The molecular diversity of the cold interstellar medium (ISM) has been recently enriched with the detection of anions. Despite growing interest, the physical & chemical processes that govern their abundance remain poorly known. Potential anions escape radio-detection because of the absence of dipole moment. To address these issues we combine astronomical observations, theory, modeling and experiments.
The main scientific aim of this project is to improve our knowledge, still very scarce, of the role of anions in the interstellar medium through their spectroscopy and their reactivity. <br /> <br />The presently proposed combination of careful state-of-the-art experimental, theoretical and observational studies will yield a synergistically enhanced understanding of the physics and chemistry of interstellar anions. A better knowledge of anion spectroscopy and reactivity, including kinetics and branching ratio determinations at the relevant, low temperature of the interstellar medium, is of crucial importance for properly modelling gaseous environments. It will also help advance our understanding of the success and limitations of the theoretical methods.
Experiments: we couple axisymmetric and planar uniform supersonic flows to state-of-the-art anion production sources and detection techniques (mass spectrometry and ultrasensitive absorption spectroscopy) in order to (1) acquire and analyze, with the help of a theoretical support, spectra of carbon chain anions that will be employed to perform astronomical search, (2) determine the rate coefficients and the branching ratios of selected reactions involving various anions (CxN-, CxH- and Cx-), as well as their temperature dependence down to 10K.
Theory: the above reactions will be investigated using capture methods, and transition state theory. The rate coefficients and branching ratios of reactions involving these anions and the most abundant atoms (H, C, N, O) will be calculated using statistical theory after characterization of adducts and transition states, by modern quantum chemistry methods. Collisional processes of anions with H2 will be investigated with quantum chemistry methods for the potential energy surface (PES) and Close Coupling calculations for the dynamics. We will use radiative transfer codes and provide the astrophysical community with a set of radiative transfer models.
Astronomical Observations: we propose to provide NIR-NUV spectral data to engage in an astronomical search of anions which have escaped detection until now because of the absence of permanent dipole moment. All the chemical data collected will be used to improve markedly the chemical models by expanding the chemical network and enhance therefore, the accuracy of the predicted abundances of the chemical species. In addition, we propose to use ALMA to map the high resolution spatial distribution of identified anions and their neutral counterparts in sources such as IRC +10216. These observations will provide an ideal test for the new photochemical models that will be developed during the project.
The development of the state-of-the-art instruments for investigating the chemistry and spectroscopy of molecular anions is underway.
At this early stage in the project, we can nevertheless highlight several advances.
We have determined the first collisional rate coefficients for the first anion detected in the interstellar medium (C6H-).
We have investigated experimentally and theoretically the reaction between the molecular anions CxN- and the polar molecule HCOOH. We found out an unusual temperature dependence of the reactions of CN- with polar molecules. This work provides new fundamental insights on prototypical reactions between polar anions and polar molecules along with critical data for astrochemical modeling.
The questions addressed are of great interest for astrophysics, physics and chemistry. Ion-neutral reactions provide much of the driving force for the chemistry of the ISM. But until recently, scientists were debating whether negative ions would survive in there. It is now time to expand our picture of interstellar ion chemistry.
Astronomical searches proposed, supported by laboratory data, will employ international facilities. ALMA will explore the coupling between chemistry and dynamics in a variety of sources. Effective and timely exploitation of these facilities will allow EU to confirm its world leadership in interstellar (IS) molecule detection.
Current chemical networks will be revisited to include anion chemistry. Models rely on accurate data recorded under relevant conditions, which is lacking. Our objective of providing kinetics and branching ratios of ion molecule reactions relevant to IS chemistry clearly answers the demands of the scientific community.
The recent detection of anions may lead to significant changes in the models whose importance are difficult to evaluate as dedicated studies are scarce. We aim to give a motivated answer to this important question. The accurate theoretical treatment of the chemistry of anions will furthermore give a new insight into the reactivity of ions of interest for plasmas.
The presence of anions within magnetized, collapsing cores may also impact star formation dynamics, through their influence on the ambipolar diffusion rate. Anions are in addition considered as plausible carriers for some of the diffuse IS bands. Anion abundances are theorized to be sensitive to electron attachment and photo-detachment rates, and may therefore be employed to determine IS electron densities and cosmic ray/photoionization rates.
The results will finally contribute to a better understanding of closer environments such as the atmosphere of Titan, in which heavy ions have been discovered at high altitudes.
K. Walker, F. Dumouchel, F. Lique, R. Dawes, The first potential energy surfaces for the C6H-–H2 and C6H-–He collisional systems and their corresponding inelastic cross sections, J. Chem. Phys., 145, 024314 (2016).
B. Joalland, N. Jamal-Eddin
The molecular diversity of cold interstellar space has been recently enriched with the detection of anions, all linear carbon chains. Usually by far less abundant than neutrals, their presence affects the density of free electrons which controls the rate of cloud collapse, and therefore of star formation. Recent studies suggest that anions could be very sensitive to atomic C, H & O, and molecular depletion. Anions could be then employed as tracers of chemical and physical conditions. Furthermore, reactive collisions with anions may constitute an effective and unique pathway towards heavier species. In spite of their role, the physics and chemistry of anions remain poorly known. In addition, whole classes of potential molecular anions escape radio-detection because of the absence of dipole moment.
To address these issues we combine state-of-the-art astronomical observations, theory, modeling and experiments with 4 scientific aims:
1) IR absorption high-resolution spectroscopy of cold carbon chain anions Cx- (x=3, 4) produced in a plasma and expanded in a planar supersonic flow. Measurements will be performed by cavity enhanced spectroscopy and the analysis will be supported by highly correlated ab initio calculations. The spectral data will then be employed to perform an astronomical search of these species towards the carbon star IRC +10216 and some other bright infrared sources.
2) Low temperature chemical reactivity of identified in the Inter Stellar Medium (ISM) or possibly present, anions. We will investigate theoretically the reactions of CxH- (x=2,4), CxN- (x=1,3) and Cx- (x=3,4) with H, C, N, & O atoms, abundant constituents of molecular clouds. These reactions will then serve as benchmark for more complex systems. A systematic determination of rate constants and branching ratios will be performed. Reactions of these anions with abundant heavy molecular species are likely to contribute to the growth of anions. Using supersonic uniform flows combined with a new selective ion source, we will study the kinetics and branching ratio of reactions of CxN- (x=3,5) , CxH- (x=2,4,6) and of, the possibly present Cx- (x=2,3,4,5) carbon chains with the abundant HC3N over the 20-300 K temperature range. These studies will also be supported by theoretical investigations.
3) Inelastic collisions of CxH- and CxN-, with H2. As the abundance of these anions and their neutral counterparts is directly linked, we will also determine the collisional rate constants for CxH and CxN + H2 - which are both endothermic reactions. The study of collisions of C3- with H2 will also be considered depending on observations during the course of the project. The most accurate methods presently available (i.e. Quantum chemistry methods for the PES and Close Coupling calculations for the dynamics) will be used. The excitation of anions for different physical conditions will be also considered. We will use radiative transfer codes and provide the astrophysical community with a set of radiative transfer models. This should help to better determine the anion abundance in IRC+10216 or TMC-1.
4) Expansion and improvement of the chemical reaction network of environments in which anions have been identified. A critical point is the estimation of branching ratios. Most of them have been arbitrarily guessed in current chemical networks leading to large uncertainties. The new data provided by this project will be incorporated in the photochemical models describing chemistry of dense clouds and IRC+10216, and are expected to improve appreciably their accuracy. In parallel, we propose to use ALMA to map the high resolution spatial distribution of identified anions and their neutral counterparts in sources such as IRC +10216.
The project will be undertaken through a partnership between 4 research teams with internationally recognized experimental, theoretical, modeling and astrophysical expertise.
Monsieur Ludovic BIENNER (Institut de Physique de Rennes)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
CSIC Consejo Superior de Investigaciones Científicas
ISM Institut des Sciences Moléculaires
LOMC Laboratoire Ondes et Milieux Complexes
IPR Institut de Physique de Rennes
Help of the ANR 498,964 euros
Beginning and duration of the scientific project: September 2014 - 48 Months