36-OPHIUCHI-AB - INCOMPATIBILITY OF THE ORBIT AND PRECISE RADIAL-VELOCITIES

The long-period (similar to 600 yr) binary system 36 Ophiuchi AB consi sts of two chromospherically active K dwarfs. If we constrain the peri od using the mass-luminosity relation and the observed parallax, the r emaining orbit parameters can be estimated from the 170 years of visua l-binary observations. In this paper, we further constrain the orbit b y precise measurements of the differential radial velocities (with arb itrary zero points) of both 36 Oph A and B and the difference in radia l velocity between 36 Oph B and A; Our best orbit gives a good lit to the visual-binary observations, the difference in velocity, and the me an radial acceleration of A; but the observed acceleration of B is a f actor of 1.64 larger than the value predicted by the orbit. This facto r is so large that no reasonable variation in the adopted sum of masse s, mass ratio, parallax, or orbit parameters will remove the B acceler ation discrepancy. The maximum companion mass allowed by the residuals from the visual-binary orbit is of order 8 Jupiter masses for assumed periods between 30 and 100 yr so the 36 Oph B acceleration discrepanc y would ordinarily make it a candidate for a substellar companion. How ever, the very high eccentricity (similar to 0.9) of the binary-star o rbit means its closest approach is of order 6 a.u,, making it unlikely that any substellar companions would form or survive with semi-major axis exceeding similar to 1.5 a.u, or period exceeding similar to 2 yr . Thus, 36 Oph B is an important counter-example which serves as a war ning that for chromospherically active stars,at !east, it is possible to have apparent radial accelerations in the absence of substellar com panions.