Mid-infrared high resolution imaging of Herbig Ae/Be stars: Exploring the geometry of circumstellar dust

Herbig Ae/Be (HAeBe) stars are emission line pre-main-sequence stars of intermediate mass. Circumstellar dust is well established as the origin of the large infrared excesses characteristic of these stars; however, the geometry of this dust has remained controversial for a long time. The sources have been divided into two groups depending on the infrared excess: Group I sources have strong near- to far-infrared excesses, and Group II sources, only have strong near-infrared excess. Previous studies associate Group II sources with optically thick flat disks and Group I sources either with optically thick flat disks surrounded by haloes or flaring disks with an inner hole at the dust sublimation radius and a puffed up inner wall. These two models predict the same overall flux at all wavelengths. An evolutionary scenario has been proposed in which grain growth, dispersion, and settling of larger grains towards the mid-plane change the geometry of circumstellar dust from flaring disks in Group I sources, to flat cold disks in Group II sources in about 1 Myr.;For this study, I imaged a sample of 20 HAeBe stars (8 Group I and 12 Group II) in the mid-infrared from the 8-meter Gemini telescopes. I resolve extended emission in all Group I sources with sizes tens to hundreds of AU. Most of the resolved emission is spherical, which could be interpreted either by a halo or the surface of an almost face-on flaring disk; however, the distribution of dust grain sizes that I derived from these observations, with larger grains at smaller radii is inconsistent with halo models. Only three of the Group II sources are extended and two of them show emission consistent with highly inclined disks. I find no correlation between stellar ages and geometry of the circumstellar material supporting an evolution from Group I sources to Group II sources, on the contrary, Group II sources are younger than Group I sources and all resolved sources presented here are older than 1 Myr.