9,027 research outputs found
Carbon line formation and spectroscopy in O-type stars
The determination of chemical abundances constitutes a fundamental
requirement for obtaining a complete picture of a star. Particularly in massive
stars, CNO abundances are of prime interest, due to the nuclear CNO-cycle and
various mixing processes which bring these elements to the surface.
We aim at enabling a reliable carbon spectroscopy for our unified NLTE
atmosphere code FASTWIND.
We develop a new carbon model atom including CII/III/IV/V, and discuss
problems related to carbon spectroscopy in O-type stars. We describe different
tests to examine the reliability of our implementation, and investigate which
mechanisms influence the carbon ionization balance. By comparing with
high-resolution spectra from six O-type stars, we check in how far
observational constraints can be reproduced by our new carbon line synthesis.
Carbon lines are even more sensitive to a variation of temperature, gravity,
and mass-loss rate, than hydrogen/helium lines. We are able to reproduce most
of the observed lines from our stellar sample, and to estimate those specific
carbon abundances which bring the lines from different ions into agreement. For
hot dwarfs and supergiants earlier than O7, X-rays from wind-embedded shocks
can impact the synthesized line strengths, particularly for CIV, potentially
affecting the abundance determination.
We have demonstrated our capability to derive realistic carbon abundances by
means of FASTWIND, using our recently developed model atom. We found that
complex effects can have a strong influence on the carbon ionization balance in
hot stars. For a further understanding, the UV range needs to be explored as
well. By means of detailed nitrogen and oxygen model atoms available to use, we
will be able to perform a complete CNO abundance analysis for larger samples of
massive stars, and to provide constraints on corresponding evolutionary models
and aspects.Comment: 22 pages, 16 figures, 6 table
Distributional approach to point interactions in one-dimensional quantum mechanics
We consider the one-dimensional quantum mechanical problem of defining
interactions concentrated at a single point in the framework of the theory of
distributions. The often ill-defined product which describes the interaction
term in the Schr\"odinger and Dirac equations is replaced by a well-defined
distribution satisfying some simple mathematical conditions and, in addition,
the physical requirement of probability current conservation is imposed. A
four-parameter family of interactions thus emerges as the most general point
interaction both in the non-relativistic and in the relativistic theories (in
agreement with results obtained by self-adjoint extensions). Since the
interaction is given explicitly, the distributional method allows one to carry
out symmetry investigations in a simple way, and it proves to be useful to
clarify some ambiguities related to the so-called interaction.Comment: Open Access link:
http://journal.frontiersin.org/Journal/10.3389/fphy.2014.00023/abstrac
Atmospheric NLTE-Models for the Spectroscopic Analysis of Blue Stars with Winds. III. X-ray emission from wind-embedded shocks
X-rays/EUV radiation emitted from wind-embedded shocks in hot, massive stars
can affect the ionization balance in their outer atmospheres, and can be the
mechanism responsible for the production of highly ionized species. To allow
for these processes in the context of spectral analysis, we have implemented
such emission into our unified, NLTE model atmosphere/spectrum synthesis code
FASTWIND.
The shock structure and corresponding emission is calculated as a function of
user-supplied parameters. We account for a temperature and density
stratification inside the post-shock cooling zones, calculated for radiative
and adiabatic cooling in the inner and outer wind, respectively. The
high-energy absorption of the cool wind is considered by adding important
K-shell opacities, and corresponding Auger ionization rates have been included
into the NLTE network.
We tested and verified our implementation carefully against corresponding
results from various alternative model atmosphere codes, and studied the
effects from shock emission for important ions from He, C, N, O, Si, and P.
Surprisingly, dielectronic recombination turned out to play an essential role
for the ionization balance of OIV/OV around Teff = 45,000 K. Finally, we
investigated the behavior of the mass absorption coefficient, kappa_nu(r),
important in the context of X-ray line formation in massive star winds.
In almost all considered cases, direct ionization is of major influence, and
Auger ionization significantly affects only NVI and OVI. The approximation of a
radially constant kappa_nu is justified for r > 1.2 Rstar and lambda < 18 A,
and also for many models at longer wavelengths. To estimate the actual value of
this quantity, however, the HeII opacities need to be calculated from detailed
NLTE modeling, at least for wavelengths longer than 18 to 20 A, and information
on the individual CNO abundances has to be present.Comment: accepted by A&
Relativistic Tunneling Through Two Successive Barriers
We study the relativistic quantum mechanical problem of a Dirac particle
tunneling through two successive electrostatic barriers. Our aim is to study
the emergence of the so-called \emph{Generalized Hartman Effect}, an effect
observed in the context of nonrelativistic tunneling as well as in its
electromagnetic counterparts, and which is often associated with the
possibility of superluminal velocities in the tunneling process. We discuss the
behavior of both the phase (or group) tunneling time and the dwell time, and
show that in the limit of opaque barriers the relativistic theory also allows
the emergence of the Generalized Hartman Effect. We compare our results with
the nonrelativistic ones and discuss their interpretation.Comment: 7 pages, 3 figures. Revised version, with a new appendix added.
Slightly changes in the styles and captions of Figures 1 and 2. To appear in
Physical Review
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