astrobites 2018

Find out the latest thinking about our universe.
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Re: A Large Impact on Earth’s Geography

Post by BDanielMayfield » Fri Mar 09, 2018 4:18 pm

geckzilla wrote:
Fri Mar 09, 2018 5:57 am
You all are right. I'm used to Astrobites being a fairly reliable source of good morsels. I let my guard down. I wonder why this was posted, now.
I'm kinda glad it was though, because it provided an opportunity for some amateur armchair astronomers to sort of have a share in the way science works in weeding out wrong ideas.

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Recoil detectives: Searching for black hole kicks using gravitational waves

Post by bystander » Sat Mar 10, 2018 3:15 pm

Recoil detectives: Searching for black hole kicks using gravitational waves
astrobites | 2018 Mar 08
Lisa Drummond wrote:
We have now entered a new era of astronomy: gravitational wave interferometers (constructed by the LIGO/VIRGO collaboration) have enabled us to observe astrophysical objects through the medium of gravitational waves. More than this – we recently detected gravitational and electromagnetic radiation from the same neutron star merger event in a historic example of multi-messenger astronomy. Gravitational wave detection is a tool we can employ to probe astrophysical phenomena that were previously invisible to us.

In today’s paper, the authors looked at a phenomenon called gravitational wave recoil. Gravitational wave emission can impart a momentum “kick” onto remnants of black hole binary mergers, and we can search for signatures of that kick directly in the form of the gravitational wave signals themselves. So, what is a black hole kick and what can we learn from them? ...

Black Hole Kicks as New Gravitational Wave Observables - Davide Gerosa, Christopher J. Moore
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Making Mountains out of…?

Post by bystander » Fri Mar 16, 2018 8:33 pm

Making Mountains out of…?
astrobites | 2018 Mar 12
Jamila Pegues wrote:
Planet or otherwise, Pluto’s not some lonely chunk of rock lurking around the edges of the Solar System – as previous astrobites, such as this one and this other one, and gorgeous pictures from the New Horizons spacecraft have shown. Pluto has not one, not two, but five satellites (natural satellites, not artificial ones), which make up its complex five-moon system. The largest of the moons is Charon, which is half the length of Pluto and one-eighth of Pluto’s mass. The other four moons, in order from closest to farthest from Pluto, are Styx, Nix, Kerberos, and Hydra. Altogether these moons make up quite a peculiar system; all five of them, for example, have orbits which are almost perfectly circular and nearly coplanar (aka, lie in the same plane). Moreover, the smaller moons’ orbital periods relative to Charon fall near some very neat intervals of 1:3:4:5:6 – meaning that the orbital periods of Styx, Nix, Kerberos, and Hydra are about 3, 4, 5, and 6 times the orbital period of Charon, respectively.

So what sort of astrophysical phenomenon led to such a nicely oriented orbiting system? The answer is still unclear. Scientists do think they have a pretty good explanation for how Charon formed. Scientists discovered Charon back in the 1970’s, and nowadays the most widely accepted idea for Charon’s formation is the intact capture scenario. This scenario says that Charon formed back when the Kuiper Belt was a lot more crowded with objects; at some point, Pluto collided with a Kuiper Belt object and captured the impactor into orbit, and that impactor became Charon. After the collision, tidal evolution (which would have slowed down Pluto’s spin and pushed Charon’s orbit outward) helped bring Pluto and Charon into the orbital system that the two have today.

Pluto’s four smaller moons, on the other hand, are recent 21st-century discoveries, and their strange orbits have yet to be explained. There are a number of proposed formation scenarios out there, but scientists are still trying to find a scenario that completely and consistently explains the complex moon system that we observe today.

The authors of today’s paper joined this very search. They focused on one scenario in particular: the early in-situ formation scenario. This scenario follows right along with the Charon-forming intact capture scenario described earlier: it states that the impact from the collision that formed Charon also produced a ring of debris at about 20 RP, where RP is the radius of Pluto. This ring spread outward over time, due to angular momentum transfer from the Pluto-Charon system. Pluto’s smaller moons then formed from the debris ring near their modern orbital distances, in orbits that were already nearly coplanar and circular as a result of the ring. ...

On the Early In Situ Formation of Pluto's Small Satellites - Man Yin Woo, Man Hoi Lee
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A long time ago in a quasar far, far away…

Post by bystander » Fri Mar 16, 2018 8:44 pm

A long time ago in a quasar far, far away…
astrobites | 2018 Mar 13
Joanna Ramasawmy wrote:
With advances in technology and more time dedicated to deep sky surveys, we are able to probe deeper and deeper into the universe. Today’s astrobite is about a new discovery of the most distant quasar – an incredibly luminous active supermassive black hole – ever observed. It’s at a redshift z = 7.54: its light has been traveling towards us for 13.1 billion years, from when the universe was only 690 million years old.

So why is this interesting? Surely as we look deeper, we’re bound to find more “most distant” objects – it’s just a matter of time. But finding objects like these is challenging, and such discoveries can reveal a lot about the early universe.

The next-furthest quasar, catchily named ULAS J1120+0641, was discovered way back in 2011, and is the only other quasar detected at a redshift greater than 7. Getting a larger sample of quasars at these high redshifts has been a high priority in research into the early universe, but they have proven hard to find. Why quasars? They’re the most luminous objects in the universe, and investigating what happens to the radiation they emit tells us about the universe around them. Why z > 7? Because then we are probing the epoch of reionization. ...

An 800 million solar mass black hole in a significantly neutral universe at redshift 7.5 - E. Bañados et al
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First Detection of the 21cm Cosmic Dawn Signal

Post by bystander » Fri Mar 16, 2018 9:31 pm

First Detection of the 21cm Cosmic Dawn Signal
astrobites | 2018 Mar 14
Joshua Kerrigan wrote:
Today’s astrobite is about the exciting detection of the 21cm neutral hydrogen emission during the Cosmic Dawn, a topic covered many times in the past (for example here, here, here, oh and here). The 21cm emission from neutral hydrogen has the ability to open up secrets of the early universe and give us some keen insight into how the first stars and galaxies formed and how the universe became reionized. You may be familiar with the cosmic microwave background (CMB) which has provided us with evidence for some of our most important cosmological understandings due to measurements by the satellites COBE, WMAP, and Planck. In much the same way the Experiment to Detect the Global Epoch of Reionization Signature or EDGES (Figure 1) has now done the same for the period between recombination and reionization. So now let’s jump into why this detection is so amazing and how there may potentially be some new underlying physics at work. ...

An absorption profile centred at 78 megahertz in the sky-averaged spectrum - Judd D. Bowman et al
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No dust ring required around Proxima Centauri

Post by bystander » Fri Mar 16, 2018 9:45 pm

No dust ring required around Proxima Centauri
astrobites | 2018 Mar 15
Emma Foxell wrote:
Warning – how you analyse your data can entirely change the interpretation of your results. Today’s paper shows the importance of multiple teams of scientists looking at the data and considering alternative explanations.

Our paper concerns Proxima Centauri, our nearest star which has attracted much interest for having a potentially rocky planet Proxima b. It uses the same radio wave data from the Atacama Large Millimetre Array (ALMA) and the Atacama Compact Array (ACA) but reaches very different conclusions to the original analysis led by Anglada (covered in astrobites previously). ...

Detection of a Millimeter Flare from Proxima Centauri - Meredith A. MacGregor et al
viewtopic.php?t=38072
viewtopic.php?p=277111#p277111
viewtopic.php?t=37725
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Testing the Limits of Gravity at Mercury

Post by bystander » Mon Mar 26, 2018 3:36 pm

Testing the Limits of Gravity at Mercury
astrobites | 2018 Mar 19
Avery Schiff wrote:
You have probably noticed: when you drop something, it falls. Of the four fundamental forces, gravity is the most present in our daily lives. Ironically, gravity is also the force we understand the least. This is partially because gravity is so weak that it is very difficult to measure. Most of what we have been able to learn about gravity outside of theory has come from either extremely precise laboratory experiments or observations of astrophysical objects that are so large they produce extraordinary gravitational activity not reproducible on Earth. Today’s paper attempts to solidify our understanding of gravity by considering data from a point that experiences some of the strongest gravitational forces in our solar system: Mercury. By analyzing seven years of data from the MESSENGER spacecraft the authors achieve high-precision measurements of some of the most fundamental parameters in the study of gravity. ...
Solar system expansion and strong equivalence principle as seen by the NASA MESSENGER mission - Antonio Genova et al
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Hazy Experiments in Exoplanet Atmospheres

Post by bystander » Mon Mar 26, 2018 3:54 pm

Hazy Experiments in Exoplanet Atmospheres
astrobites | 2018 Mar 20
Jessica Roberts wrote:
Clouds and hazes: the enemies of any observational astronomer. Many astronomers have missed observing their target simply because clouds got in the way. Yet clouds and hazes are everywhere, and not just on Earth. Every Solar System body with an atmosphere, be it planet, dwarf planet, or moon, has some form of aerosol in its atmosphere, from the thick clouds on Venus, to the optically impenetrable haze on Titan, even on Pluto! So it should come as no surprise that we have observed clouds or hazes on multiple exoplanets, especially on the cooler (<800K) and smaller (Neptune-size or smaller) planets. It is difficult to decide what is to blame for “clouding out” our observations of exoplanets from theory alone. Today’s authors set out to recreate hazy exoplanetary conditions a bit closer to home.

First Lab Simulation of Exoplanet Atmospheric Chemistry
Johns Hopkins University | 2018 Mar 07

Haze production rates in super-Earth and mini-Neptune atmosphere experiments - Sarah M. Hörst et al Haze Production in the Atmospheres of Super-Earths and Mini-Neptunes: Insights from the Lab - Sarah M. Hörst et al
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One Magnified Starburst Galaxy

Post by bystander » Mon Mar 26, 2018 4:08 pm

One Magnified Starburst Galaxy
astrobites | 2018 Mar 21
Caitlin Doughty wrote:
A crucial component of understanding how galaxies evolve is figuring out what their earliest years are like. However, this presents a problem to astronomers since most early galaxies are believed to have been small and faint, and are quite distant to boot. One simple workaround is to study nearby galaxies that we believe have similar properties to the progenitors of older galaxies like the Milky Way or Andromeda. Today’s paper covers the analysis of one such galaxy. ...

A Window on the Earliest Star Formation: Extreme Photoionization Conditions
of a High-Ionization, Low-Metallicity Lensed Galaxy at z~2
- Danielle A. Berg et al
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A “Difference” Approach to Light Curves

Post by bystander » Mon Mar 26, 2018 4:19 pm

A “Difference” Approach to Light Curves
astrobites | 2018 Mar 22
Tarini Konchady wrote:
The Transiting Exoplanet Survey Satellite (TESS) will be launching very soon (in about a month according to NASA’s launch countdown clock), and it promises to yield exciting results with a near all-sky survey for exoplanets around bright, nearby stars. Over two years, TESS will observe over 400 million stars by systematically scanning sectors of the sky for 27 days at a time (see Figure 1). About 400,000 of these stars will be studied closely, and will have light curves associated with them in data releases. The rest of the stars can be studied through full-frame images (FFIs), which have a 30-minute cadence and consist of TESS’s entire field of view. To clarify, this doesn’t mean that an observation is taken every thirty minutes. The TESS cameras take images every two seconds. To save on storage and transmission time those images are stacked to create new images with an effective exposure time of thirty minutes.

Because TESS is designed to conduct a wide survey, its pixels span a large part of the sky — 21 arcseconds per pixel to be exact. For comparison, the Wide Field Camera 3 on the Hubble Space Telescope spans 0.04 to 0.13 arcseconds per pixel depending on the detector. The large arcsecond per pixel scale means that the stars in TESS FFIs are more likely to be distorted and blurred into each other, making it difficult to measure changes in brightness (see Figure 2). How can one get around this? The authors of this paper offer up an image processing pipeline that does the trick. ...

Precision Light Curves from TESS Full-Frame Images: A Difference Imaging Approach - Ryan J. Oelkers, Keivan G. Stassun
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Dark Matter and the 21cm Cosmic Dawn Signal

Post by bystander » Mon Mar 26, 2018 4:44 pm

Why dark matter (probably) can’t explain the 21cm Cosmic Dawn signal
astrobites | 2018 Mar 23
Philippa Cole wrote:
On March 1st, the first detection of a cosmic 21cm signal by EDGES was published in Nature (see here for an astrobite that discusses the background and explains the experimental results). This got people talking for two reasons. Firstly, because it was the first direct detection of the temperature of the hydrogen gas during the Cosmic Dawn era – a time when the first sources of light were formed – and secondly, because the gas appeared much colder than everyone was expecting. Not only that, but dark matter could have been responsible for that surprise.

Less than a month later and we’ve seen a flurry of follow-up papers (for example here, here and the original theoretical companion here) with different explanations for the detection. Today’s paper agrees with claims that we shouldn’t get too excited – dark matter is probably not the answer (to this particular question anyway). ...

Signs of Dark Matter at 21-cm? - Rennan Barkana et al
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HD 59686Ab: How Did It Get There?

Post by bystander » Fri Mar 30, 2018 4:37 pm

HD 59686Ab: How Did It Get There?
astrobites | 2018 Mar 26
Mara Zimmerman wrote:
The discovery of numerous new planets has brought with it deeper scientific questions, especially those of planet formation and evolution. How do the lifetimes of stars affect their planets? And what’s more, how do these complicated systems form and stay together? In today’s paper, these questions are tackled for the system HD 59686. This is a binary star system with a circumprimary planet meaning that the planet orbit only the primary star in the system, which is also in a mutual orbit with the secondary star. I like to think of systems like these as Tatooine in a parallel universe: it has the same components as the famous binary star planet, but is rearranged in a different configuration. Figure 1 below shows a more concrete way of picturing the system.

The HD 59686 star system is fairly close (~13 AU) between the two stellar components, and the distance between the primary and the planet is also quite small (~ 1AU). Binary systems allow observers to more closely study the interactions between both stars; in this system, with the addition of the planet HD 59686 A b, the interactions between all the components are much more complicated and intense. How does a planet form and manage to stay formed in such an unusual system? ...

Dynamical Analysis of the Circumprimary Planet in the Eccentric Binary System HD 59686 - Trifon Trifonov et al
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Fantastic tidal features and where to find them

Post by bystander » Fri Mar 30, 2018 4:55 pm

Fantastic tidal features and where to find them
astrobites | 2018 Mar 27
Tomer Yavetz wrote:
What has arms, a tail, and a big shell all around it? You may be tempted to say, “a turtle” (or maybe “an unhatched crocodile”?), but an equally reasonable answer, it turns out, is “a galaxy”!

Tidal interactions between a galaxy and the objects in its vicinity lead to the formation of a variety of visually striking structures, often referred to as tidal features (see figure 1 for a few examples). Streams form when a lower-mass companion (such as a globular cluster or a smaller galaxy) orbits a more massive galaxy on a relatively circular orbit and gradually gets ripped apart and stretched out by tidal forces. Shells are the likely results of a dwarf galaxy falling radially into and merging with a bigger galaxy. Tidal arms or tails form when material from the primary galaxy gets pulled out due to an interaction with a nearby large companion, usually as a precursor or a byproduct of a major merger (check out the arms/tails in this cool comparison of simulation and observations).

In addition to providing beautiful material for your desktop background, these tidal interactions can teach us a lot about a whole slew of topics, ranging from star formation history and stellar populations to galaxy evolution and nature of dark matter. So hunting down and understanding these tidal features can be quite an (intellectually) lucrative pursuit! ...

The Origin of Faint Tidal Features around Galaxies in the RESOLVE Survey - Callie E. Hood et al
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A Little Less in the Dark

Post by bystander » Fri Mar 30, 2018 5:18 pm

A Little Less in the Dark
astrobites | 2018 Mar 28
Tarini Konchady wrote:
The Dragonfly Telephoto Array isn’t your typical detector. Consisting of 48 Canon 400mm telephoto lenses, Dragonfly has the unique ability to image dim diffuse objects, or low surface brightness objects much better than other telescopes can (check out Astrobites on some of the objects Dragonfly has imaged here, here, and here). This means that Dragonfly can probe the structures of galaxies for faint disruptions from interactions with their dark matter halos.

Relationships between the masses of dark matter and stellar matter in a galaxy have been determined but are poorly constrained for low mass galaxies in particular (low mass being 108 solar-masses or so). Low mass galaxies that are satellites to larger galaxies may hold the key to understanding galaxy formation and dark matter on small scales.

Dragonfly has already proved its usefulness in constraining the dark matter-stellar mass relationship, having found amongst other things a galaxy that is the same mass as the Milky Way (~1012 solar masses) but is mostly dark matter. In this paper however, the authors discuss a very different sort of galaxy — a galaxy with almost no dark matter at all (See Figure 2). ...

A galaxy lacking dark matter - Pieter van Dokkum et al viewtopic.php?t=38140
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The Imprint of Cosmic Reionization on Dwarf Galaxies

Post by bystander » Fri Mar 30, 2018 5:36 pm

The Imprint of Cosmic Reionization on Dwarf Galaxies
astrobites | 2018 Mar 29
Nora Shipp wrote:
More than 13 billion years ago, the Universe was dark and starless – an era referred to as the cosmic “dark ages.” Then, the first galaxies began to form. As clouds of gas cooled and condensed in their centers, the stars that were born illuminated the cosmos. These stars unleashed energetic ultraviolet (UV) photons, which bombarded the surrounding neutral hydrogen gas, separating it into charged protons and electrons, in a process called reionization (Figure 1).

... By studying dwarf galaxies in the nearby Universe, we can learn about reionization in the distant past. In particular, the nature of the dwarf galaxies we observe today depends on two things – the time when reionization occurred, and the mass cutoff below which star formation was shut off.

These are the questions that the authors of today’s paper seek to answer. In particular they look at the effect of reionization on the “luminosity function” of dwarf galaxies. The luminosity function refers to the number of galaxies as a function of brightness. Within the luminosity function, reionization produces two populations of galaxies – those that are large enough to trap hot gas and to continue forming stars, thereby increasing in brightness, and those that are too small. These two populations would appear as two peaks (as in Figure 2 below). The first, fainter peak contains the galaxies that formed stars leading up to reionization and then suddenly stopped. The second, brighter peak contains the galaxies that have continued forming stars. ...

The Imprint of Cosmic Reionisation on the Luminosity Function of Galaxies - Sownak Bose, Alis J. Deason, Carlos S. Frenk
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A white dwarf kicked out of a supernova

Post by bystander » Wed Apr 11, 2018 6:16 pm

A white dwarf kicked out of a supernova
astrobites | 2018 Mar 30
Matthew Green wrote:
I’m sure you’ve heard of Type Ia supernovae. They’re a certain type of exploding star, most famous for the fact that their brightness can be easily calculated from the other features of the explosion. If you know how bright something is, and you measure how much of the light reaches you, that tells you how far away the light source must be. (“Standard candle” is the common term for objects like this.) Type Ia supernovae are useful to astronomers who want to measure the distance to far-away galaxies, and they form one link in the cosmic distance ladder. ...

Last year, a team of astronomers found a white dwarf named LP40-365. It’s moving through the galaxy incredibly fast (about 500-800 km/s), and contains an unusual collection of elements. The authors of the discovery paper suggest that this is a leftover from a Type Ia supernova — a white dwarf that tried to go bang but survived. Today’s authors studied spectra of the star (from the Copernico telescope) in order to get a better idea what was going on with it. ...

Further insight on the hypervelocity white dwarf, LP 40-365 (GD 492):
A nearby emissary from a single-degenerate Type Ia supernova
- R. Raddi et al
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Has Anyone Found a Lost Comet?!

Post by bystander » Wed Apr 11, 2018 6:25 pm

Has Anyone Found a Lost Comet?!
astrobites | 2018 Apr 02
Kerrin Hensley wrote:
Nearly two hundred and fifty years ago, Charles Messier, renowned comet hunter and chronicler of deep-sky objects, cataloged the passage of the first known Near-Earth Object: comet D/Lexell. Calculations of its orbit revealed that the massive chunk of ice and dust had hurtled past Earth at a distance of only 1.4 million miles—just under six times the Earth-Moon distance. It was due to return within the decade but was never seen again. How did we lose a comet?!

In the following several decades, a trio of scientists independently showed that a rendezvous with Jupiter likely threw D/Lexell into a new orbit—and possibly out of the solar system entirely. While the mystery of D/Lexell’s disappearance has remained unsolved since, modern astronomical and computational methods might be able to crack this cold case. ...

Finding Long Lost Lexell's Comet: The Fate of the First Discovered Near-Earth Object - Quan-Zhi Ye, Paul A. Wiegert, Man-To Hui
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Fraught with Spots

Post by bystander » Wed Apr 11, 2018 6:34 pm

Fraught with Spots
astrobites | 2018 Apr 03
Emily Sandford wrote:
Twice in my life, I have mistakenly purchased green sweaters. “What a lovely pearl gray!” I have thought in the soothing, dim light of a Target fitting room. “This will go so nicely with my red corduroys,” I have mused under the bland fluorescent bulbs of the check-out line. “Dammit, green again?” I have sworn in the harsh sunshine of the parking lot.

Unhappily, this is neither the most important nor the most expensive mistake an astronomer can make about lighting. Misinterpreting the color of a sweater based on the wrong perception of the ambient light? Not so bad–I just had to wait in another line to return the impostor sweater. But misinterpreting the color of planets based on misunderstanding their host stars’ light? Today’s authors warn that such a mistake could lead us to false conclusions about planetary atmospheres, compositions, and habitability. ...

The Transit Light Source Effect: False Spectral Features and Incorrect Densities
for M-dwarf Transiting Planets
- Benjamin V. Rackham, Dániel Apai, Mark S. Giampapa
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Introducing ELROI: the world’s flashiest license plate

Post by bystander » Wed Apr 11, 2018 6:41 pm

Introducing ELROI: the world’s flashiest license plate
astrobites | 2018 Apr 04
Amber Hornsby wrote:
Space is a mess. From communicating with people all over the globe to navigating the roads, we have slowly become reliant on the use of satellites in our daily lives. They make us safer and more connected, which means it is no surprise that we’re launching more and more each year.

However, between retired payloads, rocket bodies and other fragments, space around Earth, is becoming increasingly cluttered. With more than 16000 objects currently being tracked*, space junk is clearly a massive problem, but dealing with it is not what the authors of today’s paper tackle. Instead, they focus on how to track objects in the first place. ...

Progress on ELROI satellite license plate flight prototypes - Rebecca M. Holmes et al ELROI: A License Plate For Your Satellite - David M. Palmer, Rebecca M. Holmes
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Spotting a planet

Post by bystander » Wed Apr 11, 2018 7:09 pm

Spotting a planet
astrobites | 2018 Apr 05
Eckhart Spalding wrote:
One technique for detecting and characterizing exoplanets is the transit method. While we never directly observe the planet, we can detect it by observing a slight dimming of the host star’s light as the planet passes in front of the star. Observing this dip in different wavelengths allows us to infer the presence of chemical species in the planet’s atmosphere. But we must be cautious, because dark or light spots on the surface of the host star can contaminate the spectrum. (Check out this bite about the problem.) We have to monitor the host star for a long time to understand the surface variations, or, as silly as it may sound, actually resolve the face of the host star during the transit.

Direct imaging has turned up a dozen or so exoplanets, out of a total of some 3,000. A dozen may not seem like a lot, but the direct imaging technique is useful for detecting massive planets on wide orbits, and for examining the planet’s thermal emission spectrum.

But what if we could apply imaging techniques to transiting systems and directly observe transits? An exciting emerging technique that bridges the imaging and transit methods is called “transit interferometry”. Interferometry involves pointing multiple telescopes to an unresolved object, measuring differences in the observed signals, and using the differences to infer something about the true image. The math yields two magical quantities called a “visibility” (Figs. 1 and 2) and an angular “phase” (Fig. 2), both of which encode information about the image. ...

The authors of today’s paper wrote a code called COMETS to calculate the interferometric observables of transiting planets, and transiting planets plus starspots, at visible wavelengths. The transiting planet is represented by a dark spot on the face of the host star’s limb-darkened disk, and the starspot is a simple disk that emits like a blackbody (Fig. 3). ...

Transiting exoplanets and magnetic spots characterized with optical interferometry - R. Ligi et al
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Statistically Confirming an Earth-Like Planet? Not So Fast!

Post by bystander » Wed Apr 11, 2018 7:27 pm

Statistically Confirming an Earth-Like Planet? Not So Fast!
astrobites | 2018 Apr 06
wrote:
Recent announcements of large batches of exoplanets discovered using the Kepler space telescope have relied on statistical methods. Rather than relying on expensive imaging and spectroscopic follow up of candidate objects these studies rely on a large number of tests meant to weed out false positives. These false positive signals could be astrophysical in nature (e.g. a background eclipsing binary) or due to noise sources inside the camera. Today’s paper calls into question the statistical confidence of these methods when used on low signal to noise ratio (SNR) candidates with long periods and specifically disputes the 99.97% confidence of Kepler-452b. ...

Kepler's Earth-like Planets Should Not Be Confirmed without
Independent Detection: The Case of Kepler-452b
- Fergal Mullally et al
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A Pebbly Barrier to Planet Formation

Post by bystander » Wed Apr 11, 2018 7:55 pm

A Pebbly Barrier to Planet Formation
astrobites | 2018 Apr 09
Peter Sinclair wrote:
Building a planet is a messy process, and astronomers are still unsure of how certain steps work. Planets are born from the same nebula that forms the star they orbit. As the nebula collapses, it flattens out, due to the gas’s angular momentum. This becomes the star’s protoplanetary disk, where the planets are born. Within this disk, small instabilities can lead to the growth of planetesimals. Some of these will continue to gain mass and will eventually become planets. How exactly they gain mass, though, is still up to debate.

The two most popular models are core accretion and disk instability. In the core accretion model, planetesimals in the protoplanetary disk collide and stick together. Eventually the core will become large enough that it begins to accrete more and more material onto itself. In the disk instability model the disk becomes unstable when it becomes massive enough. This leads to small regions undergoing local collapses, forming planets.

Combinations of the two theories have been proposed, but details are still being worked out. One large problem is that the time scale for disk instability is very short while core accretion takes a long time. There are also questions about whether or not planets remain at the distance where they formed. Discoveries like hot Jupiters raise questions about whether planets might migrate inwards or outwards from the star.

Today’s paper takes a closer look at one of the remaining problems of planetary formation – how a planet’s mass can prevent it from accreting more material. ...

How much does turbulence change the pebble isolation mass for planet formation? - S. Ataiee et al
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Too much star formation, not enough stellar mass

Post by bystander » Wed Apr 11, 2018 8:10 pm

Too much star formation, not enough stellar mass: a cosmic conundrum
astrobites | 2018 Apr 10
Christopher Lovell wrote:
The universe is a pretty efficient star making machine. Since around a billion years after the Big Bang, galaxies have been assembling and forming stars with abandon. Many of these stars have now died, particularly the most massive, which have much shorter lifetimes than less massive stars, and often end their lives in cataclysmic supernovae. But a number of low mass stars are still around from the very earliest star forming episodes. As a result, the overall density of stars in the universe has been increasing. Figure 1, from today’s paper, shows the Stellar Mass Density (SMD) from recent observations – the stellar mass density is peaked at the current day (redshift zero).

However, a key discovery over the past few decades is that the universe is not forming stars with quite the same enthusiasm as it once was. In fact, the peak of star formation activity in the universe was around ten billion years ago, or three and a half billion years after the Big Bang. Since then, the Star Formation Rate Density (SFRD, the mass in stars formed per unit volume) has been gradually decreasing. The red points in figure 2 show the observed SFRD over cosmic time.

These two properties, the SFRD and the SMD, are clearly linked: if the SFRD is high, then the SMD in the future should also be higher. Similarly, if the universe were to stop making stars, we would expect the SMD to decrease, as massive stars died and were not replaced by new stars.

Today’s paper looks at observational data on these two properties, and shows a puzzling inconsistency. The black line in figure 1 shows the SMD expected if we add up all the star formation from the SFRD measurements – it’s clearly higher than the observations. Similarly, the green line in figure 2 shows the SFRD we would expect to fit the SMD measurements, and it is much lower than the observations, by a factor of four at redshift two. ...

On the inconsistency between cosmic stellar mass density and star formation rate up to z∼8 - H. Yu, F. Y. Wang
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Let’s Call It STEVE

Post by bystander » Wed Apr 11, 2018 8:26 pm

Let’s Call It STEVE
astrobites | 2018 Apr 11
Stephanie Hamilton wrote:
You may recall the 2006 movie called “Over the Hedge” that follows the adventures of a band of forest animals as they take their first excursion, well, over the hedge. Upon first encountering the towering hedge, the animals have no idea what it is and are afraid, so one of them helpfully suggests, “Let’s call it Steve!” to make the hedge less scary (youtube link — no really, watch it!). Fast-forward now to 2016, when citizen scientists in northern Canada spotted a strange purple ribbon glowing in the sky at lower latitudes than typical auroras. With no idea what it was and no scientific classification available, the scientists called the phenomenon “Steve” (originally as a joke in the spirit of “Over the Hedge, ” but the name stuck). Now, thanks to today’s authors, we’re closer to understanding STEVE. ...

New science in plain sight: Citizen scientists lead to the discovery
of optical structure in the upper atmosphere
- Elizabeth A. MacDonald et al
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A Naked-Eye Superflare Detected from Proxima Centauri

Post by bystander » Tue Apr 17, 2018 4:54 pm

A Naked-Eye Superflare Detected from Proxima Centauri
astrobites | 2018 Apr 12
Daniel Berke wrote:
Proxima Centauri is the closest known star to the Sun at just 4.246 light-years (1.302 parsecs) away. It’s a red dwarf of spectral type M6 with about 12% of the Sun’s mass, 1.2 times the diameter of Jupiter, and 0.17% of the Sun’s luminosity. It hosts the closest known exoplanet to us, Proxima Centauri b, which was discovered in 2016 as covered in this Astrobite. Like our Sun, it’s on the main sequence, steadily fusing hydrogen into helium in its core. Yet this tiny star is way more active than the Sun is!

Red dwarfs like Proxima Centauri have interiors that are fully convective, meaning that the energy generated by fusion in their cores is transported to the surface primarily via convection. Like a pot of boiling water, you can think of it as being one giant ball of boiling plasma. This turnover of ionized gas generates powerful magnetic fields, which are carried to the surface along with the bubbles of hot plasma. When these bubbles reach the surface the energy contained in the magnetic fields can be violently released in the form of stellar flares, which can grow as large as Proxima Centauri itself and reach temperatures of up to 27 million K! (Normally its effective surface temperature is around 3,000 K.) These flares from Proxima Centauri have been observed frequently in the past (for instance in this recent Astrobite).

In this paper, the authors report the discovery of the first-known superflare from Proxima Centauri (see Figure 1 for the light curve), a flare roughly ten times more powerful than any seen before. Normally Proxima Centauri sits at a visual magnitude of 11.13, approximately 100 times fainter than the human eye can see. But during the superflare the authors calculated that it would have reached an apparent visual magnitude of 6.8 for a few minutes, just bright enough to be seen with the naked eye in extremely dark skies! (No accounts of anyone actually seeing it by eye at the time are known, though.) ...

The First Naked-Eye Superflare Detected from Proxima Centauri - Ward S. Howard et al
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