An observational study of astronomical magnetic fields and their role in star formation.

摘要

Magnetic fields are known to play an important, perhaps even crucial, role in the evolution of molecular clouds and star formation. The Zeeman effect in radio frequency spectral lines remains the most direct method for measuring magnetic field strengths in the interstellar medium (ISM). This thesis presents Zeeman effect magnetic field measurements in three different regimes: the molecular cloud complex and star forming region NGC 6334, the external galaxy Centaurus A, and H₂O masers in star forming regions. The unifying theme of these observations is the effect of magnetic fields on their environment, with particular emphasis on their role in star formation. The goal of the thesis is to significantly extend the current body of knowledge on magnetic fields. The major emphasis of this thesis is on the measurement of the Zeeman effect in H₂O masers—this is the first such comprehensive interferometric study. Significant results include: (1) Very Large Array (VLA) detections of magnetic fields ranging from 13 to 49 milligauss (mG) have been obtained for several H₂O masers in star forming regions via the Zeeman effect. H₂O masers are an unique tracer of the Zeeman effect in high density regions (n ∼ 109 cm−3 ). The observed values of the magnetic field indicate that the magnetic field-gas density relationship (B ∝ n0.47) carries over into these high density regions. For some regions, the detected fields provide a 2/3-point map of the magnetic field, indicating the nature of field variations on arcsecond scales. (2) VLBA observations of the Zeeman effect in H₂O masers (in W3 IRS5) have yielded the first ultra-high spatial resolution maps of the magnetic field—masers for which we can independently measure the magnetic field are separated by ≤25 milliaresec (mas, or 60 AU). No field reversals have been observed over a 60 mas (150 AU) region. This is the first VLBA detection of the Zeeman effect in H₂O masers. The VLBA observations also remove confusion in observed magnetic fields from closely-spaced masers. (3) The Zeeman effect in OH and H I absorption lines toward the star forming region NGC 6334 gives significant magnetic field values (∼200 μG) in at least three sources in this complex. There are also some indications of magnetic field variations in one of these sources (source A). The detected fields are of the order of the critical field needed to support the molecular cloud(s) in NGC 6334 against gravitational collapse. (4) The H I absorption profiles toward Cen A have revealed, for the first time, a broad component, which may be a blend of absorption in several clouds located in the ∼100 pc radius circumnuclear disk. Upper limits have been set on the magnetic field in Cen A in the regions traced by the H I absorption components. For T_s ≤ 50 K, magnetic fields in the narrow line width redshifted clouds observed toward the nucleus will be energetically significant if B_los ≥ 5 μG, for cloud sizes ∼ a few pc or larger.