The Wind of Rotating B Supergiants. I. Domains of Slow and Fast Solution Regimes

Abstract

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 of the 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 ionization conditions 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 the velocity law could exist for a finite interval of δ. The location and width of the gap depend on T eff and Ω. There is a smooth and continuous transition between the Ω-slow and δ-slow regimes, a single Ω δ-slow regime. We discuss different 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 A supergiants 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.

Co-existence and switching between fast and Ω-slow wind solutions in rapidly rotating massive stars

“…Higher values of α shift the location of this region towards higher values of Ω, while the width of the region is almost unchanged. For some particular values of δ, we find a gap in the steady state solutions, confirming the results of Venero et al (2016).…”

Ω-slow Solutions and Be Star Disks

“…In order to numerically solve this improved model we developed an iterative procedure to calculate the oblate finite disk correction factor. This is a more general and robust model than the one developed by Müller & Vink (2014b), because it retains the topology of the spherical m-CAK model, i.e., the model can be used to obtain either fast solutions (Friend & Abbott 1986), Ωslow solutions (Curé 2004), or δ-slow solutions Venero et al 2016). When Ω 0.75, our wind model describes a two-component wind regime similar to that obtained by Curé et al (2005).…”

“…It is worth noting that the Hydwind code can only obtain hydrodynamic solutions for some combinations of the parameters of our grid, because not all combinations have physical stationary solutions (Venero et al 2016). Figure 5 shows the density distribution (upper panel) and velocity profiles (lower panel) at the equatorial plane for hydrodynamic models with Ω = 0.90 for our three scenarios.…”

Propiedades de los vientos de las estrellas supergigantes B y estado evolutivo

“…Estos autores reportaron que un cambio en la ionización del viento, caracterizado por el parámetro δ de la fuerza de la línea, define dos regímenes diferentes de vientos estacionarios (rápido y lento). Por otra parte, Venero et al (2016) demuestran que el régimen rápido está siempre presente a altas temperaturas (>25000 K) mientras que en estrellas BSGs de tipo espectral medio podrían desarrollarse vientos rápidos o vientos lentos. Los dos regímenes están separados por una zona de inestabilidad donde existe un flujo no estacionario.…”

“…5.11, en coincidencia con nuestros resultados. Venero et al (2016) proponen que una perturbación en la densidad de la base del viento podría desencadenar un cambio de un régimen lento a uno rápido, y viceversa. Este comportamiento podría explicar la alta variabilidad del viento que encontramos en HD 99953 y 55 Cyg, que son dos estrellas que se ubican en la región de biestabilidad.…”