203 research outputs found
Measurement of resonant properties of plasmonic nanostructures in transmission and reflection mode of far-field spectroscopy
Optická spektroskopie v dalekém poli umožňuje měření lokalizovaných plazmonických rezonancí na kovových nanostrukturách různých tvarů a velikostí při jejich osvětlení spojitým světlem. Při použití mikroskopie v temném poli je možné naměřit rozptyl na lokalizovaném plazmonu excitovaném na jediné izolované nanostruktuře. Za účelem těchto měření byla sestavena aparatura založená na komerčním systému duálního mikroskopu Nanonics. Experimenty provedené v rámci této práce ukazují vysokou citlivost aparatury v jejím současném stavu, umožňující naměření plazmonické rezonance jediné nanočástice. Navržená vylepšení aparatury by měla v blízké budoucnosti vést ještě k větší citlivosti a přesnosti měření.Far-field optical spectroscopy allows for measurement of plasmonic resonances on metallic nanostructures of various shapes and sizes when illuminated by continuous light. Employing dark-field microscopy makes it possible to measure scattering on a localised plasmon excited on a single, isolated nanostructure. For the purpose of these measurements an apparatus based on commercial dual microscope system Nanonics has been put together. Experiments carried out during work on this thesis have shown a great sensitivity of the apparatus in its present form, allowing to measure a plasmonic resonance of a single nanoparticle. Proposed improvements of the apparatus should lead to even greater sensitivity and precision of measurements in the near future.
Pozorování a modelování klasických Be hvězd
Jasnost a blízkost mnoha klasických Be hvězd z nich dělá vhodné laboratoře pro studium fyziky astrofyzikálních disků. Be hvězdy jsou také mezi nejpopulárnějšími objekty studovanými optickými a infračervenými interferometry, které jsou schopny plně rozlišit jejich okolohvězdné disky, čemuž vděčíme za mnoho z nedávných pokroků v našem chápání těchto záhadných objektů. Současná shoda je, že klasické Be hvězdy vyvrhují hmotu z hvězdného povrchu do Keplerovských oběžných drah, takže dochází k tvorbě disku, jehož následný vývoj je určen turbulentní viskozitou, jež je základem tzv. viskózního dekrečního diskového (VDD) modelu. Mezi hlavními výsledky této práce je možná nejlépe specifikovaný model konkrétní Be hvězdy β CMi. Předpovědi VDD modelu byly porovnány také s radiovými pozorováními, které umožnily první určení fyzického rozsahu Be disku. Tento výsledek následně vedl k detekci hvězdného společníka, který osekává vnější části disku slapovými silami. Rozšíření vzorku o pět dalších objektů vedlo k odhalení podobné struktury vnějších disků ve všech objektech. Z možných vysvětlení je nejuspokojivější scénář, v němž jsou Be disky ovlivněny (neviditelnými) společníky mnohem častěji, než se doposud myslelo.The brightness and proximity of many classical Be stars makes them perfect laboratories for studying the physics of astrophysical disks. They are also among the most popular targets for optical/IR interferometers, which are able to fully resolve their circumstellar disks, to which much of the recent progress in our understanding of these enigmatic objects is owed. The current consensus is that classical Be stars eject material from the stellar surface into Keplerian orbits, thus forming a disk, whose subsequent evolution is governed by turbulent viscosity, which is the basis of the so-called viscous decretion disk (VDD) model. Among the main results of the present work is arguably the best-constrained model of a particular Be star β CMi. The VDD predictions were confronted also with radio observations, which allowed for the first determination of the physical extent of a Be disk. This result subsequently led to the detection of a binary companion, which is truncating the disk by tidal forces. Extending the sample to include five more targets led to revealing a similar outer disk structure in all of them. The range of explanations includes the most plausible scenario, in which the truncation of Be disks by (unseen) companions is much more common than previously thought.Astronomický ústav UKAstronomical Institute of Charles UniversityMatematicko-fyzikální fakultaFaculty of Mathematics and Physic
Population Changes and Location-Specific Differences for Otolith-Derived Age and Growth of Recreationally Harvested Spotted Seatrout (Cynoscion nebulosus) From Alabama in 2007
Spotted seatrout (Cynoscion nebulosus), is a nonmigratory game fish common in the Gulf of Mexico that is important in estuarine ecosystems. Population dynamics of spotted seatrout were examined using otolith-based age-and-growth models derived from observed and back-calculated length-at-age values. These data were used to identify sex-based differences and annular variation. Recent growth was quantified, using marginal increment analysis for comparisons between sexes and the two major bays in Alabama (Mobile Bay and Mississippi Sound). Sex ratios were also compared for these locations. Our results show that females were larger than males and that fish collected in Mobile Bay were larger than those from Mississippi Sound. Combined data from both bays resulted in a sex ratio that approached 1 : 1; however, examination of each bay individually showed that the Mississippi Sound had a female-biased population and that Mobile Bay had a male-biased population. Differences in observed length-at-age measurements became evident between males and females by age 2 with females typically larger than males, whereas the maximum age for males was greater. The oldest females were age 5 and the oldest males were age 8. Compared to previous estimates of trout growth in Alabama, results showed an increase in the modal length of fish and increased growth rates. Results suggest faster growth of the fish in the current population and decreased harvest of larger fish compared to historic estimates. This may be indicative of ecosystem-wide changes in spotted seatrout populations and highlights the need to closely monitor this population
The Orbits and Dynamical Masses of the Castor System
Castor is a system of six stars in which the two brighter objects, Castor A
and B, revolve around each other every 450 yr and are both short-period
spectroscopic binaries. They are attended by the more distant Castor C, which
is also a binary. Here we report interferometric observations with the CHARA
array that spatially resolve the companions in Castor A and B for the first
time. We complement these observations with new radial velocity measurements of
A and B spanning 30 yr, with the Hipparcos intermediate data, and with existing
astrometric observations of the visual AB pair obtained over the past three
centuries. We perform a joint orbital solution to solve simultaneously for the
three-dimensional orbits of Castor A and B as well as the AB orbit. We find
that they are far from being coplanar: the orbit of A is nearly at right angles
(92 degrees) relative to the wide orbit, and that of B is inclined about 59
degrees compared to AB. We determine the dynamical masses of the four stars in
Castor A and B to a precision better than 1%. We also determine the radii of
the primary stars of both subsystems from their angular diameters measured with
CHARA, and use them together with stellar evolution models to infer an age for
the system of 290 Myr. The new knowledge of the orbits enables us to measure
the slow motion of Castor C as well, which may assist future studies of the
dynamical evolution of this remarkable sextuple system.Comment: 17 pages in emulateapj format, including figures and tables. Accepted
for publication in The Astrophysical Journa
The Milky Way Tomography with SDSS: III. Stellar Kinematics
We study Milky Way kinematics using a sample of 18.8 million main-sequence
stars with r<20 and proper-motion measurements derived from SDSS and POSS
astrometry, including ~170,000 stars with radial-velocity measurements from the
SDSS spectroscopic survey. Distances to stars are determined using a
photometric parallax relation, covering a distance range from ~100 pc to 10 kpc
over a quarter of the sky at high Galactic latitudes (|b|>20 degrees). We find
that in the region defined by 1 kpc <Z< 5 kpc and 3 kpc <R< 13 kpc, the
rotational velocity for disk stars smoothly decreases, and all three components
of the velocity dispersion increase, with distance from the Galactic plane. In
contrast, the velocity ellipsoid for halo stars is aligned with a spherical
coordinate system and appears to be spatially invariant within the probed
volume. The velocity distribution of nearby ( kpc) K/M stars is complex,
and cannot be described by a standard Schwarzschild ellipsoid. For stars in a
distance-limited subsample of stars (<100 pc), we detect a multimodal velocity
distribution consistent with that seen by HIPPARCOS. This strong
non-Gaussianity significantly affects the measurements of the velocity
ellipsoid tilt and vertex deviation when using the Schwarzschild approximation.
We develop and test a simple descriptive model for the overall kinematic
behavior that captures these features over most of the probed volume, and can
be used to search for substructure in kinematic and metallicity space. We use
this model to predict further improvements in kinematic mapping of the Galaxy
expected from Gaia and LSST.Comment: 90 pages, 26 figures, submitted to Ap
Microstructure and Porosity of Laser Welds in Cast Ti-6Al-4V with Addition of Boron
Addition of small amounts of boron to cast Ti-6Al-4V alloy has shown to render a finer microstructure and improved mechanical properties. For such an improved alloy to be widely applicable for large aerospace structural components, successful welding of such castings is essential. In the present work, the microstructure and porosity of laser welds in a standard grade cast Ti-6Al-4V alloy as well as two modified alloy versions with different boron concentrations have been investigated. Prior-β grain reconstruction revealed the prior-β grain structure in the weld zones. In fusion zones of the welds, boron was found to refine the grain size significantly and rendered narrow elongated grains. TiB particles in the prior-β grain boundaries in the cast base material restricted grain growth in the heat-affected zone. The TiB particles that existed in the as cast alloys decreased in size in the fusion zones of welds. The hardness in the weld zones was higher than in the base material and boron did not have a significant effect on hardness of the weld zones. The fusion zones were smaller in the boron-modified alloys as compared with Ti-6Al-4V without boron. Computed tomography X-ray investigations of the laser welds showed that pores in the FZ of the boron modified alloys were confined to the lower part of the welds, suggesting that boron addition influences melt pool flow
Sequences, Annotation and Single Nucleotide Polymorphism of the Major Histocompatibility Complex in the Domestic Cat
Two sequences of major histocompatibility complex (MHC) regions in the domestic cat, 2.976 and 0.362 Mbps, which were separated by an ancient chromosome break (55–80 MYA) and followed by a chromosomal inversion were annotated in detail. Gene annotation of this MHC was completed and identified 183 possible coding regions, 147 human homologues, possible functional genes and 36 pseudo/unidentified genes) by GENSCAN and BLASTN, BLASTP RepeatMasker programs. The first region spans 2.976 Mbp sequence, which encodes six classical class II antigens (three DRA and three DRB antigens) lacking the functional DP, DQ regions, nine antigen processing molecules (DOA/DOB, DMA/DMB, TAPASIN, and LMP2/LMP7,TAP1/TAP2), 52 class III genes, nineteen class I genes/gene fragments (FLAI-A to FLAI-S). Three class I genes (FLAI-H, I-K, I-E) may encode functional classical class I antigens based on deduced amino acid sequence and promoter structure. The second region spans 0.362 Mbp sequence encoding no class I genes and 18 cross-species conserved genes, excluding class I, II and their functionally related/associated genes, namely framework genes, including three olfactory receptor genes. One previously identified feline endogenous retrovirus, a baboon retrovirus derived sequence (ECE1) and two new endogenous retrovirus sequences, similar to brown bat endogenous retrovirus (FERVmlu1, FERVmlu2) were found within a 140 Kbp interval in the middle of class I region. MHC SNPs were examined based on comparisons of this BAC sequence and MHC homozygous 1.9× WGS sequences and found that 11,654 SNPs in 2.84 Mbp (0.00411 SNP per bp), which is 2.4 times higher rate than average heterozygous region in the WGS (0.0017 SNP per bp genome), and slightly higher than the SNP rate observed in human MHC (0.00337 SNP per bp)
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