O2 signature in thin and thick O2-H2O ices

Aims. In this paper we investigate the detectability of the molecular oxygen in icy dust grain mantles towards astronomical objects. Methods. We present a systematic set of experiments with O2-H2O ice mixtures designed to disentangle how the molecular ratio affects the O2 signature in the mid- and near-infrared spectral regions. All the experiments were conducted in a closed-cycle helium cryostat coupled to a Fourier transform infrared spectrometer. The ice mixtures comprise varying thicknesses from 8 $\times$ 10$^{-3}$ to 3 $\mu$m. The absorption spectra of the O2-H2O mixtures are also compared to the one of pure water. In addition, the possibility to detect the O2 in icy bodies and in the interstellar medium is discussed. Results. We are able to see the O2 feature at 1551 cm$^{-1}$ even for the most diluted mixture of H2O : O2 = 9 : 1, comparable to a ratio of O2/H2O = 10 % which has already been detected in situ in the coma of the comet 67P/Churyumov-Gerasimenko. We provide an estimate for the detection of O2 with the future mission of the James Webb Space Telescope (JWST).Comment: 11 pages, 10 figures, article in press, to appear in A&A 201

On the evolution of the molecular line profiles induced by the propagation of C-shock waves

We present the first results of the expected variations of the molecular line emission arising from material recently affected by C-shocks (shock precursors). Our parametric model of the structure of C-shocks has been coupled with a radiative transfer code to calculate the molecular excitation and line profiles of shock tracers such as SiO, and of ion and neutral molecules such as H13CO+ and HN13C, as the shock propagates through the unperturbed medium. Our results show that the SiO emission arising from the early stage of the magnetic precursor typically has very narrow line profiles slightly shifted in velocity with respect to the ambient cloud. This narrow emission is generated in the region where the bulk of the ion fluid has already slipped to larger velocities in the precursor as observed toward the young L1448-mm outflow. This strongly suggests that the detection of narrow SiO emission and of an ion enhancement in young shocks, is produced by the magnetic precursor of C-shocks. In addition, our model shows that the different velocity components observed toward this outflow can be explained by the coexistence of different shocks at different evolutionary stages, within the same beam of the single-dish observations.Comment: 7 pages, 4 figures, accepted for publication in Ap

Gas Kinematics and Excitation in the Filamentary IRDC G035.39-00.33

Some theories of dense molecular cloud formation involve dynamical environments driven by converging atomic flows or collisions between preexisting molecular clouds. The determination of the dynamics and physical conditions of the gas in clouds at the early stages of their evolution is essential to establish the dynamical imprints of such collisions, and to infer the processes involved in their formation. We present multi-transition 13CO and C18O maps toward the IRDC G035.39-00.33, believed to be at the earliest stages of evolution. The 13CO and C18O gas is distributed in three filaments (Filaments 1, 2 and 3), where the most massive cores are preferentially found at the intersecting regions between them. The filaments have a similar kinematic structure with smooth velocity gradients of ~0.4-0.8 km s-1 pc-1. Several scenarios are proposed to explain these gradients, including cloud rotation, gas accretion along the filaments, global gravitational collapse, and unresolved sub-filament structures. These results are complemented by HCO+, HNC, H13CO+ and HN13C single-pointing data to search for gas infall signatures. The 13CO and C18O gas motions are supersonic across G035.39-00.33, with the emission showing broader linewidths toward the edges of the IRDC. This could be due to energy dissipation at the densest regions in the cloud. The average H2 densities are ~5000-7000 cm-3, with Filaments 2 and 3 being denser and more massive than Filament 1. The C18O data unveils three regions with high CO depletion factors (f_D~5-12), similar to those found in massive starless cores.Comment: 20 pages, 14 figures, 6 tables, accepted for publication in MNRA

Deuterium fractionation on interstellar grains studied with modified rate equations and a Monte Carlo approach

The formation of singly and doubly deuterated isotopomers of formaldehyde and of singly, doubly, and multiply deuterated isotopomers of methanol on interstellar grain surfaces has been studied with a semi-empirical modified rate approach and a Monte Carlo method in the temperature range 10-20 K. Agreement between the results of the two methods is satisfactory for all major and many minor species throughout this range. If gas-phase fractionation can produce a high abundance of atomic deuterium, which then accretes onto grain surfaces, diffusive surface chemistry can produce large abundances of deuterated species, especially at low temperatures and high gas densities. Warming temperatures will then permit these surface species to evaporate into the gas, where they will remain abundant for a considerable period. We calculate that the doubly deuterated molecules CHD2OH and CH2DOD are particularly abundant and should be searched for in the gas phase of protostellar sources. For example, at 10 K and high density, these species can achieve up to 10-20% of the abundance of methanol.Comment: 27 pages, 3 figures, Planetary and Space Science, in pres

Modeling of Immunosenescence and Risk of Death from Respiratory Infections: Evaluation of the Role of Antigenic Load and Population Heterogeneity

It is well known that efficacy of immune functions declines with age. It results in an increase of severity and duration of respiratory infections and also in dramatic growth of risk of death due to these diseases after age 65. The goal of this work is to describe and investigate the mechanism underlying the age pattern of the mortality rate caused by infectious diseases and to determine the cause-specific hazard rate as a function of immune system characteristics. For these purposes we develop a three-compartment model explaining observed risk-of-death. The model incorporates up-to-date knowledge about cellular mechanisms of aging, disease dynamics, population heterogeneity in resistance to infections, and intrinsic aging rate. The results of modeling show that the age-trajectory of mortality caused by respiratory infections may be explained by the value of antigenic load, frequency of infections and the rate of aging of the stem cell population (i.e. the population of T-lymphocyte progenitor cells). The deceleration of infection-induced mortality at advanced age can be explained by selection of individuals with a slower rate of stem cell aging. Parameter estimates derived from fitting mortality data indicate that infection burden was monotonically decreasing during the twentieth century, and changes in total antigenic load were gender-specific: it experienced periodic fluctuations in males and increased approximately two-fold in females

Lifetime of the embedded phase of low-mass star formation and the envelope depletion rates

Motivated by a considerable scatter in the observationally inferred lifetimes of the embedded phase of star formation, we study the duration of the Class 0 and Class I phases in upper-mass brown dwarfs and low-mass stars using numerical hydrodynamics simulations of the gravitational collapse of a large sample of cloud cores. We resolve the formation of a star/disk/envelope system and extend our numerical simulations to the late accretion phase when the envelope is nearly totally depleted of matter. We adopted a classification scheme of Andre et al. and calculate the lifetimes of the Class 0 and Class I phases (\tau_C0 and \tau_CI, respectively) based on the mass remaining in the envelope. When cloud cores with various rotation rates, masses, and sizes (but identical otherwise) are considered, our modeling reveals a sub-linear correlation between the Class 0 lifetimes and stellar masses in the Class 0 phase with the least-squares fit exponent m=0.8 \pm 0.05. The corresponding correlation between the Class I lifetimes and stellar masses in the Class I is super-linear with m=1.2 \pm 0.05. If a wider sample of cloud cores is considered, which includes possible variations in the initial gas temperature, cloud core truncation radii, density enhancement amplitudes, initial gas density and angular velocity profiles, and magnetic fields, then the corresponding exponents may decrease by as much as 0.3. The duration of the Class I phase is found to be longer than that of the Class~0 phase in most models, with a mean ratio \tau_CI / \tau_C0 \approx 1.5--2. A notable exception are YSOs that form from cloud cores with large initial density enhancements, in which case \tau_C0 may be greater than \tau_CI. Moreover, the upper-mass (>= 1.0 Msun) cloud cores with frozen-in magnetic fields and high cloud core rotation rates may have the \tau_CI / \tau_C0 ratios as large as 3.0--4.0. (Abdridged).Comment: Accepted for publication by The Astrophysical Journa

Dust in Interstellar Clouds, Evolved Stars and Supernovae

Outflows of pre-main-sequence stars drive shocks into molecular material within 0.01 - 1 pc of the young stars. The shock-heated gas emits infrared, millimeter and submillimeter lines of many species including. Dust grains are important charge carriers and play a large role in coupling the magnetic field and flow of neutral gas. Some effects of the dust on the dynamics of oblique shocks began to emerge in the 1990s. However, detailed models of these shocks are required for the calculation of the grain sputtering contribution to gas phase abundances of species producing observed emissions. We are developing such models. Some of the molecular species introduced into the gas phase by sputtering in shocks or by thermally driven desorption in hot cores form on grain surfaces. Recently laboratory studies have begun to contribute to the understanding of surface reactions and thermally driven desorption important for the chemistry of star forming clouds. Dusty plasmas are prevalent in many evolved stars just as well as in star forming regions. Radiation pressure on dust plays a significant role in mass loss from some post-main-sequence stars. The mechanisms leading to the formation of carbonaceous dust in the stellar outflows are similar to those important for soot formation in flames. However, nucleation in oxygen-rich outflows is less well understood and remains a challenging research area. Dust is observed in supernova ejecta that have not passed through the reverse shocks that develop in the interaction of ejecta with ambient media. Dust is detected in high redshift galaxies that are sufficiently young that the only stars that could have produced the dust were so massive that they became supernovae. Consequently, the issue of the survival of dust in strong supernova shocks is of considerable interest.Comment: 4 pages, to be published in the proceedings of Fifth International Conference on Physics of Dusty Plasma

Chemical differentiation in regions of high mass star formation II. Molecular multiline and dust continuum studies of selected objects

The aim of this study is to investigate systematic chemical differentiation of molecules in regions of high mass star formation. We observed five prominent sites of high mass star formation in HCN, HNC, HCO+, their isotopes, C18O, C34S and some other molecular lines, for some sources both at 3 and 1.3 mm and in continuum at 1.3 mm. Taking into account earlier obtained data for N2H+ we derive molecular abundances and physical parameters of the sources (mass, density, ionization fraction, etc.). The kinetic temperature is estimated from CH3C2H observations. Then we analyze correlations between molecular abundances and physical parameters and discuss chemical models applicable to these species. The typical physical parameters for the sources in our sample are the following: kinetic temperature in the range ~ 30-50 K (it is systematically higher than that obtained from ammonia observations and is rather close to dust temperature), masses from tens to hundreds solar masses, gas densities ~ 10^5 cm^{-3}, ionization fraction ~ 10^{-7}. In most cases the ionization fraction slightly (a few times) increases towards the embedded YSOs. The observed clumps are close to gravitational equilibrium. There are systematic differences in distributions of various molecules. The abundances of CO, CS and HCN are more or less constant. There is no sign of CO and/or CS depletion as in cold cores. At the same time the abundances of HCO+, HNC and especially N2H+ strongly vary in these objects. They anti-correlate with the ionization fraction and as a result decrease towards the embedded YSOs. For N2H+ this can be explained by dissociative recombination to be the dominant destroying process. N2H+, HCO+, and HNC are valuable indicators of massive protostars.Comment: 15 pages, 8 figure

N2H+(1-0) survey of massive molecular cloud cores

We present the results of N2H+(1-0) observations of 35 dense molecular cloud cores from the northern and southern hemispheres where massive stars and star clusters are formed. Line emission has been detected in 33 sources, for 28 sources detailed maps have been obtained. The optical depth of (23-12) component toward peak intensity positions of 10 sources is ~ 0.2-1. In total, 47 clumps have been revealed in 26 sources. Integrated intensity maps with aspect ratios < 2 have been fitted with a power-law radial distribution $r^{-p}$ convolved with the telescope beam. Mean power-law index is close to unity corresponding to the $\sim r^{-2}$ density profile provided N2H+ excitation conditions do not vary inside these regions. Line widths of the cores either decrease or stay constant with distance from the center. The ratio of rotational to gravitational energy is too low for rotation to play a significant role in the dynamics of the cores. A correlation between mean line widths and sizes of clumps has been found.Comment: 17 pages, Late