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Wednesday, May 04, 2016
An optical image of the brightest globular cluster, Omega Centauri, a group of over ten million stars older than the Sun. Astronomers have developed a new computational method to determine the abundance of oxygen in these and similar stars, and in particular in giant stars. The code finds values that are more self-consistent than previous estimates. Credit: Joaquin Polleri & Ezequiel Etcheverry, Observatorio Panameño en San Pedro de Atacama
Oxygen is the third most abundant element in the universe, after hydrogen and helium. It is an important constituent of the clouds of gas and dust in space, especially when combined in molecules with other atoms like carbon, and it is from this interstellar material that new stars and planets develop. Oxygen is, of course, also essential for life as we know it, and all known life forms require liquid water and its oxygen content. Oxygen in molecular form, especially as water, was supposed to be relatively abundant, but over the past decade considerable attention has been paid to observations suggesting that at least in molecular form oxygen is scarcer than expected, a deficit that has not yet been entirely resolved.
Atomic oxygen by contrast, seen most prominently in the light of stars, was thought to be in good agreement with expectations. The neutral oxygen atom produces strong lines that are frequently used to calculate its abundance. Models fit the line strengths by taking into account the radiation field, the star's hot gas motions, and the internal structure of the star (for example, the way the temperature and pressure change with radius). It turns out, however, that varying assumptions in these calculations can result in oxygen abundance predictions that differ significantly, and in the case of giant stars, which are larger and cooler and often have hot outer chromospheres, those abundance results can disagree with one another by as much as a factor of 15. This discrepancy has often been discounted by scientists arguing that some of the proposed stellar models are themselves unrealistic.
CfA astronomers Andrea Dupree, Eugene Avrett, and Bob Kurucz have tacked this fundamental problem with Avrett's PANDORA code for stellar atmospheres. In particular, they include the effects of a hot outer atmosphere in giant stars, something that was typically ignored. Moreover, they do not tie the excitation of oxygen atoms (and the corresponding line strengths) to the local temperature. That constraint, imposed by most previous methods in order to simplify the calculations, does not take more complex situations (like the hot atmosphere) adequately into account. The astronomers find that their new computations can resolve several outstanding issues. The lines themselves are actually as much as three times stronger than previously thought, reducing the implied oxygen abundances, and thereby also affecting details of the stellar interior models, especially for giants seen in globular clusters of stars. Similar improvements are seen in the results for stars known to be lacking other heavier elements, and even some normal, Sun-like stars. The possible implications extend to estimating more accurately the amount of oxygen present in a solar nebula when exoplanets form.
Reference(s): "Chromospheric Models and the Oxygen Abundance in Giant Stars," A. K. Dupree, E. H. Avrett, and R. L. Kurucz, ApJ 821, L7, 2016.
Artist’s impression of the ultracool dwarf star TRAPPIST-1 from close to one of its planets
Currently the best place to search for life beyond the Solar System
Astronomers using the TRAPPIST telescope
at ESO’s La Silla Observatory have discovered three planets orbiting an
ultracool dwarf star just 40 light-years from Earth. These worlds have
sizes and temperatures similar to those of Venus and Earth and are the
best targets found so far for the search for life outside the Solar
System. They are the first planets ever discovered around such a tiny
and dim star. The new results will be published in the journal Nature on
2 May 2016.
Detailed analysis showed that three planets with similar sizes to the Earth were present.
TRAPPIST-1 is an ultracool dwarf star — it is much cooler
and redder than the Sun and barely larger than Jupiter. Such stars are
both very common in the Milky Way and very long-lived, but this is the
first time that planets have been found around one of them. Despite
being so close to the Earth, this star is too dim and too red to be seen
with the naked eye or even visually with a large amateur telescope. It
lies in the constellation of Aquarius (The Water Carrier).
Emmanuël Jehin, a co-author of the new study, is excited: “This
really is a paradigm shift with regards to the planet population and
the path towards finding life in the Universe. So far, the existence of
such ‘red worlds’ orbiting ultra-cool dwarf stars was purely
theoretical, but now we have not just one lonely planet around such a
faint red star but a complete system of three planets!”
Michaël Gillon, lead author of the paper presenting the discovery, explains the significance of the new findings: "Why
are we trying to detect Earth-like planets around the smallest and
coolest stars in the solar neighbourhood? The reason is simple: systems
around these tiny stars are the only places where we can detect life on
an Earth-sized exoplanet with our current technology. So if we want to
find life elsewhere in the Universe, this is where we should start to
Astronomers will search for signs of life by studying the
effect that the atmosphere of a transiting planet has on the light
reaching Earth. For Earth-sized planets orbiting most stars this tiny
effect is swamped by the brilliance of the starlight. Only for the case
of faint red ultra-cool dwarf stars — like TRAPPIST-1 — is this effect
big enough to be detected.
Follow-up observations with larger telescopes, including the HAWK-I instrument on ESO’s 8-metre Very Large Telescope
in Chile, have shown that the planets orbiting TRAPPIST-1 have sizes
very similar to that of Earth. Two of the planets have orbital periods
of about 1.5 days and 2.4 days respectively, and the third planet has a
less well determined period in the range 4.5 to 73 days.
"With such short orbital periods, the planets are
between 20 and 100 times closer to their star than the Earth to the Sun.
The structure of this planetary system is much more similar in scale to
the system of Jupiter’s moons than to that of the Solar System," explains Michaël Gillon.
Although they orbit very close to their host dwarf star,
the inner two planets only receive four times and twice, respectively,
the amount of radiation received by the Earth, because their star is
much fainter than the Sun. That puts them closer to the star than the habitable zone
for this system, although it is still possible that they possess
habitable regions on their surfaces. The third, outer, planet’s orbit is
not yet well known, but it probably receives less radiation than the
Earth does, but maybe still enough to lie within the habitable zone.
"Thanks to several giant telescopes currently under
construction, including ESO’s E-ELT and the NASA/ESA/CSA James Webb
Space Telescope due to launch for 2018, we will soon be able to study
the atmospheric composition of these planets and to explore them first
for water, then for traces of biological activity. That's a giant step
in the search for life in the Universe," concludes Julien de Wit, a co-author from the Massachusetts Institute of Technology (MIT) in the USA.
This work opens up a new direction for exoplanet hunting,
as around 15% of the stars near to the Sun are ultra-cool dwarf stars,
and it also serves to highlight that the search for exoplanets has now
entered the realm of potentially habitable cousins of the Earth. The
TRAPPIST survey is a prototype for a more ambitious project called SPECULOOS that will be installed at ESO’s Paranal Observatory .
TRAPPIST (the TRAnsiting Planets and PlanetesImals Small Telescope)is a Belgian robotic 0.6-metre telescope operated from the University of Liège and based atESO’s La Silla Observatoryin Chile. It spends much of its time monitoring the light from around
60 of the nearest ultracool dwarf stars and brown dwarfs (“stars” which
are not quite massive enough to initiate sustained nuclear fusion in
their cores), looking for evidence of planetary transits.The target in
this case, TRAPPIST-1, is an ultracool dwarf, with about 0.05% of the
Sun’s luminosity and a mass of about 8% that of the Sun.
 This is one of the main methods
that astronomers use to identify the presence of a planet around a star.
They look at the light coming from the star, to see if some of the
light is blocked as the planet passes in front of its host star on the
line of sight to Earth —transits the star, as astronomers say. As the planet orbits around its star, we
expect to see regular small dips in the light coming from the star as
the planet moves in front of it.
 SPECULOOS is mostly funded by the
European Research Council and led also by the University of Liège. Four
1-metre robotic telescopes will be installed at the Paranal Observatory
to search for habitable planets around 500 ultra-cool stars over the
next five years.
In cosmology, one of the major challenges in next decades will be
probing the epoch of reionization in the early universe. Scientists at
MPA, the University of Oslo, and INAF have now used cosmological
hydrodynamical, radiative transfer simulations to understand the impact
of the complex distribution of neutral gas in the intergalactic medium
on distant galaxies. Combining the simulations with observations of
so-called Lyman alpha emitting galaxies they find that despite the
uncertainty, the current simulation-calibrated measurements favour a
late and rapid reionization history. The study also emphasizes that both
the large-scale distribution of ionised gas regions and the small-scale
structures of the intergalactic gas around galaxies must be understood
to derive more robust constraints on the reionization epoch.
The epoch of reionization, when early
galaxies or black holes drastically transformed the global state of the
universe from neutral to an ionized plasma, is one of the major unsolved
mysteries in modern extragalactic astronomy. Big questions remain
unanswered: What was the history of reonization? Which sources were
responsible for driving it?
One possibility to probe the physical
state of the universe at very early times is by observing distant,
high-redshift 'Lyman-alpha emitting galaxies'. These galaxies are
emitting a strong Lyman alpha line, i.e. radiation from hydrogen gas in
their interstellar medium. This strong emission line enables astronomers
to observe these objects out to very far distances, at redshifts as
high as 10. By now, hundreds of Lyman alpha galaxies have been found
beyond redshift 6.
Observations show that the apparent
demographics of Lyman alpha emitting galaxies changes over cosmic
history. Beyond redshift 6, i.e. when the universe was less than 1
billion years old, the observed population of galaxies with Lyman alpha
emission suddenly decreases. This is difficult to explain with galaxy
formation alone. From medium to high distances (redshift 2 to 6), the
fraction of star-forming galaxies that show a strong Lyman alpha
emission increases, which is partly caused by less dust in these
galaxies. Therefore, the sudden drop at very hight distances, beyond
redshift 6, seems to indicate that something is blocking this kind of
light. This drop is often interpreted as evidence of the gas in the
universe being increasingly neutral at earlier cosmic times – this means
the drop marks the time of reionization.
The idea to use Lyman alpha emitting
galaxies as a probe of reionization is based on a simple idea. With more
neutral gas along the line-of-sight to the galaxies, less Lyman alpha
flux reaches the observer. The difference between the expected flux from
a galaxy and the observed flux then tells us how much neutral gas
exists along the line-of-sight.
Kakiichi and collaborators have used this
method to infer the neutral hydrogen content of the universe at
redshift 7. They used cosmological hydrodynamical, radiative transfer
simulations of reionization (see Figure 1) to interpret observations of
Lyman-alpha emitting galaxies. The observations are then compared with
theoretical models of the apparent population of Lyman-alpha emitting
galaxies. In this way, the neutral gas fraction can be inferred from the
models that best fit the observations.
The new constraint this provides for the
reionization history is shown in Figure 2, which shows that the universe
is still very neutral at redshift 7. The present analysis therefore
seems to suggest that reionization occurred late and rapidly around
redshift 6 to 8.
This study also highlights an important
uncertainty in this simulation-calibrated measurement of the neutral
fraction. Figure 3 shows that completely different values of the neutral
fraction combined with other 'topologies' of reionization work equally
well in explaining the observed luminosity function (Figure 3). In fact,
this leads to a systematic uncertainty in the inferred neutral fraction
as high as an order of magnitude. Knowledge about the topology of
reionization, namely both the large-scale distribution of ionized
bubbles and the properties of small-scale self-shielded gas around
galaxies, is crucial to robustly infer the reionization history. Only
models containing both large and small-scale structures are able to
coherently explain the observations of the Lyman alpha forest and Lyman
alpha emitting galaxies from the reionization epoch to the
This difficultly, however, does not limit
the scope of using Lyman alpha emitting galaxies as a probe of
reionization. The uncertainties can be reduced by simultaneously using
multiple statistics such as the luminosity function and the fraction of
strong Lyman alpha line in Lyman Break Galaxies in surveys of Lyman
alpha galaxies. New survey strategies search for early galaxies in the
foreground of quasars at the reionization epoch, which will drastically
increase the scope of this method because it allows astronomers to
directly study both the state of the intergalactic gas and the
properties of Lyman alpha emitting galaxies.
Together with the increasing capability
of radiative transfer simulations, Lyman alpha emitting galaxies serve
as important beacons to probe the state of the infant universe.
Tailless Manx comet from Oort Cloud brings clues about the origin of the Solar System
Astronomers have found a unique object that appears to be made of inner Solar System material from the time of Earth’s formation, which has been preserved in the Oort Cloud far from the Sun for billions of years. Observations with ESO’s Very Large Telescope, and the Canada France Hawai`i Telescope, show that C/2014 S3 (PANSTARRS) is the first object to be discovered on a long-period cometary orbit that has the characteristics of a pristine inner Solar System asteroid. It may provide important clues about how the Solar System formed.
In a paper to be published today in the journal Science Advances, lead author Karen Meech of the University of Hawai`i’s Institute for Astronomy and her colleagues conclude that C/2014 S3 (PANSTARRS) formed in the inner Solar System at the same time as the Earth itself, but was ejected at a very early stage.
Their observations indicate that it is an ancient rocky body, rather than a contemporary asteroid that strayed out. As such, it is one of the potential building blocks of the rocky planets, such as the Earth, that was expelled from the inner Solar System and preserved in the deep freeze of the Oort Cloud for billions of years.
Karen Meech explains the unexpected observation: “We already knew of many asteroids, but they have all been baked by billions of years near the Sun. This one is the first uncooked asteroid we could observe: it has been preserved in the best freezer there is.”
C/2014 S3 (PANSTARRS) was originally identified by the Pan-STARRS1 telescope as a weakly active comet a little over twice as far from the Sun as the Earth. Its current long orbital period (around 860 years) suggests that its source is in the Oort Cloud, and it was nudged comparatively recently into an orbit that brings it closer to the Sun.
The team immediately noticed that C/2014 S3 (PANSTARRS) was unusual, as it does not have the characteristic tail that most long-period comets have when they approach so close to the Sun. As a result, it has been dubbed a Manx comet, after the tailless cat. Within weeks of its discovery, the team obtained spectra of the very faint object with ESO’s Very Large Telescope in Chile.
Careful study of the light reflected by C/2014 S3 (PANSTARRS) indicates that it is typical of asteroids known as S-type, which are usually found in the inner asteroid main belt. It does not look like a typical comet, which are believed to form in the outer Solar System and are icy, rather than rocky. It appears that the material has undergone very little processing, indicating that it has been deep frozen for a very long time. The very weak comet-like activity associated with C/2014 S3 (PANSTARRS), which is consistent with the sublimation of water ice, is about a million times lower than active long-period comets at a similar distance from the Sun.
The authors conclude that this object is probably made of fresh inner Solar System material that has been stored in the Oort Cloud and is now making its way back into the inner Solar System.
A number of theoretical models are able to reproduce much of the structure we see in the Solar System. An important difference between these models is what they predict about the objects that make up the Oort Cloud. Different models predict significantly different ratios of icy to rocky objects. This first discovery of a rocky object from the Oort Cloud is therefore an important test of the different predictions of the models. The authors estimate that observations of 50–100 of these Manx comets are needed to distinguish between the current models, opening up another rich vein in the study of the origins of the Solar System.
Co-author Olivier Hainaut (ESO, Garching, Germany), concludes: “We’ve found the first rocky comet, and we are looking for others. Depending how many we find, we will know whether the giant planets danced across the Solar System when they were young, or if they grew up quietly without moving much.”
 The Oort cloud is a huge region surrounding the Sun like a giant, thick soap bubble. It is estimated that it contains trillions of tiny icy bodies. Occasionally, one of these bodies gets nudged and falls into the inner Solar System, where the heat of the sun turns it into a comet. These icy bodies are thought to have been ejected from the region of the giant planets as these were forming, in the early days of the Solar System.
This research was presented in a paper entitled “Inner Solar System Material Discovered in the Oort Cloud”, by Karen Meech et al., in the journal Science Advances.
The team is composed of Karen J. Meech (Institute for Astronomy, University of Hawai`i, USA), Bin Yang (ESO, Santiago, Chile), Jan Kleyna (Institute for Astronomy, University of Hawai`i, USA), Olivier R. Hainaut (ESO, Garching, Germany), Svetlana Berdyugina (Institute for Astronomy, University of Hawai’i, USA; Kiepenheuer Institut für Sonnenphysik, Freiburg, Germany), Jacqueline V. Keane (Institute for Astronomy, University of Hawai`i, USA), Marco Micheli (ESA, Frascati, Italy), Alessandro Morbidelli (Laboratoire Lagrange/Observatoire de la Côte d’Azur/CNRS/Université Nice Sophia Antipolis, France) and Richard J. Wainscoat (Institute for Astronomy, University of Hawai`i, USA).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
Coincidence of a highly energetic outburst of an active galactic nucleus with a neutrino event at PeV energy
Nearly 10 billion years ago in a galaxy known as PKS B1424-418, a dramatic explosion occurred. Light from this blast began arriving at Earth in 2012. Now, an international team of astronomers, including scientists from the Max Planck Institute for Radio Astronomy in Bonn, have shown that a record-breaking neutrino seen around the same time likely was born in the same event. The results are published in "Nature Physics".
Neutrinos are the fastest, lightest and most unsociable understood fundamental particles, and scientists are just now capable of detecting high-energy ones arriving from deep space. The present work provides the first plausible association between a single extragalactic object and one of these cosmic neutrinos.
Although neutrinos far outnumber all the atoms in the universe, they rarely interact with matter, which makes detecting them quite a challenge. But this same property lets neutrinos make a fast exit from places where light cannot easily escape -- such as the core of a collapsing star -- and zip across the universe almost completely unimpeded. Neutrinos can provide information about processes and environments that simply aren't available through a study of light alone.
Recently, the IceCube Neutrino Observatory at the South Pole found first evidence for a flux of extraterrestrial neutrinos, which was named the Physics World breakthrough of the year 2013. To date, the science team of IceCube Neutrino has announced about a hundred very high-energy neutrinos and nicknamed the most extreme events after characters on the children's TV series "Sesame Street." On Dec. 4, 2012, IceCube detected an event known as Big Bird, a neutrino with an energy exceeding 2 quadrillion electron volts (PeV). To put that in perspective, it's more than a million million times greater than the energy of a dental X-ray packed into a single particle thought to possess less than a millionth the mass of an electron. Big Bird was the highest-energy neutrino ever detected at the time and still ranks second.
Where did it come from? The best IceCube position only narrowed the source to a patch of the southern sky about 32 degrees across, equivalent to the apparent size of 64 full moons. “It’s like a crime scene investigation”, says lead author Matthias Kadler, a professor of astrophysics at the University of Würzburg in Germany, “The case involves an explosion, a suspect, and various pieces of circumstantial evidence.”
Starting in the summer of 2012, NASA’s Fermi satellite witnessed a dramatic brightening of PKS B1424-418, an active galaxy classified as a gamma-ray blazar. An active galaxy is an otherwise typical galaxy with a compact and unusually bright core. The excess luminosity of the central region is produced by matter falling toward a supermassive black hole weighing millions of times the mass of our sun. As it approaches the black hole, some of the material becomes channeled into particle jets moving outward in opposite directions at nearly the speed of light. In blazars one of these jets happens to point almost directly toward Earth.
During the year-long outburst, PKS B1424-418 shone between 15 and 30 times brighter in gamma rays than its average before the eruption. The blazar is located within the Big Bird source region, but then so are many other active galaxies detected by Fermi.
The scientists searching for the neutrino source then turned to data from a long-term observing program named TANAMI. Since 2007, TANAMI has routinely monitored nearly 100 active galaxies in the southern sky, including many flaring sources detected by Fermi. Three radio observations between 2011 and 2013 cover the period of the Fermi outburst. They reveal that the core of the galaxy's jet had been brightening by about four times. “No other of our galaxies observed by TANAMI over the life of the program has exhibited such a dramatic change”, explains Eduardo Ros, from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany.
Within their jets, blazars are capable of accelerating protons to relativistic energies. Interactions of these protons with light in the central regions of the blazar can create pions. When these pions decay, both gamma rays and neutrinos are produced. "We combed through the field where Big Bird must have originated looking for astrophysical objects capable of producing high-energy particles and light," says coauthor Felicia Krauß, a doctoral student at the University of Erlangen-Nürnberg in Germany. "There was a moment of wonder and awe when we realized that the most dramatic outburst we had ever seen in a blazar happened in just the right place at just the right time."
In a paper published Monday, April 18, in Nature Physics, the team suggests the PKS B1424-418 outburst and Big Bird are linked, calculating only a 5-percent probability the two events occurred by chance alone. Using data from Fermi, NASA’s Swift and WISE satellites, the LBA and other facilities, the researchers determined how the energy of the eruption was distributed across the electromagnetic spectrum and showed that it was sufficiently powerful to produce a neutrino at PeV energies.
"Taking into account all of the observations, the blazar seems to have had means, motive and opportunity to fire off the Big Bird neutrino, which makes it our prime suspect," explains Matthias Kadler.
Francis Halzen, the principal investigator of IceCube at the University of Wisconsin–Madison, and not involved in this study, thinks the result is an exciting hint of things to come. "IceCube is about to send out real-time alerts when it records a neutrino that can be localized to an area a little more than half a degree across, or slightly larger than the apparent size of a full moon," he says. "We're slowly opening a neutrino window onto the cosmos."
"This study demonstrates the vital importance of classical astronomical observations in an era when new detection methods like neutrino observatories and gravitational-wave detectors open new but unknown skies", concludes Anton Zensus, director at MPIfR and head of its Radio Astronomy/VLBI research department, also a coauthor of the study.
TANAMI is a multiwavelength monitoring program of active galaxies in the Southern sky. It includes regular radio observations using the Australian Long Baseline Array (LBA) and associated telescopes in Chile, South Africa, New Zealand and Antarctica. When networked together, they operate as a single radio telescope more than 6,000 miles across and provide a unique high-resolution look into the jets of active galaxies.
The IceCube Neutrino Observatory, built into a cubic kilometer of clear glacial ice at the South Pole, detects neutrinos when they interact with atoms in the ice. This triggers a cascade of fast-moving charged particles that emit a faint glow, called Cerenkov light, as they travel, which is picked up by thousands of optical sensors strung throughout IceCube. Scientists determine the energy of an incoming neutrino by the amount of light its particle cascade emits.
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.
MPIfR scientists involved in the project are Eduardo Ros and J. Anton Zensus.
The elegant simplicity of NGC 4111, seen here in this
image from the NASA/ESA Hubble Space Telescope, hides a more violent
history than you might think. NGC 4111 is a lenticular, or lens-shaped, galaxy, lying about 50 million light-years from us in the constellation of Canes Venatici (The Hunting Dogs).
Lenticular galaxies are an intermediate type of galaxy between an elliptical and a spiral.
They host aged stars like ellipticals and have a disk like a spiral.
However, that’s where the similarities end: they differ from ellipticals
because they have a bulge and a thin disk, but are different from
spirals because lenticular discs contain very little gas
and dust, and do not feature the many-armed structure that is
characteristic of spiral galaxies. In this image we see the disc of NGC
4111 edge-on, so it appears as a thin sliver of light on the sky.
first sight, NGC 4111 looks like a fairly uneventful galaxy, but there
are unusual features that suggest it is not such a peaceful place.
Running through its centre, at right angles to the thin disc, is a
series of filaments, silhouetted against the bright core of the galaxy.
These are made of dust, and astronomers think they are associated with a
ring of material encircling the galaxy’s core. Since it is not aligned
with the galaxy’s main disc, it is possible that this polar ring of gas
and dust is actually the remains of a smaller galaxy that was swallowed
up by NGC 4111 long ago.
These four galaxy clusters were part of a large
survey of over 300 clusters used to investigate dark energy, the
mysterious energy that is currently driving the accelerating expansion
of the Universe, as described in our latest press release.
In these composite images, X-rays from NASA's Chandra X-ray Observatory
(purple) have been combined with optical light from the Hubble Space
Telescope and Sloan Digital Sky Survey (red, green, and blue).
Researchers used a novel technique that takes advantage of the
observation that the outer reaches of galaxy clusters, the largest
structures in the universe held together by gravity, show similarity in
their X-ray emission
profiles and sizes. That is, more massive clusters are simply scaled up
versions of less massive ones, similar to Russian dolls that nest
inside one another.
The amount of matter in the Universe, which is dominated by the unseen substance called dark matter,
and the properties of dark energy (what astronomers call cosmological
parameters) affect the rate of expansion of the Universe and, therefore,
how the distances to objects change with time. If the cosmological
parameters used are incorrect and a cluster is inferred to be traveling
away faster than the correct value, then a cluster will appear to be
larger and fainter due to this "Russian doll" property. If the cluster
is inferred to be traveling away more slowly than the correct value, the
cluster will be smaller and brighter than a cluster according to
These latest results confirm earlier studies that the amount of dark
energy has not changed over billions of years. They also support the
idea that dark energy is best explained by the "cosmological constant,"
which Einstein first proposed and is equivalent to the energy of empty
The galaxy clusters in this large sample ranged in distance from about 760 million to 8.7 billion light years
from Earth, providing astronomers with information about the era where
dark energy caused the once-decelerating expansion of the Universe to
The X-ray emission in the outer parts of galaxy clusters is faint
because the gas is diffuse there. To deal with this issue in this study,
the X-ray signal from different clusters was added together. Regions
near the centers of the clusters are excluded from the analysis because
of large differences between the properties of different clusters caused
by supermassive black hole outbursts, the cooling of gas and the formation of stars.
A paper describing these results by Andrea Morandi and Ming Sun
(University of Alabama at Huntsville) appeared in the April 11th, 2016
issue of the Monthly Notices of the Royal Astronomical Society journal
and is available online.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts,
controls Chandra's science and flight operations.
Credit: ESA/Hubble & NASA. Acknowledgement: J. Schmidt (Geckzilla)
At X-ray wavelengths, the celestial sky is dominated by two types of
astronomical objects: supermassive black holes, sitting at the centres
of large galaxies and ferociously devouring the material around them,
and binary systems, consisting of a stellar remnant – a white dwarf,
neutron star or black hole – feeding on gas from a companion star.
In both cases, the gas forms a swirling disc around the compact and
very dense central object: friction in the disc causes the gas to heat
up and emit light at many wavelengths, with a peak in X-rays.
Not all of the gas is swallowed by the central object though, and some
of it might even be pushed away by powerful winds and jets.
But an intermediate class of objects was discovered in the 1980s and is
still not well understood. Ten to a hundred times brighter than
ordinary X-ray binaries, these sources are nevertheless too faint to be
linked to accreting supermassive black holes, and in any case, are
usually found far from the centre of their host galaxy.
"We think these 'ultra-luminous X-ray sources' are somewhat special
binary systems, sucking up gas at a much higher rate than an ordinary
X-ray binary," explains Ciro Pinto from the Institute of Astronomy in Cambridge, UK.
"Some host highly magnetised neutron stars, while others might
conceal the long-sought-after intermediate-mass black holes, which have
masses around 1000 times the mass of the Sun. But in the majority of
cases, the reason for their extreme behaviour is still unclear."
Ciro is the lead author of a new study, based on observations from
ESA's XMM-Newton, revealing for the first time strong winds gusting at
very high speed from two of these exotic objects. The discovery,
published in this week's issue of the journal Nature, confirms that these sources conceal a compact object accreting matter at extraordinarily high rates.
Ciro and his colleagues delved into the XMM-Newton archives and
collected several days' worth of observations of three ultra-luminous
X-ray sources, all hosted in nearby galaxies located less than 22
million light-years from our Milky Way.
The data were obtained over several years with the Reflection Grating
Spectrometer, a highly sensitive instrument that allowed them to spot
very subtle features in the spectrum of the X-rays from the sources.
In all three sources, the scientists were able to identify X-ray
emission from gas in the outer portions of the disc surrounding the
central compact object, slowly flowing towards it.
But two of the three sources – known as NGC 1313 X-1 and NGC 5408 X-1 –
also show clear signs of X-rays being absorbed by gas that is streaming
away from the central source at an extremely rapid 70 000 km/s – almost
a quarter of the speed of light.
"This is the first time we've seen winds streaming away from ultra-luminous X-ray sources," says Ciro.
And there's more, since the very
high speed of these outflows is telling us something about the nature of
the compact objects in these sources, which are frantically devouring
While the hot gas is pulled inwards by the central object's gravity, it
also shines brightly, and the pressure exerted by the radiation pushes
it outwards. This is a balancing act: the greater the mass, the faster
it draws the surrounding gas. But this also causes the gas to heat up
faster, emitting more light and increasing the pressure that blows the
There is a theoretical limit to how much matter can be accreted by an
object of a given mass, called the 'Eddington luminosity'. It was first
calculated for stars by astronomer Arthur Eddington, but it can also be
applied to compact objects like black holes and neutron stars.
Eddington's calculation refers to an ideal case in which both the
matter being accreted onto the central object and the radiation being
emitted by it do so equally in all directions.
But the sources studied by Ciro and his collaborators are being fed
through an accretion disc that is likely being puffed up by internal
pressure of the gas flowing at a fast pace towards the central object.
In such a configuration, the material in the disc can shine 10 times or
more above the Eddington limit and, as part of the gas eludes the
gravitational grasp from the central object, very high-speed winds can
arise like the ones observed by XMM-Newton.
"By observing X-ray sources that are radiating beyond the Eddington
limit, it is possible to study their accretion process in great detail,
investigating by how much the limit can be exceeded and what exactly
triggers the outflow of such powerful winds," says Norbert Schartel, ESA XMM-Newton Project Scientist.
The nature of the compact objects hosted at the core of the sources
observed in this study is, however, still uncertain, although the
scientists suspect it might be stellar-mass black holes, with masses of
several to a few dozen times that of the Sun.
To investigate further, the team is still scrutinising the data archive
of XMM-Newton, searching for more sources of this type, and are also
planning future observations, in X-rays as well as at optical and radio
"With a broader sample of sources and multi-wavelength
observations, we hope to finally uncover the physical nature of these
powerful, peculiar objects," concludes Ciro.
Astronomers can use light echoes to measure the distance from a star to
its surrounding protoplanetary disk. This diagram illustrates how the
time delay of the light echo is proportional to the distance between the
star and the inner edge of the disk. Image credit: NASA/JPL-Caltech.› Larger image
Imagine you want to measure the size of a room, but it's completely
dark. If you shout, you can tell if the space you're in is relatively
big or small, depending on how long it takes to hear the echo after it
bounces off the wall.
Astronomers use this principle to study objects so distant they
can't be seen as more than points. In particular, researchers are
interested in calculating how far young stars are from the inner edge
of their surrounding protoplanetary disks. These disks of gas and dust
are sites where planets form over the course of millions of years.
"Understanding protoplanetary disks can help us understand some of
the mysteries about exoplanets, the planets in solar systems outside
our own," said Huan Meng, postdoctoral research associate at the
University of Arizona, Tucson. "We want to know how planets form and
why we find large planets called 'hot Jupiters' close to their stars."
Meng is the first author on a new study
published in the Astrophysical Journal using data from NASA's Spitzer
Space Telescope and four ground-based telescopes to determine the
distance from a star to the inner rim of its surrounding protoplanetary
Making the measurement wasn't as simple as laying a ruler on top of a
photograph. Doing so would be as impossible as using a satellite photo
of your computer screen to measure the width of the period at the end
of this sentence.
Instead, researchers used a method called "photo-reverberation,"
also known as "light echoes." When the central star brightens, some of
the light hits the surrounding disk, causing a delayed "echo."
Scientists measured the time it took for light coming directly from the
star to reach Earth, then waited for its echo to arrive.
Thanks to Albert Einstein's theory of special relativity, we know
that light travels at a constant speed. To determine a given distance,
astronomers can multiply the speed of light by the time light takes to
get from one point to another.
To take advantage of this formula, scientists needed to find a star
with variable emission -- that is, a star that emits radiation in an
unpredictable, uneven manner. Our own sun has a fairly stable emission,
but a variable star would have unique, detectable changes in radiation
that could be used for picking up corresponding light echoes. Young
stars, which have variable emission, are the best candidates.
The star used in this study is called YLW 16B and lies about 400
light-years from Earth. YLW 16B has about the same mass as our sun, but
at one million years old, it's just a baby compared to our
4.6-billion-year-old home star.
Astronomers combined Spitzer data with observations from
ground-based telescopes: the Mayall telescope at Kitt Peak National
Observatory in Arizona; the SOAR and SMARTS telescopes in Chile; and
the Harold L. Johnson telescope in Mexico. During two nights of
observation, researchers saw consistent time lags between the stellar
emissions and their echoes in the surrounding disk. The ground-based
observatories detected the shorter-wavelength infrared light emitted
directly from the star, and Spitzer observed the longer-wavelength
infrared light from the disk's echo. Because of thick interstellar
clouds that block the view from Earth, astronomers could not use
visible light to monitor the star.
Researchers then calculated how far this light must have traveled
during that time lag: about 0.08 astronomical units, which is
approximately 8 percent of the distance between Earth and its sun, or
one-quarter the diameter of Mercury's orbit. This was slightly smaller
than previous estimates with indirect techniques, but consistent with
Although this method did not directly measure the height of the
disk, researchers were able to determine that the inner edge is
Previously, astronomers had used the light echo technique to measure
the size of accretion disks of material around supermassive black
holes. Since no light escapes from a black hole, researchers compare
light from the inner edge of the accretion disk to light from the outer
edge to determine the disk size. This technique is also used to
measure the distance to other features near the accretion disk, such as
dust and the surrounding fast-moving gas.
While light echoes from supermassive black holes represent delays of
days to weeks, scientists measured the light echo from the
protoplanetary disk in this study to be a mere 74 seconds.
The Spitzer study marks the first time the light echo method was used in the context of protoplanetary disks.
"This new approach can be used for other young stars with planets in
the process of forming in a disk around them," said Peter Plavchan,
co-author of the study and assistant professor at Missouri State
University in Springfield.
NASA's Jet Propulsion Laboratory in Pasadena, California, manages
the Spitzer Space Telescope mission for NASA's Science Mission
Directorate in Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology in
Pasadena. Meng was a visiting researcher at Caltech during this
research. Spacecraft operations are based at Lockheed Martin Space
Systems Company in Littleton, Colorado. Data are archived at the
Infrared Science Archive housed at the Infrared Processing and Analysis
Center at Caltech. Caltech manages JPL for NASA.
Peering to the outskirts of our solar system, NASA's Hubble Space
Telescope has spotted a small, dark moon orbiting Makemake, the second
brightest icy dwarf planet — after Pluto — in the Kuiper Belt.
The moon — provisionally designated S/2015 (136472) 1 and nicknamed
MK 2 — is more than 1,300 times fainter than Makemake. MK 2 was seen
approximately 13,000 miles from the dwarf planet, and its diameter is
estimated to be 100 miles across. Makemake is 870 miles wide. The dwarf
planet, discovered in 2005, is named for a creation deity of the Rapa
Nui people of Easter Island.
The Kuiper Belt is a vast reservoir of leftover frozen material from
the construction of our solar system 4.5 billion years ago and home to
several dwarf planets. Some of these worlds have known satellites, but
this is the first discovery of a companion object to Makemake. Makemake
is one of five dwarf planets recognized by the International
The observations were made in April 2015 with Hubble's Wide Field
Camera 3. Hubble's unique ability to see faint objects near bright
ones, together with its sharp resolution, allowed astronomers to pluck
out the moon from Makemake's glare. The discovery was announced today
in a Minor Planet Electronic Circular.
The observing team used the same Hubble technique to observe the moon
as they did for finding the small satellites of Pluto in 2005, 2011,
and 2012. Several previous searches around Makemake had turned up
empty. "Our preliminary estimates show that the moon's orbit seems to
be edge-on, and that means that often when you look at the system you
are going to miss the moon because it gets lost in the bright glare of
Makemake," said Alex Parker of the Southwest Research Institute,
Boulder, Colorado, who led the image analysis for the observations.
A moon's discovery can provide valuable information on the
dwarf-planet system. By measuring the moon's orbit, astronomers can
calculate a mass for the system and gain insight into its evolution.
Uncovering the moon also reinforces the idea that most dwarf planets have satellites.
"Makemake is in the class of rare Pluto-like objects, so finding a
companion is important," Parker said. "The discovery of this moon has
given us an opportunity to study Makemake in far greater detail than we
ever would have been able to without the companion."
Finding this moon only increases the parallels between Pluto and
Makemake. Both objects are already known to be covered in frozen
methane. As was done with Pluto, further study of the satellite will
easily reveal the density of Makemake, a key result that will indicate
if the bulk compositions of Pluto and Makemake are also similar. "This
new discovery opens a new chapter in comparative planetology in the
outer solar system," said team leader Marc Buie of the Southwest
Research Institute, Boulder, Colorado.
The researchers will need more Hubble observations to make accurate
measurements to determine if the moon's orbit is elliptical or
circular. Preliminary estimates indicate that if the moon is in a
circular orbit, it completes a circuit around Makemake in 12 days or
Determining the shape of the moon's orbit will help settle the
question of its origin. A tight circular orbit means that MK 2 is
probably the product of a collision between Makemake and another Kuiper
Belt Object. If the moon is in a wide, elongated orbit, it is more
likely to be a captured object from the Kuiper Belt. Either event would
have likely occurred several billion years ago, when the solar system
The discovery may have solved one mystery about Makemake. Previous
infrared studies of the dwarf planet revealed that while Makemake's
surface is almost entirely bright and very cold, some areas appear
warmer than other areas. Astronomers had suggested that this
discrepancy may be due to the sun warming discrete dark patches on
Makemake's surface. However, unless Makemake is in a special
orientation, these dark patches should make the dwarf planet's
brightness vary substantially as it rotates. But this amount of
variability has never been observed.
These previous infrared data did not have sufficient resolution to
separate Makemake from MK 2.
The team's reanalysis, based on the new
Hubble observations, suggests that much of the warmer surface detected
previously in infrared light may, in reality, simply have been the dark
surface of the companion MK 2.
There are several possibilities that could explain why the moon would
have charcoal-black surface, even though it is orbiting a dwarf planet
that is as bright as fresh snow. One idea is that, unlike larger
objects such as Makemake, MK 2 is small enough that it cannot
gravitationally hold onto a bright, icy crust, which sublimates,
changing from solid to gas, under sunlight. This would make the moon
similar to comets and other Kuiper Belt Objects, many of which are
covered with very dark material.
When Pluto's moon Charon was discovered in 1978, astronomers quickly
calculated the mass of the system. Pluto's mass was hundreds of times
smaller than the mass originally estimated when it was found in 1930.
With Charon's discovery, astronomers suddenly knew something was
fundamentally different about Pluto. "That's the kind of transformative
measurement that having a satellite can enable," Parker said.