Uses of a Forgotten Cluster

by Paul Gilster on June 25, 2012

Astronomical surprises can emerge close to home, close in terms of light years and close in terms of time. Take NGC 6774, an open cluster of stars also known as Ruprecht 147 in the direction of Sagittarius. In astronomical terms, it’s close enough — at 800 to 1000 light years — to be a target for binoculars in the skies of late summer. In chronological terms, the cluster has had a kind of re-birth in our astronomy. John Herschel identified it in 1830, calling it ‘a very large straggling space full of loose stars’ and including it in the General Catalog of astronomical objects.

But NGC 6774 remained little studied, and it took a more intensive look by Jaroslav Ruprecht in the 1960s to give the cluster both a new name and a firmer identity. This loose group of stars had long been thought to be an asterism, a chance alignment of stars that when seen from the Earth gave the impression of being a cluster. Ruprecht realized this was no asterism, and now new work with the MMT telescope in Arizona and the Canada-France-Hawaii Telescope on Mauna Kea is telling us that this cluster is, at about 2.5 billion years old, about half the age of the Sun. In fact, it’s about the age the Sun was when multicellular life began to emerge on our planet’s surface.

Image: Penn State University astronomers have determined that 80 of the stars in this photo are members of the long-known but underappreciated star cluster Ruprecht 147. In this image, the brightest of these stars are circled in green, and the less-bright ones are circled in red. These stars were born out of the same cloud of gas and dust approximately 2-billion years ago, and now are traveling together through space, bound by the force of gravity. The astronomers have identified this cluster as a potentially important new reference gauge for fundamental stellar astrophysics. Credit: Chris Beckett and Stefano Meneguolo, Royal Astronomical Society of Canada. Annotations by Jason Curtis, Penn State University.

Jason Wright (Penn State University) has been working on NGC 6774 with graduate student Jason Curtis, who will present the new findings in an upcoming conference in Barcelona, the 17th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun. Wright sees the significance of the cluster in terms of exoplanets and the stars that host them:

“The Ruprecht 147 cluster is very unusual and very important astrophysically because it is close to Earth and its stars are closer to the Sun’s age than those in all the other nearby clusters. For the first time, we now have a useful laboratory in which to search for and study bright stars that are of similar mass and also of similar age as the Sun. When we discover planets around Sun-like and lower-mass stars, we will be able to interpret how old those stars are by comparing them to the stars in this cluster.”

Most of the other nearby clusters are much younger than the Sun, making such comparisons less helpful, and it is work of Wright’s team that has demonstrated the cluster’s age as well as distance from Sol. These observations take in the directions and velocities of the stars in the cluster, showing that they are indeed moving together through space and are not a random pattern in the sky. Thus far 100 stars have been identified as part of the cluster but as the project continues, more are expected to be found. All told, NGC 6774 may become what Wright calls “a standard gauge in fundamental stellar astrophysics,” a helpful measuring stick in our neighborhood that can help us tighten up our age estimates of Sun-like, low mass stars.

The NGC 6774 work has been submitted to the Astronomical Journal. This Penn State news release has more.

tzf_img_post

Tulse June 25, 2012 at 8:51

How close together are the stars in the cluster, compared to the general distance between stars in our galaxy? And do we know if star clusters have any impact on planetary formation?

Erik Anderson June 25, 2012 at 12:28

@ Tulse:

In response to your first question, it’s perhaps unwise to do a trigonometry problem before finishing my morning tea, but given a cluster diameter of 25 arc-minutes and a distance of 1,000 light years away, I’m coming up with a physical diameter of 23.7 light years. There are just a few other stars within a similarly-sized sphere enclosing the Sun that are at or exceed the Sun’s luminosity: Alpha Centauri A & B, Sirius, and Procyon. Compare that to the 80 stars mentioned here and the cluster is perhaps roughly 20x denser than the immediate solar neighborhood.

The second question is more complicated. I would expect some kind of impact dependent upon where within the cluster a host star forms (i.e., towards the center versus on the periphery).

FrankH June 25, 2012 at 15:05

The March 2012 issue of Sky and Telescope had a very good article on what the Sun’s birth cluster could have been like.

The Sun and our solar system probably formed in a fairly large open cluster; probably considerably larger than NGC 6774 and closer to M67 in size.

M67 has stars that are very similar to the Sun in composition and age, although it’s improbable that the Sun formed there (see http://arxiv.org/pdf/1201.0987v1.pdf)

andy June 25, 2012 at 15:19

Wonder if it will become possible to search the cluster itself for planets, the distance is quite a lot larger than that for typical targets of RV searches. It would certainly be interesting to get some actual data on how the cluster environment affects planetary systems!

From what I can tell, planet searches in open clusters generally turn up empty but are mainly targetted at finding hot Jupiters which are quite a rare class of planets anyway. There are a handful of long-period superjovians known around giant stars in open clusters (Epsilon Tau b, NGC 2423 3 b and NGC 4349 127 b), I guess the dwarfs are typically too faint or too active (in the typical young clusters) for planet searches.

Daniel Suggs June 25, 2012 at 16:02

My own personal pet peeve: Why use red and green to differentiate, when eight to ten per cent of males suffer from red/green color blindness?
Why not blue/yellow?
Done venting.

Joy June 25, 2012 at 18:06

@Erik Anderson
I assumed your diameter was correct and did my own math on the volume & density calculations and came up with the same answer. However, the press articles about the cluster are rather unclear about what the magnitude (luminosity) limit of the sample was. One wonders of the red circles in the star map represent M stars. If so, the cluster density is rather underwhelming.

Background: Around Sol, we probably now have a complete census of all luminous stars within 8 parsecs: 211 total/2144 cubic parsecs = a density of 0.1 luminous stars per cubic parsec. Only 18% of these stars are brighter than M.

If the cluster is 23.7 LY in diameter, the spherical volume would be 200 cubic parsecs. The density would be 80/200 = 0.4 stars/cubic parsec or just 4x Sol neighborhood density. We only get the ~ 20x Sol neighborhood density by assuming, as Erik did, that the authors are claiming to have detected 80 stars brighter than M in this volume (not sure about that). In any case, this is a very open cluster.

Worst of all, even in a region of space 20x more dense with stars than our solar neighborhood, the average distance between stars is only 2.7 fold less.

Rob Henry June 25, 2012 at 19:36

Daniel Suggs, I know this is off topic, but that was a great point, and even more so given that not every sufferer knows that they are colour blind. I think of traffic lights, and remember that some disasters were caused by the inversion of the order of their colours, and wonder how many calamities were, and continue to be caused be the failure of others to recognise the importance of this problem. Luckily, the above was not one of those life-or-death situations.

Erik Anderson June 26, 2012 at 11:58

Joy: a class M dwarf would be very faint @ 1,000 ly, on the order of mag. 20.

Jason Curtis June 26, 2012 at 20:48

Daniel, great point. In the future, I’ll use other color combinations when making color plots. I typically do not use yellow, because I’m often plotting on a white background, but that would have worked nicely in this case. While we have already submitted our paper on Ruprecht 147 to the Astronomical Journal, I still have time to modify this figure before it is published.

Jason Wright June 26, 2012 at 21:10

Hi, there! I’m glad to see such interest in this great cluster. Let me help with some of these questions.

Firstly, credit for realizing the cluster is real goes to astronomers named Kharchenko and Dias, who wrote a series of papers around 5 years ago that included this fact. We noticed their discovery, and followed up on it; they had the age and distance wrong, but were correct that it is old and close. They identified 41 stars, we have found over 60 more.

Also, Ruprecht did catalog the cluster, but he apparently didn’t realize it was the same object as Herschel’s and so there were two catalog entires until recently for the same cluster. He couldn’t prove it wasn’t an asterism, though.

The cluster is “straggling” so it’s hard to estimate its size. Clusters are densest in the middle and sparsest at the edges, so the distance between stars is highly variable, but its core is roughly 1 degree across, or about 1/50 of a radian. This means that it is 50 times further away than it is large, or about 20 light years across (depending on how you define its “size”). It probably has around 200 members, or one member every 40 cubic light years, which works out to very roughly a star every 3-4 light years.

We do think planets can form in Galactic clusters like this, for the reasons given earlier in this thread. Yes, I’d love to go look for planets in it! We’ve got this on our “to do” list.

Yes, M dwarfs will be around 20th magnitude in the optical, but significantly brighter in the near infrared. We’re looking for them!

Jason Curtis June 26, 2012 at 21:42

I only circled the membership down to approximately 13th magnitude, mainly because stars fainter than this are hard to see in this photograph and would distract from my main point: it is remarkable that Herschel was correct when he identified this as a galactic cluster, because the majority of bright stars in the field are unassociated with R147. We have only confirmed membership for another 10 stars fainter than this, down to about 15th magnitude, which we think corresponds to a star with 70% the mass of the Sun.

We have not yet been able to confirm membership for any M dwarfs. This is because these faint stars do not have cataloged proper motions (motions of stars across the plane of the sky over time, relative to more distant background objects). The Hipparcos mission measured proper motions for stars down to 12th mag., and while the US Naval Observatory has published proper motions for stars down to 18th mag., they really become unreliable for fainter stars (V > 15 mag).

This means we cannot yet distinguish the low mass stars that are gravitationally bound to Ruprecht 147 and are moving together through space, from the hundreds of thousands of faint stars in the field. Still, we have identified ~100 candidate K and M dwarf members, and I am traveling to Chile in 2 weeks in order to measure radial velocities for these candidates with the 6.5-meter Magellan Telescope. If they are in fact members of Ruprecht 147, then they should have velocities similar to the rest of the cluster, 41 km/s.

We have plans to measure our own proper motions for R147 in a year or two. We imaged the cluster in 2008, and at their speeds it will take 5 years for the stars to move enough in relation to more distant stars and galaxies in order to precisely measure their proper motions. This will enable us to identify all the very faintest stars in R147 – the white dwarfs (“dead” stars) and the M dwarfs.

There is some debate and speculation among astronomers whether or not R147 will contain any M dwarfs. While these stars are the most numerous in the Galaxy, their low mass means they aren’t bound as tightly to the cluster. Over time, the cluster moves through the Galaxy and the stars get gravitationally tugged on by other stars, clusters, molecular clouds, etc. These gravitational tidal forces are effective at breaking up most clusters in a few hundred million years. Only the most massive clusters, or those whose orbits are lucky enough to keep them relatively unperturbed, manage to survive. R147 was almost certainly much richer in the past, but it has been “evaporating” for about 3 billion years, and it is the lowest mass stars that tend to evaporate first.

We actually might find that R147 has retained a fair amount of M dwarf binaries, because their masses can add up to about a solar mass, and we’ve identified plenty of members of that mass (well, 30ish).

Jason Curtis June 26, 2012 at 21:59

I want to clarify this statement: ” R147 was almost certainly much richer in the past, but it has been “evaporating” for about 3 billion years, and it is the lowest mass stars that tend to evaporate first.”

Star clusters “evaporate” over time, meaning their members are dispersed into the Galaxy. And lowest mass stars are more likely to evaporate from the cluster first. I do not mean that the star itself evaporates.

Joy June 27, 2012 at 1:25

@Erik
Other researchers have used the mosaic camera of the 3.6 metre CFHT to map clusters down to magnitude 22, so I am thinking the red circles on the star map could still be M stars. The color choice, plus the ratio of red/green circles is suggestive

http://www.phy.ohiou.edu/~tss/ASTR410/Ragozzine_Uckert08/ragozzine.html

Paul Gilster June 27, 2012 at 8:24

A quick note of thanks to both Jason Wright and Jason Curtis of Penn State, whose work on NGC 6774 is under discussion here. Having the researchers on this project here to answer questions has deepened the discussion immeasurably and offered us a great deal of insight. Thanks to you both!

andy June 27, 2012 at 13:16

Thanks for the extra information. Any known blue stragglers in this cluster?

Talking of planets in open clusters, today’s arXiv batch contained a paper about a planet search in M67 which may be worth a look.

FrankH June 27, 2012 at 15:57

@ Jason x 2 If you’re taking two images of the cluster, you may be able to get a stereoscopic image, especially if you use the same telescope (or scale the images to match exposures and plate scale). J. Comas Sola did this in 1915 with an open cluster.

Jason Wright June 28, 2012 at 16:07

@Joy: The red circles around around stars we have confirmed to be members of the cluster, but all of them are too bright to be M stars. There are probably M dwarfs in that field that are also cluster members, but we have not found them yet, so they do not have circles (they are also probably too faint for this image to make out).

@FrankH: I’m not familiar with the term “stereoscopic” in this context. Usually it means to take an image from two different vantage points to get a parallax. In our case we are taking second images for a similar reason, but the primary purpose will not be to measure a parallax (which would be very small, only 0.004 arcseconds) but instead to measure the “proper motion” of the cluster across the sky (due to our motion and its motion) which is around 0.03 arcseconds per year.

@andy: There are 5 or 6 blue stragglers. They are the brightest blue stars in the image that have green circles. The bright red stars with green circles are red giants.

Rob Henry June 28, 2012 at 18:31

Jason Curtis says “While [M dwarf] stars are the most numerous in the Galaxy, their low mass means they aren’t bound as tightly to the cluster.”
And so it seems that gravity does not always exert the same force per units mass in the celestial sphere, in the way it does here on Earth!

Alternatively, we could believe that red dwarfs are just as tightly bound as other stars, but tend to build up more kinetic energy per mass from interactions with their neighbours because, in these, their velocity change is always the larger.

Rob Henry June 28, 2012 at 19:02

Jason Wright, how can there be five or six blue stragglers here. If there were that many primordial collisions represented in the few remaining stars, surely there must have been sufficient thermalisation to have broken up this cluster on a much shorter time scale that two and a half billion years. Is this evidence that blue stragglers have other causes??

Rob Henry June 29, 2012 at 1:11

I have just realised that the amazingly high proportion of blue stragglers could mean that this cluster was once very dense, even by standards of globular (let alone open) clusters, and that is why it has survived so long.

If so, are there younger examples of open clusters that are also very much larger and denser than their known contemporaries?

Jason Wright July 2, 2012 at 10:41

@Rob: The number and relative importance of the many channels to creating blue stragglers are not well constrained, as far as I know. I don’t know what you mean by “primordial collisions”; blue stragglers’ lifetimes are shorter than the age of the cluster, by definition, so they must have formed long after the cluster was born.

Open or Galactic clusters come in all sizes, from loose associations and groups to M67-sized monsters and larger, so yes, some young clusters are much larger than average. R147 was certainly richer in the past, but I don’t think that this cluster needed to once have have globular cluster densities to have survived this long or have its current binary fraction (or its current blue straggler fraction). It would be interesting to do a simulation and see.

“Thermal” evaporation is important for low mass stars but slow overall because the relaxation time for clusters is many billions of years; what is more important is that clusters are slowly teased apart by Galactic tides and passing giant molecular clouds. The rate of such disruptions depends on the cluster’s path around the Galaxy, especially its motion in the Z and R directions (i.e. non-circular motions), but is ultimately quite stochastic. It’s not clear if R147 is still around because it started large, or because it just got lucky, or both. But you are right that the high binary fraction and blue straggler fraction are evidence for the first option.

Rob Henry July 2, 2012 at 22:30

Many thanks Jason for the detailed explanation. Boy, the relaxation time of open clusters being billions of years is counterintuitive – but then it is always hard for those coming from other disciplines to comprehend how far apart these stars are, even when they are in *dense* clusters. It makes me wounder how any large scale structures manage to form in our universe in just 10 billion years.

Anyhow, I still have a problem comprehending the “not as tightly bound” comment for the following reason.

Any massive object, that interacts with other massive objects over their complete trajectory, will tend to impart slightly more velocity on the most massive ones that they pass by, because those larger stars will draw that perturbing object (very) slightly closer than smaller ones will.

The only exception is that this miniscule effect reverses for a cloud whose internal structure is such that a passage that is very slightly closer to its centre of mass actually produces less deflection (admittedly though, this is an easy structural requirement to meet).

Actually, after much thought, I can not think of any mechanism, other than passing through far more clouds than you pass close by, whereby “not as tightly bound” could have any legitimate descriptive value for explaining the preferential loss of M dwarfs whose velocity profile was similar to other stars. It would still make more sense if the little bit of thermalisation that does occur becomes the limiting factor where it is easier to tease off M’s, and the complex models with which you work hides this.

In astronomy, the sophistication of mathematical modelling is so wonderful, that the real challenge is how to communicate them. Biology has the reverse problem. This makes it possible for such nonsense, as “group selection” to come to prominence within some circles of evolutionary science, and to be supported for a decade by slogans that not one of its proponents could ever give in mathematical terms. They did not realise that something that could not be translated into a mathematical model was illogical. What I’m trying to communicate is that I have developed on over sensitivity to such matters, even though I fully understand that the question of whether an expression reflects any underlying logical truth is a far lesser problem in the eyes of an astronomer.

PS my *primordial* comment was meant to emphasise that, to me, the collisions must have occurred in an epoch when that open cluster was comparably dense to when it was first formed and not subsequent to the time. It was meant to convey that if it was much later I was baffled and intrigued – and so, would then be eager to find out more about blue stragglers in general and those ones in particular.

Jason Wright July 13, 2012 at 17:07

Hi, Rob.

The energy that binds a star to a cluster is roughly GMm/r, where m is the mass of the star, M is the mass of the cluster, and r is the size of the cluster. Low mass stars thus have a lower binding energy. Equipartition of energy states that all classes of particles will receive roughly equal shares of the energy of a system, and since the M dwarfs have lower binding energies this means that they will be more likely to escape.

For instance, in a gas all species tend to have the same temperature, which goes as kT ~ mv^2. Since k and T are the same for all species, the typical thermal speed of a species will scale as 1/sqrt(m). This is why hydrogen escapes from the top of Earth’s atmosphere, but oxygen does not. Both have thermal distributions of velocities, but the hydrogen atoms’ velocities are larger by a factor of about 4, and so a higher fraction of H atoms are in the high-velocity tail of the Maxwellian distribution above the escape speed.

A good way to imagine this is to consider gravitational interactions as being elastic collisions. Consider the limit of a high mass ratio collision — the massive object’s final velocity will not be significantly changed regardless of the impact parameter, but the lower mass object’s range of final speeds can be as high as the sum of their initial speeds or as low as the difference. The M dwarfs are essentially scattering off of the big F dwarfs, occasionally leaving the cluster. The F dwarfs can scatter each other, as well, but not as efficiently. Clusters thus evaporate from the “bottom up” in terms of mass.

Rob Henry July 14, 2012 at 18:47

Jason Wright, it is clear to me now that our differences are of a purely semantic nature, and, to me, that holds the following further interest.

You like to treat stars just the same as invisible fundamental particles, where properties such as velocity and kinetic energy are better represented as numbers than as physical reality. If a star escapes from a cluster it is because its kinetic energy exceeds its binding energy, and the fact that it is moving faster than a certain escape velocity that is common to all these particles is a trivial point.

To someone, such as myself, coming from outside, what is trivial and what is fundamental here changes place. I can see now that this is what placed me on the wrong footing as I tried to understand your previous explanation, though all is clear now as just a very different way of viewing the same solution to a problem.

ljk September 14, 2012 at 16:51

News release: 2012-289 Sept. 14, 2012

First Planets Found Around Sun-Like Stars in a Cluster

The full version of this story with accompanying images is at:

http://www.jpl.nasa.gov/news/news.php?release=2012-289&cid=release_2012-289

PASADENA, Calif. — NASA-funded astronomers have, for the first time, spotted planets orbiting sun-like stars in a crowded cluster of stars. The findings offer the best evidence yet that planets can sprout up in dense stellar environments. Although the newfound planets are not habitable, their skies would be starrier than what we see from Earth.

The starry-skied planets are two so-called hot Jupiters, which are massive, gaseous orbs that are boiling hot because they orbit tightly around their parent stars. Each hot Jupiter circles a different sun-like star in the Beehive Cluster, also called the Praesepe, a collection of roughly 1,000 stars that appear to be swarming around a common center.

The Beehive is an open cluster, or a grouping of stars born at about the same time and out of the same giant cloud of material. The stars therefore share a similar chemical composition. Unlike the majority of stars, which spread out shortly after birth, these young stars remain loosely bound together by mutual gravitational attraction.

“We are detecting more and more planets that can thrive in diverse and extreme environments like these nearby clusters,” said Mario R. Perez, the NASA astrophysics program scientist in the Origins of Solar Systems Program. “Our galaxy contains more than 1,000 of these open clusters, which potentially can present the physical conditions for harboring many more of these giant planets.”

The two new Beehive planets are called Pr0201b and Pr0211b. The star’s name followed by a “b” is the standard naming convention for planets.

“These are the first ‘b’s’ in the Beehive,” said Sam Quinn, a graduate student in astronomy at Georgia State University in Atlanta and the lead author of the paper describing the results, which was published in the Astrophysical Journal Letters.

Quinn and his team, in collaboration with David Latham at the Harvard-Smithsonian Center for Astrophysics, discovered the planets by using the 1.5-meter Tillinghast telescope at the Smithsonian Astrophysical Observatory’s Fred Lawrence Whipple Observatory near Amado, Arizona to measure the slight gravitational wobble the orbiting planets induce upon their host stars.

Previous searches of clusters had turned up two planets around massive stars but none had been found around stars like our sun until now.

“This has been a big puzzle for planet hunters,” Quinn said. “We know that most stars form in clustered environments like the Orion nebula, so unless this dense environment inhibits planet formation, at least some sun-like stars in open clusters should have planets. Now, we finally know they are indeed there.”

The results also are of interest to theorists who are trying to understand how hot Jupiters wind up so close to their stars. Most theories contend these blistering worlds start out much cooler and farther from their stars before migrating inward.

“The relatively young age of the Beehive cluster makes these planets among the youngest known,” said Russel White, the principal investigator on the NASA Origins of Solar Systems grant that funded this study. “And that’s important because it sets a constraint on how quickly giant planets migrate inward — and knowing how quickly they migrate is the first step to figuring out how they migrate.”

The research team suspects planets were turned up in the Beehive cluster because it is rich in metals. Stars in the Beehive have more heavy elements such as iron than the sun has.

According to White, “Searches for planets around nearby stars suggest that these metals act like a ‘planet fertilizer,’ leading to an abundant crop of gas giant planets. Our results suggest this may be true in clusters as well.”

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages NASA’s Exoplanet Exploration Program office. More information about exoplanets and NASA’s planet-finding program is available at:

http://planetquest.jpl.nasa.gov .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

J.D. Harrington 202-358-5241
Headquarters, Washington
j.d.harrington@nasa.gov

Comments on this entry are closed.