173 research outputs found
Barium and Yttrium abundance in intermediate-age and old open clusters
Barium is a neutron capture element, that, in open clusters, is frequently
over-abundant with respect to the Iron. A clear explanation for this is still
missing. Additionally, its gradient across the Galactic disk is poorly
constrained. We measure the abundance of yttrium and barium using the synthetic
spectrum method from UVES high-resolution spectra of eight distant open
clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75,
NGC 6192, NGC 6404, and NGC 6583. The barium abundance was estimated using NLTE
approximation. We confirm that Barium is indeed over-abundant in most clusters,
especially young clusters. Finally, we investigated the trend of yttrium and
barium abundances as a function of distance in the Galaxy and ages. Several
scenarios for the barium over-abundance are then discussed.Comment: 9 pages, 10 figure
On the correlation of elemental abundances with kinematics among galactic disk stars
We have performed the detailed analysis of 174 high-resolution spectra of FGK
dwarfs obtained with the ELODIE echelle spectrograph at the Observatoire de
Haute-Provence. Abundances of Fe, Si and Ni have been determined from
equivalent widths under LTE approximation, whereas abundances of Mg have been
determined under NLTE approximation using equivalent widths of 4 lines and
profiles of 5 lines. Spatial velocities with an accuracy better than 1 km/s, as
well as orbits, have been computed for all stars. They have been used to define
2 subsamples kinematically representative of the thin disk and the thick disk
in order to highlight their respective properties. A transition occurs at
[Fe/H]=-0.3. Stars more metal-rich than this value have a flat distribution
with Zmax<1 kpc and sigma_W<20 km/s, and a narrow distribution of [alpha/Fe].
There exist stars in this metallicity regime which cannot belong to the thin
disk because of their excentric orbits, neither to the thick disk because of
their low scale height. Several thin disk stars are identified down to
[Fe/H]=-0.80. Their Mg enrichment is lower than thick disk stars with the same
metallicity. We confirm from a larger sample the results of Feltzing et al
(2003) and Bensby et al (2003) showing a decrease of [alpha/Fe] with [Fe/H] in
the thick disk interpreted as the signature of the SNIa which have
progressively enriched the ISM with iron. However our data suggest that the
star formation in the thick disk stopped when the enrichment was [Fe/H]=-0.30,
[Mg/Fe]=+0.20, [Si/Fe]=+0.17. A vertical gradient in [alpha/Fe] may exist in
the thick disk but should be confirmed with a larger sample. Finally we have
identified 2 new candidates of the HR1614 moving group.Comment: Accepted in A&A, 16 pages, 14 figure
On the subject of the Ba overabundance in the open clusters stars
For eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583, we determined the yttrium and barium abundances using the UVES, VLT spectra (ESO, Chile). The stars of one young cluster (Ruprecht 7) demonstrate significant barium overabundance( 3c0.55 dex) that can not be due to the determination error. We have considered the Ba abundance determination errors due to LTE approach, saturation of the lines, synthetic and observed barium line fitting, and the causes of the Ba overabundance associated with the Galactic disc enrichment or the origin of open clusters. Possible explanation for this overabundance can be the origin of n-capture elements enrichment of the clusters (galactic or extragalactic) or additional sources of the Ba production
Oxygen, -element and iron abundance distributions in the inner part of the Galactic thin disc. II
We have derived the abundances of 36 chemical elements in one Cepheid star,
ASAS 181024--2049.6, located R kpc from the Galactic center.
This star falls within a region of the inner thin disc poorly sampled in
Cepheids. Our spectral analysis shows that iron, magnesium, silicon, calcium
and titanium LTE abundances in that star support the presence of a plateau-like
abundance distribution in the thin disc within 5 kpc of the Galactic center, as
previously suggested by \cite{Maret15}. If confirmed, the flattening of the
abundance gradient within that region could be the result of a decrease in the
star formation rate due to dynamic effects, possibly from the central Galactic
bar.Comment: 5 pages, 3 figure
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
The Great Eruption of Eta Carinae
During the years 1838-1858, the very massive star {\eta} Carinae became the
prototype supernova impostor: it released nearly as much light as a supernova
explosion and shed an impressive amount of mass, but survived as a star.1 Based
on a light-echo spectrum of that event, Rest et al.2 conclude that "a new
physical mechanism" is required to explain it, because the gas outflow appears
cooler than theoretical expectations. Here we note that (1) theory predicted a
substantially lower temperature than they quoted, and (2) their inferred
observational value is quite uncertain. Therefore, analyses so far do not
reveal any significant contradiction between the observed spectrum and most
previous discussions of the Great Eruption and its physics.Comment: To appear in Nature, a brief communication arising in response to
Rest et al. 2012. Submitted to Nature February 17, 201
An enigma of Przybylski's star: is there promethium on its surface?
We carried out a new attempt to check for the presence promethium lines in
the spectrum of HD101065 (Przybylski's star). The neutron capture element
promethium does not have stable isotopes and the maximum half-life time is
about 18 years. Thus its presence in this peculiar star would indicate an
ongoing process of irradiation of its surface layers with free neutrons.
Unfortunately, almost all promethium lines are heavily blended with lines of
other neutron capture elements and other species. We selected and analysed
three lines of promethium (Pm I and Pm II) and came to the conclusion that at
present it is impossible to definitely claim the presence of this element in
Przybylski's star atmosphere.Comment: Accepted for publication in Astronomische Nachrichte
The AMBRE Project: Stellar parameterisation of the ESO:FEROS archived spectra
The AMBRE Project is a collaboration between the European Southern
Observatory (ESO) and the Observatoire de la Cote d'Azur (OCA) that has been
established in order to carry out the determination of stellar atmospheric
parameters for the archived spectra of four ESO spectrographs.
The analysis of the FEROS archived spectra for their stellar parameters
(effective temperatures, surface gravities, global metallicities, alpha element
to iron ratios and radial velocities) has been completed in the first phase of
the AMBRE Project. From the complete ESO:FEROS archive dataset that was
received, a total of 21551 scientific spectra have been identified, covering
the period 2005 to 2010. These spectra correspond to ~6285 stars.
The determination of the stellar parameters was carried out using the stellar
parameterisation algorithm, MATISSE (MATrix Inversion for Spectral SynthEsis),
which has been developed at OCA to be used in the analysis of large scale
spectroscopic studies in galactic archaeology. An analysis pipeline has been
constructed that integrates spectral reduction and radial velocity correction
procedures with MATISSE in order to automatically determine the stellar
parameters of the FEROS spectra.
Stellar atmospheric parameters (Teff, log g, [M/H] and [alpha/Fe]) were
determined for 6508 (30.2%) of the FEROS archived spectra (~3087 stars). Radial
velocities were determined for 11963 (56%) of the archived spectra. 2370 (11%)
spectra could not be analysed within the pipeline. 12673 spectra (58.8%) were
analysed in the pipeline but their parameters were discarded based on quality
criteria and error analysis determined within the automated process. The
majority of these rejected spectra were found to have broad spectral features
indicating that they may be hot and/or fast rotating stars, which are not
considered within the adopted reference synthetic spectra grid of FGKM stars.Comment: 28 pages, 28 figures, 9 table
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