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

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

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

Slow Radiation-Driven Wind Solutions of A-Type Supergiants

The theory of radiation-driven winds succeeded in describing terminal velocities and mass loss rates of massive stars. However, for A-type supergiants the standard m-CAK solution predicts values of mass loss and terminal velocity higher than the observed values. Based on the existence of a slow wind solution in fast rotating massive stars, we explore numerically the parameter space of radiation-driven flows to search for new wind solutions in slowly rotating stars, that could explain the origin of these discrepancies. We solve the 1-D hydrodynamical equation of rotating radiation-driven winds at different stellar latitudes and explore the influence of ionization's changes throughout the wind in the velocity profile. We have found that for particular sets of stellar and line-force parameters, a new slow solution exists over the entire star when the rotational speed is slow or even zero. In the case of slow rotating A-type supergiant stars the presence of this novel slow solution at all latitudes leads to mass losses and wind terminal velocities which are in agreement with the observed values. The theoretical Wind Momentum-Luminosity Relationship derived with these slow solutions shows very good agreement with the empirical relationship. In addition, the ratio between the terminal and escape velocities, which provides a simple way to predict stellar wind energy and momentum input into the interstellar medium, is also properly traced.Comment: 7 Pages, 3 figures, Astrophysical Journal, Accepte

The slow winds of A-type supergiants

The line driven- and rotation modulated-wind theory predicts an alternative slow solution, besides from the standard m-CAK solution, when the rotational velocity is close to the critical velocity. We study the behaviour of the winds of A-type supergiants (Asg) and show that under particular conditions, e.g., when the δ line-force parameter is about 0.25, the slow solution could exist over the whole star, even for the cases when the rotational speed is slow or zero. We discuss density and velocity profiles as well as possible observational conterpart