VLT$/$NACO and Subaru$/$CIAOJHK-band high-resolution imaging polarimetry of the Herbig Be star R Monocerotis

Abstract

Aims. We investigate the dust properties of the disk and envelope of the Herbig Be star R Mon by means of near-infrared multiwave-length imaging polarimetry. Methods. We obtained JHK-band polarimetric images using the adaptive optics instruments NACO on the VLT, and CIAO on the Subaru telescope. Results. Our NACO JKs-band images of ∼0. 1 angular resolution resolve clearly the R Mon binary system as well as twisted string-like features in the fan-shaped lobe. The polarimetric images reveal a butterfly-shaped polarization disk with an almost constant shape and an extension of 4 in the J, H, and K bands. In this region, the polarization values are as low as Pj ∼ 7%, P H ∼ 2%, and PK ∼ 1%, and the polarization vectors are not systematically aligned along the equatorial plane. On the other hand, highly polarized scattered light is detected in the fan-shaped lobe (P j ∼ 24%, P H ∼ 33%, and P K ∼ 53%). Conclusions. Our polarimetric data suggests the presence of multiple grain populations in the R Mon nebula. From our one-dimensional single scattering modeling, the maximum grain size in the nebula at large scale is estimated to be 0.23 μm. On the other hand, the aforementioned properties of the polarization disk and a nearly spherical appearance of the nebulosity close to the central star suggests the presence of large grains (micron-size or larger) in the polarization disk.