6,332 research outputs found
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
Kinematic and morphological modeling of the bipolar nebula Sa2-237
We present [OIII]500.7nm and Halpha+[NII] images and long-slit, high
resolution echelle spectra in the same spectral regions of Sa2--237, a possible
bipolar planetary nebula. The image shows a bipolar nebula of about 34" extent,
with a narrow waist, and showing strong point symmetry about the central
object, indicating it's likely binary nature. The long slit spectra were taken
over the long axis of the nebula, and show a distinct ``eight'' shaped pattern
in the velocity--space plot, and a maximum projected outflow velocity of
V=106km/s, both typical of expanding bipolar planetary nebulae. By model
fitting the shape and spectrum of the nebula simultaneously, we derive the
inclination of the long axis to be 70 degrees, and the maximum space velocity
of expansion to be 308 km/s. Due to asymmetries in the velocities we adopt a
new value for the system's heliocentric radial velocity of -30km/s. We use the
IRAS and 21cm radio fluxes, the energy distribution, and the projected size of
Sa2-237 to estimate it's distance to be 2.1+-0.37kpc. At this distance Sa2-237
has a luminosity of 340 Lsun, a size of 0.37pc, and -- assuming constant
expansion velocity -- a nebular age of 624 years. The above radial velocity and
distance place Sa2--237 in the disk of the Galaxy at z=255pc, albeit with
somewhat peculiar kinematics.Comment: 10pp, 4 fig
Reinforcement of wood with natural fibers
This paper describes an experimental programme which examines the reinforcement in flexure of timber beams with composite materials based on natural fibers in the form of fabrics made from hemp, flax, basalt and bamboo fibers. The industrial use of natural fibers has been continuously increasing since 1990s due to their advantages in terms of production costs, pollution emissions and energy consumption for production and disposal. The technique allows the reinforcement of the intrados of beams, avoiding the dismantling of the overlying part of the structure with significant savings in terms of costs and work time. The test program consists of three phases incorporating 45 beams. The bending tests on the wooden elements made it possible to measure the increase in capacity and stiffness resulting from the composite reinforcement. This was applied to beams, creating different arrangements and using different quantities (number of layers). Despite the diversity of the various tests carried out, the results obtained in some cases showed significant increases in terms of load-carrying capacity and in deflection ductility
L’Aquila 2009 Clusters. Processi di ricostruzione e consistenza del patrimonio architettonico nelle aree minori colpite dal sisma come sistema di esperienze per la costruzione di banche dati nei territori dell’abbandono
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)
Why a Single-Star Model Cannot Explain the Bipolar Nebula of Eta Carinae
I examine the angular momentum evolution during the 1837-1856 Great Eruption
of the massive star Eta Carinae. I find that the new estimate of the mass blown
during that eruption implies that the envelope of Eta Car substantially
spun-down during the 20 years eruption. Single-star models, most of which
require the envelope to rotate close to the break-up velocity, cannot account
for the bipolar nebula (the Homunculus) formed from matter expelled in that
eruption. The kinetic energy and momentum of the Homunculus further constrains
single-star models. I discuss how Eta Car can fit into a unified model for the
formation of bipolar lobes where two oppositely ejected jets inflate two lobes
(or bubbles). These jets are blown by an accretion disk, which requires stellar
companions in the case of bipolar nebulae around stellar objects.Comment: ApJ, in press. New references and segments were adde
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
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