The young binary HD 102077: Orbit, spectral type, kinematics, and moving group membership

Abstract

The K-type binary star HD 102077 was proposed as a candidate member of the TW Hydrae Association (TWA) which is a young (5-15 Myr) moving group in close proximity (~50 pc) to the solar system. The aim of this work is to verify this hypothesis by different means. We first combine diffraction-limited observations from the ESO NTT 3.5m telescope in SDSS-i' and -z' passbands and ESO 3.6m telescope in H-band with literature data to obtain a new, amended orbit fit, estimate the spectral types of both components, and reanalyse the Hipparcos parallax and proper motion taking the orbital motion into account. Moreover, we use two high-resolution spectra of HD 102077 obtained with the fibre-fed optical echelle spectrograph FEROS at the MPG/ESO 2.2m telescope to determine the radial velocity and the lithium equivalent width of the system. The trajectory of HD 102077 is well constrained and we derive a total system mass of $2.6\pm 0.8$ M${}_{\odot }$ and a semi-major axis of $14.9\pm 1.6$AU. From the i'-z' colours we infer an integrated spectral type of K2V, and individual spectral types of K0 +/- 1 and K5 +/- 1. The radial velocity corrected for the orbital motion of the system is $17.6\pm 2$km/s. Even though the parallax determination from the Hipparcos data is not influenced by the orbital motion, the proper motion changes to ${\mu }_{\alpha }\ast \cos (\delta )=-137.84\pm 1.26$ mas/yr and ${\mu }_{\delta }=-33.53\pm 1.45$mas/yr. With the resultant space motion, the probability of HD 102077 being a member of TWA is less than 1%. Furthermore, the lithium equivalent width of $200\pm 4$mÅ is consistent with an age between 30 Myr and 120 Myr and thus older than the predicted age of TWA. In conclusion, HD 102077's age, galactic space motion, and position do not fit TWA or any other young moving group.

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