CFHT Information Bulletin Number 37, Semester 97II

Infrared Polarimetric Maps Of Herbig Ae/Be Stars

Roger Hajjar (hajjar@astro.umontreal.ca), Pierre Bastien (bastien@physcn.umontreal.ca), Daniel Nadeau (nadeau@astro.umontreal.ca), Département de Physique, Université de Montréal

Introduction

A number of models and calculations now exist to explain the observed pattern of polarization vectors in polarimetric maps of Young Stellar Objects (YSOs). We went back to an early prediction of the model by Bastien & Ménard (1988,1990), hereafter BM, in order to test its validity. A common characteristic of polarimetric disks are the two null points delimiting the parallel vector pattern, seen in many maps (see Figure 2). BM showed that the aligned polarization vectors are due to multiple scattering in the disk, with the null points marking the transition from the optically thick disk to the optically thin circumstellar material. They then predicted that this must lead to an apparent disk size decreasing with wavelength.

In order to test their prediction, we chose a sample of Herbig Ae/Be stars and other YSOs with already published optical polarization maps and disk inclination angles, as determined by BM, of 90° (as determined by Bastien & Ménard). Observations were carried out at CFHT in August 95 and June 96. In order to obtain J and K band polarimetric maps, we used MONICA (the MONtreal Infrared CAmera) (Nadeau et al 1994) coupled with a polarimetric module, consisting of a warm rotating /2 plate and a cooled fixed linear polarizer. Details of the observational procedure is given by Hajjar et al (1997).

Results

The variation of disk size as a function of

was observed for all of our seven objects. We present here (Figure 2) the J and K polarization maps of Parsamian 21, a cometary-shaped nebula already identified by Herbig (1960) as a pre-main sequence object of the Ae/Be type. The variations of the central pattern with

are obvious. We used also an already published map of Par 21 by Draper et al (1985). Measured polarimetric disk sizes are 18'', 5'' and 2.7'' at 8500 Å, J and K respectively.