B335

Figure 1.9: Near infrared images of B335, which is Class 0 source.

Figure 1.10: Line profile of CS $J=2-1$ line radio emission. Model spectra illustrated in a dashed line (Zhou 1995) are overlaid on to the observed spectra in a solid line (Zhoug et al 1993).

Figure 1.11: Explanation of blue-red asymmetry when we observe a spherical symmetric inflow motion. An isovelocity curve for the red-shifted gas is plotted in a solid line. That for the blue-shifted gas is plotted in a dashed line. Taken from Fig.14 of Lada (1999).

B335 is a dark cloud (Fig.1.9) with a distance of $D\simeq 250$pc. Inside the dark cloud, a Class 0 IR source is found. The object is famous for the discovery of gas infall motion. In Figure 1.10, the line profiles of CS $J=2-1$ line emissions are shown (Zhou et al. 1993). The relative position of the profiles correspond to the position of the beam. (9,9) represents the offset of (9",9") from the center. At the center (0,0), the spectrum shows two peaks and the blue-shifted peak is brighter than the red-shifted one. This is believed to be a sign of gas infall motion. The blue-red asymmetry is explained as follows:

1. Considering a spherical symmetric inflow of gas, whose inflow velocity $v_r$ increases with reaching the center (a decreasing function of $r$)
2. Considering a gas element at r moving at a speed of $v_r({\bf r}) < 0$, the velocity projected on a line-of-sight is equal to
   

   $$v_{\rm line-of-sight}=v_{\rm systemic}+ v_r \cos \theta,$$ (1.5)

   
   
   where $v_{\rm systemic}$ represents the systemic velocity of the cloud (line-of-sight velocity of the cloud center) and $\theta$ is the angle between the line-of-sight and the position vector of the gas element. The isovector lines, the line which connect the positions whose procession/recession velocities are the same, become like an ellipse shown in Fig.1.11.
3. An isovelocity curve for the red-shifted gas is plotted in a solid line. That for the blue-shifted gas is plotted in a dashed line. If the gas is optically thin, the blue-shifted and red-shifted gases contribute equally to the observed spectrum and the blue- and red-shifted peaks of the emission line should be the same.
4. In the case that the gas has a finite optical depth, for the red-shifted emission line a cold gas in the fore side absorbs effectively the emission coming from the hot interior. On the other hand, for the blue-shifted emission line, the emission made by the hot interior gas escapes from the cloud without absorbed by the cold gas (there is no cold blue-shifted gas).
5. As a result, the blue peak of the emission line becomes more prominent than that of the red-shifted emission. This is the explanation of the blue-red asymmetry.

In Figure 1.10, model spectra calculated with the Sobolev approximation or LVG (large-velocity gradient; Zhou 1995) are shown. These show the blue-red asymmetry (the blue line $>$ the red line).

Figure 1.12: C$^{18}$O total column density map (left) and H$^{13}$CO$^+$ channel map (right) of B335 along with the position-velocity maps along the major and minor axes. Taken from Fig.3 of Saito et al (1999).

Figure 1.13: Column density distribution $N_{\rm H}(r)$ derived from the H$^{13}$CO$^+$ and C$^{18}$O data taken by the Nobeyama 45 m telescope. Taken from Fig.9 of Saito et al (1999).

Early observation of star forming regions have revieled fast molecular outflow are often ejected from the central protostar with $\mbox{\raisebox{0.3ex}{$>$}\hspace{-1.1em} \raisebox{-0.7ex}{$\sim$}} 10{\rm km s}^{-1}$. B335 is also a typical outflow source. In Figure 1.12, distributions of high density gases traced by the C$^{18}$O and H$^{13}$CO$^+$ lines are shown as well as the bipolar outflow whose outline is indicated by a shadow (Hirano et al 1988). Comparing left and right panels, it is shown that the distribution of C$^{18}$O gas is more extended than that of H$^{13}$CO$^+$ which traces higher-density gas. And the distribution of the H$^{13}$CO$^+$ is more compact and the projected surface density seems to show the the actual distribution is spherical. And the molecular outflow seems to be ejected in the direction of the minor axis of the high-density gas. It may suggest that (1) a molecular outflow is focused or collimated by the effect of density distribution or that (2) collimation is made by the magnetic fields which run preferentially perpendicularly to the gas disk. This gas disk is observed by these high density tracers.

Combining the C$^{18}$O and H$^{13}$CO$^+$ distributions, the surface density distribution along the major axis is obtained by Saito et al (1999). From the lower panel of Figure 1.13, the column density distribution is well fitted in the range from 7,000 to 42,000 AU in radius,

$$\Sigma(r)=6.3 \times 10^{21}{\rm cm^{-2}}\left(\frac{r}{10^4 {\rm AU}}\right)^{-0.95},$$ (1.6)

where they omitted the data of $r\mbox{\raisebox{0.3ex}{$<$}\hspace{-1.1em} \raisebox{-0.7ex}{$\sim$}} 7000$ since the beam size is not negligible. Similar power-law density distributions are found by the far IR thermal dust emission.