4,122 research outputs found
Comparison of Global and Local Adaptive Coordinates for Density Functional Calculations
A globally-adaptive curvilinear coordinate formalism is shown to be easily
convertible to a class of curvilinear transformations locally optimized around
atom sites by a few parameters. Parameter transferability is established for a
demanding test case, and the results of the two methods are shown to be
comparable. Computational efficiencies realized in the local method are
discussed.Comment: 21 pages, 4 figure
Weak magnetic fields in central stars of planetary nebulae?
It is not yet clear whether magnetic fields play an essential role in shaping
planetary nebulae (PNe), or whether stellar rotation alone and/or a close
binary companion can account for the variety of the observed nebular
morphologies. In a quest for empirical evidence verifying or disproving the
role of magnetic fields in shaping PNe, we follow up on previous attempts to
measure the magnetic field in a representative sample of PN central stars. We
obtained low-resolution polarimetric spectra with FORS 2 at VLT for a sample of
twelve bright central stars of PNe with different morphology, including two
round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets
are Wolf-Rayet type central stars. For the majority of the observed central
stars, we do not find any significant evidence for the existence of surface
magnetic fields. However, our measurements may indicate the presence of weak
mean longitudinal magnetic fields of the order of 100 Gauss in the central star
of the young elliptical planetary nebula IC 418, as well as in the Wolf-Rayet
type central star of the bipolar nebula Hen2-113 and the weak emission line
central star of the elliptical nebula Hen2-131. A clear detection of a 250 G
mean longitudinal field is achieved for the A-type companion of the central
star of NGC 1514. Some of the central stars show a moderate night-to-night
spectrum variability, which may be the signature of a variable stellar wind
and/or rotational modulation due to magnetic features. We conclude that strong
magnetic fields of the order of kG are not widespread among PNe central stars.
Nevertheless, simple estimates based on a theoretical model of magnetized wind
bubbles suggest that even weak magnetic fields below the current detection
limit of the order of 100 G may well be sufficient to contribute to the shaping
of PNe throughout their evolution.Comment: 16 pages, 11 figures, 3 tables, accepted for publication in A&A;
References updated, minor correction
Nominal plurals in Sign Language of the Netherlands: Accounting for allomorphy and variation
n both signed and spoken languages, reduplication is a common process in the formation of morphologically complex structures, expressing, e.g., plurality and certain aspectual meanings. A framework in which spoken language reduplication has been formalized frequently is Optimality Theory (OT). While an important attribute of OT-constraints is their universality, to date, the question to what extent such constraints are modality-independent, and thus work for sign language reduplication as well, remains largely unanswered. In the present study, we offer the first OT-formalization of plural reduplication in Sign Language of the Netherlands (NGT). The NGT-data reveal that this language features different plural allomorphs, the choice of which depends on phonological properties of the base noun. However, we also identify variation, e.g., all noun types allow for zero marking.In our formalization, we aim to introduce constraints that are maximally modality-independent, using constraint types that have previously been proposed for spoken language reduplication. Our formalization is the first to take into account base-reduplicant faithfulness for a sign language, and also the first to account for variation in sign language data by employing stochastic OT, whereby some noise is added to the ranking value of each constraint at evaluation time. Evaluating the modality-(in)dependence of our proposed account suggests that the types of constraints we employ as well as the evaluation in the spirit of stochastic OT are not specific to a modality, while the featural implementation is inevitably modality-dependent
Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars
Context. Metal-poor massive stars are supposed to be progenitors of certain
supernovae, gamma-ray bursts and compact object mergers, potentially
contributing to the early epochs of the Universe with their strong ionizing
radiation. However, they remain mainly theoretical as individual spectroscopic
observations of such objects have rarely been carried out below the metallicity
of the SMC.
Aims. This work aims at exploring what our state-of-the-art theories of
stellar evolution combined with those of stellar atmospheres predict about a
certain type of metal-poor (0.02 Z) hot massive stars, the chemically
homogeneously evolving ones, called TWUIN stars.
Methods. Synthetic spectra corresponding to a broad range in masses (20-130
M) and covering several evolutionary phases from the zero-age
main-sequence up to the core helium-burning stage were computed.
Results. We find that TWUIN stars show almost no emission lines during most
of their {core hydrogen-burning} lifetimes. Most metal lines are completely
absent, including nitrogen. During their core helium-burning stage, lines
switch to emission and even some metal lines (oxygen and carbon, but still
almost no nitrogen) show up. Mass loss and clumping play a significant role in
line-formation in later evolutionary phases, particularly during core
helium-burning. Most of our spectra are classified as an early O type giant or
supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning
phase.
Conclusions. An extremely hot, early O type star observed in a
low-metallicity galaxy could be the outcome of chemically homogeneous evolution
and therefore the progenitor of a long-duration gamma-ray burst or a type
Ic supernova. TWUIN stars may play an important role in reionizing the Universe
due to their being hot without showing prominent emission lines during the
majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres
Wind ionization structure of the short-period eclipsing LMC Wolf-Rayet binary BAT99-129: preliminary results
BAT99-129 is a rare, short-period eclipsing Wolf-Rayet binary in the Large
Magellanic Cloud. We present here medium-resolution NTT/EMMI spectra that allow
us to disentangle the spectra of the two components and find the orbital
parameters of the binary. We also present VLT/FORS1 spectra of this binary
taken during the secondary eclipse, i.e. when the companion star passes in
front of the Wolf-Rayet star. With these data we are able to extract, for the
first time in absolute units for a WR+O binary, the sizes of the line emitting
regions.Comment: 6 pages, 5 figures, to appear in proc. of "Close Binaries in the 21st
Century: New Opportunities and Challenges", 2005 - Corrected Figure
The Wolf-Rayet stars in the Large Magellanic Cloud: A comprehensive analysis of the WN class
Aims: Following our comprehensive studies of the WR stars in the Milky Way,
we now present spectroscopic analyses of almost all known WN stars in the LMC.
Methods: For the quantitative analysis of the wind-dominated emission-line
spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By
fitting synthetic spectra to the observed spectral energy distribution and the
available spectra (ultraviolet and optical), we obtain the physical properties
of 107 stars. Results: We present the fundamental stellar and wind parameters
for an almost complete sample of WN stars in the LMC. Among those stars that
are putatively single, two different groups can be clearly distinguished. While
12% of our sample are more luminous than 10^6 Lsun and contain a significant
amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate
the luminosity range between log (L/Lsun) = 5.3...5.8. Conclusions: While the
few extremely luminous stars (log (L/Lsun) > 6), if indeed single stars,
descended directly from the main sequence at very high initial masses, the bulk
of WN stars have gone through the red-supergiant phase. According to their
luminosities in the range of log (L/Lsun) = 5.3...5.8, these stars originate
from initial masses between 20 and 40 Msun. This mass range is similar to the
one found in the Galaxy, i.e. the expected metallicity dependence of the
evolution is not seen. Current stellar evolution tracks, even when accounting
for rotationally induced mixing, still partly fail to reproduce the observed
ranges of luminosities and initial masses. Moreover, stellar radii are
generally larger and effective temperatures correspondingly lower than
predicted from stellar evolution models, probably due to subphotospheric
inflation.Comment: 17+46 pages; 10+54 figures; v2: typos corrected, space-saving layout
for appendix C, published in A&
Highly Ionized Collimated Outflow from HE 0238 - 1904
We present a detailed analysis of a highly ionized, multiphased and
collimated outflowing gas detected through O V, O VI, Ne VIII and Mg X
absorption associated with the QSO HE 0238 - 1904 (z_em ~ 0.629). Based on the
similarities in the absorption line profiles and estimated covering fractions,
we find that the O VI and Ne VIII absorption trace the same phase of the
absorbing gas. Simple photoionization models can reproduce the observed N(Ne
VIII), N(O VI) and N(Mg X) from a single phase whereas the low ionization
species (e.g. N III, N IV, O IV) originate from a different phase. The measured
N(Ne VIII)/N(O VI) ratio is found to be remarkably similar (within a factor of
~ 2) in several individual absorption components kinematically spread over ~
1800 km/s. Under photoionization this requires a fine tuning between hydrogen
density (nH) and the distance of the absorbing gas from the QSO. Alternatively
this can also be explained by collisional ionization in hot gas with T >
10^{5.7} K. Long-term stability favors the absorbing gas being located outside
the broad line region (BLR). We speculate that the collimated flow of such a
hot gas could possibly be triggered by the radio jet interaction.Comment: Minor revision (accepted for publication in MNRAS letter
Variational quantum Monte Carlo calculations for solid surfaces
Quantum Monte Carlo methods have proven to predict atomic and bulk properties
of light and non-light elements with high accuracy. Here we report on the first
variational quantum Monte Carlo (VMC) calculations for solid surfaces. Taking
the boundary condition for the simulation from a finite layer geometry, the
Hamiltonian, including a nonlocal pseudopotential, is cast in a layer resolved
form and evaluated with a two-dimensional Ewald summation technique. The exact
cancellation of all Jellium contributions to the Hamiltonian is ensured. The
many-body trial wave function consists of a Slater determinant with
parameterized localized orbitals and a Jastrow factor with a common two-body
term plus a new confinement term representing further variational freedom to
take into account the existence of the surface. We present results for the
ideal (110) surface of Galliumarsenide for different system sizes. With the
optimized trial wave function, we determine some properties related to a solid
surface to illustrate that VMC techniques provide standard results under full
inclusion of many-body effects at solid surfaces.Comment: 9 pages with 2 figures (eps) included, Latex 2.09, uses REVTEX style,
submitted to Phys. Rev.
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