Limb-Darkened Radiation-Driven Winds from Massive Stars

We calculated the influence of the limb-darkened finite disk correction factor in the theory of radiation-driven winds from massive stars. We solved the 1-D m-CAK hydrodynamical equation of rotating radiation-driven winds for all three known solutions, i.e., fast, \Omega-slow and \delta-slow. We found that for the fast solution, the mass loss rate is increased by a factor \sim 10%, while the terminal velocity is reduced about 10%, when compared with the solution using a finite disk correction factor from a uniformly bright star. For the other two slow solutions the changes are almost negligible. Although, we found that the limb darkening has no effects on the wind momentum luminosity relationship, it would affect the calculation of synthetic line profiles and the derivation of accurate wind parameters.Comment: Accepted for publication in ApJ. 19 pages, 6 figure

Properties of Regions Forming the FeII Emission Lines in Be Stars

We study FeII and Balmer hydrogen emission lines observed simultaneously of 18 southern Be stars. We use the self-absorption-curve method (SAC) to determine the optical depth regime of FeII emission lines and to derive first insights on the physical properties of their forming regions.Comment: Societe Francaise d'Astronomie et d'Astrophysique, France (2005

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

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

Open clusters: III. Fundamental parameters of B stars in NGC 6087, NGC 6250, NGC 6383 and NGC 6530. B type stars with circumstellar envelopes

Context. Stellar physical properties of star clusters are poorly known. Aims. Our goals are to perform a spectrophotometric study of the B star population in open clusters to derive accurate stellar parameters, search for the presence of circumstellar envelopes, and discuss the characteristics of these stars. Methods. The BCD spectrophotometric system is a powerful tool to obtain fundamental parameters and infer the main properties of open clusters: distance modulus, color excess, and age. We inspected the Balmer discontinuity to seek circumstellar disks and identify Be-star candidates. High resolution spectra in the H$\alpha$ region are used to confirm the Be nature. Results. We provide Teff , log g, Mv , Mbol and spectral types for a sample of 68 stars in the field of the open clusters NGC 6087, NGC 6250, NGC 6383, and NGC 6530, as well as the cluster distances, ages and reddening. Then, based on a sample of 230 B stars in the direction of the 11 open clusters studied along this series of three papers, we report 6 new Be stars, 4 blue straggler candidates, and 15 B-type stars (called Bdd) with a double Balmer discontinuity. We also find that the majority of the Be stars are dwarfs and present a maximum at the spectral type B2-B4 in young and intermediate-age open clusters. Another maximum of Be stars is observed at the spectral type B6-B8 in open clusters older than 40 Myr, where the population of Bdd stars also becomes relevant. Conclusions. Our results support previous statements that the Be phenomenon is present along the whole main sequence band and occurs in very different evolutionary states. We find clear evidence of an increase of stars with circumstellar envelopes with cluster age. The Be phenomenon reaches its maximum in clusters of intermediate age and the number of B stars with circumstellar envelopes (Be+Bdd stars) is also high for the older clusters

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

Rotation in the ZAMS: Be and Bn stars

We show that Be stars belong to a high velocity tail of a single B-type star rotational velocity distribution in the MS. This implies that: 1) the number fraction N(Be)/N(Be+B) is independent of the mass; 2) Bn stars having ZAMS rotational velocities higher than a given limit might become Be stars.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A. Okazak

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

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