Speckle observations of the nearby multiple star HR 6819 with a proposed nonaccreting black-hole component

ATel #14340; Robert Klement (CHARA Array of Georgia State University), Nic Scott (NASA Ames Research Center), Thomas Rivinius (European Southern Observatory, Chile), Dietrich Baade (European Southern Observatory, Germany), Petr Hadrava (Astronomical Institute of the Czech Academy of Sciences)
on 21 Jan 2021; 07:17 UT
Credential Certification: Joseph Anderson (janderso@eso.org)

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HR6819 is a bright multiple star containing a moderately evolved classical Be star. Rivinius et al. (2020, A&A, 637L, 3) found the system to be a triple star with the Be star in a wide orbit with an unconstrained period around an inner 40-d binary consisting of a B3III star and a nonaccreting black hole. This was challenged by Bodensteiner et al. (2020, A&A, 641A, 43) who proposed HR6819 to be a binary star with a 40-d period containing the Be star along with a low-mass envelope-stripped B star that is caught in an extremely short phase of its evolution. For the binary model with distance of 340 pc, inclination angle i = 32 deg, and the combined semi-major axis (a1 + a2) sin i = 51.4 Rsun, the maximum angular separation of the two stars amounts to about 1.3 milliarcseconds. By contrast, in the ternary model with the wide, possibly decade-long outer orbit, the separation of the two luminous components can easily be two orders of magnitude larger.

Speckle observations of HR6819 were taken on October 24, 2020 at a seeing of 0.75” with the Zorro imager on the Gemini South telescope in Cerro Pachon, Chile (Scott et al. 2018, PASP, 130, 4502, https://www.gemini.edu/instrumentation/current-instruments/alopeke-zorro). Data cubes consisting of sets of 1,000 x 60ms exposures were acquired in both the 716 and 832-nm bands for HR6819 and a point source calibration star. These data were reduced following the procedures described in Howell et al. (2011, AJ, 142, 19). Interferometric fringes were observed and the reconstructed images show a companion at a separation of 120 milliarcseconds and position angle (North through East) of 88 deg. The data-reduction pipeline did not converge on the magnitude difference. If the observed companion is not a chance superposition (and if the magnitude difference is small), this would exclude the binary model. For confirmation, further observations are needed.