4,619 research outputs found
Radiation Transfer in the Cavity and Shell of Planetary Nebulae
We develop an approximate analytical solution for the transfer of
line-averaged radiation in the hydrogen recombination lines for the ionized
cavity and molecular shell of a spherically symmetric planetary nebula. The
scattering problem is treated as a perturbation, using a mean intensity derived
from a scattering-free solution. The analytical function was fitted to Halpha
and Hbeta data from the planetary nebula NGC6537. The position of the maximum
in the intensity profile produced consistent values for the radius of the
cavity as a fraction of the radius of the dusty nebula: 0.21 for Halpha and
0.20 for Hbeta. Recovered optical depths were broadly consistent with observed
optical extinction in the nebula, but the range of fit parameters in this case
is evidence for a clumpy distribution of dust.Comment: MNRAS accepted; 10 Fig
Structure and shaping processes within the extended atmospheres of AGB stars
We present recent studies using the near-infrared instrument AMBER of the VLT
Interferometer (VLTI) to investigate the structure and shaping processes within
the extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBER
observations of the Mira variable S Ori have revealed wavelength-dependent
apparent angular sizes. These data were successfully compared to dynamic model
atmospheres, which predict wavelength-dependent radii because of geometrically
extended molecular layers. Most recently, AMBER closure phase measurements of
several AGB stars have also revealed wavelength-dependent deviations from 0/180
deg., indicating deviations from point symmetry. The variation of closure phase
with wavelength indicates a complex non-spherical stratification of the
extended atmosphere, and may reveal whether observed asymmetries are located
near the photosphere or in the outer molecular layers. Concurrent observations
of SiO masers located within the extended molecular layers provide us with
additional information on the morphology, conditions, and kinematics of this
shell. These observations promise to provide us with new important insights
into the shaping processes at work during the AGB phase. With improved imaging
capabilities at the VLTI, we expect to extend the successful story of imaging
studies of planetary nebulae to the photosphere and extended outer atmosphere
of AGB stars.Comment: 6 pages, Proc. of "Asymmetric Planetary Nebulae V", A.A. Zijlstra, F.
Lykou, I. McDonald, and E. Lagadec (eds.), Jodrell Bank Centre for
Astrophysics, Manchester, UK, 201
The Stellar Populations of NGC 3109: Another Dwarf Irregular Galaxy with a Population II Stellar Halo
We have obtained V and I-band photometry for about 17500 stars in the field
of the dwarf irregular galaxy NGC3109, located in the outskirts of the Local
Group. The photometry allows us to study the stellar populations present inside
and outside the disk of this galaxy. From the VI color-magnitude diagram we
infer metallicities and ages for the stellar populations in the main body and
in the halo of NGC3109. The stars in the disk of this galaxy have a wide
variety of ages, including very young stars with approximately 10^7 yr. Our
main result is to establish the presence of a halo consisting of population II
stars, extending out to about 4.5 arcmin (or 1.8 kpc) above and below the plane
of this galaxy. For these old stars we derive an age of > 10 Gyr and a
metallicity of [Fe/H] = -1.8 +/- 0.2. We construct a deep luminosity function,
obtaining an accurate distance modulus (m-M)_0 = 25.62 +/- 0.1 for this galaxy
based on the I-magnitude of the red giant branch (RGB) tip and adopting E(V-I)
= 0.05.Comment: Accepted for publication in the Astronomical Journal 23 pages, latex,
12 Figures (Fig 1 not available in electronic format
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
Accelerated post-AGB evolution, initial-final mass relations, and the star-formation history of the Galactic bulge
We study the star-formation history of the Galactic bulge, as derived from
the age distribution of the central stars of planetary nebulae that belong to
this stellar population. The high resolution imaging and spectroscopic
observations of 31 compact planetary nebulae are used to derive their central
star masses. The Bloecker tracks with the cluster IFMR result in ages, which
are unexpectedly young. We find that the Bloecker post-AGB tracks need to be
accelerated by a factor of three to fit the local white dwarf masses. This
acceleration extends the age distribution. We adjust the IFMR as a free
parameter to map the central star ages on the full age range of bulge stellar
populations. This fit requires a steeper IFMR than the cluster relation. We
find a star-formation rate in the Galactic bulge, which is approximately
constant between 3 and 10 Gyr ago. The result indicates that planetary nebulae
are mainly associated with the younger and more metal-rich bulge populations.
The constant rate of star-formation between 3 and 10 Gyr agrees with
suggestions that the metal-rich component of the bulge is formed during an
extended process, such as a bar interaction.Comment: accepted for publication in A&
Non-spiky density of states of an icosahedral quasicrystal
The density of states of the ideal three-dimensional Penrose tiling, a
quasicrystalline model, is calculated with a resolution of 10 meV. It is not
spiky. This falsifies theoretical predictions so far, that spikes of width
10-20 meV are generic for the density of states of quasicrystals, and it
confirms recent experimental findings. The qualitative difference between our
results and previous calculations is partly explained by the small number of k
points that has usually been included in the evaluation of the density of
states of periodic approximants of quasicrystals. It is also shown that both
the density of states of a small approximant of the three-dimensional Penrose
tiling and the density of states of the ideal two-dimensional Penrose tiling do
have spiky features, which also partly explains earlier predictions.Comment: 8 pages, 4 figures. Changes in this version: longer introduction,
details of figures shown in inset
The evolving spectrum of the planetary nebula Hen 2-260
We analysed the planetary nebula Hen 2-260 using optical spectroscopy and
photometry. We compared our observations with the data from literature to
search for evolutionary changes. The nebular line fluxes were modelled with the
Cloudy photoionization code to derive the stellar and nebular parameters. The
planetary nebula shows a complex structure and possibly a bipolar outflow. The
nebula is relatively dense and young. The central star is just starting ionization (). Comparison of our
observations with literature data indicates a 50% increase of the [OIII] 5007
\AA\ line flux between 2001 and 2012. We interpret it as the result of the
progression of the ionization of . The central star evolves to
higher temperatures at a rate of . The heating rate
is consistent with a final mass of
or for two different sets of
post-AGB evolutionary tracks from literature. The photometric monitoring of Hen
2-260 revealed variations on a timescale of hours or days. The variability may
be caused by pulsations of the star. The temperature evolution of the central
star can be traced using spectroscopic observations of the surrounding
planetary nebula spanning a timescale of roughly a decade. This allows us to
precisely determine the stellar mass, since the pace of the temperature
evolution depends critically on the core mass. The kinematical age of the
nebula is consistent with the age obtained from the evolutionary track. The
final mass of the central star is close to the mass distribution peak for
central stars of planetary nebulae found in other studies. The object belongs
to a group of young central stars of planetary nebulae showing photometric
variability.Comment: accepted for publication in A&
Reproduction of the Marsh Harrier <i>Circus aeruginosus</i> in recent land reclamations in the Netherlands
Understanding school engagement: The role of contextual continuities and discontinuities in adolescents ' learner identities
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