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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.

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: Published in 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.

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.

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. 2022]

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. 2022]

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: Published in 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 paper. The gas in these galaxies is shown in red and orange, and the AGN regions are shown in green and blue — some AGN are identified indirectly by LINER 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. 2022]

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: Published in 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: Published in 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 2022]

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 2022]

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 2022]

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.

Simulated image of a black hole warping light from background 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 Active Fraction of Massive Black Holes in Dwarf Galaxies
Authors: Fabio Pacucci, Mar Mezcua, and John A. Regan
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Published in ApJ

Dwarf galaxies, with masses less than 10 billion times the mass of the Sun (M☉), host massive black holes. Unlike the central black holes in more massive galaxies, those in dwarf galaxies may not have quite become supermassive. This presents an opportunity to study analogs of supermassive black hole “seeds,” giving us a glimpse at the growth of supermassive black holes — a process that is not quite understood yet.

Active galactic nuclei (AGN) get their name from the active accretion of the supermassive black holes that power them, but not all supermassive black holes are “active.” Some, like the one in our own galaxy, are inactive. What causes AGN to shut off or turn on is still a mystery, but there’s no denying their activity makes them easier to observe. In dwarf galaxies, this is especially useful since their central black holes are usually smaller and therefore harder to detect, so many of the observations of massive black holes in dwarf galaxies have been of low-mass AGN. This means that current observations aiming to obtain the occupation fraction — the fraction of dwarf galaxies that have massive black holes, active or inactive — could actually just be measuring the fraction of dwarfs with active massive black holes.

The authors of today’s paper developed a theoretical model to predict the fraction of dwarf galaxies that contain active massive black holes (103 M☉ < M < 107 M☉), based on galaxy properties and observational constraints. In the model, the fraction of active black holes depends on the number density of the gas in the galaxy and the available angular momentum at its center. However, these two parameters are not necessarily available from observations, so the authors use proxies instead. The stellar mass (the mass of the galaxy that comes from stars) is used as a proxy for the number density of the gas. Instead of angular momentum, the authors use rotational support, a quantity determined from the rotational velocity and velocity dispersion of gas and stars. In the analysis, a black hole is considered active if it meets the criteria for efficient accretion.

Constraining a Physical Model

Fraction of dwarf galaxies with a massive black hole detected in the X-ray versus the stellar mass of the galaxy

Figure 1: The y-axis shows the fraction of dwarf galaxies that have a massive black hole detected in the X-ray. The x-axis shows the mass of stars in the dwarf galaxy. The points with error bars show data from observations in previous works. The blue line shows the expected X-ray detected fraction as a function of stellar mass based on the model developed in this paper. [Pacucci, Mezcua & Regan 2021]

Accreting black holes emit radiation across the electromagnetic spectrum, so they have been detected in dwarf galaxies at many different wavelengths. Some of the most extensive studies have been conducted at X-ray wavelengths, encompassing both wide surveys (which cover lots of targets) and deep observations (which see faint objects). These data provide valuable constraints on the authors’ theoretical model. The authors use the X-ray observations to constrain the emission from star formation and whether the AGN are Compton-thick, or surrounded by enough hydrogen gas to absorb most of the X-rays emitted. These factors affect the detectability of the AGN in observations by outshining or obscuring (respectively) the AGN X-ray emission. By adjusting the model parameters, the authors match their calculated fraction of active massive black holes with detectable X-ray emission to what we actually observe. Their results are shown in Figure 1.

After calibrating their model to match observations in the X-ray, the authors then remove the detectability constraints to calculate the fraction of all active massive black holes, not just those that are detectable in X-rays. The results are shown in Figure 2, along with results from simulations and semi-analytical models. The model presented in today’s paper predicts the active fraction of black holes in dwarf galaxies, since that is more likely what is being measured by current observations, rather than the total occupation fraction computed by previous simulations.

Fraction of dwarf galaxies with a massive black hole vs stellar mass of the galaxy.

Figure 2: Fraction of dwarf galaxies with a massive black hole vs. stellar mass of the galaxy. The blue lines show the results of this paper, the active fraction of massive black holes as a function of mass. The solid and dotted blue lines differ in metallicity of the galaxy. The rest of the curves and colored regions show the occupation fraction of massive black holes in dwarf galaxies from observations in other work. [Pacucci, Mezcua & Regan 2021]

The range of active fractions estimated with this model is 5–22%, which is lower than the occupation fractions from other work. The authors note that this is expected, based on the definitions of the two fractions. Assuming not all massive black holes are active, the active fraction should be lower than the overall fraction of massive black holes found in dwarf galaxies.

The authors explore the effects of the galaxy’s metallicity on the predicted active fraction. Metallicity refers to the types of elements that are present in the gas: if you have mostly hydrogen and only small amounts of elements more massive than helium — what astronomers call metals — you have low metallicity. Lower-metallicity gas leads to more active star formation, which produces a lot of X-rays that can wash out the X-ray emission from the active massive black hole. Accounting for the effects of low metallicity, the calculated active fraction can be higher for this model (blue dotted line in Figure 2) — up to 30% for the most massive dwarf galaxies.

Dwarf galaxies allow us to study possible seeds of supermassive black hole growth, which is necessary to understand black hole growth and galaxy evolution. One way to check if our models of black hole growth are correct is to see if the predicted fraction of active massive black holes in galaxies matches observations. The authors of today’s paper developed a model that predicts the fraction of massive black holes in dwarf galaxies that are active, based on galaxy properties, and compare it to observations. Models like this could be useful to compare with results from upcoming observatories like JWST and Athena, among others, which could actually observe supermassive black hole seeds in the early universe.

Original astrobite edited by Jamie Sullivan.

About the author, Gloria Fonseca Alvarez:

I’m a fourth-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 spin 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.

Four bright circles appear at varying distances from a central 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: The First Dynamical Mass Measurement in the HR 8799 System
Authors: G. Mirek Brandt et al.
First Author’s Institution: University of California, Santa Barbara
Status: Published in ApJL

Astronomers have caught the first directly imaged exoplanet gravitationally tugging at its host star — thereby revealing its own weight.

Did you know that we can now take pictures of planets orbiting another star hundreds of light-years away? The HR 8799 system is famous for hosting one of the first directly imaged exoplanets. The four imaged planets are giants with masses greater than 5 times the mass of Jupiter (MJup) and wide orbits. They are named planet e, d, c, and b in order of proximity to the star. In the 13 years since their first detection back in 2008 by the Keck and Gemini telescopes in Hawaii, astronomers have tracked the planets’ orbital motions around HR 8799 with great precision (Figure 1).

This has enabled the authors of today’s article to calculate the mass of an exoplanet in this system using simple dynamics such as Newton’s and Kepler’s laws of motion. In particular, they concentrate on planet e, the one closest to HR 8799.

Four points of light slowly orbit a central point of light at varying distances

Figure 1: The HR 8799 planetary system. Blocking out the light from the central star allows us to observe and track the motions of four of its exoplanets. The given scale of 20 au, a little more than the orbit of the first planet (planet e), is equal to the distance between the Sun and Uranus. [Christian Marois (Herzberg Institute of Astrophysics) and Jason Wang (University of California, Berkeley)]

Precision Astrometry as a Weighing Scale

The measurement of planet e’s mass rides on the fact that the motion of the four planets produces minute tugs on host star HR 8799, thereby changing the star’s position in the sky ever so slightly. The process of making extremely precise measurements of a star’s sky coordinates is known as astrometry, a technique that has become especially prominent in recent years with new sky surveys like Gaia complementing their predecessor Hipparcos. The researchers used data from Gaia’s latest data release (DR3) to track the acceleration of HR 8799 caused by the net gravitational forces of all four planets.

Knowing the masses of all four major planets accurately makes it possible to predict how much they can accelerate HR 8799. In turn, precise observations of the star’s acceleration enabled the researchers to reverse this calculation and infer the exoplanet masses.

Probability curves for the likely mass of HR 8799 e for various ratios of its mass to the masses of the other planets in the system

Figure 2: The inferred probability distribution of the mass of planet e, based on varying the assumed mass ratios of other planets with respect to it (top: d, middle: c, bottom: b). The top panel shows that planet e’s mass estimation is most sensitive to the second-closest planet, planet d, being heavier (yellow curve) or lighter (teal curve). It does not depend much on the relative weights of the farther two planets. [Brandt et al. 2021]

How Heavy Is Planet e?

As the closest planet to HR 8799, planet e plays the principal role in shaking up its host star. However, the contributions from the other planets in the system are needed to determine how prominent planet e’s tugs are, and in turn how heavy the planet may be. Without knowledge of any of the planets’ masses, it is necessary to make assumptions about how much they weigh relative to each other. Figure 2 shows the authors’ best estimates for planet e’s mass, shown as probability distributions assuming different ratios of its mass to the masses of the other planets in the system.

The authors determine the resulting mass of planet e to be 9.6 times that of Jupiter, within an error bound of 2 MJup. They can say with a 95% certainty that its mass is below 13 MJup, above which it would have enough pressure at its core to start fusing deuterium and trigger nuclear fusion to become a (proto)star itself!

Implications for the Stellar System

In spite of being the planet closest to HR 8799, planet e is still three times farther away than Jupiter is from the Sun. Are there any smaller planets in the HR 8799 system, closer to the star than planet e but small enough to escape detection via direct imaging?

A shaded area indicates the mass and semi-major axis combinations that are disallowed

Figure 3: Can a hypothetical planet with a certain mass (y-axis) exist within the 16 au distance (x-axis) between HR 8799 and the orbit of planet e? The purple region represents heavier masses that are excluded since their effect is not seen in observations. [Brandt et al. 2021]

A hypothetical interior planet can additionally perturb the astrometric motion of HR 8799. But with the lack of such observed perturbations, the authors discard the presence of any planet weighing more than 6 MJup within 8 au (a little less than the distance between the Sun and Saturn), and any additional planets weighing greater than 7 MJup between a distance of 8 au and planet e’s orbit (Figure 3).

This study is a marker for how far we have come in studying exoplanets. It is a fine example of applying simple dynamics to study the gravitational dance and motions of an exoplanetary system — techniques that, until recent times, could only be applied to study our own solar system.

Original astrobite edited by Olivia Cooper.

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 beautiful barred spiral stretches out from a round, bright yellow glowing core. The stars shift from yellow to white to bluish, and dark filaments are laced with pink active star-forming regions.

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: Extragalactic Magnetism with SOFIA (Legacy Program) – II: A Magnetically Driven Flow in the Starburst Ring of NGC 1097
Authors: Enrique Lopez-Rodriguez et al.
First Author’s Institution: Kavli Institute for Particle Astrophysics & Cosmology, Stanford University
Status: Published in ApJ

Galaxies throughout the cosmos display a delightful diversity of morphologies, from elegant and complex spirals to bizarre irregular galaxies. More and more, it seems that the structure and evolution of galaxies are strongly influenced by magnetic fields. The invisible magnetic fields that permeate interstellar space have long been suspected to play a critical role in the formation of stars, buoying clouds against gravitational collapse, steering gas flows throughout galaxies, as well as feeding supermassive black holes in galactic centers. Despite their importance, these magnetic fields are notoriously difficult to measure and map. State-of-the-art instruments and techniques have opened a window into a golden age of magnetic field measurements from local molecular clouds within our Milky Way to the distant maelstroms of gas swirling around galactic nuclei.

How Do We Measure Magnetic Fields?

These magnetic fields cannot be detected directly, so we have to rely on measuring their effects on gas and dust within the environments we aim to study. One way to reveal the magnetic field in the interstellar medium is dust polarimetry. Individual grains of dust amidst the gas in molecular clouds tend to orient themselves relative to the magnetic field that is present, and these organized dust grains emit light with a certain polarization. One can measure the polarization of that light and then infer the orientation of the glowing dust grains to map out the magnetic field lines influencing the dust (Figure 1). This process requires sensitive measurements, but it’s possible using instruments like those aboard SOFIA (the Stratospheric Observatory For Infrared Astronomy).

Cartoon showing a gas cloud with a vertically oriented magnetic field. Elongated dust grains aligned parallel to the magnetic field (i.e., their long dimension is parallel to the magnetic field lines) are labeled “unlikely dust grain orientation.” Elongated dust grains aligned perpendicular to the magnetic field are labeled “likely dust grain orientation.” A sine curve representing the thermal emission from the dust grain emerges from the cloud, traveling toward a telescope.

Figure 1: A cartoon schematic demonstrating how a magnetic field in a molecular cloud influences the likely orientation of dust grains, which emit light with a polarization indicating their orientation. Measurements of this polarization can then be used to infer the original magnetic field. [H Perry Hatchfield]

The Magnetic Heart of NGC 1097

Today’s paper explores the magnetic-field structure of the center of NGC 1097, a barred spiral galaxy with an active galactic nucleus and a brilliant starburst ring fed by a pair of linear gas structures called dust lanes that are aligned with the bar. The ring of dense gas orbiting about 3,000 light-years from the galaxy’s nucleus is forming stars at more than twice the rate of the entire Milky Way! In addition to its spectacular spiral arms, prominent dust lanes, and glowing core, NGC 1097 is also known to host one of the strongest interstellar magnetic fields in a nuclear starburst ring. This galaxy provides an excellent testbed for studying the interactions of star formation, galactic-scale flows and structures, and powerful magnetic fields.

The authors use a combination of far-infrared (89-μm) polarimetry from SOFIA’s High-resolution Airborne Wideband Camera Plus (HAWC+) instrument and radio (3.5-cm and 6-cm) polarimetry from the Very Large Array (VLA) to trace the orientation of dust grains shifted by the magnetic field. They also explore the velocity structure of the gas using molecular line emission from carbon monoxide. By understanding both the gas motions and the magnetic field properties, they can see how the gas flows and magnetic structure of the galaxy might be related. The magnetic field traced by the far-infrared emission appears to have a different structure from the field revealed by the radio observations; the 89-μm dust emission seems to indicate a compressed field, while the radio observations clearly suggest a spiral structure to the field (Figure 2).

A central blob of carbon monoxide emission surrounded by a fragmented ring with a fainter spiral arm trailing off to either side. Yellow and red polarization vectors are scattered across the center of the image, as well as red and yellow contours.

Figure 2: The magnetic field structure traced by far-infrared observations from SOFIA’s HAWC+ instrument (in yellow) and 3.5-cm radio observations from the VLA (in red). In each case, the lines show the orientation of the magnetic field, and the contours show the intensity of the polarized light. The background color-scale image is the integrated carbon monoxide emission (J=2–1 transition) from ALMA. [Lopez-Rodriguez et al. 2021]

The authors interpret this apparent difference in magnetic-field morphology as the two observations tracing separate modes of the magnetic field associated with different phases of the gas: the far-infrared polarization reveals the magnetic field compressed by a shock wave crashing through the dense gas in the starburst ring, while the radio polarization shows the magnetic field being twisted in a spiral by shearing motions in the more diffuse gas. This suggests that the gas motions, whether diffuse or dense, may be guided by the strong magnetic fields. Unraveling the intricate dance of magnetic fields, kinematics, and gravity is no easy task, but multi-wavelength polarization studies like this provide an exciting window into the diversity of the magnetic structures of galaxies — and when it comes to magnetic fields, it seems like there is always more than meets the eye.

Original astrobite edited by Sasha Warren.

About the author, H Perry Hatchfield:

I’m a PhD candidate in Physics at the University of Connecticut, where I study star formation and gas structure in the Milky Way’s galactic center. I do this using radio observations of molecular clouds as well as hydrodynamic simulations, and I’m all about trying to find ways to compare these two exciting means of exploring the universe.

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