The IRAM-30 m line survey of the Horsehead PDR

Gratier, P.; Pety, J.; Guzmán, Viviana V.; Gérin, M.; Goicoechea, J. R.; Roueff, E.; Fauré, Alexandre

doi:10.1051/0004-6361/201321031

Cited by 70 publications

(81 citation statements)

References 65 publications

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“…In the previously-mentioned study of the Horsehead nebula, Gratier et al (2013) find that CH 3 CN is subthermally excited with relevant implications for the derivation of column densities. The authors compare results from large velocity gradient models with traditional rotational diagrams distinguishing three regimes: LTE, moderate-and strong subthermal excitation.…”

Section: Reliability Of Ch 3 Cn As a Thermometermentioning

confidence: 86%

“…Although the derived temperatures are reliable, column densities may be underestimated by a factor of a few, according to the discussion in Gratier et al (2013).…”

Section: Reliability Of Ch 3 Cn As a Thermometermentioning

confidence: 93%

“…Considering the uncertainties in the model and in the observations, predicted and observed abundances for CH 3 CN are in broad agreement. Gratier et al (2013) argue that bright acetonitrile lines can be associated with photon-dominated regions (PDRs), using deep spectra of the Horsehead Nebula taken at the DCO + and HCO emission peaks, associated with dense, relatively cold gas and the PDR, respectively. The authors find abundances 30 times larger in the diffuse and UV-illuminated gas than in the dense core, concluding that the detection of CH 3 CN does not automatically imply high-densities.…”

Section: Emitting Regions and Qualitative Comparison With Chemical Momentioning

confidence: 99%

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ATLASGAL-selected massive clumps in the inner Galaxy

Giannetti

Leurini

Wyrowski

et al. 2017

A&A

171

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Context. Observational identification of a solid evolutionary sequence for high-mass star-forming regions is still missing. Spectroscopic observations give the opportunity to test possible schemes and connect the phases identified to physical processes. Aims. We aim to use the progressive heating of the gas caused by the feedback of high-mass young stellar objects to prove the statistical validity of the most common schemes used to observationally define an evolutionary sequence for high-mass clumps, and characterise the sensitivity of different tracers to this process. Methods. From the spectroscopic follow-ups carried out towards submillimeter continuum (dust) emission-selected massive clumps (the ATLASGAL TOP100 sample) with the IRAM 30 m, Mopra, and APEX telescopes between 84 GHz and 365 GHz, we selected several multiplets of CH 3 CN, CH 3 CCH, and CH 3 OH emission lines to derive and compare the physical properties of the gas in the clumps along the evolutionary sequence, fitting simultaneously the large number of lines that these molecules have in the observed band. Our findings are compared with results obtained from optically thin CO isotopologues, dust, and ammonia from previous studies on the same sample. Results. The chemical properties of each species have a major role on the measured physical properties. Low temperatures are traced by ammonia, methanol, and CO (in the early phases), the warm and dense envelope can be probed with CH 3 CN, CH 3 CCH, and, in evolved sources where CO is abundant in the gas phase, via its optically thin isotopologues. CH 3 OH and CH 3 CN are also abundant in the hot cores, and we suggest that their high-excitation transitions are good tools to study the kinematics in the hot gas associated with the inner envelope surrounding the young stellar objects that these clumps are hosting. All tracers show, to different degrees according to their properties, progressive warming with evolution. The relation between gas temperature and the luminosity-to-mass (L/M) ratio is reproduced by a simple toy model of a spherical, internally heated clump. Hii regions become common, showing that dissipation of the parental clump dominates.

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Section: Reliability Of Ch 3 Cn As a Thermometermentioning

confidence: 86%

“…Although the derived temperatures are reliable, column densities may be underestimated by a factor of a few, according to the discussion in Gratier et al (2013).…”

Section: Reliability Of Ch 3 Cn As a Thermometermentioning

confidence: 93%

Section: Emitting Regions and Qualitative Comparison With Chemical Momentioning

confidence: 99%

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ATLASGAL-selected massive clumps in the inner Galaxy

Giannetti

Leurini

Wyrowski

et al. 2017

A&A

171

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“…Hence, collisions with electrons are not important for determining the CH 3 OH column density, but they are important for determining the kinetic temperature and density of the gas. The electron excitation was found to be important for determining the column density of CH 3 CN in the PDR (Gratier et al 2013).…”

Section: Ch 3 Ohmentioning

confidence: 99%

The IRAM-30 m line survey of the Horsehead PDR

Guzmán

Goicoechea

Pety

et al. 2013

A&A

Self Cite

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Context. Theoretical models and laboratory experiments show that CH 3 OH is efficiently formed on cold grain surfaces through the successive hydrogenation of CO, forming HCO and H 2 CO as intermediate species.In cold cores and low UV-field illumination photodissociation regions (PDRs) the ices can be released into the gas-phase through nonthermal processes such as photodesorption, which considerably increases their gas-phase abundances. Aims. We investigate the dominant formation mechanism of H 2 CO and CH 3 OH in the Horsehead PDR and its associated dense core.Methods. We performed deep integrations of several H 2 CO and CH 3 OH lines at two positions in the Horsehead, namely the PDR and dense core, with the IRAM-30 m telescope. In addition, we observed one H 2 CO higher-frequency line with the CSO telescope at both positions. We determined the H 2 CO and CH 3 OH column densities and abundances from the single-dish observations complemented with IRAM-PdBI high-angular resolution maps (6 ) of both species. We compared the observed abundances with PDR models including either pure gas-phase chemistry or both gas-phase and grain surface chemistry.Results. We derived CH 3 OH abundances relative to total number of hydrogen atoms of ∼1.2 × 10 −10 and ∼2.3 × 10 −10 in the PDR and dense-core positions, respectively. These abundances are similar to the inferred H 2 CO abundance in both positions (∼2 × 10 −10 ). We find an abundance ratio H 2 CO/CH 3 OH of ∼2 in the PDR and ∼1 in the dense core. Pure gas-phase models cannot reproduce the observed abundances of either H 2 CO or CH 3 OH at the PDR position. The two species are therefore formed on the surface of dust grains and are subsequently photodesorbed into the gas-phase at this position. At the dense core, on the other hand, photodesorption of ices is needed to explain the observed abundance of CH 3 OH, while a pure gas-phase model can reproduce the observed H 2 CO abundance. The high-resolution observations show that CH 3 OH is depleted onto grains at the dense core. CH 3 OH is thus present in an envelope around this position, while H 2 CO is present in both the envelope and the dense core itself. Conclusions. Photodesorption is an efficient mechanism to release complex molecules in low-FUV-illuminated PDRs, where thermal desorption of ice mantles is ineffective.

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“…Acetonitrile (CH 3 CN) is a commonly observed nitrile in the ISM, and is thought to be a good temperature probe because it is a symmetric rotor presenting metastable levels (Guesten et al 1985). The fact that CH 3 CN was observed with a higher abundance in the PDR than in the Core, as opposed to HC 3 N, made Gratier et al (2013) suggest that a surface desorption process, such as photodesorption, should be occurring to release organic molecules into the gas phase in far-UV illuminated regions. These authors developed a simple steady-state gas-phase chemical model which underestimated the CH 3 CN abundance in the PDR by approximately three orders of magnitude.…”

Section: Nitrile Casementioning

confidence: 99%

A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

Gal

Herbst

Dufour

et al. 2017

A&A

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A wide variety of molecules have recently been detected in the Horsehead nebula photodissociation region (PDR) suggesting that: (i) gas-phase and grain chemistries should both contribute to the formation of organic molecules, and (ii) far-ultraviolet (FUV) photodesorption may explain the release into the gas phase of grain surface species. In order to tackle these specific problems and more generally in order to better constrain the chemical structure of these types of environments we present a study of the Horsehead nebula gas-grain chemistry. To do so we used the 1D astrochemical gas-grain code Nautilus with an appropriate physical structure computed with the Meudon PDR Code and compared our modeled outcomes with published observations and with previously modeled results when available. The use of a large set of chemical reactions coupled with the time-dependent code Nautilus allows us to reproduce most of the observations well, including those of the first detections in a PDR of the organic molecules HCOOH, CH 2 CO, CH 3 CHO and CH 3 CCH, which are mostly associated with hot cores. We also provide some abundance predictions for other molecules of interest. Understanding the chemistry behind the detection of these organic molecules is crucial to better constrain the environments these molecules can probe.

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The IRAM-30 m line survey of the Horsehead PDR

Cited by 70 publications

References 65 publications

ATLASGAL-selected massive clumps in the inner Galaxy

ATLASGAL-selected massive clumps in the inner Galaxy

The IRAM-30 m line survey of the Horsehead PDR

A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

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