Bo Reipurth, John Bally (Colorado), Robert Fesen (Dartmouth), and David Devine (GSFC) used the NOAO Mosaic Camera at the KPNO 4-m to trace jets emerging from young stellar objects (YSOs). They extended their earlier conclusions (also using NOAO facilities) that the jets may traverse several parsecs from their points of origin, but now find an entirely new class of jets that are only made visible by the external radiation field from nearby massive stars. Previously, the Herbig-Haro objects that the jets comprise were only seen by shock-excited emission, making their characterization difficult, since their visibility depended on complex and highly non-linear processes. The external illumination provides cleaner and more direct diagnostics of the mass outflows from YSOs. One key finding is that Herbig-Haro jets now appear to be important players in the energetics of star-forming regions.
Caption:
A portion of the Mayall 4-m image showing HH 444 and HH 445,
two irradiated
jets lying about 10' east of the ionizing star, Orionis. Both jets are
predominantly one-sided and are powered by T-Tauri stars (V510 Ori and
A0976-357). This figure was produced by summing images obtained through Ha
and 6717 + 6731 Å [SII] line filters.
Most young stars produce powerful collimated winds or jets during their
birth. Optical emission lines are often seen in the vicinity of young stars
in the form of Herbig-Haro objects, which are collisionally excited nebulae
powered by jets and outflows from young stars. Until the early 1990s, it was
generally believed that HH objects and stellar jets were small-scale
phenomena with typical dimensions of 0.1 to 0.3 parsecs. Recently, however,
imagers with large fields of view allowed the jets to be followed over much
larger scales; there are now several examples of jets that can be traced on
scales of up to 10 parsecs. The new externally illuminated jets seen by
Reipurth et al. were discovered in the vicinity of the Orionis sub-group
of the Orion OB Association, which is believed to be at least 1.7 million
years old. They found about a half dozen examples in the outskirts of the
Orion Nebula on the 36' FOV images obtained at the Mayall 4-m. Since the
jets are photo-ionized, their densities can be reliably estimated.
Oddly, many irradiated jets appear to be one sided. Any counter-jets would
probably not be hidden by dust since the source regions show little evidence
of obscuration (background galaxies are visible in some cases). In the
Orionis group of irradiated jets, the source YSOs are also located more than
two parsecs from the nearest known molecular gas (the western rim of the
Orion B cloud). Furthermore, all these jets point away from the source of
ionizing radiation. Perhaps the radiation field has destroyed the
collimation region on the side of the source facing the hot star; the
outflow in this direction would be poorly collimated and experience a more
rapid decrease in density and emission measure. These irradiated jets may
for the first time be used to accurately diagnose jet densities and the mass
loss rates of their source YSOs. The appearance of jets from low mass stars
in the relatively old
Orionis sub-group of the Orion OB Association may
indicate that either the jet production phase of certain young stars may
last more than a million years or that these particular stars were the very
last to form.
The large spatial extents of YSO outflows and their clustering imply that they are a major source of energy injection into the interstellar medium that can accelerate molecular outflows, efficiently stir molecular clouds, dissociate molecules, and perhaps drive interstellar turbulence. These flows may be a vital link in the self-regulation of star formation; they may alter the chemical state of the host cloud, as well. Dissociative shocks may regulate the `chemical aging' of such clouds by periodically disrupting molecules, thereby resetting the chemical state of the host cloud to its initial conditions. Dissociative shocks may also be partly responsible for the production of CI and CII, which appear to coexist with molecules and may contribute to the relatively invariant (from cloud to cloud) CO/H2 ratio. Furthermore, since the most distant components of parsec-scale flows have dynamical ages comparable to the accretion time-scale of a typical YSO, the structure, kinematics, and symmetry properties of such flows may eventually be used to diagnose the mass loss and accretion histories of the young stars driving them (Devine et al. 1997; Reipurth, Bally, Devine 1997). The symmetries of these giant flows may provide clues to both the accretion history of the source stars and to the role that stellar multiplicity may play in the regulation of stellar outflows.
Finally, the jets and outflows from young stars may be the only class of astrophysical jet where one can determine all three components of the flow field, the chemical and physical state of the gas, and observe the time-dependent behavior of the system. Thus, Herbig-Haro objects may serve as `Rosetta Stones' for deciphering the complex behavior of accretion powered outflow systems in astrophysics.