Porous dust grains in debris disks

When modeling the density and grain size distribution in debris disks, the minimum particle size is often significantly larger than the corresponding blowout size. While the dust particles are usually modeled as compact, homogenous spheres, we instead investigate the impact of porosity. The optical properties of porous particles are determined, and the influences of porosity on the blowout size and dust temperatures investigated. Using the method of discrete dipole approximation, we calculate the scattering and absorption cross sections of porous particles and derive the blowout size and the behavior of the dust temperature. We investigate the influence on the beta-ratio. Blowout sizes are calculated for various stellar luminosities and porosities, and an approximation equation is derived to estimate the blowout size as a function of these parameters. Furthermore, we investigate the influence of the porosity on the dust equilibrium temperature. The blowout size increases with the particle porosity and stellar luminosity. The dust temperature of porous particles is lower than the one of the compact spheres, in particular the temperature of blowout grains decreases for porous particles.Comment: 10 pages, 18 figure

The circumstellar disk of FS Tau B - A self-consistent model based on observations in the mid-infrared with NACO -

Protoplanetary disks are a byproduct of the star formation process. In the dense mid-plane of these disks, planetesimals and planets are expected to form. The first step in planet formation is the growth of dust particles from submicrometer-sized grains to macroscopic mm-sized aggregates. The grain growth is accompanied by radial drift and vertical segregation of the particles within the disk. To understand this essential evolutionary step, spatially resolved multi-wavelength observations as well as photometric data are necessary which reflect the properties of both disk and dust. We present the first spatially resolved image obtained with NACO at the VLT in the L$_\text{p}$ band of the near edge-on protoplanetary disk FS Tau B. Based on this new image, a previously published Hubble image in H band and the spectral energy distribution from optical to millimeter wavelengths, we derive constraints on the spatial dust distribution and the progress of grain growth. For this purpose we perform a disk modeling using the radiative transfer code MC3D. Radial drift and vertical sedimentation of the dust are not considered. We find a best-fit model which features a disk extending from $2\,\text{AU}$ to several hundreds AU with a moderately decreasing surface density and $M_\text{disk}=2.8\,\times\,10^{-2}\,\text{M}_\odot$. The inclination amounts to $i=80^\circ$. Our findings indicate that substantial dust grain growth has taken place and that grains of a size equal to or larger than $1\,\text{mm}$ are present in the disk. In conclusion, the parameters describing the vertical density distribution are better constrained than those describing the radial disk structure.Comment: 10 pages, 9 figures, 2 table

The influence of dust grain porosity on the analysis of debris disc observations

Debris discs are often modelled assuming compact dust grains, but more and more evidence for the presence of porous grains is found. We aim at quantifying the systematic errors introduced when modelling debris discs composed of porous dust with a disc model assuming spherical, compact grains. We calculate the optical dust properties derived via the fast, but simple effective medium theory. The theoretical lower boundary of the size distribution -- the so-called 'blowout size' -- is compared in the cases of compact and porous grains. Finally, we simulate observations of hypothetical debris discs with different porosities and feed them into a fitting procedure using only compact grains. The deviations of the results for compact grains from the original model based on porous grains are analysed. We find that the blowout size increases with increasing grain porosity up to a factor of two. An analytical approximation function for the blowout size as a function of porosity and stellar luminosity is derived. The analysis of the geometrical disc set-up, when constrained by radial profiles, are barely affected by the porosity. However, the determined minimum grain size and the slope of the grain size distribution derived using compact grains are significantly overestimated. Thus, the unexpectedly high ratio of minimum grain size to blowout size found by previous studies using compact grains can be partially described by dust grain porosity, although the effect is not strong enough to completely explain the trend.Comment: accepted by MNRA

Der Einfluss poröser Staubkörner auf das Erscheinungsbild zirkumstellarer Scheiben: Beobachtungen und Scheibenmodellierungen

Zirkumstellare Scheiben sind Ansammlungen von Gas und Staub um einen Stern. Sie stellen den Entstehungsort von Planetesimalen und Planeten dar, weshalb die Untersuchung ihres Aufbaus und ihrer Zusammensetzung eine große Bedeutung besitzt. Der in der Scheibe eingebettete Staub bestimmt die Kontinuumsstrahlung der Scheibe, weshalb eine genaue Kenntnis der Zusammensetzung und Form der Staubkörner wichtig ist. In der vorliegenden Arbeit werden dazu die Eigenschaften poröser Staubkörner sowie die Auswirkungen auf das Erscheinungsbild zirkumstellarer Scheiben untersucht. Darüber hinaus werden Beobachtungen und Modellierungen zirkumstellarer Scheiben präsentiert, welche es ermöglichen, Scheiben- und Staubkorneigenschaften einzuschränken. Die Berechnung der optischen Eigenschaften wie die Absorptions- und Streufähigkeit der porösen Staubteilchen erfolgt mit der Methode der Diskreten Dipol-Approximation. Es werden Strahlungstransportsimulationen für eine parametrisierte Dichteverteilung einer protoplanetaren Scheibe durchgeführt, wobei die abgeleiteten optischen Eigenschaften poröser Staubkörner verwendet werden. Dabei werden die räumliche Temperaturverteilung der Scheibe, die spektrale Energieverteilung, Streukarten und Karten der polarisierten Streustrahlung berechnet. Die Ergebnisse unter Verwendung von porösen Staubkörnern werden mit entsprechenden Ergebnissen verglichen, welche mit kompakten, sphärischen Teilchen erzielt werden. Es zeigt sich, dass Kornporosität einen starken Einfluss auf die simulierten Größen besitzt. Weiterhin wird mit dem Wechsel der Polarisationsrichtung in einzelnen Scheibenbereichen ein Effekt festgestellt, welcher diagnos-tisches Potential zum Nachweis verschiedener Staubeigenschaften wie etwa der Porosität besitzt. Des Weiteren werden Studien zu zwei realen zirkumstellaren Scheiben durchgeführt. Dazu wird eine Beobachtung der protoplanetaren Scheibe von FS Tau B mit dem Instrument NACO am Very Large Telescope vorgestellt. In Kombination mit einer räumlich aufgelösten Beobachtung im nahen Infrarot sowie der spektralen Energieverteilung erfolgt mit Hilfe von Strahlungstransportsimulationen eine detaillierte Modellierung der Staubverteilung sowie der Staubeigenschaften der Scheibe von FS Tau B. Das ermittelte Modell kann die wesentlichen Beobachtungsdaten reproduzieren. Weiterhin wird Staubkornwachstum in der Scheibe nachgewiesen. Die zweite Studie untersucht die Scheibe HIP 22263, bei der es sich um eine im Entwicklungsstadium bereits fortgeschrittene, sogenannte Debris-Scheibe handelt. Unter Verwendung von räumlich aufgelösten Beobachtungen im fernen Infrarot sowie der spektralen Energieverteilung wird eine Scheibenmodellierung durchgeführt, die einen relativ schmalen Ring ergibt. Beide Studien geben Einblicke in die räumliche Dichteverteilung und die Staubkorneigenschaften in den Scheiben und leisten damit einen Beitrag zum Verständnis der Scheibenentwicklung und damit der Entstehung von Planeten.Circumstellar disks are accumulations of gas and dust around a star. Since they represent the birth-place of planetesimals and planets, their structure and composition is of great importance. The dust embedded in the disk determines the disk's continuum radiation. Hence a precise knowledge of the composition and shape of the dust particles is necessary. In this thesis the properties of porous dust grains are investigated and the effect of dust grain porosity on the appearance of circumstellar disks is examined. Moreover, observations and modeling of circumstellar disks are presented which enable to constrain the properties of the disks and the dust grains. The calculation of the optical properties such as absorption and scattering of the dust particles are performed using the method of discrete dipole approximation. Radiative transfer simulations are carried out for a parameterized disk-distribution, applying the derived optical properties for the porous dust grains. The spatial temperature distribution of the disk, the spectral energy distribution, scattering maps and maps of the polarized scattered radiation are calculated. The results using porous dust grains are compared with the corresponding results for compact, spherical particles. In conclusion, grain porosity has a strong influence on the quantities simulated. Moreover, an effect is observed which describes a change of the polarization direction in selected disk regions and which possesses a diagnostic potential for detection of different dust properties such as porosity. Furthermore, studies of two real circumstellar disks are conducted. An observation of the protoplanetary disk of FS Tau B using the instrument NACO at the Very Large Telescope is presented. Combining this with a spatially resolved observation in the near-infrared as well as the spectral energy distribution, a detailed modeling of the dust distribution and dust properties of the disk of FS Tau B is constructed using radiative transfer simulations. This model reproduces the essential observational data. Moreover, grain growth is detected in the disk. The second study examines the disk HIP 22263, which is already in an advanced stage of development, classified as debris disk. Using spatially resolved far-infrared observations as well as the spectral energy distribution, disk modeling is performed, which results in a relatively narrow ring. Both studies provide insights into the spatial density distribution and the dust grain properties in the disks and thus contribute to the understanding of disk evolution and the formation of planets

In-situ analysis of optically thick nanoparticle clouds

Nanoparticles grown in reactive plasmas and nanodusty plasmas gain high interest from basic science and technology. One of the great challenges of nanodusty plasmas is the in-situ diagnostic of the nanoparticle size and refractive index. The analysis of scattered light by means of the Mie solution of the Maxwell equations was proposed and used as an in-situ size diagnostic during the past two decades. Today, imaging ellipsometry techniques and the investigation of dense, i. e. optically thick nanoparticle clouds demand for analysis methods to take multiple scattering into account. We present the first 3D Monte-Carlo polarized radiative transfer simulations of the scattered light in a dense nanodusty plasma. This technique extends the existing diagnostic methods for the in-situ analysis of the properties of nanoparticles to systems where multiple scattering can not be neglected.Comment: 5 pages, 5 figure

Intrinsic polarisation of elongated porous dust grains

ALMA observations revealed recently polarised radiation of several protoplanetary disks in the (sub-)millimetre wavelength range. Besides self-scattering of large particles, thermal emission by elongated grains is a potential source for the detected polarisation signal. We calculate the wavelength dependent absorption and intrinsic polarisation of spheroidally shaped, micrometre and sub-millimetre sized dust grains using the discrete dipole approximation. In particular, we analyse the impact of dust grain porosity which appears to be present in disks when small grains coagulate to form larger aggregates. For the first time our results show that (a) the intrinsic polarisation decreases for increasing grain porosity and (b) the polarisation orientation flips by 90 degree for certain ratios of wavelength to grain size. We present a new method to constrain grain porosity and the grain size in protoplanetary disks using multi-wavelength polarisation observations in the far-infrared to millimetre wavelengths. Finally, we find that moderate grain porosities ($\mathcal{P}\lesssim0.7$) potentially explain the observed polarisation fraction in the system HD 142527 while highly porous grains ($\mathcal{P}>0.7$) fail unless the grain's axis ratio is extraordinarily large.Comment: 10 pages, 10 figure

Impact of planetesimal eccentricities and material strength on the appearance of eccentric debris disks

Context: Since circumstellar dust in debris disks is short-lived, dust-replenishing requires the presence of a reservoir of planetesimals. These planetesimals in the parent belt of debris disks orbit their host star and continuously supply the disk with fine dust through their mutual collisions. Aims: We aim to understand effects of different collisional parameters on the observational appearance of eccentric debris disks. Methods: The collisional evolution of selected debris disk configurations was simulated with the numerical code ACE. Subsequently, selected observable quantities are simulated with our newly developed code DMS. The impact of the eccentricity, dynamical excitation, and the material strength is discussed with respect to the grain size distribution, the spectral energy distribution, and spatially resolved images of debris disk systems. Results: The most recognizable features in different collisional evolutions are as follows. First, both the increase of dynamical excitation in the eccentric belt of the debris disk system and the decrease of the material strength of dust particles result in a higher production rate of smaller particles. This reduces the surface brightness differences between the periastron and the apastron sides of the disks. For very low material strengths, the "pericenter glow" phenomenon is reduced and eventually even replaced by the opposite effect, the "apocenter glow". Second, it is possible to constrain the level of collisional activity from the appearance of the disk, for example, the wavelength-dependent apocenter-to-pericenter flux ratio. Within the considered parameter space, the impact of the material strength on the appearance of the disk is stronger than that of dynamical excitation of the system. Finally, we find that the impact of the collisional parameters on the net spectral energy distribution is weak.Comment: Accepted for publication in A&A, 15 pages, 17 figure

Dust survival rates in clumps passing through the Cas A reverse shock -- II. The impact of magnetic fields

Dust grains form in the clumpy ejecta of core-collapse supernovae where they are subject to the reverse shock, which is able to disrupt the clumps and destroy the grains. Important dust destruction processes include thermal and kinetic sputtering as well as fragmentation and grain vaporization. In the present study, we focus on the effect of magnetic fields on the destruction processes. We have performed magneto-hydrodynamical simulations using AstroBEAR to model a shock wave interacting with an ejecta clump. The dust transport and destruction fractions are computed using our post-processing code Paperboats in which the acceleration of grains due to the magnetic field and a procedure that allows partial grain vaporization have been newly implemented. For the oxygen-rich supernova remnant Cassiopeia A we found a significantly lower dust survival rate when magnetic fields are aligned perpendicular to the shock direction compared to the non-magnetic case. For a parallel field alignment, the destruction is also enhanced but at a lower level. The survival fractions depend sensitively on the gas density contrast between the clump and the ambient medium and on the grain sizes. For a low-density contrast of $100$, e.g., $5\,$nm silicate grains are completely destroyed while the survival fraction of $1\,\mu$m grains is $86\,$per cent. For a high-density contrast of $1000$, $95\,$per cent of the $5\,$nm grains survive while the survival fraction of $1\,\mu$m grains is $26\,$per cent. Alternative clump sizes or dust materials (carbon) have non-negligible effects on the survival rate but have a lower impact compared to density contrast, magnetic field strength, and grain size.Comment: Accepted by MNRAS. Author accepted manuscript. Accepted on 23/01/2023. 24 pages, 21 Figure

Hot exozodis: cometary supply without trapping is unlikely to be the mechanism

Excess near-infrared emission is detected around one fifth of main-sequence stars, but its nature is a mystery. These excesses are interpreted as thermal emission from populations of small, hot dust very close to their stars (`hot exozodis'), but such grains should rapidly sublimate or be blown out of the system. To date, no model has fully explained this phenomenon. One mechanism commonly suggested in the literature is cometary supply, where star-grazing comets deposit dust close to the star, replenishing losses from grain sublimation and blowout. However, we show that this mechanism alone is very unlikely to be responsible for hot exozodis. We model the trajectory and size evolution of dust grains released by star-grazing comets, to establish the dust and comet properties required to reproduce hot-exozodi observations. We find that cometary supply alone can only reproduce observations if dust ejecta has an extremely steep size distribution upon release, and the dust-deposition rate is extraordinarily high. These requirements strongly contradict our current understanding of cometary dust and planetary systems. Cometary supply is therefore unlikely to be solely responsible for hot exozodis, so may need to be combined with some dust-trapping mechanism (such as gas or magnetic trapping) if it is to reproduce observations.Comment: 18 pages, 9 figures, accepted for publication in MNRA