The intriguing giant bow shocks near HH 131
  Using the High Dispersion Spectrograph at the Subaru Telescope, echelle spectra of two giant arcs, i.e. nebulosities Cw and L associated with HH 131 in Orion are presented. Typical emission lines of Herbig-Haro objects have been detected towards Cw. With the 2.16 m telescope of National Astronomical Observatories, spectra of Nebu. C, L and K are obtained, which also show strong [SII]6717/6731, H$\alpha$ and [NII]6583 emission lines. Position-velocity distributions of Cw and L are analyzed. The fastest radial velocity of Cw is V_r ~ -18.0 km/s. When the flow at L goes to the south, it slows down. The fastest radial velocity of L has been observed of -45.0 km/s and the slowest value is about -18.3 km/s. The similarity of the velocities and their positional connection indicate that Cw and L are physically associated. The entire flow tends to become less excited and less ionized when going further to the south (i.e., from Nebu. K, L to C). The electron densities of all the observed nebulosities are low (n_e ~ 10^2 cm^-3). Double kinematic signatures have been found in Cw from its [NII]6583 profiles while the observed H$\alpha$ profiles of Cw are almost symmetric. Bow shock models appear to agree with the observed position-velocity diagrams of the [NII spectra better than H$\alpha$ spectra. With the suggestion that these arcs are HH shocks possibly ejected out of the Orion A molecular cloud by an uncertain source, their spectra show low to intermediate excitation from their diagnostic line ratios. 
Min wang (1), Junichi Noumaru (2), Hongchi Wang (1), Ji Yang (1), and   Jiansheng Chen (3) ((1) PMO, China, (2) NAOJ, Japan,...