Find out the latest thinking about our universe.
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bystander
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by bystander » Mon Sep 08, 2014 4:11 am
What Makes Blazars Vary?
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Sep 05
A blazar is a galaxy whose central, supermassive black hole shines intensely as it accretes material from the surrounding region. Although black hole accretion happens in many galaxies and situations, in blazars the infalling material erupts into a powerful, narrow beam of high velocity charged particles that are fortuitously pointed in our direction. These particles produce gamma rays, each photon over a hundred million times more energetic than the highest energy X-ray photons seen by the Chandra X-ray Observatory. Blazars are also generally characterized by having rapid, strong, and incessant variability, among a host of effects resulting from its beam of rapidly moving electrons.
Astronomers suspect that clues to the inner workings of black holes and accretion disks can be discerned from modeling the details of the variability, but this has been a difficult task. The complexity of the variability indicates that the emitting structures are also complex, and constraining the locations and sizes of the emitting sites has been hampered by a lack of long-term, sensitive observations capable of steady monitoring of the changing activity. ...
Stochastic Modeling of the Fermi/LAT γ-ray Blazar Variability - Malgorzata A. Sobolewska
et al
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Attachments
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- An artist's conception of the Fermi spacecraft in orbit.
Astronomers have used Fermi to monitor the gamma-ray
variability of thirteen blazars, and find evidence that the
emission arises in several different zones and/or from
several mechanisms.
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bystander
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by bystander » Sat Sep 13, 2014 9:56 pm
Cold Gas in the Pipe Nebula
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Sep 12
The Pipe Nebula is a prominent dark molecular cloud located about 430 light-years from us. It contains about ten thousand solar-masses of material, making it one of the closest giant molecular complexes to us, and with dimensions of about 10 by 46 light-years, one of the largest. These properties should make it an excellent place to study star formation up close, except for one problem: there is very little star formation underway there. Instead, the Pipe Nebula has become one of the prime cases for testing ideas about star formation, since apparently an abundance of material is not enough by itself to produce new stars.
The Pipe Nebula is not uniform in density; it contains a large population of dense, low-mass cores, about 134 distinct objects. In other molecular clouds these cores evolve into young stars, but in the Pipe they remain quiescent. Astronomers have been using radio astronomy techniques to study the density and temperatures of particular molecular species in these cold cores, for example species of carbon monoxide and ammonia, whose relative line strengths can quantify these parameters.
CfA astronomers Jan Frobrich, Karin Oberg, and Charlie Lada, together with four colleagues, have now completed a study of fifty-two of the Pipe’s cores in the light from six additional molecules that are particularly useful in characterizing star formation activity, and also complemented that data with infrared images from Herschel that show the cold dust distribution. They report that the dense cores in the Pipe are very cold indeed, between 13 and 19 kelvin,with the coldest ones containing the most obscuring dust. The astronomers find that one molecule in particular, N2H+, is the only species to exclusively trace the very densest and coldest cores, making this molecule a key diagnostic for future studies to figure out why these dense cores do not form stars.
Some like it cold: molecular emission and effective dust temperatures of dense cores in the Pipe Nebula - Jan Forbrich
et al
Know the quiet place within your heart and touch the rainbow of possibility; be
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bystander
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by bystander » Tue Sep 23, 2014 3:08 am
How Far are the Pleiades, Really?
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Sep 19
The Pleiades star cluster, a brilliant collection of several hundred stars visible in the winter sky near the constellation of Orion, has been admired by people for thousands of years; it is cited in the Bible and the works of Greek authors. The Pleiades is a relatively young cluster of stars: its estimated age, only around 100 million years old, means it was born long after the Jurassic dinosaurs passed into history.
The Pleiades cluster is also relatively close by, about 439 light-years according to most measures, with an uncertainty of less than about 1%. The distance is a crucial factor (as it is in many astronomical studies) because the intrinsic luminosity of a star is determined by measuring its observed flux and then correcting for its distance. The intrinsic luminosity in turn is used to determine the star’s age and general properties. Astronomers think they understand the inner workings of stars like those in the Pleiades well enough to account precisely for the flux observed, so well in fact that even a small change in the distance to the Pleiades, at the 10% level, would modify the intrinsic values enough to throw a wrench into the stellar models. Unfortunately, in 2009, a re-analyses of data from the European Hipparcos mission, a satellite designed to measure stellar distances, announced that the true distance to the Pleiades was only 395 light-years with a 1% uncertainty. This change was enough to disrupt the current thinking not only about the stars in the Pleiades, but more generally to call into question some details about how stars work. ...
A VLBI Resolution of the Pleiades Distance Controversy - Carl Melis
et al
Toward a VLBI resolution of the Pleiades distance controversy - Carl Melis
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
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geckzilla
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by geckzilla » Tue Sep 23, 2014 3:27 am
I think I've finally decided that when astronomers use the word "flux" they mean how many times a photon of a certain wavelength lands on a detector over a certain time period from a given source.
Just call me "geck" because "zilla" is like a last name.
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MargaritaMc
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by MargaritaMc » Tue Sep 23, 2014 11:27 am
geckzilla wrote:I think I've finally decided that when astronomers use the word "flux" they mean how many times a photon of a certain wavelength lands on a detector over a certain time period from a given source.
Here you are! An excerpt from the definition of FLUX from Swinburne University of Technology's astronomy encyclopaedia:
http://astronomy.swin.edu.au/cosmos/F/Flux
Flux (or radiant flux), F, is the total amount of energy that crosses a unit area per unit time. Flux is measured in joules per square metre per second (joules/m2/s), or watts per square metre (watts/m2).
M
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
— Dr Debra M. Elmegreen, Fellow of the AAAS
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by bystander » Sun Sep 28, 2014 1:55 am
The Architecture of Planetary Systems
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Sep 26
There are 1822 confirmed exoplanets reported so far, and NASA's Kepler satellite has found evidence for more than two thousand others. Many exoplanets are expected to be in systems with multiple planets; indeed, one Kepler system is thought to contain seven or perhaps even more planets. As astronomers amass data on the characteristics of planets of all kinds, the large number of expected planetary systems allows them to study as well the nature of these systems and the stability of the orbits over time.
CfA astronomers Darin Ragozzine, John Geary, and Matt Holman, together with their colleagues, have analyzed 899 transiting planet candidates in 365 systems in an effort to understand the statistical properties of planetary systems and the extent to which our solar system might be unusual. The most complex system in their set has six planets. The sample is dominated by planets between about one and four Earth-radii in size and which orbit their stars in about ten days, making the planets hot and not Earth-like.
The astronomers found one striking feature of these exoplanetary systems: the planets seemed to lie in the same plane, to within an estimated 2.5 degrees (the team also estimates how this value might vary in time). For comparison, the solar system's planets are coplanar to about 3 degrees, with Mercury being an outlier with its angle of seven degrees; Pluto (not a planet) has an orbital angle of seventeen degrees. The team argues that this exoplanet result suggests that the individual planetary orbits are each nearly circular, a significant conclusion because it means the orbits are not likely to overlap, and hence implies (at least for systems of close-in planets) that they have long-term stability. The new paper marks continuing significant progress in unraveling the picture of planets and planetary systems in the universe.
Architecture of Kepler's Multi-transiting Systems.
II. New Investigations with Twice as Many Candidates - Daniel C. Fabrycky
et al
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by bystander » Fri Oct 03, 2014 8:56 pm
Stars: The First Three Billion Years
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Oct 03
In the last decade, the unprecedented sensitivity of astronomical observations has powered a revolution in our understanding of galaxies in the young universe. It is now possible to study directly the mechanisms and processes which formed the diverse array of galaxies we find in the local universe today. The results not only provide insight into the processes of galaxy formation, they also help us understand the role those first galaxies played in the cosmos: in particular, the ultraviolet light from hot, young stars ionized the intergalactic medium, which had contained neutral atoms ever since they formed during an earlier epoch that also produced the cosmic microwave background radiation.
One of the key questions in cosmology is the stellar composition of young galaxies that formed about two billion years after the big bang. Surveys find that a significant contribution to the reionization must have come not from sources that are seen but from faint galaxies below the current detection limits. Although faint, these galaxies and their stars also contribute to the cumulative growth of the stellar population in the cosmos. Successful models of galaxy evolution need to include all sources of star formation. ...
The mass evolution of the first galaxies: stellar mass functions and star formation
- rates at 4 < z < 7 in the CANDELS GOODS-South field - Kenneth Duncan et al
Monthly Notices of the RAS 444(3) 2960 (01 Nov 2014) DOI: 10.1093/mnras/stu1622
arXiv.org > astro-ph > arXiv:1408.2527 > 11 Aug 2014
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by bystander » Fri Oct 17, 2014 2:50 pm
The Debris Disk of a Solar-Type Star
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Oct 10
Although thousands of exoplanets and hundreds of planetary systems (stars with multiple exoplanets) are now known, astronomers still don’t know whether our solar system is typical. The distributions of known planetary system parameters are strongly affected by observational biases that are not easy to disentangle from the true distributions. Moreover, our Solar system’s architecture (small rocky inner planets, large gaseous outer planets, and an outer debris disc made of many small objects) has not yet been seen in other systems, most likely due to these same biases. Long time baselines, for example, are required to discover planets at greater than a few astronomical units (one AU is the average distance of the Earth from the Sun) with all techniques except direct imaging, but direct imaging of planets around mature stars is difficult due to the low light from planets compared to their host stars.
Debris disks, because they are spread out over large areas, are easier to see, and structures in debris discs like rings or gaps can indicate the presence of additional planets. CfA astronomer David Wilner joined with his colleagues to search for clues of planets in the debris disc around τ Ceti, a nearby solar-type star located only ten light-years from the Sun. The infrared excess towards τ Ceti has been known for nearly three decades and has been attributed to warm dust particles in a debris disk. ...
The Debris Disk of Solar Analogue τ Ceti: Herschel Observations
and Dynamical Simulations of the Proposed Multiplanet System - S. M. Lawler
et al
http://asterisk.apod.com/viewtopic.php?t=30395
Know the quiet place within your heart and touch the rainbow of possibility; be
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by bystander » Sat Oct 25, 2014 4:50 pm
Sources of the Solar Wind
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Oct 17
The solar chromosphere is the region between the Sun’s surface and its hot, million-degree corona. Within this complex interface zone, only a few thousand kilometers deep, the density of the gas drops by a factor of about one million and the temperature increases from about five thousand to one million kelvin. Almost all of the mechanical energy that drives solar activity is converted into heat and radiation within this interface. Many physical processes shape the intricate system of solar magnetic fields, energetic particles, and radiation that power the corona. The solar wind itself has long been suspected of originating in this region, perhaps in the cooler “coronal holes,” but identifying the precise origin sites required high spatial resolution observations.
In the summer of 2013, NASA launched the Interface Region Imaging Spectrograph (IRIS) mission to study this interface region in ultraviolet light; the CfA and its staff contributed its twenty centimeter telescope as well as to its science team. The latest issue of Science features IRIS results, and in it CfA astronomers Hui Tian, Ed DeLuca, Steve Cranmer, Leon Golub, Sean McKillop, Katherine Reeves, Patrick McCauley, PaolaTesta, Mark Weber, and Nicholas Murphy join with their colleagues in announcing the detection of intermittent, small-scale jets from the region and their possible contribution to the stellar wind. The paper is one of five in this special issue of
Science which they led or co-authored.
The astronomers found unambiguous evidence for these jets that are roughly about 8,000 thousand kilometers long and 300 kilometers wide, moving between about 80 to 250 kilometers per second, with gas at temperatures of at least one hundred thousand kelvin, and coming in bursts lasting between about 20 and 80 seconds. The upward flowing hot gas may provide the heated mass to the solar wind. The scientists calculate that although intermittent, the ongoing presence of these jets could provide in principle more than enough material to supply the wind. They caution, however, that much work remains to confirm the conclusions and refine the analyses and models – but in any case the newly discover jet phenomenon will need to be incorporated in any future, detailed wind models.
Prevalence of Small-scale Jets from the Networks of the Solar Transition Region and Chromosphere - H. Tian
et al
http://asterisk.apod.com/viewtopic.php?t=33979
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by bystander » Sat Oct 25, 2014 5:18 pm
Accreting Supermassive Black Holes in the Early Universe
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Oct 24
Supermassive black holes containing millions or even billions of solar-masses of material are found at the nuclei of galaxies. Our Milky Way, for example, has a nucleus with a black hole with about four million solar masses of material. Around the black hole, according to theories, is a torus of dust and gas, and when material falls toward the black hole (a process called accretion) the inner edge of the disk can be heated to millions of degrees. Such accretion heating can power dramatic phenomena like bipolar jets of rapidly moving charged particles. Such actively accreting supermassive black holes in galaxies are called active galactic nuclei (AGN).
The evolution of AGN in cosmic time provides a picture of their role in the formation and co-evolution of galaxies. Recently, for example, there has been some evidence that AGN with more modest luminosities and accretion rates (compared to the most dramatic cases) developed later in cosmic history (dubbed "downsizing"), although the reasons for and implications of this effect are debated. CfA astronomers Eleni Kalfontzou, Francesca Civano, Martin Elvis and Paul Green and a colleague have just published the largest study of X-ray selected AGN in the universe from the time when it was only 2.5 billion years old, with the most distant AGN in their sample dating from when the universe was about 1.2 billion years old.
The astronomers studied 209 AGN detected with the Chandra X-ray Observatory. They note that the X-ray observations are less contaminated by host galaxy emission than optical surveys, and consequently that they span a wider, more representative range of physical conditions. The team's analysis confirms the proposed trend towards downsizing, while it also can effectively rule out some alternative proposals. The scientists also find, among other things, that this sample of AGN represents nuclei with a wide range of molecular gas and dust extinction. Combined with the range of AGN dates, this result enables them to conclude that obscured and unobscured phases of AGN evolve in similar ways.
The Largest X-ray Selected Sample of z>3 AGNs: C-COSMOS & ChaMP - E. Kalfountzou
et al
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by bystander » Fri Oct 31, 2014 9:25 pm
Water on Earth
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Oct 31
Water, the key ingredient for life, is not only abundant on Earth, it is also ubiquitous across the solar system. Either as ice or sometimes as liquid, water has been spotted in comets, the icy moons of the giant planets, and even in the shadowed basins of Mercury. Water has left its mark in hydrated minerals in meteorites that penetrated our atmosphere, in lunar basalts retrieved by the astronauts, and in Martian melt inclusions recovered from rock samples ejected from Mars that found their way to Earth. Comets and asteroids (as traced by meteorites) remain the oldest, most primitive objects with water. They provide a natural time capsule of the conditions present during the Sun’s epoch of planet formation.
No one knows for sure when and where these ices formed. Water might have been present in the dense interstellar medium from which Sun formed or it might have been made somehow within the solar nebula after it developed. Astronomers are trying to determine which applies because the former suggests that all planet-forming systems will have abundant water ices, whereas the latter presumably means that the abundance of water can vary dramatically from stellar system to system.
Water is usually made with two atoms of hydrogen and one of oxygen, as H2O, but it can also come in deuterated form in which a deuterium atom replaces one hydrogen atom. The fraction of deuterated water in a sample is a powerful measure of the age and origin of the sample: Interstellar ices are highly enriched in the deuterated species because the chemistry of interstellar space – ionizing radiation in particular - preferentially destroys normal H2O water. Ice in interstellar space can have a two to thirty times higher fraction of deuterated water than is found on Earth. ...
The Ancient Heritage of Water Ice in the Solar System - L. Ilsedore Cleeves
et al
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MargaritaMc
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by MargaritaMc » Sat Nov 01, 2014 11:29 am
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
— Dr Debra M. Elmegreen, Fellow of the AAAS
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by bystander » Sun Nov 09, 2014 9:55 pm
The Role of Magnetic Fields in Star Formation
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Nov 07
[img3="
A schematic illustration of the magnetic field and motions found in a massive star forming cluster. The core (gray-filled ellipse) is a flattened, rotating cloud of gas and dust (blue and red arrows indicate the sense of the rotation). It is fragmenting into new stars (evidenced by the three condensations/gray dots), and is threaded by an hourglass magnetic field ("B field" green lines) largely aligned with a bipolar outflow indicated with the arrows. (Credit: Qiu et al.)"]
http://www.cfa.harvard.edu/sites/www.cf ... 201445.jpg[/img3]
Massive stars tend to form in clusters as the gas and dust in molecular clouds collapses and fragments under the influence of gravity. In the classic picture of star formation, gravity must eventually compete against the thermal pressure that develops in the collapsing core as the material heats up. Astronomers think they understand those steps, but there is a debate about the possible role of two other physical processes: turbulent motions and magnetic fields. One school argues that the turbulence that develops as the cloud shrinks leads to fragmentation and the formation of multiple young stars in a young cluster. The other camp argues that there are magnetic fields present in the original clouds, and calculates that as the cloud shrinks the fields become stronger, take on an hour-glass-shape, and produce a flattened cloud and stars with bipolar flows ejected along the direction of the field.
CfA astronomer Qizhou Zhang and five colleagues used the Submillimeter Array (SMA) to study the magnetic field in one massive young cluster known to have a flattened shape and a bipolar outflow. The region has a luminosity of about thirty thousand Suns, and is located about seventeen thousand light-years away. The team determined the properties of the magnetic field by using the SMA's ability to measure the polarization of the millimeter light from the region: Magnetic fields cause elongated dust grains in the cloud to line themselves up along the field, and arranged in this coherent pattern they scatter light preferentially in one polarization.
The scientists report detecting the clear signature of an hourglass-shaped magnetic field that is remarkably consistent with theoretical predictions of the classic paradigm. This is the first time that such an hourglass field, aligned with a well-defined outflow system, has been seen in a high-mass region. The new observations provide strong evidence that massive star and cluster formation proceeds in a way that astronomers think resembles the processes in the formation of Sun-like stars. Not least, the team notes that the magnetic field dominates over the turbulence.
Submillimeter Array Observations of Magnetic Fields in G240.31+0.07: an Hourglass in a Massive Cluster-forming Core - Keping Qiu
et al
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by bystander » Sun Nov 16, 2014 2:19 am
The Future of Neutrino Cosmology
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Nov 14
One of the most remarkable aspects of modern cosmology is that the properties of the largest physical structures in the universe reveal the properties of the smallest. The largest structures include the grand patterns seen in the cosmic microwave background radiation (CMBR) and the filamentary collections and nodes of clusters of galaxies in the early universe; the smallest include the elusive neutrinos. These hard-to-detect particles were for many decades thought to have no mass and to travel at the speed of light, like photons (quanta /particles of light). They are so hard to detect because they interact only very weakly with other kinds of matter, mostly passing through the matter unaffected.
During the past decades, scientists have discovered that there are three kinds of neutrinos and that in fact they all have some mass. The lightest kind is about a million times less massive than an electron. The reason light-weight neutrinos have such cosmological importance is that there are so many of them: The largest number of particles in the cosmos are CMBR photons, created in the later stages of the big bang, but the second largest number are neutrinos. Since the neutrinos have some slight mass while photons have none, their large number leads to a mass density (or equivalently, an energy density) today at least twenty-five times larger than that of the CMBR photons. The high density of these particles streaming past other cosmic matter without interacting with it tends to inhibit the growth of galactic structures (like those filamentary collections of galaxies). That tendency can be measured and quantified. ...
Neutrino Physics from the Cosmic Microwave Background and Large Scale Structure - K.N. Abazajian
et al
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by bystander » Mon Dec 01, 2014 9:57 pm
Measuring the Ancient Solar Nebula's Magnetic Field
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Nov 21
Astronomical observations of young protostars indicate that early planetary systems evolved from the dust in a protoplanetary disk very quickly - in under five million years. Such short timescales require very efficient mechanism(s) to transport material inward towards the central star, but the mechanism(s) that do this are uncertain. Several have been invoked, however, in which magnetic fields play a key role, either in the stellar wind or in the disk itself.
Astronomers cannot currently directly measure magnetic field strengths in planet-forming regions, but experiments on meteoritic materials in our own Solar system can potentially constrain the strength of early Solar nebular magnetic fields. Chondrules are millimeter-sized constituents of primitive meteorites that formed in brief heating events in the young solar nebula. They probably constitute a significant fraction of the mass of asteroids and even of terrestrial planet precursors. The formation of chondrules, therefore, very likely occurred during a key stage in the evolution of the early solar system. If a stable field was present during their cooling off phase, they should themselves have become slightly magnetized. Determining their magnetic fields should therefore not only constrain models of their formation, but of the disk's evolution as well.
Among the most pristine known meteorites is one called Semarkona. It contains chondrules of crystalline olivine which, due to their unique compositional and magnetic properties, can retain their primitive magnetization even over the eons since they formed and despite their subsequent histories in the solar system. ...
Solar nebula magnetic fields recorded in the Semarkona meteorite - Roger R. Fu
et al
http://asterisk.apod.com/viewtopic.php?t=34113
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by bystander » Mon Dec 01, 2014 10:11 pm
Staying Warm: The Hot Gas in Clusters of Galaxies
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Nov 28
Most galaxies lie in clusters, groupings of a few to many thousands of galaxies. Our Milky Way galaxy itself is a member of the "Local Group," a band of about fifty galaxies whose other large member is the Andromeda Galaxy about 2.3 million light-years away. The closest large cluster of galaxies to us is the Virgo Cluster, about 50 million light-years away, with about 2000 members.
The space between the galaxies in clusters is filled with very hot gas – its temperature is of order ten million kelvin, or even higher. Most of the matter in the so-called intracluster medium is in the form of this very hot gas. Hot gas should cool off, and one of the major puzzles about galaxy clusters is that the hot intracluster gas does not seem to cool. In fact, calculations based on the energy radiated indicate that the gas should cool about ten times faster than is observed. X-ray observations of the hot gas also suggest that it might be turbulent, perhaps driven by mechanical flows of matter pouring outward from the supermassive black holes at the centers of the clusters’ galaxies, perhaps creating inflating bubbles of fast-moving charged particles that stir and heat the gas. Unfortunately, current X-ray observatories do not have the ability to measure the supposed gas velocities to test this proposed solution.
A team of astronomers including CfA scientists Bill Forman and Alexey Vikhlinin have pioneered a new method to evaluate the turbulence of the hot intracluster gas. They took advantage of the superb spatial resolution and sensitive images from the Chandra X-Ray Observatory to probe small clumps distributed through two clusters, Perseus and Virgo. ...
Turbulent Heating in Galaxy Clusters Brightest in X-rays - I. Zhuravleva
et al
http://asterisk.apod.com/viewtopic.php?t=34033
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by bystander » Fri Dec 05, 2014 6:02 pm
Twinkle, Twinkle New-Born Star
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Dec 05
Stars are born in dense, cool clouds of molecular gas and dust. When the local density is high enough, the matter can gravitationally collapse to form a new star, a so-called young stellar object (YSO). In its early phases, a thick envelope dominates the infrared emission from the YSO, hiding what is going on within, but eventually the envelope flattens out into a warm circumstellar accretion disk. The disk emits more infrared than does the young star, and that excess radiation can be used to distinguish young stars from more mature stars whose disks and envelopes have disappeared. In recent years it has become possible to investigate these envelopes and disks in more detail, and astronomers have been building on these studies to address how planetary systems develop.
It turns out that an accretion disk does not extend all the way in to the central star. Instead, a gap is produced between the star and disk because the dust grains closer in are destroyed by the starlight or blown away by stellar winds. Disks can contain clumps or irregular structures which orbit around with the disk. When a YSO happens to be aligned such that we observe its light through its disk, these structures make it appear to twinkle, or more precisely, to vary in intensity. Clumps at a distance of about one astronomical unit (the average distance of the Earth from the Sun) produce flickering over times of years as they rotate, while those in the disk ten times closer in result in variations over timescales of days. ...
YSOVAR: Mid-IR variability in the star forming region Lynds 1688 - H. M. Günther
et al
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by bystander » Fri Dec 12, 2014 11:31 pm
Magnetic Fields on Solar-Type Stars
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Dec 12
The Sun rotates slowly, about once every 24 days at its equator although the hot gas at every latitude rotates at a slightly different rate. Rotation helps to drive the mechanisms that power stellar magnetic fields, and in slowly rotating solar-type stars also helps to explain the solar activity cycle. In the case of solar-type stars that rotate much faster than does the modern-day Sun, the dynamo appears to be generated by fundamentally different mechanisms that, along with many details of solar magnetic field generation, are not well understood. Astronomers trying to understand dynamos across a range of solar-type stars (and how they evolve) have been observing a variety of active stars, both slow and fast rotators, to probe how various physical parameters of stars enhance or inhibit dynamo processes.
Most techniques used to observe stellar magnetism rely on indirect proxies of the field, for example on characteristics of the radiation emitted by atoms. Surveys using these proxies have found clear dependencies between rotation and the stellar dynamo and the star’s magnetic cycles, among other things. Recent advances in instrumentation that can sense the polarization of the light extend these methods and have made it possible to directly measure solar-strength magnetic fields on other stars. ...
A Bcool magnetic snapshot survey of solar-type stars - S.C. Marsden
et al
http://asterisk.apod.com/viewtopic.php?t=10777
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bystander
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by bystander » Fri Dec 26, 2014 6:11 pm
Ultra-luminous X-Ray Sources in Starburst Galaxies
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Dec 19
Ultra-luminous X-ray sources (ULXs) are point sources in the sky that are so bright in X-rays that each emits more radiation than a million suns emit at all wavelengths. ULXs are rare. Most galaxies (including our own Milky Way) have none, and those galaxies that do host a ULX usually contain only one. ULXs are also mysterious objects. They can’t be normal stars because their huge luminosities should then tear them apart. Most astronomers think that ULXs are black holes more than about ten solar masses in size (so-called intermediate mass black holes) that are accreting matter onto a surrounding disk and emitting X-rays. Bright X-ray emission is not unusual - the nuclei of galaxies also are bright X-ray emitters - but they are super-massive black holes, while ULXs are neither super-massive nor located in galactic nuclei.
The origin of ULXs is a puzzle as well. Supernovae, the explosive deaths of massive stars, can make stellar-mass-sized black holes, but how they then grow to ULX-size is not understood. Astronomers nevertheless think that, if they are on the right track, star formation activity could be a signpost for ULXs because supernovae are short-lived young stars. Indeed, observations of ULXs indicate that they are ten times more likely to be found in star forming galaxies than in old, red galaxies.
CfA astronomers Stefano Mineo and Andy Goulding and their colleagues used the Chandra X-ray Observatory to search for ULXs in a sample of seventeen luminous infrared galaxies that are exceptionally bright because of their extreme star formation activity. If star formation does signal the presence of ULXs, or even produce them, then these objects should have many. The team discovered fifty-three ULXs (with an uncertainty of about 30% ) among the 139 X-ray sources present in this sample. They report, however, that this ULX figure is actually ten times smaller than would be expected if ULXs correlated with simple star formation activity. They offer several possible explanations for this deficiency, including a surfeit of elements heavier than helium in these galaxies (these elements can suppress the birth of black holes). But the most likely scenario, they argue, is that large amounts of gas in these galaxies are present and absorbing X-rays, with the result that many of the ULXs present are not detected. Their conclusion implies that deep X-ray surveys of galaxies must take absorbing gas into account when estimating their internal X-ray properties and how this radiation affects the galaxies' properties and evolution.
A deficit of ultraluminous X-ray sources in luminous infrared galaxies - W. Luangtip
et al
Know the quiet place within your heart and touch the rainbow of possibility; be
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bystander
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by bystander » Fri Dec 26, 2014 6:26 pm
Asteroids: Breaking up is Hard to Do
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Dec 26
Hundreds of thousands of asteroids are known to orbit our Sun at distances ranging from near the Earth to beyond Saturn. The most widely known collection of asteroids, the "main belt," contains some of the largest and brightest asteroids and lies between the orbits of Mars and Jupiter. Astronomers think that the asteroids, like the planets, formed in the early solar system from the gradual agglomeration of smaller particles but that, in the case of asteroids, their growth was interrupted by mutual collisions that caused them to fragment rather than to coalesce into planets. This is an hypothesis which astronomers are trying to test by gathering new data. Their work has some immediate repercussions: NASA is currently planning an "Asteroid Redirect Mission (ARM)" as part of America's next human spaceflight enterprise. Understanding the origins of asteroid sizes - and then identifying a good one for an astronaut to recover - are prime US goals.
The discovery rate of asteroids and comets has increased in recent years thanks to new technology detectors. The Solar System today is seen as teeming with activity, and filled with related, evolving small bodies (including asteroids) whose orbits and sizes are shaped by gravitational interactions with the giant planets, but also by other forces like collisions and radiation effects. Radiation effects include the evaporation of water ice or other volatiles, differential thermal expansion, and radiation pressure -- and they are critical when addressing the issue of asteroid sizes. Because asteroids are irregularly shaped, the pressure of incident sunlight and also effect of their own outward radiation (which is unevenly directed) can cause them to spin and, when the spinning is fast enough, to break-up.
A "catastrophic disruption" is defined as the breakup of an asteroid into fragments each smaller than half the original mass. Traditionally small asteroids have been thought to be made in collisions between a parent body and a smaller projectile body, but these events seem to be very rare, both from observations and newer modeling. Renewed attention has recently been given to non-collisional break-up mechanisms like radiation effects, especially for asteroids smaller than a few hundred meters in diameter. ...
Observational Constraints on the Catastrophic Disruption Rate of Small Main Belt Asteroids - Larry Denneau
et al
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neufer
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by neufer » Sat Nov 04, 2017 8:58 pm
bystander wrote:Binary Stars in the Globular Cluster Messier 4
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 July 25
A globular cluster is a roughly spherical ensemble of stars, as many as several million of them, gravitationally bound together in groups whose diameters can be as small as only tens of light-years. To sense the dramatic implications of this dense packing, consider that the nearest star to the Sun, Proxima Centauri, is about four light-years away. Messier 4 (M4) is the closest globular cluster to Earth at a distance of about six thousand light-years, and a puzzle to astronomers. Normal gravitational effects should, over time, redistribute the stars in a globular cluster until they are more numerous towards the center, but while M4 shows a central concentration of stars it does not show evidence for a steep central cusp even though astronomers think enough time has passed.
To understand what is going on in this globular cluster, and to help understand how these clusters evolve in general, CfA astronomer Maureen van den Berg and her collaborators have undertaken a large and unprecedented set of deep images of M4 with the Hubble Space Telescope to look for binary stars, that is stars with companions. The dynamical interactions between the densely crowded stars in a globular cluster should disrupt many such binaries, but for reasons that are not understood about fifteen percent of the stars in M4 are binaries, at least based on monitoring brightness variations (a more typical number is two percent). Whether or not this unusual abundance is connected to the lack of a central cusp in stellar density is also not understood. ...
https://en.wikipedia.org/wiki/PSR_B1620-26 wrote:
<<PSR B1620-26 is a binary star system located at a distance of 12,400 light-years in the globular cluster of Messier 4 (M4, NGC 6121) in the constellation of Scorpius. The system is composed of a pulsar (PSR B1620-26 A) and a white dwarf star (WD B1620-26 or PSR B1620-26 B). As of 2000, the system is also confirmed to have an extrasolar planet orbiting the two stars. The double system (triple including the substellar companion) is just outside the core of the globular cluster. The age of the cluster has been estimated to be about 12.2 billion years. Hence this is the age estimate for the birth of the planet, and two stars.>>
Art Neuendorffer
