How micron-sized dust particles determine the chemistry of our Universe

Abstract

In the environments where stars and planets form, about one percent of the mass is in the form of micro-meter sized particles known as dust. However small and insignificant these dust grains may seem, they are responsible for the production of the simplest (H2) to the most complex (amino-acids) molecules observed in our Universe. Dust particles are recognized as powerful nano-factories that produce chemical species. However, the mechanism that converts species on dust to gas species remains elusive. Here we report experimental evidence that species forming on interstellar dust analogs can be directly released into the gas. This process, entitled chemical desorption (fig. 1), can dominate over the chemistry due to the gas phase by more than ten orders of magnitude. It also determines which species remain on the surface and are available to participate in the subsequent complex chemistry that forms the molecules necessary for the emergence of life.

Dust as interstellar catalyst

“…Garrod et al (2007) have previously proposed a formula in their models that can be used to describe the chemical desorption (or reactive desorption). They derived the probability of A24, page 4 of 6 Bergeron et al (2008); (c) Al-Halabi & van Dishoeck (2007), Amiaud et al (2007); (d) Pirronello et al (1997); (e) Amiaud et al (2006) for low coverage; ( f ) Minissale et al (2015); (g) Dulieu et al (2013); (h) Speedy et al (1996), Fraser et al (2001 energy derived of 5800 K with pre-factor of 10 15 s −1 here corrected to have a pre-factor of 10 12 s −1 ; (i) Noble et al (2012a); ( (2)) that include division between the degree of freedom and the fraction of kinetic energy after bounce ( is mass dependent). In red we show the equal share of energy and the constant value of .…”

“…Coverage range indicates the coverage of molecular ice growth before atom irradiation; in the case of experiments performed only with atomic species, coverage range indicates the total amount of species placed on the surface. √ and X are used to indicate whether the experimental procedure (DED or TPD) was used or not; (#) CD decreases as a function of coverage; (&) experiments performed with excited particles (see Minissale 2014); (a) Chaabouni et al (2012); (b) Dulieu et al (2013); (c) Minissale et al (2013); (d) ; (e) ; ( * ) this work.…”

“…In particular, many studies have been focused on thermal desorption Noble et al 2015) and nonthermal processes, such as photo-desorption (DeSimone et al 2013;Bertin et al 2013), sputtering (Johnson et al 2013;Cassidy et al 2013), cosmic ray (CR) desorption (Leger et al 1985;Hasegawa & Herbst 1993;Ivlev et al 2015), and chemical desorption (Cazaux et al 2010;Dulieu et al 2013;.…”

Computational Modeling of High Temperature Gas-Grain Chemistry in Interstellar Medium (ISM)

“…17 To treat reactive desorption the fraction 'f' of reactions resulting in desorption is calculated by considering the competition between the rate of desorption and the rate of energy lost to the grain: 15 (2.29) f = aP 1 + aP where a= / = 1.0 and is the ratio of surface molecule bond frequency to frequency at which energy is lost. Previous models of Nautilus use a value of .01, we use a value of 1.0 assuming reactive desorption is quite efficient at high temperatures on a bare grain in accordance with work by Dulieu et al 40 P gives the probability of desorption and is based on the Rice-Rampsberger-Kessel theory and is described in more detail in Garrod 2007:…”

“…We chose this value because Dulieu et al found reactive desorption to be much more efficient in producing DO 2 on a bare graphite surface then on an icy mantle. 40 A branching ratio was assumed where a fraction 'f' of the species desorbs into the gas phase, the energy from the reaction acts to break the surface-molecule bond. In the other case this energy is lost in lateral translation along the grain surface, and 1-f stays on the grain surface.…”