Weltevreden SA
02-03-2016, 07:47 AM
Pal 2 in Auriga (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Palomar+2&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id)*
So time for the newbies. First up: Pal 2 in Auriga, only about 20° above the horizon in early evening. To locate objects, I use Alvin Huey’s guides “Globular Clusters (http://faintfuzzies.com/Files/GlobularClusters%20v4.pdf)” and “Local Group Galaxies (http://faintfuzzies.com/Files/LocalGroup%20v2.pdf)” and a set of Jose Torres B-series charts (click on the “TriAtlas project” link (http://www.uv.es/jrtorres/)). Huey shows that Pal 2 is a faint glow between a tiny kite-shaped asterism of four mag 12.5 – 13.9 stars, and nearby Mv 11.0 Tyc 2374-63-1 Aurigae. These are an easy star hop away from mag 2.7 Iota Aurigae (aka Kabdhilinan, and I can’t pronounce it either). The asterism is quickly spotted in direct vision because it is in a region with few stars. Even so, Pal 2 is no easy-pleasey. After a few minutes it became steadily more visible in averted with a few 2 – 3 second holds in direct as the position became familiar and scintillation effects revealed moments of better transparency. Careful examination revealed a thin whitish glow seen steadily in averted about 1 arcmin dia, which is just about right since Pal 2’s half-light radius is just 30 arcsec. Its central luminosity density muV is mag 19, so it may not be resolved even in large amateur instruments. (For comparison, NGC 7006’s muV is 18.72, so anyone who can resolve it might give Pal 2 a try.)
* A note about these Simbad links. If you want to follow-up on any of the objects cited here, go down the object’s Simbad page to the “References” section. Click on the “display” box. That will take you to all the professional papers published about the object since 1850, listed in descending order from the most recent to the oldest. It’s a much easier reference source to work with than astro-ph (http://arxiv.org/find/astro-ph/1) or ADS (http://adsabs.harvard.edu/).
Pal 2 is 88,600 ly out and very near the N. polar axial line of the Milky Way. It is a young halo globular which evolved about the same time as the Milky Way’s primordial bulge was being formed. “Young halo globular” is a code word in astrophysics that means a globular captured when the Milky Way disrupted an ancient dwarf galaxy. As the MW accreted smaller dwarf galaxies into the bulge and nearby globulars into what is now the inner halo, Pal 2 fell within the MW’s tidal or capture radius—but just barely. Its orbit is nearly circular, with an ellipticity of only 0.05. It is also nearly polar, which is to say that it is nearly perpendicular to the Galactic disc. Pal 2 is reddened by galactic dust, by almost 4 magnitudes visual.*
* To get actual visual extinction in our eyepieces from the Harris Catalog E(B–V) numbers, multiply by 3.2. So Harris’s E(B–V) of 1.24 for Pal 2 becomes 1.24 x 3.2 = 3.97 magnitudes as we see it.
This is surprising. Since Pal 2 is in low-dust region of the Galactic halo, why is it so reddened? We can compare it with another remote halo globular in the Galactic polar region, the Intergalactic Tramp NGC 2419, whose reddening is 2.56 magnitudes to our eyes. It turns out that 25% of Pal 2’s reddening was an internal artifact of the Hubble’s camera filtering system in the 2005-2006 era when Pal 2’s initial photometry was measured, and the rest came from dust in a large gas-dust density clump in our own Orion Spur spiral arm. This clump is one of many in the Taurus Molecular Cloud star-forming complex. If you peruse the region between Pal 2 and the Pleiades in a pair of binoculars, you will see numerous areas with very few stars adjacent to patches with quite a lot. The Taurus Molecular Cloud is a large mottled affair with many mid-density clumps of 10 to 100 atoms/cc and pre-stellar cores of 100–10,000 atoms/cc. Molecular clouds are frightfully complex affairs. Magnetic fields and supersonic turbulence compete to inhibit gravity from concentrating all that mass into stars. That is why we have regions where there are many star clusters of similar ages, and other regions where we have large numbers of unaffiliated disc stars, or few stars at all. If you are at all keen on learning more about this, Bruce Elmegreen’s 2007 (http://adsabs.harvard.edu/abs/2007ApJ...668.1064E) and 2008 (http://adsabs.harvard.edu/abs/2008ASPC..388..249E) papers are a good start. Pal 2 is a vexing cluster to astronomers because while the region surrounding it might look empty to us, to astronomers trying to make a CMD of it the foreground & background “scatter” of Milky Way stars is so thick that it is very hard to separate the cluster wheat from the Milky Way chaff. See Figure 18 on page 1666 here (http://iopscience.iop.org/article/10.1086/511979/meta).
So Pal 2 was “Gotcha” # 1 for the Mak-Newt. If object sighting reports had a dedication page like novels do, Pal 2’s dedication page would go to Mathias Wirth.
Pyxis cluster (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Pyxis+cluster&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id)
Next up (almost directly overhead, in fact) was the Pyxis cluster, which has been on my wannasee list for quite awhile. Discovered only in 1995 (http://iopscience.iop.org/article/10.1086/513301?fromSearchPage=true) by the good old-fashioned method of eyeball looking at an image, it is one of the less observable globulars in the galaxy. At 130,400 ly out and lying only 7° above galactic plane, it is reddened only 0.67 visual mag and located in a lovely but distracting star-bestrewn portion of the Perseus Arm well outside and above our own Orion Spur. Pyxis is twice as remote as the entire blanket of stars and gas in Pyxis and Vela that we must wade through to see it. It is one of the sparsest, most extended globulars in the sky, spreading a feeble 9560 solar luminosities across a patch 4 arcmins (98 ly) in diameter lying 130,400 ly out. Pyxis’s luminosity is spread over 7543 sq. light years, or 1 solar luminosity per 1.27 sq. light year. If we lived there our night sky would consist of a few dozen third magnitude stars and perhaps fifty more down to the 6th magnitude, all spread more or less evenly across the sky. In Michael Irwin’s 1995 discovery paper this globular was considered one of the outer halo objects. It is called “Pyxis cluster” because for some years astronomers were not certain that it is a globular versus a remote massive open cluster. It interests astronomers very much because Pyxis is aligned so close to the plane of the Magellanic Stream that it is thought to have been peeled loose from the LMC’s globular system during the LMC-SMC graze-by encounter with the outer halo of the Milky Way some 60 million years ago. Read all about it here (http://adsabs.harvard.edu/abs/2000PASP..112.1305P). On the other hand, it features many technical characteristics of the Palomar globulars, so could simply be yet another distant capture from a dwarf galaxy consumed by the Milky Way. For more about all this, try these papers: 1 (http://adsabs.harvard.edu/abs/1996AJ....112.2013S), 2 (http://adsabs.harvard.edu/abs/2012AAS...21943802P), 3 (http://adsabs.harvard.edu/abs/2001PhDT.........1P).
This is all very interesting, but not nearly as interesting as trying to see the thing. Looking for such a vapid object in a piffling 8-inch scope no matter who made the optics sounds like a Quixotic effort at best and a waste of time at worst. Key in “Pyxis cluster” on WikiSky and you get it, but you might have to enter the Pyxis coordinates 09 07 57, –37 13 17 on your go-to handpad. The finder field shows a pretty kite with a delicate little triplet of mags 13.2 – 14.4 near the top and a scatter of mag 11.5 to 13 stars forming a kind of waistline. Alvin Huey’s image on p.20 in his “Globular Clusters above –50°” guide shows the same field as shown in WikiSky here, minus the bright mag 7.3 star at the bottom. My APM Mak-Newt could see to mag 15.65, according to the Wiki image. During half an hour on three different nights of gazing directly at the cluster position according to Huey’s image, I saw a very faint evanescent glow whose diameter was somewhat larger than the size of the upside-down cross shown in the attached image, just twice for a few seconds each. The glimpses most resembled the globular Fornax 4 near the core of the Fornax Dwarf at mag 13.6 and 0.8 arcmin or 48 arcsec in diameter, although Pyxis was much fainter and more fleeting. Without the Wirth-Intes tenth-wave optics this observation wouldn’t have been possible.
Verdict: No “Gotcha”, but my new Terminator Mak was heard to mutter, “I’ll be back.”
So time for the newbies. First up: Pal 2 in Auriga, only about 20° above the horizon in early evening. To locate objects, I use Alvin Huey’s guides “Globular Clusters (http://faintfuzzies.com/Files/GlobularClusters%20v4.pdf)” and “Local Group Galaxies (http://faintfuzzies.com/Files/LocalGroup%20v2.pdf)” and a set of Jose Torres B-series charts (click on the “TriAtlas project” link (http://www.uv.es/jrtorres/)). Huey shows that Pal 2 is a faint glow between a tiny kite-shaped asterism of four mag 12.5 – 13.9 stars, and nearby Mv 11.0 Tyc 2374-63-1 Aurigae. These are an easy star hop away from mag 2.7 Iota Aurigae (aka Kabdhilinan, and I can’t pronounce it either). The asterism is quickly spotted in direct vision because it is in a region with few stars. Even so, Pal 2 is no easy-pleasey. After a few minutes it became steadily more visible in averted with a few 2 – 3 second holds in direct as the position became familiar and scintillation effects revealed moments of better transparency. Careful examination revealed a thin whitish glow seen steadily in averted about 1 arcmin dia, which is just about right since Pal 2’s half-light radius is just 30 arcsec. Its central luminosity density muV is mag 19, so it may not be resolved even in large amateur instruments. (For comparison, NGC 7006’s muV is 18.72, so anyone who can resolve it might give Pal 2 a try.)
* A note about these Simbad links. If you want to follow-up on any of the objects cited here, go down the object’s Simbad page to the “References” section. Click on the “display” box. That will take you to all the professional papers published about the object since 1850, listed in descending order from the most recent to the oldest. It’s a much easier reference source to work with than astro-ph (http://arxiv.org/find/astro-ph/1) or ADS (http://adsabs.harvard.edu/).
Pal 2 is 88,600 ly out and very near the N. polar axial line of the Milky Way. It is a young halo globular which evolved about the same time as the Milky Way’s primordial bulge was being formed. “Young halo globular” is a code word in astrophysics that means a globular captured when the Milky Way disrupted an ancient dwarf galaxy. As the MW accreted smaller dwarf galaxies into the bulge and nearby globulars into what is now the inner halo, Pal 2 fell within the MW’s tidal or capture radius—but just barely. Its orbit is nearly circular, with an ellipticity of only 0.05. It is also nearly polar, which is to say that it is nearly perpendicular to the Galactic disc. Pal 2 is reddened by galactic dust, by almost 4 magnitudes visual.*
* To get actual visual extinction in our eyepieces from the Harris Catalog E(B–V) numbers, multiply by 3.2. So Harris’s E(B–V) of 1.24 for Pal 2 becomes 1.24 x 3.2 = 3.97 magnitudes as we see it.
This is surprising. Since Pal 2 is in low-dust region of the Galactic halo, why is it so reddened? We can compare it with another remote halo globular in the Galactic polar region, the Intergalactic Tramp NGC 2419, whose reddening is 2.56 magnitudes to our eyes. It turns out that 25% of Pal 2’s reddening was an internal artifact of the Hubble’s camera filtering system in the 2005-2006 era when Pal 2’s initial photometry was measured, and the rest came from dust in a large gas-dust density clump in our own Orion Spur spiral arm. This clump is one of many in the Taurus Molecular Cloud star-forming complex. If you peruse the region between Pal 2 and the Pleiades in a pair of binoculars, you will see numerous areas with very few stars adjacent to patches with quite a lot. The Taurus Molecular Cloud is a large mottled affair with many mid-density clumps of 10 to 100 atoms/cc and pre-stellar cores of 100–10,000 atoms/cc. Molecular clouds are frightfully complex affairs. Magnetic fields and supersonic turbulence compete to inhibit gravity from concentrating all that mass into stars. That is why we have regions where there are many star clusters of similar ages, and other regions where we have large numbers of unaffiliated disc stars, or few stars at all. If you are at all keen on learning more about this, Bruce Elmegreen’s 2007 (http://adsabs.harvard.edu/abs/2007ApJ...668.1064E) and 2008 (http://adsabs.harvard.edu/abs/2008ASPC..388..249E) papers are a good start. Pal 2 is a vexing cluster to astronomers because while the region surrounding it might look empty to us, to astronomers trying to make a CMD of it the foreground & background “scatter” of Milky Way stars is so thick that it is very hard to separate the cluster wheat from the Milky Way chaff. See Figure 18 on page 1666 here (http://iopscience.iop.org/article/10.1086/511979/meta).
So Pal 2 was “Gotcha” # 1 for the Mak-Newt. If object sighting reports had a dedication page like novels do, Pal 2’s dedication page would go to Mathias Wirth.
Pyxis cluster (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Pyxis+cluster&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id)
Next up (almost directly overhead, in fact) was the Pyxis cluster, which has been on my wannasee list for quite awhile. Discovered only in 1995 (http://iopscience.iop.org/article/10.1086/513301?fromSearchPage=true) by the good old-fashioned method of eyeball looking at an image, it is one of the less observable globulars in the galaxy. At 130,400 ly out and lying only 7° above galactic plane, it is reddened only 0.67 visual mag and located in a lovely but distracting star-bestrewn portion of the Perseus Arm well outside and above our own Orion Spur. Pyxis is twice as remote as the entire blanket of stars and gas in Pyxis and Vela that we must wade through to see it. It is one of the sparsest, most extended globulars in the sky, spreading a feeble 9560 solar luminosities across a patch 4 arcmins (98 ly) in diameter lying 130,400 ly out. Pyxis’s luminosity is spread over 7543 sq. light years, or 1 solar luminosity per 1.27 sq. light year. If we lived there our night sky would consist of a few dozen third magnitude stars and perhaps fifty more down to the 6th magnitude, all spread more or less evenly across the sky. In Michael Irwin’s 1995 discovery paper this globular was considered one of the outer halo objects. It is called “Pyxis cluster” because for some years astronomers were not certain that it is a globular versus a remote massive open cluster. It interests astronomers very much because Pyxis is aligned so close to the plane of the Magellanic Stream that it is thought to have been peeled loose from the LMC’s globular system during the LMC-SMC graze-by encounter with the outer halo of the Milky Way some 60 million years ago. Read all about it here (http://adsabs.harvard.edu/abs/2000PASP..112.1305P). On the other hand, it features many technical characteristics of the Palomar globulars, so could simply be yet another distant capture from a dwarf galaxy consumed by the Milky Way. For more about all this, try these papers: 1 (http://adsabs.harvard.edu/abs/1996AJ....112.2013S), 2 (http://adsabs.harvard.edu/abs/2012AAS...21943802P), 3 (http://adsabs.harvard.edu/abs/2001PhDT.........1P).
This is all very interesting, but not nearly as interesting as trying to see the thing. Looking for such a vapid object in a piffling 8-inch scope no matter who made the optics sounds like a Quixotic effort at best and a waste of time at worst. Key in “Pyxis cluster” on WikiSky and you get it, but you might have to enter the Pyxis coordinates 09 07 57, –37 13 17 on your go-to handpad. The finder field shows a pretty kite with a delicate little triplet of mags 13.2 – 14.4 near the top and a scatter of mag 11.5 to 13 stars forming a kind of waistline. Alvin Huey’s image on p.20 in his “Globular Clusters above –50°” guide shows the same field as shown in WikiSky here, minus the bright mag 7.3 star at the bottom. My APM Mak-Newt could see to mag 15.65, according to the Wiki image. During half an hour on three different nights of gazing directly at the cluster position according to Huey’s image, I saw a very faint evanescent glow whose diameter was somewhat larger than the size of the upside-down cross shown in the attached image, just twice for a few seconds each. The glimpses most resembled the globular Fornax 4 near the core of the Fornax Dwarf at mag 13.6 and 0.8 arcmin or 48 arcsec in diameter, although Pyxis was much fainter and more fleeting. Without the Wirth-Intes tenth-wave optics this observation wouldn’t have been possible.
Verdict: No “Gotcha”, but my new Terminator Mak was heard to mutter, “I’ll be back.”