446 research outputs found
Non-LTE Abundances of Magnesium, Aluminum and Sulfur in OB Stars Near the Solar Circle
Non-LTE abundances of magnesium, aluminum and sulfur are derived for a sample
of 23 low-v \sin i stars belonging to six northern OB associations of the
Galactic disk within 1 kpc of the Sun. The abundances are obtained from the
fitting of synthetic line profiles to high resolution spectra. A comparison of
our results with HII region abundances indicates good agreement for sulfur
while the cepheid abundances are higher. The derived abundances of Mg show good
overlap with the cepheid results. The aluminum abundances for OB stars are
significantly below the cepheid values. But, the OB star results show a
dependence with effective temperature and need further investigation. The high
Al abundances in the cepheids could be the result of mixing. A discussion of
the oxygen abundance in objects near the solar circle suggests that the current
mean galactic oxygen abundance in this region is 8.6-8.7 and in agreement with
the recently revised oxygen abundance in the solar photosphere. Meaningful
comparisons of the absolute S, Al and Mg abundances in OB stars with the Sun
must await a reinvestigation of these elements, as well as the meteoritic
reference element Si, with 3D hydrodynamical model atmospheres for the Sun. No
abundance gradients are found within the limited range in galactocentric
distances in the present study. Such variations would be expected only if there
were large metallicity gradients in the disk.Comment: 3 figures, accepted for publication in A&A, needs aa.st
Surface abundances of light elements for a large sample of early B-type stars - IV. The magnesium abundance in 52 stars - a test of metallicity
From high-resolution spectra a non-LTE analysis of the MgII 4481.2 A feature
is implemented for 52 early and medium local B stars on the main sequence (MS).
The influence of the neighbouring line AlIII 4479.9 A is considered. The
magnesium abundance is determined; it is found that log e(Mg) = 7.67 +- 0.21 on
average. It is shown that uncertainties in the microturbulent parameter Vt are
the main source of errors in log e(Mg). When using 36 stars with the most
reliable Vt values derived from OII and NII lines, we obtain the mean abundance
log e(Mg) = 7.59 +- 0.15. The latter value is precisely confirmed for several
hot B stars from an analysis of the MgII 7877 A weak line. The derived
abundance log e(Mg) = 7.59 +- 0.15 is in excellent agreement with the solar
magnesium abundance log e_sun(Mg) = 7.55 +- 0.02, as well as with the proto-Sun
abundance log e_ps(Mg) = 7.62 +- 0.02. Thus, it is confirmed that the Sun and
the B-type MS stars in our neighbourhood have the same metallicity.Comment: 9 pages, 6 figures. Has been accepted for publication at MNRA
A cosmic abundance standard: chemical homogeneity of the solar neighbourhood and the ISM dust-phase composition
A representative sample of unevolved early B-type stars in nearby OB
associations and the field is analysed to unprecedented precision using NLTE
techniques. The resulting chemical composition is found to be more metal-rich
and much more homogeneous than indicated by previous work. A rms scatter of
~10% in abundances is found for the six stars (and confirmed by six evolved
stars), the same as reported for ISM gas-phase abundances. A cosmic abundance
standard for the present-day solar neighbourhood is proposed, implying mass
fractions for hydrogen, helium and metals of X=0.715, Y=0.271 and Z=0.014. Good
agreement with solar photospheric abundances as reported from recent 3D
radiative-hydrodynamical simulations of the solar atmosphere is obtained. As a
first application we use the cosmic abundance standard as a proxy for the
determination of the local ISM dust-phase composition, putting tight
observational constraints on dust models.Comment: 4 pages, 2 figures. Accepted for publication in ApJ Letter
Radial velocity measurements of B stars in the Scorpius-Centaurus association
We derive single-epoch radial velocities for a sample of 56 B-type stars
members of the subgroups Upper Scorpius, Upper Centaurus Lupus and Lower
Centaurus Crux of the nearby Sco-Cen OB association. The radial velocity
measurements were obtained by means of high-resolution echelle spectra via
analysis of individual lines. The internal accuracy obtained in the
measurements is estimated to be typically 2-3 km/s, but depends on the
projected rotational velocity of the target. Radial velocity measurements taken
for 2-3 epochs for the targets HD120307, HD142990 and HD139365 are variable and
confirm that they are spectroscopic binaries, as previously identified in the
literature. Spectral lines from two stellar components are resolved in the
observed spectra of target stars HD133242, HD133955 and HD143018, identifying
them as spectroscopic binaries.Comment: accepted for publication in A&
Projected Rotational Velocities and Stellar Characterization of 350 B Stars in the Nearby Galactic Disk
Projected rotational velocities (vsini) are presented for a sample of 350
early B-type main sequence stars in the nearby Galactic disk. The stars are
located within ~1.5 kpc from the Sun, and the great majority within 700 pc. The
analysis is based on high-resolution spectra obtained with the MIKE
spectrograph on the Magellan Clay 6.5-m telescope at the Las Campanas
Observatory in Chile.Spectral types were estimated based on relative
intensities of some key line absorption ratios and comparisons to synthetic
spectra. Effective temperatures were estimated from the reddening-free Q index,
and projected rotational velocities were then determined via interpolation on a
published grid that correlates the synthetic full width at half maximum of the
He I lines at 4026, 4388 and 4471 A with vsini. As the sample has been selected
solely on the basis of spectral types it contains an selection of B stars in
the field, in clusters, and in OB associations. The vsini distribution obtained
for the entire sample is found to be essentially flat for vsini values between
0-150 km/s, with only a modest peak at low projected rotational velocities.
Considering subsamples of stars, there appears to be a gradation in the vsini
distribution with the field stars presenting a larger fraction of the slow
rotators and the cluster stars distribution showing an excess of stars with
vsini between 70 and 130 km/s. Furthermore, for a subsample of potential
runaway stars we find that the vsini distribution resembles the distribution
seen in denser environments, which could suggest that these runaway stars have
been subject to dynamical ejection mechanisms.Comment: 38 pages, 11 figures. Complete sample table. AJ accepte
Sulfur Abundances in the Orion Association B Stars
Sulfur abundances are derived for a sample of ten B main-sequence star
members of the Orion association. The analysis is based on LTE plane-parallel
model atmospheres and non-LTE line formation theory by means of a
self-consistent spectrum synthesis analysis of lines from two ionization states
of sulfur, SII and SIII. The observations are high-resolution spectra obtained
with the ARCES spectrograph at the Apache Point Observatory. The abundance
distribution obtained for the Orion targets is homogeneous within the expected
errors in the analysis: A(S)=7.15+/-0.05. This average abundance result is in
agreement with the recommended solar value (both from modelling of the
photospheres in 1-D and 3-D, and meteorites) and indicates that little, if any,
chemical evolution of sulfur has taken place in the last ~4.5 billion years.
The sulfur abundances of the young stars in Orion are found to agree well with
results for the Orion nebulae, and place strong constraints on the amount of
sulfur depletion onto grains as being very modest or nonexistent. The sulfur
abundances for Orion are consistent with other measurements at a similar
galactocentric radius: combined with previous results for other OB-type stars
produce a relatively shallow sulfur abundance gradient with a slope of
-0.037+/-0.012 dex/kpc.Comment: Accepted for publication in The Astronomical Journa
Chemical abundances of fast-rotating massive stars. I. Description of the methods and individual results
Aims: Recent observations have challenged our understanding of rotational
mixing in massive stars by revealing a population of fast-rotating objects with
apparently normal surface nitrogen abundances. However, several questions have
arisen because of a number of issues, which have rendered a reinvestigation
necessary; these issues include the presence of numerous upper limits for the
nitrogen abundance, unknown multiplicity status, and a mix of stars with
different physical properties, such as their mass and evolutionary state, which
are known to control the amount of rotational mixing. Methods: We have
carefully selected a large sample of bright, fast-rotating early-type stars of
our Galaxy (40 objects with spectral types between B0.5 and O4). Their
high-quality, high-resolution optical spectra were then analysed with the
stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the
temperature of the target. Several internal and external checks were performed
to validate our methods; notably, we compared our results with literature data
for some well-known objects, studied the effect of gravity darkening, or
confronted the results provided by the two codes for stars amenable to both
analyses. Furthermore, we studied the radial velocities of the stars to assess
their binarity. Results: This first part of our study presents our methods and
provides the derived stellar parameters, He, CNO abundances, and the
multiplicity status of every star of the sample. It is the first time that He
and CNO abundances of such a large number of Galactic massive fast rotators are
determined in a homogeneous way.Comment: accepted for publication by A&
Brazilian music for youth choir by FUNARTE: exploring issues of pedagogy and authenticity in Brazilian choral music
The purpose of this study is to stimulate the dissemination of Brazilian choral repertoire, especially the pieces from the series Brazilian Music for Youth Choir by FUNARTE, and provide accurate interpretative information for authentic performance and effective teaching. Musical and pedagogical analyses of three of the pieces from the series by FUNARTE (2009) were done as means to the purpose of the study. In addition, ethnographic interview were held with the composers of each of those pieces, Paulo Malaguti, Leandro Maia, and Caio Senna, and the technical-pedagogical coordinator of the series, Eduardo Lakschevitz. Through the interviews with the composers it was possible to identify musical and pedagogical approaches relevant to accurate performance of Brazilian choral music, as well as to raise the discussion about authenticity in Brazilian music, which seemed to be a taboo among Brazilian musicians. The interview with the technical-pedagogical coordinator helped to understand the purpose of FUNARTE and the Choral Project. This study found that choral music in Brazil is a multifaceted phenomenon that does not fit the categories folk, popular, or classical music alone; so, it can be considered a style apart. The composers’ opinion about authenticity proves that it is flexible and that Brazilian musicians are open to new interpretations of Brazilian music. Complementary materials such as translation and IPA version of the pieces, and suggested reading and recording are provided to support music educators on an accurate and authentic performance of Brazilian choral repertoire
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