Kerker effect is one of the unique phenomena in modern electrodynamics. Due to overlapping of electric and magnetic dipole moments, all-dielectric particles can be invisible in forward or backward ...directions. In our paper we propose new conditions between resonantly excited electric dipole and magnetic quadrupole in ceramic high index spheroidal particles for demonstrating transverse Kerker effect. Moreover, we perform proof-of-concept microwave experiment and demonstrate dumbbell radiation pattern with suppressed scattering in both forward and backward directions and enhanced scattering in lateral directions. Our concept is promising for future planar lasers, nonreflected metasurface and laterally excited waveguides and nanoantennas.
ABSTRACT
The radiative properties of interstellar dust are affected not only by the grain size distribution but also by the grain porosity. We develop a model for the evolution of size-dependent ...grain porosity and grain size distribution over the entire history of galaxy evolution. We include stellar dust production, supernova dust destruction, shattering, coagulation, and accretion. Coagulation is assumed to be the source of grain porosity. We use a one-zone model with a constant dense gas fraction (ηdense), which regulates the balance between shattering and coagulation. We find that porosity develops after small grains are sufficiently created by the interplay between shattering and accretion (at age t ∼ 1 Gyr for star formation time-scale τSF = 5 Gyr) and are coagulated. The filling factor drops down to 0.3 at grain radii $\sim 0.03~ {\mu m}$ for ηdense = 0.5. The grains are more porous for smaller ηdense because small grains, from which porous coagulated grains form, are more abundant. We also calculate the extinction curves based on the above results. The porosity steepens the extinction curve significantly for silicate, but not much for amorphous carbon. The porosity also increases the collisional cross-sections and produces slightly more large grains through the enhanced coagulation; however, the extinction curve does not necessarily become flatter because of the steepening effect by porosity. We also discuss the implication of our results for the Milky Way extinction curve.
ABSTRACT
The properties of interstellar grains, such as grain size distribution and grain porosity, are affected by interstellar processing, in particular, coagulation and shattering, which take ...place in the dense and diffuse interstellar medium (ISM), respectively. In this paper, we formulate and calculate the evolution of grain size distribution and grain porosity through shattering and coagulation. For coagulation, we treat the grain evolution depending on the collision energy. Shattering is treated as a mechanism of forming small compact fragments. The balance between these processes are determined by the dense-gas mass fraction ηdense, which determines the time fraction of coagulation relative to shattering. We find that the interplay between shattering supplying small grains and coagulation forming porous grains from shattered grains is fundamentally important in creating and maintaining porosity. The porosity rises to 0.7–0.9 (or the filling factor 0.3–0.1) around grain radii $a\sim 0.1~\rm{\mu m}$. We also find that, in the case of ηdense = 0.1 (very efficient shattering with weak coagulation) porosity significantly enhances coagulation, creating fluffy submicron grains with filling factors lower than 0.1. The porosity enhances the extinction by 10–20 per cent at all wavelengths for amorphous carbon and at ultraviolet wavelengths for silicate. The extinction curve shape of silicate becomes steeper if we take porosity into account. We conclude that the interplay between shattering and coagulation is essential in creating porous grains in the interstellar medium and that the resulting porosity can impact the grain size distributions and extinction curves.
Fractal dimension of optical cirrus in Stripe82 Marchuk, Alexander A; Smirnov, Anton A; Mosenkov, Aleksandr V ...
Monthly Notices of the Royal Astronomical Society,
12/2021, Letnik:
508, Številka:
4
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
The geometric characteristics of dust clouds provide important information on the physical processes that structure such clouds. One of such characteristics is the 2D fractal dimension D of ...a cloud projected on to the sky plane. In previous studies, which were mostly based on infrared (IR) data, the fractal dimension of individual clouds was found to be in a range from 1.1 to 1.7 with a preferred value of 1.2–1.4. In this work, we use data from Stripe82 of the Sloan Digital Sky Survey to measure the fractal dimension of the cirrus clouds. This is done here for the first time for optical data with significantly better resolution as compared to IR data. To determine the fractal dimension, the perimeter-area method is employed. We also consider IR (IRAS and Herschel) counterparts of the corresponding optical fields to compare the results between the optical and IR. We find that the averaged fractal dimension across all clouds in the optical is $\langle D \rangle =1.69^{+0.05}_{-0.05}$ which is significantly larger than the fractal dimension of its IR counterparts $\langle D\rangle =1.38^{+0.07}_{-0.06}$. We examine several reasons for this discrepancy (choice of masking and minimal contour level, image and angular resolution, etc.) and find that for approximately half of our fields the different angular resolution (point spread function) of the optical and IR data can explain the difference between the corresponding fractal dimensions. For the other half of the fields, the fractal dimensions of the IR and visual data remain inconsistent, which can be associated with physical properties of the clouds, but further physical simulations are required to prove it.
We interpret the interstellar extinction observed toward the Galactic center (GC) in the wavelength range λ = 1-20 m. Its main feature is the flat extinction at 3-8 m whose explanation is still a ...problem for cosmic dust models. We search for the structure and chemical composition of dust grains that could explain the observed extinction. In contrast to earlier works, we use laboratory measured optical constants and consider particles of different structures. We show that a mixture of compact grains of aromatic carbon and of some silicate is better suited for reproducing the flat extinction in comparison with essentially porous grains or aliphatic carbon particles. Metallic iron should be located inside the particle, i.e., it cannot form layers on silicate grains as the extinction curves then become very peculiar. We find a model including aromatic carbonaceous particles and three-layered particles with an olivine-type silicate core, a thin, very porous layer and a thin envelope of magnetite that provides a good (but still not perfect) fit to the observational data. We suggest that such silicate dust should be fresh, i.e., recently formed in the atmospheres of late-type stars in the central region of the Galaxy. We assume that this region has a radius of about 1 kpc and produces about half of the observed extinction. The remaining part of extinction is caused by a "foreground" material being practically transparent at .
ABSTRACT
In this first paper in a series we present a study of the global dust emission distribution in nearby edge-on spiral galaxies. Our sample consists of 16 angularly large and 13 less spatially ...resolved galaxies selected from the DustPedia sample. To explore the dust emission distribution, we exploit the Herschel photometry in the range 100–500 $\mu $m. We employ Sérsic and 3D disc models to fit the observed 2D profiles of the galaxies. Both approaches give similar results. Our analysis unequivocally states the case for the presence of extraplanar dust in between 6 and 10 large galaxies. The results reveal that both the disc scale length and height increase as a function of wavelength between 100 and 500 $\mu $m. The dust disc scale height positively correlates with the dust disc scale length, similar to what is observed for the stellar discs. We also find correlations between the scale lengths and scale heights in the near- and far-infrared which suggest that the stellar discs and their dust counterparts are tightly connected. Furthermore, the intrinsic flattening of the dust disc is inversely proportional to the maximum rotation velocity and the dust mass of the galaxy: more massive spiral galaxies host, on average, relatively thinner dust discs. Also, there is a tendency for the dust-to-stellar scale height ratio to decrease with the dust mass and rotation velocity. We conclude that low-mass spiral galaxies host a diffuse, puffed-up dust disc with a thickness similar to that of the stellar disc.
ABSTRACT
Spiral structure can contribute significantly to a galaxy’s luminosity. However, only rarely are proper photometric models of spiral arms used in decompositions. As we show in the previous ...work, including the spirals as a separate component in a photometric model of a galaxy would both allow to obtain their structural parameters, and reduce the systematic errors in estimating the parameters of other components. Doing so in different wavebands, one can explore how their properties vary with the wavelength. In this paper, second in this series, we perform decomposition of M 51 in 17 bands, from the far-ultraviolet (UV) to far-infrared, using imaging from the DustPedia project. We use the same 2D photometric model of spiral structure where each arm is modelled independently. The complex and asymmetric spiral structure in M 51 is reproduced relatively well with our model. We analyse the differences between models with and without spiral arms, and investigate how the fit parameters change with wavelength. In particular, we find that the spiral arms demonstrate the largest width in the optical, whereas their contribution to the galaxy luminosity is most significant in the UV. The disc central intensity drops by a factor of 1.25–3 and its exponential scale changes by 5–10 per cent when spiral arms are included, depending on wavelength. Taking into account the full light distribution across the arms, we do not observe the signs of a long-lived density wave in the spiral pattern of M 51 as a whole.
ABSTRACT
We fit various colour–magnitude diagrams (CMDs) of the Galactic globular clusters NGC 6397 and NGC 6809 (M55) by isochrones from the Dartmouth Stellar Evolution Database (DSED) and Bag of ...Stellar Tracks and Isochrones (BaSTI) for α–enhanced α/Fe = +0.4. For the CMDs, we use data sets from Hubble Space Telescope, Gaia, Visible and Infrared Survey Telescope for Astronomy, and other sources utilizing 32 and 23 photometric filters for NGC 6397 and NGC 6809, respectively, from the ultraviolet to mid-infrared. We obtain the following characteristics for NGC 6397 and NGC 6809, respectively: metallicities Fe/H = −1.84 ± 0.02 ± 0.1 and −1.78 ± 0.02 ± 0.1 (statistic and systematic uncertainties); distances 2.45 ± 0.02 ± 0.06 and 5.24 ± 0.02 ± 0.18 kpc; ages 12.9 ± 0.1 ± 0.8 and 13.0 ± 0.1 ± 0.8 Gyr; reddenings E(B − V) = 0.178 ± 0.006 ± 0.01 and 0.118 ± 0.004 ± 0.01 mag; extinctions AV = 0.59 ± 0.01 ± 0.02 and 0.37 ± 0.01 ± 0.04 mag; and extinction-to-reddening ratio $R_\mathrm{V}=3.32^{+0.32}_{-0.28}$ and $3.16^{+0.66}_{-0.56}$. Our estimates agree with most estimates from the literature. BaSTI gives systematically higher Fe/H and lower reddenings than DSED. Despite nearly the same metallicity, age, and helium enrichment, these clusters show a considerable horizontal branch (HB) morphology difference, which must therefore be described by another parameter. This parameter must predominantly explain why the least massive HB stars (0.58–0.63 solar masses) are only found within NGC 6809. Probably they have been lost by the core-collapse cluster NGC 6397 during its dynamical evolution and mass segregation. In contrast, NGC 6809 has a very low central concentration and, hence, did not undergo this process.
Abstract
We present new isochrone fits to the colour–magnitude diagrams of the Galactic globular clusters NGC 6362 and NGC 6723. We utilize 22 and 26 photometric filters for NGC 6362 and NGC 6723, ...respectively, from the ultraviolet to mid-infrared using data sets from Hubble Space Telescope, Gaia, unWISE, and other photometric sources. We use models and isochrones from the Dartmouth Stellar Evolution Database (DSED) and Bag of Stellar Tracks and Isochrones (BaSTI) for α-enhanced α/Fe = +0.4 and different helium abundances. The metallicities Fe/H = −1.04 ± 0.07 and −1.09 ± 0.06 are derived from the red giant branch slopes in our fitting for NGC 6362 and NGC 6723, respectively. They agree with spectroscopic estimates from the literature. We find a differential reddening up to ΔE(B − V) = 0.13 mag in the NGC 6723 field due to the adjacent Corona Australis cloud complex. We derive the following for NGC 6362 and NGC 6723, respectively: distances 7.75 ± 0.03 ± 0.15 (statistic and systematic error) and 8.15 ± 0.04 ± 0.15 kpc; ages 12.0 ± 0.1 ± 0.8 and 12.4 ± 0.1 ± 0.8 Gyr; extinctions AV = 0.19 ± 0.04 ± 0.06 and 0.24 ± 0.03 ± 0.06 mag; reddenings E(B − V) = 0.056 ± 0.01 ± 0.02 and 0.068 ± 0.01 ± 0.02 mag. DSED provides systematically lower Fe/H and higher reddenings than BaSTI. However, the models agree in their relative estimates: NGC 6723 is 0.44 ± 0.04 kpc further, 0.5 ± 0.1 Gyr older, ΔE(B − V) = 0.007 ± 0.002 more reddened, and with 0.05 ± 0.01 dex lower Fe/H than NGC 6362. The lower metallicity and greater age of NGC 6723 with respect to NGC 6362 explain their horizontal branch morphology difference. This confirms age as the second parameter for these clusters. We provide lists of the cluster members from the Gaia Data Release 3.
ABSTRACT
The presence of Galactic cirrus is an obstacle for studying both faint objects in our Galaxy and low surface brightness extragalactic structures. With the aim of studying individual cirrus ...filaments in Sloan Digital Sky Survey (SDSS) Stripe 82 data, we develop techniques based on machine learning and neural networks that allow one to isolate filaments from foreground and background sources in the entirety of Stripe 82 with a precision similar to that of the human expert. Our photometric study of individual filaments indicates that only those brighter than 26 mag arcsec−2 in the SDSS r band are likely to be identified in SDSS Stripe 82 data by their distinctive colours in the optical bands. We also show a significant impact of data processing (e.g. flat-fielding, masking of bright stars, and sky subtraction) on colour estimation. Analysing the distribution of filaments’ colours with the help of mock simulations, we conclude that most filaments have colours in the following ranges: 0.55 ≤g − r ≤ 0.73 and 0.01 ≤ r − i ≤ 0.33. Our work provides a useful framework for an analysis of all types of low surface brightness features (cirri, tidal tails, stellar streams, etc.) in existing and future deep optical surveys. For practical purposes, we provide the catalogue of dust filaments.
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