Herwig Dejonghe

Keplerian discs around post-AGB stars: a common phenomenon? ,

Aims. We aim at showing that the broad-band SED characteristics of our sample of post-AGB stars are best interpreted, assuming the circumstellar dust is stored in Keplerian rotating passive discs. Methods. We present a homogeneous and systematic study of the Spectral Energy Distributions (SEDs) of a sample of 51 post-AGB objects. The selection criteria to define the whole sample were tuned to cover the broad-band characteristics of known binary post-AGB stars. The whole sample includes 20 dusty RV Tauri stars from the General Catalogue of Variable Stars (GCVS). We supplemented our own Geneva optical photometry with literature data to cover a broad range of fluxes from the UV to the far-IR. Results. All the SEDs display very similar characteristics: a large IR excess with a dust excess starting near the sublimation temperature, irrespective of the effective temperature of the central star. Moreover, when available, the long wavelength fluxes show a black-body slope indicative of the presence of a component of large mm sized grains. Conclusions. We argue that in all systems, gravitationally bound dusty discs are present. The discs must be puffed-up to cover a large opening angle for the central star and we argue that the discs have some similarity with the passive discs detected around young stellar objects. We interpret the presence of a disc to be a signature for binarity of the central object, but this will need confirmation by long-term monitoring of the radial velocities. We argue that dusty RV Tauri stars are those binaries which happen to be in the Population II instability strip.

Radiative transfer in disc galaxies — III. The observed kinematics of dusty disc galaxies

We present skirt (Stellar Kinematics Including Radiative Transfer), a new Monte Carlo radiative transfer code that allows the calculation of the observed stellar kinematics of a dusty galaxy. The code incorporates the effects of both absorption and scattering by interstellar dust grains, and calculates the Doppler shift of the emerging radiation exactly by taking into account the velocities of the emitting stars and the individual scattering dust grains. The code supports arbitrary distributions of dust through a cellular approach, whereby the integration through the dust is optimized by means of a novel efficient trilinear interpolation technique. We apply our modelling technique to calculate the observed kinematics of realistic models for dusty disc galaxies. We find that the effects of dust on the mean projected velocity and projected velocity dispersion are severe for edge-on galaxies. For galaxies which deviate more than a few degrees from exactly edge-on, the effects are already strongly reduced. As a consequence, dust attenuation cannot serve as a possible way to reconcile the discrepancy between the observed shallow slopes of the inner rotation curves of low surface brightness galaxies and the predictions of cold dark matter cosmological models. For face-on galaxies, the velocity dispersion increases with increasing dust mass owing to scattering, but the effects are limited, even for extended dust distributions. Finally, we show that serious errors can be made when the individual velocities of the dust grains are neglected in the calculations.

Dynamical models of NGC 3115

We present new dynamical models of the S0 galaxy N3115, making use of the available published photometry and kinematics as well as two-dimensional TIGER spectrography. The models are based on a detailed model of the luminosity distribution built using an MGE fit on HST/WFPC2 and ground-based photometric data. We first examined the kinematics in the central 40 arcsec in the light of two-integral f(E,J) models. Jeans equations were used to constrain the mass-to-light ratio, and the central dark mass, the existence of which was suggested by previous studies. The even part of the distribution function was then retrieved via the Hunter & Qian formalism. We thus confirmed that the velocity and dispersion profiles in the central region could be well-fitted with a two-integral model, given the presence of a central dark mass of ∼109 M⊙. However, no two-integral model could fit the h3 profile around a radius of about 25 arcsec where the outer disc dominates the surface brightness distribution. Three-integral analytical models were therefore built using a quadratic programming technique. These models showed that three-integral components do indeed provide a reasonable fit to the kinematics, including the higher Gauss--Hermite moments. Again, models without a central dark mass failed to reproduce the observed kinematics in the central arcsec. This clearly supports the presence of a nuclear black hole of at least 6.5×108 M⊙ in the centre of NGC 3115. These models were finally used to estimate the importance of the dark matter in the outer part of NGC 3115, suggested by the flat stellar rotation curve observed by Capaccioli et al. This study finally points out the difficulty of integrating independently published data in a coherent and consistent way, thus demonstrating the importance of taking into account the details of the instrumental setup and the reduction processes.

The puzzlingly large Ca II triplet absorption in dwarf elliptical galaxies

We present central CaT, PaT, and CaT* indices for a sample of 15 dwarf elliptical galaxies (dEs). Twelve of these have CaT* ~ 7 A and extend the negative correlation between the CaT* index and the central velocity dispersion σ, which was derived for bright elliptical galaxies (Es), down to 20 km s-1 < σ < 55 km s-1. For five dEs, we have independent age and metallicity estimates. Four of these have CaT* ~ 7 A, much higher than expected from their low metallicities (-1.5 < [Z/H] < -0.5). The observed anticorrelation of CaT* as a function of σ or Z is in flagrant disagreement with theory. We discuss some of the amendments that have been proposed to bring the theoretical predictions into agreement with the observed CaT* values of bright Es and how they can be extended to incorporate the observed CaT* values of dEs as well. Moreover, three dEs in our sample have CaT* ~ 5 A, as would be expected for metal-poor stellar systems. Any theory for dE evolution will have to be able to explain the coexistence of low-CaT* and high-CaT* dEs at a given mean metallicity. This could be the first direct evidence that the dE population is not homogeneous and that different evolutionary paths led to morphologically and kinematically similar but chemically distinct objects.

Hubble Space Telescope optical-near-infrared colors of nearby R1/4 and exponential bulges

We have analyzed V, H, and J Hubble Space Telescope (HST) images for a sample of early- to late-type spiral galaxies and have reported elsewhere the statistical frequency of R1/4-law and exponential bulges in our sample as a function of Hubble type and the frequency of occurrence and structural properties of the resolved central nuclei hosted by intermediate- to late-type bulges and disks (see references in the text). Here we use these data to show the following: The V-H color distribution of the R1/4 bulge peaks around V-H ~ 1.3, with a sigma Δ(V-H) ~ 0.1 mag. Assuming a solar metallicity, these values correspond to stellar ages of ≈ 6 ± 3 Gyr. In contrast, the V-H color distribution of the exponential bulges peaks at V-H ~ 0.9 and has a sigma Δ(V-H) ~ 0.4 mag. This likely implies significantly smaller ages and/or lower metallicities for (a significant fraction of the stars in) the exponential bulges compared to the R1/4-law spheroids. Most of the central nuclei hosted by the exponential bulges have V-H and J-H colors that are compatible with relatively unobscured stellar populations. Assuming no or little dust effects, ages 1 Gyr are suggested for these nuclei, which in turn imply masses of about a few 106 to a few 107 M☉, i.e., sufficient to dissolve progenitor bars with masses consistent with those inferred for the exponential bulges by their luminosities. While different bulge-nucleus pairs cover a large range of V-H colors, each bulge-nucleus pair has quite similar V-H colors and thus possibly similar stellar populations. The HST photometric analysis suggests that exponential-type bulge formation is taking place in the local universe and that this process is consistent with being the outcome of secular evolution processes within the disks. The structures that are currently formed inside the disks are quite dissimilar from the old elliptical-like spheroids that are hosted by the early-type disks.We have analysed

Inferring the mass of spherical stellar systems from velocity moments.

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A genetic algorithm for the non‐parametric inversion of strong lensing systems

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A quadratic programming technique for modeling gravitating systems

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HST Optical-NIR Colors of Nearby

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