Detection of H$α$ emission from PZ Tel B using SPHERE/ZIMPOL

Abstract

H$α$ is a powerful tracer of accretion and chromospheric activity, which has been detected in the case of young brown dwarfs and even recently in planetary mass companions (e.g. PDS70 b and c). H$α$ detections and characterisation of brown dwarf and planet companions can further our knowledge of their formation and evolution, and expanding such a sample is therefore our primary goal. We used the Zurich IMaging POLarimeter (ZIMPOL) of the SPHERE instrument at the Very Large Telescope (VLT) to observe the known $38-72$M$_{\mathrm{J}}$ companion orbiting PZ Tel, obtaining simultaneous angular differential imaging observations in both continuum and narrow H$α$ band. We detect H$α$ emission from the companion, making this only the second H$α$ detection of a companion using the SPHERE instrument. We used our newly added astrometric measurements to update the orbital analysis of PZ Tel B, and we used our photometric measurements to evaluate the H$α$ line flux. Given the estimated bolometric luminosity, we obtained an H$α$ activity (log$\mathrm{(L_{Hα}/L_{bol}})$) between $-4.16$ and $-4.31$. The H$α$ activity of PZ Tel B is consistent with known average activity levels for M dwarf of the same spectral type. Given the absence of a known gaseous disk and the relatively old age of the system (24 Myr), we conclude that the H$α$ emission around PZ Tel B is likely due to chromospheric activity.