Is there a stellar companion in hybrid star HD 81817?

  • Bang, Tae-Yang (Department of Astronomy and Atmospheric Sciences, Kyungpook National University) ;
  • Lee, Byeong-Cheol (Korea Astronomy and Space Science Institute) ;
  • Perdelwitz, V. (Hamburger Sternwarte) ;
  • Jeong, Gwang-Hui (Korea Astronomy and Space Science Institute) ;
  • Han, Inwoo (Korea Astronomy and Space Science Institute) ;
  • Oh, Hyeong-il (Department of Astronomy and Atmospheric Sciences, Kyungpook National University) ;
  • Park, Myeong-Gu (Department of Astronomy and Atmospheric Sciences, Kyungpook National University)
  • Published : 2019.10.14

Abstract

HD 81817 is known as a hybrid star. Hybrid stars have both cool stellar wind properties and UV or even X-ray emission features of highly ionized atoms in their spectra. A white dwarf companion has been suggested as the source of UV or X-ray features. HD 81817 has been observed since 2004 as a part of our radial velocity (RV) survey program to search for exoplanets around K giant stars using the Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8 m telescope of Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. We obtained 84 RV measurements between 2004 and 2018 for HD 81817 and found two periodic RV variations. The obtained amplitudes of RV periods are around 200 m/s, which are significantly lower than that expected from a white dwarf companion. Furthermore, our re-analysis of the IUE spectra used by Reimers (1984) shows that the excess in UV emission can easily be explained by a pseudo-continuum of unresolved emission lines originating in the extended chromosphere of the star. We thus conclude that there are no companions of stellar mass to HD 81817. Meanwhile, we analyzed two periodicities in RV measurements and conclude that the period of 627.9 days is caused by intrinsic stellar activities based on H alpha equivalent width (EW) variations of a similar period. On the other hand, the period of 1047.8 days is likely to be caused by substellar companion which has a minimum mass of 27.6 MJUP, a semi-major axis of 3.3 AU, and an eccentricity of 0.17 assuming the stellar mass of 4.3 M⊙ for HD 81817. The inferred mass puts HD 81817 b in the brown dwarf desert.

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