484 research outputs found
Mitigating the mass dependence in the scaling relation of red-giant stars
The masses and radii of solar-like oscillators can be estimated through the
asteroseismic scaling relations. These relations provide a direct link between
observables, i.e. effective temperature and characteristics of the oscillation
spectra, and stellar properties, i.e. mean density and surface gravity (thus
mass and radius). These scaling relations are commonly used to characterize
large samples of stars. Usually, the Sun is used as a reference from which the
structure is scaled. However, for stars that do not have a similar structure as
the Sun, using the Sun as a reference introduces systematic errors as large as
10\% in mass and 5\% in radius. Several alternatives for the reference of the
scaling relation involving the large frequency separation (typical frequency
difference between modes of the same degree and consecutive radial order) have
been suggested in the literature. In a previous paper, we presented a reference
function with a dependence on both effective temperature and metallicity. The
accuracy of predicted masses and radii improved considerably when using
reference values calculated from our reference function. However, the residuals
indicated that stars on the red-giant branch possess a mass dependence that was
not accounted for. Here, we present a reference function for the scaling
relation involving the large frequency separation that includes the mass
dependence. This new reference function improves the derived masses and radii
significantly by removing the systematic differences and mitigates the trend
with (frequency of maximum oscillation power) that exists when
using the solar value as a reference.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Uncovering hidden modes in RR Lyrae stars
The Kepler space telescope revealed new, unexpected phenomena in RR Lyrae
stars: period doubling and the possible presence of additional modes.
Identifying these modes is complicated because they blend in the rich features
of the Fourier-spectrum. Our hydrodynamic calculations uncovered that a
'hidden' mode, the 9th overtone is involved in the period doubling phenomenon.
The period of the overtone changes by up to 10 per cent compared to the linear
value, indicating a very significant nonlinear period shift caused by its
resonance with the fundamental mode. The observations also revealed weak peaks
that may correspond to the first or second overtones. These additional modes
are often coupled with period doubling. We investigated the possibilities and
occurrences of mutual resonances between the fundamental mode and multiple
overtones in our models. These theoretical findings can help interpreting the
origin and nature of the 'hidden' modes may be found in the high quality light
curves of space observatories.Comment: In proceedings of "20th Stellar Pulsation Conference Series: Impact
of new instrumentation & new insights in stellar pulsations", 5-9 September
2011, Granada, Spai
Fitting Blazhko light curves
The correct amplitude and phase modulation formalism of the Blazhko
modulation is given. The harmonic order dependent amplitude and phase
modulation form is equivalent with the Fourier decomposition of multiplets. The
amplitude and phase modulation formalism used in electronic transmission
technique as introduced by Benk\H{o}, Szab\'o and Papar\'o (2011, MNRAS 417,
974) for Blazhko stars oversimplifies the amplitude and phase modulation
functions thus it does not describe the light variation in full detail.
The results of the different formalisms are compared and documented by
fitting the light curve of a real Blazhko star, CM UMa.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
Metallic artefact reduction with monoenergetic dual-energy CT: systematic ex vivo evaluation of posterior spinal fusion implants from various vendors and different spine levels
Objectives: To evaluate optimal monoenergetic dual-energy computed tomography (DECT) settings for artefact reduction of posterior spinal fusion implants of various vendors and spine levels. Methods: Posterior spinal fusion implants of five vendors for cervical, thoracic and lumbar spine were examined ex vivo with single-energy (SE) CT (120 kVp) and DECT (140/100 kVp). Extrapolated monoenergetic DECT images at 64, 69, 88, 105keV and individually adjusted monoenergy for optimised image quality (OPTkeV) were generated. Two independent radiologists assessed quantitative and qualitative image parameters for each device and spine level. Results: Inter-reader agreements of quantitative and qualitative parameters were high (ICCâ=â0.81-1.00, Îşâ=â0.54-0.77). HU values of spinal fusion implants were significantly different among vendors (Pâ<â0.001), spine levels (Pâ<â0.01) and among SECT, monoenergetic DECT of 64, 69, 88, 105keV and OPTkeV (Pâ<â0.01). Image quality was significantly (Pâ<â0.001) different between datasets and improved with higher monoenergies of DECT compared with SECT (Vâ=â0.58, Pâ<â0.001). Artefacts decreased significantly (Vâ=â0.51, Pâ<â0.001) at higher monoenergies. OPTkeV values ranged from 123-141keV. OPTkeV according to vendor and spine level are presented herein. Conclusions: Monoenergetic DECT provides significantly better image quality and less metallic artefacts from implants than SECT. Use of individual keV values for vendor and spine level is recommended. Key Points: ⢠Artefacts pose problems for CT following posterior spinal fusion implants. ⢠CT images are interpreted better with monoenergetic extrapolation using dual-energy (DE) CT. ⢠DECT extrapolation improves image quality and reduces metallic artefacts over SECT. ⢠There were considerable differences in monoenergy values among vendors and spine levels. ⢠Use of individualised monoenergy values is indicated for different metallic hardware device
Degradation mechanisms and consequences for SOC stocks for the world's largest alpine pastoral ecosystem on the Tibetan Plateau
Approximately 1.5 million km² of the Tibetan Plateau are covered with grasslands. Thereof one third is occupied by the worldâs largest pastoral alpine ecosystem (Kobresia pastures). Paleo-records indicate the grazing-induced origin of this ecosystem since more than 8000 years or at least since yak domestication since 4000 years. Long-term moderate grazing by yak and sheep increased belowground C allocation of Kobresia pygmaea, caused the development of dense root-mats and finally lead to an accumulation of soil organic carbon (SOC) and nutrients such as nitrogen (N) and phosphorus (P) in the topsoil. These pastures, however, are increasingly affected by large-scale degradation caused by overgrazing of these highly sensitive ecosystems. Loss of the topsoil threatens several ecosystem functions: i.e. SOC and nutrient storage, biodiversity, provision of grazing-ground and supply of clean water for large parts of SE-Asia.
Here, we present a conceptual model and results of degradation processes combining anthropogenic and natural amplifications. To evaluate losses of SOC and nutrients we synthesize field observations and surveys in the highlands and validates this with own analyses in the Kobresia core area. We show that drought- and frost-induced polygonal cracking opens the root-mats, already weakened by overgrazing. This initiates a dying of the Kobresia turf, extends the surface cracks, triggers soil erosion and promotes SOC mineralization and leaching losses. Soil erosion caused further high losses of SOC and nutrients from the topsoil (i.e. 0-10 cm: ~5.1 kg C m-2), whereas SOC loss beneath the surface cracks was primary caused by both, decreasing C-input and SOC mineralization (mineralization-derived SOC loss: ~2.5 kg C m-2). The root biomass decreased with intensity of pasture degradation and lower C input constrains the ecosystem recovery. A negative δ13C shift of SOC reflected intensive decomposition and corresponded to a relative enrichment of 13C depleted lignin components. In sum, degradation triggered high SOC loss (up to 70% of intact soil in 0-30 cm: ~7.6 kg C m-2) from this ecosystem with profound consequences for carbon sequestration, atmospheric CO2, water quality and ecosystem stability
Complex dissolved organic matter (DOM) on the roof of the world â Tibetan DOM molecular characteristics indicate sources, land use effects, and processing along the fluvialâlimnic continuum
The Tibetan Plateau (TP) is the world's largest and highest plateau, comprising the earth's biggest alpine pasture system. It is sensitive to
the impacts of climate change and anthropogenic pressure. Carbon cycling on the TP is influenced by glaciation and degradation of the pasture
ecosystem. Dissolved organic matter (DOM) connects carbon reservoirs, following the hydrological continuum from glaciers and headwaters to
lakes. Due to its complexity, DOM cycling along the aquatic continuum and the impact of land use and climate change on DOM characteristics are still
not well understood. Here, we study solid phase extracted (SPE) DOM molecular characteristics using ultrahigh-resolution mass spectrometry
(FT-ICR-MS) along the TP hydrological continuum from glaciers, groundwater springs, and wetlands, including pastures and alpine steppes, to the
endorheic Lake Nam Co. Our study revealed that the SPE-DOM composition was largely influenced by local sources of glaciers, wetlands, and
groundwater springs as well as pasture degradation. Glacial meltwater SPE-DOM contained more saturated compounds suggesting microbial sources
together with aromatic compounds probably derived from aeolian deposition. In comparison, wetland and stream SPE-DOM were characterised by a higher
percentage of highly unsaturated and aromatic molecular formulae. These were likely derived from inputs of vascular plants and soils. Groundwater
spring SPE-DOM from degraded pastures differed from intact pasture samples. In degraded systems a strongly oxidised signature with the lowest counts of
PÂ heteroatoms, a lower O/CÂ ratio, and a higher aromaticity of SPE-DOM together with a high degradation index suggested a strong
transformation of SPE-DOM. SPE-DOM of the endorheic lake was richer in unsaturated molecular formulae compared to the tributaries. This suggests
algae and microbial sources and production in the lake. The SPE-DOM rich in aromatic and highly unsaturated formulae visible in the brackish zone of
the lake shore contrasted sharply with that of the lake. Aromatic molecular formulae were strongly depleted in the lake deep water
suggesting photooxidation of riverine SPE-DOM. This indicates that alpine SPE-DOM signatures are shaped by small-scale catchment properties, land
degradation, and the influence of glaciers and wetlands. The close link of alpine SPE-DOM composition to landscape properties is indicative of a
strong susceptibility of DOM characteristics to climatic and land use changes in High Asia.</p
Pulsation of the Lambda Bootis star HD 210111
CONTEXT. The Lambda Bootis stars are a small spectroscopic subgroup of
Population I A-type stars and show significant underabundances of metals. Many
are Delta Scuti pulsators.
AIMS. HD 210111 was selected for a detailed multisite pulsation study to
determine whether its pulsation properties differ from those of normal A stars.
METHODS. 262 hours of high-precision photometry were obtained at the SAAO and
SSO observatories.
RESULTS. 13 statistically significant pulsation frequencies were detected
with very small photometric amplitudes from 1 to 7 millimag in the visual. A
comparison with earlier 1994 measurements indicates a small increase in
amplitude. As a byproduct, one of the comparison stars, HD 210571, was
discovered to be a millimag variable with a frequency of 1.235 c/d and is
probably a new Gamma Doradus variable. The observed wide range of excited
frequencies from 12 to 30 c/d in HD 210111 can be explained with both the
single- and double-star hypothesis. HD 210111 is in a similar evolutionary
status to FG Vir, which also shows a wide range of excited frequencies with a
similar frequency spacing near 4 c/d. This is interpreted as successive radial
orders of the excited nonradial modes. In the double-star hypothesis previously
evoked for HD 210111, the low and the high frequencies originate in different
stars: here HD 210111 would resemble Theta^2 Tau.
CONCLUSIONS. The pulsation of the Lambda Bootis star HD 210111 does not
differ from that of normal Delta Scuti stars.Comment: 6 pages, 2 figures, 2 tables, submitted to A&
Multi-Periodic Oscillations in Cepheids and RR Lyrae-Type Stars
Classical Cepheids and RR Lyrae-type stars are usually considered to be
textbook examples of purely radial, strictly periodic pulsators. Not all the
variables, however, conform to this simple picture. In this review I discuss
different forms of multi-periodicity observed in Cepheids and RR Lyrae stars,
including Blazhko effect and various types of radial and nonradial multi-mode
oscillations.Comment: Proceedings of the 20th Stellar Pulsation Conference Series: "Impact
of new instrumentation & new insights in stellar pulsations", 5-9 September
2011, Granada, Spai
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