astrobites: Daily Paper Summaries 2019

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When Do Stars Form? Simulating Dynamic Star Formation Efficiencies

Post by bystander » Mon Nov 11, 2019 8:20 pm

When Do Stars Form? Simulating Dynamic Star
Formation Efficiencies in Giant Molecular Clouds

astrobites | Daily Paper Summaries | 2019 Nov 08
Michael Foley wrote:
Observations of Giant Molecular Clouds (GMCs) yield large scatters in star formation efficiencies. Simulations in this work show that much of this scatter may stem from the fact we observe GMCs at different evolutionary stages driven by stellar feedback.

Stars are known to form within Giant Molecular Clouds (GMCs), huge complexes of gas and dust that range in mass from roughly one thousand to ten million times the mass of our sun. These regions are much denser than their surrounding environment, causing clumps of mass within them to collapse and form stars. However, this is actually a very complicated process with many compounding factors. One of the primary influences on the star formation process is stellar feedback, a broad category that refers to all the ways that existing stars affect their environment. This includes pressure from radiation emitted by stars, deposition of mass and energy from stellar winds and outflows, and supernova explosions. Today’s paper explores the way that stellar feedback affects the star formation process within GMCs over time.

On The Nature of Variations in the Measured Star Formation Efficiency of Molecular Clouds ~ Michael Y. Grudić et al
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Testing the Limits of Galactic Clouds

Post by bystander » Mon Nov 11, 2019 8:33 pm

Testing the Limits of Galactic Clouds
astrobites | Daily Paper Summaries | 2019 Nov 11
Caitlin Doughty wrote:
The circumgalactic medium (CGM) lies outside the bright region of a galaxy and is conventionally considered to end at the virial radius. Although there are many ideas about what is happening inside of the CGM, it is not particularly well understood, and is hard to simulate to boot.

However, understanding the CGM is critical to the study of how gas cycles in and out of galaxies and by extension galaxy evolution. In addition to being a dynamic environment, the CGM is also believed to contain cool dense clouds of largely neutral gas flowing through a hotter medium, leading to a complicated “multiphase” structure. This is the generally accepted picture, but there are implications for the cool clouds. How long can these cool clouds survive being bombarded by hotter gas flowing around them, and potentially an entire host of other physical effects?

The survival of cool clouds, or the cloud crushing problem, has historically been a topic of interest in studies of the interstellar medium (ISM) because of the potential implications for star formation. However, the conditions in the CGM are quite different from those in the ISM, so astronomers need to start from scratch. Using numerical simulations of cool clouds moving through an ambient medium, the authors of today’s paper explored a range of physical conditions to examine how critical they are in determining cloud survival times. ...

On the Survival of Cool Clouds in the Circum-Galactic Medium ~ Zhihui Li et al
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The slowly cooling white dwarfs who say Ne!

Post by bystander » Sat Nov 16, 2019 3:39 pm

The slowly cooling white dwarfs who say Ne!
astrobites | Daily Paper Summaries | 2019 Nov 12
Samuel Factor wrote:
The European Space Agency’s Gaia mission has revolutionized astronomy, and will continue to do so as its mission progresses. Pristine color-magnitude (or Hertzsprung-Russell) diagrams can be made with the extremely precise distances the mission provides uncovering new features. Today’s paper focuses on explaining some interesting features discovered in the white dwarf sequence seen in Gaia’s second data release (GDR2).

A Cooling Anomaly of High-Mass White Dwarfs ~ Sihao Cheng, Jeffrey D. Cummings, Brice Ménard
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Modeling Supernovae Feedback as a Galactic Fountain

Post by bystander » Sat Nov 16, 2019 3:48 pm

Modeling Supernovae Feedback as a Galactic Fountain
astrobites | Daily Paper Summaries | 2019 Nov 13
Bryanne McDonough wrote:
Simulations are a powerful tool in astronomy. Processes that take billions of years in reality can be played out over a much smaller timescale on a computer. The exact time a simulation takes to run depends on a few things, mainly the size and resolution of the simulation, as well as the speed of the computer itself.

Resolution is a critical issue when it comes to any simulation; galaxy formation in particular involves both very large and very small processes. In the early years of galaxy formation simulations, researchers found that simulated galaxies weren’t accurately reproducing the observed rates of star formation. They realized that a relatively small process must be having a large effect, known as feedback. There are two main drivers of feedback: active galactic nuclei and supernovae. Each of these events take place on relatively small physical scales but can change galaxies in ways we are still uncovering.

The trick comes in simulating these small scale processes in large simulations. Instead of modeling the actual processes, rough approximations are made and inserted into the simulation. While these approximations are becoming increasingly better, they are still not ideal. The authors of today’s paper, Li and Tonnesen, wanted to investigate how the circumgalactic medium (a.k.a all the gas and dust surrounding a galaxy) could be affected by supernovae. ...

How do Supernovae Impact the Circumgalactic Medium?
I. Large-Scale Fountains in a Milky Way-Like Galaxy
~ Miao Li, Stephanie Tonnesen
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The Mysterious Pulsars That Switch On and Off

Post by bystander » Sat Nov 16, 2019 3:56 pm

The Mysterious Pulsars That Switch On and Off
astrobites | Daily Paper Summaries | 2019 Nov 14
Haley Wahl wrote:
Pulsars are some of the most extreme objects in the universe. With their incredible densities (a teaspoon would weigh as much as Mount Everest) and extreme magnetic fields (which are 12 orders of magnitude stronger than Earth’s), these objects are some of the strangest we have ever come across. Pulsars emit radiation like a lighthouse and sometimes they just turn off and do not emit at all. Today’s work explores this strange behavior of pulsars in order to try to understand what types of pulsars show this behavior and why. ...

Radio Pulsar Sub-Populations (I) : The Curious Case of Nulling Pulsars ~ Sushan Konar, Uddeepta Deka
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Connections on the Cosmic Web

Post by bystander » Tue Nov 19, 2019 6:29 pm

Connections on the Cosmic Web
astrobites | Daily Paper Summaries | 2019 Nov 18
Bryanne McDonough wrote:
The universe was not perfectly uniform when it began, some areas had higher density than others. As the universe evolved, these areas of high density contained most of the matter and began forming galaxies where there was the highest concentration of stuff. This large-scale structure is known as the ‘cosmic web’ and connects the observed clusters of galaxies via a series of filaments. A model of what this looks can be seen in Figure 1.

The cosmic web is a representation of the density of space, and can be traced out with the observed distribution of galaxies, connecting those closest together. This is done with similar algorithms for both observations and simulations. The densest areas are the most massive, central galaxies that are located at the crossroads of multiple filaments (called nodes), like the blue areas in Figure 1. It had been shown by another paper from different authors that the mass of a galaxy increased with the number of connected filaments (the “connectivity”). The authors of today’s paper wanted to know whether the connectivity was related to star formation and whether it helped prevent star formation from happening...

The Impact of the Connectivity of the Cosmic Web on the
Physical Properties of Galaxies at Its Nodes
~ Katarina Kraljic et al
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Combining Observations to Find EvryFlare

Post by bystander » Tue Nov 19, 2019 6:46 pm

Combining Ground and Space-based Observations to Find EvryFlare
astrobites | Daily Paper Summaries | 2019 Nov 19
Spencer Wallace wrote:
For Sun-sized and smaller stars, energy is transported to the surface by roiling columns of convection. These convective bubbles twist up the magnetic fields at the surface and drive sudden, violent releases of energy through flares. Although flare events from the Sun are relatively inconsequential, stars smaller than the Sun have been observed to produce superflares strong enough to remove a planet’s protective ozone layer and kill all but the hardiest lifeforms.

Many potentially habitable planets are being discovered around low mass stars. To better understand the viability of these words for harboring life, it its crucial to understand how and where these flare and superflare events can happen. Flares happen very quickly. Catching them as they happen requires a large number of stars to be continuously monitored. This is exactly the goal of the Evryscope, the details of which are described in a previous Astrobite. So far, Evryscope has collected several years of simultaneous brightness measurements for over 15 million stars. This long observing baseline makes relatively rare superflare events easier to catch. The tradeoff with such a wide, ground-based view is that the less energetic, but more frequent flare events go undetected due to the reduced photometric precision.

The TESS telescope, which is currently monitoring brightness variations for some of these same stars from above the Earth’s atmosphere, makes up for this limitation nicely. Although TESS only observes a given target for ~28 days, these less energetic flares are much more frequent. By combining the multi-year data from Evryscope with the much shorter, but more precise observations from TESS, the authors of today’s paper attempt to get a handle on the frequency of a wide range of flare types from a large sample of stars. An example light curve, combining the Evryscope and TESS data, is shown in Figure 1. Even though the TESS observations only add a small chunk to the light curve, the fantastic sensitivity of TESS reveals a handful of low energy flares that are lost in the noise with Evryscope...

EvryFlare. I. Long-Term Evryscope Monitoring of Flares from the
Cool Stars across Half the Southern Sky
~ Ward S. Howard et al
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Stellar Rotation with Asteroseismology

Post by bystander » Sat Nov 23, 2019 4:28 pm

You Spin me Right Round: Stellar Rotation with Asteroseismology
astrobites | Daily Paper Summaries | 2019 Nov 20
Ellis Avallone wrote:
All stars in nature rotate, including our own. However, stellar rotation over a star’s lifetime remains poorly understood. This has a profound impact on the accuracy of stellar models, which are our primary source for understanding the interiors and evolution of stars.

Today’s paper focuses on internal rotation mechanisms; specifically, how a star’s core rotates with respect to its surface. Understanding stellar core rotation can teach us a ton about internal stellar physics and long-term angular momentum transport within a star’s interior. ...

Core-Envelope Coupling in Intermediate-Mass Core-Helium Burning Stars ~ Jamie Tayar et al
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The Hidden Mechanism of Quasar Feedback

Post by bystander » Thu Nov 28, 2019 2:09 am

The Hidden Mechanism of Quasar Feedback
astrobites | Daily Paper Summaries | 2019 Nov 26
Mitchell Cavanagh wrote:
Galaxy formation and evolution is an intricate topic with many physical and chemical processes at play, from star formation to changes in morphology or shape. One of the key mysteries in galaxy evolution is how young, star-forming galaxies can be quenched, i.e starved of gas, and hence transformed into barren, so-called “red and dead” galaxies. One explanation for this transformation involves the strong negative feedback that occurs when gas accretes onto a supermassive black hole. This feedback manifests itself as strong molecular outflows that wreak havoc across the galaxy, effectively stopping star formation in its tracks. This feedback is known as quasar feedback or AGN (active galactic nuclei) feedback.

It has long been speculated that AGN feedback plays an important role in the evolution of galaxies and their central black holes, but process itself is difficult to directly observe, given that it occurs so close to the central supermassive black hole. Instead, we can observe the outflows. Modern surveys such as those making use of Herschel have been able to study these outflows in great detail, providing an indirect observation of the AGN feedback process via spectroscopy, i.e measuring light intensity over different frequencies. As such, these surveys have helped to provide insights into the inner workings of the otherwise hidden mechanism that is quasar/AGN feedback. ...

This study looked at galaxies that showed signs of recent or ongoing galaxy merging. The outflow velocity was estimated by looking at the maximum blueshift of the OH 119.233 μm line. ...

Fast Molecular Outflows In Luminous Galaxy Mergers:
Evidence For Quasar Feedback From Herschel
~ S. Veilleux et al
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Why cosmology is (probably) not behind the curve

Post by bystander » Tue Dec 10, 2019 5:34 pm

Why cosmology is (probably) not behind the curve
astrobites | Daily Paper Summaries | 2019 Nov 28
Sunayana Bhargava wrote:
Over the years, astronomers have done a pretty good job at understanding the key parameters that describe our cosmos. The resulting model, known as the Lambda-CDM model, states that the universe must contain dark matter (served cold), some component of dark energy (poured at a constant rate), and a pinch of ordinary matter.

Since our cosmological model agrees with a lot of the data we have, the burden of proof is typically on any new technique or method that finds discrepancies with LCDM. ...

In a reanalysis of data taken from the Planck satellite, today’s paper claims that there is a preference for a curved universe over a flat one. The new result states that the curvature parameter, known as Ωκ, is best described by a value of -0.0438, compared to the previously reported result of 0.001 (Planck 2018). For reference, a flat universe requires Ωκ = 0. This states the universe is approximately 4% more curved than we thought. Although this might seem like a small change, a positively curved (or closed) universe introduces statistically significant disagreements with almost every other cosmological probe. ...

Planck Evidence for a Closed Universe and a Possible Crisis for Cosmology ~ Eleonora Di Valentino et al
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A Dizzying Diversity of Dwarf Galaxies

Post by bystander » Tue Dec 10, 2019 6:41 pm

A Dizzying Diversity of Dwarf Galaxies
astrobites | Daily Paper Summaries | 2019 Nov 29
Tomer Yavetz wrote:
The known saying goes, “where there’s smoke, there’s fire.” Just like we infer the existence of a fire when we see smoke, we infer the existence of mass when we see objects moving in circles. Physics dictates that objects will revolve around a large mass — the larger the mass, the faster the rotation. When the observable matter (stars, dust, gas, etc. — collectively referred to as baryonic matter) is not sufficient to explain the rotational velocities, most astronomers conclude that there must be some form of invisible matter, or dark matter.

A successful theory needs to account for the rotational velocities as a function of radius (aka the galaxy’s “rotation curve”), which requires a precise knowledge of the density profile of baryons and dark matter. For most galaxies ... a simple model for dark matter is sufficient ...

However, the situation gets much more complicated with dwarf galaxies. Figure 1 shows the rotation curves of four dwarf galaxies that should, in theory, look similar. ...

So what is going on with these dwarf galaxies? Theorists have come up with a variety of explanations for why the rotation curves of dwarf galaxies differ from expectations. Today’s authors group these theories into four categories:
  1. Baryonic physics ...
  2. Dark Matter physics ...
  3. Baryonic acceleration laws ...
  4. Observational uncertainties ...

The main goal of today’s paper is to evaluate each of these theories against the existing observational data. The authors make use of simulations (APOSTLE and NIHAO) to test whether any of the theories are able to reproduce the observed diversity of rotation curves. In each case, they compare the expectations from theory/simulations with the observed data. ...

So which of the four theories can justify the observed rotation curves? The authors argue that the answer is ‘none of the above.’ While each theory was able to explain certain galaxies, none of the theories reproduced the full diversity of the observed rotation curves without requiring additional assumptions ...

Baryonic Clues to the Puzzling Diversity of Dwarf Galaxy Rotation Curves ~ Isabel M.E. Santos-Santos et al
  • arXiv.org > astro-ph > arXiv:1911.09116 > 20 Nov 2019 (v1), 23 Nov 2019 (v2)
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Constraining the Hubble Constant

Post by bystander » Tue Dec 10, 2019 7:03 pm

Constraining the Hubble Constant with Lensed Gravitational Wave Events
astrobites | Daily Paper Summaries | 2019 Dec 02
Kaitlyn Shin wrote:
The Hubble constant (H0) is an important cosmological parameter that governs the rate of expansion of our universe, and for which a value has not yet been universally agreed upon. This conundrum has been dubbed the “Hubble tension,” and has been covered in many previous astrobites articles (see here, here, here, here, and here). The Hubble tension boils down to how independent measurements of H0 can be made very precisely, but H0 values inferred from studying the “early” universe significantly disagree with H0 values inferred from studying the “late” universe. (What the phrases “early” universe and “late” universe mean is covered in more detail here.)

The statistically significant disagreement between H0 measurements has led to a call for further alternative and independent ways to measure H0, especially as we wait for more sensitive next-generation surveys (such as Euclid and the Square Kilometre Array, SKA) to become operational. The authors chose to study the possibility of constraining H0 by using gravitationally lensed observations of gravitational waves (GWs) and electromagnetic (EM) signals emitted by a single source. ...

High Accuracy on H0 Constraints from Gravitational Wave Lensing Events ~ Paolo Cremonese, Vincenzo Salzano
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A New, Scattered-Light View of Planet-Forming Disks

Post by bystander » Tue Dec 10, 2019 7:18 pm

A New, Scattered-Light View of Planet-Forming Disks
astrobites | Daily Paper Summaries | 2019 Dec 03
Charles Law wrote:
Ever wondered what our Solar System looked like four billion years ago? While we know that our Solar system was born out of a disk-shaped cloud of dust and gas, called the solar nebula, we want to see planet formation in “real-time”. But to do so, we need instead to look at the dense gas and dust around other young stars. These circumstellar disks provide the raw material for nascent planets and gives us a window into the earliest stages of planet formation. With the advent of new observational facilities such as ALMA, astronomers can now study the birthplaces of planets in exquisite detail. A more complete understanding of the physical and chemical properties of these disks not only lets us learn more about the birth of our Solar System, but helps to better contextualize the vast demographic diversity observed in the exoplanet population. In particular, newly-formed planets leave detectable imprints in their disks – a signature only accessible to high resolution observations. In today’s astrobite, we take a look at new scattered-light images of planet-forming disks, which reveal a variety of disk substructure, provide important constraints on planet formation, and constitute the largest sample of such observations to date. ...

Disks ARound T Tauri Stars with SPHERE (DARTTS-S) II: Twenty-one
New Polarimetric Images of Young Stellar Disks
~ Antonio Garufi et al
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Elucidating the role of gas in star-formation

Post by bystander » Tue Dec 10, 2019 7:42 pm

QUESTing for an answer: Elucidating the role of gas in star-formation
astrobites | Daily Paper Summaries | 2019 Dec 04
John Weaver wrote:
The star-formation main sequence (SFMS; Figure 1) is a key concept in modern extragalactic astrophysics. In brief, it describes the tight positive relationship between the stellar mass of a galaxy and its star-formation rate (SFR). Interpretations of the SFMS have varied over the past decade since its discovery, and although much discussion has taken place, the meaning and evolution of the SFMS remains a hotly contested subject.

Perhaps more importantly is the scatter of the SFMS, which describes the extent to which galaxies appear to have an enhanced or suppressed star-formation. If indeed the scatter is due to a real phenomenon (as opposed to noisy data), then what drives galaxies above the SFMS? Is there a larger underlying molecular gas reservoir from which more stars form, or is the star-formation itself more efficient? The answer is still unclear. ...

In this astrobite, the SFMS and its drivers are put under the microscope – or should I say, a telescope with many simultaneous integral field units! ...

The ALMaQUEST Survey: III. Scatter in the resolved star forming main
sequence is primarily due to variations in star formation efficiency
~ Sara L. Ellison et al
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Here be dragons: Mapping the Mass Gap

Post by bystander » Tue Dec 10, 2019 8:19 pm

Here be dragons: Mapping the Mass Gap
astrobites | Daily Paper Summaries | 2019 Dec 05
Sanjana Curtis wrote:
Gravitational wave astronomy is a field brimming with promise, with LIGOVirgo discovering new clues to a number of scientific mysteries, such as the origin of heavy elements and the progenitors of short gamma-ray bursts. One such mystery is the lack of observed neutron stars or black holes in the ~3 to 5 solar mass range, i.e. the ‘mass gap‘ which shows up as a conspicuously empty band in the schematic below.

We currently do not know how massive a neutron star can get before it can no longer support itself. The answer to this question depends on the unknown equation of state that describes extremely high density matter deep inside a neutron star. To date, the highest measured neutron star mass is ~2.14 solar masses, while the GW170817 gravitational wave event suggests an upper limit of ~2.2 solar masses. With the lightest black holes weighing in at ~5 solar masses, we are left with an interesting conundrum. Are there truly no neutron stars or black holes with intermediate masses, and if so, why? But if they do exist, and the mass gap is no gap at all, why haven’t we seen them? ...

The great impostors: Extremely compact, merging binary neutron stars
in the mass gap posing as binary black holes
~ Antonios Tsokaros et al
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A Comet on TESS’s Vision

Post by bystander » Tue Dec 10, 2019 8:36 pm

A Comet on TESS’s Vision
astrobites | Daily Paper Summaries | 2019 Dec 06
Tarini Konchady wrote:
The Transiting Exoplanet Survey Satellite (TESS) has been searching for exoplanets since the summer of 2018 (and will continue to till at least 2022). Its initial two year mission consists of scanning almost the entire sky. Like its predecessor Kepler, TESS searches for planets by staring at a part of the sky for an extended period of time, in the hope of catching a transit. However, while Kepler stared at one part of the sky for years, TESS moves from sector to sector on roughly a monthly basis (see Figure 1).

TESS’s observing style makes it suitable for studying objects other than planets, such as comets. Comets change rapidly, meaning that TESS’s month-long monitoring of each sector is perfect for studying them.

Comet 46P/Wirtanen zipped across TESS’s field of view while it was observing Sector 3 last year (late September to mid October). To make things even more special, Wirtanen experienced an outburst that TESS caught from start to finish — the first time a cometary outburst has been so thoroughly observed.

In the paper discussed in this Astrobite, the authors use TESS data to characterize Wirtanen and its outburst, as well as how it compares to other comets. ...

First Results from TESS Observations of Comet 46P/Wirtanen ~ Tony L. Farnham et al
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Uncovering New Sources of Dwarf Galaxy Feedback

Post by bystander » Tue Dec 10, 2019 8:44 pm

Uncovering New Sources of Dwarf Galaxy Feedback
astrobites | Daily Paper Summaries | 2019 Dec 09
Keir Birchall wrote:
Feedback doesn’t just appear on your homework or as the horrible screeching noise in poorly assembled speakers, it also helps shape the evolution of a galaxy. Feedback consists of wind or jet-driven motion of gas and dust within or without a galaxy from many different sources: supernova explosions, radiation from stars or outflows from active galactic nuclei (AGN). Winds driven by these sources blow throughout the galaxy enriching its gas with metals and regulating its star formation. Stellar sources were, until recently, believed to dominate feedback processes in dwarf galaxies, largely because it was thought such low mass galaxies couldn’t host AGN. Over the past decade, however, there have been hundreds of detections of AGN within dwarf galaxies. Motivated by this plethora of detections, today’s authors set out to identify the dominant source of feedback in nearby dwarf galaxies and report some of the first direct detections of AGN-driven outflows. ...

AGN-Driven Outflows in Dwarf Galaxies ~ Christina Manzano-King, Gabriela Canalizo, Laura V. Sales
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TESS: Not Just a Planet Hunter

Post by bystander » Tue Dec 10, 2019 8:56 pm

TESS: Not Just a Planet Hunter
astrobites | Daily Paper Summaries | 2019 Dec 10
Jessica Roberts wrote:
We expect solar mass black holes to litter our galaxy. After all, if we assume that every star over 20 solar masses creates a black hole when they die, there should be over 100 million of them out there. Yet, we have found less than 20 of them in our galaxy, with the closest about 1000 parsecs (about 3200 light years) away from us. However, models suggest that there should be a handful of solar mass black holes less than 100 parsecs (or 320 light years), with a high probability that one is only a few tens of light years from us. So, where are they? ...

Today’s paper explores the idea that we may be able to find a new population of black holes: ones that are still in binary systems, but are up to 0.3 AU away from the stellar companion. At this distance, the black hole is far enough away to prevent consumption of its stellar companion. How do the authors propose finding these black holes? The authors argue that the exoplanet hunting satellite, Transiting Exoplanet Survey Satellite (TESS) is well equipped to be a black hole hunter. ...

Prospects of Finding Detached Black Hole-Star Binaries with TESS ~ Kento Masuda, Kenta Hotokezaka
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A Black Hole at the Heart of the Archer

Post by bystander » Tue Dec 17, 2019 7:48 pm

A Black Hole at the Heart of the Archer
astrobites | Daily Paper Summaries | 2019 Dec 11
Stela Adduci Faria wrote:
Black Holes (BHs) are subjects of unending curiosity because of their mysterious nature and unknown properties. Despite their name, BHs are not really empty voids. They are astronomical objects with a gravitational pull so strong that nothing, not even light, can escape it. The “surface” of a black hole, called the event horizon, defines the boundary where the escape velocity (for any particle) exceeds the speed of light, which is the speed limit of the cosmos. Thus, matter and radiation can fall into the event horizon, but they can’t get out.

BHs can be divided in two main classes. Stellar-mass black holes, that form when a star with more than 20 solar masses “dies”. Another type are supermassive black holes (SMBHs) believed to exist in the nucleus of galaxies, although their origin is not well-understood. We also have our own SMBH at the center of the Milky Way, which is known as Sagittarius A* (Sgr A*). ...

Electromagnetic radiation propagates through free space or through a material medium in the form of electromagnetic waves with several frequencies, such as radio waves, which have low frequency, and gamma rays, which correspond to the highest frequencies of the electromagnetic spectrum, with energies above 1 million electron volts (eV). Gamma rays correspond to the energy range treated in the paper. ...

In order to explain how the observed gamma ray radiation is produced, the authors of today’s paper perform numerical simulations to model the accretion disk around the SMBH and estimate how much gamma-ray radiation is produced. ...

Very-High-Energy Emission from Magnetic Reconnection in the Radiative-
Inefficient Accretion Flow of SgrA*
~ Juan Carlos Rodríguez-Ramírez et al
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Another Interstellar Interloper

Post by bystander » Tue Dec 17, 2019 8:12 pm

Another Interstellar Interloper
astrobites | Daily Paper Summaries | 2019 Dec 12
Briley Lewis wrote:
Visitors from beyond are already here. Two years ago, the first observed interstellar asteroid, 1I/’Oumuamua, passed through our solar system. Discovered by the PanSTAARS survey, it didn’t look like anything we’d seen in our own solar system (a weird, elongated cigar shape, on a hyperbolic orbit), and some people even claimed it might be aliens scoping out our solar system. More realistically, it provided an interesting opportunity to learn about how other planetary systems formed (since it is a remnant from a distant planet’s formation), and provided our first constraint on how many of these objects we may expect to see in the future.

Flash forward in time, and we have already found a second! Enter Comet 2I/Borisov. Whereas ‘Oumuamua was a dark asteroid, Borisov is a bright trailing comet. Other than its unusual interstellar trajectory, it looks just like a comet from our solar system, as shown in Figure 1. It’s the second interstellar object we’ve seen, but the very first interstellar comet. ...

Initial Characterization of Interstellar Comet 2I/Borisov ~ Piotr Guzik et al
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They see me rolling… inflating

Post by bystander » Tue Dec 17, 2019 8:24 pm

They see me rolling… inflating
astrobites | Daily Paper Summaries | 2019 Dec 16
Philippa Cole wrote:
We’re still on the hunt for what dark matter actually is. The most popular candidates are usually some sort of particle, like the WIMP or the axion. However, an idea that was first considered by Stephen Hawking (among others), is that tiny black holes which formed right after the Big Bang could also do the trick. There have been many searches carried out for these littl’uns, known as primordial black holes, but there’s been no success just yet. However, whether there’s enough of them floating around to make up some or all of the dark matter or not, we still need to work out how they could have been produced in the first place. For that, we’ll need to go back 13.8 billions years…

Right after the Big Bang, the Universe expanded from a tiny seed by 20 orders of magnitude in approximately one second. That’d be like blowing up a pea to the size of a galaxy in that time. We call this process cosmological inflation. If everything started off really small and close together before being blown up, then we’d expect everything to look pretty homogeneous today even in regions which are really far apart, and that’s exactly the case. For example, the temperature of the Cosmic Microwave Background (CMB) has been measured to be around 3 Kelvin to one part in 100,000 in every direction across extremely large distances. This is why inflation is such a popular idea for how the Universe began.

During the expansion process, any tiny fluctuations that were there initially are able to grow, and it’s these fluctuations that make the Universe not totally smooth and boring everywhere. Once inflation is over, these inhomogeneities can then go on to continue growing and eventually gravitationally collapse to all of the structure, like stars and galaxies, that we see today. That’s exactly what the 1 part in 100,000 fluctuations that we’ve measured are responsible for. If, however, the initial fluctuations happened to be sufficiently larger in some regions, then they’d be able to seed something really dense – like a black hole.

In order to get large enough fluctuations that can collapse to form primordial black holes immediately after inflation, you need inflation to happen in a very specific way. For the Universe to expand rapidly, you need a lot of energy, and we can model how that energy is spent as a ball rolling down a hill losing potential energy. The dynamics of how this ball rolls down the hill describes the dynamics of inflation. The faster the ball rolls, the quicker the potential energy is lost and the less time fluctuations have to grow. This means that if you want really large densities to be able to grow and be left over at the end of inflation, you need to slow down that ball. Today’s authors propose a cosmological speed bump for doing this. ...

Primordial Black Holes from a Tiny Bump in the Inflaton Potential ~ Swagat S. Mishra, Varun Sahni
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Why are there so many sub-Neptune exoplanets?

Post by bystander » Fri Dec 20, 2019 8:17 pm

Why are there so many sub-Neptune exoplanets?
astrobites | Daily Paper Summaries | 2019 Dec 17
Stephanie Hamilton wrote:
In the few decades since the discovery of the first exoplanet in 1992, we’ve realized that our own solar system is just plain weird. We have no hot Jupiter gas giant planets whizzing around our star in a matter of days, nor do we have any sub-Neptune planets, the most common type of planet in the galaxy. Critically, our lack of sub-Neptunes severely hinders our understanding of the transition between Earth-like and Neptune-like planets.

The Kepler Space Telescope operated from 2009—2018 and discovered over 2600 exoplanets, nearly 1000 of which were classified as sub-Neptunes. But Neptune-like planets are considerably rarer, despite being only slightly bigger. This “radius cliff” (Fig. 1) separates sub-Neptunes (radii < 3 R⊕, where R⊕ is Earth’s radius) from Neptunes (radii > 3 R⊕). What could cause such a steep dropoff? Today’s authors explore this question. ...

Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis ~ Edwin S. Kite et al
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Spin-teresting Effects of Binary Star Formation

Post by bystander » Fri Dec 20, 2019 8:27 pm

Spin-teresting Effects of Binary Star Formation
astrobites | Daily Paper Summaries | 2019 Dec 18
Jenny Calahan wrote:
If you live at a similar latitude to me, or live in the northern hemisphere (hopefully with less cloudy snowy weather… #Michigan), you can see the big dipper on the horizon right now. It is visible right after sunset. You might be familiar with the seven bright stars that make up this asterism. But hold up! It’s actually eight stars. In the middle of the handle, there are actually two stars very close to each other called Mizar and Alcor. Normally when you look at any two bright stars next to each other in the sky, they are not physically close to each other. One could be very bright and far away, the other dimmer and closer to us. But Mizar and Alcor are actually gravitationally bound to each other, orbiting a common center of mass. This is what we call a binary star system (although Mizar and Alcor actually have some extra fainter companions, making it a multiple star system).

Turns out that there are many stars out there that are in what we call binary, or multiple star systems. We also see a wide range of separations between the stars in these systems. For example, two stars could be anywhere from just 30 AU out to 10,000 AU apart from each other. Binary star systems occur very frequently. We even see exoplanets around them. The way binary systems evolve can have tremendous insight into high energy phenomena such as x-ray binary systems, stellar evolution, and the population of field stars. This is all to say that binary systems exist, and it is very much worth understanding how they got to be there!

Looking at little baby stars, we see that even at these young ages, stars form alongside other stars forming a binary or multi-star system right from the beginning. There are two main paths to forming two stars at once. One, is turbulent fragmentation. That happens when you have a molecular core infalling and starting to form a star, but something happens like a shock wave or other source of turbulence that produces an over-density within that core, forming a sort of secondary core within a core. The second method is disk fragmentation. That happens when a protostellar disk has started to form, but perhaps it is growing in mass very quickly, and a gravitational instability can be triggered, forming a ‘seed’ of a secondary companion. In general, one might assume that turbulent fragmentation would lead to binary systems with wide separations, and disk fragmentation would lead to shorter separations. This is because turbulent fragmentation can happen over very large distances, but disk fragmentation only occurs in a disk with typically is around 200AU. You might also expect that turbulent fragmentation will lead to stars with different and random spins while stars forms from ~the same disk would have aligned spins. This paper explores the question: If turbulent fragmentation is occurring, what would the final state of binary/multi-star systems look like? ...

The Turbulent Origin of Outflow and Spin Misalignment in Multiple Star Systems - Stella S. R. Offner et al
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How do bound star clusters form?

Post by bystander » Fri Dec 20, 2019 8:33 pm

How do bound star clusters form?
astrobites | Daily Paper Summaries | 2019 Dec 20
Jessica May Hislop wrote:
Where do stars form in galaxies? We know stars form from gas, which collapses and cools. Whilst stars will form wherever gas is dense enough, regions of high stellar densities called star clusters are observed in all galaxies. From observations, we know that only around 10% of star-forming gas clumps within a galaxy become gravitationally-bound star clusters. Moreover, we observe these star clusters to form stars at continually increasing rates. This 10% of star-forming regions that do go on to form gravitationally-bound star clusters must be special in some way.

To explore this, today’s authors propose five different models for forming star clusters and evolve them analytically in order to calculate the evolution of the gas and the stars. They then compare this with observations to see which model fits best. ...

How do bound star clusters form? ~ Mark R. Krumholz, Christopher F. McKee
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Separated At Birth: Comparing Chemical Compositions of Binaries

Post by bystander » Mon Dec 30, 2019 4:48 pm

Separated At Birth: Comparing Chemical Compositions of Binaries
astrobites | Daily Paper Summaries | 2019 Dec 23
Oliver Hall wrote:
To understand our place in the Milky Way, we need to understand its history. Answering questions about the dynamical evolution of our galaxy — how its different components evolved and interacted — is the driving force behind the field of Galactic Archaeology. Using information about stellar movements, ages and chemical compositions, scientists can construct a picture of how the layout of the Milky Way has changed over the course of history.

One of the most important techniques in Galactic Archaeology is chemical tagging. As the universe ages, stars will create heavier elements and disperse these throughout their interstellar neighbourhood. As time goes on, different regions of the universe will have different chemical abundances unique to that time period. The technique of chemical tagging, then, rests on the assumption that stars with similar chemical compositions were born together. If this holds true, chemical tagging could be used to identify stellar clusters that have since dispersed, or accreted material. ...

In order to determine whether stars born in groups have the same chemical composition, todays authors performed thorough spectroscopic measurements on all 50 stars in their 25 wide binary pairs (see Figure 1). They measured 23 different elements, but focused primarily on the typically well constrained iron-to-hydrogen ratio, [Fe/H], expressed in logarithmic units of dex. ...

‘DNA’ of Twin Stars to Reveal Family History of the Milky Way
McDonald Observatory | University of Texas | 2019 Dec 20

Identical or fraternal twins? : The chemical homogeneity of wide binaries from Gaia DR2 ~ Keith Hawkins et al
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