Posted on by Nicholos Wethington

Longstanding Cepheid Mass Mystery Finally Solved

An artist's impression of the binary system OGLE-LMC-CEP0227, which contains a classical Cepheid variable star. By measuring this system, astronomers were able to settle a longstanding debate over the masses of these types of stars. Image credit: ESO/L. Calçada

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Cepheid variable stars – a class of stars that vary in brightness over time – have long been used to help measure distances in our local region of the Universe. Since their discovery in 1784 by Edward Pigott, further refinements have been made about the relationship between the period of their variability and their luminosity, and Cepheids have been closely studied and monitored by professional and amateur astronomers.

But as predictable as their periodic pulsations have become, a key aspect of Cepheid variables has never been well-understood: their mass. Two different theories – stellar evolution and stellar pulsation – have given different answers as to the masses that these stars should be. What has long been needed to correct this error was a system of eclipsing binary stars that contained a Cepheid, so that the orbital calculations could yield the mass of the star to a high degree of accuracy. Such a system has finally been discovered, and the mass of the Cepheid it contains has been calculated to within 1%, effectively ending a discrepancy that has persisted since the 1960s.

The system, named OGLE-LMC-CEP0227, contains a classical Cepheid variable (as opposed to a Type II Cepheid, which is of lower mass and takes a different evolutionary track) that varies over 3.8 days. It is located in the Large Magellanic Cloud, and as the stars orbit each other over a period of 310 days, they eclipse each other from our perspective on Earth. It was detected as part of the Optical Gravitational Lensing Experiment, and you can see from the acronym soup that this yields the first part of the name, the Large Magellanic Cloud the second, and CEP stands for Cepheid.

A team of international astronomers headed by Grzegorz Pietrzynski of Universidad de Concepción, Chile and Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland measured the spectra of the system using the MIKE spectrograph at the 6.5-m Magellan Clay telescope at the Las Campanas Observatory in Chile and the HARPS spectrograph attached to the 3.6-m telescope of the European Southern Observatory at La Silla.

The team also measured the changes in brightness and slight red and blueshift of the light from the stars as they orbited each other, as well as the pulsing of the Cepheid. By taking all of these measurements, they were able to create a model of the masses of the stars that should yield the orbital mechanics of the system. In the end, the mass predicted by stellar pulsation theory agreed much more with the calculated mass than that predicted by stellar evolution theory. In other words, stellar pulsation theory FTW!!

They published their results today in a letter to Nature, and write in the conclusion of the letter: “The overestimation of Cepheid masses by stellar evolution theory may be the consequence of significant mass loss suffered by Cepheids during the pulsation phase of their lives – such loss could occur through radial motions and shocks in the atmosphere. The existence of mild internal core mixing in the main-sequence progenitor of the Cepheid, which would tend to decrease its evolutionary mass estimate, is another possible way to reconcile the evolutionary mass of Cepheids with their pulsation mass.”

Cepheid variables take their names from the star Delta Cephei (in the constellation Cepheus), which was discovered by John Goodricke to be a variable star a few months after Pigott’s discovery in 1784. There are many different types of variable stars, and if you are interested in learning more or even participating in observing and recording their variability, the American Association of Variable Star Observers has a wealth of information.

Source: ESO, original Nature letter

Posted on by Mike Simonsen

The Furor over FUORs

FU Orionis and its associated nebula. Image cedit: ESO

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In 1937, an ordinary 16th magnitude star in the constellation Orion began to brighten steadily. Thinking it was a nova, astronomers were astounded when the star just kept getting brighter and brighter over the course of a year. Most novae burst forth suddenly and then begin to fade within weeks. But this star, now glowing at 9th magnitude, refused to fade. Adding to the puzzle, astronomers could see there was a gaseous nebula nearby shining from the reflected light of this mysterious star, now named FU Orionis. What was this new kind of star?

FU Ori has remained in this high state, around 10th magnitude ever since. Because this was a form of stellar variability never seen before and there were no other examples of this behavior, astronomers were forced to learn what they could from the only known example, or wait for another event to provide more clues.

Finally, more than 30 years later, FU Ori-like behavior appeared again in 1970 when the star now known as V1057 Cyg increased in brightness by 5.5 magnitudes over 390 days. Then in 1974, a 3rd example was discovered when V1515 Cyg rose from 17th magnitude to 12th magnitude over an interval lasting years. Astronomers began piecing the puzzle together from these clues.

FU Orionis stars, commonly called FUOrs, are pre-main sequence stars in the early stages of stellar development. They have only just formed from clouds of dust and gas in interstellar space, which occur in active star- forming regions. They are all associated with reflection nebulae, which become visible as the star brightens.

This artist's concept shows a young stellar object and the whirling accretion disk surrounding it. NASA/JPL-Caltech

Astronomers are interested in these systems because FUOrs may provide us with clues to the early history of stars and the formation of planetary systems. At this early stage of evolution, a young stellar object (YSO) is surrounded by an accretion disk, and matter is falling onto the outer regions of the disk from the surrounding interstellar cloud. Thermal instabilities, most likely in the inner portions of the accretion disk, initiate an outburst and the young star increases its luminosity. Our Sun probably went through similar events as it was developing.

One of the major challenges in studying FU Orionis stars is the relatively small number of known examples. Although approximately 20 FU Orionis candidates have been identifed, only a handful of these stars have been observed to rise from their pre-outburst state to their eruptive state.

Now, in the last year, several new FUOrs have been discovered. In November 2009, two newly discovered objects were announced. Patrick Wils, John Greaves and the Catalina Real-time Transient Survey (CRTS) collaboration had discovered them in CRTS images.

The first of these objects appeared to coincide with the infrared source IRAS 06068-0641 in Monoceros. Discovered on Nov. 10, it had been continuously brightening from at least early 2005, when it was magnitude 14.8, to its present 12.6 magnitude. A faint cometary reflection nebula was visible to the east. A spectrum taken with the SMARTS 1.5-m telescope at Cerro Tololo, on Nov. 17, confirmed it to be a YSO. The object lies inside a dark nebula to the south of the Monocerotis R2 association, and is likely related to it.

Also inside this dark nebula, a second object, coincident with IRAS 06068-0643, had been varying between mag 15 and 20 over the past few years, much like UX-Ori-type objects with very deep fades. This second object is also associated with a variable cometary reflection nebula, extending to the north.

Light curves, spectra and images can be found here.

Then, in August 2010, two new eruptive, pre-main sequence stars were discovered in Cygnus. The first object was an outburst of the star HBC 722. The object was reported to have risen by 3.3 magnitudes from May 13 to August 16, 2010. Spectroscopy reported by Ulisse Munari on August 23rd, support this object’s classification as an FU Ori star. Munari and his team reported the object at 14.04V on Aug 21, 2010.

The second object, coincident with another infrared source, IRAS 20496+4354, was discovered by K. Itagaki of Yamagata, Japan, on August 23, 2010. The object appears very faint, approximately magnitude 20, in a Digital Sky Survey image taken in 1990. Subsequent spectroscopy and photometry of this object by Munari showed that this object also has the characteristics of an FU Ori star. Munari reported the object at 14.91V on August 26, 2010.

Both these objects are now the subjects of an AAVSO observing campaign announced October 1, 2010 in AAVSO Alert Notice 425. Dr. Colin Aspin, University of Hawai’i, has requested the help of AAVSO observers in performing long-term photometric monitoring of these two new YSOs in Cygnus. AAVSO observations will be used to help calibrate optical and near-infrared spectroscopy to be obtained during the next year.

Since these stars are newly discovered, very little is known about their behavior. Their classification as FU Ori variables is based on spectroscopy, but establishing a good optical light curve and maintaining it, over the next several years, will be crucial to understanding these stars. This kind of long-term monitoring is one of the things at which amateur astronomers excel.

So after a very slow start, discoveries of new YSOs and our understanding of the dusty disk environments around them are starting to heat up. With new tools and new examples to study we are peering into the early stages of stellar and planetary formation and finding some of our models have been pretty close to the truth. We expect to find more and similar objects as new all-sky surveys begin to cover the sky, but these objects will still be relatively rare and therefore interesting, because this period in a star’s evolution is short-lived and only takes place in the active star forming regions of galaxies.

Posted on by Jon Voisey

Virtual Observatory Discovers New Cataclysmic Variable

Simulation of Intermediate Polar CV star
Simulation of Intermediate Polar CV star (Dr Andy Beardmore, Keele University)

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In my article two weeks ago, I discussed how data mining large surveys through online observatories would lead to new discoveries. Sure enough, a pair of astronomers, Ivan Zolotukhin and Igor Chilingarian using data from the Virtual Observatory, has announced the discovery of a cataclysmic variable (CV).


Cataclysmic variables are often called “novae”. However, they’re not a single star. These stars are actually binary systems in which their interactions cause large increases in brightness as matter is accreted from a secondary (usually post main-sequence) star, onto a white dwarf. The accretion of matter piles up on the surface until the it reaches a critical density and undergoes a brief but intense phase of fusion increasing the brightness of the star considerably. Unlike type Ia supernovae, this explosion doesn’t meet the critical density required to cause a core collapse.

The team began by considering a list of 107 objects from the Galactic Plane Survey conducted by the Advanced Satellite for Cosmology and Astrophysics (ASCA, a Japanese satellite operating in the x-ray regime). These objects were exceptional x-ray emitters that had not yet been classified. While other astronomers have done targeted investigations of individual objects requiring new telescope time, this team attempted to determine whether any of the odd objects were CVs using readily available data from the Virtual Observatory.

Since the objects were all strong x-ray sources, they all met at least one criteria of being a CV. Another was that CV stars often are strong emitters for Hα since the eruptions often eject hot hydrogen gas. To analyze whether or not any of the objects were emitters in this regime, the astronomers cross referenced the list of objects with data from the Isaac Newton Telescope Photometric Hα Survey of the northern Galactic plane (IPHAS) using a color-color diagram. In the field of view of the IPHAS survey that overlapped with the region from the ASCA image for one of the objects, the team found an object that emitted strongly in the Hα. But in such a dense field and with such different wavelength regimes, it was difficult to identify the objects as the same one.

To assist in determining if the two interesting objects were indeed the same, or whether they just happened to lie nearby, the pair turned to data from Chandra. Since Chandra has much smaller uncertainty in the positioning (0.6 arcsecs), the pair was able to identify the object and determine that the interesting object from IPHAS was indeed the same one from the ASCA survey.

Thus, the object passed the two tests the team had devised for finding cataclysmic variables. At this point, followup observation was warranted. The astronomers used the 3.5-m Calar Alto telescope to conduct spectroscopic observations and confirmed that the star was indeed a CV. In particular, it looked to be a subclass in which the primary white dwarf star had a strong enough magnetic field to disrupt the accretion disk and the point of contact is actually over the poles of the star (this is known as a intermediate polar CV).

This discovery is an example of how discoveries are just waiting to happen with data that’s already available and sitting in archives, waiting to be explored. Much of this data is even available to the public and can be mined by anyone with the proper computer programs and know-how. Undoubtedly, as organization of these storehouses of data becomes organized in more user friendly manners, additional discoveries will be made in such a manner.

Posted on by Mike Simonsen

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.

Posted on by Nancy Atkinson

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

Posted on by Jean Tate

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

Posted on by Nancy Atkinson

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,