astrobites: Daily Paper Summaries 2020

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A Galactic Sirocco
astrobites | Daily Paper Summaries | 2020 Mar 10

The interstellar medium (ISM) of galaxies is a surprisingly windy place. These winds are primarily driven by feedback mechanisms, such as from the galaxy’s central black hole (AGN feedback) or from the stars themselves (stellar feedback). When stars go supernova, they enrich their surroundings with heavy elements (a.k.a metals); as Carl Sagan said, “We are made of star stuff”. The term “stellar winds” describes the overall flow of materials ejected from stars. It turns out these winds don’t just contain metals; they also contain dust. These dust grains play a significant role in regulating star formation activity, especially since molecular hydrogen (stellar fuel) readily forms on the surface of dust grains (this also has a cooling effect, further influencing star formation). Ultimately, given the intricate interplay between dust and star formation, an accurate model of dusty winds is essential towards the understanding of galaxy formation and evolution.

The authors of today’s paper present the results of high-resolution hydrodynamical simulations to see how stellar feedback-driven winds can transport dust grains throughout the ISM and out beyond the outermost reaches of the galaxy’s halo. A key aspect of this simulation is that it takes into account different types of stellar feedback, from supernovae to red giants (or, more generally, AGB stars), as well as the effects of gas and metallicity. The simulation itself uses a self-consistent dust formation and destruction model. Dust is primarily produced from stellar feedback, coupled with growth from existing dust grain, but it can also be destroyed in astrophysical shocks. Simulations were conducted with models of the Milky Way (MW) and Large Magellanic Cloud (LMC). ...

Dust Entrainment in Galactic Winds ~ Rahul Kannan et al

• arXiv.org > astro-ph > arXiv:2002.01933 > 05 Feb 2020

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How It’s Made, Fast Radio Burst Edition
astrobites | Daily Paper Summaries | 2020 Mar 11

Fast radio bursts (FRBs) are probably the fastest growing and most interesting field in radio astronomy right now. These extragalactic, incredibly energetic bursts last just a few milliseconds and come in two flavors, singular and repeating. Recently the number of known FRBs has exploded with the ​Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope having discovered about 20 repeating FRBs (and also redetected the famous FRB 121102) and over 700 single bursts (hinted at here). However, despite the huge growth in the known FRB population, we still don’t know what the source(s) of these bursts is(are). Today’s paper looks at possible explanations for the properties of one FRB in particular to try to figure out what its source might be.

A number of previous astrobites have discussed the basics of FRBs (here, here, and here for example) but the FRB that the authors of this paper focus on is FRB 181112. FRB 181112 was found with the Australian Square Kilometer Array Pathfinder (ASKAP) and localized to a host galaxy about 2.7 Gpc away from us even though it has not been observed to repeat. That’s over a hundred times farther away than the closest galaxy cluster, the Virgo Cluster! One quality of FRB 181112 that makes it particularly interesting to study is that the way ASKAP records data allows the authors to study the polarization of the radio emission. Polarization of light is a measure of how much the electromagnetic (EM) wave (here the radio emission) rotates due to any magnetic fields it propagates through. The two types of polarization are linear polarization (Q for vertical/horizontal, or V for ±45°), which occurs if the EM wave rotates in a plane, and circular (either left or right handed depending on the rotation direction) if the light rotates on a circular path. By looking at the polarization of FRB 181112, shown in Figure 1, the authors can determine the strength of the magnetic field it traveled through. ...

Spectropolarimetric Analysis of FRB 181112 at Microsecond Resolution:
Implications for Fast Radio Burst Emission Mechanism ~ Hyerin Cho et al

• Astrophysical Journal Letters 891(2):L38 (2020 Mar 10) DOI: 10.3847/2041-8213/ab7824
• arXiv.org > astro-ph > arXiv:2002.12539 > 28 Feb 2020

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Emerging from the shadows: A method for detecting complex life on exoplanets
astrobites | Daily Paper Summaries | 2020 Mar 12

The challenge of positively identifying extraterrestrial life has plagued researchers since at least the time of Percival Lowell. Amidst some important contributions to astronomy, Lowell is perhaps best known for incorrectly reporting the presence of “non-natural” canals on the surface of Mars, conflating this for evidence of intelligent civilizations, which are now known (to most of us) not to exist. Yet the possibility of past or even present simple life on Mars and elsewhere remains. If, for example, the 2020 Perseverance rover finds evidence for microbial fossils on Mars, this could suggest that single-celled life is relatively common in the universe, whereas the question of multicellular life remains uncertain. Indeed, our own planet was dominated by simple lifeforms for the first ~4 billion years before the Cambrian explosion. Thus, while inventive techniques are being developed that will one day test for life on potentially habitable exoplanets, there has been little discussion of distinguishing different types of life. Today’s paper tests whether complex, upright organisms like trees could be detected from images of a distant exoplanet, even when that planet can only be seen as a single pixel!

The authors here leverage a relatively simple idea—columnar shapes (like trees) produce shadows that are unlikely to occur on a world devoid of life. Since a planet’s topography rarely includes slopes steeper than 45 degrees (<1% of Earth’s surface), when the sun is very high in the sky (low phase angle for a similarly high-angle observer) there will be little or no shadows. However, since trees extend perpendicular to the surface, they cast shadows under any circumstance where the sun is not directly overhead. Thus, we can expect more of a surface to be shadowed at low phase angles when it is forested—illustrated simply in Figure 1. Furthermore, the authors argue that an erect “tree shape” has evolutionary advantages that have allowed it to develop independently over and over throughout Earth’s history and so should be expected to occur elsewhere in the universe where complex life has developed. ...

Distinguishing Multicellular Life on Exoplanets by Testing Earth as an Exoplanet ~ Christopher E. Doughty et al

• arXiv.org > astro-ph > arXiv:2002.10368 > 24 Feb 2020

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Heavy Metal Horizons: An Ultra-Hot Jupiter That Rains Liquid Iron
astrobites | Daily Paper Summaries | 2020 Mar 14

Thousands of exoplanets have now been detected beyond our Solar System and many have challenged our expectations of what an exoplanetary system ought to look like. Before the first confirmed discoveries it was generally assumed that other planetary systems would more or less resemble our own – with small, rocky planets located close to the host star and larger gas giants residing farther out. But the discovery of the class of ‘hot Jupiters’ – large gas giants similar to Jupiter orbiting excruciatingly close to their host stars – showed that exoplanetary systems can exist in far more exotic configurations than was previously believed possible. Now an international team of researchers led by David Ehrenreich at the University of Geneva have discovered one of the most exotic exoplanets yet – an ultra-hot Jupiter where liquid iron rains from the skies.

The team observed the exoplanet known as WASP-76b as it crossed the disc of its host star using ESPRESSO, the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, at the European Southern Observatory’s Very Large Telescope (VLT) in Chile. ...

Nightside Condensation of Iron in an Ultrahot Giant Exoplanet ~ David Ehrenreich et al

• Nature (online 11 Mar 2020) DOI: 10.1038/s41586-020-2107-1
• arXiv.org > astro-ph > arXiv:2003.05528 > 11 Mar 2020

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Fundamental Oscillations: Understanding Variability in Long Period Variables
astrobites | Daily Paper Summaries | 2020 Mar 16

You usually can’t tell by looking with the naked eye, but the brightness of almost all stars varies with time. The study of oscillations in stellar brightness is called asteroseismology, and is one of the best methods we have to study the physics of different stars. If a star is exhibiting variability, it is usually due to things happening inside the star. The study of visible oscillations provides a window into a star’s internal processes.

Today’s authors were interested in asteroseismology of so-called Long Period Variables (LPVs). This class of stars includes low-temperature, evolved stars on the Asymptotic Giant Branch (stars burning helium in shells around their cores) and on the tip of the Red Giant Branch (burning hydrogen shells). These stars, which experience brightness variability with periods of longer than a few tens of days, can be classified into Semiregular Variables (SRs) and Mira variables (see Figure 1). ...

Asteroseismology of luminous red giants with Kepler I:
Long-period variables with radial and non-radial modes ~ Jie Yu et al

• Monthly Notices of the RAS 493(1):1388 (Mar 2020) DOI: 10.1093/mnras/staa300
• arXiv.org > astro-ph > arXiv:2001.10878 > 29 Jan 2020

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COMs in Cores: Complex Chemistry in Dense Cores in the Taurus Star-Forming Region
astrobites | Daily Paper Summaries | 2020 Mar 16

Astronomers have long known about the presence of complex organic molecules (COMs) in space and with the advent of more sensitive radio telescopes, discoveries of new and ever more complex molecules continue at a rapid pace. Although crucial for constraining prebiotic chemistry and questions about origins of life, an understanding of the production and distribution of COMs remains elusive. One of the principal goals of any study of COMs is to constrain where and how these prebiotic molecules form in the interstellar medium, prior to potential delivery onto a planetary surface. ...

Today’s authors used the ARO 12m telescope on Kitt Peak to observe 31 starless and prestellar cores within the L1495-B218 filament in the Taurus star-forming region, which is about 440 light years away from Earth. Source selection was based on previous analysis of an existing ammonia NH3 (1,1) intensity map of the region and excluded all objects with evidence of protostellar activity. Figure 1 shows the locations of the selected cores overlaid on a molecular hydrogen column density map of the region. ...

Prevalence of Complex Organic Molecules in Starless and Prestellar Cores
within the Taurus Molecular Cloud ~ Samantha Scibelli, Yancy Shirley

• Astrophysical Journal 891(1):73 (2020 Mar 01) DOI: 10.3847/1538-4357/ab7375
• arXiv.org > astro-ph > arXiv:2002.02469 > 06 Feb 2020

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Refurbishing a Radio Telescope: Fast Radio Burst Edition
astrobites | Daily Paper Summaries | 2020 Mar 18

Fast radio bursts (FRBs)—a mysterious, energetic, and faraway group of astrophysical phenomena—are a rapidly paced and exciting field of study. FRBs have been covered in three previous astrobites this year alone (here, here, and here), and each different astrobite (coincidentally) featured a different instrument that detected FRBs! Although instrumentation is often overlooked in press releases of scientific discoveries, many scientific advances wouldn’t have been possible without thoughtfully designed instruments. So, instrumentation-wise, what’s on the horizon for detecting FRBs? Today’s paper covers the instrumentation upgrade of the Northern Cross radio telescope in Italy, with the eventual goal of being able to survey the sky for FRBs.

The Northern Cross radio telescope is located at the Medicina Radio Astronomical Station in Italy, and is a T-shaped radio interferometer (Figure 1). The two arms that make up the “T” are aligned along the East-West and North-South directions, although today’s paper only focuses on the North-South arm. Along the North-South arm, there are 64 antennae linearly spaced 10 meters apart, with an effective total collecting area of 8000 square meters (about the area of a soccer field). Each antenna is a reflective parabolic cylinder, reflecting and then focusing the incoming radiation. This instrument has been used to survey the sky for extragalactic radio sources and operates at a central frequency of 408 MHz, a lower radio frequency than the GHz frequencies FRBs were originally detected at. ...

The Northern Cross Fast Radio Burst Project. I.
Overview and Pilot Observations at 408 MHz ~ Nicola T. Locatelli et al

• arXiv.org > astro-ph > arXiv:2003.04317 > 09 Mar 2020

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The Grand Twirl
astrobites | Daily Paper Summaries | 2020 Mar 20

The late eminent astronomer Carl Sagan once quipped “telescopes are time machines”. Indeed, one of the most celebrated milestones since Edwin Hubble’s landmark discovery of the “island universes” in 1924 is the rich collection of billions of galaxies — stretching back some 13 billion years — as revealed in ultra-deep imaging kicked off by the Hubble Space Telescope in 1995 with the spectacular Deep Field. Since then, the universe has successively provided more intriguing puzzles about the formation and evolution of the galaxies.

Within the present study of astrophysics, there is a multitude of questions about how the galaxies we see today came to be. How did galaxies get their stars and gas? How much of the stars and gas are leftover from their formation? How much gas has been accreted from pristine intergalactic molecular gas streams (of the cosmic web), how many stars were formed from this new gas, or from merging with other galaxies? What factors determine how efficiently a galaxy can turn molecular gas into stars? Do galaxies seen today reflect their “assembly history”, and could this explain the observed bimodality of blue star-forming spiral galaxies and red quiescent ellipticals, and the seemingly ubiquitous correlation of their properties?

The past two decades have witnessed phenomenal progress in this area, from the reconstruction of galaxy star-formation histories of observed galaxies to detailed simulations of the formation, growth, and interaction of thousands of galaxies over cosmic time.

Today’s astrobite addresses these questions in the context of a simulation capable of tracking a range of processes including energy injection from supernovae, the angular momentum of the gas and stars, the star-formation rate, outflowing gas, merger history, and even the properties of the parent dark matter halo in which the galaxy resides. ...

The Grand Twirl: The Epoch of Rapid Assembly of Extended and
Quiescent Discs ~ Michael Kretschmer, Oscar Agertz, Romain Teyssier

• arXiv.org > astro-ph > arXiv:2003.03368 > 06 Mar 2020

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Gravitational Wave Taxonomy
astrobites | Daily Paper Summaries | 2020 Mar 23

Researchers undecided on how to classify the first publicly announced gravitational wave detection (GW190425) from LIGO-Virgo’s third observing run

We are now in the finishing movements of the third rendition of the spacetime symphony: gravitational waves emitted by pairs of compact objects spiraling towards and smashing into one another. In January 2020, the LIGO and Virgo gravitational wave observatories publicly announced the first of many detections they have made in ‘O3’, their third joint observing run. It was announced as the second-ever confirmed detection of a binary neutron star merger – but was it? ...

This is exactly what the authors of today’s paper set out to explore. They tested the hypothesis of it being a neutron star-black hole merger by updating the priors in the Bayesian analysis. They report a mass of 2.4 M_☉ for the heavier compact object, with an error of about 0.3 M_☉ either way. They contrast this with the heaviest neutron stars we have known from the observation of pulsars.

As shown in figure 2, the mass estimate of 2.4 M_☉ for the heavier object lies significantly above the present knowledge of the maximum mass cutoff for neutron stars, as well as the error bounds of the heaviest observed neutron star. While this does not imply that it must be a black hole, the researchers verified that other inferred parameters of the source were consistent with the LIGO-Virgo analysis, and hence the NS-BH hypothesis cannot be discarded. ...

Is GW190425 Consistent with Being a Neutron Star–Black Hole Merger? ~ Ming-Zhe Han et al

• Astrophysical Journal Letters 891(1):L5 (2020 Mar 01) DOI: 10.3847/2041-8213/ab745a
• arXiv.org > astro-ph > arXiv:2001.07882 > 22 Jan 2020 (v1), 27 Feb 2020 (v2)

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Insert Tidal Here: Stretching and Squashing the Universe
astrobites | Daily Paper Summaries | 2020 Mar 24

Famously confusing to some, tides act to move part of an object relative to its center of mass. But, at least in the context of large-scale structure, we can explain that – sort of – and the authors of today’s paper can simulate it! Tidal fields, besides affecting our oceans, also play a role in the cosmic web. Tides can affect collapsing dark matter halos and the resulting galaxy formation inside them, so it is important to include them in our structure formation models!

You may have heard that on large scales (>100 Mpc), the universe is homogeneous and isotropic. However, on the scale of individual halos, gravitational tidal forces play a significant role in the dynamics by stretching and compressing matter. This deformation of matter picks out a preferred direction, introducing anisotropy. In ordinary N-body simulations, it is very challenging to separate out the effects of tidal fields from the full non-linear dynamics, as the tidal fields are constantly fluctuating. In particular, it is hard to know how much of the halo’s shape is due to the tidal field. The goal of today’s paper is to isolate the effect of the tidal field on halo shapes. ...

Measuring the Tidal Response of Structure Formation:
Anisotropic Separate Universe Simulations using TreePM ~ Jens Stücker et al

• arXiv.org > astro-ph > arXiv:2003.06427 > 13 Mar 2020

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Interstellar Travel with Sailing (Space) Ships
astrobites | Daily Paper Summaries | 2020 Mar 25

Travelling to distant stars is one of humanity’s long-term aspirations. However, interstellar travel is a challenging endeavour due to the vast distances between objects in the universe. For example, our closest stellar neighbour, Proxima Centauri, is over four light-years away. The journey there would take over 73,000 years with the Voyager 1 spacecraft – too long for a human crew. In general, travelling to other stars is only possible at relativistic speeds.

A critical factor limiting the velocity of traditional rockets is their need to carry fuel. Faster speeds require more fuel, but the weight of the fuel also slows the rocket down. Accordingly, more fuel is needed to accelerate! This dilemma, also called the tyranny of the rocket equation, implies that interstellar travel is difficult with conventional fuel-powered rockets.

The authors of today’s paper explore a different approach to space travel – sailing spaceships. Similar to sailing boats, these spaceships do not carry any fuel. Instead, they employ sails, which are accelerated by external forces, such as photon pressure and electrostatics. One project making use of such sails is Breakthrough Starshot, which intends to use a laser array to propel a spacecraft. In addition to laser arrays, astrophysical sources can also provide “wind” for the sails. Today’s paper investigates the dynamics of these sources and how they might thrust a spacecraft to relativistic speeds! ...

Propulsion of Spacecrafts to Relativistic Speeds Using Natural Astrophysical Sources ~ Manasvi Lingam, Abraham Loeb

• arXiv.org > astro-ph > arXiv:2002.03247 > 08 Feb 2020 (v1), 06 Mar 2020 (v2)

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Our Solar System: Another Planet or Another Disk?
astrobites | Daily Paper Summaries | 2020 Mar 26

As a young child, I learnt about the nine planets of the Solar System, starting at Mercury and ending with Pluto. In 2006, Pluto was notoriously (and justifiably) downgraded from full planet to dwarf planet status – and so it was that our Solar System would now have only eight planets.

The search for another ninth planet, however, has never ceased. It is now becoming one of the most exciting areas within planetary astronomy! The reason for this interest is a perplexing observation made by Mike Brown, who ironically (and aptly) is the Caltech planetary astronomer responsible for demoting Pluto in the first place. ...

Today’s paper from a graduate student at the University of Cambridge and a physicist at the University of Tel Aviv puts a novel spin on the existing Planet 9 hypothesis by invoking a ‘ring’ of small bodies in the outer reaches of the Solar System as a possible explanation for the observations. This theory could potentially be developed in conjunction with, or as an alternative to, the large Planet 9 hypothesis.

A giant disk of small bodies nearly 20 times the distance from the Sun than Neptune might sound preposterous, yet it nonetheless is not the most surprising hypothesis in the paper. The authors propose that the TNOs that we have observed in this ‘clustering’ are part of this larger, eccentric disk of TNOs – the very same disk invoked to trigger the alignment of these TNOs in the first place. While this may seem like circular reasoning, the stability of such a ‘self-consistent’ disk is backed up by mathematical and physical predictions in the paper. ...

Shepherding in a Self-Gravitating Disk of Trans-Neptunian Objects ~ Antranik A. Sefilian, Jihad R. Touma

• Astronomical Journal 157(2):59 (2019 Feb) DOI: 10.3847/1538-3881/aaf0fc
• arXiv.org > astro-ph > arXiv:1804.06859 > 18 Apr 2018 (v1), 25 Nov 2018 (v2)

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EPIC Wind-Blown Bubble?
astrobites | Daily Paper Summaries | 2020 Mar 27

Nebulae are glowing regions of ionized gas and provide some of the most beautiful images in astronomy. Two of the most commonly known types of nebulae are the result of a star ending its life: planetary nebula like the Ring Nebula, and supernova remnants like the Crab Nebula. Another type of nebula are HII regions that are ionized by surrounding stars, for example via stellar winds. The subject of today’s paper is NGC 7635 or the Bubble nebula (shown in Figure 1), an HII region surrounding the O-star BD+60°2522.

It is widely accepted that the Bubble Nebula originates from powerful winds coming from BD+60°2522. As material flows out of the star at high speeds (~2000 km/s), it heats up the interstellar medium via shocks, creating a bubble of ionized material around the star. Massive hot stars, in addition to having powerful winds, are expected to be bright X-ray sources and in some cases they produce diffuse X-ray emission in wind-blown bubbles. Due to its simple morphology, NGC 7635 is a good object for studying the effects of massive single-star winds, which is necessary to understand the effects of young stellar groups on the ISM. The authors of today’s paper use multiwavelength observations to study the Bubble Nebula and its ionizing star, and find that perhaps the bubble is not produced by simple winds. ...

The Bubble Nebula NGC 7635 -- Testing the Wind-Blown Bubble Theory ~ J.A. Toalá et al

• arXiv.org > astro-ph > arXiv:2003.06371 > 13 Mar 2020

[Ann]

EPIC Wind-Blown Bubble?
astrobites | Daily Paper Summaries | 2020 Mar 27

Nebulae are glowing regions of ionized gas and provide some of the most beautiful images in astronomy. Two of the most commonly known types of nebulae are the result of a star ending its life: planetary nebula like the Ring Nebula, and supernova remnants like the Crab Nebula. Another type of nebula are HII regions that are ionized by surrounding stars, for example via stellar winds. The subject of today’s paper is NGC 7635 or the Bubble nebula (shown in Figure 1), an HII region surrounding the O-star BD+60°2522.

It is widely accepted that the Bubble Nebula originates from powerful winds coming from BD+60°2522. As material flows out of the star at high speeds (~2000 km/s), it heats up the interstellar medium via shocks, creating a bubble of ionized material around the star. Massive hot stars, in addition to having powerful winds, are expected to be bright X-ray sources and in some cases they produce diffuse X-ray emission in wind-blown bubbles. Due to its simple morphology, NGC 7635 is a good object for studying the effects of massive single-star winds, which is necessary to understand the effects of young stellar groups on the ISM. The authors of today’s paper use multiwavelength observations to study the Bubble Nebula and its ionizing star, and find that perhaps the bubble is not produced by simple winds. ...

The Bubble Nebula NGC 7635 -- Testing the Wind-Blown Bubble Theory ~ J.A. Toalá et al

• arXiv.org > astro-ph > arXiv:2003.06371 > 13 Mar 2020

Gloria Fonseca Alvarez wrote:

The existence of substructure in the nebula, together with the high proper motion of BD+60°2522, could mean that NGC 7635 is not a simple wind-blown bubble, but rather was caused by a runaway star moving through the ISM. The lack of diffuse X-ray emission could be explained by missing instabilities that would lead to material mixing within the nebula.

That's so interesting!!

The Bubble Nebula (NGC 7635), and its central ionizing star BD+60°2522. NASA, ESA, and the Hubble Heritage Team.

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AE Aurigae and the Flaming Star Nebula. Photo: [url=https://www.reddit.com/r/Astronomy/comments/854ojn/my_image_of_the_flaming_star_nebula_consisting_of/]u/Idontlikecock[/url].

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Both BD+60°2522 and AE Aurigae are runaway stars. That is why they are asymmetrically placed inside their nebulas. But while AE Aurigae, at 23 solar masses and spectral class O9.5V, is not hot, massive or evolved enough to blow a very strong wind, simply plows through the molecular cloud it has encountered like a terribly hot massive bullet, BD+60°2522, at 44 solar masses and spectral class O6.5(f)(n)p, does blow a sufficiently strong wind to have created a bubble around itself. But look carefully, and you can see clear signs of disturbance and asymmetry around BD+60°2522, clearly suggesting that this star is not just blowing a bubble but also running away in space.

Fascinating! I might add that the runaway nature of BD+60°2522 explains why it is apparently a single star. Stars as massive as this one are almost always members of multiple star systems, unless they got a mighty kick out of their birthplace, as BD+60°2522 apparently did. And yes, AE Aurigae is a singleton, too. Runaway stars usually are.

Ann

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Come on Feel the Noise (Floor) feat. PLATO
astrobites | Daily Paper Summaries | 2020 Mar 28

In the endless search for extrasolar planets, there has been one glaring challenge: stars (and the instruments that study them) are noisy objects. While telescopes like Kepler and TESS have shown us the power of the transit method for inferring exoplanet properties, these properties aren’t nearly as precise as we would like. The uncertainties associated with the transit method become especially apparent when searching for Earth-Sun analogs. Although many Earth-like exoplanets have been previously discovered, finding one that is a true Earth twin in size and composition has remained elusive due to the uncertainties present in current datasets.

The ESA PLAnetary Transits and Oscillations (PLATO) mission is a next-generation space-based mission whose primary goal is to find transiting exoplanets orbiting around dwarf stars – like the Kepler and TESS missions. PLATO claims to be able to measure exoplanet radii from transits at a precision of 3%. This degree of precision would allow astronomers to answer many unsolved questions in exoplanet science from the evolution of planets alongside their host stars to the mechanisms behind the Fulton Gap. To validate this uncertainty, the authors of today’s paper sought to determine the limit stellar variability places on the uncertainty of exoplanet radii.

To further understand the precision claimed by PLATO, today’s paper looked towards the most well-studied star in the universe: the Sun. Using images of the full Sun from the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), the authors devised a method to efficiently simulate transits of Earth-sized planets. Their 252 simulated transit lightcurves are shown in Figure 1. ...

The Stellar Variability Noise Floor for Transiting Exoplanet Photometry with PLATO ~ Brett M. Morris et al

• Monthly Notices of the RAS 493(4):5489 (Apr 2020) DOI: 10.1093/mnras/staa618
• arXiv.org > astro-ph > arXiv:2002.08072 > 19 Feb 2020

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You Spin Me Right Round: A Magnetic Avalanche in the Solar Corona
astrobites | Daily Paper Summaries | 2020 Mar 30

Visible only as a halo around the Sun when the light from the solar disk is blotted out, for instance during a solar eclipse (Figure 1), the outer solar atmosphere (the corona) has been posing riddles to astronomers since the first eclipse observations thousands of years ago. The corona consists of hot and thin plasma. The identification of emission lines of highly ionized atoms in the coronal spectrum lead to the discovery that the temperature of the corona is over a million Kelvin while the photosphere, the visible surface of the Sun, is much cooler, with a temperature of just a few thousand Kelvin. ...

Coronal Energy Release by MHD Avalanches: Continuous Driving ~ J. Reid et al

• Astronomy & Astrophysics 615:A84 (July 2018) DOI: 10.1051/0004-6361/201732399

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You’ve Got a Friend in Me: A Hot Jupiter with a Unique Companion
astrobites | Daily Paper Summaries | 2020 Apr 01

For centuries, humankind has wondered if other planets exist outside of our own solar system, or if we are in fact unique. The first recorded attempts at observing other planets dates to around the 19th century – although they have been speculated since the 16th century – but we did not have the technology to make the detailed measurements required to detect exoplanets until the last few decades. The first detected exoplanet, 51 Pegasi b, was discovered in 1995, and since then we have learned that exoplanets are actually more of the rule than the exception. Some of the most common exoplanets that we are able to detect are called hot Jupiters – large gas giants like our Jupiter, but so close to their host stars that their orbital periods are on the order of 10 days or less – and mini-Neptunes, similar in composition to our Neptune, but smaller.

In this paper, the authors discuss a unique system called TOI-1130 which contains both a hot Jupiter and a mini-Neptune. The hot Jupiter, TOI-1130 c, has been confirmed by radial velocity measurements (see Figure 1) and is roughly 0.974 M_Jup with an orbital period of 8.4 days. Less is known about the Neptune, TOI-1130 b, since there are no radial velocity detections of it, but the authors are able to put an upper limit of 40 times the mass of the Earth on its mass. They do this by fitting the radial velocity data based on the assumption that there are two planets and determining what the largest mass for the Neptune could be based on the known mass of the hot Jupiter. But why is this system unique? TOI-1130 one of only three known systems in which a hot Jupiter-type exoplanet has another planet within its orbit around the host star, the other two being WASP-47 and Kepler-730. It is thought to be a strange occurrence both because of the small sample size, and because current migration models indicate the hot Jupiter would kick smaller planets out of its way as it settled into its current orbit, like a schoolyard bully. ...

TESS Spots a Hot Jupiter with an Inner Transiting Neptune ~ Chelsea X. Huang et al

• Astrophysical Journal Letters 892(1):L7 (2020 Mar 20) DOI: 10.3847/2041-8213/ab7302
• arXiv.org > astro-ph > arXiv:2003.10852 > 24 Mar 2020

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Searching for Red Supergiants in the Small Magellanic Cloud
astrobites | Daily Paper Summaries | 2020 Apr 02

Red Supergiants (RSGs) are evolved massive stars that begin their lives on the main sequence as 8-30M_☉ OB stars. After burning through their hydrogen, they evolve off of the main sequence, through the short lived (thousands of years) yellow supergiant phase, before cooling down to a couple thousands of degrees Kelvin and expanding to hundreds of solar radii. Here they remain until they exhaust their energy supply and end their lives as spectacular supernovae. RSGs that are bright in the night sky include Betelgeuse and Antares.

Understanding how many RSGs exist gives us more than just a number. Massive stars such as RSGs are the cosmic engines that provide important energy input and chemical enrichment on galactic scales. Their UV radiation ionizes HII regions and powers the far-infrared luminosities of distant galaxies. They are the primary sources of carbon and oxygen and their explosive deaths as supernovae produce many elements in the universe heavier than iron as they enrich the surrounding interstellar matter. Quantifying their numbers in galaxies other than our own gives us greater insight into how our universe will evolve in the future. But, given that they’re so large (in both mass and radii), they must be easy to find, right? Well, kinda …

The recent paper by Ming Yang and collaborators discusses finding these bloated evolved stars in our neighboring irregular galaxy, the Small Magellanic Cloud (SMC). While RSGs are very luminous (100,000 times the luminosity of the Sun or more), their visual magnitudes decrease quite substantially when you put them as far away as the SMC. In fact, they end up being about the same brightness as red halo giants in our own galaxy. ...

The lower luminosity RSGs are additionally contaminated by the brightest Asymptotic Giant Branch (AGB) stars which are evolved low mass stars. However, in this new paper, Yang et al. (2019) find that with the right set of magnitude-color cuts, it is possible to separate out the RSGs from the AGBs ...

Evolved Massive Stars at Low-Metallicity I. A Source Catalog for the Small Magellanic Cloud ~ Ming Yang et al

• Astronomy & Astrophysics 629:A91 (Sep 2019) DOI: 10.1051/0004-6361/201935916
• arXiv.org > astro-ph > arXiv:1907.06717 > 15 Jul 2019

[bystander]

Starburst Galaxies: Explainably Juicy
astrobites | Daily Paper Summaries | 2020 Apr 03

To truly understand the intricacies of our Universe, we must look at it in different ways, or more specifically, in different wavelengths. Most astrophysical objects are brighter at some wavelengths and dimmer in others. Looking in optical we can see galaxies and their beautiful structures. X-rays tell us about the hot gas in environment, and radio observations can teach us about relativistic jets (for more on this, check out this nifty guide).

At the highest energies (tiny wavelength, huge frequency), we call photons gamma rays. These gamma rays can reveal the secrets about different forms of particle acceleration that occur within cosmic accelerators. While some gamma rays may be produced from scattering off of high-energy electrons or emitted from electrons gyrating around magnetic field lines, they are also thought to be generated in interactions between ultra-relativistic atomic nuclei, otherwise known as ultra-high-energy cosmic rays. Detecting gamma rays from cosmic-ray interactions, so called “hadronic gamma rays,” could help unravel numerous ongoing mysteries in high-energy astrophysics.

The authors of today’s paper sought to identify this high-energy emission coming from a special type of galaxy: Star-forming galaxies. Why? Because galaxies that are rife with star formation are also thought to be riddled with ultra-high-energy cosmic rays. Although these rampant cosmic-ray interactions are reasonably well accepted, the exact magnitude and properties of the subsequently produced gamma rays is not well constrained.

Cutting right to the chase, the authors were able to detect a very convincing signal from these star-forming galaxies. While some of these objects were bright enough to stand out as significant on their own (about 10 or so), the other 500-ish couldn’t be resolved individually, but when “stacked” together, they left an incredibly strong signal for these authors to find. ...

The γ-ray Emission of Star-Forming Galaxies ~ M. Ajello et al

• arXiv.org > astro-ph > arXiv:2003.05493 > 11 Mar 2020 (v1), 01 Apr 2020 (v2)

[bystander]

Case of the missing Nitrogen on Comet 67P
astrobites | Daily Paper Summaries | 2020 Apr 04

Comets preserve information on the earliest stages of solar system formation and on the composition of its building blocks. Major advancements in our understanding of these planetary bodies have happened thanks to the Rosetta spacecraft, which gained worldwide fame for visiting the Jupiter-family comet, Comet 67-P back in 2014. One of the earliest science results of the mission came from its dust-mass spectrometer, the COSIMA instrument, which collected dust grains originating from the material blasting off of the comet’s surface, and measured their composition. COSIMA measured an average nitrogen-to-carbon ratio, or N/C ratio, of the dust grains to be 0.035 +/- 0.011. This value is much lower than the solar N/C value, which is 0.29 +/- 0.12. Since comets formed very early in the history of solar system (~ 4.5 billion years ago) and have undergone minimal chemical changes since then, we expect their elemental composition to be similar to the nebula that also formed the Sun. So, why was 67P’s nitrogen content found to be lower? Is there an unknown reservoir of nitrogen in comets? ...

The authors of today’s paper conducted laboratory experiments to produce analogs of cometary surface material and measured their reflectance spectra under comet-like conditions of low temperature and high vacuum (see Figure 2). These analog materials were mostly mixtures of opaque grains and a set of trial compounds, taken one at a time, to figure out which one produces the 3.1 and 3.2 microns absorption bands as seen in Figure 1. A thorough investigation – spanning salts, water ice grains, carboxylic acid and hydrated minerals – revealed that ammonium (NH₄⁺) salts produce the absorption bands at the right wavelengths and in the right shapes! Although the counter-ion for the salt (the negative ion), could not be constrained, the authors conclude formate (HCOO^-) is the most likely candidate, given that formic acid (HCOOH) was directly detected by Rosetta’s mass spectrometer ROSINA which analyzed the composition of the comet’s gaseous atmosphere. Hence, the authors’ most favored candidate for the mystery absorption feature in the 3 microns region of Comet 67 P’s reflectance spectrum is ammonium formate (NH₄⁺HCOO^-), whose spectrum is shown in Figure 2. ...

Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids ~ Olivier Poch et al

• Science 367(6483):aaw7462 (13 Mar 2020) DOI: 10.1126/science.aaw7462
• arXiv.org > astro-ph > arXiv:2003.06034 > 12 Mar 2020

[bystander]

Green plants on red planets: would photosynthesis work?
astrobites | Daily Paper Summaries | 2020 Apr 06

Photosynthesis is a key driving force for life on Earth. Plants absorb carbon dioxide, water, and energy from the Sun, convert this into sugar, and release oxygen into the atmosphere. This deceptively simple process dominates the ‘net primary productivity’ (NPP) of Earth, or the rate at which biomass energy is stored and made available to other organisms- in other words, photosynthesis provides the main source of accessible energy for all lifeforms which can’t produce their own (like us)! Considering photosynthesis is such a crucial process on our own planet, it stands to reason that it’d be just as important on any other planet in our galaxy that might host life. But would it work the same on alien worlds?

Some scientists believe that a particular class of star, called ‘M-dwarfs’, are the ideal places to look for Earth-like exoplanets, as their small size makes it easier to find Earth-sized planets around them and their long lifetimes could allow life enough time to appear and evolve. However, being on the cooler side of the stellar family with surface temperatures around a (comparatively chilly) 3000 degrees Kelvin means M-dwarfs are red in colour and relatively dim. As sunlight is the driving force behind photosynthesis, and therefore all life on Earth, this poses a potential problem: would planets orbiting these faint red stars be able to sustain Earth-like biospheres?

To address this question, the authors looked at the wavelengths of light which are necessary for photosynthesis. This ‘photosynthetically active radiation’ (PAR) is comprised of photons between 400 and 700 nanometres- visible light. They then calculated how much of this radiation the Earth would receive if it was orbiting different types of stars to find the PAR photon production rate for a given stellar luminosity. Compared to the Sun, M-dwarfs emit more low-energy photons and are less luminous, meaning they emit less PAR photons per second, so an Earth orbiting these stars would receive a lower PAR flux. ...

Photosynthesis on Habitable Planets around Low-Mass Stars ~ Manasvi Lingam, Abraham Loeb

• Monthly Notices of the RAS 485(4):5924 (June 2019) DOI: 10.1093/mnras/stz847
• arXiv.org > astro-ph > arXiv:1901.01270 > 04 Jan 2019 (v1), 20 Mar 2019 (v2)

viewtopic.php?p=295125#p295125

[bystander]

Black Holes Start Merging and They Don’t Always Stop Merging
astrobites | Daily Paper Summaries | 2020 Apr 07

LIGO has been up and running for a few years now, and with O3 (the third observation run) currently underway, a lot of debate still happens in the literature about the nature of LIGO sources. In general relativity, black holes are fully described using only three numbers: the charge, mass, and spin. In almost all cases, we assume that astrophysical black holes have negligible charges. However, the mass and spin of black holes can have a range of different values, and the distribution of these values in the LIGO sources we see can tell us the origin of the sources.

One explanation of how to get two black holes to merge contends that a binary star system coevolved through the main sequence, eventually becoming a black hole binary. On the other hand, this study continues a line of inquiry into what is sometimes called the dynamical channel of black hole mergers. In the extremely dense environment of star clusters, when you get a lot of things bouncing around in a small volume, you’re bound to form at least a few binaries. As a result, the binaries will look very different than those formed through normal stellar evolution. Today’s paper focuses in particular on the dynamics within globular clusters, extremely old spherical clusters of 10⁴ to 10⁶ stars. ...

Black Holes: The Next Generation – Repeated Mergers in Dense Star
Clusters and their Gravitational-Wave Properties ~ Carl L. Rodriguez et al

• Physical Review D 100(4):3027 (15 Aug 2019) DOI: 10.1103/PhysRevD.100.043027
• arXiv.org > astro-ph > arXiv:1906.10260 > 24 Jun 2019 (v1), 04 Aug 2019 (v2)

[bystander]

How to Grow a Giant Galaxy
astrobites | Daily Paper Summaries | 2020 Apr 08

How, exactly, galaxies form is still a very much open question in astrophysics. It’s not like we can watch a galaxy evolve, most are about 12 billion years old, and even the youngest we’ve discovered is about 500,000 million years old.

There are two ways to work around this problem. The first is a simple matter of looking back into time. Light takes a finite amount of time to travel to us, and so the further away we look, the older that light is. So the further a galaxy is, the younger we see it. Instead of watching a single galaxy evolve over time, we can compare further (“younger”) galaxies to closer (“older”) galaxies, and interpolate what may have happened to cause any changes.

The second way to work around our observational conundrum is to watch galaxies evolve in simulation space. The authors of today’s paper used IllustrisTNG100, part of a suite of large cosmological simulations of galaxy evolution. Figure 1 shows a subset of luminous matter in the TNG100 simulation.

The kinematic properties (how things are moving) of star-forming galaxies is strongly linked to how they gained their mass. Today’s authors compared the velocity dispersion of “younger” galaxies at redshift z=3.0–3.8 to “older” galaxies from previous studies of redshift z~2 and found that their most massive galaxies had smaller velocity dispersions than massive “older” galaxies. ...

MOSEL Survey: Tracking the Growth of Massive Galaxies at 2<z<4
using Kinematics and the IllustrisTNG Simulation ~ Anshu Gupta et al

• arXiv.org > astro-ph > arXiv:2003.01725 > 03 Mar 2020

[bystander]

Historical Flares of Sgr A*: A Polarizing Event?
astrobites | Daily Paper Summaries | 2020 Apr 09

Every so often, the supermassive black hole at the center of our Galaxy, known as Sgr A*, has a flare-up. The black hole doesn’t get zits, rather, it gives off more light than usual. These flare-ups last a few hours before they die back down, to a relatively dim state. The regular light comes from the accretion of material, and if that rate of accretion increases, so will the emission. (To read more about accretion onto Sgr A*, check out this Astrobite.)

About 120 years ago, Sgr A* had a flare that was much brighter than typical flares and lasted about 1.5 years. The event was so bright that it’s echoes can still be seen now as it reflects off of clouds. Within about 500 pc of Sgr A* is a region known as the Central Molecular Zone, which contains dense molecular clouds. The light from the flare, having traveled 120 years from our perspective (but instantaneously for the light), hits the molecular clouds and bounce back toward us.

From the X-ray emission of these clouds, we can infer when the flare happened based on how far the cloud is from the source. This is from a technique called reverberation mapping, which can be used in active galaxies to learn more about their black holes. The authors of today’s paper presented a way to learn more than just the timing of Sgr A*’s flares, using a property of light known as polarization. ...

Impact of intrinsic polarisation of Sgr A* historical flares on (polarisation)
properties of their X-ray echoes ~ Ildar Khabibullin, Eugene Churazov, Rashid Sunyaev

• arXiv.org > astro-ph > arXiv:2004.01960 > 04 Apr 2020

[Ann]

Green plants on red planets: would photosynthesis work?
astrobites | Daily Paper Summaries | 2020 Apr 06

Photosynthesis is a key driving force for life on Earth. Plants absorb carbon dioxide, water, and energy from the Sun, convert this into sugar, and release oxygen into the atmosphere. This deceptively simple process dominates the ‘net primary productivity’ (NPP) of Earth, or the rate at which biomass energy is stored and made available to other organisms- in other words, photosynthesis provides the main source of accessible energy for all lifeforms which can’t produce their own (like us)! Considering photosynthesis is such a crucial process on our own planet, it stands to reason that it’d be just as important on any other planet in our galaxy that might host life. But would it work the same on alien worlds?

Some scientists believe that a particular class of star, called ‘M-dwarfs’, are the ideal places to look for Earth-like exoplanets, as their small size makes it easier to find Earth-sized planets around them and their long lifetimes could allow life enough time to appear and evolve. However, being on the cooler side of the stellar family with surface temperatures around a (comparatively chilly) 3000 degrees Kelvin means M-dwarfs are red in colour and relatively dim. As sunlight is the driving force behind photosynthesis, and therefore all life on Earth, this poses a potential problem: would planets orbiting these faint red stars be able to sustain Earth-like biospheres?

To address this question, the authors looked at the wavelengths of light which are necessary for photosynthesis. This ‘photosynthetically active radiation’ (PAR) is comprised of photons between 400 and 700 nanometres- visible light. They then calculated how much of this radiation the Earth would receive if it was orbiting different types of stars to find the PAR photon production rate for a given stellar luminosity. Compared to the Sun, M-dwarfs emit more low-energy photons and are less luminous, meaning they emit less PAR photons per second, so an Earth orbiting these stars would receive a lower PAR flux. ...

Photosynthesis on Habitable Planets around Low-Mass Stars ~ Manasvi Lingam, Abraham Loeb

• Monthly Notices of the RAS 485(4):5924 (June 2019) DOI: 10.1093/mnras/stz847
• arXiv.org > astro-ph > arXiv:1901.01270 > 04 Jan 2019 (v1), 20 Mar 2019 (v2)

viewtopic.php?p=295125#p295125

According to Astrobiology Magazine, red dwarfs make up som 70% of all stars in the Milky Way.

If the dime-a-dozen red dwarfs are good hosts for life-bearing planets, isn't it remarkable that our own star is not a red dwarf, but rather a very much rarer G-type main sequence star?

Ann