From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio

Aims. The main goal of this work is to study element ratios that are important for the formation of planets of different masses. Methods. We study potential correlations between the existence of planetary companions and the relative elemental abundances of their host stars. We use a large sample of FGK-type dwarf stars for which precise Mg, Si, and Fe abundances have been derived using HARPS high-resolution and high-quality data. Results. A first analysis of the data suggests that low-mass planet host stars show higher [Mg/Si] ratios, while giant planet hosts present [Mg/Si] that is lower than field stars. However, we found that the [Mg/Si] ratio significantly depends on metallicity through Galactic chemical evolution. After removing the Galactic evolution trend only the difference in the [Mg/Si] elemental ratio between low-mass planet hosts and non-hosts was present in a significant way. These results suggest that low-mass planets are more prevalent around stars with high [Mg/Si]. Conclusions. Our results demonstrate the importance of Galactic chemical evolution and indicate that it may play an important role in the planetary internal structure and composition. The results also show that abundance ratios may be a very relevant issue for our understanding of planet formation and evolution.

Gibberellin Deficiency Confers Both Lodging and Drought Tolerance in Small Cereals

Tef [Eragrostis tef (Zucc.) Trotter] and finger millet [Eleusine coracana Gaertn] are staple cereal crops in Africa and Asia with several desirable agronomic and nutritional properties. Tef is becoming a life-style crop as it is gluten-free while finger millet has a low glycemic index which makes it an ideal food for diabetic patients. However, both tef and finger millet have extremely low grain yields mainly due to moisture scarcity and susceptibility of the plants to lodging. In this study, the effects of gibberellic acid (GA) inhibitors particularly paclobutrazol (PBZ) on diverse physiological and yield-related parameters were investigated and compared to GA mutants in rice (Oryza sativa L.). The application of PBZ to tef and finger millet significantly reduced the plant height and increased lodging tolerance. Remarkably, PBZ also enhanced the tolerance of both tef and finger millet to moisture deficit. Under moisture scarcity, tef plants treated with PBZ did not exhibit drought-related symptoms and their stomatal conductance was unaltered, leading to higher shoot biomass and grain yield. Semi-dwarf rice mutants altered in GA biosynthesis, were also shown to have improved tolerance to dehydration. The combination of traits (drought tolerance, lodging tolerance and increased yield) that we found in plants with altered GA pathway is of importance to breeders who would otherwise rely on extensive crossing to introgress each trait individually. The key role played by PBZ in the tolerance to both lodging and drought calls for further studies using mutants in the GA biosynthesis pathway in order to obtain candidate lines which can be incorporated into crop-breeding programs to create lodging tolerant and climate-smart crops.

The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits II. Survey description, results, and performances

Context. Young, nearby stars are ideal targets for direct imaging searches for giant planets and brown dwarf companions. After the first-imaged planet discoveries, vast efforts have been devoted to the statistical analysis of the occurence and orbital distributions of giant planets and brown dwarf companions at wide (>= 5-6 AU) orbits. Aims. In anticipation of the VLT/SPHERE planet-imager, guaranteed-time programs, we have conducted a preparatory survey of 86 stars between 2009 and 2013 to identify new faint comoving companions to ultimately analyze the occurence of giant planets and brown dwarf companions at wide (10-2000 AU) orbits around young, solar-type stars. Methods. We used NaCo at VLT to explore the occurrence rate of giant planets and brown dwarfs between typically 0.1 and 8 ''. Diffraction-limited observations in H-band combined with angular differential imaging enabled us to reach primary star-companion brightness ratios as small as 10(-6) at 1.5 ''. Repeated observations at several epochs enabled us to discriminate comoving companions from background objects. Results. During our survey, twelve systems were resolved as new binaries, including the discovery of a new white dwarf companion to the star HD8049. Around 34 stars, at least one companion candidate was detected in the observed field of view. More than 400 faint sources were detected; 90% of them were in four crowded fields. With the exception of HD8049 B, we did not identify any new comoving companions. The survey also led to spatially resolved images of the thin debris disk around HD61005 that have been published earlier. Finally, considering the survey detection limits, we derive a preliminary upper limit on the frequency of giant planets for the semi-major axes of [10, 2000] AU: typically less than 15% between 100 and 500 AU and less than 10% between 50 and 500 AU for exoplanets that are more massive than 5 M-Jup and 10 M-Jup respectively, if we consider a uniform input distribution and a confidence level of 95%. Conclusions. The results from this survey agree with earlier programs emphasizing that massive, gas giant companions on wide orbits around solar-type stars are rare. These results will be part of a broader analysis of a total of similar to 210 young, solar-type stars to bring further statistical constraints for theoretical models of planetary formation and evolution.

Uppermost Limit, Extent, and Fluctuations of the Timberline and Treeline Ecocline in the Swiss Central Alps during the past 11,500 Years

Pollen and macrofossils were analyzed at two sites above today's treeline (or tree limit) in the Swiss Central Alps (Gouillé Loéré, 2503 m a.s.l., and Lengi Egga, 2557 m a.s.l.) to test two contrasting hypotheses about the natural formation of timberline (the upper limit of closed forest) in the Alps. Our results revealed that Pinus cembra--Larix decidua forests near timberline were rather closed between 9000 and 2500 B.C. (9600-4000 ¹⁴C yr BP), when timberline fluctuations occurred within a belt 100-150 m above today's tree limit. The treeline ecocline above timberline was characterized by the mixed occurrence of tree, shrub, dwarf-shrub, and herbaceous species, but it did not encompass more than 100-150 altitudinal meters. The uppermost limit reached by timberline and treeline during the Holocene was ca. 2420 and 2530 m, respectively, i.e., about 120 to 180 m higher than today. Between 3500 and 2500 B.C. (4700-4000 ¹⁴C yr BP) timberline progressively sank by about 300 m, while treeline was lowered only ca. 100 m. This change led to an enlargement of the treeline-ecocline belt (by ca. 300 m) after 2500 B.C. (4000 ¹⁴C yr BP). Above the treeline ecocline, natural meadows dominated by dwarf shrubs (e.g., Salix herbacea) and herbaceous species (e.g., Helianthemum, Taraxacum, Potentialla, Leontodon t., Cerastium alpinum t., Cirsium spinosissimum, Silene exscapa t., and Saxifraga stellaris) have been present since at least 11,000 cal yr ago. In these meadows tree and tall shrub species (>0.5 m) never played a major role. These results support the conventional hypothesis of a narrow ecocline with rather sharp upper timberline and treeline boundaries and imply that today's treeless alpine communities in the Alps are close to a natural stage. Pollen (percentages and influx), stomata, and charcoal data may be useful for determining whether or not a site was treeless. Nevertheless, a reliable and detailed record of past local vegetation near and above timberline is best achieved through the inclusion of macrofossil analysis.