astrobites: Daily Paper Summaries 2019

Find out the latest thinking about our universe.
User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Gravitational waves through the looking glass

Post by bystander » Fri Mar 22, 2019 6:13 pm

Gravitational waves through the looking glass
astrobites | Daily Paper Summaries | 2019 Mar 20
Philippa Cole wrote:
So far, LIGO and VIRGO have detected 11 gravitational wave events – the ripples in spacetime that have traveled to us from violent collisions of black holes and neutron stars. As upgrades take place and new detectors come online, we will hopefully see tens of these types of event over the coming years. This is exciting stuff in its own right, but today’s authors have added another layer of intrigue to these future detections – they might be able to tell us about what lies betwixt us and these mergers, which could include stars, remnants of galaxies, or even primordial black holes. ...

Observational Signatures of Microlensing in Gravitational Waves at LIGO/Virgo Frequencies ~ J.M. Diego et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

U(V) Light Up My Life

Post by bystander » Fri Mar 22, 2019 6:26 pm

U(V) Light Up My Life
astrobites | Daily Paper Summaries | 2019 Mar 21
Jessica Roberts wrote:
M-dwarfs, the smallest and coolest (in temperature and by reputation) of the main sequence stars, are also the most abundant type in our galaxy. As this Astrobite post notes, Earth-sized and super-Earth sized planets are also quite common around these stars, with 0.86 of these types of planets per M-dwarf. Also, as they are cooler, the habitable zone around M-dwarfs is located much closer in. In turn, the shorter period of these habitable zone planet makes it easier to observe multiple transits. Therefore, if we want to search for a habitable planet, then we should be looking for it around an M-dwarf. Several habitable zone planets have already been discovered around M-dwarfs, Trappist-1 has 3 of them!

Unfortunately just because a planet is in the habitable zone doesn’t necessarily mean that it can support life. These stars produce a lot of ultraviolet light, especially at the shorter UV wavelengths, called extreme ultraviolet light (EUV). Not only does UV light give you a sunburn, it can wreak havoc on life, from destroying cells to stripping a planet of its water and atmosphere. Most studies agree UV is bad for business and there is a high probability that M-dwarf planets can’t support life as we know it.

The authors of today’s paper, shine a new light (pun intended) on the effect of UV on life. They note that previous laboratory studies have discovered that UV light is actually necessary to create the building blocks required for life, such as RNA, amino acids, and sugars. The creation of these building blocks is also known as prebiotic photochemistry. Could M-dwarf UV radiation actually be fueling life instead of destroying it? ...

The Surface UV Environment on Planets Orbiting M-Dwarfs: Implications for Prebiotic Chemistry
and the Need for Experimental Follow-Up
~ Sukrit Ranjan, Robin D. Wordsworth, Dimitar D. Sasselov
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Testing Einstein’s Equivalence Principle

Post by bystander » Wed Mar 27, 2019 6:53 pm

Testing Einstein’s Equivalence Principle by
Timing a Pulsar in a Stellar Triple System

astrobites | Daily Paper Summaries | 2019 Mar 25
Aaron Pearlman wrote:
One of the guiding ideas that led Einstein to develop the general theory of relativity (GR) was Einstein’s equivalence principle (EEP), which asserts that the gravitational and inertial masses of an object must be the same. Thus, it is not possible to physically distinguish between the inertial forces resulting from accelerating through space (e.g., in a rocket ship) and the force of gravity due to the curvature of space-time (e.g., near the surface of the Earth). The EEP therefore requires that objects in an external gravitational field fall identically, also known as the universality of free fall. An extension of the EEP is the strong equivalence principle (SEP), which states that the universality of free fall should also apply to self-gravitating bodies such as neutron stars. Tests of the SEP are important for constraining the nonlinearity of gravity, and there are very few theories of gravity, other than GR, that satisfy the SEP.

PSR J0337+1715 is the first millisecond pulsar (MSP) stellar triple system to be discovered out of more than 300 known MSPs in the Galaxy and globular clusters. The pulsar is in a 1.6 day orbit with a white dwarf weighing ~0.2 times the mass of our Sun, which are both also in a 327 day orbit with a ~0.4 solar mass white dwarf (see animation). Surprisingly, the orbits are nearly circular and coplanar, which suggests that this particular stellar triple system experienced an exotic and complex evolutionary history that differs from other stellar systems. Since the gravitational field of the outer binary strongly accelerates the inner binary containing the pulsar (which has strong self-gravity), PSR J0337+1715 offers an ideal laboratory for performing strong-field tests of the SEP. ...

Universality of Free Fall from the Orbital Motion of a Pulsar in a Stellar Triple System - Anne M. Archibald et al
viewtopic.php?t=38445
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

What We Don’t Know About Protoplanetary Disks

Post by bystander » Wed Mar 27, 2019 7:24 pm

What We Don’t Know About Protoplanetary Disks
astrobites | Daily Paper Summaries | 2019 Mar 26
Lauren Sgro wrote:
If you’re well-versed in exoplanets (or even the formation of your own planet), you may have heard the term protoplanetary disk. Protoplanetary disks are disks of gas and dust surrounding a fairly newborn star, although newborn here means up to several million years old. Interestingly, images of protoplanetary disks that astronomers have captured reveal gaps in the disks, or rather separate rings of material, depending on your perspective. These types of disks were once expected to be completely smooth, so why are we seeing gap-like features in essentially all resolved images of them?

The authors of today’s paper use ALMA data to explore what could be causing these gaps. They examine images of 16 different protoplanetary (see Figure 1) and transition disks (disks where the material closest to the star has been cleared out). The sample contains stars of various spectral types, each exhibiting multiple gap features. Since these gaps are present throughout the sample, some correlation between them should reveal the responsible mechanism, right? After all, we would expect that these features evolve in similar ways for most disk systems.

Before they can answer this question, the authors first determine each star + disk’s luminosity and use this information to age each star with model evolutionary tracks. The resulting age range gave them a way to classify their disks as older or younger. They also determined approximate gap locations and sizes via a type of intensity profile fitting, which essentially models the light coming from the star + disk in each image (where there is a gap, there is less light detected, etc.). ...

Protoplanetary Disk Rings and Gaps across Ages and Luminosities ~ Nienke van der Marel et al
viewtopic.php?t=38971
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Looks like the Sun, but does it spin like the Sun

Post by bystander » Wed Mar 27, 2019 7:38 pm

Looks like the Sun, but does it spin like the Sun:
Measuring differential rotation in 16 Cyg A & B

astrobites | Daily Paper Summaries | 2019 Mar 27
Oliver Hall wrote:
It’s not something you normally picture, but every star in the night sky is spinning. Some stars, like the Sun, spin relatively slowly, one rotation every 25 days. Some others, in less than a day! Rotation rates are an important and useful way of understanding stars. For example, for stars burning hydrogen in their cores like the Sun (called Main Sequence stars), the rotation rate is known to be linked to the stars colour and its age.

Rotation also plays a lead role in the most prominent theory explaining changes in magnetic activity of our Sun, which varies in a roughly-periodic 11-year cycle. The particular aspect of the Sun’s rotation that is relevant to this theory is differential rotation, the change in rotation rates of the solar material at different latitudes. It is known that the Sun rotates about 1.4 times as fast at the equator than it does at the poles, and that this holds true both for the solar surface, and the layer beneath the surface where convection happens, suggesting a possible relationship between the mechanism of convection and differential rotation. The Sun is only one star, however; if we really want to understand the impact of differential rotation on the physics of stars, we need to expand our search beyond the solar system.

The authors of today’s paper set out to expand our knowledge of differential rotation in other stars like the Sun. They studied two stars in a binary pair, 16 Cyg A & B, two solar analogues, in the sense that they have similar physical characteristics to the Sun. What makes these stars especially interesting in this study is that they have rotation rates that are similar to the Sun. This raises the question: do we observe rates of differential rotation in these stars that are similar to the Sun, or are they different, and why? ...

Latitudinal Differential Rotation in the Solar Analogues 16 Cygni A and B ~ M. Bazot et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

The “Turbulent” Relationship between Stellar Feedback and Magnetic Fields

Post by bystander » Tue Apr 02, 2019 4:34 pm

The “Turbulent” Relationship between Stellar Feedback and Magnetic Fields
astrobites | Daily Paper Summaries | 2019 Mar 28
Michael Foley wrote:
Building a star is not an easy task. We now know that a number of factors come into play, such as turbulence in the surrounding material, stellar feedback, and one of the most notoriously tricky things in astronomy – magnetic fields. Turbulence has a complicated role in the process, creating regions of high-density gas that gravitationally collapse to form stars while also preventing gas from collapsing too much on large scales. Magnetic fields are also believed to support the gas against large scale collapse and can prevent stable accretion disks from forming around protostars. Today’s paper discusses the relationship between all three aforementioned components to see how the degree of turbulence in magnetic field lines affects the young star. Let’s step back first, though, and talk about what is meant by stellar feedback. ...

The Role of Initial Magnetic Field Structure in the Launching of Protostellar Jets ~ Isabella A. Gerrard et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

A Strange Type of Matter May Lie at the Heart of Neutron Stars

Post by bystander » Tue Apr 02, 2019 4:48 pm

A Strange Type of Matter May Lie at the Heart of Neutron Stars
astrobites | Daily Paper Summaries | 2019 Mar 29
Bryanne McDonough wrote:
The types of matter we experience in every day life are simple. From a young age, we learn about the solids, liquids, and gasses around us. Plasmas are also a part of our everyday life now, from TV screens to fluorescent light bulbs. Plasma occurs when there is enough energy to strip electrons away from their atoms. What happens if we take a plasma and add even more energy? The Large Hadron Collider (LHC) at CERN can slam atoms together to produce so much energy the matter melts into a plasma of quarks. ...

Quark-gluon plasma needs extreme energy to exist. That type of energy will only exist in very hot, very dense places, like neutron stars. Neutron stars are made up of — you guessed it — neutrons and are the densest objects in the universe. Imagine compressing the mass of the sun into an area the size of Dublin. In today’s paper, the authors explored what kind of neutron stars would be able to host a core of quarks. ...

Quark-Matter Cores in Neutron Stars ~ Eemeli Annala et al
viewtopic.php?t=36218
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Distances in the Dark

Post by bystander » Tue Apr 02, 2019 5:02 pm

Distances in the Dark: Using Binary Black Holes to Study the Universe’s Expansion
astrobites | Daily Paper Summaries | 2019 Apr 01
Stephanie Hamilton wrote:
Nearly all of the galaxies we observe in the night sky are rushing away from us. Only the Andromeda galaxy is moving toward us — we are trapped in a gravitational dance that will end in a major collision about 4.6 billion years from now. The remainder of galaxies are receding due to the expansion of the universe. But how fast are the rest of the galaxies flying away from us? This is actually a difficult question to answer, partly because it is difficult to accurately measure distances across the universe. Today’s paper details a new method to measure how quickly the universe is expanding using the gravitational wave (GW) signals from binary black hole collisions. ...

First Measurement of the Hubble Constant From a Dark Standard Siren Using the Dark Energy Survey Galaxies and the
LIGO/Virgo Binary-Black-Hole Merger GW170814
~ DES Collaboration, LIGO Scientific Collaboration, Virgo Collaboration et al
viewtopic.php?t=37611
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Searching for FRBs Using Neutral Networks and Machine Learning

Post by bystander » Tue Apr 02, 2019 5:49 pm

Searching for FRBs Using Neutral Networks and Machine Learning
astrobites | Daily Paper Summaries | 2019 Apr 02
Haley Wahl wrote:
Fast radio bursts (FRBs) are currently one of the most mysterious objects in radio astronomy. They are extremely bright bursts of energy that last for only milliseconds and we currently have no idea how or why they happen, only that they seem to be coming from very far away. Identifying these bursts in a data set has proven to be a challenge, but a team at West Virginia University has developed a technique using neural networks and machine learning that could search through telescope data and detect candidates in real time. ...

Towards Deeper Neural Networks for Fast Radio Burst Detection ~ Devansh Agarwal et al
viewtopic.php?p=286130#p286130
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Dark Matter vs MOND: A Tug of War

Post by bystander » Mon Apr 08, 2019 4:39 pm

Dark Matter vs MOND: A Tug of War
astrobites | Daily Paper Summaries | 2019 Apr 03
Ali Lezeik wrote:
When Vera Rubin made her discovery on galaxy rotations back in the 70’s, she knew she was onto something that would be the center of scientific discussion for the decades to come, and still is to our modern times. ...

Rubin noticed that there is a discrepancy between observed galaxy rotation curves – A plot of the orbital speeds of visible stars or gas in a galaxy versus their radial distance from that galaxy’s center, and theoretical predictions, i.e. Newton’s law. ... And with such high speeds, stars on the edges of these spiral galaxies should fly away, getting unbounded from their orbits. ... However, we do not see stars flying away from the edges of these galaxies, hinting that there is some hidden force holding them to it. ... This exact question has led to the birth of the Dark Matter hypothesis, which suggests that there exists mysterious invisible matter that is holding galaxies from falling apart. ...

Another candidate theory doubts that Newton’s law, as we understand it, fully explains these rotation curves, claiming it needs some modification. This other candidate is known as MOND, MOdified Newtonian Dynamics. ... MOND calls for a revision of Newton’s law, and as extraordinary as this suggestion sounds, it does offer potential solutions to some otherwise troubling problems in galactic dynamics. It proposes that there is a fundamental acceleration scale a0 that should be added to the classically known Newton’s law. And with this new scale, at very large radii and small accelerations, gravity decays with distance more slowly than Newton’s inverse square law. This removes the need for dark matter, providing a clear explanation for the tight non- Newtonian correlation between visible matter and radial acceleration. ...

A new study ... examined the rotation curves of 193 disk galaxies, to see if there truly does exist such a universal fundamental acceleration scale a0. ... what was realized is that for each galaxy ... a unique scale factor was being found, a0 seems to be coming from the internal dynamics of each galaxy, meaning that this scale factor is emergent from within each galaxy and hence NOT a universal scale factor. ...

Absence of a Fundamental Acceleration Scale in Galaxies ~ Davi C. Rodrigues et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Old, New, Cyclic, and Ultra-Blue

Post by bystander » Mon Apr 08, 2019 4:56 pm

Old, New, Cyclic, and Ultra-Blue:
Visualizing the Last 400 Years of Solar Activity

astrobites | Daily Paper Summaries | 2019 Apr 04
Ellis Avallone wrote:
The solar magnetic field is continuously reshaped and regenerated through a process known as the solar cycle, or the periodic variability of the Sun’s magnetic activity on an 11 year timescale. Although this cycle repeats every 11 years, the intensity of solar radiation (which is strongly coupled to the overall magnetic activity), changes from cycle to cycle. The change in solar irradiance, or the amount of sunlight we receive here at Earth, is correlated with changes in Earth’s climate. Note: the warming trend observed since the 1980s is not caused by the Sun. The correlation between solar irradiance and its effect on Earth’s climate is of particular interest when the Sun goes through long periods of extremely low activity, the most notable of which is the Maunder Minimum which occurred between 1645 and 1715. Recent reexamination of historic solar activity data from the last 400 years has sparked further efforts to understand the abnormally low activity during the Maunder Minimum. Within this context, the authors of today’s paper focus on the limitations of using historical data to estimate and understand long-term solar variability (and events like the Maunder Minimum) and present a new way to visualize the current state of historic solar cycle data. ...

Visualization of the challenges and limitations of the long-term
sunspot number record
~ Andrés Muñoz-Jaramillo, José M. Vaquero
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Visions of the future

Post by bystander » Sat Apr 13, 2019 5:53 pm

Visions of the future: A planetary fragment orbiting a long
dead star offers a glimpse of the Solar System’s fate

astrobites | Daily Paper Summaries | 2019 Apr 09
Jamie Wilson wrote:
Nothing lasts forever. A statement as equally applicable throughout the Cosmos as it is for more earthly matters. Stars – like people – are born, age and eventually die, and in doing so they seed the Universe with the building blocks for the next generation of stars and planets.

The lifetime and ultimate fate of a star once it exhausts its supply of fuel and expands to become a red giant, depends on how much mass it started out with. Very high mass stars burn bright, but burn brief, perishing in what is surely one of nature’s most awesome and spectacular displays: a supernova explosion. Less massive stars, like the Sun, eventually shed their outer layers to form a striking and graceful planetary nebula, leaving behind only their hot, inert cores: a white dwarf. All that remains of the once bright star.

A white dwarf is extremely dense. Its mass is comparable to the mass of the Sun, but squeezed into a volume not much larger than that of the Earth. This concentration of mass produces an intense gravitational field capable of ripping apart any surviving planets that stray too close. White dwarfs represent the most common end point for the majority of stars (about 97%) in the Milky Way. Since a white dwarf no longer generates any energy through nuclear fusion it gradually cools over immense timescales, eventually fading into obscurity. But what happens to the remaining planets in these systems? Can they survive the death of their host stars? A number of white dwarf systems have been observed to host compact dust discs which are believed to be the tidally disrupted remains of planetary systems. ...

A planetesimal orbiting within the debris disc around a white dwarf star ~ Christopher J. Manser et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

The First Image of a Black Hole

Post by bystander » Sat Apr 13, 2019 6:00 pm

The First Image of a Black Hole
astrobites | Daily Paper Summaries | 2019 Apr 11
Daniel Palumbo wrote:
In April 2017, the Event Horizon Telescope (or EHT), a global interferometric network of radio dishes, observed the bright center of the galaxy M87. The EHT team studied four days of intense observations and spent months calibrating nearly a petabyte of data! What we found was astounding: an asymmetric ring-like structure consistent with the emission expected to outline the shadow of a black hole event horizon! More than six billion solar masses were confined to an area on the sky equivalent to roughly the size of an atom held at arm’s length, or a physical distance of thrice the length of our solar system.

Today we examine six papers, released together by the EHT collaboration; Paper I tells the connected story of the EHT result, while papers II-VI step through the instrument, data processing, imaging, comparison to theoretical work, and mass measurement, respectively. We’ll examine what kinds of structure we might expect to see, what the EHT actually saw, and what these results mean for Einstein’s theory of general relativity (spoiler alert: Einstein has nothing to worry about yet!).

It’s a tale as old as general relativity, so let’s take it from the top. ...

viewtopic.php?t=39338
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Understanding how massive star clusters form in our Universe

Post by bystander » Sat Apr 13, 2019 6:12 pm

Understanding how massive star clusters form in our Universe
astrobites | Daily Paper Summaries | 2019 Apr 11
Jessica May Hislop wrote:
Young massive clusters (YMCs) are the most compact, high-mass stellar systems still forming today. They have masses of around 10,000 times the mass of the sun, are less than 100 million years old and have radii of less than 3 light years. For reference, our Milky Way galaxy has a radius of 53,000 light years! The clouds from which they form from are rare due to their large initial gas reservoirs and rapid dispersal timescales due to stellar feedback. We can observe them however, and due to them being at lower redshift, they’re closer to us meaning we can resolve them better with our telescopes. This allows us to observe individual star formation and test the theories of star and cluster formation.

We know that the starting point of YMC formation is simply a large cloud of gas and dust, known as molecular clouds, shown in Figure 1. The authors in today’s paper try to answer the question of how a molecular cloud forms a typical YMC, with particular emphasis on the observed densities of these objects. ...

Young massive star cluster formation in the Galactic Centre is driven
by global gravitational collapse of high-mass molecular clouds
~ A. T. Barnes et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Re: astrobites: Daily Paper Summaries 2019

Post by bystander » Sat Apr 13, 2019 6:26 pm

When the Sky Isn’t the Limit: Simulations
of Imaging a Black Hole from Space

astrobites | Daily Paper Summaries | 2019 Apr 12
Kaitlyn Shin wrote:
Earlier this week, scientists from the Event Horizon Telescope (EHT) collaboration released the first ever images of a black hole’s shadow. Although the collaboration observed two sources — a supermassive black hole (SMBH) at the center of the galaxy Messier 87 (M87), and Sagittarius A* (Sgr A*, pronounced “Sag. A-star”), the SMBH at the heart of our own — the results released were of M87. The SMBH in M87 was targeted because of its enormous mass, but in order to see its shadow directly, an Earth-sized telescope would have to observe it at a frequency of 230 GHz (1.3 mm wavelength) or higher.

In order to build a telescope effectively the size of the Earth, the EHT collaboration used a technique called Very-Long-Baseline Interferometry (VLBI). Through VLBI, signals from the same source are collected at multiple radio telescopes; using the time difference between the arrival times of the signals at different observatories, the observed radiation is then correlated between every pair of telescopes to produce extremely high-resolution observations. On and off over multiple days in April 2017, many radio observatories across the world observed as one effectively Earth-sized telescope, an exceptional testament to the power of scientific collaboration. As impressive as this achievement already is, some scientists are already looking to take VLBI imaging of black holes to the next level: space.

The authors of today’s paper released a concept study of a space-based VLBI (SVLBI) experiment called the Event Horizon Imager (EHI), a nod to the success of EHT. SVLBI operates similarly to traditional ground-based VLBI, with the main difference being that the radio telescopes are satellites in space. The preliminary design for EHI involves two or three satellites in Medium-Earth Orbit at slightly different radii (~14,000 km) and thus orbiting at slightly different speeds, allowing for coverage that would enable high-precision image reconstructions. ...

Simulations of imaging the event horizon of Sagittarius A* from space ~ Freek Roelofs et al
viewtopic.php?t=39338
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Avoiding a (thermonuclear) supernova

Post by bystander » Tue Apr 16, 2019 9:03 pm

Avoiding a (thermonuclear) supernova
astrobites | Daily Paper Summaries | 2019 Apr 15
Sanjana Curtis wrote:
You may have read about the ongoing debate over the progenitors of Type 1a supernovae. One of the proposed scenarios for producing a Type 1a supernova is the merger of two white dwarfs. When the total mass of the merging white dwarfs exceeds the Chandrasekhar limit, the merger product can go up in flames in a thermonuclear explosion that gives rise to a Type 1a supernova. But what happens if the merger product avoids this fate? Today’s paper reports the discovery of an object that may have done just that!

A merger product that avoids going supernova is expected to form a nebula, with a hot, highly magnetized, fast-rotating central star. The nebula would be hydrogen- and helium-free, which makes sense given that white dwarfs are typically composed of carbon and oxygen. The star could proceed to survive for tens of thousands of years before its ultimate collapse, likely leaving behind a neutron star! ...

A massive white-dwarf merger product prior to collapse ~ V.V. Gvaramadze et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

A Protoplanetary Disk with a Tail

Post by bystander » Tue Apr 16, 2019 9:21 pm

A Protoplanetary Disk with a Tail
astrobites | Daily Paper Summaries | 2019 Apr 16
Charles Law wrote:
The formation of our solar system traces its history back to a swirling cloud of primordial gas and dust. New observations of protoplanetary disks – the cradles of planet formation – toward other stars have revealed unexpected chemical complexity, dust substructure in the form of rings and gaps, and even spirals arms. Although once thought to be nearly featureless and relatively homogenous, the varied nature of disks continue to surprise astronomers with unanticipated structures, orbital oddities, and collisionally-restructured systems all having been recently discovered. One such surprising disk around SU Aurigae (SU Aur) is the subject of today’s astrobite.

Today’s authors used ALMA to observe CO gas in the protoplanetary disk around SU Aur, which is located 470 lightyears away from Earth in the Taurus-Auriga star forming region. Previous scattered light observations of SU Aur – a 7 Myr old, solar-type star – revealed the presence of a long tail-like structure in small dust grains. This dust tail was found to extend more than 350 AU in length from the disk body out to the west. The discovery of this giant dust tail, nearly 7x the size of our solar system, motivated the search for a counterpart gaseous tail. To do so, today’s authors used a rotational transition of CO, a well-known gas tracer in protoplanetary disks. ...

A Tail Structure Associated with a Protoplanetary Disk around SU Aurigae ~ Eiji Akiyama et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Let’s Talk about Exoplanetary Photospheres

Post by bystander » Sat Apr 20, 2019 2:20 pm

Let’s Talk about Exoplanetary Photospheres
astrobites | Daily Paper Summaries | 2019 Apr 17
Vatsal Panwar wrote:
Exoplanets transiting or eclipsed by their host stars provide an exciting opportunity to get a spectroscopic view into their atmospheres. Measuring the dip in starlight across different wavelengths as it streams through the atmosphere of a transiting exoplanet (a method commonly known as transmission spectroscopy) can help us put constraints on the chemical composition and structure of the planet’s atmosphere. Measuring the secondary eclipse (when the planet passes behind the star) on the other hand gives us an estimate of the light emitted and reflected solely by the planet’s atmosphere. Although obtaining the reflected and emission spectrum of an exoplanet this way can be a difficult business, recent space-based secondary eclipse observations have revealed some interesting properties about these atmospheres. Models for interpreting the measured transmission spectra of exoplanets expectedly hinge around the wavelength dependence of the observed radius of the planet. However, calculations for modeling a planet’s flux as measured from its eclipse often involves assuming the planetary radius to be constant, leaving the wavelength dependence solely to the reflected and thermally emitted flux of the planet. The authors of today’s paper investigate the effect such an approximation actually has on calculating the model emission and reflected spectra of exoplanets. ...

Exploring A Photospheric Radius Correction to Model Secondary
Eclipse Spectra for Transiting Exoplanets
~ Jonathan J. Fortney et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Applying color-magnitude diagrams to semi-resolved galaxies

Post by bystander » Sat Apr 20, 2019 2:30 pm

Teaching an old dog new tricks: Applying color-
magnitude diagrams to semi-resolved galaxies

astrobites | Daily Paper Summaries | 2019 Apr 17
John Weaver wrote:
The wealth of information we have gathered about the lives of galaxies within our universe is due in part to increasingly larger and more sophisticated surveys of the night sky. Better resolution, precipitated by rapid advancement in imaging technology and telescope design, has enabled detailed studies ranging from our local group to the edges of the cosmos.

Meanwhile, the color-magnitude diagram (CMD) has remained one of the most called upon diagnostic figures in an astronomer’s tool belt. The combination of color and apparent magnitude alone is enough to map out a stellar population without much clutter, allowing for a clean-cut track of main-sequence stars burning away their hydrogen cores as well as potentially more massive stars in later stages of their lives which have moved off the main sequence. Traditionally, the CMD has enabled astronomers to obtain estimates of the ages of star clusters by determining their main-sequence turn-off point. More useful perhaps is the ability to measure dust as a shift in color (due to reddening) and magnitude (due to extinction), as well as to estimate distances by leveraging the statistical advantage of having tens to hundreds of stars to compare observed magnitudes with expected magnitudes based on theoretical CMD tracks.

While many of these techniques have been known and readily applied in the past and met with great success within our Galaxy, the authors of today’s astrobite demonstrate that this old dog can be taught new tricks. ...

Measuring Star-Formation Histories, Distances, and Metallicities with
Pixel Color-Magnitude Diagrams I: Model Definition and Mock Tests
~ B. A. Cook et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Evaporating Disks With Massive Stars

Post by bystander » Sat Apr 20, 2019 2:42 pm

Evaporating Disks With Massive Stars
astrobites | Daily Paper Summaries | 2019 Apr 19
Spencer Wallace wrote:
Planets are born from the dense, dusty regions surrounding young stars called protoplanetary disks. Recently, the ALMA telescope has provided some of the most detailed images and information about protoplanetary disks to date. Understanding the evolution of these objects is crucial to constrain how and where planets form. It is thought that planets can grow from tiny dust grains that settle toward the midplane of the disk, or even quickly materialize through gravitational instabilities.

These disks don’t stay around forever. Their viscous, fluid nature means that disk material continually migrates inward, eventually falling onto the central star. At the same time, the radiation from the central star heats and erodes the disk from the inside out until eventually nothing is left except for a few planets. Determining how long the disks stay around tells us how long planets take (at most) to form.

Stars, which host the planet forming disks, tend to form in groups and so this evaporation process can also be driven by other nearby companions. Clusters and associations usually host at least a few extremely massive stars which output colossal amounts of ultraviolet (UV) radiation. The radiation can carve huge bubbles and cavities into the natal material, in addition to deteriorating nearby protoplanetary disks. This process is called photoevaporation and is caused by the acceleration of material due to the intense heat from energetic photons.

Astronomers have recently begun asking how these high radiation environments affect the evolution of protoplanetary disks. The authors of today’s paper flip this question around and instead ask what the distribution of protoplanetary disks on the sky can tell us about the environment from which they formed. Specifically, they focus on the evolution of the stellar association Cygnus OB2, home to some of the most massive stars ever found, and try to understand how the highly UV irradiated environment affects the population of disks. ...

External photoevaporation of protoplanetary discs in Cygnus OB2:
Linking discs to star formation dynamical history
~ Andrew J. Winter et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

A Planetary Death Census

Post by bystander » Wed Apr 24, 2019 3:38 pm

A Planetary Death Census
astrobites | Daily Paper Summaries | 2019 Apr 22
Spencer Wallace wrote:
In another 6 billion or so years, the Sun will run out of fuel, expand and shed its outer layers until only a hot, dense core called a white dwarf remains. Although the expansion of the dying Sun will engulf most of the inner Solar System, it’s unclear what will happen to the terrestrial planets. They could either be engulfed by the Sun, driven to a safer, cooler location or possibly even ejected into interstellar space.

To solve this puzzle, you could wait around for the Sun to die. In a more reasonable amount of time, you could also look for elderly stars that show signs of having recently consumed planet-building materials. This can be done either by observing brightness variations from torn apart planets passing between us and the star or by searching for planet-building material sinking onto the surface of a star. The latter method has been much more successful and is what we will focus on today. White dwarf stars which show signs of infalling planetary material are called ‘polluted white dwarfs’. It turns out that the light produced as super heated planet building material sinks into the star currently provides the most accurate way of determining what faraway planets are made of.

We already have a decent idea of what it looks like when a dying star consumes a planet, but astronomers are not sure why and where it can happen. Is this a common outcome of the stellar life cycle? Will this eventually happen with the Sun? Some white dwarfs come in pairs. Is it the complex gravitational interaction between a pair of stars and a planet that subjects the planet to a firey fate? The paper we are looking at today examines a randomly selected, unbiased collection of white dwarf stars as an attempt to answer these questions. ...

The unbiased frequency of planetary signatures around single and
binary white dwarfs using Spitzer and Hubble
~ Thomas G. Wilson et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

A new, wiggly ruler for measuring the universe

Post by bystander » Wed Apr 24, 2019 3:56 pm

A new, wiggly ruler for measuring the universe
astrobites | Daily Paper Summaries | 2019 Apr 23
Kate Storey-Fisher wrote:
Baryon acoustic oscillations (BAOs) are a well-understood phenomenon in the early universe. When baryons decoupled from photons, a baryon-photon pulse shot throughout the universe at 170,000 km/s, much as sound propagates via fluctuations in air pressure—hence the name “acoustic oscillations.” At recombination, the baryons screeched to a near-halt of 6 km/s, leaving a signature at the distance they traveled in that time. This is the acoustic scale.

But there’s an epilogue to this story. After recombination, there was still a supersonic relative velocity between baryons and dark matter, of about 30 km/s (Mach number ~5). This difference in velocity also fluctuates, in turn producing acoustic oscillations. This effect is dubbed velocity-induced acoustic oscillations (VAOs).

VAOs were first pointed out in 2010, and today’s paper probes them by adding their effect to a modern, public 21-cm code. In a companion paper the author describes the results of these simulations, and in today’s paper they show that VAOs can be used as a new “standard ruler” in the early universe. ...

A Standard Ruler at Cosmic Dawn ~ Julian B. Muñoz Velocity-Induced Acoustic Oscillations at Cosmic Dawn ~ Julian B. Muñoz
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Recent Magma Heating Might Explain Water on Mars

Post by bystander » Fri Apr 26, 2019 11:24 pm

Recent Magma Heating Might Explain Water on Mars
astrobites | Daily Paper Summaries | 2019 Apr 24
Will Saunders wrote:
For years now it has been widely held that Mars had an abundance of liquid water in its early history that some scientists believe was removed through an extreme global warming scenario. In 2017 the Mars Reconnaissance Orbiter found evidence of water ice deposits under the poles of Mars. Then in 2018 the Mars Express spacecraft found, using radar, evidence of liquid deep under the ice. Liquid water requires high pressures and temperatures, not believed to be possible on Mars currently, which led the authors of this paper to hypothesize that heating deep under the Martian south pole is responsible for melting the ice.

Dissolving salts in water will lower the melting point, which is why putting salt on a frozen sidewalk melts the ice. While Mars’ surface temperature of 162 K (-111°C) is too cold to melt pure ice, salty ice would melt more easily. The authors propose that the ice under the south pole is quite salty and heated from underneath by a yet undiscovered mechanism, creating a small pool of liquid water. ...

Water on Mars, with a Grain of Salt: Local Heat Anomalies
Are Required for Basal Melting of Ice at the South Pole Today
~ Michael M. Sori, Ali M. Bramson
viewtopic.php?t=39170
viewtopic.php?t=38548
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

For habitability, two stars are not better than one

Post by bystander » Fri Apr 26, 2019 11:36 pm

For habitability, two stars are not better than one
astrobites | Daily Paper Summaries | 2019 Apr 25
Avery Schiff wrote:
Compared to other stars, our Sun is rather unremarkable. It is near the center of our classification scheme, in the middle of its life, and kind of a loner, forming without any companion stars. As it turns out, the Sun’s solitary status may have allowed for life on Earth. Plenty of past work, fiction and nonfiction, has considered the possibility of life in multi-star systems, but today’s paper explores how the rotation and magnetic activity driven in a tight binary system might be severely detrimental to the habitability of nearby planets.

At the heart of this problem is rotation. Stars form from gas falling inwards under their own self gravity. In order to conserve angular momentum, all of that infalling gas needs to continue spinning, resulting in the characteristic rotation we see in protostellar disks. As a result, all stars begin their life rotating. However, they don’t spin forever. Stellar winds carry away angular momentum, especially through magnetic braking, causing the star’s rotation to slow down as it ages. The oldest stars are therefore spinning very slowly and we can reliably predict a star’s age by its rotation period.

Unsurprisingly, if you add a second star, the picture becomes more complicated. At a sufficiently close range the stars begin to exert tidal forces on each other. In extreme cases, the stars could even collide and merge. For the purposes of this paper, the authors are especially interested in the case where tightly orbiting stars can replenish their angular momentum by sapping energy from the binary orbit. Such a process would cause the orbit to grow tighter and tighter as the stars spiral inwards. As shown in Figure 1, this can result in models of stars that are kept rotating for longer or are even accelerated by quickly rotating companions. ...

Stellar activity and planetary atmosphere evolution in tight binary star systems ~ C.P. Johnstone et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18779
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Seeking the Circumgalactic Medium with Dragonfly

Post by bystander » Fri Apr 26, 2019 11:54 pm

Seeking the Circumgalactic Medium with Dragonfly
astrobites | Daily Paper Summaries | 2019 Apr 26
Caitlin Doughty wrote:
Generally when we look at a picture of a galaxy, we see the bright central region occupied by many stars and brightly glowing gas. This can include shining arms in spiral galaxies or the clumpy splotches of light scattered around irregular galaxies. However, there is much more to a galaxy than what is brightly glowing: every galaxy is surrounded by a thin, cool, and difficult to observe cloud of gas called the circumgalactic medium (CGM). Even farther away there is a yet thinner distribution of gas, called the intergalactic medium (IGM). These structures have such low densities that the gas doesn’t emit enough light to be visible to normal telescopes. Consequently, astronomers only have a vague picture of the geometry, composition, and conditions of these components of the universe. It’s important to understand the CGM and IGM though, because they contain the majority of the baryonic matter (i.e. normal, non-dark matter) in the universe and are crucial for regulating the flow of gas onto galaxies, which allows for things like the formation of stars. Further, the CGM is currently observed mostly with absorption line studies which are restricted to directions in the sky where a background light source, such as a quasar, is available. This means that observations are often quite limited in number, making it difficult to get a comprehensive view of what is happening.

In today’s paper, the authors wrangled two tempestuous creatures: the EAGLE cosmological simulation and the Dragonfly Telephoto Array. EAGLE is a numerical code that creates a simulated chunk of the universe and Dragonfly is a 48-lensed instrument specially designed to observe emission from very dim objects. The idea is that the authors can simulate CGM and IGM around galaxies using EAGLE and predict what their emission should look like. Then, knowing the parameters of the Dragonfly array, they can determine whether this emission should be observable by such an instrument. The authors use the capabilities of a (now in-progress) upgrade to Dragonfly, involving the added capacity to use narrow-band filters). ...

On the detectability of visible-wavelength line emission
from the local circumgalactic and intergalactic medium
~ Deborah Lokhorst et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor