astrobites: Daily Paper Summaries 2020

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A Stellar M-SFR-Z Relation MOSt DEFinitely Exists at z~2.3
astrobites | Daily Paper Summaries | 2020 May 16

Galaxy evolution is a complicated thing! Our current theory is that gas comes in, stars get made & explode, the surrounding interstellar medium (ISM) heats up and gets enriched with metals, and then gas goes out. These processes are happening in different stages all across the galaxy and can make simulating and observing galaxy evolution very difficult. Thankfully, through years of observation of local galaxies, we know that some galactic properties are correlated! For example, Tremonti et al. (2004) showed that there is a relation between stellar mass (M∗) and gas-phase oxygen abundance (12 + log(O/H) or Z) in the local universe (redshift, z ~ 0). In 2008, Ellison et al. discovered that this M∗-Z relation also has a dependence on the star-formation rate (SFR), in the local universe. This relation was shown later to be more correlated than the M∗-Z relation on it’s own!

The questions that then arise are: is there evidence for a M∗-SFR-Z relation at high-z? And if so, does it agree with the one at z~0? Or does it evolve with redshift? Many have tried to answer these questions, but most of these studies were based on large samples with low signal-to-noise (S/N) or small samples with intermediate S/N and have relied on a single metallicity indicator. But not anymore! ...

The MOSDEF Survey: A Stellar Mass–SFR–Metallicity Relation Exists at z ~ 2.3 ~ Ryan L. Sanders et al

• Astrophysical Journal 858(2):99 (2018 May 10) DOI: 10.3847/1538-4357/aabcbd
• arXiv.org > astro-ph > arXiv:1711.00224 > 01 Nov 2017 (v1), 26 Jun 2018 (v2)

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The “Where’s Waldo?” of Astrochemistry
astrobites | Daily Paper Summaries | 2020 May 18

Looking for different chemicals in space is a lot like searching for Waldo in the infamous search and find series “Where’s Wally?” Only imagine that the search and find page is light years away from you and all you have is a flashlight.

As our knowledge and understanding of chemical evolution in space grows, astronomers are seeking the detection of more and more complex organic molecules (COMs). Molecules that may lead to the production of life such as prebiotic molecules that may eventually form DNA, and other larger COMs are rather difficult to detect, so oftentimes we use theoretical calculations to predict the evolution and abundance of these larger molecules.

Chemical models commonly use kinetics, how energy changes over as a reaction progresses, to determine the rate at which chemical reactions occur, and thus the rate at which more complex molecules form and how abundances vary over time. Kinetics tells us that chemical reactions typically have an energy barrier to get from reactants to products. However, space is so cold that there isn’t enough energy available to overcome energy barriers (imagine pushing a 500 pound boulder over the top of Mount Everest). So, we assume that only barrier-less reactions can occur in space. ...

One of the most important aspects of theoretical research is matching observational data. If theoretical models using activation barriers and chemical kinetics are not able to match observations, then that usually indicates that there is a physical or chemical process that we don’t know about. ...

The Case of H^₂C^₃O Isomers, Revisited: Solving the Mystery of the Missing Propadienone ~ Christopher N. Shingledecker et al

• Astrophysical Journal 878(2):80 (2019 Jun 20) DOI: 10.3847/1538-4357/ab1d4a
• arXiv.org > astro-ph > arXiv:1904.11396 > 25 Apr 2019

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Understanding the Music of Delta Scuti Stars with TESS
astrobites | Daily Paper Summaries | 2020 May 19

As stars evolve on to the main sequence, they start to burn hydrogen in their cores, producing energy. As energy is carried outwards, the heat transport can cause stars to expand and contract, causing their brightness to vary. The study of this stellar variability is called asteroseismology, and tells us about what is going on inside stars, using what we observe happening on their surface.

Stars of different masses and evolutionary stages oscillate in different ways. One class of variable stars are so-called delta Scuti (δ Scuti) stars. These include main sequence stars of masses between roughly 1.5 and 2.5 times the mass of the Sun. Observations of δ Scuti stars have allowed astronomers to construct a relationship between the period of their strongest oscillation and their luminosity, allowing them to calculate distances to these stars.

However, to really understand oscillating stars, it is important to move beyond period-luminosity relations, by characterising individual oscillation frequencies. This is commonly done for stars that oscillate in the same way as our Sun (solar-like oscillators), which exhibit oscillations at evenly spaced frequencies. If stars display oscillations of evenly spaced frequencies, then asteroseismology can be used to study a wider range of their fundamental properties. However for δ Scuti stars observed by the Kepler mission, no such clear pattern appeared. Instead, their modes of oscillation seemed to be distributed near-randomly. This has made it impossible for astronomers to probe δ Scuti stars in more detail, and find out properties such as their age. ...

Very Regular High-Frequency Pulsation Modes in Young Intermediate-Mass Stars ~ Timothy Bedding et al

• Nature 581(7807):147 (14 May 2020) DOI: 10.1038/s41586-020-2226-8
• arXiv.org > astro-ph > arXiv:2005.06157 > 13 May 2020

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Blue Lurkers and Blue Stragglers: Rapidly-
Rotating Stars and their Fountain of Youth
astrobites | Daily Paper Summaries | 2020 May 20

Star clusters are comoving, gravitationally-bound groups of stars that were born more or less around the same time. Therefore, all stars in a cluster have about the same age. However, many clusters are home to blue straggler stars (BSSs) that, curiously, appear much younger (i.e. bluer, hence the “blue” part) than their fellow cluster members. Today’s paper investigates blue lurkers, the low-luminosity end of the BSS distribution, and how to identify them in star clusters via their anomalously short rotation periods. ...

Blue Lurkers: Hidden Blue Stragglers on the M67 Main Sequence
Identified from Their Kepler/K2 Rotation Periods ~ Emily Leiner et al

• Astrophysical Journal 831(1):47 (2019 Aug 10) DOI: 10.3847/1538-4357/ab2bf8
• arXiv.org > astro-ph > arXiv:1904.02169 > 03 Apr 2019

[bystander]

Twinkle, Twinkle, Little Quasar
astrobites | Daily Paper Summaries | 2020 May 21

Reciting the rhyme “Twinkle, twinkle, little star” is the first introduction most people will have to studying astronomical variability. As light from a far-off star meets the Earth’s turbulent atmosphere, it bends away from the observer causing the infamous twinkle. Observing this ‘twinkle’ is a trick that amateur astronomers can use to differentiate between stars and other objects. Similarly, observing the time-variation in light from astronomical sources is a crucial technique in research astronomy to help identify the nature of an unknown source. Today’s paper focuses on using this technique to identifying active galactic nuclei (AGN), a supermassive black hole at the centre of a galaxy that emits huge amounts of radiation. ...

Long-term NIR variability in the UKIDSS Ultra Deep Survey:
A new probe of AGN activity at high redshift ~ E. Elmer et al

• Monthly Notices of the RAS 493(2):3026 (Apr 2020) DOI: 10.1093/mnras/staa381
• arXiv.org > astro-ph > arXiv:2002.02468 > 06 Feb 2020

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How Do You Weigh a Galaxy?
astrobites | Daily Paper Summaries | 2020 May 22

We can’t put it on a digital scale, we can’t hang it on a balance and compare it against something else, so how does one measure the mass of our home galaxy? The authors of today’s paper use measurements of globular clusters in the halo of the galaxy taken from the Gaia satellite to estimate a mass for the Milky Way. ...

Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions ~ Laura L. Watkins et al

• Astrophysical Journal 873(2):118 (2019 Mar 10) DOI: 10.3847/1538-4357/ab089f
• arXiv.org > astro-ph > arXiv:1804.11348 > 30 Apr 2018 (v1), 08 Feb 2019 (v3)

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Discovering the Building Blocks of Nuclear Star Clusters
astrobites | Daily Paper Summaries | 2020 May 23

It’s well observed that stars form in clumps, known as star clusters. Around 70% of all galaxies are observed to contain star clusters in their centre (or ‘nucleus’), known as nuclear star clusters (NSCs), but it is not clear how they actually came to be there. In addition to how NSCs actually form, a heavily debated question is where they form. The two mostly likely formation scenarios are either the formation of clusters elsewhere in the galaxy which migrate inwards, known as migration, or the in-situ formation of clusters in the centre triggered by infalling gas. This Astrobite gives an overview of these scenarios in more detail.

Today’s paper, almost inadvertently, contributes a really valuable piece of observational insight into the formation of nuclear star clusters.

Just like graduate students who enjoy leaving things to the last minute, star cluster formation thrives under high pressure. In environments such as in the nucleus (centre) of galaxies, pressures become high enough to form massive star clusters (>10^₅M_⊙), known as super star clusters (SSCs). These high pressures are further increased by other factors such as galaxy mergers or starbursts.

Today’s authors look at the compact dwarf galaxy NGC 5253, which is undergoing a very young central starburst, hosting a rich population of star clusters, including SSCs. The central starburst is likely triggered by an infalling stream of gas. ...

The Three Young Nuclear Super Star Clusters in NGC 5253 ~ Linda J. Smith et al

• Astrophysical Journal 896(1):84 (2020 Jun 10) DOI: 10.3847/1538-4357/ab8f94
• arXiv.org > astro-ph > arXiv:2004.14976 > 30 Apr 2020

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When Tides Aren’t Enough
astrobites | Daily Paper Summaries | 2020 May 25

What makes a planet a planet? Do we consider their relative size, whether they trace a path backwards in the sky when viewed from Earth, whether there’s anything around the immediate vicinity of their orbit, or whether there’s so many such objects that after encountering about eight or so we decide to stop counting? With wave after wave of successful exoplanet detections by space telescopes such as Kepler and TESS, and even ground-based telescopes such as WASP and TRAPPIST, our understanding of what a planet can be has grown tremendously.

In today’s post we will be looking at a peculiar class of exoplanets called Hot Neptunes (a.k.a warm Neptunes). These are gaseous, Neptune-like exoplanets that orbit extremely close to their host star such that a year on the planet is on the order of several days to a week. ...

Why do warm Neptunes present nonzero eccentricity? ~ A. C. M. Correia, V. Bourrier, J.-B. Delisle

• Astronomy & Astrophysics 635:A37 (Mar 2020) DOI: 10.1051/0004-6361/201936967
• arXiv.org > astro-ph > arXiv:2005.02700 > 06 May 2020

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The Eldest Sibling of Our Milky Way
astrobites | Daily Paper Summaries | 2020 May 26

The Milky Way, the galaxy we call home, has a distinguishable disk-shaped structure in its center. We refer to this type of galaxy as a disk galaxy. Billions of massive galaxies with similar morphology to our Milky Way exist, and astronomers believe that these disk-like siblings are born in the same way, i.e., through the continuous merging of dark matter halos.

When dark matter halos merge, the gas and dark matter within the halos condense into larger structures. In some dense regions, stars are born from collapsing gas, and the galaxy starts forming (See Figure 1). However, details about this process are still uncertain. This is because different physical processes have different efficiency of converting falling mass into luminosity, which results in different timescales.

Therefore, it is important to push the observing limit to search for the earliest disk galaxies, in order to put a constraint on the formation time and explore the actual physical process.

Within the constraints of current technology, in today’s paper, the authors aim at discovering the oldest and farthest disk galaxy by using the Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimetre/submillimetre Array (ALMA), which may shed some light on disk galaxy formation. ...

A Cold, Massive, Rotating Disk Galaxy 1.5 Billion Years after the Big Bang ~ Marcel Neeleman et al

• Nature 581(7808):269 (21 May 2020) DOI: 10.1038/s41586-020-2276-y
• arXiv.org > astro-ph > arXiv:2005.09661 > 19 May 2020

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Speeding up cosmological simulations
by zooming on what’s important
astrobites | Daily Paper Summaries | 2020 May 28

Modern extragalactic astronomy and cosmology rely on numerical cosmological simulations. These simulations recreate the Universe in a computer. Matter is modelled by particles, either dark matter particles or baryons, set at initial positions given by the cosmic microwave background. The particles then move and interact according to a cosmological model and the hydrodynamical equations (for more details on hydro simulations, check out this astrobite). This way, theorists can try out many different models for physical processes, such as cosmological expansion and galaxy formation. By comparing the predictions of the simulations to actual observations, astronomers can figure out which models most accurately describe the Universe.

However, creating these simulations has a huge cost. They require significant computational time and memory. For example, one run of the IllustrisTNG simulation needed 90 million hours of CPU computing time, and its output is 128 TB large. To put this in context, a typical laptop hard drive can store 500 GB, so 256 laptops would be needed to store the total simulational output. The larger the simulated volume and the higher the required resolution, the more time and memory is needed to calculate and store the positions of the particles. It is therefore expensive to run many different large volume simulations in order to test many models, which will be needed to analyze future full-sky surveys like LSST and Euclid.

But don’t worry: the authors of today’s paper have an idea on how to address this issue. Their approach? Reducing the “waste” of cosmological simulations. ...

Dynamic Zoom Simulations: a fast, adaptive algorithm for simulating lightcones ~ Enrico Garaldi, Matteo Nori, Marco Baldi

• arXiv.org > astro-ph > arXiv:2005.05328 > 11 May 2020

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Rocks Go Wild: How to Make a Solar System
astrobites | Daily Paper Summaries | 2020 May 29

Our Solar System is really unique. As far as we can tell after our first ~decade of exoplanet hunting and planet formation observations and theory, it’s quite the special snowflake. In order to answer one of the biggest questions in astronomy — “How did the Earth form?” — we first need to ask how our solar system formed. Our moon, Jupiter and Saturn, the asteroid belt, our Sun’s solar activity all provide hints as to how one can possibly form this incredibly habitable planet that we live on. Inspired by this, today’s bite answers the question: how did the terrestrial planets form? The authors argue that in order to fully understand the formation of one planet, we really should be thinking of its formation as one member of a system. ...

Constraining the Formation of the Four Terrestrial Planets in the Solar System ~ Patryk Sofia Lykawka, Takashi Ito

• Astrophysical Journal 883(2):130 (2019 Oct 01) DOI: 10.3847/1538-4357/ab3b0a
• arXiv.org > astro-ph > arXiv:1908.04934 > 14 Aug 2019 (v1), 09 Sep 2019 (v2)

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Aaand Boom Goes the Magnetar
astrobites | Daily Paper Summaries | 2020 May 30

This astrobite is about a single event that was detected by two collaborations simultaneously so we’ll discuss the results of both papers.

Fast radio bursts (FRBs) are extremely energetic radio bursts lasting only milliseconds. Some repeat and some are single bursts, never to be detected again. They are extragalactic in origin and some have now been localized to other galaxies. (For more background on FRBs, check out astrobites such as this one and this one). The number of extragalactic FRBs detected is now in the hundreds thanks to the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope, and continues to grow. Yet, despite the many existing theories, we don’t know what the sources of these bursts are, and we had yet to detect an FRB from within the Milky Way. Until now.

On April 28, 2020, both CHIME and STARE2 (an array of radio telescopes across the south-western United States) detected a bright burst of emission from the magnetar SGR 1935+2154. Magnetars (discussed more in this astrobite) are neutron stars with extremely strong magnetic fields of more than a billion Tesla. For comparison, some of the strongest magnets on Earth, neodymium magnets, only have a magnetic field strength of ~1 Tesla. This detection is exciting not only because this could be a Galactic FRB, but also because magnetars have long been thought to be a source of FRBs. ...

A bright millisecond-duration radio burst from a Galactic magnetar ~ CHIME/FRB Collaboration et al

• arXiv.org > astro-ph > arXiv:2005.10324 > 20 May 2020

A fast radio burst associated with a Galactic magnetar ~ Christopher D. Bochenek et al

• arXiv.org > astro-ph > arXiv:2005.10828 > 21 May 2020

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How Eccentric are Colliding Neutron Stars?
astrobites | Daily Paper Summaries | 2020 Jun 09

On August 17, 2017 the Laser Interferometer Gravitational-Wave Observatory, or LIGO, in conjunction with VIRGO, detected gravitational waves from a pair of merging binary neutron stars for the first time. This detection was named GW170817, and ushered in a new era of multi-messenger astronomy, where light from all wavelengths, from gamma-rays to optical to radio from the resulting kilonova were detected from the same place on the sky where the merger occurred. Since then, LIGO and VIRGO have detected one other binary neutron star merger on April 25, 2019, named GW190425. This neutron star merger was not associated with any other electromagnetic radiation, but is only the second event ever detected.

From the gravitational wave signal of these mergers, we can determine many properties of the neutron star binary system, including how far away they are, how fast they’re spinning, and their masses. One assumption made when searching the gravitational wave data for neutron star mergers is that the binary orbit has a very low eccentricity, or is fairly circular, less than 0.05 . However, binary neutron stars can have highly elliptical orbits, with eccentricities up to 0.828. While it’s predicted that neutron star binaries will be in very circular orbits when they merge, it’s important to be able to measure how elliptical these systems are when they’re close to merging to determine how much this may affect the detectability of future neutron star merger gravitational wave signals. This has led the authors of this paper to measure the eccentricity of the two detected neutron star merger events, GW170817 and GW190425. ...

Measuring the Eccentricity of GW170817 and GW190425 ~ Amber K. Lenon, Alexander H. Nitz, Duncan A. Brown

• arXiv.org > astro-ph > arXiv:2005.14146 > 28 May 2020

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Titan’s Terrain Tells Tremendous Tales
astrobites | Daily Paper Summaries | 2020 Jun 12

Though several interesting examples came earlier, the first modern geologic map (William Smith, 1815) is said to have “changed the world.” Such maps prove incredibly useful for providing context, identifying resources and hazards, interpreting spatial and temporal relationships, and reconstructing landscape histories. Nowadays, images sent back from space probes can be used to create geologic maps of planetary surfaces beyond Earth for similar purposes. A global geologic map of Mars has been a useful resource for planetary geologists, and similar efforts have been made at regional and global scales for many other bodies in the solar system. Thanks to NASA’s Cassini mission, a global map distinguishing the different dominant surface types on Saturn’s moon, Titan, is now possible. Describing today’s paper makes for a good opportunity to delve into some of the basic ideas behind planetary mapping in general as well as the broad findings that point to why Titan is such a fascinating world. ...

A Global Geomorphologic Map of Saturn’s Moon Titan ~ R. M. C. Lopes et al

• Nature Astronomy (online 18 Nov 2019) DOI: 10.1038/s41550-019-0917-6

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Spectral Line Survey Reveals New
Molecules in Two Protoplanetary Disks
astrobites | Daily Paper Summaries | 2020 Jun 13

Protoplanetary disks, which are comprised of gas and dust rotating around young stars, are the cradles of planet formation. Many efforts have focused on measuring the total amount of dust in these disks, as dust grains provide the raw materials to form terrestrial planets and the rocky cores of giant planets. However, the abundance and distribution of molecular gas within these disks also has an outsized impact on the composition of nascent comets and planetesimals. An understanding of the chemical complexity that is present at this early stage of planet formation is also directly relevant to questions of the origins of life. Moreover, molecules serve as valuable tracers of disk properties such as temperature, gas density, stellar mass, and ionization levels.

Observing this molecular gas is, however, not without its challenges. Protoplanetary disks are cold enough for the majority of molecules to freeze out onto the surfaces of dust grains and form ices, rendering them invisible to observations with radio telescopes. As only gas-phase molecules present detectable signatures from their rotational transitions, the amount of detectable emission in such disks is inherently limited. To compensate for this limitation, the majority of disk observations employ targeted studies that focus on a few specific molecules expected to be strongly-emitting. But as a result, molecular inventories toward disks remain incomplete and offer only a partial and potentially biased view of disk chemistry. In fact, only 23 different molecules have been detected in disks, which is a direct consequence of these previously narrow and inconsistent searches. To remedy this, today’s authors present an unbiased spectral line survey of two nearby protoplanetary disks, which provides us, for the first time, a comprehensive view of the chemical complexity within these disks, including the detection of five new molecules. ...

An Unbiased ALMA Spectral Survey of the LkCa 15 and MWC 480 Protoplanetary Disks ~ Ryan A. Loomis et al

• Astrophysical Journal 893(2):101 (2020 Apr 20) DOI: 10.3847/1538-4357/ab7cc8

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Transiting Brown Dwarfs from TESS
astrobites | Daily Paper Summaries | 2020 Jun 15

Brown Dwarfs (BDs) are objects with masses 13-80 times the mass of Jupiter but roughly the same radius (0.7-1.4 RJ). The lower mass limit separates them from planets, as their core fuses deuterium. On the other hand, if they get too massive (80 MJ) then their cores start to fuse hydrogen and they become a main sequence star.

Similarly to a planet, when a BD passes in front of its host star, it causes a dip in the star’s lightcurve. This allows us to detect BDs with missions like the Transiting Exoplanet Survey Satellite (TESS).

Because of their Jupiter-like radius and larger mass, BD transits should be as easy to detect as giant planets, yet there are only 23 known transiting Brown dwarfs. The lack of known transiting BDs is known as the “Brown Dwarf desert”. The answer to this problem may lie in their formation mechanism – whether they form like stars or like planets. Accurate measurements of mass, radius and orbital parameters are necessary to understand the formation and evolution of BDs.

Today’s paper reports the discovery of two new transiting BDs with reliable measurements of mass, radius, and age. With ages greater than 3 Gyr, they are the oldest transiting BDs with well-constrained measurements. ...

Two Intermediate-Mass Transiting Brown Dwarfs from the TESS Mission ~ Theron W. Carmichael et al

• arXiv.org > astro-ph > arXiv:2002.01943 > 05 Feb 2020 (v1), 09 Jun 2020 (v3)

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(Stellar) Social Distancing in the Milky Way Bulge
astrobites | Daily Paper Summaries | 2020 Jun 16

Our galactic bulge is densely populated with stars. More stars implies more planets, and hence more chances of detecting life. If you were tasked with searching for extraterrestrial life in our galaxy using a powerful enough telescope within a tight deadline, you may think your best bet would be to point it towards the central bulge. Is it really so? ...

Stellar social distancing is a good thing for the survival of planets. If not obeyed, close stellar encounters can disrupt planetary orbits and even knock them off their host star. For planets that do hold on, proximity to massive, hot-headed stars and their ensuing supernova explosions can sterilize their atmospheres due to showers of x-rays, ultraviolet radiation and gamma ray bursts. Studies have also shown that supermassive black holes, which lie at the core of most galaxies, also play a role in disrupting nearby stars and planets. The population density of stars and other agents of chaos increases as we go towards the interior of the galaxy, and hence it is hard to sustain stable planetary systems that can harbor life.

Since most exoplanet surveys have detection ranges within a few hundred parsecs, little is known about the fate of planetary systems in the Milky Way bulge. To see if they are affected by stellar dynamics, the authors of today’s paper make an effort to quantify the frequency of close encounters experienced by stars living in the galactic bulge. To achieve this, they simulate orbits of stars with varying energies and angular momenta in this environment (Figure 2), and analytically estimate their encounter rates. ...

8 in 10 Stars in the Milky Way Bulge Experience Stellar Encounters
within 1000 AU in a Gigayear ~ Moiya McTier, David Kipping, Kathryn Johnston

• Monthly Notices of the RAS 495(2):2105 (June 2020) DOI: 10.1093/mnras/staa1232
• arXiv.org > astro-ph > arXiv:2005.00026 > 30 Apr 2020

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A New Theory for an Old Siberian Explosion
astrobites | Daily Paper Summaries | 2020 Jun 17

In the morning of June 30, 1908, a large explosion occurred high in the air above Siberia, Russia. It flattened 80 million trees by one estimate and created a shock wave that measured an equivalent earthquake of magnitude 5.0. The shock wave, in fact, was detected as far away as Washington, D.C. The Tunguska phenomenon, known by some as the Great Siberian Meteor, released so much material into the atmosphere that particles of ice and dust scattered sunlight around the Earth, causing a noticeable glow at night in Europe for days following the explosion.

The Tunguska event is the largest asteroid impact in recorded history and was powerful enough to level a metropolitan area. Thankfully it occurred over one of the most sparsely occupied areas on Earth but that also means very little is known definitively about the circumstances of the asteroid explosion. The authors of today’s paper undertook an exploration of what happens to asteroids as they pass through the Earth’s atmosphere without ever directly contacting the ground, to shed some light on the Tunguska phenomenon. ...

On the Possibility of Through Passage of Asteroid Bodies across the Earth's Atmosphere ~ Daniil E Khrennikov et al

• Monthly Notices of the RAS 493(1):1344 (Mar 2020) DOI: 10.1093/mnras/staa329

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Is the Hubble Tension actually a Temperature Tension?
astrobites | Daily Paper Summaries | 2020 Jun 27

One of the most prominent issues in cosmology is the so-called “Hubble Tension”. It represents an unsolved issue in cosmology: Measurement of the Hubble constant H₀, which tells us how fast the Universe is expanding, do not agree with each other. Observations of the cosmic microwave background (CMB) made by the Planck satellite suggest a value of 67.4 ± 0.5 km/s/Mpc, while observations of supernovae and cepheids by the SH0ES team suggest a higher value of 73.5 ± 1.4 km/s/Mpc (see this astrobite as well as this one for a description of H₀ measurements with cepheids and supernovae). This tension between these measurements is a problem for the cosmological standard model In this model the two measurements should coincide. If they are not the same, the standard model might need to be adjusted. However, the authors of today’s paper discuss the possibility that the Hubble tension is actually caused by prior assumptions in the measurement analysis, in particular the assumed temperature of the CMB. ...

H₀ tension or T₀ tension? ~ Mikhail M. Ivanov, Yacine Ali-Haïmoud, Julien Lesgourgues

• arXiv.org > astro-ph > arXiv:2005.10656 > 21 May 2020

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Many Worlds: Multiple Super-Earths Discovered
around a Nearby and Unusually Quiet Red Dwarf
astrobites | Daily Paper Summaries | 2020 Jun 29

An international team of researchers have discovered two, and possibly three, ‘super-Earth’ sized exoplanets around one of our closest neighbours – a bright and unusually quiet red dwarf star. The team’s observations were obtained as part of the Red Dots #2 project, which aims to discover the nearest terrestrial sized exoplanets to the Sun which they hope will provide the best targets for investigating the atmospheres of small, rocky exoplanets and searching for signs of life outside the Solar System.

Gliese 887 is a nearby red dwarf (or M dwarf) star about half the size of our own Sun. Red dwarfs are the smallest, coolest, and by far the most common type of main sequence star in the Galaxy and are found in abundance near the Earth. At a distance of only about 11 light years, Gliese 887 is one of the 12 closest stars to the Sun and also the brightest red dwarf visible from the Earth. ...

A Multiple Planet System of Super-Earths Orbiting
the Brightest Red Dwarf Star GJ 887 ~ Sandra Jeffers et al

• Science 368(6498):1477 (26 Jun 2020) DOI: 10.1126/science.aaz0795

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Heavy-Metal Jupiters
astrobites | Daily Paper Summaries | 2020 Jun 30

Sounding like something straight out of the UK music scene in the late 70’s, complete with long flowing hair, face-melting guitar solos and amps that go all the way to 11, ‘heavy-metal’ Jupiters are the result of a violent formation history for some gas giants.

We know that gas giants are composed almost entirely of hydrogen and helium, with only a small fraction of ‘everything else’ that astronomers like to call ‘heavy metals’. Although it’s impossible to actually peer into the interior of a gas giant to confirm this idea, the picture of a small, dense core with an extended gaseous envelope seems to help fit quite a few puzzle pieces together. A popular formation model for gas giants, called the core accretion scenario, predicts that a proto-Jupiter will not start to accumulate gas until a solid core of roughly 10 Earth masses forms. Furthermore, once the gas accretion process starts, it becomes difficult for additional solids to accumulate. The core accretion idea predicts, most importantly, that all gas giants should have similar-sized cores.

Based on crude measurements of the actual solid to gas makeup of many exo-Jupiters, this universal core mass prediction might not be true. By measuring both the mass and radius of a gas giant and matching that with an evolutionary model, it’s possible to get a rough estimate for the core mass. The basic idea is that at a fixed mass, a planet with a larger radius is, on average, less dense and therefore has less of a core and more of a gaseous envelope. Putting this core size measurement technique to practice, astronomers have found some ‘heavy-metal’ Jupiters with core masses up to 100 times that of the Earth’s mass—a factor of 10 larger than what the core accretion theory predicts.

Obviously, there is something missing from the planet formation models. The key to figuring out exactly what may have to do with the fact that proto-gas giants might evolve in groups, rather than in isolation. This is not a terribly wild assumption to make, given the fact that our own solar system has multiple gas giants. If two of these planets collide and merge, the resulting body is going to have a core that’s twice as large. To work out the consequences of this phenomenon, the authors of today’s paper construct an analytic model to understand how giant planets grow when they are allowed to both accrete gas from their protoplanetary disks and interact and merge with each other. To do so, they take a number of things into consideration, including how quickly a giant will accrete gas, how densely packed proto-giants would be in a typical protoplanetary disk, and how often they might collide with each other. ...

Heavy-Metal Jupiters by Major Mergers: Metallicity vs. Mass for Giant Planets ~ Sivan Ginzburg, Eugene Chiang

• arXiv.org > astro-ph > arXiv:2006.12500 > 22 Jun 2020

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Today’s forecast? Gusty winds on a brown dwarf
astrobites | Daily Paper Summaries | 2020 Jul 02

If someone tells you to imagine Jupiter, I’d bet the image that comes to mind is a circle with orange-ish yellow stripes and a big red spot. Those stripes that we’ve seen since elementary school science classes are latitudinally-banded clouds, all rotating around the planet’s gaseous atmosphere.

Jupiter, and other gas giant planets, have these banded clouds flowing around the planet in zonal winds (that is, along latitudinal lines), and astronomers think that brown dwarfs should have them, too. Brown dwarfs are objects between the size of a giant planet and a star; they’re very cool since they aren’t massive enough to start typical fusion, but they sometimes can fuse deuterium or lithium. They’re particularly interesting for atmosphere studies since they’re like a “bridge” between stellar physics and planetary physics! Atmospheric models suggest these clouds arise based on differences in how regions of the atmosphere cool and how the atmosphere couples to the interior of the gas giant or brown dwarf. So far, astronomers have already measured wind speeds for some gas giant planets using Doppler shifts in transit spectroscopy. The catch here is that this has only been done for tidally locked planets, where one side of the planet is always facing the star; high-speed winds in this scenario come from heat redistribution between the hot star-facing side to the cooler night side. This is only one specific version of a planetary atmosphere, and isn’t representative of a “typical” planet-mass object. Today’s paper introduces a new way of measuring wind speeds on exoplanets and brown dwarfs and displays the results of testing it out on a real target! ...

A Measurement of the Wind Speed on a Brown Dwarf ~ Katelyn. N. Allers et al

• Science 368(6487):169 (10 Apr 2020) DOI: 10.1126/science.aaz2856

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[bystander]

Dwarf Galaxies at the Cutting EDGE of Galaxy Formation
astrobites | Daily Paper Summaries | 2020 Jul 03

Despite their small size, dwarf galaxies have proven to be extremely useful probes of galaxy formation. This can be attributed to the fact that dwarf galaxies are sensitive souls: the effect of feedback from either supernovae, re-ionisation, stellar winds or radiative feedback has been shown to be enough to completely extinguish their star formation.

This helps dwarf galaxies become prime candidates for constraining models of galaxy formation. Simulators have had success in reproducing a large number of the scaling relations obsreved in dwarf galaxies, often through different choices in physical models. However, there are some properties such as the stellar mass-halo mass relation and the stellar mass-metallicity relation that simulations still cannot convincingly reproduce – particularly for the faintest dwarfs. Another intriguing problem that eludes astronomers is coming up with a convincing explanation for the existence of star-forming ultra faint dwarf galaxies.

With these issues in mind, today’s authors introduce their new dwarf galaxy campaign: Engineering Dwarfs at Galaxy Formations Edge (EDGE). The goal of today’s paper is to investigate:

1. What drives the regulation of star formation in the smallest dwarf galaxies; and
2. Which dwarf galaxy observables are affected the most by changes to the physical models of galaxy formation.

EDGE: The Mass-Metallicity Relation as a Critical Test of Galaxy Formation Physics ~ Oscar Agertz et al

• Monthly Notices of the RAS 491(2):1656 (Jan 2020) DOI: 10.1093/mnras/stz3053
• arXiv.org > astro-ph > arXiv:1904.02723 > 04 Apr 2019 (v1), 09 Dec 2019 (v2)

[Ann]

Starburst dwarf galaxy IC 10 with X-ray emission from neutron stars and black holes. X-ray sources are seen in blue. Image Credit: X-ray: NASA/CXC/UMass Lowell/S.Laycock et al. Optical: Bill Snyder Astrophotography

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Fascinating! IC 10 is a dwarf galaxy with a starburst that has been going on for some time.

Wikipedia wrote:

IC 10 is the only known starburst galaxy in the Local Group of galaxies. It has many more Wolf–Rayet stars per square kiloparsec (5.1 stars/kpc2) than the Large Magellanic Cloud (2.0 stars/kpc2) or the Small Magellanic Cloud (0.9 stars/kpc2). Although the galaxy has a luminosity similar to the SMC, it is considerably smaller. Its higher metallicity compared to the SMC suggests that star formation activity has continued for a longer time period.

So even though this little dwarf has clearly experienced destructive supernova explosions, it keeps on bursting. What makes it do it?

Ann

[BDanielMayfield]

Starburst dwarf galaxy IC 10 with X-ray emission from neutron stars and black holes. X-ray sources are seen in blue. Image Credit: X-ray: NASA/CXC/UMass Lowell/S.Laycock et al. Optical: Bill Snyder Astrophotography

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Fascinating! IC 10 is a dwarf galaxy with a starburst that has been going on for some time.

Wikipedia wrote:

IC 10 is the only known starburst galaxy in the Local Group of galaxies. It has many more Wolf–Rayet stars per square kiloparsec (5.1 stars/kpc2) than the Large Magellanic Cloud (2.0 stars/kpc2) or the Small Magellanic Cloud (0.9 stars/kpc2). Although the galaxy has a luminosity similar to the SMC, it is considerably smaller. Its higher metallicity compared to the SMC suggests that star formation activity has continued for a longer time period.

So even though this little dwarf has clearly experienced destructive supernova explosions, it keeps on bursting. What makes it do it?

Ann

High mass must be the key. The combined mass of all those massive stars in a relatively small volume of space must enable this dwarf galaxy to hold onto its star making gas and dust.

P.S. And, more importantly than the stellar mass and density, it must have a higher than normal amount of dark matter. DM is like the much greater part of the iceberg below the waterline.