Stephan Schlemmer

A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA)

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar sources.

Photodesorption of CO Ice

At the high densities and low temperatures found in star forming regions, all molecules other than H2 should stick on dust grains on timescales shorter than the cloud lifetimes. Yet these clouds are detected in the millimeter lines of gaseous CO. At these temperatures, thermal desorption is negligible and hence a non-thermal desorption mechanism is necessary to maintain molecules in the gas phase. Here, the first laboratory study of the photodesorption of pure CO ice under ultra high vacuum is presented, which gives a desorption rate of 3 × 10 3 CO molecules per UV (7–10.5 eV) photon at 15 K. This rate is factors of 10 2 -10 5 larger than previously estimated and is comparable to estimates of other non-thermal desorption rates. The experiments constrains the mechanism to a single photon desorption process of ice surface molecules. The measured efficiency of this process shows that the role of CO photodesorption in preventing total removal of molecules in the gas has been underestimated. Subject headings: Molecular data — Molecular processes — ISM: abundances — Physical Data and Processes: astrochemistry — ISM: molecules

Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6–0.4

We report the detection of absorption lines by the reactive ions OH + ,H 2O + and H3O + along the line of sight to the submillimeter continuum source G10.6−0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH + at 971 GHz, H2O + at 1115 and 607 GHz, and H3O + at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6−0.4 as well as in the molecular gas directly associated with that source. The OH + spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O + lines remains lower than 1 in the same velocity range, and the H3O + line shows only weak absorption. For LSR velocities between 7 and 50 kms −1 we estimate total column densities of N(OH + ) ≥ 2.5 × 10 14 cm −2 , N(H2O + ) ∼6 × 10 13 cm −2 and N(H3O + ) ∼4.0 × 10 13 cm −2 . These detections confirm the role of O + and OH + in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH + by the H2O + column density implies that these species predominantly trace low-density gas with a small fraction of

Competition between CO and N2 Desorption from Interstellar Ices

Millimeter observations of pre- and protostellar cores show that the abundances of the gas-phase tracer molecules, C 18 O and N2H, anticorrelate with each other and often exhibit “holes” where the density is greatest. These results are reasonably reproduced by astrochemical models, provided that the ratio between the binding energies of N2 and CO, , is taken to be between 0.5 and 0.75. This Letter is the first experimental report of RBE the desorption of CO and N2 from layered and mixed ices at temperatures relevant to dense cores, studied under ultrahigh vacuum laboratory conditions using temperature programmed desorption. From control experiments with pure ices, , given K and K. In mixed R p 0.923 0.003 E (N -N ) p 790 25 E (CO-CO) p 855 25 BE b 22 b

Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction

We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschels HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1-0 transition of OH+ and the 1115 GHz 1(11)-0(00) transition of ortho-H2O+. The resultant spectra show absorption by ortho-H2O+, and strong absorption by OH+, in foreground material at velocities in the range 0 to 70 km s(-1) with respect to the local standard of rest. The inferred OH+/H2O+ abundance ratio ranges from similar to 3 to similar to 15, implying that the observed OH+ arises in clouds of small molecular fraction, in the 2-8% range. This conclusion is confirmed by the distribution of OH+ and H2O+ in Doppler velocity space, which is similar to that of atomic hydrogen, as observed by means of 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH+/H abundance ratio of a few x10(-8) suggests a cosmic ray ionization rate for atomic hydrogen of 0.6-2.4 x 10(-16) s(-1), in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H-3(+) and other species.

H3++H2 isotopic system at low temperatures: Microcanonical model and experimental study

State-to-state thermal rate coefficients for reactions of all H(3)(+) + H(2) isotopic variants are derived and compared to new experimental data. The theoretical data are also sought for astrochemical modeling of cold environments (<50 K). The rates are calculated on the basis of a microcanonical approach using the Langevin model and the conservation laws of mass, energy, angular momentum, and nuclear spin. Full scrambling of all five nuclei during the collision is assumed for the calculations and alternatively partial dynamical restrictions are considered. The ergodic principle of the collision is employed in two limiting cases, neglecting (weak ergodic limit) or accounting for explicit degeneracies of the reaction mechanisms (strong ergodic limit). The resulting sets of rate coefficients are shown to be consistent with the detailed balance and thermodynamical equilibrium constants. Rate coefficients, k(T), for the deuteration chain of H(3)(+) with HD as well as H(2)D(+)/H(3)(+) equilibrium ratios have been measured in a variable temperature 22-pole ion trap. In particular, the D(2)H(+) + HD --> D(3)(+) + H(2) rate coefficient indicates a change in reaction mechanism when going to higher temperatures. The good overall agreement between experiment and theory encourages the use of the theoretical predictions for astrophysical modeling.

Herschel observations of EXtra-Ordinary Sources (HEXOS):The present and future of spectral surveys with Herschel/HIFI

We present initial results from the Herschel GT key program: Herschel observations of EXtra-Ordinary Sources (HEXOS) and outline the promise and potential of spectral surveys with Herschel/HIFI. The HIFI instrument offers unprecedented sensitivity, as well as continuous spectral coverage across the gaps imposed by the atmosphere, opening up a largely unexplored wavelength regime to high-resolution spectroscopy. We show the spectrum of Orion KL between 480 and 560 GHz and from 1.06 to 1.115 THz. From these data, we confirm that HIFI separately measures the dust continuum and spectrally resolves emission lines in Orion KL. Based on this capability we demonstrate that the line contribution to the broad-band continuum in this molecule-rich source is ~20-40% below 1 THz and declines to a few percent at higher frequencies. We also tentatively identify multiple transitions of HD18O in the spectra. The first detection of this rare isotopologue in the interstellar medium suggests that HDO emission is optically thick in the Orion hot core with HDO/H2O ~ 0.02. We discuss the implications of this detection for the water D/H ratio in hot cores. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Figure 2 (page 6) is also available in electronic form at http://www.aanda.org

The 2014 KIDA network for interstellar chemistry

Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reacti ...

Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: an ubiquitous tracer of molecular gas

We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along the sight line to the submillimeter continuum sources W49N and W51. We have used Herschels HIFI instrument in dual beam switch mode to observe the 1232.4762 GHz J = 1-0 HF transition in the upper sideband of the band 5a receiver. We detected foreground absorption by HF toward both sources over a wide range of velocities. Optically thin absorption components were detected on both sight lines, allowing us to measure - as opposed to obtain a lower limit on - the column density of HF for the first time. As in previous observations of HF toward the source G10.6-0.4, the derived HF column density is typically comparable to that of water vapor, even though the elemental abundance of oxygen is greater than that of fluorine by four orders of magnitude. We used the rather uncertain N(CH) - N(H-2) relationship derived previously toward diffuse molecular clouds to infer the molecular hydrogen column density in the clouds exhibiting HF absorption. Within the uncertainties, we find that the abundance of HF with respect to H-2 is consistent with the theoretical prediction that HF is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen fluoride has the potential to become an excellent tracer of molecular hydrogen, and provides a sensitive probe of clouds of small H-2 column density. Indeed, the observations of hydrogen fluoride reported here reveal the presence of a low column density diffuse molecular cloud along the W51 sight line, at an LSR velocity of similar to 24 km s(-1), that had not been identified in molecular absorption line studies prior to the launch of Herschel.

Laser excited N2+ in a 22-pole ion trap:: Experimental studies of rotational relaxation processes

Abstract The laser induced charge transfer N2+ + Ar → Ar+ + N2 has been employed to measure rotationally resolved excitation spectra of N2+, held in a 22-pole ion trap. Detection of the Ar+ product ions proved to facilitate an almost background free spectroscopic method. Rotational temperatures and Doppler temperatures determined from these spectra show that the internal and translational degrees of freedom of the parent ions are very well coupled to the trap temperature via collisions with the Ar buffer gas. Laser excitation starts a complex reaction kinetics of the finite ensemble of N2+ ions. Various elementary reaction steps have been distinguished and followed in detail by varying laser duration, storage time, and target gas density. For a fixed temperature (90 K), specific rate coefficients have been determined. Pumping only one particular quantum state of ortho-N2+ (X2∑g, v″ = 0, J″ = 6.5) shows that rate coefficients for the mixing of fine structure states (F1/F2) as well as nuclear spin states (ortho/para) in collisions with Ar are negligible (smaller than 10−12 cm3 s−1). The rate coefficient for transitions between the rotational states of one subset, e.g. ortho-N2+ (F1), are surprisingly small. The results are discussed in the framework of possible dynamical constraints, e.g. imposed by the potential energy surface.