TheHerschelHIFI water line survey in the low-mass proto-stellar outflow L1448

Abstract

Aims. As part of the WISH (Water In Star-forming regions with Herschel) key project, systematic observations of H 2 O transitions in young outflows are being carried out with the aim of understanding the role of water in shock chemistry and its physical and dynamical properties. We report on the observations of several ortho-and para-H 2 O lines performed with the HIFI instrument toward two bright shock spots (R4 and B2) along the outflow driven by the L1448 low-mass proto-stellar system, located in the Perseus cloud. These data are used to identify the physical conditions giving rise to the H 2 O emission and to infer any dependence on velocity. Methods. We used a large velocity gradient (LVG) analysis to derive the main physical parameters of the emitting regions, namely n(H 2 ), T kin , N(H 2 O) and emitting-region size. We compared these with other main shock tracers, such as CO, SiO and H 2 and with shock models available in the literature. Results. These observations provide evidence that the observed water lines probe a warm (T kin ∼ 400−600 K) and very dense (n ∼ 10 6 −10 7 cm −3 ) gas that is not traced by other molecules, such as low-J CO and SiO, but is traced by mid-IR H 2 emission. In particular, H 2 O shows strong differences with SiO in the excitation conditions and in the line profiles in the two observed shocked positions, pointing to chemical variations across the various velocity regimes and chemical evolution in the different shock spots. Physical and kinematical differences can be seen at the two shocked positions. At the R4 position, two velocity components with different excitation can be distinguished, of which the component at higher velocity (R4-HV) is less extended and less dense than the low velocity component (R4-LV). H 2 O column densities of about 2 × 10 13 and 4 × 10 14 cm −2 were derived for the R4-LV and the R4-HV components, respectively. The conditions inferred for the B2 position are similar to those of the R4-HV component, with H 2 O column density in the range 10 14 −5 × 10 14 cm −2 , corresponding to H 2 O/H 2 abundances in the range 0.5−1 × 10 −5 . The observed line ratios and the derived physical conditions seem to be more consistent with excitation in a low-velocity J-type shock with strong compression rather than in a stationary C-shock, although none of these stationary models seems able to reproduce the whole characteristics of the observed emission.

High-JCO survey of low-mass protostars observed withHerschel-HIFI

“…This is in contrast with the 3-2 line, which probes the colder entrained gas. The conclusion that H 2 O and high-J CO emission go together (but not low-J CO) is consistent with recent analyses (Santangelo et al 2012;Vasta et al 2012;Tafalla et al 2013) of WISH data at outflow positions offset from the source. The CO 10-9 line seems to be the lowest J transition whose line wings probe the warm shocked gas rather than the colder entrained outflow gas (see also Sect.…”

Water distribution in shocked regions of the NGC 1333-IRAS 4A protostellar outflow

“…This secondary high-velocity (HV) emission peak appears at a velocity of about +25 km s −1 and +35 km s −1 in R1 and R2. Similar variations of water line profiles with excitation were observed at the bow-shock positions along the red lobes of the L1448 (R4) and L1157 (R) outflows by Santangelo et al (2012) and Vasta et al (2012). Figure 3 presents the H 2 O 557 GHz/1097 GHz line ratio as a function of velocity for R1 and R2.…”

“…In particular, the Water In Star-forming regions with Herschel (WISH, van Dishoeck et al 2011) key program has been dedicated to the study of the physical and dynamical properties of water and its role in shock chemistry. The H 2 O line profiles observed with the Heterodyne Instrument for the Far Infrared (HIFI, de Graauw et al 2010) at outflow shocks show several kinematic components along with variations with excitation energy Vasta et al 2012;Santangelo et al 2012). The observed H 2 O emission probes warm (>200 K) and very dense gas (n H 2 > ∼ 10 6 cm −3 ), which is associated with high-J CO emission (e.g Karska et al 2013;Santangelo et al 2013) and is not traced by other molecules seen from the ground, such as low-J CO and SiO (e.g., Vasta et al 2012;Nisini et al 2013;Tafalla et al 2013;Santangelo et al 2013).…”

The complete far-infrared and submillimeter spectrum of the Class 0 protostar Serpens SMM1 obtained withHerschel

“…We have proposed that this hot CO and H 2 O emission arises from low velocity, nondissociative shocks in the inner walls of the outflow cavity compressing the gas to very high thermal pressures (P/k = n T K 10 9−10 K cm −3 ). Based on the large gas compression factors and H 2 O line profiles seen toward several shock spots in bipolar outflows (far from the protostellar sources) Santangelo et al (2012) and Tafalla et al (in prep. ) conclude that current low velocity J-shocks models explain their observations better than stationary C-shocks models.…”