Topic: Mid March Astronomy Bulletin

[Clive]

THE WORSENING COSMIC-RAY SITUATION
Spaceweather.com

New research conducted by the University of New Hampshire has revealed that
radiation from deep space is dangerous and intensifying faster than was
previously predicted. The story begins four years ago when scientists first
sounded the alarm about cosmic rays. Analyzing data from the Cosmic Ray
Telescope for the Effects of Radiation (CRaTER) instrument onboard NASA's
Lunar Reconnaissance Orbiter (LRO), they found that cosmic rays in the
Earth--Moon system were peaking at levels never before seen in the Space
Age. The worsening radiation environment, they pointed out, was a potential
peril to astronauts, curtailing how long they could safely travel through
space. A figure from their original 2014 paper shows the number of days a
30-year old male astronaut flying in a spaceship with 10 g/cm2 of aluminium
shielding (a wall thickness of nearly 4 cm) could go before reaching NASA-
mandated radiation limits. In the 1990s, the astronaut could spend 1000
days in interplanetary space, but in 2014 only 700 days. Galactic cosmic
rays come from outside the Solar System. They are a mixture of high-energy
photons and sub-atomic particles accelerated towards the Earth by supernova
explosions and other violent events in the cosmos. Our first line of
defence is the Sun. The Sun's magnetic field and the solar wind combine to
create a porous 'shield' that fends off cosmic rays attempting to enter the
Solar System. The shielding action of the Sun is strongest during Solar
Maximum and weakest during Solar Minimum. The problem is, as the authors
note in their new paper, the shield is weakening. Over the last decade, the
solar wind has exhibited low densities and magnetic field strengths,
representing anomalous states that have not been observed previously during
the Space Age. As a result of the remarkably weak solar activity, there
have also been the highest fluxes of cosmic rays. In 2014, the team used a
leading model of solar activity to predict how bad cosmic rays would become
during the next Solar Minimum, now expected in 2019-2020. Their previous
work suggested a ~20% increase of dose rates from one solar minimum to the
next. In fact, the actual dose rates observed by CRaTER in the last 4 years
exceed the predictions by ~10%, showing that the radiation environment is
worsening even more rapidly than was expected.

The data have come from CRaTER on the LRO spacecraft in orbit around the
Moon, which is point-blank exposed to any cosmic radiation that the Sun
allows to pass. Here on Earth, we have two additional lines of defence: the
magnetic field and the atmosphere of our planet. Both mitigate cosmic rays.
But even on Earth the increase is being felt. Scientists have been
launching space-weather balloons to the stratosphere almost weekly since
2015. Sensors onboard those balloons show a 13% increase in radiation
(X-rays and gamma-rays) penetrating our planet's atmosphere. X-rays and
gamma-rays detected by the balloons are 'secondary cosmic rays', produced
by the crash of primary cosmic rays into the upper atmosphere. They trace
radiation percolating down toward our planet's surface. The energy range of
the sensors, 10 keV to 20 MeV, is similar to that of medical X-ray machines
and airport security scanners. How does that affect us? Cosmic rays
penetrate commercial airlines, dosing passengers and flight crews so much
that pilots are classified by the International Commission on Radiological
Protection as occupational radiation workers. Some research shows that
cosmic rays can seed clouds and trigger lightning, potentially altering
weather and climate. Furthermore, there are studies linking cosmic rays
with cardiac arrhythmias in the general population. Cosmic rays can be
expected to intensify even more in the years ahead as the Sun enters what
may be the deepest Solar Minimum in more than a century.


JUPITER'S JET STREAMS ARE VERY DEEP
NASA/Jet Propulsion Laboratory

Data collected by NASA's Juno mission to Jupiter indicate that the
atmospheric winds of the gas-giant planet run deep into its atmosphere and
last longer than similar atmospheric processes found here on Earth. The
findings will improve understanding of Jupiter's interior structure, core
mass and, eventually, its origin. Other Juno science results show that the
massive cyclones that surround Jupiter's poles are enduring atmospheric
features and unlike anything else encountered in the Solar System. The
depth to which the roots of Jupiter's zones and belts extend has not been
known until now. Gravity measurements collected by Juno during its close
flybys of the planet have now provided an answer. On a gas planet,
asymmetries can come only from flows deep within the planet; on Jupiter, the
visible eastward and westward jet streams are asymmetric north and south.
The deeper the jets, the more mass they contain, leading to a stronger
signal expressed in the gravity field. Thus, the magnitude of the asymmetry
in gravity is related to how deep the jet streams extend. Galileo viewed
the stripes on Jupiter more than 400 years ago. Until now, we only had a
superficial understanding of them and have been able to relate them to cloud
features along Jupiter's jets. Now, after the Juno gravity measurements,
we know how deep the jets extend and what their structure is beneath the
visible clouds. The result was a surprise for the Juno science team because
it indicated that the weather layer of Jupiter was so massive, extending
much deeper than previously expected. The Jovian weather layer, from its
very top to a depth of 3,000 kilometres, contains about one per cent of
Jupiter's mass (about 3 Earth masses). That finding is important for
understanding the nature and possible mechanisms driving the strong jet
streams. In addition, the gravity signature of the jets is entangled with
the gravity signal of Jupiter's core.

Another Juno result suggests that beneath the weather layer, the planet
rotates nearly as a rigid body. That is really unexpected, and future
measurements by Juno will help scientists understand how the transition
works between the weather layer and the rigid body below. The discovery has
implications for other objects in the Solar System and beyond. The results
imply that the outer differentially-rotating region should be at least three
times deeper in Saturn and shallower in massive giant planets and brown-
dwarf stars. Jupiter's poles are a stark contrast to the more familiar
orange and white belts and zones encircling the planet at lower latitudes.
The north pole is dominated by a central cyclone surrounded by eight
circumpolar cyclones with diameters ranging from 4,000 to 4,600 kilometres.
Jupiter's south pole also contains a central cyclone, but it is surrounded
by five cyclones with diameters ranging from 5,600 to 7,000 kilometres.
Almost all the polar cyclones, at both poles, are so densely packed that
their spiral arms are in contact with adjacent cyclones. However, as
tightly spaced as the cyclones are, they have remained distinct, with
individual morphologies, over the seven months of observations.

Juno was launched on 2011 August 5. It flies quite low over the planet's
cloud tops -- sometimes as close as about 3,500 kilometres. During such
flybys, Juno is probing beneath the obscuring cloud cover of Jupiter and
studying its aurorae to learn more about the planet's origins, structure,
weather layer and magnetosphere.


YOUNG STAR ENVELOPED BY HUGE DUST CLOUD
NASA/Goddard Space Flight Center

Astronomers have used the Hubble Space Telescope to uncover a vast, complex
dust structure, about 2.5 light-years across, enveloping the young star HR
4796A. A bright, narrow, inner ring of dust is already known to encircle the
star and may have been corralled by the gravitational pull of an unseen
giant planet. That newly discovered huge structure around the system may
have implications for what the as-yet-unseen planetary system looks like
around the 8-million-year-old star, which is in its formative years of
planet construction. The debris field of very fine dust was probably
created from collisions among developing planets near the star; that is
sugested by a bright ring of dusty debris seen 75 AU from the star. The
pressure of starlight from the star, which is 23 times more luminous than
the Sun, then expelled the dust far into space. But the dynamics don't stop
there. The puffy outer dust structure is like a doughnut-shaped tyre inner
tube that got hit by a lorry. It is much more extended in one direction
than in the other and so looks squashed on one side even after account is
taken of its inclined projection on the sky. That may be due to the motion
of the host star ploughing through the interstellar medium, or it may be
influenced by a tidal tug from the star's red-dwarf binary companion
(HR 4796 B), located at least 0.6 light-years from the primary star. Though
debris discs have long been hypothesized, the first evidence for one around
any star was not uncovered until 1983, by IRAS. Later photographs revealed
an edge-on debris disc around the southern star Beta Pictoris. In the late
1990s, Hubble's second-generation instruments, which could block out the
glare of a central star, allowed many more discs to be photographed. Now,
such debris rings are thought to be common around stars; about 40 such
systems have been imaged to date, mostly by Hubble.


EXOPLANET ATMOSPHERE OBSERVATIONS
ESA/Hubble Information Centre

An international team of scientists has used the Hubble Space Telescope to
study the atmosphere of the hot exoplanet WASP-39b. By combining the new
observations with older data they created the most complete study yet of an
exoplanet atmosphere. The atmospheric composition of WASP-39b hints that
the formation processes of exoplanets can be very different from those of
our own Solar-System giants. WASP-39b is orbiting a Sun-like star about
700 light-years away. The exoplanet is classified as a 'Hot-Saturn',
reflecting both its mass being similar to that of the planet Saturn in our
own Solar System and its distance from its parent star. The study found
that the two planets, despite having a similar mass, are profoundly
different in many ways. Not only is WASP-39b not known to have a ring
system, it also has a puffy atmosphere that is free of high-altitude clouds.
That characteristic allowed Hubble to see deep into its atmosphere. The
team found clear spectroscopic evidence for atmospheric water vapour.
In fact, WASP-39b has three times as much water as Saturn does. Although
the researchers had predicted that they would see water vapour, they were
surprised by the amount that they found. The water abundance allowed the
team to infer the presence of large amount of heavier elements in the
atmosphere. That in turn suggests that the planet was bombarded by a lot of
icy material which gathered in its atmosphere. Such a bombardment would
only be possible if WASP-39b formed much further away from its host star
than it is at present.

The analysis of the atmospheric composition and the current position of the
planet indicate that WASP-39b most likely underwent an inward migration, a
journey across its planetary system. Having made its inward journey
WASP-39b is now only an eighth as far from its parent star, WASP-39, as
Mercury is to the Sun, and it takes only four days to complete an orbit.
The planet is also tidally locked, meaning that it always shows the same
side to its star. The team measured the temperature of WASP-39b to be a
scorching 750 degrees Celsius. Although only one side of the planet faces
its parent star, powerful winds transport heat from the bright side around
the planet, keeping the dark side almost as hot. Looking ahead, the team
wants to use the James Webb Space Telescope -- scheduled to be launched in
2019 -- to capture a more complete spectrum of the atmosphere of WASP-39b.
The JWST will be able to collect data about the planet's atmospheric carbon,
which absorbs light of longer wavelengths than Hubble can observe. From the
amount of carbon and oxygen in the atmosphere, astronomers may be able to
learn more about where and how the planet formed.


DOUBT CAST ON HABITABILITY OF PROXIMA CENTAURI SYSTEM
Carnegie Institution for Science

A team of astronomers has detected a massive stellar flare -- an energetic
explosion of radiation -- from the closest star to our own Sun, Proxima
Centauri. That finding raises questions about the habitability of our Solar
System's nearest exo-planetary neighbour, Proxima b, which orbits Proxima
Centauri. The team discovered the enormous flare when it re-analyzed
observations taken last year by ALMA, a radio telescope made up of 66
antennae. At peak luminosity it was 10 times brighter than our Sun's
largest flares when observed at similar wavelengths. Stellar flares have
not been well studied at the wavelengths observed by ALMA, especially around
stars of Proxima Centauri's type, called M dwarfs, which are the most common
type in our Galaxy. The flare increased Proxima Centauri's brightness by
1,000 times over 10 seconds. It was preceded by a smaller flare; taken
together, the whole event lasted fewer than two minutes of the 10 hours that
ALMA observed the star between January and March of last year. Stellar
flares happen when a shift in the star's magnetic field accelerates
electrons to speeds approaching that of light. The accelerated electrons
interact with the highly charged plasma that makes up most of the star,
causing an eruption that produces emission across the entire electromagnetic
spectrum.

It is likely that Proxima b was blasted by high-energy radiation during the
flare. It was already known that Proxima Centauri experiences regular,
although smaller, X-ray flares. Over the thousands of millions of years
since Proxima b formed, such flares could have evaporated any atmosphere or
ocean and sterilized the surface, so we are reminded that the habitability
of a planet may involve more than just being the right distance from the
host star to have liquid water. A November paper that also used the ALMA
data on Proxima interpreted its average brightness, which included the light
output of both the star and the flare together, as being caused by multiple
discs of dust encircling the star, not unlike our own Solar System's
asteroid and Kuiper belts. The authors of that study said that the presence
of dust pointed to the existence of more planets or planetary bodies in the
stellar system. But when the team looked at the ALMA data as a function of
observing time, instead of averaging them all together, they were able to
see the transient explosion of radiation emitted from Proxima Centauri for
what it truly was. There is now no reason to think that there is a
substantial amount of dust around Proxima, nor is there any information yet
that indicates the star has a rich planetary system like ours.

 

ULTRA-LUMINOUS SOURCES OF X-RAYS
California Institute of Technology

In the 1980s, researchers began discovering extremely bright sources of
X-rays in the outer portions of galaxies, away from the supermassive black
holes that dominate their centres. At first, researchers thought that those
cosmic objects, called ultra-luminous X-ray sources, or ULXs, were hefty
black holes with more than ten times the mass of the Sun. But observations
beginning in 2014 from NuSTAR and other space telescopes are showing that
some ULXs, which glow with X-ray light equal in energy to millions of Suns,
are actually neutron stars -- the burnt-out cores of massive stars that
exploded. Three such ULXs have been identified as neutron stars so far.
Now, a team of astronomers using data from the Chandra X-ray Observatory has
identified a fourth ULX as being a neutron star -- and found new clues about
how those objects can shine so brightly.

Neutron stars are extremely dense objects. Their gravity pulls surrounding
material from companion stars onto them, and as that material is pulled in,
it heats up and glows with X-rays. But as the neutron stars 'feed' on the
matter, there comes a time when the resulting X-ray light pushes the matter
away. Astronomers call that point -- when the objects cannot accumulate
matter any faster and or give off any more X-rays -- the Eddington limit.
In the same that we can eat only so much food at a time, there are limits to
how fast neutron stars can accrete matter. But ULXs are somehow breaking
that limit to give off such incredibly bright X-rays, and we don't know why.
In the new study, the researchers looked at a ULX in M51, the Whirlpool
galaxy, which is about 28 million light-years away. They analyzed archival
X-ray data taken by Chandra and discovered an unusual dip in the ULX's
spectrum. After ruling out all other possibilities, they concluded that
the dip was from a phenomenon called cyclotron resonance scattering, which
occurs when charged particles -- either positively charged protons or
negatively charged electrons -- circle in a magnetic field. Black holes
don't have magnetic fields and neutron stars do, so the finding indicated
that that particular ULX in M51 had to be a neutron star.

Cyclotron resonance scattering creates tell-tale signatures in a star's
optical spectrum, and the presence of those signatures, called cyclotron
lines, can provide information about the strength of the star's magnetic
field -- but only if the cause of the lines, whether it be protons or
electrons, is known. The researchers have not got a sufficiently detailed
spectrum of the new ULX to say for certain. If the cyclotron line is from
protons, then we know that the magnetic fields around the neutron star are
extremely strong and may in fact be helping to break the Eddington limit.
Such strong magnetic fields could reduce the pressure from a ULX's X-rays --
the pressure that normally pushes matter away -- allowing the neutron star
to consume more matter than is typical and to shine with the extremely
bright X-rays. If the cyclotron line is from circling electrons, in
contrast, then the magnetic field strength around the neutron star would not
be exceptionally strong, and thus the field is probably not the reason that
the stars break the Eddington limit. To address the problem further, the
researchers are planning to acquire more X-ray data on the ULX in M51 and
look for more cyclotron lines in other ULXs. The discovery that those very
bright objects, long thought to be black holes with masses up to 1,000 times
that of the Sun, are powered by much less massive neutron stars, was a huge
scientific surprise. Now astronomers might actually be getting firm
physical clues as to how such relatively small objects can be so mighty.


SEARCH FOR FIRST STARS UNCOVERS DARK MATTER
American Friends of Tel Aviv University

A team of astronomers has unexpectedly stumbled upon 'dark matter', the most
mysterious building block of outer space, while attempting to detect the
earliest stars in the Universe through radio-wave signals. The idea that
those signals implicate dark matter is put forward in a paper that suggests
that the signal is proof of interactions between normal matter and dark
matter in the early Universe. The discovery offers a rather direct proof
that dark matter exists and that it is composed of low-mass particles. The
signal, recorded by a novel radio telescope called EDGES, dates to 180
million years after the Big Bang. Dark matter is thought to be the key to
determining what the Universe is made of. We know quite a bit about the
chemical elements that make up the Earth, the Sun and other stars, but most
of the matter in the Universe is invisible and known as simply as 'dark
matter'. The existence of dark matter is inferred from its gravitational
effects, but we have no idea what kind of substance it is. Hence, dark
matter remains one of the greatest mysteries in physics. To solve it, we
must travel back in time. Astronomers can see back in time, since it takes
light time to reach us. We see the Sun as it was eight minutes ago, while
the immensely distant first stars in the Universe appear to us on Earth as
they were thousands of millions of years in the past. The team reported the
detection of a radio signal at a frequency of 78 megahertz. The width of
the observed profile was largely consistent with expectations, but they also
found it had a larger amplitude (corresponding to deeper absorption) than
predicted, indicating that the primordial gas was colder than expected. It
is suggested that the gas cooled through the interaction of hydrogen with
cold, dark matter.

That surprising signal indicates the presence of two actors: the first
stars, and dark matter. The first stars in the Universe turned on the radio
signal, while the dark matter collided with the ordinary matter and cooled
it down. Extra-cold material naturally explains the strong radio signal.
Physicists expected that any such dark matter particles would be heavy, but
the discovery indicates low-mass particles. On the basis of the radio
signal, it is argued that the dark-matter particle is no more massive than
several proton masses. That insight alone has the potential to reorient the
search for dark matter. Once stars formed in the early Universe, their
light was predicted to have penetrated the primordial hydrogen gas, altering
its internal structure. That would cause the hydrogen gas to absorb photons
from the cosmic microwave background, at the specific wavelength of 21 cm,
imprinting a signature in the radio spectrum that should be observable today
at radio frequencies below 200 megahertz. The new observation matches that
prediction except for the unexpected depth of the absorption. The team
predicts that the dark matter produced a very specific pattern of radio
waves that can be detected with a large array of radio antennae. One such
array is the SKA, the largest radio telescope in the world, now under
construction. Such an observation with the SKA would confirm that the first
stars did indeed reveal dark matter.


FIRST LIGHT ON VLT INTERFEROMETER
ESO

The new MATISSE instrument on ESO's Very Large Telescope Interferometer
(VLTI) has now successfully made its first observations at the Paranal
Observatory in northern Chile. MATISSE is the most powerful interferometric
instrument in the world at mid-infrared wavelengths. It will use high-
resolution imaging and spectroscopy to probe the regions around young stars
where planets are forming as well as the regions around supermassive black
holes in the centres of galaxies. The first MATISSE observations used the
VLTI's Auxiliary Telescopes to examine some of the brightest stars in the
night sky, including Sirius, Rigel and Betelgeuse, and showed that the
instrument is working well. MATISSE (Multi AperTure mid-Infrared Spectro-
Scopic Experiment) observes infrared light of wavelengths from 3 to 13
microns. It is a second-generation spectro-interferometer instrument for
the VLT and takes advantage of the multiple telescopes coupled with the wave
nature of the light. It can produce more detailed images of celestial
objects than can be obtained with any existing or planned single telescope
at those wavelengths. The initial MATISSE observations of the red super-
giant star Betelgeuse, which is expected to explode as a supernova in a few
hundred thousand years, showed that it still has secrets to reveal. The
star appears to be of different sizes when seen at different wavelengths.
Such data will allow astronomers to study further the huge star's surround-
ings and how it is shedding material into space. MATISSE will contribute to
several fundamental research areas in astronomy, focusing in particular on
the inner regions of discs around young stars where planets are forming, the
study of stars at different stages of their lives, and the surroundings of
supermassive black holes at the centres of galaxies. MATISSE is a four-way
beam combiner, i.e. it combines the light collected from up to four of the
8.2-m VLT Unit Telescopes or up to four of the Auxiliary Telescopes (ATs)
that make up the VLTI, performing both spectroscopic and imaging observa-
tions. In doing so, MATISSE and the VLTI together possess the imaging power
of a telescope up to 200 metres in diameter, capable of producing the most
detailed images ever obtained at mid-infrared wavelengths.