The K2 M67 Study: An Evolved Blue Straggler in M67 from K2 Mission Asteroseismology

Abstract

Yellow straggler stars (YSSs) fall above the subgiant branch in optical color-magnitude diagrams, between the blue stragglers and the red giants. YSSs may represent a population of evolved blue stragglers, but none have the direct and precise mass and radius measurements needed to determine their evolutionary states and formation histories. Here we report the first asteroseismic mass and radius measurements of such a star, the yellow straggler S1237 in the open cluster M67. We apply asteroseismic scaling relations to a frequency analysis of the Kepler K2 light curve and find a mass of 2.9 $\pm$ 0.2 M${}_{\odot }$ and a radius of 9.2 $\pm$ 0.2 R${}_{\odot }$. This is more than twice the mass of the main- sequence turnoff in M67, suggesting S1237 is indeed an evolved blue straggler. S1237 is the primary in a spectroscopic binary. We update the binary orbital solution and use spectral energy distribution (SED) fitting to constrain the color-magnitude diagram (CMD) location of the secondary star. We find that the secondary is likely an upper main-sequence star near the turnoff, but a slightly hotter blue straggler companion is also possible. We then compare the asteroseismic mass of the primary to its mass from CMD fitting, finding the photometry implies a mass and radius more than 2$\sigma$ below the asteroseismic measurement. Finally, we consider formation mechanisms for this star and suggest that S1237 may have formed from dynamical encounters resulting in stellar collisions or a binary merger.

References in corpus (3)

• A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission
• Mergers and Obliquities in Stellar Triples
• The blue stragglers formed via mass transfer in old open clusters

Cited by in corpus (16)

• Surface rotation of Kepler red giant stars
• Eridanus II: A Fossil from Reionization with an Off-Center Star Cluster
• A Census of Blue Stragglers in Gaia DR2 Open Clusters as a Test of Population Synthesis and Mass Transfer Physics
• Stellar Population Astrophysics (SPA) with TNG Atmospheric parameters of members of 16 unstudied open clusters
• WIYN Open Cluster Study: The Old Open Cluster, NGC 188, and a Re-evaluation of Lithium-Richness Among Red Giants
• WIYN Open Cluster Study. LXXXVII. HST Ultraviolet Detection of Hot White Dwarf Companions to Blue Lurkers in M67
• UOCS-VIII. UV Study of the open cluster NGC 2506 using ASTROSAT
• Mergers of Equal-Mass Binaries with Compact Object Companions from Mass Transfer in Triple Star Systems
• A Triple Origin for Twin Blue Stragglers in Close Binaries
• Discovery of double BSS sequences in the old Galactic open cluster Berkeley 17
• WIYN Open Cluster Study. XC. Radial-velocity Measurements and Spectroscopic Binary Orbits in the Open Cluster NGC 2506
• The Blue Lurker WOCS 14020: A Long-Period Post-Common-Envelope Binary in M67 Originating from a Merger in a Triple System
• DUETS: Setting expectations for asteroseismic binaries and binary products with synthetic populations
• Hypercompact stellar clusters: morphological renditions and spectro-photometric models
• Variable Blue Straggler Stars in Open Cluster NGC 6819 Observed in the Kepler 'Superstamp' Field
• First direct detection of an RR Lyrae star conclusively associated with an intermediate-age cluster