astrobites: Daily Paper Summaries 2019

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Are AGN Quiescent Adolescents?

Post by bystander » Tue Aug 13, 2019 5:36 pm

Are AGN Quiescent Adolescents?
astrobites | Daily Paper Summaries | 2019 Aug 07
Keir Birchall wrote:
It has long been understood that a connection exists between galaxies and the supermassive black holes (SMBHs) that exist at their centres. Gas from the host galaxy finds its way into the centre where it is consumed by the black hole. The sated black hole then bathes the host galaxy in huge amounts of radiation that can be detected across the electromagnetic spectrum, a phenomenon known as an active galactic nucleus (AGN). The difficult question is: how does the gas find its way into the centre and what effects might this have on the host galaxy? The well-cited Alexander & Hickox (2012) paper suggests that these AGN are fuelled mainly by mergers between galaxies and is outlined in figure 1. ...

Today’s authors studied a sample of obscured quasars in this so-called ‘adolescent’ phase (figure 1; stage 3) to investigate whether AGN and their host galaxies will always follow this evolutionary path. To accomplish this the authors identified and imaged a sample of 10 FeLoBAL quasars at the lowest possible redshifts (0.6 < z < 1.1) using the Hubble Space Telescope (HST). FeLoBAL quasars are active galaxies described by broad absorption lines in their spectrum, particularly iron, indicating powerful outflows of material. A sample of 20 blue, unobscured quasars from the author’s previous work were used as a control sample. The aim was to characterise and compare the shapes of both samples of galaxies. Assuming this merger-driven theory is correct, the authors would expect an enhancement of merger signatures in the FeLoBAL quasar sample when compared to their unobscured counterparts as FeLoBAL quasars more recently experienced the initial merger event. ...

The host galaxies of FeLoBAL quasars at z ∼ 0.9
are not dominated by recent major mergers
~ C. Villforth et al
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Moonetesimals likely form relatively quickly

Post by bystander » Tue Aug 13, 2019 5:51 pm

Moonetesimals likely form relatively quickly
astrobites | Daily Paper Summaries | 2019 Aug 08
Samuel Factor wrote:
Disks of gas and dust around young stars are fairly common; they are the birthplaces of planetary systems, hence the term protoplanetary disk. In the past few years, astronomers have used the Atacama Large Millimeter/Submillimeter Array (ALMA) to image these circumstellar disks and even catch planets forming within them. But the giant planets in our solar system also have systems of moons. How do they form? Most likely in a similar process by which the planets themselves formed, though in a circumplanetary disk (or protolunar) rather than a circumstellar disk (or protoplanetary). Today’s paper combines new and old observations of young giant planets that looked for, but failed to detect, these circumplanetary disks in order to constrain the timescale of moon formation. ...

Upper limits on protolunar disc masses using ALMA
observations of directly imaged exoplanets
~ Sebastián Pérez et al
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A Proposed Moon Formation Theory: The Multiple-Impact Hypothesis

Post by bystander » Tue Aug 13, 2019 6:26 pm

A Proposed Moon Formation Theory: The Multiple-Impact Hypothesis
astrobites | Daily Paper Summaries | 2019 Aug 12
Jacob Azoulay wrote:
It’s been with us since before humans laid foot on Earth. Any person you have ever known or heard of has seen it shine bright in the night sky. Yet, researchers can only speculate about its creation.

The Moon is estimated to have formed 4.53 billion years ago, when a Mars-sized celestial body collided with Earth, spewing out debris that eventually clumped together. But what if this “giant-impact hypothesis” is not, in fact, how the Moon formed? Luckily, by using computer simulations, Robert I. Citron, Hagai B. Perets, and Oded Aharonson — the authors of today’s paper — can help us better understand our Moon’s origins. ...

The Role of Multiple Giant Impacts in the Formation of the Earth–Moon System ~ Robert I. Citron, Hagai B. Perets, Oded Aharonson
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When stretching stars with black holes gets unstable

Post by bystander » Tue Aug 13, 2019 6:40 pm

When stretching stars with black holes gets unstable
astrobites | Daily Paper Summaries | 2019 Aug 13
Elen Golightly wrote:
When a star wanders too close to a supermassive black hole at the centre of a galaxy, it can get stretched and pulled apart in what astronomers refer to as a ‘tidal disruption event’ or ‘TDE’. For this to occur the star must pass within tidal radius, the distance at which the tidal force from the black hole can overcome the self-gravity that keeps the star intact and spherical. Passing stars are not always disrupted however—those that plunge too deep within the tidal radius may be swallowed whole, while others may only be partially disrupted or miss the tidal field altogether, avoiding disruption completely.

A star that finds itself within the tidal radius begins to stretch into a stream, getting longer as it continues its orbit around the black hole. Approximately half of the elongated stream of stellar debris is then gravitationally bound to the black hole, while the other half remains unbound and escapes out into the galaxy at high velocities. The bound debris begins to fall back towards the black hole and forms an accretion disc that feeds it (see Figure 1). This accretion process can power a highly luminous and detectable flare from the disc.

The authors of this paper have modelled these TDEs computationally and were the first to simulate the full evolution of the events with realistic parameters. They simulated these events using PHANTOM, a 3D hydrodynamical code. Their system used a solar mass (1M) star, modelled as an adiabatic sphere of gas made up of one million particles, and a 106M black hole (a black hole mass between 105−108M is a reasonable choice). The authors started the star outside of the black hole’s tidal influence and ran the simulation until 90% of the disrupted material had returned to the disc, which was about 10 years after disruption started. ...

Variability in Tidal Disruption Events: Gravitationally Unstable Streams ~ Eric R. Coughlin, Chris Nixon
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The Imprint of an Invisible Giant

Post by bystander » Fri Aug 16, 2019 11:28 pm

The Imprint of an Invisible Giant
astrobites | Daily Paper Summaries | 2019 Aug 14
Spencer Wallace wrote:
Without Jupiter, the solar system might be a vastly different place. Our largest planetary companion is thought to be responsible for everything from clearing out planet-building material near the sun to throwing Neptune and Uranus to the outer solar system to delivering water to the Earth. Given the crucial role that this behemoth played in shaping our solar system, a natural question to ask is how important gas giants are for building planets around other stars.

Unfortunately, we can’t just look at stars with terrestrial planets and then determine which of those also have something resembling Jupiter. With the exception of a rather exotic and unexpected subclass of close-in giant planets known as hot Jupiters, most gas giants take far too long to orbit their host stars to be easily detectable. To illustrate this point, consider how an observer from outside the solar system would detect Jupiter. There are two ways to do this. The first involves gradually watching the sun redshift and blueshift as Jupiter’s gravity tugs on it over the course of an orbit. Alternatively, if you are extremely lucky, Jupiter might pass between you and the sun, producing a momentary dimming of the sun’s light. Both of these methods require you to potentially wait for Jupiter to complete nearly an entire orbit around the sun. This would take 12 years! Unless you’re continuously watching the sun for over a decade, you probably wouldn’t ever detect this giant.

Although extrasolar terrestrial planets are found using these same methods, the problem mentioned above is not quite so severe because these types of worlds tend to lie close to their star. This means that they complete an orbit fairly quickly and so you don’t have to wait long for them to alter the star’s light. For this reason, any large-scale exoplanet survey such as Kepler or TESS will tend to detect mostly close-in planets, while missing most of the longer period ones. This brings us to the goal of today’s paper, which is to determine how the presence of a gas giant, many of which lie on wide, long period orbits, affects the distribution of inner terrestrial worlds during the planet formation process. With a better understanding of this connection, it might be possible to use the measured properties of the more easily detectable terrestrial planets to figure out where the gas giants actually are (or aren’t). ...

Giant Planet Effects on Terrestrial Planet Formation and System Architecture ~ Anna C. Childs et al
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A Historical Nova and the First Radioactive Molecule

Post by bystander » Fri Aug 16, 2019 11:44 pm

A Historical Nova and the First Radioactive Molecule in Space
astrobites | Daily Paper Summaries | 2019 Aug 15
Charles Law wrote:
Nearly 350 years ago, two Sun-like stars collided in a spectacular explosion and formed a new type of star. The source, known as CK Vulpeculae (or more simply, CK Vul), was first seen in 1670 when observers reported the appearance of a new bright, red star. Initially visible with the naked eye, Nova CK Vul quickly faded and astronomers now need large telescopes to study the remnants left behind: a dim central star and surrounding hot, glowing gas (Figure 1). This gaseous debris, cast out into space during the violent stellar merger that created CK Vul, affords astronomers a unique opportunity to study the dense inner layers of a star, where heavy elements and radioactive isotopes are produced. In today’s astrobite, we take a look at the detection of a radioactive version of aluminum in this gaseous debris around CK Vul, which represents the first definitive detection of an unstable radioactive molecule outside of our Solar System. ...

Astronomical Detection of a Radioactive Molecule 26AlF in a Remnant of an Ancient Explosion - Tomasz Kamiński et al
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Subgiant in the Spotlight

Post by bystander » Fri Aug 16, 2019 11:57 pm

Subgiant in the Spotlight: Characterising a New Benchmark Star
astrobites | Daily Paper Summaries | 2019 Aug 16
Oliver Hall wrote:
We are currently living in the era of big data astronomy; and especially so in the field of asteroseismology – the study of stellar properties through measuring their pulsations. Between the recent Kepler, K2 and TESS missions, as well as supplementary data from Gaia, ensemble studies are often the norm, studying population effects of many stars in tandem. However in this era of large data sets it is important to continue to calibrate our methods, and this is where detailed studies of single, ‘benchmark’ stars are still very important.

The authors of today’s paper set out to study such a benchmark star, HR 7322 in great detail, using multiple complementary methods, with the intention of using the measured parameters of this star to identify problems with asteroseismic theory. The star they picked was a subgiant — a star that has burned through all the hydrogen in its core and is on its way to become a red giant. Recent results have shown a disagreement between radii of subgiant stars calculated from Gaia mission data and those calculated using asteroseismology. The authors study this subgiant to great precision in order to identify the source of this disagreement. ...

The Subgiant HR 7322 as an Asteroseismic Benchmark Star ~ Amalie Stokholm et al
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