195 research outputs found
Spectroscopic properties of a two-dimensional time-dependent Cepheid model I. Description and validation of the model
Standard spectroscopic analyses of Cepheid variables are based on hydrostatic
one-dimensional model atmospheres, with convection treated using various
formulations of mixing-length theory. This paper aims to carry out an
investigation of the validity of the quasi-static approximation in the context
of pulsating stars. We check the adequacy of a two-dimensional time-dependent
model of a Cepheid-like variable with focus on its spectroscopic properties.
With the radiation-hydrodynamics code CO5BOLD, we construct a two-dimensional
time-dependent envelope model of a Cepheid with K, , solar metallicity, and a 2.8-day pulsation period. Subsequently, we
perform extensive spectral syntheses of a set of artificial iron lines in local
thermodynamic equilibrium. The set of lines allows us to systematically study
effects of line strength, ionization stage, and excitation potential. We
evaluate the microturbulent velocity, line asymmetry, projection factor, and
Doppler shifts. The mean Doppler shift is non-zero and negative, -1 km/s, after
averaging over several full periods and lines. This residual line-of-sight
velocity (related to the "K-term") is primarily caused by horizontal
inhomogeneities, and consequently we interpret it as the familiar convective
blueshift ubiquitously present in non-pulsating late-type stars. Limited
statistics prevent firm conclusions on the line asymmetries. Our
two-dimensional model provides a reasonably accurate representation of the
spectroscopic properties of a short-period Cepheid-like variable star. Some
properties are primarily controlled by convective inhomogeneities rather than
by the Cepheid-defining pulsations
Spectroscopic properties of a two-dimensional time-dependent Cepheid model II. Determination of stellar parameters and abundances
Standard spectroscopic analyses of variable stars are based on hydrostatic
one-dimensional model atmospheres. This quasi-static approach has theoretically
not been validated. We aim at investigating the validity of the quasi-static
approximation for Cepheid variables. We focus on the spectroscopic
determination of the effective temperature , surface gravity
, microturbulent velocity , and a generic metal
abundance -- here taken as iron. We calculate a grid of 1D
hydrostatic plane-parallel models covering the ranges in effective temperature
and gravity encountered during the evolution of a two-dimensional
time-dependent envelope model of a Cepheid computed with the
radiation-hydrodynamics code CO5BOLD. We perform 1D spectral syntheses for
artificial iron lines in local thermodynamic equilibrium varying the
microturbulent velocity and abundance. We fit the resulting equivalent widths
to corresponding values obtained from our dynamical model. For the
four-parametric case, the stellar parameters are typically underestimated
exhibiting a bias in the iron abundance of \approx-0.2\,\mbox{dex}. To avoid
biases of this kind it is favourable to restrict the spectroscopic analysis to
photometric phases using additional
information to fix effective temperature and surface gravity. Hydrostatic 1D
model atmospheres can provide unbiased estimates of stellar parameters and
abundances of Cepheid variables for particular phases of their pulsations. We
identified convective inhomogeneities as the main driver behind potential
biases. For obtaining a complete view on the effects when determining stellar
parameters with 1D models, multi-dimensional Cepheid atmosphere models are
necessary for variables of longer period than investigated here.Comment: accepted for publication in Astronomy & Astrophysic
Discovery of blue companions to two southern Cepheids: WW Car and FN Vel
A large number of high-dispersion spectra of classical Cepheids were obtained
in the region of the CaII H+K spectral lines. The analysis of these spectra
allowed us to detect the presence of a strong Balmer line, H, for
several Cepheids, interpreted as the signature of a blue companion: the
presence of a sufficiently bright blue companion to the Cepheid results in a
discernible strengthening of the CaII H + Hepsilon line relative to the CaII K
line. We investigated 103 Cepheids, including those with known hot companions
(B5-B6 main-sequence stars) in order to test the method. We could confirm the
presence of a companion to WW Car and FN Vel (the existence of the former was
only suspected before) and we found that these companions are blue hot stars.
The method remains efficient when the orbital velocity changes in a binary
system cannot be revealed and other methods of binarity detection are not
efficient.Comment: 6 pages, 5 figures, 4 tables, published on MNRAS in March 201
Classical Cepheids, what else?
We present new and independent estimates of the distances to the Magellanic
Clouds (MCs) using near-infrared (NIR) and optical--NIR period--Wesenheit (PW)
relations. The slopes of the PW relations are, within the dispersion, linear
over the entire period range and independent of metal content. The absolute
zero points were fixed using Galactic Cepheids with distances based on the
infrared surface-brightness method. The true distance modulus we found for the
Large Magellanic Cloud--- mag---and the
Small Magellanic Cloud--- mag---agree quite
well with similar distance determinations based on robust distance indicators.
We also briefly discuss the evolutionary and pulsation properties of MC
Cepheids
Galactic abundance gradients from Cepheids : On the iron abundance gradient around 10-12 kpc
Context: Classical Cepheids can be adopted to trace the chemical evolution of
the Galactic disk since their distances can be estimated with very high
accuracy. Aims: Homogeneous iron abundance measurements for 33 Galactic
Cepheids located in the outer disk together with accurate distance
determinations based on near-infrared photometry are adopted to constrain the
Galactic iron gradient beyond 10 kpc. Methods: Iron abundances were determined
using high resolution Cepheid spectra collected with three different
observational instruments: ESPaDOnS@CFHT, Narval@TBL and [email protected] ESO/MPG
telescope. Cepheid distances were estimated using near-infrared (J,H,K-band)
period-luminosity relations and data from SAAO and the 2MASS catalog. Results:
The least squares solution over the entire data set indicates that the iron
gradient in the Galactic disk presents a slope of -0.052+/-0.003 dex/kpc in the
5-17 kpc range. However, the change of the iron abundance across the disk seems
to be better described by a linear regime inside the solar circle and a
flattening of the gradient toward the outer disk (beyond 10 kpc). In the latter
region the iron gradient presents a shallower slope, i.e. -0.012+/-0.014
dex/kpc. In the outer disk (10-12 kpc) we also found that Cepheids present an
increase in the spread in iron abundance. Current evidence indicates that the
spread in metallicity depends on the Galactocentric longitude. Finally, current
data do not support the hypothesis of a discontinuity in the iron gradient at
Galactocentric distances of 10-12 kpc. Conclusions: The occurrence of a spread
in iron abundance as a function of the Galactocentric longitude indicates that
linear radial gradients should be cautiously treated to constrain the chemical
evolution across the disk.Comment: 5 tables, 8 figures, Accepted in A&
Atmospheric parameters of Cepheids from flux ratios with ATHOS: I. The temperature scale
Context: The effective temperature is a key parameter governing the
properties of a star. For stellar chemistry, it has the strongest impact on the
accuracy of the abundances derived. Since Cepheids are pulsating stars,
determining their effective temperature is more complicated that in the case of
non-variable stars. Aims: We want to provide a new temperature scale for
classical Cepheids, with a high precision and full control of the systematics.
Methods: Using a data-driven machine learning technique employing observed
spectra, and taking great care to accurately phase single-epoch observations,
we have tied flux ratios to (label) temperatures derived using the infrared
surface brightness method. Results: We identified 143 flux ratios that allow us
to determine the effective temperature with a precision of a few K and an
accuracy better than 150 K, which is in line with the most accurate temperature
measures available to date. The method does not require a normalization of the
input spectra and provides homogeneous temperatures for low- and
high-resolution spectra, even at the lowest signal-to-noise ratios. Due to the
lack of a dataset of sufficient sample size for Small Magellanic Cloud
Cepheids, the temperature scale does not extend to Cepheids with [Fe/H] < -0.6
dex but nevertheless provides an exquisite, homogeneous means of characterizing
Galactic and Large Magellanic Cloud Cepheids. Conclusions: The temperature
scale will be extremely useful in the context of spectroscopic surveys for
Milky Way archaeology with the WEAVE and 4MOST spectrographs. It paves the way
for highly accurate and precise metallicity estimates, which will allow us to
assess the possible metallicity dependence of Cepheids' period-luminosity
relations and, in turn, to improve our measurement of the Hubble constant H0.Comment: 16 pages, 13 figures, accepted in A&
Formation des ions bromate dans une colonne à bulles: Effets du peroxyde d'hydrogène lors de l'ozonation
L'utilisation de l'ozone, aujourd'hui très répandue dans les filières de potabilisation, n'est pas sans effet secondaire. De nombreux sous-produits peuvent se former comme notamment les ions bromates, sous produits finaux d'oxydation des bromures contenus dans les eaux. Malheureusement, le mécanisme de production de cette espèce est complexe et dépend de nombreux paramètres difficiles à appréhender.Sur une installation pilote de type colonne à bulles fonctionnant à contre-courant, nous avons étudié l'influence de différents paramètres, comme le pH, le temps de contact, la dose d'ozone et la dose de peroxyde d'hydrogène, sur la formation des bromates et la dégradation des pesticides, représentée par l'atrazine.Les résultats de la littérature ont été confirmés lors de l'emploi unique de l'ozone. La formation des ions bromate est influencée par la présence du peroxyde d'hydrogène. Cet oxydant intervient de manière non négligeable sur la consommation des entités intermédiaires. Le couple HOBr/OBr- peut être oxydé par l'ozone moléculaire et le radical OH° mais peut également être réduit par l'ozone et par le peroxyde sous sa forme acide ou sa base conjuguée. En ce qui concerne la dégradation des pesticides, l'utilisation de peroxyde d'hydrogène couplé à l'ozone favorise l'oxydation de la molécule d'atrazine grâce à la présence plus importante de radicaux hydroxyles.Une pollution accidentelle en pesticides pourra être traitée par l'ajout ponctuel de peroxyde d'hydrogène avec une augmentation de pH, la formation des bromates sera, dans ce cas, faible. La désinfection sera alors assurée par l'étape de chloration.In drinking water treatment plants, ozonation is often used to disinfect, to remove micropollutants and to improve water taste and odour. Ozonation increases organic matter biodegradability before filtration through granular active carbon and reduces the concentration of haloform precursors that react in the final chlorination step. However, by-products that could be detrimental to human health could be formed. For example, bromates, which are classified as carcinogenic compounds by the I.A.R.C, are produced during the ozonation of bromide-containing water. The mechanism of bromate formation is complex, due to the participation of molecular ozone and radical (hydroxyl and carbonate) reactions. The optimisation of the process should allow for a good disinfection and a reduction in the levels of micropollutants, together with low by-product formation.Using a pilot-scale counter-current bubble column, we have measured the bromate concentration in relation to pesticide removal. Water spiked with bromide and atrazine was stored in a completely stirred-tank (2 m3) before being pumped to the top of the column. The inlet gaseous ozone was measured by an analyser using UV detection, the outlet gaseous ozone was monitored by the potassium iodide method, and the dissolved ozone concentration was determined by the indigo trisulfonate method. Bromides and bromates were quantified by ion chromatography with a conductimetric detector, with a sodium carbonate solution as the eluant. Samples for bromate analysis were pretreated by OnGuard-Ag and OnGuard-H cartridges in series before injection. Atrazine degradation was measured by high performance liquid chromatography with a diode array detector, with a CH3CN/H2O mixture as the eluant. The linearisation of atrazine removal allowed us to calculate the hydroxyl radical concentration in a series of a completely-stirred tank reactors and in a plug-flow reactor.We have studied the influence of several parameters on bromate formation, including pH, bromide concentration and hydrogen peroxide concentration. As bromate production is a function of bromide concentration, we have chosen to calculate the ratio between the real bromate concentration and the theoretical bromate concentration if all bromide were oxidised to bromate. The pH affects bromate formation: an increase in pH in the absence of hydrogen peroxide increases bromate production, but when this oxidant is applied bromate production decreases when the pH increases. If reaction progress is represented as a function of [O3]*TC, we note that the presence of hydrogen peroxide increases bromate formation because of the increase in hydroxyl radical concentration, which favours radical formation. Nevertheless, if we represent reaction progress as a function of [OH∘]*TC, hydrogen peroxide seems to be an initiator and a scavenger in the mechanism of bromate formation. If we calculate the rates of all the oxidation and reduction reactions for HOBr/OBr- species, the contribution to the reduction of HOBr/OBr- species by peroxide is very important in comparison to the oxidation reactions, which inhibits bromate production. Without the hydrogen peroxide, the contribution of oxidation is equal to that of the reduction reaction, and in this case bromate formation is effective. When, under the same initial operational conditions, we apply hydrogen peroxide with an increase in pH, we observe a decrease in bromate formation with a decrease of the dissolved ozone concentration, which hinders the desired disinfection. The main contribution to atrazine oxidation is from the free-radical reactions, which explains why removal is better when we apply hydrogen peroxide than when we use ozone alone. However, if we want to respect a low bromate level in drinking water, atrazine degradation should not be greater than 90% for the operational conditions on our pilot-scale.If an accidental high pesticide concentration is observed, an addition of hydrogen peroxide with a concurrent increase of pH, could treat the pollution. In this case, a subsequent chlorination step would then have to be used to assure the disinfection alone
Two-dimensional non-LTE \ion{O}{I} 777\,nm line formation in radiation hydrodynamics simulations of Cepheid atmospheres
Oxygen abundance measurements are important for understanding stellar
structure and evolution. Measured in Cepheids, they further provide clues on
the metallicity gradient and chemo-dynamical evolution in the Galaxy. However,
most of the abundance analyses of Cepheids to date have been based on
one-dimensional (1D) hydrostatic model atmospheres. Here, we test the validity
of this approach for the key oxygen abundance diagnostic, the \ion{O}{I}
~triplet lines. We carry out 2D non-LTE radiative transfer
clculations across two different 2D radiation hydrodynamics simulations of
Cepheid atmospheres, having stellar parameters of K,
solar chemical compositions, and and , corresponding to
pulsation periods of 9 and 3 days, respectively. We find that the 2D non-LTE
versus 1D LTE abundance differences range from ~dex to ~dex
depending on pulsational phase. The 2D non-LTE versus 1D non-LTE abundance
differences range from ~dex to ~dex. The abundance differences are
smallest when the Cepheid atmospheres are closest to hydrostatic equilibrium,
corresponding to phases of around to , and we recommend these phases
for observers deriving the oxygen abundance from \ion{O}{I}
triplet with 1D hydrostatic models.Comment: 9 pages, 10 figures; Published in Astronomy and Astrophysic
Deep secrets of intermediate-mass giants and supergiants: Models with rotation seem to overestimate mixing effects on the surface abundances of C, N, and Na
Recent observational results have demonstrated an increase in the surface Na
abundance that correlates with stellar mass for red giants between 2 and 3
Msun. This trend supports evolutionary mixing processes as the explanation for
Na overabundances seen in some red giants. In this same mass range, the surface
Al abundance was shown to be constant. Our main aim was to extend the
investigation of the Na and Al surface abundances to giants more massive than 3
Msun. We sought to establish accurately whether the Na abundances keep
increasing with stellar mass or a plateau is reached. In addition, we
investigated whether mixing can affect the surface abundance of Al in giants
more massive than 3 Msun. We obtained new high-resolution spectra of 20 giants
in the field of 10 open clusters; 17 of these stars were found to be members of
9 clusters. The giants have masses between 2.5 Msun and 5.6 Msun. Abundances of
C, N, and O were determined using spectrum synthesis. The abundances of Na and
Al were corrected for non-local thermodynamic equilibrium effects (non-LTE).
Moreover, to extend the mass range of our sample, we collected from the
literature high-quality C, N, O, and Na abundances of 32 Galactic Cepheids with
accurate masses in the range between 3 Msun and 14 Msun. The surface abundances
of C, N, O, Na, and Al were compared to predictions of stellar evolution models
with and without the inclusion of rotation-induced mixing. The surface
abundances of most giants and Cepheids of the sample can be explained by models
without rotation. For giants above ~ 2.5 Msun, the Na abundances reach a
plateau level of about [Na/Fe] ~ 0.20-0.25 dex (in non-LTE). Our results
support previous works that found models with rotation to overestimate the
mixing effects in intermediate-mass stars. [abridged]Comment: 17 pages, accepted for publication in A&
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