Posted on by Tammy Plotner

Revealing A Hybrid Star Cluster

NGC 6791 - The full Hubble Advanced Camera for Surveys field (top right) is full of stars estimated to be 8 billion years old. Bottom right: The blue circles identify hotter dwarfs that are 4 billion years old. The red circles identify cooler dwarfs that are 6 billion years old. - Credit: NASA, ESA, and L. Bedin (STScI)

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Almost a century ago, astronomers Shapley and Melotte began classifying star clusters. This rough, initial go-around took in the apparent number of stars and the compactness of the field – along with color. By 1927, these “classes” were again divided to include both open and globular clusters. But there are some that simply defy definition.

According to Johns Hopkins astronomer Imants Platais, there is one case which has puzzled astronomers for decades: a well-known, seemingly open star cluster in the constellation of Lyra, named NGC 6791.

“This cluster is about twice the age of the sun and is unusually metal rich (at least twice the Sun’s metallicity),” said Platais, of the Henry A. Rowland Department of Physics and Astronomy’s Center for Astrophysical Sciences. “A couple of decades ago, it was also found that NGC 6791 contains a handful of very hot but somewhat dim stars, called hot subdwarfs. The presence of such stars in an open cluster is rare, though not unique.”

Why are these hot subdwarfs an anomaly? The facts about star clusters as we know them are that globular clusters are notoriously metal poor, while open clusters are metal rich. “The massive stars that create much of the metals live for only a short time, and when they die, they spit out or eject the metals they have created.” says the team. “The expelled metals become part of the raw material out of which the next stars are formed. Thus, there is a relationship between the age of a star and how much metal it contains: old stars have a lower metallicity than do younger ones. Less massive stars live longer than higher mass stars, so low mass stars from early generations still survive today and are studied extensively.”

A team led by Platais and Kyle Cudworth from The University of Chicago’s Yerkes Observatory set out to solve the mystery of NGC 6791 by taking a census of its stars. Their findings revealed several luminous stars in the horizontal branch of the HR diagram… Stars that would normally be found in globular clusters. The hot subdwarfs were confirmed to be genuine cluster members, but they now “appear to be simply the bluest horizontal branch stars”. What’s wrong with this picture? NGC 6791 contains simultaneously both red and very blue horizontal branch stars – making it both old and metal rich. Quite simply put, studying star clusters is key to understanding stellar evolution – unless the cluster starts breaking the rules.

“Star clusters are the building blocks of galaxies and we believe that all stars, including our own sun, are born in clusters. NGC 6791 is a real oddball among about 2,000 known open and globular star clusters in the Milky Way and as such provides a new challenge and a new opportunity, to our understanding of how stars form and evolve,” said Platais, who presented this work last week at the 218th meeting of the American Astronomical Society in Boston.

So… what about star clusters in other galaxies? Three hybrids have been discovered (2005) in the Andromeda Galaxy – M31WFS C1, M31WFS C2, and M31WFS C3. They have the same basic population and metallicity of a globular cluster, but they’re expanded hundreds of light years across and are equally less dense. Are they extended? Or perhaps a dwarf spheroidal galaxy? They don’t exist (as far as we know) in the Milky Way, but there’s always a possibility these hybrid clusters may call other galaxies home.

Until then, we’ll just keep learning.

Original Story Source: John Hopkins University.

Posted on by Tammy Plotner

Old Star Clusters Shed New Light on Starbirth

NGC 3603 - Credit: NASA, ESA, R. O'Connell (University of Virginia), F. Paresce (National Institute for Astrophysics, Bologna, Italy), E. Young (Universities Space Research Association/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)

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Hovering about the galactic plane and locked in the embrace of a spiral galaxy’s arms, open star clusters usually contain up to a few hundred members and generally span around thirty light years across. Most are young, up to a few tens of millions of years old – with a few rare exceptions as old as a few billion years. We understand that over time the members of a galactic cluster slowly drift apart to form loose associations. But what we don’t understand is exactly how their stars formed.

“The net effect of this is that their stars eventually become redistributed throughout the Galaxy,” said Nathan Leigh, a PhD student at McMaster University and lead author for a study being presented this week at the CASCA 2011 meeting in Ontario, Canada. “This is how we think most of the stars in the Milky Way came to be found in their currently observed locations.”

One of the reasons we’re not able to probe deeply into the construction and evolution of galactic clusters is because they are typically hidden by a dense veil of gas and dust. Beautiful to look at… But nearly impossible to cut through in visible light. This means we can’t directly observe the process of starbirth. To help understand this process, astronomers have combined their observations of star clusters so old they date back to the beginning of the Universe itself . And, thanks to modern computing, they are also able to generate state-of-the-art simulations for stellar evolution.

“Unfortunately, most star clusters take so long to dissolve that we cannot actually see it happening. But we now understand how this process occurs, and we can look for its signatures by examining the current appearances of clusters,” said Nathan Leigh. “We have gone about this by matching up the clusters we make with our simulations to the ones we actually observe. This tells us about the conditions at the time of their formation.”

These simulations have given Leigh and collaborators the stimulus they needed to re-trace the histories of real star clusters, giving us new clues about formation. To complete their studies, they relied upon highly sophisticated observations recently taken with the Hubble Space Telescope.

“Remarkably, we are finding that all star clusters more or less share a common history, extending all the way back to their births,” said Leigh. “This came as a big surprise to us since it suggests that the problem could be much simpler than we originally thought. Our understanding of not only how stars form, but also the history of our Galaxy, just took a much bigger step forward than we were expecting.”

Source: Canadian Astronomical Society.

Posted on by Tammy Plotner

The Lone SuperStar

Credit: ESO/M.-R. Cioni/VISTA Magellanic Cloud survey. Acknowledgment: Cambridge Astronomical Survey Unit

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It hangs out in space some 160,000 light years away. Its neighborhood is the Large Magellanic Cloud. It calls the Tarantula Nebula home. It’s 150 times more massive than our Sun and it shines an astounding three million times brighter. What is it? Try a lone super star…

Utilizing the power of ESO’s Very Large Telescope, a team of international astronomers have been checking out star VFTS 682 in the Large Magellanic Cloud. Using an instrument aptly named FLAMES, they also discovered the star to be very hot, with a surface temperature of about 50 000 degrees Celsius. While most of the initial findings were rather unremarkable, when researchers cleared away the dust clouds they discovered this super star stood alone.

“We were very surprised to find such a massive star on its own, and not in a rich star cluster,” notes Joachim Bestenlehner, the lead author of the new study and a student at Armagh Observatory in Northern Ireland. “Its origin is mysterious.”

Why an enigma? For the most part, stars like VFTS 682 are found in the crowded centers of galactic clusters. They are young, hot and bright – destined to live short and explosive lifetimes. Their high mass makes them a candidate for an even more dramatic end – a long-duration gamma-ray burst, the brightest explosion in the Universe. It could happen to other super stars in the nearby stellar nursery cluster, because it has a sibling.

“The new results show that VFTS 682 is a near identical twin of one of the brightest superstars at the heart of the R 136 star cluster,” adds Paco Najarro, another member of the team from CAB (INTA-CSIC, Spain).

So how did VFTS 682 end up being solitary sun? There is speculation that it could be a “runaway”… ejected from its nest. But such a scenario is unlikely since it is doubtful such a heavy star could be thrown from the cluster by gravitational interactions.

“It seems to be easier to form the biggest and brightest stars in rich star clusters,” adds Jorick Vink, another member of the team. “And although it may be possible, it is harder to understand how these brilliant beacons could form on their own. This makes VFTS 682 a really fascinating object.”

Shine on, you crazy diamond!

Posted on by Jon Voisey

Cyanoacetylene in IC 342

IC 342 - Ken and Emilie Siarkiewicz/Adam Block/NOAO/AURA/NSF
IC 342 - Ken and Emilie Siarkiewicz/Adam Block/NOAO/AURA/NSF

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Star formation is an incredible process, but also notoriously difficult to trace. The reason is that the main constituent of stars, hydrogen, looks about the same well before a gravitational collapse begins, as it does in the dense clouds where star formation happens. Sure, the temperature changes and the hydrogen glows in a different part of the spectrum, but it’s still hydrogen. It’s everywhere!

So when astronomers want to search for denser regions of gas, they often turn to other atoms and molecules that can only form or be stimulated to emit under these relatively dense conditions. Common examples of this include carbon monoxide and hydrogen cyanide. However, a study published in 2005, led by David Meier at the University of Illinois at Urbana-Champaign, studied inner regions of the nearby face-on spiral by tracing eight molecules and determined that the full extent of the dense regions is not well mapped by these two common molecules. In particular, cyanoacetylene, an organic molecule with a chemical formula of HC3N, was demonstrated to correlate with the most active star forming regions, promising astronomers a peek into the heart of star forming regions and prompting a follow-up study.

The new study was conducted from the Very Large Array in late 2005. Specifically, it studied the emissions due to 5-4, 10-9, and 16-15 transitions which each correspond to different levels of heating and excitation. The dense regions uncovered by this study were consistent with the ones reported in 2005. One, discovered by the previous survey from another tracer molecule, was not found by this most recent study, but the new study also discovered a previously unnoticed giant molecular cloud (GMC) through the presence of HC3N.

Another technique that can be applied is examining the ratios of various levels of excitation. From this, astronomers can determine the temperature and density necessary to produce such emission. This can be performed with any type of gas, but using additional species of molecules provides independent checks on this value. For the area with the strongest emission, the team reported that the gas appeared to be a cool 40 K (-387°F) with a density of 1-10 thousand molecules per cubic centimeter. This is relatively dense for the interstellar medium, but for comparison, the air we breathe has approximately 1025 molecules per cubic centimeter. These findings are consistent with those reported from carbon monoxide.

The team also examined several of the star forming cores independently. By comparing the varying strengths of tracer molecules, the team was able to report that one GMC was well progressed in making stars while another was less evolved, likely still containing hot cores which had not yet ignited fusion. In the former, the HC3N is weaker than in the other cores explored, which the team attributes to the destruction of the molecules or dispersal of the cloud as fusion begins in the newly formed stars.

While using HC3N as a tracer is a relatively new approach (these studies of IC 342 are the first conduced in another galaxy), the results of this study have demonstrated that it can trace various features in dense clouds in similar fashions to other molecules.

Posted on by Jason Major

Kepler Discovers a Rare Triple Gem

Animation of HD 181068 (click to play)

 

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It may be visible to the naked eye, but it took the unblinking gaze of NASA’s Kepler space telescope to reveal the true triple nature of this star system.

Animation of HD 181068 (click to play)

Unofficially dubbed “Trinity”, object HD 181068 is a multiple star system comprised of three stars: a red giant more than twelve times the diameter of the Sun and two red dwarf stars each slightly smaller than the Sun. The red dwarfs orbit each other in tight rotation around a central point, which in turn orbits the red giant. The smaller stars complete a full orbit around the giant every 45.5 days and, from our point of view, pass directly in front of and behind the huge star.

The orbital eclipse events of HD 181068 last about 2 days. What’s surprising is that during these eclipses the brightness of the system is not affected very much. This is because the surface brightnesses of the three stars are very similar. The current metaphor is a “white rabbit in a snowfall”, wherein the two red dwarfs effectively become invisible when they pass in front of the red giant. It wasn’t until the Kepler mission that we had an observational tool precise enough to detect the structure of this intriguing star system, located 800 light-years away from our own.

“The intriguing nature of this unique system remained unnoticed until now despite the fact that it is nearly bright enough to be visible to the naked eye. We really needed Kepler with its unprecedentedly precise and uninterrupted photometric monitoring to uncover such a rare gem.”

– Aliz Derekas, Eotvos University and Konkoly Observatory, Budapest, Hungary

Another unexpected feature of Trinity is its “quiet” nature. Astronomers have known that red giant stars exhibit seismic oscillations, as does our own Sun. But these oscillations are not present in Trinity’s red giant. Scientists speculate that the two red dwarfs may be creating some sort of gravitational offset, effectively negating the red giant’s vibrations. More research will be needed to determine if this is in fact the case.

Find out more about HD 181068 and other recent Kepler discoveries on NASA’s mission site or in the press release issued by the Ames Research Center, or read the published report on Science.

Image credit: NASA/KASC

 

 

Posted on by Nancy Atkinson

‘Sonic Booms’ in Space Linked to Star Formation

Dense filaments of gas in the IC5146 interstellar cloud. This image was taken by ESA’s Herschel space observatory at infrared wavelengths 70, 250 and 500 microns. Stars are forming along these filaments. Credits: ESA/Herschel/SPIRE/PACS/D. Arzoumanian (CEA Saclay) for the “Gould Belt survey” Key Programme Consortium.

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Its true there is no sound in empty interstellar space, but the Herschel space observatory has observed the cosmic equivalent of sonic booms. Networks of tangled and tremendously large gaseous filaments seen within clouds of gas and dust between stars are likely to be remnants of slow shockwaves from supernovae, Herschel scientists say. And surprisingly, no matter what the length or density of these filaments are, the width is always roughly the same, about 0.3 light years across, or about 20,000 times the distance of Earth from the Sun. This consistency of the widths demands an explanation, scientists say.

And it’s possible these shockwaves could generate sound within an interstellar cloud – if something were there to hear it.

“Although the density in an interstellar cloud is lower than in a very good vacuum on Earth there are molecules in the order of 10^8 per cm^3” said Goeran Pilbratt, ESA’s Herschel mission scientist. “That should be enough for sound to propagate, apart from the fact that we do not have the instruments to measure it.”

Filaments like this have been sighted before by other infrared satellites, but they have never been seen clearly enough to have their widths measured. Herschel is seeing that the width of these filaments is nearly uniform across three nearby clouds: IC5146, Aquila, and Polaris. The Herschel team, lead by Doris Arzoumanian, Laboratoire AIM Paris-Saclay, CEA/IRFU, made observations of 90 filaments, and found all had nearly identical widths. “This is a very big surprise,” Arzoumanian said.

The network of interstellar filaments in Polaris as seen by Herschel. Credits: ESA/Herschel/SPIRE/Ph. André (CEA Saclay) for the Gould Belt survey Key Programme Consortium and A. Abergel (IAS Orsay) for the Evolution of Interstellar Dust Key Programme Consortium.

Also, newborn stars are often found in the densest parts of these filaments. One filament imaged by Herschel in the Aquila region contains a cluster of about 100 infant stars.

The Herschel team said their observations provide strong evidence for a connection between interstellar turbulence, the filaments and star formation.

“The connection between these filaments and star formation used to be unclear, but now thanks to Herschel, we can actually see stars forming like beads on strings in some of these filaments,” said Pilbratt.

Comparing the observations with computer models, the astronomers suggest that filaments are probably formed when slow shockwaves dissipate in the interstellar clouds. These shockwaves are mildly supersonic and are a result of the huge amounts of turbulent energy injected into interstellar space by exploding stars.

They travel through the dilute sea of gas found in the galaxy, compressing and sweeping it up into dense filaments as they go. As these “sonic booms” travel through the clouds, they lose energy and, where they finally dissipate, they leave these filaments of compressed material.

Interstellar clouds are usually extremely cold, about 10 degrees Kelvin above absolute zero, and this makes the speed of sound in them relatively slow at just 0.2 km/s, as opposed to 0.34 km/s in Earth’s atmosphere at sea-level.

Sound travels in waves like light or heat does, but unlike them, sound travels by making molecules vibrate. So, in order for sound to travel, there has to be something with molecules for it to travel through. On Earth, sound travels to your ears by vibrating air molecules. In deep space, the large empty areas between stars and planets, there are no molecules to vibrate.

Read the team’s paper: Characterizing Interstellar Filaments with Herschel in IC5146

Sources: ESA email exchange with Pilbratt