Water in low-mass star-forming regions with<i>Herschel</i>

Schmalzl, M.; Visser, R.; Walsh, Catherine; Albertsson, T.; Dishoeck, E. F. van; Kristensen, L. E.; Mottram, J. C.

doi:10.1051/0004-6361/201424236

Cited by 32 publications

(66 citation statements)

References 93 publications

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“…HCO + can be destroyed through reactions with H 2 O (e.g. Jørgensen et al 2013) Schmalzl et al 2014) could be suppressing the HCO + abundance in the outflow. This would explain why it is a poorer tracer of the outflow than might otherwise be expected.…”

Section: Envelopementioning

confidence: 99%

Outflows, infall and evolution of a sample of embedded low-mass protostars

Mottram

Dishoeck

Kristensen

et al. 2017

A&A

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123

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Context. Herschel observations of water and highly excited CO (J >9) have allowed the physical and chemical conditions in the more active parts of protostellar outflows to be quantified in detail for the first time. However, to date, the studied samples of Class 0/I protostars in nearby star-forming regions have been selected from bright, well-known sources and have not been large enough for statistically significant trends to be firmly established. Aims. We aim to explore the relationships between the outflow, envelope and physical properties of a flux-limited sample of embedded low-mass Class 0/I protostars. Methods. We present spectroscopic observations in H 2 O, CO and related species with Herschel HIFI and PACS, as well as ground-based follow-up with the JCMT and APEX in CO, HCO + and isotopologues, of a sample of 49 nearby (d <500 pc) candidate protostars selected from Spitzer and Herschel photometric surveys of the Gould Belt. This more than doubles the sample of sources observed by the WISH and DIGIT surveys. These data are used to study the outflow and envelope properties of these sources. We also compile their continuum spectral energy distributions (SEDs) from the near-IR to mm wavelengths in order to constrain their physical properties (e.g. L bol , T bol and M env ). Results. Water emission is dominated by shocks associated with the outflow, rather than the cooler, slower entrained outflowing gas probed by ground-based CO observations. These shocks become less energetic as sources evolve from Class 0 to Class I. Outflow force, measured from low-J CO, also decreases with source evolutionary stage, while the fraction of mass in the outflow relative to the total envelope (i.e. M out /M env ) remains broadly constant between Class 0 and I. The median value of ∼1% is consistent with a core to star formation efficiency on the order of 50% and an outflow duty cycle on the order of 5%. Entrainment efficiency, as probed by F CO /Ṁ acc , is also invariant with source properties and evolutionary stage. The median value implies a velocity at the wind launching radius of 6.3 km s −1 , which in turn suggests an entrainment efficiency of between 30 and 60% if the wind is launched at ∼1AU, or close to 100% if launched further out. L[O i] is strongly correlated with L bol but not with M env , in contrast to low-J CO, which is more closely correlated with the latter than the former. This suggests that [O i] traces the present-day accretion activity of the source while CO traces time-averaged accretion over the dynamical timescale of the outflow. H 2 O is more strongly correlated with M env than L bol , but the difference is smaller than low-J CO, consistent with water emission primarily tracing actively shocked material between the wind, traced by [O i], and the entrained molecular outflow, traced by low-J CO. L[O i] does not vary from Class 0 to Class I, unlike CO and H 2 O. This is likely due to the ratio of atomic to molecular gas in the wind increasing as the source evolves, balancing out the decrease in ...

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Section: Envelopementioning

confidence: 99%

Outflows, infall and evolution of a sample of embedded low-mass protostars

Mottram

Dishoeck

Kristensen

et al. 2017

A&A

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123

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“…The overproduction of H 2 O ice may be due to a lack of grainsurface reactions in our chemical network, which tend to drain A102, page 6 of 12 oxygen out of H 2 O and convert it to other species (Schmalzl et al 2014). N 2 H + 1-0 probably suffers from our choice of a spherically symmetric envelope model.…”

Section: Models Vs Observationsmentioning

confidence: 99%

Chemical tracers of episodic accretion in low-mass protostars

Visser

Bergin

Jørgensen

2015

A&A

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Aims. Accretion rates in low-mass protostars can be highly variable in time. Each accretion burst is accompanied by a temporary increase in luminosity, heating up the circumstellar envelope and altering the chemical composition of the gas and dust. This paper aims to study such chemical effects and discusses the feasibility of using molecular spectroscopy as a tracer of episodic accretion rates and timescales. Methods. We simulate a strong accretion burst in a diverse sample of 25 spherical envelope models by increasing the luminosity to 100 times the observed value. Using a comprehensive gas-grain network, we follow the chemical evolution during the burst and for up to 10 5 yr after the system returns to quiescence. The resulting abundance profiles are fed into a line radiative transfer code to simulate rotational spectra of C 18 O, HCO + , H 13 CO + , and N 2 H + at a series of time steps. We compare these spectra to observations taken from the literature and to previously unpublished data of HCO + and N 2 H + 6−5 from the Herschel Space Observatory. Results. The bursts are strong enough to evaporate CO throughout the envelope, which in turn enhances the abundance of HCO + and reduces that of N 2 H + . After the burst, it takes 10 3 -10 4 yr for CO to refreeze and for HCO + and N 2 H + to return to normal. The H 2 O snowline expands outwards by a factor of ∼10 during the burst; afterwards, it contracts again on a timescale of 10 2 -10 3 yr. The chemical effects of the burst remain visible in the rotational spectra for as long as 10 5 yr after the burst has ended, highlighting the importance of considering luminosity variations when analyzing molecular line observations in protostars. The spherical models are currently not accurate enough to derive robust timescales from single-dish observations. As follow-up work, we suggest that the models be calibrated against spatially resolved observations in order to identify the best tracers to be used for statistically significant source samples.

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“…We also assumed perfect symmetry, which of course is questionable in massive objects. Schmalzl et al (2014) have used a small (N reactions) chemical network (SWaN) to predict realistic water abundance profiles for low-mass protostellar envelopes, which have significantly improved the HIFI water line modeling for low-mass protostellar cores. Applying this chemistry network to our highmass protostellar objects is very difficult because several crucial input parameters are not well known or are not optimized for high-mass protostars (e.g., the large amounts of UV photons produced by the massive object).…”

Section: Abundance Structurementioning

confidence: 99%

Herschel-HIFI view of mid-IR quiet massive protostellar objects

Herpin

Chavarría

Jacq

et al. 2016

A&A

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Aims. We present Herschel/HIFI observations of 14 water lines in a small sample of Galactic massive protostellar objects: NGC 6334I(N), DR21(OH), IRAS 16272-4837, and IRAS 05358+3543. Using water as a tracer of the structure and kinematics, we individually study each of these objects with the aim to estimate the amount of water around them, but to also to shed light on the high-mass star formation process. Methods. We analyzed the gas dynamics from the line profiles using Herschel-HIFI observations acquired as part of the WISH keyproject of 14 far-IR water lines (H 2 O) and several other species. Then through modeling the observations using the RATRAN radiative transfer code, we estimated outflow, infall, turbulent velocities, and molecular abundances and investigated the correlation with the evolutionary status of each source. Results. The four sources (and the previously studied W43-MM1) have been ordered in terms of evolution based on their spectral energy distribution from youngest to older: 1) NGC 64334I(N); 2) W43-MM1; 3) DR21(OH); 4) IRAS 16272-4837; 5) IRAS 05358+3543. The molecular line profiles exhibit a broad component coming from the shocks along the cavity walls that is associated with the protostars, and an infalling (or expanding, for IRAS 05358+3543) and passively heated envelope component, with highly supersonic turbulence that probably increases with the distance from the center. Accretion rates between 6.3 × 10 −5 and 5.6 × 10 −4 M yr −1 are derived from the infall observed in three of our sources. The outer water abundance is estimated to be at the typical value of a few 10 −8 , while the inner abundance varies from 1.7 × 10 −6 to 1.4 × 10 −4 with respect to H 2 depending on the source. Conclusions. We confirm that regions of massive star formation are highly turbulent and that the turbulence probably increases in the envelope with the distance to the star. The inner abundances are lower than the expected, 10 −4 , perhaps because our observed lines do not probe deep enough into the inner envelope or because photodissociation through protostellar UV photons is more efficient than expected. We show that the higher the infall or expansion velocity in the protostellar envelope, the higher the inner abundance. This may indicate that higher infall or expansion velocities generate shocks that will sputter water from the ice mantles of dust grains in the inner region. High-velocity water must be formed in the gas phase from shocked material.

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Water in low-mass star-forming regions withHerschel

Cited by 32 publications

References 93 publications

Outflows, infall and evolution of a sample of embedded low-mass protostars

Outflows, infall and evolution of a sample of embedded low-mass protostars

Chemical tracers of episodic accretion in low-mass protostars

Herschel-HIFI view of mid-IR quiet massive protostellar objects

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