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chemical mixing

How do stars mix chemicals in their interiors, leading to the abundances we measure at their surfaces? Two scientists from the Planetary Science Institute in Arizona, Tamara Rogers (Newcastle University, UK) and Jim McElwaine (Durham University, UK), have investigated the role that internal gravity waves have in chemical mixing in stellar interiors. Internal gravity waves — not to be confused with the currently topical gravitational waves — are waves that oscillate within a fluid that has a density gradient. Rogers and McElwaine used simulations to explore how these waves can cause particles in a star’s interior to move around, gradually mixing the different chemical elements. Snapshots from four different times in their simulation can be seen below, with the white dots marking tracer particles and the colors indicating vorticity. You can see how the particles move in response to wave motion after the first panel. For more information, check out the paper below!

time snapshots of mixing

Citation

T. M. Rogers and J. N. McElwaine 2017 ApJL 848 L1. doi:10.3847/2041-8213/aa8d13

formation of rapidly rotating black hole

These stills from a simulation show the evolution (from left to right and top to bottom) of a high-mass X-ray binary over 1.1 days, starting after the star on the right fails to explode as a supernova and then collapses into a black hole. Many high-mass X-ray binaries — like the well-known Cygnus X-1, the first source widely accepted to be a black hole — host rapidly spinning black holes. Despite our observations of these systems, however, we’re still not sure how these objects end up with such high rotation speeds. Using simulations like that shown above, a team of scientists led by Aldo Batta (UC Santa Cruz) has demonstrated how a failed supernova explosion can result in such a rapidly spinning black hole. The authors’ work shows that in a binary where one star attempts to explode as a supernova and fails — it doesn’t succeed in unbinding the star — the large amount of fallback material can interact with the companion star and then accrete onto the black hole, spinning it up in the process. You can read more about the authors’ simulations and conclusions in the paper below.

Citation

Aldo Batta et al 2017 ApJL 846 L15. doi:10.3847/2041-8213/aa8506

The inset in this Solar Dynamics Observatory image shows a close-up view of a stunning coronal fan extending above the Sun’s atmosphere. These sweeping loops were observed on 7 March 2012 by a number of observatories, revealing the first known evidence of standing slow magnetoacoustic waves in cool coronal fan loops. The oscillations of the loops, studied in a recent article led by Vaibhav Pant (Indian Institute of Astrophysics), were triggered by blast waves that were generated by X-class flares from the distant active region AR 11429 (marked with the yellow box at left). The overplotted X-ray curve in the top right corner of the image (click for the full view) shows the evolution of the flares that perturbed the footpoints of the loops. You can check out the video of the action below, and follow the link to the original article to read more about what these oscillations tell us about the Sun’s activity.

 

Citation

V. Pant et al 2017 ApJL 847 L5. doi:10.3847/2041-8213/aa880f

cosmic velocity web

You may have heard of the cosmic web, a network of filaments, clusters and voids that describes the three-dimensional distribution of matter in our universe. But have you ever considered the idea of a cosmic velocity web? In a new study led by Daniel Pomarède (IRFU CEA-Saclay, France), a team of scientists has built a detailed 3D view of the flows in our universe, showing in particular motions along filaments and in collapsing knots. In the image above (click for the full view), surfaces of knots (red) are embedded within surfaces of filaments (grey). The rainbow lines show the flow motion, revealing acceleration (redder tones) toward knots and retardation (bluer tones) beyond them. You can learn more about Pomarède and collaborators’ work and see their unusual and intriguing visualizations in the video they produced, below. Check out the original paper for more information.

Citation

Daniel Pomarède et al 2017 ApJ 845 55. doi:10.3847/1538-4357/aa7f78

M33

These spectacular images are of M33, otherwise known as the Triangulum Galaxy — a spiral galaxy roughly 3 million light-years away. The views on the left and in the center are different Wide-field Infrared Survey Explorer (WISE) filters, and the view on the right is a full-resolution look at the H I gas distribution in M33’s inner disk, made with data from the Dominion Radio Astrophysical Observatory (DRAO) Synthesis Telescope and Arecibo. In a new study, a team of authors led by Zacharie Sie Kam (University of Ouagadougou, Burkina Faso; University of Montreal, Canada) uses the H I gas observations to explore how the mass is distributed throughout M33 and how the gas moves as the galaxy’s disk rotates. To read more about what they learned, check out the paper below.

Citation

S. Z. Kam et al 2017 AJ 154 41. doi:10.3847/1538-3881/aa79f3

ring galaxies

Hoag's Object

Hoag’s Object, an example of a ring galaxy. [NASA/Hubble Heritage Team/Ray A. Lucas (STScI/AURA)]

The above image (click for the full view) shows PanSTARRS observations of some of the 185 galaxies identified in a recent study as “ring galaxies” — bizarre and rare irregular galaxies that exhibit stars and gas in a ring around a central nucleus. Ring galaxies could be formed in a number of ways; one theory is that some might form in a galaxy collision when a smaller galaxy punches through the center of a larger one, triggering star formation around the center. In a recent study, Ian Timmis and Lior Shamir of Lawrence Technological University in Michigan explore ways that we may be able to identify ring galaxies in the overwhelming number of images expected from large upcoming surveys. They develop a computer analysis method that automatically finds ring galaxy candidates based on their visual appearance, and they test their approach on the 3 million galaxy images from the first PanSTARRS data release. To see more of the remarkable galaxies the authors found and to learn more about their identification method, check out the paper below.

Citation

Ian Timmis and Lior Shamir 2017 ApJS 231 2. doi:10.3847/1538-4365/aa78a3

X-ray cores

The images above show just 8 of 51 different nearby, late-type galaxies found to host X-ray cores near their centers. The main images are optical views and the insets show Chandra X-ray images of the same galaxies. The cross marks identify the near-infrared/optical nucleus of each galaxy, and the green ellipses show the source regions for the X-rays. A recent publication led by Rui She (Tsinghua University, China) presents a search for low-mass (<106 solar masses) black holes lurking in the centers of nearby late-type, low-mass galaxies. Many of the 51 X-ray cores discovered represent such hidden black holes. The authors use the statistics of this sample to estimate that at least 21% of late-type galaxies like those studied here host low-mass black holes at their centers. You can view the full set of X-ray core hosts below; for more information, check out the paper linked at the bottom of the page.

all X-ray cores

All 51 X-ray cores (displayed in 3 sets); see the article below for the originals.

Citation

Rui She et al 2017 ApJ 842 131. doi:10.3847/1538-4357/aa7634

Crab Nebula

Planning on watching fireworks tomorrow? Here’s an astronomical firework to help you start the celebrations! A new study has stunningly detailed the Crab Nebula (click for a closer look), a nebula 6,500 light-years away thought to have been formed by a supernova explosion and the subsequent ultrarelativistic wind emitted by the pulsar at its heart. Led by Gloria Dubner (University of Buenos Aires), the authors of this study obtained new observations of the Crab Nebula from five different telescopes. They compiled these observations to compare the details of the nebula’s structure across different wavelengths, which allowed them to learn about the sources of various features within the nebula. In the images above, the top left shows the 3 GHz data from the Very Large Array (radio). Moving clockise, the radio data (shown in red) is composited with: infrared data from Spitzer Space Telescope, optical continuum from Hubble Space Telescope, 500-nm optical data from Hubble, and ultraviolet data from XMM-Newton. The final two images are of the nebula center, and they are composites of the radio image with X-ray data from Chandra and near-infrared data from Hubble. To read more about what Dubner and collaborators learned (and to see more spectacular images!), check out the paper below.

Citation

G. Dubner et al 2017 ApJ 840 82. doi:10.3847/1538-4357/aa6983

Fomalhaut

ALMA and HST

ALMA continuum image overlaid as contours on the Hubble STIS image of Fomalhaut. [MacGregor et al. 2017]

This stunning image of the Fomalhaut star system was taken by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. This image maps the 1.3-mm continuum emission from the dust around the central star, revealing a ring that marks the outer edge of the planet-forming debris disk surrounding the star. In a new study, a team of scientists led by Meredith MacGregor (Harvard-Smithsonian Center for Astrophysics) examines these ALMA observations of Fomalhaut, which beautifully complement former Hubble images of the system. ALMA’s images provide the first robust detection of “apocenter glow” — the brightening of the ring at the point farthest away from the central star, a side effect of the ring’s large eccentricity. The authors use ALMA’s observations to measure properties of the disk, such as its span (roughly 136 x 14 AU), eccentricity (e ~ 0.12), and inclination angle (~66°). They then explore the implications for Fomalhaut b, the planet located near the outer disk. To read more about the team’s observations, check out the paper below.

Citation

Meredith A. MacGregor et al 2017 ApJ 842 8. doi:10.3847/1538-4357/aa71ae

composite image

This beautiful image shows two galaxies, IC 2163 and NGC 2207, as they undergo a grazing collision 114 million light-years away. The image is composite, constructed from Hubble (blue), Spitzer (green), and ALMA (red) data. In a recent study, Debra Elmegreen (Vassar College) and collaborators used this ALMA data to trace the individual molecular clouds in the two interacting galaxies, identifying a total of over 200 clouds that each contain a mass of over a million solar masses. These clouds represent roughly half the molecular gas in the two galaxies total. Elmegreen and collaborators track the properties of these clouds and their relation to star-forming regions observed with Hubble. For more information about their observations, check out the paper linked below.

CO data

A closer look at the ALMA observations for these galaxies, with the different emission regions labeled. Most of the molecular gas emission comes from the eyelids of IC 2163, and the nuclear ring and Feature i in NGC 2207. [Elmegreen et al. 2017]

Citation

Debra Meloy Elmegreen et al 2017 ApJ 841 43. doi:10.3847/1538-4357/aa6ba5

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