5,484 research outputs found
Rare White dwarf stars with carbon atmospheres
White dwarfs represent the endpoint of stellar evolution for stars with
initial masses between approximately 0.07 msun and 8-10 msun, where msun is the
mass of the Sun (more massive stars end their life as either black holes or
neutron stars). The theory of stellar evolution predicts that the majority of
white dwarfs have a core made of carbon and oxygen, which itself is surrounded
by a helium layer and, for ~80 per cent of known white dwarfs, by an additional
hydrogen layer. All white dwarfs therefore have been traditionally found to
belong to one of two categories: those with a hydrogen-rich atmosphere (the DA
spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we
report the discovery of several white dwarfs with atmospheres primarily
composed of carbon, with little or no trace of hydrogen or helium. Our analysis
shows that the atmospheric parameters found for these stars do not fit
satisfactorily in any of the currently known theories of post-asymptotic giant
branch evolution, although these objects might be the cooler counterpart of the
unique and extensively studied PG1159 star H1504+65. These stars, together with
H1504+65, might accordingly form a new evolutionary sequence that follow the
asymptotic giant branch.Comment: 7 pages, 1 figure, to appear in Nov 22nd 2007 edition of Natur
Stability control of nonlinear micromechanical resonators under simultaneous primary and superharmonic resonances
Fast effects of a slow excitation on the main resonance of a nonlinear micromechanical resonator are analytically and experimentally investigated. We show, in particular, how the bifurcation topology of an undesirable unstable behavior is modified when the resonator is simultaneously actuated at its primary and superharmonic resonances. A stabilization mechanism is proposed and demonstrated by increasing the superharmonic excitation
Rapid analyses of dry matter content and carotenoids in fresh cassava roots using a portable visible and near infrared spectrometer (Vis/NIRS)
Portable Vis/NIRS are flexible tools for fast and unbiased analyses of constituents with minimal sample preparation. This study developed calibration models for dry matter content (DMC) and carotenoids in fresh cassava roots using a portable Vis/NIRS system. We examined the effects of eight data pre-treatment combinations on calibration models and assessed calibrations on processed and intact root samples. We compared Vis/NIRS derived-DMC to other phenotyping methods. The results of the study showed that the combination of standard normal variate and de-trend (SNVD) with first derivative calculated on two data points and no smoothing (SNVD+1111) was adequate for a robust model. Calibration performance was higher with processed than the intact root samples for all the traits although intact root models for some traits especially total carotenoid content (TCC) (R2c = 96%, R2cv = 90%, RPD = 3.6 and SECV = 0.63) were sufficient for screening purposes. Using three key quality traits as templates, we developed models with processed fresh root samples. Robust calibrations were established for DMC (R2c = 99%, R2cv = 95%, RPD = 4.5 and SECV = 0.9), TCC (R2c = 99%, R2cv = 91%, RPD = 3.5 and SECV = 2.1) and all Trans β-carotene (ATBC) (R2c = 98%, R2cv = 91%, RPD = 3.5 and SECV = 1.6). Coefficient of determination on independent validation set (R2p) for these traits were also satisfactory for ATBC (91%), TCC (88%) and DMC (80%). Compared to other methods, Vis/NIRS-derived DMC from both intact and processed roots had very high correlation (>0.95) with the ideal oven-drying than from specific gravity method (0.49). There was equally a high correlation (0.94) between the intact and processed Vis/NIRS DMC. Therefore, the portable Vis/NIRS could be employed for the rapid analyses of DMC and quantification of carotenoids in cassava for nutritional and breeding purposes
Know Your Neighborhood: A Detailed Model Atmosphere Analysis of Nearby White Dwarfs
We present improved atmospheric parameters of nearby white dwarfs lying
within 20 pc of the Sun. The aim of the current study is to obtain the best
statistical model of the least-biased sample of the white dwarf population. A
homogeneous analysis of the local population is performed combining detailed
spectroscopic and photometric analyses based on improved model atmosphere
calculations for various spectral types including DA, DB, DC, DQ, and DZ stars.
The spectroscopic technique is applied to all stars in our sample for which
optical spectra are available. Photometric energy distributions, when
available, are also combined to trigonometric parallax measurements to derive
effective temperatures, stellar radii, as well as atmospheric compositions. A
revised catalog of white dwarfs in the solar neighborhood is presented. We
provide, for the first time, a comprehensive analysis of the mass distribution
and the chemical distribution of white dwarf stars in a volume-limited sample.Comment: 104 pages, 22 figures, 2 tables, accepted for publication in the
Astrophysical Journal Supplemen
Gene transfer engineering for astrocyte-specific silencing in the CNS.
Cell-type-specific gene silencing is critical to understand cell functions in normal and pathological conditions, in particular in the brain where strong cellular heterogeneity exists. Molecular engineering of lentiviral vectors has been widely used to express genes of interest specifically in neurons or astrocytes. However, we show that these strategies are not suitable for astrocyte-specific gene silencing due to the processing of small hairpin RNA (shRNA) in a cell. Here we develop an indirect method based on a tetracycline-regulated system to fully restrict shRNA expression to astrocytes. The combination of Mokola-G envelope pseudotyping, glutamine synthetase promoter and two distinct microRNA target sequences provides a powerful tool for efficient and cell-type-specific gene silencing in the central nervous system. We anticipate our vector will be a potent and versatile system to improve the targeting of cell populations for fundamental as well as therapeutic applications
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