The Orion nebula (M42) Herbig-Haro object, HH 201, within the tip of a molecular finger

Abstract

The most prominent Herbig-Haro (HH) object, HH 201, in the Orion nebula is investigated using optical spectroscopy along with optical and infrared images. An infrared Subaru H₂v= 1-0 S(1) image of the region surrounding the optical line emission of HH 201 has yielded the discovery of a faint H₂ finger associated with HH 201. The H₂ finger is seen to share a common width and orientation with the optical line emission. Therefore, it is unlikely that the association is caused by a chance superposition along the line of sight. The finger is found to precede the optical line emission, which is inconsistent with this H₂ emission being produced within a bowshock. The origin of this finger is discussed in the context of the current understanding of HH objects and the Orion molecular fingers.

Multi-epoch HST[S II] 6716- and 6731-Å images are used to measure the proper motions of the various knots present in the filamentary structure of HH 201. Tangential velocities of between 40 and 178 km s^-1 are found. Two new knots with associated proper motions are found in the wake of HH 201 and the general structure of the tangential flow is shown by means of a multi-epoch difference image.

[S II] 6716- and 6731-Å spectra taken with the RGO spectrograph are used to spectroscopically resolve the velocity profile of HH 201 into distinct components with radial velocities of between -267 and -9 km s^-1. Using the [S II] 6716- and 6731-Å line ratios, the local electron density of gas at the various velocities is found to be between 830 and 12 200 cm^-3. Combining radial and tangential velocities, the angle between the direction of motion and the line of sight is found to be approximately 30°. Assuming that the emission knots and velocity components correspond to condensations in a bowshock, a shock velocity at the head of HH 201 of 295 km s^-1 is established. Using this combination of optical and infrared data, the morphology and kinematics of HH 201 and its immediate environs are elucidated.