Origin of warm and hot gas emission from low-mass protostars:  <i>Herschel</i>-HIFI observations of CO <i>J </i>= 16–15

Kristensen, L. E.; Dishoeck, E. F. van; Mottram, J. C.; Karska, A.; Yıldız, U. A.; Bergin, Edwin A.; Bjerkeli, P.; Cabrit, S.; Doty, S. D.; Evans, Neal J.; Gusdorf, A.; Harsono, Daniel; Herczeg, Gregory J.; Johnstone, Doug; Jørgensen, J. K.; Kempen, T. A. van; Lee, Jeong Eun; Maret, S.; Tafalla, M.; Visser, R.; Wampfler, S. F.

doi:10.1051/0004-6361/201630127

Cited by 34 publications

(90 citation statements)

References 75 publications

(162 reference statements)

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“…C.2). Also, the maximum velocity v max reached by gas seen in CO is similar to that of water and independent of the CO line used, even though the line profiles change greatly from low-to high-J and their FWHM are generally smaller than that of water (Kristensen et al 2017b;San José-García et al 2016). At the outflow positions, v max is also similar among tracers such as H 2 O, CO and SiO (Santangelo et al 2012;Bjerkeli et al 2012).…”

Section: Profiles Across Mass Rangementioning

confidence: 61%

“…The first two components are also seen in CO J = 16-15 (E up = 752 K, n crit ≈ 4 × 10 6 cm −3 ) profiles observed with HIFI for low-mass protostars (Kristensen et al 2017b). Extremely high velocity features are occasionally present as well in the 16-15 lines.…”

Section: Water Emission: Two Universal Componentsmentioning

confidence: 75%

“…The absorption can also be part of a P Cygni type profile, consisting of strong emission lines with blue-shifted absorption pointing to an expanding shell of gas. Also, (b) Based on Table 1 of Kristensen et al (2017b).…”

Section: Water Emission: Two Universal Componentsmentioning

confidence: 99%

“…A key step forward in the interpretation of these two CO rotational temperature components was made by Kristensen et al (2017b) through velocity resolved CO J = 16-15 profiles, 3 4 5 ( 1 4 ) K 70 0( 35 ) K 3 2 5 ( 9 ) K 76 3( 21 ) K 2 4 3 ( 1 3 ) K 58 7( 11 8 Goicoechea et al (2012Goicoechea et al ( , 2015.…”

Section: Comparison With Co Excitationmentioning

confidence: 99%

“…It focuses on questions (i) and (iii) and the latter part of question (ii), including previously unpublished data, new models and new analyses. The physical components traced by water vapor have been addressed in a number of synthesis papers, most notably San José- García et al (2016), Herpin et al (2016), van der Tak et al (2019), Kristensen et al (2017b) and Mottram et al (2017) for WISH+, Karska et al (2018) for WISH+WILL+DIGIT, and Manoj et al (2016) for HOPS+DIGIT, with the latter paper limited to CO. In terms of question (iii), this paper describes the evolution of water from cores to disks.…”

Section: Introductionmentioning

confidence: 99%

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Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy

Dishoeck

Kristensen²,

Mottram

et al. 2021

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Context. Water is a key molecule in the physics and chemistry of star and planet formation, but it is difficult to observe from Earth. The Herschel Space Observatory provided unprecedented sensitivity as well as spatial and spectral resolution to study water. The Water In Star-forming regions with Herschel (WISH) key program was designed to observe water in a wide range of environments and provide a legacy data set to address its physics and chemistry. Aims. The aim of WISH is to determine which physical components are traced by the gas-phase water lines observed with Herschel and to quantify the excitation conditions and water abundances in each of these components. This then provides insight into how and where the bulk of the water is formed in space and how it is transported from clouds to disks, and ultimately comets and planets. Methods. Data and results from WISH are summarized together with those from related open time programs. WISH targeted ~80 sources along the two axes of luminosity and evolutionary stage: from low- to high-mass protostars (luminosities from <1 to > 105 L⊙) and from pre-stellar cores to protoplanetary disks. Lines of H2O and its isotopologs, HDO, OH, CO, and [O I], were observed with the HIFI and PACS instruments, complemented by other chemically-related molecules that are probes of ultraviolet, X-ray, or grain chemistry. The analysis consists of coupling the physical structure of the sources with simple chemical networks and using non-LTE radiative transfer calculations to directly compare models and observations. Results. Most of the far-infrared water emission observed with Herschel in star-forming regions originates from warm outflowing and shocked gas at a high density and temperature (> 105 cm−3, 300–1000 K, v ~ 25 km s−1), heated by kinetic energy dissipation. This gas is not probed by single-dish low-J CO lines, but only by CO lines with Jup > 14. The emission is compact, with at least two different types of velocity components seen. Water is a significant, but not dominant, coolant of warm gas in the earliest protostellar stages. The warm gas water abundance is universally low: orders of magnitude below the H2O/H2 abundance of 4 × 10−4 expected if all volatile oxygen is locked in water. In cold pre-stellar cores and outer protostellar envelopes, the water abundance structure is uniquely probed on scales much smaller than the beam through velocity-resolved line profiles. The inferred gaseous water abundance decreases with depth into the cloud with an enhanced layer at the edge due to photodesorption of water ice. All of these conclusions hold irrespective of protostellar luminosity. For low-mass protostars, a constant gaseous HDO/H2O ratio of ~0.025 with position into the cold envelope is found. This value is representative of the outermost photodesorbed ice layers and cold gas-phase chemistry, and much higher than that of bulk ice. In contrast, the gas-phase NH3 abundance stays constant as a function of position in low-mass pre- and protostellar cores. Water abundances in the inner hot cores are high, but with variations from 5 × 10−6 to a few × 10−4 for low- and high-mass sources. Water vapor emission from both young and mature disks is weak. Conclusions. The main chemical pathways of water at each of the star-formation stages have been identified and quantified. Low warm water abundances can be explained with shock models that include UV radiation to dissociate water and modify the shock structure. UV fields up to 102−103 times the general interstellar radiation field are inferred in the outflow cavity walls on scales of the Herschel beam from various hydrides. Both high temperature chemistry and ice sputtering contribute to the gaseous water abundance at low velocities, with only gas-phase (re-)formation producing water at high velocities. Combined analyses of water gas and ice show that up to 50% of the oxygen budget may be missing. In cold clouds, an elegant solution is that this apparently missing oxygen is locked up in larger μm-sized grains that do not contribute to infrared ice absorption. The fact that even warm outflows and hot cores do not show H2O at full oxygen abundance points to an unidentified refractory component, which is also found in diffuse clouds. The weak water vapor emission from disks indicates that water ice is locked up in larger pebbles early on in the embedded Class I stage and that these pebbles have settled and drifted inward by the Class II stage. Water is transported from clouds to disks mostly as ice, with no evidence for strong accretion shocks. Even at abundances that are somewhat lower than expected, many oceans of water are likely present in planet-forming regions. Based on the lessons for galactic protostars, the low-J H2O line emission (Eup < 300 K) observed in extragalactic sources is inferred to be predominantly collisionally excited and to originate mostly from compact regions of current star formation activity. Recommendations for future mid- to far-infrared missions are made.

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Section: Profiles Across Mass Rangementioning

confidence: 61%

Section: Water Emission: Two Universal Componentsmentioning

confidence: 75%

Section: Water Emission: Two Universal Componentsmentioning

confidence: 99%

Section: Comparison With Co Excitationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy

Dishoeck

Kristensen²,

Mottram

et al. 2021

A&A

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110

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Anatomy of the massive star-forming region S106

Schneider

Röllig

Simon

et al. 2018

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The central area (40″ × 40″) of the bipolar nebula S106 was mapped in the [O I] line at 63.2 μm (4.74 THz) with high angular (6″) and spectral (0.24 MHz) resolution, using the GREAT heterodyne receiver on board SOFIA. The spatial and spectral emission distribution of [O I] is compared to emission in the CO 16 →15, [C II] 158 μm, and CO 11 →10 lines, mm-molecular lines, and continuum. The [O I] emission is composed of several velocity components in the range from –30 to 25 km s−1. The high-velocity blue- and red-shifted emission (v = −30 to –9 km s−1 and 8 to 25 km s−1) can be explained as arising from accelerated photodissociated gas associated with a dark lane close to the massive binary system S106 IR, and from shocks caused by the stellar wind and/or a disk–envelope interaction. At velocities from –9 to –4 km s−1 and from 0.5 to 8 km s−1 line wings are observed in most of the lines that we attribute to cooling in photodissociation regions (PDRs) created by the ionizing radiation impinging on the cavity walls. The velocity range from –4 to 0.5 km s−1 is dominated by emission from the clumpy molecular cloud, and the [O I], [C II], and high-J CO lines are excited in PDRs on clump surfaces that are illuminated by the central stars. Modelling the line emission in the different velocity ranges with the KOSMA-τ code constrains a radiation field χ of a few times 104 and densities n of a few times 104 cm−3. Considering self-absorption of the [O I] line results in higher densities (up to 106 cm−3) only for the gas component seen at high blue- and red velocities. We thus confirm the scenario found in other studies that the emission of these lines can be explained by a two-phase PDR, but attribute the high-density gas to the high-velocity component only. The dark lane has a mass of ~275 M⊙ and shows a velocity difference of ~1.4 km s−1 along its projected length of ~1 pc, determined from H13CO+ 1 →0 mapping. Its nature depends on the geometry and can be interpreted as a massive accretion flow (infall rate of ~2.5 × 10−4 M⊙ yr−1), or the remains of it, linked to S106 IR/FIR. The most likely explanation is that the binary system is at a stage of its evolution where gas accretion is counteracted by the stellar winds and radiation, leading to the very complex observed spatial and kinematic emission distribution of the various tracers.

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Models of irradiated molecular shocks

Godard

Forêts

Lesaffre

et al. 2019

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134

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Context. The recent discovery of excited molecules in starburst galaxies observed with ALMA and the Herschel space telescope has highlighted the necessity to understand the relative contributions of radiative and mechanical energies in the formation of molecular lines and explore the conundrum of turbulent gas bred in the wake of galactic outflows. Aims. The goal of the paper is to present a detailed study of the propagation of low velocity (5 to 25 km s −1 ) stationary molecular shocks in environments illuminated by an external ultraviolet (UV) radiation field. In particular, we intend to show how the structure, dynamics, energetics, and chemical properties of shocks are modified by UV photons and to estimate how efficiently shocks can produce line emission. Methods. We implemented several key physico-chemical processes in the Paris-Durham shock code to improve the treatment of the radiative transfer and its impact on dust and gas particles. We propose a new integration algorithm to find the steady-state solutions of magnetohydrodynamics equations in a range of parameters in which the fluid evolves from a supersonic to a subsonic regime. We explored the resulting code over a wide range of physical conditions, which encompass diffuse interstellar clouds and hot and dense photon-dominated regions (PDR). Results. We find that C-type shock conditions cease to exist as soon as G 0 > 0.2 (n H / cm −3 ) 1/2 . Such conditions trigger the emergence of another category of stationary solutions, called C*-type and CJ-type shocks, in which the shocked gas is momentarily subsonic along its trajectory. These solutions are shown to be unique for a given set of physical conditions and correspond to dissipative structures in which the gas is heated up to temperatures comprised between those found in C-type and adiabatic J-type shocks. High temperatures combined with the ambient UV field favour the production or excitation of a few molecular species to the detriment of others, hence leading to specific spectroscopic tracers such as rovibrational lines of H 2 and rotational lines of CH + . Unexpectedly, the rotational lines of CH + may carry as much as several percent of the shock kinetic energy. Conclusions. Ultraviolet photons are found to strongly modify the way the mechanical energy of interstellar shocks is processed and radiated away. In spite of what intuition dictates, a strong external UV radiation field boosts the efficiency of low velocity interstellar shocks in the production of several molecular lines which become evident tracers of turbulent dissipation.

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Origin of warm and hot gas emission from low-mass protostars: Herschel-HIFI observations of CO J = 16–15

Cited by 34 publications

References 75 publications

Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy

Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy

Anatomy of the massive star-forming region S106

Models of irradiated molecular shocks

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