The evolution of single B-type stars with a large angular momentum content

The database of intermediate mass rotating stellar models presented in the past years by the Geneva Stellar Evolution Group can be used to build synthetic stellar populations that fully account for the effects of stellar rotation. However, up to now we still lacked stellar evolutionary tracks that rotate close to the critical limit during the whole MS phase. This occurs because the flat internal profile of rotation imposed at the Zero-Age MS (ZAMS) is modified by the action of meridional currents immediately after the ZAMS, causing the surface rotational velocity to decrease abruptly until it reaches a quasi-stationary state. We compute stellar models with non-solid rotation at the ZAMS to obtain stellar evolutionary tracks with a larger content of angular momentum, that attain rotational equatorial velocities close to the critical limit throughout their MS phase. These models have a longer MS lifetime and a higher surface chemical enrichment already at the end of the MS, particularly at Z=0.002. Stellar models with solid rotation at the ZAMS adequately represent the overall characteristics and evolution of differentially rotating models of identical angular momentum content, but with a lower initial surface rotational velocity. For these models we recommend to use as the initial rotational rate the values derived once the quasi-stationary state is reached, after the abrupt decrease in surface velocity. Interestingly, the initial equatorial rotational velocities are virtually metallicity independent for the stellar models we computed with the same mass and angular momentum content at the ZAMS. If, as some observational evidence indicates, B-type stars at Z=0.002 rotate with a higher equatorial velocity at the ZAMS than stars with Z=0.014, our finding would indicate that the angular momentum content of B-type stars in the SMC is higher than their Galactic counterparts.Comment: 15 pages, Accepted for publication in A&

The Collaboration of a Principal and a Mathematics Specialist

This article describes the importance of the collaboration between the principal, Patty Granada, and the Mathematics Specialist, Susan Garthwaite, in fostering a highly functioning mathematics program. While many aspects of the collaboration as facilitators for a mathematics program are logical, the messy aspects provide greater challenges. Through continued collaboration, they have come to embrace the messiness which has contributed to an awareness of similar belief systems in their roles. These beliefs include openness to learning and a sincere commitment to bringing out the best in students and teachers. The principal and the Mathematics Specialist share their continued journey in this collaborative relationship that is essential to the success of the mathematics program

Red supergiants and stellar evolution

We review the significant role played by red supergiants (RSGs) in stellar populations, and some challenges and questions they raise for theoretical stellar evolution. We present how metallicity and rotation modify the way stars go to the red part of the Hertzsprung- Russell diagram or come back from it, and how RSGs might keep a trace of their main-sequence evolution. We compare theoretical popu- lation ratios with observed ones.Comment: 11 pages, 5 figures, Betelgeuse workshop, November 2012, Paris. To be published in the European Astronomical Society Publications Series, editors: Pierre Kervella, Thibaut Le Bertre & Guy Perri

Radiation driven winds with rotation: The oblate finite disc correction factor

We have incorporated the oblate distortion of the shape of the star due to the stellar rotation, which modifies the finite disk correction factor (f_D) in the m-CAK hydrodynamical model. We implement a simplified version for the f_D allowing us to solve numerically the non-linear m- CAK momentum equation.We solve this model for a classical Be star in the polar and equatorial directions. The star's oblateness modifies the polar wind, which is now much faster than the spherical one, mainly because the wind receives radiation from a larger (than the spherical) stellar surface. In the equatorial direction we obtain slow solutions, which are even slower and denser than the spherical ones. For the case when the stellar rotational velocity is about the critical velocity, the most remarkable result of our calculations is that the density contrast between the equatorial density and the polar one, is about 100. This result could explain a long-standing problem on Be stars.Comment: 2 pages, to appear in the proceedings of the IAUS 272 on "Active OB stars: structure, evolution, mass loss and critical limits" (Paris, July 19-23, 2010), Cambridge University Press. Editors C. Neiner, G. Wade, G. Meynet and G. Peter

La “tradición de la casa de Elías”: Astronomía, cronología, historia

The Talmudic prophecy known as “ the tradition of Elijah’s house” concedes to the world and to history a maximum duration of something less than six thousand years, distributed in three periods of similar length according to the ‘Void’, the ‘Law’ and the ‘Messiah’. This article focuses on the diffusion of this prophecy in Protestant Europe in the sixteenth and early seventeenth centuries, mainly in Germany, in connection with contemporary eschatological expectation. It also considers the connections with astronomy (great conjunctions, celestial novelties –mainly the nova of 1572 in Cassiopeia– and the decrease of solar eccentricity), chronology and history. In the final analysis, our focus turns to the topic in the work of Helisaeus Röslin and the appropriation of Röslin’s Tabella des Welt Spiegels by Johann Heinrich Alsted in his table Speculum mundi of 1624.La profecía de origen talmúdico conocida como “tradición de la casa de Elías” concede al mundo y a la historia una extensión máxima algo inferior a los seis mil años, distribuidos en tres periodos de duración similar presididos por el ‘Vacío’, la ‘Ley’ y ‘el Mesías’. El artículo estudia la difusión de esta profecía en la Europa protestante del siglo xvi y comienzos del xvii, especialmente en Alemania, en relación con las expectativas escatológicas del momento. Se estudia también la vinculación con la problemática astronómica (grandes conjunciones, novedades celestes, especialmente la nova de 1572 en Casiopea y la disminución de la excentricidad solar), con la cronología y la historia. Finalmente se aborda la presencia del tema en Helisaeus Röslin y la apropiación de la Tabella des Welt Spiegels de Röslin por Johann Heinrich Alsted en su tabla Speculum mundi de 1624