157 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
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
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&
Chaotic Observer-based Synchronization Under Information Constraints
Limit possibilities of observer-based synchronization systems under
information constraints (limited information capacity of the coupling channel)
are evaluated. We give theoretical analysis for multi-dimensional
drive-response systems represented in the Lurie form (linear part plus
nonlinearity depending only on measurable outputs). It is shown that the upper
bound of the limit synchronization error (LSE) is proportional to the upper
bound of the transmission error. As a consequence, the upper and lower bounds
of LSE are proportional to the maximum rate of the coupling signal and
inversely proportional to the information transmission rate (channel capacity).
Optimality of the binary coding for coders with one-step memory is established.
The results are applied to synchronization of two chaotic Chua systems coupled
via a channel with limited capacity.Comment: 7 pages, 6 figures, 27 reference
Barium abundance in red giants of NGC 6752. Non-local thermodynamic equilibrium and three-dimensional effects
(Abridged) Aims: We study the effects related to departures from non-local
thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red
giant stars in Galactic globular cluster NGC 6752, to assess their influence on
the formation of Ba II lines. Methods: One-dimensional (1D) local thermodynamic
equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D
ATLAS stellar model atmospheres. The three-dimensional (3D) LTE abundances were
obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE
abundances using 3D-1D abundance corrections, i.e., the differences between the
abundances obtained from the same spectral line using the 3D hydrodynamical
(CO5BOLD) and classical 1D (LHD) stellar model atmospheres. Results: The mean
1D barium-to-iron abundance ratios derived for 20 giants are _{1D
NLTE} = 0.05 \pm0.06 (stat.) \pm0.08 (sys.). The 3D-1D abundance correction
obtained for 8 giants is small (~+0.05 dex), thus leads to only minor
adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for
the 20 giants, _{3D+NLTE} = 0.10 \pm0.06(stat.) \pm0.10(sys.). The
intrinsic abundance spread between the individual cluster stars is small and
can be explained in terms of uncertainties in the abundance determinations.
Conclusions: Deviations from LTE play an important role in the formation of
barium lines in the atmospheres of red giants studied here. The role of 3D
hydrodynamical effects should not be dismissed either, even if the obtained
3D-1D abundance corrections are small. This result is a consequence of subtle
fine-tuning of individual contributions from horizontal temperature
fluctuations and differences between the average temperature profiles in the 3D
and 1D model atmospheres: owing to the comparable size and opposite sign, their
contributions nearly cancel each other.Comment: Minor typos corrected. Accepted for publication in A&A (9 pages, 3
figures, 6 tables
The chemical composition of nearby young associations: s-process element abundances in AB Doradus, Carina-Near, and Ursa Major
Recently, several studies have shown that young, open clusters are
characterised by a considerable over-abundance in their barium content. In
particular, D'Orazi et al. (2009) reported that in some younger clusters
[Ba/Fe] can reach values as high as ~0.6 dex. The work also identified the
presence of an anti-correlation between [Ba/Fe] and cluster age. For clusters
in the age range ~4.5 Gyr-500 Myr, this is best explained by assuming a higher
contribution from low-mass asymptotic giant branch stars to the Galactic
chemical enrichment. The purpose of this work is to investigate the ubiquity of
the barium over-abundance in young stellar clusters. We analysed
high-resolution spectroscopic data, focusing on the s-process elemental
abundance for three nearby young associations, i.e. AB Doradus, Carina-Near,
and Ursa Major. The clusters have been chosen such that their age spread would
complement the D'Orazi et al. (2009) study. We find that while the s-process
elements Y, Zr, La, and Ce exhibit solar ratios in all three associations, Ba
is over-abundant by ~0.2 dex. Current theoretical models can not reproduce this
abundance pattern, thus we investigate whether this unusually large Ba content
might be related to chromospheric effects. Although no correlation between
[Ba/Fe] and several activity indicators seems to be present, we conclude that
different effects could be at work which may (directly or indirectly) be
related to the presence of hot stellar chromospheres.Comment: Accepted for publication in MNRA
A Cepheid is No More: Hubble's Variable 19 in M33
We report on the remarkable evolution in the light curve of a variable star
discovered by Hubble (1926) in M33 and classified by him as a Cepheid. Early in
the 20th century, the variable, designated as V19, exhibited a 54.7 day period,
an intensity-weighted mean B magnitude of 19.59+/-0.23 mag, and a B amplitude
of 1.1 mag. Its position in the P-L plane was consistent with the relation
derived by Hubble from a total of 35 variables. Modern observations by the
DIRECT project show a dramatic change in the properties of V19: its mean B
magnitude has risen to 19.08 +/- 0.05 mag and its B amplitude has decreased to
less than 0.1 mag. V19 does not appear to be a classical (Population I) Cepheid
variable at present, and its nature remains a mystery. It is not clear how
frequent such objects are nor how often they could be mistaken for classical
Cepheids.Comment: Accepted for publication in the Astrophysical Journal Letters.
Finding charts and photometry data can be downloaded from
http://cfa-www.harvard.edu/~kstanek/DIRECT
The ESO Large Programme First Stars
In ESO period 65 (April-September 2000) the large programme 165.N-0276, led
by Roger Cayrel, began making use of UVES at the Kueyen VLT telescope. Known
within the Team and outside as ``First Stars'', it was aimed at obtaining high
resolution, high signal-to-noise ratio spectra in the range 320 nm -- 1000 nm
for a large sample of extremely metal-poor (EMP) stars identified from the HK
objective prism survey.In this contribution we highlight the main results of
the large programme.Comment: to be published in the proceedings of the Workshop "Science with VLT
in the ELT era" 8-12 October 2007, Garching, ed. A. Moorwoo
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