1,383 research outputs found
Is the Sun Lighter than the Earth? Isotopic CO in the Photosphere, Viewed through the Lens of 3D Spectrum Synthesis
We consider the formation of solar infrared (2-6 micron) rovibrational bands
of carbon monoxide (CO) in CO5BOLD 3D convection models, with the aim to refine
abundances of the heavy isotopes of carbon (13C) and oxygen (18O,17O), to
compare with direct capture measurements of solar wind light ions by the
Genesis Discovery Mission. We find that previous, mainly 1D, analyses were
systematically biased toward lower isotopic ratios (e.g., R23= 12C/13C),
suggesting an isotopically "heavy" Sun contrary to accepted fractionation
processes thought to have operated in the primitive solar nebula. The new 3D
ratios for 13C and 18O are: R23= 91.4 +/- 1.3 (Rsun= 89.2); and R68= 511 +/- 10
(Rsun= 499), where the uncertainties are 1 sigma and "optimistic." We also
obtained R67= 2738 +/- 118 (Rsun= 2632), but we caution that the observed
12C17O features are extremely weak. The new solar ratios for the oxygen
isotopes fall between the terrestrial values and those reported by Genesis
(R68= 530, R6= 2798), although including both within 2 sigma error flags, and
go in the direction favoring recent theories for the oxygen isotope composition
of Ca-Al inclusions (CAI) in primitive meteorites. While not a major focus of
this work, we derive an oxygen abundance of 603 +/- 9 ppm (relative to
hydrogen; 8.78 on the logarithmic H= 12 scale). That the Sun likely is lighter
than the Earth, isotopically speaking, removes the necessity to invoke exotic
fractionation processes during the early construction of the inner solar
system
On The Evolution of Magnetic White Dwarfs
We present the first radiation magnetohydrodynamics simulations of the
atmosphere of white dwarf stars. We demonstrate that convective energy transfer
is seriously impeded by magnetic fields when the plasma-beta parameter, the
thermal to magnetic pressure ratio, becomes smaller than unity. The critical
field strength that inhibits convection in the photosphere of white dwarfs is
in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG
field strengths observed in magnetic white dwarfs, implying that these objects
have radiative atmospheres. We have then employed evolutionary models to study
the cooling process of high-field magnetic white dwarfs, where convection is
entirely suppressed during the full evolution (B > 10 MG). We find that the
inhibition of convection has no effect on cooling rates until the effective
temperature (Teff) reaches a value of around 5500 K. In this regime, the
standard convective sequences start to deviate from the ones without convection
owing to the convective coupling between the outer layers and the degenerate
reservoir of thermal energy. Since no magnetic white dwarfs are currently known
at the low temperatures where this coupling significantly changes the
evolution, effects of magnetism on cooling rates are not expected to be
observed. This result contrasts with a recent suggestion that magnetic white
dwarfs with Teff < 10,000 K cool significantly slower than non-magnetic
degenerates.Comment: 11 pages, 12 figures, accepted for publication in the Astrophysical
Journa
Inter-network regions of the Sun at millimetre wavelengths
The continuum intensity at wavelengths around 1 mm provides an excellent way
to probe the solar chromosphere. Future high-resolution millimetre arrays, such
as the Atacama Large Millimeter Array (ALMA), will thus produce valuable input
for the ongoing controversy on the thermal structure and the dynamics of this
layer. Synthetic brightness temperature maps are calculated on basis of
three-dimensional radiation (magneto-)hydrodynamic (MHD) simulations. While the
millimetre continuum at 0.3mm originates mainly from the upper photosphere, the
longer wavelengths considered here map the low and middle chromosphere. The
effective formation height increases generally with wavelength and also from
disk-centre towards the solar limb. The average intensity contribution
functions are usually rather broad and in some cases they are even
double-peaked as there are contributions from hot shock waves and cool
post-shock regions in the model chromosphere. Taking into account the
deviations from ionisation equilibrium for hydrogen gives a less strong
variation of the electron density and with it of the optical depth. The result
is a narrower formation height range. The average brightness temperature
increases with wavelength and towards the limb. The relative contrast depends
on wavelength in the same way as the average intensity but decreases towards
the limb. The dependence of the brightness temperature distribution on
wavelength and disk-position can be explained with the differences in formation
height and the variation of temperature fluctuations with height in the model
atmospheres.Comment: 15 pages, 10 figures, accepted for publication in A&A (15.05.07
High-order aberration compensation with Multi-frame Blind Deconvolution and Phase Diversity image restoration techniques
Context. For accurately measuring intensities and determining magnetic field
strengths of small-scale solar (magnetic) structure, knowledge of and
compensation for the point spread function is crucial. For images recorded with
the Swedish 1-meter Solar Telescope, restoration with Multi-Frame Blind
Deconvolution and Joint Phase Diverse Speckle methods lead to remarkable
improvements in image quality but granulation contrasts that are too low,
indicating additional stray light. Aims. We propose a method to compensate for
stray light from high-order atmospheric aberrations not included in MFBD and
JPDS processing. Methods. To compensate for uncorrected aberrations, a
reformulation of the image restoration process is proposed that allows the
average effect of hundreds of high-order modes to be compensated for by relying
on Kolmogorov statistics for these modes. The applicability of the method
requires simultaneous measurements of Fried's parameter r0. The method is
tested with simulations as well as real data and extended to include
compensation for conventional stray light. Results. We find that only part of
the reduction of granulation contrast in SST images is due to uncompensated
high-order aberrations. The remainder is still unaccounted for and attributed
to stray light from the atmosphere, the telescope with its re-imaging system
and to various high-altitude seeing effects. Conclusions. We conclude that
statistical compensation of high-order modes is a viable method to reduce the
loss of contrast occurring when a limited number of aberrations is explicitly
compensated for with MFBD and JPDS processing. We show that good such
compensation is possible with only 10 recorded frames. The main limitation of
the method is that already MFBD and JPDS processing introduces high-order
compensation that, if not taken into account, can lead to over-compensation.Comment: in press in Astronomy & Astrophysic
Can we trust elemental abundances derived in late-type giants with the classical 1D stellar atmosphere models?
We compare the abundances of various chemical species as derived with 3D
hydrodynamical and classical 1D stellar atmosphere codes in a late-type giant
characterized by T_eff=3640K, log g = 1.0, [M/H] = 0.0. For this particular set
of atmospheric parameters the 3D-1D abundance differences are generally small
for neutral atoms and molecules but they may reach up to 0.3-0.4 dex in case of
ions. The 3D-1D differences generally become increasingly more negative at
higher excitation potentials and are typically largest in the optical
wavelength range. Their sign can be both positive and negative, and depends on
the excitation potential and wavelength of a given spectral line. While our
results obtained with this particular late-type giant model suggest that 1D
stellar atmosphere models may be safe to use with neutral atoms and molecules,
care should be taken if they are exploited with ions.Comment: Poster presented at the IAU Symposium 265 "Chemical Abundances in the
Universe: Connecting First Stars to Planets", Rio de Janeiro, 10-14 August
2009; 2 pages, 1 figur
Entwicklung eines situationsbezogenen Konzeptes zur Regulation des Erbsenwicklers in GemĂŒse- und Körnererbsen
Das Ziel des Projektes war es, ein Konzept zur Risikobewertung des Erbsenwicklerbefalls in Anbauregionen von GemĂŒseerbsen zu entwickeln, in dem prĂ€ventive MaĂnahmen und eine bedarfsgerechte Option zur DirektbekĂ€mpfung integriert sind.
Die Datenerfassung zur Beurteilung von Risikolagen erfolgte in Erbsenanbaugebieten in Hessen und Sachsen, beide mit Schwerpunkt auf ökologischen Landbau. Die Risikobewertung umfasste die AbschĂ€tzung der SchlaggefĂ€hrdung durch den Erbsenwickler innerhalb der Anbaugebiete mittels zeitlich-rĂ€umlicher Analysen und die BerĂŒcksichtigung phĂ€nologischer Daten zum Erscheinen, FlugaktivitĂ€t und Entwicklung des Erbsenwicklers in AbhĂ€ngigkeit von Temperatur und Photoperiode. Basierend auf der Risikobewertung sollten Entscheidungen zum Einsatz ökologischer Regulierungsverfahren getroffen werden können, die in einem zweiten Projektteil bearbeitet wurden. Die Regulierung des Erbsenwicklers wurde in einem Parzellenversuch ĂŒber die prĂ€ventiven MaĂnahmen Sortenwahl und Aussaatzeitpunkt und eine bedarfsgerechte DirektbekĂ€mpfung untersucht.
Als Ergebnis konnten die wesentlichen Faktoren, die fĂŒr eine Risikobewertung zum Erbsenwicklerbefall notwendig sind, definiert werden:
a) ein zeitlich-rÀumlicher Zusammenhang zwischen den vorjÀhrigen ErbsenflÀchen und dem Erbsenwicklerauftreten im Folgejahr
b) ein Einfluss von Photoperiode und Temperatur auf die Entwicklung der Ăberwinterungsstadien von C. nigricana, sowie Erscheinen und FlugaktivitĂ€t der adulten Erbsenwickler
c) eine Steuerung der zeitlichen Koinzidenzvermeidung von empfindlichen Entwicklungsstadien der Erbsenpflanze und dem Erbsenwicklerauftreten durch Sortenwahl und Aussaatzeitpunkt.
Der Einsatz einer Pyrethrin-Rapsöl Formulierung hat eine variable Wirkung in der Regulierung des Erbsenwicklers gezeigt. Der Befall konnte nur bei geringem Befallsdruck unterhalb der sehr niedrigen Schadtoleranzgrenze von 0,5% geschÀdigter Ernteerbsen gehalten werden; bei einer starken SchÀdlingsdichte konnte keine ausreichende Befallsreduktion erreicht werden
Clinical Relevance of Transjugular Liver Biopsy in Comparison with Percutaneous and Laparoscopic Liver Biopsy
Background. Transjugular liver biopsy (TJLB) is frequently used to obtain liver specimens in high-risk patients. However, TJLB sample size possibly limits their clinical relevance. Methods. 102 patients that underwent TJLB were included. Clinical parameters and outcome of TJLB were analyzed. Control samples consisted of 112 minilaparoscopic liver biopsies (mLLBs) and 100 percutaneous liver biopsies (PLBs). Results. Fewer portal tracts were detected in TJLB (4.3 ± 0.3) in comparison with PLB (11.7 ± 0.5) and mLLB (11.0 ± 0.6). No difference regarding the specification of indeterminate liver disease and staging/grading of chronic hepatitis was observed. In acute liver failure (n = 32), a proportion of hepatocellular necrosis beyond 25% was associated with a higher rate of death or liver transplantation. Conclusions. Despite smaller biopsy samples the impact on the clinical decision process was found to be comparable to PLB and mLLB. TJLB represents a helpful tool to determine hepatocellular necrosis rates in patients with acute liver failure
Impact of granulation effects on the use of Balmer lines as temperature indicators
Balmer lines serve as important indicators of stellar effective temperatures
in late-type stellar spectra. One of their modelling uncertainties is the
influence of convective flows on their shape. We aim to characterize the
influence of convection on the wings of Balmer lines. We perform a differential
comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical
model atmospheres and 1D hydrostatic standard ones. The model parameters are
appropriate for F,G,K dwarf and subgiant stars of metallicity ranging from
solar to 1/1000 solar. The shape of the Balmer lines predicted by 3D models can
never be exactly reproduced by a 1D model, irrespective of its effective
temperature. We introduce the concept of a 3D temperature correction, as the
effective temperature difference between a 3D model and a 1D model which
provides the closest match to the 3D profile. The temperature correction is
different for the different members of the Balmer series and depends on the
adopted mixing-length parameter in the 1D model. Among the investigated models,
the 3D correction ranges from -300K to +300K. Horizontal temperature
fluctuations tend to reduce the 3D correction. Accurate effective temperatures
cannot be derived from the wings of Balmer lines, unless the effects of
convection are properly accounted for. The 3D models offer a physically well
justified way of doing so. The use of 1D models treating convection with the
mixing-length theory do not appear to be suitable for this purpose. In
particular, there are indications that it is not possible to determine a single
value of the mixing-length parameter which will optimally reproduce the Balmer
lines for any choice of atmospheric parameters.Comment: 6 pages, 3 figures, accepted for publication in A&
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