Interplay of gas and ice during cloud evolution

During the evolution of diffuse clouds to molecular clouds, gas-phase molecules freeze out on surfaces of small dust particles to form ices. On dust surfaces, water is the main constituent of the icy mantle in which a complex chemistry is taking place. We aim to study the formation pathways and the composition of the ices throughout the evolution of diffuse clouds. For this purpose, we use time-dependent rate equations to calculate the molecular abundances in both gas phase and on solid surfaces (onto dust grains). We fully consider the gas-dust interplay by including the details of freeze-out, chemical and thermal desorption, as well as the most important photo-processes on grain surfaces. The difference in binding energies of chemical species on bare and icy surfaces is also incorporated into our equations. Using the numerical code FLASH, we perform a hydrodynamical simulation of a gravitationally bound diffuse cloud and follow its contraction. We find that while the dust grains are still bare, water formation is enhanced by grain surface chemistry which is subsequently released into the gas phase, enriching the molecular medium. The CO molecules, on the other hand, tend to freeze out gradually on bare grains. This causes CO to be well mixed and strongly present within the first ice layer. Once one monolayer of water ice has formed, the binding energy of the grain surface changes significantly and an immediate and strong depletion of gas-phase water and CO molecules occur. While hydrogenation converts solid CO into formaldehyde (H$_2$CO) and methanol (CH$_3$OH), water ice becomes the main constituent of the icy grains. Inside molecular clumps formaldehyde is more abundant than water and methanol in the gas phase owing its presence in part to chemical desorption.Comment: 19 pages, 10 figures, 9 tables, 23 equations. Accepted for publication Astronomy & Astrophysics. In version 3: Language edit, added gas-phase reaction tables, title has change

The Snow Border

Context. The study of the snow line is an important topic in several domains of astrophysics, and particularly for the evolution of proto-stellar environments and the formation of planets. Aims. The formation of the first layer of ice on carbon grains requires low temperatures compared to the temperature of evaporation (T > 100 K). This asymmetry generates a zone in which bare and icy dust grains coexist. Methods. We use Monte-Carlo simulations to describe the formation time scales of ice mantles on bare grains in protostellar disks and massive protostars environments. Then we analytically describe these two systems in terms of grain populations subject to infall and turbulence, and assume steady-state. Results. Our results show that there is an extended region beyond the snow line where icy and bare grains can coexist, in both proto-planetary disks and massive protostars. This zone is not negligible compared to the total size of the objects: on the order of 0.4 AU for proto-planetary disks and 5400 AU for high-mass protostars. Times to reach the steady-state are respectively es- timated from 10^2 to 10^5 yr. Conclusions. The presence of a zone, a so-called snow border, in which bare and icy grains co- exist can have a major impact on our knowledge of protostellar environments. From a theoretical point of view, the progression of icy grains to bare grains as the temperature increases, could be a realistic way to model hot cores and hot corinos. Also, in this zone, the formation of planetesimals will require the coagulation of bare and icy grains. Observationally, this zone allows high abundances of gas phase species at large scales, for massive protostars particularly, even at low temperatures (down to 50 K).Comment: accepted in A&

The impact of freeze-out on collapsing molecular clouds

Atoms and molecules, and in particular CO, are important coolants during the evolution of interstellar star-forming gas clouds. The presence of dust grains, which allow many chemical reactions to occur on their surfaces, strongly impacts the chemical composition of a cloud. At low temperatures, dust grains can lock-up species from the gas phase which freeze out and form ices. In this sense, dust can deplete important coolants. Our aim is to understand the effects of freeze-out on the thermal balance and the evolution of a gravitationally bound molecular cloud. For this purpose, we perform 3D hydrodynamical simulations with the adaptive mesh code FLASH. We simulate a gravitationally unstable cloud under two different conditions, with and without grain surface chemistry. We let the cloud evolve until one free-fall time is reached and track the thermal evolution and the abundances of species during this time. We see that at a number density of 10$^4$ cm$^{-3}$ most of the CO molecules are frozen on dust grains in the run with grain surface chemistry, thereby depriving the most important coolant. As a consequence, we find that the temperature of the gas rises up to $\sim$25 K. The temperature drops once again due to gas-grain collisional cooling when the density reaches a few$\times$10$^4$ cm$^{-3}$. We conclude that grain surface chemistry not only affects the chemical abundances in the gas phase, but also leaves a distinct imprint in the thermal evolution that impacts the fragmentation of a star-forming cloud. As a final step, we present the equation of state of a collapsing molecular cloud that has grain surface chemistry included.Comment: Increased the number of significant digits in EQ 2. It mattered. Accepted for publication in MNRAS letter

Effective grain surface area in the formation of molecular hydrogen in interstellar clouds

In the interstellar clouds, molecular hydrogens are formed from atomic hydrogen on grain surfaces. An atomic hydrogen hops around till it finds another one with which it combines. This necessarily implies that the average recombination time, or equivalently, the effective grain surface area depends on the relative numbers of atomic hydrogen influx rate and the number of sites on the grain. Our aim is to discover this dependency. We perform a numerical simulation to study the recombination of hydrogen on grain surfaces in a variety of cloud conditions. We use a square lattice (with a periodic boundary condition) of various sizes on two types of grains, namely, amorphous carbon and olivine. We find that the steady state results of our simulation match very well with those obtained from a simpler analytical consideration provided the `effective' grain surface area is written as $\sim S^{\alpha}$, where, $S$ is the actual physical grain area and $\alpha$ is a function of the flux of atomic hydrogen which is determined from our simulation. We carry out the simulation for various astrophysically relevant accretion rates. For high accretion rates, small grains tend to become partly saturated with $H$ and $H_2$ and the subsequent accretion will be partly inhibited. For very low accretion rates, the number of sites to be swept before a molecular hydrogen can form is too large compared to the actual number of sites on the grain, implying that $\alpha$ is greater than unity.Comment: 8 pages, 5 figures in eps forma

Dust as interstellar catalyst I. Quantifying the chemical desorption process

Context. The presence of dust in the interstellar medium has profound consequences on the chemical composition of regions where stars are forming. Recent observations show that many species formed onto dust are populating the gas phase, especially in cold environments where UV and CR induced photons do not account for such processes. Aims. The aim of this paper is to understand and quantify the process that releases solid species into the gas phase, the so-called chemical desorption process, so that an explicit formula can be derived that can be included into astrochemical models. Methods. We present a collection of experimental results of more than 10 reactive systems. For each reaction, different substrates such as oxidized graphite and compact amorphous water ice are used. We derive a formula to reproduce the efficiencies of the chemical desorption process, which considers the equipartition of the energy of newly formed products, followed by classical bounce on the surface. In part II we extend these results to astrophysical conditions. Results. The equipartition of energy describes correctly the chemical desorption process on bare surfaces. On icy surfaces, the chemical desorption process is much less efficient and a better description of the interaction with the surface is still needed. Conclusions. We show that the mechanism that directly transforms solid species to gas phase species is efficient for many reactions.Comment: Accepted for publication in A&

The impact of metallicity and X-rays on star formation

Star formation is regulated through a variety of feedback processes. In this study, we treat feedback by metal injection and a UV background as well as by X-ray irradiation. Our aim is to investigate whether star formation is significantly affected when the ISM of a proto-galaxxy enjoys different metallicities and when a star forming cloud resides in the vicinity of a strong X-ray source. We perform cosmological Enzo simulations with a detailed treatment of non-zero metallicity chemistry and thermal balance. We also perform FLASH simulations with embedded Lagrangian sink particles of a collapsing molecular cloud near a massive, 10^{7} M\odot, black hole that produces X-ray radiation. We find that a multi-phase ISM forms for metallicites as small as 10^{-4} Solar at z = 6, with higher (10^{-2}Z\odot) metallicities supporting a cold ( 10^{3} cm^{-3}) phase at higher (z = 20) redshift. A star formation recipe based on the presence of a cold dense phase leads to a self-regulating mode in the presence of supernova and radiation feedback. We also find that when there is strong X-ray feedback a collapsing cloud fragments into larger clumps whereby fewer but more massive protostellar cores are formed. This is a consequence of the higher Jeans mass in the warm (50 K, due to ionization heating) molecular gas. Accretion processes dominate the mass function and a near-flat, non-Salpeter IMF results.Comment: Proceedings IAU Symposium No. 270, 2010. 4 pages, 5 figure

Dust temperature and time-dependent effects in the chemistry of photodissociation regions

When studying the chemistry of PDRs, time dependence becomes important as visual extinction increases, since certain chemical timescales are comparable to the cloud lifetime. Dust temperature is also a key factor, since it significantly influences gas temperature and mobility on dust grains, determining the chemistry occurring on grain surfaces. We present a study of the dust temperature impact and time effects on the chemistry of different PDRs, using an updated version of the Meijerink PDR code and combining it with the time-dependent code Nahoon. We find the largest temperature effects in the inner regions of high $G$$_{\mathrm{0}}$ PDRs, where high dust temperatures favour the formation of simple oxygen-bearing molecules (especially that of O$_2$), while the formation of complex organic molecules is much more efficient at low dust temperatures. We also find that time-dependent effects strongly depend on the PDR type, since long timescales promote the destruction of oxygen-bearing molecules in the inner parts of low $G$$_{\mathrm{0}}$ PDRs, while favouring their formation and that of carbon-bearing molecules in high $G$$_{\mathrm{0}}$ PDRs. From the chemical evolution, we also conclude that, in dense PDRs, CO$_2$ is a late-forming ice compared to water ice, and confirm a layered ice structure on dust grains, with H$_2$O in lower layers than CO$_2$. Regarding steady state, the PDR edge reaches chemical equilibrium at early times ($\lesssim$10$^5$ yr). This time is even shorter ($<$10$^4$ yr) for high $G$$_{\mathrm{0}}$ PDRs. By contrast, inner regions reach equilibrium much later, especially low $G$$_{\mathrm{0}}$ PDRs, where steady state is reached at $\sim$10$^6$-10$^7$ yr.Comment: 24 pages, 15 figures, 9 table

Teaching Culture in Business Studies

Nowadays, an intercultural component needs to be integrated in the development of any business course and research in the field of Intercultural Studies outlines issues directly relevant to the Business world. Research findings have underlined the complexity of concepts such as Culture and Stereotypes. In the light of these findings, future business courses incorporating an intercultural dimension should be developed