Numerical simulations of the quiet chromosphere

Numerical simulations of the solar chromosphere have become increasingly realistic over the past 5 years. However, many observed chromospheric structures and their behavior are not reproduced. Current models do not show fibrils in Ca ii 8542 Å, and neither reproduce the Ca ii 8542 Å bisector. The emergent H line core intensity computed from the models show granulation instead of chromospheric shocks or fibrils. I discuss these deficiencies and speculate about what physics should be included to alleviate these shortcomings

Time-dependent hydrogen ionisation in 3D simulations of the solar chromosphere. Methods and first results

Context. The hydrogen ionisation degree deviates substantially from statistical equilibrium under the conditions of the solar chromosphere. A realistic description of this atmospheric layer thus must account for time-dependent non-equilibrium effects. Aims. Advancing the realism of numerical simulations of the solar chromosphere by improved numerical treatment of the relevant physics will provide more realistic models that are essential for interpretation of existing and future observations. Methods. An approximate method for solving the rate equations for the hydrogen populations was extended and implemented in the three-dimensional radiation (magneto-)hydrodynamics code CO5BOLD. The method is based on a model atom with six energy levels and fixed radiative rates. It has been tested extensively in one-dimensional simulations. The extended method has been used to create a three-dimensional model that extends from the upper convection zone to the chromosphere. Results. The ionisation degree of hydrogen in our time-dependent simulation is comparable to the corresponding equilibrium value up to 500 km above optical depth unity. Above this height, the non-equilibrium ionisation degree is fairly constant over time and space, and tends to be at a value set by hot propagating shock waves. The hydrogen level populations and electron density are much more constant than the corresponding values for statistical equilibrium, too. In contrast, the equilibrium ionisation degree varies by more than 20 orders of magnitude between hot, shocked regions and cool, non-shocked regions. Conclusions. The simulation shows for the first time in 3D that the chromospheric hydrogen ionisation degree and electron density cannot be calculated in equilibrium. Our simulation can provide realistic values of those quantities for detailed radiative transfer computations

The inner rim structures of protoplanetary discs

The inner boundary of protoplanetary discs is structured by the dramatic opacity changes at the transition from the dust-containing to a dust-free zone. This paper explores the variety and limits of inner rim structures in passively heated dusty discs. For this study, we implemented detailed sublimation physics in a fast Monte Carlo radiative transfer code. We show that the inner rim in dusty discs is not an infinitely sharp wall but a diffuse region which may be narrow or wide. Furthermore, high surface densities and large silicate grains as well as iron and corundum grains decrease the rim radius, from a 2.2 AU radius for small silicates around a 47 L Herbig Ae star typically to 0.4 AU and as close as 0.2 AU. A passive disc with grain growth and a diverse dust composition must thus have a small inner rim radius. Finally, an analytical expression is presented for the rim location as a function of dust, disc and stellar properties

Full two-dimensional radiative transfer modelling of the transitional disk LkCa 15 (Research Note)

Context. With the legacy of Spitzer and current advances in (sub)mm astronomy, a considerable number of so-called “transitional” disks has been identified which are believed to contain gaps or have developped large inner holes, some filled with dust. This may indicate that complex geometries may be a key feature in disk evolution that has to be understood and modelled correctly. The disk around LkCa 15 is such a disk, with a large gap ranging from ∼5–46 AU, as identified by Espaillat et al. (2007, ApJ, 670, L135) using 1+1D radiative transfer modelling. To fit the spectral energy distribution (SED), they propose two possible scenarios for the inner

Polarization properties of real aluminum mirrors; I. Influence of the aluminum oxide layer

In polarimetry, it is important to characterize the polarization properties of the instrument itself to disentangle real astrophysical signals from instrumental effects. This article deals with the accurate measurement and modeling of the polarization properties of real aluminum mirrors, as used in astronomical telescopes. Main goals are the characterization of the aluminum oxide layer thickness at different times after evaporation, and its influence on the polarization properties of the mirror. The full polarization properties of an aluminum mirror are measured with Mueller matrix ellipsometry at different incidence angles and wavelengths. The best fit of theoretical Mueller matrices to all measurements simultaneously is obtained by taking into account a model of bulk aluminum with a thin aluminum oxide film on top of it. Full Mueller matrix measurements of a mirror are obtained with an absolute accuracy of 4.12±0.08 nm in the long term. Although the aluminum oxide layer is established to be thin, it is necessary to consider it to accurately describe the mirror's polarization properties

Why are some A stars magnetic, while most are not?

A small fraction of intermediate-mass main sequence (A and B type) stars have strong, organised magnetic fields. The large majority of such stars, however, show no evidence for magnetic fields, even when observed with very high precision. In this paper we describe a simple model, motivated by qualitatively new observational results, that provides a natural physical explanation for the small fraction of observed magnetic stars

The case for spectropolarimetry with SPEX on EJSM

We present SPEX, our Spectropolarimeter for Planetary EXploration, and its application as science payload for EJSM. SPEX’ novel spectropolarimetry method allows to simultaneously measure radiance spectra (with a resolution of 2 nm) and polarization spectra (with a resolution of 20 nm) from 0.4 to 0.8 μm. Thanks to this novel method, SPEX is small and robust

Star cluster disruption by giant molecular clouds

We investigate encounters between giant molecular clouds (GMCs) and star clusters. We propose a single expression for the energy gain of a cluster due to an encounter with a GMC, valid for all encounter distances and GMC properties. This relation is verified with N-body simulations of cluster–GMC encounters, where the GMC is represented by a moving analytical potential. Excellent agreement is found between the simulations and the analytical work for fractional energy gains of ΔE/|E0| <10 , where |E0| is the initial total cluster energy. The fractional mass loss from the cluster scales with the fractional energy gain as (ΔM/M0) =f(ΔE/|E0|) , where f≃ 0.25 . This is because a fraction 1 −f of the injected energy goes to the velocities of escaping stars, that are higher than the escape velocity. We therefore suggest that the disruption time of clusters, tdis, is best defined as the time needed to bring the cluster mass to zero, instead of the time needed to inject the initial cluster energy. We derive an expression for tdis based on the mass loss from the simulations, taking into account the effect of gravitational focusing by the GMC. Assuming spatially homogeneous distributions of clusters and GMCs with a relative velocity dispersion of σcn, we find that clusters lose most of their mass in relatively close encounters with high relative velocities (∼2σcn) . The disruption time depends on the cluster mass (Mc) and half-mass radius (rh) as tdis= 2.0 S(Mc/104 M⊙)(3.75 pc/rh)3 Gyr , with S≡ 1 for the solar neighbourhood and S scales with the surface density of individual GMCs (Σn) and the global GMC density (ρn) as S∝ (Σnρn)−1 . Combined with the observed relation between rh and Mc, that is, rh∝Mλc, tdis depends on Mc as tdis∝Mγc . The index γ is then defined as γ= 1 − 3λ . The observed shallow relation between cluster radius and mass (e.g. λ≃ 0.1 ), makes the value of the index γ= 0.7 similar to that found from observations and from simulations of clusters dissolving in tidal fields (γ≃ 0.62) . The constant of 2.0 Gyr, which is the disruption time of a 104 M⊙ cluster in the solar neighbourhood, is about a factor of 3.5 shorter than that found from earlier simulations of clusters dissolving under the combined effect of Galactic tidal field and stellar evolution. It is somewhat higher than the observationally determined value of 1.3 Gyr. It suggests, however, that the combined effect of tidal field and encounters with GMCs can explain the lack of old open clusters in the solar neighbourhood. GMC encounters can also explain the (very) short disruption time that was observed for star clusters in the central region of M51, since there ρn is an order of magnitude higher than that in the solar neighbourhoo

Imaging polarimetry of protoplanetary disks: feasibility and usability

Imaging polarimetry is one of the most promising tools to map the structure of faint protoplanetary disks. In order to assess the feasibility of imaging polarimetry of protoplanetary disks and the usability to answer the scientific questions in the field we perform numerical simulations of disks of various geometries and dust properties. We model the expected signal and detailed predictions for current and upcoming imaging polarimeters. This way we can address the question what the diagnostic value of polarimetry is for the structure of the disk and the characteristics of the grains in it

Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph

From observations collected with the ESPaDOnS spectropolarimeter, we report the discovery of magnetic fields at the surface of the mildly accreting classical T Tauri star V2129 Oph. Zeeman signatures are detected, both in photospheric lines and in the emission lines formed at the base of the accretion funnels linking the disc to the protostar, and monitored over the whole rotation cycle of V2129 Oph. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized line profiles. We reconstruct the large-scale magnetic topology at the surface of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to be rather complex, with a dominant octupolar component and a weak dipole of strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar radial field spots located at high latitudes and coinciding with cool dark polar spots at photospheric level. This large-scale field geometry is unusually complex compared to those of non-accreting cool active subgiants with moderate rotation rates. As an illustration, we provide a first attempt at modelling the magnetospheric topology and accretion funnels of V2129 Oph using field extrapolation. We find that the magnetosphere of V2129 Oph must extend to about 7R* to ensure that the footpoints of accretion funnels coincide with the high-latitude accretion spots on the stellar surface. It suggests that the stellar magnetic field succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of V2129 Oph. The magnetospheric geometry we derive produces X-ray coronal fluxes typical of those observed in cTTSs