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-Ha ro flows using the NICMOS infrared camera on board the Hubble Space Telesco pe. These Herbig-Haro energy sources, which power the HH 34, 47, 83, 111, 1 99, 300, and 454 flows, have all been previously detected in the centimeter -wavelength radio continuum. The regions were imaged in two broadband filte rs, F160W and F205W, which, in addition to stellar continuum, principally t ransmit the [Fe II] 1.644 mu m and H-2 2.122 mu m lines, respectively, of s hocks present in the images. Highly collimated infrared jets emerge from th e sources of HH 34, ill, 300, and 454. In all of these cases, the [Fe II] e mission transmitted in the F160W filter is much stronger than the H-2 emiss ion 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 b e 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 source s relative to the H-2 emission by a combination of high jet density and str ong far-ultraviolet (7.6-13.6 eV) radiation from the forming star. The HH 4 54 [Fe II] jet, which emanates from the L1551 NE source, is pointed directl y at the bright shock HH 29, providing further evidence that this young sta r, rather than L1551 IRS 5, is the source of the brightest Herbig-Haro obje ct in the L1551 outflow complex. In five regions, the driving sources are v isible at 2 mu m. Among these, the sources powering HH 47 and HH 300 are fo und to be binary stars. One other young stellar system, the source of the H H 111 protostellar jet, is triple. When combined with other studies, these results indicate that 36% of 14 Herbig-Haro energy sources observed with NI CMOS are multiple at near-infrared wavelengths with component separations g reater than 0 ".15-0 ".20. Therefore, considering incompleteness, obscurati on, and faintness of possible companions in the observed bands, the binary frequency is likely to be considerably higher among the selected systems th an among low-mass main-sequence dwarf stars. There is no obvious connection between source multiplicity and jet morphology observed on large scales. F or 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. How ever, the total mass of the observed central binary would have to be unreal istically large to make the orbital period comparable to the ejection time interval of these jet features. Finally, the properties of the near-infrare d reflection nebulae associated with the observed sources are investigated. The shapes of the cavity walls traced by the observed reflection nebulae i ndicate the presence of large-scale and highly flattened circumstellar stru ctures that are opaque at near-infrared wavelengths. Their flaring shapes e ither are the result of preferential infall of the protostellar envelope al ong the outflow axis or were carved by wide-angle winds emerging from the i nner disk and the forming star. The inclination angles of several circumste llar structures are estimated.