Exact results for hydrogen recombination on dust grain surfaces

The recombination of hydrogen in the interstellar medium, taking place on surfaces of microscopic dust grains, is an essential process in the evolution of chemical complexity in interstellar clouds. The H_2 formation process has been studied theoretically, and in recent years also by laboratory experiments. The experimental results were analyzed using a rate equation model. The parameters of the surface, that are relevant to H_2 formation, were obtained and used in order to calculate the recombination rate under interstellar conditions. However, it turned out that due to the microscopic size of the dust grains and the low density of H atoms, the rate equations may not always apply. A master equation approach that provides a good description of the H_2 formation process was proposed. It takes into account both the discrete nature of the H atoms and the fluctuations in the number of atoms on a grain. In this paper we present a comprehensive analysis of the H_2 formation process, under steady state conditions, using an exact solution of the master equation. This solution provides an exact result for the hydrogen recombination rate and its dependence on the flux, the surface temperature and the grain size. The results are compared with those obtained from the rate equations. The relevant length scales in the problem are identified and the parameter space is divided into two domains. One domain, characterized by first order kinetics, exhibits high efficiency of H_2 formation. In the other domain, characterized by second order kinetics, the efficiency of H_2 formation is low. In each of these domains we identify the range of parameters in which, the rate equations do not account correctly for the recombination rate. and the master equation is needed.Comment: 23 pages + 8 figure

Delayed response of low latitudes TEC during thirty-six geomagnetic storms from 2014 to 2017

International audienceIonospheric response to the onset of geomagnetic storms is an important aspect for developing models towards better understanding and prediction of ionospheric parameters, particularly over the equatorial and low latitude sectors that are associated with several complexities. Our paper discusses the time response of the ionosphere (∆t iono), where ∆t iono is the time elapsed from the onset of sudden storm commencement (SSC) of a magnetic storm to the absolute maximum value of DVTEC (TEC: total electron content). Over the period 2014 to 2017, thirty-six storms are reviewed, and their ∆t iono are analyzed along with the magnetic and solar parameters. We defined a threshold value of TEC to be 8 TECU. Three storms are studied in detail as a reference for the entire range of storms (March 2015, June 2015, and September 2015). The stations used are Kourou (KOUR; 5.25°N/52.80°

The feedback of massive stars on interstellar astrochemical processes

Astrochemistry is a discipline that studies physico-chemical processes in astrophysical environments. Such environments are characterized by conditions that are substantially different from those existing in usual chemical laboratories. Models which aim to explain the formation of molecular species in interstellar environments must take into account various factors, including many that are directly, or indirectly related to the populations of massive stars in galaxies. The aim of this paper is to review the influence of massive stars, whatever their evolution stage, on the physico-chemical processes at work in interstellar environments. These influences include the ultraviolet radiation field, the production of high energy particles, the synthesis of radionuclides and the formation of shocks that permeate the interstellar medium