Mapping the complex kinematics of LL objects in the Orion nebula★†

Abstract

LL Orionis-type objects (LL objects) are hyperbolic bowshocks visible around young stars in the outer Orion nebula, many of which are also associated with curved, highly collimated jets. The bowshocks are clearly due to the supersonic interaction between an outflow from the young star and an environmental flow from the core of the nebula, but the exact nature of these flows has not yet been established. We present the first high-resolution optical spectra of two of these objects, LL 1 and LL 2, together with their associated Herbig–Haro (HH) jets, HH 888 and HH 505. We combine multiple long-slit echelle spectra in the Hα 6563 Å and [N ii] 6584 Å lines to produce velocity maps of the two objects at a resolution of |$4\text{arcsec} \times 2\text{arcsec} \times 11 {\mathrm{km\ s^{-1}}}$|⁠. The gas motions within both stellar bowshocks are of rather low velocity (10–20 km s−1), but there are important differences between the two objects. LL 1 shows a high degree of symmetry, whereas LL 2 has very asymmetric kinematics that seem to follow velocity gradients in the surrounding nebula. We also measure the line-of-sight velocity for multiple knots in the HH 888 and HH 505 jets, and combine our spectroscopy with new and existing proper-motion measurements to reconstruct the three-dimensional kinematics of the jets. The knot motions in both jets are very similar: both flows are inclined at 40° to 60° from the plane of the sky, with exclusively redshifted knots to the north and exclusively blueshifted knots to the south. In both cases, one also sees a deceleration along the length of the jets, from >200 km s−1 close to the respective stars down to <100 km s−1 farther out. The marked contrasts that we find between the kinematics of the jets and the kinematics of the stellar bowshocks are evidence that the two phenomena are not causally related. Regular patterns in the dynamic ages of the HH 505 knots imply periodic ejections on three different time-scales: 50, 12 and 4 yr. We use line ratios and photometry to measure electron densities and excitation/ionization conditions in the stellar bowshocks and jet knots. The LL 1 bowshock has a bright inner shell with density ≃3000 cm−3 (compared with a local nebula density of ≃1000 cm−3) and line ratios that are consistent with equilibrium photoionization models. The bowshock also has a fainter outer rim, where the line ratios show evidence of shock excitation. Many of the jet knots also show evidence for a shock contribution to their excitation and have densities from 1000 to 8000 cm−3.