44 research outputs found
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
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
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
Jets, knots and tails in planetary nebulae: NGC 3918, K 1-2 and Wray 17-1
We analyze optical images and high-resolution, long-slit spectra of three
planetary nebulae which possess collimated, low-ionization features. NGC 3918
is composed of an inner, spindle-shaped shell mildly inclined with respect to
the plane of the sky. Departing from the polar regions of this shell, we find a
two-sided jet expanding with velocities which increase linearly with distance
from 50 to 100 km/s. The jet is probably coeval with the inner shell (with the
age of approximately 1000 D yr, where D is the distance in kpc), suggesting
that its formation should be ascribed to the same dynamical processes which
also shaped the main nebula, and not to a more recent mass loss episode. We
discuss the formation of the aspherical shell and jet in the light of current
hydrodynamical and magnetohydrodynamical theories. K 1-2 is a planetary nebula
with a close binary nucleus which shows a collimated string of knots embedded
in a diffuse, elliptical shell. The knots expand with a velocity similar to
that of the elliptical nebula (25 km/s), except for an extended tail located
out of the main nebula, which linearly accelerates up to 45 km/s. We estimate
an inclination on the line of the sight of 40 degres for the string of knots;
once the orientation of the orbit is also determined, this information will
allow us to test the prediction of current theories of the occurrence of polar
jets from close binary systems. Wray 17-1 has a complex morphology, showing two
pairs of low-ionization structures located in almost perpendicular directions
from the central star, and embedded in a large, diffuse nebula. The two pairs
show notable similarities and differences, and their origin is very puzzling.Comment: 20 pages plus 10 figures. ApJ recently published (ApJ 523, 721
(1999)
The physical parameters, excitation and chemistry of the rim, jets and knots of the planetary nebula NGC 7009
We present long-slit optical spectra along the major axis of the planetary
nebula NGC 7009. These data allow us to discuss the physical, excitation and
chemical properties of all the morphological components of the nebula,
including its remarkable systems of knots and jets. The main results of this
analysis are the following: i) the electron temperature throughout the nebula
is remarkably constant, T_e[OIII] = 10200K; ii) the bright inner rim and inner
pair of knots have similar densities of N_e = 6000cm^{-3}, whereas a much lower
density of N_e = 1500cm^{-3} is derived for the outer knots as well as for the
jets; iii) all the regions (rim, inner knots, jets and outer knots) are mainly
radiatively excited; and iv) there are no clear abundance changes across the
nebula for He, O, Ne, or S. There is a marginal evidence for an overabundance
of nitrogen in the outer knots (ansae), but the inner ones (caps) and the rim
have similar N/H values that are at variance with previous results. Our data
are compared to the predictions of theoretical models, from which we conclude
that the knots at the head of the jets are not matter accumulated during the
jet expansion through the circumstellar medium, neither can their origin be
explained by the proposed HD or MHD interacting-wind models for the formation
of jets/ansae, since the densities as well as the main excitation mechanisms of
the knots, disagree with model predictions.Comment: Figure 1 was changed because features were misidentified in the
previous version. 17 pages including 5 figures and 3 tables. ApJ in press.
Also available at http://www.iac.es/galeria/denise
Knots in the outer shells of the planetary nebulae IC 2553 and NGC 5882
We present images and high-resolution spectra of the planetary nebulae IC
2553 and NGC 5882. Spatio-kinematic modeling of the nebulae shows that they are
composed of a markedly elongated inner shell, and of a less aspherical outer
shell expanding at a considerably higher velocity than the inner one. Embedded
in the outer shells of both nebulae are found several low-ionization knots. In
IC 2553, the knots show a point-symmetric distribution with respect to the
central star: one possible explanation for their formation is that they are the
survivors of pre-existing point-symmetric condensations in the AGB wind, a fact
which would imply a quite peculiar mass-loss geometry from the giant
progenitor. In the case of NGC 5882, the lack of symmetry in the distribution
of the observed low-ionization structures makes it possible that they are the
result of in situ instabilities.Comment: 20 pages including 1 table and 6 figures. ApJ accepted. Also
available at http://andromeda.roque.ing.iac.es/~sanchez/ingpub/index2000.htm
The large-scale ionised outflow of CH Cygni
HST and ground-based [OII} and [NII] images obtained from 1996 to 1999 reveal
the existence of a ionised optical nebula around the symbiotic binary CH Cyg
extending out to 5000 A.U. from the central stars. The observed velocity range
of the nebula, derived from long-slit echelle spectra, is of 130 km/s. In spite
of its complex appearence, the velocity data show that the basic morphology of
the inner regions of the optical nebula is that of a bipolar (or conical)
outflow extending nearly along the plane of the sky out to some 2000 A.U. from
the centre. Even if the extension of this bipolar outflow and its position
angle are consistent with those of the radio jet produced in 1984 (extrapolated
to the time of our optical imagery), no obvious counterpart is visible of the
original, dense radio bullets ejected by the system. We speculate that the
optical bipolar outflow might be the remannt of the interaction of the bullets
with a relatively dense circumstellar medium.Comment: 8 text pages + 3 figures (jpeg). ApJ in press. For a full PostScript
version with figures inline see
ftp://ftp.ll.iac.es/pub/research/preprints/PP252001.ps.g
High-velocity collimated outflows in planetary nebulae: NGC 6337, He 2-186, and K 4-47
We have obtained narrow-band images and high-resolution spectra of the
planetary nebulae NGC 6337, He 2-186, and K 4-47, with the aim of investigating
the relation between their main morphological components and several
low-ionization features present in these nebulae. The data suggest that NGC
6337 is a bipolar PN seen almost pole on, with polar velocities higher than 200
km/s. The bright inner ring of the nebula is interpreted to be the "equatorial"
density enhancement. It contains a number of low-ionization knots and outward
tails that we ascribe to dynamical instabilities leading to fragmentation of
the ring or transient density enhancements due to the interaction of the
ionization front with previous density fluctuations in the ISM. The lobes show
a pronounced point-symmetric morphology and two peculiar low-ionization
filaments whose nature remains unclear. The most notable characteristic of He
2-186 is the presence of two high-velocity (higher than 135 km/s) knots from
which an S-shaped lane of emission departs toward the central star. K 4-47 is
composed of a compact core and two high-velocity, low-ionization blobs. We
interpret the substantial broadening of line emission from the blobs as a
signature of bow shocks, and using the modeling of Hartigan, Raymond, & Hartman
(1987), we derive a shock velocity of 150 km/s and a mild inclination of the
outflow on the plane of the sky. We discuss possible scenarios for the
formation of these nebulae and their low-ionization features. In particular,
the morphology of K 4-47 hardly fits into any of the usually adopted mass-loss
geometries for single AGB stars. Finally, we discuss the possibility that
point-symmetric morphologies in the lobes of NGC 6337 and the knots of He 2-186
are the result of precessing outflows from the central stars.Comment: 16 pages plus 7 figures, ApJ accepted. Also available at
http://www.iac.es/publicaciones/preprints.htm