Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........21w&link_type=abstract
PhD Dissertation, New South Wales Univ. Kensington, Australia
Astronomy and Astrophysics
Astronomy
8
Cosmic Dust, Interstellar Magnetic Fields, Stellar Magnetic Fields, Molecular Clouds, Star Formation, Early Stars, Infrared Astronomy, Accretion Disks, Infrared Spectroscopy, Astronomical Polarimetry, Stellar Envelopes, Rotating Disks
Scientific paper
One of the earliest phases of the star formation process is the bipolar outflow. It is not presently understood how the outflow is generated, but a number of theories propose that a dynamically important magnetic field, embedded in the disk and acting in concert with rotation, is able to tap the gravitational potential well of the star plus disk system to drive material off the disk surface. Spectropolarimetric observations between 8 and 13 micrometers provide information on the chemical and physical nature of dust grains, as well as on the direction of the transverse (to the line-of-sight) component of an aligning magnetic field. In this thesis, such observations toward a selection of mainly high mass young stellar objects (YSO's) are presented. The field directions inferred from the polarization position angle are compared with the axes of disks and bipolar outflows associated with the sources. A strong correlation is found such that the field tends to lie in the plane of the disk, thereby providing support for the magnetic pressure mechanism for bipolar outflows. The observed field directions are also compared with the interstellar field configuration determined from optical polarization of field stars and obtained from the literature. Two distributions are observed, one in which the difference between the position angles of the two fields is less than 30 deg, and the other for which the difference is greater than 30 deg. The existence of the second group implies that the evolution of the YSO has a significant perturbing effect on its ambient magnetic field. Together with the disk field finding, the results are discussed in terms of the initial collapse phase of the molecular cloud in which the YSO is embedded, specifically whether the cloud was supercritical or subcritical. For instance, for two high mass objects, AFGL 2591 and AFGL 989, and one low mass object, SVS13, the source and interstellar fields, and interstellar field and disk major axes, are inclined by greater than 60 deg, while the source field lies in the disk plane. This implies that the respective clouds have initially collapsed along the primitive magnetic field direction to form the disks, consistent with a subcritical collapse scenario. Within the disks there must exist a significant ionization fraction, such that the field becomes frozen-in. Contraction plus differential rotation then act to wind the field up in the disk plane, thereby magnetically braking its rotation. A further component of this thesis relates to the properties of the polarizing dust. As well as deriving a new 8-13 micrometer dust grain emissivity function, the evidence for a new absorption feature at approximately 11.15 micrometers, and its possible interpretation in terms of crystalline component, is discussed. Additionally, the 8-13 micrometer polarization profiles of a number of sources, including those embedded in molecular clouds and behind large column depths of the interstellar medium (ISM), are compared with the profile of the BN Object, usually taken as a standard. Differences between the respective profiles are interpreted in terms of differing contributions from icy grain mantles to the polarization.
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