astrobites 2018

Find out the latest thinking about our universe.
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Blurred LinesBlurred Lines: Degeneracies in Modeling Exoplanet Atmospheres

Post by bystander » Sat May 26, 2018 3:39 pm

Blurred Lines: Degeneracies in Modeling Exoplanet Atmospheres
Astrobites | 2018 May 23
Vatsal Panwar wrote:
Transmission spectroscopy is the most commonly used method to characterize the atmospheres of transiting exoplanets that have been discovered to date. The method is seemingly straightforward: photons from the host star passing through the limb of its transiting exoplanet are absorbed or transmitted by the planetary atmosphere to a varying extent in different wavelengths, which should appear as a variation in observed radius of the exoplanet with respect to wavelength. This variation in transit radius with wavelength, known as transmission spectrum, can then be interpreted by adopting either a forward or inverse modeling approach. This means that you can either start with a set of assumptions for the atmospheric properties (like chemical abundances, cloud properties etc.) and construct a physical model that can be fit to the data, or try solving the inverse problem (also known as retrieval) and infer the atmospheric properties from the measured transmission spectra itself.

However, there is a good deal of subtlety behind both approaches of constructing theoretical models for transiting exoplanet atmospheres. Today’s paper traces the assumptions and caveats involved in constructing models for observations taken by the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. In addition to deriving a validated semi-analytical approach from first principles for calculating the transmission spectra in the context of WFC3 observations, authors of today’s paper uncover a crucial and unresolved degeneracy involved in fitting a model to the transmission spectra. ...

The theory of transmission spectra revisited: a semi-analytical method for
interpreting WFC3 data and an unresolved challenge
- Kevin Heng, Daniel Kitzmann
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Gaia and the 14000 (White) Dwarfs

Post by bystander » Sat May 26, 2018 3:50 pm

Gaia and the 14000 (White) Dwarfs
Astrobites | 2018 May 24
Amber Hornsby wrote:
Unless you’ve been avoiding the internet for fear of Avengers spoilers, you may have noticed that everyone’s favourite star-tracker, Gaia, has recently released its second catalogue containing the precise location of around 1.7 billion stars in our own galaxy and beyond.

Until now, we only had access to around 250 white dwarf stars in our local galaxy, making it difficult to look for common properties and understand the population as a whole. But, thanks to Gaia, today’s authors had a sample of almost 14,000 white dwarf stars to play with – here’s what they discovered. ...

Gaia Reveals Evidence for Merged White Dwarfs - Mukremin Kilic et al
viewtopic.php?t=38230
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LGBT+ Inclusivity in Astronomy

Post by bystander » Sat May 26, 2018 4:04 pm

LGBT+ Inclusivity in Astronomy
Astrobites | 2018 May 25
Mia de los Reyes wrote:
“Science advances fastest when scientists are free to apply their intelligence and imagination to the exploration of the universe without limits and without fear.” Yet scientists who identify as LGBT+ face a number of structural and personal barriers.

What can be done to help mitigate these barriers, and to push for true equity and inclusion in astronomy? This best practices guide, a joint publication by LGBT+ Physicists and the American Astronomical Society (AAS) Committee for Sexual and Gender Minorities in Astronomy, lists plenty of ways that we can start.

In this Astrobite I’ll highlight some parts of the guide that I thought were relevant for undergraduate and graduate students. It’s worth noting that this Astrobite is really just a cursory summary, and I strongly recommend you check out the full guide (it’s a quick read!), or at least take a look at the one-page executive summary (page vii of the guide, which is also the source of the excellent quote at the start of this post). ...

LGBT+ Inclusivity in Physics and Astronomy: A Best Practices Guide, 2nd Edition - Nicole Ackerman et al
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Things that go bump in the detector

Post by bystander » Wed Jun 06, 2018 4:46 pm

Things that go bump in the detector:
Dealing with glitches in LIGO data

Astrobites | 2018 May 28
Lisa Drummond wrote:
On August 17, 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made a historic detection of a gravitational wave signal from a merging neutron star binary. This detection was interesting for many reasons, not least of all because it was the first multi-messenger detection of both gravitational waves and electromagnetic radiation from the same event.

Because gravitational wave detectors are so complicated and sensitive, they are vulnerable to transient, short-duration instrumental or environmental noise, called glitches. An interesting feature of GW170817 was that it occurred during a glitch in one of the detectors (Livingston), causing the automated system to veto the data from that detector and preventing the neutron star merger signal from being distributed immediately. Fortunately, the signal was long and the glitch was short, meaning that the glitch could easily be removed from the data. But we may not be so lucky next time! For example, if the signal itself is not well understood or even completely unknown, we need to know how to deal with detector glitches. ...

Parameter Estimation and Model Selection of Gravitational Wave Signals
Contaminated by Transient Detector Noise Glitches
- Jade Powell
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Moonlets: The Moon’s Assembly Line

Post by bystander » Wed Jun 06, 2018 5:02 pm

Moonlets: The Moon’s Assembly Line
Astrobites | 2018 May 29
Michael Hammer wrote:
“What is it you want, Earth? What do you want? You want the Moon? Hey, that’s a pretty good idea! I’ll give you the Moon!” the solar system once said to the Earth not too long after our planet formed.

The Giant Impact Hypothesis postulates that our Moon arose from a catastrophic collision between the primordial Earth and a Mars-sized planet known as Theia with about a tenth of our planet’s mass. This idea was first proposed in 1946 and has gained significant traction due to the similarity of the Moon’s composition with that of the Earth, among other reasons. Specifically, several isotope ratios on the Moon — including oxygen — are identical to those on the Earth to within 0.01%. Unfortunately, recent simulations have shown that giant impacts rarely produce two objects that are so identical because the Moon often forms mostly out of material from the impactor, not the Earth. The similarity between the Earth and the Moon that helped inspire the Giant Impact Hypothesis has now become one of the most enigmatic mysteries to unravel.

Raluca Rufu, Oded Aharonson, and Hagai Perets — the authors of today’s paper — attempt to solve this problem by tweaking the original Giant Impact Hypothesis. Instead of forming the Moon from one giant impact, they suggest it formed from a series of medium-sized impacts spread out over millions of years. ...

A multiple-impact origin for the Moon - Raluca Rufu, Oded Aharonson, Hagai Perets
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Dust n’ H2

Post by bystander » Wed Jun 06, 2018 5:36 pm

Dust n’ H2
Astrobites | 2018 May 30
Jamila Pegues wrote:
The simplest known element in the universe, hydrogen, is also the most important. Hydrogen (aka “H”) plays a crucial role for many of the awesome astrophysical phenomena that have happened (and will happen!) across the universe’s history. This element fuses in the cores of stars, lights up ionized regions in supernova remnants, and serves as a building block for other elements in the periodic table – just to name a few of its many talents.

But that’s not all this epic element can do! Its molecular form, H2, is the primary ingredient for molecular clouds, like the one shown in Figure 1. These molecular clouds are made up of both gas and dust grains. They are often found within spiral galaxies (like our own Milky Way), and they’re interesting in part because they’re the only known sites where glorious star formation occurs.

To better understand molecular clouds and their glorious star formation, scientists have long studied the timescales for these clouds to form within the less dense, diffuse interstellar medium (a collective term for all of the matter and radiation between star systems) of space.

Classic calculations of these timescales have often assumed that the diffuse interstellar medium (aka, ISM) is pretty homogeneous, even when we zoom in. In other words, they assumed that the gas and dust in the diffuse ISM are spread out uniformly, so that every bit of the diffuse ISM looks the same as every other bit. But observations from the last 50 years or so indicate that the diffuse ISM is actually very not homogeneous, with lumps and bumps even across small (relatively speaking) Solar System-sized regions.

There have been numerous studies on what could be causing this non-uniformity at such small scales. But today’s authors look at a possible mechanism that is particularly significant for H2: the formation of tightly-packed dusty clouds and equilibrium gaseous clumps in the diffuse ISM.

Compact Dusty Clouds and Efficient H2 Formation in Diffuse ISM - A.V. Ivlev et al
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An EPIC view of the Earth as an exoplanet

Post by bystander » Wed Jun 06, 2018 5:47 pm

An EPIC view of the Earth as an exoplanet
Astrobites | 2018 May 31
Joanna Ramasawmy wrote:
The search for habitable, Earth-like planets is high on the agenda of exoplanetary scientists. However, habitability is far more complicated than checking that a planet sits within the goldilocks zone of not-too-hot, not-too-cold temperatures, just right for liquid water to exist on its surface. Dozens of other factors, such as the planet’s atmosphere, seasons and surface geography, are of critical importance to sustaining a watery planet and creating somewhere suitable for life.

Many previous astrobites have covered studies investigating exoplanetary atmospheres via transmission spectroscopy, but today’s paper takes a slightly different angle by considering the possibilities of direct imaging. Using our own habitable Earth as a “proxy exoplanet”, the authors inspect multi-wavelength observations from the Deep Space Climate Observatory, or DSCOVR, to figure out what we should be looking for further from home.

DSCOVR is largely used for space weather monitoring, but its Earth-facing instrument EPIC, the Earth Polychromatic Imaging Camera, takes pictures of the sunny side of our planet from Lagrange 1 in ten wavelengths, from the ultraviolet to near-infrared (if it’s not already in your twitter feed, check out the DSCOVR:EPIC bot for a daily dose of pale blue dot). ...

Using Deep Space Climate Observatory Measurements to Study the Earth as An Exoplanet - Jonathan H. Jiang et al
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Extragalactic Magnetic Fields: Uncovering Their Origin Story

Post by bystander » Wed Jun 06, 2018 5:59 pm

Extragalactic Magnetic Fields: Uncovering Their Origin Story
Astrobites | 2018 Jun 01
Joshua Kerrigan wrote:
What do Fast Radio Bursts (FRBs), polarization, and extragalactic magnetic fields on massive scales have in relation to each other? Well to cut to the chase, by combining the polarization of FRB signals we can potentially determine the origin of extragalactic magnetic fields. This could be made possible – in simulation as of now – by some special characteristics of FRBs that wouldn’t necessarily be offered by more steady state radio sources. So welcome to today’s astrobite, where we’ll take some time to learn about how we can uncover the origin story of extragalactic magnetic fields. ...

Probing the Origin of Extragalactic Magnetic Fields with Fast Radio Bursts - F. Vazza et al
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The Biggest, Baddest Quasar of Them All

Post by bystander » Sun Jun 17, 2018 5:47 pm

The Biggest, Baddest Quasar of Them All
Astrobites | 2018 Jun 11
Lauren Sgro wrote:
If you’re reading this, chances are you have heard of black holes. These mysterious objects have long captured the interest of the public and scientists alike. Even The Simpsons have tackled this topic.

A supermassive black hole gives life to the subject of today’s paper, a quasar known as SMSS J215728.21-360215.1 (J2157-3602 for short), the brightest quasar yet discovered. Quasars are a type of Active Galactic Nuclei (AGN), which sit at the center of high redshift galaxies, meaning they have only been found at distances corresponding to the early universe. Their massive accretion disks allow them to outshine their entire host galaxy, making these astronomical bodies among the brightest objects in the sky.

So what’s with the name? ‘Quasar’ originated from the phrase ‘quasi-stellar radio source.’ These objects were identified as quasi-stellar because they appear to be a point-source, like a star. Many of the first quasars discovered emitted very strongly in radio wavelengths, but since then, it has been determined that only a fraction of quasars are ‘radio loud.’ So now, ‘quasar’ is simply in reference to ‘quasi-stellar objects,’ or ‘QSOs,’ as in the title of today’s paper. ...

Discovery of the Most Ultra-Luminous QSO Using Gaia, SkyMapper and WISE - Christian Wolf et al
viewtopic.php?t=38293
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Testing micro with macro – from quantum to the cosmos

Post by bystander » Sun Jun 17, 2018 5:54 pm

Testing micro with macro – from quantum to the cosmos
Astrobites | 2018 Jun 12
Philippa Cole wrote:
Quantum mechanics governs what goes on at mind-bogglingly small scales. So far it’s provided a really good description of microscopic systems that we’ve been able to test on earth, but today’s authors muse that it’s not really had any competition – “all theories benefit from having an alternative to serve as a foil”.

Since quantum laws should work on all scales, why not zoom all the way out and use the increasingly precise measurements we have on cosmological scales to test our quantum framework? In order to do this we need something to describe the relationship between the unfathomably large and the unimaginably small, and luckily, the leading theory of the early universe connects the two – that theory is inflation. ...

Precision test of quantum mechanics - our Universe - Julian Georg, Carl Rosenzweig
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Supernova Archeology with Radioactive Eyes

Post by bystander » Sun Jun 17, 2018 6:09 pm

Supernova Archeology with Radioactive Eyes
Astrobites | 2018 Jun 13
Maria Arias wrote:
Massive stars die as core collapse supernovae: the star can no longer produce the nuclear reactions that balance its strong gravity, and the star collapses onto its core. When this happens, large amounts of energy and neutrons are available to form elements heavier than iron. The distribution of elements produced in the deepest layers of the star as it goes supernova is key to understanding the mechanism by which the collapse of the star leads to an explosion.Radioactive decay powers the optical light emitted by the supernova ~ 50−100 days after the explosion. In fact, we can still see radioactive signatures in remnants that are hundreds of years old. In today’s paper, the authors use high energy X-ray satellite NuStar observations to study the distribution of 44Ti in the young supernova remnant Cassiopeia A (Cas A). The current distribution of radioactive elements and their decay products is linked to the local conditions in which they were synthesised when the explosion took place. Therefore, knowing where the 44Ti is now can shed light on the details of the supernova event that ended the life of Cas A’s progenitor star. ...

The distribution of radioactive 44Ti in Cassiopeia A - Brian W. Grefenstette et al
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Deflating a Planet: Helium Loss in the Atmosphere of Wasp-107b

Post by bystander » Sun Jun 17, 2018 6:26 pm

Deflating a Planet: Helium Loss in the Atmosphere of Wasp-107b
Astrobites | 2018 Jun 14
Jessica Roberts wrote:
Hydrogen and helium are the two most abundant elements in our Solar System (and the Universe as a whole). They are the main constituents in our Sun and in the atmospheres of our gas giants. Even Earth has some minor amount of helium in its upper atmosphere. Because these elements are so common, we also expect gas giant exoplanets to have large abundances of hydrogen and helium in their atmospheres. In fact, in 2000, Seager and Sasselov predicted that we should be able to observe helium and other atoms in the atmospheres of these planets in the near-future.

While we have directly detected hydrogen in the atmospheres of a handful of exoplanets, helium continues to remain elusive. This isn’t to say that these exoplanets aren’t composed of helium, helium is simply difficult to detect. As a noble gas, helium is hard to excite to different atomic energy levels, which is required for emission or absorption to occur. Even then, helium has only a few spectral lines we can observe. Furthermore, if the gas is diffuse (low-density), the signal might not be strong enough for us to observe, even if absorption/emission is occuring. Because of these difficulties we require two conditions for a helium detection:
  • A planet must receive a large amount of energetic ultraviolet (UV) photons from its star in order to populate the excited levels in a helium atom.
  • A planet must also have a large quantity of helium high up in its atmosphere where it can both interact with these photons, and be in a region of the atmosphere that is transparent. An opaque atmosphere would block any helium signal from reaching us.
Multiple exoplanets meet the above criteria, and yet, no helium has yet been detected — until Wasp-107b came along. Eighteen years after helium was first predicted in the atmospheres of exoplanets, the authors of today’s paper were finally able to make the first helium detection in the atmosphere of an exoplanet! ...

Helium in the Eroding Atmosphere of an Exoplanet - J. J. Spake et al
viewtopic.php?t=38257
viewtopic.php?t=37910#p282419
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Basaltic asteroids might not all be from Vesta

Post by bystander » Sun Jun 17, 2018 6:42 pm

Basaltic asteroids might not all be from Vesta
Astrobites | 2018 Jun 15
Peter Sinclair wrote:
The Asteroid Belt contains many objects left over from the formation of the Solar System. Studying these objects can yield insight into the conditions under which the planets formed.

There are several different kinds of asteroids in our Solar System, categorized by composition. Astronomers use the asteroid’s reflectance spectrum to determine its spectral type. C-type asteroids have large quantities of carbon and compose the majority of the asteroids in the belt. These include the carbonaceous chondrites. The second most common are S-type asteroids, rich in silicate minerals. Third place belongs to the M-type asteroids, made of metals like nickel and iron. Finally, about 6% of asteroids in the belt are V-type asteroids, or “vestoids”. These asteroids have a basalt-rich composition very similar to the dwarf planet Vesta. ...

Vesta is thought to be the source of most of these V-type asteroids. Not only do these asteroids share a similar composition to Vesta, they also have similar orbits. Moreover, images from NASA’s Dawn spacecraft revealed a massive crater 500 km wide and 19 km deep. This impact, and another similar one underneath it, would have torn millions of cubic meters of material off Vesta, more than enough to account for the V-type asteroids observed today. ...

It is unlikely, however, that Vesta is the sole source of all V-type asteroids. In particular, there is a group of asteroids beyond the 3:1 orbital resonance with Jupiter. It is extremely unlikely that an asteroid originating from Vesta would be able to break through this resonance to reach a mid-belt orbit. Some of these asteroids also exhibit sizes that seem too large to have come from Vesta’s impact basins. Moreover, it is unlikely that Vesta is the only planetesimal to undergo complete differentiation. Therefore these V-type asteroids probably have a different origin than the vestoids. Since there is no clear parent for these V-type asteroids, either they were completely broken up or our understanding of the Solar System’s formation is wrong. ...

Basaltic material in the main belt: a tale of two (or more) parent bodies? - S. Ieva et al
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Re: The Biggest, Baddest Quasar of Them All

Post by BDanielMayfield » Mon Jun 18, 2018 12:06 pm

bystander wrote:
Sun Jun 17, 2018 5:47 pm
The Biggest, Baddest Quasar of Them All
Astrobites | 2018 Jun 11
Lauren Sgro wrote:
If you’re reading this, chances are you have heard of black holes. These mysterious objects have long captured the interest of the public and scientists alike. Even The Simpsons have tackled this topic.

A supermassive black hole gives life to the subject of today’s paper, a quasar known as SMSS J215728.21-360215.1 (J2157-3602 for short), the brightest quasar yet discovered. Quasars are a type of Active Galactic Nuclei (AGN), which sit at the center of high redshift galaxies, meaning they have only been found at distances corresponding to the early universe. Their massive accretion disks allow them to outshine their entire host galaxy, making these astronomical bodies among the brightest objects in the sky.

So what’s with the name? ‘Quasar’ originated from the phrase ‘quasi-stellar radio source.’ These objects were identified as quasi-stellar because they appear to be a point-source, like a star. Many of the first quasars discovered emitted very strongly in radio wavelengths, but since then, it has been determined that only a fraction of quasars are ‘radio loud.’ So now, ‘quasar’ is simply in reference to ‘quasi-stellar objects,’ or ‘QSOs,’ as in the title of today’s paper. ...

Discovery of the Most Ultra-Luminous QSO Using Gaia, SkyMapper and WISE - Christian Wolf et al
viewtopic.php?t=38293
From the last paragraph of the astrobite article:
The first author Christian Wolf has stated in interviews that J2157-3602’s black hole is growing faster than any other black hole discovered to date, feasting on a steady diet of two Suns worth of material per day. This means that if it was sitting at the center of our Milky Way instead of our milder Saggitarius A *, it would outshine the full moon 10 times over. With Gaia’s second data release, scientists will hopefully be able to spot more curious quasars and discover how they grew so fast in so little time.
The article also stated that its intrinsic brightness was nearly 7 x 1014 times the brightness of our sun!

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The Planets in the Gaps

Post by bystander » Wed Jun 20, 2018 4:48 pm

The Planets in the Gaps
Astrobites | 2018 Jun 18
Emily Sandford wrote:
Planets form. (We know this, occupying, as we do, a planet.) And planets form out of the disks of gas and dust that surround young stars. (We know this because we see these disks around young stars, and we cannot explain where the stuff of planets comes from otherwise.) And planets form in these disks quite quickly. (We know this because the disks only last a few million years–a blink of an eye, astronomically speaking.) And planets form in these disks easily. (We know this because planets are everywhere! On average, there’s at least one planet per star.)

Planet formation, then: it’s quick, easy, commonplace, and completely mysterious. How does a sphere the size of Jupiter coalesce from a bunch of grains of dust swimming in hydrogen gas? Or a snowball like Pluto (planet, dwarf planet, don’t @ me), for that matter, or a rock like Earth? ...

Enter today’s paper. Even if we can’t see the planets directly, we can use ALMA to study the motions of the gas nearby. An orbiting planet should gather up all the gas along its orbit, opening up a gap, i.e. a narrow band of low-pressure gas in the disk. If you look at the gas pressure as you move outward along the disk (see figure 3, top left panel), you should see it dip as you reach the planet’s orbit, then rise again past the planet. ...

A Kinematical Detection of Two Embedded Jupiter Mass Planets in HD 163296 - Richard Teague et al Kinematic Evidence for an Embedded Protoplanet in a Circumstellar Disc - C. Pinte et al
viewtopic.php?t=38370
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A Paleo-Detector for Dark Matter

Post by bystander » Wed Jun 20, 2018 4:56 pm

A Paleo-Detector for Dark Matter:
How Ancient Rocks Could Help Unravel the Mystery

Astrobites | 2018 Jun 19
Thankful Cromartie wrote:
Dark matter is, by its very nature, elusive. Though we can detect its presence by observing its gravitational influence, dark matter remains invisible because it doesn’t interact electromagnetically. The most widely accepted explanation for dark matter is the existence of weakly interacting massive particles (WIMPs). WIMPs, if eventually observed, would constitute a new, massive kind of elementary particle. Their discovery would be revolutionary for particle physics and cosmology; therefore, countless direct (in labs) and indirect (observing their annihilation or decay) detection experiments are being conducted to identify them. Today’s astrobite discusses a novel proposal for direct dark matter detection that seems more fit for scientists in Jurassic Park than for particle physicists: the paleo-detector.

The authors of today’s featured paper theorize that ancient rocks could contain evidence of interactions between WIMPS and nuclei in the minerals, forming a completely natural “detector” that would allow scientists to search for evidence of the massive particles using excavated rocks. This experiment varies significantly from other direct detection efforts, as those look for evidence of WIMPs hitting Earth-based detectors in real time. The paleo-detector would instead trace nanometers-long “tracks” of chemical and physical change in the rocks as the result of WIMP-induced nuclear recoil that occurred long ago. ...

Searching for Dark Matter with Paleo-Detectors - Sebastian Baum et al
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Dating the Evaporation of Globular Clusters

Post by bystander » Wed Jun 20, 2018 5:08 pm

Dating the Evaporation of Globular Clusters
Astrobites | 2018 Jun 20
Kerrin Hensley wrote:
The most ancient stellar populations in our galaxy are being ripped apart. Globular clusters — massive gravitationally bound collections of hundreds of thousands of stars — have occupied the Milky Way halo for billions of years. Studying globular clusters can help us understand not only how our galaxy formed, but also how it has evolved over the history of the universe. As the Milky Way has evolved, its gravitational potential has changed as well — and the changes in our galaxy’s gravitational pull are recorded in the behavior of globular clusters.

As the stars in globular clusters interact gravitationally, some gain enough kinetic energy to be ejected from the cluster entirely. The shrinking of globular clusters through this process is called evaporation. When the ejected stars escape the gravitational confines of the cluster, the gravitational pull of the Milky Way starts to take over. As the cluster orbits the galactic center, it experiences tidal forces. Much like an unwitting spacefarer approaching a black hole, globular clusters get stretched out by these tidal forces, stringing those escaped stars into a tidal tail or stream (see Figure 1).

We see these tidal tails in many globular clusters, but the question remains: How can we figure out when the tidal disruption began?

Today’s paper introduces a new technique to estimate the age of tidally disrupted globular cluster streams. While the ages of the globular clusters themselves are usually determined using stellar evolution models, it can be challenging to figure out when the gravitational pull of the Milky Way began to tear them apart. ...

Dating the Tidal Disruption of Globular Clusters with GAIA Data on Their Stellar Streams - Sownak Bose, Idan Ginsburg, Abraham Loeb
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Clearing Up Stellar Streams with Gaia

Post by bystander » Sat Jun 23, 2018 4:34 pm

Clearing Up Stellar Streams with Gaia
Astrobites | 2018 Jun 21
Nora Shipp wrote:
The Gaia space telescope is revolutionizing our understanding of the Milky Way. This European satellite (Figure 1) is carefully tracking the positions of over a billion stars over five years, providing us with an evolving map of stellar locations and velocities. Just a couple months ago the second Gaia data catalog was released, including brand new information about the motions of many times more stars than in previous datasets to accuracies never before achieved, launching a scramble to see what exciting surprises this new data would reveal about our Galaxy. (For more examples of exciting Gaia science see these Astrobites.)

This new data is a powerful tool for studying the stellar halo of our galaxy – the outer region of the Milky Way system that consists of a spherical cloud of stars extending beyond the central spiral, into the outer reaches of the Milky Way’s dark matter halo. This halo of stars, and all of the systems it contains, are powerful probes of the local dark matter distribution and contain clues as to the history of the formation and evolution of our Galaxy. Today’s paper considers one particular type of system in the stellar halo – stellar streams. These groups of stars, stretched into arcs across the sky, are the remnants of small galaxies and clusters of stars that have been torn apart as they orbit around the Milky Way. Previous Astrobites have considered how stellar streams are essential tools for studying galaxy formation and the nature of dark matter, and today’s paper illustrates how powerful these streams can be with the help of Gaia. ...

Off the beaten path: Gaia reveals GD-1 stars outside of the main stream - Adrian M. Price-Whelan, Ana Bonaca
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A White Dwarf Moving With The Flow

Post by bystander » Thu Jun 28, 2018 2:15 pm

A White Dwarf Moving With The Flow
Astrobites | 2018 Jun 26
Matthew Green wrote:
In today’s astrobite, we’re talking about a paper on the AB Doradus co-moving group of stars. The paper shows that a white dwarf named GD 50 is part of the AB Dor group; we’ll be talking about how the authors arrived at this result, and what this can tell us about the history and stellar evolution of the group. ...

A Young Ultramassive White Dwarf in the AB Doradus Moving Group - Jonathan Gagné et al
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Yoga for Dark Matter

Post by bystander » Thu Jun 28, 2018 2:45 pm

Yoga for dark matter: Making the Cold Dark Matter model more flexible
Astrobites | 2018 Jun 28
Tanvi Karwal wrote:
The ΛCDM (Λ-Cold Dark Matter) model is the current standard model of cosmology and consists of normal matter, photons, neutrinos, cold dark matter and dark energy occupying a flat universe. The aptly-named Dark Sector can be described very simply: dark energy is a Cosmological Constant (Λ) and dark matter is cold (i.e. its particles move slowly compared to the speed of light). Each component of ΛCDM can be quantified by a single parameter. Dark energy drives cosmic acceleration, while dark matter clusters and seeds structure formation.

The simple ΛCDM model has done exceptionally well in describing the large-scale structure of the universe: it accurately predicts features in phenomena such as baryon acoustic oscillations, the matter power spectrum and the cosmic microwave background (CMB). Even in the local universe, measurements of dark energy do not deviate significantly from Λ and CDM has had successes explaining galaxy clusters.

Thankfully, ΛCDM might not be the whole picture (or I would be out of a job!). When a ΛCDM model is fit to the CMB, the model under-predicts the current rate of expansion of the universe (the infamous Hubble tension). Physically motivating a cosmological-constant-like dark energy has left physicists bending over backwards (even when they’re not doing yoga). Dark matter has not yet been directly detected and models of dark matter remain limited to candidates that are considered to be well-motivated theoretically. Hence, the Dark Sector remains, well, dark – despite some recent progress, our understanding is woefully incomplete.

If ΛCDM is not the whole picture, how can we construct a more complete model? One way to assemble the pieces of this cosmological puzzle is to employ a phenomenological approach, rather than deriving the model from first principles. A phenomenological model describes relationships between variables based on experiment and observations, instead of being derived from fundamental theory. ...

Structure Formation with Generalized Dark Matter - Wayne Hu
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Deciphering gamma-rays from the heart of Andromeda

Post by bystander » Thu Jul 05, 2018 4:44 pm

Deciphering gamma-rays from the heart of Andromeda
Astrobites | 2018 Jun 29
Nora Shipp wrote:
Our galaxy is full of very high energy photons known as gamma-rays. They come from pulsars, and cosmic rays; they even reach us from supermassive black holes and energetic explosions in distant galaxies. Some of the most intriguing gamma-rays, however, are not the ones that traveled billions of years to reach us, but the mysterious leftover gamma-rays, once all of these other sources are accounted for, coming from the center of our own galaxy.

The origin of these gamma-rays has been hotly debated for years, but one of the most exciting possibilities is that they come from the annihilation of dark matter particles. If that is the case, then this would be a huge discovery – the first detection of dark matter! However, right now it is impossible to claim this discovery. The centers of galaxies are complex regions full of messy physics and it is very difficult to rule out less exotic origins.

A recent update may help to clear up this mystery – a similar signal was detected coming from the center of our sister galaxy, Andromeda. Today’s paper studies this newly discovered signal and the signal from our own galaxy to determine whether they could be produced by the annihilation of dark matter. ...

A Multi-Wavelength Analysis of Annihilating Dark Matter as the Origin of the
Gamma-Ray Emission from M31
- Alex McDaniel, Tesla Jeltema, Stefano Profumo
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Have we been hit by a supernova?

Post by bystander » Thu Jul 05, 2018 4:55 pm

Have we been hit by a supernova?
Astrobites | 2018 Jul 02
Matthew Green wrote:
When we think of planetary systems and how they evolve, we generally imagine this evolution to be driven by factors within the system itself. We think about what the star is like, how much dust was around, or how the planets interact with each other. Factors outside the solar system — the local neighbourhood of the star — get a lot less press, but that doesn’t mean that they aren’t important. A couple of previous astrobites discussed a paper on the impact that nearby supernovae and gamma ray bursts can have on the habitability of a planetary system. Today, we’re talking about a paper on a similar topic: how would a nearby supernova impact the evolution of the planets themselves? Is it possible that our solar system, in its distant past, was hit by the shockwave of a supernova? ...

Today’s paper aims to model what effect a nearby supernova might have on a protoplanetary disk around a young star. To do this, they run hydrodynamic simulations of a disk of material as a nearby supernova interacts with it. They model two different forms of interaction: the radiation that comes from the supernova, and the shockwave of material that sweeps through the protoplanetary disk. For the radiation, they use two theoretical supernova lightcurves, representative of the variety of supernova lightcurves that exist. They simulate the results when the supernovae are at a range of distances from the disk, and a range of inclinations with respect to the disk (i.e., whether the shockwave will hit the disk face-on or edge-on — see Figure 1). ...

The consequences of a nearby supernova on the early Solar System - Simon Portegies Zwart et al
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A very close planet that is way out of line

Post by bystander » Thu Jul 05, 2018 5:08 pm

A very close planet that is way out of line
Astrobites | 2018 Jul 03
Emma Foxell wrote:
Our solar system is flat: all the planets orbit in the same plane as the Sun’s equator, at 90 degrees to its pole, giving inclination values close to 90°. While exoplanets orbiting at different inclinations can exist, transiting exoplanets nearly always have inclination values very close to 90°, allowing us to see them passing in front of their star.

However, the authors of today’s paper have discovered a transiting exoplanet with an inclination of 76.5°. This is possible because EPIC 248435473 b is an ultra short period planet (USP), with a period of just 0.66 days, meaning from our viewpoint the planet still transits in front of its star over a wider range of inclination angles. What makes this discovery more interesting is that the authors have found up to 5 other transiting planets in this system, all aligned with inclinations 88-90°, making EPIC248435473 b the odd one out. ...

A Compact Multi-Planet System With a Significantly Misaligned Ultra Short Period Planet - Joseph E. Rodriguez et al
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Reionizing the universe, with a quasar assist

Post by bystander » Thu Jul 05, 2018 5:18 pm

Reionizing the universe, with a quasar assist
Astrobites | 2018 Jul 04
Joanna Ramasawmy wrote:
First, a brief history of the early universe: in the beginning, there were photons, protons, neutrons and electrons (ok, not the very beginning — about 10 seconds after the Big Bang. A lot of stuff happened before that, but let’s start from there). Matter was all in a hot plasma, and stayed that way for almost 380,000 years. Finally everything cooled down enough for electrons to combine with protons to form atoms. The universe became neutral, and the dark ages began: the only photons were those left over from recombination, which form the cosmic microwave background (CMB), and for hundreds of millions of years there were no other sources of light. Once the first stars and galaxies form, they produce ultraviolet photons with energies high enough to ionize the neutral matter that the universe is composed of. This marks the epoch of reionization. ...

The authors of today’s paper investigate the role of quasars in the reionization of the universe. Instead of considering the UV photons generated by quasars themselves (quasars have been ruled out as a significant source of reionizing photons by several other authors), they look at the indirect effect that they may have. Winds and outflows produced by these supermassive-black-hole-powered engines may blow out intervening gas clouds, allowing more stellar ionizing photons to be released into the universe. ...

The Indirect Influence of Quasars on Reionization - Jacob Seiler et al
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Small black hole meets big black hole

Post by bystander » Thu Jul 05, 2018 5:28 pm

Small black hole meets big black hole
Astrobites | 2018 Jul 05
Lisa Drummond wrote:
In recent years, we have seen huge breakthroughs in the field of gravitational wave physics. Last year alone, we saw the first multi-messenger detection of a binary neutron star merger and the Nobel Prize in Physics was awarded for the discovery of gravitational waves! So far, gravitational wave discoveries have been made using ground-based observatories LIGO (in the U.S.) and VIRGO (in Europe). In the future, the space-based observatory LISA will be launched. Extreme-mass-ratio inspirals (EMRI) are black hole binaries where one of the black holes is significantly smaller than the other (by at least a factor of 10 000) – these are promising candidates for detection by LISA. If we can detect EMRIs, we will be able to map the spacetime geometry in the neighbourhood of the central (heavier) black hole as well as measure its spin and mass extremely accurately. There is one catch – in order to achieve this, we need very precise theoretical models of these systems. In this paper, the authors draw attention to the presence of transient resonances in extreme-mass-ratio systems: these effects need to be considered when building models for gravitational wave signals from EMRIs. ...

Transient Resonances in the Inspirals of Point Particles into Black Holes - Eanna E. Flanagan, Tanja Hinderer
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