The Intriguing Giant Bow Shocks near HH 131

Using the High Dispersion Spectrograph at the Subaru Telescope, echellespectra of two giant arcs, i.e. nebulosities Cw and L associated with HH 131 inOrion are presented. Typical emission lines of Herbig-Haro objects have beendetected towards Cw. With the 2.16 m telescope of National AstronomicalObservatories, spectra of Nebu. C, L and K are obtained, which also show strong[SII]6717/6731, H$\alpha$ and [NII]6583 emission lines. Position-velocitydistributions of Cw and L are analyzed. The fastest radial velocity of Cw isV_r ~ -18.0 km/s. When the flow at L goes to the south, it slows down. Thefastest radial velocity of L has been observed of -45.0 km/s and the slowestvalue is about -18.3 km/s. The similarity of the velocities and theirpositional connection indicate that Cw and L are physically associated. Theentire flow tends to become less excited and less ionized when going further tothe south (i.e., from Nebu. K, L to C). The electron densities of all theobserved nebulosities are low (n_e ~ 10^2 cm^-3). Double kinematic signatureshave 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 theobserved position-velocity diagrams of the [NII spectra better than H$\alpha$spectra. With the suggestion that these arcs are HH shocks possibly ejected outof the Orion A molecular cloud by an uncertain source, their spectra show lowto intermediate excitation from their diagnostic line ratios.Comment: 27 pages, 10 figures, accepted for publiscation in A