Seminar in Stellar Astronomy

Optical color-magnitude diagram reveal that 25% of the evolved stars in older open clusters do not fall along standard single-star evolutionary tracks. These stars include the well-known blue stragglers, but also the yellow giants and sub-subgiants.

Sub-subgiant stars (SSGs) lie red of the main-sequence and fainter than the red giant branch in a region not easily populated by standard stellar evolution pathways. While there has been speculation on what mechanisms may create these unusual stars, no well-developed theory has yet been proposed to explain their origins. This talk will focus on three hypotheses of SSG formation: (1) mass transfer in a binary system, (2) tidally induced mass loss from a subgiant star during a dynamical encounter, and (3) reduced luminosity due to magnetic fields that lower convective efficiency and produce large star spots.

Using the stellar evolution code MESA, we have developed evolutionary tracks for each of these hypotheses, and compared the expected stellar and orbital properties of these models with the six known SSGs in the two open clusters M67 and NGC 6791. We have also calculated the frequency with which each mechanism is expected to produce sub-subgiants in each cluster.

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