A simple model for heterogeneous flows of yield stress fluids

Various experiments evidence spatial heterogeneities in sheared yield stress fluids. To account for heterogeneities in the velocity gradient direction, we use a simple model corresponding to a non-monotonous local constitutive curve and study a simple shear geometry. Different types of boundary conditions are considered. Under controlled macroscopic shear stress $\Sigma$, we find homogeneous flow in the bulk and a hysteretic macroscopic stress - shear rate curve. Under controlled macroscopic shear rate $\dot{\Gamma}$, shear banding is predicted within a range of values of $\dot{\Gamma}$. For small shear rates, stick slip can also be observed. These qualitative behaviours are robust when changing the boundary conditions.Comment: 13 pages, 13 figure

Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation

We present V and I photometry of the resolved stars in the most metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young stellar population of blue main-sequence (MS) stars (age <30 Myr) and blue and red supergiants (10 Myr<age<100 Myr), but also an older evolved population of asymptotic giant branch (AGB) stars (100 Myr<age<500 Myr). We derive a distance to I Zw 18 in the range 12.6 Mpc - 15 Mpc from the brightness of its AGB stars, with preferred values in the higher range. The red giant branch (RGB) stars are conspicuous by their absence, although, for a distance of I Zw 18 <15 Mpc, our imaging data go ~ 1-2 mag below the tip of the RGB. Thus, the most evolved stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young galaxy. Several star formation episodes can be inferred from the CMDs of the main body and the C component. There have been respectively three and two episodes in these two parts, separated by periods of ~ 100-200 Myr. In the main body, the younger MS and massive post-MS stars are distributed over a larger area than the older AGB stars, suggesting that I Zw 18 is still forming from the inside out. In the C component, different star formation episodes are spatially distinct, with stellar population ages decreasing from the northwest to the southeast, also suggesting the ongoing build-up of a young galaxy.Comment: 29 pages, 13 Postscript figures, accepted for publication in the Astrophysical Journa

The Star Formation History of IZw18

The star formation history in IZw18 has been inferred from HST/WFPC2 archival data. This is done by comparing the derived V, B-V and V, V-I color-magnitude diagrams and luminosity functions with synthetic ones, based on various sets of stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the field of IZw18 allow for a lookback time up to 1 Gyr. We find that the main body is not experiencing its first episode of star formation. Instead, it has been forming stars over the last 0.5-1 Gyr, at a rate of ~ 1-2 * 10**(-2) Msol per year per kpc**2. A more intense activity of 6-16 * 10**(-2) Msol per year per kpc**2 has taken place between 15 and 20 Myr ago. For the secondary body, the lookback time is 0.2 Gyr at most and the uncertainty is much higher, due to the shallower diagrams and the small number of resolved stars. The derived range of star formation rate is 3-10 * 10**(-3) Msol per year per kpc**2. The IMF providing the best fit to the observed stellar populations in the main body has a slope 1.5, much flatter than in any similar galaxy analyzed with the same method. In the secondary body, it is peaked at 2.2, closer to Salpeter's slope (2.35).Comment: 70 pages including 18 figures, to be published in The Astronomical Journa

Multispeckle diffusing-wave spectroscopy: a tool to study slow relaxation and time-dependent dynamics

A multispeckle technique for efficiently measuring correctly ensemble-averaged intensity autocorrelation functions of scattered light from non-ergodic and/or non-stationary systems is described. The method employs a CCD camera as a multispeckle light detector and a computer-based correlator, and permits the simultaneous calculation of up to 500 correlation functions, where each correlation function is started at a different time. The correlation functions are calculated in real time and are referenced to a unique starting time. The multispeckle nature of the CCD camera detector means that a true ensemble average is calculated; no time averaging is necessary. The technique thus provides a "snapshot" of the dynamics, making it particularly useful for non-stationary systems where the dynamics are changing with time. Delay times spanning the range from 1 ms to 1000 s are readily achieved with this method. The technique is demonstrated in the multiple scattering limit where diffusing-wave spectroscopy theory applies. The technique can also be combined with a recently-developed two-cell technique that can measure faster decay times. The combined technique can measure delay times from 10 ns to 1000 s. The method is peculiarly well suited for studying aging processes in soft glassy materials, which exhibit both short and long relaxation times, non-ergodic dynamics, and slowly-evolving transient behavior.Comment: 11 pages 13 figures Accepted in Review of Scientific Instrument (june 02

The N/O Plateau of Blue Compact Galaxies: Monte Carlo Simulations of the Observed Scatter

Chemical evolution models and Monte Carlo simulation techniques have been combined for the first time to study the distribution of blue compact galaxies on the N/O plateau. Each simulation comprises 70 individual chemical evolution models. For each model, input parameters relating to a galaxy's star formation history (bursting or continuous star formation, star formation efficiency), galaxy age, and outflow rate are chosen randomly from ranges predetermined to be relevant. Predicted abundance ratios from each simulation are collectively overplotted onto the data to test its viability. We present our results both with and without observational scatter applied to the model points. Our study shows that most trial combinations of input parameters, including a simulation comprising only simple models with instantaneous recycling, are successful in reproducing the observed morphology of the N/O plateau once observational scatter is added. Therefore simulations which include delay of nitrogen injection are no longer favored over those which propose that most nitrogen is produced by massive stars, if only the plateau morphology is used as the principal constraint. The one scenario which clearly cannot explain plateau morphology is one in which galaxy ages are allowed to range below 250 Myr. We conclude that the present data for the N/O plateau are insufficient by themselves for identifying the portion of the stellar mass spectrum most responsible for cosmic nitrogen production.Comment: 41 pages, 15 figures; accepted by ApJ, to appear Aug. 20, 200

The Spatial Distribution of Atomic Carbon Emission in the Giant Molecular Cloud NGC 604-2

We have mapped a giant molecular cloud in the giant HII region NGC 604 in M33 in the 492 GHz ^3P_1 -- ^3P_0 transition of neutral atomic carbon using the James Clerk Maxwell Telescope. We find the distribution of the [CI] emission to be asymmetric with respect to the CO J=1--0 emission, with the peak of the [CI] emission offset towards the direction of the center of the HII region. In addition, the line ratio I_{[CI]}/I_{CO} is highest (~ 0.2) facing the HII region and lowest (< 0.1) away from it. These asymmetries indicate an edge-on morphology where the [CI] emission is strongest on the side of the cloud facing the center of the HII region, and not detected at all on the opposite side This suggests that the sources of the incident flux creating C from the dissociation of CO are the massive stars of the HII region. The lowest line ratios are similar to what is observed in Galactic molecular clouds, while the highest are similar to starburst galaxies and other regions of intense star formation. The column density ratio, N(C)/N(H_2) is a few times 10^{-6}, in general agreement with models of photodissociation regions.Comment: Accepted for publication in ApJ. 8 pages, 5 figures, 3 table

The Effect of Star Formation on Molecular Clouds in Dwarf Irregular Galaxies: IC 10 and NGC 6822

We have observed the 13CO J=2-1, 12CO J=2-1 and 12CO J=3-2 lines at a few locations in the dwarf irregular galaxies IC 10 and NGC 6822 using the James Clerk Maxwell Telescope. In addition, we report the first detection of the 13CO J=3-2 transition in a Local Group galaxy. These low metallicity environments appear to be porous to UV radiation and allow for more efficient heating of molecular gas by nearby HII regions. The high 12CO J=3-2/J=2-1 ratio in NGC 6822 suggests that the 12CO emission is optically thin in this region. This high line ratio is likely the result of its location inside a large HII region with low metallicity and low gas content. In IC 10 we observe structures on a variety of size scales that all appear to be gravitationally bound. This effect may help explain the rather high star formation rate in IC 10.Comment: 20 pages with 6 ps figures, accepted for publication in The Astrophysical Journa

Elastic consequences of a single plastic event : a step towards the microscopic modeling of the flow of yield stress fluids

With the eventual aim of describing flowing elasto-plastic materials, we focus on the elementary brick of such a flow, a plastic event, and compute the long-range perturbation it elastically induces in a medium submitted to a global shear strain. We characterize the effect of a nearby wall on this perturbation, and quantify the importance of finite size effects. Although for the sake of simplicity most of our explicit formulae deal with a 2D situation, our statements hold for 3D situations as well.Comment: submitted to EPJ