1,098 research outputs found
Selecting asteroids for a targeted spectroscopic survey
Asteroid spectroscopy reflects surface mineralogy. There are few thousand
asteroids whose surfaces have been observed spectrally. Determining the surface
properties of those objects is important for many practical and scientific
applications, such as for example developing impact deflection strategies or
studying history and evolution of the Solar System and planet formation.
The aim of this study is to develop a pre-selection method that can be
utilized in searching for asteroids of any taxonomic complex. The method could
then be utilized im multiple applications such as searching for the missing
V-types or looking for primitive asteroids.
We used the Bayes Naive Classifier combined with observations obtained in the
course of the Sloan Digital Sky Survey and the Wide-field Infrared Survey
Explorer surveys as well as a database of asteroid phase curves for asteroids
with known taxonomic type. Using the new classification method we have selected
a number of possible V-type candidates. Some of the candidates were than
spectrally observed at the Nordic Optical Telescope and South African Large
Telescope.
We have developed and tested the new pre-selection method. We found three
asteroids in the mid/outer Main Belt that are likely of differentiated type.
Near-Infrared are still required to confirm this discovery. Similarly to other
studies we found that V-type candidates cluster around the Vesta family and are
rare in the mid/oter Main Belt.
The new method shows that even largely explored large databases combined
together could still be further exploited in for example solving the missing
dunite problem.Comment: accepted to A
Multiwalled carbon nanotube: Luttinger liquid or not?
We have measured IV-curves of multiwalled carbon nanotubes using end
contacts. At low voltages, the tunneling conductance obeys non-Ohmic power law,
which is predicted both by the Luttinger liquid and the
environment-quantum-fluctuation theories. However, at higher voltages we
observe a crossover to Ohm's law with a Coulomb-blockade offset, which agrees
with the environment-quantum-fluctuation theory, but cannot be explained by the
Luttinger-liquid theory. From the high-voltage tunneling conductance we
determine the transmission line parameters of the nanotubes.Comment: RevTeX, 4 pages, 2 EPS-figures, submitted to Phys. Rev. Let
Construction of a flocculent brewer’s yeast strain producing an Aspergillus niger β-galactosidase
Praxis XXI (BD/11306/97).European Science Foundation (ESF).Center for International Mobility (CIMO)
Construction of a flocculent brewer's yeast strain secreting Aspergillus niger β-galactosidase
One way of improving heterologous protein
production is to use high cell density systems, one of the
most attractive being the flocculating yeast production
system. Also, lactose is available in large amounts as a
waste product from cheese production processes. The
construction of flocculent and non-flocculent brewer's
yeast strains secreting β-galactosidase and growing on
lactose is presented. A plasmid was constructed coding
for an extracellular β-galactosidase of Aspergillus niger
and having, as selective marker, the yeast CUP1 gene
conferring resistance to copper. This selective marker
allows for the transformation of wild-type yeasts. This
work represents an important step towards the study of
heterologous protein secretion by flocculent cells.Fundação para a Ciência e a Tecnologia (FCT) – PRAXIS XXI/BD/11306/97.Finland. Center For International Mobility.European Science Foundation
Construction of a flocculent Saccharomyces cerevisiae strain secreting high levels of Aspergillus niger β-galactosidase
A flocculent Saccharomyces cerevisiae strain
secreting Aspergillus niger β-galactosidase activity was
constructed by transforming S. cerevisiae NCYC869-A3
strain with plasmid pVK1.1 harboring the A. niger β-galactosidase
gene, lacA, under the control of the ADH1
promoter and terminator. Compared to other recombinant
S. cerevisiae strains, this recombinant yeast has higher
levels of extracellular β-galactosidase activity. In shakeflask
cultures, the β-galactosidase activity detected in the
supernatant was 20 times higher than that obtained with
previously constructed strains (Domingues et al. 2000a).
In bioreactor culture, with cheese-whey permeate as substrate,
a yield of 878.0 nkat/gsubstrate was obtained. The
recombinant strain is an attractive alternative to other
fungal β-galactosidase production systems as the enzyme
is produced in a rather pure form. Moreover, the use of
flocculating yeast cells allows for enzyme production
with high productivity in continuous fermentation systems
with facilitated downstream processing.Instituto de Biotecnologia e Química Fina (IBQF).Fundação para a Ciência e a Tecnologia (FCT) – PRAXIS XXI/BD/11306/97
Nanoelectronic thermometers optimised for sub-10 millikelvin operation
We report the cooling of electrons in nanoelectronic Coulomb blockade
thermometers below 4 mK. Above 7 mK the devices are in good thermal contact
with the environment, well isolated from electrical noise, and not susceptible
to self-heating. This is attributed to an optimised design that incorporates
cooling fins with a high electron-phonon coupling and on-chip electronic
filters, combined with a low-noise electronic measurement setup. Below 7 mK the
electron temperature is seen to diverge from the ambient temperature. By
immersing a Coulomb Blockade Thermometer in the 3He/4He refrigerant of a
dilution refrigerator, we measure a lowest electron temperature of 3.7 mK.Comment: 11 pages, 4 figures. (Fixed fitted saturation T_e on p9
Absolute spectral modelling of asteroid (4) Vesta
We present a new physics-based approach to model the absolute reflectance spectra of asteroid (4) Vesta. The spectral models are derived by utilizing a ray-optics code that simulates light scattering by particles large compared to the wavelength of the incident light. In the light of the spectral data obtained by the Dawn spacecraft, we use howardite powder to model Vesta's surface regolith and its particle size distribution for 10-200 mu m sized particles. Our results show that the modelled spectrum mimics well the observations. The best match was found using a power-law particle size distribution with an index 3.2. This suggests that Vesta's regolith is dominated by howardite particlesPeer reviewe
“Superconductor-Insulator Transition” in a Single Josephson Junction
VI curves of resistively shunted single Josephson junctions with different capacitances and tunneling resistances are found to display a crossover between two types of VI curves: one without and another with a resistance bump (negative second derivative) at zero bias. The crossover corresponds to the dissipative phase transition (superconductor-insulator transition) at which macroscopic quantum tunneling delocalizes the Josephson phase and destroys superconductivity. Our measured phase diagram does not agree with the diagram predicted by the original theory, but does coincide with a theory that takes into account the accuracy of voltage measurements and thermal fluctuations.Peer reviewe
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