massive, Extremely Dense Clumps of Gas
Authors: Shany Danieli et al.
First Author’s Institution: Princeton University
Status: Published in ApJL

Stars can find a home in many different places: some reside in galaxies, whereas others can reside in tightly bound star clusters, which themselves orbit galaxies. The stars that exist in the densest, oldest star clusters — globular clusters — formed in extreme conditions at very early times in regions of space with extremely high gas pressures. Globular clusters are ancient relics encoding information on conditions for star formation in the early universe.

black-on-white image of UDG1

Figure 1: V-band image of UDG1 (diffuse grey points) and its globular cluster candidates (black points). [Adapted from Danieli et al. 2022]

The authors of today’s article use Hubble Space Telescope data to investigate the globular cluster population of the ultra-diffuse galaxy NGC5846-UDG1 (UDG1 for short; image of galaxy and its globular clusters shown in Figure 1). Ultra-diffuse galaxies are a type of low-surface-brightness dwarf galaxy that can be approximately the size of the Milky Way galaxy but up to a factor of 100 less luminous (see here and here for previous bites on ultra-diffuse galaxies). Exactly how these odd galaxies form is an open question in astronomy. So, what can UDG1’s globular cluster population tell us about galaxy formation?

A Galaxy Rich in Globular Clusters

A key result of today’s article is that UDG1 has a significantly higher number of globular clusters than would be expected for a galaxy of its mass, with the tally coming in at 54 (+/- 9)! In addition to the number of globular clusters, the authors also estimate the total light from the clusters relative to the total light in the galaxy itself. The authors find that the globular clusters currently emit 13% of the total light of the galaxy, meaning that 13% of this galaxy’s stars reside in globular clusters — this is the highest fraction of stars in globular clusters known for any galaxy to date, and it’s 100 times larger than the globular cluster fraction of the Milky Way!

While the current proportion of stars in globular clusters for UDG1 is 13%, this figure was likely much higher at earlier times due to the dynamical evolution of these clusters. Globular clusters are expected to interact with their environment and lose mass (or be completely destroyed) via tidal stripping over time. The stars that are lost from the clusters may end up contributing significantly to the stellar content of the galaxy itself (see this astrobite for more).

These mass-loss processes can lead to globular cluster systems losing up to 80–90% of their stellar mass content over their lifetime. The authors use an analytical model to estimate the original cluster fraction (before any mass loss processes occurred) from the current cluster fraction of 13%. They find that the original cluster fraction is likely to have been 65%, indicating that the majority of the stars in this galaxy at present day originally formed in bound clusters from extremely dense gas clumps.

These results are summarised in Figure 2, which displays the cumulative proportion of mass in globular clusters (currently observed clusters in black; the initial cluster model in purple (before mass loss effects); and Milky Way values in orange).

plot of galaxy mass in clusters versus stellar mass in clusters

Figure 2: The cumulative fraction of galaxy mass in globular clusters for NGC5846-UDG1 in comparison to the Milky Way. The black line shows the current observed values, the purple dotted line shows the modelled values expected at the time of formation, and the orange dashed line shows the current Milky Way values. Mcl refers to the total stellar mass of the globular clusters. [Adapted from Danieli et al. 2022]

Implications for Star Formation

The large difference between the Milky Way and UDG1 globular cluster fractions highlights the rare conditions under which UDG1 originally formed. Globular cluster formation (and subsequent destruction) is very likely to have been the dominant star formation mode for UDG1, with these stars originally forming from extremely dense, high-pressure gas clumps. This idea is supported by the fact that the spatial distribution and ages of the cluster stars versus the galaxy stars in UDG1 are very similar, indicating that the galaxy’s stars likely originated from disrupted globular clusters.

It is unlikely that UDG1 is the only galaxy of its kind. The authors note that there are hints that some ultra-diffuse galaxies in the Coma cluster have similarly high globular cluster fractions. The uncertainties on these measurements are high since the galaxies in the Coma cluster are located farther away, but it’s still a promising indicator that star formation through extremely dense clumps at early times may be a viable way to build a galaxy!

Original astrobite edited by Gloria Fonesca Alvarez.

About the author, Katy Proctor:

I am a first-year PhD student at the International Centre for Radio Astronomy Research at the University of Western Australia. My research is focused on using cosmological simulations to study the build up of stellar halos. Outside of research, I can usually be found climbing up walls or playing guitar.

hubble space telescope image of the core of Starburst Galaxy NGC 1569

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Characterization of Two 2 mm detected Optically Obscured Dusty Star-forming Galaxies
Authors: Sinclaire Manning et al.
First Author’s Institution: The University of Texas at Austin and University of Massachusetts, Amherst
Status: Published in ApJ

Dusty star-forming galaxies could be important for learning what drives star formation, since they have been observed to have extremely high star-formation rates. These galaxies could also be predecessors to some quenched galaxies that have stopped forming stars at high redshift. However, due to observational challenges, it is relatively uncertain how common dusty star-forming galaxies are at high redshift.

Ultraviolet and optical light from young stars in dusty star-forming galaxies gets absorbed by large amounts of dust and re-emitted in the infrared. In some cases, there is so much dust that the optical emission from the galaxy is heavily obscured and all we can observe is the light re-emitted by the dust. When galaxies are undetectable (meaning they either can’t be observed or their presence can’t be confirmed) in both optical and near-infrared light, we call them OIR-dark galaxies.

At high redshift, dusty star-forming galaxies are observable at long wavelengths with telescopes like the Atacama Large Millimeter/submillimeter Array (ALMA). The authors of today’s article investigate two OIR-dark galaxies from the Mapping Obscuration to Reionization ALMA (MORA) survey and measure their redshifts and physical properties.

Dusty, Dark, and Distant

The two galaxies, MORA-5 and MORA-9, have been observed in multiple wavelengths with ALMA and other telescopes like the Hubble Space Telescope. What makes them interesting is that they are detected in ALMA observations but not in optical or near-infrared wavelengths, so they are considered OIR-dark. The availability of multi-wavelength observations allows the authors to study the galaxies’ spectral energy distributions (SEDs), which tell us how a galaxy’s energy is distributed over different wavelengths. The shape of a galaxy’s SED depends on properties such as the rate of star formation and gas mass, so fitting the SED can give us a clue about the physical environment of the galaxy.

Before they can determine the properties of these galaxies, the authors must obtain their redshifts. One way to measure redshift is using spectroscopy, since the wavelengths of galactic emission lines get shifted depending on their distance to us. Since spectroscopic data for these galaxies are not available, the authors use three different methods to estimate redshift from the SEDs. Figure 1 shows the SEDs of these two galaxies and the fitting results from these three methods.

Figure 1: Spectral energy distributions of MORA-5 (top) and MORA-9 (bottom). The blue line shows the fit to the OIR part of the SED from EAZY. The orange line shows the fit from MMpz to the far-infrared part of the SED. The black line shows the combination of these two models. The gray and purple lines show the MAGPHYS fitting of the entire range of the SED for 3 different redshifts. [Manning et al. 2022]

One method called MMpz fits the far-infrared part of the SED and is based on the relationship between the infrared luminosity and the wavelength at which the SED peaks. The two properties are anti-correlated, meaning that galaxies with higher infrared luminosity peak at shorter rest-frame wavelengths. This is helpful since knowing the wavelength at which the SED is expected to peak can help us figure out how much it has been redshifted. Another method uses the software EAZY to fit the (faint) OIR data. The third method, MAGPHYS, fits the entire SED (both OIR and far-infrared regions) to figure out the redshift. The best estimates from these methods place the galaxies at redshift z > 4 (within the first 1.5 billion years of the universe), potentially making them the highest redshift galaxies in the MORA survey.

Galaxy Properties

After estimating the redshifts, the authors estimate the physical properties of these galaxies. First, they once again use correlations between the infrared luminosity and other galaxy properties, but this time to estimate the star-formation rate. They estimate that in one year MORA-9 forms 200 solar masses of stars. MORA-5, on the other hand, forms roughly four times as many stars, with a star-formation rate of 830 solar masses per year! The authors also estimate the mass of the galaxy that comes from stars, gas, and dust.

Figure 2: Star-formation rate (in solar masses per year) vs. stellar mass (in solar masses) of dusty star-forming galaxies from different surveys. The blue and green diamonds represent the MORA galaxies discussed in this article. The gray shaded area represents the main sequence that galaxies follow for star formation at a redshift of z=3–5. [Manning et al. 2022]

Figure 2 shows how the star-formation rates for these two galaxies compare to other dusty star-forming galaxies, showing that the two galaxies fit in with two different populations. MORA-5 fits in with high-stellar-mass galaxies with high star-formation rates, while MORA-9 is a part of a more moderate population.

To figure out how common these galaxies are, the authors look at two other galaxies, MORA-3 and MORA-4, and estimate the volume density of OIR-dark galaxies to be 5 x 10-6 Mpc-3. The authors note that it’s unclear why some dusty star-forming galaxies are OIR-dark while others aren’t, and they suggest it could simply be dependent on their redshift. Regardless of what makes this class of galaxies OIR-dark, they provide a way to study the environments of distant star-forming galaxies that are otherwise invisible.

Original astrobite edited by Jamie Sullivan.

About the author, Gloria Fonseca Alvarez:

I’m a fifth-year graduate student at the University of Connecticut. My research focuses on the inner environments of supermassive black holes. I am currently working on measuring black hole properties from the spectral energy distributions of quasars in the Sloan Digital Sky Survey. As a Nicaraguan astronomer, I am also involved in efforts to increase the participation of Central American students in astronomy research.

CHIME radio telescope

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The First CHIME/FRB Fast Radio Burst Catalog
Authors: The CHIME/FRB Collaboration
Corresponding Author’s Institution: Massachusetts Institute of Technology
Status: Published in ApJS

Fast radio bursts are just as their name suggests — short, millisecond-long bursts observed in radio wavelengths all over the galaxy. But, despite the first burst being found in 2007, astronomers still aren’t sure what type of sources produce them. We do know that there seem to be two types of bursts — those that repeat (repeaters) and those that occur only once (non-repeaters). One of the repeating sources has recently been associated with SGR 1935+2154, a galactic magnetar — a type of neutron star with a very powerful magnetic field. Another repeating source has been associated with the M81 galaxy.

In this article from the CHIME/FRB Collaboration, the authors present a new catalog of fast radio bursts (called “Catalog 1”), which more than doubles the number of known sources. Statistical studies of the entire population are thus much more viable, and as a result, astronomers can begin to address some of the open questions about these mysterious sources.

How Can We Tell Where Fast Radio Bursts Come From?

Although one repeater has been associated with a magnetar in the Milky Way, not all fast radio bursts come from objects within our galaxy. We can see the distribution of fast radio bursts (both repeaters and non-repeaters) in Figure 1.

plot of locations of fast radio burst sources

Figure 1: The distribution of fast radio bursts across the sky seen by the CHIME telescope. CHIME, located in Canada, observes the northern sky and a small band of the southern sky near the celestial equator (declination -11 to 90 degrees). There are 474 unique non-repeating sources (blue circles) and 18 known repeaters (red triangles). [CHIME/FRB Collaboration 2021]

examples of flux observed over time for two fast radio bursts

Figure 2: 2D histograms of the amount of radio waves (flux) received in different frequency bands by CHIME for two sample fast radio bursts, with name and dispersion measure in the top right of each panel. These “waterfall plots” show what the telescope observed for each source. The plot on the left has a more pronounced “swoop” pattern and a higher dispersion measure. Plots for the rest of the catalog are here. [Adapted from CHIME/FRB Collaboration 2021]

Fast radio bursts, especially the ones outside our galaxy, travel a long way to get to us on Earth. We can look at the dispersion measure to tell how far the burst has traveled. Space is not empty, and as the burst travels through the interstellar medium it gets scattered off electrons and other particles, which causes the “swoop”-like pattern seen in the data in Figure 2. The dispersion measure is the integrated number density of particles along the path of the waves and is a quantitative way to estimate the distance that the burst has traveled to get to the telescope.

Are Repeaters and Non-Repeaters From the Same Type of Source?

One of the major open questions about fast radio bursts is what type of source produces them. The differences between repeating and non-repeating sources leave open the possibility that the two types of bursts may originate from different sources. The nature of repeating sources rules out cataclysmic scenarios (situations where one or more objects collide or explode, such as a neutron star merger), but some of these might be possible for a non-repeating source.

The authors of the article start by looking at several characteristics (including distribution across the sky, dispersion measure, signal strength, flux, temporal width, and bandwidth) of both repeater and apparent non-repeater bursts to see if it is statistically plausible that they originate from the same distribution. If so, both repeaters and non-repeaters could come from the same underlying source population.

The authors find that repeaters and non-repeaters are distributed similarly across the sky and have similar dispersion measures. Repeaters and non-repeaters have similar flux and fluence distributions, meaning they give off a similar amount of radio waves over the course of the burst (fluence here refers to the integral of the flux over the duration of the burst).

histogram of the number of fast radio bursts as a function of duration

Figure 3: Distribution of (temporal) widths for the bursts. The distribution in blue shows the single non-repeater bursts, while the orange distribution chooses only the first observed burst for each repeater. These distributions do not look at all similar and showcase one of the differences seen between the types of fast radio burst sources. [Adapted from CHIME/FRB Collaboration 2021]

However, repeaters and non-repeaters also show some differences. Their temporal widths and bandwidths appear to differ, which is shown in Figure 3 (as also previously reported with lower statistics in two articles [1] [2]). The fact that the widths differ between the two types of bursts is interesting, because it seems that repeater bursts last longer than non-repeaters on average. The bandwidth (the spread of radio frequencies observed for the burst) is also different between the two types, which is an additional indicator that repeaters and non-repeaters may come from different types of sources.

So, What Is the Major Takeaway From This Article?

One of the most exciting aspects of this article is the increase in the number of sources and bursts now available for fast radio burst research. With higher statistics, studies of the entire population are more meaningful. Differences between repeaters and non-repeaters (with regards to their temporal width and bandwidth) leave open the possibility that these come from different populations. The authors estimate that 820 bursts occur over the full sky per day, which means that statistics will continue to increase over time, more sources may be found to repeat, and more pieces of the fast radio burst puzzle will continue to fall into place.

Original astrobite edited by Konstantin Gerbig and Ali Crisp.

About the author, Jessie Thwaites:

Jessie is a PhD student at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. She studies possible astrophysical sources for high-energy neutrinos through multimessenger astrophysics. Outside of physics, she plays horn and enjoys spending time outdoors, especially skiing and biking.

photograph of an engineer standing in front of JWST mirror segments

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope
Authors: Caprice Phillips et al.
First Author’s Institution: The Ohio State University
Status: Published in ApJ

Across the World, Astronomers Are Excited…

…because the latest “Great Observatory” was successfully launched into space! The space telescope known as JWST has become infamous amongst space professionals for long and frequent delays in its deployment since its conception in 1996. Though its timeline and price tag are daunting, they are but sxmall consequences of the telescope’s exquisite engineering and mind-blowing potential. Most often cited as the instrument that will allow us to look deeper into the past than ever before, JWST might also be able to search for potential signatures of life on planets outside our solar system! The authors of today’s article explain how.

A Portrait of an (Exo)World and Its Atmosphere

On Earth, we are protected from the adverse effects of our solar system (like flying rocks and too much radiation) by our atmosphere. While our rocky home has just enough gravity to hold on to an atmosphere of mostly nitrogen (N), planets slightly larger than Earth but smaller than Neptune would likely live in a bubble of hydrogen (H). Known as gaseous dwarf planets, these potential homes are easier to identify and study thanks to their larger sizes. Because of their hydrogen-dominated atmospheres, chemical reactions with hydrogen that produce molecules like ammonia (NH3) are expected to be common on these planets.

However, ammonia doesn’t occur spontaneously; it requires either extremely high temperatures and pressures or the existence of a reaction catalyst, potentially one that arose to support life-sustaining chemical processes. Furthermore, ammonia is easy to destroy in volcanic environments, meaning that the presence of ammonia suggests the presence of a process that regularly replenishes it. Therefore, in these worlds ammonia is a possible biosignature: a clue that the planet in question might be hosting life!

Unfortunately, the presence of ammonia alone isn’t enough to detect life outside the solar system, but it would set the stage for follow-up observations and even more amazing science. The authors of today’s paper begin by considering seven close-by exoplanets of the right size and temperature range to have gaseous envelopes and be observable by JWST. In this article, we’ll focus on one of the seven: TOI-270 c, which the authors find to be best suited for future JWST observations.

How Would We Search for Ammonia?

One method for determining the contents of a planet’s atmosphere is transmission spectroscopy: measuring the spectrum of the planet’s host star as its light filters through the planet’s atmosphere on its way to the detector. Ideally, the measured spectrum will reveal the tell-tale dips and peaks of absorption and emission spectra that are associated with individual elements and molecules. An example of the signature of ammonia from planet TOI-270 c is given in Figure 1. Note that there are several ammonia features we could potentially look for (at 1.0–1.5, 2.0, 2.3, and 3.0, as well as 5.5–6.5 and 10.3–10.8 µm, which are not shown) and that the height of the peaks depends on how much ammonia there is in the atmosphere in parts per million (ppm). The more ammonia there is, the easier it will be to detect!

ammonia spectrum

Figure 1: The spectrum of ammonia in the atmosphere of TOI-270 c for different ammonia concentrations in parts per million (ppm). The height of the peak is given in terms of the ratio of the radius of the planet squared and the radius of the star squared (i.e., how much of the light is blocked). Note that there are at least six ammonia features to look for! [Phillips et al. 2021]

Another lever controlling the spectrum of TOI-270 c’s ammonia signature is the presence of a cloud deck in its atmosphere. Whether the planet is enveloped in clouds — and at what height they occur — will significantly change its ammonia signature, as shown in Figure 2. The authors show that the most identifiable signature will come from a cloudless atmosphere, or one with a cloud deck at high pressure (read: low to the ground). On the other hand, a cloud deck at 0.01 bar (really high up!) would flatten the signature almost completely, making it incredibly hard to detect ammonia.

spectra of ammonia

Figure 2: The spectrum of ammonia in the atmosphere of TOI-270 c as above, but now for different cloud decks. The most noticeable features emerge from an atmosphere with no clouds or with a cloud deck at 1.0 bar. Smaller features are noticeable in an atmosphere with a cloud deck at 0.1 bar, while a cloud deck at 0.01 bar flattens the spectrum to almost a straight line. [Phillips et al. 2021]

Could JWST See These Signals?

There are several instruments aboard JWST that could be used to hunt for ammonia-filled atmospheres; the authors find that Near-Infrared Spectrograph (NIRSpec) and the Near-Infrared Imager and Slitless Spectrograph (NIRISS) are particularly suitable. Together, these instruments cover the wavelength range of the ammonia features illustrated in Figures 1 and 2 above. The question is: are they sensitive enough to make the detection?

I won’t keep you in suspense: the authors’ results suggest that the answer is yes! The authors used two of their simulated spectra from above — assuming two different percentages of hydrogen in the atmosphere of TOI-270 c — and modeled what JWST observations of such an atmosphere might look like for 10 transits (see Figures 3 and 4). As might be expected, an atmosphere that is richer in hydrogen will produce a higher signal-to-noise ratio (S/N).

simulated spectrum of ammonia

Figure 3: Top: The spectrum of ammonia in the atmosphere of TOI-270 c for an atmosphere rich in hydrogen is shown in orange, while the different black points show simulated JWST observations with NIRISS and two NIRSpec filters (G235M and G395). The authors also show the wavelength range of each of the instruments below the spectrum. Bottom: The signal-to-noise ratio of the simulated spectrum and observations. [Phillips et al. 2021]

simulated spectrum of ammonia

Figure 4: The same figure as above, but this time with an atmosphere that is poorer in hydrogen. Note that this results in a lower signal-to-noise ratio than in Figure 3. [Phillips et al. 2021]

Conclusion

Under the right circumstances (hydrogen-rich atmosphere, lots of ammonia, no cloud deck), JWST is well-equipped to detect ammonia on gas dwarf planets — an exciting start on a long road towards determining whether we truly are alone in the universe!

Original astrobite edited by Lili Alderson.

About the author, Luna Zagorac:

I am a PhD candidate in the Physics Department at Yale University. My research focus is ultra-light (or fuzzy) dark matter in simulations and observations. I’m also a Franke Fellow in the Natural Sciences & Humanities at Yale working on a project on Egyptian archaeoastronomy, another passion of mine. When I’m not writing code or deciphering glyphs, I can usually be found reading, doodling, or drinking coffee.

artist's impression of an exoplanet

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Chaotic History of the Retrograde Multi-planet System in K2-290A Driven by Distant Stars
Authors: Sergio Best and Cristobal Petrovich
First Author’s Institution: Pontifical Catholic University of Chile
Status: Published in ApJL

Most planets orbit their stars in the same direction the stars spin. Why are some rare systems misaligned?

The K2-290 triple star system hosts a pair of planets with seriously misaligned orbits. Planets b and c orbit the system’s central star — K2-290A — with an obliquity (relative tilt) of 124° from their host star’s spin axis. An obliquity of greater than 90° is considered retrograde, as the planets move opposite the direction that the star spins.

A previous study concluded that this tilt was caused by interactions between star A and its stellar companion, star B, while star A’s planetary system was still in the protoplanetary disk phase of development. In this scenario, the disk was knocked out of alignment when the planets were in resonance with star B, when the closest or farthest orbital points align. However, as the authors of this earlier work stated, “there may be more than one way to misalign a disk.” Today’s article explores another possible cause of this misalignment involving the system’s third stellar member, K2-290C.

Schematic of the K2-290 system

Figure 1: Diagram of the K2-290 system (distances not to scale). The two planets share an orbital plane, which is inclined ~124° relative to star A’s spin axis. The distant stellar companions have some unknown mutual inclination relative to one another (labeled iBC on this diagram). Stellar distances are projected onto the observable plane of the system from Earth, at some unknown inclination, meaning that the orbital separations may be larger by some unknown factor. [Best & Petrovich 2022]

How to Misalign Your Planet

The authors of today’s article simulated the interactions between the five bodies in the K2-290 system to see what sort of initial conditions would result in the strange alignment observed today. When more than two bodies interact gravitationally, dynamical systems can be quite chaotic. When a distant third star (C, in this case) orbits a closer binary pair (A and B) at an angle, it can cause the inner binary pair’s orbital eccentricity and inclination to oscillate through a three-body interaction called the Zeipel-–Kozai–Lidov mechanism. These oscillations provide a possible mechanism for planetary orbit misalignment.

When star B is on a highly elongated orbit, it causes the planets’ orbital precession (slow tilting of spin axis) to change in frequency, knocking them out of star A’s equatorial plane. With star C in the picture to mess with star B’s orbit, a wide array of initial conditions can lead to this strongly misaligned end result.

fractions of systems reaching 124 degree obliquity from various starting conditions

Figure 2: The fraction of systems with initial conditions that result in planetary orbits with an obliquity of ~124°. These panels examine star A’s initial spin period, star A’s quadrupole moment (deviation from spherical shape), the mutual inclination of stars B and C, planet b’s mass, star B’s semi-major axis, and star C’s semi-major axis. The red points illustrate how simulations without star C don’t create the observed misalignment for reasonable estimations of star A’s internal structure. [Best & Petrovich 2022]

The authors tested ~50,000 possible initial conditions for the system after its formation. They found that 56% of their scenarios reached the 124° obliquity observed within the star’s estimated age range of 3.2–5.6 billion years. In 17% of trials, the planetary system was destroyed, and in 27% of trials it remained intact but without high obliquity. When star C’s effect on the system is neglected, only 12% of the simulations end up reaching the observed obliquity.

So, what really happened in the K2-290 system? Ultimately, the authors show that star C’s influence could cause the observed retrograde planetary orbits seen around K2-290A. Does this mean that previous findings about primordial misalignment in K2-290A’s protoplanetary disk is wrong? No, but it shows that it isn’t a requirement to get this result when a third star is in play.

Some unanswered questions about this system remain. So far, K2-290A is only known to host two planets, but its radial-velocity measurements show that another planet with a longer orbital period is possible and could increase the level of interaction between the planets and star B. Putting better constraints on the orbits of stars B and C could further test this dynamical theory.

What Does This Mean for Other Planetary Systems?

K2-290 was the first system found to present strong evidence of primordial disk misalignment, with star B at the right distance to cause the theorized effect. But, considering star C, these large obliquities can form after the protoplanetary disk phase from a wide array of different initial conditions. The primordial misalignment theory depends on some highly uncertain gas dynamics and evaporation, while the components of the tertiary-driven mechanism presented in today’s article are physically pretty well understood. The two mechanisms could work together in some systems since the mechanism presented in this article is not highly dependent on the initial configuration of the system.

This 124° obliquity is not particularly special; this mechanism could drive systems all the way to 180° — a perfectly retrograde orbit. Additionally, in other systems, a Jupiter-sized planet at a tenth of the distance of star B could mimic star B’s effect, helping to drive the misalignment of inner planets. It’ll be fascinating to see what sorts of weird misaligned systems exoplanet hunters find next!

Original astrobite edited by Roan Haggar.

About the author, Macy Huston:

I am a fourth-year graduate student at Penn State University studying astronomy and astrophysics. My current work focuses on technosignatures, also referred to as the Search for Extraterrestrial Intelligence (SETI). I am generally interested in exoplanet and exoplanet-adjacent research. In the past, I have performed research on planetary microlensing and low-mass star and brown dwarf formation.

3D rendering of the proto-neutron star produced in a simulation of a 25 solar mass star undergoing core-collapse.

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: On the Origin of Pulsar and Magnetar Magnetic Fields
Authors: Christopher J. White et al.
First Author’s Institution: Princeton University
Status: Published in ApJ

Magnetic fields at the surface of a neutron star can reach up to one quadrillion times the strength of those on Earth — that’s a factor of one followed by fifteen zeros. If you were to step foot on a neutron star, the scrambling of your credit card data would be the least of your worries. At these magnetic field strengths, the supposed emptiness of space starts to refract light and light itself splits into matter and antimatter. If a frog starts levitating in fields one hundred thousand times stronger than Earth’s, who knows what would happen to a frog on a neutron star!

So, how did these magnetic fields get so strong?

Cooking Up a Neutron Star Core

First, let’s step back a bit and review how neutron stars form. In short, neutron stars are the remnants of old, massive stars. When a massive main-sequence star (typically between 8 and 25 times the mass of the Sun) runs out of fuel for fusion in its core, the core becomes degenerate, meaning that peculiar quantum mechanical effects prevent the core from collapsing. However, as more matter piles onto the degenerate core and the core exceeds the Chandrasekhar mass, the core contracts and heats up to billions of kelvin, forcing electrons and protons to combine into neutrons, releasing a deluge of high-energy neutrinos in the process. This “proto-neutron star” core continues to contract until it reaches the density of an atomic nucleus. Meanwhile, the infalling outer regions of the original star bounce off the degenerate core and are expelled by the outgoing neutrinos, resulting in the violent explosion of the star via a core-collapse supernova. When the dust settles, all that’s left is the neutron star core.

Well, then, how does this core-collapse process relate to the extreme magnetic fields possessed by neutron stars? One explanation is that the number of magnetic field lines threading the surface of the contracting proto-neutron star must stay fixed, so, as it shrinks, the field lines bunch together and amplify the total field strength. However, this model fails to capture the dichotomy between radio pulsars — rapidly rotating neutron stars with relatively lower magnetic field strengths (typically between 1011 and 1013 times as strong as Earth’s field) — and magnetars — slowly rotating neutron stars with exceedingly powerful magnetic fields (reaching up to 1015 times as strong as Earth’s). With this dichotomy in mind, the authors of today’s article propose a new avenue for the growth of neutron star fields: convective dynamos in collapsing proto-neutron star cores.

cartoon of a dynamo

Figure 1: A cartoon illustration of dynamo action inside Earth. The motion of the convecting liquid iron of the outer core is organized into coils by Earth’s rotation, thus generating large-scale magnetic fields. [Andrew Z. Colvin (via Wikipedia)]

Stirring Up Magnetic Fields

Dynamos are responsible for generating and maintaining the magnetic fields of a wide array of astronomical objects, from young protostars and old, dim M dwarfs to the gas giants of the outer solar system to Earth itself (see Figure 1). If an astrophysical body is filled with electrically conducting fluid that’s continually undergoing convection (like the liquid iron in Earth’s outer core), the rotation of the body will cause these convecting electrical currents to twist into elongated coils, thus producing large-scale magnetic fields — this is the fundamental mechanism of dynamo action.

We can quantify the strength of a convective dynamo via the Rossby number, the ratio of the speed of fluid convection to the speed of the body’s rotation; a slowly rotating body containing vigorously convecting fluid will have a high Rossby number, while a fast-rotating body with weaker convection will have a low Rossby number. A natural dichotomy in magnetic field strength arises from a threshold value for the Rossby number: dynamos with Rossby numbers below this threshold tend to produce stronger dipolar fields (like Earth’s) that scale with the strength of convection, while dynamos with Rossby numbers above the threshold tend to produce weaker fields with multiple poles. Since proto-neutron stars are expected to undergo convection during core-collapse, dynamo action and this Rossby number dichotomy could explain the split between pulsars and magnetars.

Today’s authors thus turn to simulations of proto-neutron star formation in core-collapse supernovae to test this hypothesis. In a suite of 12 supernova simulations with non-rotating progenitor stars ranging from 9 to 25 solar masses (plus one simulation featuring a rotating 9-solar-mass progenitor), the authors look at the convective power and the geometry of the fluid flow in the interiors of the developing proto-neutron stars; from these observations, the authors can infer the magnetic field strengths of the resulting neutron stars. Figure 2 shows the convective power as a function of radius for each of the simulations without rotation, demonstrating that convection becomes stronger with larger progenitor mass. Meanwhile, Figure 3 shows a comparison of the flow geometry between a simulation with a rotating progenitor vs. one without rotation, demonstrating the relative complexity of the flow in a rotating proto-neutron star. The fact that the interiors of rotating proto-neutron stars are capable of sustaining vigorous convection and complex fluid flows implies that dynamos in proto-neutron stars should be able to generate strong magnetic fields.

simulated convective power versus radius for proto-neutron stars of vayring masses

Figure 2: Convective power vs. radius in the 12 supernova simulations (with non-rotating progenitor stars) analyzed by the authors. The color gradient represents the time since the infalling outer region of the progenitor has “bounced” off the degenerate core. These plots show a general trend of increasingly vigorous convection with increasing progenitor mass and with increasing time since bounce. [White et al. 2022]

Completing the Recipe

complexity of the fluid flow for a rotating and non-rotating 9 solar mass star

Figure 3: Complexity of the fluid flow (or, in hydrodynamics jargon, the “kinetic helicity”) near the surface of the proto-neutron star in supernova simulations with a 9-solar-mass non-rotating progenitor (top) and with a 9-solar-mass rotating progenitor (bottom). The proto-neutron star in the simulation with the rotating progenitor has developed more extreme, complex structures. [White et al. 2022]

The authors conclude that convection in proto-neutron stars could play a vital role in setting the properties of newborn neutron stars. If a proto-neutron star rotates sufficiently quickly, then dynamo action will endow it with a strong dipolar magnetic field that will naturally reduce the star’s rotation speed over short time scales, yielding a slowly rotating, highly magnetized neutron star — in other words, a magnetar. On the other hand, a relatively slow-rotating proto-neutron star will develop weaker magnetic fields and will thus retain its nascent rotational velocity over a longer period of time — thus explaining the formation of pulsars.While the simulations analyzed by the authors capture the fluid dynamics and neutrino physics of core-collapse supernovae in remarkable detail, they are missing one important ingredient: magnetic fields! Incorporating magnetic fields into these highly sophisticated simulations will be computationally challenging, but well worth it: with fully realistic 3D simulations of core-collapse supernovae, we’ll finally be able to complete the recipe for turning massive stars into compact, magnetized neutron stars.

Original astrobite edited by Suchitra Narayanan.

About the author, Ryan Golant:

I am a second-year astronomy Ph.D. student at Columbia University. My current research involves the use of particle-in-cell simulations to study magnetic field growth in gamma-ray burst afterglows and closely related plasma systems. I completed my undergraduate at Princeton University, and I’m originally from Northern Virginia. Outside of astronomy, I enjoy learning about art history, playing violin and video games, and watching cat videos on the internet.

comparison of solar granules and the solar magnetic field

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Mapping the Hidden Magnetic Field of the Quiet Sun
Authors: J. C. Trelles Arjona, M. J. Martínez González, and B. Ruiz Cobo
First Author’s Institution: Instituto de Astrofísica de Canarias (IAC), Spain
Status: Published in ApJL

The Sun’s magnetism holds the key to solving a well-known mystery: what makes the temperature of its outermost atmosphere, or corona, several hundred times hotter than its surface? When the Sun is not silent, we can observe and measure magnetic forces at work that produce sunspots, giant solar flares, and coronal mass ejections — fiery processes that can inject heat into the corona.

Over the past few years, the Sun has been at the quiet end of its cycle, displaying little or no surface activity. Yet the solar corona remains heated to over a million degrees even when the Sun is silent. Without the telltale signs of periods of high solar activity, measuring the surface magnetism that may be driving this heating is extremely difficult. In a new study, astronomers have now achieved this measurement using special techniques to analyze sunlight.

Solar Magnetic Cartography

demonstration of Zeeman splitting due to a sunspot

Figure 1: An example of the Zeeman splitting of spectral lines of light coming from a sunspot due to its strong magnetic field. During phases of strong solar activity, sunspots can have magnetic fields as high as 4,000 Gauss, which is several thousand times stronger than Earth’s magnetic field. [NSO/AURA/NSF]

Sunlight seen through a spectrograph reveals dark lines in its rainbow-like continuous spectrum. These lines represent individual elements that absorb light in the Sun’s atmosphere. In the presence of a magnetic field, these lines split into multiple components, a phenomenon called the Zeeman effect. Figure 1 shows Zeeman splitting by the strong magnetic field in a sunspot.

However, in the quiet Sun’s weaker magnetic fields, the Zeeman splitting is small and there are other physical processes that can contaminate its measurement. In order to distinguish these contaminations from the effects of the magnetic field, the researchers in today’s Astrobite studied high-resolution polarized light images from the GREGOR Solar Telescope by restricting the electromagnetic oscillations of sunlight to certain orientations.

This enabled the astronomers to obtain a high-resolution map of the variation in the magnetic field of the quiet Sun covering an area spanning roughly 112 times the landmass of the contiguous United States (Figure 2).

map of solar magnetic field strength

Figure 2: The map of the quiet Sun’s magnetic field. Dark blue regions overlap with granules and have weak fields, while their boundaries (red) have stronger magnetic fields. [Arjona et al 2021]

Revealing the Hidden Field

image of solar granules

Figure 3: Granules representing plasma convective cells on the Sun’s surface. Each individual granule is roughly the size of Texas. [NSO/AURA/NSF]

The most striking outcome of the map is that the magnetic field variation closely matches solar granules: features on the Sun’s surface representing convective plasma cells (Figure 3). The field is weak within a granule and stronger along the boundaries. On average, the researchers found the magnetic field of the quiet Sun to be 46 Gauss, comparable to that of a refrigerator magnet. While these fields are much weaker than what is observed in a solar maximum, they are still strong enough to heat the solar corona through small-scale nanoflares.

Detailed studies of the Sun’s magnetism, both when it is roaring and when it is relaxed, are vital to make better models of the solar cycle and possibly predict the intensity of future solar storms that can threaten catastrophic damage to our telecommunications systems.

Original astrobite edited by Pratik Gandhi.

About the author, Sumeet Kulkarni:

I’m a third-year PhD candidate at the University of Mississippi. My research revolves around various aspects of gravitational wave astrophysics as well as noise characterization of the LIGO detectors. It involves a lot of coding, and I like to keep tapping my fingers on a keyboard even in my spare time, creating tunes instead of bugs. I run a science cafe featuring monthly public talks for the local community here in Oxford, MS, and I also love writing popular science articles. My other interests include reading, cooking, cats, and coffee.

composite X-ray, optical, and millimeter image of supernova 1987A

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Final Moments I: Precursor Emission, Envelope Inflation, and Enhanced Mass loss Preceding the Luminous Type II Supernova 2020tlf
Authors: Wynn Jacobson-Galán et al.
First Author’s Institution: University of California, Berkeley
Status: Published in ApJ

Massive stars end their lives in energetic explosions known as core-collapse supernovae. Before exploding, however, some stars get the chance to say their final words! In the months leading up to the explosion, stars can expel some of their outer layers, which causes their brightness to increase dramatically. Today’s article describes a star that showed this “precursor emission” prior to the final supernova explosion.

SN 2020tlf: The Star That Spoke Before It Died

The supernova SN 2020tlf was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey on 16 September 2020 in the galaxy NGC 5731. The authors of today’s article noticed that this galaxy had also been observed by the Pan-STARRS telescope regularly since 18 January 2020 as part of the Young Supernova Experiment. The authors examined the data and voila! They found significant activity at the location of SN 2020tlf for more than a hundred days prior to the explosion. Figure 1 shows this precursor activity of SN 2020tlf.

magnitude versus time for SN 2020tlf

Figure 1: Pre-explosion activity in SN 2020tlf lasting for 130 days (solid symbols), before the SN exploded. Symbols with arrows indicate upper limits on the brightness. [Adapted from Jacobson-Galán et al. 2022]

After this discovery, the authors obtained additional spectroscopic and photometric observations to identify the nature of the supernova. Their spectroscopic observations indicated that SN 2020tlf is a Type II-P supernova. Type II-P supernovae are exploding red supergiant stars that are characterized by a long plateau in their brightness lasting for a few months after the explosion. Type II-P supernovae are pretty common in the universe, but SN 2020tlf is the first Type II-P supernova that shows pre-explosion brightening. From the post-explosion data, the authors calculated that the exploding star was a red supergiant with a mass of 10–12 solar masses and a radius of ~1,100 solar radii.

Precursor Emission = Signs of Mass Loss

The authors noted that the pre-explosion activity is detected only in the redder photometric r-, i-, and z-band observations, but it is absent from the bluer g- and cyan-band observations. This suggests that the precursor emission is red, suggesting that it comes from a “cold” surface (recall blackbody physics — bluer is hotter and redder is colder) with a temperature of 5000K and a radius of ~1,000 times the radius of the Sun. This is consistent with our understanding of a red supergiant star that is shedding its outer layers. From the precursor emission, the authors calculated that the red supergiant was surrounded by at least 0.3 solar mass of dense circumstellar material in its final days.

There are signs of this circumstellar material in the post-explosion observations as well. A spectrum taken just a few days after the explosion shows several narrow emission lines of hydrogen, helium, neon, and carbon, which are characteristic of a large amount of circumstellar material around the star. In addition, the early light curve shows signs that it is dominated by interaction of the supernova ejecta with this circumstellar material. From the light curve and spectra, the authors determined that the red supergiant was surrounded by 0.05–0.07 solar mass of circumstellar material, which is smaller than the mass derived from the precursor emission. The authors were not able to resolve this discrepancy satisfactorily, but they noted that it could be resolved if the precursor emission mechanism was super-Eddington in nature. Regardless of the exact quantity of circumstellar material, the red supergiant was losing mass at a rate of 0.01 solar mass per year in the months prior to explosion. This is significantly larger than mass-loss rates expected in normal stars (the Sun loses 10-14 solar mass per year).

What Caused the Mass Loss?

The leading theory for the enhanced mass loss is that it was triggered by an instability inside the star. One possibility is that the mass loss was driven by waves in the envelope of the star that were caused by the pulsations in the core. Such mass loss is possible for stars with masses less than 14 solar masses. However, in this model, the mass loss is expected to last for significantly longer than the 130 days observed for SN 2020tlf. A second model suggests that the mass loss was a result of some sudden energy deposition in the star’s envelope. If the deposited energy is equal to the binding energy of the star’s envelope, it can produce a detectable precursor emission that lasts for a few hundred days and has temperatures and luminosities roughly consistent with that of SN 2020tlf. This energy could be deposited by burning of oxygen and neon or silicon in the core of the star. However, additional studies are required to understand the exact reason for the precursor emission.

The authors noted that such precursor emission should be fairly common in Type II-P supernovae. However, as this emission is intrinsically faint, it is challenging to detect. Future, more sensitive surveys such as the Vera Rubin Observatory will be equipped to detect the final words of the stars.

Disclaimer: Today’s editor is in the same research group as first-author Jacobson-Galán but was not involved in this project. Jacobson-Galán is also an active Astrobites author but was not involved in the publication of today’s bite.

Original astrobite edited by Huei Sears.

About the author, Viraj Karambelkar:

I am a second-year graduate student at Caltech. My research focuses on infrared time-domain astronomy. I study dusty explosions and dust-enshrouded variable stars using optical and infrared telescopes. I mainly work with data from the Zwicky Transient Facility and the Palomar Gattini-IR telescopes. I love watching movies and plays, playing badminton and am trying hard to improve my chess and crossword skills.

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