We have used the Atacama Pathfinder Experiment 12 m telescope (APEX) to carry out an absorption study of submillimeter wavelength rotational ground-state lines of H 35 Cl, H 37 Cl, 13 CH + , and, for the first time, of the SH + radical (sulfoniumylidene or sulfanylium). We detected the quartet of ground-state hyperfine structure lines of SH + near 683 GHz with the CHAMP+ array receiver against the strong continuum source Sagittarius B2, which is located close to the center of our Galaxy. In addition to absorption from various kinematic components of Galactic center gas, we also see absorption at the radial velocities belonging to intervening spiral arms. This demonstrates that SH + is a ubiquitous component of the diffuse interstellar medium. We do not find clear evidence for other SH + lines we searched for, which is partially due to blending with lines from other molecules. In addition to SH + , we observed absorption from H 35 Cl, H 37 Cl, and 13 CH + . The observed submillimeter absorption is compared in detail with absorption in 3 mm transitions of H 13 CO + and c-C3H2 and the CO J = 1− 0a nd 3−2 transitions.

/pdf/submillimeter-absorption-from-sh-a-new-widespread-4b58do793a.pdf

Submillimeter absorption from SH+, a new widespread interstellar radical, 13CH+ and HCl

We have used the Atacama Pathfinder Experiment 12 m telescope (APEX) to carry out an absorption study of submillimeter wavelength rotational ground-state lines of H35Cl, H37Cl, 13CH+, and, for the first time, of the SH+ radical (sulfoniumylidene or sulfanylium). We detected the quartet of ground-state hyperfine structure lines of SH+ near 683 GHz with the CHAMP+ array receiver against the strong continuum source Sagittarius B2, which is located close to the center of our Galaxy. In addition to absorption from various kinematic components of Galactic center gas, we also see absorption at the radial velocities belonging to intervening spiral arms. This demonstrates that SH+ is a ubiquitous component of the diffuse interstellar medium. We do not find clear evidence for other SH+ lines we searched for, which is partially due to blending with lines from other molecules. In addition to SH+, we observed absorption from H35Cl, H37Cl, and 13CH+. The observed submillimeter absorption is compared in detail with absorption in 3 mm transitions of H13CO+ and c-C3H2 and the CO J = 1-0 and 3-2 transitions.

/pdf/submillimeter-absorption-from-sh-a-new-widespread-3b8rhrpywu.pdf

Context. C3 is the smallest pure carbon chain detected in the dense environment of star-forming regions, although diatomic C2 is detected in diffuse clouds. Measurement of the abundance of C3 and the chemistry of its formation in dense star-forming regions has remained relatively unexplored. Aims. We aim to identify the primary C3 formation routes in dense star-forming regions following a chemical network producing species like CCH and c-C3H2 in the star-forming cores associated with DR21(OH), a high-mass star-forming region. Methods. We observed velocity resolved spectra of four ro-vibrational far-infrared transitions of C3 between the vibrational ground state and the low-energy ν2 bending mode at frequencies between 1654–1897 GHz using HIFI on board Herschel, in DR21(OH). Several transitions of CCH and c-C3H2 were also observed with HIFI and the IRAM 30 m telescope. Rotational temperatures and column densities for all chemical species were estimated. A gas and grain warm-up model was used to obtain estimates of densities and temperatures of the envelope. The chemical network in the model was used to identify the primary C3 forming reactions in DR21(OH). Results. We detected C3 in absorption in four far-infrared transitions, P(4), P(10), Q(2), and Q(4). The continuum sources MM1 and MM2 in DR21(OH), though spatially unresolved, are sufficiently separated in velocity to be identified in the C3 spectra. All C3 transitions are detected from the embedded source MM2 and the surrounding envelope, whereas only Q(4) and P(4) are detected toward the hot core MM1. The abundance of C3 in the envelope and MM2 is ∼6 × 10 −10 and ∼3 × 10 −9 , respectively. For CCH and c-C3H2, we only detect emission from the envelope and MM1. The observed CCH, C3 and c-C3H2 abundances are most consistent with a chemical model with nH2 ∼ 5 × 10 6 cm −3 , a post-warm-up dust temperature Tmax = 30 K, and a time of ∼ 0.7–3 Myr. Conclusions. Post-warm-up gas phase chemistry of CH4 released from the grain at t ∼ 0.2 Myr and lasting for 1 Myr can explain the observed C3 abundance in the envelope of DR21(OH), and no mechanism involving photodestruction of PAH molecules is required. The chemistry in the envelope is similar to the warm carbon chain chemistry found in lukewarm corinos. We interpret the observed lower C3 abundance in MM1 as compared to MM2 and the envelope to be due to the destruction of C3 in the more evolved MM1. The timescale for the chemistry derived for the envelope is consistent with the dynamical timescale of 2 Myr derived for DR21(OH) in other studies.

/pdf/chemistry-of-c-3-and-carbon-chain-molecules-in-dr21-oh-50sbh3i4nq.pdf

Chemistry of C 3 and carbon chain molecules in DR21(OH)

In previous warm-up chemical models of the low-mass star-forming region L1527, we investigated the evolution of carbon-chain unsaturated hydrocarbon species when the envelope temperature is slightly elevated to T 30?K. These models demonstrated that enhanced abundances of such species can be explained by gas-phase ion-molecule chemistry following the partial sublimation of methane from grain surfaces. We also concluded that the abundances of hydrocarbon radicals such as the C n H family should be further enhanced as the temperatures increase to higher values, but this conclusion stood in contrast with the lack of unambiguous detection of these species toward hot core and corino sources. Meanwhile, observational surveys have identified C2H, C4H, CH3CCH, and CH3OH toward hot corinos (especially IRAS 16293?2422) as well as toward L1527, with lower abundances for the carbon-chain radicals and higher abundances for the other two species toward the hot corinos. In addition, the Herschel Space Telescope has detected the bare linear chain C3 in 50?K material surrounding young high-mass stellar objects. To understand these new results, we revisit previous warm-up models with an augmented gas-grain network that incorporated reactions from a gas-phase network that was constructed for use with increased temperature up to 800?K. Some of the newly adopted reactions between carbon-chain species and abundant H2 possess chemical activation energy barriers. The revised model results now better reproduce the observed abundances of unsaturated carbon chains under hot corino (100?K) conditions and make predictions for the abundances of bare carbon chains in the 50?K regions observed by the Herschel HIFI detector.

http://iopscience.iop.org/article/10.1088/0004-637X/743/2/182/pdf

Carbon-chain species in warm-up models

We present the photoelectron spectra of C3Hx (x = 0–3) formed in a microwave discharge flow-tube reactor by consecutive H abstractions from C3H4 (C3Hx + F → C3Hx−1 + HF (x = 1–4)), but also from F + CH4 schemes by secondary reactions. The spectra were obtained combining tunable VUV synchrotron radiation with double imaging electron/ion coincidence techniques, yielding mass-selected threshold photoelectron spectra. The obtained results complement not only existing ones, but for the first time the photoelectron spectra of C3, cyclic and linear C3H (c,l-C3H) as well as of the excited states of C3H3 are reported. In the case of c-C3H, l,t-C3H2 and C3H3, Franck–Condon simulations have been performed in order to assign the vibrational structure. The adiabatic ionization energies of these radicals are reported and compared to ab initio calculated values as well as to theoretical values using known enthalpies of formation.

Valence shell threshold photoelectron spectroscopy of C3Hx ( x = 0–3)

We present spectrally resolved observations of triatomic carbon (C-3) in several ro-vibrational transitions between the vibrational ground state and the low-energy nu(2) bending mode at frequencies between 1654-1897 GHz along the sight-lines to the submillimeter continuum sources W31C and W49N, using Herschel's HIFI instrument. We detect C-3 in absorption arising from the warm envelope surrounding the hot core, as indicated by the velocity peak position and shape of the line profile. The sensitivity does not allow to detect C-3 absorption due to diffuse foreground clouds. From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature (T-rot) of similar to 50-70 K, which is a good measure of the kinetic temperature of the absorbing gas, as radiative transitions within the vibrational ground state are forbidden. It is also in good agreement with the dust temperatures for W31C and W49N. Applying the partition function correction based on the derived T-rot, we get column densities N(C-3) similar to 7-9 x 10(14) cm(-2) and abundance x(C-3) similar to 10(-8) with respect to H-2. For W31C, using a radiative transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find that a model with x(C-3) = 10(-8), T-kin = 30-50 K, N(C-3) = 1.5 x 10(15) cm(-2) fits the observations reasonably well and provides parameters in very good agreement with the simple excitation analysis.

/pdf/excitation-and-abundance-of-c-3-in-star-forming-cores-1sibtp7vcq.pdf

Excitation and abundance of C-3 in star forming cores: Herschel/HIFI observations of the sight-lines to W31C and W49N

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Excitation and abundance of C-3 in star forming cores: Herschel/HIFI observations of the sight-lines to W31C and W49N