SAO: Science Updates 2019

Find out the latest thinking about our universe.
User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

The Low Density of Some Exoplanets is Confirmed

Post by bystander » Mon Jun 24, 2019 7:05 pm

The Low Density of Some Exoplanets is Confirmed
SAO Science Updates | 2019 Jun 21
The Kepler mission and its extension, called K2, discovered thousands of exoplanets. It detected them using the transit technique, measuring the dip in light intensity whenever an orbiting planet moved across the face of its host star as viewed from Earth. Transits can not only measure the orbital period, they often can determine the size of the exoplanet from the detailed depth and shape of its transit curve and the host star’s properties. The transit method, however, does not measure the mass of the planet. The radial velocity method, by contrast, which measures the wobble of a host star under the gravitational pull of an orbiting exoplanet, allows for the measurement of its mass. Knowing a planet's radius and mass allows for the determination of its average density, and hence clues to its composition.

About fifteen years ago, CfA astronomers and others realized that in planetary systems with multiple planets, the periodic gravitational tug of one planet on another will alter their orbital parameters. Although the transit method cannot directly measure exoplanet masses, it can detect these orbital variations and these can be modeled to infer masses. Kepler has identified hundreds of exoplanet systems with transit-timing variations, and dozens have been successfully modeled. Surprisingly, this procedure seemed to find a prevalence of exoplanets with very low densities. The Kepler-9 system, for example, appears to have two planets with densities respectively of 0.42 and 0.31 grams per cubic centimeter. (For comparison, the rocky Earth’s average density is 5.51 grams per cubic centimeter, water is, by definition, 1.0 grams per cubic centimeter, and the gas giant Saturn is 0.69 grams per cubic centimeter.) The striking results cast some doubt on one or more parts of the transit timing variation methodology and created a long-standing concern.

CfA astronomers David Charbonneau, David Latham, Mercedes Lopez-Morales, and David Phillips, and their colleagues tested the reliability of the method by measuring the densities of the Kepler-9 planets using the radial velocity method, its two Saturn-like planets being among a small group of exoplanets whose masses can be measured (if just barely) with either technique. They used the HARPS-N spectrometer on the Telescopio Nazionale Galileo in La Palma in sixteen observing epochs; HARPS-N can typically measure velocity variations with an error as tiny as about twenty miles an hour. Their results confirm the very low densities obtained by the transit-timing method, and verify the power of the transit-variation method.

HARPS-N Radial Velocities Confirm the Low Densities of the Kepler-9 Planets ~ L. Borsato et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

How Shiny are Near Earth Objects?

Post by bystander » Sun Jul 14, 2019 9:31 pm

How Shiny are Near Earth Objects?
SAO Science Updates | 2019 Jun 28
Near Earth Objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth. NEOs are consequently potential collision threats, but scientists are also interested in them because they offer keys to the composition, dynamics and environmental conditions of solar system and its evolution. Most meteorites for example, one of the key sources of knowledge about the early solar system, come from NEOs. The large majority NEOs were discovered in optical searches, and today the total number of known NEOs exceeds 20,000. The crucial NEO parameter of interest for most problems, including the possible dangers from an impact, is the size, but unfortunately optical detections usually cannot determine size. This is because an NEO's optical light is reflected sunlight, and the object could be bright either because it is large or because it has a high reflectivity (albedo).

CfA astronomers Joe Hora, Howard Smith, and Giovanni Fazio helped lead the team that was the first to undertake the systematic measurement of NEO sizes using their infrared brightnesses. An NEO's infrared signal is the result of its thermal emission, and that provides an independent measure of its size. The team used Spitzer IRAC infrared observations of NEOs together with optical data and their sophisticated thermal model to break the size/albedo degeneracy and determine the sizes of NEOs. (The NASA WISE mission and its NEOWISE team subsequently also undertook infrared size determinations.) So far, infrared measurements have been made on over 3000 NEOs, the vast majority of them using IRAC. The smallest NEO characterized this way, so far, is only about twelve meters in diameter (with about a 20% uncertainty). But strangely, the results also suggest an abundance of high-albedo objects, nearly eight times more than had been expected based on the current thinking about the population distribution.

The scientists had previously analyzed and published the variations of NEO brightness that resulted as their non-spherical bodies rotated in space (their light-curves). They wondered whether the large apparent excess of high-albedo objects was the result of an inadequate correction for light-curve variations. They performed a statistical analysis using Monte-Carlo simulations to estimate what might be expected of a population of rotating, non-spherical NEOs. They conclude that while light-curve variations could indeed be the cause of the large high-albedo excess, the excess is also consistent with a real – and still unexplained – overabundance of shiny objects. They also concluded that whatever the explanation, it is unlikely that NEOs have albedos exceeding 50%. Additional observations of full NEO light-curves are need to resolve the uncertainties.

Spitzer Albedos of Near-Earth Objects ~ Annika Gustafsson et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Powering the Extreme Jets of Active Galaxies

Post by bystander » Sun Jul 14, 2019 9:38 pm

Powering the Extreme Jets of Active Galaxies
SAO Science Updates | 2019 Jul 05
An active galaxy nucleus (AGN) contains a supermassive black hole that is vigorously accreting material. It typically ejects jets of particles that move at close to the speed of light, radiating across many wavelengths, in particular the X-ray, in processes are among the most energetic phenomena in the universe. The jets are often also highly collimated and extend far beyond their host galaxy, and if they happen to be pointed along our line of sight they are the most spectacular class of this phenomenon: blazars.

A few years ago astronomers noticed that some types of blazars have jet powers that appear to exceed the power provided by the accretion. Two ideas were put forward to explain the difference: the jets are also extracting power from the spin of the black hole or from the magnetic flux around the object. How either process happens – if indeed they do happen - is hotly debated, but one popular line of argument asserts that the processes are somehow related to the mass of the supermassive black hole, with the most massive cases (more than a hundred million solar-masses) being the most anomalous. Recently the Fermi Gamma-Ray Space Telescope detected gamma-rays (even more energetic photons than X-rays) coming from jets in a class of galaxies called Seyferts, spiral galaxies with relatively small supermassive black hole masses, typically about ten million solar-masses. Astronomers speculated that these relatively low-mass yet powerful emission engines might provide keys to sorting out the various sources of jet power.

CfA astronomer Mislav Balokovic and his colleagues completed a multi-wavelength study of the bright blazar-like Seyfert galaxy PKSJ1222+0413 and included data from the gamma-ray to the radio, both archival and new observations, including new results from the NuSTAR space observatory They then undertook a complete modeling of this source, the most distant one of its type known - its light has been traveling towards us for about eight billion years. They detected the pronounced signature of an accretion disk, and estimated the mass of the supermassive black hole from the widths and strengths of the emission lines to be about two hundred million solar-masses, about ten times higher than most other Seyferts of its type. The jet luminosity is only about half the accretion luminosity, unlike cases like galaxies whose jet power exceeds the accretion. But the object nonetheless clearly falls into a transition regime for jet strengths, enabling future studies to study in more detail the origins of jet power both Seyfert galaxies and in blazars.

The Relativistic Jet of the γ-ray Emitting Narrow-Line Seyfert 1 Galaxy PKS J1222+0413 ~ Daniel Kynoch et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

BDanielMayfield
Don't bring me down
Posts: 2069
Joined: Thu Aug 02, 2012 11:24 am
AKA: Bruce
Location: East Idaho

Re: The Low Density of Some Exoplanets is Confirmed

Post by BDanielMayfield » Mon Jul 15, 2019 6:40 pm

bystander wrote:
Mon Jun 24, 2019 7:05 pm
The Low Density of Some Exoplanets is Confirmed
SAO Science Updates | 2019 Jun 21
The Kepler mission and its extension, called K2, discovered thousands of exoplanets. It detected them using the transit technique, measuring the dip in light intensity whenever an orbiting planet moved across the face of its host star as viewed from Earth. Transits can not only measure the orbital period, they often can determine the size of the exoplanet from the detailed depth and shape of its transit curve and the host star’s properties. The transit method, however, does not measure the mass of the planet. The radial velocity method, by contrast, which measures the wobble of a host star under the gravitational pull of an orbiting exoplanet, allows for the measurement of its mass. Knowing a planet's radius and mass allows for the determination of its average density, and hence clues to its composition.

About fifteen years ago, CfA astronomers and others realized that in planetary systems with multiple planets, the periodic gravitational tug of one planet on another will alter their orbital parameters. Although the transit method cannot directly measure exoplanet masses, it can detect these orbital variations and these can be modeled to infer masses. Kepler has identified hundreds of exoplanet systems with transit-timing variations, and dozens have been successfully modeled. Surprisingly, this procedure seemed to find a prevalence of exoplanets with very low densities. The Kepler-9 system, for example, appears to have two planets with densities respectively of 0.42 and 0.31 grams per cubic centimeter. (For comparison, the rocky Earth’s average density is 5.51 grams per cubic centimeter, water is, by definition, 1.0 grams per cubic centimeter, and the gas giant Saturn is 0.69 grams per cubic centimeter.) The striking results cast some doubt on one or more parts of the transit timing variation methodology and created a long-standing concern.

CfA astronomers David Charbonneau, David Latham, Mercedes Lopez-Morales, and David Phillips, and their colleagues tested the reliability of the method by measuring the densities of the Kepler-9 planets using the radial velocity method, its two Saturn-like planets being among a small group of exoplanets whose masses can be measured (if just barely) with either technique. They used the HARPS-N spectrometer on the Telescopio Nazionale Galileo in La Palma in sixteen observing epochs; HARPS-N can typically measure velocity variations with an error as tiny as about twenty miles an hour. Their results confirm the very low densities obtained by the transit-timing method, and verify the power of the transit-variation method.

HARPS-N Radial Velocities Confirm the Low Densities of the Kepler-9 Planets ~ L. Borsato et al
Nice confirmation of transit timing variation as a planetary density estimation tool.

As so many of the Kepler planets are in star hugging orbits, it shouldn't be all that surprising that many of them have low density. They are inflated by heat from the star they orbit.

Bruce
"Happy are the peaceable ... "

BDanielMayfield
Don't bring me down
Posts: 2069
Joined: Thu Aug 02, 2012 11:24 am
AKA: Bruce
Location: East Idaho

Re: How Shiny are Near Earth Objects?

Post by BDanielMayfield » Mon Jul 15, 2019 6:59 pm

bystander wrote:
Sun Jul 14, 2019 9:31 pm
How Shiny are Near Earth Objects?
SAO Science Updates | 2019 Jun 28
Near Earth Objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth. NEOs are consequently potential collision threats, but scientists are also interested in them because they offer keys to the composition, dynamics and environmental conditions of solar system and its evolution. Most meteorites for example, one of the key sources of knowledge about the early solar system, come from NEOs. The large majority NEOs were discovered in optical searches, and today the total number of known NEOs exceeds 20,000. The crucial NEO parameter of interest for most problems, including the possible dangers from an impact, is the size, but unfortunately optical detections usually cannot determine size. This is because an NEO's optical light is reflected sunlight, and the object could be bright either because it is large or because it has a high reflectivity (albedo).

CfA astronomers Joe Hora, Howard Smith, and Giovanni Fazio helped lead the team that was the first to undertake the systematic measurement of NEO sizes using their infrared brightnesses. An NEO's infrared signal is the result of its thermal emission, and that provides an independent measure of its size. The team used Spitzer IRAC infrared observations of NEOs together with optical data and their sophisticated thermal model to break the size/albedo degeneracy and determine the sizes of NEOs. (The NASA WISE mission and its NEOWISE team subsequently also undertook infrared size determinations.) So far, infrared measurements have been made on over 3000 NEOs, the vast majority of them using IRAC. The smallest NEO characterized this way, so far, is only about twelve meters in diameter (with about a 20% uncertainty). But strangely, the results also suggest an abundance of high-albedo objects, nearly eight times more than had been expected based on the current thinking about the population distribution.

The scientists had previously analyzed and published the variations of NEO brightness that resulted as their non-spherical bodies rotated in space (their light-curves). They wondered whether the large apparent excess of high-albedo objects was the result of an inadequate correction for light-curve variations. They performed a statistical analysis using Monte-Carlo simulations to estimate what might be expected of a population of rotating, non-spherical NEOs. They conclude that while light-curve variations could indeed be the cause of the large high-albedo excess, the excess is also consistent with a real – and still unexplained – overabundance of shiny objects. They also concluded that whatever the explanation, it is unlikely that NEOs have albedos exceeding 50%. Additional observations of full NEO light-curves are need to resolve the uncertainties.

Spitzer Albedos of Near-Earth Objects ~ Annika Gustafsson et al
Don't worry, be happy. (Really, no sarcasm intended.) This study shows that the threat from a NEO asteroid is considerably less than what has been advertised. Since they are on average brighter than expected they would also be less massive than expected as well.

I still hope we develop impact warning and avoidance methods of course, but this isn't the dire threat it is played up to be in the popular press.

Bruce
"Happy are the peaceable ... "

User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Tracer Galaxies Probe the Cosmic Background

Post by bystander » Fri Aug 16, 2019 4:16 pm

Tracer Galaxies Probe the Cosmic Background
SAO Science Updates | 2019 Jul 12
The universe, perhaps surprisingly, is not comprised of galaxies randomly distributed throughout space; that is, it is not very homogeneous. Instead, its galaxies are clustered into distinct structures, typically gigantic filaments separated by vast voids -- the "large-scale structure (LSS)," an architecture whose discovery and mappings were pioneered by CfA astronomers about thirty years ago. Astronomers since have combined LSS maps with results from the cosmic microwave background radiation (CMBR) and ideas about the inflationary big bang to assemble a remarkably consistent picture of the universe, its origins and its evolution.

Mysteries remain, for example dark matter, which is also expected to gather in large-scale structures. CfA astronomers David James and Tony Stark were members of a large international team that used photons from galaxies in the early universe ("tracer galaxies") to probe the LSS in more detail. As these photons traverse the universe on their way to us, their paths are perturbed by the gravitational influences of the LSS, including in particular the effects of gravitational lensing. The apparent placements of young galaxies as projected on the sky and their statistical distributions are sensitive both to the current and the evolving geometry and structure of matter in the universe.

The astronomers recognized that although the details of the projected mass distribution are extremely complex, using the ratios of some parameters could obviate some uncertainties, enabling them to obtain important constraints on the current models of cosmic evolution. The team combined observations from the Dark Energy Survey (an optical survey that has mapped millions of galaxies), the South Pole Telescope (a submillimeter-wave facility studying the CMBR and early galaxies), and the Planck mission (a far infrared and millimeter survey spacecraft). One particularly valuable advantage of this approach is that it does not require knowing the distances to the tracer galaxies (distances would require their being able to measure the faint spectroscopic redshifts). The scientists were able to obtain constraints with a precision of about ten percent on some of the detailed parameters of current cosmological models, and they forecast that with further research these techniques will even enable them to constrain some of the essential features of dark matter, like its equatrion of state, and properties that have so-far remained elusive.

Cosmological Lensing Ratios with DES Y1, SPT, and Planck ~ DES Collaboration, SPT Collaboration, J. Prat et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Modeling Exoplanet Atmospheres

Post by bystander » Fri Aug 16, 2019 4:22 pm

Modeling Exoplanet Atmospheres
SAO Science Updates | 2019 Jul 19
All atoms and molecules emit distinctive spectral lines across the spectrum, the details of which depend on the internal structures of the species (for example, the vibration and rotation properties of molecules) and how they are excited by their environments. Measurements of the features' brightnesses, relative intensities, and shapes enable astronomers, at least in principle, to reconstruct most of the essential properties of these environments, including species abundances, temperatures, densities, and motions. But in order to be successful, scientists need to know quantitatively exactly how the temperature, density, and so forth, affect the excitation of each atom or molecule, and then how each species emits light in response. A collision between oxygen and nitrogen molecules, for example, will affect an oxygen molecule differently than its collision with hydrogen.

CfA astronomers develop and maintain the HITRAN (High Resolution Transmission) database, a compilation of diagnostic spectroscopic parameters that is the worldwide standard for calculating atmospheric molecular radiation from the microwave through the ultraviolet region of the spectrum. HITRAN has acquired particular new importance in recent years with the discovery of thousands of exoplanets and the steadily improving technology to detect their atmospheres and measure their compositions. HITRAN is commonly used to model these exotic atmospheres. Molecular oxygen absorption stimulated by collisions between oxygen molecules is thought, for example, to be an important biomarker on potentially habitable exoplanets, but the detection of this absorption feature is not enough: it needs an interpretation.

CfA astrophysicists Tijs Karman, Iouli Gordon, Bob Kurucz, Larry Rothman, and Kang Sun led a team of colleagues in updating HITRAN with many of the essential collision-induced absorption properties of the molecules needed for modeling exoplanet atmospheres. Key molecular species include nitrogen, oxygen, methane, carbon dioxide, and hydrogen. The numerical parameters were gleaned from a wide collection of recent laboratory and theoretical papers and incorporated into the HITRAN database after being validated. The updated compilation goes a long way towards addressing the current needs, but the authors note that additional laboratory and theoretical work is needed to include other effects, water for example, as well as the isotopic variations of the currently included species.

Update of the HITRAN Collision-Induced Absorption Section ~ Tijs Karman et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Imaging a Young, Planet-Forming Disk

Post by bystander » Fri Aug 16, 2019 4:43 pm

Imaging a Young, Planet-Forming Disk
SAO Science Updates | 2019 Jul 26
Protoplanetary disks are a natural product of the star formation process. As material in a pre-stellar condensation collapses to form the central star, angular momentum conservation prompts it to develop a circumstellar disk. Planets form out of the material in these disks, whose structure and evolution are therefore keys to understanding the planet building process. Two main scenarios dominate in current theories. In the core accretion model, planets assemble through the aggregation of the dust grains, eventually forming planetesimals followed by a balance between their growth and fragmentation as they collide. In the alternative scenario, gravitational instabilities develop during the initial stages of the disk evolution and the associated density perturbations grow until they form into planetesimals.

In either of these two scenarios, young massive planets are expected to imprint their signatures on the structure of their parent disks, carving out cavities, gaps or other asymmetries which should be detectable. Infrared observations of young stars have uncovered a set whose emission seems to lack a contribution from warm dust. The implication is that the hottest dust closest to the star is absent, perhaps because a cavity has been carved out by an unseen, orbiting planet. Checking these ideas, and identifying examples of young stars in the midst of their planet birthing process, are key goals of modern exoplanetary research.

CfA astronomers Sean Andrews and David Wilner were members of a team that used the ALMA millimeter array to image the disk around CQ Tau, a young star about 530 light-years away that is known from infrared observations to have a cavity in its massive circumstellar disk, estimated to contain about .03 solar-masses of material. The dramatic new images have a spatial resolution of about twenty-four astronomical units (AU), more than enough to resolve the roughly face-on disk which is over 175 AU across (in our solar system, Pluto’s farthest passage is about fifty AU from the Sun). The ALMA images clearly reveal some details of a cavity in both the gas and dust components which the astronomers find to be between about 25 and 40 AU in radius. The team concludes that a massive planet roughly eight Jupiter-masses in size located at twenty AU can produce some (though not all) of the cavity's dimensions, and additional observations and modeling are needed to refine the picture. The new results, coupled with other ALMA observations by the team, offer ground-breaking details about the early stages of planet formation.

A Dust and Gas Cavity in the Disc around CQ Tau Revealed by ALMA ~ M. Giulia Ubeira Gabellini et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Looking for Warm Dark Matter

Post by bystander » Fri Aug 16, 2019 5:50 pm

Looking for Warm Dark Matter
SAO Science Updates | 2019 Aug 02
su201930-ls.jpg
Two simulations of galaxy formation at the epoch when the universe was only about
one billion years old. The left (CDM) shows clumps and filaments of young galaxies
using a conventional treatment of non-interacting dark matter, while the right (sDAO)
shows the slightly different - but measurable - differences that occur if dark matter
instead could interact with some particles. Astronomers show that future precise
measurements of large-scale galaxy structures could help constrain the nature of
the mysterious dark matter in the universe. Credit: Bose et al. 2019 MNRAS

In the last century, astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize that most of the matter in the universe was not visible. About 84% of the matter in the cosmos is dark, emitting neither light nor any other known kind of radiation. Hence it is called dark matter. One of its other primary qualities is that it only interacts with other matter via gravity: it carries no electromagnetic charge, for example. Dark matter is also "dark" because it is mysterious: it is not composed of atoms or their usual constituents like electrons and protons. Particle physicists have imagined new kinds of matter, consistent with the known laws of the universe, but so far none has been detected or its existence confirmed. The Large Hadron Collider's discovery of the Higgs boson in 2012 prompted a burst of optimism that dark matter particles would soon be discovered, but so far none has been seen and previously promising classes of particles now seem to be long-shots.

Astronomers realize that dark matter is the dominant component of matter in the universe. Whatever its nature, it profoundly influenced the evolution of galactic structures and the distribution of the cosmic microwave background radiation (CMBR). The remarkable agreement between the values of key cosmic parameters (like the rate of expansion) derived from observations of two completely different kinds of large-scale cosmic structures, galaxies and the CMBR. lend credence to inflationary big bang models that include the role dark matter.

Current models of dark matter presume it is "cold," that is, that it does not interact with any other kinds of matter or radiation - or even with itself – beyond the influences of gravity. This version of cosmology is therefore called the cold dark matter scenario. But cosmologists wonder whether more precise observations might be able to exclude even small levels of interactions. CfA astronomer Sownak Bose led a team of colleagues in a study of one very popular (if speculative) "dark matter" particle, one that has some ability to interact with very light particles that move close to the speed of light. This version forms one of several possible warm dark matter (perhaps more accurately called interacting dark matter) scenarios. In particular, the hypothetical particles are allowed to interact with neutrinos (neutrinos are expected to be extremely abundant in the hot early universe).

The scientists used state-of-the-art cosmological simulations of galaxy formation to a model universe with this kind of warm dark matter. They find that for many observations the effects are too small to be noticeable. However, the signature of this warm dark matter is present in some distinct ways, and in particular in the way distant galaxies are distributed in space, something that can be tested by mapping galaxies by looking at their hydrogen gas. The authors conclude that future, highly sensitive observations should be able to make these tests. Detailed new maps of the distribution of hydrogen gas absorption could be used to support -- or exclude -- this warm dark matter possibility (see the figure), and shed light on this mysterious cosmic component.

ETHOS – an Effective Theory of Structure Formation: Detecting
Dark Matter Interactions through the Lyman-α Forest
~ Sownak Bose et al
  • Monthly Notices of the RAS 487(1):522 (July 2019) DOI: 10.1093/mnras/stz1276
  • arXiv.org > astro-ph > arXiv:1811.10630 > 26 Nov 2018 (v1), 30 May 2019 (v2)
You do not have the required permissions to view the files attached to this post.
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Modeling a Core Collapse Supernova

Post by bystander » Fri Aug 16, 2019 6:16 pm

Modeling a Core Collapse Supernova
SAO Science Updates | 2019 Aug 09
su201931ls.jpg
Before and after – optical images of the field of stars around the core collapse
supernova ASASSN-15oz in the relatively nearby galaxy HIPASSJ1919-33.
Astronomers studying the photometric and spectral lightcurves have concluded
that the progenitor star was a red supergiant star with a substantial shell of
previously ejected material. Credit: Bostroem et al. 2019 MNRAS

Stars greater than eight solar-masses end their lives spectacularly -- as supernovae. These single-star supernovae are called core collapse supernovae because when their dense cores (at this stage composed primarily of iron) are no longer able to withstand the inward pressure of gravity they collapse inward before exploding. Core collapse supernovae with strong hydrogen emission lines are thought to result from the explosions of red supergiant stars, massive stars that have evolved beyond their principle hydrogen burning stage and grown in radius. Until recently, astronomers thought these stars were relatively quiescent until their final demise, but evidence has been accumulating that they actually experience strong mass loss before exploding. In some models, emission resulting when ejecta from the supernovae encounter these envelopes produces the observed variations in core collapse supernova.

CfA astronomer Griffin Hosseinzadeh was a member of a team of astronomers testing these ideas by studying the core collapse supernova ASASSN-15oz. He assisted in the multiband observations, which included X-ray, UV, optical, infrared, and radio measurements. ASASSN-15oz exploded almost exactly four years ago, around 31 August 2015, and is located in the relatively nearby galaxy HIPASSJ1919-33, about one hundred million light-years away. The astronomers were able to obtain spectra and photometric lightcurves of the object over a period of about 750 days. They successfully modeled the event as the explosion of a red supergiant star that had ejected material in a wind for most of its later evolution and underwent an extreme eruption just prior to its demise. They estimate that about 1.5 solar-masses of material was ejected in total. The new analysis is consistent with the idea that this class of core collapse supernova is indeed surrounded by a substantial circumstellar shell that was the result of eruptive mass loss from the red supergiant progenitor.

Signatures of Circumstellar Interaction in the Type IIL Supernova ASASSN-15oz ~ K. Azalee Bostroem et al
You do not have the required permissions to view the files attached to this post.
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

User avatar
bystander
Apathetic Retiree
Posts: 18664
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Spotting Merging Galaxies

Post by bystander » Fri Aug 16, 2019 6:35 pm

Spotting Merging Galaxies
SAO Science Updates | 2019 Aug 16
su201932.jpg
A Hubble image of a suspected galaxy merger about seven hundred million
light-years away. Might it actually be a single spiral galaxy? A new paper
proposes an algorithm to decide. The method was developed with computer
training techniques applied to a million simulated merging galaxy images.
Credit: NASA/Hubble; Kim et al. 2013

Over thirty years ago, the Infrared Astronomy Satellite discovered that the universe contained many extremely luminous galaxies, some more than a thousand times brighter than our own Milky Way, but which are practically invisible at optical wavelengths. These galaxies are powered by bursts of star formation buried deep within clouds of dust and gas. The dust absorbs the ultraviolet light while radiating at infrared wavelengths. In many cases the hyperactivity was triggered by a collisional encounter between galaxies that facilitated the collapse of interstellar gas into new stars.

Collisions between galaxies are common. Indeed, most galaxies have probably been involved in one or more encounters during their lifetimes, making these interactions an important phase in galaxy evolution and the formation of stars in the universe. The Milky Way, for example, is bound by gravity to the Andromeda galaxy and is approaching it at a speed of about 50 kilometers per second; we are expected to meet in another billion years or so. In the local universe about five percent of galaxies are currently in a merger, and mergers usually can be easily identified by the visible morphological distortions they produce such as tidal tails sweeping out from the galactic discs.

Not all infrared luminous galaxies show such distortions, however, and the issue of identifying (and classifying) mergers becomes especially problematic for studies of earlier cosmic epochs when the star formation rates were much higher than today, and when the merger rate of galaxies was also higher. (Moreover, such systems are preferentially discovered in deep galaxy surveys precisely because they are so luminous.) But galaxies in the distant cosmos are too remote to detect spatial signatures like tidal arms (at least with current telescopes). It is possible that other processes besides merger-induced star formation are lighting up some of these bright galaxies, for example accreting supermassive black holes can emit copious amounts of ultraviolet radiation. Because of such cases, estimates of star formation in the early universe based on luminosity measurements alone could be incorrect.

CfA astronomer Lars Hernquist is a pioneer in the development of computer simulations of merging galaxies. Several years ago he and a team of colleagues produced a massive new simulation of the formation and evolution of galaxies in the universe, called Illustris. In a new paper based on Ilustris simulated images of merger galaxies, the astronomers present a way to help identify when imaged systems are mergers. They created about one million synthetic Hubble and James Webb Space Telescope images from their simulated mergers, and then looked for common morphological indicators of merging. They developed an algorithm that successfully identified mergers at roughly a seventy percent level of completeness out to distances of as much as eighty-five billion light-years (the current distance value), corresponding to light dating from the epoch about 2 billion years after the big bang. Results from the algorithm indicated that spatial features associated with strong central concentrations (or bulges) were most important for selecting past mergers, while double nuclei and asymmetries were most important for selecting future mergers (that is, sometime in the next 250 million years). The new algorithm will be particularly valuable when applied to future Webb images of very distant mergers.

Automated Distant Galaxy Merger Classifications from
Space Telescope Images using the Illustris Simulation
~ Gregory F. Snyder et al
You do not have the required permissions to view the files attached to this post.
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor