Weltevreden SA
27-03-2014, 09:30 AM
I'm reposting this in modified form from the IIC Observing Reports forum at the suggestion of Robert. I hope this gets a good discussion going, as the LMC and SMC halo regions don't get a lot of attention from either us or the professionals. Robert has pointed out the same level of tepid interest in the Milky Way and Andromeda halo regions. I guess haloes just aren't very sexy. I'm heading out to the dark site now, where I won't have Internet and cell phone service till I get back in a week. I really look forward to what this topic stimulates.
The span between the western (leading) fringe of the LMC and the shallow triangle of Mensa’s base—basically the two-degree band between 74° and 76° South, and spanning alpha to eta Mensae—contains a boggling 35 faint globulars and open clusters. None are marked on my 1988 edition of Uranometria (charts 462 & 463). The only downloadable charts that give any clue of the profusion of faint LMC globulars in Mensa are the 10 shown on Chart 217 of José Torrés B set (http://www.uv.es/jrtorres/TriAtlas_B_Index.pdf) (DSOs down to visual mag 14), and Chart 558 of Torrés’s chart set C (http://www.uv.es/jrtorres/TriAtlas_C_Index.pdf) (DSOs to mag 15.5). I’ve attached a clip of the area from the Torres C set.
The LMC is a terrible place to plan for a long, quiet holiday. A simple sweep at 30x using an 80mm refractor unveils a dazzling variety of every kind of object except empty space. There are over 1800 star clusters alone. Broaden your definition of ‘interesting’ and you’ll wish you hadn’t: the Bica et al survey (http://fr.arxiv.org/abs/astro-ph/9810266) done in 1998 lists 7847 ‘extended objects’. That doesn’t include binaries.
Paddy, IIS’s resident guru on matters Magellanic, has done a magnificent job of making sense out of the Magellanics. Recently I was looking for interesting things to check out and came across the Torres C chart, which gave positions for 10 globular and open clusters far to the south of Paddy’s limits. They turned out to be some of the more demanding objects I’ve ever tried. Six hours over four nights using a 180mm Mak and all I had to show for it was a dozen tick marks on a list of objects and an empty tube of mozzie repellant.
Seen in isolation, the 10 clusters are piffles and a puffball amid the grand Magellanic scheme of things. They have been little studied, yet they have the potential to add a new facet to globular cluster lore: the chemical history of delayed adolescence. What’s more, two of them—NGC 1777 and 2209—are open clusters, not globulars, even though they look like globulars. Indeed, most Magellanic clusters look much the same to a visual observer. Cluster categories for objects as remote as the Magellanics is more a matter of astrophysical definition than eyepiece appearance. Faint and fuzzy is what we see, no matter what name the picky physics folks give it.
Several days after I observed them I perused the literature on their NGC and IC numbers and discovered I had stumbled upon what we non-PhD lot regard as an ocular Holy Grail—something very few people have ever noticed, and even fewer have studied. The NED and Simbad data on the 10 makes pretty skimpy reading. An ADS search for ‘LMC halo globular cluster’ brings up Kenneth Freeman’s name in items #3 and #4—in papers dated 1983 and 1987. Those were the days when the word Macintosh meant a red fruit that grows on trees. As a category of study, the Magellanic halo clusters are terra practically incognita.
Start with the difference between a globular and an open cluster. Do the Magellanic halo clusters follow different set of rules than the Milky Way’s?
Yes. Of the 10 Mensa clusters that we can see using a good scope on a good night, all are between 300 million and 3 billion years old. This age bracket is typical for many LMC clusters, but it is a mere 1/4 to 1/3 the age of most Milky Way globulars. Why should a 3 billion year old cluster be all that different from an 11 billion year cluster? By 3 billion years the only stars left began life less than 2.1 times the Sun's mass when fusion reactions started in their cores. When we think ‘globular cluster’ we automatically think of wizened decabillionarians glowing faintly into their dotage in serene loneliness. But the youngest fully accepted globular cluster is NGC 1818 in the LMC, at 40 million years. (This fact has been featured 8 times on APOD since 1997.) The technical distinction between an open and a globular cluster rests on the finer points of astrophysics—core luminosity, tidal radius, mass-luminosity ratio, binarism, relaxation parameters, evaporation mass loss rate, and several magnitude spreads on the cluster's colour-magnitude diagrams, e.g. the difference between the main sequence turnoff and the stars in the horizontal branch directly above the MSTO. It all gets a bit arcane. The bottom line is that globulars can hang together for many billions of years while very few open clusters remain bound longer than half a billion years. The open clusters that survive are on large looping orbits that keep them v-e-r-y far away from the chewy habits of the Milky Way disc.
What does remoteness do to a cluster? In the Magellanics, we simply don't know. In our galaxy, only a few globulars reside in the outer halo, and most of them are the cores of former dwarf galaxies whose outer stars were stripped away and now reside in the Milky Way's halo (many in retrograde orbits, a dead giveaway as to their origins). As for globulars near a galaxy's core, the Milky Way bulge hosts over 20 contrarian GCs that were born before the bulge, watched their neighbourhood turn into a boxy peanut, and survived to tell the tale. Haute Province 1 (Ophiuchus) is presently the nearest to the galactic core (1630 light years) and yet is roughly 13 billion years old. Core concentration? Nope there, too: Six Milky Way's massive young clusters are far denser than the old globular Pal 3 in Sextans—we can see three of them in our scopes as NGC 3603, Westerlund 1, and Westerlund 2. Some Class XII globulars such as NGC 7942 Aquarius are looser and less massive than a young hotshot like the Jewel Box.
Core or bulge globulars are tough critters. The same can't be said for remote globulars that cross horns with our spiral disc every couple of billion years or so. Poor NGC 7942 is falling headlong into its likely last passage through the Milky Way disc. Presently the loosest known globular, it will end up a swarm of dissociated stars in roughly half a billion more years. (For more details about what happens to a globular after a few disc crossings, see ‘Cluster’s Last Stand (http://adsabs.harvard.edu/abs/2001AJ....121..935S)’ which relates the fate of Palomar 13.)
As hinted above, professional astronomers have a different notion of the term ‘globular cluster’ than our airy eyepiece impressions of a beautiful speckled ball. They are more impressed with the shape and contents of the cluster’s horizontal branch in the Color Magnitude Diagram. Sporting a blue tail is is a prized quality in the horizontal branch’s celestial kennel show. Astronomers also fancy other indicators such as the magnitude difference between a cluster’s red giant tip and the horizontal branch red clump; this is a measure of hydrogen envelope loss as the core fuses helium into carbon and oxygen. Arrival at the horizontal red clump signifies a star’s heaviest mass loss phase is over. This data point helps determine the age and internal dynamics of the entire cluster. Astronomers naturally have a name for this, the d(V-I) index. Graduate students, be forewarned: it means mathematics nightmares till you get through it.
Astronomers also wring their hankies over whether a cluster exhibits an anti-correlation between its oxygen and sodium abundances, an innocuous-sounding phrase that carries a big stick when it comes to defining a globular cluster’s status. The [Na/O] reciprocal arises in connection with two less-known cyclical catalytic processes that affect an aging star’s atmospheric chemistry, the Neon-Sodium and Magnesium-Aluminium-cycles. This is a worthy topic because our lives depend upon it: the richness of earthly life forms we enjoy daily originated in complex nuclear reactions in the horizontal branches and AGB phases of stars long since vanished into the cosmic graveyard. (You can read all about the importance of the horizontal branch in globular cluster studies here (http://fr.arxiv.org/abs/0911.2469).)
We could read papers on family squabbles between galaxies and star clusters for the rest of our lives and never get through just the papers published up till now. However, when it comes to the Magellanic clusters, all these uncountable pages of astrophysical jargon have a very obvious shortcoming: what we know about the minute-by-minute pulsebeat of globular clusters comes almost entirely from Milky Way studies. The most detailed and digestible paper devoted to our Magellanic friends is a 2007 study (http://adsabs.harvard.edu/cgi-bin/bib_query?2007A%26A...462..139K) of 15 of the LMC’s middle-age globulars. I don't want to go about chiding professional astronomers, but really fellas, let's all get on the train here.
So MW globulars are very old and very chemically evolved compared with the youngsters in the Magellanic Clouds. You'd never know it just by looking at them. I observed 10 of the lot. They are faint, elusive, demand patience and attention, and for all that give in return a dozen measly check marks on a to-see list. That leaves 1790 clusters left to go. I'm off to the dark site for a week. With luck I'll whittle that down to 1780. That is likely to be about the number as swatted mosquitoes. We must be daft to do this, yet I can hardly wait till dusk.
=Dana in S Africa
The span between the western (leading) fringe of the LMC and the shallow triangle of Mensa’s base—basically the two-degree band between 74° and 76° South, and spanning alpha to eta Mensae—contains a boggling 35 faint globulars and open clusters. None are marked on my 1988 edition of Uranometria (charts 462 & 463). The only downloadable charts that give any clue of the profusion of faint LMC globulars in Mensa are the 10 shown on Chart 217 of José Torrés B set (http://www.uv.es/jrtorres/TriAtlas_B_Index.pdf) (DSOs down to visual mag 14), and Chart 558 of Torrés’s chart set C (http://www.uv.es/jrtorres/TriAtlas_C_Index.pdf) (DSOs to mag 15.5). I’ve attached a clip of the area from the Torres C set.
The LMC is a terrible place to plan for a long, quiet holiday. A simple sweep at 30x using an 80mm refractor unveils a dazzling variety of every kind of object except empty space. There are over 1800 star clusters alone. Broaden your definition of ‘interesting’ and you’ll wish you hadn’t: the Bica et al survey (http://fr.arxiv.org/abs/astro-ph/9810266) done in 1998 lists 7847 ‘extended objects’. That doesn’t include binaries.
Paddy, IIS’s resident guru on matters Magellanic, has done a magnificent job of making sense out of the Magellanics. Recently I was looking for interesting things to check out and came across the Torres C chart, which gave positions for 10 globular and open clusters far to the south of Paddy’s limits. They turned out to be some of the more demanding objects I’ve ever tried. Six hours over four nights using a 180mm Mak and all I had to show for it was a dozen tick marks on a list of objects and an empty tube of mozzie repellant.
Seen in isolation, the 10 clusters are piffles and a puffball amid the grand Magellanic scheme of things. They have been little studied, yet they have the potential to add a new facet to globular cluster lore: the chemical history of delayed adolescence. What’s more, two of them—NGC 1777 and 2209—are open clusters, not globulars, even though they look like globulars. Indeed, most Magellanic clusters look much the same to a visual observer. Cluster categories for objects as remote as the Magellanics is more a matter of astrophysical definition than eyepiece appearance. Faint and fuzzy is what we see, no matter what name the picky physics folks give it.
Several days after I observed them I perused the literature on their NGC and IC numbers and discovered I had stumbled upon what we non-PhD lot regard as an ocular Holy Grail—something very few people have ever noticed, and even fewer have studied. The NED and Simbad data on the 10 makes pretty skimpy reading. An ADS search for ‘LMC halo globular cluster’ brings up Kenneth Freeman’s name in items #3 and #4—in papers dated 1983 and 1987. Those were the days when the word Macintosh meant a red fruit that grows on trees. As a category of study, the Magellanic halo clusters are terra practically incognita.
Start with the difference between a globular and an open cluster. Do the Magellanic halo clusters follow different set of rules than the Milky Way’s?
Yes. Of the 10 Mensa clusters that we can see using a good scope on a good night, all are between 300 million and 3 billion years old. This age bracket is typical for many LMC clusters, but it is a mere 1/4 to 1/3 the age of most Milky Way globulars. Why should a 3 billion year old cluster be all that different from an 11 billion year cluster? By 3 billion years the only stars left began life less than 2.1 times the Sun's mass when fusion reactions started in their cores. When we think ‘globular cluster’ we automatically think of wizened decabillionarians glowing faintly into their dotage in serene loneliness. But the youngest fully accepted globular cluster is NGC 1818 in the LMC, at 40 million years. (This fact has been featured 8 times on APOD since 1997.) The technical distinction between an open and a globular cluster rests on the finer points of astrophysics—core luminosity, tidal radius, mass-luminosity ratio, binarism, relaxation parameters, evaporation mass loss rate, and several magnitude spreads on the cluster's colour-magnitude diagrams, e.g. the difference between the main sequence turnoff and the stars in the horizontal branch directly above the MSTO. It all gets a bit arcane. The bottom line is that globulars can hang together for many billions of years while very few open clusters remain bound longer than half a billion years. The open clusters that survive are on large looping orbits that keep them v-e-r-y far away from the chewy habits of the Milky Way disc.
What does remoteness do to a cluster? In the Magellanics, we simply don't know. In our galaxy, only a few globulars reside in the outer halo, and most of them are the cores of former dwarf galaxies whose outer stars were stripped away and now reside in the Milky Way's halo (many in retrograde orbits, a dead giveaway as to their origins). As for globulars near a galaxy's core, the Milky Way bulge hosts over 20 contrarian GCs that were born before the bulge, watched their neighbourhood turn into a boxy peanut, and survived to tell the tale. Haute Province 1 (Ophiuchus) is presently the nearest to the galactic core (1630 light years) and yet is roughly 13 billion years old. Core concentration? Nope there, too: Six Milky Way's massive young clusters are far denser than the old globular Pal 3 in Sextans—we can see three of them in our scopes as NGC 3603, Westerlund 1, and Westerlund 2. Some Class XII globulars such as NGC 7942 Aquarius are looser and less massive than a young hotshot like the Jewel Box.
Core or bulge globulars are tough critters. The same can't be said for remote globulars that cross horns with our spiral disc every couple of billion years or so. Poor NGC 7942 is falling headlong into its likely last passage through the Milky Way disc. Presently the loosest known globular, it will end up a swarm of dissociated stars in roughly half a billion more years. (For more details about what happens to a globular after a few disc crossings, see ‘Cluster’s Last Stand (http://adsabs.harvard.edu/abs/2001AJ....121..935S)’ which relates the fate of Palomar 13.)
As hinted above, professional astronomers have a different notion of the term ‘globular cluster’ than our airy eyepiece impressions of a beautiful speckled ball. They are more impressed with the shape and contents of the cluster’s horizontal branch in the Color Magnitude Diagram. Sporting a blue tail is is a prized quality in the horizontal branch’s celestial kennel show. Astronomers also fancy other indicators such as the magnitude difference between a cluster’s red giant tip and the horizontal branch red clump; this is a measure of hydrogen envelope loss as the core fuses helium into carbon and oxygen. Arrival at the horizontal red clump signifies a star’s heaviest mass loss phase is over. This data point helps determine the age and internal dynamics of the entire cluster. Astronomers naturally have a name for this, the d(V-I) index. Graduate students, be forewarned: it means mathematics nightmares till you get through it.
Astronomers also wring their hankies over whether a cluster exhibits an anti-correlation between its oxygen and sodium abundances, an innocuous-sounding phrase that carries a big stick when it comes to defining a globular cluster’s status. The [Na/O] reciprocal arises in connection with two less-known cyclical catalytic processes that affect an aging star’s atmospheric chemistry, the Neon-Sodium and Magnesium-Aluminium-cycles. This is a worthy topic because our lives depend upon it: the richness of earthly life forms we enjoy daily originated in complex nuclear reactions in the horizontal branches and AGB phases of stars long since vanished into the cosmic graveyard. (You can read all about the importance of the horizontal branch in globular cluster studies here (http://fr.arxiv.org/abs/0911.2469).)
We could read papers on family squabbles between galaxies and star clusters for the rest of our lives and never get through just the papers published up till now. However, when it comes to the Magellanic clusters, all these uncountable pages of astrophysical jargon have a very obvious shortcoming: what we know about the minute-by-minute pulsebeat of globular clusters comes almost entirely from Milky Way studies. The most detailed and digestible paper devoted to our Magellanic friends is a 2007 study (http://adsabs.harvard.edu/cgi-bin/bib_query?2007A%26A...462..139K) of 15 of the LMC’s middle-age globulars. I don't want to go about chiding professional astronomers, but really fellas, let's all get on the train here.
So MW globulars are very old and very chemically evolved compared with the youngsters in the Magellanic Clouds. You'd never know it just by looking at them. I observed 10 of the lot. They are faint, elusive, demand patience and attention, and for all that give in return a dozen measly check marks on a to-see list. That leaves 1790 clusters left to go. I'm off to the dark site for a week. With luck I'll whittle that down to 1780. That is likely to be about the number as swatted mosquitoes. We must be daft to do this, yet I can hardly wait till dusk.
=Dana in S Africa