Water depletion in the disk atmosphere of Herbig AeBe stars

TL;DR

This study uses high-resolution spectroscopy to investigate water and hydroxyl emissions in disks around Herbig AeBe stars, revealing a depletion of hot water vapor compared to T Tauri stars and linking OH emission to disk geometry.

Contribution

It provides the first detailed spectroscopic analysis of water and hydroxyl in Herbig AeBe disks, highlighting differences from T Tauri disks and suggesting water depletion in these environments.

Findings

OH emission detected in 4 disks with Keplerian profiles

No hot water vapor detected at similar sensitivity levels

OH/H2O column density ratio significantly higher than in T Tauri disks

Abstract

We present high resolution (R = 100,000) L-band spectroscopy of 11 Herbig AeBe stars with circumstellar disks. The observations were obtained with the VLT/CRIRES to detect hot water and hydroxyl radical emission lines previously detected in disks around T Tauri stars. OH emission lines are detected towards 4 disks. The OH P4.5 (1+,1-) doublet is spectrally resolved as well as the velocity profile of each component of the doublet. Its characteristic double-peak profile demonstrates that the gas is in Keplerian rotation and points to an emitting region extending out to ~ 15-30 AU. The OH, emission correlates with disk geometry as it is mostly detected towards flaring disks. None of the Herbig stars analyzed here show evidence of hot water vapor at a sensitivity similar to that of the OH lines. The non-detection of hot water vapor emission indicates that the atmosphere of disks around Herbig AeBe stars are depleted of water molecules. Assuming LTE and optically thin emission we derive a lower limit to the OH/H2O column density ratio > 1 - 25 in contrast to T Tauri disks for which the column density ratio is 0.3 -- 0.4.