On the alignment between the circumstellar disks and orbital planes of Herbig Ae/Be binary systems

Abstract

Context : The majority of the intermediate mass, pre-main-sequence Herbig Ae/Be stars reside in binary systems. As these systems are young, their properties may contain an imprint of the star formation process at intermediate masses (2-15M). However, these systems are generally spatially unresolved, making it difficult to probe their circumstellar environment to search for manifestations of their formation process, such as accretion disks. Aims. Here we investigate the formation mechanism of Herbig Ae/Be (HAe/Be) binary systems by studying the relative orientation of their binary orbits and circumstellar disks. Methods : We present linear spectropolarimetric observations of HAe/Be stars over the Hα line, which are used to determine the orientation of their circumstellar disks. In conjunction with data from the literature, we obtain a sample of 20 binaries with known disk position angles (PAs). We subsequently compare our disk PA data to a model to investigate whether HAe/Be binary systems and their disks are co-planar. Moreover, in the light of a relatively recent suggestion that some HAe/Be star spectropolarimetric signatures may not necessarily be related to circumstellar disks, we re-assess the relationship between spectropolarimetric signatures and disk PAs. We do this by comparing spectropolarimetric and high spatial resolution observations of young stellar objects (both HAe/Be and T Tauri stars). Results : We find that spectropolarimetric observations of pre-main-sequence stars do indeed trace circumstellar disks. This finding is significant above the 3σ level. In addition, our data are entirely consistent with the situation in which HAe/Be binary systems and circumstellar disks are co-planar, while random orientations can be rejected at the 2.2σ level. Conclusions : The conclusive alignment (at more than 3σ) between the disk PAs derived from linear spectropolarimetry and high spatial resolution observations indicates that linear spectropolarimetry traces disks. This in turn allows us to conclude that the orbital planes of HAe/Be binary systems and the disks around the primaries are likely to be co-planar, which is consistent with the notion that these systems form via monolithic collapse and subsequent disk fragmentation.