astrobites 2018

Find out the latest thinking about our universe.
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Milky Way’s Stellar Streams

Post by bystander » Mon Aug 20, 2018 5:21 pm

Do the Milky Way’s Stellar Streams have that Fuzzy (Dark Matter) Feeling?
Astrobites | 2018 Aug 16
Tomer Yavetz wrote:
It is often said that people fear what they do not understand. In astrophysics, there are many things we do not understand, and perhaps, at least subconsciously, we begin to fear them. Take Dark Matter: it makes up roughly 80% of the mass in the Universe, and our humble galactic home, the Milky Way, is surrounded by a Dark Matter Halo that is likely over 10 times as massive as all of the stars, planets, dust, and gas in the galaxy combined. Yet we still have no clue what it really is, or what kind of particles it is made of. Ostensibly, we call it ‘Dark’ because we cannot see it, but there is no escaping the fact that the name gives off a creepy, sinister vibe.

However, as astrophysicists, it is our job to conquer our fears and study this mysterious entity. And what better way to face our fear than to give it funny names? Cue a long list of adjectives and acronyms that have been used to describe Dark Matter: cold, warm, hot, sticky, self-interacting, WIMPs, MACHOs, and the list goes on. The topic of today’s paper involves a particularly friendly sounding version: Fuzzy Dark Matter. ...

First Constraints on Fuzzy Dark Matter from the Dynamics
of Stellar Streams in the Milky Way
~ Nicola C. Amorisco, Abraham Loeb
viewtopic.php?t=37413
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What’s the powerhouse of the AGN?

Post by bystander » Thu Aug 23, 2018 5:35 pm

What’s the powerhouse of the AGN?
Astrobites | 2018 Aug 20
Mia de los Reyes wrote:
Although nearly every massive galaxy has a supermassive black hole at its center (even our own Milky Way has one), active galactic nuclei (AGN) like to show theirs off the most. There are many different kinds of AGN, but we think they’re all just different views of the same thing: a supermassive black hole at the center of a galaxy that accretes large amounts of gas, creating a superheated accretion disk and powering incredibly energetic jets.

The physics of how AGN actually work are still active (ha!) areas of research. Although the model described above is conceptually simple, it’s hard to explain some of the details. For instance, huge amounts of gas are needed to sufficiently power the AGN—how exactly does that much gas get to the center of a galaxy? ...

In today’s paper, Alonso and his team aim to compare the relative importance of galactic bars and galaxy interactions in fueling AGN. To do this, they use data from the Sloan Digital Sky Survey to identify a sample of AGN hosted in barred spiral galaxies, and a sample of AGN hosted in pairs of galaxies. They then identify a control sample of AGN in unbarred, isolated galaxies. To control for other properties (redshift, magnitude, stellar mass, color, and stellar age population), Alonso et al. make sure their control sample has distributions of these properties that are similar to the distributions of the two other populations. Figure 1 shows examples from these three different galaxy samples. ...

The impact of bars and interactions on optically selected AGNs in spiral galaxies ~ Sol Alonso et al
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Ideal cosmic yardstick

Post by bystander » Thu Aug 23, 2018 5:47 pm

Ideal cosmic yardstick: neutron star black hole mergers as standard sirens
Astrobites | 2018 Aug 21
Lisa Drummond wrote:
A little over a year ago, on the 17 August, LIGO (Laser Interferometer Gravitational Wave Observatory) detected a binary neutron star merger (known as GW170817). This was groundbreaking for a number of reasons. First of all, it was a multi-messenger detection – meaning we observed ripples in spacetime (gravitational waves) traveling through the LIGO detector and, around two seconds later, saw light across the electromagnetic spectrum from the same collision.

In addition, because we detected the collision through two different physical mechanisms (light and gravitation) it was possible to combine these pieces of information to get a new estimate for the Hubble constant. The Hubble constant is an extremely important number in cosmology – it describes how fast Universe is expanding and is needed to determine the age and size of our universe. Gravitational wave events with coincident electromagnetic signals are new yardsticks for measuring our universe.

But binary neutron star mergers are not the only type of mergers we expect to see. Neutron star black hole (NSBH) mergers (which are likely to be more rare) could also produce both gravitational and electromagnetic radiation that we could use to deduce the Hubble constant. Indeed, this may be a better method, as is discussed in today’s paper. ...

Measuring the Hubble Constant with Neutron Star Black Hole Mergers ~ Salvatore Vitale, Hsin-Yu Chen
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Giant Planets, don’t let a windy day ruin your childhood

Post by bystander » Thu Aug 23, 2018 6:01 pm

Giant Planets, don’t let a windy day ruin your childhood
Astrobites | 2018 Aug 22
Michael Hammer wrote:
Do you take Jupiter for granted? When you were taking your dog for a walk last night, did you stop and think about how your dog could have been hit by a meteorite had Jupiter not been ejecting asteroids from the solar system for the last few billion years – just to protect life on Earth from frequent giant asteroid impacts? Or when you were eating breakfast this morning, did you stop and appreciate how you would not have been able to have breakfast if it weren’t for the fact that Jupiter migrated into the inner solar system shortly after it formed, in order to dump excess planetesimals into the Sun – just to prevent the Earth from growing past its current size and becoming uninhabitable?

We may have Jupiter to thank for shaping the conditions that allow life to thrive on our planet. However, surveys of exoplanets in other star systems have found giant planets to be rare (albeit a new study is more optimistic). In smaller protoplanetary disks (such as those around smaller stars), the lack of gas giants is easy to understand as there simply may not have been enough planet-forming material in the disk to form such a large planet. However even in larger disks, there is a longstanding unanswered theoretical question of how can planets of this size form so quickly before the disk fades away in a few million years. If real protoplanetary disks cannot solve this problem, Jupiter-sized planets may indeed be few and far between.

It was long thought that the rocky cores of gas giant planets (5 to 10 times the mass of the Earth) formed from city-sized planetesimals (> 1 km) merging together, but this takes too long. The reason this process is so slow is that there is a limited supply of planetesimals in any given area of the disk. In the last decade, it has been suggested that the largest dust particles (1 cm to 1 m) called “pebbles” can act as a catalyst to speed up the gas giant growth process because they do not stay where they formed. Pebbles drift towards their stars faster than any other sized object. This change of location makes it possible for pebbles all the way near the outer edge of the disk to reach a rocky core much further inwards and help it grow. Even though pebbles are small, they also make up a large fraction of the mass in a disk – allowing them to readily speed up a planet’s growth process.

However, today’s paper by Mohsen Shadmehri et al. suggests pebbles may not be a good solution to this problem in all cases. ...

On the Dynamics of Pebbles in Protoplanetary Disks with Magnetically Driven Winds ~ M. Shadmehri, F. Khajenabi, M.E. Pessah
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Filling Gaps in our Knowledge of Planet Formation

Post by bystander » Thu Aug 23, 2018 6:18 pm

Filling (Dust) Gaps in our Knowledge of Planet Formation
Astrobites | 2018 Aug 23
Jamila Pegues wrote:
It’s a little too late for anyone to witness how Earth, and its planetary neighbors, came to be – at least, not without some sort of time machine. That’s one reason scientists study protoplanetary disks out in space: to learn more about how planets form.

A protoplanetary disk is a fluffy disk of dust and gas orbiting around a young star. These disks are believed to be sites of planet formation. The Solar System we live in was once a protoplanetary disk, waaay back in the day before its planets formed.

In the past, it’s been extremely difficult to directly catch disks in the act of forming planets. But powerful new instruments of this (and the next) decade are helping to change that. With the awesome might of one such instrument, known as the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, scientists have very recently been able to observe protoplanetary disks at higher resolution than ever before. Their observations show all sorts of substructures in disks, like holes, spirals, and gaps. Figure 1 gives a taste of these intricate substructures in a handful of disks, which were all observed with ALMA a few years ago. ...

Today’s authors set out to more fully explore the relationship between a growing planet and the gaps that it can form. Using simulations of disks with evolving planets, they studied how gap characteristics vary with disk and planet parameters. They also determined how scientists can use these gap characteristics to infer properties of planets from observed disk substructures – assuming that those observed substructures were caused by planets to begin with! ...

Multiple Disk Gaps and Rings Generated by a Single Super-Earth:
II. Spacings, Depths, and Number of Gaps, with Application to Real Systems
~ Ruobing Dong et al Multiple Disk Gaps and Rings Generated by a Single Super-Earth ~ Ruobing Dong et al
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Re: Ideal cosmic yardstick

Post by BDanielMayfield » Fri Aug 24, 2018 1:18 pm

bystander wrote:
Thu Aug 23, 2018 5:47 pm
Ideal cosmic yardstick: neutron star black hole mergers as standard sirens
Astrobites | 2018 Aug 21
Lisa Drummond wrote:
A little over a year ago, on the 17 August, LIGO (Laser Interferometer Gravitational Wave Observatory) detected a binary neutron star merger (known as GW170817). This was groundbreaking for a number of reasons. First of all, it was a multi-messenger detection – meaning we observed ripples in spacetime (gravitational waves) traveling through the LIGO detector and, around two seconds later, saw light across the electromagnetic spectrum from the same collision.

In addition, because we detected the collision through two different physical mechanisms (light and gravitation) it was possible to combine these pieces of information to get a new estimate for the Hubble constant. The Hubble constant is an extremely important number in cosmology – it describes how fast Universe is expanding and is needed to determine the age and size of our universe. Gravitational wave events with coincident electromagnetic signals are new yardsticks for measuring our universe.

But binary neutron star mergers are not the only type of mergers we expect to see. Neutron star black hole (NSBH) mergers (which are likely to be more rare) could also produce both gravitational and electromagnetic radiation that we could use to deduce the Hubble constant. Indeed, this may be a better method, as is discussed in today’s paper. ...

Measuring the Hubble Constant with Neutron Star Black Hole Mergers ~ Salvatore Vitale, Hsin-Yu Chen
This method would rely on a black hole's ability to shread a very close orbiting neutron star before it crosses the BH's event horizon. Wouldn't many of such mergers just have the NS swallowed whole with no disruption?

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The strange case of quiescent and dusty

Post by bystander » Mon Aug 27, 2018 8:15 pm

The strange case of quiescent and dusty
Astrobites | 2018 Aug 24
Joanna Ramasawmy wrote:
Today’s astrobite is a story of two papers, two research groups – and perhaps, two galaxies.

It starts with this previously-astrobitten Nature paper, which announced the discovery of a massive, quiescent (i.e. no longer star-forming) galaxy at a surprisingly high redshift of z = 3.7. Surprising, because in order to form a red-and-dead galaxy at such high z, it must have undergone a significant and sudden burst of star formation at a very early time in the history of the universe. This, the authors claimed, challenges current models of galaxy evolution, in which we don’t expect to see galaxies like this until much later.

This is already controversial — as the previous astrobite points out, theorists quickly leapt to defend their models, showing that while these galaxies are rare, it is not at all impossible to create them in cosmological simulations.

However, shortly after publication, another group of observational astronomers (Simpson et al.) suggested that in fact, the authors of the original paper hadn’t seen a quiescent galaxy at all. Rather, they just hadn’t accounted for the fact that the galaxy also contained a lot of dust, only visible in long-wavelength submillimetre observations. ...

In response, the authors of the original paper published a follow-up (Schreiber et al.) including the submillimetre observations of this object. With the new data, they identify two distinct galaxies, one quiescent and one dusty (which they somewhat ironically call Jekyll and Hyde — unarguably, though, an improvement on the original mouthful, ZF-COSMOS-20115). ...

An imperfectly passive nature: Bright sub-millimeter emission from
dust-obscured star formation in the z=3.717 "passive" system, ZF20115
~ J.M. Simpson et al Jekyll & Hyde: quiescence and extreme obscuration in
a pair of massive galaxies 1.5 Gyr after the Big Bang
~ C. Schreiber et al
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A Radio Bright Quasar at the Edge

Post by bystander » Mon Aug 27, 2018 8:31 pm

A Radio Bright Quasar at the Edge
Astrobites | 2018 Aug 27
Joshua Kerrigan wrote:
Quasars are some of the most interesting astronomical objects, able to provide us with information across both astrophysics and cosmology. When a quasar with unique properties, such as being exceptionally luminous is discovered, it’s particularly eye-catching. This is because it can provide an opportunity to make measurements that wouldn’t be possible otherwise, as we will explore in today’s astrobite.

Astronomers look to quasars because of the extreme conditions surrounding their existence. They are exceptionally bright and emit across the entire span of the electromagnetic spectrum. These luminous emissions are thought to be due to the accretion of gas onto a supermassive black hole at the center. So you might be asking yourself, if these are so bright, why do we even need telescopes to see them? Well that’s because most quasars are at incredible distances, located at redshifts of z > 0.1, meaning relative to anything local to us, they’ll appear very dim. In fact the closest quasar to us, Markarian 231, is still at a measurable redshift of z=0.04.

What we want to highlight today are quasars on the opposite end of that spectrum, those that are very far away, at high redshift. These types of quasars can give us direct probes to intermediate periods in the Universe’s history, like a certain epoch that may have left the Universe reionized. One way a quasar can tell us about the timeline of the Universe is through the Lyman alpha Forest, which provides us with hints about neutral hydrogen content in the intervening intergalactic medium (IGM). This happens when higher energy UV emissions (shorter wavelength than the Lyman Alpha line) from high redshift galaxies get redshifted into the Lyman Alpha absorption range. This leaves dips in the flux density when observed here on Earth, like in the example in Figure 1. We can directly relate these dips in the flux density to the redshift location of neutral hydrogen. ...

A Powerful Radio-loud Quasar at the End of Cosmic Reionization ~ Eduardo Banados et al viewtopic.php?t=38502
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Can we predict if planets are lonely?

Post by bystander » Sat Sep 01, 2018 6:08 pm

Can we predict if planets are lonely?
Astrobites | 2018 Aug 28
Emma Foxell wrote:
Congratulations! You’ve just found your first exoplanet transiting a star. You may wonder if there are any additional planets in that stellar system. This information is important in deciding which targets are worth spending more time observing at the telescope. Can we use the known stellar and planet properties to predict whether more transiting planets may be found?

The authors of today’s paper investigated if there were any differences between the properties of systems with only one known transiting exoplanet (single planet systems) and systems where multiple transiting exoplanets orbit the same star (multi-planet systems). ...

The California-Kepler Survey. VI: Kepler Multis and Singles Have
Similar Planet and Stellar Properties Indicating a Common Origin
~ Lauren M. Weiss et al
viewtopic.php?t=37917
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Sorting Stars with a Light Touch

Post by bystander » Sat Sep 01, 2018 6:26 pm

Sorting Stars with a Light Touch
Astrobites | 2018 Aug 29
Tarini Konchady wrote:
Asymptotic Giant Branch (AGB) stars are bright but cool (if a few thousand Kelvin can be considered cool). A star with a mass of 8M ☉ or less will usually evolve into an AGB star towards the end of its life. AGB stars are named so because of the region they occupy on the Hertzsprung-Russell diagram (see Figure 1, and this Astrobite for details on how they get to that region in the first place). At this point in its life, a star has burned through all of the helium in its core and appears as a red giant.

AGB stars play a significant role enriching the metal content of their host galaxies, and can serve as reliable distance indicators by being standard candles. So it makes sense that we should try to detect and characterize them as far out as we can, in order to better understand the local universe.

AGB stars readily show up in multiband observations. Large surveys such as the MACHO Project, the OGLE survey, and (as this paper demonstrates) the Gaia mission have proven invaluable in creating large samples of AGB stars for further study. In this paper, the authors explore how subclasses of AGB stars can be identified using Gaia and 2MASS photometry. ...

A new method to identify subclasses among AGB stars using Gaia and 2MASS photometry ~ T. Lebzelter et al
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Zoos, Swamplands and Cosmology

Post by bystander » Sat Sep 01, 2018 6:40 pm

Zoos, Swamplands and Cosmology
Astrobites | 2018 Aug 30
Philippa Cole wrote:
What do zoos and swamplands have to do with Cosmology? There’s a zoo of possibilities for how inflation happened, and there’s a landscape of possible string theories surrounded by a swampland of implausible ones… and we need to find ways to narrow them down. ...

Inflation describes how the Universe expanded extremely rapidly within the first second of its existence. It explains and solves some big problems in Cosmology, for example the flatness problem and the horizon problem, so it’s one of the most popular theories for the early history of the Universe. However, inflation can only describe how things worked in the early Universe up to a certain energy scale (although we’re not sure exactly what that scale is), so ideally we’d like to find an overarching theory that is compatible with inflation, but that also tells us how things work at arbitrarily high energies.

String theory offers examples of such high-energy theories. However, for these candidates to be successful, the low-energy limit of such a theory needs to predict a well-defined inflationary model as well as explaining the high energy behaviour. Those that can do both are known as residing on the string ‘landscape’, and those that can’t are banished to the stringy ‘swampland’.

Today’s authors have added to the mix by claiming that theories that are happily on the string ‘landscape’ (and safe from the swampland) are inconsistent with current cosmological data that constrain theories of inflation. ...

The Zoo Plot Meets the Swampland: Mutual (In)Consistency of
Single-Field Inflation, String Conjectures, and Cosmological Data
~ William H. Kinney, Sunny Vagnozzi, Luca Visinelli
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Our M-dwarf Models Just Aren’t Good Enough

Post by bystander » Fri Sep 07, 2018 3:44 pm

Our M-dwarf Models Just Aren’t Good Enough
Astrobites | 2018 Sep 03
Avery Schiff wrote:
There is a decent chance that the first habitable exoplanet we discover will be orbiting an M-dwarf star. This is not because M-dwarf stars (named for their spectral classification and sometimes referred to as red dwarfs) create particularly hospitable environments for life. Rather, it is partly because they are so common, and partly because their size and dimness are ideal for exoplanet detection. A small, rocky Earth-like planet is large enough, relative to an M-dwarf, to significantly dim the star as it transits or to gravitationally “tug” on the star. In order to study these potentially habitable exoplanets, however, we need to know as much about the host stars as possible. And as the authors of today’s paper point out, it is unfortunately sometimes difficult to determine one of the most important stellar characteristics for exoplanetology: the star’s radius.

Precise measurements of an exoplanet’s radius are necessary to distinguish between an icy and a rocky planet. We often constrain the radius of a planet by measuring how much of the star’s light is blocked when the planet moves in front of the star, and the star’s radius is a crucial ingredient for our analysis. We can derive a spherical estimate for a star’s radius with its color, distance, and brightness, but more precise estimates for distant, faint stars require comparisons to stellar evolution models (models that simulate a star throughout its live by numerically solving the equations of stellar structure).

M-dwarf radii frequently exceed model predictions. Stars with these large radii are called “over-inflated.” While over-inflation sometimes indicates strong magnetic fields or rapid rotation, the authors point out that neither guarantees over-inflation, and over-inflation can occur without either of those conditions. They therefore seek some relationship between over-inflation and other fundamental stellar properties. ...

The scatter of the M dwarf mass-radius relationship ~ S. G. Parsons et al
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Pebbly Planets

Post by bystander » Fri Sep 07, 2018 3:58 pm

Pebbly Planets
Astrobites | 2018 Sep 04
Jamila Pegues wrote:
One of the great questions of astronomy today is also a deceptively simple one:

How do planets form?

It’s an old question, to say the least. But even in modern times, scientists continue to search for the answer.

One popular theory for how planets form – although there are other theories, like those described here and also here – is known as the core accretion model. This model says that a planet is born through the following two steps:
  1. A core forms through the accretion, or buildup, of solids.
  2. The core grows massive enough that, using its own gravity, it can accrete an envelope, or layer, of gas around it.
The core accretion model is meant to explain, for example, how gas giants like Jupiter form in protoplanetary disks. A protoplanetary disk is a disk of gas and dust orbiting about a young star. For the core accretion model to work, a core must form before the gas in the disk can disperse with disk age. Otherwise, there wouldn’t be any gas left for the core to accrete a gas envelope in the first place.

While the core accretion model has evolved a lot over the years, today’s authors focus on one scenario, which says planet cores grow through the accretion of pebbles. In this case, pebbles are bits of dust that are about a centimeter in size (or about half the length of a penny). These pebbles orbit along with the protoplanetary disk around the central star. But because of pressure in the disk, the gas in the disk is actually orbiting at a slower speed than the dust in the disk. So the poor pebbles, due to their small sizes, feel something like a headwind from the disk gas, which pushes back against their movement. These pebbles thus feel a drag force from the gas, which allows a growing core to accrete the pebbles more easily. And that’s good, because we want the core to grow massive quickly enough, so that it can accrete a gas envelope before the gas in the disk disperses.

Today’s authors set out to explore this pebble-based core accretion scenario more fully. Using theoretical modeling, they investigated what sort of planets this scenario could produce. ...

Metallicity effect and planet mass function in pebble-based planet formation models ~ Natacha Brügger et al
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Huntsman Spiders and “Boring” Millisecond Pulsars

Post by bystander » Fri Sep 07, 2018 4:20 pm

Huntsman Spiders and the Race to Discover the Origins of “Boring” Millisecond Pulsars
Astrobites | 2018 Sep 05
Thankful Cromartie wrote:
Millisecond pulsars (MSPs) — our favorite fast-spinning, ultra-dense cosmic laboratories — are well known as exotic compact objects that can be exploited as probes of general relativity and nuclear physics. Today’s astrobite explores a darker side of MSPs: their arachnid-like cannibalism.

MSPs are thought to be neutron stars that have attained their rapid rotation through angular momentum transfer from a binary companion. Once this period of mass accretion ends, MSPs are stable rotators (capable of more precise time keeping than atomic clocks). Before their eventual settling, however, a period of vicious cannibalism can reign. In systems with short orbital periods and relatively low-mass companions, the highly energetic MSP can ablate (in this analogy, “eat”) its binary companion. This destructive behavior, analogous to some arachnids’ sexual cannibalism, prompted the naming of two spider pulsar classifications: black widows, whose companions are lighter than 0.1 solar masses, and redbacks, with slightly heavier companion stars. Three of these redback pulsars have displayed unprecedented, relatively fast transitional behavior, demonstrating periods of both active accretion and relative quiescence. The ability to watch these changes in real time is an invaluable tool for understanding the lifecycle of MSPs.

Today’s featured article introduces another species of spider MSP: the huntsman. The new classification was fueled by the discovery of 1FGL J1417.7–4407, a Fermi gamma-ray MSP with a companion that was discovered at optical wavelengths. Like its redback cousins, the huntsman is a gamma-ray-emitting MSP in orbit with a (non-degenerate) stellar companion. This new spider, however, is in a wide binary orbit with a larger partner, and therefore does not display similarly predatory, ablating behavior.

While this may sound like a more boring class of binary MSP, J1417 (as it is called by the authors) is unique in what it might reveal about binary MSP evolution. Unlike redbacks and black widows, the huntsman J1417 may be the first-ever progenitor of what will become a normal MSP binary with a white dwarf companion and a long orbital period. As surprising as it may seem, a glimpse into the life of the most typical MSPs is one of the evolutionary links we’ve been most desperately lacking. ...

A multi-wavelength view of the neutron star binary 1FGL J1417.7—4402:
A progenitor to canonical millisecond pulsars
~ Samuel J. Swihart et al
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Corralling the Solar Corona

Post by bystander » Fri Sep 07, 2018 4:29 pm

Corralling the Solar Corona
Astrobites | 2018 Sep 07
Stephanie Hamilton wrote:
The total solar eclipse on 21 August 2017 was possibly the most-viewed celestial phenomenon in history (look at us, still talking about it!). Not only was it a wondrous sight for hundreds of millions of observers, it was also a rare chance to study one of the least understood parts of the Sun — its corona. Unfortunately, the corona is also the region that gives rise to phenomena like solar flares that could have extremely damaging effects on our tech-based society. As a consequence, a lot of money and resources go into trying to forecast when the next major solar flare might occur. Today’s authors capitalized on the rare opportunity afforded by the 2017 eclipse to study the Sun’s corona by comparing predictions of the corona to what was actually observed on the day of the eclipse (spoiler: the simulations actually did reasonably well!) ...

Predicting the Corona for the 21 August 2017 Total Solar Eclipse ~ Zoran Mikić et al
viewtopic.php?t=38638
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How heavy is our Galaxy?

Post by bystander » Thu Sep 13, 2018 5:25 pm

How heavy is our Galaxy?
Astrobites | 2018 Sep 10
Nora Shipp wrote:
Weighing our Galaxy is a difficult task. The Milky Way is a complicated mess of stars and gas, and – to make things even more difficult – the majority of our Galaxy is invisible in the form of dark matter (Figure 1). Today’s paper states that current calculations of the mass range from about 500 billion to 2.5 trillion times the mass of our sun. The difference between these two huge numbers is only a factor of a few, but a precise measurement of the mass of the Galaxy is essential for understanding the physics of galaxy formation and for unraveling fundamental cosmological mysteries like the nature of dark matter.

This number is very important to pin down because our Galaxy is such a unique laboratory for studying the mysteries of the Universe. It is the only galaxy that we can observe from up-close, and thereby collect detailed information on the structure of a galaxy and the complicated physical processes that occur within it. This snapshot of a single galaxy can be generalized to overarching physical theories when combined with observations of distant galaxies and compared to theoretical predictions from numerical simulations. This generalization, however, depends on our ability to place the Milky Way in the context of the general galaxy population – and this requires a precise measurement of the mass of our Galaxy.

So, how is it possible to weigh a Galaxy from within? The authors of today’s paper measure the mass of the Milky Way based on the motions of orbiting satellite galaxies (Figure 2). Just like we can use the orbital speeds of the planets in our solar system, along with Newton’s law of gravitation, to infer the mass of the Sun, we can use measurements of the orbits of small satellite galaxies around the Milky Way to weigh our Galaxy. ...

The mass of the Milky Way from satellite dynamics ~ Thomas Callingham et al
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Can clouds make the sky brighter?

Post by bystander » Thu Sep 13, 2018 5:30 pm

Can clouds make the sky brighter?
Astrobites | 2018 Sep 12
Peter Sinclair wrote:
Light pollution is a large problem facing astronomy in the modern age. The rise of urban areas and the preponderance of electric lighting means that it has become increasingly difficult to find truly dark skies. Astronomers are forced to set up observatories in remote locations to avoid any skyglow which might interfere with their observations. But research by Jechow et al. suggest that observatories may be affected by distant cities if it is cloudy. ...

How dark can it get at night? Examining how clouds darken the sky via all-sky
differential photometry
~ Andreas Jechow, Franz Hölker, Christopher C. M. Kyba
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Solving Cepheid Mysteries with the Magellanic Clouds

Post by bystander » Thu Sep 13, 2018 5:43 pm

Solving Cepheid Mysteries with the Magellanic Clouds
Astrobites | 2018 Sep 13
wrote:
Cepheid variables are definitely a gift to astronomers. They are a class of variable stars that are useful as distance indicators (objects that can help us measure distances because we know what their brightness ought to be if they were say 10 parsecs away from us). Henrietta Swan Leavitt was the first to notice that Cepheids that took longer to go from their brightest to their dimmest and back—that is, they had longer periods—tended to be brighter than Cepheids with shorter periods (see Figure 1). This relationship between period and brightness is called a period-luminosity relationship (PLR). We continue to use Cepheids to measure distances even though we still don’t yet know everything about them.

Cepheids are broken up into two classes, generally speaking—Type I or Classical Cepheids, and Type II Cepheids. Type I Cepheids are younger than Type II Cepheids, and have a more structured light curve (see Figure 2) as well. There is also a third class of Cepheids that don’t quite fit into either of the previous two classes called Anomalous Cepheids. The peculiarities of Type II Cepheids and Anomalous Cepheids have made understanding their origin and evolution an interesting problem. In this paper, the authors attempt to make headway on this issue by studying the spatial distribution of Cepheids and other variable stars called RR Lyrae in the Magellanic Clouds (Large: LMC, and Small: SMC, two of the largest satellites of the Milky Way).

The authors work with data taken of the Magellanic Clouds from the Optical Gravitational Lensing Experiment (OGLE). The data span 25 years, making the Clouds a hotbed for variable star identification. In this paper, nearly 10,000 Classical Cepheids and 50,000 RR Lyrae (another type of variable star that is a distance indicator) are used to constrain the positions of just under 350 Type II Cepheids and about 250 Anomalous Cepheids across both Clouds. ...

The three-dimensional distributions of Type II Cepheids and Anomalous Cepheids in the Magellanic Clouds.
Do these stars belong to the old, young or intermediate-age population?
~ P. Iwanek et al
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Re: How heavy is our Galaxy?

Post by Ann » Fri Sep 14, 2018 6:56 am

bystander wrote:
Thu Sep 13, 2018 5:25 pm
How heavy is our Galaxy?
Astrobites | 2018 Sep 10
Nora Shipp wrote:
Weighing our Galaxy is a difficult task. The Milky Way is a complicated mess of stars and gas, and – to make things even more difficult – the majority of our Galaxy is invisible in the form of dark matter (Figure 1). Today’s paper states that current calculations of the mass range from about 500 billion to 2.5 trillion times the mass of our sun. The difference between these two huge numbers is only a factor of a few, but a precise measurement of the mass of the Galaxy is essential for understanding the physics of galaxy formation and for unraveling fundamental cosmological mysteries like the nature of dark matter.

This number is very important to pin down because our Galaxy is such a unique laboratory for studying the mysteries of the Universe. It is the only galaxy that we can observe from up-close, and thereby collect detailed information on the structure of a galaxy and the complicated physical processes that occur within it. This snapshot of a single galaxy can be generalized to overarching physical theories when combined with observations of distant galaxies and compared to theoretical predictions from numerical simulations. This generalization, however, depends on our ability to place the Milky Way in the context of the general galaxy population – and this requires a precise measurement of the mass of our Galaxy.

So, how is it possible to weigh a Galaxy from within? The authors of today’s paper measure the mass of the Milky Way based on the motions of orbiting satellite galaxies (Figure 2). Just like we can use the orbital speeds of the planets in our solar system, along with Newton’s law of gravitation, to infer the mass of the Sun, we can use measurements of the orbits of small satellite galaxies around the Milky Way to weigh our Galaxy. ...

The mass of the Milky Way from satellite dynamics ~ Thomas Callingham et al
This is absolutely hugely interesting! The conclusion of Thomas Callingham et al is that the mass of the Milky Way is about a trillion times the mass of the Sun.

A big question to me is how the mass of the Milky Way compares with the mass of the Andromeda galaxy, which will be very important when the two "mass centers" of the Local Group collide. Is Andromeda a lot more massive than the Milky Way? Or are these two galaxies at least relatively equal in size?
Perseus A in the Perseus cluster. Photo: R. Jay GaBany.
Arp 271, NGC 5426 and NGC 5427.
Photo: VIMOS of ESO’s Very Large Telescope.

























If the Milky Way is a flyweight galaxy compared with Andromeda's heavyweight, the collision between our two galaxies will likely scatter the Milky Way all over the place while leaving Andromeda moderately intact, perhaps somewhat similar to the collision between a smallish spiral and a giant elliptical that created the massive radio galaxy Perseus A. But if our two galaxies are relatively equal in mass, perhaps like the two components of Arp 271, NGC 5426 and NGC 5427, then the meeting of our two galaxies might be more like a meeting of equals. Of course, the results could be equally messy, if not more so.

According to Wikipedia, the mass of Andromeda may be about 150% of the mass of the Milky Way, if the mass of our own galaxy is one trillion solar masses:
Wikipedia wrote:

Mass estimates for the Andromeda Galaxy's halo (including dark matter) give a value of approximately 1.5×1012 M (or 1.5 trillion solar masses) compared to 8×1011 M for the Milky Way. This contradicts earlier measurements, that seem to indicate that Andromeda Galaxy and the Milky Way are almost equal in mass.
Well, as we say in Swedish, "Den som lever får se", loosely translated as "The one who survives will live to see what happens". I guess few of us will be around to see the merging of the Milky Way and M31.

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What You Get When You Throw a Dead Star Against Another

Post by bystander » Tue Sep 18, 2018 3:51 pm

What You Get When You Throw a Dead Star Against Another
Astrobites | 2018 Sep 14
Antonio Bernardo wrote:
To understand the origin of elements in the universe is a very challenging quest. There is a myriad of phenomena responsible for the creation and annihilation of nuclei. The lightest elements were forged in the first minutes of the universe and constitute 98% of the matter in the cosmos; the heaviest atoms depend on the curiosity and ingenuity of scientists to be made and exist for less than seconds. Here we will see something in the middle, the nucleosynthesis of elements like gold and ​lanthanides​. ...

Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event ~ Daniel Kasen et al
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The Curious Case of the Mysterious Over-Luminous Brown Dwarf

Post by bystander » Tue Sep 18, 2018 4:06 pm

The Curious Case of the Mysterious Over-Luminous Brown Dwarf
Astrobites | 2018 Sep 17
Jessica Roberts wrote:
Our story begins on a bright and not-so stormy brown dwarf. Now brown dwarfs are themselves mysterious objects, but CWW 89Ab is especially odd. But let’s start at the beginning; why are brown dwarfs mysterious? It is theorized that brown dwarfs form similarly to main-sequence stars; a gas cloud collapses, heats up, and eventually ignites. However, brown dwarfs do not have a high enough mass to start fusing hydrogen like a star and instead are left fusing deuterium. Over time, these objects cool off and start to dim and fade away. ...

The authors of today’s paper used Spitzer to observe CWW 89Ab passing behind its star, which is known as the secondary eclipse. The depth of the eclipse represents how much light from the brown dwarf is blocked, allowing the authors to determine its luminosity at 3.6 and 4.5 micron wavelengths. From these observations, the authors discovered that this brown dwarf is 16x brighter than predicted by evolutionary models! In other words, from the secondary eclipse depths, the authors found that the brown dwarf must have a brightness temperature of 1700K. However, brown dwarf evolution models suggest that this object should instead have an interior temperature of 850K. Figure 1 highlights just how extreme this luminosity difference is. ...

A Significant Over-Luminosity in the Transiting Brown Dwarf CWW 89Ab ~ Thomas G. Beatty et al
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The Tortoise and the Star

Post by bystander » Tue Sep 18, 2018 4:22 pm

The Tortoise and the Star
Astrobites | 2018 Sep 18
Emily Sandford wrote:
The Sun is whizzing around the Milky Way at about 500,000 miles per hour. A Galápagos tortoise, which is the opposite of the Sun, ambles along at about 0.2 miles per hour. One might reasonably conclude that the speed of a Galápagos tortoise is completely irrelevant compared to the speed of the Sun–if the Sun were to speed up or slow down by a tortoise-speed or two, what difference would it really make? ...

The problem is that tortoise-speed velocity shifts are not necessarily the result of orbiting planets. Stellar surfaces are bubbling, boiling messes—hot “granules” of plasma bubble up from the sweltering depths, then cool and sink back down again. Meanwhile, the bubbling in the upper layers is forceful enough that the entire surface reverberates, expanding and shrinking to the bubbling beat.

All of that bubbling and oscillating can wobble the star more than an orbiting planet does, and confuse us in our attempts to find other Earths. Today’s authors ask: How well do we actually understand the radial velocity of a stellar surface? In particular, how well do we understand the radial velocity signals coming from the center of a star, compared to those coming from the edge? ...

Stellar Surface Magneto-Convection as a Source of Astrophysical Noise II. Center-to-Limb
Parameterisation of Absorption Line Profiles and Comparison to Observations
~ H. M. Cegla et al Stellar Surface Magneto-Convection as a Source of Astrophysical Noise I.
Multi-component Parameterization of Absorption Line Profiles
~ H. M. Cegla et al
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One (solar system) catalogue to aid them all

Post by bystander » Sat Sep 22, 2018 4:14 pm

One (solar system) catalogue to aid them all
Astrobites | 2018 Sep 19
Amber Hornsby wrote:
As of today, we have confirmed the existence of 3,774 planets orbiting stars outside our the solar system. Even with the Kepler spacecraft running very low on fuel, new observatories such as TESS (Transiting Exoplanet Survey Satellite) will ensure the number of known exoplanets keeps rising. For many scientists, the next step is not to find more exoplanets but to characterise their physical properties. Are they icy like Enceladus? Or are they rocky like Mars? Do they have an atmosphere capable of supporting life?

The authors of today’s paper have collated the spectra (and albedo) of 19 bodies in our solar system. Using colours alone, they set out to answer an important question – can we distinguish between the three main types of planetary body we see in our solar system: rocky, icy and gaseous? ...

A Catalog of Spectra, Albedos, and Colors of Solar System Bodies for Exoplanet Comparison ~ J. H. Madden, Lisa Kaltenegger
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Studying the Earth’s Interior with (atmospheric) Neutrinos

Post by bystander » Sat Sep 22, 2018 4:25 pm

Studying the Earth’s Interior with (atmospheric) Neutrinos
Astrobites | 2018 Sep 20
Aaron Tohuvavohu wrote:
The IceCube Neutrino Observatory, an instrumented cubic kilometer of ice at the South Pole, occupies a unique position at the cross-section of particle physics, astrophysics, and glaciology. Today’s paper demonstrates how IceCube can extend its scope to geophysics and use neutrinos to probe the Earth’s interior using a technique known as neutrino tomography.

Neutrino Tomography of the Earth ~ Andrea Donini et al
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Directly measuring the mass of an imaged planet

Post by bystander » Sat Sep 22, 2018 4:42 pm

Directly measuring the mass of an imaged planet
Astrobites | 2018 Sep 21
wrote:
The mass of a directly imaged planet is a tricky quantity to measure. Normally all you have is the brightness of the planet in comparison to its host star. Since giant planets do not produce their own energy (e.g. through fusion like a star) they simply cool with time. Their initial brightness depends on their mass: the gravitational potential energy of all the material collapsing down to form the planet. Therefore, the brightness of the planet, combined with the age of the system and models of planet formation, can be used to back out the mass of the planet. A prominent question in the field of planet formation is the initial conditions for these models: how much of the gravitational potential energy goes into the planet (heating it up) and how much is dissipated before the planet forms? This uncertainty leads to a spectrum of different initial conditions ranging from “hot” to “cold” start models.

β Pictoris b is one of the first directly imaged planets. It is a gas giant (4-17 times the mass of Jupiter, though likely closer to 13) orbiting a nearby (19 pc) star in the young (~10-20 Myr) β Pictoris moving group. It has been well studied in the last decade since its discovery, including determining that its orbit is nearly edge on, though non-transiting. Figure 1 shows the first image of the planet and some more recent observations showing its orbital motion. Today’s paper presents the measurement of the mass of β Pic b using astrometry. Since the measurement is model independent, it can be used to calibrate the initial conditions of the models. ...

The Mass of the Young Planet Beta Pictoris b through the Astrometric Motion of Its Host Star ~ Ignas Snellen, Anthony Brown
viewtopic.php?t=38622
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