Previously, the observation was made that BSs with non zero eccentricities are preferentially made from binaries with a period range of about 1,000 to 10,000 days. The next question that this study attempted to answer is, are the formation pathways which produced those BSs also related to the eccentricity, period, or perhaps both? A great advantage of the BSE is the documentation of the formation pathways of each of the BSs generated through various mechanisms of binary evolution. When the specific type of mass transfer that allowed the accreting star to be classified as a BSs is paired with its orbital parameters, particularly the e versus log p plot, yet another striking trend emerges.
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In the above left pane one will see the simuated e log p plot, where the black columns represents clumps of selected BSs accumulated from all 1,000 runs. It appears straightway that there are some divisions between BSs as they appear to arrange themselves in columns according to a given period range, especially in the "BSs Before Simulation" plot. In fact, when we assign a color for each case of the aforementioned mass transfers and then highlight each binary BSs according to the mass transfer case that produced it with that color, then the divisions of BSs, or as it turns out the formation pathways, become quite apparent. The red strip that is seen in this plot highlights only the BSs that were formed through merger. From this, it seems evident that BSs formation is more dependent upon the orbital period as opposed to the eccentricity. In the right pane, one will see the regions on the CMD where all of the BSs generated in 1,000 runs lie in terms of their formation pathway.
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While only those BSs formed through binary mergers are shown above, in the diagrams that follow (below) note how for certain formation pathways, such as the AGB-MS and wind accretion especially, the mass transfer type regionalizes on the CMD very near to the MS turnoff. If the BSE is accurate, this trend may be used along with the oberserved orbital parameters as a way to approximate the formation pathways of observed BSs.
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Illustrating the Formation Pathway's Dependence on BSs Production
BSs Formation By MS-MS RLOF- Occurs mostly in close period binaries
- For those BSs remaining in binaries, all have short period and circular orbits
- This formation type produced about 1% of BSs
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BSs Formation By Giant-MS RLOF- Occurs in slightly wider binaries than MS-MS RLOF
- Forms BSs that are in short to medium period binaryies, all with circular orbits
- This formation type produced about 8% of BSs
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BSs Formation By AGB-MS RLOF- Occurs in medium-wide binaries
- Generally produces BSs in long period binaries (nearing 1,000 days) with circular orbits
- Increasingly regionalized on the CMD
- This formation type produced about 23% of BSs
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BSs Formation By RGB-MS Wind- Occurs in wide binaries with periods greater than about 1,000 days
- Produces BSs in binaries with similar periods though with various eccentricities, not necessarily zero
- Regionalized on CMD
- This formation type produced about 22% of BSs
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BSs Formation By Supergiant Wind- Occurs in wide binaries with periods greater than about 1,000 days (same as RGB-MS wind)
- Produces BSs in a similar period range, again with various eccentricities, not necessarily zero
- Highly regionalized on CMD
- This formation type produced about 3% of BSs
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The Big Prediction
At this point binary evolution makes its most blatant declaration. While all mergers and Roche Lobe Overflow formation pathways tend to produce BSs in tidally circularized binaries (previously known), one now must note the spike in the e log p plot (at right - bottom) in the 1,000 10,000 day period range. Keeping in mind that this peak is recognized in observation, the results of the simulation seemingly declare that all BSs observed at this period and with non zero eccentricity, were formed through wind accretion from a Hertzsprung Gap (HG), Red Giant Branch (RGB), sub-giant, or supergiant star as opposed to merger or a type of Roche Lobe Overflow. Stated another way, if this model of stellar evolution is correct, then after 7 Gyr of evolution in an open cluster, we may predict that the only BSs found in binaries with an eccentricity other than zero, must be created by a form of wind accretion.
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The verification or falsification of this prediction places a significant constraint on the legitimacy of creating BSs through binary evolution alone. If it can be observed that BSs created through Roche Lobe Overflow have a variety of eccentricities on a consistent basis throughout clusters of similar ages to NGC 188, then their orbits must necessarily have been perturbed by an outside influence owing to the cluster environment and not to any mechanism currently included in the BSE. Thus, to explain the full panoply of BSs parameters at that point, one would need to evoke cluster dynamics.
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