The Multiple System HD 27638

Abstract

We report spectroscopic observations of HD 27638B, the secondary in a visual binary in which the physically associated primary (separation approximately 19 arcsec) is a B9V star. The secondary shows strong Li 6708 absorption suggesting youth, and has attracted attention in the past as a candidate post-T Tauri star although this has subsequently been ruled out. It was previously known to be a double-lined spectroscopic binary (F8+G6) with a period of 17.6 days, and to show velocity residuals indicating a more distant massive third companion with a period of at least 8 years. Based on our radial velocity measurements covering more than two cycles of the outer orbit, along with other measurements, we derive an accurate triple orbital solution giving an outer period of 9.447 +/- 0.017 yr. The third object is more massive than either of the other two components of HD 27638B, but is not apparent in the spectra. We derive absolute visual magnitudes and effective temperatures for the three visible stars in HD 27638. Isochrone fitting based on those properties gives an age of 200 +/- 50 Myr for the system. We infer also an inclination angle of about 53.3 degrees for the inner orbit of HD 27638B. We detect a near-infrared excess in HD 27638B which we attribute to the third star being a close binary composed of late-type stars. This explains its large mass and lack of a visible signature. Modeling of this excess allows us to infer not only the masses of the components of the unseen companion, but also the inclination angle of the outer orbit (approximately 73 deg). The HD 27638 system is thus at least quintuple.