Constraining the Circumbinary Envelope of Z CMa via imaging polarimetry

Z CMa is a complex binary system, composed of a Herbig Be and an FU Ori star. The Herbig star is surrounded by a dust cocoon of variable geometry, and the whole system is surrounded by an infalling envelope. Previous spectropolarimetric observations have reported a preferred orientation of the polarization angle, perpendicular to the direction of a large, parsec-sized jet associated with the Herbig star. The variability in the amount of polarized light has been associated to changes in the geometry of the dust cocoon that surrounds the Herbig star. We aim to constrain the properties of Z CMa by means of imaging polarimetry at optical wavelengths. Using ExPo, a dual-beam imaging polarimeter which operates at optical wavelengths, we have obtained imaging (linear) polarimetric data of Z CMa. Our observations were secured during the return to quiescence after the 2008 outburst. We detect three polarized features over Z CMa. Two of these features are related to the two jets reported in this system: the large jet associated to the Herbig star, and the micro-jet associated to the FU Ori star. Our results suggest that the micro-jet extends to a distance ten times larger than reported in previous studies. The third feature suggests the presence of a hole in the dust cocoon that surrounds the Herbig star of this system. According to our simulations, this hole can produce a pencil beam of light that we see scattered off the low-density envelope surrounding the system.Comment: Accepted for publication in A\&

The color dependent morphology of the post-AGB star HD161796

Context. Many protoplanetary nebulae show strong asymmetries in their surrounding shell, pointing to asymmetries during the mass loss phase. Questions concerning the origin and the onset of deviations from spherical symmetry are important for our understanding of the evolution of these objects. Here we focus on the circumstellar shell of the post-AGB star HD 161796. Aims. We aim at detecting signatures of an aspherical outflow, as well as to derive the properties of it. Methods. We use the imaging polarimeter ExPo (the extreme polarimeter), a visitor instrument at the William Herschel Telescope, to accurately image the dust shell surrounding HD 161796 in various wavelength filters. Imaging polarimetry allows us to separate the faint, polarized, light from circumstellar material from the bright, unpolarized, light from the central star. Results. The shell around HD 161796 is highly aspherical. A clear signature of an equatorial density enhancement can be seen. This structure is optically thick at short wavelengths and changes its appearance to optically thin at longer wavelengths. In the classification of the two different appearances of planetary nebulae from HST images it changes from being classified as DUPLEX at short wavelengths to SOLE at longer wavelengths. This strengthens the interpretation that these two appearances are manifestations of the same physical structure. Furthermore, we find that the central star is hotter than often assumed and the relatively high observed reddening is due to circumstellar rather than interstellar extinction.Comment: Accepted for publication in A&

The Extreme Polarimeter: Design, Performance, First Results & Upgrades

Well over 700 exoplanets have been detected to date. Only a handful of these have been observed directly. Direct observation is extremely challenging due to the small separation and very large contrast involved. Imaging polarimetry offers a way to decrease the contrast between the unpolarized starlight and the light that has become linearly polarized after scattering by circumstellar material. This material can be the dust and debris found in circumstellar disks, but also the atmosphere or surface of an exoplanet. We present the design, calibration approach, polarimetric performance and sample observation results of the Extreme Polarimeter, an imaging polarimeter for the study of circumstellar environments in scattered light at visible wavelengths. The polarimeter uses the beam-exchange technique, in which the two orthogonal polarization states are imaged simultaneously and a polarization modulator swaps the polarization states of the two beams before the next image is taken. The instrument currently operates without the aid of Adaptive Optics. To reduce the effects of atmospheric seeing on the polarimetry, the images are taken at a frame rate of 35 fps, and large numbers of frames are combined to obtain the polarization images. Four successful observing runs have been performed using this instrument at the 4.2 m William Herschel Telescope on La Palma, targeting young stars with protoplanetary disks as well as evolved stars surrounded by dusty envelopes. In terms of fractional polarization, the instrument sensitivity is better than 10^-4. The contrast achieved between the central star and the circumstellar source is of the order 10^-6. We show that our calibration approach yields absolute polarization errors below 1%

The inner environment of Z~CMa: High-Contrast Imaging Polarimetry with NaCo

Context. Z\,CMa is a binary composed of an embedded Herbig Be and an FU Ori class star separated by $\sim100$ au. Observational evidence indicate a complex environment in which each star has a circumstellar disk and drives a jet, and the whole system is embedded in a large dusty envelope. Aims. We aim to probe the circumbinary environment of Z\,CMa in the inner 400 au in scattered light. Methods. We use high contrast imaging polarimetry with VLT/NaCo at $H$ and $K_s$ bands. Results. The central binary is resolved in both bands. The polarized images show three bright and complex structures: a common dust envelope, a sharp extended feature previously reported in direct light, and an intriguing bright clump located 0\farcs3 south of the binary, which appears spatially connected to the sharp extended feature. Conclusions.We detect orbital motion when compared to previous observations, and report a new outburst driven by the Herbig star. Our observations reveal the complex inner environment of Z\,CMa with unprecedented detail and contrast.Comment: Accepted for publication in A&A Letter

Evolution annuelle du peuplement zooplanctonique dans un lagunage à haut rendement et incidence du temps de séjour

Proposé au début des années soixante par W.J. OSWALD, le lagunage dit « à haut rendement », utilisé pour l'épuration d'eaux usées domestiques, représente un écosystème hypereutrophe. Il se distingue considérablement du lagunage naturel par des temps de séjour très courts (2 à 12 jours), de très faibles profondeurs (< 1 m) et par une agitation constante de façon à favoriser le développement algal.Cette étude a pour objectifs de : 1) caractériser les successions saisonnières du zooplancton dans un écosystème où celles-ci n'ont pratiquement pas été étudiées et 2) étudier l'impact, sur la structure du peuplement et au fil des saisons, du temps de séjour des eaux à traiter.Le suivi du peuplement zooplanctonique a été réalisé pendant une période de deux ans dans deux bassins à haut rendement (nommés A et B) de même configuration alimentés en eaux usées. Les temps de résidence des eaux à traiter sont constants et de huit jours dans l'un, variable et ajusté selon l'ensoleillement et la charge entrante dans l'autre.Les résultats obtenus sont analysés par une classification hiérarchique avec contrainte de contiguïté temporelle. Les groupes obtenus sont ensuite testés par l'analyse discriminante. Les résultats sont regroupés chronologiquement : en six groupes pour le bassin B et en dix groupes pour le bassin A. L'analyse discriminante montre que les Crustacés et les Rotifères sont respectivement responsables des regroupements dans les bassins B et A. Pendant la période de démarrage du lagunage ainsi qu'en automne et en hiver, le zooplancton est principalement composé de Protozoaires : Ciliés libres ou fixés en fortes densités (jusqu'à 2,7.107 ind. 1-1) et Rhizopodes (Amibes). Au printemps et en été, lorsque le rayonnement solaire devient important, la production primaire étant optimale, les Protozoaires sont remplacés par des brouteurs herbivores métazoaires. La production de Rotifères et de Daphnies dont les exigences en nourriture (microalgues) sont considérables est importante au printemps ou bien en été uniquement dans le bassin à temps de séjour court. Le développement des Crustacés Copépodes et Ostracodes est favorisé en été dans le bassin à temps de séjour plus long.Tous ces organismes participent activement à réparation. ils se nourrissent pour la plupart en filtrant les éléments en suspension dans le milieu et participent directement à l'élimination des MES et de la DCO (PIZAY-PARENTY, 1985).II devient donc envisageable, par suite du rôle non négligeable joué par le zooplancton et surtout par Daphnia magna, de concevoir une gestion du lagunage qui favoriserait le développement de ce Cladocère. Cette biomasse facilement récupérable par tamisage peut être valorisée en aquaculture.High rate ponds for wastewater treatment have been the focus of much attention since their creation by W.J. OSWALD in the early 1960's. These aquatic ecosystems are hypereutrophic because of massive nutrient introduction by wastewater influents. These kinds of ponds are very different front natural biological ponds because of short residence times (2 to 12 days), shallow depths (<1 m) and constant mechanical mixing which improve alga : growth.This study presents the following objectives : 1) to characterize seasonal successions of zooplankton in this ecosystem where they are unknown and 2) to study residence time influence on zooplanktonic successions in function of seasons. Our investigation consists in studying two similar high rate ponds of 48 m2 surface area and 35 cm depth constantly stirred by paddle wheels.Only residence time is different. The first pond has eight days residence time and in the second one, it changes with solar radiations and organic matter concentrations. The influents come from a primary pond of eight days residence time, fed continuously with domestic influents front a small town, Mèze. The aim of this experiment is to compare seasonal successions in these two high rate ponds (called A and B) with different residence times. Zooplanktonic investigation has been carried out for two years. One sample was collected twice a week in primary pond effluent and two others samples collected once a week into the two high rate ponds near the outflow areas. The results of chronological clustering take inter account the discontinuities of zooplanktonic structure. We obtain six sequences in B pond and ten groups in A pond. Stepwise discriminant analysis shows that Crustaceans and Rotifers are respectively responsible for chronological clustering in B and in A pond. During the sterling period of wastewater treatment and in autumn and in winter, photosynthesis is low and zooplankton is composed mainly of Protozoa : free-living or fixed Ciliates in great concentrations (until 2,7.107 ind. l-1) and Rhizopods (Amæba sp.). These organisms have important food needs (bacteria and algal detritus). During spring and summer, as solar radiations become important, Protozoa are replaced by metazoan grazers. Spring or summer with short residence times increase Rotifers or Daphnia magna production white summer with longer residence times increase Cyclops sp and cypris ovum production. All these organisms are active components of wastewater treatment : the consumption of particulates by zooplankton Increases TSS and COD removal efficiencies (PIZAY-PARENTY, 1985).Therefore, because of the filtering activity of Crustaceans (especially of Daphnia magna), it seems possible to conceive a new pond managment, with residence time changes, favourable to exponential algal growth and consequently to crustacean biomass production. It is easier to harvest this biomass (with nets) than to harvest algal biomass, which still presents problems

The First Science Results from SPHERE: Disproving the Predicted Brown Dwarf around V471 Tau

Variations of eclipse arrival times have recently been detected in several post common envelope binaries consisting of a white dwarf and a main sequence companion star. The generally favoured explanation for these timing variations is the gravitational pull of one or more circumbinary substellar objects periodically moving the center of mass of the host binary. Using the new extreme-AO instrument SPHERE, we image the prototype eclipsing post-common envelope binary V471 Tau in search of the brown dwarf that is believed to be responsible for variations in its eclipse arrival times. We report that an unprecedented contrast of 12.1 magnitudes in the H band at a separation of 260 mas was achieved, but resulted in a non-detection. This implies that there is no brown dwarf present in the system unless it is three magnitudes fainter than predicted by evolutionary track models, and provides damaging evidence against the circumbinary interpretation of eclipse timing variations. In the case of V471 Tau, a more consistent explanation is offered with the Applegate mechanism, in which these variations are prescribed to changes in the quadrupole moment within the main-sequence sta

Direct imaging of a massive dust cloud around R Coronae Borealis

We present recent polarimetric images of the highly variable star R CrB using ExPo and archival WFPC2 images from the HST. We observed R CrB during its current dramatic minimum where it decreased more than 9 mag due to the formation of an obscuring dust cloud. Since the dust cloud is only in the line-of-sight, it mimics a coronograph allowing the imaging of the star's circumstellar environment. Our polarimetric observations surprisingly show another scattering dust cloud at approximately 1.3" or 2000 AU from the star. We find that to obtain a decrease in the stellar light of 9 mag and with 30% of the light being reemitted at infrared wavelengths (from R CrB's SED) the grains in R CrB's circumstellar environment must have a very low albedo of approximately 0.07%. We show that the properties of the dust clouds formed around R CrB are best fitted using a combination of two distinct populations of grains size. The first are the extremely small 5 nm grains, formed in the low density continuous wind, and the second population of large grains (~0.14 {\mu}m) which are found in the ejected dust clouds. The observed scattering cloud, not only contains such large grains, but is exceptionally massive compared to the average cloud.Comment: 8 pages, 7 figures published in A&