1,447 research outputs found
Measuring angular diameters of extended sources
When measuring diameters of partially resolved sources often a technique
called gaussian deconvolution is used. This technique yields a gaussian
diameter which subsequently has to be multiplied with a conversion factor to
obtain the true angular diameter of the source. This conversion factor is a
function of the FWHM of the beam or point spread function and also depends on
the intrinsic surface brightness distribution of the source.
In this paper conversion factors are presented for a number of simple
geometries: a circular constant surface brightness disk and a spherical
constant emissivity shell, using a range of values for the inner radius. Also
more realistic geometries are studied, based on a spherically symmetric
photo-ionization model of a planetary nebula. This enables a study of optical
depth effects, a comparison between images in various emission lines and the
use of power law density distributions. It is found that the conversion factor
depends quite critically on the intrinsic surface brightness distribution,
which is usually unknown. The uncertainty is particularly large if extended
regions of low surface brightness are present in the nebula. In such cases the
use of gaussian or second moment deconvolution is not recommended.
As an alternative, a new algorithm is presented which allows the
determination of the intrinsic FWHM of the source using only the observed
surface brightness distribution and the FWHM of the beam. Tests show that this
implicit deconvolution method works well in realistic conditions, even when the
signal-to-noise is low, provided that the beam size is less than roughly 2/3 of
the observed FWHM and the beam profile can be approximated by a gaussian.Comment: 11 pages, 7 figures, accepted for publication in MNRA
Photo-ionization modelling of planetary nebulae -- II. Galactic bulge nebulae, a comparison with literature results
We have constructed photo-ionization models of five galactic bulge planetary
nebulae using our automatic method which enables a fully self-consistent
determination of the physical parameters of a planetary nebula. The models are
constrained using the spectrum, the IRAS and radio fluxes and the angular
diameter of the nebula. We also conducted a literature search for physical
parameters determined with classical methods for these nebulae. Comparison of
the distance independent physical parameters with published data shows that the
stellar temperatures generally are in good agreement and can be considered
reliable. The literature data for the electron temperature, electron density
and also for the abundances show a large spread, indicating that the use of
line diagnostics is not reliable and that the accuracy of these methods needs
to be improved. Comparison of the various abundance determinations indicates
that the uncertainty in the electron temperature is the main source of
uncertainty in the abundance determination. The stellar magnitudes predicted by
the photo-ionization models are in good agreement with observed values.Comment: Accepted for publication in MNRA
Micromachined Polycrystalline Sige-Based Thermopiles for Micropower Generation on Human Body
This paper presents a polycrystalline silicon germanium (poly-SiGe)
thermopile specially designed for thermoelectric generators used on human body.
Both the design of the single thermocouple and the arrangement of the
thermocouple array have been described. A rim structure has been introduced in
order to increase the temperature difference across the thermocouple junctions.
The modeling of the thermocouple and the thermopile has been performed
analytically and numerically. An output power of about 1 W at an output
voltage of more than 1 V is expected from the current design of thermopiles in
a watch-size generator. The key material properties of the poly-SiGe have been
measured. The thermopile has been fabricated and tested. Experimental results
clearly demonstrate the advantage of the rim structure in increasing output
voltage. In presence of forced convection, the output voltage of a non-released
thermopile can increase from about 53 mV/K/cm2 to about 130 mV/K/cm2 after the
rim structure is formed. A larger output voltage from the thermopile is
expected upon process completion.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/EDA-Publishing
CKVul: evolving nebula and three curious background stars
We analyse the remnants of CK Vul (Nova Vul 1670) using optical imaging and
spectroscopy. The imaging, obtained between 1991 and 2010, spans 5.6% of the
life-time of the nebula. The flux of the nebula decreased during the last 2
decades. The central source still maintains the ionization of the innermost
part of the nebula, but recombination proceeds in more distant parts of the
nebula. Surprisingly, we discovered two stars located within 10 arcsec of the
expansion centre of the radio emission that are characterized by pronounced
long term variations and one star with high proper motion. The high proper
motion star is a foreground object, and the two variable stars are background
objects. The photometric variations of two variables are induced by a dusty
cloud ejected by CK Vul and passing through the line of sight to those stars.
The cloud leaves strong lithium absorption in the spectra of the stars. We
discuss the nature of the object in terms of recent observations.Comment: Published in MNRAS, available at
http://mnras.oxfordjournals.org/cgi/content/abstract/stt426
The H II Region/PDR Connection: Self-Consistent Calculations of Physical Conditions in Star-Forming Regions
We have performed a series of calculations designed to reproduce infrared
diagnostics used to determine physical conditions in star forming regions. We
self-consistently calculate the thermal and chemical structure of an H II
region and photodissociation region (PDR) that are in pressure equilibrium.
This differs from previous work, which used separate calculations for each gas
phase. Our calculations span a wide range of stellar temperatures, gas
densities, and ionization parameters. We describe improvements made to the
spectral synthesis code Cloudy that made these calculations possible. These
include the addition of a molecular network with ~1000 reactions involving 68
molecular species and improved treatment of the grain physics. Data from the
Spitzer First Look Survey, along with other archives, are used to derive
important physical characteristics of the H II region and PDR. These include
stellar temperatures, electron densities, ionization parameters, UV radiation
flux, and PDR density. Finally, we calculate the contribution of the H II
region to PDR emission line diagnostics, which allows for a more accurate
determination of physical conditions in the PDR.Comment: 60 pages, 35 figures, to be published in the Astrophysical Journal.
Version with full resolution is available at
http://www.pa.uky.edu/~nicholas/hii_pdr_high_res.pd
Voluntary Environmental Agreements in Developing Countries: The Colombian Experience
According to proponents, voluntary agreements (VAs) negotiated with polluters sidestep weak institutions and other barriers to conventional environmental regulation in developing countries. Yet little is known about their effectiveness. We examine VAs in Colombia, a global leader in the use of these policies. We find that the main motive for using VAs has been to build capacity needed for broader environmental regulatory reform. Their additional effect on environmental performance has been questionable. These findings suggest that in developing countries, VAs may be best suited to capacity building, not environmental management per se.voluntary environmental agreement, pollution, Colombia
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