astrobites 2018

Find out the latest thinking about our universe.
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Clearing up the Redshifts of Obscured AGNs

Post by bystander » Tue Jul 10, 2018 4:15 pm

Clearing up the Redshifts of Obscured AGNs
Astrobites | 2018 Jul 09
Jamila Pegues wrote:
One of the biggest mysteries of astronomy focuses on, quite fittingly, the absolute biggest astronomical phenomenon we know of: the universe, how it came to be, and where it could possibly be going. To learn more about the evolutionary timeline of the universe, scientists measure the redshifts of far-away objects like black holes and galaxies. In a surreal (and really cool) way, these far-away objects and their redshifts give us snapshots in time of how the universe looked long, long, long ago.

In today’s astrobite, we focus on one particular group of these far-away objects: active galactic nuclei. An active galaxy is a galaxy that has a central core giving off substantial amounts of energy. This core is what’s known as an active galactic nucleus – or AGN for short.

The authors of today’s paper highlighted two ways that scientists measure the redshifts of typical, not-active galaxies. One way is through spectroscopy, which uses the observed spectrum of a galaxy to measure the redshift. Unfortunately this way is pretty costly, and doesn’t work well on a galactic spectrum that is faint at near-infrared and optical wavelengths. The more common way is through photometry, which counts the number of photons, within a given energy band or range, that emit from the galaxy and are collected by a telescope. Given enough different energy bands, scientists can use photometry to construct a low-resolution spectrum of a galaxy and measure the redshift. ...

XZ: Deriving redshifts from X-ray spectra of obscured AGN - Charlotte Simmonds et al
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Spiral Galaxies Show Their Metal

Post by bystander » Tue Jul 10, 2018 4:34 pm

Spiral Galaxies Show Their Metal
Astrobites | 2018 Jul 10
Mia de los Reyes wrote:
To astronomers, the world is composed of hydrogen, helium, and… other stuff. And because astronomers are The Worst At Naming Things™, we call all the other stuff “metals.”

Almost all metals in the universe were produced in stars through nuclear fusion. As stars died, they released these metals into their surrounding environment. These heavy elements were then incorporated into later generations of stars, which eventually exploded as supernovae and released more metals into their surroundings, and so on.

So the abundance of metals in the interstellar medium of a galaxy (the gas-phase metallicity) can be used to study how that galaxy evolved over time! ...

But can we get even more specific? Instead of just considering how metallicities change as a function of radius, can we start to consider how metallicities change around the disk of a galaxy?

Today, we’ll take a look at two papers that consider this question for two different disk galaxies, NGC 1365 and NGC 2997 (Figure 1). ...

The Chemical Evolution Carousel of Spiral Galaxies: Azimuthal Variations of Oxygen Abundance in NGC 1365 - I-Ting Ho et al Azimuthal Variations of Gas-Phase Oxygen Abundance in NGC 2997 - I-Ting Ho et al
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Satellite Galaxies All in a Row—How So?

Post by bystander » Thu Jul 12, 2018 7:44 pm

Satellite Galaxies All in a Row—How So?
Astrobites | 2018 Jul 11
Daniel Berke wrote:
Imagine going for a walk in the summer and coming upon a swarm of bees in flight. But instead of buzzing around in a randomly-distributed cloud, many of the bees are flying in a tightly correlated disk or plane like the rings around Saturn. Not exactly what you’d expect to see, right? (See Figure 1)

Something similar is happening in the universe. (Except with dwarf galaxies instead of bees.) Large galaxies like our Milky Way are surrounded by swarms of smaller satellite galaxies. In simulations of galaxy formation made using the standard model of cosmology, the Lambda Cold Dark Matter model (ΛCDM), these satellite galaxies end up orbiting mostly randomly. That is to say, you generally wouldn’t expect to find multiple satellite galaxies having orbits similar to each other and orbiting in the same direction. However, observations of the Milky Way, the Andromeda Galaxy (M31), and the relatively nearby galaxy Centaurus A have revealed the presence of large, coherent planes of satellite galaxies orbiting around all three. And not only are many of the satellites orbiting in a plane, but in each case the majority of the galaxies in the plane are also orbiting in the same direction. (See Figure 2 for an overview of the various planar structures around the three galaxies.) ...

The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open Questions - Marcel S. Pawlowski
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Multi-messenger observations of a flaring blazar coincident with an IceCube neutrino

Post by bystander » Thu Jul 12, 2018 7:57 pm

Multi-messenger observations of a flaring blazar coincident with an IceCube neutrino
Astrobites | 2018 Jul 12

Interesting article about the multi-messenger observations surrounding neutrino IC-170922A and blazar TXS 0506+056

viewtopic.php?t=38508
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The speeding binary that shouldn’t exist

Post by bystander » Fri Jul 13, 2018 5:37 pm

The speeding binary that shouldn’t exist
Astrobites | 2018 Jul 13
Jennifer Scora wrote:
Hyper-velocity stars (HVS) are so named because they speed through our galaxy really, really fast. So fast, in fact, that they can escape the gravitational pull of our galaxy (about 300 km/s for the Milky Way). This means that either they are flying in from somewhere else altogether, maybe a dwarf galaxy that collided with us, or they were accelerated to such high velocities by a dramatic event within our own galaxy. There are two widely accepted theories about what this event could be, and HVS stars are often divided into two categories based on which origin story they fit best. One theory proposes that when a binary star system gets too close to the supermassive black hole at the center of the Milky Way it is ripped apart by the strong gravitational forces, leaving one star in a close orbit around the black hole and ejecting the other. However, some HVS have been seen travelling from other parts of the galaxy. These can also be explained with binary star systems, but in this case it is the force of the supernova explosion of one star that ejects its companion and accelerates it to high speeds.

To learn more about the supernova theory, the authors picked a star from the Sloan Digital Sky Survey (SDSS) called PB 3877 to be part of a follow-up study of HVS stars. PB 3877 was observed at a speed of 713 ± 140 km/s with respect to the galaxy, putting it well above the requirements for a HVS. They also traced its trajectory backwards and found it did not originate from the black hole at the centre of our galaxy, ruling out the other main theory (see Figure 1). In order to better classify the star and to learn more about the star’s composition and rotation, the authors used the Keck and European Southern Observatory Very Large Telescope (ESO-VLT) to take higher resolution spectra of the star. ...

An extremely fast halo hot subdwarf star in a wide binary system - Péter Németh et al
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Creating a more general deep learning algorithm for galaxies

Post by bystander » Tue Jul 17, 2018 5:09 pm

Creating a more general deep learning algorithm for galaxies
Astrobites | 2018 Jul 16
Avery Schiff wrote:
Between Gaia, LSST, COSMOS, and several other large surveys, astronomers are drowning in far more data than any human could ever hope to handle. As a result machine learning, a branch of computer science that “teaches” a program to rapidly process data, is a very popular topic here at Astrobites. One area of research that previously turned to creative approaches in data analysis–from machine learning to crowdsourcing–is galaxy morphology: what shapes are the hundreds of billions observed galaxies? The authors of today’s paper recently released a massive catalog of galaxy morphologies determined by a deep learning algorithm. In the companion paper discussed here, the authors also explore whether the algorithm can be applied to the data from an entirely different survey. ...

Knowledge Transfer of Deep Learning for Galaxy Morphology from One Survey to Another - H. Domínguez Sánchez et al
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Stealing from the Solar System: the Effects of a Stellar Fly-By

Post by bystander » Thu Jul 19, 2018 5:07 pm

Stealing from the Solar System: the Effects of a Stellar Fly-By
Astrobites | 2018 Jul 17
Tomer Yavetz wrote:
A star is born from the gravitational collapse of a cloud of gas and dust. Yet not all of the material ends up in the star, and instead forms a flat protoplanetary disk that surrounds the new star. Over time, the materials in this disk coalesce to form planets, moons, asteroids, and most other objects you might expect to find near a typical star.

Since protoplanetary disks are flat, the expectation is that all of the planets and objects orbiting a star that formed out of a protoplanetary disk should orbit on a single plane. So when we find stars with planets that orbit at multiple different inclinations, this raises questions. A recent astrobite discussed such a case, where an exoplanet was observed orbiting on a completely different plane than the other exoplanets in that same system. But we needn’t look that far to find deviations like this – our very own Solar System exhibits several features that don’t line up, so to speak. ...

Outer Solar System Possibly Shaped by a Stellar Fly-By - Susanne Pfalzner et al
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Best exoplanet destinations for mountain climbers

Post by bystander » Thu Jul 19, 2018 5:16 pm

Best exoplanet destinations for mountain climbers
Astrobites | 2018 Jul 18
Sanjana Curtis wrote:
What’s the first question that comes to mind when someone tells you they have discovered an exoplanet? Very likely, you want to know if it looks anything like the Earth. To actually answer that question, we need to not only find exoplanets but also characterize them, figuring out properties like the size of the planet, whether it has liquid water, and its atmospheric composition.

Yet another aspect one might consider is the surface topography of the planet. The Earth and other rocky bodies in the Solar System have mountains, valleys, craters, and volcanoes. Is it at all possible to detect the presence of such features on exoplanets? Apart from how mind-bogglingly amazing it would be to detect mountains on another world (think about that for a second), it would also tell us something about the planet’s internal processes, such as volcanism and tectonic plate movement. Today’s paper proposes a novel method for detecting and quantifying topographical features on the surfaces of exoplanets by using transit light curves. ...

Finding Mountains with Molehills: The Detectability of Exotopography - Moiya A.S. McTier, David M. Kipping
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Getting WISE-r about Variable Stars

Post by bystander » Thu Jul 19, 2018 5:32 pm

Getting WISE-r about Variable Stars
Astrobites | 2018 Jul 19
Tarini Konchady wrote:
Despite being such temperamental objects, variable stars can be reliable tools for probing the universe. By measuring the period of a variable star, we can learn about its composition, age, and brightness, broadly speaking. Applying that knowledge to large populations of variable stars can tell us about the structure of the Milky Way and our nearby Universe (shoutout to Henrietta Swan Leavitt and the Cepheid period-luminosity relation) as well as stellar evolution.

Given how useful they are, it stands to reason that creating a large, reliable sample of variable stars would be a great help for astronomers. The Wide-field Infrared Survey Explorer (WISE) is an infrared space telescope whose purpose is to image “the entire sky in the infrared” (see this Astrobite for an early overview of WISE’s goals and accomplishments). Between its initial mission (2009-2011) and its revival mission NEOWISE (2013-present), WISE has examined all sorts of objects, from asteroids to distant galaxies. In this paper, the authors searched for variable stars in the trove of WISE data to create a catalog of variable stars spanning the Milky Way. ...

Wide-field Infrared Survey Explorer (WISE) Catalog of Periodic Variable Stars - Xiaodian Chen et al
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A metal rich exo-Mercury

Post by bystander » Fri Jul 27, 2018 3:30 pm

A metal rich exo-Mercury
Astrobites | 2018 Jul 23
Shang-Min Tsai wrote:
Earth, Mars, and Venus are close siblings in our Solar System. They have similar bulk compositions, consisting of about 30% iron core and 70% silicate mantle. Mercury, being the smallest guy, is composed of about 70% metallic core and 30% silicate mantle. The evolution scenarios that can make the metal-rich composition of Mercury are still under debate. To make the story even more intriguing, the authors of today’s paper report the discovery of a planetary system K2-229, in which the inner planet has an Earth-like radius but Mercury-like composition! ...

An Earth-sized exoplanet with a Mercury-like composition - A. Santerne et al
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Measuring Masses with Microlensing: A White Dwarf Magnifying Glass

Post by bystander » Fri Jul 27, 2018 3:48 pm

Measuring Masses with Microlensing: A White Dwarf Magnifying Glass
Astrobites | 2018 Jul 24
Tomer Yavetz wrote: ...
The authors of today’s paper used the Tycho-Gaia Astrometric Solution catalogue (aka TGAS) which provides very high-accuracy velocity data for potential lens stars, and cross matched it with all of the stars in the Gaia Data Release 1 (DR1) source catalog to find any microlensing events that would happen within Gaia’s remaining lifetime. After combing through over 13,000 candidate lens stars, the authors were able to find one promising microlensing event for Gaia to detect! It turns out that we’re in for a treat on November 11th, 2019: LAWD 37, one of the closest known WDs to the Sun at only 4.6 parsecs away, will pass within a fraction of an arcsecond of a background star (with the somewhat less catchy name: Gaia Source 5332606346467258496), as shown in figure 3.

The authors calculated that this encounter should cause the apparent position of the background source to shift by about 3 milliarcseconds (see figure 4) – a shift big enough for Gaia to detect (if you’re thinking that 3 milliarcseconds sounds really small – you’re right. It’s about how big a typical pickup truck would look from Earth if you parked it on the Moon). With this data, we should be able to determine the mass of LAWD 37 to within 3%, providing us with our desired test of the MRR of WDs in isolation. ...

A Predicted Astrometric Microlensing Event by a Nearby White Dwarf - Peter McGill et al
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The Elder Generation of Taurus

Post by bystander » Fri Jul 27, 2018 3:56 pm

The Elder Generation of Taurus
Astrobites | 2018 Jul 25
Eckhart Spalding wrote:
If only our eyes were sensitive enough, the night sky would be ablaze with nearby astronomical structures. The Andromeda Galaxy would stretch across the sky about six times the angular width of the full Moon. The Magellanic Clouds would be fireballs tens of Moons across. The reddened molecular filaments of the nearby star-forming region Taurus-Auriga, glittering with young stars, would stretch over an area about 30 Moons across.

Taurus-Auriga is almost too close to us for its own good. It’s the nearest substantial star-forming region, lying only 145 pc (about 470 light-years) away. Much of it is extremely young, perhaps less than 5 Myr old. Its age is invaluable for studying star formation, but its size across the sky makes it a tricky business to make a full accounting of its members. But this accounting is unavoidable if we want to establish, among other things, the timescales of star formation or molecular cloud dissipation, and the system’s initial mass function (IMF). ...

In fact, previous attempts suggest that the Taurus IMF is a bit weird, because it’s heavy on the low-mass end. Is this indicative of deeper physics of the star formation process? Or are we just being hoodwinked and bamboozled by a masquerade of stellar interlopers? How are we to tell apart Taurus-Auriga’s serious card-carrying members from the masked ball of harlequins and mountebanks? At 30 Moons across, it’s quite a carnival. ...

The Greater Taurus–Auriga Ecosystem: I. There is a Distributed Older Population - Adam L. Kraus et al
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Discovery of water ice in the shadows of Ceres

Post by bystander » Fri Jul 27, 2018 4:03 pm

Discovery of water ice in the shadows of Ceres
Astrobites | 2018 Jul 26
Emma Foxell wrote:
Despite being located in the asteroid belt, the dwarf planet Ceres is more like a low density version of Earth than its asteroid neighbours. This lower density has led scientists to believe that it contains a large fraction of water ice, like the icy moons in the outer solar system. Following the discovery of water vapour from Ceres by the Herschel telescope and the probable detection of exposed surface water ice by NASA’s Dawn spacecraft, the authors of today’s paper searched for water in the permanent shadows of the many craters in Ceres’ northern hemisphere. ...

Surface water-ice deposits in the northern shadowed regions of Ceres - T. Platz et al
viewtopic.php?p=265135#p265135
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Photons from the “Dark Side”

Post by bystander » Sat Jul 28, 2018 2:33 pm

Photons from the “Dark Side” of Neutron Stars Could Help Unravel a Supernuclear Mystery
Astrobites | 2018 Jul 27
Thankful Cromartie wrote:
Can you see all sides of a sphere simultaneously? While an emphatic “no” is a reasonable answer, you shouldn’t feel quite so confident if the object in question is a compact neutron star. Today’s featured article explores how emission from a very compact pulsar might elucidate the mysterious nature of neutron star interiors. Hang on tight; we’re in for a geometrically challenging ride. ...

Pulse profiles of highly compact pulsars in general relativity - Hajime Sotani, Umpei Miyamoto
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Seeking Dyson Spheres with Gaia

Post by bystander » Wed Aug 01, 2018 5:00 pm

SETI on the Side: Seeking Dyson Spheres with Gaia
Astrobites | 2018 Jul 30
Kerrin Hensley wrote:
Signs of extraterrestrial intelligence don’t appear in the astrophysical literature very often. One of the most well-known signposts of advanced spacefaring civilizations, a Dyson sphere (see Figure 1), named after physicist Freeman Dyson, is a theorized structure surrounding a star, through which a highly technologically advanced civilization could harness the full energy output of its star.

Most Dyson sphere searches to date have looked for excess infrared radiation. Since a large portion of the star is covered, the amount of visible light emitted drops sharply. However, the emission from the Dyson sphere itself, which has an estimated temperature between 50 and 1000 K, peaks in the infrared. So far, searches for infrared excesses have come up empty.

In today’s paper, Zackrisson and coauthors looked for Dyson spheres with little or no infrared excess, just the sort that would have been overlooked by past searches. Specifically, they considered the case of a Dyson sphere made of a gray absorber — a material that dims the star’s light equally at all wavelengths. An observer will see the same overall shape of the star’s spectrum, but the flux will be lower everywhere. ...

SETI with Gaia: The Observational Signatures of Nearly Complete Dyson Spheres - Erik Zackrisson et al
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A different kind of planetary ring

Post by bystander » Wed Aug 01, 2018 5:13 pm

A different kind of planetary ring
Astrobites | 2018 Jul 31
Eckhart Spalding wrote:
Transit and occultation photometry have given us a lot of insight into exoplanet atmospheres by allowing us to identify atmospheric absorbing species (like potassium or sodium), Rayleigh scattering (which makes our own sky blue), and clouds and hazes.

The tiny signals are difficult to make out, because they easily become washed out by the scatter in the measured photometry. But astronomers have pushed the noise floors further and further down, to the point where a number of higher-order science signals have begun to surface. In a small handful of cases, there have been detections of phase curves that exhibit off-center bumps which indicate off-center hotspots on the planet due to winds, “ellipsoidal” variations due to the rotation of oblate host stars, and phase curves that reveal the changing amount of stellar light reflected towards us as a planet traces out its orbit.

Additional effects have been theoretically studied in preparation for super-precise observations with the next generation of facilities, like JWST. These include a relativistic “Doppler beaming” effect whereby emitted light is beamed in the direction of the planet’s motion, and the refraction of host star light through the exoplanet’s atmosphere. Today’s paper considers another effect called “forward scattering”. ...

Exoplanet phase curves at large phase angles. Diagnostics for extended hazy atmospheres - A. García Muñoz, J. Cabrera
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The Jupiter Analog Companions to Super-Earths

Post by bystander » Fri Aug 03, 2018 5:16 pm

The Jupiter Analog Companions to Super-Earths
Astrobites | 2018 Aug 01
Catherine Clark wrote:
Various studies throughout the years have established the fact that the gas giants significantly influenced the formation and evolution of our solar system. Jupiter in particular is thought to have played a considerable role in shaping the formation of the inner solar system. Astronomers believe that during the formation of the solar system, Jupiter blocked material from flowing into the inner disk, altered the velocity distribution of this material, and disrupted planet formation within several AU of the sun, leaving our solar system with no planets out to 0.39 AU. Because the gas giants played such an integral role in the formation of our own solar system, scientists are now interested in whether similar processes occur in exoplanetary systems.

The authors of today’s paper are particularly interested in how long-period gas giants, or Jupiter analogs, disrupt planet formation close to the host star. In particular, the authors are interested in exoplanetary systems containing both Jupiter analogs and super-Earths, or planets with masses larger than Earth’s, but significantly less than that of Uranus or Neptune (see Figure 1). In today’s paper, the authors use radial velocity (RV) observations to search for such systems. ...

An Excess of Jupiter Analogs in Super-Earth Systems - Marta L. Bryan et al
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What happened to the Milky Way’s sister galaxy?

Post by bystander » Fri Aug 03, 2018 5:33 pm

What happened to the Milky Way’s sister galaxy?
Astrobites | 2018 Aug 02
Stephanie Hamilton wrote:
Cosmic cannibalism is incredibly common — we believe that all large galaxies grew through mergers with smaller galaxies. Our own Milky Way is guilty of eating its smaller siblings and will continue to do so, possibly consuming the Large and Small Magellanic Clouds at some point billions of years in the future. Our closest neighbor galaxy and the largest galaxy in the Local Group, Andromeda, will also eventually consume us! (Luckily, we’ll be long gone by the time that happens in about 4 billion years.) However, today’s paper isn’t about future mergers of galaxies in our Local Group. Instead, it discusses a significant past merger event between Andromeda (aka M31) and what used to be the third-largest galaxy in the Local Group, M32. ...

The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor - Richard D’Souza, Eric F. Bell viewtopic.php?t=38540
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More Informative Mapping of Exoplanetary Peekaboos

Post by bystander » Thu Aug 09, 2018 5:41 pm

More Informative Mapping of Exoplanetary Peekaboos
Astrobites | 2018 Aug 06
Vatsal Panwar wrote:
During a recent visit to the Dutch National Maritime Museum I came across the world map from 1648 AD by cartographer Joan Blaeu. In addition to being the first world map of the Earth that adhered to the Copernican worldview, Blaeu’s map is an extremely detailed document of the knowledge of the world at the time sourced solely from the information gathered by explorers from their travels through ages to the far reaches of the world. It’s inaccuracies and subjectivity to the western view of the world are quite amusing, but there is still a level of high detail and surprising accuracy. Through these details, it is clear that cartographers must meticulously piece together information from a large number of sources of varying reliability. Creating maps of exoplanets, also beginning to be known as Exocartography, is an equally challenging observational and mathematical problem which is addressed by today’s paper.

Creating maps of exoplanets can give an idea of the presence of climatic features in the planetary atmosphere or surface which when compared to predictions from general circulations models can give us an insight into physical processes in play in their atmospheres. Observations in this context comprise of continuous monitoring of the brightness of a planetary system as different regions of the planet come into view. A schematic of these occultations is shown in Figure 1. Disk integrated flux (total flux from the planet’s surface which is visible as a circular disk) of the planet observed as a function of time in this way can then be converted to flux (more easily so for a tidally locked planet) as a function of spatial locations on the planet (which are coming into view as a function of time) to render a 2D brightness map of the planet for the chosen wavelength band.

However, this technique can be sensitive to the choice of map structure and uncertainties in the orbital parameters of the system and hence has only been reliably attempted for a few well-studied hot Jupiters(for example HD189733 and hot super-Earths to date. Today’s paper proposes a method that maximizes the information in the flux maps retrieved from phase curves and secondary eclipse observations while accounting for the effect of orbital parameter uncertainties. ...

A More Informative Map: Inverting Thermal Orbital Phase
and Eclipse Lightcurves of Exoplanets
- E. Rauscher, V. Suri, N. B. Cowan
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The Loudest Planets in our Sky: Gravitational Waves from Super-Jupiters

Post by bystander » Thu Aug 09, 2018 6:09 pm

The Loudest Planets in our Sky: Gravitational Waves from Super-Jupiters
Astrobites | 2018 Aug 07
Aaron Tohuvavohu wrote:
Since the first direct detection of gravitational waves in 2015, there’s been a large focus on the gravitational waves produced by binary systems due to their loudness and characteristic signature. Most of this attention has focused on binary systems composed of compact objects like black holes, neutron stars and white dwarfs as these are naturally the loudest events, the only binaries we know to exist in the frequency bands of ground-based interferometers…and compact objects are really cool. However, the coming space-based interferometer LISA will open up an entirely new frequency band for gravitational wave detection, and with it new progenitor systems to probe. Previous Astrobites have focused on some of these systems, such as supermassive black hole binaries, neutron star-white dwarf binaries, and even galactic binaries like AM Canum Venaticorum-type systems. However all these systems are still composed of compact and stellar, or larger, sized objects. Today’s paper takes a close look at a very different kind of binary system, an ultra-short period exoplanet, and shows that some of them should produce detectable gravitational wave signatures.

Thousands of exoplanets have been discovered to date, the vast majority within the past twenty years. In comparison with more massive and compact systems composed of black holes and neutron stars, the intrinsic gravitational wave luminosity of exoplanet systems is quite weak, but for select systems that are close to Earth their proximity can make up for the relatively weak signal and can be loud enough to be detected above the gravitational wave background. In order for such exoplanet systems to be detectable in the frequency range of space-based interferometers they must have extremely short periods of order an hour or less, and to be loud enough the planet has to be quite large.

The authors of today’s paper identified three known exoplanet systems whose characteristics fit these criteria: V396 Hya b, J1433b and GP Com b. All three exoplanet systems are within 230 parsecs, have periods of approximately one hour, and the planets have masses ranging from 18-57 times the mass of Jupiter. The authors demonstrate that the gravitational radiation produced by these three systems is detectable by both direct and indirect means. Following the example of the classic measurement of the decreasing orbital period of the Hulse-Taylor binary, one can track the energy being carried away from the system by gravitational waves, as shown in Figure 1 below. The authors claim that indirect measurements of the gravitational radiation can be performed with currently existing instrumentation and techniques similar to those used for pulsars and white-dwarf binaries. ...

Gravitational Waves From Ultra Short Period Exoplanets - J. V. Cunha, F. E. Silva, J. A. S. Lima
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Seeing Red (and Blue): Two Sub-Populations of Type Ia Supernovae?

Post by bystander » Thu Aug 09, 2018 6:18 pm

Seeing Red (and Blue): Two Sub-Populations of Type Ia Supernovae?
Astrobites | 2018 Aug 08
Daniel Berke wrote:
Type Ia (pronounced “one-A”) supernovae are powerful explosions caused by a stellar remnant known as a white dwarf undergoing runaway nuclear fusion so violent that it blows the star apart. They are an important part of astronomy, as they can help astronomers estimate distances to far-away galaxies. (In fact, observations of Type Ia supernovae led to the 2011 Nobel Prize in physics for the discovery of the acceleration of the expansion of the universe.)

Type Ia supernovae are useful because we can use them as standardizable candles—objects whose inherent luminosity we can figure out based on various properties such as how long it takes for them to fade after brightening. Once we know their luminosity (how much light they actually emit) we can calculate their distance based on measuring their brightness (how much light we measure from them here on Earth) and applying the inverse-square law. Actually figuring out their luminosity requires some care, however. We think that Type Ia supernovae can happen in at least two different ways (see this bite from 2012 for an explanation), and astronomers using them for distance measurements need to make various empirical corrections in order to do so.

Today’s paper offers another potential factor to consider, by looking at thirteen supernovae that were discovered very early after the initial explosion (within a few days, sooner after the explosion than most supernovae are discovered). The authors compared the light curves of the supernovae after normalizing for reddening caused by intervening dust and time-dilation caused by the expansion of the universe and found what appear to be two distinct populations. Based on their colors (in the astronomical sense of measuring the difference in brightness of their light between two standard filters) the two populations were named “Red” and “Blue.” Interestingly, after about five days the light curves of both populations were mostly indistinguishable; the difference was only seen prior to that time. ...

Red vs Blue: Early observations of thermonuclear supernovae reveal two distinct populations? - Maximilian D. Stritzinger et al
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Probing the Surroundings of Tycho’s Supernova

Post by bystander » Sat Aug 11, 2018 2:30 pm

Probing the Surroundings of Tycho’s Supernova
Astrobites | 2018 Aug 10
Matthew Green wrote:
In the last 2000 years, only 8 supernovae have occurred within our galaxy that were bright enough to be recorded by humans. Among these is SN 1572, which was seen in the year 1572. It was observed around the world, but is perhaps most famously associated with the Danish astronomer Tycho Brahe, who wrote a small book about it titled De Nova Stella. As an aside, the title of that book is where the modern-day terms ‘nova’ and hence ‘supernova’ come from. The appearance of a new star in the sky helped to challenge the old Aristotelian understanding that the heavens were unchanging. Even today, there is a lot that we can learn from this supernova.

At the site where SN 1572 occurred, we see today a supernova remnant — a cloud of gas that was thrown off by the supernova (see Figure 1). In fact, the gas shell is still visibly expanding, as you can see in this video. By studying light emitted by the supernova and reflected from surrounding material, researchers in 2008 were able to tell that SN 1572 was a type Ia supernova. Supernovae of this type are used to measure the distances to far-away galaxies, because of their unique feature that each explosion has almost the same luminosity. We know that Type Ia supernovae are caused by exploding white dwarfs, but we don’t fully understand what triggers the explosion. There are two important models: either the white dwarf collides with another white dwarf, or it grows in mass by pulling in material from a companion star.

In the second of those two models, the companion star should survive the explosion and be flung away at a relatively high speed. Apart from having a high velocity, it would look just like a normal star near the supernova remnant. Therefore, by looking for a star near the supernova remnant that might be a surviving companion, researchers can hope to tell which of the two models triggered the explosion in SN 1572.

The team behind today’s paper set out to do just this using data from Gaia. Regular readers will no doubt have heard of Gaia by now, but a brief refresher: Gaia is a satellite that measures the positions of stars, the movements of stars due to their own innate velocity (the “proper motion” of a star), and the apparent movements of the stars created by the Earth’s movement around the sun (the “parallax” of a star, which effectively tells you the distance of the star from the Earth). Combining these gives you information on each star’s position in 3D, and its velocity in 2D — the third component, its radial velocity (towards and away from the Earth), can be found from spectroscopy. ...

Tycho’s Supernova: the View from Gaia - Pilar Ruiz-Lapuente et al
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Disk-Bearing Binaries & Potential Tatooines

Post by bystander » Thu Aug 16, 2018 3:22 pm

Disk-Bearing Binaries & Potential Tatooines
Astrobites | 2018 Aug 13
Lauren Sgro wrote:
If you’ve ever seen Star Wars (the original saga), you probably remember that scene in A New Hope with the two suns. You know, Luke Skywalker is whining about being stuck on his home planet Tatooine and stomps off into the binary sunset. Two suns hover over the horizon in a way that we Earthlings never witness in our single star system.

Well, this scene in the science fiction masterpiece is not so far off from reality. Planets have been found to exist around binary star systems. Whether or not there are droids and Jedi on those planets is a different story.

Unfortunately, today’s paper is not about Star Wars. But, it is about those binary star systems and how likely these twin sun systems are to host planets. Planets are thought to form in disks of gas and dust surrounding their parent star, called protoplanetary disks. These disks go through many stages of their life cycle — they can look very different at different stages of planet formation — so any hint of disk material around a star may indicate that exoplanets are present. ...

The authors of today’s paper examine the Upper Scorpius (US) and Upper Centaurus-Lupus (UCL) regions of the Sco-Cen association, a group of stars in Scorpius and Centaurus that formed together and still move through space together. The Sco-Cen association is the nearest region to us that hosts massive star formation, so it is a good place to look for stars with circumstellar disks. ...

Multiplicity of Disc-Bearing Stars in Upper Scorpius and Upper Centaurus-Lupus ~ Rajika L. Kuruwita et al
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Spinning Away From the Main Sequence

Post by bystander » Thu Aug 16, 2018 3:37 pm

Spinning Away From the Main Sequence
Astrobites | 2018 Aug 14
Sanjana Curtis wrote:
Today’s paper examines the phenomenon of extended main sequence turn-off (eMSTO) in a galactic open clusterNGC 2818A. To do this, the authors make use of Gaia DR2, the data gift that keeps on giving, along with archival VLT/FLAMES spectroscopy.

Let’s first unpack the acronym, eMSTO, before we delve into the new results. Think of your standard Hertzsprung-Russell (HR) diagram. The main sequence is a distinctive band of stars that appears on the HR diagram. Stars evolve away from the main sequence when they exhaust the hydrogen fuel at their cores. The point at which this occurs is called the main sequence turnoff. In general, the more massive a star is, the faster it leaves the main sequence. If we look at the HR diagram of a stellar cluster, assuming all the stars in the cluster were created during one burst of star formation, heavier stars will evolve away from the main sequence as the cluster ages. This gives us a way of estimating the age of a stellar cluster by looking for the turnoff point! If we were to see, for example, an HR diagram for a cluster where no O-type stars are present on the main sequence but a lot of B-M type stars are still on it, we would know that it is a very young cluster.

That’s all about main sequence turnoffs, but what about the “extended” bit? An extended MSTO is an observational feature where the MSTO of a cluster is more extended than what you would expect for a simple population of stars. An example is shown in Fig. 1, which also appears in this astrobite along with a nice explanation!

So the question is: What is behind this effect? Large actual age spreads within the clusters were suggested as the origin of eMSTOs. Another suggestion links the eMSTO width to the cluster mass, broader for higher mass clusters. However, there is also observational evidence that stellar rotation is related to position within the MSTO. This is the explanation favored by today’s paper. ...

Extended Main Sequence Turnoffs in Open Clusters as Seen by Gaia:
I. NGC 2818 and the Role of Stellar Rotation
~ N. Bastian et al
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We cannot see stars form but we can hear black holes collide

Post by bystander » Thu Aug 16, 2018 3:49 pm

We cannot see stars form but we can hear black holes collide
Astrobites | 2018 Aug 15
Avery Schiff wrote:
Our first detection of gravitational waves with LIGO changed everything for astronomy. For the first time, we are able to not just look out into the Universe, but “listen” to it as well. So far, we’ve heard the collisions of the remnants of massive stars: black holes and neutron stars. The resulting data has had massive implications in fields from general relativity to nucleosynthesis. Today’s paper proposes a new use for detections of binary black hole (BBH) mergers. Vitale & Farr (2018) make the argument that if we know how often black holes are colliding, we should be able to determine how often stars form to produce these black holes.

The star formation rate (SFR) in a galaxy is a crucial variable for interpreting the brightness of a galaxy. Different galaxies that formed at the same time will have similar SFRs, but as the universe ages the star formation rate changes. In the earliest stages of the Universe, stars have not had time to form yet. After too long, however, star formation is “quenched” by various processes that eject the necessary gas from the galaxy. Figure 1 shows that the star formation rate peaked at redshift z=2 (about 10 billion years ago) but it also demonstrates that we have significantly less data at higher redshifts (further back in time). Part of the difficulty in measuring distant SFRs is the large amount of dust in the way. Since gravitational waves are unaffected by dust, the authors of today’s paper argue that gravitational waves have a significant advantage in probing the early universe. ...

Measuring the star formation rate with gravitational waves from binary black holes ~ Salvatore Vitale, Will M. Farr
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