Topic: Mid April Astronomy Bulletin

[Clive]

SPUN UP ASTEROID COMING APART
NASA/Goddard Space Flight Center

A small asteroid has been caught in the process of spinning so fast that
it is throwing off material, according to new data from the Hubble Space
Telescope. Images show two narrow, comet-like tails of dusty debris
streaming from the asteroid (6478) Gault. Each tail represents an episode
in which the asteroid gently shed its material -- key evidence that Gault is
beginning to come apart. Discovered in 1988, the 4-kilometre-wide asteroid
has been observed repeatedly, but the debris tails are the first evidence of
disintegration. Gault is 344 million kilometres from the Sun. For the
roughly 800,000 known asteroids between Mars and Jupiter, astronomers
estimate that this type of event in the asteroid belt is rare, occurring
roughly once a year. Watching an asteroid become unglued gives astronomers
the opportunity to study the makeup of such space rocks without sending a
spacecraft to sample them. Gault is only the second asteroid whose
disintegration has been strongly linked to a process known as the YORP
effect. (YORP stands for "Yarkovsky-O'Keefe-Radzievskii-Paddack", the names
of four scientists who contributed to the concept.) When sunlight heats an
asteroid, infrared radiation escaping from its warmed surface carries off
angular momentum as well as heat. That process creates a tiny torque that
can cause the asteroid continually to spin faster. When the resulting
centrifugal force starts to overcome gravity, the asteroid's surface becomes
unstable, and landslides may send dust and rubble drifting into space at a
couple of miles per hour. The researchers estimate that Gault could have
been slowly spinning up for more than 100 million years.


SATURN'S RINGS COAT TINY MOONS
NASA

New findings have emerged about five tiny moons nestled in and near Saturn's
rings. The new research, from data gathered by six of Cassini's instruments
before its mission ended in 2017, is a clear confirmation that dust and ice
from the rings accretes onto the moons embedded within and near the rings.
Scientists also found the moon surfaces to be highly porous, further
confirming that they were formed in multiple stages as ring material settled
onto denser cores that might be remnants of a larger object that broke
apart. The porosity also helps explain their shape: rather than being
spherical, they are blobby and ravioli-like, with material stuck around
their equators. Of the satellites studied, the surfaces of those closest
to Saturn -- Daphnis and Pan -- are the most altered by ring materials.
The surfaces of the moons Atlas, Prometheus and Pandora, farther out from
Saturn, have ring material as well -- but they are also coated with the
bright icy particles and water vapour from the plume spraying out of
Enceladus. (A broad outer ring of Saturn, known as the E ring, is formed
by the icy material that fans out from Enceladus' plume.)

The key puzzle piece was a data set from Cassini's Visible and Infrared
Mapping Spectrometer (VIMS), which collected light visible to the human eye
and also infrared light of longer wavelengths. It was the first time that
Cassini was close enough to create a spectral map of the surface of the
innermost moon Pan. By analyzing the spectra, VIMS was able to learn about
the composition of materials on all five moons. VIMS saw that the ring
moons closest to Saturn appear the reddest, similar to the colour of the
main rings. Scientists don't yet know the composition of the material
that appears red, but they believe that it's likely to be a mix of organics
and iron. The moons just outside the main rings, on the other hand, appear
more blue, similar to the light from Enceladus' icy plumes.


MOST PROMISING EXOPLANETS IN HUNT FOR LIFE
Cornell University :

NASA's new Transiting Exoplanet Survey Satellite (TESS) is designed to
ferret out habitable exoplanets, but with hundreds of thousands of sunlike
and smaller stars in its camera views, which of those stars could host
planets like our own? TESS will observe 400,000 stars across the whole sky
to catch a glimpse of a planet transiting across the face of its star, one
of the primary methods by which exoplanets are identified. A team of
astronomers from Cornell University, Lehigh University and Vanderbilt
University has identified the most promising targets for this search in the
new 'TESS Habitable Zone Star Catalogue', published in Astrophysical Journal
Letters. The catalogue identifies 1,822 stars for which TESS is sensitive
enough to observe Earth-like planets just a bit larger than the Earth that
receive radiation from their star equivalent to what Earth receives from the
Sun. For 408 stars, TESS can glimpse a planet just as small as Earth, with
similar irradiation, in one transit alone. Confirming that an exoplanet has
been observed and determining the distance between it and its star requires
detecting two transits across the star. The 1,822 stars the researchers
have identified in the catalogue are ones from which TESS could detect two
planetary transits during its mission. Those orbital periods place them
squarely in the habitable zone of their star. The habitable zone is the
area around a star at which water can be liquid on a rocky planet's surface,
therefore considered ideal for sustaining life. As the researchers note,
planets outside the habitable zone could certainly harbour life, but it
would be extremely difficult to detect any signs of life on such frozen
planets without flying there.

The catalogue also identifies a subset of 227 stars for which TESS can not
only probe for planets that receive the same irradiation as Earth, but for
which TESS can also probe out farther, covering the full extent of the
habitable zone all the way to cooler Mars-like orbits. That will allow
astronomers to probe the diversity of potentially habitable worlds around
hundreds of cool stars during the TESS mission's lifetime. The stars
selected for the catalogue are bright, cool dwarfs, with temperatures
roughly between 2,700 and 5,000 degrees Kelvin. The stars in the catalogue
are selected because of their brightness; the closest are approximately 6
light-years from Earth. A total of 137 stars in the catalogue are within
the continuous viewing zone of NASA's James Webb Space Telescope, now under
construction. Webb will be able to observe them to characterize planetary
atmospheres and search for signs of life in their atmospheres. Planets
that TESS identifies may also make excellent targets for observations by
ground-based extremely large telescopes currently being built, the
researchers note, as the brightness of their host stars would make them
easier to characterize.


ARE BROWN DWARFS FAILED STARS OR SUPER PLANETS?
University of Heidelberg

Brown dwarfs fill the 'gap' between stars and the much smaller planets --
two very different types of astronomical objects. But how they originate
has yet to be fully explained. Astronomers have discovered that the star v
Ophiuchi in the Milky Way is being orbited by two brown dwarfs, which in all
probability formed along with the star from a gas and dust disc, just as
planets do. Brown dwarfs orbit either one star or travel in isolation in
the vast expanse of the Milky Way. Their masses -- at least 13 times more
than that of the Jupiter -- is sufficient to generate, at least temporarily,
energy in their cores through nuclear fusion. They are not sufficiently
massive, however, to ignite hydrogen in their cores and hence to create
their own light. The heat they continue to radiate after formation is how
astronomers are able to locate them. It is estimated that up to 100 billion
brown dwarfs make their home in the Milky Way. Yet it remains unclear how
they form -- whether they are 'failed' stars or possibly even super-planets.

The recent discoveries could provide an answer. Astronomers analysed the
variations in radial velocity of the star v Ophiuchi for 11 years. The star
has a mass slightly greater than two and half times that of the Sun, and is
located approximately 150 light years from Earth in the constellation
Ophiuchus. The team noticed a certain pattern in the measurements, similar
to those caused by orbiting planets or binary stars, which is usually
nothing out of the ordinary. But in this case, in-depth analysis of the data
revealed something extraordinary: apparently, v Ophiuchi is being orbited by
two brown dwarfs with an orbital period of approximately 530 and 3,185 days,
which puts them in a 6:1 resonant configuration. So the brown dwarf closer
to v Ophiuchi orbits its star exactly six times while the other, more
distant brown dwarf completes only one orbit. This discovery sheds
completely new light on the evolution of brown dwarfs. Do they develop
exclusively like normal stars in interstellar clouds, or can they also form
in the so-called protoplanetary disc of gas and dust that surrounds the
parent star in the early phase of its formation? The 6:1 resonance is a
strong indication for the latter scenario. Only then could the orbits of
the newly developing brown dwarfs adjust to a stable resonance over millions
of years. That is what the extensive dynamic analyses for possible
configurations of the vOphiuchi system suggest. This superplanetary system
is the first of its kind as well as the first sure sign that brown dwarfs
can form in a protoplanetary disc and the team hope for other such
discoveries that may one day allow astronomers to clarify how many of the
'failed stars' are actually more massive siblings of Jupiter and Saturn.


DARK MATTER NOT MADE FROM TINY BLACK HOLES
Kavli Institute for the Physics and Mathematics of the Universe

Scientists know that 85 per cent of the matter in the Universe is made up
of dark matter. Its gravitational force prevents stars in our Milky Way
from flying apart. However, attempts to detect such dark-matter particles
by underground experiments, or accelerator experiments including the
world's largest accelerator, the Large Hadron Collider, have so far failed.
That has led scientists to consider Hawking's 1974 theory of the existence
of primordial black holes, born shortly after the Big Bang, and his
speculation that they could make up a large fraction of the elusive dark
matter that scientists are trying to discover today. An international
team of researchers has used the gravitational lensing effect to look for
primordial black holes between the Earth and the Andromeda galaxy.
Gravitational lensing, an effect first suggested by Einstein, manifests
itself as the bending of light rays coming from a distant object such as
a star due to the gravitational effect of an intervening massive object
such as a primordial black hole. In extreme cases, such light bending
causes the background star to appear much brighter than it really is.
However, gravitational lensing effects are very rare events because it
requires a star in the Andromeda galaxy, a primordial black hole acting
as the gravitational lens, and an observer on Earth to be exactly in
line with one another. So to maximize the chances of capturing an event,
the researchers used the Hyper Suprime-Cam digital camera on the Subaru
telescope in Hawaii, which can image the whole of the Andromeda galaxy
in one shot.

Taking into account how fast primordial black holes are expected to move
in interstellar space, the team took multiple images to try to catch the
flicker of a star as it brightens for a period of a few minutes to hours
due to gravitational lensing. From 190 consecutive images of the Andromeda
galaxy taken over seven hours during one clear night, the team scoured the
data for potential gravitational-lensing events. If dark matter consists of
primordial black holes of a given mass, in this case masses lighter than the
Moon, the researchers expected to find about 1000 events. But after careful
analyses, they could identify only one case. The team's results showed that
primordial black holes can contribute no more than 0.1 per cent of all dark-
matter mass. Therefore it is unlikely that the theory is true. The
researchers are now planning to develop their analysis of the Andromeda
galaxy further. One new theory that they will investigate is to find
whether binary black holes discovered by the gravitational-wave detector
LIGO are in fact primordial black holes.


BLACK HOLE IMAGE MAKES HISTORY
NASA

A black hole and its shadow have been captured in an image for the first
time, an historic feat, by an international network of radio telescopes
called the Event Horizon Telescope (EHT). A black hole is an extremely
dense object from which no light can escape. Anything that comes within a
black hole's 'event horizon', its point of no return, will be consumed,
never to re-emerge, because of the black hole's unimaginably strong
gravity. By its very nature, a black hole cannot be seen, but the hot disc
of material that encircles it shines bright. Against a bright backdrop,
such as that disc, a black hole appears to cast a shadow. The image shows
the shadow of the supermassive black hole in the centre of Messier 87 (M87),
an elliptical galaxy some 55 million light-years away. That black hole is
6.5 US-billion times the mass of the Sun. Catching its shadow involved
eight ground-based radio telescopes around the globe, operating together
as if they were one telescope the size of our entire planet. While NASA
observations did not directly trace out the historic image, astronomers
used data from NASA's Chandra and NuSTAR satellites to measure the X-ray
brightness of M87's jet. Scientists used that information to compare their
models of the jet and disc around the black hole with the EHT observa-
tions. Other insights may come as researchers continue to pore over these
data.

There are many remaining questions about black holes that the coordinated
NASA observations may help to answer. Mysteries linger about why particles
get such a huge energy boost around black holes, forming dramatic jets that
surge away from the poles of black holes at nearly the speed of light.
When material falls into the black hole, where does the energy go? NASA
space telescopes have previously studied a jet extending more than 1,000
light-years away from the centre of M87. The jet is made of particles
travelling near the speed of light, shooting out at high energies from close
to the event horizon. The EHT was designed in part to study the origin of
that jet and others like it. A blob of matter in the jet called HST-1,
discovered by Hubble astronomers in 1999, has undergone a mysterious cycle
of brightening and dimming. Chandra, NuSTAR and Swift, as well as NASA's
Neutron star Interior Composition Explorer (NICER) experiment on the Inter-
national Space Station, also looked at the black hole at the centre of our
own Milky Way galaxy, called Sagittarius A*, in coordination with EHT.