263 research outputs found
Zinc abundances of planetary nebulae
Zinc is a useful surrogate element for measuring Fe/H as, unlike iron, it is
not depleted in the gas phase media. Zn/H and O/Zn ratios have been derived
using the [Zn IV] emission line at 3.625um for a sample of nine Galactic
planetary nebulae, seven of which are based upon new observations using the
VLT. Based on photoionization models, O/O++ is the most reliable ionisation
correction factor for zinc that can readily be determined from optical emission
lines, with an estimated accuracy of 10% or better for all targets in our
sample. The majority of the sample is found to be sub-solar in [Zn/H]. [O/Zn]
in half of the sample is found to be consistent with Solar within
uncertainties, whereas the remaining half are enhanced in [O/Zn]. [Zn/H] and
[O/Zn] as functions of Galactocentric distance have been investigated and there
is little evidence to support a trend in either case.Comment: Accepted MNRAS, 11 pages, 8 figure
Catalog of Lunar Craters I
This catalog gives the selenographic coordinates of all craters observable on a selected portion of the moon's surface. The diameter of the crater together with comments on shape are also given. Approximately 25 per cent of the craters have been measured previously by other observers. The catalog gives the position found in the present series of measurements and the name adopted by the International Astronomical Union
Chemical compositions and plasma parameters of planetary nebulae with Wolf-Rayet and wels type central stars
Aims: Chemical compositions and other properties of planetary nebulae around
central stars of spectral types [WC], [WO], and wels are compared with those of
`normal' central stars, in order to clarify the evolutionary status of each
type and their interrelation. Methods: We use plasma diagnostics to derive from
optical spectra the plasma parameters and chemical compositions of 48 planetary
nebulae. We also reanalyze the published spectra of a sample of 167 non-WR PN.
The results as well as the observational data are compared in detail with those
from other studies of the objects in common. Results: The central star's
spectral type is clearly correlated with electron density, temperature and
excitation class of the nebula, [WC] nebulae tend to be smaller than the other
types. All this corroborates the view of an evolutionary sequence from cool [WC
11] central stars inside dense, low excitation nebulae towards hot [WO 1] stars
with low density, high excitation nebulae. The wels PN, however, appear to be a
separate class of objects, not linked to WRPN by evolution, --abridged--Comment: 17 pages, 28 figures, Accepted in A&A. Accepted in A&
Three-dimensional laser Doppler anemometer measurements of a jet in a crossflow
A three-dimensional laser Doppler anemometer (3D-LDA) was used in a wind tunnel to measure a jet in a crossflow. Measurements were made in the vicinity of a 5-cm-diam jet which issued normally into a 10.65 m/sec wind tunnel crossflow; the velocity ratio Vjet/Vinf was 8. Detailed lateral surveys were made at two elevations (z = cm and 2 cm); both elevations were within the region affected by the boundary layer on the plate. The results are believed to provide reliable velocity field information in the boundary layer of the jet in a crossflow. Turbulence information also is available and believed to be roughly correct, although it may be subject to broadening effects for the lower values of turbulence. A weak vortex pair was observed in the wake at the plate surface. This structure existed in the boundary layer and built confidence because the 3D-LDA was, indeed, able to resolve fine detail in the wake. The capabilities of the 3D-LDA not only allow the making of the velocity surveys, but can be utilized to follow mean streamlines in the flow
Planetary nebulae in the inner Milky Way: new abundances
The study of planetary nebulae in the inner-disk and bulge gives important
information on the chemical abundances of elements such as He, N, O, Ar, Ne,
and on the evolution of these abundances, which is associated with the
evolution of intermediate-mass stars and the chemical evolution of the Galaxy.
We present accurate abundances of the elements He, N, S, O, Ar, and Ne for a
sample of 54 planetary nebulae located towards the bulge of the Galaxy, for
which 33 have the abundances derived for the first time. The abundances are
derived based on observations in the optical domain made at the National
Laboratory for Astrophysics (LNA, Brazil). The data show a good agreement with
other results in the literature, in the sense that the distribution of the
abundances is similar to those works.Comment: Accepted for publication in RevMexAA (29 pages, 15 figures, 7 tables,
uses rmaa.cls
PAH Formation in O-rich Planetary Nebulae
Polycyclic aromatic hydrocarbons (PAHs) have been observed in O-rich
planetary nebulae towards the Galactic Bulge. This combination of oxygen-rich
and carbon-rich material, known as dual-dust or mixed chemistry, is not
expected to be seen around such objects. We recently proposed that PAHs could
be formed from the photodissociation of CO in dense tori. In this work, using
VISIR/VLT, we spatially resolved the emission of the PAH bands and ionised
emission from the [SIV] line, confirming the presence of dense central tori in
all the observed O-rich objects. Furthermore, we show that for most of the
objects, PAHs are located at the outer edge of these dense/compact tori, while
the ionised material is mostly present in the inner parts of these tori,
consistent with our hypothesis for the formation of PAHs in these systems. The
presence of a dense torus has been strongly associated with the action of a
central binary star and, as such, the rich chemistry seen in these regions may
also be related to the formation of exoplanets in post-common-envelope binary
systems.Comment: 14, accepted for publication in the MNRAS Journa
Angular diameters, fluxes and extinction of compact planetary nebulae: further evidence for steeper extinction towards the Bulge
We present values for angular diameter, flux and extinction for 70 Galactic
planetary nebulae observed using narrow band filters. Angular diameters are
derived using constant emissivity shell and photoionization line emission
models. The mean of the results from these two models are presented as our best
estimate. Contour plots of 36 fully resolved objects are included and the low
intensity contours often reveal an elliptical structure that is not always
apparent from FWHM measurements. Flux densities are determined, and for both
H-alpha and O[III] there is little evidence of any systematic differences
between observed and catalogued values. Observed H-alpha extinction values are
determined using observed H-alpha and catalogued radio fluxes. H-alpha
extinction values are also derived from catalogued H-alpha and H-beta flux
values by means of an Rv dependent extinction law. Rv is then calculated in
terms of observed extinction values and catalogued H-alpha and H-beta flux
values. Comparing observed and catalogue extinction values for a subset of
Bulge objects, observed values tend to be lower than catalogue values
calculated with Rv = 3.1. For the same subset we calculate = 2.0,
confirming that toward the Bulge interstellar extinction is steeper than Rv =
3.1. For the inner Galaxy a relation with the higher supernova rate is
suggested, and that the low-density warm ionized medium is the site of the
anomalous extinction. Lowvalues of extinction are also derived using dust
models with a turnover radius of 0.08 microns.Comment: Accepted by MNRAS. 17 pages, 9 figures (including 36 contour plots of
PNe), 5 Tables (including 2 large tables of angular diameters, fluxes and
extinction
Unveiling shocks in planetary nebulae
The propagation of a shock wave into a medium is expected to heat the
material beyond the shock, producing noticeable effects in intensity line
ratios such as [O III]/Halpha. To investigate the occurrence of shocks in
planetary nebulae (PNe), we have used all narrowband [O III] and Halpha images
of PNe available in the HST archive to build their [O III]/Halpha ratio maps
and to search for regions where this ratio is enhanced. Regions with enhanced
[O III]/Halpha emission ratio can be ascribed to two different types of
morphological structures: bow-shock structures produced by fast collimated
outflows and thin skins enveloping expanding nebular shells. Both collimated
outflows and expanding shells are therefore confirmed to generate shocks in
PNe. We also find regions with depressed values of the [O III]/Halpha ratio
which are found mostly around density bounded PNe, where the local contribution
of [N II] emission into the F656N Halpha filter cannot be neglected.Comment: 13 pages, 9 figures, 3 tables; To appear in Astronomy & Astrophysic
- …