Rotational Evolution During Type I X-Ray Bursts

The rotation rates of six weakly-magnetic neutron stars accreting in low-mass X-ray binaries have most likely been measured by Type I X-ray burst observations with RXTE. The nearly coherent oscillations detected during the few seconds of thermonuclear burning are most simply understood as rotational modulation of brightness asymmetries on the neutron star surface. We show that, as suggested by Strohmayer and colleagues, the frequency changes of 1-2 Hz observed during bursts are consistent with angular momentum conservation as the burning shell hydrostatically expands and contracts. We calculate how vertical heat propagation through the radiative outer layers of the atmosphere and convection affect the coherence of the oscillation. We show that the evolution of the rotational profile depends strongly on whether the burning layers are composed of pure helium or mixed hydrogen/helium. Our results help explain the absence (presence) of oscillations from hydrogen-burning (helium-rich) bursts that was found by Muno and collaborators. We investigate angular momentum transport within the burning layers and the recoupling of the burning layers with the star. We show that the Kelvin-Helmholtz instability is quenched by the strong stratification, and that mixing between the burning fuel and underlying ashes by the baroclinic instability does not occur. However, the baroclinic instability may have time to operate within the differentially rotating burning layer, potentially bringing it into rigid rotation.Comment: To appear in The Astrophysical Journal; minor corrections made to tables and figure

Convergent development of low-relatedness supercolonies in Myrmica ants.

Many ant species have independently evolved colony structures with multiple queens and very low relatedness among nestmate workers, but it has remained unclear whether low-relatedness kin structures can repeatedly arise in populations of the same species. Here we report a study of Danish island populations of the red ant Myrmica sulcinodis and show that it is likely that such repeated developments occur. Two microsatellite loci were used to estimate genetic differentiation (F(ST)) among three populations and nestmate relatedness within these populations. The F(ST) values were highly significant due to very different allele frequencies among the three populations with relatively few common alleles and relatively many rare alleles, possibly caused by single queen foundation and rare subsequent immigration. Given the isolation of the islands and the low investment in reproduction, we infer that each of the populations was most likely established by a single queen, even though all three extant populations now have within-colony relatedness 95%), and the genetic differentiation of nests showed a significantly positive correlation with the distance between them. Both male-biased sex-ratio and genetic viscosity are expected characteristics of populations where queens have very local dispersal and where new colonies are initiated through nest-budding. Based on a comparison with other M. sulcinodis populations we hypothesise a distinct succession of population types and suggest that this may be a possible pathway to unicoloniality, ie, development towards a complete lack of colony kin structure and unrelated nestmate workers

Reconstructing extreme AMOC events through nudging of the ocean surface: a perfect model approach

While the Atlantic Meridional Overturning Circulation (AMOC) is thought to be a crucial component of the North Atlantic climate, past changes in its strength are challenging to quantify, and only limited information is available. In this study, we use a perfect model approach with the IPSL-CM5A-LR model to assess the performance of several surface nudging techniques in reconstructing the variability of the AMOC. Special attention is given to the reproducibility of an extreme positive AMOC peak from a preindustrial control simulation. Nudging includes standard relaxation techniques towards the sea surface temperature and salinity anomalies of this target control simulation, and/or the prescription of the wind-stress fields. Surface nudging approaches using standard fixed restoring terms succeed in reproducing most of the target AMOC variability, including the timing of the extreme event, but systematically underestimate its amplitude. A detailed analysis of the AMOC variability mechanisms reveals that the underestimation of the extreme AMOC maximum comes from a deficit in the formation of the dense water masses in the main convection region, located south of Iceland in the model. This issue is largely corrected after introducing a novel surface nudging approach, which uses a varying restoring coefficient that is proportional to the simulated mixed layer depth, which, in essence, keeps the restoring time scale constant. This new technique substantially improves water mass transformation in the regions of convection, and in particular, the formation of the densest waters, which are key for the representation of the AMOC extreme. It is therefore a promising strategy that may help to better constrain the AMOC variability and other ocean features in the models. As this restoring technique only uses surface data, for which better and longer observations are available, it opens up opportunities for improved reconstructions of the AMOC over the last few decades

Advances in reconstructing the AMOC using sea surface observations of salinity

The Atlantic meridional overturning circulation (AMOC) is one of the main drivers of climate variability at decadal and longer time scales. As there are no direct multi-decadal observations of this key circulation, the reconstruction of past AMOC variations is essential. This work presents a step forward in reconstructing the AMOC using climate models and time-varying surface nudging of salinity and temperature data, for which independent multi-decadal observed series are available. A number of nudging protocols are explored in a perfect model framework to best reproduce the AMOC variability accommodating to the characteristics of SST and SSS available products. As reference SST products with sufficient space and time coverage are available, we here choose to focus on the limitations associated to SSS products with the goal of providing protocols using independent salinity products. We consider a global gridded dataset and, additionally, a coarser SSS dataset restricted to the Atlantic and with a quite low spatial resolution (order of 10 degrees vs. 2 for the model grid). We show how, using the latter, we can improve the efficiency of the nudging on the AMOC reconstruction by adding a high-resolution annual cycle to the coarse resolution SSS product as well as a spatial downscaling to account for SSS gradient. The final protocol retained for the coarse SSS data is able to reconstruct a 100-year long AMOC period (average of 10.18 Sv and a standard deviation of 1.39 Sv), with a correlation of 0.76 to the target and a RMSE of 0.99 Sv. These values can be respectively compared to 0.85 and 0.75 Sv when using the global salinity surface observations. This work provides a first step towards understanding the limitations and prospects of historical AMOC reconstructions using different sea surface salinity datasets for the surface nudging