astrobites 2017

[bystander]

Great News for Impatient Scientists!
astrobites | 2017 May 15

Discoveries of exoplanets happen quite often these days, so much so that the discovery alone is not enough to satisfy collective scientific curiosity. Discovery with direct imaging, in particular, does not usually reveal much about the planet, other than its existence. However, unlike the transit method and radial velocity measurements, direct imaging allows us to observe exoplanets with very long periods, which is an under-sampled population in the list of currently known exoplanets. Still, this double-edged method of measurement cannot give us full orbital parameters of the planetary system. This population of exoplanets cannot be easily observed by any other method but direct imaging, so the question arises—how can we find the orbital properties of this planetary system with the measurements we have?

The authors of this paper use a new rejection sampling method to quickly find the orbits of these exoplanets, called Orbits for the Impatient (OFTI) . This method generates random orbital fits from astrometric measurements, then scales and rotates the orbits, and then reject orbits too unlikely. ...

Orbits for the Impatient: A Bayesian Rejection-Sampling Method
for Quickly Fitting the Orbits of Long-period Exoplanets - Sarah Blunt et al

• Astronomical Journal 153(5):229 (2017 May) DOI: 10.3847/1538-3881/aa6930
  arXiv.org > astro-ph > arXiv:1703.10653 > 30 Mar 2917

[bystander]

Hunting for ECOs: Gravitational Wave “Smoking Guns”
astrobites | 2017 May 16

An exotic compact object (ECO) consists of matter that is not electrons, neutrons, protons or muons. There have been numerous “exotic” astronomical objects proposed (for example, quark stars, boson stars and preon stars) but none of these hypothetical stars have been detected. Up until recently, detecting objects that do not radiate electromagnetically has been challenging for astronomers and only accomplished indirectly. With the advent of the emerging field of gravitational wave astronomy, we have the ability to directly detect ECOs (if they exist) – we just need to know the gravitational wave “smoking gun” to look for!

But why study ECOs at all, given we don’t know if they exist? Even as purely hypothetical objects, they are useful as tractable toy models for testing consequences of general relativity. In addition, they could play a role in solving some of the biggest mysteries in the Universe – boson stars, for example, have been considered as potential dark matter candidates (see here). And if they do exist, we need to be able to know how to distinguish their gravitational wave signals from those of objects that have already been observed.

This brings us to today’s astrobite. Firstly, the authors simulate bouncing a wave packet off the gravitational potential of several different models of ECOs and observe that, qualitatively, ultra-compact objects have a universal signature in their response. Secondly, the authors investigate the complementary problem of boson stars colliding. Boson stars are chosen because they are ECOs whose formation can potentially occur in dynamical scenarios and they are relatively simple to treat numerically. ...

Gravitational-wave signatures of exotic compact objects
and of quantum corrections at the horizon scale - Vitor Cardoso et al

• Physical Review D 94(8):4031 (15 Oct 2016) DOI: 10.1103/PhysRevD.94.084031
  arXiv.org > gr-qc > arXiv:1608.08637 > 30 Aug 2016 (v1), 13 Oct 2016 (v2)

[bystander]

It’s the Remix to Star Formation
astrobites | 2017 May 17

Estimating physical properties of galaxies is a messy business. We can measure the light from galaxies, usually in the form of spectra, colors, or magnitudes. To convert these directly-observable measurements to physical properties, we have to use models of galaxies.

Sometimes these conversions are relatively straightforward—for instance, the ultraviolet flux of a galaxy can be plugged into commonly-used formulas to find the star formation rate of a galaxy (the mass of stars that a galaxy forms per year). Sometimes these conversions are more complicated—one way to measure the stellar mass of a galaxy (the mass of stars in a galaxy) is to compare the galaxy’s spectral energy distribution to a library of simulated models and to find the closest match.

Today’s paper presents a new way to find simple conversions from observables to physical properties. ...

Quenching or Bursting: Star Formation Acceleration --
A New Methodology for Tracing Galaxy Evolution - D. Christopher Martin et al

• arXiv.org > astro-ph > arXiv:1705.03514 > 09 May 2017

[bystander]

Things that go “chirp” in the night
astrobites | 2017 May 18

One of the biggest scientific accomplishments of the last few years was the discovery of gravitational waves by the LIGO Collaboration, which you can read about on Astrobites here. There is, of course, still plenty of work to be done in this field. For example, no experiment has definitively detected an electromagnetic counterpart (which would give off radiation somewhere in the electromagnetic spectrum) to a gravitational wave, although the Fermi Gamma-ray Burst Monitor may have seen hints of one. Detecting such a component would be scientifically interesting for many reasons. The authors of today’s paper give us two such reasons: first, LIGO currently only has a rudimentary ability to localize where in the sky gravitational waves are coming from. Identifying the specific galaxy that produced the gravitational wave would allow us to constrain certain astrophysical models. Second, we could possibly combine an electromagnetic (EM) counterpart with a sub-threshold gravitational wave signal (one that is not statistically significant on its own) to gleam more information about astrophysical events. ...

The authors used a Monte Carlo radiation transport code to calculate the light curves and the resulting spectra on Earth. Free parameters included the masses of the neutron star and black hole, along with the separation between them. ...

Electromagnetic Chirps from Neutron Star-Black Hole Mergers - Jeremy D. Schnittman et al

• arXiv.org > astro-ph > arXiv:1704.07886 > 25 Apr 2017

[bystander]

Supernovae Made Way for Cosmic Reionization
astrobites | 2017 May 19

The epoch of reionization may not be a familiar time period to all, but it gave rise to the luxury we have to be able to see galaxies, stars, and planets far far away! Let’s have a one-minute review on cosmic history. As the Universe expanded after the Big Bang, the hot dense soup of energetic particles cooled down. After about 380,000 years, it cooled down enough where free electrons and protons could find each other and reunite into neutral hydrogen (it did happen, we took pictures!) Hydrogen atoms are extremely good at blocking light. If the Universe stayed neutral, we wouldn’t be able to see so far. So something must have managed to ionize these neutral hydrogen atoms back again for the second time since the first ionization during the Big Bang. Indeed we call the time before such reionization the cosmic dark ages.

We aren’t entirely sure what provided the photons responsible for reionizing the Universe. To this end, we astrophysicists are hard-working and creative. Black holes and massive stars, the merging of binary stars, and the low-mass and faint galaxies at high redshifts were found to be the potential suppliers of the ionizing photons. In particular, recent studies have argued in favor of the dominant role of low-mass star-forming galaxies, but there are much more to it. Photons have to escape the galaxies in order to contribute to reionization, on their ways they might be absorbed by gas or dust. What fraction of the ionizing photons could actually leave the galaxies? This is quantified by an important number known as the escape fraction, commonly denoted as f_esc. ...

Today’s paper aims not just to calculate f_esc, but to take a step forward in understanding the escape of ionizing photons from galaxies and what regulates f_esc. Starting from three low-mass galaxies (with dark matter masses ranging from a tenth to a few billion solar masses) assembled in a cosmological simulation, the authors performed zoomed-in, high-resolution simulations of star formation. Supernova explosions were set to go off 10 Myr after the stars formed. In the course of the simulation, the propagation of ionizing photons was followed and the amount escaping was measured. In this bite we will focus on the representative results of the most massive galaxy out of the three simulated. ...

Fluctuating feedback-regulated escape fraction of ionizing radiation
in low-mass, high-redshift galaxies - Maxime Trebitsch et al

• Monthly Notices of the RAS (online 03 May 2017) DOI: 10.1093/mnras/stx1060
  arXiv.org > astro-ph > arXiv:1705.00941 > 02 May 2017

[bystander]

The stellar evolution conspiracy, part II
astrobites | 2017 May 20

Because we know the Sun so well, we use it and other similar stars as templates to understand the physics of other objects in the universe. Thus the theory and modelling of stellar evolution play a key role on our understanding of the universe: from the habitability of exoplanets to the nature of dark matter, you name it! The problem is that it is extremely complicated and different authors use different treatments, which leads to some inconsistencies, especially for stars that diverge from the solar standard (see part I). With that in mind, let us ask ourselves again, this time with feeling: how much should we trust our understanding of stellar evolution? ...

The Influence of Atomic Diffusion on Stellar Ages and Chemical Tagging - Aaron Dotter et al

• Astrophysical Journal 840(2):99 (2017 May 10) DOI: 10.3847/1538-4357/aa6d10
  arXiv.org > astro-ph > arXiv:1704.03465 > 11 Apr 2017

[bystander]

Will the real Protoplanetary Disk Mass please stand up?
astrobites | 2017 May 22

The clock is ticking. Time is running out. If you placed a disk with 10 times the mass of Jupiter around a star that was very much like the Sun, how quickly could you build a gas giant planet? Would you have enough gaseous material to form a planet of that size? Or would the disk fade away before you had time to finish assembling your magnificent planetary masterpiece? ...

Using Ice and Dust Lines to Constrain the Surface Densities of Protoplanetary Disks - Diana Powell, Ruth Murray-Clay, Hilke E. Schlichting

• Astrophysical Journal 840(2):93 (10 May 2017) DOI: 10.3847/1538-4357/aa6d7c
  arXiv.org > astro-ph > arXiv:1704.04693 > 15 Apr 2017

http://asterisk.apod.com/viewtopic.php?p=270982#p270982

[bystander]

Strength in numbers: a smarter way to find habitable worlds
astrobites | 2017 May 23

The discovery of the first exoplanet around a main sequence star in 1995 brought a great deal of attention to the search for life beyond the Solar System (i.e., The Big Question). Since then, we have found thousands of extrasolar planets. As humans, our first instinct is to comb over this huge sample to find the best candidates for habitability (such as TRAPPIST-1 and Proxima b), and then painstakingly characterize them to check if they really are habitable. But this process is very resource expensive and it hasn’t really paid off thus far. What if there are other, more effective ways to answer The Big Question? ...

A Statistical Comparative Planetology Approach to the Hunt
for Habitable Exoplanets and Life Beyond the Solar System - Jacob L. Bean, Dorian S. Abbot, Eliza M.-R. Kempton

• Astrophysical Journal Letters 841(2):L24 (2017 Jun 01) DOI: 10.3847/2041-8213/aa738a
  arXiv.org > astro-ph > arXiv:1705.06288 > 17 May 2017

[bystander]

Wonderful Miras in the Heart of the Galaxy
astrobites | 2017 May 24

Mira-type variables are stars at the very end of their lifetimes: giant stars in the process of throwing off their outer layers. They consist of a dense core not much bigger than the Earth, several onion-like layers of burning hydrogen and helium, and a puffed-up outer layer which is only loosely bound to the star. They pulsate on periods lasting several months or years, and cast away a bit more of their atmosphere on each pulsation. The first known member of this class, Mira itself (known as Mira ‘the wonderful’ to early astronomers because of the way it seemed to appear and disappear annually), is followed by an amazing, comet-like tail of matter stretching thirteen lightyears behind it (see for yourself in Figure 1). Once the atmosphere is completely stripped, all that is left will be the dense core which will evolve into a white dwarf.

Astronomers love to subdivide their classifications, and so Miras are split into two subclasses: oxygen-rich and carbon-rich. The ignition of helium burning during a star’s evolution can trigger a ‘dredge-up‘ of carbon from the core to the surface; if the star becomes a Mira while this carbon is still at the surface it will be carbon-rich, whereas if it does so once the carbon has settled it will be oxygen-rich. Carbon-rich Miras are therefore generally younger. ...

Discovery of carbon-rich Miras in the Galactic bulge - Noriyuki Matsunaga et al

• Monthly Notices of the RAS (online 18 May 2017) DOI: 10.1093/mnras/stx1213
  arXiv.org > astro-ph > arXiv:1705.05485 > 15 May 2017

[bystander]

Astrophysical Classics: Joy in One Hundred and Twenty Particles
astrobites | 2017 May 25

Sometimes it’s hard to draw a beautiful picture from an astrophysical concept.

Even when the subject is stunning the image can still be pragmatically plain, completely byzantine or, while handsome, simply do not engender a deeper understanding.

I was going to write a bite about the scientific impact of this classic paper, one often cited as the first in a long lineage of a wholly new take on galaxy evolution. I’ll draw from that source, but looking carefully at it, there’s a deeper joy in the eyes of the reader, and of the authors. ...

Galactic Bridges and Tails - Alar Toomre, Juri Toomre

• Astrophysical Journal 178:123 (1972 Dec 15) DOI: 10.1086/151823

[bystander]

Citizen scientists in search of failed stars
astrobites | 2017 May 29

Not everyone can be a star. Brown dwarfs, for example, have failed on their attempt. These objects have masses below the necessary amount to reach pressure and temperature high enough to burn hydrogen into helium in their cores and thus earn the classification “star”. It’s not very long since we’ve learned of their existence. They were proposed in the 1960s by Dr. Shiv S. Kumar, but the first one was only observed many years later, in 1988 – and we are not even sure it is in fact a brown dwarf! We’ve only reached a substantial number of known brown dwarfs with the advent of infrared sky surveys, such as the Two Micron All Sky Survey (2MASS) and the Wide-field Infrared Survey Explorer (WISE).

Discovering and characterising cold brown dwarfs in the solar neighbourhood is one of the primary science goals for WISE. There are two ways of doing that: 1) identifying objects with the colours of cold brown dwarfs; 2) identifying objects with significant proper motion. Brown dwarfs are relatively faint objects, so they need to be nearby to be detected. We can detect the movement of such nearby targets against background stars, which are so distant that they appear to be fixed on the sky. This movement is called proper motion. As the signal-to-noise ratio is not very good for such faint objects, the second method is the preferred one. However, single exposure WISE images are not deep enough to find most brown dwarfs. This is where today’s paper enters. The authors have launched a citizen science project called “Backyard Worlds: Planet 9” to search for high proper motion objects, including brown dwarfs and possible planets orbiting beyond Pluto, in the WISE co-add images. Co-add images are simply a sum of the single exposures images taking into account corrections to possible shifts between them. This increases signal-to-noise ratio and helps to detect faint targets. On today’s paper, they report the first discovery of their project: a new brown dwarf in the solar neighbourhood, which was identified only six days after the project was launched! ...

The First Brown Dwarf Discovered by the Backyard Worlds: Planet 9 Citizen Science Project - Marc J. Kuchner et al

• Astrophysical Journal Letters 841(2):L19 (2017 Jun 01) DOI: 10.3847/2041-8213/aa7200
  arXiv.org > astro-ph > arXiv:1705.02919 > 08 May 2017

http://asterisk.apod.com/viewtopic.php?t=37004

[bystander]

Blown away by Black Holes: Losing Planetary Atmospheres to Quasar Radiation
astrobites | 2017 May 30

Life on Earth: beautiful, wondrous, and utterly at the mercy of the environment. There are a number of crucial qualities – liquid water, breathable air, temperate sunlight – that we Earth-born life forms need to survive. Without them, we’d have a truly frightening apocalypse on our hands (worse than any other apocalypse you might have come across).

In light of that, astronomers searching for life (as we know it) beyond Earth look for exoplanets that have these same crucial qualities. But for an exoplanet to house Earth-like life, there’s quite a lot that needs to be “just right”. The exoplanet must be in some sort of habitable zone, for example, where liquid water can exist, and the exoplanet must also have some form of hospitable atmosphere. And just as things can go “just right” to cultivate these qualities, other things can go very wrong.

The authors of today’s astrobite look into one of these characteristics that could go very wrong: the loss of a planet’s atmosphere. They explore how quasars can rather literally blow these atmospheres away. ...

Evaporation of Planetary Atmospheres due to XUV Illumination by Quasars - John C. Forbes, Abraham Loeb

• arXiv.org > astro-ph > arXiv:1705.06741 > 18 May 2017

[bystander]

What do you have to do to get a water covered planet around here?
astrobites | 2017 Jun 01

Astronomers (and Astrobites!) like exoplanets, and we particularly like those that may have water—because water holds the potential for life. Water has a unique ability to participate in biochemical processes: if we are looking for life as we know it, finding water is a good place to start.

Today’s paper targets one such exoplanet, – the memorably named Kepler-62f. It’s the outermost planet in a five-planet system 1,200 light-years from Earth. Like all good candidates for little green men, Kepler-62f lies within its host star’s habitable zone. This sweet spot for planets, also known as the goldilocks zone, is where liquid water can be supported on the planet’s surface. Not too close to the star, where it’s too hot and the water boils away, not too far out, where it’s too cold and the water freezes, but just right. However, before we send out the search parties, it’s important to remember that a favourable distance to the star is just one piece of the puzzle. In this paper, the authors explore some of the other key factors at play, such as different orbital configurations and climates, to study how these might affect the habitability of Kepler-62f. ...

The Effect of Orbital Configuration on the Possible Climates and Habitability of Kepler-62f - Aomawa L. Shields et al

• Astobiology 16(6):443 (2016 Jun 06) DOI: 10.1089/ast.2015.1353
  arXiv.org > astro-ph > arXiv:1603.01272 > 03 Mar 2016

http://asterisk.apod.com/viewtopic.php?t=35991

[bystander]

It’s a tough life for a small galaxy
astrobites | 2017 Jun 02

It’s a jungle out there, and it’s all about survival of the biggest.

As our understanding of how galaxies form has developed, we’ve come to realise that galaxies tend to start small and grow over time by gobbling up smaller galaxies: a kind of cosmic food chain. The biggest galaxies – the predators of this vicious ecosystem – can usually look after themselves. It’s pretty rare for them to encounter an equally big and bad galaxy and for the most part they barely pause as they tear unfortunate smaller galaxies to shreds.

(Quite literally – no exaggeration here! When the galaxies are similar sizes it gets pretty messy…)

In the end, most massive galaxies generally end up all looking fairly similar, because there’s not much that the galaxies around them can do to make them evolve differently to how they would if they were left alone. But for smaller galaxies it’s a very different story. ...

The hELENa project: I. Stellar populations of early-type galaxies
linked with local environment and galaxy mass - Agnieszka Sybilska et al

• Monthly Notices of the RAS (online 11 May 2017) DOI: 10.1093/mnras/stx1138
  arXiv.org > astro-ph > arXiv:1706.00014 > 31 May 2017

[bystander]

A first glimpse into deep Jupiter
astrobites | 2017 Jun 12

When we gaze upon the night sky, we can often spot Jupiter. We have also launched several missions to explore Jupiter, like Pioneer, Voyager, and Galileo. Yet all we have “seen” stops at the cloud tops. Now our understanding is going to change by NASA’s Juno mission, which provides us the privilege to have a glimpse through the clouds. The primary science goal of Juno is to measure the deep composition and internal structure, so that we can better understand the formation and evolution of Jupiter and planetary formation in general. Juno takes elliptical orbits around Jupiter to minimize the damage from the radiation belt. Once every 53 days, Juno accomplishes a close flyby and takes as many photos as possible. Today’s paper brings some exciting new results from first few close passes of Juno probe. ...

Jupiter’s interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft - S. J. Bolton et al

• Science 356(6340):821 (26 May 2017) DOI: 10.1126/science.aal2108

[bystander]

These aren’t the bursts you’re looking for
astrobites | 2017 Jun 13

Using the intermediate Palomar Transient Factory (iPTF), a broadlined type Ic supernova (Bl-Ic SN) was accidentally uncovered during follow up observations of the newest gravitational wave in town – GW170104 . Further investigation of iPTF17cw suggests it is the first discovery of a candidate relativistic BL-Ic SN discovered independently of a gamma ray trigger. Today’s bite presents the discovery, classification and follow-up observations of this fascinating supernova. ...

iPTF17cw: An engine-driven supernova candidate discovered independent of a gamma-ray trigger - A. Corsi et al

• arXiv.org > astro-ph > arXiv:1706.00045 > 31 May 2017

[bystander]

It’s a gas gas gas: understanding gas motions surrounding galaxies
astrobites | 2017 Jun 14

For galaxies, gas is a pretty big deal. Without it, they’re unable to form new stars — which is pretty much their only job. Once they get hold of some though, stars of all shapes and sizes start forming. Some of these will be massive, rapidly burning up their fuel and going supernovae. If the supernovae is energetic enough, this can eject a load of precious gas and shut down the star formation again. Understanding this precarious galactic balancing act, between inflows and outflows of gas, is crucial for modelling the properties of galaxies. ...

Today’s paper uses observations from the Keck Baryonic Structure Survey (KBSS), which studies the gas around 854 star forming galaxies at a redshift of two (around 3 billion years after the big bang) using background quasars. The authors compare the observations with mock spectra from the EAGLE simulation, a computer model of galaxy formation and evolution that matches various galaxy properties. The mock spectra are designed to mimic KBSS as closely as possible, so that comparisons can be made between the two. For this study they measure the optical depth (which roughly corresponds to the amount) of three elements: neutral hydrogen, ionised carbon and ionised silicon. ...

A comparison of observed and simulated absorption from HI, CIV, and SiIV around z≈2
star-forming galaxies suggests redshift-space distortions are due to inflows - Monica L. Turner et al

• arXiv.org > astro-ph > arXiv:1703.00086 > 28 Feb 2017

[bystander]

Clouds over the sunlit arch
astrobites | 2017 Jun 15

Having grown up on Earth, we tend to associate cloudy skies with brisk weather. Even gloomy weather. As Robert Frost said, if it’s the month of May and a “cloud comes over the sunlit arch” you’ll find yourself “two months back in the middle of March”. How different are clouds on alien worlds? Is there such a thing as searingly hot clouds, suspended high above in skies so bright they make your eyes ache when you shut them tight? ...

The authors of today’s paper push the boundaries of knowledge into the most scalding area of parameter space: the realm of sizzling “super-hot” Jupiters, with temperatures of more than 1800 K. ...

High temperature condensate clouds in super-hot Jupiter atmospheres - Hannah R. Wakeford et al

• Monthly Notices of the RAS 464(4):4247 (Feb 2017) DOI: 10.1093/mnras/stw2639
  arXiv.org > astro-ph > arXiv:1610.03325 > 11 Oct 2017

[bystander]

Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line
astrobites | 2017 Jun 16

How is it possible for planets to exist? Even though we know planets must have formed from planetesimals that are tens of kilometers in size, the most basic models of protoplanetary disks have trouble forming planetesimals from the micron to centimeter-sized dust that populates these disks. For dust particles to grow into planetesimals, they need to be able to clump together enough to reach roughly the same level of concentration as the gas in the disk – which can be difficult since there is 100 times more gas than dust. ...

Djoeke Schoonenberg and Chris Ormel, the authors of today’s paper, set out to improve our understanding of whether the snow line can trigger the formation of planetesimals by creating a more rigorous model that better captures the dynamic structure of the disk and of the dust grains themselves. ...

Planetesimal formation near the snowline: in or out? - Djoeke Schoonenberg, Chris W. Ormel

• Astronomy & Astrophysics 602:A21 (June 2017) DOI: 10.1051/0004-6361/201630013
  arXiv.org > astro-ph > arXiv:1702.02151 > 07 Feb 2017

[bystander]

Could we detect Europa-quakes?
astrobites | 2017 Jun 19

Early images of Europa, Jupiter’s fourth-largest moon, revealed an icy surface scarred with reddish stripes. Later observations by the Galileo spacecraft of distortions of Jupiter’s magnetic field near Europa hinted at the presence of a global water ocean beneath the ice shell. But how thick is the ice shell? And just how deep is the ocean? A Europa lander equipped with a seismometer might be able to answer these questions. A seismometer measures how much the ground moves as a result of seismic activity like earthquakes or volcanic eruptions. In this paper, the authors use statistical models of seismic activity and thermodynamical models of planetary interiors to estimate the seismic noise on Europa. With these estimates, we can begin to set basic requirements for the instruments that could one day measure the seismic rumbles of Europa’s ice shell. ...

The seismic noise environment of Europa - Mark P. Panning et al

• arXiv.org > astro-ph > arXiv:1705.03424 > 09 May 2017 (v1), 16 May 2017 (v2)

[bystander]

Galactic Archaeology of the Sagittarius Stream
astrobites | 2017 Jun 20

Small satellite galaxies and star clusters orbiting the Milky Way are often torn apart by its gravitational potential, leaving behind trails of stars stretched out across the sky (Figure 1). These stellar streams (more info in this astrobite!) contain information about the history of their parent object and can be studied to learn about the evolution of the Milky Way in an approach referred to as “Galactic Archaeology.” Like archaeologists who study ancient history on Earth, Galactic archaeologists search for present-day remnants of historical events. The spatial structure and the properties of the stars that make up these stellar streams tell a story of the parent object’s evolution and eventual destruction. ...

The star formation history of the Sagittarius stream - T.J.L. de Boer, V. Belokurov, S. Koposov

• Monthly Notices of the RAS 451(4):3489 (21 Aug 2015) DOI: 10.1093/mnras/stv946
  arXiv.org > astro-ph > arXiv:1505.00787 > 04 May 2017

[bystander]

Telescopes Don’t Make Catalogs
astrobites | 2017 Jun 21

What is a catalog? As far as many of the latest and largest astronomical surveys are concerned (SDSS, 2MASS, USNO-B, etc.), a catalog is essentially a list of sources that includes measurements of a variety of properties, such as source positions and fluxes. These catalogs are undeniably useful, not least because they make the sky searchable and facilitate large statistical studies of whole populations of different objects. But catalogs are not the fundamental data product of a survey, since telescopes make images, not catalogs.

Catalogs are made by people (or perhaps rather the algorithms they invent), who have to make hard decisions about the specifics of raw image calibration and source detection. In this process of catalog-making, some potentially valuable information will almost inevitably get lost. Reluctant to accept this, the authors of today’s paper experiment with a novel representation of the imaging data, a probabilistic catalog that encapsulates more of the available information and thus enables more precise tests of models against the underlying source population. ...

Probabilistic Catalogs for Crowded Stellar Fields - Brendon J. Brewer, Daniel Foreman-Mackey, David W. Hogg

• Astronomical Journal 146(1):7 (2013 July) DOI: 10.1088/0004-6256/146/1/7
  arXiv.org > astro-ph > arXiv:1211.5805 > 25 Nov 2012 (v1), 20 Apr 2013 (v2)

Telescopes Don’t Make Catalogs! - David W. Hogg, Dustin Lang

• EAS Publications 45:351 (2010 Jun 07) DOI: 10.1051/eas/1045059
  arXiv.org > astro-ph > arXiv:1008.0738 > 04 Aug 2010

[bystander]

Particle Acceleration in the Galactic Center
astrobites | 2017 Jun 22

Particle astrophysics is a rapidly growing field that draws from both physics and astronomy, and one of the big open questions in the field is how exactly cosmic rays (composed of astrophysical charged particles such as protons) are accelerated, along with where they originate from. We know most Galactic cosmic rays are associated with supernova remnants, but those can’t accelerate cosmic rays up to the highest energies. Many theories point to activity in and around the Galactic Center as an important region in solving this mystery.

Since cosmic rays are charged particles, they bend in magnetic fields during the journey to Earth and therefore do not point back to their source. One of the ways to get around this conundrum is to study gamma rays (high-energy photons) instead. They are associated with cosmic ray sites, but being neutral, it is possible to trace their path back to their origin. Gamma rays are produced at these sites via processes such as inverse Compton scattering and synchrotron radiation. ...

Characterising the VHE diffuse emission in the central 200 parsecs of our Galaxy with H.E.S.S - H.E.S.S. Collaboration

• arXiv.org > astro-ph > arXiv:1706.04535 > 14 Jun 2017

[bystander]

The grass might be redder on the other side
astrobites | 2017 Jun 23

The explosion in the number of discovered exoplanets, especially some interesting systems with terrestrial planets in the habitable zone, has attracted a lot of attention. We are moving one step closer to the ultimate question — are we alone? Today’s paper brought us to look at certain distinctive spectral features that could be caused by “extraterrestrial plants”, or even crazier: advanced civilizations. ...

Natural and Artificial Spectral Edges in Exoplanets - Manasvi Lingam, Abraham Loeb

• arXiv.org > astro-ph > arXiv:1702.05500 > 17 Feb 2017 (v1), 25 May 2017 (v2)

[bystander]

A New Glow in the Eye of Sauron
astrobites | 2017 Jun 26

The Fomalhaut debris disk is one of the most recognizable circumstellar disks—mainly because images of it bear an uncanny resemblance to a certain tyrant of Middle Earth. If you are completely unfamiliar with The Lord of the Rings, just click this link, straight from the Wikipedia page on the Eye of Sauron, and observe how oddly similar this movie screencap is to observations of a debris disk roughly 7.6 parsecs away. ...

In this paper, the authors use observations from the ALMA (Atacama Large Millimeter/submillimeter Array) observatory to reveal the structure of the Fomalhaut disk, and observe an unusual glow in the disk corresponding to the apocenter of the system. The apocenter of an eccentric orbit is the point where the orbiting object is furthest from the star; the pericenter is when the orbiting object is at its closest approach. ...

A Complete ALMA Map of the Fomalhaut Debris Disk - Meredith A. MacGregor et al

• Astrophysical Journal 842(1):8 (2017 Jun 10) DOI: 10.3847/1538-4357/aa71ae
  arXiv.org > astro-ph > arXiv:1705.05867 > 16 May 2017

http://asterisk.apod.com/viewtopic.php?t=37192