High-pressure, low-abundance water in bipolar outflows

Abstract

Context. Water is a potential tracer of outflow activity because it is heavily depleted in cold ambient gas and is copiously produced in shocks.Aims. We present a survey of the water emission in a sample of more than 20 outflows from low-mass young stellar objects with the goal of characterizing the physical and chemical conditions of the emitting gas. Methods. We used the HIFI and PACS instruments on board the Herschel Space Observatory to observe the two fundamental lines of ortho-water at 557 and 1670 GHz. These observations were part of the "Water In Star-forming regions with Herschel" (WISH) key program, and have been complemented with CO and H 2 data. Results. The emission of water has a different spatial and velocity distribution from that of the J = 1−0 and 2−1 transitions of CO. On the other hand, it has a similar spatial distribution to H 2 , and its intensity follows the H 2 intensity derived from IRAC images. This suggests that water traces the outflow gas at hundreds of kelvins that is responsible for the H 2 emission, and not the component at tens of kelvins typical of low-J CO emission. A warm origin of the water emission is confirmed by a remarkable correlation between the intensities of the 557 and 1670 GHz lines, which also indicates that the emitting gas has a narrow range of excitations. A radiative transfer analysis shows that while there is some ambiguity in the exact combination of density and temperature values, the gas thermal pressure nT is constrained within less than a factor of 2. The typical nT over the sample is 4×10 9 cm −3 K, which represents an increase of 10 4 with respect to the ambient value. The data also constrain the water column density within a factor of 2 and indicate values in the sample between 2 × 10 12 and 10 14 cm −2 . When these values are combined with estimates of the H 2 column density, the typical water abundance is only 3 × 10 −7 , with an uncertainty of a factor of 3. Conclusions. Our data challenge current C-shock models of water production through the combination of wing-line profiles, high gas compressions, and low abundances.

First spectrally-resolved H2observations towards HH 54

“…Our H 2 O abundance estimate in HH 54 confirms the values recently found by Herschel in outflows from Class 0 sources (e.g. Bjerkeli et al 2012;Santangelo et al 2013;Tafalla et al 2013;Busquet et al 2014), which are based on the assumption that H 2 O traces the same gas as traced by the low-J H 2 emission. An estimate of the H 2 O abundance at the C knot can also be derived using the PACS map of H 2 O (2 12 −1 01 ) by Bjerkeli et al (2011).…”

“…The H 2 0−0 S(4) component appears to be associated with H 2 O and high-J CO, as expected from the comparison between the spatial distributions (e.g. Nisini et al 2010;Tafalla et al 2013;Santangelo et al 2013). We note that in the low-J CO an additional gas component around the systemic velocity is detected.…”

“…Tafalla et al 2013;Santangelo et al 2013;Busquet et al 2014). We used the radex molecular LVG radiative transfer code (van der Tak et al 2007) to model the observed H 2 O (2 12 −1 01 ) emission.…”

“…Codella et al 2010;Kristensen et al 2012;Santangelo et al 2012;Vasta et al 2012) and showed that H 2 O emission probes warm ( 300 K) and dense (n H 2 > 10 5 cm −3 ) gas with spatial distribution that resembles the H 2 emission (e.g. Nisini et al 2010;Tafalla et al 2013;Santangelo et al 2013). Low H 2 O abundances are derived in outflows for warm shocked gas, ranging from a few ×10 −6 to a few ×10 −5 (e.g.…”

Water in star-forming regions withHerschel(WISH)

“…Those outflows extend well beyond the PACS field of view mapped here and their active outflow hot spots are not covered in our data. The excitation conditions for high-J CO and H 2 O emission in the outflow shocks have been determined recently in several studies based on PACS and HIFI observations (e.g., Santangelo et al 2012;Vasta et al 2012;Benedettini et al 2012;Bjerkeli et al 2012;Lefloch et al 2012;Codella et al 2012;Tafalla et al 2013). All studies conclude that only a combination of high density and temperature can account for the observed intensities.…”