Globsmacked | Bad Astronomy

Globsmacked | Bad Astronomy

There are 1 comment on the Bad Astronomy Blog story from Jul 27, 2012, titled Globsmacked | Bad Astronomy. In it, Bad Astronomy Blog reports that:

Globular clusters are some of the most stunning objects in the sky. Composed of hundreds of thousands of stars, over 150 of these compact beehives orbit our Milky Way galaxy alone.

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“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Jul 30, 2012
Like the reference to direct collisional mergers making blue stragglers.

The blue stragglers seen today are probably more likely to be tidally driven mergers of close binaries of W Uma type. Orbits of a day or so would be stable in most globulars but interactions with close passes will eventually rob the binaries of angular momentum in the same way as the distant 3rd companion in a classic W Uma system.

In dense globulars direct collisional mergers would occur and this would be a fast process in the initial chaos of large numbers of near transparent and hot main sequence low metallicity stars. The most massive originally condensed star would be about 3 solar masses in a low metallicity cluster with a star forming wave extending outward from the star rich core as the gas continues to fall into it. The gas inflow therefore directs all newly formed stars toward the globular cluster core while leaving the formative gas behind in the surrounding shell shockfront.
The result is an interstellar traffic jam with direct collisional mergers most likely among the initial stars to form in the cluster closest to the core. The result of a collisional merger is a larger star and easier target with the result being a runaway producing a very massive star in the core of a dense globular in a short time period.

The result is a supergiant still being dumped on by low mass stars as it evolves through its nuclear burning stages and the end result is a gamma ray burst and black hole. The black hole then continues to get dumped on. 3 Globular clusters to my knowledge so far (Omega centauri, M15, and M22,{not sure about M22, was mentioned in passing with no reference to its mass}) host central black holes. The same direct collisional process will also account for the occasional neutron stars in low metallicity globular clusters where the progenitor would be more massive than the possible maximum mass capable of being condensed from a nebula.

The interest here is that in a globular cluster, a stellar mass black hole foremed from a supermassive collisional remnant going supernova at around 10 to 20 solar masses would still be more massive then the bulk of the other stars and would sink into the core ultimately occupying the core position. In a globular cluster of high metallicity such as R136 in the LMC, the black hole formed would be less massive than the neighbouring stars and may be footballed out of the cluster by its companion stars soon after it formed. The metal rich globular cluster R136 may eventually only contain neutron stars or low mass stellar mass black holes as it ages as a result of its most massive stars booting out all the compact remnants or transferring them to the energetic orbit outer shell. A situation similar to the X ray clusters of the starburst galaxy M82.

Of interest here of course is that low mass close binaries may also be formed from grazing, or "pinball machine" near collisions in the core, with orbits then circularised by tidal interactions with neighbours, hence the occasional millisecond pulsars seen in the clusters.

I will keep watching that space, there may be black holes in the core of other dense globular clusters M5 may be just a little bit too small, and 47 Tucanae a mite too loose (large diameter & uncondensed @ the core). The mode of formation may also have a lot to do with the number of neutron stars versus the presence of a black hole in a globular.


Have a nice day: Ag

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