Interstellar Carbon in Translucent Sight Lines

Sofia, U. J.; Lauroesch, J. T.; Meyer, David M.; Cartledge, Stefan I. B.

doi:10.1086/382592

Cited by 229 publications

(223 citation statements)

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“…we adopt the value of 3.2 (Jensen et al 2007;Sofia et al 2004) and the values for N(C + ) are given in Table 3 T k = 8000 K. The exact kinetic temperature is not critical as the column density is only weakly dependent on temperature through the collisional de-excitation rate coefficient, resulting in n cr (e) ∼ 50T 4 0.37 cm −3 , where T 4 is the kinetic temperature in units of 10 4 K. The results are given in ). This quantity,…”

Section: Resultsmentioning

confidence: 99%

Ionized gas at the edge of the central molecular zone

Langer

Goldsmith

Pineda

et al. 2015

A&A

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Context. The edge of the central molecular zone (CMZ) is the location where massive dense molecular clouds with large internal velocity dispersions transition to the surrounding more quiescent and lower CO emissivity region of the Galaxy. Little is known about the ionized gas surrounding the molecular clouds and in the transition region. Aims. We determine the properties of the ionized gas at the edge of the CMZ near Sgr E using observations of N + and C + .Methods. We observed a small portion of the edge of the CMZ near Sgr E with spectrally resolved [C ii] 158 μm and [N ii] 205 μm fine structure lines at six positions with the GREAT instrument on SOFIA and in [C ii] using Herschel HIFI on-the-fly strip maps. We use the [N ii] spectra along with a radiative transfer model to calculate the electron density of the gas and the [C ii] maps to illuminate the morphology of the ionized gas and model the column density of CO-dark H 2 .Results. We detect two [C ii] and [N ii] velocity components, one along the line of sight to a CO molecular cloud at −207 km s −1 associated with Sgr E and the other at −174 km s −1 outside the edge of another CO cloud. From the [N ii] emission we find that the average electron density is in the range of ∼5 to 21 cm −3 for these features. This electron density is much higher than that of the disk's warm ionized medium, but is consistent with densities determined for bright diffuse H ii nebula. The column density of the CO-dark H 2 layer in the −207 km s −1 cloud is ∼1−2× 10 21 cm −2 in agreement with theoretical models. The CMZ extends further out in Galactic radius by ∼7 to 14 pc in ionized gas than it does in molecular gas traced by CO. Conclusions. The edge of the CMZ likely contains dense hot ionized gas surrounding the neutral molecular material. The high fractional abundance of N + and high electron density require an intense EUV field with a photon flux of order 10 6 to 10 7 photons cm −2 s −1 , and/or efficient proton charge exchange with nitrogen, at temperatures of order 10 4 K, and/or a large flux of X-rays. Sgr E is a region of massive star formation as indicated by the presence of numerous compact H ii regions. The massive stars are potential sources of the EUV radiation that ionizes and heat the gas. In addition, X-ray sources and the diffuse X-ray emission in the CMZ are candidates for ionizing nitrogen.

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Section: Resultsmentioning

confidence: 99%

Ionized gas at the edge of the central molecular zone

Langer

Goldsmith

Pineda

et al. 2015

A&A

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“…Indications of a carbon deficit are also found towards other planetary systems (Jura 2008). In the diffuse ISM, A&A 541, A91 (2012) about 40−60% of the carbon is locked into amorphous carbon grains (Savage & Sembach 1996;Sofia et al 2004Sofia et al , 2011. To be consistent with the Earth's composition, these grains need to have been destroyed before planet formation.…”

Section: Introductionmentioning

confidence: 93%

“…At the earliest stage, in diffuse clouds, the carbon gas-phase abundance can be measured directly by the UV absorption of C ii, without referring to a dust model (e.g. Sofia et al 2004Sofia et al , 2011. The measured amount of volatile carbon, which is not bound in dust, is constrained to be between ∼40% and 60% (δ C = 0.4 to 0.6), with some differences between the measurements of the strong 1334 Å feature (Sofia et al 2011) and the weak intersystem feature at 2325 Å (Sofia et al 2004).…”

Section: The Carbon Budget Of the Disk Atmospherementioning

confidence: 99%

“…Sofia et al 2004Sofia et al , 2011. The measured amount of volatile carbon, which is not bound in dust, is constrained to be between ∼40% and 60% (δ C = 0.4 to 0.6), with some differences between the measurements of the strong 1334 Å feature (Sofia et al 2011) and the weak intersystem feature at 2325 Å (Sofia et al 2004). Assuming that the refractory material does not change as the gas evolves to cores and finally disks, these micron-sized carbon grains quickly settle down to the midplane of the disk.…”

Section: The Carbon Budget Of the Disk Atmospherementioning

confidence: 99%

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The warm gas atmosphere of the HD 100546 disk seen byHerschel

Bruderer

Dishoeck

Doty

et al. 2012

A&A

254

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Context. With the Herschel Space Observatory, lines of simple molecules (C + , O, and high-J lines of CO, J up 14) have been observed in the atmosphere of protoplanetary disks. When combined with ground-based data on [C i], all principle forms of carbon can be studied. These data allow us to test model predictions for the main carbon-bearing species and verify the presence of a warm surface layer. The absence of neutral carbon [C i], which is predicted by models to be strong, can then be interpreted together with ionized carbon [C ii] and carbon monoxide. Aims. We study the gas temperature, excitation, and chemical abundance of the simple carbon-bearing species C, C + , and CO, as well as O by the method of chemical-physical modeling. Using the models, we explore the sensitivity of the lines to the entering parameters and constrain the region from which the line radiation emerges. Methods. Numerical models of the radiative transfer in the lines and dust are used together with a chemical network simulation and a calculation of the gas energetics to obtain the gas temperature. We present our new model, which is based on our previous models but includes several improvements that we report in detail, together with the results of benchmark tests. Results. A model of the disk around the Herbig Be star HD 100546 is able to reproduce the CO ladder together with the atomic finestructure lines of [O i] and either [C i] or [C ii]. We find that the high-J lines of CO can only be reproduced by a warm atmosphere with T gas T dust . The low-J lines of CO, observable from the ground, are dominated by the outer disk with a radius of several 100 AU, while the high-J CO observable with Herschel-PACS are dominated from regions within some tens of AU. The spectral profiles of high-J lines of CO are predicted to be broader than those of the low-J lines. We study the effect of several parameters including the size of the disk, the gas mass of the disk, the PAH abundance and distribution, and the amount of carbon in the gas phase. Conclusions. The main conclusions of our work are (i) only a warm atmosphere with T gas T dust can reproduce the CO ladder. (ii) The CO ladder together with [O i] and the upper limit to [C i] can be reproduced by models with a high gas/dust ratio and a low abundance of volatile carbon. These models however produce too small amounts of [C ii]. Models with a low gas/dust ratio and more volatile carbon also reproduce CO and [O i], are in closer agreement with observations of [C ii], but overproduce [C i]. Owing to the uncertain origin of the [C ii] emission, we prefer the high gas/dust ratio models, indicating a low abundance of volatile carbon.

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“…Most studies of this phase have been made to date with optical and UV absorption observations of low extinction clouds, as pioneered by the Copernicus satellite, and extended by the Hubble Space Telescope (e.g. Sofia et al 2004). However, this approach traces only lines of sight A&A 561, A122 (2014) towards bright UV sources and is largely limited to local clouds (<1 kpc).…”

Section: Introductionmentioning

confidence: 99%

AHerschel[C ii] Galactic plane survey

Langer

Velusamy

Pineda

et al. 2014

A&A

154

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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Interstellar Carbon in Translucent Sight Lines

Cited by 229 publications

References 40 publications

Ionized gas at the edge of the central molecular zone

Ionized gas at the edge of the central molecular zone

The warm gas atmosphere of the HD 100546 disk seen byHerschel

AHerschel[C ii] Galactic plane survey

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