arXiv subjects

Th. Henning

Publications and source records attributed to Th. Henning.

At least 613 records · Page 34Linked to original sources

arXiv2003

NACO Polarimetric Differential Imaging of TW Hya: A Sharp Look at the Closest T Tauri Disk

We present high-contrast imaging data on the disk of the classical T Tauri star TW Hya. The images were obtained through the polarimetric differential imaging technique with the adaptive optics system NACO. Our commissioning data show the presence of polarized disk emission between 0.1" and 1.4" from the star. We derive the first Ks-band radial polarized intensity distribution. We show that the polarized intensity compares well to shorter wavelengths surface brightness observations and confirm the previously reported gradual slope change around 0.8". These results show the potential of the new polarimetric differential imaging technique at 8m-class telescopes to map the inner regions of protoplanetary disks.

astro-ph↗

arXiv2003

Rosseland and Planck Mean Opacities for Protoplanetary Discs

In this paper, we present mean gas and dust opacities relevant to the physical conditions typical of protoplanetary discs. As the principal absorber for temperatures below ~1,500 K, we consider spherical and aggregate dust particles of various sizes, chemical structure, and porosity, consisting of ice, organics, troilite, silicates, and iron. For higher temperatures, ions, atoms, molecules, and electrons are included as the main opacity sources. Rosseland and Planck mean opacities are calculated for temperatures between 5 K and 10,000 K and gas densities ranging from 10^{-18} g/ccm to 10^{-7} g/ccm. The dependence on the adopted model of dust grains is investigated. We compare our results with recent opacity tables and show how different opacity models affect the calculated hydrodynamical structure of accretion discs.

astro-ph↗

arXiv2003

Database of optical properties of cosmic dust analogues (DOP)

We present a database containing information on different aspects of calculation and usage of the optical properties of small non-spherical particles -- cosmic dust analogues. The main parts of the DOP are a review of available methods of the light scattering theory, collection of light scattering codes, special computational tools, a graphic library of the optical properties (efficiencies, albedo, asymmetry factors, etc.), a database of the optical constants for astronomy, a bibliographic database of light scattering works, and links to related Internet resources. The general purpose of the DOP having the address http://www.astro.spbu.ru/DOP is twofold -- to help scientists to apply the light scattering theory in astronomy and to give students and beginners a possibility quickly to get necessary knowledge on the subject.

astro-ph↗

arXiv2003

First Detection of Millimeter Dust Emission from Brown Dwarf Disks

We report results from the first deep millimeter continuum survey targeting Brown Dwarfs (BDs). The survey led to the first detection of cold dust in the disks around two young BDs (CFHT-BD-Tau 4 and IC348 613), with deep JCMT and IRAM observations reaching flux levels of a few mJy. The dust masses are estimated to be a few Earth masses assuming the same dust opacities as usually applied to TTauri stars.

astro-ph↗

arXiv2003

The First Detailed Look at a Brown Dwarf Disk

The combination of mid-infrared and recent submm/mm measurements allows us to set up the first comprehensive spectral energy distribution (SED) of the circumstellar material around a young Brown Dwarf. Simple arguments suggest that the dust is distributed in the form of a disk. We compare basic models to explore the disk parameters. The modeling shows that a flat disk geometry fits well the observations. A flared disk explains the SED only if it has a puffed-up inner rim and an inner gap much larger than the dust sublimation radius. Similarities and differences with disks around T Tauri stars are discussed.

astro-ph↗

arXiv2003

A new model of composite interstellar dust grains

The approach to model composite interstellar dust grains, using the exact solution to the light scattering problem for multi-layered spheres as suggested by Voshchinnikov & Mathis (1999), is further developed. Heterogeneous scatteres are represented by particles with very large number of shells, each including a homogeneous layer per material considered (amorphous carbon, astronomical silicate and vacuum). The applicability of the effective medium theory (EMT) mostly utilized earlier to approximate inhomogeneous interstellar grains is examined on the basis of the new model. It is shown that the EMT rules generally have an accuracy of several percent in the whole range of particle sizes provided the porosity does not exceed about 50%. For larger porosity, the EMT rules give wrong results. Using the model, we reanalyze various basic features of cosmic dust -- interstellar extinction, scattered radiation, infrared radiation, radiation pressure, etc. As an example of the potential of the model, it is applied to reproduce the extinction curves in the directions to $ζ$ Oph and $σ$ Sco using subsolar cosmic abundances. We also conclude that metallic iron even in negligible amount ($\la 1$ % by the volume fraction) is unlikely to form a layer on or inside a grain because of peculiar absorption of radiation by such particles.

astro-ph↗

arXiv2002

Reduction of chemical networks. I. The case of molecular clouds

We present a new method to analyse and reduce chemical networks and apply this technique to the chemistry in molecular clouds. Using the technique, we investigated the possibility of reducing the number of chemical reactions and species in the UMIST 95 database simultaneously. In addition, we did the same reduction but with the ``objective technique'' in order to compare both methods. We found that it is possible to compute the abundance of carbon monoxide and fractional ionisation accurately with significantly reduced chemical networks in the case of pure gas-phase chemistry. For gas-grain chemistry involving surface reactions reduction is not worthwhile. Compared to the ``objective technique'' our reduction method is more effective but more time-consuming as well.

astro-ph↗

arXiv2002

Migration and Accretion of Protoplanets in 2D and 3D Global Hydrodynamical Simulations

Planet evolution is tightly connected to the dynamics of both distant and close disk material. Hence, an appropriate description of disk-planet interaction requires global and high resolution computations, which we accomplish by applying a Nested-Grid method. Through simulations in two and three dimensions, we investigate how migration and accretion are affected by long and short range interactions. For small mass objects, 3D models provide longer growth and migration time scales than 2D ones do, whereas time lengths are comparable for large mass planets.

astro-ph↗

arXiv2002

Interferometric Detection of Planets/Gaps in Protoplanetary Disks

We investigate the possibility to find evidence for planets in circumstellar disks by infrared and submillimeter interferometry. Hydrodynamical simulations of a circumstellar disk around a solar-type star with an embedded planet of 1 Jupiter mass are presented. On the basis of 3D radiative transfer simulations, images of this system are calculated. These intensity maps provide the basis for the simulation of the interferometers VLTI (equipped with the mid-infrared instrument MIDI) and ALMA. While ALMA will provide the necessary basis for a direct gap and therefore indirect planet detection, MIDI/VLTI will provide the possibility to distinguish between disks with or without accretion on the central star on the basis of visibility measurements.

astro-ph↗

arXiv2002

High Resolution Mid-infrared Imaging of W3(OH)

We present results of our diffraction-limited mid-infrared imaging of the massive star-forming region W3(OH) with SpectroCam-10 on the 5-m Hale telescope at wavelengths of 8.8, 11.7, and 17.9 micron. The thermal emission from heated dust grains associated with the ultracompact HII region W3(OH) is resolved and has a spatial extent of ~2 arcsec in the N band. We did not detect the hot core source W3(H_2O) which implies the presence of at least 12 mag of extinction at 11.7 micron towards this source. These results together with other data were used to constrain the properties of W3(OH) and W3(H_2O) and their envelopes by modelling the thermal dust emission.

astro-ph↗

arXiv2002

Probing Dust around Brown Dwarfs: The naked LP 944-20 and the Disk of Cha Ha2

We present the first mid-infrared (MIR) detection of a field brown dwarf (BD) and the first ground-based MIR measurements of a disk around a young BD candidate. We prove the absence of warm dust surrounding the field BD LP 944-20. In the case of the young BD candidate Cha Ha2, we find clear evidence for thermal dust emission from a disk. Surprisingly, the object does not exhibit any silicate feature as previously predicted. We show that the flat spectrum can be explained by an optically thick flat dust disk.

astro-ph↗

arXiv2002

Multiple scattering of polarized radiation by non-spherical grains: first results

We present the first numerical radiative transfer simulation of multiple light scattering in dust configurations containing aligned non-spherical (spheroidal) dust grains. Such models are especially important if one wants to explain the circular polarization of light, observed in a variety of astronomical objects. The radiative transfer problem is solved on the basis of the Monte Carlo method. Test simulations, confirming the correct numerical implementation of the scattering mechanism, are presented. As a first application, we investigate the linear and circular polarization of light coming from a spherical circumstellar shell. This shell contains perfectly aligned prolate or oblate spheroidal grains. The most remarkable features of the simulated linear polarization maps are so-called polarization null points where the reversal of polarization occurs. They appear in the case when the grain alignment axis is perpendicular to the line of sight. The maps of circular polarization have a sector-like structure with maxima at the ends of lines inclined to the grain alignment axis by $\pm 45\degr$.

astro-ph↗

arXiv2002

Detecting planets in protoplanetary disks: A prospective study

We investigate the possibility to find evidence for planets in circumstellar disks by infrared and submillimeter interferometry. We present simulations of a circumstellar disk around a solar-type star with an embedded planet of 1 Jupiter mass. The three-dimensional (3D) density structure of the disk results from hydrodynamical simulations. On the basis of 3D radiative transfer simulations, images of this system were calculated. The intensity maps provide the basis for the simulation of the interferometers VLTI (equipped with the mid-infrared instrument MIDI) and ALMA. While MIDI/VLTI will not provide the possibility to distinguish between disks with or without a gap on the basis of visibility measurements, ALMA will provide the necessary basis for a direct gap detection.

astro-ph↗

arXiv2001

Effects of Grain Morphology and Impurities on the Infrared Spectra of Silicon Carbide Particles

In this paper we demonstrate that distinguishing between the polytypes of silicon carbide by means of infrared features in small-grain spectra is impossible. Therefore, the infrared spectra of carbon stars, unfortunately, do not provide a means for drawing comparisons between the crystal structures of grains condensed in these environments and found in meteorites. This is proven first by comparing theoretical band profiles calculated for ellipsoidal particles, which show clearly a strong dependence on the axis ratio of the ellipsoids but negligible differences for the two most common polytypes. Second, spectra measured on submicron particle samples in the laboratory do not show any obvious correlation of band position or shape to the polytype. However, we demonstrate by measurements on SiC whiskers that grain shape is able to determine the spectrum completely. A further strong systematic influence on the band profile can be exerted by plasmon-phonon coupling due to conductivity of the SiC material. The latter fact probably is responsible for the confusion in the astronomical literature about spectral properties of SiC grains. We show that, although the conductivity seems to be a common property of many SiC laboratory samples, it is, however, independent of the polytype.

astro-ph↗

arXiv2001

Infrared Observation of Hot Cores

We report on mid-infrared imaging of hot cores performed with SpectroCam-10 and TIMMI2. The observations aimed at the detection of thermal emission presumably associated with the hot cores. Mid-infrared flux measurements are required to improve the luminosity and optical depth estimates for these sources. Results are presented for W3(H$_2$O), G9.62+0.19, G10.47+0.03, and the possible hot core candidate G232.620+0.996. They illustrate that the morphology of these sources cannot be described by simple geometries. Therefore, line-of-sight effects and considerable extinction even at mid-infrared wavelengths must not be neglected.

astro-ph↗

arXiv1999

Extinction and polarization of radiation by absorbing spheroids: shape/size effects and benchmark results

We use the separation of variables and T-matrix methods to calculate the optical properties of homogeneous spheroids with refractive indices from m = 1.3+0.0i up to 3+4i. It is found that the extinction cross-sections for highly absorbing spheroids are normally 1.5 -- 2 times larger than those for spheres of the same volume. The albedo of the non-spherical particles rather slightly depends on the particle shape and is mainly determined by the imaginary part of the refractive index. Beginning at some size, the spheroidal particles do not polarize the transmitted radiation independent of their shape. We also suggest a new approach for axisymmetric particles which would combine the strong aspects of both methods mentioned above and give several values of the cross-sections as benchmarks in tabular form.

astro-ph↗

arXiv1999

The temperature of non-spherical interstellar grains

A model of spheroidal particles is used to calculate the steady-state temperature of dust grains immersed in the interstellar radiation field. It is found that the temperature of non-spherical grains with the aspect ratios a/b <= 2 deviates from that of spheres less than 10%. More elongated or flattened particles are usually cooler than spheres of the same mass and in some cases the temperatures may differ by even about a factor of 2. The shape effects increase with the infrared absorptivity of the grain material and seem to be more important in dark interstellar clouds.

astro-ph↗

arXiv1999

The composition and nature of the dust shell surrounding the binary AFGL 4106

We present infrared spectroscopy and imaging of AFGL~4106. The 2.4-5 micron ISO-SWS spectrum reveals the presence of a cool, luminous star (T_eff ~ 3750 K) in addition to an almost equally luminous F star (T_eff ~ 7250 K). The 5-195 micron SWS and LWS spectra are dominated by strong emission from circumstellar dust. We find that the dust consists of amorphous silicates, with a minor but significant contribution from crystalline silicates. The amorphous silicates consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be excluded but if present they contain much less Fe than the amorphous olivines. Comparison with laboratory data shows that the pure Mg-end members of the crystalline olivine and pyroxene solid solution series are present. In addition, we find strong evidence for simple oxides (FeO and Al2O3) as well as crystalline H2O ice. Several narrow emission features remain unidentified. Modelling of the dust emission using a dust radiation transfer code shows that large grains (~1 micron) must be present and that the abundance of the crystalline silicates is between 7 and 15% of the total dust mass, depending on the assumed enstatite to forsterite ratio, which is estimated to be between 1 and 3. The amorphous and crystalline dust components in the shell do not have the same temperature, implying that the different dust species are not thermally coupled. We find a dust mass of ~3.9 x 10^-2 M_sol expelled over a period of 4 x 10^3 years for a distance of 3.3 kpc. The F-star in the AFGL~4106 binary is likely a post-red-supergiant in transition to a blue supergiant or WR phase.

astro-ph↗