247 research outputs found
Reddenings of FGK supergiants and classical Cepheids from spectroscopic data
Accurate and homogeneous atmospheric parameters (Teff, log (g), Vt, [Fe/H])
are derived for 74 FGK non-variable supergiants from high-resolution, high
signal-to-noise ratio, echelle spectra. Extremely high precision for the
inferred effective temperatures (10-40 K) is achieved by using the line-depth
ratio method. The new data are combined with atmospheric values for 164
classical Cepheids, observed at 675 different pulsation phases, taken from our
previously published studies. The derived values are correlated with unreddened
B-V colours compiled from the literature for the investigated stars in order to
obtain an empirical relationship of the form: (B-V)o = 57.984 - 10.3587(log
Teff)^2 + 1.67572(log Teff)^3 - 3.356(log (g)) + 0.0321(Vt) + 0.2615[Fe/H] +
0.8833((log (g))(log Teff)). The expression is used to estimate colour excesses
E(B-V) for individual supergiants and classical Cepheids, with a precision of
+-0.05 mag. for supergiants and Cepheids with n=1-2 spectra, reaching +-0.025
mag. for Cepheids with n>2 spectra, matching uncertainties for the most
sophisticated photometric techniques. The reddening scale is also a close match
to the system of space reddenings for Cepheids. The application range is for
spectral types F0--K0 and luminosity classes I and II.Comment: accepted for publication (MNRAS
Abundance gradients in the Milky Way for alpha elements, Iron peak elements, Barium, Lanthanum and Europium
We model the abundance gradients in the disk of the Milky Way for several
chemical elements (O, Mg, Si, S, Ca, Sc, Ti, Co, V, Fe, Ni, Zn, Cu, Mn, Cr, Ba,
La and Eu), and compare our results with the most recent and homogeneous
observational data. We adopt a chemical evolution model able to well reproduce
the main properties of the solar vicinity. We compute, for the first time, the
abundance gradients for all the above mentioned elements in the galactocentric
distance range 4 - 22 kpc. The comparison with the observed data on Cepheids in
the galactocentric distance range 5-17 kpc gives a very good agreement for many
of the studied elements. In addition, we fit very well the data for the
evolution of Lanthanum in the solar vicinity for which we present results here
for the first time. We explore, also for the first time, the behaviour of the
abundance gradients at large galactocentric distances by comparing our results
with data relative to distant open clusters and red giants and select the best
chemical evolution model model on the basis of that. We find a very good fit to
the observed abundance gradients, as traced by Cepheids, for most of the
elements, thus confirming the validity of the inside-out scenario for the
formation of the Milky Way disk as well as the adopted nucleosynthesis
prescriptions.Comment: 11 pages, 9 figures, accepted for publication in A&
The origin of abundance gradients in the Milky Way: the predictions of different models
We aim at studying the abundance gradients along the Galactic disk and their
dependence upon several parameters: a threshold in the surface gas density
regulating star formation, the star formation efficiency, the timescale for the
formation of the thin disk and the total surface mass density of the stellar
halo. We test a model which considers a cosmological infall law. This law does
not predict an inside-out disk formation, but it allows to well fit the
properties of the solar vicinity. We study several cases. We find that to
reproduce at the same time the abundance, star formation rate and surface gas
density gradients along the Galactic disk it is necessary to assume an
inside-out formation for the disk. The threshold in the gas density is not
necessary and the same effect could be reached by assuming a variable star
formation efficiency. A cosmologically derived infall law with an inside-out
process for the disk formation and a variable star formation efficiency can
indeed well reproduce all the properties of the disk. However, the cosmological
model presented here does not have sufficient resolution to capture the
requested inside-out formation for the disk.Comment: 13 pages, 17 figures and 2 tables. Accepted for publication in
Astronomy & Astrophysic
First Stars XIV. Sulfur abundances in extremely metal-poor (EMP) stars
Sulfur is important: the site of its formation is uncertain, and at very low
metallicity the trend of [S/Fe] against [Fe/H] is controversial. Below
[Fe/H]=-2.0, [S/Fe] remains constant or it decreases with [Fe/H], depending on
the author and the multiplet used in the analysis. Moreover, although sulfur is
not significantly bound in dust grains in the ISM, it seems to behave
differently in DLAs and in old metal-poor stars. We aim to determine precise S
abundance in a sample of extremely metal-poor stars taking into account NLTE
and 3D effects. NLTE profiles of the lines of the multiplet 1 of SI have been
computed using a new model atom for S. We find sulfur in EMP stars to behave
like the other alpha-elements, with [S/Fe] remaining approximately constant for
[Fe/H]<-3. However, [S/Mg] seems to decrease slightly as a function of [Mg/H].
The overall abundance patterns of O, Na, Mg, Al, S, and K are best matched by
the SN model yields by Heger & Woosley. The [S/Zn] ratio in EMP stars is solar,
as found also in DLAs. We obtain an upper limit on the abundance of sulfur,
[S/Fe] < +0.5, for the ultra metal-poor star CS 22949-037. This, along with a
previous reported measurement of zinc, argues against the conjecture that the
light-element abundances pattern in this star, and, by analogy, the hyper
metal-poor stars HE 0107-5240 and HE 1327-2326, are due to dust depletion.Comment: 10 pages, accepted for publication in A&
The influence of chemical composition on the properties of Cepheid stars I - Period-Luminosity relation vs iron abundance
We have assessed the influence of the stellar iron content on the Cepheid
Period-Luminosity (PL) relation by relating the V band residuals from the
Freedman et al (2001) PL relation to [Fe/H] for 37 Galactic and Magellanic
Clouds Cepheids. The iron abundances were measured from FEROS and UVES
high-resolution and high-signal to noise optical spectra. Our data indicate
that the stars become fainter as metallicity increases, until a plateau or
turnover point is reached at about solar metallicity. Our data are incompatible
with both no dependence of the PL relation on iron abundance, and with the
linearly decreasing behavior often found in the literature (e.g. Kennicutt et
al 1998, Sakai et al 2004). On the other hand, non-linear theoretical models of
Fiorentino et al (2002) provide a fairly good description of the data.Comment: 5 pages, 2 figures. Accepted for pubblication in Astronomy &
Astrophysics Letters (Table 1 will only appear in electronic form). Revised
version (23.11.04) to fix typo in Table 1 (the V magnitudes for Galactic
stars were wrong
Classical Cepheid Pulsation Models. XI. Effects of convection and chemical composition on the Period-Luminosity and Period-Wesenheit relations
In spite of the relevance of Classical Cepheids as primary distance
indicators, a general consensus on the dependence of the Period-Luminosity (PL)
relation on the Cepheid chemical composition has not been achieved yet. From
the theoretical point of view, our previous investigations were able to
reproduce some empirical tests for suitable assumptions on the helium to metal
relative enrichment, but those results relied on specific assumptions
concerning the Mass-Luminosity relation and the efficiency of the convective
transfer in the pulsating envelopes. In this paper, we investigate the effects
of the assumed value of the mixing length parameter l/Hp on the pulsation
properties and we release the assumption of a fixed Mass-Luminosity relation.
As a whole, we show that our pulsation relations appear fully consistent with
the observed properties of Galactic and Magellanic Cloud Cepheids, supporting
the predicted steepening and brightening of the PL relations when moving from
metal-rich to metal-poor variables. Moreover, we show that the distances
inferred by the predicted PW relations agree with recently measured
trigonometric parallaxes, whereas they suggest a correction to the values based
on the Infrared Surface Brightness technique, as already found from an
independent method. Finally, also the pulsation metal contents suggested by the
predicted PW relations appear in statistical agreement with spectroscopic
[Fe/H] measurements.Comment: 20 pages, 15 figure
Controlled Synchronization of One Class of Nonlinear Systems under Information Constraints
Output feedback controlled synchronization problems for a class of nonlinear
unstable systems under information constraints imposed by limited capacity of
the communication channel are analyzed. A binary time-varying coder-decoder
scheme is described and a theoretical analysis for multi-dimensional
master-slave systems represented in Lurie form (linear part plus nonlinearity
depending only on measurable outputs) is provided. An output feedback control
law is proposed based on the Passification Theorem. It is shown that the
synchronization error exponentially tends to zero for sufficiantly high
transmission rate (channel capacity). The results obtained for synchronization
problem can be extended to tracking problems in a straightforward manner, if
the reference signal is described by an {external} ({exogenious}) state space
model. The results are applied to controlled synchronization of two chaotic
Chua systems via a communication channel with limited capacity.Comment: 8 pages, 2 figure
Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood
A number of studies of abundance gradients in the galactic disk have been
performed in recent years. The results obtained are rather disparate: from no
detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The
present study concerns the abundance gradient based on the spectroscopic
analysis of a sample of classical Cepheids. These stars enable one to obtain
reliable abundances of a variety of chemical elements. Additionally, they have
well determined distances which allow an accurate determination of abundance
distributions in the galactic disc. Using 236 high resolution spectra of 77
galactic Cepheids, the radial elemental distribution in the galactic disc
between galactocentric distances in the range 6-11 kpc has been investigated.
Gradients for 25 chemical elements (from carbon to gadolinium) are derived...Comment: 28 pages, 14 postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002) also available at
http://www.iagusp.usp.br/~maciel/index.htm
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