The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region

Putaud, Thomas; Michaut, X.; Petit, Franck; Roueff, E.; Lis, D. C.

doi:10.1051/0004-6361/201935402

Cited by 20 publications

(22 citation statements)

References 86 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Consequently, water lines emission from this object are sensitive to the absolute photodesorption yields employed in the disk model (Kamp et al 2013). Finally, photodesorption of water has also been considered to explain the ortho-to-para ratio (OPR) observed from water emission lines around TW Hydrae (Hogerheijde et al 2011;Salinas et al 2016) or in the Orion photon dominated region (Putaud et al 2019).…”

Section: Introductionmentioning

confidence: 99%

Vacuum-UV photodesorption from compact Amorphous Solid Water : photon energy, isotopic and temperature effects

Fillion¹,

Dupuy²,

Féraud³

et al. 2021

Preprint

View full text Add to dashboard Cite

Context. Vacuum-UV (VUV) photodesorption from water-rich ice mantles coating interstellar grains is known to play an important role on the gas-to-ice ratio in stars and planets formation regions. Quantitative photodesorption yields from water ice are crucial for astrochemical models. Aims. The aims is to provide the first quantitative photon-energy dependent photodesorption yields from water ices in the VUV. This information is important for the understanding of the photodesorption mechanisms and to account for the variation of the yields under interstellar irradiation conditions. Methods. Experiments have been performed on the DESIRS beamline at the SOLEIL synchrotron facility (St Aubin, France), delivering tunable VUV radiation, using the ultra-high SPICES (Surface Processes and ICES) vacuum chamber. Thick compact amorphous solid water ice (H 2 O and D 2 O) grown onto a cold Au substrate have been irradiated from 7 to 13.5 eV. Quantitative yields have been obtained by detection into the gas phase with mass-spectrometry for sample temperatures ranging from 15 K to 100 K. Results. Photodesorption spectra of H 2 O (D 2 O), OH (OD), H 2 (D 2 ) and O 2 peak around 9-10 eV and decrease at higher energies. Average photodesorption yields of intact water at 15 K are 5 × 10 −4 molecule/photon for H 2 O and 5 × 10 −5 molecule/photon for D 2 O over the 7-13.5 eV range. The strong isotopic effect can be explained by a differential chemical recombination between OH (OD) and H (D) photofragments originating from lower kinetic energy available for the OH photofragments upon direct water photodissociation and/or possibly by an electronic relaxation process. It is expected to contribute to water fractionation during the building-up of the ice grain mantles in molecular clouds and to favor OH-poor chemical environment in comet-formation regions of protoplanetary disks. The yields of all the detected species except OH (OD) are enhanced above (70 ±10) K, suggesting an ice restructuration at this threshold temperature.

show abstract

Section: Introductionmentioning

confidence: 99%

Vacuum-UV photodesorption from compact Amorphous Solid Water : photon energy, isotopic and temperature effects

Fillion¹,

Dupuy²,

Féraud³

et al. 2021

Preprint

View full text Add to dashboard Cite

show abstract

“…Hence, this temperature might be also compatible with s-H 2 S formation 9 in warm grains if T spin T d upon formation is preserved in the gas-phase after photodesorption (e.g., Guzmán et al 2013). Interestingly, the H 2 O OTP ratio derived from observations of the Orion Bar is 2.8 ± 0.1 (Putaud et al 2019) and implies T spin (H 2 O) = 35 ± 2 K. This value is compatible with T spin (H 2 S) and might reflect the similar T d of the PDR layers where most s-H 2 O and s-H 2 S form and photodesorb. Nevertheless, laboratory experiments have chal- 9 Crockett et al (2014) inferred N(o-H 2 S)/N(p-H 2 S) = 2.5 ± 0.8 in the hot core of Orion KL using LTE rotational digrams.…”

Section: Line Intensity Comparison and H 2 S Ortho-to-para Ratiomentioning

confidence: 81%

Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains

Goicoechea,

Aguado,

Cuadrado

et al. 2021

Preprint

View full text Add to dashboard Cite

Hydride molecules lie at the base of interstellar chemistry, but the synthesis of sulfuretted hydrides is poorly understood and their abundances often crudely constrained. Motivated by new observations of the Orion Bar photodissociation region (PDR) -1 resolution ALMA images of SH + ; IRAM 30m detections of bright H 32 2 S, H 34 2 S, and H 33 2 S lines; H 3 S + (upper limits); and SOFIA/GREAT observations of SH (upper limits) -we perform a systematic study of the chemistry of sulfur-bearing hydrides. We self-consistently determine their column densities using coupled excitation, radiative transfer as well as chemical formation and destruction models. We revise some of the key gas-phase reactions that lead to their chemical synthesis. This includes ab initio quantum calculations of the vibrational-state-dependent reactions SH + + H 2 (v) H 2 S + + H and S + H 2 (v) SH + H. We find that reactions of UV-pumped H 2 (v ≥ 2) molecules with S + ions explain the presence of SH + in a high thermal-pressure gas component, P th /k ≈ 10 8 cm −3 K, close to the H 2 dissociation front (at A V < 2 mag). These PDR layers are characterized by no or very little depletion of elemental sulfur from the gas. However, subsequent hydrogen abstraction reactions of SH + , H 2 S + , and S atoms with vibrationally excited H 2 , fail to form enough H 2 S + , H 3 S + , and SH to ultimately explain the observed H 2 S column density (∼2.5×10 14 cm −2 , with an ortho-to-para ratio of 2.9 ± 0.3; consistent with the high-temperature statistical value). To overcome these bottlenecks, we build PDR models that include a simple network of grain surface reactions leading to the formation of solid H 2 S (s-H 2 S). The higher adsorption binding energies of S and SH suggested by recent studies imply that S atoms adsorb on grains (and form s-H 2 S) at warmer dust temperatures (T d < 50 K) and closer to the UV-illuminated edges of molecular clouds. We show that everywhere s-H 2 S mantles form(ed), gas-phase H 2 S emission lines will be detectable. Photodesorption and, to a lesser extent, chemical desorption, produce roughly the same H 2 S column density (a few 10 14 cm −2 ) and abundance peak (a few 10 −8 ) nearly independently of n H and G 0 . This agrees with the observed H 2 S column density in the Orion Bar as well as at the edges of dark clouds without invoking substantial depletion of elemental sulfur abundances.

show abstract

“…There have been some claims of ortho/para ratios lower than 3 in other sources (e.g., Hogerheijde et al 2011;Choi et al 2014;Dionatos et al 2018) but differences in optical depth of the ortho and para lines can also mimic such low ratios if not properly accounted for (Salinas et al 2016). In fact, a more detailed analysis of many water lines observed with HIFI in the Orion Bar shows values close to 3 (Putaud et al 2019).…”

Section: Ortho/para Ratiomentioning

confidence: 98%

“…The only exception are Photon Dominated Regions (PDRs), that is, clouds exposed to enhanced UV radiation where water has been observed to have a single narrow emission line component. Examples are the Orion Bar PDR (Choi et al 2014;Putaud et al 2019), the Orion Molecular Ridge (Melnick et al 2020) and the extended ρ Oph cloud (see Fig. 1 in Bjerkeli et al 2012).…”

Section: Water Emission: Two Universal Componentsmentioning

confidence: 99%

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

Self Cite

114

View full text Add to dashboard Cite

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.

show abstract

The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region

Cited by 20 publications

References 86 publications

Vacuum-UV photodesorption from compact Amorphous Solid Water : photon energy, isotopic and temperature effects

Vacuum-UV photodesorption from compact Amorphous Solid Water : photon energy, isotopic and temperature effects

Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains

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

Contact Info

Product

Resources

About