The effect of grain drift on the structure of (Post-) AGB winds

We have developed an implementation for the momentum transfer force in numerical two fluid hydrodynamics. This form of the frictional coupling between gas and grains is consistent with the microscopic interactions between the two components. The coupling force gives rise to a drift velocity of the grains with respect to the gas. We apply this mechanism to the outflow of (Post-) AGB objects. Our numerical hydrodynamics code calculates self consistently the dynamics of these outflows, as well as the nucleation and growth of grains and equilibrium chemistry of the gas. Grain nucleation and growth are processes that depend strongly on the rate of gas-grain collisions. Hence, the drift velocity becomes an important variable. The tight connection between grain chemistry and drift causes the system to become extremely sensitive to small changes in almost any parameter. This may be a cause for deviation from (spherical) symmetry and structure.Comment: To appear in "Asymmetrical Planetary Nebulae II: from Origins to Microstructures" ASP Conference Series, J.H. Kastner, N. Soker, & S.A. Rappaport, ed

Capturing the oxidation of silicon carbide in rocky exoplanetary interiors

Theoretical models predict the condensation of silicon carbide around host stars with C/O ratios higher than 0.65 (cf. C/O$_{\mathrm{Sun}}$ = 0.54), in addition to its observations in meteorites, interstellar medium and protoplanetary disks. Consequently, the interiors of rocky exoplanets born from carbon-enriched refractory material are often assumed to contain large amounts of silicon carbide. Here we aim to investigate the stability of silicon carbide in the interior of carbon-enriched rocky exoplanets and to derive the reaction leading to its transformation. We performed a high-pressure high-temperature experiment to investigate the reaction between a silicon carbide layer and a layer representative of the bulk composition of a carbon-enriched rocky exoplanet. We report the reaction leading to oxidation of silicon carbide producing quartz, graphite, and molten iron silicide. Combined with previous studies, we show that in order to stabilize silicon carbide, carbon saturation is not sufficient, and a complete reduction of Fe$^{2+}$ to Fe$^{0}$ in a planetary mantle is required, suggesting that future spectroscopic detection of Fe$^{2+}$ or Fe$^{3+}$ on the surface of rocky exoplanets would imply the absence of silicon carbide in their interiors.Comment: Accepted for publication in Astronomy & Astrophysic

Obscuring Supersoft X-ray Sources in Stellar Winds

We investigate the possibility of obscuring supersoft X-ray sources in the winds of companion stars. We derive limits on the amount of circumstellar material needed to fully obscure a 'canonical' supersoft X-ray source in the Large Magellanic Cloud, as observed with the Chandra X-ray Observatory.Comment: Conference proceedings for IAU Symposium no. 281 "Binary Paths to Type Ia Supernova Explosions", 5-10 July 2011, Padova, Ital

Formation of dust grains with impurities in red giant winds

Among the several proposed carriers of diffuse interstellar bands (DIB's) are impurities in small dust grains, especially in iron oxide grains (Huffman 1977) and silicate grains (Huffman 1970). Most promising are single ion impurities since they can reproduce the observed band widths (Whittet 1992). These oxygen-rich grains are believed to originate mostly in the mass flows from red giants and in supernovae ejecta (e.g. Gehrz 1989). A question of considerable impact for the origin of DIB's is therefore, whether these grains are produced as mainly clean crystals or as some dirty materials. A formalism has been developed that allows tracking of the heterogeneous growth of a dust grain and its internal structure during the dust formation process. This formalism has been applied to the dust formation in the outflow from a red giant star

How large are the monomers of dust aggregates in planet-forming disks?: Insights from quantitative optical and near-infrared polarimetry

Context: The size of the constituent particles (monomers) of dust aggregates is one of the most uncertain parameters directly affecting collisional growth of aggregates in planet-forming disks. Despite its importance, the monomer size has not yet been meaningfully constrained by disk observations. Aims: We attempt to derive the monomer size from optical and near-infrared (IR) polarimetric observations of planet-forming disks. Methods: We perform a comprehensive parameter survey on the degree of linear polarization of light scattered by dust aggregates, using an exact numerical method called the $T$-matrix method. We investigate the effect of the monomer size, aggregate size, porosity, and composition on the degree of polarization. The obtained results are then compared with observed polarization fractions of several planet-forming disks at optical and near-IR wavelengths. Results: It is shown that the degree of polarization of aggregates depends sensitively on the monomer size unless the monomer size parameter is smaller than one or two. Comparing the simulation results with the disk observations, we find that the monomer radius is no greater than $0.4~\mu$m. The inferred monomer size is therefore similar to subunit sizes of the solar system dust aggregates and the maximum size of interstellar grains. Conclusions: Optical and near-IR quantitative polarimetry will provide observational grounds on the initial conditions for dust coagulation and thereby planetesimal formation in planet-forming disks.Comment: 17 pages, 12 figures, 1 table; Accepted for publication in A&

Probing the turbulent mixing strength in protoplanetary disks across the stellar mass range: no significant variations

Dust settling and grain growth are the first steps in the planet-formation process in protoplanetary disks. These disks are observed around stars with different spectral types, and there are indications that the disks around lower mass stars are significantly flatter, which could indicate that they settle and evolve faster, or in a different way. We aim to test this assumption by modeling the median spectral energy distributions (SEDs) of three samples of protoplanetary disks: around Herbig stars, T Tauri stars and brown dwarfs. We focus on the turbulent mixing strength to avoid a strong observational bias from disk and stellar properties that depend on stellar mass. We generated SEDs with the radiative transfer code MCMax, using a hydrostatic disk structure and settling the dust in a self-consistent way with the alpha-prescription to probe the turbulent mixing strength. We are able to fit all three samples with a disk with the same input parameters, scaling the inner edge to the dust evaporation radius and disk mass to millimeter photometry. The Herbig stars require a special treatment for the inner rim regions, while the T-Tauri stars require viscous heating, and the brown dwarfs lack a good estimate of the disk mass because only few millimeter detections exist. We find that the turbulent mixing strength does not vary across the stellar mass range for a fixed grain size distribution and gas-to-dust ratio. Regions with the same temperature have a self-similar vertical structure independent of stellar mass, but regions at the same distance from the central star appear more settled in disks around lower mass stars. We find a relatively low turbulent mixing strength of alpha = 10^(-4) for a standard grain size distribution, but our results are also consistent with alpha = 0.01 for a grain size distribution with fewer small grains or a lower gas-to-dust ratio.Comment: 13 pages, 16 figures, accepted by A&

A tunnel and a traffic jam: How transition disks maintain a detectable warm dust component despite the presence of a large planet-carved gap

We combined hydrodynamical simulations of planet-disk interactions with dust evolution models that include coagulation and fragmentation of dust grains over a large range of radii and derived observational properties using radiative transfer calculations. We studied the role of the snow line in the survival of the inner disk of transition disks. Inside the snow line, the lack of ice mantles in dust particles decreases the sticking efficiency between grains. As a consequence, particles fragment at lower collision velocities than in regions beyond the snow line. This effect allows small particles to be maintained for up to a few Myrs within the first astronomical unit. These particles are closely coupled to the gas and do not drift significantly with respect to the gas. For lower mass planets (1$M_{\rm{Jup}}$), the pre-transition appearance can be maintained even longer because dust still trickles through the gap created by the planet, moves invisibly and quickly in the form of relatively large grains through the gap, and becomes visible again as it fragments and gets slowed down inside of the snow line. The global study of dust evolution of a disk with an embedded planet, including the changes of the dust aerodynamics near the snow line, can explain the concentration of millimetre-sized particles in the outer disk and the survival of the dust in the inner disk if a large dust trap is present in the outer disk. This behaviour solves the conundrum of the combination of both near-infrared excess and ring-like millimetre emission observed in several transition disks.Comment: Accepted for publication in A&A (including acknowledgments

Adaptive modification and flexibility of the proteasome system in response to proteasome inhibition

AbstractThe highly conserved ubiquitin–proteasome system is the principal machinery for extralysosomal protein degradation in eukaryotic cells. The 26S proteasome, a large multicatalytic multisubunit protease that processes cell proteins by limited and controlled proteolysis, constitutes the central proteolytic component of the ubiquitin–proteasome system. By processing cell proteins essential for development, differentiation, proliferation, cell cycling, apoptosis, gene transcription, signal transduction, senescence, and inflammatory and stress response, the 26S proteasome plays a key role in the regulation and maintenance of basic cellular processes. Various synthetic and biologic inhibitors with different inhibitory profiles towards the proteolytic activities of the 26S proteasome have been identified recently. Such proteasome inhibitors induce apoptosis and cell cycle arrest preferentially in neoplastic cells. Based on these findings proteasome inhibitors became useful in cancer therapy. However, under the pressure of continuous proteasome inhibition, eukaryotic cells can develop complex adaptive mechanisms to subvert the lethal attack of proteasome inhibitors. Such mechanisms include the adaptive modification of the proteasome system with increased expression, enhanced proteolytic activity and altered subcomplex assembly and subunit composition of proteasomes as well as the expression of a giant oligomeric protease complex, tripeptidyl peptidase II, which partially compensates for impaired proteasome function. Here we review the adaptive mechanisms developed by eukaryotic cells in response to proteasome inhibition. These mechanisms reveal enormous flexibility of the proteasome system and may have implications in cancer biology and therapy