The physical and chemical structure of hot molecular cores

Nomura, Hideko; Millar, T. J.

doi:10.1051/0004-6361:20031646

Cited by 143 publications

(165 citation statements)

References 63 publications

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“…Unlike H 2 CO, for which efficient gas-phase synthetic pathways have been studied in the laboratory, analogous reactions that might form thioformaldehyde do not occur. As an example, the chemical model of Nomura & Millar (2004) cannot reproduce, by several order of magnitudes, the observed N(H 2 CO)/N(H 2 CS) abundance ratio in hot cores.…”

Section: H 2 Csmentioning

confidence: 97%

A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

Tercero

Cernicharo

Pardo

et al. 2010

A&A

155

250

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We perform a sensitive (line confusion limited), single-side band spectral survey towards Orion KL with the IRAM 30 m telescope, covering the following frequency ranges: 80−115.5 GHz, 130−178 GHz, and 197−281 GHz. We detect more than 14 400 spectral features of which 10 040 have been identified up to date and attributed to 43 different molecules, including 148 isotopologues and lines from vibrationally excited states. In this paper, we focus on the study of OCS, HCS + , H 2 CS, CS, CCS, C 3 S, and their isotopologues. In addition, we map the OCS J = 18−17 line and complete complementary observations of several OCS lines at selected positions around Orion IRc2 (the position selected for the survey). We report the first detection of OCS ν 2 = 1 and ν 3 = 1 vibrationally excited states in space and the first detection of C 3 S in warm clouds. Most of CCS, and almost all C 3 S, line emission arises from the hot core indicating an enhancement of their abundances in warm and dense gas. Column densities and isotopic ratios have been calculated using a large velocity gradient (LVG) excitation and radiative transfer code (for the low density gas components) and a local thermal equilibrium (LTE) code (appropriate for the warm and dense hot core component), which takes into account the different cloud components known to exist towards Orion KL, the extended ridge, compact ridge, plateau, and hot core. The vibrational temperature derived from OCS ν 2 = 1 and ν 3 = 1 levels is 210 K, similar to the gas kinetic temperature in the hot core. These OCS high energy levels are probably pumped by absorption of IR dust photons. We derive an upper limit to the OC 3 S, H 2 CCS, HNCS, HOCS + , and NCS column densities. Finally, we discuss the D/H abundance ratio and infer the following isotopic abundances: 12 C/ 13 C = 45 ± 20, 32 S/ 34 S = 20 ± 6, 32 S/ 33 S = 75 ± 29, and 16 O/ 18 O = 250 ± 135.

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Section: H 2 Csmentioning

confidence: 97%

A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

Tercero

Cernicharo

Pardo

et al. 2010

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155

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“…Commonly-suggested formation routes include various gas-phase reactions involving evaporated CH 3 OH ices, atom-addition reactions on dust grains, and UV-and cosmic rayinduced chemistry in the granular ices (Charnley et al 1992;Nomura & Millar 2004, Herbst & van Dishoeck 2009. Recently, Appendices are only available in electronic form at http://www.aanda.org the focus has shifted to an ice formation pathway (e.g.…”

Section: Introductionmentioning

confidence: 99%

Formation rates of complex organics in UV irradiated CH₃OH-rich ices

Öberg¹,

Garrod²,

Dishoeck

et al. 2009

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426

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Context. Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes, so-called hot cores. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. There is, however, a general lack of quantitative data on UV-induced complex chemistry in ices. Aims. This study aims to experimentally quantify the UV-induced production rates of complex organics in CH 3 OH-rich ices under a variety of astrophysically relevant conditions. Methods. The ices are irradiated with a broad-band UV hydrogen microwave-discharge lamp under ultra-high vacuum conditions, at 20-70 K, and then heated to 200 K. The reaction products are identified by reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), through comparison with RAIRS and TPD curves of pure complex species, and through the observed effects of isotopic substitution and enhancement of specific functional groups, such as CH 3 , in the ice. Results. Complex organics are readily formed in all experiments, both during irradiation and during the slow warm-up of the ices after the UV lamp is turned off. The relative abundances of photoproducts depend on the UV fluence, the ice temperature, and whether pure CH 3 OH ice or CH 3 OH:CH 4 /CO ice mixtures are used. C 2 H 6 , CH 3 CHO, CH 3 CH 2 OH, CH 3 OCH 3 , HCOOCH 3 , HOCH 2 CHO and (CH 2 OH) 2 are all detected in at least one experiment. Varying the ice thickness and the UV flux does not affect the chemistry. The derived product-formation yields and their dependences on different experimental parameters, such as the initial ice composition, are used to estimate the CH 3 OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. At 20 K, the pure CH 3 OH photodesorption yield is 2.1(±1.0) × 10 −3 per incident UV photon, the photo-destruction cross section 2.6(±0.9) × 10 −18 cm 2 . Conclusions. Photochemistry in CH 3 OH ices is efficient enough to explain the observed abundances of complex organics around protostars. Some complex molecules, such as CH 3 CH 2 OH and CH 3 OCH 3 , form with a constant ratio in our ices and this can can be used to test whether complex gas-phase molecules in astrophysical settings have an ice-photochemistry origin. Other molecular ratios, e.g. HCO-bearing molecules versus (CH 2 OH) 2 , depend on the initial ice composition and temperature and can thus be used to investigate when and where complex ice molecules form.

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“…In these regions, the grains are being warmed during the evolution towards the main sequence. Thus hot cores are richer than quiescent molecular clouds in gas-phase saturated organic molecules (Nomura & Millar 2004). These evaporated gas-phase molecules provide a record of both the accretion taking place during the collapse phase of star formation and of the solid-state chemistry that occurs within the ice mantle.…”

Section: Introductionmentioning

confidence: 99%

The desorption of H₂CO from interstellar grains analogues

Noble

Theulé

Mispelaer

et al. 2012

A&A

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Context. Much of the formaldehyde (H 2 CO) is formed from the hydrogenation of CO on interstellar dust grains, and is released in the gas phase in hot core regions. Radio-astronomical observations in these regions are directly related to its desorption from grains. Aims. We study experimentally the thermal desorption of H 2 CO from bare silicate surfaces, from water ice surfaces and from bulk water ice in order to model its desorption from interstellar grains. Methods. Temperature-programmed desorption experiments, monitored by mass spectrometry, and Fourier transform infrared spectroscopy are performed in the laboratory to determine the thermal desorption energies in: (i.) the multilayer regime where H 2 CO is bound to other H 2 CO molecules; (ii.) the submonolayer regime where H 2 CO is bound on top of a water ice surface; (iii.) the mixed submonolayer regime where H 2 CO is bound to a silicate surface; and (iv.) the multilayer regime in water ice, where H 2 CO is embedded within a H 2 O matrix. Results. In the submonolayer regime, we find the zeroth-order desorption kinetic parameters ν 0 = 10 28 mol cm −2 s −1 and E = 31.0 +/− 0.9 kJ mol −1 for desorption from an olivine surface. The zeroth-order desorption kinetic parameters are ν 0 = 10 28 mol cm −2 s −1 and E = 27.1 +/− 0.5 kJ mol −1 for desorption from a water ice surface in the submonolayer regime. In a H 2 CO:H 2 O mixture, the desorption is in competition with the H 2 CO + H 2 O reaction, which produces polyoxymethylene, the polymer of H 2 CO. This polymerization reaction prevents the volcano desorption and co-desorption from happening. Conclusions. H 2 CO is only desorbed from interstellar ices via a dominant sub-monolayer desorption process (E = 27.1 +/− 0.5 kJ mol −1 ). The H 2 CO which has not desorbed during this sub-monolayer desorption polymerises upon reaction with H 2 O, and does not desorb as H 2 CO at higher temperature.

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The physical and chemical structure of hot molecular cores

Cited by 143 publications

References 63 publications

A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

Formation rates of complex organics in UV irradiated CH₃OH-rich ices

The desorption of H₂CO from interstellar grains analogues

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The physical and chemical structure of hot molecular cores

Cited by 143 publications

References 63 publications

A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

Formation rates of complex organics in UV irradiated CH3OH-rich ices

The desorption of H2CO from interstellar grains analogues

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Product

Resources

About

Formation rates of complex organics in UV irradiated CH₃OH-rich ices

The desorption of H₂CO from interstellar grains analogues