The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No.394 2015 March 1
Here is the latest round-up of news from the Society for Popular Astronomy. The SPA is arguably Britain's liveliest astronomical society, with members all over the world. We accept subscription payments online at our secure site and can take credit and debit cards. You can join or renew via a secure server or just see how much we have to offer by visiting http://www.popastro.com/
UNUSUAL COMET SURVIVES CLOSE PASSAGE PAST THE SUN
Astronomers are puzzling over a comet that passed 'insanely close' to the Sun on February 19. At first glance it appeared to be a small object, doomed to disintegrate in the fierce heat. Instead, it has emerged apparently intact and is actually brightening as it recedes from the Sun. Unofficially, the icy visitor is being called SOHO 2875 because it is SOHO's 2,875th comet discovery. What is unusual about it is that it does not appear to be related to any other comet or comet family that we have on record. Most comets that SOHO sees belong to the Kreutz family. Kreutz sungrazers are fragments from the
break-up of a single giant comet many centuries ago. They get their name from the 19th-century German astronomer Heinrich Kreutz, who studied them in detail. SOHO 2875, however, is not such a fragment. Non-group comets of that sort appear a few times a year, so in that sense it is not too unusual, but this one is relatively bright. The comet has brightened dramatically and now is showing an increasingly impressive tail. Visibility from the ground in a few weeks is no longer out of the question.
MARS PLUME BAFFLES SCIENTISTS
European Space Agency
Plumes seen reaching high above the surface of Mars are troubling scientists who are studying the Martian atmosphere. On two separate occasions in 2012 March and April, amateur astronomers reported definite plume-like features developing on the planet. The plumes
were seen rising to altitudes of over 250 km above the same region of Mars on both occasions. By comparison, similar features seen in the past have not exceeded 100 km. At 250 km, there is little difference between the atmosphere and outer space, so the reported plumes are
extremely unexpected. The features developed in less than 10 hours, covering an area of up to 1000 x 500 km, and remained visible for around 10 days, changing their structure from day to day. None of the spacecraft orbiting Mars saw the features because of their viewing geometries and illumination conditions at the time. However, HubbleTelescope images taken between 1995 and 1999 and data-bases of amateur images spanning 2001 to 2014 revealed occasional clouds at the limb of Mars, albeit usually only up to 100 km in altitude. But one set of Hubble images from 1997 May 17 revealed an abnormally high plume, similar to that observed by the amateur astronomers in 2012. Scientists are now trying to determine the nature and cause of the plumes. One idea is that they are caused by a reflective cloud of water ice, carbon dioxide ice, or dust particles, but it would require exceptional deviations from standard atmospheric-circulation models to explain cloud formations at such high altitudes. Another idea is that they are related to auroral emission, and indeed aurorae have been previously observed at those locations, linked to a known region on the surface where there is a large anomaly in the crustal magnetic field. Further insights may be be possible after the arrival at Mars of a proposed ESA spacecraft that is scheduled for launch in 2016.
DIM STAR PASSED THROUGH OORT CLOUD 70,000 YEARS AGO
University of Rochester
A group of astronomers from the US, Europe, Chile and South Africa has suggested that 70,000 years ago a recently discovered dim star nicknamed Scholz's star is likely to have passed through the Solar System's distant cloud of comets, the Oort Cloud. No other star is known ever to have approached the Solar System so closely -- five times closer than the currently closest star, Proxima Centauri. Its trajectory suggests that 70,000 years ago it passed roughly 0.8 light-years from the Sun. The astronomers say that they are 98% certain that it went through what is known as the 'outer Oort Cloud' -- a
region at the edge of the Solar System where there are ever so many comets a mile or more across that are thought to give rise to long-period comets orbiting the Sun after their orbits are perturbed. Scholz's star has an unusual mix of characteristics: despite being fairly close ('only' 20 light-years away), it shows very slow tangential motion, that is, motion across the sky. The radial-
velocity measurements, however, show the star to be moving away from the Solar System at a considerable speed. Most stars so 'nearby' show much larger tangential motion. The small tangential motion and
proximity initially indicated that the star was most likely to be either moving towards a future close encounter with the Solar System, or it had 'recently' come close and was now moving away. Then the radial-velocity measurements showed that it is running away from the
Sun's vicinity -- so it must have had a close fly-by in the past.
To determine its trajectory the astronomers needed both pieces of data, the tangential velocity and the radial velocity. The measurements were carried out with spectrographs on large telescopes in both South Africa and Chile -- the Southern African Large Telescope (SALT) and the Magellan telescope at Las Campanas Observatory, respectively. The researchers traced the trajectory back in time to its position 70,000 years ago, when their models indicated that it came closest to
the Sun. Until now, the best candidate for the closest passage of a star to the Solar System was HIP 85605, which was predicted to come close to us in 240,000 to 470,000 years from now. But astronomers have since demonstrated that the originally supposed 20-light-year
distance of HIP 85605 was probably under-estimated by a factor of ten. From its more likely current distance of about 200 light-years, HIP 85605's newly calculated trajectory would not bring it within the Oort Cloud.
Researchers computed 10,000 trial orbits for Scholz's star, taking into account the star's position, distance, and velocity, the Milky Way galaxy's gravitational field, and the statistical uncertainties in all of those measurements. Of the 10,000 simulations, 98% showed the star passing through the outer Oort cloud, but only one of the
simulations brought the star within the inner Oort cloud, where it could trigger a 'comet shower'. While the close fly-by of Scholz's star probably had little impact on the Oort Cloud, other dynamically important Oort-Cloud perturbers may be lurking among nearby stars. The recently launched Gaia satellite is hoped to measure the distances and velocities of a thousand million stars. With the Gaia data, astronomers will be able to tell which other stars may have had a close encounter with us in the past or will do so in the distant future. Currently, Scholz's star appears as a small, dim red dwarf in the constellation Monoceros. However, at the closest point in its fly-by, it would have been a 10th-magnitude star. It shows magnetic activity, however, which can cause stars to flare and briefly become thousands of times brighter, so it is possible that the star may have been visible to the unaided eyes of our ancestors 70,000 years ago for minutes or hours at a time during rare flaring events. The star is
part of a binary system, consisting of the red-dwarf star (with mass about 8% that of the Sun) and a brown-dwarf companion (with mass about 6% of the Sun). Brown dwarfs are considered 'failed stars'; their masses are too low for them to fuse hydrogen in their cores like a star, but they are still much more massive than gas-giant planets like Jupiter. The formal name of the star is 'WISE J072003.20-084651.2'; however it has been nicknamed 'Scholz's star' after its discoverer -- astronomer Ralf-Dieter Scholz of the Leibniz-Institut fur Astrophysik
Potsdam (AIP) in Germany -- who first reported it in late 2013. The 'WISE' part of the designation refers to the Wide-field Infrared Survey Explorer mission, which mapped the entire sky in infrared light in 2010 and 2011, and the 'J-number' part refers to the star's celestial coordinates.
THE MISSING BROWN DWARF
Some pairs of stars consist of two normal stars with slightly different masses in a close orbit. When the star of slightly higher mass ages and expands to become a red giant, material is transferred to the other star and ends up surrounding both stars in a huge cloud of gas. When that cloud disperses the two move closer together and form a very tight pair with one white dwarf and one more normal star. One such stellar pair is called V471 Tauri. It is a member of the Hyades star cluster in Taurus and is estimated to be around 600 million years old and 163 light-years away. The two stars are very close and orbit one another every 12 hours. Twice per orbit one star
passes in front of the other, the eclipses causing regular changes in the brightness of the pair.
A team of astronomers used the ULTRACAM system on the New Technology Telescope at ESO to measure the brightness changes very precisely. The times of the eclipses were measured with an accuracy of better
than two seconds -- a big improvement on earlier measurements. The eclipse timings were not regular, but could be explained well by assuming that, orbiting both stars, there was a brown dwarf whose gravitational pull was disturbing the orbits of the stars. They also found hints that there might be a second small companion object. Up till now it has been impossible actually to image a faint brown dwarf so close to much brighter stars, but the new 'SPHERE' instrument on the Very Large Telescope allowed the team to look exactly where the brown-dwarf companion was expected to be. But they saw nothing, even though the images from SPHERE should easily have revealed it. There are many papers suggesting the existence of such circumbinary objects,
but the results in this case provide damaging evidence against them. If there is no orbiting object, then what is causing the odd changes to the orbit of the binary? Several theories have been proposed, and, while some of them have already been ruled out, it is possible that
the effects are caused by magnetic-field variations in the larger of the two stars, somewhat analogous to the smaller changes seen in the Sun.
KEPLER 432b IS A DENSE, MASSIVE PLANET
Two research teams have independently discovered a rare type of planet. The planet, called Kepler 432b, is one of the most dense and massive planets known so far. It has six times the mass of Jupiter, but is about the same size. The shape and the size of its orbit are also unusual for such a planet that is revolving around a giant star.
The majority of known planets associated with giant stars have orbits that are large and circular. With its small and highly eccentric orbit, Kepler 432b is a real maverick among such planets. The star around which Kepler 432b is orbiting has already exhausted the nuclear fuel in its core and is gradually expanding. Its radius is already
four times that of our Sun and it will get even larger in the future.
The eccentric orbit brings Kepler 432b very close to its host star at some times and much farther away at others, thus creating enormous temperature differences over the course of the planet's year, which corresponds to 52 Earth days. During the winter season, the temperature on Kepler 432b is roughly 500 degrees C; in the short
summer season, it can increase to nearly 1,000 degrees. Kepler 432b was identified as a transiting-planet candidate by the Kepler satellite. From our vantage point, a transiting planet passes in front of its host star, periodically dimming the received stellar light. Both groups of researchers used the 2.2-m telescope at Calar
Alto in Spain, and one also used the Nordic Optical Telescope on La Palma, to acquire the measurements needed to determine the planet's mass. The days of Kepler 432b are numbered, though, because in less
than 200 million years it will be swallowed by its continually expanding host star. The reason why we do not find other planets like Kepler 432b may be that, astronomically speaking, their lives are very short.
MISMATCHED BINARY STARS
Harvard-Smithsonian Center for Astrophysics
Many stars come in pairs. In particular, the most massive stars usually have companions. The twins tend to be somewhat equal partners when it comes to mass -- but not always. In a quest to find mismatched star pairs known as extreme-mass-ratio binaries, astronomers
have discovered a new class of binary stars. One star is fully formed while the other is still in its infancy. Astronomers have identified 18 extreme-mass-ratio binaries in the Large Magellanic Cloud. The more massive stars are 6 to 16 times the mass of the Sun, while the less-massive ones are 1 to 2 solar masses. The more massive a star is, the brighter it shines. That makes it difficult to identify extreme-mass-ratio binaries because the more massive star greatly outshines, and thereby hides, its companion. To combat that effect astronomers looked for eclipsing systems. When the fainter star
eclipses the brighter one, their combined light drops detectably. Such systems are rare because they require rather precise alignment as seen from here. The 18 systems so far recognized have orbits with periods of 3 to 9 days. A clue to the young nature of the systems came from an unusual feature in the data. The fainter star shows illumination phases, just like phases of the Moon, as the two stars orbit each other, indicating that the companion is reflecting the light of the brighter, more massive star. We only see phases because the fainter and less massive companion is not yet a fully-fledged
star. Astronomers describe it as being 'pre-main-sequence'.
A star forms when a giant clump of gas pulls together under its own gravity, growing denser and hotter until nuclear fusion ignites. That process happens faster for more massive stars. In the young systems this research identified, the more-massive star is already on the main
sequence, while the less-massive companion is not. As a result, that companion is puffier than it should be, since it is still contracting. That effectively lets the pre-main-sequence star act as a mirror, reflecting the brilliance of its partner. The discovery of the systems may offer insight into the formation and evolution of massive stars, close binaries, and star formation. The 18 systems were culled
from millions of stars observed in the Large Magellanic Cloud by the Optical Gravitational Lensing Experiment. Owing to their rarity, finding examples in our own galaxy will probably require an extensive survey with instruments such as the upcoming Large Synoptic Survey Telescope.
WORLD'S BIGGEST SOLAR TELESCOPE
Queen's University, Belfast
The Daniel K. Inouye Solar Telescope (DKIST), due to be completed in 2019, is being constructed by the US National Solar Observatory on Haleakala mountain in Maui, Hawaii. With a 4-m primary mirror, the telescope will be able to pick up unprecedented detail on the surface of the Sun -- the equivalent of being able to examine a penny from 100 km away. It is hoped that DKIST will address fundamental questions at the core of contemporary solar physics. It will do so by high-speed
(sub-second time-scales) spectroscopic and magnetic measurements of the solar photosphere, chromosphere and corona. Solar activity is of significance beyond astronomy -- it drives 'space weather' and has profound effects on global climate and communications.
COLLIDER HOPES FOR 'SUPER RESTART'
Researchers at the Large Hadron Collider say that a new particle that is even more exciting than the Higgs boson could be detected this year. The accelerator is due to come back into service this month after an upgrade that has given it a big boost in energy. That could cause the first so-called super-symmetric particle to appear in the
machine, with the most likely candidate being the gluino. Its detection would give scientists direct pointers to 'dark matter'. Super-symmetry is an addition to the Standard Model, which describes nature's fundamental particles and their interactions. Susy, as it is sometimes known, fills some gaps in the model and provides a basis to
unify nature's forces. It predicts each of the particles to have more massive partners. So the particle that carries light -- the photon -- would have a partner called the photino. The quark, the building block of an atom's protons and neutrons, would have a partner called
the squark. But when the LHC was colliding matter at its pre-upgrade energies, no sign of such super-particles was seen in the debris, which led to some consternation among theorists. Now, with the accelerator about to re-start in the coming weeks, there is high hope that the first evidence of Susy can be found. The machine is going to
double the collision energy, taking it into a domain where the theorists say the gluino really ought to emerge in sufficient numbers to be noticed. The gluino is the super-partner of the gluon, which 'glues' the quarks together inside protons and neutrons. The LHC's detectors would not see it directly. What they would track is its decay, which scientists would then have to reconstruct. But
importantly, the decay products should include the lightest and most stable super-particle, known as the neutralino -- the particle that researchers have proposed to makes up dark matter, the missing mass in the cosmos that gravitationally binds galaxies together on the sky but
which cannot be seen directly with telescopes. So, not only would super-symmetry proponents be elated because they would have their first super-particle, but science in general would have a firm foot on the road to understanding dark matter.
VARIABLE-STAR SECTION REPORT 2014
By Tony Markham, SPA Variable-Star Section Director
2014 was a very interesting year for many of the stars on the SPA Variable-Star Section's programme. The highlights of 2014 included:
- R Coronae Borealis brightening back from its very long and deep minimum. By October it had become visible in binoculars. It is not quite back to maximum yet, but it is getting close.
- An unusually faint maximum for the Mira-type variable Chi Cygni. In 2013, it produced a particularly bright maximum, reaching mag 3.8, but the 2014 maximum (in early July) only reached about mag 6.4 - possibly the faintest ever seen. The next maximum is due in 2015 August.
- Another eventful year for R Scuti. It still shows no sign of reverting back to its 'standard' pattern of alternate shallow and deep minima -- most minima continue to be of the deep variety. The latest minimum was under way by mid-February.
A selection of light-curves from 2014 for stars on the Variable-Star Section's programme can be found at:
Bulletin compiled by Clive Down
(c) 2015 the Society for Popular Astronomy
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