Multi-modal atmospheric characterization of ${\hat{I}}^2$ Pictoris b: Adding high-resolution continuum spectra from GRAVITY

Abstract

We present the first VLTI/GRAVITY observations at R${}_{\hat{I}}»\sim 4000$ of ${\hat{I}}^2$ Pic b. These four high S/N ($\sim$20) K-band spectra conserve both the pseudo-continuum and molecular absorption patterns. We analyze them with four self-consistent forward model grids (Exo-REM, ATMO, BT-Settl, Sonora) exploring $T_{\mathrm{eff}}$, log(g), metallicity, C/O, and ${}^{12}$CO/${}^{13}$CO ratio. We also upgrade our forward modeling code ForMoSA to account for the data multi-modality and combine the GRAVITY epochs with published 1-5 $\hat{I}¼$m photometry, low- to medium-resolution spectra (0.9-7 $\hat{I}¼$m), and high-resolution echelle spectra (2.1-5.2 $\hat{I}¼$m). Sonora and Exo-REM are statistically preferred. Exo-REM yields $T_{\mathrm{eff}}$ $=1607.45_{-6.20}^{+4.85}$ K and log(g) $=4.46_{-0.04}^{+0.02}$ dex from GRAVITY alone, and $T_{\mathrm{eff}}$ $=1502.74_{-2.14}^{+2.32}$ K and log(g) $=4.00\pm 0.01$ dex when including all datasets. Archival data significantly affect the retrieved parameters. C/O remains solar ($0.552_{-0.002}^{+0.003}$) while [M/H] reaches super-solar values (0.50$\pm$0.01). We report the first tentative constraint on log(${}^{12}$CO/${}^{13}$CO) $\sim$1.12, though this remains inconclusive due to telluric residuals. Additionally, we estimate the luminosity to be log(L/L${}_{\odot }$) $=-4.01_{-0.05}^{+0.04}$, implying a heavy-element content of up to $\sim$5% (20-80 M${}_{\oplus }$) given the system age and dynamical mass measurements. Access to both continuum and molecular lines at K-band significantly impacts the metallicity, possibly owing to collision-induced absorption shaping the continuum. Echelle spectra do not dominate the final fit with respect to lower resolution data. Future multi-modal frameworks should include weighting schemes reflecting bandwidth and central wavelength coverage.

Accepted for publication in A&A