Water in star-forming regions with Herschel: highly excited molecular emission from the NGC 1333 IRAS 4B outflow

Abstract

During the embedded phase of pre-main sequence stellar evolution, a disk forms from the dense envelope while an accretion-driven outflow carves out a cavity within the envelope. Highly excited (E' = 1000 - 3000 K) H₂O emission in spatially unresolved Spitzer/IRS spectra of a low-mass Class 0 object, NGC 1333 IRAS 4B, has previously been attributed to the envelope-disk accretion shock. However, the highly excited H₂O emission could instead be produced in an outflow. As part of the survey of low-mass sources in the Water in Star Forming Regions with Herschel (WISH-LM) program, we used Herschel/PACS to obtain a far-IR spectrum and several Nyquist-sampled spectral images to determine the origin of excited H₂O emission from NGC 1333 IRAS 4B. The spectrum has high signal-to-noise in a rich forest of H₂O, CO, and OH lines, providing a near-complete census of far-IR molecular emission from a Class 0 protostar. The excitation diagrams for the three molecules all require fits with two excitation temperatures. The highly excited component of H₂O emission is characterized by subthermal excitation of ~1500 K gas with a density of ~3 × 10⁶ cm^-3, conditions that also reproduce the mid-IR H₂O emission detected by Spitzer. On the other hand, a high density, low temperature gas can reproduce the H₂O spectrum observed by Spitzer but underpredicts the H₂O lines seen by Herschel. Nyquist-sampled spectral maps of several lines show two spatial components of H₂O emission, one centered at ~5'' (1200 AU) south of the central source at the position of the blueshifted outflow lobe and a heavily extincted component centered on-source. The redshifted outflow lobe is likely completely obscured, even in the far-IR, by the optically thick envelope. Both spatial components of the far-IR H₂O emission are consistent with emission from the outflow. In the blueshifted outflow lobe over 90% of the gas-phase O is molecular, with H₂O twice as abundant than CO and 10 times more abundant than OH. The gas cooling from the IRAS 4B envelope cavity walls is dominated by far-IR H₂O emission, in contrast to stronger [O I] and CO cooling from more evolved protostars. The high H₂O luminosity may indicate that the shock-heated outflow is shielded from UV radiation produced by the star and at the bow shock.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are only available in electronic form at http://www.aanda.org