arXiv subjects

S. Cazaux

Publications and source records attributed to S. Cazaux.

47 records · Page 3Linked to original sources

arXiv2011

Interstellar ices as witnesses of star formation: selective deuteration of water and organic molecules unveiled

Observations of star forming environments revealed that the abundances of some deuterated interstellar molecules are markedly larger than the cosmic D/H ratio of 10-5. Possible reasons for this pointed to grain surface chemistry. How- ever, organic molecules and water, which are both ice constituents, do not enjoy the same deuteration. For example, deuterated formaldehyde is very abundant in comets and star forming regions, while deuterated water rarely is. In this article, we explain this selective deuteration by following the formation of ices (using the rate equation method) in translucent clouds, as well as their evolu- tion as the cloud collapses to form a star. Ices start with the deposition of gas phase CO and O onto dust grains. While reaction of oxygen with atoms (H or D) or molecules (H2) yields H2O (HDO), CO only reacts with atoms (H and D) to form H2CO (HDCO, D2CO). As a result, the deuteration of formaldehyde is sensitive to the gas D/H ratio as the cloud undergoes gravitational collapse, while the deuteration of water strongly depends on the dust temperature at the time of ice formation. These results reproduce well the deuterium fractionation of formaldehyde observed in comets and star forming regions and can explain the wide spread of deuterium fractionation of water observed in these environments.

astro-ph.SR↗

arXiv2011

When sticking influences H2 formation

Aims. Interstellar dust grains, because of their catalytic properties, are crucial to the formation of H2, the most abundant molecule in the Universe. The formation of molecular hydrogen strongly depends on the ability of H atoms to stick on dust grains. In this study we determine the sticking coefficient of H atoms chemisorbed on graphitic surfaces, and estimate its impact on the forma- tion of H2. Methods. The sticking probability of H atoms chemisorbed onto graphitic surfaces is obtained using a mixed classical-quantum dynamics method. In this, the H atom is treated quantum- mechanically and the vibrational modes of the surface are treated classically. The implications of sticking for the formation of H2 are addressed by using Kinetic Monte Carlo simulations that follow how atoms stick, move and associate with each other on dust surfaces of different temper- ature. Results. In our model, molecular hydrogen forms very efficiently for dust temperatures lower than 15 K through the involvement of physisorbed H atoms. At dust temperatures higher than 15 K and gas temperatures lower than 2000 K, H2 formation differs strongly if the H atoms com- ing from the gas phase have to cross a square barrier (usually considered in previous studies) or a barrier obtained by DFT calculations to become chemisorbed. The product of sticking times efficiency can be increased by many orders of magnitude when realistic barriers are considered. If graphite phonons are taken into account in the dynamics calculations, then H atoms stick better on the surface at high energies, but the overall H2 formation efficiency is only slightly affected. Our results suggest that H2 formation can proceed efficiently in photon dominated regions, X-ray dominated regions, hot cores and in the early Universe when the first dust is available.

astro-ph.SR↗

arXiv2011

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).

astro-ph.EP↗

arXiv2010

Water formation on bare grains: When the chemistry on dust impacts interstellar gas

Context. Water together with O2 are important gas phase ingredients to cool dense gas in order to form stars. On dust grains, H2 O is an important constituent of the icy mantle in which a complex chemistry is taking place, as revealed by hot core observations. The formation of water can occur on dust grain surfaces, and can impact gas phase composition. Aims. The formation of molecules such as OH, H2 O, HO2, H2 O2, as well as their deuterated forms and O2 and O3 is studied in order to assess how the chemistry varies in different astrophysical environments, and how the gas phase is affected by grain surface chemistry. Methods. We use Monte Carlo simulations to follow the formation of molecules on bare grains as well as the fraction of molecules released into the gas phase. We consider a surface reaction network, based on gas phase reactions, as well as UV photo-dissociation of the chemical species. Results. We show that grain surface chemistry has a strong impact on gas phase chemistry, and that this chemistry is very different for different dust grain temperatures. Low temperatures favor hydrogenation, while higher temperatures favor oxygenation. Also, UV photons dissociate the molecules on the surface, that can reform subsequently. The formation-destruction cycle increases the amount of species released into the gas phase. We also determine the time scales to form ices in diffuse and dense clouds, and show that ices are formed only in shielded environments, as supported by observations.

astro-ph.SR↗

arXiv2009

HD and H2 formation in low-metallicity dusty gas clouds at high redshift

Context: The HD and H2 molecules play important roles in the cooling of primordial and very metal-poor gas at high redshift. Aims: Grain surface and gas phase formation of HD and H2 is investigated to assess the importance of trace amounts of dust, 10^{-5}-10^{-3} Zo, in the production of HD and H2. Methods: We consider carbonaceous and silicate grains and include both physisorption and chemisorption, tunneling, and realistic grain surface barriers. We find, for a collapsing gas cloud environment with coupled chemical and thermal balance, that dust abundances as small as 10^{-5} solar lead to a strong boost in the H2 formation rate due to surface reactions. As a result of this enhancement in H2, HD is formed more efficiently in the gas phase through the D+ + H2 reaction. Direct formation of HD on dust grains cannot compete well with this gas phase process for dust temperatures below 150 K. We also derive up-to-date analytic fitting formulae for the grain surface formation of H2 and HD, including the different binding energies of H and D. Results: Grain surface reactions are crucial to the availability of H2 and HD in very metal-poor environments. Above metallicities of 10^{-5} solar, the grain surface route dominates the formation of H2, which in turn, drives the formation of HD in the gas phase. At dust temperatures above 150 K, laboratory experiments and theoretical modelling suggest that H2 formation on grains is suppressed while HD formation on grains is not.

astro-ph.CO↗

arXiv2008

The role of carbon grains in the deuteration of H2

Aims: The production of molecular hydrogen and its deuterated forms onto carbonaceous dust grains is investigated in detail. The goal of this study is to estimate the importance of the chemistry occuring on grain surfaces for the deuteration of H2. Furthermore, we aim to find a robust and general surface chemical model which can be used in different astrophysical environments. Methods: Surface processes are described for the cases of graphitic and amorphous--carbon grains, where laboratory work is available. Langmuir--Hinshelwood as well as Eley--Rideal surface chemistries are included in the model and their relative contributions are highlighted. Analytic expressions are derived for H2, HD, and D2 formation efficiencies for both type of grains. Rate equations are tested against stochastic methods. Results: As expected, rate equations and stochastic methods diverge for grain sizes lower than a critical value acrit. For grain sizes below this critical value, D2 formation decreases to favour HD formation. The formation efficiencies of H2 and D2 can be calculated by adding a correction factor to the rate equations methods. We found that because of the presence of chemisorbed sites, which can store atoms to form molecules up to high grain temperatures, the formation efficiency of HD and D2 is very high compared to models where only physisorption sites are taken into account. When considering a realistic distribution of dust grains, we found that the formation rate of H2 and HD is enhanced by an order of magnitude if small grains are taken into account. The processes described in this paper, that allow a strong enhancement of the deuterated forms of molecular hydrogen, could explain the high degree of deuterium fractionation observed in protostellar environments.

astro-ph↗

arXiv2005

A low background Micromegas detector for the CAST experiment

A low background Micromegas detector has been operating on the CAST experiment at CERN for the search of solar axions during the first phase of the experiment (2002-2004). The detector operated efficiently and achieved a very low level of background rejection ($5\times 10^{-5}$ counts keV$^{-1}$cm$^{-2}$s$^{-1}$) thanks to its good spatial and energy resolution as well as the low radioactivity materials used in the construction of the detector. For the second phase of the experiment (2005-2007), the detector will be upgraded by adding a shielding and including focusing optics. These improvements should allow for a background rejection better than two orders of magnitude.

physics.ins-det↗

arXiv2005

Molecular hydrogen formation on grain surfaces

We reconsider H2 formation on grain surfaces. We develop a rate equation model which takes into account the presence of both physisorbed and chemisorbed sites on the surface, including quantum mechanical tunnelling and thermal diffusion. In this study, we took into consideration the uncertainties on the characteristics of graphitic surfaces. We calculate the H2 formation efficiency with the Langmuir Hinshelwood and Eley Rideal mechanisms, and discuss the importance of these mechanisms for a wide range of grain and gas temperatures. We also develop a Monte Carlo simulation to calculate the H2 formation efficiency and compare the results to our rate equation models. Our results are the following: (1) Depending on the barrier against chemisorption, we predict the efficiency of H2 formation for a wide range of grain and gas temperatures. (2) The Eley-Rideal mechanism has an impact on the H2 formation efficiency at high grain and gas temperatures. (3) The fact that we consider chemisorption in our model makes the rate equation and Monte Carlo approaches equivalent.

astro-ph↗

arXiv2004

HDO abundance in the envelope of the solar-type protostar IRAS16293-2422

We present IRAM 30m and JCMT observations of HDO lines towards the solar-type protostar IRAS 16293-2422. Five HDO transitions have been detected on-source, and two were unfruitfully searched for towards a bright spot of the outflow of IRAS 16293-2422. We interpret the data by means of the Ceccarelli, Hollenbach and Tielens (1996) model, and derive the HDO abundance in the warm inner and cold outer parts of the envelope. The emission is well explained by a jump model, with an inner abundance of 1e-7 and an outer abundance lower than 1e-9 (3 sigma). This result is in favor of HDO enhancement due to ice evaporation from the grains in theinner envelope. The deuteration ratio HDO/H2O is found to be f_in=3% and f_out < 0.2% (3 sigma) in the inner and outer envelope respectively and therefore, the fractionation also undergoes a jump in the inner part of the envelope. These results are consistent with the formation of water in the gas phase during the cold prestellar core phase and storage of the molecules on the grains, but do not explain why observations of H2O ices consistently derive a H2O ice abundance of several 1e-5 to 1e-4, some two orders of magnitude larger than the gas phase abundance of water in the hot core around IRAS 16293-2422.

astro-ph↗

arXiv2004

Molecular hydrogen formation on dust grains in the high redshift universe

We study the formation of molecular hydrogen on dust grain surfaces and apply our results to the high redshift universe. We find that a range of physical parameters, in particular dust temperature and gas temperature, but not so much dust surface composition, influence the formation rate of H$_2$. The H$_2$ formation rate is found to be suppressed above gas kinetic temperatures of a few hundred K and for dust temperatures above 200-300 K and below 10 K. We highlight the differences between our treatment of the H$_2$ formation process and other descriptions in the literature. We also study the relative importance of H$_2$ formation on dust grains with respect to molecular hydrogen formation in the gas phase, through the H$^-$ route. The ratio of formation rates of these two routes depends to a large part on the dust abundance, on the electron abundance, and also on the relative strength of the FUV (extra-)galactic radiation field. We find that for a cosmological evolution of the star formation rate and dust density consistent with the Madau plot, a positive feedback effect on the abundance of H$_2$ due to the presence of dust grains can occur at redshifts $z\ge 3$. This effect occurs for a dust-to-gas mass ratio as small as 10$^{-3}$ of the galactic value.

astro-ph↗

arXiv2002

Molecular hydrogen formation in the interstellar medium

We have developed a model for molecular hydrogen formation under astrophysically relevant conditions. This model takes fully into account the presence of both physisorbed and chemisorbed sites on the surface, allows quantum mechanical diffusion as well as thermal hopping for absorbed H-atoms, and has been benchmarked versus recent laboratory experiments on H2 formation on silicate surfaces. The results show that H2 formation on grain surface is efficient in the interstellar medium up to some 300K. At low temperatures (<100K), H2 formation is governed by the reaction of a physisorbed H with a chemisorbed H. At higher temperatures, H2 formation proceeds through reaction between two chemisorbed H atoms. We present simple analytical expressions for H2 formation which can be adopted to a wide variety of surfaces once their surfaces characteristics have been determined experimentally.

astro-ph↗