The IRAM-30m line survey of the Horsehead PDR: IV. Comparative chemistry of H2CO and CH3OH

Aims. We investigate the dominant formation mechanism of H2CO and CH3OH in the Horsehead PDR and its associated dense core. Methods. We performed deep integrations of several H2CO and CH3OH lines at two positions in the Horsehead, namely the PDR and dense core, with the IRAM-30m telescope. In addition, we observed one H2CO higher frequency line with the CSO telescope at both positions. We determine the H2CO and CH3OH column densities and abundances from the single-dish observations complemented with IRAM-PdBI high-angular resolution maps (6") of both species. We compare the observed abundances with PDR models including either pure gas-phase chemistry or both gas-phase and grain surface chemistry. Results. We derive CH3OH abundances relative to total number of hydrogen atoms of ~1.2e-10 and ~2.3e-10 in the PDR and dense core positions, respectively. These abundances are similar to the inferred H2CO abundance in both positions (~2e-10). We find an abundance ratio H2CO/CH3OH of ~2 in the PDR and ~1 in the dense core. Pure gas-phase models cannot reproduce the observed abundances of either H2CO or CH3OH at the PDR position. Both species are therefore formed on the surface of dust grains and are subsequently photodesorbed into the gas-phase at this position. At the dense core, on the other hand, photodesorption of ices is needed to explain the observed abundance of CH3OH, while a pure gas-phase model can reproduce the observed H2CO abundance. The high-resolution observations show that CH3OH is depleted onto grains at the dense core. CH3OH is thus present in an envelope around this position, while H2CO is present in both the envelope and the dense core itself. Conclusions. Photodesorption is an efficient mechanism to release complex molecules in low FUV-illuminated PDRs, where thermal desorption of ice mantles is ineffective.Comment: 12 pages, 5 tables, 7 figures; Accepted for publication in A&

Correlation between electric-field-induced phase transition and piezoelectricity in lead zirconate titanate films

We observed that electric field induces phase transition from tetragonal to rhombohedral in polycrystalline morphotropic lead zirconate titanate (PZT) films, as reported in 2011 for bulk PZT. Moreover, we evidenced that this field-induced phase transition is strongly correlated with PZT film piezoelectric properties, that is to say the larger the phase transition, the larger the longitudinal piezoelectric coefficient d 33,eff . Although d 33,eff is already comprised between as 150 to 170 pm/V, our observation suggests that one could obtain larger d 33,eff values, namely 250 pm/V, by optimizing the field-induced phase transition thanks to composition fine tuning

Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample

We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs in the UV spectra of quasars selected from the SDSS Early Data Release. These quasars have strong intervening MgII-FeII systems which are known signatures of high column density neutral gas. In total, UV observations of Ly-alpha absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW) W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36 lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M = 0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is constant for 0.5<z<5.0 to within the uncertainties. This is larger than Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution does not change significantly with redshift. However, the low redshift distribution is marginally flatter due to the higher fraction of high N(HI) systems in our sample. (6) Finally, using the precision of MgII survey statistics, we find that there may be evidence of a decreasing Omega_{DLA} from z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very high columns of neutral gas might be missed by DLA surveys because of their very small cross sections, and therefore, that Omega_{DLA} might not include the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure

Free-energy distribution of the directed polymer at high temperature

We study the directed polymer of length $t$ in a random potential with fixed endpoints in dimension 1+1 in the continuum and on the square lattice, by analytical and numerical methods. The universal regime of high temperature $T$ is described, upon scaling 'time' $t \sim T^5/\kappa$ and space $x = T^3/\kappa$ (with $\kappa=T$ for the discrete model) by a continuum model with $\delta$-function disorder correlation. Using the Bethe Ansatz solution for the attractive boson problem, we obtain all positive integer moments of the partition function. The lowest cumulants of the free energy are predicted at small time and found in agreement with numerics. We then obtain the exact expression at any time for the generating function of the free energy distribution, in terms of a Fredholm determinant. At large time we find that it crosses over to the Tracy Widom distribution (TW) which describes the fixed $T$ infinite $t$ limit. The exact free energy distribution is obtained for any time and compared with very recent results on growth and exclusion models.Comment: 6 pages, 3 figures large time limit corrected and convergence to Tracy Widom established, 1 figure changed

Chemical Evolution of Damped Ly alpha galaxies: The [S/Zn] abundance ratio at redshift z > 2

Relative elemental abundances, and in particular the alpha/Fe ratio, are an important diagnostic tool of the chemical evolution of damped Ly alpha systems (DLAs). The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the alpha/Fe ratio. We report 6 new determinations of sulphur abundance in DLAs at zabs greater than or equal to 2 with already known zinc abundances. The combination with extant data from the literature provides a measure of the S/Zn abundance ratio for a total of 11 high redshift DLA systems. The observed [S/Zn] ratios do not show the characteristic [alpha/Fe] enhancement observed in metal-poor stars of the Milky Way at comparable level of metallicity ([Zn/H] ~ -1). The behaviour of DLAs data is consistent with a general trend of decreasing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the first evidence of the expected decrease of the alpha/Fe ratio in the course of chemical evolution of DLA systems. However, in contrast to what observed in our Galaxy, the alpha/iron-peak ratio seems to attain solar values when the metallicity is still low ([Zn/H] < -1) and to decrease below solar values at higher metallicities. The behaviour of the alpha/Fe ratio challenges the frequently adopted hypothesis that high redshift DLAs are progenitors of spiral galaxies and favours instead an origin in galaxies characterized by low star formation rates, in agreement with the results from imaging studies of low redshift DLAs, where the candidate DLA galaxies show a variety of morphological types including dwarfs and LSBs and only a minority of spirals.Comment: ApJ (accepted

Si and Mn Abundances in Damped Lya Systems with Low Dust Content

We have measured the abundances of Zn, Si, Mn, Cr, Fe, and Ni in three damped Lyman alpha systems at redshifts z < 1 from high resolution echelle spectra of QSOs recorded with the Keck I telescope. In all three cases the abundances of Cr, Fe, and Ni relative to Zn indicate low levels of dust depletions. We propose that when the proportion of refractory elements locked up in dust grains is less than about 50 percent, it is plausible to assume an approximately uniform level of depletion for all grain constituents and, by applying a small dust correction, recover the intrisic abundances of Si and Mn. We use this approach on a small sample of damped systems for which it is appropriate, with the aim of comparing the metallicity dependence of the ratios [Si/Fe] and [Mn/Fe] with analogous measurements in Milky Way stars. The main conclusion is that the relative abundances of both elements in distant galaxies are broadly in line with expectations based on Galactic data. Si displays a mild enhancement at low metallicities, as expected for an alpha-capture element, but there are also examples of near-solar [Si/Fe] at [Fe/H] < -1. The underabundance of Mn at low metallicities is possibly even more pronounced than that in metal-poor stars, and no absorption system has yet been found where [Mn/Fe] is solar. The heterogeneous chemical properties of damped Lyman alpha systems, evident even from this limited set of measurements, provide further support for the conclusion from imaging studies that a varied population of galaxies gives rise to this class of QSO absorbers.Comment: 29 pages, LaTex, 7 Postscript Figures. Accepted for Publication in the Astrophysical Journa

Modeling and Optimization of Lactic Acid Synthesis by the Alkaline Degradation of Fructose in a Batch Reactor

The present work deals with the determination of the optimal operating conditions of lactic acid synthesis by the alkaline degradation of fructose. It is a complex transformation for which detailed knowledge is not available. It is carried out in a batch or semi-batch reactor. The ‘‘Tendency Modeling’’ approach, which consists of the development of an approximate stoichiometric and kinetic model, has been used. An experimental planning method has been utilized as the database for model development. The application of the experimental planning methodology allows comparison between the experimental and model response. The model is then used in an optimization procedure to compute the optimal process. The optimal control problem is converted into a nonlinear programming problem solved using the sequencial quadratic programming procedure coupled with the golden search method. The strategy developed allows simultaneously optimizing the different variables, which may be constrained. The validity of the methodology is illustrated by the determination of the optimal operating conditions of lactic acid production

Influence possible des protozoaires sur le taux de mortalité des bactéries autotrophes nitrifiantes

Le modèle de l'IAWQ du processus de boues activées représente les mécanismes endogènes de la biomasse nitrifiante par le décès des micro-organismes (équation d'ordre 1 par rapport à la biomasse). La constante de décès, ou taux de mortalité bA, est aujourd'hui encore mal connue, et en particulier les facteurs influants sur sa valeur. De récentes études ont montré que la prédation par la microfaune pourrait être un facteur déterminant sur la valeur de bA. Cette étude se propose donc de quantifier l'effet de la prédation sur la valeur de bA. Deux réacteurs maintenus sans alimentation en substrat ont été caractérisés en parallèle: l'un a reçu une dose d'antibiotique spécifique aux eucaryotes (cycloheximide) afin de diminuer la quantité d'organismes de la microfaune, alors que l'autre n'a reçu aucun antibiotique (témoin). Les résultats obtenus montrent que le cycloheximide inhibe la plupart des organismes de la microfaune sauf les amibes; celles-ci semblent plutôt stimulées par cet antibiotique. En ce qui concerne la nitrification, un ralentissement de la production de nitrate dans le réacteur traité à l'antibiotique est observé à partir du sixième jour. Cette diminution de production de nitrate est probablement causée par une réduction de l'azote nitrifiable (qui est mobilisé par les amibes) couplée à une prédation des organismes nitrifiants par les amibes. D'ailleurs, l'augmentation de la prédation par les amibes à partir du jour 6 a diminué l'activité nitrifiante également mesurée par respirométrie (rO2 Nmax). Cette diminution du taux de respiration indique une augmentation du taux de mortalité (bA) des organismes nitrifiants. En effet, la valeur du taux de mortalité mesurée dans le réacteur témoin est de 0.08 d-1 alors que selon la microfaune présente dans le réacteur inhibé au cycloheximide, la valeur de ce taux de mortalité a varié entre 0.05 d-1 et 0.15 d-1.Designing biological wastewater treatment plants with the aid of the model developed by the IAWQ requires the knowledge of biological kinetic parameters. For nitrifying activated sludge, these parameters are related to nitrifying bacteria: maximum autotrophic growth rate µAmax, yield coefficient YA and the autotrophic decay rate bA. Although variables influencing µAmax and YA values are well known, this is not the case for bA. MARTINAGE and PAUL (2000) have recently shown that the bA value is strongly influenced by the influent quality, leading to the assumption that influent quality has a strong effect on microfauna composition, and consequently on the grazing rate of microfauna on nitrifying bacteria. In fixed-film processes, protozoan grazing reduces the bacterial population considerably (NATUSCKA and WELANDER, 1994). However, although many data are available concerning the grazing rates of different protozoa, the effect of microfaunal grazing on nitrification is still a matter of debate (RATSAK et al., 1994) and its effect on the bA value is still unknown. These two topics are investigated here.Nitrifying activated sludges were grown in two identical batch reactors, but in one, cycloheximide was added to inhibit eucaryotic growth (MAURINES CARBONEILL et al., 1998). Microfauna organism numbers were quantified in both reactors by microscopic observations of flagellated protozoa (>8 µm), amoebae, ciliates, rotifers and higher invertebrates (Fig. 3). Microbial counts were then correlated with the bA value. The latter was determined using the procedure proposed by SALZER (1992) which consists of characterising the time behaviour of the maximum nitrification rate measured by respirometry of activated sludge under substrate starvation. Under these conditions bacteria die and organic nitrogen is released into the bulk phase. This nitrogen is ammonified, and nitrifying bacteria use this substrate to produce nitrate, and then autotrophic bacterial growth occurs. This method takes this growth into account by characterising nitrate production during the experiment (Fig. 2).The effect of cycloheximide on nitrification was first determined to make sure that this compound is not inhibitory toward nitrifiers. Results obtained (Table 1) show that cycloheximide was not inhibitory toward nitrate production or the maximum nitrification oxygen uptake rate (rO2 N) after 4 hours of contact with nitrifying biomass. Cycloheximide addition in the activated sludge had an important impact on rotifers and flagellates but no effect on ciliates; it also seemed to stimulate amoebae growth. In both reactors, flagellates were mainly Peranama, attached ciliates were mainly Opercularia and Epistylis and a few Vorticella. Free ciliates like Aspidisca and Euplotes were found in both reactors.Variation with time of the abundance of microfauna organisms is shown in Figures 4 and 5 for both reactors. In the reference reactor the number of microfauna organisms decreased with time (Fig. 4) probably due to substrate starvation. Microfauna composition remained however diversified. For the inhibited reactor (Fig. 5), three periods were observed. During period I, the microfauna was mainly composed of ciliates and the number of microfaunal organisms decreased rapidly. During period II, an important growth of amoebae was observed. Cycloheximide was then added during this period to reduce their number. This growth of amoebae seems to be caused by the resistance of these micro-organisms toward inhibiting compounds (SRIKANTH et BERK, 1993). During period III, the number of microfaunal organisms was lower than during period II, and microfauna was mainly composed of ciliates.Nitrate concentration behaviour, necessary for bA calculation, is shown on Figure 6. In the reference reactor, nitrate concentrations varied linearly. For the inhibited reactor, the linear pattern was not observed during period II. This result was probably caused by an important nitrogen assimilation need of amoebae (ELDRIGE and JACKSON, 1993). Because organic nitrogen released by bacterial decay is consumed by amoebae assimilation, less nitrogen is available for the ammonification process and therefore for nitrification. Ammonia concentrations remained below 0.2 mg N·l-1 during all the experiment for both reactors. When amoebae disappeared from the inhibited reactor (period III) nitrate concentration varied linearly again.Variations of the maximum nitrification oxygen uptake rate (rO2 Nmax) with time are presented in Figure 7 (A&B) for both reactors. Two curves are plotted on each figure. Empty squares represent the measured rO2 N and black points represent the maximum nitrification rate that would have been measured if no growth on ammonification products had occurred. For the reference reactor (Fig. 7A), a value for bA of 0.08 d-1 can be calculated and can be considered constant for a constant microfauna composition.Three bA values can be estimated for the reactor inhibited with cycloheximide (Fig. 7B), corresponding to the three periods observed for microfauna composition. During period I, the bA value is 0.05 d-1 : a decrease in the microfaunal organism numbers implies a decrease of the bA value. During period II, when a development of amoebae is observed, the bA value increases and reaches 0.15 d-1. During period III with reduced grazing, the bA value is 0.13 d-1. Since during periods I and III the microfauna is mainly composed of ciliates, this difference between bA values is likely due to the observed difference in floc size between periods I and III.The results obtained during this study tend to prove (1) that the use of cycloheximide reduces microfaunal populations but can lead to a development of amoebae, and (2) that microfauna grazing seems to have an influence on the bA value, which can vary from 0.05 to 0.15 d-1 depending on microfaunal composition and abundance

Dynamics of particles and manifolds in a quenched random force field

We study the dynamics of a directed manifold of internal dimension D in a d-dimensional random force field. We obtain an exact solution for $d \to \infty$ and a Hartree approximation for finite d. They yield a Flory-like roughness exponent $\zeta$ and a non trivial anomalous diffusion exponent $\nu$ continuously dependent on the ratio $g_{T}/g_{L}$ of divergence-free ($g_{T}$) to potential ($g_{L}$) disorder strength. For the particle (D=0) our results agree with previous order $\epsilon^2$ RG calculations. The time-translational invariant dynamics for $g_{T} >0$ smoothly crosses over to the previously studied ultrametric aging solution in the potential case.Comment: 5 pages, Latex fil

The First Galaxies: Clues from Element Abundances

It has recently become possible to measure directly the abundances of several chemical elements in a variety of environments at redshifts up to z = 5. In this review I summarise the latest observations of Lyman break galaxies, damped Lyman alpha systems and the Lyman alpha forest with a view to uncovering any clues which these data may offer to the first episodes of star formation. The picture which is emerging is one where the universe at z = 3 already included many of the components of today's galaxies--even at these early times we see evidence for Populations I and II stars, while the `smoking gun' for Population III objects may be hidden in the chemical composition of the lowest density regions of the IGM, yet to be deciphered.Comment: 15 pages, LaTex, 8 Postscript Figures. To appear in the Philosophical Transactions of The Royal Society, Series