Detection of H$\hat{I}\pm$ emission from PZ Tel B using SPHERE/ZIMPOL

Abstract

H$\hat{I}\pm$ 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$\hat{I}\pm$ 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$\hat{I}\pm$ band. We detect H$\hat{I}\pm$ emission from the companion, making this only the second H$\hat{I}\pm$ 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$\hat{I}\pm$ line flux. Given the estimated bolometric luminosity, we obtained an H$\hat{I}\pm$ activity (log$({\mathrm{L}}_{\mathrm{H}\hat{\mathrm{I}}\pm }/{\mathrm{L}}_{\mathrm{bol}})$) between $-4.16$ and $-4.31$. The H$\hat{I}\pm$ 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$\hat{I}\pm$ emission around PZ Tel B is likely due to chromospheric activity.

9 pages, 4 figures, to be published in Astronomy & Astrophysics