Introducing BSs & Hypothesized Formation Pathways

Why are BSs Important for Study?

Evidence shows that Blue Straggler Stars (BSs) in possibly all but the youngest systems result from interactions involving binary systems, thus BSs point to the binary populations in the cluster or dwarf galaxy. Since binaries represent a significant percentage of all the stars in the sky, BSs studies provide an opportunity to learn how interacting binary systems affect stellar evolution. The standard stellar evolution model of an isolated star predicts that once sufficient core hydrogen has been depleted, it will evolve away from the main sequence (MS) toward the giant branch. Proper motion studies concluded that the observed BSs were indeed members of the clusters, as opposed to field stars residing outside the cluster. Interestingly, the ages of the clusters can be measured by knowing the color and magnitude of the turn-off of the main sequence as its luminosity decreases and color reddens with time.

Hypothesized Formation Pathways

Though not applicable for this study, an interesting postulate for the formation f a single BSs (especially in young clusters) is by internal mixing of its hydrogen fuel. In 1980, Wheeler suggested that single stars could become BSs if an internal mechanism supported by fast rotation or by higher than normal magnetism would bring in more gas to the core from the outer layers to rejuvenate the life of the star.

Around this time, two additional pathways for BSs formation were accepted that involved the evolution of binary systems: (1) mass transfer in a close binary system rejuvenates the hydrogen fuel; and (2) angular momentum loss in a close binary system resulting in coalescence of the components. Evidence supporting the evolution of binaries was supported by the work of Mathieu and Lathman (1986, 1987) who found the concentration of BSs was distributed in a similar way to known binary stars, especially toward the cluster centers. This apparent mass segregation suggests that BSs are either binaries or massive single stars (masses at least 1.5 solar mass).

Illustration by hubblesite.org

In contrast to a BSs being formed through a mergers from binary evolution, BSs were also believed to be created through dynamical collisions (as illustrated above). Collisional mergers owe to direct stellar collisions of cluster stars that occur as they orbit the cluster’s center of gravity. Binary evolution, therefore, does not encompass BSs created in this way. Stryker (1993) provides the following scenario of binary evolution leading to the potential formation of a BSs.

W Ursae Majoris (W UMa) stars are unique and exciting since not only are they in binaries, they are also in contact with one another. W UMas are two main sequence stars that evolve by transferring mass from the original secondary to the primary and in time will undergo “deep contact”, or coalescence. If the stars rapidly coalescence, one expects the material from the secondary to mix throughout the envelope of the final merged product, replenishing the used hydrogen and thus rejuvenating the star. Physically, the merged product will be as a younger star – its appearance, that of a blue straggler. According to Stryker, the number of close binaries observed to date among well-studied BSs in intermediate to old open and globular clusters may be consistent with the merger hypothesis.

W UMa Contact Binaries Diagram by Carla Maceroni ©

Young age cluster: <2 X 10^8 yrs

• W UMa stars are not observed

• Internal mixing within a single star supported by rapid rotation seems a possible way to create BSs (Stryker 1993)

Intermediate age clusters: 3 X 10^8 to 10^9 yrs

• BSs in this group typically have slower rotations but stronger magnetic fields, and it is noted that moderate magnetic fields (10^3 Gauss) could internally mix the material of the star prolonging its main sequence lifetime (Stryker 1993).

Old age clusters: 3 – 8 X 10^9 yrs

• BSs are numerous and compose nearly 10% of the visual (V) light of their clusters

• BSs with masses > 2.5 magnitudes above the MS turn-off have been observed (see the next section for the significance of this observation)

• A high binary frequency is observed in a sample of BSs and in open clusters at ages around 4 – 5 Gyr, there appears to be a large formation rate of contact binaries (Kaluzny et al. 1993).

• Mathys (1988) infers that BSs in old open clusters do not generally possess large magnetic fields, thus magnetic fields do not appear to contribute to BSs formation in old clusters. Since the cluster of this study is NGC 188, its old age therefore places internal mixing outside of the range of consideration for possible formation pathways.