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

A Herschel study of Planetary Nebulae

We present Herschel PACS and SPIRE images of the dust shells around the planetary nebulae NGC 650, NGC 6853, and NGC 6720, as well as images showing the dust temperature in their shells. The latter shows a rich structure, which indicates that internal extinction in the UV is important despite the highly evolved status of the nebulae.Comment: 2 pages, 1 figure, 2012, proceedings IAU Symposium 283 Planetary Nebulae: An Eye to the Futur

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