Winds from Hot Stars

Abstract

This review deals with the winds from "normal" hot stars such as O-stars, Band A-supergiants and Central Stars of Planetary Nebulae with O-type spectra. The advanced diagnostic methods of stellar winds including an assessment of the accuracy of the determinations of global stellar wind parameters (terminal velocities, mass-loss rates, wind momenta and energies) are introduced and scaling relations as a function of stellar parameters are provided. Observational results are interpreted in the framework of the stationary, one-dimensional theory of line driven winds. Systematic effects caused by non-homogeneous structures, time dependence and deviations from spherical symmetry are discussed. The review finishes with a brief description of the rôle of stellar winds as extragalactic distance indicators and as tracers of the chemical composition of galaxies at high redshift.

News on the X-ray emission from hot subdwarf stars

“…They allowed us to detect and investigate the X-ray emission of four additional sdO stars: BD +37 ∘ 442, BD +37 ∘ 1977, Feige 34, and BD+28 ∘ 4211 (Mereghetti & La Palombara 2016). BD +37 ∘ 442 and BD +37 ∘ 1977 are very similar to HD 49798, since they have high luminosity (L BOL ∼ 10 4 L ⊙ ) and low surface gravity (log g ∼ 4); however, both stars are at a distance above 2 kpc, while the estimated distance of HD 49798 is 650 pc (Kudritzki & Simon 1978). On the contrary, Feige 34 and BD+28 ∘ 4211 are 'compact' subdwarf stars, with low luminosity (L BOL < 10 3 L ⊙ ) and high surface gravity (log g > 5); they are also much nearer than the previous stars.…”

“…Stellar winds are a common feature of hot stars, irrespective of their wide range of luminosities, masses, and chemical compositions: among the massive stars, strong mass losses have been observed in Wolf-Rayet, O-type, and B-type stars, while among hot low-mass stars stellar winds have been revealed in the central stars of planetary nebulae and in a few extreme helium (EHe) and O-type subdwarf (sdO) stars (Hamann 2010). According to the radiative line-driven wind theory (Castor et al 1975;Kudritzki & Puls 2000), these winds are accelerated by the photons emitted by the star, which transfer part of their momentum to the wind matter through line absorption and reemission. However, it is well established that these winds are neither steady-state nor homogeneous; they seem to be highly structured on a broad range of spatial scales and several observational results suggest a clumped structure of the stellar winds .…”

Hot Stars: Old-Fashioned or Trendy?

“…With respect to the soft X-ray emission of O stars (detected by Seward et al 1979 andHarnden et al 1979), Cassinelli and Olson (1979) on the ionization structure. Finally, Lucy and White (1980) and Owocki, Castor, and Rybicki (1988) have distinguished themselves with basic theoretical investigations of time-dependent radiation hydrodynamics which describe the creation and development of shocks (for more details about the role of X-rays in the atmospheres of Hot Stars see the reviews of Pauldrach et al 1994b andKudritzki andPuls 2000).…”

“…This relation is based on two important facts: The first one is that, due to the driving mechanism of hot stars, Puls et al 1996, and squares that of CSPNs by Kudritzki et al 1997. Figure from Kudritzki and Puls 2000. the mechanical momentum of the wind flow (v ∞Ṁ ) is mostly a function of photon momentum (L/c) and is therefore related to the luminosity. The second one is that the expression v ∞Ṁ R 1/2 is an almost directly observable quantity.…”

“…Moreover, it is important to note that as a by-product of this relation it can be used as an independent tool for measuring extragalactic distances up to Virgo and Fornax, since it is independent of the observationally unknown masses (for a recent and comprehensive review on this subject see Kudritzki and Puls 2000). Figure 23 shows that the observed behavior of the wind momentum luminosity relation is represented quite well by our improved realistic models, particularly in the case of the two supergiants (ζ Puppis and α Cam) analyzed in Section 7.2.…”