The Intriguing Giant Bow Shocks near HH 131

Abstract

Using the High Dispersion Spectrograph (HDS) 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 (HH) objects have been detected toward nebulosity Cw with the broadband filter KV 408. With the low-dispersion spectrograph at the National Astronomical Observatories (NAO) 2.16 m telescope, spectra of nebulosities C, L, and K are obtained, which also show strong [S II] lambda 6717/lambda 6731, H alpha, and [N II] lambda 6583 emission lines. Position-velocity distributions of Cw and L are analyzed from the long-slit spectra observed with the HDS H alpha narrowband filter. The fastest radial velocity of Cw is V-r similar to -18.0 km s(-1). When the flow at L goes to the south, it slows down. The fastest radial velocity of L has been observed at -45.0 km s(-1), and the slowest value is about -18.3 km s(-1); the radial velocity gradient is about 200 km s(-1) pc(-1). The similarity of the fastest radial velocity of Cw to the slowest value of L and their positional connection indicate that they are physically associated. There is a tendency for the entire flow to become less excited and less ionized when going further to the south (i.e., from nebulosities K to L and C), where the most extended ( and presumably evolved) objects are seen. The electron densities of all the observed nebulosities are low (n(e) similar to 10(2) cm(-3)). Double-peaked kinematic signatures have been found in Cw from its [N II] lambda 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 [N II] spectra better than H alpha spectra, and a bow shock with its wing, apex, and postshock has been possibly revealed near Cw from the [N II] emission. 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.