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    • Keywords


      Open star clusters (1160); blue straggler stars (168); extreme horizontal branch stars (513); B subdwarf stars (129); ultraviolet astronomy (1736); spectral energy distribution (2129); binary stars (154).

    • Abstract


      King 2, one of the oldest clusters in the Milky Way, with an age of $\sim$6 Gyr and distance of $\sim$5700 pc, has been observed with UVIT payload on the AstroSat. With membership information derived from Gaia EDR3, the cluster is found to have 39 blue straggler stars (BSSs). We created multi-wavelength spectra-energy distributions (SED) of all the BSSs. Out of 10 UV detected BSSs, 6 bright ones were fitted with double component SEDs and were found to have hotter companions with properties similar to extreme horizontal branch (EHB)/subdwarf B (sdB) stars, with a range in luminosity and temperature, suggesting a diversity among the hot companions. We suggest that at least 15% of BSSs in this cluster are formed via mass-transfer pathway. When we compared their properties to EHBs and hotter companions to BSS in open and globular clusters, we suggest that EHB/sdBs like companions can form in binaries of open clusters as young as 6 Gyr.

    • Author Affiliations



      1. Indian Institute of Astrophysics, Sarjapur Road, Koramangala, Bangalore 560 034, India.
      2. Joint Astronomy Programme and Department of Physics, Indian Institute of Science, Bangalore 560 012, India.
      3. Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 002, India
    • Dates

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