Carbon monoxide in low-mass dwarf stars

We compare high-resolution infrared observations of the CO 2–0 bands in the 2.297–2.310 μm region of M dwarfs and one L dwarf with theoretical expectations. We find a good match between the observational and synthetic spectra throughout the 2000–3500 K temperature regime investigated. None the less, for the 2500–3500 K temperature range, the temperatures that we derive from synthetic spectral fits are higher than expected from more empirical methods by several hundred kelvin. In order to reconcile our findings with the empirical temperature scale, it is necessary to invoke warming of the model atmosphere used to construct the synthetic spectra. We consider that the most likely reason for the back-warming is missing high-temperature opacity due to water vapour. We compare the water vapour opacity of the Partridge–Schwenke line list used for the model atmosphere with the output from a preliminary calculation by Barber & Tennyson. While the Partridge–Schwenke line list is a reasonable spectroscopic match for the new line list at 2000 K, by 4000 K it is missing around 25 per cent of the water vapour opacity. We thus consider that the offset between empirical and synthetic temperature scales is explained by the lack of hot water vapour used for computation of the synthetic spectra. For our coolest objects with temperatures below 2500 K, we find best fits when using synthetic spectra which include dust emission. Our spectra also allow us to constrain the rotational velocities of our sources, and these velocities are consistent with the broad trend of rotational velocities increasing from M to L.     

An Auxiliary Tool to Determine the Height of the Boundary Layer

Results from radiosoundings, performed both over land and over sea, show that the ascent rate of a radiosounding balloon, the vertical velocity of the balloon, can be used to determine the height of the boundary layer. In many cases the balloon has a higher ascent rate in the boundary layer and a lower, less variable, ascent rate above. The decrease in ascending velocity appears as a jump at the top of the boundary layer. Two examples of potential temperature profiles for unstable stratification and one profile for stable conditions are shown with the corresponding ascent rates. A comparison between the boundary-layer height determined from potential temperature profiles and from ascent rates is presented for a larger dataset. The different ascent rates of the balloon in the boundary layer and above can be explained by a decrease in drag on the balloon in combination with a lowering of the critical Reynolds number in the boundary layer caused by turbulence. Hence, by simply logging the time from release of a radiosonde, it is possible to obtain additional information that can be used to estimate the height of both the unstable and stable boundary layers.     

Dehydration reactions and micro/nanostructures in experimentally-deformed serpentinites

High-T torsion experiments on lizardite + chrysotile serpentinites produced mineralogical and micro/nanostructural changes, with important implications in rheological properties. High-resolution TEM showed that specimens underwent ductile [by microkinking and (001) interlayer glide] and brittle deformation (by microfracturing), together with dehydration and break-down reactions. Lizardite is affected by polytypic disorder and microkinking [kink axial planes at high angle with respect to (001) planes], that were not present in the initial ordered 1T-lizardite. Chrysotile fibres are deformed, resulting in elliptical cross-sections, with strong loss of interlayer cohesion. Both lizardite and chrysotile break down to a fine intergrowth of olivine (up to 200 nm), talc (up to 30 nm) and poorly-crystalline material. Lizardite-out reaction preferentially occurs at kink axial planes, representing sites of preferential strain and enhanced reactivity; conversely, chrysotile break-down is a bulk process, resulting in large healed olivine aggregates, up to micrometric in size. Overall observations suggest that dehydration and break-down reactions are more advanced in chrysotile than in lizardite.