0000000000771075
AUTHOR
Laurent Pagani
Ortho-H2 and the age of prestellar cores
Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but they are currently difficult to distinguish. One major difference is the timescale involved in forming the prestellar cores: some theories advocate nearly free-fall speed via, e.g., rapid turbulence decay, while others can accommodate much longer periods to let the gas accumulate via, e.g., ambipolar diffusion. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based…
Ortho−ParaH2Conversion by Proton Exchange at Low Temperature: An Accurate Quantum Mechanical Study
We report extensive, accurate fully quantum, time-independent calculations of cross sections at low collision energies, and rate coefficients at low temperatures for the ${\mathrm{H}}^{+}+{\mathrm{H}}_{2}(v=0,j)\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{H}}_{2}(v=0,{j}^{\ensuremath{'}})$ reaction. Different transitions are considered, especially the ortho-para conversion ($j=1\ensuremath{\rightarrow}{j}^{\ensuremath{'}}=0$) which is of key importance in astrophysics. This conversion process appears to be very efficient and dominant at low temperature, with a rate coefficient of $4.15\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\ma…
H-2, H-3(+) and the age of molecular clouds and prestellar cores
Measuring the age of molecular clouds and prestellar cores is a difficult task that has not yet been successfully accomplished although the information is of paramount importance to help in understanding and discriminating between different formation scenarios. Most chemical clocks suffer from unknown initial conditions and are therefore difficult to use. We propose a new approach based on a subset of deuterium chemistry that takes place in the gas phase and for which initial conditions are relatively well known. It relies primarily on the conversion of H 3 + into H 2D + to initiate deuterium enrichment of the molecular gas. This conversion is controlled by the ortho/para ratio of H2 that i…
Quantum mechanical study of the proton exchange in the ortho-para H2 conversion reaction at low temperature
Ortho-para H2 conversion reactions mediated by the exchange of a H+ proton have been investigated at very low energy for the first time by means of a time independent quantum mechanical (TIQM) approach. State-to-state probabilities and cross sections for H+ + H 2 (v = 0, j = 0,1) processes have been calculated for a collision energy, Ec, ranging between 10-6 eV and 0.1 eV. Differential cross sections (DCSs) for H+ + H2 (v = 0, j = 1) → H+ + H2 (v′ = 0, j′ = 0) for very low energies only start to develop a proper global minimum around the sideways scattering direction ( ≈ 90°) at Ec = 10-3 eV. Rate coefficients, a crucial information required for astrophysical models, are provided between 10…