2,036 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
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
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