Weltevreden SA
15-02-2014, 10:59 AM
E3, a most curious globular
I posted a visual sighting of the elusive globular E3 in the Observing Reports forum. As I looked into it, this globular became as fun to read about as to observe.
You can see E3 in a 6 inch scope if you have very dark skies. As always, more aperture is better—unless you have my budget, in which case 'more' means more time at the eyepiece.
But once we see E3, what are we looking at? If E3 seems a curiosity to us, it’s all the more so to professional astronomers. It is a very loose (probably Class X to XII) star-poor cluster, and its lusterless mass-to-luminosity ratio make it one of the intrinsically faintest globulars we can see in a small scope. Yet E3 has bequeathed us a unique class of binary star that was identified only in 1994, ‘yellow stragglers’. E3’s color magnitude differs from most globular clusters. It lies 8540 light years beneath the Galactic disk, which put it firmly in the Galactic halo. Open clusters do not form that far out into the halo. Infalling giant molecular clouds will not encounter enough galactic gas density to enter into collapse condition until they reach about 800 light years from the disc.
In 1984 and 1985 astronomers noticed an odd jump in E3’s main sequence at the turn-off point where a star’s hydrogen core burning slows to a stop. The star’s helium-ash core is no longer dense enough to support hydrogen fusion any more. In E3’s CMD (http://adsabs.harvard.edu/abs/1985PASP...97..665M) there is a curious ‘platform-shoes’ kink straight up for nearly a full magnitude directly above the main sequence. It looks like these stars got a second wind just as they were wheezing their last for lack of hydrogen. As it turns out, that’s exactly what happened.
In the CMD, the normal main sequence begins to turn off at the intersection of V=20 to 21 and B-V=0.9 to 1.0. The spread is because of the slightly different star masses involved and camera photometric spread. Main sequence stars cross over to the red giant branch to the right. Instead, most of E3’s main sequence turn-off stars (a grand total of about 21) head straight up to V=19 before turning rightward and redder. An turn-off jump of one magnitude may not thrill you to bits, but it had professional astronomers re-checking their computer models and scratching their heads.
There’s a lot to be gleaned from E3’s crossover stage between turn-off and the red giant branch. For one, the profile closely emulates the similarly composed but far more populous red giant branch of 47 Tucanae. (See Hesser 1984, Fig. 9. (http://adsabs.harvard.edu/abs/1984PASP...96..406H)) Behind the dramatic difference between E3, one of the faintest globulars in the sky, and 47 Tuc, one of the most luminous, is the common property that globulars evolve much the same way and pass through the same stages at roughly the same time. But here, E3 throws us yet another curve: it has almost no horizontal branch or asymptotic giant branch—less than one-tenth the numbers and mass of typical globulars. So where did all E3’s old, highly evolved stars go?
Astronomers don’t know, in part because there is so little data and partly because they are so interested in E3’s yellow stragglers. This star class doesn’t come up very often in cluster studies. Mostly it arises in connection with small, star-poor clusters like E3. While we often hear of ‘blue stragglers’, it’s not always clear what the word ‘straggler’ means. Stragglers are members of the stellar exotica club, along with neutron stars and black holes. But where did they straggle from and where are they straggling to?
By this time in a globular’s life, the cluster has already lost as many as half of its lightweight low-mass outer stars. This mass loss served mainly to tighten up the core, but it also made the cluster more vulnerable to disruption from massive objects that it nears. The stars remaining in the core are rather alike in mass, luminosity, composition, and age—between 0.2 to 1.6 times the Sun’s mass. Blue stragglers involve mergers of unequal binaries—typically one star is several times as massive as the other. The large star reaches the MS turn-off point before the small star, but they end up merging and both their evolutions become a single evolution. The slow pace and free exchange of atmospheric gas turns the merger into a very gradual equilibration as their orbits tighten around each other. It’s not the big fissile blast in the sky we all wish we could witness.
The merged star gets hotter and larger, and hence brighter. The combined mass enables the star to remain on the main sequence longer—trending blueward in other words. Blue stragglers show up on a CMD as a sprinkle of stars on the main sequence which continue past the normal right-turn point for stars like their unmerged pals. Stragglers continue up and to the left as if they were more massive stars to begin with. But more massive stars reached that point on the main sequence long ago; no cluster stars should be in that part of the CMD any more. Hence the moniker ‘blue’ and ‘straggler’.
But what happens when a pair of equal-mass stars merge? Already they are ‘hard’—so close we don’t see them as binaries either optically or spectroscopically. They ascend the main sequence together, but look brighter because we see the combined light of two stars of the same mass and age. Only when they turn off the main sequence together can we ID them as binaries; their combined luminosity is almost a magnitude brighter, but their colour remains the same. They become . . . yellow stragglers. They would look a lot like the Sun.
One would think the mishmash of star orbits (http://www.noao.edu/outreach/aop/observers/nbody.html) in a globular cluster would militate against binaries hanging together. In fact, it’s just the opposite: globulars encourage binary formation—40% or more GC stars are binaries. Binaries have a tough life in globulars. Hence they get tough themselves. Astronomers have coined the term ‘bully binaries’ (BB) to describe the behaviour of binaries in the core of a globular.
At the beginning of a globular’s life, the tumult of so many stars forming nearly at once disrupts most if not all the binaries that naturally form in clouds of collapsing gas. Then the primordial massive stars explode as supernovae within the cluster’s first 10 million years, ejecting the formation gas with them. The gas is almost completely expelled by the first 100 million years, though there is sometimes enough to fuel a second main sequence before it empties completely. But that gas had a lot of mass itself—almost as much mass as the stars—leaving the cluster all but gas-free and seriously out of equilibrium. With no ability to create new stars, it has no choice but to collapse until the kinetic energy of the high-mass stars in the core reaches a new balance with the gravity of the whole system. Eventually most of the low-mass stars escape the cluster altogether. It’s as if the lightweight stars in a globular take a long look at the barroom brawl going on in the core and decide to leave town for good.
This takes place on a quantifiable time scale: 10 or more crossing times. A ‘crossing time’ is the time it takes for a typical star to cross from the tidal radius on one side of a cluster to the same radius on the other. This can take hundreds of millions to over a billion years in a globular. (One to 10 million years in an open cluster.) The tidal radius is the surface where the cluster’s gravity exactly balances the gravity of the huge galaxy that the cluster lies within. ‘Crossing time’ is a calculated, not a real, event. Stars do not move in straight lines anywhere, much less across the middle of a star cluster. The term ‘crossing time’ is an mathematical clock used to analyze the star motions in a specific cluster.
Astronomers have a term for the end result we see today: the half-mass radius. When I first glimpsed E3 I had calculated what it should look like to my eye by adding its listed half-light radius of 2.1 arcmin from its overall listed magnitude of 11.35, or mag 13.6. For casual visual observers like us, the half-light radius and half-mass radius are about the same. The light from inside the half-light radius is half the total light produced by the cluster. Astronomers go through a considerable weeding-out process to deduct the light contributed by field stars in front of or behind the cluster. They usually do this spectroscopically because cluster stars will all absorb wavelengths in specific bands that can be easily distinguished from non-cluster stars.
Billions of years after the cluster formed, the end result of all this seething history is a more tightly packed cluster of stars that has lost most of its mass into space. The name of the overall process is ‘mass-segregation induced evaporative dissolution‘ and the name for the end result we see in our eyepieces as E3 is ‘post core-collapse equilibrium’. We amateurs are OK with more casual descriptions, so E3 can be pictured as what happens when the big bullies take over the town and turf out the lightweights. In a way this is a blessing for the halo stars of a globular cluster (or galaxy). The halo is a giant retirement village for unattached singles—lonely but unstressed and blissfully far from the bullies dominating the places where all the action is. All this plays out on such a stately timescale that we don’t see it for the violent process it is.
If we could call stars astute, halo stars are wise indeed. Those bullies in the core have created a dreadful life of endless gravitational squabbles and they are stuck with it till they wheeze into white dwarfs. Life in the Mafia is a breeze compared with life in a globular, it’s just a lot faster. The early binaries coupled in loose orbits and became ‘soft’ binaries that can be easily disrupted. And, like hot young things anywhere coupling without any strings attached, soft binaries have the same problem humans do: messy breakups. When a third star comes too close to a soft binary, the heavier two stars will end up binding more tightly and the third will be ejected; it usually ends up in the halo. (Sore loser.) This ‘hardens’ the binary, meaning its stars are in tighter orbits that only really massive interlopers can disrupt. Naturally, this eventually happens, too. If a hard binary comes too close to another hard binary, they end up in an awful slugging match. The end result is as human as it is stellar: the harder binary throws the not-so-hard binary out of the cluster completely. In the end, only the lawyers—gravity—come out ahead. Given the lifestyle of a star cluster, gravity becomes a very pricey lawyer. Poor hapless E3 will learn this the hard way—it is on a collision course with the Milky Way. It was originally a prosperous globular with respectably starry assets. Its legal problems with gravity have proven ruinous. It has lost most of its stars. Now it has one of the least dense cores in the Galaxy. One look in the eyepiece tells us that.
E3's miseries have only begun. The rest of E3's fate is described in the following post.
=Dana in SA
I posted a visual sighting of the elusive globular E3 in the Observing Reports forum. As I looked into it, this globular became as fun to read about as to observe.
You can see E3 in a 6 inch scope if you have very dark skies. As always, more aperture is better—unless you have my budget, in which case 'more' means more time at the eyepiece.
But once we see E3, what are we looking at? If E3 seems a curiosity to us, it’s all the more so to professional astronomers. It is a very loose (probably Class X to XII) star-poor cluster, and its lusterless mass-to-luminosity ratio make it one of the intrinsically faintest globulars we can see in a small scope. Yet E3 has bequeathed us a unique class of binary star that was identified only in 1994, ‘yellow stragglers’. E3’s color magnitude differs from most globular clusters. It lies 8540 light years beneath the Galactic disk, which put it firmly in the Galactic halo. Open clusters do not form that far out into the halo. Infalling giant molecular clouds will not encounter enough galactic gas density to enter into collapse condition until they reach about 800 light years from the disc.
In 1984 and 1985 astronomers noticed an odd jump in E3’s main sequence at the turn-off point where a star’s hydrogen core burning slows to a stop. The star’s helium-ash core is no longer dense enough to support hydrogen fusion any more. In E3’s CMD (http://adsabs.harvard.edu/abs/1985PASP...97..665M) there is a curious ‘platform-shoes’ kink straight up for nearly a full magnitude directly above the main sequence. It looks like these stars got a second wind just as they were wheezing their last for lack of hydrogen. As it turns out, that’s exactly what happened.
In the CMD, the normal main sequence begins to turn off at the intersection of V=20 to 21 and B-V=0.9 to 1.0. The spread is because of the slightly different star masses involved and camera photometric spread. Main sequence stars cross over to the red giant branch to the right. Instead, most of E3’s main sequence turn-off stars (a grand total of about 21) head straight up to V=19 before turning rightward and redder. An turn-off jump of one magnitude may not thrill you to bits, but it had professional astronomers re-checking their computer models and scratching their heads.
There’s a lot to be gleaned from E3’s crossover stage between turn-off and the red giant branch. For one, the profile closely emulates the similarly composed but far more populous red giant branch of 47 Tucanae. (See Hesser 1984, Fig. 9. (http://adsabs.harvard.edu/abs/1984PASP...96..406H)) Behind the dramatic difference between E3, one of the faintest globulars in the sky, and 47 Tuc, one of the most luminous, is the common property that globulars evolve much the same way and pass through the same stages at roughly the same time. But here, E3 throws us yet another curve: it has almost no horizontal branch or asymptotic giant branch—less than one-tenth the numbers and mass of typical globulars. So where did all E3’s old, highly evolved stars go?
Astronomers don’t know, in part because there is so little data and partly because they are so interested in E3’s yellow stragglers. This star class doesn’t come up very often in cluster studies. Mostly it arises in connection with small, star-poor clusters like E3. While we often hear of ‘blue stragglers’, it’s not always clear what the word ‘straggler’ means. Stragglers are members of the stellar exotica club, along with neutron stars and black holes. But where did they straggle from and where are they straggling to?
By this time in a globular’s life, the cluster has already lost as many as half of its lightweight low-mass outer stars. This mass loss served mainly to tighten up the core, but it also made the cluster more vulnerable to disruption from massive objects that it nears. The stars remaining in the core are rather alike in mass, luminosity, composition, and age—between 0.2 to 1.6 times the Sun’s mass. Blue stragglers involve mergers of unequal binaries—typically one star is several times as massive as the other. The large star reaches the MS turn-off point before the small star, but they end up merging and both their evolutions become a single evolution. The slow pace and free exchange of atmospheric gas turns the merger into a very gradual equilibration as their orbits tighten around each other. It’s not the big fissile blast in the sky we all wish we could witness.
The merged star gets hotter and larger, and hence brighter. The combined mass enables the star to remain on the main sequence longer—trending blueward in other words. Blue stragglers show up on a CMD as a sprinkle of stars on the main sequence which continue past the normal right-turn point for stars like their unmerged pals. Stragglers continue up and to the left as if they were more massive stars to begin with. But more massive stars reached that point on the main sequence long ago; no cluster stars should be in that part of the CMD any more. Hence the moniker ‘blue’ and ‘straggler’.
But what happens when a pair of equal-mass stars merge? Already they are ‘hard’—so close we don’t see them as binaries either optically or spectroscopically. They ascend the main sequence together, but look brighter because we see the combined light of two stars of the same mass and age. Only when they turn off the main sequence together can we ID them as binaries; their combined luminosity is almost a magnitude brighter, but their colour remains the same. They become . . . yellow stragglers. They would look a lot like the Sun.
One would think the mishmash of star orbits (http://www.noao.edu/outreach/aop/observers/nbody.html) in a globular cluster would militate against binaries hanging together. In fact, it’s just the opposite: globulars encourage binary formation—40% or more GC stars are binaries. Binaries have a tough life in globulars. Hence they get tough themselves. Astronomers have coined the term ‘bully binaries’ (BB) to describe the behaviour of binaries in the core of a globular.
At the beginning of a globular’s life, the tumult of so many stars forming nearly at once disrupts most if not all the binaries that naturally form in clouds of collapsing gas. Then the primordial massive stars explode as supernovae within the cluster’s first 10 million years, ejecting the formation gas with them. The gas is almost completely expelled by the first 100 million years, though there is sometimes enough to fuel a second main sequence before it empties completely. But that gas had a lot of mass itself—almost as much mass as the stars—leaving the cluster all but gas-free and seriously out of equilibrium. With no ability to create new stars, it has no choice but to collapse until the kinetic energy of the high-mass stars in the core reaches a new balance with the gravity of the whole system. Eventually most of the low-mass stars escape the cluster altogether. It’s as if the lightweight stars in a globular take a long look at the barroom brawl going on in the core and decide to leave town for good.
This takes place on a quantifiable time scale: 10 or more crossing times. A ‘crossing time’ is the time it takes for a typical star to cross from the tidal radius on one side of a cluster to the same radius on the other. This can take hundreds of millions to over a billion years in a globular. (One to 10 million years in an open cluster.) The tidal radius is the surface where the cluster’s gravity exactly balances the gravity of the huge galaxy that the cluster lies within. ‘Crossing time’ is a calculated, not a real, event. Stars do not move in straight lines anywhere, much less across the middle of a star cluster. The term ‘crossing time’ is an mathematical clock used to analyze the star motions in a specific cluster.
Astronomers have a term for the end result we see today: the half-mass radius. When I first glimpsed E3 I had calculated what it should look like to my eye by adding its listed half-light radius of 2.1 arcmin from its overall listed magnitude of 11.35, or mag 13.6. For casual visual observers like us, the half-light radius and half-mass radius are about the same. The light from inside the half-light radius is half the total light produced by the cluster. Astronomers go through a considerable weeding-out process to deduct the light contributed by field stars in front of or behind the cluster. They usually do this spectroscopically because cluster stars will all absorb wavelengths in specific bands that can be easily distinguished from non-cluster stars.
Billions of years after the cluster formed, the end result of all this seething history is a more tightly packed cluster of stars that has lost most of its mass into space. The name of the overall process is ‘mass-segregation induced evaporative dissolution‘ and the name for the end result we see in our eyepieces as E3 is ‘post core-collapse equilibrium’. We amateurs are OK with more casual descriptions, so E3 can be pictured as what happens when the big bullies take over the town and turf out the lightweights. In a way this is a blessing for the halo stars of a globular cluster (or galaxy). The halo is a giant retirement village for unattached singles—lonely but unstressed and blissfully far from the bullies dominating the places where all the action is. All this plays out on such a stately timescale that we don’t see it for the violent process it is.
If we could call stars astute, halo stars are wise indeed. Those bullies in the core have created a dreadful life of endless gravitational squabbles and they are stuck with it till they wheeze into white dwarfs. Life in the Mafia is a breeze compared with life in a globular, it’s just a lot faster. The early binaries coupled in loose orbits and became ‘soft’ binaries that can be easily disrupted. And, like hot young things anywhere coupling without any strings attached, soft binaries have the same problem humans do: messy breakups. When a third star comes too close to a soft binary, the heavier two stars will end up binding more tightly and the third will be ejected; it usually ends up in the halo. (Sore loser.) This ‘hardens’ the binary, meaning its stars are in tighter orbits that only really massive interlopers can disrupt. Naturally, this eventually happens, too. If a hard binary comes too close to another hard binary, they end up in an awful slugging match. The end result is as human as it is stellar: the harder binary throws the not-so-hard binary out of the cluster completely. In the end, only the lawyers—gravity—come out ahead. Given the lifestyle of a star cluster, gravity becomes a very pricey lawyer. Poor hapless E3 will learn this the hard way—it is on a collision course with the Milky Way. It was originally a prosperous globular with respectably starry assets. Its legal problems with gravity have proven ruinous. It has lost most of its stars. Now it has one of the least dense cores in the Galaxy. One look in the eyepiece tells us that.
E3's miseries have only begun. The rest of E3's fate is described in the following post.
=Dana in SA