Astrobites RSS

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: Accepted to 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. 2021]

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. 2021]

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: Accepted to 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 supernova exploded. Symbols with arrows indicate upper limits on the brightness. [Adapted from Jacobson-Galán et al. 2021]

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.

a single radio dish points at the sky, with gnarled trees and shrubs in the foreground

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: Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder
Authors: Ziteng Wang et al.
First Author’s Institution: The University of Sydney, Australia
Status: Published in ApJ

There seems to be a never-ending list of exciting radio sources within and outside of our galaxy. There are pulsars: a type of rapidly rotating neutron star. There are magnetars: neutron stars with extraordinarily powerful magnetic fields. There are flare stars: stars whose brightness will rapidly vary or “flare” in just a few minutes. There are supernovae and jets in active galactic nuclei. There are the galactic center radio transients: transient radio sources with unknown origins located toward the center of our galaxy. There are fast radio bursts: extremely energetic bursts of radio emission originating at extragalactic distances. On top of all of these, today’s authors present a possible new type of radio source: ASKAP J173608.2−321635.

What’s So Cool About This Source?

ASKAP J173608.2−321635 was first detected by a group of astronomers using the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey, a survey that focuses on finding transient objects whose emission changes on timescales down to ~5 seconds. After the initial detection, the team continued to monitor the source and detected it a total of six times over a nine-month period (see Fig. 1). They found that the flux was fairly steady on timescale of a few hours and persistent yet variable on the longer timescale of a few weeks. They then observed the source with the radio telescope MeerKAT in South Africa, which allowed them to search for pulsar-like emission (pulsed emission) and continuum (constant) emission. Since MeerKAT operates in a different frequency regime, this also allowed them to study the frequency dependence of the source.

six views of the field of view containing the source, showing detections by ASKAP and MeerKAT

Figure 1: Detections of ASKAP J173608.2−321635 with ASKAP and MeerKAT. Both “on” (i.e., pointed at the source), “off” (i.e., pointed away from the source), and Stokes V (which quantifies the circular polarization of the source) images are shown. [Wang et al. 2021]

In their first five observations with MeerKAT, the team didn’t detect anything (sad!). However, they hit gold on their sixth observation, finding strong continuum emission but no pulsed emission. The continuum emission that they detected had a number of interesting features:

  1. significant circular polarization and high levels of linear polarization
  2. a steep, negative spectrum, meaning that the flux decreases sharply as frequency increases
  3. a rapidly decreasing flux with a characteristic timescale of decay of ~26 hours (see lower right panel of Fig. 2)
  4. no variability on the timescale of ~minutes

In addition to radio observations with ASKAP and MeerKAT, the authors also looked for non-radio emission from the source in archival X-ray and near-infrared data, as well as in their own observations using the Neil Gehrels Swift Observatory and the Chandra X-ray Observatory. They didn’t find any significant emission in their searches, though.

flux density versus time

Figure 2: Observed fluxes for observations at different frequencies and with different radio telescopes. Upper limits on emission are shown with downward errors. Note in the lower right panel how the emission decreases with time for the MeerKAT observations, and how the flux spans almost two orders of magnitude. [Wang et al. 2021]

What Could It Be…

This source seems to have a lot of really interesting features, but there are already a lot of interesting radio sources out there. Let’s take a look at the similarities and differences between this source and other radio sources to figure out a possible origin. Could it be…

  • A star: Stars are one of a few sources that can emit polarized emission. However, X-ray and radio luminosities are typically correlated for these objects, so we would expect to detect X-rays from the source. Additionally, near-infrared emission would also likely be significant and detectable. So, unless our source has a really really small ratio of X-ray/infrared to radio emission, we can probably rule out a star. Buh bye stars!
  • A pulsar: The steep spectrum and circular polarization of our source are very reminiscent of pulsars. However, the source shows no pulsed emission. It’s possible that it’s a highly scattered pulsar, a pulsar with an ultra-long period, or a pulsar in an eccentric binary system, but it doesn’t like look it could be your everyday pulsar.
  • A magnetar: Magnetars tend to have very flat spectra (e.g., the intensity of the emission does not vary a lot as a function of frequency), which is very unlike our source! Magnetars also typically have periods of ~1–10 seconds, which is ruled out by the MeerKAT periodicity searches. However, there is the possibility that it is a special ultra-long period magnetar.
  • A jet, gamma-ray burst, supernova, or tidal disruption event: Again, not looking so likely. The levels of circular and linear polarization, the steep spectra, and the short timescale over which it decayed make all of these very unlikely.
  • A galactic center radio transient: We actually might have something here! Galactic center radio transients similarly have steep spectra, are highly polarized, and have no X-ray emission. However, only three galactic center radio transients have been detected and their origins, if they are even the same, are still uncertain. Additionally, the timescale over which the emission varies for ASKAP J173608.2−321635 doesn’t match the timescales of other galactic center radio transients.

What Should Our Takeaway Be?

It seems like this source is definitely not a typical pulsar, magnetar, or star. The authors seem to be leaning toward a new type of source, which might be related to galactic center radio transients, or might be the first of its kind. The only way to find out is to keep observing!

Original astrobite edited by Ishan Mishra.

About the author, Alice Curtin:

I’m a PhD student at McGill University studying fast radio bursts and pulsars using the Canadian Hydrogen Mapping Experiment (CHIME). My work mainly focuses on characterizing radio frequency interference, investigating possible relationships between gamma-ray bursts and FRBs, and using pulsars as calibrators of future radio instruments. When not doing research, I typically find myself teaching physics to elementary school students, spending time with friends, or doing something active outside.

multicolored lines twist and curve around a simulated planet

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: Exploring the Effects of Active Magnetic Drag in a GCM of the Ultra-Hot Jupiter WASP-76b
Authors: Hayley Beltz et al.
First Author’s Institution: University of Michigan
Status: Accepted to AJ

Love it or hate it, magnetism can influence a whole host of observable astronomical phenomena, especially when it comes to hot gas giant planets. The most extreme of these planets, ultra-hot Jupiters, are tidally locked and have temperatures over 2000K — hot enough to thermally ionise chemical species within the planet’s atmosphere. This process leaves plenty of charged particles ready to interact with the planet’s magnetic field as they get carried by the planet’s strong winds. While there have yet to be any direct measurements of exoplanetary magnetic fields, the fields of ultra-hot Jupiters are expected to be comparable to (if not even greater than) that of Jupiter. With a field strength at its surface of ~4 gauss (G), Jupiter’s magnetic field is already strong enough to extend its magnetosphere beyond the orbit of Saturn, so magnetohydrodynamics should be an important consideration. For example, the Lorentz force created as the ions move through the magnetic fields can result in a drag effect on the circulation within the atmosphere. This magnetic drag can change wind patterns, impacting how easily energy is transported between the ultra-hot Jupiter’s permanent day and night sides.

However, modelling exoplanet atmospheres is a complicated task even before considering magnetohydrodynamics. One of the simplest ways to model magnetic drag forces is via a uniform drag applied to the entire model atmosphere, but this method ignores the potential impact that a planet’s hotter dayside and cooler nightside would have on the forces at play. Today’s article instead uses a locally calculated active magnetic drag, taking into account factors such as the temperature to explore its potential effects.

Modelling Magnetism

For their work, the authors choose to focus on one particular ultra-hot Jupiter, WASP-76b. To model the planet, they make use of a general circulation model to simulate temperature and wind patterns under the influence of magnetic fields with strengths of 0, 0.3, 3, and 30 G. After running the simulations for 2,000 orbits of WASP-76b — enough to allow the atmosphere to reach a steady state — snapshots are taken of the planet so that the magnetic field strengths can be compared.

In the case where no field is applied (0 G), WASP-76b looks like a typical ultra-hot Jupiter, with a strong equatorial wind blowing to the east and a 1500K temperature difference between the day and night sides. The planet’s hottest region is also shifted to the east of its sub-stellar point (the part of the planet closest to the star and therefore receiving the most stellar energy) as expected due to those supersonic winds. This is demonstrated in the first column of Figure 1.

Maps of the atmosphere of WASP-76b under 4 different magnetic field strengths (left to right) at 5 different pressures (top to bottom). Each map is centred of 0 degrees longitude.

Figure 1: Maps of WASP-76b showing the temperature and wind patterns for the different field strengths at different pressures. Lower pressures higher in the atmosphere are shown at the top, while higher pressures deeper in the atmosphere are at the bottom. The thickness of the wind arrows demonstrates the speed of the winds. Each map is centred on the dayside of the planet. [Beltz et al. 2021]

When the planet has a magnetic field, however, the general circulation model snapshot shows a very different picture. For all field strengths, winds high in the atmosphere on WASP-76b’s dayside flow over the poles — a complete change from the 0 G case! The low pressures and high temperatures here cause a drag that counteracts the traditional east–west flow, leaving only the weaker north and south winds. At higher pressures the field strength becomes more important, with the strongest fields able to disrupt the wind deeper into the atmosphere. This poleward flow isn’t a behaviour seen when using the simpler uniform drag method and could have some observable consequences for the planet.

The Answer Lies in the Phase Curves

The reduction in the eastward winds in strong magnetic fields means that WASP-76b’s hotpot would have a smaller shift from the sub-stellar point than otherwise expected. The change in wind flow also reduces the amount of heat that moves between the two sides of the planet, increasing the contrast in temperature between the day and night. Helpfully, both of these effects can be tested by observing the planet’s phase curve, which measures the changing brightness of different faces of the planet as it orbits its star.

As shown in Figure 2, the modeled phase curves of WASP-76b have varying amplitudes and peaks at different offsets from an orbital phase of 0.5, when the planet is perfectly behind the star. Stronger magnetic fields cause the phase curve to peak closer to 0.5 due to the lack of hotspot shift. Meanwhile, the phase curve amplitude increases as the reduced winds prevent efficient transport of heat between the day and night sides, increasing the temperature difference and hence the difference in emitted flux.

Left: a plot of flux against orbital phase showing the phase curves of 4 different active magnetic drag effects and 2 uniform drag effects. Right: a plot of amplitude against orbital phase centred on the flux peak, highlighting the difference between field strengths.

Figure 2: Left: Simulated phase curves of WASP-76b at the four different magnetic field strengths studied in the article (solid lines). To show the difference between the active and uniform drag method, phase curves are also plotted using two different uniform drags, shown with dashed lines. Right: The amplitudes of each phase curve, given by the difference in the maximum and minimum fluxes normalised by the maximum flux. The stronger magnetic fields have phase curves with peaks closer to 0.5 and larger amplitudes. [Beltz et al. 2021]

Published Spitzer phase curves of WASP-76b are consistent with the 3 and 30 G models due to their small hotspot offsets, but the authors caution against making direct comparisons. The atmospheric models that were used neglect several important effects such as the presence of clouds and the dissociation and recombination of hydrogen molecules, both of which could reduce the impacts of magnetism on the upper atmosphere.

Despite this, today’s article clearly shows how important it is to consider magnetohydrodynamics when modelling exoplanet atmospheres and provides a great starting point for future studies.

Original astrobite edited by Abygail Waggoner.

About the author, Lili Alderson:

Lili Alderson is a second-year PhD student at the University of Bristol studying exoplanet atmospheres with space-based telescopes. She spent her undergrad at the University of Southampton with a year in research at the Center for Astrophysics | Harvard-Smithsonian. When not thinking about exoplanets, Lili enjoys ballet, film, and baking.

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: Exploring the AGN-ram pressure stripping connection in local clusters
Authors: Giorgia Peluso et al.
First Author’s Institution: INAF-Padova Astronomical Observatory, Padova, Italy
Status: Accepted to ApJ

Current theories suggest that most galaxies — if not all — contain a supermassive black hole in their centre, with masses anywhere from millions to billions of times that of our Sun. This theory was bolstered in 2019, when the Event Horizon Telescope took the first ever photograph of a black hole in the centre of the nearby elliptical galaxy M87, and in 2020, when Reinhard Genzel and Andrea Ghez were awarded the Nobel Prize in Physics for discovering a supermassive black hole at the centre of our own galaxy.

With their enormous density and gravitational fields, black holes are some of the most extreme objects in the universe. Consequently, material that is pulled towards a supermassive black hole can be accelerated almost to the speed of light. If enough material is present, it can form an accretion disk — an incredibly hot structure that feeds material into a black hole and converts this mass into energy, which is emitted as light. A supermassive black hole surrounded by an accretion disk is known as an active galactic nucleus (AGN) as shown by the artist’s impression in Figure 1.

Artist's impression of an AGN. In the centre is a black sphere, representing the black hole, surrounded by a flat, orange, rotating disk of material. A thin blue jet is pointing upwards from the black hole, perpendicular to the disk.

Figure 1: Artist’s impression of an AGN, with a central black hole surrounded by a flat, rotating accretion disk. Some AGNs emit narrow, powerful jets of material — this can be seen above the black hole. [NASA/JPL-Caltech]

AGNs are incredibly efficient engines. Around 10% of the mass that is accreted by an AGN is converted into energy (this efficiency is about 0.7% in nuclear fusion, and just one billionth of a percent in typical combustion). As a result, AGNs emit huge amounts of light and are some of the brightest objects in the universe. But, like all engines, AGNs need an ignition: a trigger that can cause a dormant supermassive black hole to start accreting material and become an AGN. Today’s article explores one mechanism, which could be responsible for switching on AGNs in galaxy clusters.

It’s Tough to Stay Cool Under Pressure

Galaxy clusters are huge objects containing up to several thousand galaxies. Between these is a sea of hot gas called the intracluster medium. As galaxies sail through this ocean, drag forces from the intracluster medium can cause gas in the galaxies to be stripped away, leading to tails of gas streaming out in their wake. This process is known as ram-pressure stripping. In this article, the authors investigate whether these stripped galaxies are more likely to have an AGN in their centres.

To do this, they use observations of 115 galaxies that are currently undergoing ram-pressure stripping within clusters, taken from multiple different astronomical surveys. These galaxies are compared to a control sample of 782 star-forming galaxies taken from field regions (i.e., not in clusters) of the MaNGA survey. These galaxies are then tested to see whether they contain AGNs by looking at their emission of light at different wavelengths and placing them on a BPT diagram (a commonly used method for identifying AGNs).

Two panels, each showing a map of the star forming gas, and other emission from a galaxy. Each panel shows an elliptical contour, representing the disk of the galaxy. Offset from each of these is a cloud of gas that has been stripped. In the centre of each galaxy is a small patch -- the galaxy on the left has a central LINER emission region, and the galaxy on the right has AGN emission.

Figure 2: Examples of two ram-pressure stripped galaxies from today’s article. The gas in these galaxies is shown in red and orange, and the AGN regions are shown in green and blue — some AGNs are identified indirectly by low-ionization nuclear emission-line region emission. The red contours show the edge of the galactic disks, demonstrating how the gas has been stripped away and is trailing behind the galaxies. [Adapted from Peluso et al. 2021]

There are two main findings in this article. Firstly, ram-pressure stripped galaxies are roughly 1.5 times more likely to host an AGN in their centre than the control sample (see Figure 2 for two example galaxies). Additionally, larger galaxies are far more likely to contain AGNs, particularly those with stellar masses greater than 1010 solar masses. The authors find that 27% of their stripped galaxies contain AGNs, compared to just 18% of the field galaxies. When just looking at large galaxies, AGNs are found in 51% and 35% of their stripped and control samples, respectively.

Fire Up the AG-Engines!

These results are exciting, and tell us that there is a close connection between AGNs and ram-pressure stripping. One potential explanation comes from the fact that the intracluster medium, which causes the stripping of a galaxy’s gas, can also increase the external pressure on a galaxy. This pressure can compress the gas in the galaxy, triggering star formation and causing gas to spiral inwards towards the galaxy centre, resulting in the formation of a luminous accretion disk. Furthermore, triggering an AGN can result in gas being thrown out of a galaxy, leading to the tails of jettisoned gas that are attributed to ram-pressure stripping.

There are several caveats in this article. For example, this work only compares field galaxies to a very specific type of cluster galaxies — those with clear signs of ram-pressure stripping. The authors explain that future studies involving larger numbers of galaxies will be able to constrain their results even further.

Since one third of the non-stripped galaxies also contain AGNs, ram-pressure stripping clearly isn’t the only possible cause of AGN ignition. However, the impact of ram-pressure stripping shown in this article is a valuable clue that will help us move towards a better understanding of what causes these huge cosmic light bulbs to switch on.

Original astrobite edited by Lili Alderson.

About the author, Roan Haggar:

I’m a PhD student at the University of Nottingham, working with hydrodynamical simulations of galaxy clusters to study the evolution of infalling galaxies. I also co-manage a portable planetarium that we take round to schools in the local area. My more terrestrial hobbies include rock climbing and going to music venues that I’ve not been to before.

x-ray image of the Milky Way center, featuring bright emission at the center and two lobes of gas to either side,

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: Stars Lensed by the Supermassive Black Hole in the Center of the Milky Way: Predictions for ELT, TMT, GMT, and JWST
Authors: Michał J. Michałowski and Przemek Mróz
First Author’s Institution: Adam Mickiewicz University, Poland
Status: Published in ApJ

The Milky Way Galaxy is known to host a supermassive black hole at its center. First detected as a strong radio source called Sagittarius A* (Sgr A*), we have still not seen it directly through optical telescopes. However, it made its presence known at the turn of the millennium, when astronomers caught its immense gravitational pull whirling around a number of stars in the galactic core in slingshot orbits. The motion of these “S stars” implied the presence of a central mass weighing a whopping four million times the mass of the Sun, concentrated within a sphere of radius equivalent to the Earth–Sun distance. Such a concentration of mass meant it could only be a black hole — an observation that won Andrea Ghez and Reinhard Genzel the 2020 Nobel Prize in Physics.

The coming decades will see the next generation of optical telescopes pushing our frontiers of observational astronomy. These include the successor to the Hubble Space Telescope — the James Webb Space Telescope (JWST), due for launch at the end of 2021 — and planned ground-based telescopes such as the Thirty-Meter Telescope (TMT), the Giant Magellan Telescope (GMT), and the Extremely Large Telescope (ELT). At a distance of 26,000 light-years and smaller than our solar system, the Milky Way’s supermassive black hole is impossible to detect with our current telescopes, but the authors of today’s article say detecting it with future telescopes would be elementary.

A Supermassive Black Hole Lens

Gravitational lensing occurs when a dense, massive clump of matter — the gravitational lens — distorts a distant light source either by magnifying and extending it into rings called Einstein rings, or by generating multiple copies of it. Observations of this phenomenon commonly involve a large lens, such as a galaxy cluster, lensing an extended object like a distant galaxy. Very few observations have been made of point-like stars being gravitationally lensed. This is where Sgr A*, the Milky Way’s supermassive black hole, comes into the picture.

The light coming from stars directly behind the Milky Way’s supermassive black hole can get lensed as it makes its way to Earth. This causes the resulting image to split as illustrated in Figure 1, giving rise to a second image of the same star, diametrically opposite with respect to the location of Sgr A*. This splitting is greatest for light coming from directly behind the black hole along its axis and becomes tinier for stars away from this axis. To detect these secondary images, a telescope must have high sensitivity (being able to see faint sources), and high resolution (being able to differentiate between two very close sources). The most important characteristic of the next generation of telescopes will be their enhanced sensitivity to stars that are dimmer than 24th magnitude. (The magnitude scale is flipped, with lower numbers indicating brighter stars, and each increment makes a star 2.5 times dimmer. For reference, the human eye can only see stars up to 6th magnitude in perfect conditions.)

Illustration of a black hole at the center, splitting light from a star to the right to form two apparent images at a telescope on the left

Figure 1: An illustration of the image of a star behind a black hole getting split into two as seen by a telescope on the other side. [NASA Roman Space Telescope]

Prospects for Next-Generation Giant Eyes to the Skies

Knowing the expected sensitivity and resolution of each of the next-generation telescopes enabled the researchers to estimate the number of lensed stars they will detect. This is shown in Figure 2. The authors calculated how many stars, from just behind the black hole to the edge of the galaxy, are bright enough for their split lensed image to be visible and well-separated in five hours of observations by each telescope. The authors found that the ELT, GMT, and TMT will each be able to resolve over a hundred such lensed star images, but JWST will largely be limited by confusion, where a different star from the dense galactic core is mistaken to be the lensed image of another star. Most lensing detections will come from stars within 16,000 light-years of Sgr A*.

number of lensed stars versus distance from the galactic center for each telescope

Figure 2: The cumulative number of lensed stars detectable by various next-generation optical telescopes is plotted here against increasing distance in kiloparsecs (kpc) behind the supermassive black hole at the center at 0 kpc. The x-axis scale is set by the radius of the Milky Way, around 15 kpc. [Michałowski & Mróz 2021]

Putting Einstein’s General Relativity Under the Lens

The observation of lensed background stars provides us with another way of studying our neighborhood supermassive black hole, but that’s not all! Not only are the lensed objects point-like, but in this case, the gravitational lens itself is point-like. This distinguishes it from previous gravitational lensing observations where the lenses comprised large-scale structures like galaxies and galaxy clusters. Mathematically, a point-like source and point-like lens gives the clearest and most complete description of gravitational lensing through Einstein’s theory of general relativity. As such, we can use observations of this phenomenon to put Einstein’s theory itself under the test! A century since its inception, general relativity has withstood tests from planetary and spacecraft orbits, binary pulsars, X-ray observations, gravitational waves, and other forms of gravitational lensing. Will it be able to hide behind a supermassive black hole?

Original astrobite edited by Huei Sears.

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.

a dark, dusty, sinuous gloud against a glowing backdrop of distant dust clouds and stars

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 ALMA Survey of 70 µm Dark High-mass Clumps in Early Stages (ASHES). IV. Star formation signatures in G023.477
Authors: Kaho Morii et al.
First Author’s Institution: The University of Tokyo, Japan
Status: Accepted to ApJ

Despite attempts to probe the early stages of massive star formation, those critical phases are still shrouded in mystery. Massive stars (greater than 8 solar masses) enrich the interstellar medium with heavy elements expelled through stellar winds and eventually collapse and release even more material through supernovae. That’s why understanding their formation is essential for understanding the entire feedback cycle within the universe.

Massive stars form out of dense “cores” of material. One promising area for hosting such cores is the infrared dark cloud IRDC 18310-4. In today’s article, the authors use data from the Atacama Large Milllimeter/submillimeter Array (ALMA) Survey of 70-µm dark High-mass clumps in Early Stages (ASHES) to look at the cloud and what may be hidden within.

Getting to the Core(s) of the Problem

The authors begin by examining an ALMA image of the cloud (see Figure 1), which shows the continuum dust emission at 1.3 mm. The main cloud is in the brightest part of the image near the center, running from the upper left to the lower right. The white contours trace the emission and are concentrated in the areas where it is brightest.

Using this image and specific requirements for brightness and spatial separation, the authors determine that there are 11 cores within the cloud, which are marked in Figure 1 by the cyan ellipses. They label the brightest cores ALMA1–ALMA8 and the other cores sub1–sub3. These cores are important because they’re the most likely sites for future (or maybe even current!) massive star formation.

An image of the dark cloud, with distinct clumps of dust that have higher fluxes.

Figure 1: The ALMA dust continuum emission at 1.3 mm. Contours show the increasing flux in the cloud, which runs diagonally from the upper left to lower right. Cyan ellipses mark the 11 cores (labeled ALMA1–ALMA8 and sub1–sub3) that may host the earliest stages of massive star formation. [Morii et al. 2021]

CO and Important Outflow

The authors also looked at different molecular emission lines in the cloud, which allowed them to estimate the properties of each core. Emission lines help determine gas temperature because the presence of more energetic emission lines means the gas is hotter. Core mass was also determined using the velocity dispersion of the molecular lines. Additionally, past studies have found that molecules like CO and SiO trace outflows and shocks.

To find outflows in the image, the team separated the redshifted and blueshifted components of the CO emission. Figure 2 shows the same dust continuum map as Figure 1, but in grayscale. Green crosses mark the locations of the highest continuum emission. A few of the cores are labeled. The red and blue contours highlight the integrated redshifted and blueshifted CO emission. The black contours demonstrate the areas with the greatest SiO emission.

An image of the same region with molecular gas contours showing outflows from some of the cores.

Figure 2: The same image as Figure 1, but in grayscale. The red and blue contours show the red- and blue-shifted CO emission. Black contours trace SiO emission. The arrows show the inferred outflows of material present in ALMA2, ALMA3, ALMA4, and ALMA8. [Morii et al. 2021]

ALMA2, ALMA3, ALMA4, and ALMA8 all have outflows of material, illustrated with the red and blue arrows. ALMA3 has the longest and the fastest outflow, which is horizontal in Figure 2. After examining the cores with individual position–velocity diagrams, the authors found all of these active cores, except ALMA8, have signs of episodic outflows. That means an important fraction of massive protostars may undergo distinct episodes of accreting and ejecting material.

Sorting Cores

The authors of today’s article sort the cores into three main groups based on their measurements of molecular emission lines.

  1. Protostellar cores that have both high temperatures and outflows: ALMA2, ALMA3, ALMA4, and ALMA8.
  2. Protostellar core candidates that have high temperatures but no outflows: ALMA1 and ALMA5.
  3. Prestellar core candidates that have neither high temperatures nor outflows: ALMA6, ALMA7, and sub1–sub3.

Basically, the higher the temperature, the older the protostar, since it warms the surrounding material in the core as it evolves. ALMA8 has the highest temperature of the cores, so it’s likely the most evolved. The protostellar core candidates might represent an extremely early phase of star formation before outflows begin. The prestellar core candidates might one day start to form protostars, but for now they’re probably just clumps of gas and dust.

Looking Ahead with ASHES

While this is a study of only one cloud, the complete ASHES project will explore many similar structures and may enable statistical studies of the earliest stages of massive star formation. That will help constrain how and where massive stars form, a crucial aspect of understanding how the universe evolves.

Original astrobite edited by Luna Zagorac.

About the author, Ashley Piccone:

I am a third-year PhD student at the University of Wyoming, where I use polarimetry and spectroscopy to study the magnetic field and dust around bowshock nebulae. I love science communication and finding new ways to introduce people to astronomy and physics. In addition to stargazing at the clear Wyoming skies, I also enjoy backpacking, hiking, running, and skiing.

overlapping rings of panels surround a star

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: Evolutionary and Observational Consequences of Dyson Sphere Feedback
Authors: Macy Huston and Jason Wright
First Author’s Institution: The Pennsylvania State University
Status: Accepted to ApJ

Are we truly alone? Or rather, where is everybody? After all, there are many billions of stars and exoplanets in the Milky Way, and our galaxy has had billions of years to evolve. Although the search for extraterrestrial life has so far yielded nothing, we may still be able to detect signs of past or present intelligent life within our galaxy by detecting artificial megastructures.

Technosignatures

In order for a civilisation to make the technological leap to conquer the stars, it must be able to acquire and harness sufficient energy. This is the basis of the Kardeshev scale, which ranks technological advancement based on energy utilisation. One way to harness a massive amount of energy is to partially or even fully surround a star with solar panels or solar collectors. This type of megastructure is known as a Dyson sphere, named after physicist Freeman Dyson who proposed it as a thought experiment, arguing that such megastructures were a logical next step to meet the energy needs of a space-faring civilisation. Dyson postulated that it may be possible to detect the presence of megastructures by looking for changes in a star’s electromagnetic spectrum. A change in a measurable property, like a star’s spectrum, due to the presence of some artificial structure is referred to as a technosignature. So, what is the technosignature of a Dyson sphere?

So far, studies have mostly focused on changes to the spectrum of the star — particularly in the infrared — and/or light dimming as the megastructure rotates around the star. However, there is a more subtle effect at play too: the radiative feedback from the Dyson sphere itself. Today’s article examines this feedback and the subsequent effects on the star’s evolution, and whether such effects constitute a detectable technosignature.

Feedback

The goal of a Dyson sphere is to collect as much energy as possible from a star, but it is likely that some of this energy will be reflected back onto the star. Additionally, the material of the Dyson sphere will ultimately heat up and emit thermal radiation. As such, Dyson proposed conducting observations in the infrared to try to detect this heat signature. We can gain further insight into this signature by understanding how the star behaves when it is subjected to Dyson sphere feedback. The authors of today’s article use the open-source Modules for Experiments in Stellar Astrophysics (MESA) tool to simulate the evolution of a star subject to external irradiation — in this case, the radiative feedback from the surrounding Dyson sphere. The authors simulate several stars with different masses and with different degrees of feedback, modelled as the fraction of the star’s luminosity that is reflected back.

As seen in Figure 1, feedback from the Dyson sphere generally results in a decrease in nuclear luminosity (the rate of energy production due to nuclear fusion) and an increase in radius. In other words, the star cools and expands. The effect is considerably more pronounced for the 0.4-solar-mass star as it is primarily convective (energy transfer is dominated by convection), whereas the 1-solar-mass star is mostly radiative with only a small outer convection layer. The drop in luminosity also means that the star lasts longer on the main sequence before exhausting its supply of hydrogen.

radius and nuclear luminosity as a function of the age of the star

Figure 1: The nuclear luminosity (left panels) and stellar radius (right panels) as a function of the age of the star for a 0.4-solar-mass star (top row) and 1-solar-mass star (bottom row). Different coloured lines correspond to different levels of Dyson sphere feedback, with the blue line corresponding to the star before the construction of the megastructure. [Adapted from Huston & Wright 2021]

Dyson Sphere Program

The feedback from Dyson spheres is thus able to change the properties of the host star, with more substantial changes occurring in low-mass stars. How does this translate into observations? To find out, the authors use the AGENT formalism, which characterises a Dyson sphere with five parameters. These include the power of the intercepted starlight, denoted α, and the characteristic temperature of the waste heat, T. The authors consider two types of Dyson spheres: hot Dyson spheres, which are coloured black and absorb all starlight, and cold, mirrored Dyson spheres, which reflect all starlight without heating up.

In Figure 2, we see mock colour–magnitude diagrams for two instruments, ESA’s Gaia spacecraft and NASA’s WISE spacecraft. GBP, GRP and G-W4 refer to the blue and red Gaia filters and the infrared WISE filter, respectively. At high feedback levels, the temperature of the sphere is close to that of the star, so it appears bluer. At low feedback levels, the Dyson sphere is cooler and contributes to the dimming and reddening of the star. For cold, mirrored spheres, the reflected light makes the star appear bluer, but its overall luminosity is unchanged.

magnitude versus color

Figure 2: Colour–magnitude diagrams for potential Dyson spheres systems around a 1-solar-mass star for Gaia observations (left panel) and WISE observations (right panel). Coloured lines denote different fractions of feedback, with the black lines denoting Dyson spheres at 0.1 au and 1 au. [Adapted from Huston & Wright 2021]

To Change a Star

Low-mass stars are significantly affected by Dyson sphere feedback; only 1.3% feedback is required to alter the nuclear luminosity of a 0.4-solar-mass star by 1%, while 45% feedback is required to produce the same change in a solar-mass star. In general, the Dyson spheres themselves must also have extremely high temperatures in order to generate sufficient feedback. Figure 3 shows that even low-mass stars would require temperatures well in excess of 1000K to result in even a slight change of nuclear luminosity. There is no significant change in nuclear luminosity for Dyson spheres with temperatures in the hundreds of kelvin.

fraction of captured starlight versus the effective temperature of the sphere

Figure 3: The fraction of captured starlight versus the effective temperature (in kelvin, K) of the Dyson sphere. Solid lines correspond to the values required for a 1% change in nuclear luminosity, and dotted lines correspond to a 1% change in the effective temperature of the star. [Huston & Wright 2021]

Astroengineering

This study has demonstrated that in extreme cases, the feedback from Dyson spheres can directly influence a star’s evolution: it cools, reddens, expands, and its lifetime on the main sequence is extended. The authors suggest that advanced civilisations could therefore use Dyson spheres as part of stellar engineering projects to extend a star’s life or siphon material (star lifting). The search for technosignatures has ramped up in recent years thanks to improvements in instrumentation. Modern instruments are sensitive enough to measure a star’s light dimming, most notably in Boyajian’s star, where one explanation proposed for its unusual light fluctuations is a transiting megastructure. Today’s article shows that Dyson spheres can result in measurable changes to stellar properties. Megastructures have long been confined to science fiction, imagination, and certain video games. However, if there are indeed Dyson spheres out there waiting to be found, we could soon be in a position to find them.

Original astrobite edited by Luna Zagorac.

About the author, Mitchell Cavanagh:

Mitchell is a PhD student in astrophysics at the University of Western Australia. His research is focused on the applications of machine learning to the study of galaxy formation and evolution. Outside of research, he is an avid bookworm and enjoys gaming, languages, and code jams.

1 2 3 4 5 31