astrobites 2017

Find out the latest thinking about our universe.
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FRBs are of UTMOST importance

Postby bystander » Fri Jun 30, 2017 3:00 pm

FRBs are of UTMOST importance
astrobites | 2017 Jun 27
Joshua Kerrigan wrote:
Fast Radio Bursts more commonly known as FRBs, have been a hot topic in recent years. These loud bursts of radio emissions appear in radio observation data at seemingly random times from random positions on the sky (see Fig. 1). Ever since the first FRB was found in archival data from the Parkes Observatory in Australia by Duncan Lorimer and his undergraduate student David Narkovic, we’ve seen an explosion in searches and resources devoted to finding more FRBs. The initial discovery of FRBs was highly scrutinized because unfortunately even astronomers can make mistakes. As of today, radio astronomers are trying to collect as much data on FRBs as possible to constrain all of the hypothesized origins of these extragalactic bursts of radio emissions. ...

The First Interferometric Detections of Fast Radio Bursts - M. Caleb et al

viewtopic.php?t=37038
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Definitive discovery of a dead distant disc

Postby bystander » Fri Jun 30, 2017 3:07 pm

Definitive discovery of a dead distant disc
astrobites | 2017 Jun 28
Paddy Alton wrote:
Today’s article is all about an unusual discovery, a galaxy that appears to be lost in time and space.

I’ve written before about gravitational lensing and how this promises to help us make new discoveries. It’s not necessary to rehash the gory details here; suffice to say that, just as a glass lens in a telescope or microscope can focus light to help us see things in more detail, so too can a very strong gravitational field. And if you’re really lucky, a gravitational lens might just line up with something interesting…

A Massive, Dead Disk Galaxy in the Early Universe - Sune Toft et al

viewtopic.php?t=37314
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Is there a Planet Nine?

Postby bystander » Fri Jun 30, 2017 3:16 pm

Is there a Planet Nine?
astrobites | 2017 Jun 28
Bhawna Motwani wrote:
Dubbed as one of the “Wildest Alien Planet Discoveries of 2016“, Planet Nine was a truly exciting prospect for all planet lovers alike. Unfortunately, the line of evidence for this elusive planet lurking in the outer reaches of our Solar System has been called into question recently by the study discussed in today’s bite. ...

OSSOS VI. Striking Biases in the detection of large semimajor axis Trans-Neptunian Objects - Cory Shankman et al

viewtopic.php?t=35573
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A Ring of Ice and Glows (around Fomalhaut)

Postby bystander » Fri Jun 30, 2017 3:28 pm

A Ring of Ice and Glows (around Fomalhaut)
astrobites | 2017 Jun 30
Michael Hammer wrote:
Welcome back to Fomalhaut, a star system we cover a lot here at Astrobites! Fomalhaut is famous for having a debris disk that looks like the Eye of Sauron, and for Fomalhaut b – a possible planet that may or may not lie at the edge of that disk. Residing in Piscis Austrinus (the Southern Fish), Fomalhaut is named after the Arabic “fum al-hut,” meaning the “mouth of the whale” because of its location in the constellation. It is also the 18th brightest star in the sky, and one of the ten closest stars more massive than the Sun.

Unlike many of the other stars near our solar system, Fomalhaut is still young enough at 440 Myr old to have a prominent debris disk – a thin ring of dust that is similar to our solar system’s own Kuiper Belt, except 1000 times more massive! This makes Fomalhaut an excellent testbed for studying if and how planets are still sculpting the size and shape of debris disks at this middle stage of solar system evolution when planets have already formed, but may still not have settled onto their final orbits.

When the Atacama Large Millimeter/sub-mm Array (ALMA) first turned on in 2011, one of the first scientific results published was an image of half of Fomalhaut’s debris disk that inspired this Astrobite. That was taken in the very first observing cycle when ALMA only had 16 antennas operational. In today’s twin papers, Meredith MacGregor, Luca Matrà, and their collaborators take advantage of ALMA’s 43 currently operational antennas to take a much higher resolution image of the full debris disk to uncover two new exciting results about the Fomalhaut system. ...

A Complete ALMA Map of the Fomalhaut Debris Disk - Meredith A. MacGregor et al
Detection of Exocometary CO Within the 440-Myr-old Fomalhaut Belt:
A Similar CO+CO2 Ice Abundance in Exocomets and Solar System Comets
- Luca Matrà et al

viewtopic.php?p=272467#p272467
viewtopic.php?t=37192
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Adventures in watchmaking for cool stars

Postby bystander » Sat Jul 08, 2017 2:47 pm

Adventures in watchmaking for cool stars
astrobites | 2017 Jul 03
Leonardo dos Santos wrote:
Among humans, it is kind of rude to ask for somebody’s age if you’re not well-acquainted with them, but at least we generally understand basic human physiology well enough to make educated guesses about how old they are. We can do something similar if we want to know the age of a star. Just by asking them? No, that would be silly! Instead we can guess how old a star is based on how it looks and acts (although it is sometimes a bit misleading).

Also like humans, stars get less active as they get older, and we can measure that in different ways. One of them is by assessing their rotation, which slows down as stars age; because rotational rates correlate with magnetic activity, the latter also grows weaker with time. If we know how fast a star rotates or how active it is (i.e., the frequency of flares, coronal mass ejections and size of starspots), we can estimate its age — this is known as gyrochronology (see Fig. 1). This method is not as precise as measuring the ages of open clusters using isochrones, but we do understand the magnetic and rotational evolution of stars well enough to apply gyrochronology with satisfactory results when no better option is available. ...

An Improved Age-Activity Relationship for Cool Stars older than a Gigayear - R. S. Booth et al
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The Farthest Star Ever Seen

Postby bystander » Sat Jul 08, 2017 3:00 pm

The Farthest Star Ever Seen
astrobites | 2017 Jul 04
Gourav Khullar wrote:
I write this astrobite in my living room, assisted heavily by the lenses in my spectacles. It’s quite a marvelous thing – a pair of glasses. For the last decade or so, a pair of glasses have been necessary for me to marvel at the wonders of the world (and universe, now that studying astrophysics allows me to do that at leisure). Then, imagine my surprise when I came across the cosmic equivalent of my glasses and what a marvel they are! Gravitational lenses are some of the most massive structures in the universe, teaching us about the most distant and the most massive objects in the cosmos. ...

It is important to note that since a galaxy cluster can act as a gravitational lens, just like a magnifying glass it also has the capacity to ‘magnify’ background objects. Imagine a faint galaxy that we are not able to detect with regular telescopes. Now imagine the same galaxy when it happens to lie in the line of sight of a galaxy cluster. Based on its location and distance from the ‘lens’, a faint background galaxy can get magnified by several times, allowing its minimal flux to reach us at last. It is common for astrophysicists in this field to study large objects like faint blue galaxies at redshifts of 2-3 that get magnified by nearby galaxy clusters, which would not have been possible otherwise. This advancing field is only getting better with improved understanding of galaxy clusters as lenses – the calculations of their masses, and how this mass may be distributed throughout the cluster. Today’s study has gone one step further, into uncharted territory. Today’s study has observed a gravitationally lensed star! ...

An individual star at redshift 1.5 extremely magnified by a galaxy-cluster lens - Patrick L. Kelly et al

viewtopic.php?t=34519
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Revisiting the Universe-making Recipes

Postby bystander » Sat Jul 08, 2017 3:10 pm

Revisiting the Universe-making Recipes
astrobites | 2017 Jul 05
Benny Tsang wrote:
We astrophysicists should give thanks to our computers for making our groundbreaking endeavors possible. Without them, we wouldn’t be able to operate telescopes in space, mine gigantic observational datasets, or even simulate the whole Universe from nearly nothing! As it turns out, we are doing a pretty good job at simulating the Universe on computers. For example, in the recent Illustris simulation, we have successfully reproduced the observed structures of galaxies based on initial conditions shortly after the Big Bang. Simulations that aim to study the formation of large-scale structures (e.g. galaxies, clusters of galaxies) of the Universe are known as cosmological simulations, and the ones that include also the motions of baryons (e.g. gas, stars, black holes) are called hydrodynamic simulations.

Despite the success, we have to constantly remind ourselves not to be complacent about our achievements. Earlier we have reflected on the general approach of simulations in astrophysics. Today’s paper concerns a rather technical aspect of hydrodynamic simulations – the way gas cooling is followed numerically. Since stars and galaxies owe their existence to gravitational collapses of gas clouds, and the collapses would not sustain without the gas being efficiently cooled, gas cooling plays a key role in the study of galaxy formation. ...

Gas Cooling in Hydrodynamic Simulations with an Exact Time Integration Scheme - Qirong Zhu, Britton Smith, Lars Hernquist
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Wobbling Galaxies: Evidence of Dark Matter Interactions?

Postby bystander » Sat Jul 08, 2017 3:18 pm

Wobbling Galaxies: Evidence of Dark Matter Interactions?
astrobites | 2017 Jul 06
Nora Shipp wrote:
The identity of the phantom dark matter particle that dominates the Universe around us remains elusive. Scientists have searched for signs of dark matter in deep mines, designed to keep out all but the weakly interacting dark matter particle (e.g. the LUX experiment). They have searched at the Large Hadron Collider for signs of dark matter particles created in energetic collisions (check out this Particlebite). However, these methods of detecting or creating dark matter have yet to reveal the hidden identity of these ghostly particles, leaving us with the original dark matter laboratory – the astrophysical universe – as the best place to search for clues to the particle nature of dark matter. ...

However, there are many things we still don’t know about dark matter particles (in addition to the big one – what are they?), including the question addressed in today’s paper – how do dark matter particles interact with each other? Do they pass through each other like they have passed through our detectors? Or can they bump into and scatter off of each other?

David Harvey and his coauthors use simulations and observations of galaxy clusters to study possible self-interactions between dark matter particles. In particular, they consider how the central galaxy of a galaxy cluster (referred to as the Brightest Cluster Galaxy or BCG), would behave if the surrounding dark matter particles did not quietly pass by each other, but instead bounced off of each other transferring energy and momentum, like colliding billiard balls. ...

A Detection of Wobbling Brightest Cluster Galaxies within Massive Galaxy Clusters - David Harvey et al
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Finding the Brightest Exoplanet Hosts with MASCARA

Postby bystander » Tue Jul 11, 2017 2:55 pm

Finding the Brightest Exoplanet Hosts with MASCARA
astrobites | 2017 Jul 10
Matthew Green wrote:
Before we start: the system discussed in this astrobite was discovered separately by two teams and presented simultaneously. The other paper, by the KELT team, can be found here. This astrobite will focus on the results of the MASCARA team.

It’s clear that there are a lot of exoplanets out there. While large surveys like K2 continue to bring in hundreds of new planets, other projects are filling in the gaps that these surveys miss. The relatively new project MASCARA intends to find planets around the brightest host stars yet. They are targeting stars with magnitudes less than 8.4 (remember that fainter stars have higher magnitudes). For comparison, that’s still fainter than the human eye can see (magnitude 6 or less), but it’s a fair bit brighter than the Kepler space telescope can see (Kepler saturates on stars brighter than about 11th magnitude). There are currently only 14 exoplanet host stars known that are brighter than 8.4th magnitude, with the brightest being KELT-9 at a magnitude of 7.56. These exoplanets around bright stars are interesting because it’s so much easier to do follow-up observations on them. In particular, in-depth studies of exoplanet atmospheres — which require collecting starlight that has passed through the exoplanet atmosphere, and studying how the atmosphere has affected the starlight — are much easier when the exoplanet orbits bright stars like these, simply because there are so many more photons that reach us. ...

MASCARA-2b is the second exoplanet to be discovered by this method, but the first to be published (MASCARA-1b is also in the works, but 2b was pushed ahead in the queue because of a simultaneous discovery by another team). From the MASCARA data in Figure 2, a clear transit can be seen every 3.47 days. To follow this up, the team observed transits with the NITES, IAC80 and SONG telescopes. To emphasise how bright this star is compared to the usual astronomical targets: these are small telescopes — NITES in particular is only 40cm in diameter. Even these telescopes however had to be kept deliberately out-of-focus, blurring the resulting image and spreading the star’s light over more pixels, because otherwise there would be a danger of saturating the image. This practise is not uncommon for larger telescopes, but it’s surprising to see it necessary on these rather smaller telescopes. ...

MASCARA-2b: A hot Jupiter transiting a mV=7.6 A-star - G.J.J. Talens et al
KELT-20b: A giant planet with a period of P~ 3.5 days transiting the V~ 7.6 early A star HD 185603 - Michael B. Lund et al
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Extreme variability quasars

Postby bystander » Tue Jul 11, 2017 3:04 pm

Extreme variability quasars
astrobites | 2017 Jul 11
Suk Sien Tie wrote:
Active galactic nuclei (AGNs), the central active regions of supermassive black holes, have many masks. They span a large range of luminosities from roughly ten billion to ten thousand Milky Ways (even at their dimmest, they are still one of the brightest objects in the Universe). They have varying radio brightnesses and the presence of radio jets is not a luxury to be had by all. When scrutinized with a spectrograph, they reveal telltale signs of different anatomies. Some exhibit broad emission lines, others narrow, and still others both. Therefore, AGNs carry a myriad of different names, such as Seyferts, blazars, and quasars. However, the multifaceted appearances of AGNs are deceiving — the AGN unification theory postulates that which type of AGN you see depends on your viewing angle and the wavelength of light you’re looking in. Otherwise, you’re simply looking at one and the same object, the central bright region of a supermassive black hole. ...

Extreme variability quasars from the Sloan Digital Sky Survey and the Dark Energy Survey - Nick Rumbaugh et al
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What’s your Cosmology? Ask CMB Maps

Postby bystander » Mon Jul 17, 2017 2:29 pm

What’s your Cosmology? Ask CMB Maps
astrobites | 2017 Jul 12
Gourav Khullar wrote:
There are papers that talk of groundbreaking discoveries. There are papers which review the current status of the field, akin to bringing you up-to-date with what’s going on. And then there are papers that open up portals to new sub-fields, with the clarity of their message and the precision of the questions they pose. Today’s paper is one such publication, which in 1996 started an interesting journey in the world of Cosmic Microwave Background (CMB) and Observational Cosmology. ...

Cosmological-Parameter Determination with Microwave Background Maps - Gerard Jungman et al
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A New Dark Force? Clues in the Smallest Galaxies

Postby bystander » Mon Jul 17, 2017 2:38 pm

A New Dark Force? Clues in the Smallest Galaxies
astrobites | 2017 Jul 14
Stacy Kim wrote:
It’s a curious thing that the two classes of matter that make up our universe behave so differently. Baryonic matter, the matter out of which we and most things we interact with on a daily basis are made of, obey four fundamental forces. In comparison, dark matter seems to obey only one: gravity.

The appearance of a single dark force may only be a guise—it’s possible that other forces are at work but, because of our inability to see dark matter, we are blind to them. Undeterred, physicists have dreamed up a multitude of possible new interactions between dark matter particles. Some of these models were put forward to explain observations that gravity-only dark matter (called cold dark matter, or CDM, for short) could not explain. These problems included the existence of galaxies with lower densities than expected at their centers (the core-cusp problem) and the dearth of small galaxies orbiting the Milky Way. One particularly promising model envisions that dark matter particles interact in such a way that causes them to scatter like billiard balls off of each other—this is known as self-interacting dark matter, or SIDM for short.

The authors of today’s paper show that there’s newfound hope for detecting such interactions. When dark matter particles scatter off of each other, they exchange momentum, which on a grander scale causes the dark matter densities at the centers of galaxies to be lower than with CDM. In fact, while CDM galaxies tend to have densities that continue to increase towards their centers, the density at the centers of SIDM galaxies rise far less and possess a nearly uniform density “core”. Thus measuring low central densities in galaxies is a dead ringer for SIDM—except for the fact that plain old baryonic matter can also produce similarly depressed densities. Baryonic processes such as stars going supernovae can blast matter out of the centers of galaxies and generate cores. We thus had a tricky problem on our hands—were the cores we observed due to baryons, or dark matter self-interactions? ...

SIDM on FIRE: Hydrodynamical Self-Interacting Dark Matter simulations of low-mass dwarf galaxies - Victor H. Robles et al
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How well do we measure the radii of white dwarfs?

Postby bystander » Mon Jul 17, 2017 2:47 pm

How well do we measure the radii of white dwarfs?
astrobites | 2017 Jul 17
Ingrid Pelisoli wrote:
Look outside your window. Can you see the Sun? If it’s night-time, just pick a random star instead. Our Sun one day will become a white dwarf star, and the chance that the random star you’ve picked will follow the same path is over 95%. White dwarfs are by far the most common final evolutionary state for a star. The famous supernovas actually only occur when a star is massive enough to burn elements heavier than helium in its core, and that is usually not the case. What happens instead is that the star can only produce elements up to carbon and oxygen, and then nuclear reactions in the core cease to occur. With no release of energy to counteract the gravitational force, the carbon-oxygen core will contract more and more until it becomes degenerate. This degenerate core is essentially the white dwarf, which becomes visible when the outer layers of the star are ejected on its final breadth of hydrogen burning in outer shells. ...

Testing the white dwarf mass-radius relationship with eclipsing binaries - S. G. Parsons et al
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Re: The Farthest Star Ever Seen

Postby Ann » Tue Jul 18, 2017 8:27 pm

bystander wrote:The Farthest Star Ever Seen
astrobites | 2017 Jul 04
Gourav Khullar wrote:
I write this astrobite in my living room, assisted heavily by the lenses in my spectacles. It’s quite a marvelous thing – a pair of glasses. For the last decade or so, a pair of glasses have been necessary for me to marvel at the wonders of the world (and universe, now that studying astrophysics allows me to do that at leisure). Then, imagine my surprise when I came across the cosmic equivalent of my glasses and what a marvel they are! Gravitational lenses are some of the most massive structures in the universe, teaching us about the most distant and the most massive objects in the cosmos. ...

It is important to note that since a galaxy cluster can act as a gravitational lens, just like a magnifying glass it also has the capacity to ‘magnify’ background objects. Imagine a faint galaxy that we are not able to detect with regular telescopes. Now imagine the same galaxy when it happens to lie in the line of sight of a galaxy cluster. Based on its location and distance from the ‘lens’, a faint background galaxy can get magnified by several times, allowing its minimal flux to reach us at last. It is common for astrophysicists in this field to study large objects like faint blue galaxies at redshifts of 2-3 that get magnified by nearby galaxy clusters, which would not have been possible otherwise. This advancing field is only getting better with improved understanding of galaxy clusters as lenses – the calculations of their masses, and how this mass may be distributed throughout the cluster. Today’s study has gone one step further, into uncharted territory. Today’s study has observed a gravitationally lensed star! ...

An individual star at redshift 1.5 extremely magnified by a galaxy-cluster lens - Patrick L. Kelly et al

viewtopic.php?t=34519


Gourav Khullar wrote:
Image of the initial SN Refsdal observations via the Hubble Space Telescope,
as well as the newly discovered magnified star LS1.


From Hubble observations of a multiple-imaged supernova via gravitational lensing (see this astrobite for more details on Supernova Refsdal), Kelly et al. were successful in observing another extremely fascinating object too – a blue unresolved source of light, whose flux kept fluctuating. By taking spectra of that object, they concluded that it was a B-type star whose light was magnified approximately 2000 times on its way to us! In comparison, a typical galaxy is expected to be magnified by ~50 times.


Stellar spectra. Note the Balmer break at 3650 Å for spectral classes A5V and B5V.
Source: http://www.jb.man.ac.uk/distance/life/sample/stars/

What makes this object undoubtedly a star are specific features seen in the spectra – namely the huge drop in flux around a rest-frame wavelength of ~3650 Angstroms called the Balmer break.
...
It turns out that this star, called LS1, is at a redshift of ~1.5, the farthest star ever discovered. For context, 99% of the stars you see in the night sky, are within the Milky Way.
...
The light travel distance from LS1, on the other hand, is 9 billion light years !
...
Other properties of the star have also been observed – its temperature (~11000 K), its transverse velocity or motion in the sky (~1000 km/s) and the reason why this star magnified to the extent it did.
...
...there are certain surfaces around the lens that are called ‘caustics’. These surfaces correspond to a ‘mathematically infinite’ magnification in ideal lens models, and if an object is present on these unique surfaces, even a very faint object can pop out in lens observations!


Sizes of stars. Source: Wikimedia Commons user Kieff.



















I find all of this quite amazing! LS1 is an "infinitely magnified" - well, at least 2,000 times magnified - B-type star 9 billion light-years away, whose temperature is 11,000K. What kind of a B-type star is that? Well, the strong Balmer break and the relatively low temperature - low for a B-type star - tells us that it is a rather late B-type star. Prominent Rigel in Orion is a B8Ia-type blue supergiant, whose temperature is 11,500K. The magnified star LS1 is slightly cooler, so its spectral class might be B8 or B9.

But it certainly matters if LS1 is a supergiant star like Rigel (85,000 solar luminosities) or a B8V main sequence star like 18 Tauri (73 solar luminosities). Look at the picture above right showing typical sizes of stars. I don't feel convinced that the picture is absolutely accurate in all its details - I distrust the yellow color of the F-type star, for example - but I believe that the sizes of the stars are basically correct for main sequence stars. But what about supergiant stars? If two stars are the same temperature, but one is ~700 times brighter than the other one, the brighter star would have to be ~25 times bigger than the other one, or at least its radius would have to be ~25 times bigger than the radius of the smaller star. It would most definitely be easier for any cosmic lens to magnify a terrifically distant Rigel into visibility than to do the same thing for an equally distant 18 Tauri, if Rigel is 700 times brighter than 18 Tauri!

Supergiant B-type stars of some 18 solar masses like Rigel are extremely rare. Cool B-type main sequence stars of some 3 solar masses like 18 Tauri are so much more common, but even they are really rare compared with all the little main sequence K- and M-type stars that must have been present at the same redshift. So why was it a B-type star that got magnified, even though B-type stars are rare?

Probably because all the little K-and M-type main sequence stars 9 billion light-years away would have been too faint to be detected this way, even if they had been "infinitely magnified".

Well, the whole thing is utterly amazing and marvelously fascinating, in any case! :D

Ann
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Black Hole Mergers in Nuclear Star Clusters

Postby bystander » Fri Jul 21, 2017 3:01 pm

Black Hole Mergers in Nuclear Star Clusters
astrobites | 2017 Jul 18
Philipp Plewa wrote:
Thanks to the efforts of the LIGO and VIRGO collaborations, we now have direct evidence of gravitational waves (GWs) produced by three or possibly even four pairs of merging stellar-mass black holes (BHs). This spectacular discovery has given us a first glimpse of a yet unexplored realm of the universe, so it is not surprising that the astrophysical origins of these BH mergers are still under much discussion.

For example, mergers of BHs formed by some of the very first stars have been suggested as candidate sources for these GW signals, but dynamical processes happening in dense star clusters such as globular clusters could also cause BHs formed there to merge. The authors of today’s paper investigate how stellar-mass BHs bound in binary systems evolve inside of nuclear star clusters, which are located at the cores of galaxies and are known to host another kind of BH, a (super-)massive black hole (MBH), at their centers. ...

Black Hole Mergers in Galactic Nuclei Induced by the Eccentric Kozai-Lidov Effect - Bao-Minh Hoang et al
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An Unconventional Solar Fountain

Postby bystander » Fri Jul 21, 2017 3:14 pm

An Unconventional Solar Fountain
astrobites | 2017 Jul 19
Amber Hornsby wrote:
Our local star, the Sun, is an active star. It regularly sends streams of highly energetic particles hurtling towards our home planet, causing dazzling auroral displays at the poles, and occasionally we notice emissions in the radio regime. Back in July 2013, an unusual and very bright burst of radio wave energy was observed by the Low Frequency Array (LOFAR) based in the Netherlands, with our Sun being the likely culprit. Today’s bite will illustrate how J-bursts, an unconventional type of jet from the Sun, differ from the so-called type III bursts commonly observed. Also, we will describe their likely origin and the proposed mechanism to explain their odd characteristics. ...

The Association of a J-burst with a Solar Jet - D. E. Morosan et al

Additional information:

Imaging Spectroscopy of Type U and J Solar Radio Bursts with LOFAR - Hamish A. S. Reid, Eduard P. Kontar
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An Astrophysical Event (Probably) Won’t Kill Every Species on Earth

Postby bystander » Fri Jul 21, 2017 3:23 pm

An Astrophysical Event (Probably) Won’t Kill Every Species on Earth
astrobites | 2017 Jul 20
Thankful Cromartie wrote:
Humans, despite our ingenuity and unrelenting will to live, are among Earth’s more fragile creatures. Though our susceptibility to extinction may seem like a pressing issue on a personal level, today’s astrobite delves into a more interesting thought experiment: which astrophysical events might be capable of wiping out every single organism on Earth. ...

The Resilience of Life to Astrophysical Events - David Sloan, Rafael Alves Batista, Abraham Loeb
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I see skies of blue and clouds of white

Postby bystander » Sun Jul 23, 2017 1:55 pm

I see skies of blue and clouds of white
astrobites | 2017 Jul 21
Shang-Min Tsai wrote:
Most astronomers hate clouds, except for those who study them. Not only our clouds on the sky ruin observing nights, clouds on other planets obscure everything underneath, too. When observing extra-solar planets during transit, different atoms, molecules or particles absorb light at certain wavelengths, making the apparent radii of the planets change as viewed with different colors — known as transmission spectroscopy. Although it is a powerful tool to infer the atmospheric composition of exoplanets, these clouds often hinder our efforts to understand such alien worlds. With clouds present at high altitude, only the atmosphere above the clouds can be seen and it is too thin to provide any spectral features (see this popular example). It turns out that a useful strategy for dealing with a problem is to avoid the problem. Valuable space telescope time could be saved if we could filter cloud-free objects from ground-based measurements. Today’s paper presents an index to quantify the degree of cloudiness for transiting planets. ...

A Cloudiness Index for Transiting Exoplanets Based on the Sodium and Potassium Lines:
Tentative Evidence for Hotter Atmospheres Being Less Cloudy at Visible Wavelengths
- Kevin Heng
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Where are the IceCube neutrinos coming from? (part 2)

Postby bystander » Fri Jul 28, 2017 5:51 pm

Where are the IceCube neutrinos coming from? (part 2)
astrobites | 2017 Jul 24
Kelly Malone wrote:
Back in 2013, the IceCube Collaboration published a paper announcing their discovery of astrophysical neutrinos, i.e. ones that have an origin outside our Solar System (Astrobites coverage). Since this discovery, scientists have been busily working to develop theories as to the origin of these neutrinos. The original paper noted some clustering in the area of the center of our Galaxy, but it was not statistically significant. Since then, both Galactic and extragalactic origins have been proposed. Star-forming galaxies have been suggested as one possible origin, which Astrobites has covered papers arguing both for and against (here and here). Other theories involve radio galaxies, transients, and dark matter.

In today’s paper, the IceCube Collaboration has analyzed more of their data and set limits on the percentage of the diffuse neutrino flux that can come from Galactic sources. Theoretically, some neutrinos should be created in the Galactic plane: we know that this area emits gamma rays from pion decay, and neutrinos are created in the same types of interactions that create the gamma rays. ...

Constraints on Galactic Neutrino Emission with Seven Years of IceCube Data - IceCube Collaboration
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Dark Matter in the Milky Way: ‘A Matter of Perspective’

Postby bystander » Fri Jul 28, 2017 5:59 pm

Dark Matter in the Milky Way: ‘A Matter of Perspective’
astrobites | 2017 Jul 25
Nora Shipp wrote:
Dark matter dominates the Universe around us, far exceeding the amount of everyday baryonic matter that makes up humans, the Earth, and the entire visible Milky Way. Our galaxy is embedded in an invisible cloud of dark matter, which contains smaller dark matter clouds that orbit around us like satellites. These satellites do not contain big spiral galaxies like the Milky Way and, although they may contain smaller galaxies, they are made up of almost entirely dark matter, which means that they are very sensitive to the precise nature of the dark matter particle.

Today’s paper investigates whether two of the Milky Way’s largest satellite galaxies (Fornax and Sculptor, Figure 1) conflict with the leading theory of Cold Dark Matter (CDM), potentially requiring a complete reconsideration of our understanding of the evolution of the Universe. Don’t get too excited, though. I will break the suspense and say that, as usual, the answer is “not yet” – we don’t know enough about these mini galaxies to throw away CDM. There is still a lot of work to be done if we want to break this paradigm. ...

The core-cusp problem: a matter of perspective - Anna Genina et al
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And now, the smallest star ever

Postby bystander » Fri Jul 28, 2017 6:05 pm

And now, the smallest star ever
astrobites | 2017 Jul 26
Bhawna Motwani wrote:
Continuing on our spree of reporting discoveries of superlative celestial objects (see this, for example), in today’s bite, I talk about the sighting of yet another one of those, the smallest star ever observed in our universe. ...

The EBLM Project III. A Saturn-size Low-Mass Star at the Hydrogen-Burning Limit - Alexander von Boetticher et al

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Bald Black Holes Wearing Wigs

Postby bystander » Fri Jul 28, 2017 6:23 pm

Bald Black Holes Wearing Wigs
astrobites | 2017 Jul 27
Lisa Drummond wrote:
The No-Hair Theorem is a conjecture about the simplicity of black holes. They are called “bald” to reflect the paucity of information necessary to characterise their spacetime – only three parameters at most are needed. The theorem is formulated to describe isolated black holes. A realistic black hole will almost invariably be distorted by its astrophysical environment, e.g. a binary companion, surrounding plasma, accretion disks, or nearby jets. It seems possible that in these cases, the black hole could grow hair and the No-Hair Theorem would break down.

However, this paper shows that the distortions from the surrounding environment do not imply the black hole now has hair. Although our observations of the spacetime an infinite distance from the black hole do change, this is solely due to the external neighbourhood around the black hole, not the distorted black hole itself. In the words of Norman Gürlebeck, the author of this paper: "Thus, even though the black hole might put on a wig it still looks bald." The No-Hair Theorem remains valid in this more general context: even when engulfed by a complex environment, the black hole resists growing hair. ...

No-Hair Theorem for Black Holes in Astrophysical Environments - Norman Gürlebeck

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What are Mars’ moons made of?

Postby bystander » Fri Jul 28, 2017 6:34 pm

What are Mars’ moons made of?
astrobites | 2017 Jul 28
Kerrin Hensley wrote:
Phobos and Deimos, Mars’ two small moons, were initially believed to be the result of interplanetary kidnapping. Many moons in the Solar System appear to be captured objects, and the featureless reflectance spectra of Phobos and Deimos hint that they might be D-type asteroids. However, captured objects tend to have highly eccentric orbits, and both Phobos and Deimos orbit Mars in a nearly circular fashion. More recently, it has been proposed that both moons are the result of a massive impact 4.3 billion years ago—instead of being captured from interplanetary space, they could coalesce from the debris disk generated by the impact. Past research has shown that the masses and orbits of Phobos and Deimos can be explained by this method. This theory could also explain the presence of Borealis basin, an extended low-altitude region spanning Mars’ north pole, which can be seen in Figure 1.

In this paper, the authors use smoothed particle hydrodynamics (SPH) simulations to learn more about the thermodynamical and structural properties of the debris disk generated in the proposed impact. SPH is a common method used when simulating astrophysical fluids, or systems with a large number of particles that can be treated as a fluid, like stars in colliding galaxies. In SPH (which has been detailed in previous Astrobites like this one and this one), the properties at a given location within a fluid are extracted by weighting the properties of the particles near that point using a smoothing function (sometimes called a smoothing kernel), which is often simply a Gaussian. ...

On the Impact Origin of Phobos and Deimos I: Thermodynamic and Physical Aspects - Ryuki Hyodo et al
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The Measure of Things

Postby bystander » Wed Aug 09, 2017 6:41 pm

The Measure of Things
astrobites | 2017 Jul 31
Emily Sandford wrote:
Today’s recent astronomy paper is a 200-year-old, 500-page book about heat flow. This arguably stretches the bounds of what’s astrobiteable, even for our Classics section, but I say it counts because a) 200 years ago is recent compared to the age of the Universe, and b) books are printed on paper, so no one can tell me this isn’t a paper.

And if you’re looking for important developments in astronomy, it’s hard to do better than this! It’s the first major work by Joseph Fourier, who made all kinds of important contributions to math, physics, and, strangely, Egyptology. In chapter 4 of this book, he writes down an equation for heat flow that is still the first partial differential equation that most physicists ever learn to analyze. In chapter 3, he straight-up invents Fourier series, which astronomers use to study signals that change over time, from pulsating stars to black holes circling each other.

But I’m actually going to leave all that, and focus instead on a tiny slice of chapter 2. Four pages, presenting an idea so simple it’s hard to believe anyone had to come up with it. But it’s an idea right at the heart of physics. ...

The Analytical Theory of Heat - Joseph Fourier
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Detecting Exoplanet Life in Our Proximity

Postby bystander » Wed Aug 09, 2017 6:59 pm

Detecting Exoplanet Life in Our Proximity
astrobites | 2017 Aug 01
Mara Zimmerman wrote:
This summer marks the one-year anniversary of the detection of an exoplanet orbiting our solar system’s nearest stellar neighbor, Proxima Centauri. This Earth sized exoplanet, with the oh-so-imaginative name Proxima Centauri b, lies in the habitable zone of its M-dwarf star. As we’ve previously discussed, having such an Earth-like planet in our stellar backyard is really exciting, and astronomers are keen to explore the system as thoroughly as possible for potential signs of life. ...

Since the discovery of this close planet, researchers have been studying methods that might detect the presence of life on Proxima Centauri b. Today’s paper by devising a method to detect CO2 in the planet’s atmosphere. They focus on this particular molecule because it is one of the four main biomarkers used in evaluating habitability of exoplanets; water, methane, carbon dioxide, and oxygen are primarily produced during biological processes, so their presence in the atmosphere implies life. In addition to being a biomarker molecule, carbon dioxide (CO2) has many distinguishable features that are visible on the 15 micron band, which JWST is equipped to look at. ...

Detecting Proxima b's Atmosphere with JWST Targeting CO2
at 15 μm Using a High-Pass Spectral Filtering Technique
- I. A. G. Snellen et al
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