Molecular hydrogen in the circumstellar environments of Herbig Ae/Be stars probed by FUSE

Abstract

We observed molecular hydrogen around a sample of pre-main sequence stars in order to better characterize their gaseous CS environments. We analyzed the FUSE (Far Ultraviolet Spectroscopic Explorer) spectra of a sample of Herbig Ae/Be stars (HAeBes) covering a broad spectral range, including the main-sequence A5 star Beta-Pictoris. To better diagnose the origin of the detected H2 and its excitation conditions, we used a model of a photodissociation region. Our analysis demonstrates that the excitation of H2 is clearly different around most of the HAeBes compared to the interstellar medium. Moreover, the characteristics of H2 around Herbig Ae and Be stars give evidence for different excitation mechanisms. For the most massive stars of our sample (B8 to B2 type), the excitation diagrams are reproduced well by a model of photodissociation regions (PDR). Our results favor an interpretation in terms of large CS envelopes, remnants of the molecular clouds in which the stars were formed. On the other hand, the group of Ae stars (later than B9 type) known to possess disks is more inhomogeneous. In most cases, when CS H2 is detected, the lines of sight do not pass through the disks. The excitation conditions of H2 around Ae stars cannot be reproduced by PDR models and correspond to warm and/or hot excited media very close to the stars. In addition, no clear correlation has been found between the ages of the stars and the amount of circumstellar H2. Our results suggest structural differences between Herbig Ae and Be star environments. Herbig Be stars do evolve faster than Ae stars, and consequently, most Herbig Be stars are younger than Ae ones at the time we observe them. It is thus more likely to find remnants of their parent cloud around them.

23 pages (including on-line material), accepted for publication in A&A

References in corpus (4)

• The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk
• CalFUSE v3: A Data-Reduction Pipeline for the Far Ultraviolet Spectroscopic Explorer
• TEXES Observations of Pure Rotational H2 Emission From AB Aurigae
• Detection of warm molecular hydrogen in the circumstellar disk around the Herbig Ae star HD97048

Cited by in corpus (7)

• Models of turbulent dissipation regions in the diffuse interstellar medium
• Exocometary gas structure, origin and physical properties around $\beta$ Pictoris through ALMA CO multi-transition observations
• Diagnosing Mass Flows Around Herbig Ae/Be Stars Using the He I 10830 Line
• CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk
• The Immediate Environments of Two Herbig Be Stars: MWC 1080 and HD 259431
• Towards Understanding The B[e] Phenomenon: VI. Nature and Spectral Variations of HD 85567
• Searching for molecular hydrogen mid-infrared emission in the circumstellar environments of Herbig Be stars