A JWST/MIRI view of ąppa,Andromedae b : Refining its mass, age, and physical parameters

ąppa,And,b is a substellar companion with a mass near the planet--brown dwarf boundary orbiting a B9IV star at ∼50-$100$,au. Estimates of its age and mass vary, which has fueled a decade-long debate. Additionally, the atmospheric parameters (mathrm T_ eff $1650-2050$,K and log(g) $3.5-5.5$,dex) remain poorly constrained. The differences in atmospheric models and inhomogeneous datasets contribute to the varied interpretations. We aim to refine the characterization of ąppa,And,b by using mid-infrared data to capture its full bolometric emission. Combined with near-infrared (NIR) measurements, we aim to constrain mathrm T_ eff log(g), and the radius to narrow down the uncertainties in age and mass. We obtained JWST/MIRI coronagraphic data in the F1065C F1140C and F1550C filters and recalibrated existing NIR photometry using an updated ATLAS stellar model. We used MIRI color-magnitude diagrams to probe the likelihood of species (e.g. and silicates). We compared the H and F1140C colors and magnitudes of the companion to isochrones to constrain the age and mass. We then modeled its spectral energy distribution with atmospheric models to refine the estimates of mathrm T_ eff radius, and log(g) and to constrain age and mass using evolutionary models. Cloudy atmosphere models fit the spectral energy distribution of ąppa,And,b best. This is consistent with its L0/L2 spectral type and its position near silicate-atmosphere field objects in the MIRI color-magnitude diagram. We derived an age of $47±7$,Myr and a mass of $17.3±1.8 M_ Jup $ by weight-mean combining the models. Atmospheric modeling yielded mathrm T_ eff =,$1791±68$,K and a radius of $1.42±0.06 R_ Jup $. This improves the precision by ∼30% over previous estimates. Log(g) was constrained to $4.35±0.07$,dex, which is an improvement in the precision by ∼70% relative to the most precise literature value of $4.75 ± 0.25$ dex. Our new mass estimate places ąppa,And,b slightly above the planet--brown dwarf boundary determined by the deuterium-burning limit. Our age estimate is ∼75% more precise than previous values and aligns the object with the Columba association ($42$,Myr). The derived mathrm T_ eff suggests silicate clouds, but this needs to be confirmed spectroscopically. MIRI data were crucial to refine the radius and temperature, which led to stronger constraints on the age and mass (both dependent on the model) and improved the overall characterization of ąppa,And,b.