Asymmetric, variable H$\alpha$ line profile in planetary mass object SR 12 c

Abstract

Young, forming planetary-mass objects often exhibit clear signatures of ongoing mass accretion and are thought to accrete material through processes analogous to those operating in young stars. In this study, we present high-spectral-resolution observations of asymmetric and time-variable H$\alpha$ line profiles from the planetary-mass companion SR~12~c. The H$\alpha$ line was observed at a resolving power of $R \sim 49,000$--40{,}000 (corresponding to 6.1--7.5~km~s$^{-1}$) using the High Dispersion Spectrograph (HDS) on the 8.2 m Subaru Telescope. Strong H$\alpha$ emission is clearly detected, while higher-order Balmer lines (H$\beta$, H$\gamma$, and H$\delta$) are not detected due to their faintness. The H$\alpha$ line profiles are well spectrally resolved and exhibit blueshifted emission peaks, which can be interpreted as arising from either (a) emission partially absorbed by redshifted accreting material along the line of sight and/or (b) geometric occultation by the inner circumplanetary disk. Moreover, the H$\alpha$ flux shows significant variability at 43.6~$\pm$~6.4~\% relative to the peak flux on hourly timescales. During a continuous 2.5-hour observing sequence, the emission component peaking at approximately $-30$~km~s$^{-1}$ weakened over the first hour. Subsequently, an emission component centered near $-10$~km~s$^{-1}$ became dominant and remained stable for the remaining 1.5 hours. We discuss possible interpretations of this behavior. Overall, these results support that magnetospheric accretion is operating in the planetary-mass object SR~12~c while a scenario combining boundary-layer accretion with a failed wind cannot be ruled out.