A-type stars: evolution, rotation and binarity

We discuss the internal structure of stars in the mass range 1.5 to 4 M_sun from the PMS to the subgiant phase with a particular emphasis on the convective core and the convective superficial layers. Different physical aspects are considered such as overshooting, treatment of convection, microscopic diffusion and rotation. Their influence on the internal structure and on the photospheric chemical abundances is briefly described. The role of binarity in determining the observed properties and as a tool to constrain the internal structure is also introduced and the current limits of theories of orbital evolution and of available binary data--sets are discussed. keywords{stars: evolution, stars: binaries: general, stars: rotation}Comment: 11 pages, 7 figures, conference: The A-star Puzzle, IAU Simp. 224, 200

The self-enrichment of galactic halo globular clusters : a clue to their formation ?

We present a model of globular cluster self-enrichment. In the protogalaxy, cold and dense clouds embedded in the hot protogalactic medium are assumed to be the progenitors of galactic halo globular clusters. The massive stars of a first generation of metal-free stars, born in the central areas of the proto-globular cluster clouds, explode as Type II supernovae. The associated blast waves trigger the expansion of a supershell, sweeping all the material of the cloud, and the heavy elements released by these massive stars enrich the supershell. A second generation of stars is born in these compressed and enriched layers of gas. These stars can recollapse and form a globular cluster. This work aims at revising the most often encountered argument against self-enrichment, namely the presumed ability of a small number of supernovae to disrupt a proto-globular cluster cloud. We describe a model of the dynamics of the supershell and of its progressive chemical enrichment. We show that the minimal mass of the primordial cluster cloud required to avoid disruption by several tens of Type II supernovae is compatible with the masses usually assumed for proto-globular cluster clouds. Furthermore, the corresponding self-enrichment level is in agreement with halo globular cluster metallicities.Comment: 12 pages, 7 figures. Accepted for publication in Astronomy and Astrophysic

Purchase of dunes: the first step towards nature restoration along the Flemish coast

In 1997 only 1022ha of the remaining 3800ha of coastal dunes was owned by the Flemish Region. The other 2778ha were mostly private property of real estate development firms, large landowners, individuals and, to a lesser degree, public owned by water collection companies, the Ministry of Defence and a couple of municipalities. The then public owned dunes of the Flemish Region were divided as follows among the different administrations of the Ministry of the Flemish Community: 522ha under competence of the Nature Division, 350ha under that of the Waterways and Coast Division and 150ha under that of the Forestry Division. Most of the areas owned by the Nature Division were already purchased between 1956 and 1990. Lack of personnel, funds and strategic perspective prevented the Flemish Region to pursue an active policy of land purchase along the coast. In 1996 an 'Acquisition Plan for the Coastal Dunes' was drawn up by the Group for Applied Ecology of the University of Antwerp under the supervision of the Nature Division. Parliamentary initiatives, following a political debate organised in the frame of the Life nature project 'ICCI', led to the creation of an 'instrument for the acquisition of coastal dunes' by decision of the Flemish government of 3 February 1998. Since 1998 the 'Instrument for the acquisition of coastal dunes' consists of two members of staff, that were added to the Nature Division, and a special article on the budget of the Flemish government. This budgetary article received an initial annual endowment of EUR 1,735,255 in 1998, EUR 3,222,616 in 1999 and EUR 4,462,083 for each year between 2000 and 2004. That initial endowment has been reduced to EUR 2,546,000 in 2005. A weakness in this financing system is that the possibly annually remaining budget cannot be transferred to the budget of the next year, so that no strategic fund can be built up. The active prospecting by the staff of the Acquisition Instrument has allowed the Nature Division to purchase 480ha of dunes between 1998 and 2004. Nearly all these acquisitions were realised with agreement of the former owner. In execution of the Decree of 21 October 1997 concerning Nature Conservation and the Natural Environment, the right of pre-emption of the Flemish Region has been introduced in most of the legally protected areas of Flanders. In the coastal zone however this right of pre-emption has not led to spectacular results, because of a very strongly fragmented property structure and high ground prices due to land speculation. Although the Acquisition Instrument has obtained very good results, a long way still has to be gone before the goal of public ownership of all remaining coastal dunes will be achieved. Essential improvements of the financial and legal instruments for the purchase of dunes should be the creation of a strategic financial fund, an actualisation of the since long outdated expropriation act and improvement of the right of pre-emption for conservation purposes to be able to fend off land speculation

Symbol-asynchronous transmission in multibeam satellite user down-link : rate regions for novel superposition coding schemes

We consider the forward link of a multibeam satellite system with high spectral reuse and the novel low-complexity transmission and detection strategies from [1]. More specifically, we study the impact of a time offset between the antenna beams that cooperate to simultaneously serve a given user. Assuming Gaussian signaling, we provide closed-form expressions for the achievable rate region. It is demonstrated that, in the absence of timing information at the gateway, this region is not affected by a time offset. Our numerical results further show that, in case timing is known at the gateway, an offset of half a symbol period at both user terminals is optimal in terms of spectral efficiency.Grant numbers : Satellite Network of Experts IV. © 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

Non-coherent rate-splitting for multibeam satellite forward link : practical coding and decoding algorithms

Non-Coherent Rate-Splitting (NCRS) was recently proposed as a practical multiuser coding and decoding scheme to increase the spectral efficiency of multibeam satellite communication systems. In this paper, we further study the practical realization of NCRS. We propose a modified coding scheme (NCRS*) that is robust to a nonzero time offset among beams. In NCRS*, as opposed to NCRS, the beams send independently channel encoded and modulated waveforms. We assess the performance of NCRS* in terms of the achievable rate region. It is shown that NCRS* performs worse than NCRS, but better than or comparable to other competing schemes, which, as opposed to NCRS*, require flexible bandwidth allocation or perfect synchronization at the transmitter. We also propose a new N-MAP algorithm for the practical implementation of NCRS* receivers. Similar to the existing UMAP algorithm, N-MAP takes into account the modulation used by, and the time offset between, the signals received from the different beams. In most cases, however, N-MAP has a significantly lower complexity than U-MAP

An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry

The O9V star HD 46202, which is a member of the young open cluster NGC 2244, was observed by the CoRoT satellite in October/November 2008 during a short run of 34 days. From the very high-precision light curve, we clearly detect beta Cep-like pulsation frequencies with amplitudes of ~0.1 mmag and below. A comparison with stellar models was performed using a chi^2 as a measure for the goodness-of-fit between the observed and theoretically computed frequencies. The physical parameters of our best-fitting models are compatible with the ones deduced spectroscopically. A core overshooting parameter alpha_ov = 0.10 +- 0.05 pressure scale height is required. None of the observed frequencies are theoretically excited with the input physics used in our study. More theoretical work is thus needed to overcome this shortcoming in how we understand the excitation mechanism of pulsation modes in such a massive star. A similar excitation problem has also been encountered for certain pulsation modes in beta Cep stars recently modelled asteroseismically.Comment: Accepted for publication in Astronomy and Astrophysics on 17/12/2010, 9 pages, 7 figures, 4 table

Periodic mass loss episodes due to an oscillation mode with variable amplitude in the hot supergiant HD50064

We aim to interpret the photometric and spectroscopic variability of the luminous blue variable supergiant HD\,50064 ($V=8.21$).CoRoT space photometry and follow-up high-resolution spectroscopy, with a time base of 137\,d and 169\,d, respectively, was gathered, analysed and interpreted using standard time series analysis and light curve modelling methods as well as spectral line diagnostics.The space photometry reveals one period of 37\,d, which undergoes a sudden amplitude change with a factor 1.6. The pulsation period is confirmed in the spectroscopy, which additionally reveals metal line radial velocity values differing by $\sim 30\,$km\,s$^{-1}$ depending on the spectral line and on the epoch. We estimate \teff$\sim$13\,500\,K, \logg$\sim$1.5 from the equivalent width of Si lines. The Balmer lines reveal that the star undergoes episodes of changing mass loss on a time scale similar to the changes in the photometric and spectroscopic variability, with an average value of $\log\dot{\rm M}\simeq-5$ (in M$_\odot$\,yr$^{-1}$). We tentatively interpret the 37\,d period as due to a strange mode oscillation.Comment: 4 pages, accepted for publication in Astronomy & Astrophysics Letter