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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