Ultraviolet Survey of CO and H<sub>2</sub>in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships

Sheffer, Yaron; Rogers, M.; Federman, S. R.; Abel, N. P.; Gredel, R.; Lambert, David L.; Shaw, Gargi

doi:10.1086/591484

Cited by 272 publications

(439 citation statements)

References 152 publications

(339 reference statements)

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“…In comparison with analogous studies, our estimates are different compared to the ratio of ∼2 × 10 −4 found for ∼50 K gas in Lacy et al (1994) but fall within the limits of log(CO/H 2 ) between -7.58 and -4.68 as measured by Sheffer et al (2008) for diffuse gas.…”

Section: Relation Between Co and Hcontrasting

confidence: 94%

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

Dionatos

Jørgensen

Green

et al. 2013

A&A

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Context. Mid-and far-infrared observations of the environment around embedded protostars reveal a plethora of high excitation molecular and atomic emission lines. Different mechanisms for the origin of these lines have been proposed, including shocks induced by protostellar jets and radiation or heating by the embedded protostar of its immediate surroundings. Aims. By studying of the most important molecular and atomic coolants, we aim at constraining the physical conditions around the embedded protostars SMM3 and SMM4 in the Serpens molecular cloud core and measuring the CO/H 2 ratio in warm gas. The excitation conditions for molecular species are assessed through rotational diagrams. Emission lines from major coolants are compared to the results of steady-state C-and J-type shock models. Results. Line emission tends to peak at distances of ∼10-20 from the protostellar sources with all but [CII] peaking at the positions of outflow shocks seen in near-IR and sub-millimeter interferometric observations. The [CII] emission pattern suggests that it is most likely excited from energetic UV radiation originating from the nearby flat-spectrum source SMM6. Excitation analysis indicates that H 2 and CO originate in gas at two distinct rotational temperatures of ∼300 K and 1000 K, while the excitation temperature for H 2 O and OH is ∼100-200 K. The morphological and physical association between CO and H 2 suggests a common excitation mechanism, which allows direct comparisons between the two molecules. The CO/H 2 abundance ratio varies from ∼10 −5 in the warmer gas up to ∼10 −4 in the hotter regions. Shock models indicate that C-shocks can account for the observed line intensities if a beam filling factor and a temperature stratification in the shock front are considered. C-type shocks can best explain the emission from H 2 O. The existence of J-shocks is suggested by the strong atomic/ionic (except for [CII]) emission and a number of line ratio diagnostics. Dissociative shocks can account for the CO and H 2 emission in a single excitation temperature structure. Conclusions. The bulk of cooling from molecular and atomic lines is associated with gas excited in outflow shocks. The strong association between H 2 and CO constrain their abundance ratio in warm gas. Both C-and J-type shocks can account for the observed molecular emission; however, J-shocks are strongly suggested by the atomic emission and provide simpler and more homogeneous solutions for CO and H 2 . The variations in the CO/H 2 abundance ratio for gas at different temperatures can be interpreted by their reformation rates in dissociative J-type shocks, or the influence of both C and J shocks.

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Section: Relation Between Co and Hcontrasting

confidence: 94%

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

Dionatos

Jørgensen

Green

et al. 2013

A&A

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“…In Fig. C.3 we show our model results for N(CH) versus N(H 2 ) together with the observational data (Rachford et al 2002;Magnani et al 2005Magnani et al , 2006Sheffer et al 2008;Qin et al 2010;Gerin et al 2010;Suutarinen et al 2011).…”

Section: D2 Column Densitymentioning

confidence: 67%

“…The column densities in our model are consistent with values for TMC-1 given by Suutarinen et al (2011) as well as with diffuse cloud observations presented by Gerin et al (2010) which give N(CH) ≈ 1−26 × 10 13 cm −2 for total H 2 columns between 10 21 −10 22 cm −2 . Sheffer et al (2008) present column densities along 42 diffuse molecular Galactic sight lines. They find total columns between 10 12 and 10 14 cm −2 and a very strong correlation between column densities of CH and H 2 .…”

Section: D2 Column Densitymentioning

confidence: 99%

Carbon fractionation in photo-dissociation regions

Röllig

Ossenkopf

2013

A&A

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We upgraded the chemical network from the UMIST Database for Astrochemistry 2006 to include isotopes such as 13 C and 18 O. This includes all corresponding isotopologues, their chemical reactions and the properly scaled reaction rate coefficients. We study the fractionation behavior of astrochemically relevant species over a wide range of model parameters, relevant for modelling of photo-dissociation regions (PDRs). We separately analyze the fractionation of the local abundances, fractionation of the total column densities, and fractionation visible in the emission line ratios. We find that strong C + fractionation is possible in cool C + gas. Optical thickness as well as excitation effects produce intensity ratios between 40 and 400. The fractionation of CO in PDRs is significantly different from the diffuse interstellar medium. PDR model results never show a fractionation ratio of the CO column density larger than the elemental ratio. Isotope-selective photo-dissociation is always dominated by the isotope-selective chemistry in dense PDR gas. The fractionation of C, CH, CH + and HCO + is studied in detail, showing that the fractionation of C, CH and CH + is dominated by the fractionation of their parental species. The light hydrides chemically derive from C + , and, consequently, their fractionation state is coupled to that of C + . The fractionation of C is a mixed case depending on whether formation from CO or HCO + dominates. Ratios of the emission lines of [C ii], [C i], 13 CO, and H 13 CO + provide individual diagnostics to the fractionation status of C + , C, and CO.

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“…For the diffuse low column density molecular clouds (no or very little CO) in the solar neighborhood, the H 2 layer containing C + is observed to have thermal pressures of order a few × 10 3 K cm −3 (cf. Sheffer et al 2008;Goldsmith 2013). Densities and temperatures, and therefore thermal pressures, have been determined from observations of absorption lines in the optical and UV for nearby low column density clouds.…”

Section: Pressure In the Co-dark H 2 Gasmentioning

confidence: 99%

“…Densities and temperatures, and therefore thermal pressures, have been determined from observations of absorption lines in the optical and UV for nearby low column density clouds. For example, Sheffer et al (2008) analyzed UV absorption lines of H 2 in diffuse molecular clouds with N(H 2 ) column densities up to ∼10 21 cm −2 (or A V ∼ 1 mag.) and derived T kin ranging from 50 to 120 K, with an average of 76 K in the molecular portion of the cloud.…”

Section: Pressure In the Co-dark H 2 Gasmentioning

confidence: 99%

AHerschel[C ii] Galactic plane survey

Langer

Velusamy

Pineda

et al. 2014

A&A

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184

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Context. H i and CO large scale surveys of the Milky Way trace the diffuse atomic clouds and the dense shielded regions of molecular hydrogen clouds, respectively. However, until recently, we have not had spectrally resolved C + surveys in sufficient lines of sight to characterize the ionized and photon dominated components of the interstellar medium, in particular, the H 2 gas without CO, referred to as CO-dark H 2 , in a large sample of interstellar clouds.Aims. We use a sparse Galactic plane survey of the 1.9 THz (158 μm) [C ii] spectral line from the Herschel open time key programme, Galactic Observations of Terahertz C+ (GOT C+), to characterize the H 2 gas without CO in a statistically significant sample of interstellar clouds.Methods. We identify individual clouds in the inner Galaxy by fitting the [C ii] and CO isotopologue spectra along each line of sight. We then combine these spectra with those of H i and use them along with excitation models and cloud models of C + to determine the column densities and fractional mass of CO-dark H 2 clouds.Results. We identify1804 narrow velocity [C ii] components corresponding to interstellar clouds in different categories and evolutionary states. About 840 are diffuse molecular clouds with no CO, ∼510 are transition clouds containing [C ii] and 12 CO, but no 13 CO, and the remainder are dense molecular clouds containing 13 CO emission. The CO-dark H 2 clouds are concentrated between Galactic radii of ∼3.5 to 7.5 kpc and the column density of the CO-dark H 2 layer varies significantly from cloud to cloud with a global average of 9 × 10 20 cm −2 . These clouds contain a significant fraction by mass of CO-dark H 2 , that varies from ∼75% for diffuse molecular clouds to ∼20% for dense molecular clouds.Conclusions. We find a significant fraction of the warm molecular ISM gas is invisible in H i and CO, but is detected in [C ii]. The fraction of CO-dark H 2 is greatest in the diffuse clouds and decreases with increasing total column density, and is lowest in the massive clouds. The column densities and mass fraction of CO-dark H 2 are less than predicted by models of diffuse molecular clouds using solar metallicity, which is not surprising as most of our detections are in Galactic regions where the metallicity is larger and shielding more effective. There is an overall trend towards a higher fraction of CO-dark H 2 in clouds with increasing Galactic radius, consistent with lower metallicity there.

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Ultraviolet Survey of CO and H₂in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships

Cited by 272 publications

References 152 publications

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

Carbon fractionation in photo-dissociation regions

AHerschel[C ii] Galactic plane survey

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Ultraviolet Survey of CO and H2in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships

Cited by 272 publications

References 152 publications

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

Carbon fractionation in photo-dissociation regions

AHerschel[C ii] Galactic plane survey

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Ultraviolet Survey of CO and H₂in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships