414 research outputs found
C II abundances in early-type stars: solution to a notorious non-LTE problem
We address a long-standing discrepancy between non-LTE analyses of the
prominent C II 4267 and 6578/82 A multiplets in early-type stars. A
comprehensive non-LTE model atom of C II is constructed based on critically
selected atomic data. This model atom is used for an abundance study of six
apparently slow-rotating main-sequence and giant early B-type stars.
High-resolution and high-S/N spectra allow us to derive highly consistent
abundances not only from the classical features but also from up to 18 further
C II lines in the visual - including two so far unreported emission features
equally well reproduced in non-LTE. These results require the stellar
atmospheric parameters to be determined with care. A homogeneous (slightly)
sub-solar present-day carbon abundance from young stars in the solar vicinity
(in associations and in the field) of log C/H +12= 8.29+/-0.03 is indicated.Comment: 8 pages, 5 figure
Disorder-induced double resonant Raman process in graphene
An analytical study is presented of the double resonant Raman scattering
process in graphene, responsible for the D and D features in the
Raman spectra. This work yields analytical expressions for the D and
D integrated Raman intensities that explicitly show the dependencies
on laser energy, defect concentration, and electronic lifetime. Good agreement
is obtained between the analytical results and experimental measurements on
samples with increasing defect concentrations and at various laser excitation
energies. The use of Raman spectroscopy to identify the nature of defects is
discussed. Comparison between the models for the edge-induced and the
disorder-induced D band intensity suggests that edges or grain boundaries can
be distinguished from disorder by the different dependence of their Raman
intensity on laser excitation energy. Similarly, the type of disorder can
potentially be identified not only by the intensity ratio
, but also by its laser energy
dependence. Also discussed is a quantitative analysis of quantum interference
effects of the graphene wavefunctions, which determine the most important
phonon wavevectors and scattering processes responsible for the D and
D bands.Comment: 10 pages, 4 figure
Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars
It is generally accepted that the atmospheres of cool/lukewarm stars of
spectral types A and later are described well by LTE model atmospheres, while
the O-type stars require a detailed treatment of NLTE effects. Here model
atmosphere structures, spectral energy distributions and synthetic spectra
computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid
method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE
are compared. Their ability to reproduce observations for effective
temperatures between 15000 and 35000 K are verified. Strengths and weaknesses
of the different approaches are identified. Recommendations are made as to how
to improve the models in order to derive unbiased stellar parameters and
chemical abundances in future applications, with special emphasis on Gaia
science.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics:
Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er
Modelo anual de la dinámica sedimentaria en una marisma mareal mediterránea.
Se ha detectado un patrón estacional en la dinámica sedimentaria en zonas de marisma baja, relacionado
con la distribución anual de las precipitaciones caracterÃstica del clima mediterráneo, que induce cambios en el
caudal de los rÃos y en la cantidad y distribución de los aportes de partÃculas finas que entran en el estuario. En
este escenario se pone de manifiesto cómo la presencia de vegetación (Spartina maritima) favorece la acreción y
estabilización de sedimentos. En planicies desprovistas de vegetación, los mayores niveles de acreción se registran
durante primavera y principios de verano. Las lluvias otoñales determinan un aumento del caudal de los rÃos,
con un incremento en el aporte de sedimentos y removilización de éstos en las zonas desnudas. Parte de estos sedimentos
quedarán entonces retenidos en las zonas provistas de vegetación. Durante este periodo de otoño es cuando
esta vegetación alcanza sus mayores valores de biomasa, maximizando asà su papel como estabilizador de sedimentos.We have found a seasonal pattern operating on the lower areas of a tidal marsh, in relation to the characteristic
annual distribution of the rainfall in a Mediterranean climate which induces changes in river flows and in
quantity and distribution of loam sediments entering the estuary. Also, the presence of vegetation (S. maritima)
favours the accretion rate and the sediment stabilization. Bare muds record the maximum values of accretion during
Spring and early Summer. Autumn rainfalls increase the river flow what means a new input of sediments and the
movilization of the old ones from the bare muds; the vegetation raises the maximum biomass and it traps and stabilizes
these sediments.Dirección General de Investigación CientÃfica y Técnica. Gobierno de España-PB94-145
Pristine CNO abundances from Magellanic Cloud B stars II. Fast rotators in the LMC cluster NGC 2004
We present spectroscopic abundance analyses of three main-sequence B stars in
the young Large Magellanic Cloud cluster NGC 2004. All three targets have
projected rotational velocities around 130 km/s. Techniques are presented that
allow the derivation of stellar parameters and chemical abundances in spite of
these high v sin i values. Together with previous analyses of stars in this
cluster, we find no evidence among the main-sequence stars for effects due to
rotational mixing up to v sin i around 130 km/s. Unless the equatorial
rotational velocities are significantly larger than the v sin i values, this
finding is probably in line with theoretical expectations. NGC 2004/B30, a star
of uncertain evolutionary status located in the Blue Hertzsprung Gap, clearly
shows signs of mixing in its atmosphere. To verify the effects due to
rotational mixing will therefore require homogeneous analysis of statistically
significant samples of low-metallicity main-sequence B stars over a wide range
of rotational velocities.Comment: 12 pages, 5 figures, 2 tables; accepted for publication in ApJ (vol.
633, p. 899
B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid Nebula, one of the youngest star forming regions
Recent magnetic field surveys in O- and B-type stars revealed that about 10%
of the core-hydrogen-burning massive stars host large-scale magnetic fields.
The physical origin of these fields is highly debated. To identify and model
the physical processes responsible for the generation of magnetic fields in
massive stars, it is important to establish whether magnetic massive stars are
found in very young star-forming regions or whether they are formed in close
interacting binary systems.
In the framework of our ESO Large Program, we carried out low-resolution
spectropolarimetric observations with FORS2 in 2013 April of the three most
massive central stars in the Trifid nebula, HD164492A, HD164492C, and
HD164492D. These observations indicated a strong longitudinal magnetic field of
about 500-600G in the poorly studied component HD164492C. To confirm this
detection, we used HARPS in spectropolarimetric mode on two consecutive nights
in 2013 June.
Our HARPS observations confirmed the longitudinal magnetic field in
HD164492C. Furthermore, the HARPS observations revealed that HD164492C cannot
be considered as a single star as it possesses one or two companions. The
spectral appearance indicates that the primary is most likely of spectral type
B1-B1.5V. Since in both observing nights most spectral lines appear blended, it
is currently unclear which components are magnetic. Long-term monitoring using
high-resolution spectropolarimetry is necessary to separate the contribution of
each component to the magnetic signal. Given the location of the system
HD164492C in one of the youngest star formation regions, this system can be
considered as a Rosetta Stone for our understanding of the origin of magnetic
fields in massive stars.Comment: Five pages, six figures, accepted for publication in A&
Role of twin boundaries on the vortex dynamics in YBaCuO
By means of a novel technique of rotating the applied current we have
directly measured the influence of twin boundaries on the vortex motion in a
YBaCuO single crystal. The results indicate that the effect of twin
planes on the vortex dynamics starts to develop below a certain temperature,
being responsible for an anisotropic viscosity in the vortex liquid state and a
guided motion in the solid state.Comment: 4 pages, 4 figure
Summary of IAU GA SpS 5 II: Stellar and Wind Parameters
The development of infrared observational facilities has revealed a number of massive stars in obscured environments throughout the Milky Way and beyond. The determination of their stellar and wind properties from infrared diagnostics is thus required to take full advantage of the wealth of observations available in the near and mid infrared. However, the task is challenging. This session addressed some of the problems encountered and showed the limitations and successes of infrared studies of massive stars
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