Detailed spectroscopic and photometric analysis of the remarkable planet-hosting wide binary system HD 202772A/B

TL;DR

This paper presents a high-precision, strictly differential chemical analysis of the planet-hosting wide binary HD 202772A/B, using Gemini-GRACES spectra and multi-epoch photometry from TESS and ground-based facilities to refine stellar and planetary properties. It finds HD 202772A hotter and more evolved than B, with a small but significant enhancement in refractory elements (notably for $T_c>1400$ K) relative to B, a pattern partially consistent with atomic diffusion. The hot Jupiter HD 202772A b is characterized with improved orbital and physical parameters, and a comprehensive search finds no additional transiting planets and no significant RV signals for other companions, though detectability limits leave room for small or long-period planets. The authors argue that atomic diffusion provides the most plausible explanation for the observed chemical differences, while other scenarios like planet engulfment or primordial inhomogeneities are disfavored, making this system a valuable testbed for star–planet chemical connections and planetary evolution in binary systems.

Abstract

We conducted a detailed spectroscopic and photometric characterization of the planet-hosting wide binary HD 202772A/B. No planet has been detected around HD 202772B, whereas HD 202772A, more evolved than its companion and near the end of its main-sequence (MS) phase, hosts a transiting hot Jupiter. The system has one of the hottest components ($T_{\mathrm{eff}; A} \sim 6440$ K) and one of the largest surface gravity differences between components ($Δ\log g_{A-B} \sim 0.4$ dex) among MS planet-hosting wide binaries. Using a global fit including our stellar parameters, radial velocities, and new TESS data, we derive refined properties of the planet orbiting HD 202772A, finding it to be the most irradiated hot Jupiter known in a wide binary. We also constrain the presence of additional transiting planets around HD 202772A and new transiting planets around HD 202772B using TESS photometry. We derive high-precision, strictly differential abundances for 27 species based on Gemini-GRACES spectra. HD 202772A shows lower lithium abundance (by 0.45 dex) relative to B, consistent with their stellar parameter differences. We also detect a small but significant enhancement in refractory elements in HD 202772A, particularly those with condensation temperatures above 1400 K (+0.018 $\pm$ 0.004 dex). We explored several scenarios to explain the observed chemical anomalies. Our analysis suggests that rocky planet engulfment, primordial inhomogeneities, and $δ$ Scuti-related effects are unlikely to fully account for the chemical pattern. Instead, the differences observed in certain refractory elements might support atomic diffusion as the most plausible explanation.

Figures (15)

• Figure 1: Location of HD 202772A (squares) and HD 202772B (circles) in the T$_{\mathrm{eff}}$--$\log g$ diagram. The red and black symbols indicates the parameters derived in this work. In particular, the black ones represent the T$_{\mathrm{eff}}$ and $\log g$ values derived from the global analysis performed with EXOFASTv2 whilst those the in red indicate the adopted differential spectroscopic parameters. The solid lines mark MIST isochrones corresponding to ages of 1.0, 1.6, 1.8, and 2.0 Gyr (left to right) for [Fe/H] = 0.23 dex.
• Figure 2: Best fit obtained between the synthetic (red line) and the observed (blue circles) GRACES spectra of HD 202772A (upper panel) and HD 202772B (bottom panel) around the 6707.8 Å lithium line. The colored vertical bands indicate the positions and contribution of different species in the region of the 6707.8 Å Li I line.
• Figure 3: Differential abundances (A$-$B) derived in this work (red circles) in comparison to those reported by BE23 (green squares) for common elements. The dotted line mark identical composition.
• Figure 4: Chemical composition difference between HD 202772A and HD 202772B as a function of atomic number. The dashed line corresponds to identical composition and the dotted line represents the weighted average of $\Delta$[X/H]$_{A-B}$ considering all the elements but those measured from only one line (see text for more details). Red vertical lines connect two species of the same chemical element (e.g., Sc i and Sc ii) and squares show the species measured from only one line (Li i, Sr i, Zr ii, and Ce ii).
• Figure 5: Chemical composition difference between HD 202772A and HD 202772B versus condensation temperature. Volatile elements (T$_{c}$$\leq$ 1000 K) are shown in red, refractory elements with 1000 $<$ T$_{c}$$\leq$ 1400 K are shown in green, while those with T$_{c}$$>$ 1400 K are shown in black. The gray dot-dashed line is the weighted linear least-squares fit to all the elements measured from more than one line, whilst the solid one is for the refractory elements only (T$_{c}$$>$ 1000 K). Squares show the species measured from only one line (Li, Ce, Sr, and Zr). As before, the black dashed line corresponds to identical composition.