Hubble Space Telescope NICMOS images of Herbig-Haro energy sources: [Fe II] jets, binarity, and envelope cavities

We have observed seven regions surrounding the driving sources of Herbig-Haro flows using the NICMOS infrared camera on board the Hubble Space Telescope. These Herbig-Haro energy sources, which power the HH 34, 47, 83, 111, 199, 300, and 454 flows, have all been previously detected in the centimeter-wavelength radio continuum. The regions were imaged in two broadband filters, F160W and F205W, which, in addition to stellar continuum, principally transmit the [Fe II] 1.644 mu m and H-2 2.122 mu m lines, respectively, of shocks present in the images. Highly collimated infrared jets emerge from the sources of HH 34, ill, 300, and 454. In all of these cases, the [Fe II] emission transmitted in the F160W filter is much stronger than the H-2 emission in the F205W band. Combined with previously published NICMOS images of the bright [Fe II] HH 1 jet, these data suggest that [Fe II] emission may be as important a tracer of shocked jets in the infrared as [S II] emission is in the optical. [Fe II] emission may be enhanced near the driving sources relative to the H-2 emission by a combination of high jet density and strong far-ultraviolet (7.6-13.6 eV) radiation from the forming star. The HH 454 [Fe II] jet, which emanates from the L1551 NE source, is pointed directly at the bright shock HH 29, providing further evidence that this young star, rather than L1551 IRS 5, is the source of the brightest Herbig-Haro object in the L1551 outflow complex. In five regions, the driving sources are visible at 2 mu m. Among these, the sources powering HH 47 and HH 300 are found to be binary stars. One other young stellar system, the source of the HH 111 protostellar jet, is triple. When combined with other studies, these results indicate that 36% of 14 Herbig-Haro energy sources observed with NICMOS are multiple at near-infrared wavelengths with component separations greater than 0 .15-0 .20. Therefore, considering incompleteness, obscuration, and faintness of possible companions in the observed bands, the binary frequency is likely to be considerably higher among the selected systems than among low-mass main-sequence dwarf stars. There is no obvious connection between source multiplicity and jet morphology observed on large scales. For example, the beam of the HH 47 jet exhibits quasi-periodic wiggles that might be one indication of jet modulation at the binary orbital period. However, the total mass of the observed central binary would have to be unrealistically large to make the orbital period comparable to the ejection time interval of these jet features. Finally, the properties of the near-infrared reflection nebulae associated with the observed sources are investigated. The shapes of the cavity walls traced by the observed reflection nebulae indicate the presence of large-scale and highly flattened circumstellar structures that are opaque at near-infrared wavelengths. Their flaring shapes either are the result of preferential infall of the protostellar envelope along the outflow axis or were carved by wide-angle winds emerging from the inner disk and the forming star. The inclination angles of several circumstellar structures are estimated.