Low-ionization structures in planetary nebulae - I: physical, kinematic and excitation properties

Though the small-scale, low-ionization knots, filaments and jets (LISs) of planetary nebulae (PNe) are known for ~30yr, some of their observational properties are not well established. In consequence our ability to include them in the wider context of the formation and evolution of PNe is directly affected. Why most structures have lower densities than the PN shells hosting them? Is their intense emission in low-ionization lines the key to their main excitation mechanism? Therefore, if considered altogether, can LISs line ratios, chemical abundances and kinematics enlighten the interplay between the different excitation and formation processes? Here we present a spectroscopic analysis of five PNe that possess LISs confirming that all nebular components have comparable electron temperatures, whereas the electron density is systematically lower in LISs than in the surrounding nebula. Chemical abundances of LISs versus other PN components do not show significant differences as well. By using diagnostic diagrams from shock models, we demonstrate that LISs' main excitation is due to shocks, whereas the other components are mainly photo-ionized. We also propose new diagnostic diagrams involving a few emission lines ([NII], [OIII], [SII]) and $\rm{log}$(f$_{shocks}$/f$_{\star}$), where f$_{shocks}$ and f$_{\star}$ are the ionization photon fluxes due to the shocks and the central star ionizing continuum, respectively. A robust relation differentiating the structures is found, with the shock-excited clearly having $\rm{log}$(f$_{shocks}$/f$_{\star}$)$>$-1; while the photo-ionized show $\rm{log}$(f$_{shocks}$/f$_{\star}$)$<$-2. A transition zone, with -2$<\rm{log}$(f$_{shocks}$/f$_{\star}$)$<$-1 where both mechanisms are equally important, is also defined.Comment: 26 pages, 11 figures, MNRAS accepte

On the nitrogen abundance of FLIERs: the outer knots of the planetary nebula NGC 7009

We have constructed a 3D photoionisation model of a planetary nebula (PN) similar in structure to NGC 7009 with its outer pair of knots (also known as FLIERs --fast, low-ionization emission regions). The work is motivated by the fact that the strong [N II]6583A line emission from FLIERs in many planetary nebulae has been attributed to a significant local overabundance of nitrogen. We explore the possibility that the apparent enhanced nitrogen abundance previously reported in the FLIERs may be due to ionization effects. Our model is indeed able to reproduce the main spectroscopic and imaging characteristics of NGC 7009's bright inner rim and its outer pairs of knots, assuming homogeneous elemental abundances throughout the nebula, for nitrogen as well as all the other elements included in the model. Because of the fact that the (N+/N)/(O+/O) ratio predicted by our models are 0.60 for the rim and 0.72 for the knots, so clearly in disagreement with the N+/N=O+/O assumption of the ionization correction factors method (icf), the icfs will be underestimated by the empirical scheme, in both components, rim and knots, but more so in the knots. This effect is partly responsible for the apparent inhomogeneous N abundance empirically derived. The differences in the above ratio in these two components of the nebula may be due to a number of effects including charge exchange --as pointed out previously by other authors-- and the difference in the ionization potentials of the relevant species --which makes this ratio extremely sensitive to the shape of the local radiation field. Because of the latter, a realistic density distribution is essential to the modelling of a non-spherical object, if useful information is to be extracted from spatially resolved observations, as in the case of NGC 7009.Comment: 12 pages including 8 tables and 2 figures. MNRAS in pres

Qualitative evaluation of a conformal velocity vector display for use at high angles-of-attack in fighter aircraft

A piloted simulation study was conducted to evaluate the utility of a display device designed to illustrate graphically and conformally the approximate location of a fighter aircraft's velocity vector. The display device consisted of two vertical rows of light emitting diodes (LED's) located toward the center of the cockpit instrument panel to each side of the control stick. The light strings provided a logical extension of the head up display (HUD) velocity vector symbol at flight path angles which exceeded the HUD field-of-view. Four test subjects flew a modified F/A-18 model with this display in an air-to-air engagement task against an equally capable opponent. Their responses to a questionnaire indicated that the conformal velocity vector information could not be used during the scenarios investigated due to the inability to visually track a target and view the lights simultaneously

Do we really know how to derive the basic PNe parameters?

How well do we know the physical/chemical properties of PNe? 1D (CLOUDY) and 3D (MOCASSIN) photoionisation codes are used in this contribution to model the PNe K 4-47 and NGC 7009 as an attempt to question whether or not the high Te (higher than 21,000K) of the K 4-47's core and the N overabundance of the outer knots in NGC 7009 are real. These are very basic parameters, obtained for Galactic PNe, e.g. nearby objects, even though with large uncertainties. Based on the comparison of the modelling with, mainly, optical images and long-slit spectroscopic data, it is suggested here that K 4-47 high Te can be explained if its core is composed of a very dense and small inner region --that matches the radio measurements-- and a lower density outer core --matching the optical observations. This approach can account for the strong auroral emission lines [OIII]4363A and [NII]5755A observed, and so for the high temperatures. This teaches us that the assumption of a homogeneous distribution of the gas is completely wrong for the core of such PN. In the case of NGC 7009 a simple 3D model that reproduces the observed geometry of this nebula is constructed. The aim of this modelling was to explore the possibility that the enhanced [NII] emission observed in the outer knots may be due to ionisation effects instead to a local N overabundance. Here it is discussed the model that can best reproduce the observations employing a homogeneous set of abundances throughout the nebula, not only for nitrogen but also for all the other elements considered.Comment: 4 pages; talk presented in the international conference Planetary Nebulae as Astronomical Tools; June 28 - July 02, 2005; Gdansk, Poland; Eds: R. Szczerba, G. Stasinska, and S. K. Gorny; AIP Conference Proceeding

Survey of currently available high-resolution raster graphics systems

Presented are data obtained on high-resolution raster graphics engines currently available on the market. The data were obtained through survey responses received from various vendors and also from product literature. The questionnaire developed for this survey was basically a list of characteristics desired in a high performance color raster graphics system which could perform real-time aircraft simulations. Several vendors responded to the survey, with most reporting on their most advanced high-performance, high-resolution raster graphics engine

Simulator comparison of thumball, thumb switch, and touch screen input concepts for interaction with a large screen cockpit display format

A piloted simulation study was conducted comparing three different input methods for interfacing to a large screen, multiwindow, whole flight deck display for management of transport aircraft systems. The thumball concept utilized a miniature trackball embedded in a conventional side arm controller. The multifunction control throttle and stick (MCTAS) concept employed a thumb switch located in the throttle handle. The touch screen concept provided data entry through a capacitive touch screen installed on the display surface. The objective and subjective results obtained indicate that, with present implementations, the thumball concept was the most appropriate for interfacing with aircraft systems/subsystems presented on a large screen display. Not unexpectedly, the completion time differences between the three concepts varied with the task being performed, although the thumball implementation consistently outperformed the other two concepts. However, pilot suggestions for improved implementations of the MCTAS and touch screen concepts could reduce some of these differences

Seasonal adjustment and the detection of business cycle phases

To date, there has been little investigation of the impact of seasonal adjustment on the detection of business cycle expansion and recession regimes. We study this question both analytically and through Monte Carlo simulations. Analytically, we view the occurrence of a single business cycle regime as a structural break that is later reversed, showing that the effect of the linear symmetric X-11 filter differs with the duration of the regime. Through the use of Markov switching models for regime identification, the simulation analysis shows that seasonal adjustment has desirable properties in clarifying the true regime when this is well underway, but it distorts regime inference around turning points, with this being especially marked after the end of recessions and when the one-sided X-11 filter is employed. JEL Classification: E32, C22, C80business cycles, Markov switching models, Seasonal adjustment

Does Seasonality Change over the Business Cycle? An Investigation using Monthly Industrial Production Series

This paper examines the proposition that the business cycle affects seasonality in industrial production, with output being switched to the traditionally low production summer months when recent (annual) growth has been strong. This is investigated through the use of a restricted threshold autoregressive model for the monthly growth rate in a total of 74 industries from 16 OECD countries. Approximately one third of the series exhibit significant nonlinearity, with this nonlinearity predominantly associated with changes in the seasonal pattern. Estimates show that the summer slowdown in many European countries is substantially reduced when recent growth has been high.

The first near infrared detection of XTE J1118+480 in quiescence

We report the first quiescent detection of KV UMa, the optical counterpart of XTE J1118+480 at near infrared. The observed magnitudes and colours are consistent with a K7-M0 V star, at the distance 1.4 +/- 0.2 kpc. The light curve shows strong orbital modulation with possible contamination from a superhump detected in the quiescent optical light curves.Comment: To appear in MNRAS Letters; 4 pages, 3 figure