Circumstellar disks around Herbig Be stars

T. Alonso-Albi et al., A&A

This paper presents the last of the data from a survey of Herbig Be stars using the Very Large Array (VLA) and the Plateau de Bure (PdB). The point of the survey was to investigate the properties of intermediate mass stars to determine the occurrence, lifetime, and evolution of the disks surrounding them.

Herbig Be, along with Herbig Ae, stars are pre-main sequence. Herbig Ae stars have strong infrared excess have disks that are similar to T Tauri stars. Herbig Be stars tend to be more modest their infrared excess and their disks are flatter than Herbig Ae stars. While no one is quite sure what causes this divergence in disk geometry T. Alonso-Albi et al. put forth the idea that Herbig Be stars lose a large portion of their mass before they hit the pre-main sequence phase.

T. Alonso-Albi et al. report the results of 6 objects studied in mm wavelengths. They chose mm wavelengths for their observations since optical-NIR and mid-IR observations are limited in only providing information about the disk surface and cannot give the disk mass. Also, observations at mm wavelengths allowed them to determine the size and properties of the large grains of dust that partially make up the disk.

The authors claim that a two component model is necessary to fit the SED  since the envelop surrounding the disk influenced so much of the observed flux. Out of the six objects observed, four were found to have disks. They found that the disk mass was usually < 10% of the entire envelope and 5-10 times lower than the disks around Herbig Ae stars.

They propose that photoevaporation is the cause of the dissipation of the disks. From what I understand their argument is that this happens with HAe stars as well, it’s just that the time scale is shorter. I’m not entirely clear why there would be such a significant difference. I’m actually fuzzy on physics of how these star/disk/envelope work on a detailed level. As I work on this project I think it will be necessary to learn more about the astrophysics in order to make any substantial and novel claims.