A Century of Radial Velocity and Astrometric Monitoring of 70 Oph AB: New PFS Data and Constraints on Planetary Companions

Abstract

At a distance of 5.1 pc, the 70 Oph AB binary star system is one of the most favorable targets for future direct imaging and astrometry missions surveying mature, terrestrial planets. We present new radial velocities (RVs) obtained with the Planet Finder Spectrograph (PFS) on the 6.5\,m Magellan II Clay Telescope in Chile. We collected 499 measurements of 70 Oph A and 334 measurements of 70 Oph B during 2023--2025. Combining these data with decades of archival RVs and astrometry, we derive an updated orbital solution for the binary and dynamical masses of $0.88 \pm 0.004\,M_\odot$ and $0.73 \pm 0.003\,M_\odot$ for the primary and secondary components, respectively. We find that the long-term RV variability of both components is consistent with stellar activity modulated by rotation periods, and we detect no coherent planetary signals in either component. We place upper limits on any planets orbiting in the plane of the binary. The 27 yr RV baseline for 70 Oph A excludes Jupiter-mass planets interior to 5 au and reaches a sensitivity of $0.3\,M_{\rm Jup}$ at 1 au or $0.5\,M_{\rm Jup}$ at 2 au. For 70 Oph B, with PFS data we rule out planets more massive than $0.25$--$0.3\,M_{\rm Jup}$ inside 0.5 au. We show that stable S-type orbits around 70 Oph A extend to $\sim2.5$ au, covering the habitable zone. Thus, Saturn-mass planets or smaller on stable orbits in the habitable zone of 70 Oph A are allowed. Overall, our results provide important guidance for future planet searches around this stellar system.

Figures (12)

• Figure 1: Nightly binned RVs of 70 Oph A (48 nights) and 70 Oph B (44 nights) derived from PFS spectra, shown as a function of time. Each night's data are represented by a single weighted-average measurement, with weights computed as the inverse square of the RV uncertainties. The time of each binned point corresponds to the weighted average of the observation times. Error bars are smaller than the symbol size and are therefore not visible.
• Figure 2: The left panel shows the relative astrometric orbit of 70 Oph B around 70 Oph A. Red points denote archival visual and micrometric measurements from the WDS/ORB6 catalog, while the solid black curve shows the maximum-likelihood joint orbital solution. 50 orbits color-coded by companion mass are randomly drawn from the posterior and plotted, but they are indistinguishable from the black curve because of the tight constraint on the orbit. The dashed line marks the line of nodes, and the star indicates the primary's position. The right panel shows the RV variation of 70 Oph A measured by eight instruments. Colors correspond to individual RV datasets as indicated in the legend. Residuals relative to the best-fit model are shown.
• Figure 3: The left panel displays the relative position angle vs. time, shown with the maximum-likelihood orbital solution from the joint astrometric and RV fit (black curve). Residuals relative to the best-fit model are shown. The right panel shows relative separation vs. time, again compared with the maximum-likelihood orbital model. 50 random orbits drawn from the posterior distribution are overplotted, which are colored by companion mass, and they are hidden behind the maximum-likelihood orbit.
• Figure 4: Posterior distributions of key orbital parameters for the 70 Oph AB binary from the joint orvara fit. The panels show marginalized one- and two-dimensional posteriors for the primary and secondary masses ($M_{\rm pri}$, $M_{\rm sec}$), semimajor axis ($a$), eccentricity ($e$), and inclination ($i$). Contours represent 1, 2, and 3$\sigma$ credible regions, while vertical dashed lines mark the median and 68% confidence intervals.
• Figure 5: Generalized Lomb-Scargle periodograms of the nightly binned PFS radial velocities (top panels), $H_{\alpha}$ indices (middle panels), and spectral window functions (bottom panels) for 70 Oph A (left) and 70 Oph B (right). Note that the long-period binary orbital trend is not subtracted here. The dark-gray dotted and light-gray dashed horizontal lines mark the $0.1\%$ and $1\%$ false-alarm probability (FAP) levels, respectively. Vertical orange shaded bands indicate significant peaks in the RV periodograms above the $0.1\%$ FAP.