CFHT Information Bulletin Number 37, Semester 97II

Infrared Polarimetric Maps Of Herbig Ae/Be Stars

Roger Hajjar (, Pierre Bastien (, Daniel Nadeau (, Département de Physique, Université de Montréal


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).


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.


Based on the prediction of BM that null points are located where the line of sight optical depth = 1, we devised a way to extract disk density information from the measurements. This method is detailed in Hajjar & Bastien (1997). We used two grain models (Henning & Stognienko 1996 and Pollack et al 1994), with opacities calculated for both by Henning & Stognienko, to determine the disk densities, and we assumed the usual interstellar gas to dust ratio of 100. The line of sight data give the column density through the disk. An inversion procedure allows one to obtain actual disk densities. To obtain the variation of density as a function of disk radius, we plotted all column density measurements of all seven YSOs, giving column density relative to a normalized disk size. We fitted the profiles with power laws (Figure 3). These values of the power law yield a disk density law (r) r-(1+) r-1.7 for the Henning & Stognienko dust model, or (r)r-1.4 for the Pollack dust model. This is very close to the standard value used for the protosolar nebula, -1.5.


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CFHT Information Bulletin Number 37, Semester 97II
Copyright © 1997, Canada-France-Hawaii Telescope