Unprecedented Eruption Catches Astronomers By Surprise

Artists rendering of a symbiotic recurrent nova. Image credit: David A. Hardy & PPARC

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An alert was raised March 11 when Japanese amateur astronomers announced what might have been the discovery of a new 8th magnitude nova in the constellation of Cygnus. It was soon realized that this eruption was not what it appeared to be. It was actually the unexpected nova-like erruption of a known variable star, V407 Cygni. Typically varying between 12th and 14th magnitude, V407 Cyg is a rather mundane variable star. So what caused this well-behaved star to suddenly go ballistic?

V407 Cyg is a symbiotic variable. These are close, interacting binary pairs usually containing a red giant and a hotter, smaller white dwarf. They orbit a common center of gravity inside a shared nebulosity. A typical symbiotic variable consists of an M type giant transferring matter to a hot white dwarf via its stellar wind. This wind is ionized by the white dwarf, giving rise to the symbiotic nebula.

Symbiotic variables are complex systems with many sources of variability. They can vary periodically due to the binary motion, the red giant can vary due to pulsation, the stars may be obscured by circumstellar dust, or the light emitted my change due to the formation of giant star spots. The white dwarf component may glow more or less constantly as it accretes material from the red giant and heats it up at a steady rate, or the material may form an accretion disk around the white dwarf, like in dwarf novae. Mass accreted onto the white dwarf can result in flickering and quasi-periodic oscillations. If there is a sudden increase in the rate of accretion, or the material in the accretion disk reaches a point of instability and crashes down onto the surface of the white dwarf the symbiotic system may undergo a nova-like eruption.

About 20% of symbiotics consist of a Mira-type variable as the giant of the pair. These binaries reside in much dustier envelopes. V407 Cyg is one of these dusty, Mira-type symbiotics. Its typical variation of a few magnitudes is due mainly to the pulsation of the Mira component of the system. Astronomers had never before witnessed a nova-like outburst of this interacting binary. You can imagine their surprise when Japanese amateurs, searching for novae along the galactic plane, suddenly detected this mild mannered, dusty Mira, symbiotic variable glowing nearly 100 times brighter than ever before.

That was just the beginning of the story. The first new spectra taken of the system, on March 13th, was different from any ever recorded for this star or any other symbiotic Mira variable in outburst. The normal absorption spectra of the Mira star was completely overwhelmed by the blue continuum of the outbursting white dwarf. The characteristics of the emission spectra revealed two distinct types of activity. One was the relatively slow ionized wind of the Mira star. The other looked like the fast expanding ejecta of a nova outburst. In fact, the spectrum looked remarkably similar to the symbiotic recurrent novae, RS Ophiuchi.

Typical outbursts of known symbiotic binaries, and symbiotic Miras in particular, usually exhibit a very slow rise to maximum, taking months, and no real significant mass ejection. This appears to be a much more quickly evolving and violent event, more like the eruptions of the recurrent novae RS Oph and T CrB. V407 Cyg may join this rare class of symbiotic recurrent novae.

As if that weren’t enough, another twist was added to the story on March 19th, when the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope detected the star in gamma-rays, something never observed in a symbiotic system before. The gamma-rays could be caused by shock driven acceleration of the ejected material, and its capture by strong magnetic fields within the system.

Like many novae and recurrent novae outbursts, this eruption may last for weeks or months and the variation in light output could be quite complex and interesting. Because the giant secondary is losing mass, the system is likely to have a large amount of circumstellar material. The ejected shell from the nova explosion on the white dwarf will interact with this material as the shell propagates outward, and will likely produce a wide variety of variable phenomena.

V407 Cyg has our attention now, and professional and amateur astronomers will be keeping a close eye on it from now on.

High School Students Get Published in Astrophysics Journal

From the left: Klaus Beuermann (group leader), Jens Diese (back,teacher), and the high-school students Joshua Zachmann (front), Alexander-Maria Ploch (back), Sang Paik (front). JD, JZ, and AMP are from the Max-Planck-Gymnasium, SP is from the Felix-Klein-Gymnasium.

High school students from Germany have now done what many scientists strive for: had their research work published by a science journal. The Astronomy & Astrophysics science journal published a paper co-authored by three students who observed the light variations of the faint (19th magnitude) cataclysmic variable EK Ursae Majoris (EK UMa) over two months. Led by astronomer Klaus Beuermann from the University of Göttingen, and the students’ high school physics teacher, the team made use of a remotely-controlled 1.2-meter telescope in Texas. Astronomy & Astrophysics says the team “presents an accurate, long-term ephemeris,” and that “they participated in all the steps of a real research program, from initial observations to the publication process, and the result they obtained bears scientific significance.”

The students, Joshua Zachmann, Alexander-Maria Ploch, Sang Paik and their teacher, Jens Diese, made observations, analyzed the CCD images, produced and interpreted light curves, and looked at archival satellite data. Beuermann, the astronomer they worked with said, “Although it is fun to perform one’s own remote observations with a professional telescope from the comfort of a normal school classroom, it is even more satisfying to be involved in a project that provides new and publishable results rather than to perform experiments with predictable outcomes.”

Cataclysmic variable research is a field where the contributions of small telescopes has a long tradition. Cataclysmic variables are extremely close binary systems containing a low-mass star whose material is being stripped off by the gravitational pull of a white dwarf companion. Due to the transfer of matter between the stars, these systems vary dramatically in brightness on timescales in the whole range between seconds and years. This largely unpredictable variability makes them ideal targets for school projects, particularly since professional observatories are generally unable to provide enough observation time for regular monitoring.

An accurate ephemeris is needed to keep track of the orbital motions of the two stars, but none was available because EK UMa is faint in the optical range and requires a long-term observation of the light variations. The strong magnetic field of the white dwarf turns the light of the hot matter striking the surface of the white dwarf into two “lighthouse” beams. By measuring the times of the minimum between the beams, the group was able to determine an orbital period accurate enough to keep track of the eclipse that took place in 1985, over 100 000 cycles earlier. By combining their own measurements with those made by the Einstein, ROSAT, and EUVE satellites, they estimated the orbital period over 137 000 cycles to an accuracy of a tenth of a millisecond. Surprisingly, the orbital period is extremely stable, although the period of such very close binaries is expected to vary due to the presence of third bodies and magnetic activity cycles on the companion star.

The team’s paper: (not yet available) A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris, by K. Beuermann, J. Diese, S. Paik, A. Ploch, J. Zachmann, A.D. Schwope, and F.V. Hessman.

Source: Astronomy & Astrophysics

Betelgeuse

Betelgeuse. Image credit: Hubble

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Betelgeuse is the ninth brightest star in the sky, and the second brightest in the constellation of Orion (it’s the red one, on the opposite side of the Belt from Rigel, which is the blue one, and the brightest).

With a mass of some 20 sols (= the mass of 20 Suns), Betelgeuse is evolving rapidly, even though it’s only a few million years old. It’s now a red supergiant, burning helium in a shell, and (very likely) burning carbon in another shell (closer to the nucleus), and (possibly) oxygen, silicon, and sulfur in other nested shells (like Russian dolls).

Betelgeuse is enormous … if it were where the Sun is, all four inner planets would be inside it! Because it’s so big, and is only approx 640 light-years away, Betelgeuse appears to about 1/20 of an arcsecond in size; this made it an ideal target for optical interferometry. And so it was that in 1920 Michelson and Pease used the 100″ Mt Wilson telescope, with a 20 m interferometer attached to the front, to measure Betelgeuse’s diameter.

The Hubble Space Telescope imaged Betelgeuse directly, in 1995, in the ultraviolet (see above). Why the UV? Because ground-based telescopes can’t make such observations, and because the Hubble’s resolution is greatest in the UV.

Since the 1920s Betelgeuse has been observed, from the ground, by many different optical interferometers, at many wavelengths. Its diameter varies somewhat, as does its brightness (Herschel is perhaps the first astronomer to describe its variability, in 1836). It also has ‘hotspots’, which are ginormous.

Betelgeuse is also shedding mass in giant plumes that stretch to over six times its diameter. Although these plumes will certainly cause it to ‘slim down’, they won’t be enough to stop its core turning to iron (when the silicon there is exhausted, if it hasn’t already done so). Not long afterwards, perhaps within the next thousand years or so, Betelgeuse will go supernova … making it the brightest and most spectacular supernova visible from Earth in perhaps a million years. Fortunately, because we are not looking directly down on its pole, when Betelgeuse does go bang, we won’t be fried by a gamma ray burst (GRB) which may occur (while a core collapse supernova can cause one kind of GRB, it is not yet known if all such supernovae produce GRBs; in any case, such a GRB is one of a pair of jets which rip through the poles of the dying star).

AAVSO has an excellent article on Betelgeuse, and COAST’s (Cambridge Optical Aperture Synthesis Telescope) webpage on its observations of Betelgeuse gives a good summary of one interferometric technique (and some great images too!).

Universe Today has many stories on just about every aspect of Betelgeuse, from its varying size (The Curious Case of the Shrinking Star), the bubbles it’s blowing and its plumes (Closest Ever Look at Betelgeuse Reveals its Fiery Secret), featured in What’s Up This Week, to the bow shock it creates in the interstellar medium (The Bow Shock of Betelgeuse Revealed).

Astronomy Cast’s The Life of Other Stars is a whole episode on the evolution of stars other than the Sun.

References:
http://en.wikipedia.org/wiki/Betelgeuse
http://www.solstation.com/x-objects/betelgeuse.htm

Help Solve the Mystery of Epsilon Aurigae with Citizen Sky

ESO Online Digitized Sky Survey

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We’ve written about Epsilon Aurigae before, but this mysterious star is just now beginning to dim, so we wanted to remind everyone that they can be involved in real science and help solve a mystery! The variable star Epsilon Aurigae is now beginning its puzzling transformation that happens every 27 years. “That means the last time Epsilon Aurigae had an eclipse we were all rockin’ big hair and sporting shoulder pads in all of our clothes,” said Rebecca Turner, coordinator for a special project for the IYA organized by the American Association of Variable Star Observers (AAVSO). Astronomers can’t figure out why this mysterious star dims on a regular basis, so to help solve the mystery they are calling for assistance from thousands of citizen scientists.

That means you can help contribute to real astronomical research!

Since its discovery in 1821, the supergiant star Epsilon Aurigae has dipped in brightness like clockwork every 27.1 years as it is eclipsed by a very large companion object. But based on the shape of the lightcurve and the spectra that have taken of the system, astronomers can’t figure out what exactly what kind of object is eclipsing the star. Another strange feature of the lightcurve is that there is a slight brightening in the middle of the eclipse.

“The leading theory is that the secondary is surrounded by a large opaque disk,” said Turner, on the July 7 episode of the 365 Days of Astronomy podcast. “This would explain why light from the secondary doesn’t seem to be showing up in spectra. The disk seems to have a hole in the center, which would account for the mid-eclipse brightening. Current thinking is that perhaps the center of the disk is home to 2 less luminous, tightly orbiting stars. This tight orbit could create what astronomers are calling a gravitational eggbeater effect – creating that hole in the disk. Theories of a large planet falling into the stars at the center of the disk have also been introduced recently.”

Sky map of Epsilon Aurigae
Sky map of Epsilon Aurigae

Epsilon Aurigae is a bright star that can be seen with the unaided eye even in bright urban areas of the Northern Hemisphere from fall to spring. But it is also too bright for most professional telescopes to observe, so this is where the public comes in.

“It’s not just amateurs with fancy telescopes and CCDs or photoelectric photometers that are needed for this experiment,” said AAVSO’s Mike Simonsen. “People with just their eyes or a pair of binoculars can contribute to understanding this weird star by observing epsilon Aurigae over the next two years and reporting their observations to AAVSO.”

A diagram of the most popular model of the epsilon Aurigae system, by Jeff Hopkins:
A diagram of the most popular model of the epsilon Aurigae system, by Jeff Hopkins:

For this project, a new website has been launched called “Citizen Sky”, and all you need are a good pair of eyes, and a finder chart, which can be found on the website. No previous astronomical experience is necessary.

The project is supported by a three-year grant from the National Science Foundation to recruit, train, and coordinate public participation in this project. What makes this project different from previous citizen science projects is its emphasis on participation in the full scientific method. Participants are not being asked simply to collect data. They will also be trained to analyze data, create and test their own hypotheses, and to write papers for publication in professional astronomy journals. Participants can work alone on all phases of the project or they can focus on one stage and team up with others.

Epsilon Aurigae is just now beginning to dim. It will remain faint during all of 2010 before slowly regaining its normal brightness by the summer of 2011.

The lead astronomer for this project is Dr. Robert Stencel, the William Herschel Womble Professor of Astronomy at Denver University. Dr. Bob, as the amateur astronomy community knows him, studied the last event in 1982-84 while working at NASA. “This is truly an amazing star system. It contains both a supergiant star and a mysterious companion. If the supergiant was in our solar system, its diameter would extend to Earth, engulfing us,” Stencel said. “The companion only makes its presence known every 27 years and is a type of ‘dark matter’ in that we indirectly detect its presence but don’t know what it is.

“To make things even more fun, we also have some evidence of a substantial mass, perhaps a large planet, spiraling into the mysterious dark companion object. Observations during the upcoming eclipse will be key to understanding this and predicting what will happen if the putative planet does eventually fall into the star,” Dr. Bob added.

Here’s a video with Rebecca Turner explaining more about Citizen Sky.

For more on Epsilon Aurigae, see this page from AAVSO
Citizen Sky

Sources: 365 Days of Astronomy,