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three images of simulated dust grains

What do you see when you look at these images? Clouds? Islands? Legos? Think smaller: simulated dust particles! Modelers often approximate dust grains as spheres, but real dust grains likely come in a range of shapes. A new publication led by Jessica Arnold (Army Research Laboratory) explores the impact of dust shapes on models of debris disks around young stars. Irregularly shaped dust grains, like those shown above, have been used to reproduce spectra of dust grains in a lab setting as well as those of dusty comets, making them a promising tool for modelers. Arnold and collaborators modeled dust grains three ways — as solid spheres, porous spheres, and randomly generated irregular shapes — to constrain the properties of the debris disk surrounding AU Microscopii. The three shapes yielded different best-fitting dust grain size distributions for AU Microscopii’s disk, suggesting that future modeling should account for the uncertainties introduced by grain shapes. To learn more about how astronomers decipher distant dust, be sure to read the full article below!


“Stumbling over Planetary Building Blocks: AU Microscopii as an Example of the Challenge of Retrieving Debris-disk Dust Properties,” Jessica A. Arnold et al 2022 ApJ 930 123. doi:10.3847/1538-4357/ac63a9

optical image of the bow shock around a black widow pulsar

Though it’s not uncommon for stars to snack on their stellar companions from time to time, it’s rare for them to eat their associates entirely. This is the scenario proposed for black widow pulsars — tiny, dense stellar remnants that rotate rapidly, produce beams of radio emission and fierce winds, and, like the spiders from which they get their name, eventually consume their companions. Black widow pulsars may be the missing link in the creation of single millisecond pulsars — those that rotate hundreds to thousands of times a second but lack a neighboring star to lend them the angular momentum to do so. In a new article, a team led by Roger Romani (Stanford University) used images and spectra to study the first known black widow pulsar, PSR J1959+2048. The image above (click for the full view) reveals the shock created by the pulsar’s outflowing wind colliding with interstellar material. Romani and collaborators used this rare sight — there are only nine known pulsar wind nebulae with associated shocks — to begin the investigation into whether this pulsar will lose too much energy to fully consume its companion. To learn more about this unusual object, check out the full article below!


“The Bow Shock and Kinematics of PSR J1959+2048,” Roger W. Romani et al 2022 ApJ 930 101. doi:10.3847/1538-4357/ac6263

high-resolution radio image of abell 2256

composite radio and x-ray image of abell 2256

New radio observations from uGMRT (red) highlight the detailed structure of the radio relic that curves around the diffuse X-ray emission seen by the Chandra X-ray Observatory (blue). Various regions of the radio relic are labeled. Click for high-resolution version. [Rajpurohit et al. 2022]

Galaxy cluster Abell 2256 is home to an incredible variety of structures traced by radio emission. The most prominent structure is an extended radio relic: a region of diffuse radio emission found on the outskirts of a cluster of galaxies. The precise cause of these massive radio relics is unknown, though shocks are expected to play a central role; the acceleration, re-acceleration, or compression of plasma by a shock wave could all cause the observed emission. Using new deep observations by the Giant Metrewave Radio Telescope (uGMRT) — shown above and to the right — a team led by Kamlesh Rajpurohit (University of Bologna, Italy; National Institute of Astrophysics, Italy; Thuringian State Observatory, Germany) investigated the cause of the striking radio emission surrounding Abell 2256. The new high-resolution images and spectra suggest that the surface of the radio relic traces a shock front, which is jumbled and twisted by interactions with the hot, turbulent plasma that suffuses the space between the galaxies in the cluster. For more fantastic images of the Abell 2256 radio relic, be sure to read the full article below!


“Deep Low-frequency Radio Observations of A2256. I. The Filamentary Radio Relic,” K. Rajpurohit et al 2022 ApJ 927 80. doi:10.3847/1538-4357/ac4708

grid of 8 images of galaxy clusters with contours indicating mass, distance from the brightest cluster galaxy, and magnification

grid of 20 images of galaxy clusters with contours indicating mass, distance from the brightest cluster galaxy, and magnification

Composite-color images of 20 of the galaxy clusters from the study. The contour lines show levels of mass density (white), magnification (cyan), and distance from the brightest galaxy in the cluster (green). Click for the high-resolution version. [Fox et al. 2022]

The points of light in the images above and to the right are not stars but rather galaxies in distant galaxy clusters — the largest gravitationally bound structures in the universe. These clusters are so massive that they can act as gravitational lenses, bending the light from background objects into arcs and circles. Comparisons of observations and cosmological models reveal that we see far more galaxies distorted into arcs than predicted, suggesting that we don’t yet fully understand the connections between the properties of a galaxy cluster, its ability to lens distant objects, and cosmology. In a new article, a team led by Carter Fox (University of Michigan) studied dozens of galaxy clusters to understand the connection between the properties of a cluster and its lensing strength. Fox and collaborators identified properties that correlate with the cluster’s lensing strength, like the amount of mass concentrated near the cluster’s brightest galaxy. The team’s results should guide the search for galaxy clusters with strong lensing properties, helping astronomers study galaxies in the early universe and constrain cosmological models. To learn more about how astronomers study gravitational lensing, check out the full article below.


“The Strongest Cluster Lenses: An Analysis of the Relation between Strong Gravitational Lensing Strength and the Physical Properties of Galaxy Clusters,” Carter Fox et al 2022 ApJ 928 87. doi:10.3847/1538-4357/ac5024

composite image of symbiotic star R Aquarii

two composite images of R aquarii

Left: XMM-Newton X-ray observations (blue) overlaid on optical observations (red and green). Right: X-ray emission detected by the Chandra X-ray Observatory (blue) overlaid on the same optical image. Click to enlarge. [Toalá et al. 2022]

A symbiotic star is a close binary system containing a red giant and a white dwarf. One such system, R Aquarii, has been the subject of extensive investigations due to its location — just 1,255 light-years away — and the intriguing filamentary structure of the nebula that surrounds it. The blue areas in the images above and to the right represent the nebula’s 0.3–0.7 kiloelectronvolt X-ray emission, while the green and red areas show the optical emission. Early observations of this object found that the X-ray emission was concentrated at the center of the nebula as well as in clumps of material arranged along a jet-like structure, but a new analysis of archival X-ray Multi-Mirror Mission (XMM-Newton) observations led by Jesús Toalá (National Autonomous University of Mexico, Morelia Campus) has revealed extended X-ray emission associated with the nebula for the first time. Toalá and collaborators suggest that the diffuse X-ray emission arises when outflowing jets create regions of hot gas that are later disrupted, similar to the behavior of hot bubbles of gas blown by the supermassive black holes at the centers of galaxies. To learn more about the complex structure of the gas surrounding R Aquarii, check out the full article below.


“An XMM-Newton EPIC X-Ray View of the Symbiotic Star R Aquarii,” Jesús A. Toalá et al 2022 ApJL 927 L20. doi:10.3847/2041-8213/ac589d

representative-color hubble image of a planetary nebula

two panel hubble image of NGC 6302

Two composite images of NGC 6302 constructed from Hubble data. The top panel is composed of narrowband images centered on emission lines of hydrogen (red), oxygen (green), and neon (blue), while the colors in the bottom panel show emission from iron (red), sulfur (green), and neon (blue). Click for full-size image. [Kastner et al. 2022]

The representative-color images above and to the right showcase the intricate structures in planetary nebula NGC 6302, known as “the Butterfly.” Planetary nebulae are shells of gas and dust that are lofted into space at the end of a 0.8–8 solar-mass star’s life, set aglow by ultraviolet rays from the star as it evolves into a white dwarf. A team led by Joel Kastner (Rochester Institute of Technology) has used Hubble data to explore the structures in NGC 6302. The new images reveal clumps, knots, and filaments of gas, as well as evidence of shocks generated by fast-flowing winds from the central star. NGC 6302’s central star has been hard to track down: these new observations reveal that the previously identified central star is in fact not associated with the nebula at all! What’s more, some models suggest that bilobed planetary nebulae can only arise if the central star has a binary companion, no hint of which has been found yet for NGC 6302. Clearly, planetary nebulae represent a challenge for modelers and observers alike! For more information on the latest investigation of this intriguing object, check out the full article below.


“Panchromatic HST/WFC3 Imaging Studies of Young, Rapidly Evolving Planetary Nebulae. I. NGC 6302,” Joel H. Kastner et al 2022 ApJ 927 100. doi:10.3847/1538-4357/ac51cd

composite image of the Sun

How does the Sun accelerate particles to relativistic energies? The answer may lie in the shocks that form between clashing plasma structures high in the Sun’s atmosphere, in its corona. The composite image above shows the Sun’s disk in extreme-ultraviolet light overlaid on a white-light image of the wispy corona captured by blocking the Sun from view. A massive eruption of plasma called a coronal mass ejection emerges from the lower-left side, while faint, streaky coronal streamers point outward in multiple directions. The white arrow indicates a streamer that has been deflected by the passage of the coronal mass ejection. Space-based observatories viewed the event from multiple perspectives, allowing a team led by Federica Frassati (National Institute for Astrophysics, Astrophysical Observatory of Turin, Italy) to reconstruct a three-dimensional view of the shock front as it expanded. The team’s results show, for the first time, that the interaction between an advancing shock front and background solar streamers can accelerate particles up to a whopping 100 megaelectronvolts. To learn more, check out the full article below.


“Acceleration of Solar Energetic Particles through CME-driven Shock and Streamer Interaction,” Federica Frassati et al 2022 ApJ 926 227. doi:10.3847/1538-4357/ac460e

simulations of the Sun's magnetic fields

simulations of the Sun's magnetic fields

Simulated magnetic fields under high solar activity (top row) and low solar activity (bottom row) conditions. The left column shows the results with no outflow, while the right column shows the resulting magnetic field when a solar wind with a velocity of 150 kilometers per second is included. Click to enlarge. [Rice & Yeates 2021]

Solar physicists have a magnetic flux problem: current models of the magnetic field threaded through the Sun’s million-degree upper atmosphere, or corona, struggle to reproduce the observed amount of open magnetic flux — the field lines that extend from the Sun’s surface into the solar system, carrying solar plasma far afield. Now, Oliver Rice and Anthony Yeates (Durham University, UK) have found a way to lessen the discrepancy between models and observations without increasing the computational cost by incorporating an outward-flowing solar wind into their model. As the simulation results above and to the right show, introducing a solar wind component draws the magnetic field lines outward, increasing the number of magnetic field lines that extend outward into the solar system and decreasing the number of loops that double back toward the Sun’s surface. While the addition of a solar wind component doesn’t fully relieve the tension between models and observations, the authors are hopeful that future developments, such as incorporating more realistic spatial variations of the solar wind, will further improve the outcome. To learn more, see the full article below.


“Global Coronal Equilibria with Solar Wind Outflow,” Oliver E. K. Rice and Anthony R. Yeates 2021 ApJ 923 57. doi:10.3847/1538-4357/ac2c71

Hubble image of Sh2-106 star forming region

The birth of a massive star is anything but calm and peaceful — a fact that is dramatically illustrated in this false-color Hubble image of the star-forming region Sh2-106 (click for the full view, or see below for the original image with scale). This spectacular nebula hides the young massive star S106 IR. Birthed from the tumultuous collapse of its parent molecular cloud, S106 IR grew via rapid accretion until a tremendous explosion rocked the region roughly 3,500 years ago, according to new research led by John Bally (University of Colorado Boulder).

Hubble image of Sh2-106 star forming region

False-color Hubble image of the Sh2-106 star-forming region. [Bally et al. 2021]

Bally and collaborators have used both new and archival observations of Sh2-106 spanning 16 years to analyze the supersonic motion of the gas sent streaming in the powerful explosion. The authors’ work provides unique insight into the fireworks possible during the final stages of a massive star’s birth. For more information (and more stunning images of Sh2-106!), check out the original article below.


“Supersonic Expansion of the Bipolar H II Region Sh2-106: A 3500 Year Old Explosion?,” John Bally et al 2022 ApJ 924 50. doi:10.3847/1538-4357/ac30de

simulation results in the x-y plane

hydrodynamic simulations at different angles

Simulations of the Be star (larger green circle) and neutron star (smaller green circle) disks. Lighter colors indicate denser material. Results are shown after 29 (top), 33 (middle), and 38 (bottom) orbital periods of the binary have passed. After the initial mass transfer, an accretion disk forms around the neutron star and elongates. Click to enlarge. [Franchini & Martin 2021]

Alessia Franchini (University of Milano-Bicocca, Italy) and Rebecca Martin (University of Nevada, Las Vegas) study binary systems made up of a neutron star and a Be star — a rapidly rotating high-mass star with hydrogen emission lines in its spectrum. Because the Be star (the letters are pronounced separately as “B e”) spins so quickly, it flings away some of its mass into a decretion disk, from which the neutron star can siphon material. Be star–neutron star binaries tend to emit bursts of X-rays once per orbital period as well as on irregular timescales, with these irregular bursts often occurring in pairs — though it’s not yet clear why. Franchini and Martin use hydrodynamic simulations, shown above and to the right, to understand the behavior of these extreme systems. Their simulations show that the first of the paired X-ray outbursts can be caused by the transfer of mass from the Be star to the neutron star, while the second outburst might be driven by changes in the eccentricity of the neutron star’s disk, though higher-resolution simulations are needed to confirm this result. To learn more, see the full article below.


“Eccentric Neutron Star Disk Driven Type II Outburst Pairs in Be/X-ray Binaries,” Alessia Franchini and Rebecca G. Martin 2021 ApJL 923 L18. doi:10.3847/2041-8213/ac4029

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