Astronomy and Astrophysics, volume 540A, 84-84 (2012/4-1)
Water in star-forming regions with Herschel: highly excited molecular emission from the NGC 1333 IRAS 4B outflow.
HERCZEG G.J., KARSKA A., BRUDERER S., KRISTENSEN L.E., VAN DISHOECK E.F., JORGENSEN J.K., VISSER R., WAMPFLER S.F., BERGIN E.A., YILDIZ U.A., PONTOPPIDAN K.M. and GRACIA-CARPIO J.
Abstract (from CDS):
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 ∼3x106cm–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'' (1200AU) 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 [OI] 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.
infrared: ISM - ISM: jets and outflows - stars: protostars - molecular processes - stars: individual: NGC 1333 IRAS 4B
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