10,749 research outputs found
The Calibration of Stromgren uvby-Hbeta Photometry for Late-Type Stars -- a Model Atmosphere Approach
We aim to test the power of theoretical calibrations based on a new
generation of MARCS models by comparisons with observational photomteric data.
We calculate synthetic uvby-Hbeta colour indices from synthetic spectra. A
sample of 388 field stars as well as stars in globular clusters is used for a
direct comparison of the synthetic indices versus empirical data and for
scrutinizing the possibilities of theoretical calibrations for temperature,
metallicity and gravity. We show that the temperature sensitivity of the
synthetic (b-y) colour is very close to its empirical counterpart, whereas the
temperature scale based upon Hbeta shows a slight offset. The theoretical
metallicity sensitivity of the m1 index (and for G-type stars its combination
with c1) is somewhat larger than the empirical one, based upon spectroscopic
determinations. The gravity sensitivity of the synthetic c1 index shows a
satisfactory behaviour when compared to obervations of F stars. For stars
cooler than the sun a deviation is significant in the c1-(b-y) diagram. The
theoretical calibrations of (b-y), (v-y) and c1 seem to work well for Pop II
stars and lead to effective temperatures for globular cluster stars supporting
recent claims by Korn et al. (2007) that atomic diffusion occurs in stars near
the turnoff point of NGC 6397. Synthetic colours of stellar atmospheres can
indeed be used, in many cases, to derive reliable fundamental stellar
parameters. The deviations seen when compared to observational data could be
due to incomplete linelists but are possibly also due to effects of assuming
plane-parallell or spherical geometry and LTE
Exploring wind-driving dust species in cool luminous giants III. Wind models for M-type AGB stars: dynamic and photometric properties
Stellar winds observed in asymptotic giant branch (AGB) stars are usually
attributed to a combination of stellar pulsations and radiation pressure on
dust. Shock waves triggered by pulsations propagate through the atmosphere,
compressing the gas and lifting it to cooler regions, which create favourable
conditions for grain growth. If sufficient radiative acceleration is exerted on
the newly formed grains through absorption or scattering of stellar photons, an
outflow can be triggered. Strong candidates for wind-driving dust species in
M-type AGB stars are magnesium silicates (MgSiO and MgSiO). Such
grains can form close to the stellar surface, they consist of abundant
materials and, if they grow to sizes comparable to the wavelength of the
stellar flux maximum, they experience strong acceleration by photon scattering.
We use a frequency-dependent radiation-hydrodynamics code with a detailed
description for the growth of MgSiO grains to calculate the first
extensive set of time-dependent wind models for M-type AGB stars. The resulting
wind properties, visual and near-IR photometry and mid-IR spectra are compared
with observations.We show that the models can produce outflows for a wide range
of stellar parameters. We also demonstrate that they reproduce observed
mass-loss rates and wind velocities, as well as visual and near-IR photometry.
However, the current models do not show the characteristic silicate features at
10 and 18 m as a result of the cool temperature of MgSiO grains in
the wind. Including a small amount of Fe in the grains further out in the
circumstellar envelope will increase the grain temperature and result in
pronounced silicate features, without significantly affecting the photometry in
the visual and near-IR wavelength regions.Comment: 11 pages, 14 figure
Exploring wind-driving dust species in cool luminous giants II. Constraints from photometry of M-type AGB stars
The heavy mass loss observed in evolved asymptotic giant branch (AGB) stars
is usually attributed to a two-stage process: atmospheric levitation by
pulsation-induced shock waves, followed by radiative acceleration of newly
formed dust grains. The dust transfers momentum to the surrounding gas through
collisions and thereby triggers a general outflow. Radiation-hydrodynamical
models of M-type AGB stars suggest that these winds can be driven by photon
scattering -- in contrast to absorption -- on Fe-free silicate grains of sizes
0.1--1\,m. In this paper we study photometric constraints for wind-driving
dust species in M-type AGB stars, as part of an ongoing effort to identify
likely candidates among the grain materials observed in circumstellar
envelopes. To investigate the scenario of stellar winds driven by photon
scattering on dust, and to explore how different optical and chemical
properties of wind-driving dust species affect photometry we focus on two sets
of dynamical models atmospheres: (i) models using a detailed description for
the growth of MgSiO grains, taking into account both scattering and
absorption cross-sections when calculating the radiative acceleration, and (ii)
models using a parameterized dust description, constructed to represent
different chemical and optical dust properties. By comparing synthetic
photometry from these two sets of models to observations of M-type AGB stars we
can provide constraints on the properties of wind-driving dust species.
Photometry from wind models with a detailed description for the growth of
MgSiO grains reproduces well both the values and the time-dependent
behavior of observations of M-type AGB stars, providing further support for the
scenario of winds driven by photon scattering on dust.Comment: Accepted for publication in A&A. 15 pages, 14 figure
Diplomats or Defendants? Defining the Future of Head-of-State Immunity
Fluorescence nanoscopy provides means to discernthe finer details of protein localization and interaction in cells by offeringan order of magnitude higher resolution than conventional optical imagingtechniques. However, these super resolution techniques put higher demands onthe optical system as well as on the fluorescent probes, making multicolorfluorescence nanoscopy a challenging task. Here we present a new and simpleprocedure which exploits the photostability and excitation spectra of dyes toincrease the number of simultaneous recordable targets in STED nanoscopy. Weuse this procedure to demonstrate four color STED imaging of platelets with ≤40 nm resolution and low crosstalk. Platelets can selectively store, sequesterand release a multitude of different proteins, and in a manner specific fordifferent physiological and disease states. By applying multicolor nanoscopy tostudy platelets, we can achieve spatial mapping of the protein organizationwith a high resolution, for multiple proteins at the same time and in the samecell. This provides a means to identify specific platelet activation states fordiagnostic purposes and to understand the underlying protein storage andrelease mechanisms. We studied the organization of the pro- and anti-angiogenicproteins VEGF and PF-4 together with fibrinogen and filamentous actin, andfound distinct features in their respective protein localization. Further,colocalization analysis revealed only minor overlap between the proteins VEGFand PF-4 indicating that they have separate storage and release mechanisms,corresponding well with their opposite rules as pro- and anti-angiogenicproteins, respectively.Updated from "Submitted" to "Published". QC 20140630</p
The Abundance of Elements in Cool Stars, as Determined from High-Resolution, 1-5 Micron Spectroscopy
We review the field of abundance determinations of elements in cool stars,
with special interest paid to determinations based on analyses of
high-resolution, 1-5 micron spectra. We discuss the current status, problems,
and challenges of exploring high-resolution, near-infrared spectra. In
particular, advantages and drawbacks are pointed out. A few examples of
current, chemical-abundance determinations are high-lighted and, finally, we
discuss the developmentComment: To appear in the proceedings of the ESO Workshop on High Resolution
Infrared Spectroscopy in Astronomy held in Garching, Germany, 18-21 November
200
The effect of multiple paternity on genetic diversity during and after colonisation
In metapopulations, genetic variation of local populations is influenced by
the genetic content of the founders, and of migrants following establishment.
We analyse the effect of multiple paternity on genetic diversity using a model
in which the highly promiscuous marine snail Littorina saxatilis expands from a
mainland to colonise initially empty islands of an archipelago. Migrant females
carry a large number of eggs fertilised by 1 - 10 mates. We quantify the
genetic diversity of the population in terms of its heterozygosity: initially
during the transient colonisation process, and at long times when the
population has reached an equilibrium state with migration. During
colonisation, multiple paternity increases the heterozygosity by 10 - 300 % in
comparison with the case of single paternity. The equilibrium state, by
contrast, is less strongly affected: multiple paternity gives rise to 10 - 50 %
higher heterozygosity compared with single paternity. Further we find that far
from the mainland, new mutations spreading from the mainland cause bursts of
high genetic diversity separated by long periods of low diversity. This effect
is boosted by multiple paternity. We conclude that multiple paternity
facilitates colonisation and maintenance of small populations, whether or not
this is the main cause for the evolution of extreme promiscuity in Littorina
saxatilis.Comment: 7 pages, 5 figures, electronic supplementary materia
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