Weltevreden SA
15-02-2014, 11:11 AM
Poor old E3. First it was internal punch-ups, now this.
If you think the squabbly little stars in a cluster have sticky outcomes after a brush with gravity, imagine what happens when they collide with a huge spiral like ours. If globulars can be thought of as feisty neighbourhoods, a galaxy is a giant city filled with tower blocks. Just as suburban commuters have to endure the traffic jams of downtown, globulars have to endure high-stress galaxy disc crossings. Remember the last time you crossed downtown Sydney at 5:15 on Friday afternoon?
Galaxy discs are tidal stress fields of disastrous proportions. Go with the flow of the disc around the centre and life is bearable. Ancient but sparse open clusters like Collinder 261 and NGC 188 illustrate what happens when a cluster is gravitationally stable and keeps its distance. But plunge though a galaxy from high above or below? While we have no firm velocity data on E3, we know that it is 8470 light years out. A few well-known halo globulars are nearing the end of their cohesive rope because of galactic crossings—NGC 2298 in Puppis is 9780 light years out yet has lost around 77% of its stars. The notoriously tough visual challenge object NGC 6749 in Aquila is 975 light years from the disk and about to endure a make-or-break crossing through the Scutum bar of the Milky Way. It lucklessly entered one of the most tumultuous regions in a galaxy disc, the junction where a bar smears backward into a spiral arm.
Pal 5 has almost completely disrupted—its former stars are spread along an arc 10° long on either side of the core; their total mass is 1.2 times the mass of the core still remaining. I'd like to lose weight, too, but not 60% of what I have. E3 is looking at a future something like Pal 5’s (http://arxiv.org/pdf/astro-ph/0209555v1.pdf). Moral: globulars shouldn’t mess with a galaxy.
E3 has been so little studied that we don’t know its full orbit. We can’t calculate backwards to its crossing history. We don’t know its mass-to-luminosity ratio or metallicity, which are key to knowing its age and therefore how many times it has crossed our galaxy. No matter: all globulars are effected by a tidal crossing much the same way. The stars on the fore side of the cluster are pulled ahead of their orbits, but not so much they actually escape. Then the whole cluster is bow-shocked as it passes through the disc. Bow shock automatically brings ionization fronts as dust, free electrons, and protons in the Galaxy are brusquely shoved aside. Molecular clouds are disrupted; they respond with an angry red HII glow. Magnetic fields are not one bit happy; they spit back with even more turbulence than they were creating before the cluster arrived. As the cluster crosses the disc, its stars on the fore and aft sides are pulled at by the mass of the disc. Some slip away into the galaxy disc. The rest eventually assume new and more chaotic orbits around the cluster. The cluster core shrinks into new equilibrium. The process then reverses as the cluster pulls away. The cluster loses more stars on departure than it did on arrival. Image #3 attached is a diagram of what tidal stresses did to a cluster similar to E3, Haute Province 1 in Ophiuchus. HP1 is moving to the NW in the direction of the shaded arrow. It can lose stars in all directions, but most of the loss will be on the trailing side. (Source: Ortolani 2011, Fig. 6 (http://arxiv.org/pdf/1106.2725v1.pdf).)
There is evidence that E3 has endured Galaxy crossings before. Earlier we mentioned that E3 is notably deficient in horizontal branch and AGB stars. Those star classes are usually the majority tenants of a globular’s halo because of their low mass and age. ER3’s luminosity function drops off sharply about 2.5 mag below the main sequence turnoff. This is to be expected in a cluster severely truncated by earlier tidal encounters. Another clue is the very small value of E3’s ratio of tidal radius (2.1 arcmin) to core radius (1.85 arcmin)—89%. Similarly distant NGC 2808’s ratio is 32%.
So all in all, E3 is a bit player on the Galactic stage. It barely has a walk-on part. It has lost most of its stars. It has been badly mangled by the Milky Way. It is sparse, vulnerable, and barely visible in our telescopes. It’s practically unknown and no one is much interested. Life in the limelight can be a bummer.
But there’s a happy ending in all this. It’s not about E3, it’s about us. Pruned of a bit of technical arcana, the photometry for the 1984 and 1985 papers that coined the term ‘yellow straggler’ is described this way in one of them (McClure 1985): ‘The CMDs of E3 were recorded with a CCD chip with a size of 492 X 284 pixels, each 0.59 arc sec square, or about 5 arc min E-W by 3 arc min N-S on the sky.’
Now think about that. The biggest and best astro camera available in 1985 had one-quarter of the resolution of a low-budget cellphone today. In 1985, using 3 and 4 meter scopes, it took hours to build up enough E3 photons for one CMD. This week I spotted it using a 150mm Mak-Newt. We’ve come a long way.
If you think the squabbly little stars in a cluster have sticky outcomes after a brush with gravity, imagine what happens when they collide with a huge spiral like ours. If globulars can be thought of as feisty neighbourhoods, a galaxy is a giant city filled with tower blocks. Just as suburban commuters have to endure the traffic jams of downtown, globulars have to endure high-stress galaxy disc crossings. Remember the last time you crossed downtown Sydney at 5:15 on Friday afternoon?
Galaxy discs are tidal stress fields of disastrous proportions. Go with the flow of the disc around the centre and life is bearable. Ancient but sparse open clusters like Collinder 261 and NGC 188 illustrate what happens when a cluster is gravitationally stable and keeps its distance. But plunge though a galaxy from high above or below? While we have no firm velocity data on E3, we know that it is 8470 light years out. A few well-known halo globulars are nearing the end of their cohesive rope because of galactic crossings—NGC 2298 in Puppis is 9780 light years out yet has lost around 77% of its stars. The notoriously tough visual challenge object NGC 6749 in Aquila is 975 light years from the disk and about to endure a make-or-break crossing through the Scutum bar of the Milky Way. It lucklessly entered one of the most tumultuous regions in a galaxy disc, the junction where a bar smears backward into a spiral arm.
Pal 5 has almost completely disrupted—its former stars are spread along an arc 10° long on either side of the core; their total mass is 1.2 times the mass of the core still remaining. I'd like to lose weight, too, but not 60% of what I have. E3 is looking at a future something like Pal 5’s (http://arxiv.org/pdf/astro-ph/0209555v1.pdf). Moral: globulars shouldn’t mess with a galaxy.
E3 has been so little studied that we don’t know its full orbit. We can’t calculate backwards to its crossing history. We don’t know its mass-to-luminosity ratio or metallicity, which are key to knowing its age and therefore how many times it has crossed our galaxy. No matter: all globulars are effected by a tidal crossing much the same way. The stars on the fore side of the cluster are pulled ahead of their orbits, but not so much they actually escape. Then the whole cluster is bow-shocked as it passes through the disc. Bow shock automatically brings ionization fronts as dust, free electrons, and protons in the Galaxy are brusquely shoved aside. Molecular clouds are disrupted; they respond with an angry red HII glow. Magnetic fields are not one bit happy; they spit back with even more turbulence than they were creating before the cluster arrived. As the cluster crosses the disc, its stars on the fore and aft sides are pulled at by the mass of the disc. Some slip away into the galaxy disc. The rest eventually assume new and more chaotic orbits around the cluster. The cluster core shrinks into new equilibrium. The process then reverses as the cluster pulls away. The cluster loses more stars on departure than it did on arrival. Image #3 attached is a diagram of what tidal stresses did to a cluster similar to E3, Haute Province 1 in Ophiuchus. HP1 is moving to the NW in the direction of the shaded arrow. It can lose stars in all directions, but most of the loss will be on the trailing side. (Source: Ortolani 2011, Fig. 6 (http://arxiv.org/pdf/1106.2725v1.pdf).)
There is evidence that E3 has endured Galaxy crossings before. Earlier we mentioned that E3 is notably deficient in horizontal branch and AGB stars. Those star classes are usually the majority tenants of a globular’s halo because of their low mass and age. ER3’s luminosity function drops off sharply about 2.5 mag below the main sequence turnoff. This is to be expected in a cluster severely truncated by earlier tidal encounters. Another clue is the very small value of E3’s ratio of tidal radius (2.1 arcmin) to core radius (1.85 arcmin)—89%. Similarly distant NGC 2808’s ratio is 32%.
So all in all, E3 is a bit player on the Galactic stage. It barely has a walk-on part. It has lost most of its stars. It has been badly mangled by the Milky Way. It is sparse, vulnerable, and barely visible in our telescopes. It’s practically unknown and no one is much interested. Life in the limelight can be a bummer.
But there’s a happy ending in all this. It’s not about E3, it’s about us. Pruned of a bit of technical arcana, the photometry for the 1984 and 1985 papers that coined the term ‘yellow straggler’ is described this way in one of them (McClure 1985): ‘The CMDs of E3 were recorded with a CCD chip with a size of 492 X 284 pixels, each 0.59 arc sec square, or about 5 arc min E-W by 3 arc min N-S on the sky.’
Now think about that. The biggest and best astro camera available in 1985 had one-quarter of the resolution of a low-budget cellphone today. In 1985, using 3 and 4 meter scopes, it took hours to build up enough E3 photons for one CMD. This week I spotted it using a 150mm Mak-Newt. We’ve come a long way.