Characterization of the Host Binary of the Directly Imaged Exoplanet HD 143811 AB b

TL;DR

HD 143811 AB b is a directly imaged planet in a close 18-day binary host within Sco-Cen. The authors fit a precise SB2 orbit from archival FEROS spectra, obtaining $P = 18.59090 \pm 0.00007$ days, $q = 0.886 \pm 0.003$, and component masses $M_A = 1.30^{+0.03}_{-0.05} M_\odot$, $M_B = 1.15^{+0.03}_{-0.04} M_\odot$, with an orbital inclination $i_{AB} = 22.9^{+0.3}_{-0.2}$ degrees and $a = 0.1854^{+0.0014}_{-0.0024}$ au. Combining SB2 results with unresolved Gaia/2MASS photometry and MIST+ATLAS9 evolutionary models yields Teffs around $6750$ K and $6350$ K for A and B, and an age near $13 \pm 4$ Myr, though spectroscopic fits indicate a higher Teff for the A component; lithium is detected in both stars, consistent with youth. The planet lies at ~60 au, and while current data permit a coplanar configuration, the 3D orientation remains poorly constrained, necessitating future interferometric measurements (e.g., VLTI/GRAVITY) and Gaia DR4 to test coplanarity. Altogether, this work provides crucial constraints on planet formation in binary systems and sets the stage for refined dynamical modeling and disk-planet interactions in the HD 143811 AB system.

Abstract

HD~143811~AB is the host star to the directly imaged planet HD~143811~AB~b, which was recently discovered using data from the Gemini Planet Imager and Keck NIRC2. A member of the Sco-Cen star-forming region with an age of $13 \pm 4$ Myr, HD~143811~AB is somewhat rare among hosts of directly imaged planets as it is a close stellar binary, with an $\sim$18 day period. Accurate values for the orbital and stellar parameters of this binary are needed to understand the formation and evolutionary history of the planet in orbit. We utilize archival high-resolution spectroscopy from FEROS on the MPG/ESO 2.2-meter telescope to fit the orbit of the binary, and combine with unresolved photometry to derive the basic stellar properties of the system. From the orbit, we derive precise values of orbital period of $18.59098 \pm 0.00007$ days, and mass ratio of $0.885 \pm 0.003$. When combined with stellar evolutionary models, we find masses of both components of $M_A = 1.30^{+0.03}_{-0.05}$ M$_\odot$ and $M_B = 1.15^{+0.03}_{-0.04}$ M$_\odot$. While the current data are consistent with the planet and stellar orbits being coplanar, the 3D orientations of both systems are currently poorly constrained, with additional observations required to more rigorously test for coplanarity.

Figures (9)

• Figure 1: FEROS spectra of HD 143811 AB taken on 2018-04-22 (black points with error bars), showing two sets of well-resolved lines. Red tracks are draws from the posterior (lowest chi-square solution in dark red) of a two-star model that best fits the data. The blue and yellow lines are draws from single star models for A and B, respectively. Given the high resolution and SNR of these data we are able to recover both the radial velocities of both components as well as $v$sin($i$).
• Figure 2: Radial velocity measurements of both HD 143811 A and HD 143811 B with orbits drawn from the posteriors, with the darkest blue and red lines representing the lowest $\chi^2$ orbit. The Observed-Calculated (O-C) panels show the residuals with respect to the lowest $\chi^2$ orbit (black line at 0), as well as the offset of other draws from the posterior with respect to the best-fitting orbit. The $\sim$18 day orbit is an excellent fit to the data, and the combination of multiple observation over 2 weeks and a multi-year baseline strongly constrain the orbital parameters.
• Figure 3: Posterior on the SB2 radial velocity orbit of HD 143811 AB. The orbit is very well constrained by the FEROS data, with orbital period known to 6 second precision. Direct measurement of both semi-amplitudes allows a measurement of mass ratio at the sub-1% level: $0.886 \pm 0.003$.
• Figure 4: The results of a two-star fit to the unresolved optical and near-infrared photometry of the system from Gaia and 2MASS. Plotted are the spectral energy distribution of the blended system (black) and the two components (blue, yellow) drawn from the posterior distributions estimated using MCMC. Also shown are the synthetic photometry of the blended system derived from the median SED (black squares), and the observed photometry (circles).
• Figure 5: We are generally able to accurately recover effective temperature and metallicity when applying our method to well-characterized F and G stars from worley2012 However, the results for stars hotter than $\gtrsim$6000 K show a systematic offset in temperature and a larger scatter in metallicity compared to G stars. The solid black line is the 1-to-1 line, representing identical results.