Title: Extended Main-Sequence Turnoff and Red Clump in intermediate-age star clusters: A study of NGC 419
Authors: F. Dresbach, D. Massari, B. Lanzoni, et al.
First Author’s Institution: Keele University
Status: Published in Astronomy & Astrophysics [open access]
by Zachary Pleska

This guest post was written by Zachary Pleska, a sophomore at Lycoming College in Williamsport, Pennsylvania. Zachary is pursuing a double major in Astrophysics and Mathematics with a minor in Spanish. Zachary has been mesmerized by the cosmos since he was in middle school when he discovered a “Cosmology and Astronomy” course on Khan Academy. Upon graduating, Zachary aspires to attend graduate school for cosmological astrophysics. Other than academics, Zachary is a baseball player and enjoys playing the flute, working out, playing chess, and being involved in Lycoming’s Code Warriors Club and the Society of Physics Students.
Astronomers always want to know why the universe is the way it is. When focusing on stellar astrophysics, there is always a question of why stars evolve the way they do. Equations of state set a foundation for being able to map out where the stars are, where they came from, and where they are going to be. Two influential graphs in stellar evolution are the observational color-magnitude diagram and the theoretical Hertzsprung-Russel Diagram. This research article focuses on the color-magnitude diagram and attempts to explain deviations with extended main-sequence turnoff and the extended red clump. Without explanation for these deviations in the expected pathways of stellar evolution, it will continue to challenge our understanding about star formation and development in clusters.
These studies are based on an intermediate age cluster, NGC 419, with data found using the Hubble Space Telescope. Hubble uses visible-ultraviolet wavelengths to search for radial distributions of the members in different subregions throughout the cluster to represent an adequate stellar population sample. The search aims for uniformity throughout the cluster by relating the segregation of star color, chemical composition, and position on the color-magnitude diagram.

The researchers observed odd trends in the diagram when using these stars to represent the cluster. One oddity that they found was the splitting of the main sequence versus the extended main-sequence turnoff. This concept similarly corresponds to an extended red clump as the stars continue to evolve.

With these patterns, there is still an absence of an explanation as to why some stars have deviated from the expected evolutionary tracks. The NGC 419, among other clusters, has been studied in the past, and the best explanation was that the birthing period of the cluster occurred over vast spans of time, causing a drastic difference in star age and affecting the evolution of the cluster.
The researchers returned to that theory to see if they could use the data they compiled to support previous hypotheses. The explanation for these extended branches was that the orientation of the stars was non-uniform throughout their lives. Prior researchers attributed these differences to changes in internal structure due to internal mixing, the stars’ greatly varying ages, and how the accretion and influence of other stars could affect their evolutionary path.
The authors of this paper believed differently, as they proposed that the rotational kinematics of the star are a crucial factor in being able to predict how these stars change over time. The focus of their research is on the rotational behaviors of the star: the limits of solar rotation, potential mass loss or extermination from rotation, and the effects on convection and fields. Rotation can have effects including reduced surface gravity, which decreases the luminosity and temperature, causing a dimmer, redder star. Another factor to consider is the angular momentum of the star: as the star expands, it is going to spin much more slowly, causing additional cooling.
From their simulations, they were able to conclude with some level of confidence that these extensions were not solely based on prior theory. They discovered that the absence of segregations of stars in these zones does not support prolonged periods of star formation. However, at the same time, they also proved that their theory could not exist by itself either. A big problem with their theory is that the rotational characteristics of these stars did not display direct correlations between them to the extent that a hypothesis could be made.
After encountering this dilemma, the team tried running simulations with their data, testing a variety of different rotational speeds and characteristics; however, they were still unable to explain these extended branches. They also attempted to use helium abundances and exotic mass loss for these stars to see if that would greatly influence these paths. Although it made a small difference, the change was not significant enough to declare that either was a major contributor. Conclusively, after eliminating the causes of rotational velocity and birthing age spreads for these extended branches, it leaves the possibility that these have originated due to a different process, or, perhaps, that there are shortcomings in our observations and definitions of these stellar objects.
Astrobite edited by: Ali Crisp
Featured image credit: NASA