Topic: Late January Astronomy Bulletin

[Clive]

CITIZEN SCIENTISTS DISCOVER EXOPLANET
NASA

Using data from the Kepler space telescope, citizen scientists have
discovered a planet roughly twice the size of the Earth located within its
star's habitable zone (the range of orbital distances where liquid water may
exist on the planet's surface). The new world, known as K2-288Bb, could be
rocky or could be a gas-rich planet similar to Neptune. Its size is rare
among exoplanets (planets beyond our Solar System). Located 226 light-years
away in the constellation Taurus, the planet lies in a stellar system known
as K2-288, which contains a pair of dim, cool M-type stars separated by
about 8.2 billion kilometres -- roughly six times the distance between
Saturn and the Sun. The brighter star is about half as massive and large as
the Sun, while its companion is about one-third the Sun's mass and size.
The new planet, K2-288Bb, orbits the smaller, dimmer star every 31.3 days.
Examining data from the fourth observing campaign of Kepler's K2 mission,
the team noticed two probable planetary transits in the system. But
scientists require a third transit before claiming the discovery of a
candidate planet, and there wasn't a third signal in the observations they
reviewed. As it turned out, though, the team wasn't actually analyzing all
of the data. In Kepler's K2 mode, which ran from 2014 to 2018, the space-
craft repositioned itself to point at a new patch of sky at the start
of each three-month observing campaign. Astronomers were initially
concerned that that repositioning would cause systematic errors in the
measurements. To deal with that, early versions of the software that was
used to prepare the data for planet-finding analysis simply ignored the
first few days of observations -- and that's where the initial transit was
hiding. As scientists learned how to correct for the systematic errors,
that trimming step was eliminated -- but the early K2 data had been clipped.
The re-processed data were posted directly to Exoplanet Explorers, a project
where the public searches Kepler's K2 observations to locate new transiting
planets. In 2017 May, volunteers noticed the third transit and began an
excited discussion about what was then thought to be an Earth-sized
candidate in the system, which caught the attention of the team. Estimated
to be about 1.9 times the Earth's size, K2-288Bb is half the size of
Neptune. That places it within a recently discovered category called the
Fulton gap, or radius gap. Among planets that orbit close to their stars,
there's a curious dearth of worlds between about 1.5 and two times the
Earth's size. That is likely to be a result of intense starlight breaking
up atmospheric molecules and eroding away the atmospheres of some planets
over time, leaving behind two populations. Since K2-288Bb's radius places
it in that gap, it may provide a case study of planetary evolution within
that size range.


TESS DISCOVERS ITS THIRD NEW PLANET
Massachusetts Institute of Technology

NASA's Transiting Exoplanet Survey Satellite, TESS, has discovered a third
small planet outside our Solar System. The new planet, named HD 21749b,
orbits a bright, nearby dwarf star about 53 light-years away, in the
constellation Reticulum, and appears to have the longest orbital period of
the three planets so far identified by TESS. HD 21749b journeys around its
star in a relatively leisurely 36 days, compared to the two other planets --
Pi Mensae b, a 'super-Earth' with a 6.3-day orbit, and LHS 3844b, a rocky
world that speeds around its star in just 11 hours. All three planets were
discovered in the first three months of TESS observations. The surface of
the new planet is probably at around 150 C -- relatively cool, given its
proximity to its star, which is almost as bright as the Sun. The planet
is about three times the size of the Earth. Surprisingly, it is also a
whopping 23 times as massive as Earth. But it is unlikely that the planet
is rocky and therefore habitable; it is more likely to be made of gas, of a
kind that is much denser than the atmospheres of either Neptune or Uranus.
Serendipitously, the researchers have also detected evidence of a second
planet, though not yet confirmed, in the same planetary system, with a
shorter, 7.8-day orbit. If it is confirmed as a planet, it could be the
first Earth-sized planet discovered by TESS. Since it was launched in 2018
April, TESS has been monitoring the sky, sector by sector, for momentary
dips in the light of about 200,000 nearby stars. Such dips are likely to
represent planets passing in front of the stars.

 
SUB-SATURN GIANT PLANETS ARE COMMON
W. M. Keck Observatory

Astronomers have found a new exoplanet that could alter the standing theory
of planet formation. With a mass that is between those of Neptune and
Saturn, and its location beyond the 'snow line' of its host star, an alien
world of that scale was supposed to be rare. Using the Near-Infrared
Camera, second generation (NIRC2) instrument on the 10-m Keck II telescope
on Mauna Kea, Hawaii, and the Wide-Field Camera 3 (WFC3) instrument on the
Hubble Space Telescope, the researchers took simultaneous high-resolution
images of the exoplanet, named OGLE-2012-BLG-0950Lb, allowing them to
determine its mass. They were surprised to find the mass come out right in
the middle of the predicted intermediate-giant-planet mass gap. In an
uncanny timing of events, another team of astronomers published a
statistical analysis at almost the same time showing that such sub-Saturn-
mass planets are not rare after all. OGLE-2012-BLG-0950Lb was among the
sub-Saturn planets in the statistical study; all were detected through
microlensing, the only method currently sensitive enough to detect planets
with less than Saturn's mass in Jupiter-like orbits. Microlensing arises as
a consequence of Einstein's general theory of relativity: the bending and
magnification of light near a massive object like a star, producing a
natural lens in the sky. In the case of OGLE-2012-BLG-0950Lb, the light
from a distant background star was magnified by OGLE-2012-BLG-0950L (the
exoplanet's host star) over the course of two months as it passed close to
perfect alignment in the sky with the background star. By carefully
analyzing the light during the alignment, an unexpected dimming with a
duration of about a day was observed, revealing the presence of
OGLE-2012-BLG-0950Lb by its own influence on the lensing.

What is unique about the microlensing method is its sensitivity to
sub-Saturn planets like OGLE-2012-BLG-0950Lb that orbit beyond the 'snow
line' of their host stars. The snow line, or frost line, is the distance in
a young solar system, (a.k.a. a protoplanetary disc) at which it is cold
enough for water to condense into ice. At and beyond the snow line there is
a dramatic increase in the amount of solid material needed for planet
formation. According to the core accretion theory, the solids are thought
to build up into planetary cores first through chemical and then gravita-
tional processes. A key process of the core-accretion theory is called
'runaway gas' accretion. Giant planets are thought to start their formation
process by collecting a core mass of about 10 times the Earth's mass in rock
and ice. At that stage, a slow accretion of hydrogen and helium gas begins
until the mass has doubled. Then, the accretion of hydrogen and helium is
expected to speed up exponentially in a runaway gas-accretion process. That
process stops when the supply is exhausted. If the supply of gas is stopped
before runaway accretion stops, we get 'failed Jupiter' planets with masses
of 10-20 Earth-masses (like Neptune). The runaway gas-accretion scenario of
the core-accretion theory predicts that planets like OGLE-2012-BLG-0950Lb
are expected to be rare. At 39 times the mass of the Earth, planets that
size are thought to be continuing through a stage of rapid growth, ending in
a much more massive planet. This new result suggests that the runaway-
growth scenario may need revision. The discovery has not only called into
question an established theory, it was made using a new technique that will
be a key part of NASA's next big planet-finding mission, the Wide-Field
Infra-Red Survey Telescope (WFIRST), which is scheduled to be launched into
orbit in the mid-2020s.


A GALAXY WITHOUT NEIGHBOURS
University of Michigan

Researchers have long known that the Milky Way has about 10 smaller
satellite galaxies surrounding it, each with at least a million stars, and
up to more than a billion, such as the Magellanic Clouds. Now, with
the Subaru telescope, astronomers can observe galaxies five or ten times the
distance from the Milky Way, such as M94. They can then use the physics of
how satellite galaxies form around the Milky Way to predict how many
satellite galaxies a similar-sized galaxy such as M94 may have. When
astronomers examined M94, they expected to find a similar number of
satellite galaxies. However, they detected just two galaxies near M94, with
very few stars each. The results have implications for the current
understanding of how galaxies form -- which is in much larger haloes of dark
matter. Those halos of dark matter surrounding galaxies have immense
gravitational force, and can pull in gas from their immediate vicinity.
Large galaxies like the Milky Way generally form in haloes of about the same
mass. But smaller satellite galaxies, which form in smaller 'subhaloes' are
not nearly so dependable. The production rate of high-mass stars in such
satellite galaxies actually modulates their growth. If, for example, the
nascent satellite galaxy forms too many high-mass stars at one time, their
eventual supernova explosions might expel all its gas and halt all further
growth. But astronomers are unsure at what size halo such 'scatter' in
galaxy formation becomes important. M94 indicates that galaxy formation in
intermediate-sized dark haloes may be much more uncertain than previously
thought. To observe the number of satellite dwarf galaxies around M94, the
researchers took a composite image of the large galaxy. The image covered
about 12 square degrees of the night sky. That kind of image includes
layers and layers of 'noise', including cosmic rays and scattered light,
which make faint dwarf galaxies difficult to detect. To make sure they
weren't missing satellite galaxies, the team engineered artificial galaxies
back into the image and recovered them using the same methods as for real
satellites. With that technique, the researchers confirmed that there were
no more than two galaxies around M94.


X-RAY PULSE DETECTED NEAR EVENT HORIZON
Massachusetts Institute of Technology

On 2014 Nov. 22, astronomers observed a rare event in the night sky: a
supermassive black hole at the centre of a galaxy, nearly 300 million light
years away, ripping apart a passing star. The event, known as a tidal-
disruption flare, for the black hole's massive tidal pull that tears a star
apart, created a burst of X-ray activity near the centre of the galaxy.
Since then, many observatories have trained their sights on the event, in
the hope of learning more about how black holes feed. Now researchers have
pored through data from multiple telescopes' observations of the event, and
discovered a curiously intense, stable, and periodic pulse, or signal, of
X-rays, across all datasets. The signal appears to emanate from an area
very close to the black hole's event horizon -- the point beyond which
material is swallowed inescapably by the black hole. The signal appears
periodically to brighten and fade every 131 seconds, and persisted over at
least 450 days. The researchers believe that whatever is emitting the
periodic signal must be orbiting the black hole, just outside the event
horizon, near the Innermost Stable Circular Orbit, or ISCO -- the smallest
orbit in which a particle can safely travel around a black hole. Given the
signal's stable proximity to the black hole, and the black hole's mass,
which researchers previously estimated to be about 1 million times that of
the Sun, the team has calculated that the black hole is spinning at about
half the speed of light. The findings are the first demonstration of a
tidal-disruption flare being used to estimate a black hole's spin. Most
supermassive black holes are dormant and do not usually emit much in the
way of X-ray radiation. Only occasionally will they release a burst of
activity, such as when stars get close enough for black holes to devour
them. Such tidal-disruption flares can be used to estimate the spin of
supermassive black holes -- a characteristic that has been, up until now,
incredibly tricky to pin down. Events where black holes shred stars that
come too close to them could help us map out the spins of several super-
massive black holes that are dormant and otherwise hidden at the centres of
galaxies. That could ultimately help us understand how galaxies evolved
over cosmic time.

Theoretical models of tidal-disruption flares show that when a black hole
shreds a star apart, some of that star's material may stay outside the event
horizon, circling, at least temporarily, in a stable orbit such as the ISCO,
and giving off periodic flashes of X-rays before ultimately being fed by the
black hole. The periodicity of the X-ray flashes thus encodes key
information about the size of the ISCO, which itself is dictated by how fast
the black hole is spinning. Astronomers thought that if they could see such
regular flashes very close to a black hole that had undergone a recent tidal
disruption event, those signals could give them an idea of how fast the
black hole was spinning. They focused their search on ASASSN-14li, the
tidal-disruption event that astronomers identified in 2014 November, using
the ground-based All-Sky Automated Survey for SuperNovae (ASASSN). The
team looked through archived datasets from three observatories that collected
X-ray measurements of the event since its discovery: the European Space
Agency's XMM-Newton space observatory, and NASA's space-based Chandra
and Swift observatories. On the basis of the properties of the signal, and the
mass and size of the black hole, the team estimated that the black hole is
spinning at at least half the speed of light. Once the team discovered the
periodic signal, it was up to the theorists to find an explanation for what
may have generated it. The team came up with various scenarios, but the one
that seems the most likely to generate such a strong, regular X-ray flare
involves not just a black hole shredding a passing star, but also a smaller
white-dwarf star, orbiting close to the black hole. Such a white dwarf may
have been circling the supermassive black hole, at ISCO -- the innermost
stable circular orbit -- for some time. Alone, it would not have been
enough to emit any sort of detectable radiation. For all intents and
purposes, the white dwarf would have been invisible to telescopes as it
circled the relatively inactive, spinning black hole. Sometime around 2014
Nov. 22, a second star passed close enough to the system that the black hole
tore it apart in a tidal-disruption flare that emitted an enormous amount of
X-ray radiation, in the form of hot, shredded stellar material. As the
black hole pulled that material inward, some of the stellar debris fell into
the black hole, while some remained just outside, in the innermost stable
orbit -- the very same orbit in which the white dwarf circled. As the white
dwarf came in contact with that hot stellar material, it probably dragged it
along as a luminous overcoat of sorts, illuminating the white dwarf in an
intense amount of X-rays each time it circled the black hole, every 131
seconds. The scientists admit that such a scenario would be incredibly rare
and would last only for several hundred years at most -- a blink of an eye
on the cosmic scale. The chances of detecting such a scenario would be
exceedingly slim. Estimating spins of several black holes from the
beginning of time to now would be valuable in terms of estimating whether
there is a relationship between the spin and the age of black holes.


BIRTH OF BLACK HOLE OR NEUTRON STAR
NASA

A brief and unusual flash observed in the night sky on 2018 June 16 puzzled
astronomers and astrophysicists across the globe. The event -- called
AT2018cow and nicknamed 'the Cow' after the coincidental final letters in
its official name -- is unlike any celestial outburst ever seen before,
prompting multiple theories about its source. Over three days, the Cow
produced a sudden explosion of light at least 10 times brighter than a
typical supernova, and then it faded over the next few months. The unusual
event occurred inside or near a star-forming galaxy known as CGCG 137-068,
located about 200 million light-years away in the constellation Hercules.
The object was first observed by the Asteroid Terrestrial-impact Last Alert
System telescope in Hawaii. So exactly what is the Cow? Using data from
multiple NASA missions, including the Neil Gehrels Swift Observatory and the
Nuclear Spectroscopic Telescope Array (NuSTAR), two groups are publishing
papers that provide possible explanations for the Cow's origins. One paper
argues that the Cow is a monster black hole shredding a passing star. The
second paper hypothesizes that it is a supernova that gave birth to a black
hole or a neutron star. One potential explanation of the Cow is that a star
has been ripped apart in what astronomers call a 'tidal disruption event'.
Just as the Moon's gravity causes Earth's oceans to bulge, creating tides, a
black hole has a similar but more powerful effect on an approaching star,
ultimately breaking it apart into a stream of gas. The tail of the gas
stream is flung out of the system, but the leading edge swings back round
the black hole, collides with itself and creates an elliptical cloud of
material. According to one research team using data spanning from infrared
radiation to gamma rays from Swift and other observatories, that transform-
ation best explains the Cow's behaviour.

Other astronomers think that the shredded star was a white dwarf -- a hot,
roughly Earth-sized stellar remnant marking the final state of stars like
our Sun. They also calculated that the black hole's mass ranges from
100,000 to 1 million times the Sun's, almost as large as the central black
hole of its host galaxy. It is unusual to see black holes of that scale
outside the centre of a galaxy, but it is possible that the Cow occurred in
a nearby satellite galaxy or a globular star cluster whose older stellar
populations could have a higher proportion of white dwarfs than average
galaxies. A third team of scientists was able to gather data on the Cow
over an even broader range of wavelengths, spanning from radio waves to
gamma rays. On the basis of those observations, the team suggests that a
supernova could be the source of the Cow. When a massive star dies, it
explodes as a supernova and leaves behind either a black hole or an
incredibly dense neutron star. The Cow could represent the birth of one
such stellar remnant. The team used high-energy X-ray data to show that the
Cow has characteristics similar to a compact body like a black hole or
neutron star consuming material. But on the basis of what we saw in other
wavelengths, we think that this was a special case and that we may have
observed -- for the first time -- the creation of a compact body in real
time. The team analyzed data from multiple observatories, including NASA's
NuSTAR, ESA's XMM-Newton and INTEGRAL satellites, and the National
Science Foundation's Very Large Array. The team proposes that the bright
optical and ultraviolet flash from the Cow signalled a supernova and that
the X-ray emissions that followed shortly after the outburst arose from gas
radiating energy as it fell onto a compact object. Typically, a supernova's
expanding debris cloud blocks any light from the compact object at the
centre of the blast. Because of the X-ray emissions, astronomers suggest
that the original star in this scenario may have been relatively low in mass,
producing a comparatively thinner debris cloud through which X-rays from
the central source could escape. If we are seeing the birth of a compact object
in real time, this could be the start of a new chapter in our understanding
of stellar evolution.


DARK MATTER ON THE MOVE
RAS

Scientists have found evidence that dark matter can be heated up and moved
around, as a result of star formation in galaxies. The findings provide the
first observational evidence for the effect known as 'dark matter heating',
and give new clues as to what makes up dark matter. In the new work,
scientists set out to hunt for evidence for dark matter at the centres of
nearby dwarf galaxies. Dwarf galaxies are small, faint galaxies that are
typically found orbiting larger galaxies like our own Milky Way. Dark
matter is thought to make up most of the mass of the Universe. However,
since it does not interact with light in the same way as normal matter, it
can only be observed through its gravitational effects. The key to studying
it may however lie in how stars are formed in these galaxies. When stars
form, strong winds can push gas and dust away from the heart of the galaxy.
As a result, the galaxy's centre has less mass, which affects how much
gravity is felt by the remaining dark matter. With less gravitational
attraction, the dark matter gains energy and migrates away from the centre,
an effect called 'dark matter heating'. The team of astrophysicists
measured the amount of dark matter at the centres of 16 dwarf galaxies with
very different star-formation histories. They found that galaxies that
stopped forming stars long ago had higher dark-matter densities at their
centres than those that are still forming stars today. That supports the
theory that the older galaxies had less dark-matter heating. The findings
provide a new constraint on dark-matter models: dark matter must be able to
form dwarf galaxies that exhibit a range of central densities, and those
densities must relate to the amount of star formation. The team hopes to
expand on that work by measuring the central dark-matter density in a larger
sample of dwarfs, pushing to even fainter galaxies, and testing a wider
range of dark-matter models.


RADIO SIGNALS FROM DEEP SPACE
BBC Science

Astronomers have revealed details of mysterious signals emanating from a
distant galaxy, picked up by a telescope in Canada. The precise nature and
origin of the blasts of radio waves is unknown. Among the 13 bursts of fast
radio waves, known as FRBs, was a very unusual repeating signal, coming from
the same source about 1.5 billion light-years away. Such an event has only
been reported once before, by a different telescope. The CHIME observatory,
located in British Columbia's Okanagan Valley, consists of four 100-metre
semi-cylindrical antennae, which scan the entire northern sky each day. The
telescope only got up and running last year, detecting 13 of the radio
bursts almost immediately, including the repeater. FRBs are short, bright
flashes of radio waves, which appear to be coming from almost halfway across
the Universe. So far, scientists have detected about 60 single fast radio
bursts and two that repeat. They believe there could be as many as a
thousand FRBs in the sky every day. There are a number of theories about
what could be causing them. They include a neutron star with a very strong
magnetic field that is spinning very rapidly or two neutron stars merging
together.


GALAXY ZOOMS IN ON BEGINNING OF TIME
Association of Universities for Research in Astronomy (AURA)

Observations from Gemini Observatory identify a key fingerprint of an
extremely distant quasar, allowing astronomers to sample light emitted from
the dawn of time. Astronomers happened upon this deep glimpse into space
and time thanks to an unremarkable foreground galaxy acting as a
gravitational lens, which magnified the quasar's ancient light. The Gemini
observations provide critical pieces of the puzzle in confirming the object
as the brightest-appearing quasar so early in the history of the Universe,
raising hopes that more sources like it will be found. Before the cosmos
reached its billionth birthday, some of the very first cosmic light began
a long journey through the expanding Universe. One particular beam of
light, from a quasar, serendipitously passed near an intervening galaxy,
whose gravity bent and magnified the quasar's light and refocused it in our
direction, allowing telescopes like Gemini North to probe the quasar in
great detail. The Gemini observations provided key pieces of the puzzle by
filling a critical hole in the data. The Gemini North telescope on Mauna
Kea, Hawaii, utilized the Gemini Near-InfraRed Spectrograph (GNIRS) to
dissect a significant swath of the infrared part of the light's spectrum.
The Gemini data contained the tell-tale signature of magnesium which is
critical for determining how far back in time we are looking. The Gemini
observations also led to a determination of the mass of the black hole
powering the quasar. When astronomers combined the Gemini data with
observations from multiple observatories on Mauna Kea, the Hubble Space
Telescope, and other observatories around the world, they were able to paint
a complete picture of the quasar and the intervening galaxy. That picture
reveals that the quasar is located extremely far back in time and space --
shortly after what is known as the Epoch of Reionization -- when the very
first light emerged from the Big Bang. This is one of the first sources to
shine as the Universe emerged from the cosmic dark ages. Before then, no
stars, quasars, or galaxies had been formed, until objects like this
appeared like candles in the dark.

The foreground galaxy that enhances our view of the quasar is especially
dim, which is extremely fortuitous. If that galaxy were much brighter,
astronomers would not have been able to differentiate it from the quasar.
The intense brilliance of the quasar, known as J0439+1634, also suggests
that it is fuelled by a supermassive black hole at the heart of a young
forming galaxy. The broad appearance of the magnesium fingerprint captured
by Gemini also allowed astronomers to measure the mass of the quasar's
supermassive black hole at 700 million times that of the Sun. The
supermassive black hole is most likely surrounded by a sizeable flattened
disc of dust and gas. That torus of matter -- known as an accretion disc --
most likely continually spirals inward to feed the black hole powerhouse.
Observations at sub-millimetre wavelengths with the James Clerk Maxwell
Telescope on Mauna Kea suggest that the black hole is not only accreting gas
but may be triggering star birth at a prodigious rate -- which appears to be
up to 10,000 stars per year; for comparison, our Milky Way Galaxy makes one
star per year. However, because of the boosting effect of gravitational
lensing, the actual rate of star formation could be much lower. Quasars are
extremely energetic sources powered by huge black holes thought to have
resided in the very first galaxies to form in the Universe. Because of
their brightness and distance, quasars provide a unique glimpse into the
conditions in the early Universe. This quasar has a redshift of 6.51, which
translates to a distance of 12.8 billion light-years, and appears to
shine with a combined light of about 600 million million Suns, boosted by
the gravitational lensing magnification. The foreground galaxy which bent
the quasar's light is about half that distance away, at a mere 6 billion
light-years from us. The first follow-up spectroscopic observations,
conducted with the Multi-Mirror Telescope in Arizona, confirmed the object
as a high-redshift quasar. Subsequent observations with the Gemini North
and Keck I telescopes in Hawaii confirmed the MMT's finding, and led to
Gemini's detection of the crucial magnesium fingerprint -- the key to
nailing down the quasar's fantastic distance. However, the foreground
lensing galaxy and the quasar appear so close that it is impossible to
separate them with images taken from the ground owing to blurring by the
Earth's atmosphere. It took the Hubble Space Telescope to reveal that the
quasar image is split into three components by a faint lensing galaxy.
The quasar is ripe for future scrutiny. Astronomers also plan to use the
Atacama Large Millimetre/submillimetre Array, and eventually the James
Webb Space Telescope, to look within 150 light-years of the black hole and
directly detect the influence of the gravity from the black hole on gas
motion and star formation in its vicinity. Any future discoveries of very
distant quasars like J0439+1634 will continue to teach astronomers about the
chemical environment and the growth of massive black holes in the early
Universe.