Dynamics of liquid crystalline domains in magnetic field

We study microscopic single domains nucleating and growing within the coexistence region of the Isotropic (I) and Nematic (N) phases in magnetic field. By rapidly switching on the magnetic field the time needed to align the nuclei of sufficiently large size is measured, and is found to decrease with the square of the magnetic field. When the field is removed the disordering time is observed to last on a longer time scale. The growth rate of the nematic domains at constant temperature within the coexistence region is found to increase when a magnetic field is applied.Comment: 10 pages, 5 figures, unpublishe

Crystal nuclei and structural correlations in two-dimensional colloidal mixtures: experiment versus simulation

We examine binary mixtures of superparamagnetic colloidal particles confined to a two-dimensional water-air interface both by real-space experiments and Monte-Carlo computer simulations at high coupling strength. In the simulations, the interaction is modelled as a pairwise dipole-dipole repulsion. While the ratio of magnetic dipole moments is fixed, the interaction strength governed by the external magnetic field and the relative composition is varied. Excellent agreement between simulation and experiment is found for the partial pair distribution functions including the fine structure of the neighbour shells at high coupling. Furthermore local crystal nuclei in the melt are identified by bond-orientational order parameters and their contribution to the pair structure is discussed

Multiple light scattering in nematic liquid crystals

We present a rigorous treatment of the diffusion approximation for multiple light scattering in anisotropic random media, and apply it to director fluctuations in a nematic liquid crystal. For a typical nematic material, 5CB, we give numerical values of the diffusion constants $D_{\|}$ and $D_{\perp}$. We also calculate the temporal autocorrelation function measured in Diffusing Wave Spectroscopy.Comment: 5 pages RevTeX, 1 postscript figure, to be published in Phys. Rev. E (Rapid Communication

The elusive archaeology of Kongo urbanism: the case of Kindoki, Mbanza Nsundi (Lower Congo, DRC)

We present here results, analyses and an in-depth historical contextualisation of the fieldwork undertaken in 2012 and 2013 at the Kindoki site in the Lower Congo (DRC). This site is linked with Mbanza Nsundi, one of the Kongo Kingdom's provincial capitals, which turns out to be archaeologically 'elusive'. Pinpointing its location proved to be particularly challenging. To this end, a historically-informed excavation methodology was developed that was never implemented in Central Africa before. We combined a strategy of systematic test pits with a large-scale 50 m grid approach. A cemetery was identified on Kindoki Hill with distinct but contemporaneous quarters of a 16th-17thcenturies settlement on both sides. The cemetery itself contains mainly 18th-century burials, in all likelihood of successive Nsundi rulers. The foreign, especially Portuguese, ceramics excavated on the hilltop and the hundreds of Venetian and likely Bavarian beads found in the graves are indicative of Mbanza Nsundi's connection to trade routes linking the Atlantic coast with the Pool region. The most striking discovery is that of a previously unknown type of comb-impressed pottery, from a pit with a calibrated radiocarbon date AD 1294-1393 (2 sigma). This suggests that a settlement had been developing at Kindoki since at least the 14th century, which allows us, for the very first time, to spatially bridge Kongo history and 'prehistory'. For the entire Lower Congo region only three 14C dates posterior to AD 1000 were available before the start of the KongoKing project, twelve have been added for just Kindoki

Detection of 6.7 GHz methanol absorption towards hot corinos

Methanol masers at 6.7 GHz have been found exclusively towards high-mass star forming regions. Recently, some Class 0 protostars have been found to display conditions similar to what are found in hot cores that are associated with massive star formation. These hot corino sources have densities, gas temperatures, and methanol abundances that are adequate for exciting strong 6.7 GHz maser emission. This raises the question of whether 6.7 GHz methanol masers can be found in both hot corinos and massive star forming regions, and if not, whether thermal methanol emission can be detected. We searched for the 6.7 GHz methanol line towards five hot corino sources in the Perseus region using the Arecibo radio telescope. To constrain the excitation conditions of methanol, we observed thermal submillimeter lines of methanol in the NGC1333-IRAS 4 region with the APEX telescope. We did not detect 6.7 GHz emission in any of the sources, but found absorption against the cosmic microwave background in NGC1333-IRAS 4A and NGC1333-IRAS 4B. Using a large velocity gradient analysis, we modeled the excitation of methanol over a wide range of physical parameters, and verify that the 6.7 GHz line is indeed strongly anti-inverted for densities lower than 10^6 cm^-3. We used the submillimeter observations of methanol to verify the predictions of our model for IRAS 4A by comparison with other CH3OH transitions. Our results indicate that the methanol observations from the APEX and Arecibo telescopes are consistent with dense (n ~ 10^6 cm^-3), cold (T ~ 15-30 K) gas. The lack of maser emission in hot corinos and low-mass protostellar objects in general may be due to densities that are much higher than the quenching density in the region where the radiation field is conducive to maser pumping.Comment: Accepted by A&

Strongly induced collapse in the Class 0 protostar NGC 1333 IRAS 4A

The onset of gravitational collapse in cluster-forming clouds is still poorly known. Our goal is to use the Class 0 protostar IRAS 4A, which is undergoing collapse in the active molecular cloud NGC 1333, to set constraints on this process. In particular we want to measure the mass infall rate and investigate whether the collapse could have been triggered by a strong external perturbation. We analyze existing continuum observations to derive the density structure of the envelope, and use our new molecular line observations done with the IRAM 30m telescope to probe its velocity structure. We perform a detailed comparison of this set of data with a numerical model of collapse triggered by a fast external compression. Both the density and velocity structures of the envelope can be well fitted by this model of collapse induced by a fast external compression for a time elapsed since point mass formation of 1-2 x 10**4 yr. We deduce a large mass infall rate of 0.7-2 x 10**-4 Msun/yr. The momentum required for the perturbation to produce this large mass infall rate is of the same order as the momenta measured for the NGC 1333 numerous outflows. Our analysis shows also that the turbulence is highly non uniform in the envelope, dropping from supersonic to subsonic values toward the center. The inner subsonic turbulence is most likely a relic of the conditions prevailing in the dense core before the onset of collapse. The vigorous collapse undergone by IRAS 4A was triggered by a fast external compression, probably related to the expansion of a nearby cavity, which could have triggered the collapse of the nearby Class 0 protostar IRAS 4B simultaneously. This cavity could have been generated by an outflow but we have not found a good protostellar candidate yet.Comment: Accepted by A&A. 12 pages, 10 figure

Water in low-mass star-forming regions with Herschel: HIFI spectroscopy of NGC1333

'Water In Star-forming regions with Herschel' (WISH) is a key programme dedicated to studying the role of water and related species during the star-formation process and constraining the physical and chemical properties of young stellar objects. The Heterodyne Instrument for the Far-Infrared (HIFI) on the Herschel Space Observatory observed three deeply embedded protostars in the low-mass star-forming region NGC1333 in several H2-16O, H2-18O, and CO transitions. Line profiles are resolved for five H16O transitions in each source, revealing them to be surprisingly complex. The line profiles are decomposed into broad (>20 km/s), medium-broad (~5-10 km/s), and narrow (<5 km/s) components. The H2-18O emission is only detected in broad 1_10-1_01 lines (>20 km/s), indicating that its physical origin is the same as for the broad H2-16O component. In one of the sources, IRAS4A, an inverse P Cygni profile is observed, a clear sign of infall in the envelope. From the line profiles alone, it is clear that the bulk of emission arises from shocks, both on small (<1000 AU) and large scales along the outflow cavity walls (~10 000 AU). The H2O line profiles are compared to CO line profiles to constrain the H2O abundance as a function of velocity within these shocked regions. The H2O/CO abundance ratios are measured to be in the range of ~0.1-1, corresponding to H2O abundances of ~10-5-10-4 with respect to H2. Approximately 5-10% of the gas is hot enough for all oxygen to be driven into water in warm post-shock gas, mostly at high velocities.Comment: Accepted for publication in the A&A HIFI special issu

Molecular line survey of the high-mass star-forming region NGC 6334I with Herschel/HIFI and the SMA

We aim at deriving the molecular abundances and temperatures of the hot molecular cores in the high-mass star-forming region NGC 6334I and consequently deriving their physical and astrochemical conditions. In the framework of the Herschel guaranteed time key program CHESS, NGC 6334I is investigated by using HIFI aboard the Herschel Space Observatory. A spectral line survey is carried out in the frequency range 480-1907 GHz, and auxiliary interferometric data from the SMA in the 230 GHz band provide spatial information for disentangling the different physical components contributing to the HIFI spectrum. The spectral lines are identified with the aid of former surveys and spectral line catalogs. The observed spectrum is then compared to a simulated synthetic spectrum with XCLASS, assuming local thermal equilibrium, and best fit parameters are derived using the model optimization package MAGIX. A total of 46 molecules are identified, with 31 isotopologues, resulting in about 4300 emission and absorption lines. High- energy levels of the dominant emitter methanol and vibrationally excited HCN are detected. The number of unidentified lines remains low with 75, or less than 2 percent of the lines detected. The modeling suggests that several spectral features need two or more components to be fitted properly. Other components could be assigned to cold foreground clouds or to outflows, most visible in the SiO emission. A chemical variation between the two embedded hot cores is found, with more N-bearing molecules identified in SMA1 and O-bearing molecules in SMA2. Spectral line surveys give powerful insights into the study of the interstellar medium. Different molecules trace different physical conditions like the inner hot core, the envelope, the outflows or the cold foreground clouds. The derived molecular abundances provide further constraints for astrochemical models.Comment: 30 pages including appendix, 49 figures, accepted for publication in Astronomy and Astrophysic

Multiple light scattering in anisotropic random media

In the last decade Diffusing Wave Spectroscopy (DWS) has emerged as a powerful tool to study turbid media. In this article we develop the formalism to describe light diffusion in general anisotropic turbid media. We give explicit formulas to calculate the diffusion tensor and the dynamic absorption coefficient, measured in DWS experiments. We apply our theory to uniaxial systems, namely nematic liquid crystals, where light is scattered from thermal fluctuations of the local optical axis, called director. We perform a detailed analysis of the two essential diffusion constants, parallel and perpendicular to the director, in terms of Frank elastic constants, dielectric anisotropy, and applied magnetic field. We also point out the relevance of our results to different liquid crystalline systems, such as discotic nematics, smectic-A phases, and polymer liquid crystals. Finally, we show that the dynamic absorption coefficient is the angular average over the inverse viscosity, which governs the dynamics of director fluctuations.Comment: 23 pages, 12 ps figures, to be published in Phys. Rev.