Error Analysis for Dual-Beam Optical Linear Polarimetry

In this paper we present an error analysis for polarimetric data obtained with dual-beam instruments. After recalling the basic concepts, we introduce the analytical expressions for the uncertainties of polarization degree and angle. These are then compared with the results of Monte-Carlo simulations, which are also used to briefly discuss the statistical bias. Then we approach the problem of background subtraction and the errors introduced by a non-perfect Wollaston prism, flat-fielding and retarder plate defects. We finally investigate the effects of instrumental polarization and we propose a simple test to detect and characterize it. The application of this method to real VLT-FORS1 data has shown the presence of a spurious polarization, which is of the order of 1.5% at the edges of the field of view. The cause of this effect has been identified with the presence of rather curved lenses in the collimator, combined with the non complete removal of reflections by the coatings. This problem is probably common to all focal-reducer instruments equipped with a polarimetric mode. An additional spurious and asymmetric polarization field, whose cause is still unclear, is visible in the B band.Comment: 13 pages, 11 figures. Accepted for publication in PAS

VLT Spectropolarimetry of the fast expanding Type Ia SN2006X

Using VLT-FORS1 we performed optical spectropolarimetric observations of the Type Ia SN2006X on 7 pre-maximum epochs (day -10 to day -1) and one post-maximum epoch (+39 days). The SN shows strong continuum interstellar polarization reaching about 8% at 4000A, characterized by a wavelength dependency that is substantially different from that of the Milky-Way dust mixture. Several SN features, like SiII 6355A and the CaII IR triplet, present a marked evolution. The CaII near-IR triplet shows a pronounced polarization (~1.4%) already on day -10 in correspondence with a strong high-velocity feature (HVF). The SiII polarization peaks on day -6 at about 1.1% and decreases to 0.8% on day -1. By day +39 no polarization signal is detected for the SiII line, while the CaII IR triplet shows a marked re-polarization at the level of 1.2%. As in the case of another strongly polarized SN (2004dt), no polarization was detected across the OI 7774A absorption. The fast-expanding SN2006X lies on the upper edge of the relation between peak polarization and decline rate, and it confirms previous speculations about a correlation between degree of polarization, expansion velocity, and HVF strength. The polarization of CaII detected in our last epoch, the most advanced ever obtained for a Type Ia SN, coincides in velocity with the outer boundary of the Ca synthesized during the explosion (15,000-17,000 km/s) in delayed-detonation models. This suggests a large scale chemical inhomogeneity as produced by off-center detonations, a rather small amount of mixing, or a combination of both effects. In contrast, the absence of polarization at the inner edge of the Ca-rich layer (8000-10,000 km/s) implies a substantial amount of mixing in these deeper regions.Comment: 28 pages, 39 figures, accepted for publication in Astronomy and Astrophysic

Bow shocks, Wiggling Jets, and Wide-Angle Winds: A High Resolution Study of the Entrainment Mechanism of the PV Ceph Molecular (CO) Outflow

We present a new set of high-resolution molecular line maps of the gas immediately surrounding various Herbig-Haro (HH) knots of the giant HH flow HH 315, from the young star PV Cephei. The observations, aimed at studying the entrainment mechanism of the 2.6 pc-long HH 315 flow, include IRAM 30m maps of the 12CO(2-1), 12CO(1-0), and 13CO(1-0) lines, with beam sizes of 11'', 21'', and 22'', respectively. We compare the morphology and the kinematics of the outflow gas, as well as the temperature and momentum distribution of the molecular outflow with those predicted by different entrainment models. With our detailed study we are able to conclude that jet bow shock entrainment by an episodic stellar wind, with a time-varying axis, produces most of the high-velocity molecular outflow observed far from the source. In addition, near PV Cephei we find evidence for a poorly collimated, wide-angle, molecular outflow and a collimated wiggling jet-like molecular outflow. We propose that the poorly collimated component is entrained by a wide-angle wind, and the collimated component is entrained by a variable jet with internal working surfaces. If this picture is true, then a stellar wind model which allows for the coexistence of a wide-angle component and a collimated (jet-like) stellar wind component is needed to explain the observed properties of the PV Ceph outflow. The wiggling axis of the redshifted molecular outflow lobe indicates that the outflow ejection axis is changing over time. We find that the time-scale of the axis variation shown by the molecular outflow lobe is about a factor of 10 less than that shown by the large-scale optical HH knots.Comment: 41 pages, including 7 tables. 18 figures included separately. Version with embedded full-resolution figures available at http://www.astro.caltech.edu/~harce/papers . Accepted by The Astrophysical Journa

Erratum to: 36th International Symposium on Intensive Care and Emergency Medicine

[This corrects the article DOI: 10.1186/s13054-016-1208-6.]

Prediction of extreme temperatures in a reactor using measurements affected by control action.

In a nuclear reactor, the coolant temperatures at 4,000 channel outlets are important for risk assessment. Temperatures measurements are made at only one in nine channels, and predictions of the temperatures are required at the remaining channels. The measured temperatures do not exceed an upper limit imposed by control action. Their histogram therefore provides no direct information about the distribution above this limit of the temperatures at the remaining channels. We predict the unmeasured temperatures by spatial interpolation of a surface fitted to the measured temperatures and modeling of the distribution of the residuals from this fit