astrobites: Daily Paper Summaries 2019

Find out the latest thinking about our universe.
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astrobites: Daily Paper Summaries 2019

Post by bystander » Fri Jan 04, 2019 5:33 pm

astrobites: Daily Paper Summaries

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy since 2010. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

<< astrobites 2018
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Growing once, growing twice … or three times?

Post by bystander » Fri Jan 04, 2019 5:40 pm

Growing once, growing twice … or three times?
astrobites | Daily Paper Summaries | 2019 Jan 02
Philippa Cole wrote:
We’re pretty sure that the Universe has undergone two phases of accelerated expansion. Firstly, during a period known as inflation immediately after the Big Bang, and secondly – right now! It was realised in the 90s that the Universe is expanding today, and that it is doing so at an increasing rate. Measuring exactly how fast it’s expanding has opened up one of the biggest disagreements in cosmology today: the Hubble tension*. The Hubble constant, which is the quantity that tells us how fast the Universe is expanding today, has been measured in two different ways… with two different results.

While some would argue that the answers aren’t different enough to warrant worrying about too much (yet), many people are starting to believe that there is a real problem brewing. It could be that experimental uncertainties haven’t been accounted for fully, or, it could be that we’re missing something in our current model of cosmology and that we need new physics to explain it. Today’s authors offer an explanation for the discrepancy by suggesting the existence of a third phase of accelerated expansion in the early Universe. Messing around with how the Universe expanded at early times is a fragile task, but they’ve found a model that seems to do the trick. ...

Early Dark Energy Can Resolve The Hubble Tension ~ Vivian Poulin et al
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Where did the eMSTO go?

Post by bystander » Fri Jan 04, 2019 5:51 pm

Where did the eMSTO go?
astrobites | Daily Paper Summaries | 2019 Jan 03
Sanjana Curtis wrote:
In a previous astrobite, I wrote about the curious phenomenon of extended main sequence turn-off (eMSTO) seen in young and intermediate age stellar clusters. The eMSTO is a feature in the color-magnitude diagram (CMD) of these clusters where the main sequence turn-off looks unusual – far more spread out than it should be for a normal set of stars with the same age. One possible reason for this phenomenon is stellar rotation and the findings I discussed last time supported that explanation.

In today’s bite, we will follow the thread even further. The main idea is this: if rotation causes the eMSTO feature, then removing it should make the feature disappear. That is why the authors of today’s paper investigate magnetic braking, which slows down rotating stars. Using many sets of rotating stellar models, they predict the cluster age when we should no longer see eMSTOs. If rotation is indeed the correct explanation, their predictions should agree with observations. Read on to find out whether or not they do! ...

Disappearance of the extended main sequence turn-off in intermediate
age clusters as a consequence of magnetic braking
~ C. Georgy et al
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Monsters in Miniature

Post by bystander » Tue Jan 15, 2019 4:29 pm

Monsters in Miniature
astrobites | Daily Paper Summaries | 2019 Jan 14
Joanna Ramasawmy wrote:
Cosmological simulations using our best model of the Universe, ΛCDM, do a pretty good job of reproducing large-scale structure. But when you look closely, on smaller scales, things start to get complicated. Dwarf galaxies, the building blocks of larger structure, present a number of problems. Such significant problems in fact, that many of them have names (and are listed on Wikipedia’s unsolved problems in physics page). The “missing satellite problem” describes the fact that simulations predict many more dwarfs to exist around massive galaxies like the Milky Way than are actually observed. A proposed solution to this is that, while many dwarf satellites exist, only the most massive actually have enough stars to be visible. Unfortunately this gives rise to yet another challenge, dubbed the “too-big-to-fail” problem: these visible satellites would, therefore, have to be much more massive than those observed in the local group. To resolve the problem, some additional physics is required to suppress the formation of the largest dwarf galaxies.

These are just two of ten issues arising from inconsistencies between observations and simulations of dwarf galaxies that Joseph Silk lists in this somewhat speculative paper from 2017. As he remarks, there have been a multitude of proposed solutions. ...

The paper tackled in today’s astrobite is one of several more rigorous follow-ups to the ideas that Silk presented earlier. Here, the authors investigate the energetics of IMBH feedback, to find out if it is a plausible contender for that missing ingredient. Feedback in this case means the process by which the energetic output of an active galactic nucleus (AGN) — a black hole accreting matter — drives an outflow, blasting out the gas and preventing further star formation, thus shutting down the growth of the galaxy. ...

AGN Feedback in Dwarf Galaxies? ~ Gohar Dashyan et al
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Sunspots and Space Storms

Post by bystander » Fri Jan 18, 2019 7:22 pm

Sunspots and Space Storms
astrobites | Daily Paper Summaries | 2019 Jan 15
Ellis Avallone wrote:
The Sun, Earth’s closest stellar neighbor, conducts many processes we still don’t fully understand, many of which affect Earth directly. Space weather events are short-term changes in near-Earth space that are driven by solar activity. These events impact Earth and can negatively affect the technology we use (e.g., telecommunications satellites). Space climate refers to long-term variations in solar activity (i.e., the solar cycle, or the change in solar magnetic activity every 11 years). These long-term variations can also affect Earth, most notably our terrestrial climate.

To observationally study space weather and space climate, astronomers observe structures on the solar surface that affect short-term and long-term activity. The authors of today’s paper focus on structures known as active regions. Solar active regions are rooted in concentrations of strong magnetic field, are a major source of activity that contributes to both space weather and space climate, and are an ideal environment in which to observe these variations. ...

Different Contributions to Space Weather and Space Climate
from Different Big Solar Active Regions
~ Jie Jiang et al
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Where’s Lucy Going? Studying Asteroid Mission Targets

Post by bystander » Fri Jan 18, 2019 7:36 pm

Where’s Lucy Going? Studying Asteroid Mission Targets
astrobites | Daily Paper Summaries | 2019 Jan 16
Briley Lewis wrote:
Asteroids, meteoroids, meteors, meteorites. Usually when we talk about these small chunks of debris and rock in the solar system, it’s about another possible apocalypse scenario. Studies of rocky objects that may pass near Earth’s orbit (Near Earth Objects, or NEOs) are of obvious importance for the safety of humanity, but they are only one minor subset of the small bodies in our solar system. Most of the asteroids in our neighborhood live in the Asteroid Belt, a region between the orbits of Mars and Jupiter, and are referred to as “main belt asteroids”. There are also large populations trailing Jupiter in its orbit (the Trojan asteroids) and floating out in the outer solar system near Neptune (the Centaur asteroids). ...

A new asteroid mission has begun preparation as well, targeting multiple asteroids in the further-out Trojan group near Jupiter. The Lucy Discovery mission plans to visit multiple Trojans (actually, the largest number of independently orbiting objects ever visited by a single probe), including Leucus and Polymele, whose flybys are scheduled for September 2027 and April 2028 respectively. Until that date, though, astronomers are busy preparing for the mission and trying to gather all the data on these objects that we can from Earth. The authors specifically investigate Leucus and Polymele, using their light curves to tease out information about their color, composition, orbit, and reflectivity. ...

Light Curves of Lucy Targets: Leucus and Polymele ~ Marc W. Buie et al
viewtopic.php?t=38842
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How to find exoplanets and ‘listen’ to their stars with TESS

Post by bystander » Fri Jan 18, 2019 7:51 pm

How to find exoplanets and ‘listen’ to their stars with TESS
astrobites | Daily Paper Summaries | 2019 Jan 17
Emma Foxell wrote:
NASA’s space mission TESS is currently hunting for new exoplanets in the southern hemisphere sky. But while its primary aim is to find 50 small planets (with radii less than 4 Earth radii) with measurable mass, there is a lot of other interesting science to do. Today’s paper presents the discovery of a new exoplanet that is quite precisely characterised thanks to the complementary technique of asteroseismology used on the same data. ...

A Hot Saturn Orbiting An Oscillating Late Subgiant Discovered by TESS ~ Daniel Huber et al
viewtopic.php?t=31141
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Observational consequences of the beaming effect for gravitational waves

Post by bystander » Fri Jan 18, 2019 8:07 pm

Observational consequences of the beaming effect for gravitational waves
astrobites | Daily Paper Summaries | 2019 Jan 18
Aaron Tohuvavohu wrote:
The direct detection of compact binary mergers like binary black hole and binary neutron star systems via their gravitational radiation have come via decades of development in inteferometric detector technology and data analysis techniques. One of the most important components to the success of the enterprise has been the breakthrough achievements in numerical relativity, which is necessary to reproduce the inspiral, merger, and ringdown gravitational waveforms produced in compact binary mergers and produce template waveform banks for comparison with the data. However, these numerical relativity simulations are computationally expensive, which limits template banks to use component masses and spins of the compact objects, instead of perhaps a larger battery of parameters. The authors of today’s paper argue that another component, the velocity of the source relative to the observer (us), can have a substantial impact on the inferred gravitational wave amplitude. ...

Detecting the Beaming Effect of Gravitational Waves ~ Alejandro Torres-Orjuela et al
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Forecast calls for clear skies on WASP-96b

Post by bystander » Wed Jan 23, 2019 5:35 pm

Forecast calls for clear skies on WASP-96b
astrobites | Daily Paper Summaries | 2019 Jan 21
Jamie Wilson wrote:
The field of Exoplanetary science, the study of planets orbiting stars other than the Sun, is a relatively new and exciting area of Astrophysics. Ever since the first detection of an extrasolar planet, 51 Peg b, scientists have made rapid progress in the discovery of these new worlds, with over 3900 confirmed exoplanets found to date. More recently we have moved beyond mere discovery and have begun to investigate the chemical composition, physical processes and evolutionary histories of exoplanetary atmospheres. One major hurdle in the interpretation of these observations is the presence of clouds and hazes which act to weaken or even completely obscure atmospheric features. Now, for the first time, astronomers have discovered an exoplanet that appears to be completely cloud-free, marking a significant step forward in attempts to characterise new worlds beyond our solar system.

WASP-96b is an inflated ‘hot Saturn’ with a mass roughly similar to Saturn but a radius about 20% larger than Jupiter. It periodically transits a Sun-like star around 980 light years away in the southern constellation Phoenix and due to its proximity to its host star has a searing temperature of 1300K. Its extended atmosphere means that it provides an excellent target for atmospheric characterisation studies. ...

An Absolute Sodium Abundance for a Cloud-Free ‘Hot Saturn’ Exoplanet - Nikolay Nikolov et al
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Satellites of satellites should exist. Let’s go look for them.

Post by bystander » Wed Jan 23, 2019 5:54 pm

Satellites of satellites should exist. Let’s go look for them.
astrobites | Daily Paper Summaries | 2019 Jan 22
Jenny Calahan wrote:
Dwarf galaxies hold the key to our understanding of the make-up of the universe. Dwarf galaxies are similar to galaxies like our own Milky Way, except that they are much less massive and have a higher ratio of dark matter to stellar matter. The dark matter of a galaxy lives in its ‘dark matter halo’. The Milky Way and our nearby galactic neighbor, Andromeda, live in massive dark matter halos that contain sub-halos which can host dwarf galaxies. Based on the ‘ΛCDM’ model of our universe and simulations using the rules of ΛCDM, the more massive dark matter sub-halos should have sub-halos themselves. For example, Andromeda has a dwarf galaxy called M33, and M33 is the third most massive galaxy in our local group. The dark matter halo that M33 resides in should be massive enough to not only have sub-halos, but super-ultra-faint dwarf galaxies that reside in those sub-halos. These galaxies are so faint because they may only contain as few as a couple thousand stars. It just so happens that super-ultra-faint objects are super-ultra-hard to detect. Before we devote all our telescope time to survey the M33 region, we want to find out how many galaxies we would expect to find around M33 and what we can learn from them. Predicting the number of dwarfs around M33 depends on many factors, such as M33’s mass, distance from us, its orbital history, and even the laws that govern our universe.

These dwarf galaxies that reside in ‘sub-sub-halos’ are going to be tiny (a few thousand to a few tens of thousand solar masses, while the M33 is ~ 1 billion solar masses). M33 has already been partially surveyed by the PAndAS survey. This survey was not sensitive enough to detect the faintest types of galaxies, and only surveyed ~30% of the M33’s dark matter halo. PAndAS found just one dwarf galaxy possibly associated with M33, called ‘And XXII’. Follow up observations need to be done to be sure that And XXII is gravitationally bound to M33 as opposed to Andromeda. But how many associated dwarf galaxies of M33 would we have expected to find in the PAndAS survey? And how many satellites should M33 have? And how will we find them? ...

ΛCDM Predictions for the Satellite Population of M33 ~ Ekta Patel et al
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Small, but Mighty! A galaxy robbed of its stars but contains a supermassive secret.

Post by bystander » Wed Jan 23, 2019 6:17 pm

Small, but Mighty! A galaxy robbed of its stars but contains a supermassive secret.
astrobites | Daily Paper Summaries | 2019 Jan 23
Jessica May Hislop wrote:
Galaxies come in many different shapes, sizes, colours and ages, and understanding what drives galaxies to evolve the way that they do puzzles astronomers every day. But occasionally, we come across a galaxy that makes everyone go ‘huh?’, which is the case for one of the less impressive-looking galaxies seen in Figure 1. The image shows two large well-known galaxies: NGC 4647 and M60 (also known as NGC 4649). These galaxies are members of the Virgo Cluster, which lies around 54 million light-years from our own solar system.

Now, M60 has a very interesting little friend called M60-UCD1. This galaxy is called an ‘ultra-compact dwarf’ (UCD) because it’s very small and dense as far as galaxies generally go. The effective radius (the radius which contains half of the total light of the galaxy) of M60-UCD1 is only 24 parsecs (78 light-years), but packs a mass of 140 million solar masses. Astronomers are not entirely sure how these objects form, and there may be multiple ways that these objects assemble. For this object, however, the most likely scenario is that this was once a much larger galaxy. The authors suggest that M60-UCD1 has been tidally stripped due to a close call with its giant friend M60. This means that the galaxies have interacted with each other and the gravitational force of M60 has disrupted the stars, particularly in the outer parts of M60-UCD1. The evidence that motivates this scenario is that something pretty extraordinary is going on in the center of this tiny galaxy. ...

A Supermassive Black Hole in an Ultracompact Dwarf Galaxy - Anil C. Seth et al
viewtopic.php?t=32141
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Finding a supermassive black hole’s stellar-mass friends

Post by bystander » Mon Jan 28, 2019 6:23 pm

Finding a supermassive black hole’s stellar-mass friends
astrobites | Daily Paper Summaries | 2019 Jan 24
Kaitlyn Shin wrote:
Many massive galaxies are believed to host a supermassive black hole, or SMBH, at their centers. For decades, theorists have thought there should also be thousands of stellar-mass black holes surrounding a SMBH, forming a “density cusp.” Since our own Milky Way has a SMBH at its center—Sagittarius A*, or Sgr A* (“Sag. A-star”) for short—many observational attempts had previously been made to search for such a “cusp” of smaller black holes around Sgr A*.

The amount of gas and dust around Sgr A* makes it a perfect environment for the birth and death of massive stars, which often end up as black holes. Additionally, stellar-mass black holes may settle close to Sgr A* through a process called dynamical friction, where a massive object is slowed down as it moves through less massive objects. Some of these black holes remain isolated, but others may bind with passing stars, accreting (gravitationally pulling in) material from these stars and then releasing X-rays, forming “X-ray binaries.” These binaries occasionally emit bursts of X-rays so bright that they could be visible from Earth, but such bursts only occur every 100 to 1,000 years. Previous attempts to search for stellar-mass black holes near Sgr A* have focused on searching for these bright bursts.

Rather than search again for bright X-ray bursts, the authors decided instead to search for X-ray signals that were fainter, but more consistent, such as those resulting from the binding of black holes and low-mass stars. Using archival Chandra data spanning twelve years, they discovered 12 new sources within three light years of Sgr A* that had X-ray signals suggestive of quiescent (inactive, and relatively tame) black-hole low-mass X-ray binaries, abbreviated tongue-twistingly as qBH-LMXBs. ...

A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy - Charles J. Hailey et al
viewtopic.php?t=38206
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Judging a book by its cover

Post by bystander » Fri Feb 01, 2019 4:18 pm

Judging a book by its cover: estimating red supergiant masses from their surface abundance
astrobites | Daily Paper Summaries | 2019 Jan 28
Tomás Müller wrote:
Core-Collapse supernovae (CCSNe, see this animation) are explosions coming from massive stars (above 8 solar masses) when they reach the end of their life. A Type II-P Supernova (SN II-P) is a common type of CCSN which shows a “plateau” in its light curve, driven by a hydrogen-rich envelope. We know that the progenitors of SN II-P are Red Supergiant (RSG) stars by looking what was at the explosion site beforehand. Astronomers can make several predictions about the star by comparing these observations with stellar evolution models. The initial progenitor mass (i.e. the main sequence mass) can be estimated by: i) comparing the luminosity with model predictions, ii) measuring the mass of the hydrogen-rich envelope by modelling the light curve and making some assumptions about the core mass, or iii) measuring spectral lines of some elements, like oxygen, during the nebular phase. This phase refers to epochs from a few months to a few years after the explosion, where the material is optically thin and the spectrum shows mainly emission lines, which correlate with the initial mass in some models.

Unfortunately, these methods do not generally agree, so we cannot accurately estimate the initial mass of the progenitor of a SN II-P. Bearing this in mind, the authors of today’s article proposed a different way of estimating the initial mass by measuring the surface composition (or surface abundance) of the progenitor star at early epochs (less than 1 day after the explosion). The structure of RSG stars consist of several layers of burning material. Shallower layers are composed of lighter elements but some mixing occurs between the different layers, the amount of mixing depending mainly on the stellar mass. Looking at the early composition of a SN II-P can give us an idea of the progenitor star’s mass. The benefits of looking at early epochs are: firstly, at this stage some spectral features are easy to identify, and secondly, the surface abundance is not expected to suffer from explosive mixing at early times, which would erase any link to the progenitor mass.

The authors test this using the stellar evolution code, MESA, to model stars of different initial masses evolving. The evolution across the Hertzsprung-Russell (H-R) diagram is key to understanding the different processes and phases a star goes through. Figure 1 shows that less massive stars cross the H-R diagram more rapidly than more massive stars. This means that more massive stars have more time to dredge up material from their inner layers into the surface (mixing the abundances) before the end of the RSG phase. Additionally, more massive stars lose more mass than the least massive ones, so their outer envelopes are thinner compared to the total size of the star, hence their surface abundances suffer from more mixing. ...

The surface abundances of Red Supergiants at core-collapse ~ Ben Davies, Luc Dessart
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Stirring Up Rocky Worlds

Post by bystander » Fri Feb 01, 2019 4:31 pm

Stirring Up Rocky Worlds
astrobites | Daily Paper Summaries | 2019 Jan 29
Spencer Wallace wrote:
The catalog of known exoplanets has absolutely exploded in the last decade. Out of these discoveries have come two profound pieces of information. The first is that there are at least as many planets in our galaxy as stars. More unexpectedly, many of these extrasolar planetary systems appear totally exotic compared to our own. Astronomers have found everything from hot Jupiters to systems of tightly packed inner planets (a well-known example being TRAPPIST-1) to gas giant worlds with inclined orbits.

Amidst all of these exciting discoveries, it is important to keep in mind that our detection methods are biased. The vast majority of exoplanets so far have been discovered with either the transit or the radial velocity method. Both of these techniques are sensitive to large planets on short period orbits. In fact, the very first exoplanet discovered around a Sun-like star was a Jupiter-sized world on a 10 day orbit! It is very likely that there are more planets in many of these systems that are simply evading detection.

Rocky exoplanets are small and dim. Because of this, astronomers must often turn to computer simulations in order to understand how and where they can form. The authors of today’s paper are interested in whether these worlds can form and coexist alongside inclined giant planets. That is, planets whose orbits don’t lie in a nice, flat plane like we see in the Solar System. Specifically, the authors ask whether this environment can produce terrestrial planets that also have inclined orbits. ...

Formation of terrestrial planets in eccentric and inclined giant planet systems ~ Sotiris Sotiriadis et al
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Where Did All the Dark Matter Go?

Post by bystander » Fri Feb 01, 2019 4:45 pm

Where Did All the Dark Matter Go?
astrobites | Daily Paper Summaries | 2019 Jan 30
Tomer Yavetz wrote:
Last March, a team of astronomers led by Pieter van Dokkum announced the discovery of a galaxy with almost no dark matter at all (for a great summary of the original discovery, check out this astrobite). The leading theories of galaxy formation rely on a significant dark matter component, known as a dark matter halo, to create a galaxy, so this unexpected observational result naturally sparked a healthy amount of debate and discussion, both in the Twittersphere and in subsequent publications (and we have an astrobite for that too!). Since then, the original results have been scrutinized, criticized, and defended, and multiple follow-up studies have been conducted to try and provide additional evidence for or against the discovery. Today we discuss the latest exciting development in this ongoing saga: the discovery of a second galaxy that is also severely deficient in dark matter. ...

A Second Galaxy Missing Dark Matter in the NGC 1052 Group ~ Pieter van Dokkum et al
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How to Read (LSS)Tea Leaves

Post by bystander » Fri Feb 01, 2019 5:07 pm

How to Read (LSS)Tea Leaves to Better Understand Dark Energy
astrobites | Daily Paper Summaries | 2019 Jan 31
wrote:
The Large Synoptic Survey Telescope (LSST) will spend ten years observing the Universe from its site in Chile. It can take large images (over forty full moons worth of the sky at once), while also detecting faint objects, enabling it to create rich visions of the entire sky every few nights. The LSST has enormous science potential, and one of its goals is to aid our understanding of dark energy and dark matter.

The LSST survey strategy will impact subfields in astronomy to different degrees. Individual observations will be taken as visits—a pair of 15-second exposures of the same 10 degrees of sky taken in succession. How and when these visits will occur over the decade of operation is still being discussed. The paper in this Astrobite comes out of the LSST Dark Energy Science Collaboration (DESC) and outlines their recommendations to maximize the LSST’s benefit to cosmology ...

Optimizing the LSST Observing Strategy for Dark Energy Science:
  • DESC Recommendations for the Wide-Fast-Deep Survey ~ Michelle Lochner et al
  • DESC Recommendations for the Deep Drilling Fields and other Special Programs ~ Daniel M. Scolnic et al
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Connecting galaxy properties to star-formation histories

Post by bystander » Sat Feb 02, 2019 5:13 pm

Connecting galaxy properties to their star-formation histories
astrobites | Daily Paper Summaries | 2019 Feb 01
John Weaver wrote:
For over a century astronomers have continued to identify fainter and more distant galaxies. In the years since Hubble’s landmark illustration of galaxy types with his eponymous “tuning fork”, incredible progress has been made in understanding the demographics and evolution of galaxies. This is due in large part to the wealth of multi-wavelength observations made possible by incredibly deep surveys, analyzed in concert with high-resolution galaxy simulations. A consensus has slowly emerged: most galaxies live in one of two populations, and evolve from one to the other primarily by ceasing their star-formation. ...

Featured in today’s Astrobite, the Caplar & Tacchella present the first paper in a series which proposes a single underlying model to generate stochastic star-formation histories in order to elucidate the nature of the intrinsic scatter in the main-sequence of star-formation. ...

Stochastic modeling of star-formation histories I:
The scatter of the star-forming main sequence
~ Neven Caplar, Sandro Tacchella
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Can we a-void the Hubble tension with local voids?

Post by bystander » Wed Feb 06, 2019 5:58 pm

Can we a-void the Hubble tension with local voids?
astrobites | Daily Paper Summaries | 2019 Feb 04
Kate Storey-Fisher wrote:
Feeling a bit tense these days? So is the value of the Hubble constant. This parameter, written as H0, governs the rate of expansion of the universe, caused by some unknown dark energy. Despite cosmologists’ best efforts at massaging out the knots, H0 has long suffered from a tension between competing measurements. Today’s paper uses more precise data to reevaluate one possible cause of this tension. ...

Today’s paper considers what would happen if our distance measurements to nearby galaxies were off. This could be the case if the Milky Way were at the center of a local void (an under-density of galaxies), also known as a Hubble Bubble. This would cause the surrounding galaxies to be more strongly drawn towards higher-density regions, away from us. The extra pull would make the value for H0 that we measure locally higher than the true value; fixing this would bring it closer to the CMB measurement. ...

The Local Perspective on the Hubble Tension: Local Structure Does Not Impact
Measurement of the Hubble Constant
~ W. D'Arcy Kenworthy, Dan Scolnic, Adam Riess
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Simulating an Extreme TNO on the Road to Planet Nine

Post by bystander » Wed Feb 06, 2019 6:17 pm

Simulating an Extreme TNO on the Road to Planet Nine
astrobites | Daily Paper Summaries | 2019 Feb 05
Will Saunders wrote:
Three years ago, Mike Brown and Konstantin Batygin published their seminal paper predicting a massive, undiscovered Planet Nine. The publication was quickly picked up by the media and reinvigorated the study of objects beyond the orbit of Neptune, appropriately called trans-Neptunian objects (TNOs). Among the TNOs are Kuiper Belt objects, which have well-behaved orbits, and “extreme” TNOs (ETNOs), which have highly eccentric and inclined orbits. ...

This paper reports on DES’ discovery of 2015 BP519, the “most extreme” TNO yet, because it has a highly elliptical orbit with an eccentricity of 0.92 and is inclined a whopping 54 degrees out of the plane of the planets. ...

Discovery and Dynamical Analysis of an Extreme Trans-Neptunian Object with a High Orbital Inclination ~ J. C. Becker et al
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Three new pictures of the ancient Universe

Post by bystander » Wed Feb 06, 2019 6:32 pm

Three new pictures of the ancient Universe
astrobites | Daily Paper Summaries | 2019 Feb 06
Avery Schiff wrote:
Stars flood our night sky, enough to stagger even the most casual of stargazers. Still, telescopes are necessary to reveal just how numerous stars are in the Universe. One striking example of this phenomenon is globular clusters (GCs) which appear to the naked eye as a single bright point in the sky but are actually hundreds of thousands or millions of tightly bound stars (see figure 1). Today’s article is a letter that succinctly announces the discovery of three new globular clusters in the bulge of the Milky Way that may be nearly as old as the Universe. ...

Three candidate globular clusters discovered in the Galactic bulge ~ Denilso Camargo, Dante Minniti
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Reading the Epic of Reionization

Post by bystander » Fri Feb 08, 2019 9:48 pm

Reading the Epic of Reionization
astrobites | Daily Paper Summaries | 2019 Feb 07
Caitlin Doughty wrote:
The epoch of reionization (EoR) refers to a period in the universe’s history in which the element hydrogen, which to this day constitutes the majority of the baryonic matter in the universe, transitioned from being mostly neutral to mostly ionized (Figure 1). This transition occurred by a fairly intuitive process. Early on in the universe’s history, since there were no stars or galaxies to produce light, there were no energetic photons present to dislodge the electrons orbiting the nuclei of hydrogen atoms. However, once stars and galaxies began to form, there was an increase in the availability of such photons and once the universe was roughly 1 billion years old, nearly 100% of hydrogen atoms had been ionized.

Studying reionization presents an interesting puzzle for astronomers because we know the starting point (almost all hydrogen is neutral) and the ending point (almost all hydrogen is ionized), but have little idea of what path the universe took on its journey. Did reionization start early or late in the Universe’s history? Did it progress quickly or slowly? When did the various sources of ionizing photons (quasars, galaxy clusters, Population III stars, etc.) become abundant enough to have a significant effect on the transition?

One reason why these seemingly basic questions are still open is that astronomers mainly study light, and neutral hydrogen is notorious for absorbing lots of it. In fact, any photons with wavelengths less than 912 Angstroms, the Lyman limit, will be absorbed by neutral hydrogen. Effectively, the neutral hydrogen that defines the EoR prevents it from being studied. This has forced astronomers to devise creative ways of studying this period of time.

Today’s paper covers one team’s strategy to measure the neutral hydrogen fraction, a value that is used to characterize the amount of remaining neutral hydrogen compared to the total. If this fraction equals one, then all the hydrogen in the universe is neutral. If it equals 0, then all the hydrogen in the universe is ionized. The larger the neutral hydrogen fraction, the deeper into the EoR measurements are probing. ...

Constraining the neutral fraction of hydrogen in the IGM at redshift 7.5 ~ Austin Hoag et al
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A protoplanetary disk on its side

Post by bystander » Wed Feb 13, 2019 5:14 pm

A protoplanetary disk on its side
astrobites | Daily Paper Summaries | 2019 Feb 11
Samuel Factor wrote:
Binary stars are a common outcome of star formation, yet most of what we know about planet formation comes from studies of disks around single stars. In fact, almost all of the “pretty pictures” of protoplanetary disks you have seen are around single stars (with at least one notable exception). In the simplest case, the rotation of the disk, host stars, and their orbits would all be aligned since rotation on small scales is inherited from the bulk rotation of the parent cloud. Yet, we have seen planets which go against this idea, usually indicating that something interesting is going on. Today’s bite presents the first observations of a disk in exactly the opposite configuration: a disk in a polar orientation around a young binary star system. ...

A circumbinary protoplanetary disk in a polar configuration ~ Grant M. Kennedy et al
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Looking for a dark matter needle in an X-ray stack

Post by bystander » Wed Feb 13, 2019 5:28 pm

Looking for a dark matter needle in an X-ray stack
astrobites | Daily Paper Summaries | 2019 Feb 12
Sunayana Bhargava wrote:
Dark matter is frustratingly elusive. Despite the wealth of indirect astronomical evidence for its existence, dark matter has continued to escape direct detection. To date, teams of particle physicists have not yet been able to trap a dark matter particle in laboratory experiments based on the properties predicted from leading theories. ...

One exciting example of an indirect dark matter search has come from searching the sky in the X-ray part of the spectrum. Specifically, the authors present a discovery of an unidentified emission line in the X-ray spectra of multiple galaxy clusters. This line is present at the same energy in each individual spectrum, at approximately 3.5 kiloelectronvolts (keV).

Galaxy clusters contain hundreds of galaxies, trillions of stars, and an energetic plasma between galaxies, which shines in X-rays. When we look at the composition of this X-ray gas using spectroscopy, we can observe emission lines that tell us about metals and charged particle interactions within the cluster. The authors claim that the 3.5 keV emission line is not the result of any known charged particle transitioning between energy states and therefore has an unidentified origin. They go on to suggest this line could be a signal from a dark matter candidate known as the sterile neutrino. ...

Detection of an Unidentified Emission Line in the Stacked X-Ray Spectrum of Galaxy Clusters - Esra Bulbul et al
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Tiny Stars in a Tiny Orbit: A New Ultracompact Binary

Post by bystander » Wed Feb 13, 2019 5:45 pm

Tiny Stars in a Tiny Orbit: A New Ultracompact Binary
astrobites | Daily Paper Summaries | 2019 Feb 13
Aaron Pearlman wrote:
What does a ballerina doing a pirouette and a millisecond pulsar (MSP) have in common? Just as the ballerina will spin faster and faster as she brings her arms in toward her body, a MSP will also rotate more rapidly as the star’s radius is reduced by gravitational compression. These swiftly spinning stars are neutron stars – one of the possible end stages of stellar life. Neutron stars are born during a core-collapse supernova, which occurs once a massive star can no longer withstand its own gravity after fusion has ceased. These exotic compact objects are among the densest stars in the universe. They have 1.4 times the mass of our Sun crammed into a sphere that is roughly 20 km in diameter (the size of a large city!). As these neutron balls rotate, they can produce various types of electromagnetic radiation near their magnetic poles. If the magnetic axis and rotation axis of the neutron star are misaligned, we typically observe periodic pulses of radiation as the star rotates (if the radiation is beamed towards our telescopes) – hence the nickname “pulsar”! MSPs are particularly special compared to ordinary pulsars. They are typically spun-up by accreting material from a nearby orbiting star through angular momentum transfer and rotate hundreds of times per second (in some cases, even faster than your kitchen blender).

In today’s astrobite, we cover the discovery of the orbit of an ultracompact X-ray binary, IGR J17062–6143 (depicted in Figure 1), which harbors an accreting MSP. Using the newly commissioned Neutron Star Interior Composition Explorer (NICER) X-ray instrument on board the International Space Station, Strohmayer et al. showed that the system consists of a very low mass white dwarf in a record-setting 38-minute orbit with a pulsar that rotates at about 9,800 revolutions per minute. ...

NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062–6143 - T. E. Strohmayer et al
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Looking Deeper at Milky Way Satellite Candidates

Post by bystander » Wed Feb 13, 2019 6:05 pm

Looking Deeper at Milky Way Satellite Dwarf Galaxy Candidates
astrobites | Daily Paper Summaries | 2019 Feb 13
Amy Miller wrote:
Dwarf galaxies are believed to be the fundamental building blocks of larger galaxies like our Milky Way and are the most abundant galaxies in our known universe. Astronomers use the terms “satellite” or “companion” galaxy to denote less massive galaxies that orbit around a larger galaxy’s center of mass. Two of the most well-known dwarf galaxies are the Large and Small Magellanic Clouds—satellite galaxies of the Milky Way. Though vitally important to galaxy formation and evolution, astronomers have a difficult time producing adequate numbers of them in cosmological simulations. Computer simulations that test the Lambda Cold Dark Matter (ΛCDM) model—the standard model of Big Bang Cosmology—predict that there should be at least 100, and possibly up to a 1000, satellite galaxies orbiting the Milky Way. However, there are only about 50 known Milky Way companions. This huge discrepancy between theory and observation has been dubbed the “missing satellites problem.” Solutions to this problem are a hot topic of debate in the astronomy community.

Meanwhile, increasingly sensitive data are revealing more Milky Way dwarf satellite candidates. Recent work done in two separate papers led by K. Bechtol and A. Drlica-Wagner revealed 16 new Milky Way dwarf satellite candidates in ~5000 square degrees of the southern sky. These candidates were found by analyzing astronomical data catalogs from the Dark Energy Survey and identifying overdensities of stellar populations. Although these 16 overdensities were previously unidentified, it is still unclear as to whether they are dwarf satellite galaxies or star clusters. By gathering more light to see even fainter stars, the subject of today’s Astrobite uses “deeper” images than the Dark Energy Survey to further investigate the exact nature of four of these stellar populations. These deeper data give more information to the authors and allow them to make better estimations as to whether they are dwarf galaxies or star clusters.

On the Nature of Ultra-faint Dwarf Galaxy Candidates.
III. Horologium I, Pictor I, Grus I, and Phoenix II
~ Helmut Jerjen et al
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