weltevredenkaroo
April 8th, 2015, 12:57 AM
04/07/2015:
Thanks to an offhand comment about runaway O stars, I looked into the literature on this and got a surprise. It turns out that an eye-brow-raising 20% of all the O stars that astronomer have catalogued are loners, seemingly out there with no identifiable cluster nearby. Out of 43 isolated O stars within 60 parsecs (200 ly), only 5 could be identified as within a cluster of very faint stars. Fifty-three percent of the rest were runaways. Runaways are an O star that was ejected from a young, dense cluster. These can travel up to 65 parsecs (211 light years) before it reaches the inevitable day when it goes supernova. They follow what is called a random-walk chain of close encounters with other massive stars that each fortuitously adds to the star’s escape velocity. Considering what a beehive a young, compact cluster is, the odds are that 53% of solo O stars nearby are runaways. Escapee Exhibit A is Wd20a, a pair of 80-plus solar-mass stars orbiting each other in under 4 days. These were ejected from one of the Galaxy’s massive superclusters, Westerlund 2 in Carina. Wd2 is not an easy find on the first try because it’s so unclusterly-looking and faint. My best 7-inch scopes at 300x and above can pull put no more than five stars in a straight line, inside a bit of the white glow that hints of numerous unresolved stars below a scope’s limit.
In the sample of 43 above, deduct the 5 that are in very faint clusters, and 47% of the rest leaves 16 candidates for true Solo Mio. It seems implausible that O stars could actually form in splendid isolation. The cluster process usually originates in a dense collapsing molecular cloud core. The densest core in the Coal Sack (the darkest blob near A Crucis) is a candidate for this type of cluster. The Coal Sack core is somewhat spherical and does not have much rotation as a single unit. At some point, less-than-massive, pre-main sequence Herbig AeBe (HAeBe) protostars will condense from the collapse. These will grow mass more quickly than smaller blobs around them. In this case, the massive early O stars contribute to the total mass that attracts the other gas blobs. The whole things grows in mass as it shrinks in size. A cluster as we know it forms. The beautiful Jewel Box cluster may one day be joined by a new bauble from the Coal Sack. Put another way, as the mass of the most massive object increases, the richness of the associated cluster of lower mass stars also increases (Testi, Palla, & Natta 1999 (http://adsabs.harvard.edu/abs/1999A%26A...342..515T)).
But if the collapsing cloud core is filamentary, lozenge-shaped, or is rotating rapidly as a unit, it will not end up a spherical ball but rather an accretion disc, rather like a gigantic version of the accretion disc that formed around the Sun and eventually became us. (Bonnell & Clarke 1999 (http://adsabs.harvard.edu/abs/1999MNRAS.309..461B)). When that happens, the O star ingests most of the disc spiraling inward around it, leaving little left for other stars. Burp—O Solo Mio. Surprisingly, many of those 43 isolated O stars around us, most are easily visible at between magnitude 5.1 and 9.2 in a pair of binoculars or modest scope. (Zeta Puppis is closer, at mag 2.2) Here’s where to find them: de Wit et al 2004 (http://arxiv.org/pdf/astro-ph/0405348v1.pdf) (Table 2 on p.3) and 2005 (http://arxiv.org/pdf/astro-ph/0503337v1.pdf).
The 2004 paper has many beautiful images showing both the stars and isopleth maps of their fields, demonstrating that they exhibit no evidence of clustering. Most of these stars are identified by the old HD (Henry Draper) numbers. See Table 2 on page 3 of the de Wit et al 2004 paper above. Plug any of these HD numbers into the Aladin Previewer (http://aladin.u-strasbg.fr/java/alapre.pl) or Simbad (http://simbad.u-strasbg.fr/simbad/sim-fid) and they’ll take you right to the star. With pictures, even.
Alas for me, it is **c~l^o*ud•y** A-G-A-I-N. When I poke my nose outside I can’t see my shoes. It’s been an awful summer here, just dreadful. So if all this ignites the urge to go yank the cover off your scope and switch on the go-to, tell me what you find.
=Dana
Thanks to an offhand comment about runaway O stars, I looked into the literature on this and got a surprise. It turns out that an eye-brow-raising 20% of all the O stars that astronomer have catalogued are loners, seemingly out there with no identifiable cluster nearby. Out of 43 isolated O stars within 60 parsecs (200 ly), only 5 could be identified as within a cluster of very faint stars. Fifty-three percent of the rest were runaways. Runaways are an O star that was ejected from a young, dense cluster. These can travel up to 65 parsecs (211 light years) before it reaches the inevitable day when it goes supernova. They follow what is called a random-walk chain of close encounters with other massive stars that each fortuitously adds to the star’s escape velocity. Considering what a beehive a young, compact cluster is, the odds are that 53% of solo O stars nearby are runaways. Escapee Exhibit A is Wd20a, a pair of 80-plus solar-mass stars orbiting each other in under 4 days. These were ejected from one of the Galaxy’s massive superclusters, Westerlund 2 in Carina. Wd2 is not an easy find on the first try because it’s so unclusterly-looking and faint. My best 7-inch scopes at 300x and above can pull put no more than five stars in a straight line, inside a bit of the white glow that hints of numerous unresolved stars below a scope’s limit.
In the sample of 43 above, deduct the 5 that are in very faint clusters, and 47% of the rest leaves 16 candidates for true Solo Mio. It seems implausible that O stars could actually form in splendid isolation. The cluster process usually originates in a dense collapsing molecular cloud core. The densest core in the Coal Sack (the darkest blob near A Crucis) is a candidate for this type of cluster. The Coal Sack core is somewhat spherical and does not have much rotation as a single unit. At some point, less-than-massive, pre-main sequence Herbig AeBe (HAeBe) protostars will condense from the collapse. These will grow mass more quickly than smaller blobs around them. In this case, the massive early O stars contribute to the total mass that attracts the other gas blobs. The whole things grows in mass as it shrinks in size. A cluster as we know it forms. The beautiful Jewel Box cluster may one day be joined by a new bauble from the Coal Sack. Put another way, as the mass of the most massive object increases, the richness of the associated cluster of lower mass stars also increases (Testi, Palla, & Natta 1999 (http://adsabs.harvard.edu/abs/1999A%26A...342..515T)).
But if the collapsing cloud core is filamentary, lozenge-shaped, or is rotating rapidly as a unit, it will not end up a spherical ball but rather an accretion disc, rather like a gigantic version of the accretion disc that formed around the Sun and eventually became us. (Bonnell & Clarke 1999 (http://adsabs.harvard.edu/abs/1999MNRAS.309..461B)). When that happens, the O star ingests most of the disc spiraling inward around it, leaving little left for other stars. Burp—O Solo Mio. Surprisingly, many of those 43 isolated O stars around us, most are easily visible at between magnitude 5.1 and 9.2 in a pair of binoculars or modest scope. (Zeta Puppis is closer, at mag 2.2) Here’s where to find them: de Wit et al 2004 (http://arxiv.org/pdf/astro-ph/0405348v1.pdf) (Table 2 on p.3) and 2005 (http://arxiv.org/pdf/astro-ph/0503337v1.pdf).
The 2004 paper has many beautiful images showing both the stars and isopleth maps of their fields, demonstrating that they exhibit no evidence of clustering. Most of these stars are identified by the old HD (Henry Draper) numbers. See Table 2 on page 3 of the de Wit et al 2004 paper above. Plug any of these HD numbers into the Aladin Previewer (http://aladin.u-strasbg.fr/java/alapre.pl) or Simbad (http://simbad.u-strasbg.fr/simbad/sim-fid) and they’ll take you right to the star. With pictures, even.
Alas for me, it is **c~l^o*ud•y** A-G-A-I-N. When I poke my nose outside I can’t see my shoes. It’s been an awful summer here, just dreadful. So if all this ignites the urge to go yank the cover off your scope and switch on the go-to, tell me what you find.
=Dana