Jorge Sahade: First Latin American IAU President

Prof. Jorge Sahade (1915-2012) was the first Latin American President of the International Astronomical Union (1985-1988). From then on, he had a very active participation as president, vice-president, and organizing committee member of several Commissions and Divisions of the IAU, related to stellar astrophysics and exchange of astronomers. Prof. J. Sahade was born in Argentina and was one of the first students graduated in astronomy at the National University of La Plata. He served as director of the Astronomical Observatory of Córdoba (1953-1955) and of the Observatory of La Plata (1968-1969). He was the first Dean of the Faculty of Exact Sciences of the National University of La Plata. He promoted the purchase of a 2.15-m diameter telescope, today located in the Complejo Astronómico El Leoncito, San Juan, Argentina. He founded the Institute of Astronomy and Physics of Space (IAFE) in Buenos Aires and was its first director (1971-1974). He was also director of the "Comisión Nacional de Actividades Espaciales" (the Argentina Space Activity Agency) and promoted the inclusion of Argentina as a partnership of the Gemini Observatory. Prof. Sahade also focused on the development of the astronomy in Latin America and this led to the creation of the "Liga Latinoamericana de Astronomía" (nowadays LIADA).Fil: Cidale, Lydia Sonia. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaProceedings of the International Astronomical UnionVienaAustriaInternational Astronomical Unio

Diagnosis of stellar winds and temperature structures in Be stars through the analysis of Mg II lines

We compute non-LTE Mg II line profiles for Be stars by considering 12 energy-level atoms and supposing that the circumstellar medium can be described by an expanding, spherically symmetric flow in which we assume the presence of a chromosphere. The line radiative transfer equation was solved rigorously in spherical coordinates and in the comoving frame. The Mg II line profiles predicted by this model coincide with those observed in some Be stars. The calculation was performed for a range of effective temperatures representative of the B spectral type. We have also analyzed the influence on the line profiles of different temperature and velocity distributions in the circumstellar material. The line spectrum variability of a Be star is interpreted as the result of a variable outward mass flux.Facultad de Ciencias Astronómicas y Geofísica

Estrellas Be: formación de líneas de Mg II en un viento estelar

We have worked out the equations of conservation of mass, momentum and energy for the case of steady outflows in a rotating magnetic field, introducing a polytropic-type relation between pressure and density, with a variable index. We have determined sets of parameters that bring up the critical points that are needed to select a unique solution for the wind and finally, we have chosen those sets that can reproduce better the conditions observed in Be stars. Each solution depends on the choice of six non-dimensional parameters which are related to the fluxes of energy at the base of the wind and at infinity, to the gravitational field, to the effective temperature of the star, to rotation and to the magnetic field. The corotation induced by the magnetic field increases the centrifugal force and accelerates the wind. The choice of a particular functional form for the polytropic index allows the formation of a heating-accelerating region where the temperature increases with expansion and where larger temperatures and larger expansion values are obtained for strongly magnetized flows. Outside this region, temperature and rotation decay and the magnetic field yields two solutions of different asymptotic behaviour; in one solution the velocity accelerates and in the other the velocity decelerates, until they reach constant values.Asociación Argentina de Astronomí

Discussion: winds and magnetic fields of active OB stars

The discussion on winds and magnetic fields of active OB stars was carried out by S. Owoki, G. Wade, M. Cantiello, O. Kochukhov, M. Smith, C. Neiner, T. Rivinius, H. Henrichs and R. Townsend. The topics were the ability to detect small and large scale magnetic fields in massive stars and the need to consider limits on photometric variability of the star surface brightness.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Discussion: winds and magnetic fields of active OB stars

The discussion on winds and magnetic fields of active OB stars was carried out by S. Owoki, G. Wade, M. Cantiello, O. Kochukhov, M. Smith, C. Neiner, T. Rivinius, H. Henrichs and R. Townsend. The topics were the ability to detect small and large scale magnetic fields in massive stars and the need to consider limits on photometric variability of the star surface brightness.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Constraints on the wind structure of O-B stars from theoretical He II lines.

Theoretical profiles of He ii lines in OB stars with an expanding, spherically symmetric atmosphere are computed. The extended atmospheric model is formed by a classical photosphere, characterized by the effective temperature and the surface gravity, and superimposed layers that have different velocity and temperature structure. We solve rigorously the radiative transfer equation, simultaneously with the statistical equilibrium equations for multilevel atoms, by making use of Feautrier’s method in the comoving frame. We discuss the influence on the He ii lines of the hydrodynamic and thermodynamic structure of the atmosphere, paying special attention to those configurations that give rise to emission lines. The main conclusions of our work can be summarized as follows: 1. The shape of the profiles is determined by the velocity gradient at the base of the wind whenever a positive temperature gradient occurs. 2. In O-type stars, the emission-line intensity depends quite sensitively on log g, in agreement with the Walborn luminosity criterion. 3. In addition, we are able to produce emission and absorption profiles that are in qualitative agreement with those observed in O and B stars. 4. We also confirm the previous Cidale & Ringuelet and Venero, Cidale, & Ringuelet results that showed that a warm, extended, and rapidly expanding atmosphere is sufficient to give rise to emission components in the line profiles.Fil: Venero, Roberto Oscar José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica la Plata; ArgentinaFil: Cidale, Lydia Sonia. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Ringuelet, Adela Emilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica la Plata; Argentin

Analytical solutions for radiation-driven winds in massive stars, I: the fast regime

Accurate mass-loss rate estimates are crucial keys in the study of wind properties of massive stars and for testing different evolutionary scenarios. From a theoretical point of view, this implies solving a complex set of differential equations in which the radiation field and the hydrodynamics are strongly coupled. The use of an analytical expression to represent the radiation force and the solution of the equation of motion has many advantages over numerical integrations. Therefore, in this work, we present an analytical expression as a solution of the equation of motion for radiation-driven winds in terms of the force multiplier parameters. This analytical expression is obtained by employing the line acceleration expression given by Villata and the methodology proposed by Müller & Vink. On the other hand, we find useful relationships to determine the parameters for the line acceleration given by Müller & Vink in terms of the force multiplier parameters.Facultad de Ciencias Astronómicas y Geofísica

The wind of rotating B supergiants, I : domains of slow and fast solution regimes

In the scenario of rotating radiation-driven wind theory for massive stars, three types of stationary hydrodynamic solutions are currently known: the classical ( fast) m-CAK solution, the Ω-slow solution that arises for fast rotators, and the so-called δ-slow solution if high values of the δ line-force parameter are allowed independently of the rotation speed. Compared to the fast solution, both ?slow solutions? have lower terminal velocities. As the study ofthe parameter domain for the slow solution is still incomplete, we perform a comprehensive analysis of the distinctive flow regimes for B supergiants that emerge from a fine grid of rotation values, Ω, and various ionizationconditions in the wind (δ) parameter. The wind ionization defines two domains: one for fast outflowing winds and the other for slow expanding flows. Both domains are clear-cut by a gap, where a kink/plateau structure of thevelocity law could exist for a finite interval of δ. The location and width of the gap depend on Teff and Ω. There is a smooth and continuous transition between the Ω-slow and δ-slow regimes, a single Ω δ-slow regime. We discussdifferent situations where the slow solutions can be found and the possibility of a switch between fast and slow solutions in B supergiant winds. We compare the theoretical terminal velocity with observations of B and Asupergiants and find that the fast regime prevails mostly for early B supergiants while the slow wind regime matches better for A and B mid- and late-type supergiants.Fil: Venero, Roberto Oscar José. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Curé, M.. Universidad de Valparaíso; ChileFil: Cidale, Lydia Sonia. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Araya, I.. Universidad de Valparaíso; Chil

Pérdida de masa y pulsaciones estelares : Factores claves en la evolución de las supergigantes azules

Las estrellas supergigantes azules son objetos masivos (M_inicial >10 M_Sol) que se encuentran quemando helio en el núcleo e hidrógeno en las capas más externas. Son consideradas objetos claves en la evolución estelar y en la evolución de sus galaxias huéspedes, ya que poseen fuertes vientos estelares, es decir, partículas cargadas como electrones y protones, que escapan de la superficie de la estrella e interactúan con el medio interestelar circundante. Si bien la teoría de vientos impulsados por radiación (Castor, Abbott y Klein, 1975, ApJ, 195, 157, CAK , Pauldrach et al. 1986, A&A, 164, 86, Curé, M. 2004, ApJ, 614, 929, Curé, Cidale & Granada, 2011, ApJ, 737, 18) describe de manera general el proceso de pérdida de masa, aún se desconocen los mecanismos responsables de sus variaciones y las escalas de variación de las mismas. Se sospecha que esta variabilidad puede estar vinculada al acoplamiento de diferentes modos de pulsación (Aerts, C., Lefever, K., Baglin, A., et al. 2010, A&A, 513, L11) y se desconoce completamente el papel que tienen las pulsaciones en la región del salto de bi-estabilidad en temperatura, ubicado alrededor de los 21000 K, responsable del cambio en el régimen del viento y la velocidad de rotación de las estrellas. El objetivo general del plan de trabajo posdoctoral es la búsqueda de evidencias observacionales que permitan víncular a las pulsaciones estelares con la variabilidad en la tasa de pérdida de masa, y el comportamiento del viento de las estrellas ubicadas en el salto de bi-estabilidad. El entendimiento del fenómeno de pérdida de masa en las estrellas supergigantes azules, junto al análisis de sus causas en el escenario evolutivo adecuado, y de su variabilidad, tendrá impacto en el desarrollo de áreas temáticas vinculadas al estudio de poblaciones estelares, evolución estelar, brotes de formación estelar, supernovas, nucleosíntesis, producción de polvo y cosmología.Universidad Nacional de La Plat

Pérdida de masa y pulsaciones estelares : Factores claves en la evolución de las supergigantes azules

Las estrellas supergigantes azules son objetos masivos (M_inicial >10 M_Sol) que se encuentran quemando helio en el núcleo e hidrógeno en las capas más externas. Son consideradas objetos claves en la evolución estelar y en la evolución de sus galaxias huéspedes, ya que poseen fuertes vientos estelares, es decir, partículas cargadas como electrones y protones, que escapan de la superficie de la estrella e interactúan con el medio interestelar circundante. Si bien la teoría de vientos impulsados por radiación (Castor, Abbott y Klein, 1975, ApJ, 195, 157, CAK , Pauldrach et al. 1986, A&A, 164, 86, Curé, M. 2004, ApJ, 614, 929, Curé, Cidale & Granada, 2011, ApJ, 737, 18) describe de manera general el proceso de pérdida de masa, aún se desconocen los mecanismos responsables de sus variaciones y las escalas de variación de las mismas. Se sospecha que esta variabilidad puede estar vinculada al acoplamiento de diferentes modos de pulsación (Aerts, C., Lefever, K., Baglin, A., et al. 2010, A&A, 513, L11) y se desconoce completamente el papel que tienen las pulsaciones en la región del salto de bi-estabilidad en temperatura, ubicado alrededor de los 21000 K, responsable del cambio en el régimen del viento y la velocidad de rotación de las estrellas. El objetivo general del plan de trabajo posdoctoral es la búsqueda de evidencias observacionales que permitan víncular a las pulsaciones estelares con la variabilidad en la tasa de pérdida de masa, y el comportamiento del viento de las estrellas ubicadas en el salto de bi-estabilidad. El entendimiento del fenómeno de pérdida de masa en las estrellas supergigantes azules, junto al análisis de sus causas en el escenario evolutivo adecuado, y de su variabilidad, tendrá impacto en el desarrollo de áreas temáticas vinculadas al estudio de poblaciones estelares, evolución estelar, brotes de formación estelar, supernovas, nucleosíntesis, producción de polvo y cosmología.Universidad Nacional de La Plat