Infrared Polarization Studies of Bok Globules

by Dimitri I. Klebe

Under the supervision of Professor Terry Jones

ABSTRACT

Near-infrared photometry and polarimetry and optical polarimetry of field stars shining through the Bok globules B118, B133, and B361 are presented. Three distinct density distributions are found and mass estimates of the inner regions of these globules are given. Near-infrared polarimetry was done using the Minnesota Infrared Polarimeter whose construction, operation, and signal to noise performance are also discussed. The polarizoation vectors show no indication that the magnetic field in the globules is being compressed due to gravitational collapse. The general direction of the polarization vectors is parallel to the plain of the Galaxy in all three globules. The polarization vectors are bent in the southwestern corner of B361 and are consistent with a collision between the globule and the intercloud medium in this region. The presence of the aligned grains in the isolated and cold environments of these globules demands an efficient grain alignment mechanism.No dependence of polarization efficiency Pk/E(J-K) on temperature, density, and magnetic field strength (inferred from regions of different optical depth) is found. Polarization efficiency does appear to be lower in regions with more random position angles.
 
A model describing the polarizing properties of the interstellar medium (both cloud and intercloud medium) is presented. The model is based on the premise that the mechanism responsible for grain alignment is sufficient to provide 100% alignment of all polarization grains with their local magnetic fields independent of field strength. In this model, variations in the polarization efficiency Pk/Tk are due to departures in the magnetic field direction from a unidirectional field (assumed to be perpendicular to the line of sight) rather than due to partially aligned grains. The departures in the field direction are thought to originate in the propagation of transverse magnetic waves (Alfven waves) along the field or by turbulent motions of gas and dust transverse to the field. In either case, the model predicts an equipartition of the magnetic and turbulent or kinetic energy densities in order to fit the available polarization data for our Galaxy. The model requires that along any given line of sight, that the interstellar medium be composed of segments in which a random component of the magnetic field direction is uncorrelated with the field direction in the preceding or following segment. The model requires that the optical depth of an individual segment be 0.1 at K to best represent conditions in the interstellar medium, particularly for optical depths less than unity.