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

TL;DR

This study reports the detection of Hα emission from the substellar companion PZ Tel B using SPHERE/ZIMPOL, providing insights into its chromospheric activity and orbital parameters, and expanding the sample of such detections.

Contribution

First detection of Hα emission from PZ Tel B with SPHERE/ZIMPOL, updating its orbital analysis and characterizing its activity level in a relatively old system.

Findings

Hα emission detected from PZ Tel B, indicating chromospheric activity.

Updated orbital parameters of PZ Tel B based on new astrometry.

Hα activity level consistent with M dwarf stars of similar spectral type.

Abstract

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