Ultracool dwarf binaries

We review here the multiplicity properties of ultracool dwarfs (spectral type later than M6) observed in three different environments and at three different ages: in the field, where the objects are relatively old (1–5 Gyrs) and isolated, in the Pleiades young (∼120 Myr) open cluster, and in the young (∼5 Myr) Upper Scorpius OB association (USco). While the field and Pleiades populations seem to have very similar properties, the preliminary results obtained in USco might show significant differences. If confirmed, it would mean that the phenomena responsible for the “final” properties of ultracool dwarfs multiplicity are still at work at the age of USco, but are already over at the age of the Pleiades. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical modelling of optical and IR spectra of brown dwarfs and ultracool dwarfs

The current state of theoretical modelling of the spectra of cool dwarfs and ultracool objects are discussed. Fits of synthetical spectral energy distributions (SEDs) in the optical and IR spectral regions to observed spectra show problems of theoretical modelling. Problems of interpretation of observed spectra are more obvious for the case of L and T dwarfs. Still even for late‐M dwarfs the situation is far from being perfect. Some examples of the .ne analysis of the spectra of ultracool dwarfs are presented. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A search for short time-scale I-band variability in ultracool dwarfs

76 hours of CCD photometry of 11 L dwarfs and one T dwarf is described. Three objects, including the T dwarf ε Indi B, showed convincing evidence for variability. A periodicity of 8 h is clearly visible in the light curve of 2MASS J1155395−372735.     

Accretion in brown dwarfs down to nearly planetary masses

We show that in accreting ultra low‐mass stars and brown dwarfs, the CaII λ 8662 emission line flux correlates remarkably well with the mass accretion rate ( ), just as it does in higher mass classical T Tauri stars (CTTs). A straightforward measurement of the CaII flux thus provides an easier determination technique than detailed modeling of the Hα emission line profile (except at the very lowest accretion rates, where CaII does not appear to be in emission for ultra low‐mass objects, and Hα modeling is required). Using optical high‐resolution spectra, we infer from CaII emission for young ultra low‐mass objects down to nearly the deuterium‐burning (planetary‐mass) limit. Our results, in combination with previous determinations of in CTTs, illustrate that the accretion rate declines steeply with mass, roughly as ∝ M_*² (albeit with considerable scatter). A similar relationship has been suggested by previous studies; we extend it down to nearly the planetary regime. The physical reason for this phenomenon is not yet clear; we discuss various possible mechanisms. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Cenozoic Lower Rhine Basin – rifting, sedimentation, and cyclic stratigraphy

In the Cenozoic, the Lower Rhine Basin formed as a rift at the southeastern terminus of the Dutch German Central Graben, while the Rhenish Massif was uplifted. The study focusses on the marginal marine and fluvial fill of the Lower Rhine Basin. A basin model is developed. Support for this study was given by extensive industry outcrop and well data, by new stratigraphical and sedimentological observations. The ingression and subsequent regression of the Cenozoic North Sea is analysed using the concept of base level cyclicity. As the geohistory of the basin was complex, a subsidence curve is constructed. Furthermore, an attempt is made to trace the simultaneous uplift of the Rhenish Massif.