Water in star-forming regions withHerschel(WISH)

Abstract

Context. Water is a key tracer of dynamics and chemistry in low-mass star-forming regions, but spectrally resolved observations have so far been limited in sensitivity and angular resolution, and only data from the brightest low-mass protostars have been published. Aims. The first systematic survey of spectrally resolved water emission in 29 low-mass (L < 40 L ) protostellar objects is presented. The sources cover a range of luminosities and evolutionary states. The aim is to characterise the line profiles to distinguish physical components in the beam and examine how water emission changes with protostellar evolution. Methods. H 2 O was observed in the ground-state 1 10 -1 01 transition at 557 GHz (E up /k B ∼ 60 K) as single-point observations with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel in 29 deeply embedded Class 0 and I low-mass protostars. Complementary far-IR and sub-mm continuum data (including PACS data from our programme) are used to constrain the spectral energy distribution (SED) of each source. H 2 O intensities are compared to inferred envelope properties, e.g., mass and density, outflow properties and CO 3-2 emission. Results. H 2 O emission is detected in all objects except one (TMC1A). The line profiles are complex and consist of several kinematic components tracing different physical regions in each system. In particular, the profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not in the quiescent envelope. Several sources show multiple shock components appearing in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in seven sources (six Class 0, one Class I) indicative of infalling envelopes, and regular P-Cygni profiles in four sources (three Class I, one Class 0) indicative of expanding envelopes. Molecular "bullets" moving at > ∼ 50 km s −1 with respect to the source are detected in four Class 0 sources; three of these sources were not known to harbour bullets previously. In the outflow, the H 2 O/CO abundance ratio as a function of velocity is nearly the same for all line wings, increasing from 10 −3 at low velocities (<5 km s −1 ) to > ∼ 10 −1 at high velocities (>10 km s −1 ). The water abundance in the outer cold envelope is low, > ∼ 10 −10 . The different H 2 O profile components show a clear evolutionary trend: in the younger Class 0 sources the emission is dominated by outflow components originating inside an infalling envelope. When large-scale infall diminishes during the Class I phase, the outflow weakens and H 2 O emission all but disappears.

The first ALMA view of IRAS 16293-2422

“…supersonic infall. The infall velocity and derived infall rates are consistent with the values derived by Kristensen et al (2012) for a small sample of low-mass protostars. It is important to note that for source A we could not identify an inverse P-Cygni profile, if present, because of the steep velocity gradient.…”

“…Since the front absorbing layer is probably subthermally excited, we assumed a conservative value for the excitation front layer of T f = 3 K. The continuum temperature, T c , was chosen such that the continuum radiation temperature, J c (T c ), matches the peak continuum flux in the image. The continuum source beam filling fraction, Φ, which is not well constrained, was set to 0.3 to use the same value as Di Francesco et al (2001), who modeled interferometric observations with a similar angular resolution as we did; this value is consistent with the line modeling results of coarse angular resolution Herschel data Kristensen et al (2012). Although the value of Φ used here is arbitrary, we have also performed line profile fits with higher values of Φ and the obtained infall velocity and velocity dispersion are unchanged, while the values for T r and τ 0 are increased and decreased, respectively.Similarly, if a higher value of T c is used, T r and τ 0 are increased and decreased, respectively, while we obtain the same results for V in and σ v .…”

“…C.1). The blue excess in the line profiles is not fitted, and therefore the fit is mostly constrained by the absorption feature, see Di Francesco et al (2001); Kristensen et al (2012) for similar procedures. Despite its simplicity, the two-layer model contains more parameters than can be fully constrained because of degeneracies in the model, but allows a robust determination of the velocity information.…”

“…Each inverse P-Cygni profile was fitted using the simple twoslab model described by Myers et al (1996) with the modification introduced by Di Francesco et al (2001) to take into account the continuum source, see also Kristensen et al (2012). The model assumes that the central continuum source is optically thick, emitting as a blackbody of temperature T c , and filling a fraction of the beam, Φ.…”

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

“…More recently, water emission observed with HIFI (Santangelo et al 2012) was suggested to originate in J-shocks, which is not consistent with the findings from the shock model comparisons above. However, Kristensen et al (2012) has shown that the water line profiles resolved with HIFI are complex, tracing simultaneously more than one processes.…”