Research Note

Searching for molecular hydrogen mid-infrared emission in the circumstellar environments of Herbig Be stars

¹ Laboratoire d'Astrophysique de Grenoble, CNRS, Université Joseph-Fourier, UMR 5571, 38041 Grenoble Cedex 09, France e-mail: claire.martin-zaidi@obs.ujf-grenoble.fr
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Laboratoire AIM, CEA/DSM, CNRS, Université Paris Diderot, DAPNIA/Service d'Astrophysique, Bât. 709, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 14 February 2008
Accepted: 30 June 2008

Abstract

Context. Molecular hydrogen (H₂) is the most abundant molecule in the circumstellar (CS) environments of young stars, and is a key element in giant planet formation. The measurement of the H₂ content provides the most direct probe of the total amount of CS gas, especially in the inner warm planet-forming regions of the disks.

Aims. Most Herbig Be stars (HBes) are distant from the Sun and their nature and evolution are still debated. We therefore conducted mid-infrared observations of H₂ as a tracer of warm gas around HBes known to have gas-rich CS environments.

Methods. We report a search for the H₂ S(1) emission line at 17.0348 μm in the CS environments of 5 HBes with the high resolution spectroscopic mode of VISIR (ESO VLT Imager and Spectrometer for the mid-InfraRed).

Results. No source shows evidence for H₂ emission at 17.0348 μm. Stringent 3σ upper limits on the integrated line fluxes are derived. Depending on the adopted temperature, limits on column densities and masses of warm gas are also estimated. These non-detections constrain the amount of warm (>150 K) gas in the immediate CS environments of our target stars to be less than ∼.