Magnetic Field $\sim$1000AU Scale

Figure 1.24: SMA observation low-mass (NGC1333 IRAS 4A) star formation region. (middle) The total intensity of 879 $\mu$m thermal dust emission (contour lines), that of polarized intensity (false color) and polarization E-vector (red bars) are shown. (right) The polarization E-vector of the dust thermal emissions is perpendicular to the direction of magnetic field (Fig.1.21). The direction of magnetic field (red bars) is overlaid on the total intensity distribution (false color). $1''=300$AU. This clearly shows that the magnetic field looks like a hour-glass within $\sim 1000$ AU scale.

Single dish observation with JCMT can reveal a structure larger than 5000-10000AU (Fig.1.23). Smaler-scale configuration of magnetic field is able to be reached with interferometric observation such as SMA. Figure 1.24 indicates a 1000 AU-scale configuration of magnetic field for a low-mass star forming region NGS 1333 IRAS 4A (Girart, Rao & Marrone 2006). Although this is a complex region, a core extends from the upper-left to the lowe-right. The magnetic field (Fig.1.24(right)) runs from the upper-right to the lower-left and this clearly shows 'hour-glass' shape. This means that the magnetic field is squeezed by the effect of contraction in the star formation process.

They fitted the magnetic field line by a polynomial as

$$y=g(1+Cx^2),$$ (1.12)

where $x$ and $y$, distances along the symmetic axis and along the major axis of the disk, respectively. They obtained $C=0.12\pm 0.06{\rm sec}^{-1}$.

Figure 1.25: SMA observation high-mass (G31.41+0.31) star formation region. (left) The total thermal dust emission at 879 $\mu$m is shown with contour lines, while the intensity of polarized intensity is shown with false-color. $1''=7000$AU. Directions of magnetic field are illustrated with solid thick bars. (middle) Directions of magnetic field is overlaid on the flux-weighted velocity map of the CH$_3$OH 14$_7$-15$_6$ line, which indicates the core has a systematic velocity gradient.

SMA observation for G31.41+0.31 (Fig. 1.25) also reviels that the magnetic field configuration of $10000$AU scale for high-mass star forming region is ``hour-glass'' shape (Girart et al. 2009). This may indicate the magnetic field is draged by the infall motion. However, this can not be fitted with the configuration of the magnetic field line obtained with single dish observations, since Fourier component with the small wave-number $k$ is not sufficiently picked up in the interferometric observation. In other words, large-scale (more or less uniform) magnetic field is underestimated in the interferometric observations.