121 research outputs found
Chemical composition of intermediate mass stars members of the M6 (NGC 6405) open cluster
We present here the first abundance analysis of 44 late B, A and F-type
members of the young open cluster M6 (NGC 6405, age about 75 Myrs). Spectra,
covering the 4500 to 5800 \AA{} wavelength range, were obtained using the
FLAMES/GIRAFFE spectrograph attached to the ESO Very Large Telescopes (VLT). We
determined the atmospheric parameters using calibrations of the Geneva
photometry and by adjusting the profiles to synthetic ones. The
abundances of up to 20 chemical elements, were derived for 19 late B, 16 A and
9 F stars by iteratively adjusting synthetic spectra to the observations. We
also derived a mean cluster metallicity of dex
from the iron abundances of the F-type stars. We find that, for most chemical
elements, the normal late B and A-type stars exhibit larger star-to-star
abundance variations than the F-type stars do probably because of the faster
rotation of the B and A stars. The abundances of C, O, Mg, Si and Sc appear to
be anticorrelated to that of Fe, while the opposite holds for the abundances of
Ca, Ti, Cr, Mn, Ni, Y, and Ba about as expected if radiative diffusion is
efficient in the envelopes of these stars. In the course of this analysis, we
discovered five new peculiar stars: one mild-Am, one Am, and one Fm star (HD
318091, CD-32 13109, GSC 07380-01211), one HgMn star (HD 318126), and one
He-weak P-rich (HD 318101) star. We also discovered a new spectroscopic binary,
most likely a SB2. We performed a detailed modelling of HD 318101,the new
He-weak P-rich CP star, using the Montr\'eal stellar evolution code XEVOL which
treats self-consistently all particle transport processes. Although the overall
abundance pattern of this star is properly reproduced, we find that detailed
abundances (in particular the high P excess) resisted modelling attempts even
when a range of turbulence profiles and mass loss rates were considered.Comment: Accepted for publication in Astronomical Journal (Oct.15,2015), 115
pages, 15 figure
Parallélogrammes Galoisiens
RésuméWe generalize the notion of a Galois extension by that of a Galois parallelogram; a Galois extension is just a flat Galois parallelogram. The existence of Galois parallelograms is proved in the number field case. We state numerous general properties of Galois parallelograms which give, in particular, the usual properties of classical Galois theory. Then, we introduce a “two-dimensional Galois theory” by generalizing to Galois parallelograms the classical Galois bijection for finite Galois extensions
A study of the elements copper through uranium in Sirius A: Contributions from STIS and ground-based spectra
We determine abundances or upper limits for all of the 55 stable elements
from copper to uranium for the A1 Vm star Sirius. The purpose of the study is
to assemble the most complete picture of elemental abundances with the hope of
revealing the chemical history of the brightest star in the sky, apart from the
Sun. We also explore the relationship of this hot metallic-line (Am) star to
its cooler congeners, as well as the hotter, weakly- or non-magnetic
mercury-manganese (HgMn) stars. Our primary observational material consists of
{\em Hubble Space Telescope} () spectra taken with the Space Telescope
Imaging Spectrograph (STIS) in the ASTRAL project. We have also used archival
material from the %\citep/{ayr10}. satellite, and from the
Goddard High-Resolution Spectrograph (GHRS), as well as ground-based spectra
from Furenlid, Westin, Kurucz, Wahlgren, and their coworkers, ESO spectra from
the UVESPOP project, and NARVAL spectra retrieved from PolarBase. Our analysis
has been primarily by spectral synthesis, and in this work we have had the
great advantage of extensive atomic data unavailable to earlier workers. We
find most abundances as well as upper limits range from 10 to 100 times above
solar values. We see no indication of the huge abundance excesses of 1000 or
more that occur among many chemically peculiar (CP) stars of the upper main
sequence. The picture of Sirius as a hot Am star is reinforced.Comment: With 6 Figures and 4 Tables; accepted for publication in Ap
The SIMBAD astronomical database
Simbad is the reference database for identification and bibliography of
astronomical objects. It contains identifications, `basic data', bibliography,
and selected observational measurements for several million astronomical
objects. Simbad is developed and maintained by CDS, Strasbourg. Building the
database contents is achieved with the help of several contributing institutes.
Scanning the bibliography is the result of the collaboration of CDS with
bibliographers in Observatoire de Paris (DASGAL), Institut d'Astrophysique de
Paris, and Observatoire de Bordeaux. When selecting catalogues and tables for
inclusion, priority is given to optimal multi-wavelength coverage of the
database, and to support of research developments linked to large projects. In
parallel, the systematic scanning of the bibliography reflects the diversity
and general trends of astronomical research.
A WWW interface to Simbad is available at: http://simbad.u-strasbg.fr/SimbadComment: 14 pages, 5 Postscript figures; to be published in A&A
Chemical composition of A and F dwarf members of the Coma Berenices open cluster
Abundances of 18 chemical elements have been derived for 11 A (normal and Am)
and 11 F dwarfs members of the Coma Berenices open cluster in order to set
constraints on evolutionary models including transport processes (radiative and
turbulent diffusion)calculated with the Montreal code. A spectral synthesis
iterative procedure has been applied to derive the abundances from selected
high quality lines in high resolution high signal-to-noise echelle spectra
obtained with ELODIE at the Observatoire de Haute Provence. The chemical
pattern found for the A and F dwarfs in Coma Berenices is reminiscent of that
found in the Hyades and the UMa moving group. In graphs representing the
abundances [X/H] versus the effective temperature, the A stars often display
abundances much more scattered around their mean values than the F stars do.
Large star-to-star variations are detected for A stars in their abundances
which we interpret as evidence of transport processes competing with radiative
diffusion. The F stars have solar abundances for almost all elements except for
Mg, Si, V and Ba. The derived abundances patterns, [X/H] versus atomic number,
for the slow rotator HD108642 (A2m) and the moderately fast rotator HD106887
(A4m) were compared to the predictions of self consistent evolutionary model
codes including radiative and different amounts of turbulent diffusion. None of
the models reproduces entirely the overall shape of the abundance pattern.
While part of the discrepancies between derived and predicted abundances may be
accounted for by non-LTE effects, the inclusion of competing processes such as
rotational mixing in the radiative zones of these stars seems necessary to
improve the agreement between observed and predicted abundance patterns.Comment: 25 pages, 20 figure
The BTC40 Survey for Quasars at 4.8 < z < 6
The BTC40 Survey for high-redshift quasars is a multicolor search using
images obtained with the Big Throughput Camera (BTC) on the CTIO 4-m telescope
in V, I, and z filters to search for quasars at redshifts of 4.8 < z < 6. The
survey covers 40 sq. deg. in B, V, & I and 36 sq. deg. in z. Limiting
magnitudes (3 sigma) reach to V = 24.6, I = 22.9 and z = 22.9. We used the
(V-I) vs. (I-z) two-color diagram to select high-redshift quasar candidates
from the objects classified as point sources in the imaging data. Follow-up
spectroscopy with the AAT and CTIO 4-m telescopes of candidates having I < 21.5
has yielded two quasars with redshifts of z = 4.6 and z = 4.8 as well as four
emission line galaxies with z = 0.6. Fainter candidates have been identified
down to I = 22 for future spectroscopy on 8-m class telescopes.Comment: 27 pages, 8 figures; Accepted for publication in the Astronomical
Journa
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