Acquisition and analysis of adaptive optics imaging polarimetry data

The process of data taking, reduction and calibration of near-infrared imaging polarimetry data taken with the ESO Adaptive Optics System ADONIS is described. The ADONIS polarimetric facility is provided by a rotating wire grid polarizer. Images were taken at increments of 22.5 degrees of polarizer rotation from 0 to 180 degrees, over-sampling the polarization curve but allowing the effects of photometric variations to be assessed. Several strategies to remove the detector signature are described. The instrumental polarization was determined, by observations of stars of negligible polarization, to be 1.7% at J, H and K bands. The lack of availability of unpolarized standard stars in the IR, in particular which are not too bright as to saturate current IR detectors, is highlighted. The process of making polarization maps is described. Experiments at restoring polarimetry data, in order to reach diffraction limited polarization, are outlined, with particular reference to data on the Homunculus reflection nebula around Eta Carinae.Comment: 20 pages, A&A LaTeX2e, 11 figures. To appear in Astronomy & Astrophysics, Supplement Serie

Flow and air-entrainment around partially submerged vertical cylinders

In this study, a partially submerged vertical cylinder is moved at constant velocity through water, which is initially at rest. During the motion, the wake behind the cylinder induces free-surface deformation. Eleven cylinders, with diameters from $D=1.4$ to 16 cm, were tested at two different conditions: (i) constant immersed height $h$ and (ii) constant $h/D$. The range of translation velocities and diameters are in the regime of turbulent wake with experiments carried out for $4500<Re<240 \,000$ and $0.2<Fr<2.4$, where $Re$ and $Fr$ are the Reynolds and Froude numbers based on $D$. The focus here is on drag force measurements and relatively strong free-surface deformation up to air-entrainment. Specifically, two modes of air-entraiment have been uncovered: (i) in the cavity along the cylinder wall and (ii) in the wake of the cylinder. A scaling for the critical velocity for air-entrainment in the cavity has been observed in agreement with a simple model. Furthermore, for $Fr>1.2$, the drag force varies linearly with $Fr$

Studies on electrochromatic materials and devices

This thesis investigates electrochromic thin films needed to construct a variable transmission electro chromic device. Such a device is made of 5 layers sandwiched between 2 pieces of glass: two electronic transparent conducting layers, an optically active electro chromic layer (W03), a ion-conducting polymer electrolyte and an ionstorage layer (NiOx, TiOx, VOx, VzTiyOx) . Electrochromic NiOx thin films were produced by R.F. magnetron sputtering and electrodeposition techniques and studied under proton intercalation. A visible transmittance modulation of 0.70 and 0.80 and a visible coloration efficiency of 35 and 100 cm2.C-1 for a thickness of 300 and 200 nm were obtained for sputtered and chemically-deposited NiOx films respectively. Anodic films are extremely porous and soft. Under the mechanical stresses of ionic insertion/extraction they degrade more quickly than the compact nanostructure of physically deposited films. When studied under lithium intercalation, sputtered NiOx films exhibit a nucleation loop observed in cyclic voltammetry indicating the growth of a new phase and are seen to degrade quickly. NiOx films were not seen to be potential candidates for EC applications using Lt intercalation. W03, TiOx and VOx thin films were deposited by R.F. magnetron sputtering and studied under Lt intercalation/deintercalation. Optimised W03 films exhibited good electro chromic properties: a visible transmittance modulation of 0.82 and a visible coloration efficiency of 49 cm2 . C-l for a thickness of 450 nm. Electrochromic properties of TiOx films were seen to not strongly depend on the sputtering process parameters whereas VOx films showed a stronger dependence. TiOx films are able to store a limited quantity of charge Q = 13 mC.cm-2 for thicknesses greater than 13 nm. They are transparent in both charged and uncharged states T V,u and Tv,ch> 0.80, and are stable upon charge insertion/extraction. VOx films can store a much larger quantity of charge Q = 35 mC.cm-2 for a thickness of 70 nm. They are yellow in the uncharged state and bluish in the charged state: Tv,u and Tv,ch > 0.70, and the charge insertion/extraction process is seen to evolve during the initial cycles. Both TiOx and VOx films did not show all the required electrochromic properties for EC applications. The main achievement of this work was the development of highly durable vanadium/titanium mixed oxide thin films. Work was carried out on different VITi ratios using specific deposition techniques developed for that purpose. Films with a vanadium to titanium ratio of about 50 % showed optimum performance characteristics for passive ion storage layer applications. Such layers deposited on ITO exhibited high visible transmittance: Tv,ch > 0.62, and a relatively low visible modulation (0.20), with high storage capacity Q > 40 mC.cm-2 for a thickness of80 nm. The laminated W03IPAAUAlVzTiyOx EC device was assembled and exhibited under specific switching conditions encouraging properties: a visible transmittance modulation > 0.50 over more than 105 cycles

Direct current plasma spraying of mechanofused alumina-steel particles

Stainless steel particles (60 $\mu$m in mean diameter) cladded with an alumina shell (2 $\mu$m thick and manufactured by mechanofusion) were sprayed with an Ar-H2 (53-7 slm) d.c. plasma jet (I = 500 A, P = 28 kW, \rho_th = 56 %). Two main types of particles were collected in flight, as close as 50 mm downstream of the nozzle exit: particles with a steel core with pieces of alumina unevenly distributed at their surface and those consisting of a spherical stainless steel particle with an alumina cap. The plasma flow was modeled by a 2D steady parabolic model and a single particle trajectory by using the 3D Boussinesq-Oseen-Basset equation. The heat transfer, within the two-layer, stainless steel cladded by alumina, particle, considered the heat propagation phenomena including phase changes. The models allowed determining the positions, along the particle trajectory, where the convective movement could occur as well as the entrainment of the liquid oxide to the leading edge of the in-flight particles. The heat transfer calculations showed the importance of the thermal contact resistance TCR between alumina and steel

Young stars and dust in AFGL437: NICMOS/HST polarimetric imaging of an outflow source

We present near infrared broad band and polarimetric images of the compact star forming cluster AFGL437 obtained with the NICMOS instrument aboard HST. Our high resolution images reveal a well collimated bipolar reflection nebulosity in the cluster and allow us to identify WK34 as the illuminating source. The scattered light in the bipolar nebulosity centered on this source is very highly polarized (up to 79%). Such high levels of polarization implies a distribution of dust grains lacking large grains, contrary to the usual dust models of dark clouds. We discuss the geometry of the dust distribution giving rise to the bipolar reflection nebulosity and make mass estimates for the underlying scattering material. We find that the most likely inclination of the bipolar nebulosity, south lobe inclined towards Earth, is consistent with the inclination of the large scale CO molecular outflow associated with the cluster, strengthening the identification of WK34 as the source powering it.Comment: 26 pages, 10 figues. Accepted for publication in the Astrophysical Journa

Optical and infrared properties of V1647 Orionis during the 2003-2006 outburst. II. Temporal evolution of the eruptive source

The occurrence of new FU Orionis-like objects is fundamental to understand the outburst mechanism in young stars and their role in star formation and disk evolution. Our work is aimed at investigating the properties of the recent outburst of V1647 Ori. Using optical and mid infrared long slit spectroscopy we monitored V1647 Ori in outburst between February 2004 and January 2006. The optical spectrum is characterized by Halpha and Hbeta in P-Cygni profile and by many weak FeI and FeII emission lines. Short timescale variability was measured in the continuum and line emission. On January 2006 we detected for the first time forbidden emission lines ([OI], [SII] and [FeII]). These lines are likely produced by an Herbig-Haro object driven by V1647 Ori. The mid infrared the spectrum of V1647 Ori is flat and featureless at all epochs. The SED changed drastically: the source was much redder in the early outburst than in the final phase. The magnitude rise and the SED of V1647 Ori resembles that of a FUor while the duration and recurrence of the outburst resemble that of a EXor. The optical spectrum is clearly distinct from either the absorption line spectrum of a FUor or the T Tauri-like spectrum of an EXor. Our data are consistent with a disk instability event which led to an increase of the mass accretion rate. The data also suggest the presence of a circumstellar envelope around the star+disk system. The peculiar N band spectrum might be explained by dust sublimation in the outer layers of the disk. The presence of the envelope and the outburst statistics suggest that these instability events occur only in a specific stage of a Class I source (e.g. in the transition phase to an optically visible star surrounded by a protoplanetary disk). We discuss the outburst mechanisms in term of the thermal instability model.Comment: 12 pages, 7 figures, accepted for publication in A&