Images of Shocks in the Orion Molecular Ridge

Ka Chun Yu, John Bally, David Devine (all at Colorado), and Bo Reipurth (ESO) used IR cameras on the KPNO 2.1-m and CTIO 1.5-m telescopes to map the Orion molecular cloud sources OMC-2 and OMC-3 in the 2.12 m line of shocked molecular Hydrogen (H₂). They find that the OMC-2/3 region is one of the most active sites of ongoing low to intermediate mass star formation known. The discovery of nearly a dozen collimated outflows from young stellar objects (YSOs) and more than 80 individual H₂ emitting shocks demonstrates that outflows from young stars are churning this molecular cloud.

OMC-2 and OMC-3 are two major star forming cloud cores that form a molecular ridge bounded by the Orion Nebula to the south and NGC 1977 to the north. Previous infrared investigations have shown that the majority of stars on the line of sight to OMC-2 are likely young members of the Orion OB association. Images of continuum emission by dust at 1.3 mm show a remarkable north-south chain of over a dozen class-0 protostars embedded in a thin dust filament extending from OMC-2 to OMC-3 (see Chini et al., 1997, ApJL, 474, L135).

The figures show a continuum-subtracted 2.12 m line image of a 24' × 11' field in the OMC-2/3 region, and an optical narrowband S [II] image of the same region, taken with the CTIO 1.5-m. The locations of known IRAS (circles), far-infrared (triangles), and mm continuum (diamonds) sources are shown in the near-infrared difference image. Also identified are previously known HH objects to the east, the Haro 5a/6a reflection nebula (containing HH294), and a slew of new HH objects. The S [II] image also shows in a white outline the area covered in the difference mosaic.

The mm cores, the far-infrared sources, and the sources for all of the YSO outflows fall on a narrow molecular ridge that runs roughly N-S. The suspected YSOs are so deeply embedded that they are invisible in the S [II] image, where only a fraction of the shocks are visible as HH objects. In fact, the most well known set of visible shocks to the east (HH41, HH42, HH128, and HH129) are well away from the deepest extinction. Without imaging with an IR shock-excited tracer, it would be extremely difficult if not impossible to connect shocks with individual outflows and sources. Also seen are scores of individual H₂ knots, which show up as black in the difference image, which delineate flow axes. The flow morphologies include chains of H₂ knots, bright jets, bow shocks, and streamers, frequently coinciding with known HH objects or millimeter emission sources. Other structures, such as the faint fan-shaped filamentary structures to the far north in the center of the mosaic, may mark molecular gas excited by UV-induced fluorescence rather than shocks.

The longest suspected single outflow in the image is a remarkable E-W flow (highlighted by the horizontal line in the figures) that connects with HH128 to the east and HH295 to the west, for a length of 19', or at the distance to Orion, 2.6 pc. It is the first parsec-scale YSO outflow to be discovered in the infrared, although the suspected source is so deeply embedded that it is not seen at 2 m and is not associated with a mm or far-infrared core.

The total 2.12 m H₂ emission line luminosity within the field is ~ 0.5 . For an excitation temperature of T_e ~ 2000 K, the total luminosity for H₂ emission is 10 times that of the 2.12 m line alone. Assuming a 2.2 m extinction of 1 mag, the extinction-corrected H₂ luminosity is then ~ 12 . This is a strong lower limit on the rate at which mechanical energy is injected into the molecular cloud since H₂ is but one of many important coolants that radiate away thermal energy in shocks.

The Orion Nebula has produced on the order of 500 to 1,000 young stars, including a number of high mass ones in the last million years. Although much less luminous, the OMC-2 and 3 cores and the dense ridge of molecular gas that extends north of the Orion Nebula appear to be an extremely active site of on-going star formation that contains over a dozen class-0 protostars, dozens of active outflows, and perhaps hundreds of more evolved young stars that have formed within the last few million years. The large number of active flows and sub-mm sources indicates that this region is continuing to form stars at a high rate. Assuming that the depth of this star forming ridge is about the same as its projected width, 4' or 0.5 pc, then the effective volume where most of the young stars are located is on the order of several cubic parsecs. If the duration of the phase during which an outflow is only observable by means of its H₂ or CO emission (as opposed to shocks visible as Herbig-Haro objects) is about 3 X 10⁴ years, comparable to the duration of the lifetime of class­0 YSOs, then the formation rate must be about 30 to 40 per 10⁵ years. The cumulative effect of the jets and outflows from such sustained star formation must be an important source of dissociating shocks and turbulent motions, and must play a crucial role in the dynamics, chemistry, and evolution of star formation within the cloud.