Formation of blue straggler stars revealed through Hubble Space Telescope observations

by Natalie Gosnell | Graduate student, UW-Madison
Posted Feb 27, 2014

Artist's cartoon of a blue straggler star (front) accreting gas from its companion (back). Later, the companion will turn into a white dwarf star, while the blue straggler will appear too bright and too blue compared to other stars in its immediate neighborhood.

A figure showing the color and brightness of three blue straggler companion stars and contours of where the blue stragglers should be if they did not have a companion white dwarf (lower right). This figure confidently demonstrates the presence of white dwarfs in these systems.

Blue straggler stars are originally defined to be anomalous stars more luminous and bluer than the hydrogen-burning stars on the main sequence of a star cluster.  They are found in open star clusters, globular star clusters, the Galactic field, and dwarf spheroidal galaxies, yet their formation has been a puzzle for almost six decades.  How do these stars obtain the extra mass necessary to be brighter than they should be? Several formation pathways have been identified in the past decade, but recent work at UW-Madison has observationally confirmed the formation of a specific population of blue stragglers for the first time.

Writing in the Astrophysical Journal Letters on February 13, UW-Madison graduate student Natalie Gosnell, along her advisor Bob Mathieu and a team of international researchers, used the Hubble Space Telescope to identify three young white dwarf companions to blue stragglers stars in the old open cluster NGC 188.  These three white dwarfs are definitive proof that the blue stragglers in NGC 188 are dominated by mass transfer formation.

Using a novel observational approach, Gosnell detected the white dwarf companions through far-ultraviolet photometry using the Solar Blind Channel on the Advanced Camera for Surveys.  "Since NGC 188 is 7 billion years old the blue stragglers are not too hot.  Their temperatures are just above that of our Sun," says Gosnell.  "The blue straggler does not emit very much radiation in the ultraviolet, giving us a window to detect the emission from the much hotter, but smaller, white dwarf companion."  The three detections found by the team are consistent with all the binary blue stragglers forming through mass transfer.  "The white dwarfs cool as they age," explains Gosnell, "so we can only detect the youngest white dwarfs using this observational method."  

These binaries will aid future mass transfer modeling efforts.  Mathieu and collaborators have been observing NGC 188 for almost 15 years using the WIYN 3.5m Telescope.  Since these binaries exist in such a well-studied cluster environment researchers can constrain the binary parameters before the mass transfer begins, as well as know what the binary looks like after mass transfer is completed.   

Improving our understanding of the physics of mass transfer is key to understanding the evolved stellar populations of clusters. Combined with yellow giants and sub-sub giants, other types of stars thought to form through mass transfer, blue stragglers comprise 25% of the evolved population of NGC 188.  "Blue stragglers are challenging our understanding of stellar astrophysics," says Gosnell, "but these observations broadly answer how the blue stragglers came to be, and that is an important place to start."