2010
Posted in: Astronomy by Mike Simonsen (No Comments ») 
Symbiotic variables are binary pairs in orbit around each other inside a common envelope. Credit: NASA
November 23rd,
astronomers from the Asiago Novae and Symbiotic
Stars collaboration announced recent changes in the symbiotic variable
star, AX Persei, could indicate the onset of a rare
eruption of this system. The last major eruption took place between 1988 and1992. In the (northern hemisphere) spring of 2009, AX Per underwent a short outburst that was the first time since 1992 this star had experienced a bright phase. Now AX Per is on the rise again. This has tempted astronomers to speculate that another major eruption could be in the making.
Symbiotic
variable stars are
binary systems whose members are a hot compact
white dwarf in a wide
orbit around a cool
giant star. The orbital periods of symbiotic variables are between 100 and 2000 days. Unlike
dwarf novae, compact
binaries whose periods are measured in hours, where mass is transferred directly via an
accretion disk around the white dwarf, siphoned directly from the surface of the secondary, in symbiotic variables the pair orbit each other far enough away that the mass exchanged between them comes from the strong stellar wind blowing off the
red giant. Both stars reside within a shared cloud of gas and dust called a common envelope.
When astronomers look at the spectra of these systems they see a very complex picture. They see the spectra of a hot compact object superimposed on the spectra of a cool giant star tangled up with the spectrum of the common envelope. The term “symbiotic” was coined in 1941 to describe stars with this combined spectrum.
Typically, these systems will remain quiescent or undergo slow, irregular changes in brightness for years at a time. Only occasionally do they undergo large outbursts of several magnitudes. These outbursts are believed to be caused either by abrupt changes in the
accretion flow of gas onto the primary, or by the onset of thermonuclear burning of the material piled up on the surface of the white dwarf. Whatever the cause, these major
eruptions are rare and unpredictable.
The AAVSO light curve of AX Persei from 1970 to November 2010. In the middle is the eruption of 1988-1992. The precursor outburst is the sudden narrow brightening left of the larger eruption. To the right of the light curve you can see the 2009 brightening event. Is this a precursor to a coming major eruption? Credit: AAVSO
AX Per underwent a short-duration
flare about one year before the onset of the major 1988-1992 outburst. Now astronomers are tempted to speculate. Could the 2009 short outburst be a similar precursor type event? The present rise in brightness by AX Per might be the onset of a major outburst event similar to that in 1988-1992. The watch begins now, and professional and amateur
variable star observers will be keeping a close eye on AX Per in the coming months.
Ranging from 8.5 to 13th magnitude, AX Persei is visible to anyone with an 8-inch
telescope, and if it erupts to maximum it will be visible in
binoculars. You can monitor this interesting star and report your observations to the
American Association of Variable Star Observers (AAVSO). Charts with comparison stars of known brightness can be plotted and printed using the
AAVSO’s Variable Star Chart Plotter, VSP.
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2010
Posted in: Astronomy by Mark Thompson (7 Comments ») 
Artists impression of a binary star system (courtesy NASA)
have recently discovered an exotic
star system which has shed some light on the mass and age of one of the systems rare stellar components. Using data from World’s largest optical
telescope, the
Very Large Telescope (VLT) in Chile, the team has had a new insight into the properties of the unusual T-
dwarf stars. Its believed there are around 200 of these stars in
our Galaxy but this is the first one to be discovered as part of a
binary star system which has given astronomers an extra special insight into their properties.
The system, that has been dubbed the ‘Rosetta Stone’ for T-dwarf stars, was studied by a team led by Dr Avril
Day-Jones of the Universidad de Chile and included Dr David Pinfield of the University of Hertfordshire and other astronomers from the University of Montreal. They first identified the
dwarf star, which has a
temperature of around 1000 degrees compared to our
Sun at 5500 degrees, in the UKIRT Infra-red Deep Sky Survey while searching for the coolest objects in the
Galaxy. They found to their surprise, that the T-dwarf star was joined by a companion
blue star, later revealed to be a cool
white dwarf. The pair have now been given the ‘memorable‘ name of 1459+0857 A and B.
The
binary system is the first of its type to be discovered as, whilst both
types of stars have been identified individually, they have never been found gravitationally bound to one another. The two stars are about 0.25
light years apart (compared to our
nearest star at just over 4 light years away) but despite the distance and the weak gravitational interaction between the stars, they remain in
orbit and will do so until the two stars slowly fizzle out to a dark and cool death.
The T-dwarf stars are an exotic breed which lie on the border between
a star and a
planet, much like our own
Solar System giant,
the planet Jupiter. They are not massive enough for nuclear reactions to take place in the
core so from their birth, they simply cool and fade. The presence of methane too is a pointer to their cool nature as it gets destroyed at higher temperatures and so is not found in fully fledged stars. The companion star, the white dwarf, is a star at the end of its life. When average stars like
the Sun die, their outer layers will blow off into
space, leaving behind a planetary
nebula and a cooling, dying stellar core. With the new
binary system, the
white dwarf star lost a significant amount of matter and so its gravitational pull weakened, slowly increasing the distance between the two companions. The planetary nebula has long since dissipated and from looking at the white dwarf, we can tell that this weak, fragile system has existed for several billions of year.
The
discovery of this binary system has allowed the team to test the
physics of cool stellar atmospheres that exist on these strange, failed stars and to measure its mass and age, providing an opportunity for astronomers to study other low mass objects. “The discovery is an important stepping stone to improve astronomers ability to measure the properities of low-mass star like objects (brown
dwarfs). ” Dr Pinfield told
Universe Today. “Only be accurately measuring these properties will we be able to understand how these objects form and evolve over time. Brown dwarfs are just as numerous as stars in
the Milky Way, but their nature is not yet well understood. As such, this new discovery is helping astronomers interpret an important but mysterious population of objects that are quite common in our
Galactic backyard.”
2010
Posted in: Cassini, Enceladus by Nancy Atkinson (3 Comments ») 
At least four distinct plumes of water ice spew out from the south polar region of Saturn's moon Enceladus. Credit: NASA/JPL/Space Science Institute
What is going on inside
Saturn’s
moon Enceladus and what powers the icy geysers and jets? A pair of upcoming
flybys by the Cassini
spacecraft could help answer those questions. Radio instruments on board will measure the gravity field of Enceladus and focus particularly on the very intriguing south polar
hot spot.
Of course, the success of these flybys hinges on the Cassini mission controllers being able to wake up the
dormant spacecraft which has been in safe mode since November 2. Teams will attempt to get Cassini up and running again tomorrow, November 24, and they don’t anticipate any problems.
Cassini went into the protective standby mode and the likely cause of the problem was a faulty program code line, or a flipped bit in the spacecraft’s command and data system computer.
The upcoming flybys of Enceladus will put Cassini very close – about 48 kilometers (30 miles) above the surface. The first will take place on November 30. Pairing this
flyby with one on April 28, should provide scientists enough information to determine the nature of the interior right under the hot spot. The next flyby on December 21, Cassini will make 50-kilometer pass over the north pole of Enceladus. The fields and particles instruments will be trying to “sniff” anything coming from
the moon.
There will be two three-hour “wing” observations before and after closest-approach (from five to eight hours from closest approach on either side), and then three more hours centered directly around closest approach. The Cassini team is throwing almost the entire gamut of instruments into the flyby program, between radio science (RSS) observations, the imaging science system (
ISS) and composite
infrared spectrometer (CIRS) which will observe this moon on the inbound leg, and CIRS and the visible and infrared mapping spectrometer (VIMS) which will take data on the outbound leg, with other optical remote sensing and fields, particles and waves instruments also taking data.
2010
One of the greatest potentials of transiting exoplanets is the ability to monitor the spectra and examine the composition of the
planet‘s
atmosphere. This has been done already for
HD 18733b and
HD 209458b. In a new article by a team of
astronomers at Keele University in the UK,
absorption spectroscopy has been applied to the unusual
exoplanet WASP-17b, which is known to
orbit retrograde.
Not only does the spectra tell astronomers the atmospheric composition, but can also give an understanding of the the composition, but can also be indicative of how
the atmosphere absorbs the light from the
star and how heat is transferred around
the planet. Additionally, since the atmosphere will absorb differently at different wavelengths, this gives differences in the timing of the eclipse and can be used to probe the radius of the planet more tightly as well as potentially examining the layering of the atmosphere.
For their investigation, the team concentrated on the sodium doublet lines at 5889.95 and 5895.92 Å. Observations were taken by the Very Large
Telescope in Chile to observe 8 transits of the planet in June of 2009. The planet itself has a short
orbit of 3.74 days.
Applying these spectroscopic techniques to WASP-17b, the team discovered the presence of sodium in the atmosphere. Yet the absorption wasn’t as strong as expected based on models using formation mechanisms from a
nebula with
solar composition and forming a planet with a cloudless atmosphere. Instead, the team describes 17b’s atmosphere as “sodium-depleted” similar to HD 209458b.
An additional observation was that the depth of seeing dropped off when using certain filters with different bandwidths (ranges of allowed wavelengths). The team noted that at bandwidths greater than 3.0 Å, the amount of sodium absorption seen nearly disappeared. Since this property is related to how much atmosphere the light travels through, this allowed the team to speculate that this may be indicative of clouds in the upper layers of the atmosphere.
Lastly, the team speculated as to the reason on the lack of sodium in the atmosphere. They proposed that energy from the star ionizes sodium on the
day side. The motion of the atmosphere carrying it to the night side would then allow it to condense and be removed from the atmosphere. Since giant exoplanets in such tight
orbits would likely be tidally locked, the sodium would have little chance to return to the day side and be brought back into the atmosphere.
While the examination of
extrasolar atmospheres is undoubtedly new and will certainly be revised as the number of explored atmospheres increases, these pioneering studies are among the first that can allow astronomers directly test predictions of planetary atmospheres which, until recently have been solely based on observations of our own
solar system. More generally, this will allow us to develop a fuller understanding of how
planets evolve.
nya ia
