Comoving frame models of hot star winds

Kubát

Krtičková

2016

A&A

Self Cite

1Ú stav teoretické fyziky a astrofyziky, Masarykova univerzita, Kotlářská 2, CZ-611 37 Brno, Czech Republic 2 Astronomickýústav, Akademie vědČeské republiky, Fričova 298, CZ-251 65 Ondřejov, Czech Republic Received ABSTRACTContext. Mass-loss rate is one of the most important stellar parameters. Mass loss via stellar winds may influence stellar evolution and modifies stellar spectrum. Stellar winds of subluminous hot stars, especially subdwarfs, have not been studied thoroughly. Aims. We aim to provide mass-loss rates as a function of subdwarf parameters and to apply the formula for individual subdwarfs, to predict the wind terminal velocities, to estimate the influence of the magnetic field and X-ray ionization on the stellar wind, and to study the interaction of subdwarf wind with mass loss from Be and cool companions. Methods. We used our kinetic equilibrium (NLTE) wind models with the radiative force determined from the radiative transfer equation in the comoving frame (CMF) to predict the wind structure of subluminous hot stars. Our models solve stationary hydrodynamical equations, that is the equation of continuity, equation of motion, and energy equation and predict basic wind parameters. Results. We predicted the wind mass-loss rate as a function of stellar parameters, namely the stellar luminosity, effective temperature, and metallicity. The derived wind parameters (mass-loss rates and terminal velocities) agree with the values derived from the observations. The radiative force is not able to accelerate the homogeneous wind for stars with low effective temperatures and high surface gravities. We discussed the properties of winds of individual subdwarfs. The X-ray irradiation may inhibit the flow in binaries with compact components. In binaries with Be components, the winds interact with the disk of the Be star. Conclusions. Stellar winds exist in subluminous stars with low gravities or high effective temperatures. Despite their low mass-loss rates, they are detectable in the ultraviolet spectrum and cause X-ray emission. Subdwarf stars may lose a significant part of their mass during the evolution. The angular momentum loss in magnetic subdwarfs with wind may explain their low rotational velocities. Stellar winds are especially important in binaries, where they may be accreted on a compact or cool companion.

Section: Description Of the Cmf Wind Modelsmentioning

confidence: 99%

Stellar wind models of subluminous hot stars

Kubát

Krtičková

2016

A&A

Self Cite

“…To predict main-sequence B star mass-loss rates we applied the NLTE wind models of Krtička & Kubát (2010) with a comoving frame (CMF) line force. Our wind models assume stationary and spherically symmetric flow.…”

Section: Description Of Modelsmentioning

confidence: 99%

Mass loss in main-sequence B stars

2014

A&A

115

133

We calculate radiatively driven wind models of main-sequence B stars and provide the wind mass-loss rates and terminal velocities. The main-sequence mass-loss rate strongly depends on the stellar effective temperature. For the hottest B stars the mass-loss rate amounts to 10 −9 M year −1 , while for the cooler ones the mass-loss rate is lower by more than three orders of magnitude. Mainsequence B stars with solar abundance and effective temperatures lower than about 15 000 K (later than spectral type B5) do not have any homogeneous line-driven wind. We predict the wind mass-loss rates for the solar chemical composition and for the modified abundance of heavier elements to study the winds of chemically peculiar stars. The mass-loss rate may either increase or decrease with increasing abundance, depending on the importance of the induced emerging flux redistribution. Stars with overabundant silicon may have homogeneous winds even below the solar abundance wind limit at 15 000 K. The winds of main-sequence B stars lie below the static limit, that is, a static atmosphere solution is also possible. This points to an important problem regarding the initiation of these winds. We discuss the implications of our models for rotational braking, filling the magnetosphere of Bp stars, and for chemically peculiar stars.

“…We used the recent version of our non-local thermodynamic equilibrium (NLTE) wind models (Krtička & Kubát 2010) with a radiative force calculated consistently from the solution of the comoving frame (CMF) radiative transfer equation with actual opacities. Our models are able to predict wind mass-loss rates from the stellar parameters, that is, the effective temperature, mass, radius, and arbitrary chemical composition.…”

Section: Wind Modelsmentioning

confidence: 99%

Effect of rotational mixing and metallicity on the hot star wind mass-loss rates

Kubát

2014

A&A

Self Cite

Hot star wind mass-loss rates depend on the abundance of individual elements. This dependence is usually accounted for assuming scaled solar chemical composition. However, this approach may not be justified in evolved rotating stars. The rotational mixing brings CNO-processed material to the stellar surface, increasing the abundance of nitrogen at the expense of carbon and oxygen, which potentially influences the mass-loss rates. We study the influence of the modified chemical composition resulting from the rotational mixing on the wind parameters, particularly the wind mass-loss rates. We use our non-local thermodynamic equilibrium wind code to predict the wind structure and compare the calculated wind mass-loss rate for the case of scaled solar chemical composition and the composition affected by the CNO cycle. We show that for a higher mass-fraction of heavier elements Z/Z 0.1 the change of chemical composition from the scaled solar to the CNO-processed scaled solar composition does not significantly affect the wind mass-loss rates. The missing line force caused by carbon and oxygen is compensated for by nitrogen line force. However, for a very low-mass fraction of heavier elements Z/Z 0.1 the rotational mixing significantly affects the wind mass-loss rates. Moreover, the decrease of the mass-loss rate with metallicity is stronger at such low metallicities. We study the relevance of the wind momentumluminosity relationship for different metallicities and show that for a metallicity Z/Z 0.1 the relationship displays a large scatter, which depreciates the use of this relationship at the lowest metallicities.