Topic: Early December Astronomy Bulletin

[Clive]

CRATER LAKES CARVED CANYONS ACROSS MARS
University of Texas at Austin

Today, most of the water on Mars is locked away in frozen ice caps. But
billions of years ago it flowed freely across the surface, forming rushing
rivers that emptied into craters, forming lakes and seas. New research has
found evidence that sometimes the lakes would accumulate so much water that
they overflowed and burst from their basins, creating catastrophic floods
that carved canyons very rapidly, perhaps in a matter of weeks. From
studies of rock formations from satellite images, scientists know that
hundreds of craters across the surface of Mars were once filled with water.
More than 200 of those 'paleo-lakes' have outlet canyons, tens to hundreds
of kilometres long and several kilometres wide, carved by water flowing from
the ancient lakes. However, until this study, it was unknown whether the
canyons were gradually carved over millions of years or carved rapidly by
single floods.

Using high-resolution photos taken by the Mars Reconnaissance Orbiter
satellite, the researchers examined the topography of the outlets and the
crater rims and found a correlation between the size of the outlet and the
volume of water expected to be released during a large flooding event.
If the outlet had instead been gradually whittled away over time, the
relationship between water volume and outlet size probably would not hold.
In total, the researchers examined 24 paleo-lakes and their outlet canyons
across the Red Planet. One of the paleo-lakes examined in the study, Jezero
Crater, is a potential landing site for NASA's Mars 2020 rover mission to
look for signs of past life. The team proposed the crater as a landing site
on the basis of prior studies that found that it held water for long periods
in Mars' past. While massive floods flowing from Martian craters might
sound like a scene from a science-fiction novel, a similar process occurs on
Earth when lakes dammed by glaciers break through their icy barriers. The
researchers found that the similarity is more than superficial. As long as
gravity is accounted for, floods create outlets with similar shapes whether
on Earth or Mars. Although big floods on Mars and Earth are governed by the
same mechanics, they fit into different geological paradigms. On Earth, the
slow-and-steady motion of tectonic plates dramatically changes the planet's
surface over millions of years. In contrast, the lack of plate tectonics on
Mars means that cataclysmic events -- like floods and asteroid impacts --
quickly create changes that can amount to near-permanent changes in the
landscape.


SPITZER REVEALS MORE ABOUT INTERSTELLAR VISITOR
NASA

In 2017 November, scientists pointed the Spitzer Space Telescope at the
object known as 'Oumuamua -- the first known interstellar object to visit
the Solar System. The infrared Spitzer was one of many telescopes pointed
at 'Oumuamua in the weeks after its discovery that October. 'Oumuamua was
too faint for Spitzer to detect when it looked more than two months after
the object's closest approach to the Earth in early September. However,
the 'non-detection' puts a new limit on how large the strange object can
be. The new size limit is consistent with the findings of a research paper
published earlier this year, which suggested that outgassing was responsible
for the slight changes in 'Oumuamua's speed and direction as it was tracked
last year. The authors of that paper concluded that the expelled gas acted
like a small thruster gently pushing the object. That determination was
dependent on 'Oumuamua being relatively smaller than typical Solar-System
comets. (The conclusion that 'Oumuamua experienced outgassing suggested
that it was composed of frozen gases, similar to a comet.)

'Oumuamua was first detected by the University of Hawaii's Pan-STARRS 1
telescope on Haleakala, Hawaii (the object's name is a Hawaiian word meaning
'visitor from afar arriving first), in 2017 October while the telescope was
surveying for near-Earth asteroids. Subsequent detailed observations
conducted by multiple ground-based telescopes and the Hubble Space Telescope
detected the sunlight reflected off 'Oumuamua's surface. Large variations
in its brightness suggested that 'Oumuamua is highly elongated and probably
less than 800 metres in its longest dimension. But Spitzer tracks asteroids
and comets using the infrared energy, or heat, that they radiate, which can
provide more specific information about an object's size than optical
observations of reflected sunlight alone would.

The fact that 'Oumuamua was too faint for Spitzer to detect sets a limit on
the object's total surface area. However, since the non-detection can not
be used to infer shape, the size limits are presented as what 'Oumuamua's
diameter would be if it were spherical. Using three separate models that
make different assumptions about the object's composition, Spitzer's
non-detection limited 'Oumuamua's 'spherical diameter' to 440 metres, 140
metres or perhaps as little as 100 metres. The wide range of results stems
from the assumptions about 'Oumuamua's composition, which influences how
visible (or faint) it would appear to Spitzer were it a particular size.
The new study also suggests that 'Oumuamua may be up to 10 times more
reflective than the comets in the Solar System -- a surprising result,
according to the paper's authors. Because infrared light is largely heat
radiation produced by 'warm' objects, it can be used to determine the
temperature of a comet or asteroid; in turn, that can be used to determine
the reflectivity of the object's surface -- what scientists call albedo.
An object with low reflectivity retains more heat than an object with high
reflectivity, so a lower temperature means a higher albedo. A comet's
albedo can change throughout its lifetime. When it passes close to the Sun,
a comet's ice warms and turns directly into a gas, sweeping dust and dirt
off the comet's surface and revealing more reflective ice. 'Oumuamua had
been travelling through interstellar space for millions of years, far from
any star that could refresh its surface. But it may have had its surface
refreshed through such 'outgassing' when it made an extremely close approach
to the Sun, a little more than five weeks before it was discovered. In
addition to sweeping away dust and dirt, some of the released gas may have
covered the surface of 'Oumuamua with a reflective coat of ice and snow --
a phenomenon that has also been observed in comets in the Solar System.
'Oumuamua is now on its way out of the Solar System -- almost as far from
the Sun as Saturn's orbit -- and is well beyond the reach of any existing
telescopes.


PLANET ORBITING BARNARD'S STAR
ESO

The nearest single star to the Sun hosts an exo-planet at least 3.2 times
as massive as the Earth -- a so-called super-Earth. One of the largest
observing campaigns to date using data from a world-wide array of tele-
scopes, including the planet-hunting HARPS instrument, have revealed this
frozen, dimly lit world. The newly discovered planet is the second-closest
known exo-planet to the Earth. Barnard's star is the fastest-moving star in
the night sky. The planet, designated Barnard's Star b, now steps in as the
second-closest known exo-planet to the Earth. The gathered data indicate
that the planet could be a super-Earth, having a mass at least 3.2 times
that of the Earth, and orbits its host star in roughly 233 days. Barnard's
Star, the planet?s host star, is a red dwarf, a cool, low-mass star, which
only dimly illuminates this newly-discovered world. Light from Barnard's
Star provides its planet with only 2% of the energy the Earth receives from
the Sun. Despite being relatively close to its parent star -- at a distance
only 0.4 times that between Earth and the Sun -- the exoplanet lies close to
the snow line, the region where volatile compounds such as water condense
into solid ice. That freezing, shadowy world could have a temperature of
170 K, making it inhospitable for life as we know it. Named for astronomer
E. E. Barnard, Barnard's Star is the closest single star to the Sun. While
the star itself is ancient -- probably twice the age of our Sun -- and
relatively inactive, it also has the fastest apparent motion of any star in
the night sky. Super-Earths are the most common type of planet to form
around low-mass stars such as Barnard's Star, lending credibility to its
newly discovered planetary candidate. Furthermore, current theories of
planetary formation predict that the snow line is the ideal location for
such planets to form.

Previous searches for a planet around Barnard's Star have had disappointing
results -- this recent breakthrough was possible only by combining measure-
ments from several high-precision instruments mounted on telescopes all over
the world. The astronomers used the Doppler effect to find the exo-planet
candidate. While the planet orbits the star, its gravitational pull causes
the star to wobble. HARPS can detect changes in the star's velocity as
small as 3.5 km/h -- about walking pace. That radial-velocity method has
never been used previously to detect a similar super-Earth-type exo-planet
in such a large orbit around its star.


GRAVITATIONAL WAVES FROM MERGED HYPER-MASSIVE NEUTRON STAR
RAS

For the first time astronomers have detected gravitational waves from a
merged, hyper-massive neutron star. Gravitational waves were predicted by
Albert Einstein in his General Theory of Relativity in 1915. The waves
are disturbances in space-time, generated by rapidly moving masses, which
propagate out from the source. By the time the waves reach the Earth,
they are incredibly weak and their detection requires extremely sensitive
equipment. It took scientists until 2016 to announce the first observation
of gravitational waves, using the Laser Interferometer Gravitational Wave
Observatory (LIGO) detector. Since that seminal result, gravitational waves
have been detected on a further six occasions. One of those, GW170817,
resulted from the merger of two stellar remnants known as neutron stars.
Those objects form after stars much more massive than the Sun explode
as supernovae, leaving behind cores of material packed to extraordinary
densities. At the same time as the burst of gravitational waves from the
merger, observatories detected emission in gamma rays, X-rays, ultraviolet,
visible light, infrared and radio waves -- an unprecedented observing
campaign that confirmed the location and nature of the source. The initial
observations of GW170817 suggested that the two neutron stars merged into a
black hole, an object with a gravitational field so powerful that not even
light can escape its grasp. Astronomers set out to check that, using a
novel technique to analyze the data from LIGO and the Virgo gravitational
wave detector sited in Italy. Their detailed analysis shows the H1 and L1
detectors in LIGO, which are separated by more than 3,000 kilometres,
simultaneously picked up a descending 'chirp' lasting around 5 seconds.
Significantly, the chirp started between the end of the initial burst of
gravitational waves and a subsequent burst of gamma rays. Its low frequency
(less than 1 KHz, reducing to 49 Hz) suggests that the merged object spun
down to become a larger neutron star, rather than a black hole. There are
other objects like that, with their total mass matching known neutron-star
binary pairs. But the team has now confirmed their origin. Gravitational-
wave astronomy, and eking out the data from every detection, will take a
further step forward next year, when the Japanese Kamioka Gravitational-Wave
Detector (KAGRA) comes online.


ELUSIVE STAR BEHIND SUPERNOVA FOUND
NASA/Goddard Space Flight Center

Astronomers may have finally uncovered the long-sought progenitor to a
specific type of exploding star by sifting through Hubble Space Telescope
archival data. The supernova, called a Type Ic, is thought to detonate
after its massive-star precursor has shed or been stripped of its outer
layers of hydrogen and helium. Such stars could be among the most massive
known -- at least 30 times more massive than the Sun. Even after shedding
some of their material late in life, they are expected to be big and bright.
So it was a mystery why astronomers had not been able to find any such stars
in pre-explosion images. Finally, in 2017, astronomers got lucky. A nearby
star ended its life as a Type Ic supernova. Two teams of astronomers pored
through the archive of Hubble images to uncover the putative precursor star
in pre-explosion photos taken in 2007. The supernova, catalogued as SN
2017ein, appeared near the centre of the 'nearby' spiral galaxy NGC 3938,
located roughly 65 million light-years away. That potential discovery could
yield insight into stellar evolution, including how the masses of stars are
distributed when they are born in batches. An analysis of the object's
colours shows that it is blue and extremely hot. On the basis of that
assessment, both teams suggest two possibilities for the source's identity.

The progenitor could be a single massive star between 45 and 55 times the
mass of the Sun. Another idea is that it could have been a massive binary-
star system in which one of the stars was between 60 and 80 solar masses and
the other roughly 48 Suns. In that latter scenario, the stars are orbiting
closely and interact with one another. The more massive star is stripped of
its hydrogen and helium layers by the close companion, and eventually
explodes as a supernova.

The possibility of a massive double-star system is a surprise. Expectations
on the identity of the progenitors of Type Ic supernovae have been a puzzle.
Astronomers have known that the supernovae were deficient in hydrogen and
helium, and initially proposed that some massive stars shed material in a
strong wind (a stream of charged particles) before they exploded. When they
did not find the progenitor stars, which should have been extremely massive
and bright, they suggested a second method to produce the exploding stars
that involves a pair of close-orbiting, lower-mass binary stars. In that
scenario, the more massive star is stripped of its hydrogen and helium by
its companion. But the 'stripped' star is still massive enough eventually
to explode as a Type Ic supernova. Type Ic supernovae are just one class
of exploding star. They account for about 20 per cent of massive stars that
explode from the collapse of their cores. Astronomers caution that they
won't be able to confirm the source's identity until the supernova fades in
about two years. The astronomers hope to use either Hubble or the upcoming
NASA James Webb Space Telescope to see whether the candidate progenitor star
has disappeared or has significantly dimmed. They will also be able to
separate the supernova's light from that of stars in its environment to
calculate a more accurate measurement of the object's brightness and mass.