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Title: Formation and Evolution of Blue Stragglers in 47 Tucanae
Authors: J. Parada, H. Richer, J. Heyl, J. Kalirai, R. Goldsbury
First Author’s Institution: Department of Physics & Astronomy, University of British Columbia, Canada
Status: Published in ApJ
Just by looking at a star’s colour, astronomers can learn a few things about its properties. The most direct one is its temperature: red stars like Betelgeuse are cool, blue stars like Rigel are hot. Another property we can estimate by eye is the apparent magnitude, a concept idealised by the Greek Hipparchus almost 2000 years ago. He ranked stars from brightest to faintest in a scale of one to six, one denoting the twenty brightest stars he could see, and six denoting those he could barely spot with the naked eye. This scheme was adapted into a logarithmic scale by N. R. Pogson in 1856, and is still in use nowadays. The apparent magnitude depends on the star’s luminosity and distance, so if we know the latter, we can estimate the former using the measured magnitude. Knowing the luminosity and the temperature we’ve estimated from the star’s colour, we can place it in a Hertzprung-Russell (HR) diagram and infer other properties, like evolutionary stage, mass, and radius. Unfortunately, distance is a very hard thing to measure accurately in astronomy. However, there’s a way to overcome that: studying stars in clusters. In a star cluster, all stars are basically at the same distance from us, so we can ignore the effect of the distance and compare the apparent magnitude of these stars directly. We can then build a colour-magnitude diagram (CMD), the observational version of the HR diagram. Lots of our knowledge about stellar evolution came from comparing theoretical evolutionary models to this kind of diagram.With the development of high resolution imaging, CMDs revealed the presence of some unexpected stellar populations. A prominent population is blue straggler stars (BSS). These stars appear as a blue extension of the main sequence (MS) in globular clusters, right above the so-called turnoff point, where stars are about to leave the main sequence. They are unexpected because, given the cluster’s age and metallicity, stars with their properties should already have evolved off the main sequence. Blue stragglers are still there though, like they were too lazy to leave the MS. How they are formed and where they go when they finally evolve are two questions still up for debate (bite1, bite2). The authors of today’s paper studied the blue stragglers in the cluster 47 Tucanae with the aim of shedding some light on the subject.
Blue stragglers: what we know so far
Combining theory and observations: what 47 Tucanae can teach us
Next the authors relied on evolutionary models calculated with the code MESA (Modules for Experiments in Stellar Astrophysics) to identify the region occupied by evolved blue straggler stars (see Fig. 2). Calculating the cumulative radial distribution, they noted that what they call evolved blue stragglers (eBSS) follow a similar distribution to the blue stragglers, suggesting they are indeed linked. Moreover, they found an excess of stars in the RGB and the horizontal branch (HB) when compared to the expected number considering only single evolution. According to their estimates, this excess can be explained by stars evolving from blue stragglers into these regions. So it appears that the blue stragglers have a post-MS evolution comparable to that of a normal star of the same mass. There’s still some disagreement between the lifetimes the authors estimated and others found in the literature, indicating a more detailed study of individual blue straggler properties is in order to better constrain these values. Future studies using high quality spectra may help with that.
In short, the authors verified that different mechanisms leading to blue stragglers can in fact be identified within a cluster. Interactions in multiple systems seem to dominate in the central regions, while binary evolution seems to be the dominant mechanism in the cluster outskirts. The former leads to more massive, brighter objects, while fainter blue stragglers are explained by the latter. The evolution of the blue stragglers seems to be similar to simple MS stars with same mass, which makes it easier to model their evolution. However, this still has some discrepancies with other results, so more detailed studies, focused on individual objects, are needed. The blue stragglers have not shown all their true colours yet.
About the author, Ingrid Pelisoli:
I am a second year PhD student at Universidade Federal do Rio Grande do Sul, in Brazil. I study white dwarf stars and (try to) use what we learn about them to understand more about the structure and evolution of our Galaxy. When I am not sciencing, I like to binge-watch sci-fi and fantasy series, eat pizza, and drink beer.