Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations

Wiel, M. H. D. van der; Tak, F. F. S. van der; Ossenkopf, V.; Spaans, Marco; Roberts, H.; Fuller, G. A.; Plume, R.

doi:10.1051/0004-6361/200811391

Cited by 50 publications

(52 citation statements)

References 24 publications

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“…The H 2 S 2 12 → 1 01 line at ∼407 μm is detected, while other fainter H 2 S lines at shorter wavelengths are only marginally detected. Some features related to the emission of HCO + , HCN, CN and C 2 H are observed as expected (e.g., Hogerheijde et al 1995;Simon et al 1997;Young Owl et al 2000;Teyssier et al 2004;van Der Wiel et al 2009), but to help 1 The obliquity effect is important at the highest frequencies, where a significant error in the line position is introduced. distinguish the spectral confusion for fainter lines or unresolved k-ladder transitions from species such as methanol, the actual signal-to-noise ratio will be improved as the SPIRE FTS response is better understood and, scheduled deeper observations will also help.…”

Section: Detected Gas Linessupporting

confidence: 68%

SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

Habart¹,

Dartois²,

Abergel³

et al. 2010

A&A

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Aims. We present observations of the Orion Bar photodissociation region (PDR) obtained with the SPIRE instrument on-board Herschel. Methods. We obtained SPIRE Fourier-transform spectrometer (FTS) sparse sampled maps of the Orion bar. Results. The FTS wavelength coverage and sensitivity allow us to detect a wealth of rotational lines of CO (and its isotopologues), fine structure lines of C and N + , and emission lines from radicals and molecules such as CH + , CH, H 2 O or H 2 S. For species detected from the ground, our estimates of the column densities agree with previously published values. The comparison between 12 CO and 13 CO maps shows particularly the effects of optical depth and excitation in the molecular cloud. The distribution of the 12 CO and 13 CO lines with upper energy levels indicates the presence of warm (∼100-150 K) CO. This warm CO component is a significant fraction of the total molecular gas, confirming previous ground based studies.

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Section: Detected Gas Linessupporting

confidence: 68%

SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

Habart¹,

Dartois²,

Abergel³

et al. 2010

A&A

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“…These observations will be able to address the origin of the velocity structure seen in the JCMT observations as well as determine whether different components of the emission are spatially segregated due to differences in excitation or chemistry. Together with the analysis of the more complete set of molecular species probing other physical and chemical environments which will be available in the full JCMT SLS-observations of W49A and the Orion Bar ( Van der Wiel et al 2009), such studies of W49A can provide more insight into the relationship between the molecular emission, the structure and the star formation in star forming regions in other galaxies.…”

Section: Discussionmentioning

confidence: 99%

Dense molecular gas towards W49A: a template for extragalactic starbursts?

Roberts

Tak

Fuller

et al. 2010

A&A

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Context. The HCN, HCO + , and HNC molecules are commonly used as tracers of dense star-forming gas in external galaxies, but such observations are spatially unresolved. Reliably inferring the properties of galactic nuclei and disks requires detailed studies of sources whose structure is spatially resolved. Aims. To understand the origin of extragalactic molecular line emission, we compare the spatial distributions and abundance ratios of HCN, HCO + , and HNC in W49A, the most massive and luminous star-forming region in the Galactic disk. We use maps of the integrated intensity and line-profiles to pick out regions of the source to study in more detail. We compare column densities and abundance ratios towards these regions with each other and with predictions from gas-phase chemical models.Results. The kinematics of the molecular gas in W49A appears complex, with a mixture of infall and outflow motions. Both the line profiles and comparison of the main and rarer species show that the main species are optically thick. Two "clumps" of infalling gas that we look at in more detail appear to be at ∼40 K, compared to ≥100 K at the source centre, and may be ∼10× denser than the rest of the outer cloud. The chemical modelling suggests that the HCN/HNC ratio probes the current gas temperature, while the HCN/HCO + ratio and the deuterium fractionation were set during an earlier, colder phase of evolution. Conclusions. The similarity in the derived physical conditions in W49A and those inferred for the molecular gas in external galaxies suggest that W49A is an appropriate analogue of an extragalactic star forming region. Our data show that the use of HCN/HNC/HCO + line ratios as proxies for the abundance ratios is incorrect for W49A, suggesting that using these line ratios as abundance ratios in galactic nuclei is invalid too. On the other hand, our observed isotopic line ratios such as H 13 CN/H 13 CO + approach our modeled abundance ratios quite well in W49A. Second, the 4−3 lines of HCN and HCO + are much better tracers of the dense star-forming gas in W49A than the 1−0 lines, confirming similar indications for galactic nuclei. Finally, our observed HCN/HNC and HCN/HCO + ratios in W49A are inconsistent with homogeneous PDR or XDR models, indicating that irradiation does not strongly affect the gas chemistry in W49A. Overall, the W49A region appears to be a useful template for starburst galaxies.

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“…To compare the line intensities of the transitions detected with HIFI with different beam sizes, we convert all the observed line intensities to a common ∼47 resolution. We derive conversion factors between the original beam sizes and ∼47 based on the integrated intensity map of the HCN 4−3 transition from the James Clerk Maxwell Telescope (JCMT) Spectral Legacy Survey ( Van der Wiel et al 2009). For this, we assume the intensity distribution of the HCN 4−3 transition to trace the spatial structure of the Orion Bar.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

“…The column density of C + is approximately 10 18 cm −2 (Ossenkopf et al 2013) and the H 2 column density is approximately 10 22 cm −2 (e.g. Habart et al 2010; Van der Wiel et al 2009). This implies an electron abundance of 10 −4 and using n(H 2 ) = 10 5 cm −3 , an electron density of 10 cm −3 .…”

Section: Physical Conditions Traced By Ch + and Sh +mentioning

confidence: 99%

“…The Orion Bar is located at a distance of 414 pc (Menten et al 2007). Its stratified structure has been the subject of many previous studies (such as Van der Wiel et al 2009 and references therein). The mean density of the Orion Bar is about 10 5 cm −3 , the mean molecular gas temperature 85 K (Hogerheijde et al 1995), and the impinging radiation field is (1−4) × 10 4 in Draine units (Draine field: χ = 2.7 × 10 −3 erg s −1 cm −2 for the energy range 6 < hν < 13.6 eV; Draine 1978).…”

Section: Introductionmentioning

confidence: 99%

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The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺

Nagy

Tak

Ossenkopf

et al. 2013

A&A

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Context. The abundances of interstellar CH + and SH + are not well understood as their most likely formation channels are highly endothermic. Several mechanisms have been proposed to overcome the high activation barriers, including shocks, turbulence, and H 2 vibrational excitation. Aims. Using data from the Herschel Space Observatory, we studied the formation of ions, in particular CH + and SH + in a typical high UV-illumination warm and dense photon-dominated region (PDR), the Orion Bar. Methods. The HIFI instrument on board Herschel provides velocity-resolved line profiles of CH + 1-0 and 2-1 and three hyperfine transitions of SH + 1 2 −0 1 . The PACS instrument provides information on the excitation and spatial distribution of CH + by extending the observed CH + transitions up to J = 6-5. We compared the observed line intensities to the predictions of radiative transfer and PDR codes. Results. All CH + , SH + , and CF + lines analyzed in this paper are seen in emission. The widths of the CH + 2-1 and 1-0 transitions are of ∼5 km s −1 , significantly broader than the typical width of dense gas tracers in the Orion Bar (∼2-3 km s −1 ) and are comparable to the width of species that trace the interclump medium such as C + and HF. The detected SH + transitions are narrower compared to CH + and have line widths of ∼3 km s −1 , indicating that SH + emission mainly originates in denser condensations. Non-LTE radiative transfer models show that electron collisions affect the excitation of CH + and SH + and that reactive collisions need to be taken into account to calculate the excitation of CH + . Comparison to PDR models shows that CH + and SH + are tracers of the warm surface region (A V < 1.5) of the PDR with temperatures between 500 and 1000 K. We have also detected the 5-4 transition of CF + at a width of ∼1.9 km s −1 , consistent with the width of dense gas tracers. The intensity of the CF + 5-4 transition is consistent with previous observations of lower-J transitions toward the Orion Bar.Conclusions. An analytic approximation and a numerical comparison to PDR models indicate that the internal vibrational energy of H 2 can explain the formation of CH + for typical physical conditions in the Orion Bar near the ionization front. The formation of SH + is also likely to be explained by H 2 vibrational excitation. The abundance ratios of CH + and SH + trace the destruction paths of these ions, and indirectly, the ratios of H, H 2 , and electron abundances as a function of depth into the cloud.

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Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations

Cited by 50 publications

References 24 publications

SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

Dense molecular gas towards W49A: a template for extragalactic starbursts?

The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺

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Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations

Cited by 50 publications

References 24 publications

SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

Dense molecular gas towards W49A: a template for extragalactic starbursts?

The chemistry of ions in the Orion Bar I. – CH+, SH+, and CF+

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Product

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The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺