An overlooked brown dwarf neighbour (T7.5 at d~5pc) of the Sun and two additional T dwarfs at about 10pc

Although many new brown dwarf (BD) neighbours have recently been discovered thanks to new sky surveys in the mid- and near-infrared (MIR, NIR), their numbers are still more than five times lower than those of stars in the same volume. Our aim is to detect and classify new BDs to eventually complete their census in the immediate Solar neighbourhood. We combined multi-epoch data from sky surveys at different wavelengths to detect BD neighbours of the Sun by their high proper motion (HPM). We concentrated at relatively bright MIR (w2<13.5) BD candidates from WISE expected to be so close to the Sun that they may also be seen in older NIR (2MASS, DENIS) or even red optical (SDSS i- and z-band, SSS I-band) surveys. With low-resolution NIR spectroscopy we classified the new BDs and estimated their distances and velocities. We have discovered the HPM (pm~470mas/yr) T7.5 dwarf, WISE J0521+1025, which is at d=5.0+-1.3pc from the Sun the nearest known T dwarf in the northern sky, and two early-T dwarfs, WISE J0457-0207 (T2) and WISE J2030+0749 (T1.5), with proper motions of ~120 and ~670mas/yr and distances of 12.5+-3.1pc and 10.5+-2.6pc, respectively. The last one was independently discovered and also classified as a T1.5 dwarf by Mace and coworkers. All three show thin disk kinematics. They may have been overlooked in the past owing to overlapping images and because of problems with matching objects between different surveys and measuring their proper motions.Comment: 7 pages, incl. 6 figures and 1 table, accepted for publication in Astronomy and Astrophysics, minor changes in title, abstract, and conclusion

A spectroscopic survey of the WNL stars in the large magellanic cloud : general properties and binary status

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal

Evolution and fate of very massive stars

There is observational evidence that supports the existence of very massive stars (VMS) in the local universe. First, VMS (Mini ≲ 320 M⊙) have been observed in the Large Magellanic Clouds (LMC). Secondly, there are observed supernovae (SNe) that bear the characteristics of pair creation supernovae (PCSNe, also referred to as pair instability SN) which have VMS as progenitors. The most promising candidate to date is SN 2007bi. In order to investigate the evolution and fate of nearby VMS, we calculated a new grid of models for such objects, for solar, LMC and Small Magellanic Clouds (SMC) metallicities, which covers the initial mass range from 120 to 500 M⊙. Both rotating and non-rotating models were calculated using the GENEVA stellar evolution code and evolved until at least the end of helium burning and for most models until oxygen burning. Since VMS have very large convective cores during the main-sequence phase, their evolution is not so much affected by rotational mixing, but more by mass loss through stellar winds. Their evolution is never far from a homogeneous evolution even without rotational mixing. All the VMS, at all the metallicities studied here, end their life as WC(WO)-type Wolf-Rayet stars. Because of very important mass losses through stellar winds, these stars may have luminosities during the advanced phases of their evolution similar to stars with initial masses between 60 and 120 M⊙. A distinctive feature which may be used to disentangle Wolf-Rayet stars originating from VMS from those originating from lower initial masses would be the enhanced abundances of Ne and Mg at the surface of WC stars. This feature is however not always apparent depending on the history of mass loss. At solar metallicity, none of our models is expected to explode as a PCSN. At the metallicity of the LMC, only stars more massive than 300 M⊙ are expected to explode as PCSNe. At the SMC metallicity, the mass range for the PCSN progenitors is much larger and comprises stars with initial masses between about 100 and 290 M⊙. All VMS in the metallicity range studied here produce either a Type Ib SN or a Type Ic SN but not a Type II SN. We estimate that the progenitor of SN 2007bi, assuming a SMC metallicity, had an initial mass between 160 and 175 M⊙. None of models presented in this grid produces gamma-ray bursts or magnetars. They lose too much angular momentum by mass loss or avoid the formation of a black hole by producing a completely disruptive PCSN

The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit

Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high spatial resolution near-IR photometry. We derive high T* for the WN stars in NGC 3603 (T*~42+/-2 kK) & R136 (T*~53+/-3 kK) plus clumping-corrected dM/dt ~ 2-5x10^-5 Msun/yr which closely agree with theoretical predictions. These stars make a disproportionate contribution to the global budget of their host clusters. R136a1 alone supplies ~7% of N(LyC) of the entire 30 Dor region. Comparisons with stellar models calculated for the main-sequence evolution of 85-500 Msun suggest ages of ~1.5 Myr & M_init in the range 105 - 170 Msun for 3 systems in NGC 3603, plus 165-320 Msun for 4 stars in R136. Our high stellar masses are supported by dynamical mass determinations for the components of NGC 3603 A1. We consider the predicted L_X of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars in such a dense environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa IMF, both clusters are consistent with a tentative upper mass limit of ~300 Msun. The Arches cluster is either too old, exhibits a deficiency of very massive stars, or more likely stellar masses have been underestimated - M_init for the most luminous stars in the Arches cluster approach 200 Msun according to contemporary stellar & photometric results. The potential for stars greatly exceeding 150 Msun within metal-poor galaxies suggests that such pair-instability SNe could occur within the local universe, as has been claimed for SN 2007bi (abridged).Comment: 20 pages, 14 figures, accepted for MNRAS. Version with higher resolution figures is available from http://pacrowther.staff.shef.ac.uk/R136.pdf See also http://www.eso.org/public/news/eso1030/ from Wed 21 from noon (CEST

Plasmacytoid Dendritic Cells Capture and Cross-Present Viral Antigens from Influenza-Virus Exposed Cells

Among the different subsets of dendritic cells (DC), plasmacytoid dendritic cells (PDC) play a unique role in secreting large amounts of type I interferons upon viral stimulation, but their efficiency as antigen-presenting cells has not been completely characterized. We show here, by flow cytometry, with human primary blood PDC and with a PDC cell line, that PDC display poor endocytic capacity for soluble or cellular antigens when compared to monocyte-derived myeloid DC. However, immature PDC efficiently take up cellular material from live influenza-exposed cells, subsequently mature and cross-present viral antigens very efficiently to specific CD8+ T cells. Therefore, during viral infection PDC not only secrete immunomodulatory cytokines, but also recognize infected cells and function as antigen cross-presenting cells to trigger the anti-viral immune response

The After-School-Program of the Erziehungsdepartement Basel-Stadt - Evaluation (1)

In einem Mehrebenendesign wird die Einführung von freiwilligen und modularen Tagesschulangeboten im Kanton Basel-Stadt evaluiert. Dieser im Februar 2009 bereitgestellte Zwischenbericht (Projektlaufzeit 2007 - 2011) liefert Befunde und datenbasierte Einschätzungen zu folgenden Themen: (1) Organisatorische und sozio-demografische Strukturen der Angebote und ihrer Nutzung; (2) Motivation und Einstellungen der beteiligten Fachkräfte (Lehrerinnen/Lehrer; Leitungskräfte der ausserunterrichtlichen Angebote); (3) Kooperation der verschiedenen Funktionsgruppen; (4) Praktische Umsetzung der Tagesschulangebote(u.a. auf der Basis von Beobachtungen); (5) Einschätzungen der Eltern (standardisierte Befragung) und Einschätzungen der Kinder (Interviews und Gruppendiskussionen). Die Autorinnen und Autoren plädieren abschliessend unter anderem dafür, die Professionalisierung des Angebots fortzusetzen und mit einem übergreifenden Qualitätsmanagement zu verbinden, das Angebot konsequent weiter als ein "Angebot-für-alle" auszugestalten sowie Anregungsqualität des Angebots und Partizipationsmöglichkeiten der Kinder zu steigern. (Autor