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bow shock nebulae

These dynamic infrared images (click for the full view!) reveal what are known as “bow shock nebulae” — nebulae that form at the interface between the interstellar medium and the stellar wind from a high-speed star zipping through the galaxy (the arrows show the direction of motion of the star). When the relative speed between the two is supersonic, an arc-shaped bow shock forms ahead of the star, like the six prototypical ones pictured here. A team of scientists led by Henry Kobulnicky (University of Wyoming) has recently searched through survey data from the Spitzer Space Telescope and the Wide Field Infrared Explorer (WISE) to build a catalog of more than 700 such bow-shock nebula candidates, the vast majority of which are new discoveries. To find out more about their sample, check out the paper below!

Citation

Henry A. Kobulnicky et al 2016 ApJS 227 18. doi:10.3847/0067-0049/227/2/18

grand design spirals

In this figure, the top panels show three spiral galaxies in the Virgo cluster, imaged with the Sloan Digital Sky Survey. The bottom panels provide a comparison with three morphologically similar galaxies generated in simulations. The simulations — run by Marcin Semczuk, Ewa Łokas, and Andrés del Pino (Nicolaus Copernicus Astronomical Center, Poland) — were designed to examine how the spiral arms of galaxies like the Milky Way may have formed. In particular, the group explored the possibility that so-called “grand-design spiral arms” are caused by tidal effects as a Milky-Way-like galaxy orbits a cluster of galaxies. The authors show that the gravitational potential of the cluster can trigger the formation of two spiral arms each time the galaxy passes through the pericenter of its orbit around the cluster. Check out the original paper below for more information!

Citation

Marcin Semczuk et al 2017 ApJ 834 7. doi:10.3847/1538-4357/834/1/7

This series of images (click for the full view!), taken by the Solar and Heliospheric Observatory satellite (SOHO) in August 2015, reveals a tremendous outburst of plasma and magnetic field from the Sun: a coronal mass ejection (CME). If you look closely, you’ll note that as the CME expands from the Sun’s surface, it passes in front of a dot highlighted in yellow. This dot marks the location of a distant background pulsar, PSR B0950+08. In a recent study led by Tim Howard (Southwest Research Institute), a team of scientists studied the change observed in the radio emission of this pulsar as the CME passed by in the foreground. The team used these observations to estimate the CME’s density and magnetic field — measurements that can tell us more about the nature of the magnetic field in the Sun’s corona and the solar wind.

You can check out the animation of this CME, also taken with SOHO’s LASCO instrument, below (the CME starts around 20 seconds in), or you can find out more from the original paper!

Citation

T. A. Howard et al 2016 ApJ 831 208. doi:10.3847/0004-637X/831/2/208

comet 67P's coma

This series of images (click for the full view!) features the nucleus of comet 67P/Churymov-Gerasimenko. The images were taken with the Wide Angle Camera of Rosetta’s OSIRIS instrument as Rosetta orbited comet 67P. Each column represents a different narrow-band filter that allows us to examine the emission of a specific fragment species, and the images progress in time from January 2015 (top) to June 2015 (bottom). In a recent study, Dennis Bodewits (University of Maryland) and collaborators used these images to analyze the comet’s inner coma, the cloud of gas and dust produced around the nucleus as ices sublime. OSIRIS’s images allowed the team to explore how the 67P’s inner coma changed over time as the comet approached the Sun — marking the first time we’ve been able to study such an environment at this level of detail. To read more about what Bodewits and collaborators learned, you can check out their paper below!

Citation

D. Bodewits et al 2016 AJ 152 130. doi:10.3847/0004-6256/152/5/130

core-collapse SN simulation

This stunning snapshot (click for a closer look!) is from a simulation of a core-collapse supernova. Despite having been studied for many decades, the mechanism driving the explosions of core-collapse supernovae is still an area of active research. Extremely complex simulations such as this one represent best efforts to include as many realistic physical processes as is currently computationally feasible. In this study led by Luke Roberts (a NASA Einstein Postdoctoral Fellow at Caltech at the time), a core-collapse supernova is modeled long-term in fully 3D simulations that include the effects of general relativity, radiation hydrodynamics, and even neutrino physics. The authors use these simulations to examine the evolution of a supernova after its core bounce. To read more about the team’s findings (and see more awesome images from their simulations), check out the paper below!

Citation

Luke F. Roberts et al 2016 ApJ 831 98. doi:10.3847/0004-637X/831/1/98

NGC 5907

This negative image of NGC 5907 (originally published in Martinez-Delgado et al. 2008; click for the full view!) reveals the faint stellar stream that encircles the galaxy, forming loops around it — a fossil of a recent merger. Mergers between galaxies come in several different flavors: major mergers, in which the merging galaxies are within a 1:5 ratio in stellar mass; satellite cannibalism, in which a large galaxy destroys a small satellite less than a 50th of its size; and the in-between case of minor mergers, in which the merging galaxies have stellar mass ratios between 1:5 and 1:50. These minor mergers are thought to be relatively common, and they can have a significant effect on the dynamics and structure of the primary galaxy. A team of scientists led by Seppo Laine (Spitzer Science Center – Caltech) has recently analyzed the metallicity and age of the stellar population in the stream around NGC 5907. By fitting these observations with a stellar population synthesis model, they conclude that this stream is an example of a massive minor merger, with a stellar mass ratio of at least 1:8. For more information, check out the paper below!

Citation

Seppo Laine et al 2016 AJ 152 72. doi:10.3847/0004-6256/152/3/72

Venus transits

This stunning image was captured by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly in June 2012 during the transit of Venus across the face of the Sun. A recent study led by Masoud Afshari (University of Palermo, Italy and INAF-OAPA) presents an analysis of high-energy imaging of Venus’s silhouette as it crossed the Sun. This imaging reveals X-ray and ultraviolet emission coming from the dark side of Venus during the transit — which Afshari and collaborators conclude is not due to instrumental scattering, but instead has an origin directly related to the planet. The authors suggest that this light could be emitted from the Sun and then scattered by Venus’s very long magnetotail, and they wonder if such an effect might be important for exoplanets as well! For more information, check out the paper below.

Citation

M. Afshari et al 2016 AJ 152 107. doi:10.3847/0004-6256/152/4/107

DISCO tests

Creating the codes that are used to numerically model astrophysical systems takes a lot of work — and a lot of testing! A new, publicly available moving-mesh magnetohydrodynamics (MHD) code, DISCO, is designed to model 2D and 3D orbital fluid motion, such as that of astrophysical disks. In a recent article, DISCO creator Paul Duffell (University of California, Berkeley) presents the code and the outcomes from a series of standard tests of DISCO’s stability, accuracy, and scalability.

From left to right and top to bottom, the test outputs shown above are: a cylindrical Kelvin-Helmholtz flow (showing off DISCO’s numerical grid in 2D), a passive scalar in a smooth vortex (can DISCO maintain contact discontinuities?), a global look at the cylindrical Kelvin-Helmholtz flow, a Jupiter-mass planet opening a gap in a viscous disk, an MHD flywheel (a test of DISCO’s stability), an MHD explosion revealing shock structures, an MHD rotor (a more challenging version of the explosion), a Flock 3D MRI test (can DISCO study linear growth of the magnetorotational instability in disks?), and a nonlinear 3D MRI test.

Check out the gif below for a closer look at each of these images, or follow the link to the original article to see even more!

test outputs

Citation

Paul C. Duffell 2016 ApJS 226 2. doi:10.3847/0067-0049/226/1/2

debris disk around solar analog

HD 207917

The Hubble image of a second circumstellar debris disk, HD 207917, and its best-fit model.

This is a new deep observation made by Hubble’s Space Telescope Imaging Spectrograph of the tilted debris disk surrounding the star HD 207129. In a recent study led by Glenn Schneider (Seward Observatory, University of Arizona), three known, nearby circumstellar disks were imaged by Hubble in order to gain a better understanding of the disks’ ring-like structure. The three central stars of these disks are all G-type solar analogs, and the debris rings bear many similarities to our own Kuiper belt. Imaging of debris disks like these can help us to learn more about how solar systems form around stars like our own. For more information, check out the paper below!

Citation

Glenn Schneider et al 2016 AJ 152 64. doi:10.3847/0004-6256/152/3/64

WISE Milky Way

X-shaped bulge

The X-shaped bulge is even more evident in this image, wherein a simple exponential disk model has been subtracted off. [Adapted from Ness & Lang 2016]

This contrast-enhanced image of the Milky Way, observed by the Wide-Field Infrared Survey Explorer (WISE), clearly reveals that the bulge of stars at the center of our galaxy is shaped like a large “X”. The boxy nature of the Milky Way’s bulge was revealed by satellite image in 1995, but in recent years, star counts along the line of sight toward the bulge have suggested that the bulge may be X-shaped. It was unclear whether this apparent morphology was due to the difference in the distributions of different stellar populations, or if the actual physical structure of the bulge was X-shaped. But these new WISE images, produced by astronomers Melissa Ness (Max Planck Institute for Astronomy) and Dustin Lang (University of Toronto and University of Waterloo), now provide firm evidence that the Milky Way’s bulge actually is X-shaped, supplying clues as to how our galaxy’s center may have formed. This morphology is not uncommon; observations of other barred galaxies reveal similar X-shaped profiles. To learn more, check out the paper below!

Citation

Melissa Ness and Dustin Lang 2016 AJ 152 14. doi:10.3847/0004-6256/152/1/14

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